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Seismotectonics of British Columbia Rogers, Garry Colin 1983

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SEISMOTECTONICS OF BRITISH COLUMBIA by GARRY COLIN ROGERS B.Sc, The University of B r i t i s h Columbia, 1967 M.Sc, The Un i v e r s i t y of Hawaii, 1972 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Geophysics and Astronomy) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA January 1983 © G a r r y C o l i n Rogers, 1983 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 DE-6 (.3/81) ABSTRACT A comprehensive seismotectonic model i s developed to e x p l a i n the s e i s m i c i t y of B r i t i s h Columbia. In order to do t h i s extensive r e v i s i o n s are made to l o c a t i o n and magnitude parameters i n the Canadian Earthquake Data F i l e . F a u l t plane s o l u t i o n s are c a l c u l a t e d f o r a l l earthquakes p o s s i b l e and a l l mechanisms p r e v i o u s l y c a l c u l a t e d are examined and upgraded where necessary. I t i s proposed that the s u b c r u s t a l s u i t e of earthquakes i n the Puget Sound and southern Vancouver I s l a n d region are a r e s u l t of s t r a i n caused by phase changes i n the descending oceanic l i t h o s p h e r e of the subducting Juan de Fuca p l a t e . The c r u s t a l earthquakes above the deeper s e i s m i c i t y can be explained w i t h an oblique subduction model. The d i s t r i b u t i o n of s e i s m i c i t y , the amount of s e i s m i c i t y and the f o c a l mechanisms support these i n t e r p r e t a t i o n s . The l a r g e earthquakes of c e n t r a l Vancouver I s l a n d are probably a r e s u l t of the i n t e r a c t i o n of the E x p l o r e r P l a t e w i t h the o v e r r i d i n g America P l a t e . In the southern Queen C h a r l o t t e Islands t h r u s t i n g components i n the f a u l t plane s o l u t i o n s confirm there i s an element of convergence across the p a c i f i c / A m e r i c a boundary i n t h i s r e g i o n . The d i s t r i b u t i o n of s e i s m i c i t y suggests a l l r e l a t i v e p l a t e motion i s p r e s e n t l y o c c u r r i n g along the Queen C h a r l o t t e f a u l t . The Quaternary volcanoes of B r i t i s h Columbia show l i t t l e c o r r e l a t i o n w i t h the s e i s m i c i t y p a t t e r n except f o r the eastern end of the Anahim v o l c a n i c b e l t . - i i i -TABLE OF CONTENTS I . INTRODUCTION 1 I I . SEISMICITY OF THE VANCOUVER ISLAND - PUGET SOUND REGION 9 A. INTRODUCTION 9 B. HISTORICAL SEISMICITY 12 1) Comments on the data set 12 2) Completeness 13 3) Locations 16 4) Magnitude Revisions 17 5) Comments on r e v i s i o n s of some key earthquakes 22 5) Revised data set 29 C. MODERN SEISMICITY 31 1) Problems w i t h the data set 31 2) The Georgia S t r a i t C r u s t a l model 32 3) Revised data set 35 4) Deeper earthquakes 35 D. CONTEMPORARY SEISMICITY 37 1) Problems w i t h the data set 37 2) Depth d i s t r i b u t i o n 41 3) Shallow earthquakes 43 4) Deeper earthquakes 43 E. RECURRENCE RELATIONSHIPS 47 1) C e n t r a l Vancouver I s l a n d and Puget Sound ..47 2) Shallow and deep earthquakes of Puget Sound 47 F. FOCAL MECHANISMS 50 1) Shallow earthquakes 50 2) Deeper earthquakes 58 G. CONTINENTAL SHELF SEISMICITY 62 H. CONCLUSIONS 68 I I I . SEISMOTECTONICS OF PUGET SOUND - SOUTHERN VANCOUVER ISLAND 70 A. INTRODUCTION 70 B. DEEP SEISMICITY 71 1) Main Seismic Zone 72 2) S t r u c t u r e of the Subducted Slab 73 3) Maximum depth of earthquakes 77 4) Space Problem 78 (a) overlap model 82 (b) shortening model 84 (c) bending model 85 - i v -(d) phase change model 86 (e) space problem above and below the earthquake zone ....97 5) F o c a l Mechanisms 99 6) Other Subduction Zones 103 7) S e i s m i c i t y Outside the Puget Sound Region 105 8) Phase changes and the development of f o r e a r c basins 107 C. SHALLOW SEISMICITY I l l T) D i s t r i b u t i o n of shallow earthquakes 112 2) F o c a l Mechanisms 112 3) The Oblique Subduction Model 114 D. CONCLUSIONS 122 IV. SEISMOTECTONICS OF CENTRAL VANCOUVER ISLAND 125 A. INTRODUCTION 125 B. THE LARGER EARTHQUAKES 125 1) Tectonic S e t t i n g 125 2) H o r i z o n t a l Motion Vectors 128 3) Aftershock P r o p e r t i e s 129 4) Amount of S e i s m i c i t y 132 C. DEEPER GEORGIA STRAIT EVENTS 134 D. CONCLUSIONS 140 V. SEISMICITY AND SEISMOTECTONICS OF THE QUEEN CHARLOTTE REGION 140 A. INTRODUCTION 145 B. EARTHQUAKE LOCATIONS 145 1) Larger earthquakes 145 2) Completeness and Accuracy 149 3) Smaller earthquakes 1951-1980 152 (a) C a l i b r a t i n g events 153 (b) Sandspit F a u l t 156 (c) Queen C h a r l o t t e Sound 159 4) F o c a l depth of Queen C h a r l o t t e earthquakes 163 C. THE LARGEST EARTHQUAKES 165 T) The May 2b, Vil^i earthquake 165 2) The August 22, 1949 earthquake 169 . 3) The June 24, 1970 earthquake 170 D. SEISMIC GAPS 173 1) The northern seismic gap 173 2) The southern seismic gap 175 3) The expected earthquakes 180 - v -E. FOCAL MECHANISMS 181 1) The August 22, 1949 earthquake, M s = 8.1 184 2) The June 24, 1970 earthquake, M s = 7 186 3) June 24, 1970: Foreshock and Aftershock 190 4) February 23, 1976 earthquake, M s = 6.0 190 5) Summary of F a u l t Plane S o l u t i o n s 192 F. CONCLUSIONS 195 VI. SEISMICITY OF BRITISH COLUMBIA'S VOLCANIC REGIONS 199 A. INTRODUCTION 199 B. OBSERVED SEISMICITY 199 T) The S t i k i n e B e l t 199 2) The G a r i b a l d i B e l t 203 3) The Anahim B e l t 203 C. CONCLUSIONS 209 V I I . A SUMMARY OF CONCLUSIONS 210 BIBLIOGRAPHY 214 APPENDIX 1: Revised Parameters f o r earthquakes i n the Vancouver I s l a n d - Puget Sound Region (1900-1950) 228 APPENDIX 2: Revised parameters f o r earthquakes i n the Vancouver I s l a n d - Puget Sound Region (1951-1969) 232 APPENDIX 3: Revised parameters f o r earthquakes i n the Vancouver I s l a n d - Puget Sound Region (1970-1979) 235 APPENDIX 4: Revised parameters f o r earthquakes i n the Queen C h a r l o t t e I s l a n d s Region (1900-1979) 238 - v i -LIST OF TABLES I Completeness of the r e v i s e d data set 15 I I F e l t area and magnitudes 19 I I I Toppozada's f e l t area r e l a t i o n s h i p s 21 IV Georgia S t r a i t C r u s t a l Model 34 V D e t a i l s of earthquakes with f a u l t plane s o l u t i o n s 52 VI Explosions at sea 63 VII Moment rat e s 98 V I I I Tension axes and p l a t e motion 102 IX Aftershocks of Vancouver i s l a n d earthquakes 131 X Completness and accuracy of the r e v i s e d data set 150 XI Residuals from southern Queen C h a r l o t t e c a l i b r a t i n g events ....157 X I I Residuals from northern Queen C h a r l o t t e c a l i b r a t i n g events ....158 X I I I pP depths f o r Queen C h a r l o t t e f a u l t earthquakes 166 XIV Revised parameters f o r 1949 earthquake and aftershocks 171 XV Magnitude f a u l t area/length r e l a t i o n s h i p s 176 XVI Published f a u l t plane s o l u t i o n s 183 - v i i -LIST OF FIGURES 1 The t e c t o n i c s e t t i n g of western North America 2 2 Earthquakes i n B r i t i s h Columbia 5 3 The t e c t o n i c s e t t i n g of Canada's west coast 6 4 D i s t r i b u t i o n of seismograph s t a t i o n s through time 10 5 I s o s e i s m a l maps of some major west coast earthquakes 20 6 Revisions to key h i s t o r i c a l earthquakes 23 7 I s o s e i s m a l map of 1909 earthquake 25 8 Various ep i c e n t r e s of December 6, 1918 earthquake 26 9 Various e p i c e n t r e s of June 23, 1946 earthquake 28 10 Larger h i s t o r i c a l earthquakes 1900-1950 30 11 Georgia S t r a i t c r u s t a l model 33 12 Revised data set 1951-1969 36 13 Deeper earthquakes 1951-1969 38 14 Revised data set 1971-1979 40 15 Depth d i s t r i b u t i o n between 48ON and 49°N 42 16 Cross s e c t i o n through V i c t o r i a and Mount Baker 44 17 Shallow earthquakes 1971-1979 45 18 Deeper earthquakes 1971-1979 46 19 Vancouver I s l a n d and Puget Sound recurrence r e l a t i o n s h i p s 48 20 Shallow and deep recurrence r e l a t i o n s h i p s 49 21 L o c a t i o n of shallow f a u l t plane s o l u t i o n s 51 22 F a u l t plane s o l u t i o n s of shallow s t r i k e - s l i p earthquakes 52 23 F a u l t plane s o l u t i o n s of shallow t h r u s t earthquakes 53 24 Pressure axes of shallow earthquakes 57 25 T e l e s e i s m i c f a u l t plane s o l u t i o n s of 1965 and 1976 earthquakes...59 26 Pressure and t e n s i o n axes of deep earthquakes 60 27 1976 f a u l t plane s o l u t i o n 61 28 L o c a t i o n of deep sea explosions 64 29 L o c a t i o n of e x p l o s i o n s , earthquake, and seismograph s t a t i o n s ....65 30 Seismograms of underwater explosions 66 31 Seismograms of reference earthquake 67 32 Recurrence r e l a t i o n s h i p s f o r Puget Sound earthquakes 72 33 Deeper events i n Puget Sound 1970-1979 74 34 Cross s e c t i o n through c e n t r a l Puget Sound 75 35 Cross s e c t i o n through V i c t o r i a and Mt Baker 76 36 Inland extent of deeper earthquakes 79 37 R i g i d p l a t e model of subducting Juan de Fuca P l a t e 80 38 North-south cross s e c t i o n through subducting p l a t e 81 39 Geometry of overlap i n subducting p l a t e 83 40 Density changes i n subducting oceanic c r u s t 87 41 Phase changes i n subducting p y r o l i t i c mantle 88 42 Shallow phase changes i n subducting p l a t e 89 43 Deep phase changes i n subducting p l a t e 92 44 Moment r a t e f o r deep earthquakes ..95 45 Cartoon of subduction zone 100 46 Depth to magma sources 101 - v i i i -47 Mantle phase changes i n cross s e c t i o n 106 48 Georgia S t r a i t - Puget Sound - Willamette V a l l e y lowlands 109 49 Shallow earthquakes 1970-1978 113 50 Oblique subduction model 115 51 Locked and Unlocked subduction i n New Zealand 117 52 Earthquakes of c e n t r a l Vancouver I s l a n d 126 53 S l i p v e c t o r s f o r Vancouver I s l a n d earthquakes 130 54 S u b c r u s t a l earthquakes 1951-1979 135 55 Composite f o c a l mechanism f o r Texada I s l a n d earthquakes 136 56 Sine f u n c t i o n model of Juan de Fuca p l a t e 138 57 Tectonic s e t t i n g of Queen C h a r l o t t e I s l a n d s 141 58 Major f a u l t s of the Queen C h a r l o t t e Islands 142 59 Queen C h a r l o t t e I s l a n d s earthquakes M _ 5 144 60 E p i c e n t r e s f o r the May 26, 1929 earthquake 147 61 Epi c e n t r e s i n l a n d of the Queen C h a r l o t t e f a u l t 154 62 Sandspit f a u l t event r a t e 160 63 Revisions of e p i c e n t r e s i n Queen C h a r l o t t e Sound 161 64 Queen C h a r l o t t e Islands earthquakes si n c e 1965 164 65 Major earthquakes and after s h o c k zones 168 66 D e t a i l s of southern Queen C h a r l o t t e Islands seismic gap 179 67 F a u l t plane s o l u t i o n of August 22, 1949 earthquake 185 68 F a u l t plane s o l u t i o n of June 24, 1970 earthquake 187 69 S l i p v e c t o r s , f a u l t o r i e n t a t i o n and p l a t e motion 189 70 Foreshock and after s h o c k of June 24, 1970 earthquake 191 71 F a u l t plane s o l u t i o n of February 23, 1976 earthquake 193 72 P o s i t i o n of w e l l defined f a u l t plane s o l u t i o n s 194 73 Queen C h a r l o t t e f a u l t t e c t o n i c model 197 74 Three zones of Quaternary volcanoes i n B r i t i s h Columbia 200 75 Microearthquake a c t i v i t y i n the S t i k i n e v o l c a n i c b e l t 202 76 G a r i b a l d i v o l c a n i c b e l t and seismograph s t a t i o n s 204 77 Anahim v o l c a n i c b e l t and s e i s m i c i t y 205 78 Ea s t e r n end of the Anahim b e l t and seismograph s t a t i o n s 208 - i x -ACKNOWLEDGEMENTS Edu c a t i o n a l leave and f i n a n c i a l support during the period of residence at the U n i v e r s i t y of B r i t i s h Columbia was provided by the E a r t h Physics Branch of the f e d e r a l Department of Energy, Mines and Resources. The manuscript has been g r e a t l y improved as a r e s u l t of the c o n s t r u c t i v e c r i t i c i s m of Dr. R.M. E l l i s and Dr. R.M. Clowes. - 1 -I . INTRODUCTION Some of the pioneering work of p l a t e t e c t o n i c s began o f f the west coast of Canada. A n a l y s i s of the h i s t o r i c Raff and Mason (1961) magnetic survey (Vine and Wilson, 1965; Wilson, 1965b; Vine, 1966) made the area west of Vancouver I s l a n d one of the f i r s t regions i n the world where ocean f l o o r magnetic anomalies were recognized as an a c t i v e spreading ocean ridge system. The f i r s t comprehensive p l a t e motion s t u d i e s were done on the North P a c i f i c (McKenzie and Parker, 1967; Atwater, 1970; S i l v e r , 1971) and lea d to the c o n c l u s i o n that subduction under Vancouver I s l a n d and western North America was a necessary consequence of the p l a t e motion geometry (Figu r e 1 ) . In h i s landmark paper on transform f a u l t s , Wilson (1965a) used the Queen C h a r l o t t e f a u l t boundary as a c l a s s i c example. The t e c t o n i c model of Canada's west coast appeared to be simple, yet as the study of the west coast has progressed over the past decade i t has become c l e a r that both the Juan de Fuca subduction regime and the Queen C h a r l o t t e transform boundary are not t y p i c a l examples (see Keen and Hyndman, 1979 f o r a r e v i e w ) . The subduction regime l a c k s the t y p i c a l s e i s m i c i t y signatures of most subduction zones: a deep B e n i o f f zone of earthquakes and l a r g e t h r u s t earthquakes on the subduction i n t e r f a c e . The t r i p l e j u n c t i o n region between the subduction and transform boundaries i s complicated by the presence of the E x p l o r e r P l a t e , a small subplate that has become detached from the northern end of the Juan de Fuca. This has a profound e f f e c t on both the o f f s h o r e and onshore s e i s m i c i t y . The Queen C h a r l o t t e transform has an element of convergence across i t and thus i t i s not a pure transform, again a f f e c t i n g the s e i s m i c i t y . Complete understanding of these - 2 -F i g u r e 1 The t e c t o n i c s e t t i n g of western North America. Most of the western margin i s dominated by r i g h t l a t e r a l shear between the P a c i f i c and America P l a t e s along the Queen C h a r l o t t e and San Andreas f a u l t systems. Between these f a u l t s the Juan de Fuca P l a t e system i s subducting under North America i n a northeast d i r e c t i o n . The vector t r i a n g l e i n the lower l e f t shows how the Juan de Fuca/America d i r e c t i o n (J/A) i s r e s o l v e d from the Juan de F u c a / P a c i f i c (J/P) and P a c i f i c / A m e r i c a (P/A) v e c t o r s . Vector lengths are c a l i b r a t e d by the s c a l e . - 3 -p l a t e I n t e r a c t i o n c o m p l e x i t i e s can only come about w i t h a d e t a i l e d study of the s e i s m i c i t y and the c o n s t r u c t i o n of seismotectonic models that can be t e s t e d . Good seismotectonic models are a l s o important f o r the assessment of seism i c r i s k . Current p r a c t i s e f o r a c h i e v i n g the best r i s k estimates are to d i v i d e the earthquakes i n t o a number of seismic source zones, each with i t s own seismotectonic model and then to use the combined e f f e c t of these zones to make p r o b a b i l i s t i c estimates of f u t u r e ground motion ( C o r n e l l , 1968; Basham et a l . , 1979; Rogers, 1981). The present Canadian seismic zoning map, adopted i n 1970 (Whitham et a l . , 1970), i s due to be replaced i n the 1980's by a more robust v e r s i o n c a l c u l a t e d by the above method. For t h i s the best current estimate of the seismotectonics of B r i t i s h Columbia i s e s s e n t i a l . I t i s a l s o e s s e n t i a l i n order to c a l c u l a t e s i t e s p e c i f i c r i s k i n such areas as southwestern B r i t i s h Columbia where the population and s e i s m i c i t y p a t t e r n combine to make i t the region i n Canada where the greate s t p o p u l a t i o n i s exposed to the great e s t r i s k . The widespread search f o r new energy resources has a l s o made accurate r i s k assessment e s s e n t i a l i n regions p r e v i o u s l y thought of as f r o n t i e r areas. For a l l of these a seismotectonic understanding i s imperative. The purpose of t h i s d i s s e r t a t i o n i s f i r s t to upgrade the knowledge of earthquakes i n B r i t i s h Columbia and adjacent regions to the point where the s e i s m i c i t y can be used to put e f f e c t i v e c o n s t r a i n t s on t e c t o n i c modelling and then to use the s e i s m i c i t y f o r the development of a comprehensive seismotectonic model f o r B r i t i s h Columbia. Earthquakes i n the Canadian Earthquake Data F i l e f o r western Canada are examined i n d e t a i l and s e v e r a l thousand r e v i s i o n s are made to l o c a t i o n and magnitude parameters. A number of key earthquakes are studi e d i n d e t a i l . F i n a l l y the c h a r a c t e r i s t i c s of - 4 -the s e i s m i c i t y i n the subduction, transform and inland regions are discussed and i n each case a seismotectonic model i s proposed to explain the earthquakes. Previous reviews of s e i s m i c i t y i n western Canada were presented by Milne (1963, 1967), Milne et a l . (1970) and Milne et a l . (1978). The most recent study (Milne et a l . , 1978) was a comprehensive review of the s e i s m i c i t y information i n the Canadian Earthquake Data f i l e to the end of 1975 (Figure 2). That study pointed out the l i m i t a t i o n s of the data set i n terms of completeness, magnitude values, l o c a t i o n s , f o c a l depths and f o c a l mechanisms and i s the s t a r t i n g point for t h i s study. Milne et a l . (1978) emphasized that the earthquakes coincided with the tectonic pattern but the l i m i t a t i o n s of the data set prevented anything more than acknowledging a general agreement. This study c l a r i f i e s the r e l a t i o n s h i p of the s e i s m i c i t y to the tectonic model and uses the s e i s m i c i t y to r e f i n e the model. The tectonic s e t t i n g of western North America i s dominated by r i g h t l a t e r a l motion between the P a c i f i c and America Plates along the San Andreas and Queen Charlotte f a u l t systems (Figure 1). Between these huge transform f a u l t s i s the subduction regime of the Juan de Fuca Plate and i t s associated subplates. The subduction i s i n a northeast d i r e c t i o n r e l a t i v e to the America Plate (Atwater, 1970). The northern part of the Juan de Fuca Plate has become detached along the Nootka Fault zone (Figure 3) and i s known as the Explorer Plate (Riddihough, 1977; Hyndman et a l . , 1979). This small plate moves independently and i n t e r a c t s with the America Plate at a much slower rate and i n a d i f f e r e n t d i r e c t i o n than the Juan de Fuca Plate (Riddihough, 1977). It i s possible i t has ceased to subduct into the mantle (Riddihough, 1981). A complex t r i p l e junction region e x i s t s at the northern end of the Explorer Plate (Riddihough, 1977; Riddihough et a l . , - 5 -F i g u r e 2 The d i s t r i b u t i o n of earthquakes w i t h magnitudes greater or equal to 3.0 i n and around B r i t i s h Columbia from 1899 to 1975. Earthquakes l e s s than magnitude 4.0 are marked w i t h an 'x'. The f i g u r e i s from Milne et a l . 1978. - 6 -F i g u r e 3 D e t a i l of the t e c t o n i c s e t t i n g of Canada's west coast. The E x p l o r e r P l a t e i s an independently moving p l a t e on the no r t h end of the Juan de Fuca P l a t e system separated from the main p l a t e along the Nootka f a u l t zone. Arrows show i n t e r a c t i o n d i r e c t i o n r e l a t i v e to the America P l a t e - 7 -1980; Davis and Riddihough, 1982) which i s marked by intense o f f s h o r e s e i s m i c i t y (Milne et a l . , 1978; Hyndman and Rogers, 1981). North of the t r i p l e j u n c t i o n i s the Queen C h a r l o t t e f a u l t system which r e f l e c t s P a c i f i c / A m e r i c a i n t e r a c t i o n . The s e i s m i c i t y data used here to study the seismotectonics of B r i t i s h Columbia are r e s t r i c t e d mainly to earthquakes on land and on the adjacent c o n t i n e n t a l s h e l f . The deep ocean earthquakes are not considered i n d e t a i l except that i t was necessary to examine the q u a l i t y of a l a r g e number of o f f s h o r e e p i c e n t r e s to determine the number of onshore events that were mislocated o f f s h o r e and v i s a versa. For d i s c u s s i o n the s e i s m i c i t y data set i s d i v i d e d i n t o three subsets based on s i g n i f i c a n t changes i n data q u a l i t y , mainly r e l a t e d to changes i n the d i s t r i b u t i o n of seismograph s t a t i o n s . F i r s t there i s the h i s t o r i c a l s e i s m i c i t y . This i s defined as p r i o r to 1951 when the f i r s t network of h i g h g a i n seismographs appeared on the west coast. The second period covers the years from 1951 to 1970 when there were a l i m i t e d number of modern s t a t i o n s on the west coast. The t h i r d d i v i s i o n , 1970 and l a t e r i s marked by the appearance of the f i r s t dense network of seismograph operations on the west coast, the Puget Sound array operated by the U n i v e r s i t y of Washington. In Chapter I I the s e i s m i c i t y of the subduction regime i s described and analyzed. Then, i n the next chapter a comprehensive seismotectonic model i s proposed r e l a t i n g the c o n c e n t r a t i o n of s e i s m i c i t y i n the southern Vancouver I s l a n d - Puget Sound r e g i o n to the subduction process. The f o l l o w i n g chapter proposes a model to e x p l a i n the presence and character of the l a r g e earthquakes i n c e n t r a l Vancouver I s l a n d . I n Chapter V the s e i s m i c i t y p a t t e r n and seismotectonic model f o r the Queen C h a r l o t t e transform f a u l t r e g i o n i s discussed and i n Chapter VI the s e i s m i c i t y - 8 -a s s o c i a t e d w i t h B r i t i s h Columbia b e l t s of Quaternary volcanoes i s discussed. The main conclusions of the study are summarized i n the f i n a l chapter. Four Appendices c o n t a i n d e t a i l s of the r e v i s i o n s made to the Canadian Earthquake Data F i l e during the course of t h i s study. - 9 -I I . SEISMICITY OF THE VANCOUVER ISLAND - PUGET SOUND REGION A. INTRODUCTION The s e i s m i c i t y data i n the Vancouver I s l a n d - Puget Sound region can be d i v i d e d i n t o three subsets based on s i g n i f i c a n t changes i n data q u a l i t y , mainly r e l a t e d to changes i n the d i s t r i b u t i o n of seismograph s t a t i o n s ( F i g u r e 4). F i r s t there i s the h i s t o r i c a l s e i s m i c i t y . This i s classed as p r i o r to 1951 when the f i r s t network of high gain seismographs appeared on the west coast. The second period covers the years from 1951 to 1970 when there were a l i m i t e d number of modern s t a t i o n s on the west coast. The t h i r d d i v i s i o n , 1970 and l a t e r i s marked by the appearance of the f i r s t dense network of seismograph operations on the west coast, the Puget Sound array operated by the U n i v e r s i t y of Washington. The h i s t o r i c a l data has been l i m i t e d to data 1900 and l a t e r . The f i r s t seismograph appeared on the west coast i n V i c t o r i a i n 1898 and t h i s i s one of the reasons f o r s t a r t i n g the study at the turn of the century. Most e a r l i e r e p i c e n t r e s , and a l l of those before 1917 when the I n t e r n a t i o n a l S e i s m o l o g i c a l Summary (ISS) s t a r t e d up, are based on f e l t r e p o r t s . From 1917 to 1950 there are ep i c e n t r e s f o r many of the l a r g e r west coast events i n the ISS and i n l a t e r years (1940+) from the P r e l i m i n a r y Determination of E p i c e n t r e (PDE) s e r v i c e of the United States Coast and Geodetic Survey (USCGS). Gutenberg and R i c h t e r (1949) a l s o c a l c u l a t e d epicentres f o r many e a r l i e r earthquakes. F e l t r e p o r t s s t i l l play a l a r g e r o l e i n l o c a t i o n a f t e r 1917 because t h i s was before the time of computer processing of s e i s m i c i t y data ( t h a t s t a r t e d i n the mid 1960's) and the philosophy was to - 10 -1900-1950 1951-1969 1970 1978 igure 4 D i s t r i b u t i o n of seismograph s t a t i o n s through time showing s t a t i o n s a c t i v e during the three d i v i s i o n s of the data set discussed i n t h i s chapter. Numbers i n d i c a t e the year the s t a t i o n was e s t a b l i s h e d . The f i r s t seismograph appeared i n 1898, the f i r s t seismograph s u i t a b l e f o r studying small l o c a l earthquakes appeared i n 1948 and the f i r s t dense l o c a l network appeared i n 1970. - 11 -use only a few of the data a v a i l a b l e to l o c a t e the earthquake i n the c o r r e c t part of the globe r a t h e r than to obt a i n the best estimate p o s s i b l e w i t h the data a v a i l a b l e . Thus, many of the published epicentres which have been incorporated i n t o the Canadian Earthquake Data F i l e are inaccurate by as much as 100 km even though data e x i s t s to c a l c u l a t e a more accurate e p i c e n t r e . This time period i s a l s o before the period of r o u t i n e magnitude r e p o r t i n g , so that many earthquakes have no magnitudes or the values present i n the Canadian Earthquake Data F i l e are estimates based on very l i m i t e d data. I n 1951 a t r i a n g l e of s e n s i t i v e short period seismograph s t a t i o n s was e s t a b l i s h e d at V i c t o r i a , Horseshoe Bay, and A l b e r n i (Figure 4) and a s e i s m o l o g i s t , W.G. M i l n e , was t r a n s f e r r e d to V i c t o r i a to s t a r t a study of l o c a l earthquakes on the west coast. The study c a r r i e d on w i t h the same s t a t i o n s f o r 10 years b u i l d i n g up a body of data on which Milne based h i s f i r s t summary of western Canadian s e i s m i c i t y ( M i l n e , 1963). In 1960, Horseshoe Bay s t a t i o n was closed and a new s t a t i o n opened at P e n t i c t o n , 500 km to the east. The coverage of the whole province improved as P e n t i c t o n was a su p e r i o r s t a t i o n , but the l o c a t i o n and d e t e c t i o n c a p a b i l i t y on the coast was l e f t poorer f o r a 15 year period u n t i l a s t a t i o n was e s t a b l i s h e d at Haney (HYC), j u s t east of Vancouver, i n 1975. Thus, the period from 1950 to 1970 represents a mixed q u a l i t y of data. Good s t a t i o n geometry e x i s t e d i n the e a r l y years f o r c o a s t a l s t u d i e s , but t h i s was i n the in f a n c y of the program when instruments were not standardized, instrument problems were common and processing procedures v a r i e d . During the 1960's the s t a t i o n geometry was poorer but the s t a t i o n operation and data handling procedures were much b e t t e r . The f i n a l d i v i s i o n of the data set s t a r t s i n 1970 when the U n i v e r s i t y of Washington i n s t a l l e d a high d e n s i t y a r r a y i n the Puget Sound re g i o n j u s t - 12 -south of the border. For the f i r s t time, determination of f o c a l depth was p o s s i b l e f o r p a r t of southwest B r i t i s h Columbia c l o s e to the border and f o c a l mechanism s t u d i e s could be attempted f o r earthquakes as small as magnitude 4. Data was never exchanged on a re g u l a r b a s i s between the Earth Physics Branch (EPB) and the U n i v e r s i t y of Washington (UW) when l o c a t i n g earthquakes f o r the annual catalogues of Canadian earthquakes and Washington State earthquakes. Depth f o r almost a l l events i n the Canadian catalogues was f i x e d at 18 km i n standard processing. Thus, even f o r the more recent data i n the Canadian Earthquake Data F i l e there i s considerable room f o r improvement. The d e t a i l s of upgrading the three data sets i n the Vancouver I s l a n d - Puget Sound r e g i o n and the r e s u l t s are discussed i n t h i s chapter. B. HISTORICAL SEISMICITY 1) Comments on the Data Set The h i s t o r i c a l data represents the bulk of the data which i s a v a i l a b l e to analyze the s e i s m i c i t y of B r i t i s h Columbia. F i r s t l y , because i t represents the l a r g e s t time p e r i o d , but secondly because most of the major earthquakes i n the re g i o n occurred before 1950. Thus, much of the impression we have of the s e i s m i c i t y p a t t e r n and many of the s t a t i s t i c a l c a l c u l a t i o n s depend h e a v i l y on the h i s t o r i c a l data s e t . I t i s ther e f o r e v i t a l that i t s q u a l i t y and i t s l i m i t a t i o n s be understood and that i t i s as accurate as p o s s i b l e . The problems d e a l i n g w i t h t h i s data set can be d i v i d e d i n t o three c a t e g o r i e s . The f i r s t i s how complete the l i s t i s , the second i s how - 13 -accurate the l o c a t i o n s are and the t h i r d i s how accurate the magnitudes are. Although i t i s not p o s s i b l e to be p r e c i s e , a l l three of these problems are discussed i n t h i s s e c t i o n and estimates are made where p o s s i b l e . I t was decided not to attempt to work back before 1900 f o r two reasons. The establishment of the seismograph s t a t i o n i n V i c t o r i a i n 1898 helped the documentation of the occurrence of the l a r g e r earthquakes i n the r e g i o n , but more important, the population d i s t r i b u t i o n was very sparse before 1900 and much of the i n f o r m a t i o n that i s a v a i l a b l e on e a r l y earthquakes comes from f e l t r e p o r t s . There i s , however, a s i g n i f i c a n t body of i n f o r m a t i o n on earthquakes o c c u r r i n g before 1900. 2) Completeness The f i r s t step i n assessing the completness of the data set was to cross check the Canadian Earthquake Data F i l e w i t h a l l a v a i l a b l e l i s t s and catalogues. These included the summaries of M i l n e (1956) and Rasmussen (1967), the e a r l i e r works of McAdie (1907), Bradford (1935), Townley and A l l a n (1939) and Coombs (1953), the I n t e r n a t i o n a l S e i s m o l o g i c a l Summary, the PDE l i s t i n g of the United States Coast and Geodetic Survey, the annual p u b l i c a t i o n United States Earthquakes which s t a r t e d i n 1928 and the summary p u b l i c a t i o n of Coffman and von Hake (1973) e n t i t l e d Earthquake H i s t o r y of the United Sta t e s. A number of events were discovered that were not i n the Canadian Earthquake Data F i l e and a number of non e x i s t e n t events were a l s o found. Most non e x i s t e n t events were d u p l i c a t i o n s , events that had i n c o r r e c t i n f o r m a t i o n a s s o c i a t e d w i t h them and were l i s t e d i n both the c o r r e c t and i n c o r r e c t form. Once the t o t a l l i s t of earthquakes f o r the region was assembled and the best estimates of l o c a t i o n and magnitude were made (discussed i n the f o l l o w i n g s e c t i o n s ) , the completness of the data set at each magnitude - 14 -l e v e l was i n v e s t i g a t e d . Any earthquake of magnitude 7 or greater would have been f e l t throughout the region even i n e a r l y years when the p o p u l a t i o n was sparse. The f i r s t r e g u l a r newspapers i n the region s t a r t e d up i n the l a t e 1850's, thus 1860 i s chosen as the f i r s t complete year f o r magnitude 7 events. The establishment of the V i c t o r i a seismograph s t a t i o n i n the f a l l of 1898 makes 1899 the f i r s t year of seismograph recording i n the r e g i o n . Even though the s t a t i o n was very low gain i t i s u n l i k e l y that any events i n the region greater than 6 were missed since 1899. There are few events of magnitude 5 1/2 l i s t e d i n the Canadian Earthquake Data F i l e before 1917, the year when the ISS s t a r t e d s y s t e m a t i c a l l y gathering data on earthquakes. A f t e r 1917 there does not seem to be a notable increase i n the number of events w i t h time, thus the data set i s l i k e l y complete f o r events of t h i s s i z e s i n c e 1917. For events of magnitude 5 and greater there i s an increase i n the number of events i n the Canadian Earthquake Data F i l e from an average of about one per year to an average of about two per year that occurs between 1935 and 1940. There were no s i g n i f i c a n t changes i n the d i s t r i b u t i o n of seismograph s t a t i o n s i n the region so the in c r e a s e i n events i s l i k e l y due to the i n s t a l l a t i o n of short period B e n i o f f seismographs i n the western United States s t a r t i n g w i t h Berkeley i n 1934. 1940 i s chosen as a sure date f o r the complete data set of magnitude 5 events but i t may have been a few years sooner. Completeness at the 4.5 l e v e l c o i n c i d e s w i t h the f i r s t year of r o u t i n e magnitude c a l c u l a t i o n s i n western Canada and completeness at the 4.0 l e v e l r e f l e c t s the f i r s t complete year of operation of the P e n t i c t o n (PNT) s t a t i o n . The completeness l e v e l s f o r the Vancouver I s l a n d - Puget Sound region are summarized i n Table I . - 15 -TABLE I Completeness of the Data Set i n the Vancouver I s l a n d Puget Sound Area Magnitude Year Complete 7 1860 6.5 1899 6.0 1899 5.5 1917 5.0 1940 4.5 1955 4.0 1963 - 16 -3) L o c a t i o n The most s i g n i f i c a n t step i n improving l o c a t i o n s of the h i s t o r i c a l earthquakes i n the Canadian Earthquake Data F i l e comes from r e p l a c i n g e p i c e n t r e s that o r i g i n a t e d w i t h the ISS. Some of the ISS epicentres were hundreds of ki l o m e t e r s i n e r r o r . Because a l l ISS c a l c u l a t i o n s were done by hand, the f u l l data set assembled was u s u a l l y not used, but only a few s t a t i o n s at v a r y i n g azimuths and distances were used to c a l c u l a t e the ep i c e n t r e . Often earthquakes were assigned the epicent r e of an e a r l i e r event i n the same region r a t h e r than computing a new ep i c e n t r e . Thus, some ISS e p i c e n t r e s are very poor even though s u f f i c i e n t data e x i s t s f o r a much b e t t e r l o c a t i o n . ISS epice n t r e s improve through the years, becoming much more r e l i a b l e a f t e r the mid 1930's. Some ISS epi c e n t r e s were replaced by those of Gutenberg and R i c h t e r (1949) who published e p i c e n t r e s f o r most of the l a r g e r earthquakes i n the world. They used both P and S data, o r i g i n a l seismograms from C a l i f o r n i a and s e l e c t e d data from s t a t i o n s known to be r e l i a b l e . In other instances published e p i c e n t r e s c a l c u l a t e d w i t h ISS data processed by a modern computer program were used (e.g. K e l l e h e r and Savino, 1975) or the ISS values were run through the t e l e s e i s m i c e p i c e n t r e program EPDET (Weichert and Newton, 1970) to achieve a b e t t e r s o l u t i o n . For the cases of the two l a r g e s t earthquakes on Vancouver I s l a n d , December 19, 1918 (M = 7) and June 23, 1946 (M = 7.3) extensive experimentation was done w i t h the ISS data set to achieve the most r e l i a b l e e p i c e n t r e (see Rogers and Hasegawa, 1978). F e l t r e p o r t s a l s o played a l a r g e r o l e i n r e l o c a t i n g e p i c e n t r e s . The epi c e n t r e s of a l l major earthquakes were compared w i t h f e l t r e p o r t s . This was o f t e n a very t e l l i n g comparison and numerous epic e n t r e s were r e l o c a t e d on the ba s i s of f e l t i n f o r m a t i o n . F e l t r e p o r t s e i t h e r came from o r i g i n a l d e s c r i p t i o n s of the events i n the catalogues of McAdie (1907), Bradford - 17 -(1935), Townley and A l l a n (1939), Coombs (1953), Milne (1956) and Rasmussen (1967) or were compiled from newspaper research done i n archives of the Province of B r i t i s h Columbia i n V i c t o r i a or i n the extensive newspaper m i c r o f i l m c o l l e c t i o n s of the l i b r a r i e s of the U n i v e r s i t y of B r i t i s h Columbia and the U n i v e r s i t y of Washington. For a number of events, a n a l y s i s of newspaper r e p o r t s from surrounding communities allowed a more accurate e p i c e n t r e to be assigned. F e l t r e p o rts were p a r t i c u l a r l y u s e f u l f o r those that had poor instru m e n t a l e p i c e n t r e s and had been assigned an ep i c e n t r e at the l o c a t i o n of a town that had reported f e e l i n g the earthquake, or at a convenient l a t i t u d e and lo n g i t u d e . The d e t a i l s of the changed epi c e n t r e s are documented i n Appendix 1. A l l r e v i s e d e p i c e n t r e s have an u n c e r t a i n t y of + 50 km or l e s s . Changes of some more s i g n i f i c a n t events are discussed b r i e f l y l a t e r i n t h i s chapter. 4) Magnitude Revi s i o n s The t h i r d major l i n e of i n v e s t i g a t i o n was to check the magnitudes of earthquakes. I t was obvious from cross checking w i t h the f e l t r e p o r ts of some events that some magnitudes were s e r i o u s l y i n e r r o r . An i n v e s t i g a t i o n was made to see where the magnitude f o r each event came from, what k i n d of magnitude had been computed ( M ^ M s or m b), how much data had been used and thus estimate how r e l i a b l e the value was. In many cases the magnitudes were based on only one reading or were estimates or were based on maximum reported i n t e n s i t y . Because o r i g i n a l seismograms from before the 1960's are not r e a d i l y a v a i l a b l e i t was decided that the best defined parameter, common to a l l earthquakes, was the f e l t i n f o r m a t i o n , and of the f e l t i n f o r m a t i o n the t o t a l f e l t area was one of the e a s i e s t pieces of in f o r m a t i o n to o b t a i n . Recent successes i n using f e l t i n f o r m a t i o n to ob t a i n accurate magnitude estimates (e.g. Evernden, 1975; Toppozada, 1975; - 18 -Malone and Bor, 1979) were i n c e n t i v e s f o r using t h i s technique. The t o t a l f e l t area was estimated f o r the major earthquakes i n the data set (Table I I ) . In the United States much d e t a i l e d f e l t i n f o r m a t i o n was published i n the annual p u b l i c a t i o n United States Earthquakes which s t a r t e d i n 1928. For earthquakes o c c u r r i n g before t h i s p u b l i c a t i o n s t a r t e d , newspapers were searched f o r f e l t i n f o r m a t i o n . In Canada very l i t t l e d e t a i l e d f e l t i n f o r m a t i o n had been documented and a l l major earthquakes r e q u i r e d newspaper searches to construct rough i s o s e i s m a l maps. Where is o s e i s m a l s were clo s e to being c i r c u l a r , the f e l t area i n s i d e a contour was estimated by c o n s t r u c t i n g s i m i l a r c i r c l e s and c a l c u l a t i n g the area. Where the i s o s e i s m a l contours were more complex, a planimeter was used to o b t a i n the area i n s i d e contours. Appropriate e x t r a p o l a t i o n s to the t o t a l f e l t area were made f o r those events that d i d not have complete i s o s e i s m a l maps because t h e i r i s o s e i s m a l s extended i n t o the ocean. Some sample maps are shown i n F i g u r e 5. Once the estimates of the maximum f e l t areas and areas w i t h i n other contours were assembled, these were t r a n s l a t e d i n t o magnitude values by using the e m p i r i c a l r e l a t i o n s h i p s of Toppozada (1975) l i s t e d i n Table I I I , which were derived from data i n C a l i f o r n i a and western Nevada . I t was decided to use Toppozada's (1975) r e l a t i o n s h i p s as they were derived w i t h more data than some e a r l i e r r e l a t i o n s h i p s i n the l i t e r a t u r e . Malone and Bor (1979) showed that Washington State can be d i v i d e d i n t o two regions w i t h s l i g h t l y d i f f e r e n t a t t e n u a t i o n p r o p e r t i e s f o r seismic waves. S i m i l a r p r o p e r t i e s were found f o r regions of C a l i f o r n i a and Nevada by Evernden (1975). Since most of the i s o s e i s m a l s considered here are i n the c o a s t a l zone of Washington and B r i t i s h Columbia and s i n c e Toppozoda (1975) derived one e m p i r i c a l r e l a t i o n s h i p that a p p l i e d to the same region where Evernden (1975) had derived two a t t e n u a t i o n curves, i t was decided to use - 19 -TABLE I I Larger H i s t o r i c Vancouver I s l a n d - Puget Sound Earthquakes : Their F e l t Area and R e s u l t i n g Magnitude PREVIOUSl FELT AREA2 DATE COORDINATES FELT AREA MAGNITUDE MAGNITUDE 1872 DEC 14 48.6 121.4 1010 ,000 — 7.3 1903 MAR 14 47.7 122.2 26 ,000* 4.3 4.9 1904 MAR 17 47.8 123.0 50 ,000* 6.0 5.3 1909 JAN 11 48.7 122.8 150 ,000 5.6 6.0 1911 SEP 29 48.8 122.7 8 ,000 4.3 4.1 1913 DEC 25 47.7 122.5 20 000* 4.3 4.7 1915 AUG 18 48.5 121.4 77 ,000* 5.5 4.6 1918 DEC 06 49.5 125.9 650 ,000 7.0 7.0 1920 JAN 24 48.6 123.0 70 ,000 5.0 5.5 1923 FEB 12 49.0 122.7 8 ,000 4.3 4.1 1926 DEC 04 48.5 123.0 30 ,000 4.3 5.0 1928 FEB 09 49.0 125.3 120 ,000 3.7 5.8 1931 APR 18 48.7 122.2 13 ,000* 4.3 4.4 1931 DEC 31 47.5 123.0 26 ,000* 5.0 4.9 1932 JAN 05 48.0 121.8 4 ,000* 4.3 3.6 1932 JUL 18 48.0 121.8 36 000* 4.3 5.1 1933 OCT 05 49.0 124.0 23 ,000 - 4.8 1934 MAY 05 48.0 123.0 26 000* 4.3 4.9 1934 NOV 03 48.0 121.0 26 000* 4.0 4.9 1939 NOV 13 47.4 122.6 200 000 5.7 6.2 1940 OCT 27 48.2 122.5 38 000 4.6 5.1 1943 NOV 29 48.4 122.9 23 000* 5.0 4.8 1945 JUN 15 49.0 123.5 23 ,000 4.2 4.8 1946 FEB 15 47.3 122.9 269 ,000 5.7 6.4 1946 JUN 23 49.8 125.3 1096 ,000 7.3 7.3 1949 APR 13 47.1 122.7 549 000 7.0 6.9 1950 APR 14 48.0 122.5 18 000* 4.5 . 4.6 1965 APR 14 47.4 122.3 500 000 6.5 6.8 * F e l t area from Earthquake H i s t o r y of the United States by Coffman and von Hake (1973). iMagnitude i n the Canadian Earthquake Data F i l e . 2 c a l c u l a t e d using Toppozada's (1975) r e l a t i o n s h i p s (Table I I I ) . - 20 -Figure 5 Isoseismal maps of major earthquakes. Heavy l i n e s show how maps were divided to calculate t o t a l area when isoseismals extended into the ocean. Areas on land were measured with a planimeter and then m u l t i p l i e d by 4/3 or 2 to obtain t o t a l area. - 21 -TABLE I I I Toppozada's (1975) F e l t Area R e l a t i o n s h i p s * \ = -1.88 + 1.53 l o g A \ = 0.86 + 1.09 l o g A v \ = 2.56 + 0.85 l o g A V i * I n order to avoid complications w i t h deeper earthquakes i n the Puget Sound r e g i o n these r e l a t i o n s h i p s are best used at distances greater than 150 km. This r e s t r i c t i o n e f f e c t i v e l y l i m i t s the s i z e of earthquakes to greater than magnitude 5.5. Thus values i n Table I I l e s s than 5.5 may r e q u i r e a depth c o r r e c t i o n . - 22 -Toppozada's (1975) r e l a t i o n s h i p s over the whole region considered here. This means that the f e l t areas are being compared to w e l l defined values f o r s i m i l a r f e l t areas i n C a l i f o r n i a and western Nevada. Topozada (1975) s t a t e s the values are g e n e r a l l y w i t h i n 1/2 magnitude u n i t of the values c a l c u l a t e d from seismograms. I n most instances i t appears from Toppozada's (1975) data set and the few events i n the Vancouver I s l a n d area which have both w e l l defined magnitudes and f e l t areas that the f e l t area estimates are c o n s i s t e n t to + 1/4 magnitude u n i t . While the absolute magnitude l e v e l can be debated, the most obvious f e a t u r e that the use of a f e l t area versus magnitude r e l a t i o n s h i p p o ints out i s that there are some se r i o u s d i s c r e p a n c i e s between the r e l a t i v e magnitudes of the l a r g e r events i n Puget Sound. For example the Puget Sound earthquakes of November 12, 1939; February 14, 1946; A p r i l 13, 1949 and A p r i l 29, 1965 have epi c e n t r e s that are very c l o s e together and have i s o s e i s m a l maps which suggest there i s only about 1/2 magnitude u n i t range over a l l 4 events. However, they were o r i g i n a l l y assigned magnitudes of 5.75, 5.75, 7.1 and 6.5 r e s p e c t i v e l y (see Table I I ) . 5) Comments on Revisions of Some Key Earthquakes (Figure 6) a) March 16, 1904 In the Canadian Earthquake Data F i l e the ep i c e n t r e f o r t h i s event was l o c a t e d on the west s i d e of the Olympic Peninsula and i t was assigned a magnitude of 6. In Earthquake H i s t o r y of the United States i t i s given the l o c a t i o n of V i c t o r i a , B.C. and assigned an i n t e n s i t y value of V. A study of f e l t l e v e l s from newspaper r e p o r t s suggests the earthquake has an ep i c e n t r e along the west s i d e of Puget Sound, south of Port Townsend and that the magnitude i s about 5. The c o n f i r m a t i o n that t h i s earthquake was not near the west coast of the Olympic Peninsula i s important as no l a r g e - 23 -Figure 6 Some revisions to the Canadian Earthquake Data F i l e for the locations and magnitudes of some key h i s t o r i c a l earthquakes. Open c i r c l e s represent previous locations and magnitudes. The 1927 event relocates near Brooks Peninsula on northern Vancouver Island. - 24 -earthquakes have yet been l o c a t e d along the coast. b) January 11, 1909 F e l t r e p o r t s from newspapers l i m i t t h i s e p i c e n t r e to the San Juan Is l a n d s region where s e v e r a l instances of damage occurred. F e l t i n f o r m a t i o n a l s o c l e a r l y shows that the s i z e of the i s o s e i s m a l maps i s between that f o r the 1965 S e a t t l e earthquake and the 1976 Gulf Islands earthquake (Figure 7). The magnitude i s about 6 according to Toppozada's (1975) f e l t area r e l a t i o n s h i p . Lack of aftershocks and l a c k of higher i n t e n s i t i e s i n the e p i c e n t r a l region suggest t h i s event belongs to the deeper s u i t e of earthquakes. This event i s s i g n i f i c a n t because i t points out that l a r g e earthquakes i n the deeper s u i t e can a l s o occur north of Puget Sound. c) December 6, 1918 This earthquake has s e v e r a l e p i c e n t r e s i n the l i t e r a t u r e which are shown i n Figure 8. I n v e s t i g a t i o n s w i t h the set of P a r r i v a l times l i s t e d i n the ISS place the ep i c e n t r e on Vancouver I s l a n d near the west coast, south of the present town of Gold R i v e r which d i d not e x i s t i n 1918. Because of the q u a l i t y of 1918 a r r i v a l times t h i s e p i c e n t r e has an u n c e r t a i n t y of the order of + 50 km. F e l t i n f o r m a t i o n c o l l e c t e d by Dennison (1919) and supplemented by newspaper i n v e s t i g a t i o n s suggest the magnitude of 7 (M g) c a l c u l a t e d by Gutenberg and R i c h t e r (1949) i s c o r r e c t . d) February 9, 1928 F e l t i n f o r m a t i o n and a 100 mile distance issued from the V i c t o r i a seismograph s t a t i o n (VGZ), presumably from the S-P i n t e r v a l , i n d i c a t e s the earthquake was i n the v i c i n i t y of Barkely Sound and had a magnitude of about 5-3/4. This event was i n the Canadian Earthquake Data F i l e but was lo c a t e d i n the S t r a i t of Juan de Fuca w i t h a magnitude of 3.7. I t i s s i g n i f i c a n t that an earthquake of t h i s s i z e occurred i n the Barkely Sound - 25 -Isoseismal map f o r 1909 earthquake compiled from newspaper r e p o r t s . The f e l t area i s e q u i v a l e n t to a magnitude 6 earthquake. Two s u b s c r u s t a l events i n the area are shown f o r comparison. - 26 -Figure 8 Epicentres of the December 6, 1918 earthquake (M = 7). The e r r o r of the p r e f e r r e d e p i c e n t r e may approach +50km because of the q u a l i t y of the data. - 27 -r e g i o n as there have been no events above magnitude 4 i n that r e g i o n s i n c e 1928. e) September 17, 1926 and May 7, 1927 Both of these events were assigned e p i c e n t r e s by the ISS at the l o c a t i o n p r e v i o u s l y c a l c u l a t e d f o r the e p i c e n t r e of the December 6, 1918 earthquake (Fi g u r e 8). Because of the p r o x i m i t y of the c i t i e s of Nanaimo and V i c t o r i a and the d i s t r i b u t i o n of population c l o s e to the ISS e p i c e n t r a l r e g i o n , these earthquakes could not have been lo c a t e d there as they would have been f e l t . With the data l i s t e d i n the ISS and f e l t reports they were l o c a t e d elsewhere. The 1927 event occurred i n the v i c i n i t y of Brooks Pen i n s u l a on Vancouver I s l a n d and the 1926 event occurred on the mainland no r t h of Vancouver (see Appendix 1). f ) November 12, 1939 This earthquake was o r i g i n a l l y assigned a magnitude of 5.75 by Gutenberg and R i c h t e r (1949), but when Toppozada's (1975) r e l a t i o n s h i p i s a p p l i e d to the f e l t area, a magnitude of 6.2 i s suggested. g) February 14, 1946 This event i s w e l l l o c a t e d i n southern Puget Sound but has p r e v i o u s l y been assigned a magnitude of 5-3/4 (Gutenberg and R i c h t e r , 1949). Rassmusen et a l . (1974) r e a l i z e d t h i s was an underestimate and assigned i t a magnitude of 6.3 based on readings from the seismographs at the U n i v e r s i t y of Washington i n S e a t t l e (Rasmussen et a l . , 1974). The f e l t area r e l a t i o n s h i p used here (Toppozada, 1975) y i e l d s an M^ value of 6.4. h) June 23, 1946 This earthquake has s e v e r a l epicentres i n the l i t e r a t u r e , probably the most f r e q u e n t l y quoted has been that of Hodgson and Milne (1951) which i s l o c a t e d i n the S t r a i t of Georgia (Figure 9). Extensive experimentation w i t h the data set l i s t e d i n the ISS confirms the earthquake to be l o c a t e d - 28 -Figure 9 i - 29 -on Vancouver I s l a n d i n the v i c i n i t y of the Beaufort Range f a u l t . F e l t i n f o r m a t i o n was compiled and an i s o s e i s m a l map drawn (see Figure 5). The magnitude computed using Toppozada's (1975) r e l a t i o n s h i p f o r f e l t area i s 7.3, the same as the M g magnitude c a l c u l a t e d by Gutenberg and R i c h t e r (1949). The d e t a i l s of the work on t h i s earthquake have been combined w i t h the surface wave a n a l y s i s and the near f i e l d deformation a n a l y s i s of Hasegawa and published elsewhere (Rogers and Hasegawa, 1979). i ) A p r i l 13, 1949 The l o c a t i o n of t h i s earthquake i s w e l l defined ( N u t t l i , 1952). The magnitude assigned i n the p u b l i c a t i o n United States Earthquakes i s 7.1 and that by Gutenberg and R i c h t e r (1949) i s 7.0. The w e l l - d e f i n e d i s o s e i s m a l maps suggests the event i s s i g n i f i c a n t l y smaller than the 1946 Vancouver I s l a n d earthquake and not too much l a r g e r than the 1965 S e a t t l e earthquake ( F i g u r e 5). The maximum f e l t area r e l a t i o n s h i p of Toppozada (1975) gives a value of 6.9 f o r the magnitude. Averaging the values f o r the IV, V and VI i n t e n s i t y contours gives a value of 6-3/4 which seems c o n s i s t e n t w i t h the comparison of i s o s e i s m a l maps of other large earthquakes i n the r e g i o n . This small adjustment i n magnitude makes a considerable d i f f e r e n c e to the s i z e of the moment c a l c u l a t e d f o r t h i s earthquake, a point which i s discussed i n the next chapter. 6) The Revised Data Set The r e v i s e d parameters f o r the l a r g e r earthquakes i n the Puget Sound/Vancouver I s l a n d r e g i o n are l i s t e d i n Table I I and d i s p l a y e d i n F i g u r e 10. D e t a i l s and a d d i t i o n a l earthquakes are l i s t e d i n Appendix 1. One notable f e a t u r e of the r e v i s e d data set i s the c o n c e n t r a t i o n of earthquakes i n the Puget Sound and C e n t r a l Vancouver I s l a n d regions w i t h l i t t l e s e i s m i c i t y elsewhere. The l a c k of s e i s m i c i t y i n the S t r a i t of Figure 10 Larger h i s t o r i c a l earthquakes i n the Puget Sound - Vancouver I s l a n d r e g i o n 1900-1950 which are l i s t e d i n Table I I . Open c i r c l e i s large 1872 event. - 31 -Georgia i s r e a l as the p o p u l a t i o n d i s t r i b u t i o n around the S t r a i t during t h i s time p e r i o d would not have allowed any s i z e a b l e event to go undetected. The main c o n c e n t r a t i o n of events beneath the Puget Sound Lowlands i n a s t r i p about 100 km wide and dying out to the north and to the south i s very s i m i l a r to the p a t t e r n of small earthquakes observed i n recent years. The presence of the l a r g e earthquakes i n c e n t r a l Vancouver I s l a n d however, i s not r e f l e c t e d i n the p a t t e r n of smaller events o c c u r r i n g since 1950. C. MODERN SEISMICITY (1951-1970) 1) Problems With The Data Set The f i r s t modern high g a i n seismograph, a short period B e n i o f f model, was set up i n June of 1948 at the V i c t o r i a seismograph s t a t i o n (VIC). I t was not u n t i l mid 1951 that two other s t a t i o n s were set up i n the region at Horseshoe Bay and A l b e r n i (see Figure 4). The instruments used had a very high frequency response and were not s u i t a b l e f o r standard amplitude magnitude determinations. From 1951 to 1954 no magnitudes were c a l c u l a t e d f o r earthquakes but a system of c a t e g o r i z i n g earthquakes by using I , I I , or I I I depending whether they were recorded by 1, 2 or 3 seismograph s t a t i o n s was used along w i t h a general estimate of the maximum Modified M e r c a l l i I n t e n s i t y l e v e l i f the earthquake was f e l t . When the Canadian Earthquake Data F i l e was set up a l l these numbers were i n t e r p r e t e d as Modified M e r c a l l i I n t e n s i t i e s and turned i n t o magnitudes by using an e m p i r i c a l r e l a t i o n s h i p of R i c h t e r (1958). Thus, magnitude values from 1951 to 1954 are not very r e l i a b l e and these years should not be used f o r s t a t i s t i c a l c a l c u l a t i o n s . Beginning i n 1955 magnitudes were c a l c u l a t e d w i t h the - 32 -nomogram of Gutenberg and R i c h t e r (1942) f o r as many s t a t i o n s as were a v a i l a b l e and the f i n a l value determined by a simple average. In 1960 Horseshoe Bay s t a t i o n closed down and f o r the decade from 1960 to 1970 the c a p a b i l i t y of the network i n the Georgia S t r a i t - Vancouver I s l a n d - Puget Sound r e g i o n was s i g n i f i c a n t l y diminished. Some smaller events went unlocated and the u n c e r t a i n t y of l o c a t i o n s went from about + 20 km to more than + 30 km i n some cases. The other problem with the data set was that a l l events were hand l o c a t e d drawing arcs on a map assuming a simple one l a y e r c r u s t a l model and surface focus. To be able to compare t h i s data, p a r t i c u l a r l y the 1950-1960 data, w i t h data a f t e r 1975 when the s t a t i o n d i s t r i b u t i o n f i n a l l y regained the c a p a b i l i t y that was present i n the 1950's, i t was necessary to r e l o c a t e 1950's earthquakes w i t h a modern computer program and a more appropriate c r u s t a l model and to be able to experiment w i t h f o c a l depth. A l l phases of earthquakes l o c a t e d from 1950 to 1960 were thus punched on cards and the earthquakes run through the program HYPOELLIPSE (Lahr, 1978). 2) The Georgia S t r a i t C r u s t a l Model Numerous P v e l o c i t y models (Milne and White, 1960; White and Savage, 1965; Tseng, 1967; Crosson, 1972 and 1976) and one S v e l o c i t y model (Wickens, 1977) have been published f o r the Georgia S t r a i t - Vancouver I s l a n d - Puget Sound r e g i o n . A s i n g l e h o r i z o n t a l l y layered P v e l o c i t y s t r u c t u r e that approximated the common features of these models was s e l e c t e d ( F i g u r e 11 and Table I V ) . The model was tes t e d by l o c a t i n g quarry b l a s t s on Texada I s l a n d . Varying s t a t i o n combinations a l l produced e p i c e n t r e s w i t h i n 5 km of the true l o c a t i o n . The program HYPOELLIPSE accepts only one Poisson's r a t i o f o r the whole c r u s t . Thus, to decide on the best e f f e c t i v e P v e l o c i t y to S v e l o c i t y - 33 -P-WAVE VELOCITY (km/s) 5 7 9 I i I — 20 E CL LU Q 40H 60-1 i : ! :i i i i i Georgia Strait Model White & Savage 1965 Tseng 1968 Berry & Forsyth 1975 Crosson 1976 Figure 11 Georgia S t r a i t c r u s t a l model used to l o c a t e earthquakes i n t h i s study. I t i s based on previous st u d i e s a l s o shown i n the diagram. - 34 -TABLE IV Georgia S t r a i t C r u s t a l Model V e l o c i t y (Km/s) Depth 5.0 0 6.0 1.0 6.7 6.0 7.1 30.0 7.75 45.0 - 35 -r a t i o two experiments were performed. F i r s t some deeper events were l o c a t e d w i t h P waves only, then S waves were added at zero weight, w i t h the P to S v e l o c i t y r a t i o held at s e v e r a l values. Residuals f o r S waves increased as the r a t i o was moved away from 1.73 (Poisson's r a t i o = 1/4). The Texada I s l a n d quarry b l a s t s were a l s o l o c a t e d w i t h both P and S data v a r y i n g the P to S v e l o c i t y r a t i o . The choice here was between a s o l u t i o n that reproduced the P wave s o l u t i o n or one that l o c a t e d c l o s e s t to the quarry. A value of 1.73 most f a i t h f u l l y reproduced the P wave s o l u t i o n w h i l e s l i g h t l y higher values of 1.77 or 1.78 produced epicentres c l o s e r to the quarry. While these t e s t s were not c o n c l u s i v e , they showed no trend that suggested the e f f e c t i v e Poisson's r a t i o should be d i f f e r e n t from 1/4 ( i . e . P to S v e l o c i t y r a t i o of 1.73). Thus f o r a l l c a l c u l a t i o n s done i n t h i s s e c t i o n and the f o l l o w i n g s e c t i o n the c r u s t a l model depicted i n Figure 11 and l i s t e d i n Table IV was used w i t h a Poisson's r a t i o of 1/4. 3) The Revised Data Set The s e i s m i c i t y represented by t h i s data i s more d i f f u s e , but some of the f e a t u r e s of the previous data set are repeated here (see Figure 12). The northern end of the Puget Sound s e i s m i c i t y stops near 49° and t h i s s e i s m i c i t y i s concentrated beneath the Puget Sound - Gulf I s l a n d s lowland r e g i o n . Georgia S t r a i t e x h i b i t s very l i t t l e s e i s m i c i t y although there i s a co n c e n t r a t i o n of small events near the south end of Texada I s l a n d (Figure 12). The c e n t r a l Vancouver I s l a n d r e g i o n that contained the very l a r g e earthquakes i n the f i r s t h a l f of the century has another l a r g e event (a magnitude 6 event on Dec. 16, 1957) but very l i t t l e minor s e i s m i c i t y . One of the most i n t e r e s t i n g f e a t u r e s of the r e v i s e d data set i s the f o c a l depth i n f o r m a t i o n . Experimentation w i t h 3 s t a t i o n s (ALB, VIC and HYC) and the contemporary data set (next s e c t i o n ) was c a r r i e d out to Figure 12 - 37 -estimate the depth c a p a b i l i t y of the 3 s t a t i o n network (ALB, VIC and HBC) that e x i s t e d i n the 1950's. While depth r e s o l u t i o n i s poor, as long as 4 phases are w e l l recorded the network can re s o l v e between shallow (depth of approximately 25 km) and deep events (depth the order of 60 km) i n some regio n s . The 1950's data was thus processed w i t h a f r e e f o c a l depth to see the r e s u l t s (Figure 13). The deeper earthquakes near Texada I s l a n d are a s i g n i f i c a n t d i scovery. D. CONTEMPORARY SEISMICITY (1970-1979) 1) Problems With The Data Set As w i t h other time p e r i o d s , the problems can be sorted i n t o those d e a l i n g w i t h l o c a t i o n and those d e a l i n g w i t h magnitude. Dealing f i r s t w i t h l o c a t i o n : the accuracy of the epice n t r e s v a r i e s through time w i t h the changing d i s t r i b u t i o n of seismograph s t a t i o n s ( F i g u r e 4). In general, accurate ( + 1 0 km) epice n t r e s and f o c a l depths could be computed f o r most events i n western Washington State during t h i s time period (Crosson, 1972; Peters and Crosson, 1972), p a r t i c u l a r l y a f t e r 1972 i n the northern part of the s t a t e when the s t a t i o n s MCW and MBW were e s t a b l i s h e d . E p i c e n t r a l accuracy was much poorer on the Canadian side (+ 20 km) and no r o u t i n e depth c a p a b i l i t y e x i s t e d . The s i t u a t i o n was p a r t i c u l a r l y poor from 1972 to 1975 when ALB s t a t i o n i n the centre of Vancouver I s l a n d was not oper a t i n g . U n c e r t a i n t i e s i n the Vancouver I s l a n d r e g i o n probably exceed + 30 km f o r some events when ALB was not i n operation. A f t e r the beginning of 1976 the ex i s t e n c e of a 4 s t a t i o n telemetered array permitted good l o c a t i o n s ( b e t t e r than + 20 km) and some degree of depth c o n t r o l around the Gulf Islands r e g i o n i n the southern S t r a i t of Georgia. With t h i s network the r e s t of Figure 13 Deeper earthquakes 1951-1969. The depths of these earthquakes are not w e l l determined but the r e s o l u t i o n i s s u f f i c i e n t to say that they are s u b c r u s t a l . - 39 -southwest B r i t i s h Columbia returned to the monitoring c a p a b i l i t y of the 1950's. I n 1972 EPB s t a r t e d to process a l l events i n Canada with the same s i n g l e l a y e r c r u s t a l model and a r e s t r i c t e d f o c a l depth of 18 km. There was l i t t l e exchange of data between EPB and UW except f o r l a r g e r events. There are two main improvements that were made to the data set of t h i s time p e r i o d . F i r s t , a l l EPB events were run with the Georgia S t r a i t c r u s t a l model and a f r e e f o c a l depth to see whether some depth i n f o r m a t i o n might be e x t r a c t e d . Second, data from the EPB and UW networks were combined f o r events i n the border region ( g e n e r a l l y from 48°N to 49°N) to give good depth c o n t r o l where the c a p a b i l i t y of each network d e t e r i o r a t e s . A l l P and S a r r i v a l times from each network were t r a n s f e r r e d to punched cards and run through the program HYPOELLIPSE. The r e s u l t s of r e l o c a t i n g a l l events are shown i n Figure 14. The magnitude problems i n the data set stem from d i f f e r e n t methods used by EPB and UW. EPB used the o r i g i n a l ^ d e f i n i t i o n (e.g. R i c h t e r , 1958) based on the maximum amplitude on the seismogram, but EPB a p p l i e d the formula to the v e r t i c a l component only. UW used a magnitude c a l c u l a t e d from the d u r a t i o n of the earthquake s i g n a l (e.g. Tsumura, 1967) and c a l i b r a t e d w i t h magnitudes from a few events recorded on the p a i r of h o r i z o n t a l Wood-Anderson seismographs at UW (Crosson, 1972). The two magnitude systems are not e x a c t l y compatable, i n most cases the UW procedure g i v i n g somewhat higher values. A study was c a r r i e d out comparing both magnitude formulas computed on an EPB data set gathered over a period of 3 years (Rogers and Muraro, 1981). The d u r a t i o n magnitudes proved to be more i n t e r n a l l y c o n s i s t e n t , having an average standard d e v i a t i o n of 0.15 versus 0.25 f o r the values based on v e r t i c a l amplitudes. Comparison of each set of values with magnitudes of 9 earthquakes that were l a r g e enough to record on the p a i r of - 1*0 -Figure 14 Revised data set 1971-1979 f o r the Gulf Islands r e g i o n . AB i s the cross s e c t i o n shown i n Figure 16. 1975 events are only known events i n southern Georgia S t r a i t . - 41 -h o r i z o n t a l Wood Anderson seismographs at VIC and had magnitudes c a l c u l a t e d i n the t r a d i t i o n a l manner, revealed that the values c a l c u l a t e d from the d u r a t i o n method were more c o n s i s t e n t w i t h the o r i g i n a l M^ d e f i n i t i o n and t h a t magnitudes c u r r e n t l y published as by the EPB are s y s t e m a t i c a l l y 0.4 magnitude u n i t s too low i n the Gulf I s l a n d s r e g i o n . The discrepancy i n the EPB magnitudes l i k e l y r e s u l t s from a p p l y i n g the o r i g i n a l Mj^  formula too c l o s e to the e p i c e n t r e , which emphasizes the d i f f e r e n c e i n depth d i s t r i b u t i o n between C a l i f o r n i a and Gulf Islands earthquakes. Thus, w i t h evidence suggesting UW magnitudes were more i n t e r n a l l y c o n s i s t e n t and w i t h no exact way of a d j u s t i n g EPB data without going back to a l l of the o r i g i n a l seismograms, i t was decided not to adjust EPB values and to accept s t a t i s t i c a l c a l c u l a t i o n s done on the UW data set (Crosson 1972, 1981) as r e p r e s e n t a t i v e of the southern Vancouver I s l a n d - Puget Sound r e g i o n . A cut o f f l e v e l of magnitude 2 on the UW s c a l e was used i n meshing the two data s e t s . 2) Depth D i s t r i b u t i o n The combination of EPB and UW data extends the good depth c o n t r o l i n c e n t r a l Puget Sound (Crosson, 1972; Peters and Crosson, 1972) i n t o the southern Vancouver I s l a n d and southern Georgia S t r a i t r e g i o n . A p l o t of the depth d i s t r i b u t i o n of the r e v i s e d hypocentres f o r events north of 48° ( F i g u r e 15) shows a d i s t r i b u t i o n very s i m i l a r to that of Crosson (1981) f o r c e n t r a l Puget Sound. The zone of c r u s t a l s e i s m i c i t y i s about 30 km t h i c k w i t h a peak of a c t i v i t y at about 25 km depth. There i s a secondary peak at about 15 km s i m i l a r to the data of Crosson (1981) which shows a secondary peak centred on a 10 km depth. A l u l l i n a c t i v i t y separates the shallow and deep events. A cross s e c t i o n through southern Vancouver I s l a n d and Mt. Baker shows the c l a s s i c - kz -No. of Earthquakes i i 70* i Figure 15 Depth d i s t r i b u t i o n between 48ON and 49 ° N and 122°W and 124oW. - 43 -B e n i o f f zone s t r u c t u r e d i p p i n g at 12 to the northeast (Figure 16). This i s a l s o seen to the south i n c e n t r a l Puget Sound (Crosson, 1981). 3) Shallow Earthquakes The shallow s u i t e of earthquakes are d i s p l a y e d i n Figure 17. They tend to d i e out north of 49°N with a l u l l i n c e n t r a l Georgia S t r a i t except f o r one sequence of events that s t a r t e d w i t h a magnitude 4-1/2 event on November 30, 1975. This event was shallow (10 + 10 km) and had a t h r u s t f a u l t i n g mechanism on an east-west o r i e n t e d f a u l t plane (see s e c t i o n F on f o c a l mechanisms). I t occurred c l o s e to some major east-west trending f a u l t s on the f l o o r of Georgia S t r a i t . The aftershock sequence of t h i s earthquake went on f o r s i x months and i s the longest ever observed i n southwestern B r i t i s h Columbia . Normal aftershock sequences f o r an event of t h i s s i z e l a s t a few days. Because deeper earthquakes have very few aft e r s h o c k s (Page, 1968; Robinson et a l . , 1975), the converse may be true and t h i s prolonged sequence may be evidence of an extremely shallow f o c a l depth. 4) Deeper Earthquakes The sequence of deep earthquakes centred on Puget Sound dies o f f rat h e r a b r u p t l y at about 49° l a t i t u d e . (Figure 18). I t appears l i m i t e d i n east-west extent to about 100 km. One of the most notable features are three deep events near the southern t i p of Texada I s l a n d i n 1979. The depth c o n t r o l was not good here i n 1979 but i t was s u f f i c i e n t to say the events are s u b c r u s t a l . There are d e f i n i t e l y no events under c e n t r a l S t r a i t of Georgia. The p a t t e r n i s very s i m i l a r to that f o r the l a r g e r h i s t o r i c a l earthquakes (Figure 10). _ -_ L*. l o w l a n d 6 x50\ o . • • • • • • v*. • . ••• IOO KM 1001 Figure 16 Cross section through V i c t o r i a and Mount Baker (see Figure 14). Dashed l i n e s are hypothetical p o s i t i o n of subducted plate (see Chapter I I I ) . Figure 17 Shallow earthquakes 1971-1979. 1975 events are only known events i n southern Georgia S t r a i t . Compare Figure 10 and Figure 12. Figure 18 E. RECURRENCE RELATIONSHIPS 1) C e n t r a l Vancouver I s l a n d and Puget Sound The r e l a t i o n s h i p between earthquake magnitude and the frequency of occurrence, known as the recurrence r e l a t i o n s h i p , i s a u s e f u l way of c a t a g o r i z i n g groups of earthquakes (Gutenberg and R i c h t e r , 1949). The s t a t i s t i c a l parameter used f o r comparison i s the slope of a l i n e a r f i t to the data commonly c a l l e d the 'b value'. The b value v a r i e s w i t h t e c t o n i c regime and may be r e l a t e d to the heterogeneity (Mogi, 1967) or the s t r e s s l e v e l i n the f r a c t u r i n g m a t e r i a l ( S c h o l z , 1968). The recurrence r e l a t i o n s h i p s f o r earthquakes i n the c e n t r a l Vancouver I s l a n d r e g i o n and the Puget Sound region (Figure 19) were c a l c u l a t e d using the completeness i n f o r m a t i o n i n Table I and the method of Weichert (1980). The b values are very d i f f e r e n t ( F i g u r e 19) suggesting d i f f e r e n t t e c t o n i c environments. The low value of 0.38 f o r c e n t r a l Vancouver I s l a n d i s lower than the m a j o r i t y of values reported i n the l i t e r a t u r e . However, i t c o r r e c t l y r e f l e c t s the observed data as there are very few smaller magnitude events f o r a region that has experienced s e v e r a l l a r g e earthquakes. 2) Shallow and Deep Earthquakes of Puget Sound Because of the magnitude problems i n meshing the contemporary EPB and UW data set (see S e c t i o n II-D) the recurrence r e l a t i o n s h i p s c a l c u l a t e d by Crosson (1981) which i n c l u d e most of the data are taken as r e p r e s e n t a t i v e of a l l the data i n the shallow and deeper regimes of the Puget Sound -southern Vancouver I s l a n d r e g i o n ( F i g u r e 20). The two s u i t e s of earthquakes have very d i f f e r e n t b values suggesting two d i f f e r e n t t e c t o n i c environments. The b value f o r the deeper" s u i t e has a b value s i m i l a r to Figure 19 Recurrence r e l a t i o n s h i p s for central Vancouver Island and Puget Sound 1900-1978. The b values are s i g n i f i c a n t l y d i f f e r e n t . - 49 -SHALLOWER THAN 33 KM MAGNITUDE Recurrence r e l a t i o n s h i p s for shallow and deep suites of earthquakes of Puget Sound 1970-1978 taken from Crosson (1981). The b values are s i g n i f i c a n t l y d i f f e r e n t . The b value for the deep events i s s i m i l a r to that for a l l the data i n the Puget Sound region (Figure 19). - 50 -that f o r a l l the data i n the Puget Sound region ( F i g u r e 19). Because the b value f o r the t o t a l data set i s determined by much l a r g e r earthquakes t h i s suggests that the m a j o r i t y of the l a r g e r earthquakes belong to the deeper s u i t e . F. FOCAL MECHANISMS 1) Shallow Earthquakes A search was made through a l l r e l e v a n t earthquake catalogues f o r earthquakes i n the Vancouver I s l a n d r e g i o n that were recorded by a s u f f i c i e n t number of seismograph s t a t i o n s to a l l o w a study of the mechanism by the P nodal method. F i v e earthquakes were found. Their l o c a t i o n s are d i s p l a y e d i n Figure 21 with the d e t a i l s of the s o l u t i o n s shown i n Figure 22 and 23 and l i s t e d i n Table V. A p r e l i m i n a r y P nodal s o l u t i o n was c a l c u l a t e d f o r each earthquake using only published f i r s t a r r i v a l p o l a r i t i e s , then a d d i t i o n a l data were sought from seismograph s t a t i o n s at c r i t i c a l azimuths and distances that d i d not have f i r s t motion p o l a r i t i e s published i n r o u t i n e catalogues and b u l l e t i n s . Where o r i g i n a l records or copies could be conveniently obtained, published values were checked f o r accuracy and q u a l i t y . Readings e x t r a c t e d from b u l l e t i n s were given one-half weight during f i n a l computer processing because b u l l e t i n s can c o n t a i n a high percentage of e r r o r s (Hodgson and Adams 1958). The phases used were almost a l l f i r s t a r r i v i n g P or PKP phases, though o c c a s i o n a l l y r e f l e c t e d pP or PcP a r r i v a l s were in c l u d e d i f they were impulsive and unambiguous. C a r e f u l a t t e n t i o n was paid to phase r e v e r s a l of pP a r r i v a l s (Ingram and Hodgson 1956). The computer program employed to c a l c u l a t e the P nodal s o l u t i o n s i s - 51 -Figure 21 L o c a t i o n of shallow f a u l t plane s o l u t i o n s c a l c u l a t e d during t h i s study. The c i r c l e s represent the lower h a l f of the f o c a l spheres and the shaded areas i n d i c a t e the quadrants of compressional a r r i v a l s . C i r c l e s i z e i s r e l a t e d to magnitude. The dotted l i n e i n d i c a t e s the edge of the c o n t i n e n t a l s h e l f . For d e t a i l s of earthquakes see Table V. - 52 -, o \ » ® strike N48E dip 85 NW 0.98 strike slip 0.20 normal 1946 strike N4IW dip 79NE I.00 strike slip 0.09 normal strike N68E dip 78 NW 0.96 strike slip 0.28 thrust 1957 strike N 25W dip 75 SW 0.98 strike slip 0.22 thrust strike N64E dip 73 NW I.00 strike slip 0.03 thrust 1972 strike N 26W dip 88 SW 0.95 strike slip 0.30 thrust strike N77E dip 48 NW 0.99 strike slip e 0.I6 thrust 1975a strike NI9W dip 83 SW 0.74 strike slip 0.67 thrust Figure 22 D e t a i l s of shallow s t r i k e - s l i p earthquakes. A l t e r n a t e but lower s c o r i n g s o l u t i o n s are a l s o shown. The p r o j e c t i o n i s the lower h a l f of the f o c a l sphere; s o l i d c i r c l e s are compressional a r r i v a l s and open c i r c l e s are d i l a t i o n a l a r r i v a l s , l a r g e r symbols are more r e l i a b l e . Pressure and tens i o n axes are i n d i c a t e d as P and T. - 53 -N 5 N 5 Figure 23 D e t a i l s of the shallow t h r u s t earthquakes i n Georgia S t r a i t . A l t e r n a t e but lower s c o r i n g s o l u t i o n i s a l s o shown. Notations as i n Figure 22. - 54 -TABLE V D e t a i l s of F a u l t Plane S o l u t i o n Earthquakes Date O r i g i n Time Coordinates Depth Magnitude June 23, 1946 17 13 26 49.8N 30 km 7.3 (M s) 125.3W Dec 16, 1957 17 27 51 49.8N 0 km 6.0 (M s) 126.5W Apr 29, 1965 15 28 44 47.4N 59 km 6.5 (Mb) 123.3W J u l y 5, 1972 10 16 39 49.5N 25 km 5.7 (M b) 127.2W Mar 31, 1975 05 48 38 49.3N 18 km 5.4 (M L) 126.0W Nov 30, 1975 10 48 21 49.2N 10 km 4.9 (M L) 123.6W May 16, 1976 08 35 15 48.8N 62 km 5.4 (M L) 123.3W - 55 -that l i s t e d i n the p u b l i c a t i o n of Wickens and Hodgson (1967). Extended di s t a n c e t a b l e s r e q u i r e d by the program were those of Hodgson and Storey (1953) and Hodgson and A l l e n (1954a,b). The computer program seeks a s o l u t i o n by t r y i n g s e v e r a l thousand t r i a l s o l u t i o n s and r a t i n g them by a s c o r i n g system based on the t h e o r e t i c a l r a d i a t i o n p a t t e r n expected from the focus. A number of the top s c o r i n g s o l u t i o n s a u t o m a t i c a l l y undergo r e f i n e d processing and are subjected to t e s t s that assess r e l i a b i l i t y . Only the highest s c o r i n g s o l u t i o n f o r each earthquake i s discussed here, although a l t e r n a t e , but lower s c o r i n g s o l u t i o n s are a l s o shown i n Figures 22 and 23. The only a l t e r a t i o n to the computing procedure described by Wickens and Hodgson (1967) was to r e s t r i c t the energy assumed to be a r r i v i n g as P n from c r u s t a l earthquakes to an assigned angle of 60° ( r e l a t i v e to v e r t i c a l ) l e a v i n g the f o c a l sphere (see Sutton and Berg 1958). Nodal planes f o r s e v e r a l of the s o l u t i o n s are not w e l l defined; some are constrained by only one value or have a range of p o s s i b l e o r i e n t a t i o n s before they meet a c o n s t r a i n i n g value and a l l have a l t e r n a t e (but lower sc o r i n g ) p o s i t i o n s (see Figures 22 and 23). Thus, the general features of the group of s o l u t i o n s should be noted and the exact o r i e n t a t i o n of the nodal planes i s secondary. There are a l t e r n a t e s o l u t i o n s published f o r the 1946 earthquake (Hodgson and Milne 1951; Wickens and Hodgson 1967) and the 1972 earthquake (Chandra 1974) which suggest l a r g e components of normal f a u l t i n g . However, when more complete data sets are used and processed as discussed above, the s t r i k e - s l i p s o l u t i o n s i n d i c a t e d here emerge as the more l i k e l y ones (Rogers 1976; Rogers and Hasegawa 1978). Viewing Figures 21, 22, and 23 i t i s apparent that s t r i k e - s l i p f a u l t i n g i s the dominant type. The o r i e n t a t i o n s of the nodal planes are remarkably s i m i l a r and the sense of motion i s the same f o r each s o l u t i o n : d e x t r a l on - 56 -the northwest s t r i k i n g plane or s i n i s t r a l on the northeast plane. U n f o r t u n a t e l y , i t i s impossible to decide from the f a r f i e l d P (or S) r a d i a t i o n p a t t e r n which nodal plane i s the f a u l t plane, because the f a r f i e l d r a d i a t i o n p a t t e r n from a double couple source i s symmetric about each nodal plane (except f o r rupture propagation e f f e c t s which are very s m a l l ) . Only f o r the 1946 earthquake i s there other evidence to suggest that the northwest s t r i k i n g plane i s the p r e f e r r e d one (Rogers and Hasegawa, 1978; Slawson and Savage, 1979). Perhaps the most i n t e r e s t i n g c o l l e c t i v e property of the mechanism s o l u t i o n s f o r the earthquakes i s the s i m i l a r i t y of the o r i e n t a t i o n s of the pressure axes that are c a l c u l a t e d from the P nodal s o l u t i o n s (Figure 24). A l l are o r i e n t e d i n a near h o r i z o n t a l north-south d i r e c t i o n . However there are some d i f f e r e n c e s which may be s i g n i f i c a n t . The three earthquakes near the centre of Vancouver I s l a n d (1946, 1957 and 1972) have pressure axes which average s l i g h t l y east of nor t h . This i s s i m i l a r to the d i r e c t i o n Riddihough (1977) p r e d i c t e d f o r i n t e r a c t i o n between the Expl o r e r and America P l a t e s and q u i t e d i f f e r e n t from the i n t e r a c t i o n d i r e c t i o n between the Juan de Fuca and American P l a t e s (Figure 24). The 1975a event which i s w e l l south of the Explorer/American i n t e r a c t i o n region (Figure 3) has a pressure a x i s that i s much c l o s e r to the Juan de Fuca/America i n t e r a c t i o n d i r e c t i o n (Figure 24). The 1975b t h r u s t event i n Georgia S t r a i t has a pressure a x i s which i s s l i g h t l y west of no r t h , s i m i l a r to the north-south and s l i g h t l y west of north o r i e n t a t i o n s f o r pressure axes founed by Crosson (1972, 1981) j u s t to the south i n Puget Sound. Thus, although d i f f e r e n c e s are s l i g h t , these mechanism s o l u t i o n s may be r e f l e c t i n g three d i f f e r e n t s t r e s s regimes. - 57 -Pressure axes of shallow earthquakes shown i n Figure 21. The symbols are p l o t t e d where the axes i n t e r s e c t the lower h a l f of the focal sphere. C i r c l e s are s t r i k e - s l i p s o l u t i o n s ; squares are t h r u s t s o l u t i o n s . The s o l i d arrows i n d i c a t e the Explorer/America i n t e r a c t i o n d i r e c t i o n suggested by Riddihough (1977) and the open arrows i n d i c a t e the Juan de Fuca i n t e r a c t i o n suggested by Riddihough (1977). - 58 -2) Deeper Earthquakes There have been only two deeper earthquakes i n recent times that have been l a r g e enough to record on enough seismograph s t a t i o n s to enable w e l l constrained t e l e s e i s m i c f a u l t plane s o l u t i o n s to be c a l c u l a t e d . These are the A p r i l 29, 1965 S e a t t l e earthquake and the May 16, 1976 earthquake i n the Canadian Gulf I s l a n d s . The s o l u t i o n s are shown i n Figure 25. The 1965 s o l u t i o n i s that of Isacks and Molnar (1971) and the 1976 s o l u t i o n i s c a l c u l a t e d here using t e l e s e i s m i c data only. These two earthquakes are at opposite ends of the s u i t e of deeper earthquakes but the t e l e s e i s m i c f a u l t plane s o l u t i o n s are almost i d e n t i c a l . They both have te n s i o n axes dipping to the east. This i s the d i r e c t i o n the subducting p l a t e i s moving and i s c o n s i s t e n t w i t h downdip t e n s i o n i n the subducting l i t h o s p h e r e seen i n other subduction zones (Isacks and Molnar, 1971). Crosson (1981) has presented f o c a l mechanisms of s e v e r a l smaller deep Puget Sound earthquakes. Viewing the d i s t r i b u t i o n of pressure and t e n s i o n axes from h i s s o l u t i o n s (Figure 26) i t can be seen that two of the events are a l s o dominated by the same downdip t e n s i o n as the l a r g e r events, but that there i s a d i v e r s i t y amongst the f o c a l mechanisms of s m a l l earthquakes. The i m p l i c a t i o n s of the deep f o c a l mechanisms are discussed i n the next chapter. I n Figure 25 the P nodal s o l u t i o n f o r the 1976 earthquake computed w i t h t e l e s e i s m i c data i s compared w i t h the s o l u t i o n f o r the 1965 S e a t t l e earthquake as there were few seismograph s t a t i o n s c l o s e to the S e a t t l e earthquake. When the a v a i l a b l e near f i e l d data i s combined w i t h the t e l e s e i s m i c data f o r the 1976 earthquake the s o l u t i o n i s s l i g h t l y d i f f e r e n t and i s the p r e f e r r e d s o l u t i o n ( F i g u r e 27). - 59 -N 1 S Figure 25 Teleseismic f a u l t plane solutions of 1965 Seattle earthquake and 1976 Gulf Island earthquake. Notation as i n Figure 22. Not a l l of the data are plotted to avoid confusion. The 1965 solution i s that of Isacks and Molnar (1971) and the 1976 solution i s calculated during this study using teleseismic data only. The solutions are remarkably s i m i l a r . For d e t a i l s of earthquakes see Table V. - 60 -1974-78; MAG .GE, 3.0; DEPTHS .GT. 35 KM LOWER HEMISHPERE; EQUAL ANGLE Figure 26 Pressure and tension axes of small magnitude deep earthquakes from Crosson 1981. Two ten s i o n axes a l i g n w i t h those of the large events i n Figure 25 but there i s a d i v e r s i t y of s o l u t i o n s present. - 61 -Figure 27 1976 f a u l t plane s o l u t i o n s . (a) Teleseismic data only used to compare with 1965 s o l u t i o n i n Figure 25. (b) P r e f e r r e d s o l u t i o n using l o c a l data (squares) and t e l e s e i s m i c data. - 62 -G. CONTINENTAL SHELF SEISMICITY Other than a few minor events ( a l l l e s s than magnitude 4) there i s no s e i s m i c i t y under the c o n t i n e n t a l s h e l f or c o n t i n e n t a l slope except where the Nootka f a u l t zone subducts under Vancouver I s l a n d . Most a c t i v e subduction zones have t h r u s t events i n t h i s r e g i o n . There are three s i z e a b l e events i n the Canadian Earthquake Data F i l e l o c a t e d on the c o n t i n e n t a l slope due west of the entrance to the S t r a i t of Juan de Fuca. However, when examined c l o s e l y they appear to be l a r g e explosions at sea (Table VI and Figure 28). I t was not p o s s i b l e to confirm from o f f i c i a l sources that these events were explosions but a l l three occur c l o s e together i n a r e l a t i v e l y aseismic area at about the same l o c a t i o n (48.5°N, 126.5°W) and the same time of day. The events are r i c h i n short p e r i o d energy but do not have any signature on the long period seismograms. This i s a t y p i c a l c h a r a c t e r i s t i c of l a r g e explosions at sea (Buchbinder, 1971) and i s c o n s i s t e n t w i t h the M : m, r a t i o of S D underground explosions (e.g. M a r s h a l l and Basham, 1972). Nearby earthquakes of s i m i l a r magnitude show t y p i c a l p a r t i t i o n of short and long p e r i o d energy that i s expected f o r earthquakes of t h i s s i z e . Examples of short and long period seismograms of one of the explosions and a nearby earthquake recorded at s t a t i o n s PNT, SES and FSJ shown i n Figure 29, are reproduced i n Figures 30 and 31. These events are s i m i l a r i n s i z e and character to the CHASE s e r i e s of ordinance d i s p o s a l explosions (e.g. Lomnitz and B o l t , 1967) which are l i s t e d as explosions i n earthquake catalogues. However, the three events noted here have been l i s t e d as earthquakes i n the United States Department of Commerce p u b l i c a t i o n P r e l i m i n a r y Determination of E p i c e n t r e s , the B u l l e t i n of the I n t e r n a t i o n a l S e i s m o l o g i c a l Centre and the Department of Energy, Mines - 63 -TABLE VI Hypocetiter S o l u t i o n s For Explosions And Earthquakes* EXPLOSIONS DEPTH MAGNITUDE COMPRES- DILATA-DATE TIME POSITION (km) (m^ ,) SIONS- TIONS 1969 AUG 13 16:12:18 48.5N 33 4.6 4 6 126.5W 1969 OCT 01 17:11:11 48.5N 23 4.6 6 7 126.5W 1970 MAY 28 17:38:33 48.5N 3 4.9 8 5 126.7W EARTHQUAKE 1968 NOV 17 21:11:34 49.ON 6 4.4 3 9 128.8W * from B u l l e t i n of the I n t e r n a t i o n a l S e i s m o l o g i c a l Centre - 64 -Regional s e i s m i c i t y showing l o c a t i o n of explosions and the earthquake used f o r comparison. - 65 -Figure 29 L o c a t i o n of the exposions and earthquake r e l a t i v e to the seismograph s t a t i o n s that recorded the seismograms shown i n Figure 30 and 31. - 66 -FSJ A=tV; LPZ _f 17=40 — "^~V\W>W,' \' _ E___ S P Z_; --17/41— SES A =10.5° • • « A • » • t • » • • V • • " • • • 1 LPZ • » 17=41 • . / t • J • • • t 4 t gure 30 Short p e r i o d v e r t i c a l and long period v e r t i c a l seismograms of underwater e x p l o s i o n w i t h equivalent magnitude of mb=4.9. Note the absence of energy on long period seismograms. - 67 -_ PNT A=6° ' 21=13 ' SES "A-11.5° 21=14 F i g u r e 31 Short p e r i o d v e r t i c a l and long p e r i o d v e r t i c a l seismograms of an mb=4.4 earthquake from the same region as the ex p l o s i o n s . Note the w e l l developed surface waves on the long period seismograms. - 68 -and Resources of Canada P u b l i c a t i o n Canadian Earthquakes. I t i s i n t e r e s t i n g to note that the f i r s t motion d i r e c t i o n s reported i n the B u l l e t i n of the I n t e r n a t i o n a l S e i s m o l o g i c a l Centre i n d i c a t e both compressions and d i l a t a t i o n s were observed from the explosions (see Table V I ) . In f a c t f o r the 1970 event, the compressions and d i l a t a t i o n s are c l e a r l y d i v i d e d i n t o quadrants. Since f o r an e x p l o s i o n , a l l s t a t i o n s should show compressions, the presence of numerous d i l a t a t i o n s i n the ISC catalogue emphasizes the d i f f i c u l t y i n p i c k i n g f i r s t motions f o r events of t h i s s i z e and casts doubt on P nodal s o l u t i o n s f o r earthquakes of t h i s s i z e that r e l y on data from r o u t i n e l y published b u l l e t i n s (e.g. see Hodgson and Adams, 1958). H. CONCLUSIONS Data sets from three d i f f e r e n t time p e r i o d s , 1900-1950, 1951-1969 and 1970-1978 show a co n c e n t r a t i o n of s e i s m i c i t y around the Puget Sound lowlands w i t h a t r u n c a t i o n of the s e i s m i c i t y i n the north at almost the 49th p a r a l l e l s i m i l a r to the t r u n c a t i o n that occurs at the south end of Puget Sound. This change i n the s e i s m i c i t y p a t t e r n i s not a f u n c t i o n of seismograph s t a t i o n d i s t r i b u t i o n or popu l a t i o n d i s t r i b u t i o n i n the e a r l y years. On the other hand, the three l a r g e earthquakes that have occurred i n c e n t r a l Vancouver I s l a n d do not have an as s o c i a t e d c o n c e n t r a t i o n of minor s e i s m i c i t y . The only c o n c e n t r a t i o n of events on the c o n t i n e n t a l s h e l f or c o n t i n e n t a l slope i s o f f the end of the Nootka F a u l t zone and adjacent to the l a r g e c e n t r a l Vancouver I s l a n d events. F o c a l mechanism st u d i e s of the l a r g e r shallow earthquakes show that - 69 -strike-slip faulting is most common and north-south orientation of the pressure axes is a common feature. However, there are slight differences in orientation of pressure axes which may reflect different stress regimes. Large earthquakes of the deeper suite have tension axes dipping to the east. Some smaller deep events also show this but there is a diversity in the mechanisms of the small earthquakes in the deeper site. -70-I I I . SEISMOTECTONICS OF PUGET SOUND - SOUTHERN VANCOUVER ISLAND A. INTRODUCTION The Puget Sound - southern Vancouver I s l a n d s e i s m i c i t y presents a challenge to seismotectonic modelling. The observed s e i s m i c i t y does not express the s i g n a t u r e of a t y p i c a l subduction zone and there i s s t i l l c o n t i n u i n g debate as to whether subduction i s p r e s e n t l y o c c u r r i n g and i f i t i s , whether i t i s happening i n an aseismic manner (Crosson, 1972; Riddihough and Hyndman, 1976; Keen and Hyndman, 1979; Ando and B a l a z s , 1979; Savage et a l . , 1981; Hyndman and Weichert, 1982; Riddihough et a l . , 1982). Among the problems yet to be explained s a t i s f a c t o r i l y are: 1) why there i s a c o n c e n t r a t i o n of s e i s m i c i t y centred on the Puget Sound region and very l i t t l e elsewhere along the convergent p l a t e margin; 2) the meaning and cause of the deeper earthquakes; 3) the l a c k of t h r u s t earthquakes on the subduction i n t e r f a c e that c h a r a c t e r i z e most subduction zones; 4) the small amount of s e i s m i c i t y compared to the high convergence r a t e . This chapter addresses these problems. The increased s e i s m i c i t y i n the Puget Sound - southern Vancouver I s l a n d r e g i o n stands out on any e p i c e n t r e p l o t of the r e g i o n (e.g. Figure 2). Studies of the small earthquakes which have occurred during the l a s t decade by Crosson (1972, 1981) and i n Chapter I I of t h i s work show that the earthquakes occur i n two depth ranges: a shallow s u i t e from the surface to about 30 km and a deeper s u i t e from 40 to 70 km, separated by an aseismic -71-r e g i o n . The shallow earthquakes are more numerous, but the deeper s u i t e contains more l a r g e r events and l i b e r a t e s more energy (Crosson, 1981). This d i v i s i o n can be seen c l e a r l y i f the magnitude versus frequency r e l a t i o n s h i p s of Crosson (1981) f o r shallow and deep s u i t e s of small earthquakes are superimposed ( F i g u r e 32). The two l i n e s cross at about magnitude 4 1/2. I f the whole magnitude range of shallow and deep events f o l l o w s these curves then a higher p r o p o r t i o n of smaller events are shallow and a higher p r o p o r t i o n of events l a r g e r than magnitude 4 1/2 are deeper. The i n f e r e n c e that most of the l a r g e r events are deeper ones can be checked i n two ways. F i r s t , i t has been observed i n t h i s r e gion as w e l l as elsewhere (e.g. Page, 1968; Robinson et a l . , 1975) that the deeper events are accompanied by very few, i f any, a f t e r s h o c k s . Thus, a f t e r making a search through f e l t r e p o r t s and newspaper reports f o r mention of aftershocks of a l l s i z e a b l e events i t i s noted that very few have any a f t e r s h o c k s r e p o r t e d , suggesting that they are deep (see Chapter I I ) . Second, the "b" value computed using j u s t the l a r g e events f o r the e n t i r e time p e r i o d i s 0.68. This i s almost i d e n t i c a l to that f o r the smaller deep events (Figure 32). Thus i t seems reasonable that the depth d i s t r i b u t i o n observed i n the s m a l l e r earthquakes of the l a s t decade can be a p p l i e d to the whole s u i t e of earthquakes observed s i n c e the t u r n of the century. Any s a t i s f a c t o r y t e c t o n i c model put forward f o r t h i s r e gion must e x p l a i n not only t h i s bimodal d i s t r i b u t i o n and the co n c e n t r a t i o n of s e i s m i c i t y i n Puget Sound, but the amount of s e i s m i c i t y and the f o c a l mechanisms observed i n both the deep and shallow s u i t e s . The model I propose here provides explanations f o r a l l of these p o i n t s . - 72 -F i g u r e 32 Recurrence r e l a t i o n s h i p s c a l c u l a t e d by Crosson (1981) f o r earthquakes i n the Puget Sound region from 1970 - 1978. The two curves cross at magnitude 4 1/2 i n d i c a t i n g that the m a j o r i t y of earthquakes l a r g e r than t h i s magnitude are deeper events. -73-B. DEEP SEISMICITY (40 km to 70 km) 1) Main Seismic Zone The main zone of deep s e i s m i c i t y i n the Puget Sound region as defined by earthquakes i n the l a s t decade i s shown i n Figure 33. I t extends about 250 km i n a north-south d i r e c t i o n from the Gulf Islands i n the north to j u s t past the terminus of Puget Sound i n the south. The width of the zone i s roughly 100 km. I t i s d e l i n e a t e d by both the w e l l l o c a t e d small earthquakes o c c u r r i n g i n the l a s t decade and by the l a r g e r earthquakes that have occured s i n c e the tu r n of the century (Fi g u r e 10). Work done here ( F i g u r e 15) and that of others ( N u t t l i , 1952; Algermissen and Harding, 1965; Crosson, 1972, 1981) show that the earthquakes i n t h i s zone range i n depth from about 40 km to 70 km. A cross s e c t i o n through c e n t r a l Puget Sound i n d i c a t e s a s u i t e of deeper earthquakes w i t h a gentle eastward d ip of about 12° (see Figure 34). 2) S t r u c t u r e of the Subducted P l a t e The deeper s u i t e of earthquakes must be i n the subducted p l a t e because the p l a t e contains the only m a t e r i a l i n that depth range that has a s u f f i c i e n t l y low temperature to support shear s t r e s s at the depths of the earthquakes. Thus a few w e l l l o c a t e d earthquakes can be used to f i x the l o c a t i o n of the p l a t e ( F i g u r e 35). The t o t a l t hickness of the p l a t e as i t s t a r t s to subduct i s c a l c u l a t e d to be about 30 km using Oldenburg's (1975) square root of age formula ( t h i c k n e s s (km) = 9.5yage (m.y.)). This assumes a melti n g temperature of about 1200°C on the bottom edge of the l i t h o s p h e r e . The b r i t t l e t hickness where earthquakes can occur must be much t h i n n e r , as earthquakes can only occur i n the p o r t i o n of the s l a b that i s c o l d enough to support e l a s t i c s t r e s s . Estimates f o r the maximum temperature of the l i t h o s p h e r e that can support earthquake s t r e s s range from 300°C to 700°C (e.g. Brace and Byerl e e , 1970; Burr and Solomon, - 74 -Figure 3 3 Deeper events i n Puget Sound from 1970 to 1980. AB and A'B' are locations of cross sections i n Figures 3 5 and 3 6 . - 75 -X - SECTION. FILL QUAKES .CE. MAG 2-0. APERTURE 300 KM AZIMUTH OF PROJECTION ; 60 DEGREES ! 1 . . . ,., 1 L L « w - - U J L - L » O o t f T 0 oo3> ° O S o Ml (So eO ° S ° o °#oSXl> -5° " > f e CD . O o e 0 o o 0 ° oo - o J S ° Vs.;: 6 , 7 ' 0 ^ _ o ° O o o 2.0iM<3.0 O 3.0sM<4.0 O 4.0iM ( H 1 0 10 20 30 40 50 60 70 80 90 100 KM Fxgure 34 Cross s e c t i o n through c e n t r a l Puget Sound i n the d i r e c t i o n the subducted p l a t e i s moving along s e c t i o n A'-B' i n Figure 33 ( a f t e r Crosson, 1981). - 76 -A I B Figure 35 Cross s e c t i o n through V i c t o r i a and Mt. Baker along s e c t i o n A-B of Figure 33. Eight w e l l l o c a t e d earthquakes are p r o j e c t e d on to t h i s plane. The dip of the p l a t e (12°) i s c o n t r o l l e d by the l o c a t i o n of these earthquakes which must occur near i t s upper s u r f a c e . The bend to a steeper angle i s suggested by the t e n s i o n axes of l a r g e r earthquakes ( d i s c u s s i o n i n s e c t i o n B5) and the potassium r a t i o s i n the lavas of the Cascade volcanoes ( D i c k i n s o n , 1970). No v e r t i c a l exageration. -77-1978; C a l d w e l l and T u r c o t t e , 1979). I n Figure 35 the b r i t t l e thickness i s depicted as 10 km at the s t a r t of subduction based on the order of seismic rupture depth expected f o r oceanic earthquakes (Burr and Salomon, 1978). This estimate i s supported by depths of l e s s than 10 km found f o r microearthquakes i n the oceanic l i t h o s p h e r e west of Vancouver I s l a n d (Hyndman and Rogers, 1981). The b r i t t l e p o r t i o n w i l l become thinner due to heating as the s l a b reaches greater depth (e.g. Toksoz et a l . , 1972; Keen and Hyndman, 1979). The bend i n the s l a b where i t dips at a steeper angle (Figure 35) has been drawn to be c o n s i s t e n t w i t h the depths to the subducting slab under the Cascade volcanoes suggested by Dickinson (1970) on the basis of the geochemistry of the lavas and w i t h the average worldwide depth of about 100 km (e.g. Isacks and Barazangi, 1977). There are subduction zones where the depth to the top of the subducted s l a b i s l e s s than 100 km (e.g. the North I s l a n d of New Zealand as shown by Reyners, 1980 and the northeastern Japan area as shown by Hasegawa et a l . , 1978). Thus, i t i s p o s s i b l e a bend may not be r e q u i r e d here. 3) Maximum Depth of Earthquakes The absence of a B e n i o f f zone of deeper earthquakes extending beneath the Cascade volcanoes has been c i t e d as an argument against a c t i v e subduction of the Juan de Fuca p l a t e (e.g. Crosson, 1972). The B e n i o f f zone does e x i s t ( F i g u r e 34) but i t does not extend very deep or very f a r i n l a n d . As Riddihough and Hyndman (1976) point out, earthquakes i n the subducted Juan de Fuca p l a t e should not be expected to be much deeper than 70 km because earthquakes w i t h i n the subducted p l a t e can only occur i n that r e g i o n which remains c o l d enough and the r e f o r e b r i t t l e enough to support shear s t r e s s . I t has been shown on a worldwide basis that the maximum -78-depth of earthquakes depends on the age of the m a t e r i a l when i t was subducted and the subduction ra t e (Deffeyes, 1972). E x t r a p o l a t i n g those data to the young age of the Juan de Fuca p l a t e , i t can be shown, as Riddihough and Hyndman (1976) have done, that earthquakes i n the subducting Juan de Fuca p l a t e should not be deeper than about 70 km. This i n f o r m a t i o n can be turned i n t o an estimate of the i n l a n d extent of the deeper s u i t e of earthquakes (Figure 36). Because the spreading r i d g e has remained r e l a t i v e l y s t a t i c o f f the west coast of North America f o r the past 10 m i l l i o n years and spreading r a t e s are q u i t e w e l l known (Riddihough, 1977), the age at which the l i t h o s p h e r e subducted can be estimated f o r any po i n t along the margin. Then by using the e m p i r i c a l r e l a t i o n s h i p from Deffreys (1972), the maximum depth, and thus the maximum i n l a n d extent, can be c a l c u l a t e d since the cross s e c t i o n a l geometry of the subducted p l a t e has already been determined (Figure 35). The maximum observed depth of 70 km serves as a c a l i b r a t i n g point f o r the e m p i r i c a l r e l a t i o n s h i p and thus the r e l a t i v e i n l a n d extent of p o t e n t i a l f o r deeper earthquakes can be d i s p l a y e d ( F i g u r e 36). The much shallower regime opposite the Ex p l o r e r p l a t e i s immediately obvious. 4) Space Problem I n the r e g i o n of the deeper earthquakes of the Puget Sound and southern Vancouver I s l a n d r egion there i s a space problem i n the subducted p l a t e because the bend a x i s p a r a l l e l s the c o a s t l i n e and thus changes from north-south o f f Washington and Oregon to northwest-southeast o f f Vancouver I s l a n d (Figure 37). The l a t e r a l shortening t h a t must occur i n a subducting p l a t e encountering a convex bend has been discussed by Isacks and Molnar (1971). I suggest there are f o u r p o s s i b l e ways that the excess of m a t e r i a l created by the subducting geometry can be d e a l t w i t h ( F i g u r e 38). F i r s t - 79 -OLYMPIC MOUNTAINS MAXIMUM INLAND EXTENT OF EARTHQUAKES WITHIN gTB'E SUBDUCTED PLATE Figure 36 The maximum inland extent of the deeper suite of earthquakes assuming the empirical r e l a t i o n s h i p from Deffeyes (1972) on plate age and subduction v e l o c i t y i s correct and that the deepest earthquakes i n Puget Sound (70 km) are used as a c a l i b r a t i n g point. - 80 -A r i g i d p l a t e model of the subducting Juan de Fuca P l a t e d e p i c t i n g the subducted p o r t i o n of the p l a t e sheared and overlapped as a r e s u l t of the change i n o r i e n t a t i o n of the subduction zone from north-south o f f Oregon to northwest-southeast o f f Vancouver I s l a n d (adapted from Keen and Hyndman, 1979). Large arrows d e p i c t r e l a t i v e motion of overlapped p o r t i o n s . Small arrows d e p i c t p l a t e motion r e l a t i v e to the North America P l a t e from Riddihough (1977,1980). - 81 -a. S H E A R a n d O V E R L A P d. V O L U M E C H A N G E F i g u r e 38 A north-south cross s e c t i o n through the subducting p l a t e i n the Puget Sound region showing 4 ways the e x t r a m a t e r i a l r e s u l t i n g from the change i n the trench a x i s could be handled: (a) the p l a t e could be sheared and overlapped, (b) i t could be shortened and thickened by a s e r i e s of low angle f a u l t s (c) i t could assume gentle f o l d s or (d) the volume could change due to phase changes i n the l i t h o s p h e r e . - 8 2 -the p l a t e can be sheared and overlapped as has been modelled by Keen and Hyndman, (1979) and as shown i n Figure 37. Second, the shortening can take place i n the m a t e r i a l along a s e r i e s of l i s t r i c f a u l t s which must r e s u l t i n a net increase i n volume i n the a f f e c t e d region. T h i r d , the m a t e r i a l can bend i n t o a s e r i e s of undulations (much as happens to a t a b l e c l o t h when i t i s draped over the corner of a t a b l e ) . Fourth, the volume can be decreased by a change i n phase. The l a t t e r seems the most l i k e l y i n the r e g i o n of the observed earthquakes and w i l l be discussed more f u l l y i n the next s e c t i o n . a) Overlap Model This can be r e j e c t e d immediately i n the region of the deep earthquakes as the overlap should show up i n t h e i r depth d i s t r i b u t i o n . One s u i t e of deep events should be d i s p l a c e d the whole thickness of the l i t h o s p h e r e (about 30 km) higher or lower than the other s u i t e . There i s no evidence f o r t h i s i n the d e t a i l e d set of hypocentres c o l l e c t e d i n the l a s t decade. A l s o , one would expect at the onset of the overlap a shearing mechanism along a v e r t i c a l plane approximately perpendicular to the coast l i n e . There i s no evidence f o r t h i s i n the f o c a l mechanisms of the deep earthquakes. However u n r e a l i s t i c the overlap model may be, i t does provide a simple way to c a l c u l a t e the amount of e x t r a m a t e r i a l that must be d e a l t w i t h i n the subducted p l a t e . The amount of shortening needed can be found by c a l c u l a t i n g the amount of overlap. In Figure 39 the overlap i s q u a n t i t y "x" where x = 2a tan&( The distance "a" i s the amount of surface p r o j e c t i o n l o s t due to downbending and the angle " Gt" i s one h a l f the angle beyond 90° at the corner (p( = 22 1/2° f o r t h i s case where the change i n the d i r e c t i o n of - 83 -a. PLAN VIEW OF O V E R L A P MODEL b. C R O S S S E C T I O N ALONG AXIS O F SY M M E T R Y Figure 39 The geometry of c a l c u l a t i n g the amount of overlap "x" i n the overlap model shown i n Figure 37. -84-the c o a s t l i n e i s 45°). Viewing the diagram i n cross s e c t i o n a = b t a n ^ where "b" i s the depth of subduction (70 km maximum i n the earthquake region) and "^ g " i s the angle of subduction (about 12° through the zone of earthquakes). This gives a value of 14.8 km f o r "a" and a value of 11.3 km f o r "x" at the deepest part of the seismic zone. This distance w i l l decrease l i n e a r l y towards the o r i g i n of the f o l d and so f o r the western side of the region of earthquakes, which i s about one t h i r d the distance from the deepest earthquakes to the trench, the overlap "X" w i l l be reduced by about one t h i r d or have a value of 7.6 km. I f these distances are considered as percentages of the t o t a l north-south length of about 250 km f o r the s e i s m i c i t y zone, then f o r the deepest part of the seismic zone i t i s 4.5% (11.3 / 250) and f o r the shallowest part 3.0% (7.6 / 250). This i s the percentage of shortening i n the zone of s e i s m i c i t y that must be d e a l t w i t h i n the remaining models, b) Shortening Model The shortening model would have the l i t h o s p h e r e shorten i n a north-south d i r e c t i o n by i n t e r n a l f a u l t i n g to form a s l i g h t l y t h i c k e r l i t h o s p h e r e . Although t h i s seems f e a s i b l e i n terms of observed s t r u c t u r a l deformation preserved i n the geology i n various places around the world, i t i s r u l e d out on the b a s i s of the observed f o c a l mechanisms of the deep earthquakes. Although there i s a d i v e r s i t y i n the observed f o c a l mechanisms, there i s no evidence of north-south compression or north-south t h r u s t f a u l t i n g i n any mechanisms (e.g. see Figures 25 and 26). This would be necessary as 3% to 4.5% of shortening must take place i n the b r i t t l e r e g i o n where the earthquakes are o c c u r r i n g . The f o c a l mechanisms would have to r e f l e c t north-south compression i f that i s the main body force i n -85-t h i s p o r t i o n of the subducted p l a t e , c) Bending Model The idea that the downgoing p l a t e might deform as a gentle f o l d or s e r i e s of gentle f o l d s as i t bends down and around a corner may be a reasonable one. I f a s t i f f p l a t e i s bent i n a viscous medium, there i s an e m p i r i c a l r e l a t i o n s h i p (L/T 27) between the thickness of the p l a t e (T) and the wavelength of deformation (L) that p e r s i s t s through a l l s i z e s of g e o l o g i c a l s t r u c t u r e s from centimetres to tens of ki l o m e t e r s ( C u r r i e e t a l . , 1962). Thus, a wavelength of about 250 km i s produced by bending a l a y e r w i t h a st r e n g t h member about 10 km t h i c k . I f a l l the deformation were to take place i n one f o l d the amplitude of the f o l d can be c a l c u l a t e d using the equations f o r bending an e l a s t i c beam which have been used by C u r r i e et a l . (1962) f o r the development of f o l d s i n sedimentary s t r a t a . Since we know the shortening (S) from the overlap model and the wavelength (L) i s the l e n g t h of the seismic zone, the amplitude (a) i s : Thus, f o r the p l a t e i n the region of the deepest s e i s m i c i t y the amplitude would be about 40 k i l o m e t r e s . This means that the earthquakes i n the subducted p l a t e i n c e n t r a l Puget Sound should be 40 kil o m e t e r s shallower (or deeper) than those at the north and south ends of the zone. There i s no evidence f o r t h i s . The o v e r a l l u n c e r t a i n t y i n the depths of w e l l l o c a t e d Puget Sound events i s f o r the most part l e s s than 10 km (Peters and Crosson, 1972). Thus the amplitude of a bend must be at l e a s t 10 km before i t can be sure of being detected. This would n e c e s s i t a t e three or more f o l d s to take up the m a t e r i a l at the eastern side of the deep seismic zone and keep the amplitude below 10 km. The main segment of deep earthquakes a = 2SL_ _ * /l-3*250 - 42. -86-appears to be only i n Puget Sound. Thus three equal amplitude f o l d s cannot be t a k i n g place; otherwise there would be three groups of deep earthquakes of equal i n t e n s i t y spread about 250 km apart. Thus gentle f o l d i n g of the subducting p l a t e cannot be t a k i n g up a l l of the shortening necessary i n the re g i o n of deep earthquakes, d) Phase Change Model As the oceanic l i t h o s p h e r e descends i n t o the mantle i t must undergo s e v e r a l phase changes (e.g. Anderson, 1967; Ringwood, 1972; Toksoz et a l . , 1973). The deep mantle phase changes and t h e i r c h a r a c t e r i s t i c s have been discussed i n many papers. I t i s , however, the shallow phase changes that are of importance here. F i r s t , there are the changes i n the descending oceanic c r u s t from b a s a l t and gabbro to e c l o g i t e (Ringwood and Green, 1966; Green and Ringwood, 1967). This i n v o l v e s a l a r g e d e n s i t y change from about 3 g/cm to 3.5 g/cm which i s equivalent to a volume change of about 17% (Green and Ringwood, 1967). This phase change takes place i n the depth range of 30 km to 70 km (Grow and Bowin, 1975; Ahrens and Schubert, 1975) (Figu r e 40). A l s o , i n t h i s depth range the oceanic mantle undergoes phase changes ( W y l l i e , 1971; Grow and Bowin, 1975). P l a g i o c l a s e p e r i d o t i t e c o l l a p s e s to a s p i n e l p e r i d o t i t e and then to garnet p e r i d o t i t e (Figure 41), a t o t a l volume change of about 4.5 % f o r the mantle composition suggested by W y l l i e (1971). A l l of the phase changes are depicted i n Figure 42. The mantle p o r t i o n of the subducted p l a t e i s the t h i c k e s t and i t has the s m a l l e s t volume change. Once i t has contracted about 4.5% to garnet p e r i d o t i t e i t w i l l c o n t r a c t no f u r t h e r because there are no more known phase changes between 80 and 300 km (Grow and Bowin, 1975). As i t turns out, the volume change a v a i l a b l e i s e x a c t l y the amount needed (4.5 %) to compensate f o r the p l a t e bending around the corner at the depth of the earthquakes. The volume changes necessary w i t h i n the seismic zone vary from - 8 7 -3.0 3.2 3.4 3.6 i i — i — i — i — i — i — r Figure 40 Several experimentally measured curves of v a r i a t i o n i n density as gabbro i s transformed to e c l o g i t e i n the oceanic crust as i t descends to 70 km(adapted from Wyllie, ,1971). - 88 -TEMPERATURE °C PERIDOTITE PHASE DIAGRAM Figure 41 Phase changes i n a p y r o l i t i c mantle i n the 30 km to 70 km depth-range (adapted from W y l l i e , 1971). Both phase changes can be expected to take place i n the depth range of the earthquakes. They have volume changes of 1.8% and 2.3% as s o c i a t e d w i t h them. In order to be c o o l enough f o r b r i t t l e f r a c t u r e the geotherm i n the subduction zone must be s e v e r a l hundred degrees c o o l e r (to the l e f t ) than the oceanic geotherm i n the 60-70 km depth range. - 89 -Figure 42 Summary diagram of phase changes that must take place and h they f i t i n t o the subducting p l a t e i n the Puget Sound r e g i o n . Base diagram i s the same as Figure 35 . -90-3.0 % i n the western part of the seismic zone to 4.5% at the eastern side where the deepest earthquakes are (see s e c t i o n 4a of t h i s chapter). Thus the phase changes that are l i k e l y to happen i n t h i s depth range, i f they occur completely by 70 km depth, and the c o n t r a c t i o n takes place i n a h o r i z o n t a l d i r e c t i o n , w i l l take care of any space problem without n e c e s s i t a t i n g any f o l d i n g , bending or crumpling of the subducted p l a t e — at l e a s t i n the 30 to 70 km depth range. Bending i s the most l i k e l y mode of deformation shallower and deeper than the phase changes (see s e c t i o n 4e of t h i s c h a pter). The next question to be addressed i s : could the phase changes be r e s p o n s i b l e f o r the observed s e i s m i c i t y ? The suggestion that deep phase changes o c c u r r i n g i n a c a t a s t r o p h i c f a s h i o n might be the cause of deep earthquakes has been debated i n the l i t e r a t u r e f o r many years (e.g. Bridgman, 1945; B i r c h , 1952; Griggs and Handin, 1960; R a n d a l l , 1964; E v i s o n , 1967). The c o n c l u s i o n seems to be that most deep earthquakes are caused by shear f a i l u r e and are f i t t e d w e l l by a double couple model ra t h e r than an implosive one, although some events may have an implosive source superimposed on a shearing source (E v i s o n , 1967). The p o s s i b i l i t y that a phase change need not occur c a t a s t r o p h i c a l l y to cause an earthquake was f i r s t discussed by Ringwood (1972). He suggested that c o n t r a c t i o n of a volume of mantle i n the s i n k i n g s l a b due to a phase change may occur sl o w l y but would probably cause the development of l a r g e s t r e s s e s i n the r e g i o n surrounding the c o n t r a c t i n g volume. These s t r e s s e s would then cause shear f a i l u r e producing earthquakes. Woodward (1977) has shown w i t h numerical modeling that the s t r e s s e s caused by phase changes can be s e v e r a l k i l o b a r s greater than the ambient h y d r o s t a t i c pressure. This should be ample to cause earthquakes since the s t r e s s drops observed f o r most earthquakes are l e s s than 100 bars (Kanamori and Anderson, 1975). -91-McGarr (1977) showed very c o n v i n c i n g l y that most deep mantle earthquakes are as s o c i a t e d w i t h the phase changes i n a c t i v e l y subducting p l a t e s ( F i g u r e 43). He added up the t o t a l moment of earthquakes i n the region of each phase change ( s e i s m i c moment i s displacement times f a u l t a r e a ) . He then used h i s r e l a t i o n s h i p between seismic moment and volume change (McGarr, 1976) to show that f o r each phase change the t o t a l moment observed was equ i v a l e n t to that expected from the amount of volume change. The moment r a t e was thus p r o p o r t i o n a l to the rate at which the p l a t e was descending i n t o the mantle . McGarr d i d not de a l w i t h the shallower phase changes discussed here. However, h i s equations can be a p p l i e d i n the same way as he a p p l i e d them to the deeper phase changes. His b a s i c equation i s where _> M„ i s the cummulative moment rate of earthquakes i n the depth range of the phase change;yd i s the modulus of r i g i d i t y f o r mantle depths (taken to be 7 x lO-H dyne/cm^); L i s the length of the seismic zone along s t r i k e ( t h i s i s the 250 km north-south extent of the earthquake zone); T i s the thickness of the l a y e r where the phase change i s t a k i n g p l a c e ; (—) i s the f r a c t i o n a l volume change a n d / 2 ^ u b i s the subduction The subduction v e l o c i t y i s the rate at which the p l a t e descends i n t o the mantle and thus i s p r o p o r t i o n a l to the absolute motion of the subducting p l a t e not the r e l a t i v e convergence r a t e at the subduction zone. For the Juan de Fuca p l a t e the absolute v e l o c i t y i s about 2 cm per year (Riddihough, 1981) as compared w i t h about 4.5 cm per year f o r the convergence v e l o c i t y (Atwater, 1970). The d i r e c t i o n i s almost the same. However, because the subduction angle of the Juan de Fuca p l a t e (about 12° i n c r e a s i n g to 30°) i s much l e s s than those i n the subduction zones v e l o c i t y . - 92 -F i g u r e 4 3 d i s c u s s e d by McGarr (averaging about 45°), and because the v e r t i c a l r a t e i n t o the mantle i s the important parameter, an a d d i t i o n a l f a c t o r of 1/2 i s introduced i n t o the McGarr's equation f o r use w i t h the Juan de Fuca p l a t e . For the oceanic c r u s t , during the gabbro/basalt to e c l o g i t e phase change the d e n s i t y changes from about 3.0 g/cm3 to 3.5 g/cm3, or about 17 percent (Ringwood and Green, 1966). Thus, f o r a t y p i c a l 7 km t h i c k oceanic c r u s t ( H a r r i s o n and B o n a t t i , 1981) the t o t a l moment a v a i l a b l e from phase changes i n a 7 km t h i c k oceanic c r u s t i s : £ M Q = 1/2 x 7 x 1 0 1 1 dyne/cm 2 x 250 km x 7 km x 0.143 x 2 cm/yr = 0.018 x 1 0 2 6 dyne cm/yr. For the mantle p o r t i o n of the subducting l i t h o s p h e r e , i f the bulk composition of W y l l i e (1971) i s used, there i s the change from p l a g i o c l a s e p e r i d o t i t e to s p i n e l p e r i d o t i t e corresponding to a den s i t y change of 2.5% , and a f u r t h e r change from s p i n e l p e r i d o t i t e to garnet p e r i d o t i t e corresponding to a de n s i t y change of 1.8% Together these make a volume change of 4.3 % or a ^ o f 0.041. Thus, the t o t a l moment a v a i l a b l e from phase changes i n the oceanic mantle of thickness 23 km i s : M0 = 1/2 x 7 x 1 0 1 1 dyne/cm 3 x 250 km x 23 km x 0.041 x 2 cm/yr. = 0.016 x 1 0 2 6 dyne-cm/yr. The t o t a l moment a v a i l a b l e i f a l l volume changes go i n t o earthquakes i s 0.034 x 1 0 2 6 dyne-cm/yr. ( i ) Moment Rate The moment r a t e p r e d i c t e d from phase changes can be compared with the observed moment r a t e . Since the Puget Sound earthquake l i s t i s complete f o r l a r g e r earthquakes si n c e 1900, the simplest way to estimate the observed moment r a t e i s to add up the moment of each earthquake and d i v i d e by the observation time of 80 years. The problem w i t h t h i s i s that the t o t a l moment i s dominated by the l a r g e s t earthquakes and because of the -94-u n c e r t a i n t y i n the magnitude of the l a r g e s t event, the A p r i l 13, 1949 earthquake (see Chapter I I ) , there i s a l s o u n c e r t a i n t y i n i t s moment. The moment of the second l a r g e s t earthquake ( A p r i l 29, 1965) has been c a l c u l a t e d by Langston and Blum (1977) to be 1.2 x 102** dyne-cm. (This i s e q u i v a l e n t to a magnitude 6.9 earthquake using the most common moment-magnitude r e l a t i o n s h i p s , e.g. Kanamori and Anderson, 1975). The moments of the other earthquakes are computed from the Kanamori and Anderson (1975) formula (assuming an average s t r e s s drop of 30 bars) and the magnitudes c a l c u l a t e d here, which are based on f e l t area (Table I I ) . I f the 1949 earthquake i s assumed to have the 6.75 magnitude c a l c u l a t e d from i s o s e i s m a l areas, then the moment rate i s 0.030 x 10 dyne-cm/year. I f the published value of magnitude 7.1 i s used then the moment rate i s 0.0525 x 10 2^ dyne-cm/year. A b e t t e r way to estimate the moment r a t e i s to p l o t the cummulative sum of the moments of i n d i v i d u a l earthquakes and attempt to approximate i t w i t h a l i n e a r f u n c t i o n ( F i g u r e 44). Again, the u n c e r t a i n t y of the magnitude of the l a r g e s t event i s the biggest source of e r r o r . I f a magnitude of 6.75 i s used then the moment r a t e i s 0.031 x 10 2^ dyne-cm/year and i f a magnitude of 7 i s used the r a t e i s 0.0415 x 10 2^ dyne-cm/year. Approximating a few l a r g e step f u n c t i o n s by a s t r a i g h t l i n e i s not a very s a t i s f a c t o r y process, but accepting the moments c a l c u l a t e d here, i t i s d i f f i c u l t to see how the moment rat e s can be too much d i f f e r e n t from the value presented i n Figure 44. I f the 0.031 x 10 2^ dyne cm/year rat e i s accepted then the r a t e i s almost e x a c t l y equal to the moment rate expected from phase changes using the McGarr (1977) r e l a t i o n s h i p . Given the dates of the l a r g e r earthquakes, even a l l o w i n g f o r considerable v a r i a t i o n i n the moment-magnitude r e l a t i o n s h i p (say a f a c t o r of two) when c a l c u l a t i n g the moments other than the 1965 event, the moment r a t e c a l c u l a t e d i n t h i s way - 95 -F i g u r e 44 The dashed step f u n c t i o n represents the cumulative sum of moments f o r published magnitude values f o r Puget Sound c a l c u l a t e d earthquakes using the moment - magnitude r e l a t i o n s h i p of Kanamori and Anderson (1975). The s o l i d ste f u n c t i o n uses magnitude values c a l c u l a t e d here i n Chapter I I . The s t r a i g h t l i n e i s an estimate of moment r a t e . - 9 6 -cannot vary more than about 15%. However, because there are so few l a r g e events and t h e i r occurence should be random i n time, the u n c e r t a i n t y i n the moment r a t e determined i n t h i s way i s q u i t e l a r g e . ( i i ) Moment Rate from Recurrence R e l a t i o n s h i p s Another way of e s t i m a t i n g the moment r a t e i s by using the magnitude recurrence r e l a t i o n s h i p of Gutenberg and R i c h t e r (1949) which i s u s u a l l y expressed a: LogN = a - bM where N i s e i t h e r the cumulative number of earthquakes greater than a given magnitude or the number of earthquakes i n a given magnitude range. The constants "a" and "b" are determined from the data s e t . Because t h i s r e l a t i o n s h i p i s u s u a l l y defined from a l a r g e body of data i t i s s t a t i s t i c a l l y s t a b l e . Smith (1976) f i r s t suggested using t h i s k i n d of approach and used i t to estimate maximum magnitude. Molnar (1979) and Anderson (1979) used the same idea to estimate earthquake recurrence r e l a t i o n s h i p s from g e o l o g i c a l and p l a t e t e c t o n i c data. F o l l o w i n g Anderson's (1979) paper, i f the earthquake recurrence r e l a t i o n s h i p i s combined w i t h a moment-magnitude r e l a t i o n s h i p (e.g. that of Thatcher and Hanks, 1973), LogM Q = cf = 3/2M + 16 the r e s u l t i n the n o t a t i o n of Anderson (1979) i s : N( ) = 1 0 c _ d y where d = 2/3b and c = a + 16d - logjQ(3/2). The moment r a t e i s then: dt if min , \0C f | 0 ( i - c O V m a . y ( l - d)£n/0 'a" i n Anderson's (1979) equations i s defined f o r an i n t e r v a l type of -97-recurrence r e l a t i o n s h i p whereas the value u s u a l l y c a l c u l a t e d i s f o r a cumulative r e l a t i o n s h i p " a c " . Thus the number f o r Anderson's equations i s : a = a c + l o g 1 0 ( 2 . 3 0 3 b ) (see Hyndman and Weichert, 1982). Using "a" and "b" values from Crosson's (1981) recurrence r e l a t i o n s h i p ( F i g u r e 20) and assuming a maximum magnitude of 7 1/4, the moment rate i s c a l c u l a t e d to be 0.0194 x 10 2^ dyne cm/year. This compares favourably w i t h the moment r a t e a v a i l a b l e from phase changes (Table V I I ) . e) Space Problem Above and Below the Earthquake Zone Although the volume changes take care of the space problem i n the 30 km t o 70 km depth range, at shallower depths i n the surrounding s l a b few, i f any earthquakes occur, and thus i t can be assumed the phase changes are not yet t a k i n g place. A space problem s t i l l e x i s t s but i t i s probably accommodated by s l a b bending. This i s suggested because there i s no co n c e n t r a t i o n of s e i s m i c i t y to i n d i c a t e shearing or shortening and the topography i s l o c a l l y u p l i f t e d between the trench and Puget Sound to form the topographic high of the Olympic Mountains (e.g. see Figure 36). The Olympic Mountains g r a d u a l l y i n c r e a s e i n height from west to east u n t i l they are truncated by Puget Sound, which i s what would be expected i f they were caused by an underlying f o l d i n the subducting p l a t e that i s g r a d u a l l y i n c r e a s i n g i n amplitude. Deeper than 70 km there i s a l s o the same space problem (Figure 37) as there are no more phase changes to reduce the volume u n t i l the 400 km depth range (e.g. Ringwood, 1972; McGarr 1977). The problem i s more acute here as the s l a b i s probably di p p i n g at a steeper angle of 30°. Thus again, the p l a t e can e i t h e r be overlapping, i n t e r n a l l y deforming or bending i n t o g e n t l e f o l d s . Below 70 km there are no earthquake f o c a l mechanisms to help -98-TABLE V I I Moment Rates ( x l O 2 ^ dyne-cm/yr) A v a i l a b l e from 0.034 Phase changes Observed: 1) Cumulative 0.031 2) Recurrence 0.019 -99-s o r t out the choices but there i s some evidence from the volcanism suggesting that g e n t l e f o l d s , such as those shown i n Figure 45, are the mode of deformation. Dickinson (1970) c a l c u l a t e d the depth to the magma source f o r the major Cascade volcanoes from potassium r a t i o s i n the l a v a s . He contoured the depth of the source zones and h i s f i g u r e i s reproduced i n Figure 46. The wavelength and amplitude are c l o s e to what would be expected i f a s l i c e were taken through Figure 45 at 100 km depth. 5) F o c a l Mechanisms There have been only two deeper earthquakes that have been l a r g e enough to record on enough seismograph s t a t i o n s to enable w e l l constrained t e l e s e i s m i c f a u l t plane s o l u t i o n s to be c a l c u l a t e d . These are the 1965 S e a t t l e earthquake and the 1976 earthquake i n the Canadian Gulf I s l a n d s . These earthquakes are at extreme ends of the s u i t e of deeper earthquakes but t h e i r f a u l t plane s o l u t i o n s are remarkably s i m i l a r ( F i g u r e 25). They are c o n s i s t e n t w i t h downdip extension seen elsewhere i n subducting l i t h o s p h e r e (Isacks and Molnar, 1971) and the azimuth and dip d i r e c t i o n of the t e n s i o n axes are c o n s i s t e n t w i t h that p r e d i c t e d by the steeper angle f o r the subducted p l a t e of 25° to 30° to reach a depth of 100 km underneath the volcanoes (Table V I I I ) . Crosson (1981) has presented f o c a l mechanisms of s e v e r a l smaller deep Puget Sound earthquakes. Only a few of the smaller earthquakes have mechanisms s i m i l a r to the 1965 and 1976 events, but these two earthquakes are much l a r g e r than the earthquakes i n Crosson's s t u d i e s and l i k e l y f r a c t u r e d a l l the way through the b r i t t l e p o r t i o n of the subducted l i t h o s p h e r e (see Figure 35). Thus, the two l a r g e r events probably r e f l e c t the s t r e s s regime a c t i n g on the whole l i t h o s p h e r e . The smaller events may r e f l e c t more complex s t r e s s e s i n v o l v e d i n the phase change process. - 100 -Figure 45 Cartoon of model based on c a l c u l a t i o n of a c t u a l amplitudes needed to accommodate the e f f e c t of the 45° change i n d i r e c t i o n of the subduction zone. Part "a" shows l o c a t i o n of f o l d s r e l a t i v e to o v e r l y i n g p l a t e ; "b" i s hatchered i n the r e g i o n of the deeper Puget Sound earthquakes. Before the second kneebend, gentle f o l d i n g i s unnecessary as volume changes due to phase changes accomodate the e x t r a m a t e r i a l . Below the bend, however, the p l a t e may look l i k e the cartoon. - 101 -Figure 46 From Dic k i n s o n (1970) showing that depth to magma source i s c o n s i s t e n t w i t h the sources being on a convoluted surface. Black dots represent major Cascades volcanoes and depth numbers i n d i c a t e depth of the Juan de Fuca p l a t e . -102-TABLE V I I I Comparison of Tension Axes From Deep Earthquakes  With Motion of Subducting P l a t e EARTHQUAKE TENSION AXES PLATE MOTION3 AZ DIP AZ DIP 1965 1 63 26 63 30 1976 2 61 30 1 S o l u t i o n of Isacks and Molnar (1971). 2 T e l e s e i s m i c data s o l u t i o n c a l c u l a t e d here; shown i n Figure 25. 3 D i r e c t i o n of absolute p l a t e motion of Juan de Fuca p l a t e under Puget Sound c a l c u l a t e d using pole of Riddihough (1982). -103-Woodward (1977) d i d extensive modelling of phase changes i n the subducting l i t h o s p h e r e . He found that s t r e s s e s due to the changes i n phase induced i n a descending l i t h o s p h e r i c p l a t e are an order of magnitude l a r g e r than those due to the negative buoyancy of the s l a b i n the asthenosphere. In a l l cases he found there were l a r g e t e n s i o n a l s t r e s s e s i n the centre of the s l a b p a r a l l e l to i t s edges and smaller compressional stresses at the edges. He c a l c u l a t e d the h o r i z o n t a l components of the st r e s s e s to be about equal to the h y d r o s t a t i c s t r e s s e s a p p l i e d to the boundaries. Thus, h i s t h e o r e t i c a l s t u d i e s p r e d i c t downdip extension i s the predominant e f f e c t of phase changes which i s e x a c t l y what i s observed i n the Puget Sound-southern Vancouver I s l a n d region. Woodward (1977) a l s o found that the st r e s s e s induced by the phase changes themselves are s u f f i c i e n t l y l a r g e to a l t e r the e q u i l i b r i u m of the phases f o r a given depth. Thus, the phase t r a n s i t i o n s can migrate from the t h e o r e t i c a l e q u i l i b r i u m p o s i t i o n to change the d e n s i t y to that r e l e v a n t to the new mean s t r e s s . He found that a t r a n s i t i o n zone could be extended by more than 30 km by the s t r e s s e s induced by the re d u c t i o n i n the volume as the phase change proceeds. Ahrens and Schubert (1975) found that the presence of water made the phase changes very pressure s e n s i t i v e . These observations have important consequences f o r the Puget Sound region as the phase changes may have migrated up the s l a b to e l i m i n a t e the space problem at the corner. 6) Other subduction zones Because phase changes seem to q u a l i t a t i v e l y and q u a n t i t a t i v e l y e x p l a i n the deeper s e i s m i c i t y i n the southern Vancouver I s l a n d - Puget Sound r e g i o n , the i m p l i c a t i o n i s that the phenomenon i s u n i v e r s a l and earthquakes due to phase changes should be observed i n a l l subduction zones where -104-oceanic c r u s t i s being consumed. The problem i s that most subduction zones are dominated by l a r g e events on the t h r u s t plane and intense s e i s m i c i t y i n the o v e r l y i n g p l a t e which w i l l completely mask events i n the phase change r e g i o n , which could be best described as a second order e f f e c t . However, i n s e c t i o n s of New Zealand, Japan and A l a s k a , dense l o c a l seismic networks e x i s t to i d e n t i f y events i n t h i s depth range. The p r o p e r t i e s to look f o r are a d i p p i n g zone of small earthquakes w i t h depths up to 70 km, magnitudes i n the 2 to 4 range and a "b" value of 0.7. F o c a l mechanisms should be g e n e r a l l y confused f o r the small events w i t h down dip extension or p o s s i b l y v e r t i c a l compression being a u n i f y i n g f e a t u r e amongst a small subset of the events. Occasional l a r g e r events should e x i s t and have downdip te n s i o n axes and have very few a f t e r s h o c k s . I n New Zealand, Robinson (1978) stu d i e d small earthquakes i n a di p p i n g zone i n the 20 - 40 km depth range which he suggested were i n the upper p o r t i o n of the subducting P a c i f i c P l a t e . He found very d i v e r s e f o c a l mechanisms w i t h the only c o n s i s t e n t p a t t e r n being downdip extension amongst a few events. Reyners (1980) found an absence of an obvious p a t t e r n i n the f i r s t motions of microearthquakes i n the P a c i f i c P l a t e beneath New Zealand i n the 40 km to 70 km depth range. He suggested such a d i v e r s e p a t t e r n would be c o n s i s t e n t w i t h s t r e s s e s induced by a phase change. He found downdip extension i n the 70 to 100 km depth range. Larger New Zealand earthquakes i n t h i s depth range were found to have downdip tension axes by H a r r i s (1975). In Japan, Anderson et a l . (1980) found that i n the 40 - 70 km depth range, earthquakes had a "b" value of 0.7 and they show a downdip extension mechanism i n one of t h e i r diagrams, although they do not mention i t s p e c i f i c a l l y . They a t t r i b u t e the low "b" value to low pore pressure i n the subducting c r u s t caused by advection of f r e e water from the oceanic c r u s t . -105-They a l l u d e to phase changes as a p o s s i b l e cause of the earthquakes. In Alaska i n the Shumagin Isl a n d s r e g i o n , Reyners and Coles (1980) studi e d the double B e n i o f f zone. The depth range up to 70 km incl u d e s mainly events i n the upper zone. They found that the events i n the 40 to 60 km range had a l a r g e d i v e r s i t y i n f o c a l mechanisms but below t h i s and up to 100 km downdip extension was the r u l e . Thus, earthquakes w i t h the same f o c a l mechanisms and s t a t i s t i c a l p r o p e r t i e s have been observed i n the depth range of the phase changes i n other subduction zones. Most authors have t r i e d to e x p l a i n them by bending s t r e s s e s but s e v e r a l authors have a l l u d e d to phase changes i n the region or even suggested phase changes as a cause. None has d e a l t w i t h t h i s i n any d e t a i l . 7) S e i s m i c i t y outside the Puget Sound Region Since the phase changes o f f e r a reasonable explanation f o r the deeper earthquakes of the Puget Sound — southern Vancouver I s l a n d r e g i o n , the problem i s changed from e x p l a i n i n g the presence of earthquakes i n Puget Sound to e x p l a i n i n g the absence of earthquakes at that depth range i n the r e s t of the Juan de Fuca p l a t e . The phase changes must be happening everywhere, and i f they are causing earthquakes i n Puget Sound and i n other subduction zones why are they not causing earthquakes i n the r e s t of the subducting Juan de Fuca plate? The answer to t h i s must be that i n most of the Juan de Fuca p l a t e the s t r e s s e s a s s o c i a t e d w i t h the phase change r e g i o n do not extend i n t o the p o r t i o n of the p l a t e that i s s t i l l c o ol enough to support earthquakes. This i s depicted i n Figure 47. I t i s only because of the space problem at the corner and the metastable p r o p e r t i e s of the phase changes that a l l o w them to run beyond the immediate pressure and temperature c o n d i t i o n s (Woodward, 1977) by c r e a t i n g t h e i r own s t r e s s f i e l d - 106 -Figure 47 Cross section i n Figure 35 with mantle phase changes from Figure 41 superimposed. A s i m i l a r diagram could be constructed for c r u s t a l phase changes. Earthquakes w i l l only occur when the phase change has migrated far enough up the slab to encounter the b r i t t l e zone. -107-as they go that has allowed them to migrate f a r t h e r up the p l a t e i n t o the b r i t t l e r e g ion under Puget Sound. Also the presence of water profoundly a l t e r s the p r o p e r t i e s of the phase changes, speeding up the r e a c t i o n time and making the process very pressure s e n s i t i v e (Ahrens and Schubert, 1975). In the r e s t of the Juan de Fuca p l a t e the same phase changes must s t i l l be happening, but they are occuring a s e i s m i c a l l y because the phase change f r o n t i s s l i g h t l y deeper where the descending l i t h o s p h e r e i s above the temperature that can support shear s t r e s s . 8) The r o l e of phase changes i n the development of f o r e a r c basins One of the most s t r i k i n g features of the Puget Sound s e i s m i c i t y , both deep and shallow, i s that i t c o r r e l a t e s s p a t i a l l y w i t h the Georgia S t r a i t — Puget Sound lowlands. The d i f f e r e n t i a l volume change between the c r u s t and mantle of the underlying subducting oceanic l i t h o s p h e r e may be the reason f o r the c o r r e l a t i o n . As mentioned p r e v i o u s l y , the phase changes i n the mantle m a t e r i a l r e s u l t i n a c o n t r a c t i o n of the order of 4% while those i n the c r u s t a l m a t e r i a l r e s u l t i n a volume change of 17%. Since the mantle i s t h i c k e r i t w i l l dominate the bulk c o n t r a c t i o n of the subducting s l a b and thus, the remaining 13% volume change i n the oceanic c r u s t w i l l l i k e l y take place i n the d i r e c t i o n normal to the surface of the p l a t e unless there i s a shear between the c r u s t and mantle i n the subducting l i t h o s p h e r e . This represents a v e r t i c a l c o n t r a c t i o n of the order of a kilometer f o r a t y p i c a l oceanic c r u s t of 7 km thickness (13% x 7 km = 0.91 km). Since the aseismic l a y e r between the o v e r l y i n g North American p l a t e and the Juan de Fuca p l a t e i s q u i t e t h i n ( l e s s than 10 km - see Figure 34) i t i s l i k e l y that the p l a t e s are i n contact and that the v e r t i c a l c o n t r a c t i o n w i l l be t r a n s m i t t e d to the o v e r l y i n g p l a t e r e s u l t i n g i n a down drop. I n i t i a l l y t h i s would be i n the form of a gra b e n - l i k e s t r u c t u r e but because the phase change f r o n t -108-i n the subducting p l a t e Is i n a dynamic p o s i t i o n , i t i s l i k e l y that i t w i l l move back and f o r t h w i t h changes i n the r a t e of p l a t e motion and changes i n the subduction angle, thus weakening a broad region of the o v e r l y i n g p l a t e w i t h normal f a u l t s . This k i n d of mechanism would be more important i n regimes such as the Juan de Fuca p l a t e where the o v e r l y i n g p l a t e i s o v e r r i d i n g the subduction zone and f o r c i n g the subducting p l a t e to be at an ever decreasing shallow descent angle and i n c r e a s i n g the area of the subduction i n t e r f a c e . As i n the formation of any b a s i n , once i t i s i n i t i a t e d , the p o t e n t i a l f o r i s o s t a t i c subsidence due to sediment l o a d i n g i s t h e re, and the ba s i n continues to develop i f i t has an adequate sediment supply. The f a u l t s under the b a s i n , although o r i g i n a l l y normal f a u l t s roughly p a r a l l e l to the margin w i l l l a t e r act as a zone of weakness i n other s t r e s s f i e l d s . S p e c i f i c a l l y , i n the case of oblique subduction they w i l l form a zone of weakness i n the o v e r l y i n g p l a t e where s t r i k e s l i p f a u l t i n g can take place (see d i s c u s s i o n on oblique subduction i n next s e c t i o n ) . Another piece of c i r c u m s t a n t i a l evidence f o r t h i s hypothesis i s that the Georgia S t r a i t , - Puget Sound - Willamette V a l l e y lowland that p a r a l l e l s the coast only e x i s t s over the subduction path of the present day Juan de Fuca p l a t e ( F i g u r e 48). Immediately to the north of t h i s topographic low, the Johnson s t r a i t topographic high appears over the subducted p o r t i o n of the independent E x p l o r e r P l a t e (Riddihough, 1977). This p l a t e has ceased to subduct i n the l a s t two m i l l i o n years (Riddihough, 1981) and thus the dynamic e q u i l i b r i u m c o n d i t i o n s that hold the phase change f r o n t i n place w i l l have ceased to e x i s t and the whole region should be coming to i s o s t a t i c e q u i l i b r i u m . This r e s u l t s i n the u p l i f t of the topographic low to the same e l e v a t i o n as the surrounding region. U p l i f t r a t e s i n the Johnson s t r a i t r e g i o n , based on the a n a l y s i s of t i d e gauge - 1 0 9 - -OLYMPIC HIGH LOWLANDS ABOVE MM PHASER CHANGE REGION ^ v c ^ PLATE MlNS^a i l l vtfe^-111 I v Figure 48 The Georgia S t r a i t - Puget Sound - Willamette v a l l e y lowlands occur only above the phase change region i n the Juan de Fuca p l a t e . Contour i n t e r v a l i s 500 m on land. -110-data, are at l e a s t one mm per year (one km per m i l l i o n years) higher i n t h i s r e g i o n than i n the Georgia s t r a i t lowland area above the subducting Juan de Fuca P l a t e (Vanicek and Nagy, 1981; Wigen and Stephenson, 1980; Riddihough, 1982). At the south end of the Georgia S t r a i t - Puget Sound - Willamette V a l l e y lowland the same p a t t e r n appears. Above the subducted p o r t i o n of small Gorda p l a t e , which a l s o appears to have stopped subducting (Riddihough, 1980), the lowland i s replaced by the Klamath Mountains and surrounding high. The topographic r e l a t i o n s h i p here i s complicated by a pronounced change i n geology which undoubtedly plays a r o l e i n the height of the Klamath Mountains. The core of the Klamath Mountains are made up of much ol d e r r o c k s , l i k e l y an e x o t i c terrane r a f t e d i n from elsewhere (eg. Coney et a l . , 1980). To the south of the Klamath Mountains the Great V a l l e y of C a l i f o r n i a appears to c a r r y on the topographic low even though today there i s no a c t i v e subduction under C a l i f o r n i a . The Great V a l l e y could w e l l be a f o s s i l phase change downdrop, preserved because the o v e r l y i n g l i t h o s p h e r e has adjusted to the s l a b window e f f e c t l e f t behind i n the asthenosphere by the northward m i g r a t i o n of the Juan de Fuca P l a t e system (D i c k i n s o n and Snyder, 1979). A p a r t i a l t e s t f o r t h i s hypothesis i s to f i n d other examples of ba s i n formation or topographic lows immediately above the 30 to 80 km depth range i n the downgoing s l a b where the phase changes discussed here take p l a c e . This e f f e c t would only be seen where the o v e r l y i n g and subducting pl a t e s are i n compression across the subduction i n t e r f a c e and where the subduction i n t e r f a c e transcends the whole phase change region . These c r i t e r i o n are necessary to e l i m i n a t e s i t u a t i o n s where asthenospheric flow can compensate f o r the volume change. Compressional subduction regimes are found to - I l l -c o r r e l a t e w i t h the subduction of oceanic l i t h o s p h e r e younger than 50 m i l l i o n years i n age (Molnar and Atwater, 1978). This occurs mainly i n the subduction zones of the eastern P a c i f i c . Long, low-angle subduction zones are a r e s u l t of the r o l l - b a c k r a t e of the subduction hinge due to the s i n k i n g l i t h o s p h e r e being l e s s than the motion of the o v e r r i d i n g p l a t e towards the trench l i n e (Dewey, 1980) or the accumulation of a l a r g e a c c r e t i o n a r y prism which may c o n s i s t i n part of e x o t i c terranes r a f t e d from elsewhere ( K a r i g et a l . , 1976). I n a d d i t i o n to the subduction of the Juan de Fuca P l a t e under western North America, the subduction of the P a c i f i c P l a t e under southern Alaska and the Nazca P l a t e under Peru and C h i l e meet these c r i t e r i a and have topographic lows above the phase change region i n the subducting p l a t e . The Cook I n l e t low i n southern Alaska and the Great C e n t r a l V a l l e y - Gulf of Corcovado low i n C h i l e appear to be f o r e a r c basins that are the equivalent of the Georgia S t r a i t - Puget Sound - Willamette V a l l e y lowland above the Juan de Fuca p l a t e . This suggested mode of i n i t i a t i o n of f o r e a r c basins d i f f e r s from the more t r a d i t i o n a l ideas ( r e c e n t l y reviewed by Dickinson and Seely, 1979) which demand the presence of oceanic c r u s t under the basin or r e q u i r e more obscure methods of i n i t i a t i o n i n c o n t i n e n t a l c r u s t such as extension over r i s i n g plutons or magma withdrawal at depth. Recently the r o l e of asthenosphere flow i n subduction zones has been i n v e s t i g a t e d (Thatcher and Rundle, 1979) and i t may a l s o be a v i a b l e mechanism f o r the i n i t i a t i o n of f o r e a r c basins under some circumstances. -112-C. SHALLOW SEISMICITY (SURFACE TO 30 Km) 1) D i s t r i b u t i o n Whereas the deeper events are l i k e l y a r e s u l t of the absolute v e l o c i t y of the subducting p l a t e and i t s unique geometry, the shallow s u i t e must have t h e i r o r i g i n i n the i n t e r a c t i o n of the two p l a t e s , i . e . s t r a i n from s t r e s s coupled across the convergent boundary. The shallow s u i t e of earthquakes i s centred over the c o n c e n t r a t i o n of deeper earthquakes but covers a wider area (Figure 49). The l o c a t i o n above the deep earthquakes i s probably caused by an increased c o e f f i c i e n t of f r i c t o n here due to incomplete compensation by the phase change process or an i r r e g u l a r upper surface due to the shear f a i l u r e r e l a t e d to the s e i s m i c i t y . The depths of the shallow events range from near surface to about 30 km w i t h the highest incidence of occurrence near 25 km (Figure 15). They are separated from the deeper earthquakes by a zone c o n t a i n i n g almost no earthquakes (presumably a zone of low strength) which i s the order of 10 km t h i c k (see Figures 15, 16 and 34). The shallow events have d i f f e r e n t s t a t i s t i c a l p r o p e r t i e s (Figure 32) and d i f f e r e n t f o c a l mechanisms from the deep events ( F i g u r e s 21 and 25). 2) F o c a l Mechanisms A l l of the observed mechanisms of shallow earthquakes have been e i t h e r s t r i k e - s l i p or t h r u s t f a u l t i n g . However, there i s a general north-south alignment of the pressure axes both w i t h i n Puget Sound (Crosson 1972, 1981) and the surrounding r e g i o n (see a l s o Chapter I I and Malone et a l . , 1975). This i s c o n s i s t e n t w i t h the region being under north-south compression or r i g h t l a t e r a l shear. There i s a high incidence of f a u l t planes o r i e n t e d i n a north-northwest d i r e c t i o n , s p e c i f i c a l l y about N20°W, f o r s e v e r a l recent sequences ( Y e l i n & Crosson, 1981; Weaver and Smith, 1982). - 113 -Figure 49 The shallow earthquakes of Puget Sound from 1970-1978 (from Crosson 1981). -114-3) The Oblique Subduction Model The idea of l o c k i n g a subduction t h r u s t plane and t r a n s f e r r i n g the accumulated p o t e n t i a l energy of p l a t e convergence to e l a s t i c s t r a i n i n the o v e r l y i n g l i t h o s p h e r e was discussed e x t e n s i v e l y by F i t c h and Scholz (1971). F i t c h (1972) extended t h i s concept to i n c l u d e oblique subduction and showed that once the subduction zone was locked, the component of convergence perpendicular to the s t r i k e of the subduction zone stored e l a s t i c energy i n the o v e r l y i n g p l a t e f o r the next l a r g e t h r u s t earthquake, while the component p a r a l l e l to the trench would r e s u l t i n s t r i k e - s l i p f a u l t i n g i n a shear zone p a r a l l e l to the s t r i k e of the trench (Figure 50). He c i t e d s e v e r a l examples of oblique subduction zones where l a r g e s t r i k e s l i p f a u l t zones had developed p a r a l l e l to the s t r i k e of the trench. Walcott (1978) i n a study of deformation i n New Zealand c a r r i e d these ideas f u r t h e r . He showed that deformation at the o b l i q u e l y convergent New Zealand margin was not one of c o n t i n u a l compression but of e p i s o d i c compressional and e x t e n s i o n a l movements normal to the boundary together w i t h a c o n t i n u i n g r e g i o n a l shear p a r a l l e l to the boundary. He explained the continued shear by p o i n t i n g out that the subduction zone need not be locked to transmit s t r e s s across the boundary. The only requirement i s that the f o r c e r e q u i r e d to overcome dynamical f r i c t i o n on the subduction t h r u s t plane i s greater than that r e q u i r e d to deform the o v e r l y i n g l i t h o s p h e r e i n s t r i k e s l i p motion. I f t h i s i n e q u a l i t y i s s a t i s f i e d then the component of p l a t e motion p a r a l l e l to the s t r i k e of the subduction t h r u s t w i l l produce f a i l u r e i n the s t r i k e s l i p zone of the o v e r l y i n g l i t h o s p h e r e . The Juan de Fuca/America i n t e r a c t i o n f o r the most part i s an oblique subduction s i t u a t i o n (except f o r the short s e c t i o n on Vancouver I s l a n d ) ; thus along the coast of Washington and Oregon an oblique subduction model - 115 -The oblique subduction model of F i t c h (1972) and Walcott (1978) where continuous shear develops i n l a n d and p a r a l l e l the trench. Motion perpendicular to the trench i s i n the form of compression and r e l a x a t i o n before and a f t e r l a r g e earthquakes. -116-should apply. I f the subduction t h r u s t becomes locked or the c o e f f i c i e n t of dynamic f r i c t i o n i ncreases s u f f i c i e n t l y , s t r e s s coupled across the boundary can be expected to show up as s t r a i n i n the o v e r l y i n g p l a t e . This s t r a i n can be expected to take the form of deformation and/or e l a s t i c s t r a i n accumulation perpendicular to the coast. F o r t u n a t e l y there i s a measurement of s t r a i n accumulation across the seismic zone i n Puget Sound (Savage et a l . , 1981). Because they observed compressional s t r a i n w i t h i n about 15° of the o r i e n t a t i o n of the long term p l a t e i n t e r a c t i o n vector they i n t e r p r e t e d t h i s as a buildup of e l a s t i c s t r a i n due to the upper part of the subduction boundary being locked. While t h i s i n t e r p r e t a t i o n may be c o r r e c t i t i s not the only i n t e r p r e t a t i o n that can be made from the data and as Savage et a l . (1981) remark, i t i s an i n t e r p r e t a t i o n that does not seem c o n s i s t e n t w i t h the f o c a l mechanisms of shallow earthquakes i n the r e g i o n . The oblique subduction boundary of New Zealand can serve as a comparison f o r the Juan de Fuca P l a t e r e g i o n as there i s both a locked and unlocked s e c t i o n , both w i t h s t r a i n measurements (Walcott, 1978) and f o c a l mechanism s t u d i e s (Reyners, 1980). For the locked s e c t i o n i n New Zealand the measured azimuth of the p r i n c i p a l a x i s of compression i s about 25° from the azimuth of the p l a t e motion vector (see F i g u r e 51). This i s s i m i l a r to the Juan de Fuca s i t u a t i o n where the compression a x i s measured by Savage et a l . (1981) i s about 15° d i f f e r e n t from the p l a t e i n t e r a c t i o n v e c t o r . This i s not of concern because the p l a t e v e c t o r i s a long term average measured over m i l l i o n s of years while the s t r a i n measurement i s measured only over a few years or few tens of years. Where the two regimes d i f f e r i s i n the earthquake f o c a l mechanisms. In the New Zealand s i t u a t i o n the pressure axes from the f o c a l mechanisms of l a r g e r earthquakes and composite f o c a l mechanism s o l u t i o n s of microearthquakes have a t i g h t - 117 -NEW ZEALAND LOCKED Measured Deformation Earthquake Pressure Axes (within 5° ) Measured Deformation Earthquake Pressure Axes NEW ZEALAND UNLOCKED onal Earthquake Pressure Axes Average Earthquake Pressure Axes PUGET SOUND Some Recent Earthquake Pressure Axes Measured Deformation Figure 51 c Ui ro Comparison of r e l a t i v e plate motion, measured deformation and earthquake pressure axis d i r e c t i o n s of locked and unlocked portions of the oblique subduction New Zealand margin with the Puget Sound s i t u a t i o n . Puget Sound earthquake mechanisms are s i m i l a r to the unlocked s i t u a t i o n i n New Zealand whereas the one deformation measurement i s s i m i l a r to the locked regime i n New Zealand. -118-c l u s t e r around the measured s t r a i n measurement compression axes. T h i s holds f o r earthquakes r i g h t across New Zealand, i n t o the back arc region s e v e r a l hundred k i l o m e t e r s from the trench. The Juan de Fuca region shows almost u n i l a t e r a l north-south o r i e n t a t i o n f o r compression axes from mechanisms of earthquakes both w i t h i n the Puget Sound and i n surrounding r e g i o n s . Weaver and Smith (1982) suggest that a recent group of events are i n f a c t compatible w i t h a locked regime. However, these have an average compression a x i s of about N30E which i s more than 40° away from the azimuth of the measured a x i s of compression (N71°E+6°) of Savage et a l . (1981). This i s u n l i k e the s i t u a t i o n i n New Zealand where the f o c a l mechanism pressure axes are c l u s t e r e d very t i g h t l y around the measured or i n f e r r e d a x i s of compression. I t seems more l i k e l y that these events are r e s t r i c t e d to o c c u r r i n g along e x i s t i n g f a u l t planes and that the o r i e n t a t i o n s of t h e i r pressure axes are not s i g n i f i c a n t . McKenzie (1969) points out that where f a u l t s e x i s t i n a region under s t r a i n the earthquakes w i l l tend to occur on these f a u l t s r a t h e r than i n i t i a t e f r e s h breaks and thus the pressure a x i s i s not a good t e c t o n i c i n d i c a t o r . The f o c a l mechanism p a t t e r n i n the Juan de Fuca subduction regime i s much more l i k e the unlocked segment of the New Zealand margin (Reyners, 1980), where the m a j o r i t y of f a u l t plane s o l u t i o n s have P axes p a r a l l e l to the margin. This i s p a r a l l e l to the maximum compressive a x i s of the s t r a i n f i e l d . Both the f o c a l mechanisms and s t r a i n f i e l d s have l o c a l inhomogenities. Walcott (1978) e x p l a i n s t h i s as a tendency f o r the earthquakes and deformation to f o l l o w i n d i v i d u a l f a u l t s , something which doesn't happen as much when the f a u l t s are under compressive s t r e s s and many of them are locked. Comparison of the expected and observed s e i s m i c i t y rates can a l s o shed l i g h t on the source of s t r a i n f o r the shallow earthquakes. Hyndman and -119-Weichert (1982) showed that the earthquake r a t e f o r a l l of Puget Sound was l e s s than that expected from the convergence r a t e . However, si n c e i t has been shown here that the deeper earthquakes are a product of subduction r a t h e r than convergence, a more p r e c i s e estimate of the convergence s e i s m i c i t y r a t e can be c a l c u l a t e d . The moment r a t e of the convergence s e i s m i c i t y can be estimated w i t h Anderson's (1979) equation used i n s e c t i o n B of t h i s chapter. I f Crosson's (1981) w e l l defined "b" value of 1.02 (Figur e 32) f o r the shallow s u i t e of earthquakes i s used w i t h an estimate of 6 f o r the maximum magnitude the moment r a t e i s : X 1 (j-d) bxtO M - 12 x l O 3 _ 0.OO2.3 x / O 2 6 c A j n e - O r y V I f an estimate of 7.25 f o r the maximum magnitude i s used the r a t e i s 0.01 x 10^6 dyne-cm/yr. The convergence r a t e between the Juan de Fuca and America P l a t e s i s approximately 4 cm/yr (Riddihough, 1977). I f i t i s assumed the subduction zone i s locked i n the v i c i n i t y of Puget Sound, then a l l of t h i s convergence r a t e should be going i n t o deforming the o v e r l y i n g p l a t e . Anderson (1979) and Molnar (1979) both show how to c a l c u l a t e the s e i s m i c i t y expected when a volume i s deformed under compression. Following Anderson's (1979) n o t a t i o n , the moment rate expected i s : M o - ^-a^AAL/^ where 4/3 i s an e m p i r i c a l constant (Anderson, 1979), yu" i s the modulus of r i g i d i t y , "A" i s the cross s e c t i o n a l area and "AL/yr" i s the deformation -120-r a t e . S u b s t i t u t i n g i n the values f o r the Puget Sound convergence regio n : This i s about three orders of magnitude greater than the observed moment ra t e or equivalent to a continuous r a t e of at l e a s t two magnitude 7 earthquakes per year. C l e a r l y t h i s i s not the case! Thus, i t must be assumed that the subduction zone i s not locked, that the convergence r a t e i s much s m a l l e r , that we are observing at an anomalous po i n t i n time or that f o r some reason Anderson's (1979) equation i s not a p p l i c a b l e . I f i t i s assumed that the subduction zone i s not locked but that a dynamic c o e f f i c i e n t of f r i c t i o n i s causing s t r a i n to couple across the subduction zone then the t o t a l convergence r a t e of 4 cm/year need not apply. The measured shortening r a t e of 1.3 cm/year across the shallow s e i s m i c i t y zone (Savage et a l . , 1981) could be considered as the e f f e c t i v e convergence r a t e . This would reduce the moment r a t e caused by convergence to : This i s s t i l l about two orders of magnitude greater than the observed s e i s m i c i t y r a t e . Thus, si n c e there i s an a c t u a l measurement of deformation the only options l e f t to e x p l a i n t h i s discrepancy are that we are observing at an anomalous po i n t i n time or that Anderson's (1979) equation i s not a p p l i c a b l e to t h i s s i t u a t i o n . I f i n s t e a d of using a d i r e c t convergence model, as envisaged by ^- X £ * 3.3 * /O^cA/ne/cm^ * 30 Jen, X 2 50Je*, x ^Cm/yr -121-Anderson (1979), an oblique subduction model i s used, such as that proposed by F i t c h (1972) and Walcott (1978), then only the component of the 1.3 cm/yr p a r a l l e l to the coast would be expected to cause s e i s m i c i t y i n the Puget Sound r e g i o n . I f the w e l l defined azimuths of recent f a u l t i n g of about N20°W ( Y e l i n and Crossn, 1981; Weaver and Smith, 1982) are assumed to be the azimuth of the s t r i k e s l i p zone, then the compression a x i s of Savage et a l . (1981) (71° + 6°) i s almost perpendicular to the s t r i k e s l i p zone. The s t r a i n causing the s e i s m i c i t y should be the 1.3 cm/yr shortening r a t e observed by Savage et a l . (1981) m u l t i p l i e d by the sine of a very small angle ( l e s s than 5°). The amount of s e i s m i c i t y expected can be c a l c u l a t e d from Brune's (1968) formula f o r moment r a t e from a s t r i k e s l i p zone: were i s the modulus of r i g i t y , "A" i s the area of the f a u l t zone and "s" i s the s l i p r a t e i n the s t r i k e s l i p zone. S u b s t i t u t i n g i n the values f o r the Puget Sound r e g i o n . This i s much c l o s e r to the moment rates c a l c u l a t e d above from the s e i s m i c i t y (0.0023 x 1 0 2 6 dyne-cm/yr to 0.01 x 1 0 2 6 dyne-cm/yr) and could be d i r e c t l y comparable i f an angle smaller than 5° i s used i n the c a l c u l a t i o n . Thus, the oblique subduction model can produce the c o r r e c t amount of s e i s m i c i t y and i f a steady s t a t e s i t u a t i o n e x i s t s , i t i s yV[ 0 = 33 */O" dyne/cm* X J O km Y Z50^ y Sl'ne.(S°) * /• 3 Om/gy. -122-c o n s i s t e n t w i t h an unlocked subduction zone. Thus the parodox remains, Savage et a l . (1981) i n t e r p r e t the s i n g l e measurement of the s t r a i n f i e l d as evidence f o r a locked subduction zone i n the Puget Sound r e g i o n while numerous f o c a l mechanisms and the amount of s e i s m i c i t y observed are c o n s i s t e n t w i t h an unlocked regime. One way out of the dilemma i s to consider the Puget Sound s t r a i n measurements as temporally and s p a t i a l l y anomalous, comparable to such regions observed i n New Zealand (Walcott, 1978). The way to check t h i s i s to make more deformation measurements along the Juan de Fuca/America subducting margin. Another way to r a t i o n a l i z e the observations i s to suggest that the subduction zone i s locked but has only r e c e n t l y become locked. Thus, deformation perpendicular to the margin i s s t i l l aseismic deformation while the earthquakes are a r e s u l t of continuous coupling of the p a r a l l e l motion to the margin. I f t h i s i s the case then there should be a s h i f t i n time to f o c a l mechanisms w i t h pressure axes p a r a l l e l to the compression d i r e c t i o n i n the s t r a i n f i e l d . D. CONCLUSIONS 1) Volume changes, t h a t accompany phase changes i n subducting l i t h o s p h e r e , cause l a r g e s t r e s s e s i n the region surrounding the c o n t r a c t i n g volume which r e s u l t s i n shear f a i l u r e and earthquakes. Phase changes that must occur i n the subducting o c e a n i c , l i t h o s p h e r e i n the f i r s t 100 km of depth can e x p l a i n q u a l i t a t i v e l y and q u a n t i t a t i v e l y the s u b c r u s t a l earthquakes i n the southern Vancouver I s l a n d - Puget Sound region . This confirms the Juan de Fuca p l a t e i s c u r r e n t l y subducting at a rat e comparable to the 2.5 cm/yr that has been c a l c u l a t e d from magnetic anomaly -123-s t u d i e s . 2) Subduction zones i n New Zealand, Alaska and Japan a l s o have a s u i t e of earthquakes of s i m i l a r c h a r a c t e r i n the depth range where the shallow phase changes can be expected. 3) The con c e n t r a t i o n of deeper s e i s m i c i t y i n the southern Vancouver I s l a n d - Puget Sound region i s due to the a b i l i t y of the boundary between the high d e n s i t y and low d e n s i t y m a t e r i a l to migrate i n t o adjacent regions due to the s t r e s s e s induced by the r e d u c t i o n i n volume as the phase change proceeds (Woodward, 1977). Thus the geometry of t h i s p a r t i c u l a r subduction s i t u a t i o n ( i e . the 45° change i n the azimuth of the s t r i k e of the subduction zone) r e q u i r e s a volume change that i s s a t i s f i e d by the phase change moving f a r t h e r up the subducting p l a t e than i n surrounding r e g i o n s . The phase changes i n the surrounding regions are thus being d i s s i p a t e d by creep r a t h e r than b r i t t l e f r a c t u r e . 4) Phase changes i n the subducting l i t h o s p h e r e may be the reason f o r the p o s i t i o n and s i z e of the Georgia S t r a i t - Puget Sound - Willamette V a l l e y Lowlands and may be an important mechanism i n the development of f o r e a r c b a s i n s . S i m i l a r basins occur i n Alaska and C h i l e above the phase change region i n the subducting p l a t e . 5) The presence of c r u s t a l s e i s m i c i t y i n the southern Vancouver I s l a n d - Puget Sound region i s due to s t r e s s being coupled across the subduction i n t e r f a c e . The c o e f f i c i e n t of f r i c t i o n on the subduction i n t e r f a c e must be higher above the phase change region d e l i n e a t e d by the zone of deeper earthquakes. This i s probably because the phase change process does not compensate p e r f e c t l y the necessary volume change i n the subducting p l a t e and there i s s t i l l l i k e l y some upward pressure here as w e l l as a p o s s i b l e i r r e g u l a r surface due to the shear f a i l u r e i n d i c a t e d by the s e i s m i c i t y . 6) The shallow s e i s m i c i t y can q u a l i t a t i v e l y and q u a n t i t a t i v e l y be -124-described by an oblique subduction model as o r i g i n a l l y proposed by F i t c h (1972) and l a t e r modified by Walcott (1978). 7) The l a c k of subduction plane t h r u s t earthquakes, the north-south o r i e n t a t i o n of the pressure axes of f a u l t plane s o l u t i o n s and the amount of s e i s m i c i t y observed s i n c e the t u r n of the century suggest the subduction i n t e r f a c e i s p r e s e n t l y unlocked and subduction Is proceeding a s e i s m i c a l l y . The one piece of d i s s e n t i n g evidence that suggests the subduction zone i s now locked and s t r a i n i s accumulating i s the recent s t r a i n measurements of Savage et a l . (1981). These can be r e c o n c i l e d i f we assume the subduction i n t e r f a c e has j u s t r e c e n t l y become locked or experienced a s i g n i f i c a n t i n c r e a s e i n the dynamic c o e f f i c i e n t of f r i c t i o n . I f the i n t e r f a c e i s locked then l a r g e subduction i n t e r f a c e earthquakes are a p o s s i b i l i t y . A change i n the s e i s m i c i t y p a t t e r n from that observed i n the past 80 years, r e f l e c t i n g a change i n the near surface s t r a i n f i e l d , should precede any l a r g e subduction earthquake. - 125 -IV. SEISMOTECTONICS OF CENTRAL VANCOUVER ISLAND A. INTRODUCTION Three l a r g e earthquakes i n a span of 40 years have occurred i n c e n t r a l Vancouver I s l a n d ( F i g u r e 52). They are a magnitude 7 event i n 1918 (Denison, 1919), a magnitude 7.3 event i n 1946 (Hodgson, 1946; Rogers and Hasegawa, 1978) and a magnitude 6 event i n 1957 (Milne and Lucas, 1961). I f t h i s i s r e p r e s e n t a t i v e of the ongoing r a t e of s e i s m i c i t y , c e n t r a l Vancouver I s l a n d i s one of the most seismic areas i n North America. In t h i s chapter a t e c t o n i c hypothesis i s proposed to e x p l a i n the occurrence of these major earthquakes. A l s o discussed i s a small pocket of s u b c r u s t a l earthquakes i n Georgia S t r a i t that has been discovered during the course of t h i s study (see Figures 13 and 18). Whether these events are r e l a t e d to the main zone of deeper Puget Sound s e i s m i c i t y (Figure 18) or have a separate cause has important t e c t o n i c and seismic r i s k i m p l i c a t i o n s . B. THE LARGER EARTHQUAKES 1) Tectonic S e t t i n g The trend of earthquakes i n the deep ocean that marks the Nootka F a u l t zone (Hyndman et a l . 1979) (Figure 28) i n t e r s e c t s Vancouver I s l a n d near the centre adjacent to the re g i o n where l a r g e earthquakes have occurred. I t i s tempting to r e l a t e t h i s band of s e i s m i c i t y to the la r g e events of c e n t r a l Vancouver I s l a n d . The Nootka f a u l t zone marks the re g i o n of shear between - 126 -Figure 52 Locations of the large earthquakes of the c e n t r a l Vancouver I s l a n d r e g i o n and an i s o l a t e d pocket of s u b c r u s t a l earthquakes discussed i n t e x t . . - 127 -the Juan de Fuca p l a t e which subducts under southern Vancouver I s l a n d and the E x p l o r e r P l a t e which subducts under northern Vancouver I s l a n d (Figure 3). This zone i s marked by earthquakes w i t h l e f t l a t e r a l s t r i k e - s l i p f a u l t plane s o l u t i o n s and l a r g e numbers of a f t e r s h o c k s . As the zone subducts under the c o n t i n e n t a l s l o p e , c o n t i n e n t a l s h e l f and Vancouver I s l a n d , the c h a r a c t e r of the s e i s m i c i t y changes. There are fewer earthquakes, they are l a r g e r and they have very few a f t e r s h o c k s . I t i s not p o s s i b l e to t e l l whether earthquakes on the c o n t i n e n t a l s h e l f are i n the underlying oceanic p l a t e or i n the o v e r l y i n g c o n t i n e n t a l c r u s t of Vancouver I s l a n d . I t i s a l s o not p o s s i b l e to t e l l whether earthquakes are o c c u r r i n g on northeast s t r i k i n g l e f t l a t e r a l f a u l t s or northwest s t r i k i n g r i g h t l a t e r a l f a u l t s (e.g. Rogers, 1976b). When the most eastern earthquake of c e n t r a l Vancouver I s l a n d i s considered, the June 23, 1946 (M = 7.3) event, i t has been shown to be i n the o v e r l y i n g c o n t i n e n t a l c r u s t and occurred on a r i g h t l a t e r a l northwest trending f a u l t , perpendicular to the Nootka f a u l t zone (Rogers and Hasegawa, 1978). Thus, although the Nootka f a u l t zone i n the deep ocean i n t e r s e c t s c e n t r a l Vancouver I s l a n d i t i s not obvious that the earthquakes of c e n t r a l Vancouver I s l a n d are r e l a t e d to motion on the underlying Nootka f a u l t zone. The argument pursued here i s t h a t , indeed, they are not d i r e c t l y r e l a t e d , but the earthquakes of c e n t r a l Vancouver I s l a n d are a r e s u l t of the s t r e s s regime generated by the i n t e r a c t i o n of the E x p l o r e r P l a t e w i t h the America P l a t e . The subduction of the Juan de Fuca p l a t e under southern Vancouver I s l a n d seems to be proceeding a s e i s m i c a l l y at present and the drop o f f i n s e i s m i c i t y immediately north of the Puget Sound region suggests there i s l i t t l e c o u pling between the subducting and o v e r l y i n g p l a t e on southern Vancouver I s l a n d . However, once the region over the E x p l o r e r P l a t e i s approached l a r g e earthquakes occur, p o s s i b l y because - 128 -there i s an increased coupling between the E x p l o r e r P l a t e and the o v e r l y i n g l i t h o s p h e r e of Vancouver I s l a n d . This may be due to the f a c t the E x p l o r e r p l a t e has slowed (Riddihough, 1977) or perhaps stopped subducting i n an absolute sense and s t a r t e d to revolve around an i n t e r n a l pole (Riddihough, 1981). The r e s u l t a n t change of the upper surface of the subducted p l a t e due to the phase changes seeking a new e q u i l i b r i u m p o s i t i o n , may cause upward pressure on the c r u s t . This upward pressure would increase the dynamic c o e f f i c i e n t of f r i c t i o n . The higher u p l i f t r a t e and higher topography i n c e n t r a l Vancouver I s l a n d may be evidence of t h i s process (see d i s c u s s i o n i n Chapter I I I ) . Any such e f f e c t would be expected to die out to the n o r t h as the t r i p l e j u n c t i o n i s approached, because the E x p l o r e r p l a t e t h i n s and thus looses s t r e n g t h . I f the E x p l o r e r P l a t e does have an i n t e r n a l pole as suggested by Riddihough (1981) then the s e i s m i c i t y would a l s o d i e out to the n o r t h as the pole i s approached. I t was noted i n Chapter I I that the average d i r e c t i o n of the pressure axes of the f a u l t planes corresponds to the Explorer/America i n t e r a c t i o n d i r e c t i o n suggested by Riddihough (1977) which i s s i g n i f i c a n t l y d i f f e r e n t from the Juan de Fuca/America i n t e r a c t i o n d i r e c t i o n ( F i g u r e 24). I t i s a l s o s i g n i f i c a n t l y d i f f e r e n t from the northeast-southwest o r i e n t a t i o n of compressive s t r e s s preserved i n the Cenozoic geology ( T i f f i n et a l . 1972). This s t r o n g l y suggests that the earthquakes are responding to a new s t r e s s caused by the present i n t e r a c t i o n of the E x p l o r e r and America p l a t e s . 2) H o r i z o n t a l Motion Vectors Although the preceding d i s c u s s i o n seems c o n s i s t e n t w i t h earthquakes r e s u l t i n g from r e g i o n a l north-south compression due to the Explorer/America i n t e r a c t i o n , McKenzie (1969) p o i n t s out that the p r i n c i p a l s t r e s s e s of mechanism s o l u t i o n s do not n e c e s s a r i l y correspond to r e g i o n a l t e c t o n i c - 129 -s t r e s s . He suggests that s i n c e most earthquakes are not new f r a c t u r e s , but occur on e x i s t i n g f a u l t s or zones of weakness, i t i s the h o r i z o n t a l component of the s l i p v ector which w i l l be the c o n s i s t e n t parameter amongst a group of earthquakes, because i t should r e f l e c t r e l a t i v e p l a t e motions. While t h i s hypothesis seems to hold f o r earthquakes along p l a t e boundaries (McKenzie and Parker, 1967), the c o r r e l a t i o n of p r i n c i p a l s t r e s s d i r e c t i o n s seems to be a more c o n s i s t e n t parameter f o r i n t r a p l a t e earthquakes (Sykes and Sbar, 1973). The h o r i z o n t a l p r o j e c t i o n s of the s l i p v e c t o r s f o r both p o s s i b l e f a u l t planes f o r each of the earthquakes are p l o t t e d i n Figure 53. The vectors form two c l u s t e r s , one i n d i c a t i n g d e x t r a l movement w i t h a northwest o r i e n t a t i o n , the other s i n i s t r a l motion w i t h a southwest o r i e n t a t i o n . The s c a t t e r i n o r i e n t a t i o n i s about the same f o r each set of vectors and, indeed, i t i s about the same as the s c a t t e r i n pressure axes shown i n Figure 24. Thus, contrary to the experience of McKenzie and Parker (1967), the ambiguity i n choosing which s l i p d i r e c t i o n i s appropriate does not disappear when the two a l t e r n a t i v e s are examined. This might be considered as evidence against the i n t e r p l a t e ( i . e . Nootka f a u l t ) i n t e r p r e t a t i o n f o r these earthquakes. 3) Aftershock P r o p e r t i e s One way of separating i n t e r p l a t e and i n t r a p l a t e earthquakes i s by the s i z e of the s t r e s s drops during the earthquakes. Kanamori and Anderson (1975) have shown that s t r e s s drop i s g e n e r a l l y higher f o r i n t r a p l a t e earthquakes and Gibowicz (1973) has suggested that a higher s t r e s s drop r e s u l t s i n fewer and smaller a f t e r s h o c k s . The f i v e earthquakes, which are l a r g e r than magnitude 5 and f o r which i t i s p o s s i b l e to provide aftershock i n f o r m a t i o n , are l i s t e d i n Table IX. For the two e a r l i e s t earthquakes the - 130 -PLATE MARGIN EXPLORER/AMERICA (b) Figure 53 H o r i z o n t a l p r o j e c t i o n of the s l i p vectors on the two p o s s i b l e f a u l t planes, (a) northwest and (b) northeast, f o r each of the P nodal s o l u t i o n s f o r the shallow earthquakes of c e n t r a l Vancouver I s l a n d . Large arrows i n d i c a t e o r i e n t a t i o n s of Explorer/America and Explorer/Juan de Fuca (Nootka f a u l t ) p l a t e i n t e r a c t i o n s ( a f t e r Riddihough, 1977). Open arrow at the top i n d i c a t e s the approximate o r i e n t a t i o n s of p l a t e margin i n t h i s r e g i o n . - 131 -TABLE IX Aftershocks of Larger Vancouver I s l a n d Earthquakes Magnitude Detected of Largest Minimum Date Magnitude Aftershocks Aftershock Magnitude Dec. 6, 1918 7 10a 4 . 5 b 4c June 23, 1946 7.3 3* 4*5b 4c Dec. 16, 1957 6 1 2^8 2.3 J u l y 5, 1972 5.7 6 3 ^ 2*.4d March 31, 1975 5.1 0 ? 2.1 a From f e l t r e p o r ts only b Estimate based on f e l t r e p o r t s c Estimate based on population d i s t r i b u t i o n ^ Deployment of p o r t a b l e seismographs lowered the d e t e c t i o n c o m p a t a b i l i t y to M L = 3 f o r an 8 day period (Rogers 1976a) - 132 -in f o r m a t i o n i s based on f e l t r e p o r t s (Dennison, 1919; Hodgson, 1946; Rogers and Hasegawa, 1978), but the p a t t e r n i s c l e a r . For each earthquake, very few af t e r s h o c k s have been detected, and the l a r g e s t aftershock i n the sequence i s at l e a s t two magnitude u n i t s smaller than the main shock. This i s l a r g e r than the worldwide average f o r shallow earthquakes which i s a d i f f e r e n c e of 1.2 magnitude u n i t s between the mainshock and l a r g e s t a f t e r s h o c k (Bath, 1965). Gibowicz (1973) suggests that such low magnitudes i n an af t e r s h o c k sequence may be i n d i c a t i v e of higher than normal s t r e s s drop i n the main shock. Although the aftershock patterns are not what could be considered d i r e c t evidence, they do tend to favour an i n t r a p l a t e i n t e r p r e t a t i o n f o r these earthquakes and thus the Explorer/America i n t e r a c t i o n as the cause of the earthquakes. 4) Amount of S e i s m i c i t y Another way of c o n s i d e r i n g whether the Explorer/America i n t e r a c t i o n or the Nootka F a u l t zone i s causing the earthquakes i s to consider the amount of s e i s m i c i t y . The moment r a t e p r e d i c t e d from the Nootka f a u l t zone co u p l i n g through the o v e r l y i n g c r u s t of Vancouver I s l a n d and the moment r a t e c a l c u l a t e d f o r the Explorer/America i n t e r a c t i o n can be compared with the moment r a t e from the observed s e i s m i c i t y which Hyndman and Weichert 26 (1982) have estimated as 0.14 x 10 dyne-cm/yr. F i r s t , c o n s i d e r i n g the reg i o n above the subducted Nootka F a u l t , the len g t h from the edge of the c o n t i n e n t a l s h e l f to the eastern edge of Vancouver I s l a n d i s about 150 km. The depth of rupture i n the o v e r l y i n g c r u s t can be expected to be up to 30 km (Rogers and Hasegawa, 1978). Thus, as i n chapter I I I , using the equation of Brune (1968) the moment r a t e i s : - 133 -Second, c o n s i d e r i n g the Explorer/American i n t e r a c t i o n , the length of the convergence zone from the Nootka F a u l t zone to Brooks Peninsula i s about 100 km. I f complete coupling i s assumed then a l l of the convergence r a t e should go i n t o deformation of the o v e r l y i n g p l a t e by shortening. Riddihough (1977) estimated a r a t e of about 2 cm/yr convergence i n a north-south d i r e c t i o n . More recent a n a l y s i s (Riddihough, 1981) i n d i c a t e s that the pole of motion f o r the Ex p l o r e r P l a t e may be w i t h i n the p l a t e which would make 2 cm/yr a maximum, decreasing as the pole i s approached. As i n chapter I I I , using the equation of Anderson (1979) the moment r a t e p r e d i c t e d from the shortening r a t e i s : The maximum moment rat e s c a l c u l a t e d f o r each t e c t o n i c regime are about the same and both are only a f a c t o r of 3 d i f f e r e n t from the rate estimated from the s e i s m i c i t y . Considering the assumptions i n v o l v e d , a l l three estimates are probably not s i g n i f i c a n t l y d i f f e r e n t . Thus, the amount of s e i s m i c i t y cannot help to determine which t e c t o n i c model i s appropriate except to say there i s s u f f i c i e n t p o t e n t i a l i n e i t h e r system to produce the earthquakes observed. A/I. 4. x £ * 33 x 10"clyn<L/cmz * /OOjcm X JOAm *2crr>/yr - 134 -C. DEEPER GEORGIA STRAIT EARTHQUAKES The small group of s u b c r u s t a l earthquakes i n the v i c i n i t y of Texada I s l a n d ( F i g u r e 54) are an i n t e r e s t i n g d i s c o v e r y of t h i s a n a l y s i s . There are at l e a s t three ways i n which they might be ex p l a i n e d . F i r s t , they may be r e l a t e d to the subducted Nootka f a u l t system; second, they may be part of the Puget Sound co n c e n t r a t i o n but separated by a seismic gap; or t h i r d , they may be a smaller s c a l e analogy to the Puget Sound s e i s m i c i t y and thus a r e s u l t of phase changes that have l o c a l l y moved i n t o the b r i t t l e p o r t i o n of the subducting l i t h o s p h e r e . I t seems u n l i k e l y that the events are r e l a t e d to the subducted Nootka f a u l t zone as they are too f a r south of the projected trend of the present Nootka f a u l t zone. They could be on a p a r a l l e l splay f a u l t i n the subducted l i t h o s p h e r e , but there i s no evidence f o r any northeast-southwest alignment amongst the e p i c e n t r e s or any c o n t i n u a t i o n under Vancouver I s l a n d towards the o f f s h o r e Nootka s e i s m i c i t y . A l s o , the composite f o c a l mechanism of the events (Figure 55) does not suggest any r e l a t i o n to the Nootka f a u l t zone. Although the composite f o c a l mechanism i s not w e l l d efined the data do not seem c o n s i s t e n t w i t h a simple l e f t l a t e r a l f a u l t as would be expected on the Nootka f a u l t zone. The second p o s s i b i l i t y i s that these events are a contiguous part of the deeper Puget Sound s e i s m i c i t y separated from the main group by a major seis m i c gap. The composite f o c a l mechanism i s c o n s i s t e n t w i t h t h i s i n t e r p r e t a t i o n . However, the deeper s e i s m i c i t y i n Puget sound and i n the phase change regions of other subduction zones i s f a i r l y continuous over a pe r i o d of tens of years, thus the p e r s i s t a n c e of a major seismic gap f o r any l e n g t h of time i n t h i s environment seems u n l i k e l y (although not i m p o s s i b l e ) . A l s o , abundant s e i s m i c i t y occurs i n the regions of the l a r g e Figure 54 - 136 -Figure 55 The.P-nodal s o l u t i o n f o r the May 16, 1976 earthquake (Figure 27) i s superimposed on composite data from three deep 1979 events near Texada I s l a n d . Large symbols are those read from UW and EPB seismograms, small symbols are those reported by UW. Although there are some i n c o n s i s t e n c i e s , the o v e r a l l p a t t e r n i s c o n s i s t e n t w i t h the 1976 s o l u t i o n . The p r o j e c t i o n i s of the lower hemisphere. - 137 -1949 and 1965 s u b c r u s t a l earthquakes of southern Puget Sound, i n d i c a t i n g that the phase change process i s ongoing even a f t e r a major earthquake has r e l i e v e d much of the accumulated s t r a i n . The t h i r d p o s s i b i l i t y seems the most l i k e l y . The deep Texada I s l a n d earthquakes, l i k e the deeper ones i n Puget Sound, are probably the r e s u l t of a phase change f r o n t that has migrated i n t o the b r i t t l e p o r t i o n of the subducted l i t h o s p h e r e . The composite f o c a l mechanism (Figure 55) i s c o n s i s t e n t w i t h t h i s i n t e r p r e t a t i o n and the concept of other zones of deep s e i s m i c i t y i s c o n s i s t e n t w i t h the subducted p l a t e deforming i n t o three f o l d s as shown i n Figure 45. The region of s e i s m i c i t y under Texada I s l a n d i s much smaller than the region under Puget Sound. I f t h i s i s r e l a t e d to the amplitude of the f o l d then the amplitude under the v o l c a n i c region must a l s o be sm a l l e r . Thus, w i t h t h i s i n mind the f o l d i n g i n the subducting p l a t e i s modelled as a sine f u n c t i o n w i t h the Texada s e i s m i c i t y opposite one of the secondary lobes. The sine f u n c t i o n f i t s the volcano depths of Dicki n s o n (1970) very w e l l ( F i g u r e 56) and i s c o n s i s t e n t w i t h the smaller amount of deep s e i s m i c i t y under Texada I s l a n d . One of the consequences of the s i n e f u n c t i o n model i s that i f the downgoing l i t h o s p h e r e i s folded symmetrically as depicted i n Figure s 45 and 56 then there should be a corresponding pocket of deep s e i s m i c i t y south of Puget Sound near the Washington Oregon border. I t w i l l be i n t e r e s t i n g to see i f the increased number of seismograph s t a t i o n s i n s t a l l e d i n 1981 i n that border region detect any deep s e i s m i c i t y . D. CONCLUSIONS The l a r g e earthquakes of c e n t r a l Vancouver I s l a n d are not t y p i c a l of - 138 -Figure 56 Sine f u n c t i o n model of the downgoing p l a t e . The l i n e represents a s l i c e through the model i n Figure 45 at 100 km depth. Volcano source depths are from Dickinson (1970). - 139 -earthquakes at a p l a t e boundary: they do not have t h r u s t mechanisms, nor do they a l i g n to suggest a major s t r i k e - s l i p f a u l t , the d i r e c t i o n s of the h o r i z o n t a l motion v e c t o r s cannot be i n t e r p r e t e d unambiguously, and the a f t e r s h o c k sequences are not t y p i c a l of i n t e r p l a t e events. These observations are supportive of the argument that the earthquakes are a r e s u l t of Explorer/America i n t e r a c t i o n . The a l t e r n a t e e x p l a n a t i o n , that the earthquakes are a r e s u l t of l e f t l a t e r a l motion on the underlying Nootka f a u l t zone coupling through the o v e r l y i n g l i t h o s p h e r e cannot be r u l e d out by these observations, but i t seems l e s s l i k e l y . Appealing s o l e l y to the change i n i n t e r a c t i o n r a t e s across the u n d e r l y i n g f a u l t zone as the cause of the earthquakes, when normal subduction immediately south of the Nootka f a u l t zone does not seem to couple s u f f i c i e n t s t r e s s to cause l a r g e earthquakes, i s not a p a r t i c u l a r l y s a t i s f a c t o r y argument. The s u b c r u s t a l earthquakes of c e n t r a l Georgia S t r a i t do not appear to be r e l a t e d to the subducted Nootka f a u l t zone. They are probably caused by phase changes and are a smaller s c a l e analogy of the main Puget Sound c o n c e n t r a t i o n of s u b c r u s t a l earthquakes discussed i n chapter I I I . This i n t e r p r e t a t i o n suggests that the region of the subducted Juan de Fuca p l a t e u n d e r l y i n g southern Georgia S t r a i t i s aseismic r a t h e r than being a seismic gap w i t h a p o t e n t i a l f o r l a r g e s u b c r u s t a l earthquakes. - 140 -V. SEISMICITY AND SEISMOTECTONICS OF THE QUEEN CHARLOTTE ISLAND REGION A. INTRODUCTION The t e c t o n i c s e t t i n g of the Queen C h a r l o t t e Islands and southeast Alaska i s dominated by the p r o x i m i t y of the a c t i v e boundary between the P a c i f i c P l a t e and the America P l a t e , g e n e r a l l y r e f e r r e d to as the Queen C h a r l o t t e f a u l t ( F i g u r e 57). There are two major seismotectonic problems to be addressed i n the Queen C h a r l o t t e Islands r e g i o n . The f i r s t i s the exact l o c a t i o n of the s e i s m i c i t y . A p l o t of epicentres from the Canadian Earthquake Data F i l e shows most of them concentrated around the postu l a t e d t r a c e of the Queen C h a r l o t t e f a u l t , but some events are s i g n i f i c a n t d i s t a n c e s from the main grouping (eg. Figure 2, taken from Milne et a l . , 1978). Thus, i s a l l the s e i s m i c i t y along the f a u l t or are earthquakes o c c u r r i n g elsewhere i n the region? More s p e c i f i c a l l y , could one of the main i n l a n d f a u l t s such as Rennell Sound - Louscoone I n l e t f a u l t or Sandspit f a u l t ( F i g u r e 58) be a c t i v e or are there any a c t i v e f a u l t s under Hecate S t r a i t or Queen C h a r l o t t e Sound? The second major questions to be addressed i s that of the earthquake mechanisms. G l o b a l t e c t o n i c models of the re g i o n (Minster et a l . , 1974; Minste r et a l . , 1978; Chase, 1978) suggest that i n the region of the Queen C h a r l o t t e I s l a n d s , the P a c i f i c p l a t e and the American p l a t e i n t e r a c t at an angle that i s oblique to the s t r i k e of the Queen C h a r l o t t e f a u l t (Figure 58). This angle i s very small at the north end of the f a u l t i n southeast A l a s k a , but becomes more pronounced i n the Queen C h a r l o t t e Islands region and i s most oblique i n the southern Queen C h a r l o t t e I s l a n d s , south of 53° - lkl -Figure 57 Tectonic s e t t i n g , geographic features and key seismograph s t a t i o n s i n the Queen C h a r l o t t e Islands r e g i o n . - 142 -Figure 58 Major f a u l t s , plate i n t e r a c t i o n d i r e c t i o n and bathymetry. - 143 -N, where the s h e l f break s t r i k e s s i g n i f i c a n t l y more to the east. These p l a t e i n t e r a c t i o n models suggest there i s convergence i n the Queen C h a r l o t t e I s l a n d s r e g i o n which must be accommodated i n some way. I t may be that the Queen C h a r l o t t e Islands are o v e r r i d i n g the P a c i f i c P l a t e . Seismic r e f l e c t i o n p r o f i l e s ( S r i v a s t a v a , 1978; Chase et a l . , 1975) suggest that some element of underthrusting may be present. Thus the second question to be asked i s i f any net convergence i n t h i s r e gion can be detected i n the earthquake f o c a l mechanisms. To i n v e s t i g a t e earthquake l o c a t i o n s i n the Canadian Earthquake Data F i l e f o r the Queen C h a r l o t t e I s l a n d s r e g i o n , e p i c e n t r e s were examined f o r the p e r i o d from 1900 to 1980. A l i t e r a t u r e search was performed to seek a l l p u b lished e p i c e n t r e s other than those i n standard catalogues. Two key papers (Tobin and Sykes, 1968; K e l l e h e r and Savino, 1975) were found which r e l o c a t e d a number of earthquakes i n the region by recomputing e a r l i e r e p i c e n t r e s using the raw data published by the I n t e r n a t i o n a l S e i s m o l o g i c a l Summary ( I S S ) . These ep i c e n t r e s were evaluated against the epic e n t r e s i n the data f i l e and s u b s t i t u t e d i n many cases. In a d d i t i o n to these e p i c e n t r e s , a l l other events f o r which s u f f i c i e n t data e x i s t e d i n the ISS, USCGS b u l l e t i n s or Canadian b u l l e t i n s were r e c a l c u l a t e d . The r e s u l t s f o r a l l earthquakes of magnitude 5 and greater are shown i n Figure 59 and a l l r e v i s i o n s are l i s t e d i n Appendix 4. I n a d d i t i o n to e p i c e n t r e s , a l l p r e v i o u s l y published f a u l t plane s o l u t i o n s i n the region were compiled and examined i n d e t a i l . Their accuracy and relevance to the t e c t o n i c s e t t i n g i s d iscussed. Epicentres magnitude 5 and greater (1900 to 1980) with r e v i s i o n s from Appendix 4. - 145 -B. EARTHQUAKE LOCATIONS 1) Larger Earthquakes The c o n t i n e n t a l s h e l f break i s very sharp adjacent to the Queen C h a r l o t t e I s l a n d s (see Figure 59) and appears to mark the tra c e of the Queen C h a r l o t t e f a u l t w i t h i n a few k i l o m e t e r s . The r e v i s e d epicentres f o r events l a r g e r than magnitude 5 show much more of a concentration along the Queen C h a r l o t t e f a u l t than d i d the epicentres i n the Canadian Earthquake Data F i l e (compare Figures 2 and 59). Opposite the Queen C h a r l o t t e I s l a n d s a second lower scarp e x i s t s about 20 km seaward of the main scarp (eg. Hyndman and E l l i s , 1981). Although i t cannot be r u l e d out, there i s no suggestion here that the lower scarp i s s e i s m i c a l l y a c t i v e . Because the s h e l f break i s so sharp the ep i c e n t r e s of earthquakes i n t h i s r e g i o n , more than i n any other i n t h i s study, give a f e e l i n g f o r how accurate earthquake l o c a t i o n s are. For i n s t a n c e , most of the l a r g e earthquakes that are l o c a t e d from a good d i s t r i b u t i o n of world wide data have epicentres w i t h i n a few k i l o m e t e r s of the s h e l f break (Fi g u r e 59). As events become s m a l l e r , the d i s t r i b u t i o n of s t a t i o n s r e p o r t i n g becomes l e s s evenly d i s t r i b u t e d i n azimuth and east-west accuracy becomes a problem. In e a r l i e r years, o f t e n Saskatoon, Saskatchewan (SAS), 1600 km d i s t a n t , was the only s t a t i o n p r o v i d i n g east-west c o n t r o l unless the event was l a r g e enough to record at Ottawa (OTT) and s t a t i o n s along the east coast of United States. I n more recent y e a r s , even though readings are very accurate, the l a r g e number of s t a t i o n s i n North America at a narrow range of azimuths make the epicentres s u s c e p t i b l e to e r r o r s i n the e a r t h model, again a f f e c t i n g east-west c o n t r o l . The drawing of the epic e n t r e s to the east of t h e i r true l o c a t i o n i s a common r e s u l t . A good example of t h i s i s shown by Hyndman et a l . (1978). - 146 -When t r y i n g to estimate the best e p i c e n t r e s f o r earthquakes that occurred before 1950, there are three main choices: the ISS e p i c e n t r e s , the e p i c e n t r e s published by Gutenberg and R i c h t e r (1949) and epicentres computed using P a r r i v a l times l i s t e d i n the ISS. When the earthquake i s l a r g e and has been recorded by s t a t i o n s w e l l d i s t r i b u t e d i n azimuth and dist a n c e then the three e p i c e n t r e s are a l l reasonably c l o s e together ( i . e . w i t h i n 100 km of each o t h e r ) . When fewer data are a v a i l a b l e epicentres can vary by hundreds of k i l o m e t e r s . For example consider the d i f f e r e n t e p i c e n t r e s f o r the May 26, 1929 event (M = 7) and the J u l y 1, 1930 event (M = 5.7) shown i n Figure 60. I n almost a l l cases the ISS s o l u t i o n s can be considered to be the poorest. They were c a l c u l a t e d using a r r i v a l times mailed from v a r i o u s s t a t i o n s and the aim was to l o c a t e the earthquakes i n the c o r r e c t part of the globe r a t h e r than pursue the best e p i c e n t r e . Often e p i c e n t r e s were assigned to one p r e v i o u s l y computed i n the region r a t h e r than computing a new e p i c e n t r e . T r a v e l time curves and computational methods improved through time and thus the l a t e r ISS epice n t r e s are more accurate. Even though M i l n e , s e l e c t e d Gutenberg and R i c h t e r (1949) epicentres f o r most Queen C h a r l o t t e s events i n h i s report on h i s t o r i c a l west coast s e i s m i c i t y ( M i l n e , 1956), ISS epi c e n t r e s are the ones that were most commonly compiled i n t o the present Canadian Earthquake Data F i l e . There appears to be no reason f o r t h i s other than the a v a i l a b i l i t y of the ISS catalogues. Gutenberg and R i c h t e r produced epi c e n t r e s f o r most major earthquakes i n the world i n t h e i r volume S e i s m i c i t y of the E a r t h , f i r s t published i n 1949. They had the b e n e f i t of ISS e p i c e n t r e s , worked w i t h o r i g i n a l records from C a l i f o r n i a s t a t i o n s and used readings from s e l e c t e d seismograph s t a t i o n s known to have r e l i a b l e time c o n t r o l . They used both P and S in f o r m a t i o n and had a good set of t r a v e l time curves to work w i t h . Thus, - 14? -Figure 60 Various epicentres for the May 26, 1929 (M=7) earthquake (squares) and the J u l y 1, 1930 (M = 5.7) earthquake. ISS -International Seismological Summary; GR - Gutenberg and Richter (1949); KS - Kelleher and Savino (1975): M - Milne (1963). - 148 -t h e i r e p i c e n t r e s are u s u a l l y more r e l i a b l e than those published by the ISS. They quote t h e i r e p i c e n t r e s i n most cases to the nearest 1/2 degree or 1/4 degree ( i . e . about + 50 km or + 25 km). To be weighed against the Gutenberg and R i c h t e r epicentres are ep i c e n t r e s that have been computed w i t h a modern computer program using P a r r i v a l s l i s t e d i n the ISS. A l l P a r r i v a l data can be used, but a l l data i s not e q u a l l y r e l i a b l e and thus erroneous values may severely p r e j u d i c e the s o l u t i o n . Low gain seismographs o f t e n made i d e n t i f i c a t i o n of the f i r s t onset of P a r r i v a l s d i f f i c u l t and r e s u l t e d i n many l a t e p i c k s . Slow paper speed (commonly 10 or 15 mm per minute) and d i f f i c u l t y i n maintaining accurate time o f t e n made timing e r r o r s l a r g e . For example, the two c l o s e s t s t a t i o n s to the Queen C h a r l o t t e Islands r e g i o n , S i t k a and V i c t o r i a , which have considerable i n f l u e n c e on most s o l u t i o n s , are t y p i c a l . S i t k a had a Bosh-Omori seismograph from 1904 to 1932 which had a s t a t i c m a g n i f i c a t i o n of 10 and a paper speed of 15 mm per minute. In 1932 i t was replaced by Wenner instruments w i t h a s t a t i c m a g n i f i c a t i o n of 1000. I t was not u n t i l the 1960's that a modern high gain short period s t a t i o n w i t h 60 mm per minute paper speed was e s t a b l i s h e d . In a 1920 b u l l e t i n the timing accuracy of S i t k a was l i s t e d as + 10 seconds and d i d not improve to the order of + 1 second u n t i l some time i n the 1930's. V i c t o r i a had good time c o n t r o l of + 1 second almost from the beginning but the f a s t e s t paper speed on any instrument was 8 mm per minute u n t i l the f i r s t high gain short period s t a t i o n was e s t a b l i s h e d i n J u l y of 1948. The f i r s t seismograph at V i c t o r i a , a Milne i n s t a l l e d i n l a t e 1898, had a s t a t i c m a g n i f i c a t i o n of about 7. A Weichert ( s t a t i c m a g n i f i c a t i o n of 70) was added i n 1917 and Milne-Shaw seismographs ( s t a t i c m a g n i f i c a t i o n of 300) were added i n 1922. Epi c e n t r e s f o r many earthquakes i n the region have been computed by - 149 -Tobin and Sykes (1968) and K e l l e h e r and Savino (1975). For t h i s study a l l earthquakes p r i o r to 1948 i n the Queen C h a r l o t t e s r e gion were recomputed as w e l l as any l a t e r event that had not been p r e v i o u s l y recomputed or that showed s i g n i f i c a n t d e v i a t i o n ( g e n e r a l l y greater than 50 km) from the Gutenberg and R i c h t e r (1949) e p i c e n t r e s . The program EPDET (Weichert and Newton, 1970) was used w i t h the JB t r a v e l time t a b l e s . Considerable experimentation was done to determine the s t a b i l i t y of the s o l u t i o n and the dependence on c e r t a i n s t a t i o n s i n order to assess the r e l i a b i l i t y of the e p i c e n t r e c a l c u l a t e d . This was done by e l i m i n a t i n g the l a r g e s t r e s i d u a l s one by one, paying a t t e n t i o n to the f a c t that l a t e p i c k s are more l i k e l y on low g a i n seismograms. I t was u s u a l l y found that by the time a l l the remaining s t a t i o n s had r e s i d u a l s which were l e s s than about 3 seconds, adding or s u b t r a c t i n g a few s t a t i o n s made very l i t t l e d i f f e r e n c e to the l o c a t i o n or the o r i g i n time. T h i s was then considered to be a s t a b l e s o l u t i o n . Some s o l u t i o n s would not converge to one l o c a t i o n and i n these cases the Gutenberg and R i c h t e r s o l u t i o n was chosen as the best estimate over the computer c a l c u l a t e d s o l u t i o n because they had the advantage of S i n f o r m a t i o n to r e s t r i c t the o r i g i n time. A l l changes are documented i n Appendix 4. 2) Completeness and Accuracy Some comment on the completeness and accuracy of the r e v i s e d Queen C h a r l o t t e s data set i s warranted. A summary of these c h a r a c t e r i s t i c s i s made i n Table X, but the t a b l e should only be used as a guide as there are exceptions and some boundaries are not w e l l d e f i n e d . Before the ISS s t a r t e d i t s annual summary i n 1917, e p i c e n t r e s were very poorly defined and many earthquakes went unlocated. However no earthquakes l a r g e r than magnitude 7 could have escaped d e t e c t i o n i n the Queen C h a r l o t t e s region - 150 -TABLE X Completeness and Accuracy of the Revised Data Set, Completeness  Magnitude t h r e s h o l d 7 6.5 6 5.5 4.5 4 Year 1899 1917 1917 1940+ 1951+ 1965 1965 Accuracy* + 100 ki l o m e t e r s + 50 ki l o m e t e r s + 50 km (east-west) + 25 km (north-south) + 25 km *Accuracy estimates apply only to events above the magnitude threshold f o r complete d e t e c t i o n . - 151 -s i n c e the founding of the V i c t o r i a seismograph s t a t i o n i n l a t e 1898. A f t e r 1917 ISS processing should i n s u r e that a l l earthquakes of magnitude 6 and g r e a t e r should have been detected and l o c a t e d i n the r e g i o n . As short p e r i o d B e n i o f f seismometers were deployed around North America i n the 1930's the l o c a t i o n threshold moved down to the 5.5 l e v e l . C e r t a i n l y t h i s was the case by 1940 but the year 1940 i s not s i g n i f i c a n t and the 5.5 t h r e s h o l d may have been reached a few years sooner. The accuracy during t h i s time period i s roughly + 50 km. The establishment of a modern high gain seismograph i n V i c t o r i a i n 1948 made an improvement i n the l o c a t i o n c a p a b i l i t y i n the Queen C h a r l o t t e s area although, because of s t a t i o n r e l i a b i l i t y , i t was not u n t i l two companion s t a t i o n s were e s t a b l i s h e d at A l b e r n i (ALB) and Horseshoe Bay (HBC) i n 1951 that a reading from the southern Vancouver I s l a n d region could be guaranteed. The e f f e c t of having at l e a s t one high q u a l i t y a r r i v a l from the southern Vancouver I s l a n d region decreased the e r r o r along the Queen C h a r l o t t e f a u l t (roughly i n a-northwest-southeast d i r e c t i o n ) to b e t t e r than + 25 km. The e r r o r perpendicular to the f a u l t s t i l l remained the order of 50 km f o r a l l but the l a r g e s t ( g r e a t e r than M = 6.5) events. The l o c a t i o n t h r e s h o l d was a l s o dropped at t h i s time to about magnitude 5 f o r the Queen C h a r l o t t e Islands although some magnitude 5 events may s t i l l have not been l o c a t e d immediately to the north i n southeast Alaska even though they would have been recorded on s e v e r a l s t a t i o n s . The most s i g n i f i c a n t improvement f o r earthquake l o c a t i o n i n the region was made when the F o r t St. James seismograph s t a t i o n (FSJ) was e s t a b l i s h e d i n c e n t r a l B r i t i s h Columbia i n 1965. This lowered the complete d e t e c t i o n t h r e s h o l d to magnitude 4 and reduced the east-west e r r o r to the same order as the north-south e r r o r (about + 25 km). I t must be emphasized again that t h i s estimate i s only f o r events above the completeness t h r e s h o l d . While - 152 -smaller events may have been l o c a t e d from time to time, u n c e r t a i n t y i n phase i d e n t i f i c a t i o n can give r i s e to much l a r g e r e r r o r s (see d i s c u s s i o n i n next s e c t i o n ) . The establishment of a s t a t i o n on the Queen C h a r l o t t e Islands i n 1970 increased the d e t e c t i o n c a p a b i l i t y but d i d not s i g n i f i c a n t l y decrease the l o c a t i o n t h r e s h o l d below the 1965 l e v e l as c l e a r readings at FSJ and PHC are s t i l l the l i m i t i n g f a c t o r . There are only three earthquakes l o c a t e d s i g n i f i c a n t l y east of the Queen C h a r l o t t e f a u l t i n Figure 59, two are poorly l o c a t e d aftershocks of the 1949 earthquake (see Figure 65) and the t h i r d i s the December 21, 1936 (M=6) earthquake which appears to be s i g n i f i c a n t l y east of the Queen C h a r l o t t e f a u l t (near 53N, 132W i n Figure 59). With an e r r o r of + 50 km t h i s earthquake could a c t u a l l y be on the Queen C h a r l o t t e f a u l t . The l o c a t i o n f o r t h i s event i s s u s p i c i o u s , f o r although the s o l u t i o n appears to be w e l l defined at the given l o c a t i o n , a l i k e l y a f t e r s h o c k , o c c u r r i n g 24 minutes l a t e r , l o c a t e s w i t h the same degree of p r e c i s i o n immediately to the west on the Queen C h a r l o t t e f a u l t scarp. I t should be emphasized that the e r r o r estimates i n Table X apply only to events over the magnitude thresholds i n d i c a t e d and smaller events, though they may o c c a s i o n a l l y have enough readings to be l o c a t e d , can have conside r a b l y l a r g e r u n c e r t a i n t y . For example, the Nov. 16, 1923 earthquake (M=5) shown at 53.5N, 133W i n Figure 59 probably has an u n c e r t a i n t y the order of + 100 km. 3) Smaller Earthquakes 1951-1980 Up u n t i l now, I have been d i s c u s s i n g l a r g e r events that are l o c a t e d w i t h i n t e r n a t i o n a l networks, g e n e r a l l y events l a r g e r than magnitude 5. A f t e r 1951 some earthquakes smaller than magnitude 5 were l o c a t e d i n the Queen C h a r l o t t e I s l a n d s r e g i o n but the l o c a t i o n s have l a r g e u n c e r t a i n t i e s - 153 -(+ 50 km or more i n some cases). When Fort St. James (FSJ) s t a t i o n was e s t a b l i s h e d i n c e n t r a l B r i t i s h Columbia i n 1965, i t became p o s s i b l e i n c o n j u n c t i o n w i t h P o r t Hardy (PHC) on northern Vancouver I s l a n d , to r o u t i n e l y a s s i g n e p i c e n t r e s to events magnitude 4 and l a r g e r . The accuracy of these ep i c e n t r e s was improved w i t h establishment of a seismograph s t a t i o n on the Queen C h a r l o t t e Islands i n 1970. E p i c e n t r e s f o r smaller events could be c a l c u l a t e d i n some in s t a n c e s , although t h i s i s dependent on the noise c o n d i t i o n s at the i n d i v i d u a l seismograph s t a t i o n s . Most epi c e n t r e s a f t e r 1965 l o c a t e along the Queen C h a r l o t t e f a u l t , but three s m a l l earthquakes had e p i c e n t r e s s i g n i f i c a n t distances to the east of the Queen C h a r l o t t e f a u l t (Figure 61). These three events were subjected to a d e t a i l e d study, a) C a l i b r a t i n g Events I n order to i n v e s t i g a t e whether the small events on the Queen C h a r l o t t e I s l a n d s were a c t u a l l y l o c a t e d there or were mislocated Queen C h a r l o t t e f a u l t events, s e v e r a l l a r g e r w e l l l o c a t e d reference events were used to understand what phases were l i k e l y to be v i s i b l e at each of the 3 c l o s e s t seismograph s t a t i o n s and what time v a r i a t i o n s could be expected from the standard t r a v e l time curves f o r the a r r i v a l time of each phase at each s t a t i o n . In order to i n s u r e w e l l l o c a t e d c a l i b r a t i n g events, only ones w i t h e p i c e n t r e s c l o s e to the Queen C h a r l o t t e f a u l t scarp and s o l u t i o n s w i t h s m a l l RMS r e s i d u a l s were s e l e c t e d . The seismograms of these events were examined to e s t a b l i s h the character of a r r i v a l s at each s t a t i o n , a c c u r a t e l y p i c k Pn, Pg, Sn and Sg a r r i v a l s and e s t a b l i s h s t a t i o n c o r r e c t i o n s at the 3 c l o s e s t s t a t i o n s that recorded the s m a l l e r events. The seismograms of the s m a l l e r events were then examined to see i f i n c o r r e c t p i c k s could have been made when the o r i g i n a l processing was done. The earthquakes were then r e l o c a t e d using the s t a t i o n c o r r e c t i o n s from the c a l i b r a t i n g events (see - 154 -Figure 61 Epicentres i n l a n d of the Queen C h a r l o t t e f a u l t since 1965. Open c i r c l e s i n d i c a t e o r i g i n a l e p i c e n t r e , s o l i d c i r c l e s i n d i c a t e r e v i s e d e p i c e n t r e s . Stars are c a l i b r a t i n g events used to generate s t a t i o n c o r r e c t i o n s and to i d e n t i f y phases. - 155 -Figure 61). The c a l i b r a t i n g events were s e l e c t e d so that they were l a r g e enough to have a l l phases recorded at PHC and FSJ but small enough that the onset of S phases would not be l o s t at PHC and FSJ. This e f f e c t i v e l y l i m i t e d the magnitude range from 3 3/4 ( i f the background noise was low) to about 4 3/4. Another c r i t e r i o n used to ensure w e l l l o c a t e d events were chosen was that the e p i c e n t r e s were near the Queen C h a r l o t t e f a u l t and that the 3 - s t a t i o n s o l u t i o n (QCC, PHC, FSJ) was not too d i f f e r e n t from the published EPB s o l u t i o n . A r r i v a l s were picked by comparing seismograms from s e v e r a l earthquakes w i t h the same e p i c e n t r a l area. For the northern Queen C h a r l o t t e s Pn, Pg and Sg are i d e n t i f i e d r e a d i l y on FSJ whereas only Pn and Sn can be picked on PHC. Moving to the southern Queen C h a r l o t t e s , again Pn, Pg and Sg are the phases that can be picked on FSJ but the a r r i v a l p icked as Pg, though always i m p u l s i v e , seems to be u n r e l i a b l e . This v a r i a b i l i t y of a r r i v a l times of Pg by s e v e r a l seconds compared to other phases was not j u s t r e s t r i c t e d to the three c a l i b r a t i n g events used, but was a l s o n o t i c e d i n s e v e r a l other events that were considered f o r c a l i b r a t i n g events. The reason i s not c l e a r but could be caused by v a r i a t i o n i n c r u s t a l s t r u c t u r e or f o c a l depth. The southern c a l i b r a t i n g events show Pn and Sn c l e a r l y on PHC but Sg i s a recognizeable i n c i d e n t a r r i v a l as w e l l and i s u s u a l l y the l a r g e s t phase on the seismograms. Moving even f a r t h e r south to the r e g i o n of the aftershocks of the magnitude 7 June 24, 1970 earthquake, Sn becomes i n d i s t i n c t and there are s e v e r a l P phases a f t e r Pn so that Pn and Sg are the only phases that can be picked r e l i a b l y . This change of phases w i t h the l a t i t u d e of the e p i c e n t r e has been a source of e r r o r i n c o r r e c t l y i d e n t i f y i n g phases on PHC f o r Queen C h a r l o t t e f a u l t zone earthquakes, e s p e c i a l l y f o r smaller earthquakes where only the l a r g e r phases are v i s i b l e . - 156 -The c a l i b r a t i n g events were used to i n v e s t i g a t e the e f f e c t of c r u s t a l model on e p i c e n t r e s o l u t i o n s . The standard EPB model c o n s i s t s of a 36 km l a y e r of 6.2 km/s m a t e r i a l over an 8.2 km/s h a l f s p a c e , both w i t h a Poisson's r a t i o of approximately 0.25. A seismic experiment was conducted i n 1970 s e t t i n g o f f explosions i n B i r d Lake on the Queen C h a r l o t t e Islands and recorded on the mainland. The r e s u l t i n g model (F o r s y t h et a l . , 1974) c o n s i s t s of an e f f e c t i v e Pn v e l o c i t y of 8.0 km/s, an e f f e c t i v e Pg v e l o c i t y at the d i s t a n c e of FSJ of 6.2 and a c r u s t a l thickness of about 30 km. A Poisson's r a t i o of 0.25 was assumed and the model was used to l o c a t e the c a l i b r a t i n g events. In most cases, the RMS e r r o r was reduced s l i g h t l y (by 0.1 or 0.2 s) and the e p i c e n t r e moved by l e s s than 2 km. Since the improvements w i t h the B i r d Lake model were not great, i t was decided to use the standard model and generate s t a t i o n c o r r e c t i o n s equivalent to the r e s i d u a l s observed. (Tables XI and X I I ) . b) Sandspit F a u l t There are two events occuring i n 1974 and 1975 which have epicentres s i g n i f i c a n t l y east of the Queen C h a r l o t t e f a u l t ( F i g u r e 61). They were i n v e s t i g a t e d to see i f they were mislocated Queen C h a r l o t t e F a u l t events or p o s s i b l y Sandspit f a u l t events. The r e v i s e d e p i c e n t r e s , c a l c u l a t e d using the generated s t a t i o n c o r r e c t i o n s and only high q u a l i t y a r r i v a l s picked a f t e r examining the o r i g i n a l seismograms, are a l s o l o c a t e d i n the same v i c i n i t y , s i g n i f i c a n t l y east of the extension of the Sandspit f a u l t ( F i g u r e 61). The S and P a r r i v a l s at both QCC and FSJ are such that the events could be l o c a t e d as i n d i c a t e d or on the Queen C h a r l o t t e f a u l t near Tasu. The earthquakes are too small (^ = 3.3, 3.7) to be recorded w e l l at PHC and only one phase stands out on the record. By analogy w i t h the northern Queen C h a r l o t t e s c a l i b r a t i n g events (Table X I I ) the l a r g e s t phase should be Sn. This i n t e r p r e t a t i o n i s used to give the l o c a t i o n s shown i n Figure 61. - 157 -TABLE XI Residuals from Southern Queen C h a r l o t t e s  C a l i b r a t i n g Events STN Pn P1 S n S i 1) QCC +0.4 +0.4 PHC -2.2 +0.5 -1.3 +2.3 FSJ +1.0 +5.9 -1.0 2) QCC -1.5 +1.8 PHC -2.1 +2.3 -1.6 +1.6 FSJ +0.5 +0.5 -1.5 3) QCC not recording PHC -2.7 -1.1 +2.0 FSJ +1.3 +2.4 -1.9 Average R e s i d u a l s * QCC (-0.6) (+1.1) PHC -2.3 -1.5 +2.0 FSJ +0.9 (+2.7) -1.5 Three s t a t i o n s o l u t i o n s : 1) 1975 Feb 14 12 15 04.8 52.67 132.07 3.8 2) 1976 May 13 07 I I 42.6 52.79 132.31 4.8 3) 1978 J u l 11 03 04 50.0 52.64 132.00 4.1 *Values considered u n r e l i a b l e i f s t a t i o n c o r r e c t i o n s are bracketed. - 158 -TABLE X I I Residuals from Northern Queen C h a r l o t t e s  C a l i b r a t i n g Events STN Pn P i S n 4) QCC -2.1 PHC -1.4 +2.5 FSJ +2.3 -0.4 -1.0 5) QCC -3.2 PHC +2.6 FSJ +0.8 +0.9 -1.1 Average R e s i d u a l s * QCC -2.7 PHC (-1.4) +2.6 FSJ +1.6 (+0.2) -1.1 S l Three s t a t i o n s o l u t i o n s : 4) 1972 Jun 17 23 50 24.4 54.29 133.55 4.3 5) 1976 Oct 15 20 29 30.7 54.36 133.86 3.8 *Values considered u n r e l i a b l e i f s t a t i o n c o r r e c t i o n s are bracketed. - 159 -However, as there are no other earthquakes located near the east coast of the Queen Charlotte Islands and QCC and FSJ suggest locations either near the east coast or near Queen Charlotte fault, some uncertainty must be attached to epicentres that depend on an unconfirmed phase. Unlocated events recorded at Queen Charlotte Island stations were routinely reported from 1970 to 1976, f i r s t at Sandspit station (SSC) and after mid-1971 at Queen Charlotte City station (QCC). A l l events which had S-P times that indicated epicentres inland of the Queen Charlotte fault (see circles of detection in Figure 62) were tabulated to give an indication of possible seismicity along the Sandspit fault. Because the central Queen Charlotte Islands are an active logging area, the total number of events were expected to include a number of road construction blasts. The events were thus plotted to indicate time of day and as can be seen in Figure 62, the daytime hours are heavily blast contaminated. Four small events were detected in 6 years outside of working hours ranging in magnitude from 0.8 to 1.8. If this rate i s extrapolated to include the daytime hours, an event rate of about 1 microearthquake per year is indicated. This i s lower than or equal to the background level of microseismicity observed at most seismograph stations in the Canadian Cordillera and does not suggest any activity on the Sandspit fault, c) Queen Charlotte Sound The Canadian Earthquake data f i l e l i s t s locations for 21 earthquakes on the continental shelf of Queen Charlotte Sound since 1951. These events were examined in detail to see i f their epicentres were the best estimate possible. In most cases the earthquakes were found to be significantly mislocated and were relocated elsewhere. The problem was usually one of east-west control. The original epicentres and the revised epicentres are shown in Figure 63. - 160 -POSSIBLE SANDSPIT FAULT EVENTS TIME OF DAY L O C A L (GMT) N O ° N Figure 62 Sandspit f a u l t event rate. Histogram shows events that have S-P i n t e r v a l s i n d i c a t i n g they are inland of the Queen Charlotte Fault as indicated by the c i r c l e s around the seismograph s t a t i o n s . Numbers above histogram are magnitudes of events. - 161 -Revisions f o r epi c e n t r e s i n Queen C h a r l o t t e Sound. Open c i r c l e s are o r i g i n a l e p i c e n t r e s , s o l i d c i r c l e s are r e v i s e d e p i c e n t r e s . Three poorly located events do not have r e v i s e d e p i c e n t r e s but based on appearance they are b e l i e v e d to be part of a swarm i n the deep ocean. A l l of the r e v i s e d e p i c e n t r e s o c c u r r i n g before 1965 (12 of the 21 events) are poorly c o n s t r a i n e d . - 162 -For a l l events o r i g i n a l work sheets were examined to see the s t a t i o n s and phases used to l o c a t e the earthquake. For events p r i o r to 1965 USCGS b u l l e t i n s were a l s o checked f o r any a d d i t i o n a l data and the combined data set s were processed w i t h the same c r u s t a l model and computer program (CANSES) t h a t has been used by the E a r t h P h y s i c s Branch to c a l c u l a t e e p i c e n t r e s f o r the annual earthquake catalogues s i n c e 1972 i n western Canada. The main problem i s that i n t h i s p e r i o d , p r i o r to the establishment of FSJ, east-west c o n t r o l i s poor. The three c l o s e s t s t a t i o n s PHC, ALB and VIC (and only ALB and VIC p r i o r to 1962) are almost i n a s t r a i g h t l i n e p o i n t i n g at the re g i o n . Only when events are l a r g e enough to record on P e n t i c t o n (PNT) or s t a t i o n s i n the northwestern United States i s some degree of east-west c o n t r o l a v a i l a b l e . The ep i c e n t r e s of most pre-1965 events moved out of Queen C h a r l o t t e Sound when lo c a t e d w i t h the CANSES computer program and the standard model, but the ep i c e n t r e s of 3 small events i n September, 1963 remained. These events a l l have P. and S phases i d e n t i f i e d at PHC and P a r r i v a l s only i d e n t i f i e d at VIC, ALB and PNT. The data as presented on the work sheets suggested l i t t l e room f o r improvement. However, when the seismograms were examined, the a r r i v a l s were found to be so s m a l l as to be u n c e r t a i n by s e v e r a l 10's of seconds i n some cases. At Port Hardy (PHC) these events appear s i m i l a r and have s i m i l a r S-P i n t e r v a l s to events i n a la r g e swarm that occurred west of Vancouver I s l a n d from the period Aug. 30 to Sept. 10 (see Milne and Smith, 1966) and thus were removed from Queen C h a r l o t t e Sound. A l l of the ep i c e n t r e s a f t e r 1965 move out of Queen C h a r l o t t e Sound when r e l o c a t e d . The main problem here i s c o r r e c t l y i d e n t i f y i n g phases on FSJ, p a r t i c u l a r l y the S phases. An S phase i f c o r r e c t l y i d e n t i f i e d , even i f i t s onset i s u n c e r t a i n by s e v e r a l seconds, i s u s u a l l y s u f f i c i e n t to provide east-west c o n t r o l , and f o r a l l of the cases here, move the epicen t r e to the - 163 -west out of Queen C h a r l o t t e Sound. The c o r r e c t i d e n t i f i c a t i o n of phases i s complicated as events from the Queen C h a r l o t t e f a u l t have P and S as g g the most prominent phases on the FSJ seismogram, while events on the Revere-Dellwood F r a c t u r e zone have P^ and S n as the most prominent phases w i t h P and S very d i f f i c u l t to i d e n t i f y . When de a l i n g w i t h earthquakes near the background noise l e v e l i t i s d i f f i c u l t to p i c k phases c o r r e c t l y without a l a r g e r reference event from the r e g i o n . FSJ seismograms and i n some cases PHC seismograms were examined f o r most of the post 1965 events w i t h the Queen C h a r l o t t e s c a l i b r a t i n g events and w e l l l o c a t e d o f f s h o r e events used as reference events. This helped c o n s i d e r a b l y i n c o r r e c t phase i d e n t i f i c a t i o n . Some post 1965 epicentres s t i l l have l a r g e r r e s i d u a l s than w e l l l o c a t e d events i n the region and thus may not yet be the best s o l u t i o n s . I t would probably be necessary to re-examine a l l seismograms to i n s u r e optimum s o l u t i o n s , however the main concern here was c o r r e c t east-west c o n t r o l to determine i f there was any s e i s m i c i t y i n Queen C h a r l o t t e Sound. This o b j e c t i v e has been s a t i s f i e d and a l l earthquakes p r e v i o u s l y thought to be i n the Sound have been r e l o c a t e d to the r e g i o n of the steep c o n t i n e n t a l slope or to the deep ocean. Other than the r e v i s e d events discussed i n t h i s s e c t i o n on smaller earthquakes, most events i n the reg i o n of the Queen C h a r l o t t e I s l a n d s s i n c e 1965 are expected to be w e l l l o c a t e d ( i e . b e t t e r than +25 km) i f they are magnitude 4 or l a r g e r (Table X). These events are p l o t t e d i n Figure 64 and, s i m i l a r to Figure 59, show most epicentres l o c a t e d c l o s e to the scarp marking the Queen C h a r l o t t e f a u l t . 4) F o c a l Depth of Queen C h a r l o t t e Islands Earthquakes There i s very l i t t l e i n f o r m a t i o n on f o c a l depths f o r Queen C h a r l o t t e I s l a n d s ' earthquakes because there are no c l o s e seismograph s t a t i o n s to Figure 64 A l l events since 1965, magnitude 4 and gre a t e r from the Canadian Earthquake data f i l e . Only a few of these events have, been r e l o c a t e d . Epicentres are expected to be w i t h i n 25 km of true l o c a t i o n s except f o r those i n the bottom r i g h t of the f i g u r e where phase m i s i d e n t i f i c a t i o n can lead to l a r g e r e r r o r s . - 165 -c a l c u l a t e depths d i r e c t l y . A microearthquake survey (Hyndman and E l l i s , 1981) suggested depths of 20-25 km f o r s e v e r a l small events along the Queen C h a r l o t t e f a u l t . Routine processing of l a r g e r earthquakes by the ISC suggests shallow depths, g e n e r a l l y l e s s than 30 k i l o m e t e r s , but r e l i a b l e depth cannot be c a l c u l a t e d i n t h i s way unless the data are very c a r e f u l l y s e l e c t e d . Depth c a l c u l a t i o n s using the r e f l e c t i o n o f f the earth's surface pP can be very accurate and thus a search was made through the ISC f o r any events having w e l l defined pP depth c a l c u l a t i o n s . Only 4 were found and they are l i s t e d i n Table X I I I . These should be considered as maximum depths as pP phases i d e n t i f i e d at some s t a t i o n s may w e l l be pwP, r e f l e c t i o n o f f the surface of the ocean, which would give deeper depths because the low water v e l o c i t y i s not taken i n t o account i n standard pP depth c a l c u l a t i o n s by the ISC (e.g. see Mendiguren, 1971; F r o l i c h , 1982). This i s p a r t i c u l a r l y l i k e l y f o r events south of the Queen C h a r l o t t e i s l a n d s that are surrounded by water such as the 1970 and 1976 events l i s t e d i n Table X I I I . C. THE LARGEST EARTHQUAKES 1) The May 26, 1929 earthquake The magnitude of the 1929 earthquake was computed by Gutenburg and R i c h t e r (1954) to be 7, which i s confirmed i f the f e l t area estimated from newspaper r e p o r t s i s used to compute a magnitude from Toppozada's (1975) r e l a t i o n s h i p . The e p i c e n t r e i s constrained by a world-wide d i s t r i b u t i o n of seismograph s t a t i o n s w i t h a good range of d i s t a n c e and azimuth. The main source of e r r o r s i n the s o l u t i o n are the timing and measuring u n c e r t a i n t i e s a s s o c i a t e d w i t h seismograms of 1929 vintage. The e p i c e n t r e was recomputed - 166 -TABLE X I I I pP depths f o r Queen C h a r l o t t e F a u l t earthquakes* Date Time Lat(N) Long(W) Mag Depth(km) 1970 Jun 24 13 09 11.3 51.8 130.8 7 22 + 0.7 1976 Feb 23 15 14 15.1 51.4 130.6 6.0 6 + 0.8 1978 J u l 11 02 55 50.0 52.7 132.0 5.1 10 + 1.4 1979 J u l 11 12 28 04.0 55.2 134.0 5.1 9 + 1.0 *Depths and standard e r r o r s are from the ISC. These should be considered maximum depths as pP phases may a c t u a l l y be pwP, the r e f l e c t i o n of the ocean surface. - 167 -using P a r r i v a l times l i s t e d i n the ISS and considerable experimentation was done v a r y i n g the combinations of s t a t i o n s . A l l reasonable s o l u t i o n s f a l l w i t h i n a r e c t a n g l e defined by + 50 km perpendicular to the Queen C h a r l o t t e f a u l t and + 25 km along the f a u l t from the p r e f e r r e d e p i c e n t r e which i s i d e n t i f i e d i n Figure 65. This places the e p i c e n t r e to the south of the 1970 magnitude 7 earthquake i n t h i s region (Figure 65). There are no l o c a t e d aftershocks f o r t h i s event but they would have had to have been at l e a s t magnitude 5 1/2 to be r o u t i n e l y reported by the ISS i n 1929. Milne estimated the e p i c e n t r e f o r the 1929 earthquake to be i n Hecate S t r a i t near the c e n t r a l Queen C h a r l o t t e I s l a n d s on the basis of f e l t r e p o r t s ( M i l n e , 1956) and some c a l c u l a t i o n s ( M i l n e , 1963). There i s no evidence i n the c a l c u l a t i o n s done here to support t h i s and a l l other published e p i c e n t r e s (ISS, Gutenberg and R i c h t e r , 1949; K e l l e h e r and Savino, 1975) are a l s o south of the Queen C h a r l o t t e I s l a n d s . The d e s c r i p t i v e i n f o r m a t i o n l i s t e d by Milne (1956) can be misleading i f the M o d i f i e d M e r c a l l i s c a l e (e.g. R i c h t e r , 1958) i s used to i n t e r p r e t the l a r g e l a n d s l i d e s i n the c e n t r a l region of the Queen C h a r l o t t e s . Very high M o d i f i e d M e r c a l l i i n t e n s i t i e s r e s u l t suggesting proximity to the e p i c e n t r a l r e g i o n . A d e t a i l e d study of the l a n d s l i d e s a s s o c i a t e d w i t h the 1946 Vancouver I s l a n d earthquake (M g =7.3) has shown that i n the steep and h i g h r a i n f a l l westcoast t e r r a i n , l a r g e l a n d s l i d e s can occur i n regions of M o d i f i e d M e r c a l l i VI and greater (Mathews, 1979). The only observation reported by Milne (1956) that i s d e f i n i t e l y i n d i c a t i v e of an i n t e n s i t y higher than VI i s at Rose Harbour near the southern t i p of the Queen C h a r l o t t e I s l a n d s where a chimney was knocked down (Modified M e r c a l l i V I I ) . A l l of the f e l t and damage observations as reported by M i l n e (1956) are c o n s i s t e n t w i t h a magnitude 7 earthquake south of the Queen C h a r l o t t e Figure 65 Major earthquakes along the Queen Charlotte Fault Zone and the extent of t h e i r aftershock zones. C i r c l e s are possible 1949 aftershocks. Shaded c i r c l e s are poorer s o l u t i o n s . Two possible seismic gaps are i d e n t i f i e d . The northern gap may not e x i s t as the northern end of the 1949 aftershock zone i s not well defined. The southern gap has not experienced any major earthquakes i n at least 80 years. - 169 -Is l a n d s at the l o c a t i o n i n d i c a t e d here. 2) The August 22, 1949 Earthquake The August 22, 1949 earthquake, which had a magnitude of 8.1, i s the l a r g e s t earthquake that has occurred i n Canada i n h i s t o r i c times. Gutenberg and R i c h t e r (1949) computed the magnitude (M g) f o r the earthquake and i t was judged to be w e l l defined by G e l l e r and Kanamori (1977) who examined the o r i g i n a l work sheets. Tobin and Sykes (1968) r e l o c a t e d the l a r g e r aftershocks of t h i s event. The earthquake ruptured the Queen C h a r l o t t e f a u l t f o r at l e a s t 250 kilometres as i n d i c a t e d by the l e n g t h of the zone defined by the b e t t e r l o c a t e d aftershocks of Tobin and Sykes (1968). The rupture l e n g t h may have been up to 470 km i f a poorly l o c a t e d event near 56°N on August 26 and two earthquakes a l s o near 56°N o c c u r r i n g on October 31, more than two months a f t e r the mainshock are considered to be aftershocks (Figure 65). The l o c a t i o n s of the aftershocks of t h i s earthquake were i n v e s t i g a t e d w i t h the t e l e s e i s m i c e p i c e n t r e program EPDET, s t a r t i n g w i t h ISS P a r r i v a l s and using JB t a b l e s and zero f o c a l depth as Tobin and Sykes (1968) had done. Improved e p i c e n t r e s o l u t i o n s f o r s e v e r a l events were found by e l i m i n a t i n g s t a t i o n s w i t h the l a r g e s t r e s i d u a l s , but s e v e r a l very poorly l o c a t e d e p i c e n t r e s remain ( a f t e r s h o c k s are p l o t t e d i n Figure 65). The main problem f o r these events i s that the recording s t a t i o n s are a l l i n a narrow azimuth range to the southeast which gives very poor c o n t r o l perpendicular to the Queen C h a r l o t t e F a u l t . Stable s o l u t i o n s cannot be found as adding or s u b t r a c t i n g a s t a t i o n from the data set changes the l o c a t i o n of the e p i c e n t r e by tens of k i l o m e t e r s . Because i t was found that most t e l e s e i s m i c s o l u t i o n s become s t a b l e when a l l r e s i d u a l s above 3 seconds were e l i m i n a t e d , a 3 second c u t o f f was used here as w e l l to determine the - 170 -s o l u t i o n s l i s t e d i n Appendix 4. Magnitudes ( M ^ were estimated f o r most of the aftershocks from V i c t o r i a (VIC) short period v e r t i c a l B e n i o f f seismograph using the nomogram of Gutenberg and R i c h t e r (1942). The day of the mainshock and the day f o l l o w i n g are missing from EPB seismogram storage so magnitudes f o r events on these days were estimated from the number of P a r r i v a l s i n the ISS by comparing the number of P a r r i v a l s w i t h aftershocks f o r which seismograms were a v a i l a b l e . Hodgson, i n unpublished notes made when examining the o r i g i n a l seismograms of t h i s earthquake, scanned the records f o r a f t e r s h o c k s up to the end of September and i d e n t i f i e d nineteen. I t i s i n t e r e s t i n g to note that he d i d not i n c l u d e i n h i s l i s t the event on August 26 at 05:25 near 56°N nor the event on September 20 at 12:18 that has a p o o r l y defined e p i c e n t r e s i g n i f i c a n t l y east of the Queen C h a r l o t t e f a u l t near l a t i t u d e 53°N (see Figure 65). The September 20 event looks very d i f f e r e n t from the other a f t e r s h o c k s on the V i c t o r i a seismogram. A complete l i s t of p o s s i b l e aftershocks w i t h r e v i s e d l o c a t i o n s and magnitudes i s given i n Table XIV. A l l events above about magnitude 4 1/2 would have been i d e n t i f i e d on the V i c t o r i a seismograms. The r e v i s e d a f t e r s h o c k l o c a t i o n s suggest the aftershock zone extends a l i t t l e f a r t h e r south than that i n d i c a t e d by Tobin and Sykes (1968) and K e l l e h e r and Savino (1975) and that the rupture zone i s about 300 km long i f the events at 56°N are considered to be unrelated (Figure 65). 3) The June 24, 1970 Earthquake This earthquake has magnitude (M g = 7) reported by USCGS on 2 observations and a magnitude (M g = 7.4) reported by Moscow on 25 observations. Berkeley (M = 6.25 - 6.5) and Pasadena (M = 6.5 - 7) r e p o r t somewhat lower values. The f e l t area estimated from i n t e n s i t y reports - 171 -TABLE XIV Revised Parameters f o r 1949 Earthquake and P o s s i b l e Aftershocks ( F o c a l depth r e s t r i c t e d to zero i n a l l c a l c u l a t i o n s ) Date Time Lat(N) Long(W) Magnitude 1 Q u a l i t y 2 Aug 22 04 01 12.2 53.62 133.27 8.1(M S) Aug 22 09 15 21.4 54.96 133.43 4.5 P Aug 22 12 22 05 not l o c a t e d Aug 22 13 40 20 not l o c a t e d Aug 23 02 59 06.1 55.08 134.01 5.0 P Aug 23 19 37 33.0 52.42 131.87 5.0 Aug 23 19 43 35.0 52.64 132.10 5.0 Aug 23 20 24 31.1 52.69 132.23 6.4(MS) Aug 24 02 37 21 not lo c a t e d Aug 24 09 20 00 not l o c a t e d Aug 24 12 42 39 not l o c a t e d Aug 24 21 51 41 not l o c a t e d 5.0 Aug 24 22 37 13.1 52.78 132.11 4.9 Aug 26 05 25 57.5 56.08 135.27 4.9 P Aug 26 22 39 37.2 54.67 133.88 5.1 Aug 27 21 30 40.7 53.05 132.74 5.3 Sep 02 01 31 15.5 54.22 133.61 4.6 P Sep 05 06 54 10.0 53.62 132.97 4.9 Sep 12 08 36 03.5 54.87 134.32 4.9 P Sep 12 14 37 48.6 55.16 132.57 5.0 P Sep 18 11 59 00 not l o c a t e d 4.8 Sep 20 12 18 06.4 52.87 131.32 5.1 P Oct 31 01 39 29.5 56.05 135.69 6.25(M S) Oct 31 02 32 11.3 56.02 135.91 5.1 1 Magnitudes a f t e r Aug 24 at 21 hrs are M L values estimated from V i c t o r i a seismograms. Seismograms were not a v a i l a b l e before t h i s f o r e a r l i e r events but magnitudes were estimated from the number of P a r r i v a l s i n the ISS where p o s s i b l e . P i n d i c a t e s a poorly constrained e p i c e n t r e s o l u t i o n - 172 -(Horner et a l . , 1975) suggests a magnitude of about 6 3/4, but t h i s i s u n r e l i a b l e because the earthquake occurred at 6 a.m. l o c a l time and many people may have been asleep and thus d i d not observe the low i n t e n s i t y l e v e l s that are necessary to a c c u r a t e l y d e f i n e the t o t a l f e l t area. The o r i g i n a l seismograms from the c l o s e s t s t a t i o n s , Sandspit (SSQ), Port Hardy (PHC) and F o r t St. James (FSJ) were examined f o r the aftershocks of t h i s event to see i f more p r e c i s e l o c a t i o n s could be c a l c u l a t e d to define the f a u l t area. U n f o r t u n a t e l y , t h i s i s not p o s s i b l e f o r most aftershocks f o r s e v e r a l reasons: there are some key events that have t h e i r onsets buried i n the coda of previous events, PHC a r r i v a l s are very emergent and the S a r r i v a l s at FSJ cannot be picked f o r the l a r g e r aftershocks which have r e l i a b l e P phases at PHC. However, f o r three a f t e r s h o c k s , a r r i v a l times f o r both Pn and Sg phases could be picked at SSQ, PHC and FSJ. These events were l o c a t e d using the s t a t i o n c o r r e c t i o n s derived from the southern Queen C h a r l o t t e s c a l i b r a t i n g events (see s e c t i o n on C a l i b r a t i n g events and Table XI) and l o c a t e about 40 k i l o m e t r e s south of the e p i c e n t r e s l i s t e d i n the Canadian Earthquake Data f i l e (which are ISC e p i c e n t r e s ) . R e l a t i v e l o c a t i o n s were done between the mainshock and these aftershocks using only Pn a r r i v a l s at SSQ, PHC and FSJ and assuming a l l d i f f e r e n c e s i n a r r i v a l times were due to distance d i s t r i b u t i o n along the f a u l t . This shows an aftershock zone extending about 20 km southwest from the mainshock e p i c e n t r e . S-P i n t e r v a l s of unlocated aftershocks at SSQ show a v a r i a t i o n from 20 to 24 seconds which i s e q uivalent to an a f t e r s h o c k zone of about 35 k i l o m e t e r s i n l e n g t h , extending mainly south of the e p i c e n t r e . Both these observations suggest the rupture was south from the e p i c e n t r e and not b i l a t e r a l . - 173 -D. SEISMIC GAPS Most of the seismic s l i p along major p l a t e boundaries occurs i n earthquakes of magnitude 7 and greater (Brune, 1968). Earthquakes s m a l l e r than t h i s may be regarded as noise f o r the purpose of modelling p l a t e motion (McCann et a l . , 1979). The rupture zones of adjacent l a r g e earthquakes, defined by the extent of t h e i r a f t e r s h o c k zones, tend to abut r a t h e r than overlap (Fedotov, 1965; Mogi, 1968; Sykes, 1971) and thus regions of s e i s m i c a l l y a c t i v e p l a t e margins which have not r e c e n t l y experienced rupture can be considered as seismic gaps, where fu t u r e l a r g e earthquakes can be expected. The s i z e of the gap d e l i n e a t e s the s i z e of the zone which must rupture and thus defines the maximum magnitude the impending earthquake can have, assuming the whole gap ruptures at once. The seismic gap hypothesis i s w e l l e s t a b l i s h e d (Fedotov, 1965; A l l e n et a l . , 1965; Mogi, 1968; Tobin and Sykes, 1968; K e l l e h e r , 1972; K e l l e h e r et a l . , 1973; K e l l e h e r and Savino, 1975; McCann et a l . , 1979) and has now been used to s u c c e s s f u l l y f o r e c a s t the l o c a t i o n s of twelve l a r g e i n t e r p l a t e earthquakes (McCann et a l . 1979). Two such seismic gaps may c u r r e n t l y be present along the Queen C h a r l o t t e f a u l t (Figure 65). 1) The Northern Seismic Gap The p o s s i b i l i t y of a gap between the 1949 (M = 8.1) and 1972 (M = 7.6) earthquakes has been pointed out by K e l l e h e r and Savino (1975). They based the length of the rupture during the 1949 earthquake on the aftershocks l o c a t e d by Tobin and Sykes (1968). The u n c e r t a i n t y comes from whether to consider as aftershocks a po o r l y l o c a t e d event near 56°N on August 23, 1949 and two w e l l l o c a t e d earthquakes (M = 6 and M = 5.5) a l s o near 56°N that occurred on October 31, 1949, more than two months a f t e r the - 174 -mainshock. I f the northernmost events are a f t e r s h o c k s then the rupture l e n g t h i s 470 km; i f not i t i s about 300 km (see Figure 65). Although no d e f i n i t e c r i t e r i a have been e s t a b l i s h e d f o r i d e n t i f i c a t i o n of aftershocks at t e l e s e i s m i c d i s t a n c e s , K e l l e h e r (1972), when studying South American rupture zones, d i d not consider an event an af t e r s h o c k i f i t occurred more than 2 months a f t e r the mainshock or i f i t was i s o l a t e d by 50 km or more from other a f t e r s h o c k s . On t h i s b a s i s these events would not be considered as a f t e r s h o c k s and the s h o r t e r rupture l e n g t h of 300 km would be favoured. For t h i s study the aftershocks of the 1949 earthquake were r e l o c a t e d from Tobin and Sykes (1968) and magnitudes were c a l c u l a t e d f o r most events from the V i c t o r i a (VIC) short period B e n i o f f seismograph using the nomogram of Gutenberg and R i c h t e r (1942). F o l l o w i n g Tobin and Sykes (1968) f o c a l depths were r e s t r i c t e d to zero, only f i r s t a r r i v a l P and PkP a r r i v a l times were considered and the J e f f r e y s - B u l l e t i n t r a v e l time tables were used. S o l u t i o n s f o r some of the e p i c e n t r e s were improved over those of Tobin and Sykes (1968) by e l i m i n a t i n g s t a t i o n s w i t h the l a r g e s t r e s i d u a l s , but s e v e r a l very poorly l o c a t e d e p i c e n t r e s remain. The main problem i s that the r e c o r d i n g s t a t i o n s are a l l i n a narrow azimuth range to the southeast and give very poor east-west c o n t r o l . The b e t t e r l o c a t e d aftershocks that occurred w i t h i n a few weeks of the mainshock d e f i n e a zone which extends i n both d i r e c t i o n s from the mainshock f o r a t o t a l l e n g t h of about 300 km. Relocated aftershocks are p l o t t e d i n Figure 65. Rupture l e n g t h can a l s o be estimated from the magnitude of the earthquake. The magnitude (M g) of 8.1 f o r the Queen C h a r l o t t e Islands earthquake was computed by Gutenberg and R i c h t e r (1949) and was judged to be w e l l defined by G e l l e r and Kanamori (1977) who examined the o r i g i n a l worksheets. However, a w e l l defined magnitude does not n e c e s s a r i l y d e f i n e the extent of the rupture zone as magnitude versus f a u l t length and - 175 -magnitude versus f a u l t area r e l a t i o n s h i p s vary c o n s i d e r a b l y (Acharya, 1979). A s e l e c t i o n of o f t e n quoted r e l a t i o n s h i p s give estimates which more than cover the range of 300 km or 470 km suggested by the two i n t e r p r e t a t i o n s of the afters h o c k zone (Table XV). The Utsu and Seki (1965) r e l a t i o n s h i p i s known to over estimate f a u l t area (Kanamori and Anderson, 1975) and the Bath and Duda (1964) r e l a t i o n s h i p which gives an even l a r g e r estimate f o r the f a u l t area i s based on the study of only s i x earthquakes. Probably the most r e l i a b l e f a u l t length estimates f o r t h i s r e g i o n are those of Tocher (1958) and the western United States r e l a t i o n s h i p of Archarya (1979) as these are based on data derived mainly from l a r g e s t r i k e - s l i p earthquakes r a t h e r than l a r g e subduction zone t h r u s t events which dominate the other r e l a t i o n s h i p s . These both suggest a f a u l t l e n g t h of about 300 km f o r the 1949 event. This would a l s o favour the sh o r t e r i n t e r p r e t a t i o n f o r the afters h o c k zone and leave a seismic gap of about 150 k i l o m e t e r s , or so, no r t h of the Queen C h a r l o t t e I s l a n d s . 2) The Southern Seismic Gap The evidence f o r the southern gap i s more concrete. C e r t a i n l y no earthquake of magnitude 7 or greater could have occurred here without d e t e c t i o n since the establishment of a seismograph s t a t i o n at V i c t o r i a i n 1898. The zone i s bounded on the north by the southernmost aftershocks of the 1949 earthquake (M = 8.1). These are w e l l l o c a t e d i n a north-south sense and there does not appear to be any unlocated aftershocks above magnitude 4-1/2 on the V i c t o r i a seismograms that could be i n the gap reg i o n . The gap i s bounded on the south by the ep i c e n t r e of the mainshock of the 1970 earthquake (M = 7). The range of S-P i n t e r v a l s of unlocated a f t e r s h o c k s f o r the 1970 earthquake at the seismograph s t a t i o n on the Queen C h a r l o t t e Islands i n d i c a t e s a rupture zone extending about 35 km south of - 176 -TABLE XV Magnitude-Fault Area R e l a t i o n s h i p s (S=km2) 8.1 7.75 7.5 7.0 Log S = 1.02 M - 4.1 (Utsu and S e k i , 1954) 14500 6400 3500 1100 Log S = 1.21 M - 5.05 (Bath and Duda, 1964) 56400 21300 10600 2600 *Log S = 1.0 M - 4.53 ( G e l l e r , 1976) 3700 1700 900 300 Magnitude-Fault Length R e l a t i o n s h i p s (L=km) 8.1 7.75 7.5 7.0 M = 0.98 Log L + 5.65 (Tocher, 1958) 316 139 77 24 M = 0.76 Log L + 6.05 ( l i d a , 1965) 503 173 81 18 M = 1.06 Log L + 5.53 (Press, 1967) 265 124 72 24 M = 1.17 Log L + 5.2 (Acharya, 1979) 301 151 92 35 * P r e f e r r e d r e l a t i o n s h i p . When used w i t h a nominal f a u l t width of 10 km f o r oceanic l i t h o s p h e r e i t gives s i m i l a r values to the magnitude-fault length r e l a t i o n s h i p s . - 177 -the mainshock e p i c e n t r e . R e l o c a t i o n of three of the l a r g e r aftershocks of t h i s event using a r e l a t i v e l o c a t i o n technique i n d i c a t e s a rupture zone extending at l e a s t 20 km to the southeast of the mainshock e p i c e n t r e . Since these rupture l e n g t h estimates are of the order of f a u l t length that i s expected f o r a magnitude 7 earthquake (see Table XV), the rupture probably propagated south from the mainshock epicen t r e and d i d not extend s i g n i f i c a n t l y i n t o the gap r e g i o n . Sykes (1971) was the f i r s t to suggest that a seismic gap may e x i s t south of the 1949 Queen C h a r l o t t e I s l a n d s earthquake but he d i d not pursue the matter. K e l l e h e r and Savino (1975) noted that there was a 200 k i l o m e t e r r e g i o n c o n t a i n i n g some magnitude 7 earthquakes between the a f t e r s h o c k zone of the 1949 earthquake and the intense s e i s m i c i t y which marks the beginning of the r e g i o n of a c t i v e spreading centres. They d i d not, however, comment on the gap between the 1970 earthquake and the 1949 a f t e r s h o c k zone. McCann et a l . (1979) who summarized seismic gaps around the P a c i f i c appear to have i n c o r r e c t l y i n t e r p r e t e d K e l l e h e r and Savino (1975) when they suggest that most earthquakes south of the 1949 a f t e r s h o c k zone are expected to be l e s s than magnitude 7 because of l o c a l l y t h i n l i t h o s p h e r e i n c l o s e p r o x i m i t y to spreading centres. The t r i p l e j u n c t i o n r e g i o n between the P a c i f i c , America and Juan de Fuca P l a t e s has r e s i d e d i n the v i c i n i t y of Brooks Pe n i n s u l a on Vancouver I s l a n d (about 300 km south of the seismic gap) f o r most of the past 10 m i l l i o n years ( T i f f i n et a l . , 1972; Riddihough, 1977). I t i s only w i t h i n the l a s t m i l l i o n years that the t r i p l e j u n c t i o n has jumped about 200 kilometers north to where the Delwood K n o l l s and Tuzo Wilson K n o l l s appear to be new spreading centres that have broken through older l i t h o s p h e r e (Davis and Riddihough, 1982). Thus the l i t h o s p h e r e i n the v i c i n i t y of the seismic gap would be about 25 km t h i c k according to the square root formula of Oldenburg (1975). This i s not - 178 -s i g n i f i c a n t l y t h i n n e r than the l i t h o s p h e r e w i t h i n the rupture zone of 1949 earthquake immediately to the north. Routine p u b l i c a t i o n of the I n t e r n a t i o n a l S e i s m o l o g i c a l Summary (ISS) which s t a r t e d i n 1917 insures that a l l earthquakes i n t h i s region of magnitude 6 and greater since that time would have been detected and l o c a t e d . There appears to have been no earthquakes of t h i s s i z e i n the gap s i n c e 1917 w i t h the p o s s i b l e exception of a magnitude 6.0 event i n 1936 l o c a t e d at the northern end of the gap (at 52-l/2°N and 131-1/2°W) and a magnitude 6.1 event i n 1929 l o c a t e d at the southern end of the gap (at 51-1/2°N and 130-3/4°W). The epicentres of these earthquakes are l o c a t e d outside the gap but the u n c e r t a i n t y a s s o c i a t e d w i t h epicentres of events of that vintage (perhaps as l a r g e as + 50 km) precludes p o s i t i v e l y a s s i g n i n g them l o c a t i o n s outside the gap. I n any case, these events are not l a r g e enough to have f i l l e d the seismic gap even i f they occurred w i t h i n the gap. With the establishment of a modern high gain seismograph i n V i c t o r i a i n 1948 i t became p o s s i b l e to r o u t i n e l y l o c a t e earthquakes of magnitude 5 and g r e a t e r i n southern the Queen C h a r l o t t e Islands r e g i o n . The i n s t a l l a t i o n of a s t a t i o n at F o r t St. James (FSJ) i n c e n t r a l B r i t i s h Columbia east of the Queen C h a r l o t t e I s l a n d s i n 1965 moved the l o c a t i o n threshold down to the magnitude 4 l e v e l (see Table X). Magnitude 5 and greater events s i n c e 1950 and magnitude 4 and greater events since 1965 are p l o t t e d i n Figure 66 r e v e a l i n g the p e r s i s t e n c e of the seismic gap at the moderate earthquake l e v e l through that time p e r i o d . Except f o r an earthquake i n 1954 of magnitude 5.4, the gap r e g i o n has not experienced any s i g n i f i c a n t s e i s m i c i t y . Some microearthquakes were detected along the Queen C h a r l o t t e f a u l t w i t h i n the gap region during a b r i e f r e cording period i n 1979 (Hyndman and - 179 -54°-, yj53°H => H fe 52^ 1954 —2 MAGNITUDE 4- 4.9 • 5- 5.9 • 6- 6.9 T SEISMIC GAP i 1950 T T 1960 1970 1980 TIME (YEARS) Figure 66 D e t a i l s of the southern Queen Ch a r l o t t e Islands seismic gap. The l a t i t u d e s of earthquakes along the Queen C h a r l o t t e f a u l t are p l o t t e d against time. Earthquakes greater than magnitude 5 are p l o t t e d s i n c e 1950 and earthquakes greater than magnitude 4 s i n c e 1965. - 180 -E l l i s , 1981). About the same l e v e l of microearthquake a c t i v i t y was observed i n 1969 by Rogers (1976a) and i n 1971-1972 by K e l l e h e r and Savino (1975) i n the seismic gap that ruptured during the magnitude 7.6 S i t k a earthquake i n 1972. 3) The Expected Earthquakes Some estimates can be made of the s i z e and c h a r a c t e r i s t i c s of the earthquakes that could f i l l the seismic gaps. The northern gap i s about 150 km i n length (Figure 65). I f t h i s were a l l to rupture during one earthquake the magnitude-fault length r e l a t i o n s h i p s of I i d a (1965), Press (1967), Tocher (1958) and Acharya (1979) and the magnitude-fault area r e l a t i o n s h i p of G e l l e r (1976) suggest a magnitude 7-3/4 event would occur (see Table XV). The f a u l t i n g would be expected to be almost pure s t r i k e s l i p , s i m i l a r to the August 22, 1949 earthquake immediately to the south and the July 30, 1972 earthquake immediately to the north (see s e c t i o n E on Fo c a l Mechanisms). The l e n g t h of the southern gap appears to be about 75 km; r e q u i r i n g an earthquake of magnitude about 7-1/2 to completely f i l l i t (see Table XV). The Queen C h a r l o t t e f a u l t i s not a pure transform f a u l t i n t h i s r e gion but there i s an element of convergence r e f l e c t i n g the l o c a l r e l a t i v e motion between the P a c i f i c and America p l a t e s (Minster et a l . , 1978; Chase, 1978). The earthquake can thus be expected to have a combined s t r i k e - s l i p and t h r u s t i n g mechanism w i t h a minor t h r u s t component s i m i l a r to the magnitude 7 1970 earthquake that occurred immediately to the south (see s e c t i o n E on F o c a l Mechanisms). This could be s i g n i f i c a n t because v e r t i c a l motion can mean the p o s s i b i l i t y of tsunami generation. However, a major tsunami seems u n l i k e l y because the t h r u s t component i s small compared to the s t r i k e s l i p motion and there i s not a la r g e c o n t i n e n t a l s h e l f i n v o l v e d - 181 -i n t h i s r e g i o n . Combining the mechanism of the 1970 earthquake w i t h the t o t a l displacement estimated using Chinnery's (1969) magnitude — displacement r e l a t i o n s h i p , v e r t i c a l motion during the 1970 earthquake i s estimated to be the order of 0.25 meter and that f o r a magnitude 7.5 earthquake w i t h the same mechanism, the order of 0.7 meter. Small waves were noted l o c a l l y during the 1929, 1949, 1970 events (Milne 1956, Horner et a l . , 1970) but none were l a r g e enough to record on the Tofino t i d e gauge recorder ( p e r s o n a l communication 1982, S.0. Wigen, P a t r i c i a Bay I n s t i t u t e of Ocean Sciences.) The amount of s t r a i n that could have been stored i n the seismic gaps by constant p l a t e motion since the establishment of the V i c t o r i a seismograph s t a t i o n i s approximately 4.5 meters (84 years X 5.5 cm/yr). This i s equivalent to approximately a 7 3/4 magnitude earthquake (e.g. Chinnery, 1969). Thus, i n the northern region the stored s t r a i n appears to equal that expected to be released during a complete rupture and i n the southern sei s m i c gap the stored s t r a i n may exceed the t o t a l displacement expected during complete rupture. Taking these numbers at face value suggests that i n the southern gap s t r a i n may be being stored over a s l i g h t l y l a r g e r area or that some i s being taken up as aseismic creep. However, given the crudeness of the e m p i r i c a l r e l a t i o n s h i p s between earthquake magnitude and the v a r i o u s f a u l t parameters, the most reasonable c o n c l u s i o n i s simply that the time i s approaching when a s i g n i f i c a n t earthquake (or earthquakes) can be expected to occur In both of Queen C h a r l o t t e f a u l t seismic gaps. E. FOCAL MECHANISMS Fo c a l mechanisms that have been published f o r earthquakes i n the region - 182 -of the Queen C h a r l o t t e Islands are l i s t e d i n Table XVI. To the nort h , along southeast Alaska p r i m a r i l y s t r i k e - s l i p and t h r u s t i n g events are observed. The sense of r e l a t i v e motion c a l c u l a t e d from the mechanism s o l u t i o n s corresponds c l o s e l y w i t h expected r e l a t i v e p l a t e motion i n the r e g i o n (Perez and Jacob, 1980). I n the Queen C h a r l o t t e s area f o c a l mechanisms have been published f o r the great 1949 earthquake (M = 8.1) (Hodgson and M i l n e , 1951; Wickens and Hodgson, 1967), the 1970 earthquake (M = 7) south of the C h a r l o t t e s and an aftershock (Chandra, 1974), and a 1976 earthquake (M = 6) a l s o south of the C h a r l o t t e s (Wetmiller and Horner, 1978). Other than f o r the 1949 event, the published s o l u t i o n s f o r these earthquakes do not correspond i n azimuth to the l o c a l s t r i k e of the Queen C h a r l o t t e f a u l t or i n d i r e c t i o n of motion to the motion p r e d i c t e d by p l a t e t e c t o n i c s (Chase, 1978; Minster and Jordan, 1978). A l l of these mechanisms are reexamined i n t h i s s e c t i o n . To the south of the Queen C h a r l o t t e f a u l t , i n the complex t r i p l e j u n c t i o n r egion l e a d i n g to the E x p l o r e r spreading centre, the published mechanisms are not w e l l constrained but suggest s t r i k e - s l i p and a combination of s t r i k e - s l i p and normal f a u l t i n g (Hodgson and Storey, 1954; Chandra, 1974). There are a l s o mechanisms f o r some smaller earthquakes i n t h i s r e g i o n presented by Gallagher (1969) i n h i s Ph.D. t h e s i s . However, they are so poorly constrained that they are not u s e f u l f o r t e c t o n i c modelling. The l o c a t i o n of the Queen C h a r l o t t e I s l a n d s r e l a t i v e to the d i s t r i b u t i o n of seismographs on the globe e f f e c t i v e l y l i m i t s the s i z e of earthquake f o r which a w e l l d e f i n e d f o c a l mechanism can be c a l c u l a t e d by the P nodal method. Because of the o r i e n t a t i o n of the Queen C h a r l o t t e f a u l t , the southern extension of the f a u l t plane i s u s u a l l y w e l l c o nstrained because i t b i s e c t s the network of s t a t i o n s i n C a l i f o r n i a . The - 183 -TABLE XVI Published F a u l t Plane S o l u t i o n s f o r Earthquakes i n the V i c i n i t y  of the Queen Ch a r l o t t e F a u l t North of the Queen C h a r l o t t e I s l a n d s 1927 Oct 24 Stauder, 1959; Wickens and Hodgson, 1967 s t r i k e - • s l i p 1949 Oct 31 Hodgson and Storey, 1954; Wickens and Hodgson, 1967 th r u s t 1958 J u l 10 Stauder, 1960; Wickens and Hodgson, 1967 s t r i k e - • s l i p 1972 J u l 30 Chandra, 1974; Perez and Jacob, 1980 s t r i k e -• s l i p *1972 Aug 15 Perez and Jacob, 1980 s t r i k e -• s l i p *1972 Aug 04 Chandra, 1974 th r u s t 1973 J u l 01 Chandra, 1974; Perez and Jacob, 1980 t h r u s t 1973 J u l 03 Chandra, 1974; Perez and Jacob, 1980 s t r i k e -• s l i p Queen Ch a r l o t t e s I s l a n d s Region 1949 Aug 22 Hodgson and M i l n e , 1951; Wickens and Hodgson, 1967 s t r i k e -• s l i p *1970 Jun 24 Chandra, 1974 s t r i k e -- s l i p *1970 Jun 24 Chandra, 1974 s t r i k e -• s l i p *1976 Feb 23 Wetmiller and Horner, 1978 s t r i k e -• s l i p T r i p l e J u n c t i o n Region, south of the Queen C h a r l o t t e I s l a n d s *1948 Dec 31 Hodgson and Storey, 1954; Wickens and Hodgson, 1967 s t r i k e -- s l i p *1964 Mar 31 Tobin and Sykes, 1968; Chandra, 1964 s t r i k e -- s l i p *1971 Mar 13 Chandra, 1974 s t r i k e -- s l i p * P o orly constrained s o l u t i o n s - 184 -northern extension which passes through Alaska i s not w e l l c o n s t r a i n e d . The only operating s t a t i o n i n c e n t r a l Alaska f o r much of t h i s century was College (COL) and i n recent times i t i s the only one w i t h long period data. Thus, there i s o f t e n only one s t a t i o n to r e s t r i c t the p o s i t i o n of the f a u l t plane and s i n c e i t i s q u i t e close to the nodal plane i t o f t e n does not have an impulsive P a r r i v a l . To a c c u r a t e l y define the azimuth of the f a u l t i n g and to give i n f o r m a t i o n on the d i p , the earthquake must be l a r g e enough to record w e l l i n Europe. This i s i l l u s t r a t e d i n Figure 67. The minimum magnitude i s about 6 and even t h i s i s pushing the l i m i t i f there i s a high microseismic noise l e v e l i n Europe. A l l events l i k e l y to produce s o l u t i o n s are examined here. 1) The August 22, 1949 Earthquake (M g = 8.1) The f o c a l mechanism of the 1949 earthquake was s t u d i e d o r i g i n a l l y by Hodgson and Milne (1951). Hodgson l a t e r gathered together a l a r g e number of seismograms to study t h i s event. He re-examined previous f i r s t motions and t h i r t e e n new f i r s t motions were read. E i g h t of the p o l a r i t i e s read f o r the o r i g i n a l s o l u t i o n were reversed due to i n f o r m a t i o n about r e v i s e d seismograph p o l a r i t i e s or because of a reading d i f f e r e n c e w i t h Milne who read most of the o r i g i n a l records. Three readings were added from b u l l e t i n s and t h i s data set was processed by computer program and published as a r e v i s e d s o l u t i o n i n the Wickens and Hodgson (1967) catalogue. Extensive notes made by Hodgson on the seismograms as w e l l as t r a c i n g s of the beginings of most of them were examined to o b t a i n a f e e l i n g f o r the q u a l i t y of the data. Those readings which were impulsive and f o r which there was no doubt were given f u l l weight i n the processing. A l l d o u b t f u l readings f o r whatever reason, as w e l l as a l l readings from b u l l e t i n s were given one h a l f weight. Three more readings from b u l l e t i n s were found and Figure 67 August 22, 1949, Mechanism s o l u t i o n lower hemisphere p r o j e c t i o n . P o s i t i o n of key s t a t i o n s are i n d i c a t e d on the f o c a l sphere. - 186 -added to the Hodgson data s e t . The data were processed using a v e r s i o n of Wickens 1 o r i g i n a l program (Wickens and Hodgson, 1967) w i t h a l l a r r i v a l s i n the Pn d i s t a n c e range r e s t r i c t e d to an angle of emergence from the f o c a l sphere of 55°. While r e s t r i c t i n g the angle of emergence of rays i n the pn d i s t a n c e range i s c o r r e c t procedure f o r most earthquakes (because the rupture surface i s above the Moho) i t i s not c l e a r that t h i s i s appropriate f o r l a r g e earthquakes on oceanic transforms (Burr and Solomon, 1978) which may a l s o have some seismic rupture i n the mantle p o r t i o n of the l i t h o s p h e r e . In any case, changing the p o s i t i o n s of the P r rays on the f o c a l sphere does not a l t e r the s o l u t i o n s of any of the Queen C h a r l o t t e f a u l t events s u f f i c i e n t l y to a f f e c t any of the conclusions drawn here. The f a u l t plane s o l u t i o n f o r the 1949 event (Fi g u r e 67) i s almost i d e n t i c a l to that of Wickens and Hodgson (1967). The azimuth of the northwest s t r i k i n g plane corresponds e x a c t l y to the s t r i k e of the f a u l t at the l a t i t u d e of the e p i c e n t r e . The dip of the f a u l t i s very steep and w e l l c o n s t r a i n e d by a number of European s t a t i o n s that have good q u a l i t y readings. The motion i s mainly s t r i k e - s l i p w i t h a small t h r u s t component. The net h o r i z o n t a l motion during t h i s earthquake i s s i g n i f i c a n t l y d i f f e r e n t (15°) from the l a t e s t estimates of P a c i f i c - A m e r i c a n motion i n the region p r e d i c t e d by p l a t e i n t e r a c t i o n models (Minster et a l , 1978, Chase 1978). I f the p l a t e i n t e r a c t i o n models are c o r r e c t , then there i s a component of convergence along the f a u l t at t h i s l a t i t u d e not taken up by t h i s earthquake, that must be taken up by some other means. 2) The June 24, 1970 Earthquake (M = 7) ' • ' • • • ' - f t / The P nodal s o l u t i o n f o r the June 24, 1970 earthquake j u s t south of the Queen C h a r l o t t e Islands i s shown i n Figure 68. A s o l u t i o n f o r t h i s earthquake was p r e v i o u s l y published by Chandra (1970) who used a Figure 68 June 24, 1970 compressional n e c e s s i t a t e a mechanism s o l u t i o n . A large number a r r i v a l s i n Europe and the Canadian t h r u s t component i n the s o l u t i o n . of a r c t i c - 188 -combination of P f i r s t motion and S p o l a r i z a t i o n data. I t appears that S p o l a r i z a t i o n angles, which are d i f f i c u l t to d e f i n e p r e c i s e l y , are e x e r t i n g undue i n f l u e n c e on h i s s o l u t i o n . A s o l u t i o n can be found using h i s P data alone which has fewer i n c o r r e c t P f i r s t motions and i s not s i g n i f i c a n t l y d i f f e r e n t i n S wave r a d i a t i o n p a t t e r n . The a d d i t i o n of 66 a d d i t i o n a l short p e r i o d P p o l a r i t i e s from the ISC (weighted at 1/2 weight i n the s o l u t i o n ) confirm the P o n ly s o l u t i o n (Figure 68). Superior P only s o l u t i o n s have been found f o r s e v e r a l of Chandra's s o l u t i o n s where he has used h i s P and S a l g o r i t h m (e.g. Rogers, 1976b; Perez and Jacob, 1980) suggesting the r e l a t i v e weights he used f o r P and S data may need to be r e v i s e d . The northwest-southeast f a u l t plane f o r t h i s earthquake represents a combination of s t r i k e - s l i p and t h r u s t f a u l t i n g w i t h s t r i k e - s l i p being the dominant type. The s t r i k e of the f a u l t plane i n the optimum s o l u t i o n produced by the computer program has a more north-south o r i e n t a t i o n (by 19°) than the l o c a l s t r i k e of the c o n t i n e n t a l s h e l f break of N40°W (presumably the Queen C h a r l o t t e f a u l t ) . The maximum dip of the f a u l t i s w e l l c o n s t r a i n e d demanding a s i g n i f i c a n t t h r u s t component. The f a u l t plane can be r o t a t e d counterclockwise 19° to correspond to the s t r i k e of the c o n t i n e n t a l s h e l f by making f i v e a d d i t i o n a l s t a t i o n p o l a r i t i e s i n c o r r e c t . A l l of these s t a t i o n s are near the nodal plane and thus the q u a l i t y of the s o l u t i o n does not degrade s i g n i f i c a n t l y , but the h o r i z o n t a l p r o j e c t i o n of the f a u l t motion vector becomes s i g n i f i c a n t l y d i f f e r e n t from the h o r i z o n t a l motion p r e d i c t e d by p l a t e t e c t o n i c s (Figure 69). The h o r i z o n t a l motion vector depends on the azimuth of the f a u l t plane and sweeps out a range from N14°W f o r the optimum s o l u t i o n to N27°W f o r the s o l u t i o n constrained to the azimuth of the s h e l f break. The p r e d i c t e d motion ve c t o r f o r the P a c i f i c - A m e r i c a i n t e r a c t i o n at the e p i c e n t r e from recent p l a t e models (Minster et a l . , 1978; Chase, 1978) i s N18°W, i n the - 189 -Figure 69 Dashed l i n e represents shelf break (presumeably the or i e n t a t i o n of the Queen Charlotte Fault) and dashed arrow represents plate i n t e r a c t i o n d i r e c t i o n . S o l i d arrow i s hor i z o n t a l p r o j e c t i o n of r e l a t i v e motion vector from P nodal s o l u t i o n of 1970 earthquake and s o l i d l i n e i s surface i n t e r s e c t i o n of the f a u l t plane. (a) i s the optimum computer s o l u t i o n , (b) adjusts the hori z o n t a l motion vector to coincide with the plate motion vector with the same data d i v i s i o n as in (a), (c) r e s t r i c t s the azimuth of the f a u l t plane to have the same o r i e n t a t i o n as the shelf break but forces some observations to be in c o r r e c t . - 190 -middle of the range. I f the h o r i z o n t a l motion vector i s made to correspond to the p l a t e i n t e r a c t i o n v e c t o r , as Mackenzie (1969) suggests i t should f o r i n t e r p l a t e earthquakes, then a s o l u t i o n can be found w i t h i n the same minimum chosen by the computer program. This i s thus the p r e f e r r e d s o l u t i o n as i t i s both the optimum d i v i s i o n of the data set and s a t i s f i e s the p l a t e t e c t o n i c c o n s t r a i n t . The surface p r o j e c t i o n of the f a u l t plane i s s t i l l about 10° from the l o c a l o r i e n t a t i o n of the s h e l f break, but i s very s i m i l a r to the trend of microearthquakes recorded by ocean bottom seismographs immediately to the south of t h i s earthquake (Hyndman and Rogers, 1981). The dip of the f a u l t plane i s 50° (Figure 68). 3) June 24, 1970: Foreshock and Aftershock Chandra (1974) published a poorly constrained s o l u t i o n f o r the l a r g e s t a f t e r s h o c k (M = 5.2) of the June 24, 1970 earthquake. A l s o , a foreshock (M = 5) o c c u r r i n g 5 and one h a l f hours before the mainshock has 38 f i r s t motions l i s t e d i n the ISC. As mentioned before, because of the r e l a t i v e l o c a t i o n of the Queen C h a r l o t t e Islands w i t h respect to the d i s t r i b u t i o n of world wide seismograph s t a t i o n s , these events are too small to provide w e l l c o nstrained s o l u t i o n s . However, when the s o l u t i o n f o r the main shock i s superimposed on these data sets I t can be seen that they are l i k e l y to have s i m i l a r mechanisms i n v o l v i n g r i g h t l a t e r a l s t r i k e s l i p f a u l t i n g w i t h a s i g n i f i c a n t t h r u s t component (Figu r e 70). 4) The February 23, 1976 Earthquake (M^ _ 6.0) This earthquake i s a magnitude 6 event immediately to the south of the 1970 earthquake sequence. A very p o o r l y constrained s o l u t i o n w i t h 20 percent i n c o r r e c t data was published by Wetmiller and Horner (1978) using f i r s t motions read from short p e r i o d s t a t i o n s i n the Canadian network and - 191 -FORESrioct< Figure 70 June 24, 1970 mechanism s o l u t i o n superimposed on data of a foreshock and an aftershock. Note that the data although i n s u f f i c i e n t to produce well constrained solutions i s consistent with a thrusting component i n the s o l u t i o n . - 192 -s e l e c t e d data published i n the NEIS Earthquake Data Report. In order to b e t t e r d e f i n e the s o l u t i o n , m i c r o f i l m was examined f o r a l l a v a i l a b l e short and long p e r i o d seismograms from the World Wide Standard (WWSS) network and a l l Canadian long period s t a t i o n s . This r e s u l t e d i n 27 f i r s t motions of high r e l i a b i l i t y . A l s o , data from the U n i v e r s i t y of C a l i f o r n i a (Berkeley) B u l l e t i n of Seismograph S t a t i o n s was included (at one h a l f weight) as i t showed a c l e a r d i v i s i o n i n the C a l i f o r n i a s t a t i o n s which unambiguously spans a nodal plane. The s o l u t i o n i s shown i n Fi g u r e 71. Adding readings from the ISC and lower q u a l i t y p o l a r i t i e s read from the WWSS network does not r e s u l t i n a c l e a r e r d e f i n i t i o n of the dip of the nodal planes. This earthquake i s too small to read r e l i a b l e f i r s t motions at most European s t a t i o n s and as such i s j u s t below the magnitude l i m i t necessary to o b t a i n a w e l l c o n s t r a i n e d s o l u t i o n . Nevertheless, there i s a s u f f i c i e n t d i s t r i b u t i o n of s t a t i o n s w i t h r e l i a b l e readings to see that t h i s earthquake cannot have a f a u l t plane d i p p i n g to the east at an angle as shallow as i n the 1970 events (Figures 68 and 70). This earthquake has a much shallower f o c a l depth than the 1970 event (Table X I I I ) and thus probably represents f a u l t i n g i n the upper c r u s t . 5) Summary of F a u l t Plane S o l u t i o n s The only w e l l defined f a u l t plane s o l u t i o n s i n the Queen C h a r l o t t e I s l a n d s r e g i o n are those f o r the August 22, 1949 earthquake (M s =8.1) and the June 24, 1970 earthquake (M s = =7). These are shown i n the Figure 72 superimposed on the s e i s m i c i t y map of the re g i o n . The f a u l t plane s o l u t i o n of the 1949 earthquake (Figure 67) represents almost pure s t r i k e - s l i p f a u l t i n g on a near v e r t i c a l plane. The s t r i k e i s c o n s i s t e n t w i t h the l o c a l s t r i k e of the Queen C h a r l o t t e f a u l t but the h o r i z o n t a l motion vector i s s i g n i f i c a n t l y d i f f e r e n t (15°) from the l a t e s t estimates of the - 193 -Figure 71 February 23, 19 76 mechanism s o l u t i o n . The dip of the f a u l t plane i s not w e l l c o n t r o l l e d , but i t cannot have a dip to the east as shallow as the 1970 earthquake i n Figure 68. - 194 5m m felt' 130° Figure 72 P o s i t i o n of the only w e l l defined f a u l t plane s o l u t i o n s i n the Queen C h a r l o t t e Islands r e g i o n . The northern s o l u t i o n i s f o r the August 22, 1949 (M-8.1) earthquake (Figure 67) and the southern one i s f o r the June 24, 1970 (M=7) earthquake (Figure 68). M - 195 -P a c i f i c / A m e r i c a i n t e r a c t i o n d i r e c t i o n (Minster et a l . , 1978; Chase, 1978). On the other hand, the 1970 earthquake has a combined s t r i k e - s l i p and t h r u s t mechanism (Figure 68) which i s c o n s i s t e n t w i t h the pr e d i c t e d P a c i f i c / A m e r i c a motion, but the s t r i k e of the 1970 f a u l t plane i s about 10° d i f f e r e n t from the probable trend of the Queen C h a r l o t t e f a u l t . The f a c t t h a t the h o r i z o n t a l motion vec t o r of the 1970 earthquake i s c o n s i s t e n t w i t h the recent p l a t e i n t e r a c t i o n models suggests the models are r e p r e s e n t a t i v e of P a c i f i c / A m e r i c a motion i n the Queen C h a r l o t t e I s l a n d s area. The i m p l i c a t i o n of t h i s i n the region of the 1949 earthquake i s that there i s a s i g n i f i c a n t amount of convergence perpendicular to the s t r i k e of the Queen C h a r l o t t e f a u l t that was not taken up during the rupture of that earthquake and must be taken up i n some other way. F. CONCLUSIONS The r e v i s e d s e i s m i c i t y p a t t e r n shows a strong c o r r e l a t i o n w i t h the Queen C h a r l o t t e f a u l t scarp w i t h l i t t l e , i f any, s e i s m i c i t y on other major f a u l t systems of the Queen C h a r l o t t e I s l a n d s . There have been no earthquakes confirmed to be i n Hecate S t r a i t or Queen C h a r l o t t e Sound. These observations i n d i c a t e that a l l P a c i f i c / A m e r i c a motion occurs along the Queen C h a r l o t t e f a u l t . The f a c t that the observed s e i s m i c i t y r a t e along the f a u l t corresponds c l o s e l y to the r a t e expected from P a c i f i c / A m e r i c a motion (Hyndman and Weichert, 1982) a l s o supports t h i s c o n c l u s i o n . The d i s t r i b u t i o n of l a r g e earthquakes along the Queen C h a r l o t t e f a u l t suggests that two major seismic gaps are present. F a u l t plane s o l u t i o n s f o r the region have been r e c a l c u l a t e d w i t h a d d i t i o n a l data and show a combination of t h r u s t i n g and s t r i k e - s l i p f a u l t i n g i n the south, - 196 -w i t h h o r i z o n t a l motion p a r a l l e l to the P a c i f i c - A m e r i c a n v e c t o r , changing to a predominantly s t r i k e - s l i p environment i n the north. The t h r u s t components i n the f a u l t plane s o l u t i o n s of earthquakes at the southern end of the Queen C h a r l o t t e I s l a n d s support the p l a t e motion models that suggest a component of convergence between the P a c i f i c and America P l a t e s i n the region. However, t h i s i s hard to r a t i o n a l i z e w i t h the almost pure s t r i k e - s l i p f a u l t plane s o l u t i o n of the great 1949 earthquake which ruptured the f a u l t adjacent to the northern Queen C h a r l o t t e I s l a n d s — s t i l l w i t h i n the region of expected convergence. The f a u l t plane s o l u t i o n , which represents i n i t i a l r upture, may not be r e p r e s e n t a t i v e of the t o t a l motion during 1949 earthquake or the convergence component may be taken up i n some other way. I f the convergence component i n the region has r e s u l t e d i n oblique u n d e r t h r u s t i n g of the Queen C h a r l o t t e I s l a n d s then the concentration of s e i s m i c i t y along the present Queen C h a r l o t t e f a u l t , w i t h l i t t l e i f any s e i s m i c i t y on i n l a n d f a u l t s , suggests that the present f a u l t must be shearing through the subducting l i t h o s p h e r e immediately below the Queen C h a r l o t t e f a u l t . This i s l i k e the model described by Hyndman et a l . (1982) ( F i g u r e 73). I f there was a s i g n i f i c a n t underthrust p o r t i o n moving o b l i q u e l y under the Queen C h a r l o t t e I s l a n d s then the i n l a n d s e i s m i c i t y seen i n other o b l i q u e subduction zones would be expected here. Thus, t h i s i s perhaps how oblique subduction i s accomplished i n t h i s r e g i o n , f i r s t by a s e r i e s of great s t r i k e - s l i p earthquakes, then by oblique convergence u n t i l the c o u p l i n g between the o v e r r i d i n g and underlying p l a t e i s so great that i t exceeds the stre n g t h of the oceanic l i t h o s p h e r e and another sequence of great s t r i k e - s l i p events i s p r e c i p i t a t e d . This i s a c t u a l l y a s p e c i a l case of the oblique subduction model proposed by F i t c h (1972) (Figure 50) where the c o u p l i n g on the subduction i n t e r f a c e i s so great that shear f a i l u r e - 197 -I 10-g20-Q.C. Trough Possible old fault or Deformation Front Terrace Coast Q.C. Islands 30H discontinuity — CONTINENTAL ^xCRUST:::: Earthquakes: No vertical exaggeration Main / / 7 % f / / ^ wi Transcurrent ™ « / * f Fault Faults ' % Figure 73 A possible model of the Queen Charlotte f a u l t a f t e r Hyndman et a l . (1982). The analysis presented here i s consistent with t h i s model. It i s a s p e c i a l case of the oblique convergence model of F i t c h (1972) (Figure 50) where s t r i k e - s l i p motion ruptures the subducting p l a t e . - 198 -occurs elsewhere at the weakest p o i n t . In the case of the Queen C h a r l o t t e f a u l t the weakest point i s i n the subducting p l a t e rather than i n the o v e r l y i n g p l a t e as F i t c h (1972) proposed. -199-VI. SEISMICITY OF BRITISH COLUMBIA'S VOLCANIC REGIONS A. INTRODUCTION One of the most prominent and dynamic features of the recent t e c t o n i c h i s t o r y of B r i t i s h Columbia are the zones of Quaternary and Recent volcanoes. There are three main zones, the S t i k i n e , the Anahim and the G a r i b a l d i ( F i g u r e 74). The S t i k i n e appears to be a gian t tension r i f t a s s o c i a t e d w i t h P a c i f i c / A m e r i c a n i n t e r a c t i o n as i t i s or i e n t e d at an appropriate angle f o r t e n s i o n from the mega shear at the c o n t i n e n t a l margin. The Anahim b e l t may be due to North America d r i f t i n g over a hot spot (Bevier et a l , 1979). The G a r i b a l d i B e l t seems to be a product of a c t i v e subduction of the Juan de Fuca P l a t e . The S t i k i n e and Anahim b e l t s are made up of mainly eruptions of f l u i d b a s a l t s (Souther, 1970; Campbell and Tipper, 1971; Souther, 1977; Bevier et a l . , 1979) w i t h most of the volume i n a few l a r g e s h i e l d volcanoes. The most recent eruptions have been small cindercones and small e r u p t i v e centres of f l u i d b a s a l t . The G a r i b a l d i b e l t i s s i m i l a r to e r u p t i v e centres found i n subduction zones. The lavas are i n the c a l c a l k a l i n e s u i t e and the er u p t i v e s t y l e has v a r i e d from f l u i d b a s a l t flows to h i g h l y e x p l o s i v e ash eruptions (Souther, 1977; Naysmith et a l . , 1975). B. OBSERVED SEISMICITY 1) S t i k i n e B e l t There i s no obvious c o r r e l a t i on of the S t i k i n e b e l t w i t h the s e i s m i c i t y - 200 -Figure 74 The three main zones of Quaternary volcanoes i n western Canada. -201-p a t t e r n although i t should be emphasized that the l o c a t i o n threshold has only been below the magnitude 5 l e v e l since the establishment of the s t a t i o n at Fort St James (FSJ) i n 1965. Complete d e t e c t i o n to the magnitude 4 l e v e l has only been p o s s i b l e since 1971 w i t h the establishment of Whitehorse (WHC) and Queen C h a r l o t t e C i t y (QCC) s t a t i o n s . Since 1965 no events have been detected i n the region w i t h the Canadian seismograph network. Between 1968 and 1972 a t o t a l of 579 days of recording w i t h high gain seismographs was c a r r i e d out i n the S t i k i n e V o l c a n i c B e l t , a l l w i t h i n 30 km of Mt. E d z i z a , the most prominent volcano i n the b e l t . P r e v i o u s l y published r e s u l t s from 1968 and 1969 (Rogers, 1976a) are combined w i t h unpublished data from 1971 and 1972 and d i s p l a y e d i n Figure 75. Very l i t t l e s e i s m i c i t y was detected i n the v o l c a n i c b e l t and none could be i d e n t i f i e d w i t h the volcano i t s e l f . Events detected were confined to the Mess Creek graben which u n d e r l i e s Mt. E d z i z a . The earthquakes were a l l very s m a l l , the l a r g e s t being magnitude 1.3. The event r a t e of very much l e s s than one microearthquake per day i s c o n s i d e r a b l y l e s s than the r a t e s observed during other surveys done w i t h equipment operating at comparable m a g n i f i c a t i o n i n what might be c a l l e d s i m i l a r e x t e n s i o n a l t e c t o n i c environments. For example, i n Nevada ( O l i v e r et a l , 1966), i n Iceland (Ward et a l . , 1969), and i n Kenya (Tobin et a l . , 1969) microearthquake occurrence r a t e s of s e v e r a l events to s e v e r a l tens of events per day were observed at most l o c a t i o n s . Although fewer s i t e s were occupied during the surveys discussed here (Milne et a l , 1970; Rogers, 1976a), the low r a t e s of nearby microearthquakes and the l a c k of any microearthquakes l a r g e enough to be l o c a t e d w i t h the temporary arrays suggest that the s e i s m i c i t y p a t t e r n of l a r g e r earthquakes c o r r e c t l y i d e n t i f i e s the v o l c a n i c zone as being a s e i s m i c a l l y i n a c t i v e feature at the - 202 -Figure 75 Microearthquake a c t i v i t y detected i n the Stikine volcanic b e l t during 579 days of high gain monitoring i n the region of Mt. Edziza at the s i t e s indicated by the s o l i d c i r c l e s . The microearthquake event rate i s no higher than most of the Canadian C o r d i l l e r a . No events were located on Mt. Edziza and a l l but one event indicated here were too small to be located. This figure combines data from Rogers (1976a) with previously unpublished data. -203-present time. This i m p l i e s minimal s t r a i n r e l e a s e and that most of the i n t e r a c t i o n between the P a c i f i c and North American p l a t e i s t a k i n g place at the margin. 2) The G a r i b a l d i B e l t Regular s t a t i o n s of the Canadian seismograph network have monitored the G a r i b a l d i b e l t at about the magnitude 2.5 l e v e l from 1951 to 1960 and since 1975. During these periods an average of 5 events per year have been detected, none a s s o c i a t e d w i t h any p a r t i c u l a r volcano. This i s a much lower r a t e than i n the f o r e a r c region of Georgia S t r a i t and Puget Sound. In 1974 and 1975, 122 days of high gain monitoring were c a r r i e d out i n the v i c i n i t y of Meager Mountain i n conjunction w i t h geothermal i n v e s t i g a t i o n s (Figure 76). No a c t i v i t y was detected on the mountain and only one small event, too small to be l o c a t e d (M = 1.5), was detected i n the v o l c a n i c r e g i o n . 3) The Anahim B e l t The whole of the Anahim b e l t has been monitored at the magnitude 4 l e v e l s i n c e the i n s t a l l a t i o n of FSJ i n 1965. There i s no obvious c o r r e l a t i o n of the s e i s m i c i t y p a t t e r n w i t h the volcanoes. However, there i s a c o n c e n t r a t i o n of earthquake occurences roughly 100 km from the easternmost i d e n t i f i e d v o l c a n i c centre (Figure 77). Although the developing Canadian seismograph network has only been capable of l o c a t i n g earthquakes l a r g e r than magnitude 3.5 i n the above region since 1963 and events l a r g e r than 2.5 since the establishment of the Mica Creek s t a t i o n (MCC) i n 1966 ( E l l i s et a l . , 1976; Milne et a l . , 1978) a c l e a r c o n c e n t r a t i o n of events has already emerged. In a d d i t i o n , the p o s t u l a t e d p o s i t i o n of the l a r g e (M = 6) 1918 earthquake i n eastern B r i t i s h Columbia - 204 -c3 1 GOLD BRIDGE V -X MEAGER MTN ^ - A V ^ L O n ^ - IN PEMBERTON ^MT. CAYLEY^ : j y 0 « WHISTLER ^O^-BLACK TUSK JMT. PRICE ! X,MT. GARIBALDI ^•SQUAMISH Figure 76 Shaded areas i n d i c a t e quaternary v o l c a n i c s i n the G a r i b a l d i b e l t . Routine earthquake monitoring i n southwest B r i t i s h Columbia since 1951 i n d i c a t e s about 5 smal1 earthquakes a year occur i n the v i c i n i t y of the v o l c a n i c b e l t . 122 days of high gain monitoring at the s i t e s i n d i c a t e d by the t r i a n g l e s detected no microearthquake a c t i v i t y i n the v i c i n i t y of Meager Mountain. - 205 -Figure 77 T r i a n g l e s are v o l c a n i c centres i n d i c a t e d by Souther (1970). Numbers are the ages (Ma) suggested by Bevier et a l . (1979) f o r the i n i t i a t i o n of volcanism i n each regio n . Black dots are earthquakes: a l l events greater than magnitude 3 s i n c e 1963 and l a r g e r events since 1900. I t i s suggested here that the c o n c e n t r a t i o n of earthquakes from the end of the v o l c a n i c b e l t to the A l b e r t a border may be r e l a t e d to a mantle hotspot proposed by Bevier et a l . (1979). -206-has been r e c e n t l y r e l o c a t e d to t h i s r e g ion (Rogers and E l l i s , 1979). Because of the c o n s t r u c t i o n of the Mica Creek Dam and the formation of a l a r g e r e s e r v o i r , McNaughton Lake, a l o c a l array of seismograph s t a t i o n s was e s t a b l i s h e d around the r e s e r v o i r area i n 1972 p r i o r to the commencement of r e s e r v o i r f i l l i n g . E l l i s et a l . (1976) and E l l i s and Chandra (1981) have shown that there i s considerable m i c r o s e i s m i c i t y i n the region and discussed i t s c h a r a c t e r . There was a s i g n i f i c a n t amount of s e i s m i c i t y before the r e s e r v o i r was f i l l e d and the s e i s m i c i t y since the commencement of f i l l i n g does not appear to be r e l a t e d to the r e s e r v o i r . One of t h e i r i n t e r e s t i n g f i n d i n g s i s that most of the earthquakes have occurred i n swarms of hundreds of small shallow events, a c h a r a c t e r i s t i c of earthquakes i n v o l c a n i c and r i f t i n g areas and u n c h a r a c t e r i s t i c of any other observed s e i s m i c i t y i n the Canadian C o r d i l l e r a . A d e t a i l e d study of the l a r g e s t recent earthquake i n the r e g i o n (Rogers et a l . , 1980) shows the f a u l t i n g was a combination of t h r u s t and s t r i k e - s l i p motion and probably occurred along a p r e - e x i s t i n g shallow d i p p i n g f a u l t . This i s s i m i l a r to Yellowstone (Smith and Sbar, 1974; Trimble and Smith, 1975; P i t t et a l . , 1979) and Hawaii (Koyanagi and Endo, 1976), the two best studied examples of suggested i n t r a p l a t e hotspots, where much of the s e i s m i c i t y i s not r e l a t e d to l o c a l v o l c a n i c processes. Earthquake a c t i v i t y occurs along previous f a u l t s , or zones of weakness, presumably a c t i v a t e d by the r e g i o n a l u p l i f t due to the thermal expansion of the c r u s t i n the hotspot region. I f the v o l c a n i c b e l t and the s e i s m i c i t y are r e l a t e d to a hotspot, then the 100 km s e p a r a t i o n between the most recent volcanoes at the eastern end of the Anahim b e l t and the centre of the seismic a c t i v i t y has to be explained. I f the hotspot i s underneath the s e i s m i c i t y then the volcanism, which i s the surface expression of the hotspot, must be lagging 100 km or about 4 Ma (at a r a t e of 2.5 cm/yr) behind the passage of the c r u s t over -207-the hotspot. A l t e r n a t i v e l y , i f the hotspot i s underneath the e a s t e r n volcanoes the s t r a i n f i e l d surrounding the hotspot must precede i t by some 100 km. A combination of the two e f f e c t s may w e l l provide a reasonable e x p l a n a t i o n . At present there i s no way of l i m i t i n g the region where a hotspot might be. Studies i n the area of surface wave d i s p e r s i o n (Wickens, 1977), seismic r e f r a c t i o n (Berry and F o r s y t h , 1975), geomagnetic i n d u c t i o n (Dragert, 1973), and repeated p r e c i s e l e v e l i n g (Lambert and Vanicek, 1979), which might provide geophysical evidence i n d i c a t i n g a hotspot, have u n f o r t u n a t e l y not been l o c a t e d i n the appropriate places to do t h i s . E xtension of these s t u d i e s i n t o the McNaughton Lake region might w e l l provide c r i t i c a l data to t e s t the hotspot hypothesis. In the meantime, there appears to be enough evidence to suggest that the r e l a t i o n s h i p between the McNaughton Lake earthquakes and the Anahim v o l c a n i c b e l t may be more than c a s u a l geographic chance. Because of the p r o x i m i t y of the Mica seismic a r r a y around McNaughton Lake ( F i g u r e 78), the eastern end of the Anahim b e l t w i t h i n Wells Grey Park has been e f f e c t i v e l y monitored at the magnitude 2 l e v e l since the end of 1972 (the order of 2000 days.). Although earthquakes were not r o u t i n e l y t a b u l a t e d i n that r e g i o n , a search through unpublished e p i c e n t r e s c a l c u l a t e d by the Department of Geophysics and Astronomy at U.B.C. re v e a l s an average of 2 events per year i n the Wells Grey Park region. Only 5 events are entered i n t o the E a r t h Physics Branch data f i l e f o r that time period ( F i g u r e 77). Although t h i s i s nowhere near the l e v e l of s e i s m i c i t y i n the v i c i n i t y of McNaughton Lake i t i s somewhat higher than the surrounding r e g i o n . Though there have been no earthquakes recorded at the western end of the Anahim b e l t , a swarm of earthquakes during 1943 and 1944 caused concern amongst B e l l a Coola r e s i d e n t s ( M i l n e , 1956). The events were too small to - 208 -Figure 78 Shaded areas i n d i c a t e Quaternary v o l c a n i c s at the eastern end of the Anahim b e l t . An array of telemetered seismographs around the McNaughton Lake r e s e v o i r , since l a t e 1972 ( i n d i c a t e d by the t r i a n g l e s ) and a temporary s t a t i o n at Blue R i v e r has detected an average of 2 small earthquakes per year west of the 119th meridian i n the region of the volcanoes. -209-record on the Milne-Shaw seismographs at V i c t o r i a ( e f f e c t i v e l y below magnitude 4 1/2). The swarm continued f o r about 3 months t o t a l l i n g at l e a s t a dozen f e l t events. This has a s i m i l a r character to events observed i n v o l c a n i c regions where viscous magma has moved but not reached the s u r f a c e . However there i s no way of confirming t h i s . C. CONCLUSIONS B r i t i s h Columbia v o l c a n i c zones have a very low r a t e of earthquake a c t i v i t y even at the microearthquake l e v e l . The eastern end of the Anahim b e l t i n Wells Grey park may e x h i b i t a s l i g h t l y higher r a t e than the surrounding regions. Immediately to the east of the Anahim B e l t a c o n c e n t r a t i o n of s e i s m i c i t y may be r e l a t e d to an underlying mantle hotspot due to the thermal expansion of the c r u s t r e a c t i v a t i n g o l d e r f a u l t s and zones of weakness. - 210 -VII A SUMMARY OF CONCLUSIONS In t h i s d i s s e r t a t i o n a comprehensive seismotectonic model has been proposed to e x p l a i n the main features of the s e i s m i c i t y of B r i t i s h Columbia. Many concepts are used and developed to achieve t h i s end but the ideas of McGarr (1976) on phase changes and those of F i t c h (1972) on oblique subduction, although extended and modified here, form a foundation f o r much of the work. A synopsis f o l l o w s . The deeper s u i t e of earthquakes i n the Puget Sound and southern Vancouver I s l a n d r e g i o n are a r e s u l t of phase changes i n the descending oceanic l i t h o s p h e r e . The phase changes, p a r t i c u l a r l y those i n the oceanic c r u s t , are accompanied by l a r g e volume changes which cause high s t r a i n r a t e s i n the adjacent m a t e r i a l r e s u l t i n g i n shear f a i l u r e and earthquakes. These phase changes are extremely pressure s e n s i t i v e and the phase change f r o n t i s capable of moving higher than i t s normal e q u i l i b r i u m p o s i t i o n i f excess c o n f i n i n g pressure occurs. Such i s the case i n the region below Puget Sound, where the subducting p l a t e deforms around a corner because the trench a x i s f o l l o w s the bend i n the coast l i n e . As the p l a t e s t a r t s to deform opposite the corner i t bulges i n t o a gentle f o l d f o r c i n g up the Olympic Mountains i n Washington St a t e . The bulge p e r s i s t s u n t i l a depth of about 30 km i s reached where phase changes occur compensating f o r the need to bulge and leave the p l a t e r e l a t i v e l y h o r i z o n t a l to a depth of 70 km, where the phase changes are complete and the p l a t e must deform again by bending. A sine f u n c t i o n approximation to the bend surface s a t i s f i e s both the depth of the magma sources under a l l of the major Cascade volcanoes and a secondary lobe of deep s e i s m i c i t y under Texada I s l a n d i n northern - 211 -Georgia S t r a i t . Because the volume change i n the oceanic c r u s t i s much greater than i n the mantle there i s a net v e r t i c a l c o n t r a c t i o n i n the oceanic c r u s t i n the re g i o n of the phase change. In t h i s low angle subduction zone, where the o v e r r i d i n g l i t h o s p h e r e i s i n contact w i t h the subducting p l a t e over the phase change r e g i o n , asthenospheric flow cannot compensate f o r the volume change and there i s a down drop of the o v e r r i d i n g l i t h o s p h e r e above the phase change r e g i o n . This r e s u l t s i n the Georgia S t r a i t - Puget Sound - Willamette v a l l e y topographic low which i s probably u n d e r l a i n by abundant normal f a u l t s caused during the formation process. The f a u l t s are not p r e s e n t l y a c t i v e as normal f a u l t s because the reg i o n i s i n dynamic e q u i l i b r i u m . However, they provide planes of weakness f o r the contemporary s t r e s s regime i n the c o n t i n e n t a l p l a t e . Because the phase change process probably does not compensate p e r f e c t l y f o r the p l a t e bend at the corner under Puget Sound some upward pressure may r e s u l t . A l s o , shear f a i l u r e of the upper surface of the subducted p l a t e under Puget Sound, as compared w i t h volume change by creep i n adjacent nonseismic regions, probably r e s u l t s i n a rougher upper s u r f a c e . Both of these e f f e c t s tend to increase the dynamic c o e f f i c i e n t of f r i c t i o n on the subduction i n t e r f a c e above the deep s e i s m i c i t y . S t r a i n w i l l thus couple across the boundary and r e s u l t i n shallow earthquakes. This s t r a i n i s r e l i e v e d as small earthquakes on the many planes of f a i l u r e p a r a l l e l to the coast formed during the downdrop of Puget Sound. In Puget Sound because of the oblique subduction s i t u a t i o n , only the component of convergence p a r a l l e l to the coast r e s u l t s i n shallow earthquakes. The component perpendicular to the coast i s e i t h e r stored as e l a s t i c s t r a i n and then r e l e a s e d as l a r g e earthquakes along t h r u s t s i n the subduction zone or reaches the l i m i t of p l a s t i c deformation and i s released a s e i s m i c a l l y . I n c e n t r a l Vancouver I s l a n d where s e v e r a l l a r g e earthquakes have taken - 212 -place there i s a change i n t e c t o n i c regime from the Juan de Fuca/America regime to the Explorer/America regime. The E x p l o r e r subduction r a t e has slowed consid e r a b l y and may have stopped i n an absolute sense, although convergence s t i l l e x i s t s because the American p l a t e i s moving southwestward. The phase change f r o n t i n the subducted p l a t e i s i n the process of r e a d j u s t i n g i t s e q u i l i b r i u m p o s i t i o n and the topography i s rebounding. This upward movement of the top surface of the subducted p l a t e d r a s t i c a l l y increases the dynamic c o e f f i c i e n t of f r i c t i o n i n the region of the E x p l o r e r p l a t e r e s u l t i n g i n l a r g e earthquakes responding to the s t r e s s generated by the Explorer/America i n t e r a c t i o n . These earthquakes occur near the southern p l a t e boundary because the p l a t e t h i n s to the north as i t approaches the t r i p l e j u n c t i o n and cannot transmit as much s t r a i n across the subduction i n t e r f a c e . I n the region of the southern Queen C h a r l o t t e Islands t h r u s t components i n the f a u l t plane s o l u t i o n s confirm there i s a convergence component across the P a c i f i c / A m e r i c a boundary as p r e d i c t e d by p l a t e i n t e r a c t i o n models. However, the Queen C h a r l o t t e f a u l t appears to have ruptured completely through the subducted P a c i f i c P l a t e l e a v i n g any m a t e r i a l u n d e r l y i n g the Queen C h a r l o t t e Islands separated from the main P a c i f i c p l a t e and no longer subducting o b l i q u e l y . The near v e r t i c a l s t r i k e - s l i p f a u l t s of some major earthquakes, the narrowness of the band of earthquakes along the Queen C h a r l o t t e f a u l t and the l a c k of any s e i s m i c i t y east of the f a u l t t e s t i f y to t h i s . The S t i k i n e V o l c a n i c b e l t of northern B r i t i s h Columbia, once a region of e x t e n s i o n a l t e c t o n i c s i s not p r e s e n t l y s e i s m i c a l l y a c t i v e . The subduction r e l a t e d volcanoes of the G a r i b a l d i b e l t are s e i s m i c a l l y i n a c t i v e , as are a l l other Cascade volcanoes i n non e r u p t i v e s t a t e s . The only c o n c e n t r a t i o n of s e i s m i c i t y away from the c o a s t a l region of B r i t i s h - 213 -Columbia occurs o f f the eastern end of the Anahim v o l c a n i c b e l t and may be a r e s u l t of the proximity of an underlying mantle hotspot that may have caused the v o l c a n i c b e l t . One of the main problems addressed here remains unanswered. I t i s s t i l l not c l e a r whether the subduction i n t e r f a c e i n the Juan de Fuca p l a t e r e g i o n i s i n a locked or unlocked c o n d i t i o n , and thus i t i s not c l e a r whether the region i s l i k e l y to experience l a r g e subduction zone earthquakes. A l l the s e i s m i c i t y evidence presented here suggests the subduction i n t e r f a c e i s i n an unlocked s t a t e and subduction i s proceeding a s e i s m i c a l l y . However, the only study of h o r i z o n t a l s t r a i n i n the region suggests that e l a s t i c s t r a i n i s accumulating i n the c r u s t , which i s most e a s i l y explained by a locked subduction zone. Thus, e i t h e r the s t r a i n measurement i s anomolous or the zone has j u s t r e c e n t l y become locked and the e l a s t i c deformation has not yet r e s u l t e d i n earthquakes i n the o v e r l y i n g p l a t e . 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Yount, e d i t o r , USGS Open F i l e Report, Menlo Park, C a l i f o r n i a . - 228 -APPENDIX 1 REVISED PARAMETERS FOR EARTHQUAKES IN THE  VANCOUVER ISLAND - PUGET SOUND REGION (1900-1950) (The f i r s t l i n e contains the present Data F i l e s o l u t i o n , the second l i n e contains the r e v i s e d parameters) YEAR DAY TIME LAT. LONG. MAG. 1903 Mar. 14 02 15 00.0 47.7 122.2 4.3 02 15 00.0 47.7 122.2 4.9 L o c a t i o n from 'Earthquake H i s t o r y of the United States'. Magnitude c a l c u l a t e d from f e l t area. 1904 Mar. 17 04 21 00.0 47.7 124.0 6.0 04 21 00.0 47.8 123.0 5.3 L o c a t i o n from 'Earthquake H i s t o r y of the United S t a t e s ' . Magnitude c a l c u l a t e d from f e l t area. 1909 Jan. 11 23 49 00.0 49.0 122.7 5.6 23 49 00.0 48.7 122.8 6.0 Lo c a t i o n based on f e l t r e p o r t s . Magnitude c a l c u l a t e d from f e l t area. 1911 Sep. 29 02 39 00.0 48.8 122.7 4.3 02 39 00.0 48.8 122.7 4.1 Lo c a t i o n from 'Earthquake H i s t o r y of the United S t a t e s ' . Magnitude c a l c u l a t e d from f e l t area. 1913 Dec. 25 14 40 00.0 47.7 122.5 4.3 14 40 00.0 47.7 122.5 4.7 Lo c a t i o n from 'Earthquake H i s t o r y of the United S t a t e s ' . Magnitude from f e l t area. 1915 Aug. 18 14 05 00.0 48.53 121.43 5.5 14 05 00.0 48.5 121.4 4.6 Lo c a t i o n from 'Earthquake H i s t o r y of the United States'. Magnitude from f e l t area. 1918 Dec. 06 08 41 05.8 49.75 126.5 7.0 08 41 05.8 49.62 125.92 7.0 Rec a l c u l a t e d w i t h ISS data. Magnitude from Gutenberg and R i c h t e r (1949) and f e l t area. - 229 -Oct. 10 01 07 20.0 48.3 124.3 5.5 01 07 16.5 48.63 127.15 5.5 Recalculated w i t h ISS data. Magnitude estimated from the number of P a r r i v a l s . Jan. 24 07 10 00.0 48.7 123.0 5.0 07 10 00.0 48.6 123.0 5.5 L o c a t i o n from 'Earthquake H i s t o r y of the United States'. Magnitude from f e l t area. Feb. 12 18 30 00.0 49.0 122.7 4.3 18 30 00.0 49.0 122.7 4.1 L o c a t i o n from 'Earthquake H i s t o r y of the United States'. Magnitude from f e l t area. Sep. 07 22 14 36.0 49.0 124.0 5.5 Sep. 17 23 14 40.0 50.0 123.0 5.5 Recalculated w i t h ISS data. Magnitude estimated from the number of P a r r i v a l s . Dec. 04 13 55 00.0 48.5 123.5 4.3 13 55 00.0 48.5 123.0 5.0 L o c a t i o n based on f e l t r e p o r t s . Magnitude from f e l t area. May 08 14 00 00.0 49.0 124.0 5.5 May 07 21 56 00.0 50.15 127.85 5.5 Gonzales (VGZ) S-P and f e l t r e p o r ts are s i m i l a r to 1978 Brooks Peninsula events, thus t h i s e p i c e n t r e i s used. Magnitude from f e l t area. Feb. 09 11 05 00.0 48.5 125.0 3.7 11 05 00.0 49.0 125.3 5.8 L o c a t i o n on b a s i s of Gonzales (VGZ) S-P and f e l t r e p o r t s . Magnitude from f e l t area. Apr. 18 04 00 00.0 48.75 122.25 4.3 04 00 00.0 48.75 122.25 4.4 L o c a t i o n from 'Earthquake H i s t o r y of the United S t a t e s ' . Magnitude from f e l t area. Dec. 31 07 25 00.0 47.5 123.0 5.0 07 25 00.0 47.5 123.0 4.9 L o c a t i o n from 'Earthquake H i s t o r y of the United States'. Magnitude from f e l t area. - 230 -- 231 -1946 Jun. 23 17 13 19.0 49.9 124.9 7.3 17 13 25.8 49.8 125.3 7.3 Re c a l c u l a t e d w i t h ISS data. Magnitude from surface wave data and f e l t area. 1946 J u l . 05 02 41 18.0 49.9 124.9 4.5 02 41 18.4 49.83 125.50 4.5 Re c a l c u l a t e d w i t h ISS data. Magnitude from V i c t o r i a (VIC) seismograms. 1949 Apr. 13 19 55 36.0 47.2 122.6 7.0 19 55 36.0 47.1 122.7 6.9 L o c a t i o n from 'Earthquake H i s t o r y of the United States'. Magnitude from f e l t area. 1950 Apr. 14 11 03 49.0 48.0 122.5 4.5 11 03 49.0 48.0 122.5 4.6 L o c a t i o n from 'Earthquake H i s t o r y of the United S t a t e s ' . Magnitude from f e l t area. 1950 Dec. 02 17 57 00.0 48.0 122.3 4.3 Dec. 03 01. 57 00.0 48.0 122.3 2.5 Lo c a t i o n from 'Earthquake H i s t o r y of the United S t a t e s ' . Magnitude from f e l t area and l a c k of s i g n a l at VIC. 232 -APPENDIX 2 REVISED PARAMETERS FOR EARTHQUAKES IN THE VANCOUVER ISLAND-PUGET SOUND REGION (1951-1969) D M E ORIGIN TIME SE »G( V LAT SE 550111 10 20 10 .8211 .15 ) N47 .987112) 550211 21 15 2 2 . 6 8 ( 1 .11 ) N 4 8 . 1 3 5 ( 1 1 ) 550271 10 .00 50 .59 ) . 2 7 ) ] N47 .908 ) 2) 5S0J11 22 12 52 .3612 . 3 2 ) 1 N48 .282 )21 ) 550311 01 .55 13.151 . 5 0 ) 1 N 1 9 . 8 5 8 ) 1) 550321 22 10 28 .571 . 5 8 ) 1 N19 .373 ) 3) 550326 06 55 50 .301 . 6 5 ) 1 N 1 7 . 9 6 9 ) 1 0 ) 550327 00 05 12.391 .82 ) 1 N19 .001 I 2) 550107 23 15 55 .79 1 . 1 0 ) 1 N 1 8 . 9 7 K 3) 55011 1 07 35 1 7 . 6 5 ( . 5 8 ) ] N48 .91S) 6) 550126 00 13 1 0 . 6 7 1 .81 ) 1 N18 .708) 9) 55050J 21 21 2 7 . 9 3 ( 3 . 7 0 ) 1 N 1 8 . 3 5 1 ) 3 3 ) 550513 19 19 33 .7319 .99 ) ] N 1 8 . 1 5 2 ( 1 7 ) 550516 03 01 2 3 . 6 7 ( 9 . 9 9 1 ] N47 .699) 7) 550603 20 21 6 . 1 3 ( 1 .11 ) 1 N19 .123 ( 8) 550603 21 31 1 7 . 2 6 ( . 58 ) 1 N49 .362) 3) 550603 21 13 1 6 . 2 8 ( . 58 ) 1 N19 .110( 3) 550606 15 23 3 2 . 6 6 ( .58 ) ] NIB.7011 3) 55070S 07 52 5 . 8 2 ( 1 . 5 5 ) 1 N 1 8 . 1 3 0 ( 1 2 ) 550722 06 51 1 7 . 2 8 ( 1 .21 ) 1 N 1 7 . 8 7 2 ( 1 7 ) 550723 19 02 3 1.81( .72 ) 1 N17 .529 ( S) 550912 15 09 1.311 .98 1 1 N18 .357 ( 6) 550911 13 03 . 23 (1 . 1 6 ) 1 N 1 7 . 3 2 7 ( 1 1 ) 551009 11 21 2 7 . 0 7 11 . 0 8 ) 1 N 1 8 . 510 (10 ) 551027 18 09 3 1 . 9 1 ( . 58 ) 1 N 1 8 . 0 7 5 ( 1 1 ) 551103 01 10 2 8 . 7 5 ( 2 . 0 6 ) 1 N 4 8 . 0 3 2 ) 2 1 ) 551107 01 21 12 .18 13 .19 ) ] N 1 9 . 1 7 7 ) 2 0 ) 5S1117 23 39 1 9 . 7 0 ( 2 .91 ) 1 N 1 9 . 3 3 2 ( 1 7 ) 551121 22 19 S O . 3 6 ( .11 ) 1 N18 .132 ) 3) 551206 03 21 1 9 . 8 1 ( .38 ) 1 N50 .616113) 551230 19 15 2 1 . 1 8 11 .11 ) 1 N 1 8 . 6 3 9 ) 1 0 ) 560107 01 29 3 5 . 0 8 ( . 1 7 ) 1 N 1 7 . 3 1 K 2) 560121 10 11 2 2 . 2 3 1 . 8 6 ) 1 N18 .097 ) 9) 560126 02 17 1 3 . 2 7 1 .98 ) 1 N19 .116) 8) 560206 11 35 5 3 . 1 1 1 9 .99 ) 1 N 1 7 . 8 1 0 ( 1 8 ) ' 560209 01 01 1 8 . 3 1 ( 1 . 0 5 ) I N18 .299) 9) 560209 01 18 5 7 . 8 8 ( . 7 2 ) 1 N18 .353( 6) 560209 01 28 3 6 . 9 6 ( . 5 8 ) 1 N18 .352) 1) 560209 01 36 1 2 . 1 5 ( . 8 7 ) ] N 4 8 . 3 4 3 ) 7) 560211 13 50 2 2 . 5 0 ( 1 . 5 2 ) 1 N18 .121115 ) 560229 17 29 3 2 . 8 9 ( . 2 5 ) 1 N19 .075 ( 3) 560308 17 32 1 0 . 9 1 ( 1 . 0 0 ) 1 N18 .639 ( 8) 560327 08 26 3 7 . 2 0 ( . 92 ) 1 N49 .192 ) 4) 560327 15 31 2 3 . 3 7 ( 1 .11 ) 1 N19 .203) 6) 560327 15 31 3 2 . 7 0 ( 1 .13 ) 1 N 1 9 . 2 0 K 6) 560327 15 3S 2 . 6 1 ( 1 . 1 3 ) 1 N19 .201) 6) 560106 00 31 6 . 1 7 ( . 5 s i i N17 .902) 6) 560108 22 28 1 2 . 2 6 ( . 1 2 ) 1 N I B . 121) 3) 560111 23 01 5 3 . 3 0 ( 1 . I S ) 1 N49 .266) 6) 560112 15 10 9 .82 ( .58 ) 1 N49 .243 ) 2) 560126 16 18 2 2 . 5 6 ( 1 . 0 1 ) 1 N 4 7 . 5 1 6 ) 1 2 ) 560127 16 16 3 8 . 8 2 ) . 5 8 ) 1 N19 .179) 9) LONG SE • 1 2 3 . 6 7 2 ( 1 1 ) W122.519(10) W122.992I 3) W123 .166(15) W123.564) 2) M122.614I 5) W121.7131 8) H 2 3 . 2 4 9 ) 15) h l 2 2 . 0 0 0 ( 2 2 ) .W126 .232 (86 ) W 1 2 2 . 1 5 K 8) W123.389(10) W123.4561 5) W125 .502 (49 ) W123.553I 6) W123.700( 4) W123.699I 4) M123.326) 4) W123.30S)15) W123.637) 9) W123.267) 5) U124 .684 ) 5) W122.053119) U123.881) 7) K123 .259 ) 4) V 1 2 1 . 9 0 5 ( 2 0 ) U 1 2 2 . 5 7 4 ( 3 3 ) W122.755(24) M122.117) 4) U123 .504 I 3) y i 2 3 . 6 4 5 1 7) W 1 2 2 . 2 9 6 1 ' 2 ) W123.1311 7) W121.00S( 5) W124.273(12) W122.452( 9) W122.S15( 61 W 1 2 2 . 4 1 K 8) U122 .388 ( 7) M122.794(141 W125.813) 2) W122.986( 8) U122 .770 ) 8) U 1 2 2 . 7 8 0 ) 1 0 ) U122 .776 ) 9) M122 .754 )10 ) W121.838) 5) W123.180I 4) W123.355( 7) W123.316I 3) W122.285)12) W123.995) 2) DEPTH SE 37 (71 ) 20 59 6 7 5 33 29 5 5 29 40 5 11 48 19 17 58 41 5 36 37 52 57 5 15 35 24 33 5 17 22 22 70 6 8 26 32 24 15 23 30 19 23 7 17 11 69 18 20 19 48 (12 ) ( 5 ) I 99 ) (99 ) (99 ) ( 16 ) ( 14 ) (43 ) (99 ) (10 > (28 ) (99 ) (99 ) (16 ) ( 18 ) (20 ) (10 ) (11 ) (99 ) ( 7 ) (35 ) (10 ) ( 16 ) (99 ) ( 18 ) (55 ) (41 ) ( 4 ) (99 ) (35 ) ( 1 ) 117) (13) (99 ) ( 11 ) ( 6 ) ( 7 ) I 8 ) (71 ) ( 3 ) ( 16 ) ( 19 ) ( 19 ) (99 ) (27 ) ( 11 ) 111 ) (21 • (13 ) ( 6 ) ( 9 ) UST PH 1/ 6( 3/ 6( 4 / 81 3/ 6( 3 / 6( 3/ 51 4 / 71 3 / 41 3/ 61 3 / 5( 3/ 6( 3/ 6( 3 / 6< 21 4) 3 / 6) 3 / 5( 3 / 5( 3 / SI 3 / 61 3 / 6( 3 / 6 I 3/ 6( 3/ 61 3/ 6( 3/ 51 3 / 6( 3 / 6( 3 / 6( 3 / 6( 3 / 61 3 / 6( 3 / 6( 3/ 6( 3 / 6( 3 / 61 3 / 6) 3 / 61 3 / 51 3/ 6) 3/ 6) 3 / 61 3/ 6( 3 / 6( 3 / 61 3 / 6( 3 / 6( 3 / 5) 4 / 6) 3 / 6( 3 / 5( 3 / 61 3 / 51 RMS .61 ) . 29 ) . 1 7 ) . 6 0 ) .24 ) . 42 ) .24 ) . 00 ) . 1 5 ) . 73 ) .24 ) .96 ) . 72 ) . 1 7 ) . 2 9 ) .68 ) . 6 3 ) . 3 9 ) .10 ) . 85 ) .25 ) . 2 5 ) . 3 0 ) .28 I . 5 5 ) . 53 ) .90 ) .75 ) .11 ) . 2 3 ) . 36 ) .01 ) . 2 2 ) . 25 ) . 6 9 ) . 3 2 ) . 1 9 ) . 09 ) . 22 ) . 1 3 ) .06 I . 2 6 ) .21 I . 2 9 ) . 2 9 ) . 2 9 ) . 1 2 ) . 18 ) . 3 0 ) .22 ) . 2 7 ) . 2 9 ) - 233 -560430 00 : 58 560508 15 45 560715 06 01 560725 02 13 560815 23 21 560829 04 42 560904 10 17 560912 2 2 . 44 5 b l 0 0 3 00 45 561020 13 . 27 561027 0 3 . 57 561103 19 01 561104 04- 44 5 b l 1 1 8 14- 42 561122 • 0 23 561201 08 26 561205 23 03 561207 12 46 561213 13 42 561226 15 10 561226 20 54 570102 13 47 570103 06 06 570105 13 58 570110 01 59 570126 01 16 570201 09 38 570205 19 22 570307 23 31 570314 11 15 570322 01 57 570322 02 23 570325 08 07 570328 04 45 570424 22 56 S70425 20 06 570505 21 44 570506 18 35 570515 22 .05 570529 09 34 570708 05 24 570820 1 1 .23 570821 03 .46 570902 01 50 570905 01 .36 570906 12 33 570912 23 03 570912 23 .03 570912 23 .07 570912 23 09 570913 01 .47 570913 14 .29 570914 02 :54 570917 06 : 37 570921 12 .55 570921 23 .22 570924 11 :12 570929 16 :43 571020 22 :04 571022 20 :03 571022 20 :05 571101 21 :23 571104 22 :02 571113 20 :11 571114 03 :54 571115 18 :0b 571211 22 :47 571211 23 :05 571222 09 :54 580108 19 :24 580121 20 :59 580121 21 :22 580210 10 :51 580216 22 :14 580301 19 :08 580302 14 :38 580303 19 :49 580331 22 :12 2 7 . 0 0 ( 1 . 3 7 1 43 .031 . 5 8 ) 3.421 . 0 7 ) 6 .53 ( . 2 1 ) 4 .411 . 8 2 ) 53 .581 . 5 8 ) 2 9 . 3 9 ( . 5 8 ) 4 3.011 . 3 9 ) 47 .91 I .88 I 54 .291 . 2 8 ) 48 .741 . 3 1 ) 3 8 . 9 2 ( . 1 9 ) .011 . 8 5 ) 56 .61 (1 .08 ) 38 .181 . 5 8 ) 1 1 . 2 6 ( 1 . 1 5 ) 42 .831 .58 ) 4 7 . 9 K . 5 8 ) 5 1 . 1 1 ( 9 . 9 9 ) 53 .401 . 6 4 ) 2 9 . 4 5 ( . 2 5 ) 2 8 . 0 5 ( . 5 8 ) 2 5 . 2 2 ( . 5 8 ) 1 8 . 7 9 ( . 5 8 ) 29 .641 . 4 9 ) 11 .051 . 5 8 ) 1 3 . 4 b ( . 56 ) 5 4 . 0 5 ( . 2 3 ) 2 .491 . 5 8 ) 5 2 . 1 1 ( 1 . 0 2 ) 5 9 . 8 8 ( . 5 8 ) 38 .181 . 5 8 ) 3 3 . 4 0 ( . 6 1 ) 2 3 . 5 1 ( 1 . 0 1 ) 9 . 0 4 ( 1 . 0 3 ) 6 . 9 3 ( . 3 0 ) 27 .051 . 9 1 ) 2 7 . 2 8 ( . 1 9 ) 1 7 . I K .31 ) 5 9 . 6 3 d . 40) 5 6 . 4 9 ( 1 . 3 0 ) 5 7 . 5 9 ( . 8 3 ) 1 6 . 2 1 ( 2 . 6 7 ) 2 8 . 3 2 ( 9 . 9 9 ) 1 . 0 5 ( . 8 4 ) 37 .031 . 0 2 ) 2 2 . 5 3 ( . 5 8 ) 2 3 . 0 3 ( . 9 4 ) 5 7 . 2 0 ( 1 . 2 3 ) 31 .141 . 7 0 ) 4 3 . 7 0 ( . 7 7 ) 1 8 . 6 1 ( 1 . 1 9 ) 5 3 . 1 B ( . 5 8 ) 9 . 3 9 ( 1 . 7 1 ) 5 8 . 7 3 ( . 1 8 ) 41 .431 . 5 6 ) 9 . 9 6 ( . 1 9 ) 10.411 .92 ) 1 3 . 8 b ( . 47 ) 3 0 . 4 6 ( 1 . 1 9 ) 1 9 . 4 4 ( . 2 9 ) 1 6 . 1 5 ( 1 . 0 4 ) 3 9 . 0 1 ( . 8 2 ) 38 .801 . 4 2 ) 2 B . 6 0 I . 5 8 ) 51 .461 . 5 8 ) 2 3 . 9 6 ( . 5 8 ) 2 5 . 9 1 ( . 58 ) 4 3 . 0 2 ( 9 . 9 9 ) 6 . 9 5 ( 9 . 9 9 ) 5 . 3 3 ( 1 . 3 6 ) 1 0 . 8 7 ( 1 . 7 0 ) 2 1 . 4 0 ( 1 . 2 4 ) 19.301 . 8 2 ) 3 8 . 2 4 ( 1 . 8 5 ) 4 . 36 ( .46 ) 4 7 . 1 4 ( \ S 8 ) 3 9 . 1 0 ( . 4 3 ) N 4 8 . 2 8 2 ( 1 5 ) N49.315I S) N48 .309 I 1) N49 .052 ( 1) N 4 8 . 1 6 0 ( 1 8 ) N4B .S02 ( 3) N49 .501110) N49.388I 2) N48 .185( 10) N48 .314( 3) N49 .045( 2) N48 .216 I 41 N47 .687 ( 9) N47 .955( 16) N48 .816( 4) N 4 7 . 9 1 8 ( 1 0 ) N49.390I 3) N 5 0 . 2 4 b ( 1 2 ) N48.593I 6) N47 .617 ( 7) N49 .159 ( 2) N49 .689 ( 4 ) : N49 .689 I 7) N49 .b74 ( 4) N48 .50b ( 3) NM8.123I 2) N49 .375 ( 4 ) . N48 .225 ( 2) N49 .438 ( 4) N48 .768 ( 6) N49 .578 I 4) N49 .614 ( 4) N48 .413 ( 5) N 4 8 . 1 9 8 ( 2 7 ) N48 .986( 7) N48 .164( 3 ) . N 4 8 . 0 3 K 8) N48 .521 I 1) N49 .156 I 2) N 4 7 . 5 9 4 ( 1 4 ) N47 .342 (1S>; N 4 8 . 1 9 K 8) N 4 8 . 7 6 2 ( 1 5 ) N 4 8 . 0 7 4 ( 1 3 ) N 4 7 . 6 0 5 I 1 0 ) N49 .313 ( .0) N48.604I 3) N48.6121 S) N48.627 ( 8 ) N48 .614 ( 4) N48 .629( 5) N48 .419( 9) N48 .704 ( 3) N 5 1 . 1 5 0 ( 2 1 ) N48 .896 ( 1) N48 .464 ( 4) N47 .780 ( 2) N48.548I 8) N48.60O( 3) N 4 8 . 1 9 9 ( 1 4 ) N47 .972 ( 3) N 4 8 . 1 7 4 ( 1 1 ) N48.582I 5) N48 .624 ( 3) N48.980I 3) N48.132I 8) N49 .544( 4) N49 .42b( 3) N 4 7 . 8 5 2 ( 1 3 ) N49.441I 7) N 4 8 . 3 0 3 ( 1 3 ) N 4 8 . 2 9 6 ( 1 7 ) N 4 8 . 8 2 9 ( 7) N49 .364 I 6 ) ' N48 .691110 ) N48 .638 ( 3 ) . N 4 9 . 7 4 H <!»' N48 .188 ( 91 W123.443( 9) « 1 2 3 . 6 2 B ( 2) W122.757( 1) H122 .139 ( 2) W123.199( 8) V122 .434 I 6) W126.794 I 34) W122.598I 3) H 1 2 2 . 4 2 5 ( 1 0 ) W123.184I 2) W122.130I 3) U123 .524 I 1) M123.984I 9) M121 .788 I18 ) * 1 2 2 . 2 1 2 ( 5) W124.627I 8) W122.594I 7) U123 .864 ( 3) U125 .157 ( 8) U124.3431 7) W122.046( 7) W123.670( 2) W123.605( 2) W123.518I 2) W 1 2 2 . 4 6 K 5) W123.040I 2) U123 .963 ( 2) W122.784I 2) U123 .982 ( 3) M122 .235 (24 ) W123.759( 4) U123 .643 ( 5) U123 .424 I 4) W123.566( 6) M121;931(12> U 1 2 3 . 7 0 M 2) U 1 2 4 . 6 5 K 5) U123 .238 ( 1) WI22.077I 9) W123.2S2I17) W122.135(211 W123.244( 7) M123 .809 (12 ) W l 2 4 . 3 7 9 ( 8) U 1 2 1 . 2 0 1 ( 1 3 ) W123.929I 0) U123 .007 ( 4) W123.0521 7) U123 .028 I 9) W123.043( 5) W123.018I 6) U 1 2 2 . 3 B 3 ( 1 0 ) U125.1471 3) W124.985(17) W 1 2 3 . 4 7 K 2) N122 .807 ( 5) W123.292( 2) W123.456I 7) N l 2 3 . 0 1 0 ( 3) W124.118I 5) W124.195I 2) M123.482( 8) W125.01BI 6) W 1 2 3 . 6 3 K 2) W122.779(10) W123.478( 4) U123 .659 ( 2) W123.699I 2) K 1 2 2 . 0 8 7 ( 1 1 ) W122.433I12) W123.6371 8) M123 .533 (12 ) W122 .384(13) U123 .629 ( 2) M 1 2 4 . 9 6 8 » 1 3 ) W123.115( 3) W123.565( 3) W123.574( 4) 0 131 ) 3 / 8 (70 ) 3/ 27 ( 1 ) 3/ 23 ( 2 I 3/ 5 (99 ) 3 / 5 (99 ) 3 / 5 (99 1 3 / 10 ( 29 ) 3 / 4 ( 99 ) 3 / 49 ( 2) 3 / 24 ( 3 ) 3 / 6 (50 ) 3 / 40 ( 9) 3 / 2 (99 ) 3 / 23 ( 7 ) 3 / 21 (94 ) 3 / 29 ( 6 ) 3 / 10 (99 ) 3 / 6 ( 99 ) 3 / 39 ( 9 ) 3 / 5 1 36 ) 3/ 7 (99 ) 3 / 29 ( 7 ) 3/ 15 (22 > 3 / 5 ( 38 ) 3 / 21 (43 ) 3 / 64 ( 8 ) 3 / 26 ' ( 2 ) 3 / 60 ( 11 ) 3 / 5 (73 » 3 / 40 ( 10 1 3 / 66 ( 8 ) 3 / 40 ( 5 ) 3 / 6 ( 99 ) 3 / 11 (99 1 3 / 7 (88 ) 3 / 39 ( 8 1 3 / 28 ( 1 ) 3 / 6 (42 ) 3/ 40 (12) 3/ 53 ( 12 ) 3 / 15 (18 1 3 / 7 (99 ) 3 / 6 ( 99 ) 3 / 4 116) 3/ 66 ( 0) 3 / 33 ( 11 ) 3 / 28 ( 7 ) 3 / 12 (42 ) 3 / 22 ( 10 ) 3 / 8 (62 ) 3 / 23 (11 ) 3 / 8 ( 99 ) 3 / 1 ( 37 ) 4 / 45 1 4 ) 3 / 7 (99 1 3 / 42 ( 1 1 3/ 50 (11 ) 3 / 10 120 ) 3 / 5 ( 39 ) 4 / 32 ( 17 ) 3 / 7 ( 99 ) 3 / 5 1 30 ) 3 / 16 ( 10 ) 3 / 60 ( 6 ) 3 / 8 189 ) 3 / 39 (11 ) 3 / 18 (20 ) 3 / 5 (99 ) 3 / 10 (99 ) 3 / 2 I 22 ) 3 / 18 ( 24 ) 3 / 31 (99 ) . 3 / 14 ( 33 ) 3 / 41 (15 ) 3 / 15 (10 ) 3 / 29 ( 4 ) 3 / 7 (99 ) 3 / 61 .41 ) 5( . 4 8 ) 6( . 0 2 ) 6( . 0 5 ) 4 1 .51 ) 5( . 2 3 ) 5(1 . 00 I 6( . 45 ) b( . 29 ) 6( . 0 7 ) 6( .08 ) b< . 2 7 ) 6( . 22 ) 6( . 2 9 ) 51 . 1 7 ) 6( .31 ) 5( .26 ) 5( .69 ) 6( .44 ) 61 . 1 7 ) 6( . 3 4 ) 5( . 86 I 5( . 0 5 ) 5( . 1 2 ) 6( . 1 4 ) 5(1 . 0 2 ) 61 . 1 5 ) 6( .06 ) 51 . 10 ) 6( . 8 5 ) 5( . 42 ) 5( .00 ) 6( . 16 I 61 . 6 3 ) 61 . 2 7 ) 6( .32 ) 61 . 2 3 ) 6( . 0 5 ) 6* .24 ) 61 . 3 6 ) 6( .34 ) fal .21 ) 6( . 76 ) 6( . 56 ) 6( . 22 ) 6( .00 > 5( . 18 ) fal .24 ) 6( . 3 2 ) 6( . 18 ) fal .20 ) 61 .31 ) 5( . 3 0 ) 7( .64 ) fal . 0 5 ) fal . 1 5 ) fal . 0 5 ) fal . 2 4 ) fal . 1 2 ) 7( .71 ) b( . 0 7 ) b( .54 ) 4 ( . 55 ) 61 .11 ) SI . 2 5 ) 5( . 9 5 ) 51 .02 ) SI . 15 ) fal .63 ) 31 .36 > fal . 3 5 ) 6( .44 ) ,6( . 3 3 ) 4( . .00 ) 6( .48 ) b( . 1 2 ) 5( . 35 ) b( .561 - 234 -580402 10: 12 6 . 7 5 ( 1 . 7 5 ) ] N 4 8 . 811 (11 ) U125 .340116 ) 7 ( 99 ) 3 / 6( . 5 9 ) 580428 0 2 : 10 1 .03( . 5 8 ) 1 N48 .312 ( 5) U 1 2 2 . 6 6 K 5) 35 ( 4 ) 3 / 6( . 1 5 ) 580429 20 : 05 19 .411 . 5 8 ) N48 .182 I 5) W123.591I 3) 11 I 39 ) 3 / 5( .58 1 580429 20 : 14 54 .081 . 5 8 ) ] N48.3461 8) • 1 2 3 . 5 0 0 1 3) 1 1 32 ) 3 / 5( .50 > 580501 18: 25 13 .041 . 7 2 ) 1 N49.3031 5) U 1 2 3 . 9 4 K 3) 66 ( 11 ) 3/ 6( . 1 9 ) 580507 11 . 03 2 6 . 2 9 ( . 0 8 ) N48 .5731 1) U122.328 I 1) 8 ( 24 ) 3 / 6( . 20 ) 580530 2 1 : 23 1 9 . 7 0 ( 1 . 3 1 ) N48 .004114 ) U 1 2 3 . 6 5 9 I 1 0 ) 7 (99 ) 3 / 61 . 3 6 ) 580602 2 1 : 20 32 .391 .39 ) N48 .661 ( 3) W123.5771 2) 23 ( 7 ) 3 / 6( . 10 ) 580619 2 1 . 50 28 .521 .58 ) I N48 .871 I 7.1 U124 .955 I B) 44 ( 8 ) 3 / 5( . 0 6 ) 580628 10 18 8 . 0 2 ( 1 . 7 1 ) N 4 7 . 7 6 0 ( 3 5 ) W122.486(22) 5 (99 ) 3 / 6( .58 1 580703 11: 11 2 4 . 6 4 ( . 5 8 ) 1 N48.7371 3) • 1 2 2 . 0 S 3 ( 6) 21 ( 9 ) 3 / 5( . 1 7 ) 580703 2 0 . 10 44 . 451 . 5 4 ) N48.1901 5) W 1 2 3 . 4 6 K 4) 6 (99 ) 3 / 61 .61 ) 580704 0 5 : 56 4 9 . 2 K . 7 4 ) N47.8921 8 ) y l 2 2 . 0 6 2 1 8) 25 ( b ) 3 / 6( . 1 9 ) 580709 17: 32 43 .431 . 7 8 ) N48.S901 5) U123.0071 6) 22 ( 12 ) 3 / 6( . 20 ) 580710 04 23 1 9 . 1 4 ( . 1 7 ) N48 .833 ( 2) • 1 2 2 . 2 0 7 1 21 24 ( 3 ) 4 / 7( . 1 3 ) 580710 14. 51 33 .141 . 4 1 ) 1 N48.187I 4 ) • 1 2 2 . 4 8 3 1 4) 20 I 4 ) 3 / 6( .11 ) 580710 2 0 . 06 9.48(9 .99) N48.87S1 4) • 1 2 2 . 1 3 8 1 8) 6 (99 ) 3 / 41 .04 ) 580713 01 41 52 .491 . 9 5 ) I N 4 7 . 7 1 1 ( 1 0 ) • 1 2 2 . 5 7 2 1 1 3 ) 20 110) 3 / b( . 25 ) 580721 05 51 3 4 . 5 K .58 ) N48.86S1 3) U122 .100110 ) 30 (28 ) 3 / 5( .01 ) 580729 21 14 16 .671 . 5 8 ) N48 .6151 3) • 1 2 3 . 0 1 9 1 6) 12 1 20 > 3 / 5( . 12 ) 580731 07 22 1 2 . 5 2 ( 1 . 8 1 ) 1 N 4 8 . 6 6 9 ( 2 1 ) N I23 .456110 ) 5 (99 ) 3 / 61 . 4 7 ) 580808 18 04 5 5 . 5 9 ( . 5 8 ) 1 N4B.1801 7) W123.677I 4) 11 (41 ) 3 / 5( . 18 ) 580814 21 21 3 8 . 9 1 ( . 8 2 ) N48 .445 I 8) U123.0671 6) 27 1 4 ) 3 / 4( . 00 ) 580911 00 35 49.90(9 .99) 1 NOB.9121 4) • 1 2 1 . 9 0 8 1 6) 6 (99 ) 3 / 6( .22 ) 580915 14 25 3 2 . 8 2 ( 2 . 5 1 ) 1 N 4 7 . 8 4 3 ( 2 4 ) U 1 2 4 . 6 5 1 ( 1 7 ) 38 (24 ) 3 / 6( . 65 ) 580919 02 18 33.641 . 6 5 ) 1 N 4 7 . 7 5 0 ( 1 5 ) U122 .604 I 6) S (99 ) 3 / b( . 49 ) 581003 00 OS 4 9 . O K . 3 3 ) 1 N47.3831 3) y l 2 3 . 3 4 l ( 4) 40 ( 3) 3/ b( . 0 9 ) 5B1012 22 30 5 9 . 8 6 ( . 5 8 ) N48 .699 ( 4 ) y l 2 4 . 9 7 5 ( 6) 26 ( b ) 3/ 5( .08 ) 581102 22 14 4 0 . 3 3 ( 3 . 3 4 > 1 N48 .525128 ) y l 2 3 . 8 1 1 ( 1 8 ) 14 (99 ) 3 / 6( . 8 6 ) 581123 14 40 58 .391 . 5 8 ) 1 N49.3191 5) y l 2 3 . 6 1 7 ( 3) 12 (25 ) 3/ 5( . 36 ) 581128 22 32 47 .501 . 5 8 ) 1 . N48 .592 I 3) • 1 2 3 . 1 2 3 1 6) 8 1 3b ) 3 / 5( . 18 ) 581204 17 45 44 .061 . 8 2 ) 1 N49.6491 6) y l 2 3 . 8 6 2 ( 4) 50 (20 ) 3 / 4( . 0 0 ) 581204 18 13 17 .751 .82 ) 1 N49 .584 I 9) y i 2 3 . 8 7 3 ( 41 39 (121 3 / 4( . 0 0 ) 581206 21 09 57 .261 . 6 3 ) I N49.0751 3) U122 .690 ( 6) 8 (99 ) 3 / 6( .44 ) 581207 14 42 58 .311 . 5 8 ) 1 N49.0871 3) y l 2 2 . 7 S l ( 5) 29 I 5 > 3 / 5( . 13 1 581207 20 23 6 .641 .58 ) 1 N49.1051 3) y l 2 2 . 7 5 7 ( 5) 29 ( 5 ) 3 / 51 . 2 3 ) 581219 00 33 17.931 . 4 1 ) 1 N49.110I 2) • 1 2 2 . 6 7 8 1 4) 7' 199 ) 3 / b( . 3 3 ) 581219 06 43 8 . 5 3 1 9 . 9 9 ) 1 N49 .138 I 6) • 1 2 2 . 6 1 5 1 1 3 ) 6 (99 ) 3/ 6( . 5 3 ) 581220 06 .42 2 . 2 4 ( 1 . 9 2 ) 1 N 4 B . 5 0 6 ( 1 0 ) • 1 2 4 . 6 6 4 1 9) 8 (99 ) 3 / 6( . 60 ) 581228 08 .02 5 1 . 7 7 1 1 . 1 6 ) 1 N 4 B . 6 2 0 ( 1 2 ) • 1 2 3 . 0 0 9 1 7) 1 (61 ) 3 / b( . 3 9 ) 581228 15 50 11 .851 . 8 9 ) 1 N 4 8 . 6 0 9 ( 1 0 ) • 1 2 3 . 0 5 6 1 5) 2 (4b ) 3 / b( . 25 ) 581228 19 .58 1 1 . 4 6 1 1 . 5 8 ) 1 N48 .589111 ) • 1 2 3 . 0 2 7 ( 1 2 ) 7 (99 ) 3 / b( .41 ) 590217 03 :08 35 .11 1 1 .08 I 1 N 4 9 . 6 5 7 ( 1 1 ) • 1 2 3 . 9 7 9 1 4) 35 (20 ) 3 / 6( . 28 ) 590217 20 :25 2 3 . 2 1 1 1 . 6 9 ) 1 N 4 9 . 2 0 8 ( 1 2 ) • 1 2 4 . 0 8 7 1 8) 69 ( 2b ) 3/ 6( . 4 4 ) 590314 19 :58 2 3 . 2 7 ( 9 . 9 9 ) 1 N48 .969 ( 6 ) • 1 2 1 . 9 9 4 1 9) 6 (99 ) 3 / 6( . 3 5 ) 590404 02 .04 5 7 . 9 7 ( . 5 8 ) 1 N48 .633 ( 6 ) • 1 2 3 . 0 1 4 1 4) 25 (11 ) 3/ 5( .04 ) 590404 20 25 3 9 . 1 9 ( . 5 8 ) 1 N48.S621 7) • 1 2 3 . 4 4 0 1 6) 44 ( 7 ) 3/ 5( .28 > 590414 21 •55 49 .241 . 9 1 ) 1 N47 .736( 3 ) • 1 2 1 . 0 8 2 1 1 1 ) 3 122 1 5 / 8( . 4 0 ) 590420 00 .27 22 .131 .58 ) 1 N48 .694( 6) • 1 2 3 . 0 4 0 1 4) 11 ( 34 ) 3 / 5( .01 ) 590420 01 55 3 1 . 7 0 ( . 5 8 ) 1 N48 .680 ( 2) • 1 2 3 . 1 1 7 1 5) 28 ( 4 > 3/ 5( .01 ) 590422 07 .14 4 2 . 2 2 ( 1 . 1 5 ) 1 N48 .656 I 6 ) • 1 2 2 ; 9 S 4 ( 9) 19 1 24 ) 3 / 6( . 3 0 ) 590502 20 .35 47 .601 . 5 8 ) 1 N 4 8 . 5 9 K 3 ) • 1 2 3 . 1 5 2 1 6) 29 ( b ) 3/ 51 . 1 1 ) 590509 00 .24 49 .131 . 2 5 ) 1 N47 .367 ( 3) • 1 2 2 . 4 0 7 1 5) 19 ( 8 ) 5 / 81 .40 ) 590510 02 :04 13.521 . 5 8 ) 1 N48 .622 ( 5) • 1 2 3 . 0 1 0 1 4) 29 1 b ) 3 / 5( . 31 ) 590704 05 .27 22 .811 . 2 6 ) 1 N48.112I 1 ) • 1 2 2 . 3 5 8 1 2) 36 ( 5 ) 5 / 7( .11 ) 590721 21 .24 49 .881 . 3 4 ) 1 N48 .643 ( 2) • 1 2 3 . 6 4 7 1 2) 17 ( 8 1 3/ 61 . 0 9 ) 590802 09 :35 51 .151 . 8 2 ) 1 N47.799I 7 ) • 1 2 6 . 7 7 9 1 7) 10 1 31 ) 3 / 4( .41 ) 590814 21 .28 3 1 . 8 8 ( 9 . 9 9 ) 1 N49 .354( 6) • 1 2 3 . 4 9 K 5) 10 (99 ) 3 / 3( . 00 ) 590822 23 :42 5 2 . 6 4 ( 1 . 1 9 ) 1 N48 .402( 9) • 1 2 2 . 6 2 7 1 1 1 ) 13 ( 76 ) 3 / 6( . 5 7 ) 590823 23 :11 1 4 . S K . 5 8 ) 1 N48 .314 ) 8 ) • 1 2 2 . 4 5 5 1 4) 27 1 5 ) 1/ 5( . 0 9 ) 590824 17 :29 1 6 . 8 6 ( 2 . 2 6 ) 1 N 4 7 . 9 5 7 ( 2 5 ) • 1 2 4 . 3 5 3 ( 1 9 ) 45 (99 ) 3/ 6( . 58 ) 590825 17 :12 2 0 . 8 0 ( 1 . 4 2 ) 1 N 4 8 . 3 4 9 ( 1 1 ) • 1 2 2 . 4 2 0 ( 1 2 ) 22 (13 1 3 / 6( . 3 7 ) 590904 20 :57 30 .111 . 5 8 ) 1 N48.6261 3 ) • 1 2 2 . 8 4 5 1 8) 40 ( 5 ) 3 / 5( . 35 ) 590917 05 :48 4 5 . 6 3 ( . 8 2 ) 1 N48.3631 7) • 1 2 2 . 4 5 8 1 7) 3b I 7 1 3/ 6( .21 ) 591015 04 .51 9 . 9 2 1 1 . 0 1 ) I N48 .808 ( 6 ) • 1 2 5 . 1 2 2 1 9) 42 ( 7 ) 3 / b( . 2 6 ) 591015 23 :15 9 . 8 7 ( . 8 2 ) 1 N49.B031 9) • 1 2 3 . 3 9 2 1 5) 5 199 ) 3/ 4 ( . 89 ) 591111 02 :02 3 9 . 6 0 ( 1 . 2 2 ) 1 N4B.29S111) • 1 2 2 . 5 0 9 1 1 1 ) 39 ( 9 1 3 / 61 . 32 ) 591118 23 :48 3 1 . 1 7 ( 1 . 5 4 ) 1 N 4 8 . 2 2 8 ( 1 3 ) • 1 2 2 . 5 7 4 ( 1 3 ) 24 113) 3 / b< .40 ) 591209 20 .54 41 .121 .121 I N48 .630( 1 ) • 1 2 2 . 9 1 5 1 1) 56 ( 1 ) 3/ b l . 03 ) 591213 21 .32 42 .24 11.63 ) 1 N 4 B . 6 1 K 7) y l 2 3 . 0 O O ( 1 2 ) 6 148 ) 3/ b l .64 ) 591230 02 .05 21 .54 ( .58 ) 1 N48.634I 2) y l 2 5 . 0 0 7 ( 3 ) 5 ( 48 ) 3 / 51 .54 ) 600112 07 .52 51 .571 . 8 2 ) 1 N48.1451 6 ) • 1 2 4 . 8 0 5 ( 5) 6 (99 ) 3 / 4 1 . 5 7 ) 600210 16 :48 18.031 . 7 6 ) 1 N 4 8 . 4 9 K 8 ) • 1 2 3 . 4 8 0 1 9) 35 ( 14 ) 4 / b l .41 ) 600322 01 .13 4 6 . 6 0 ( 9 . 9 9 ) 1 N49.0051 6) • 1 2 2 . 0 4 0 1 1 0 ) 6 ( 99 ) 3 / 61 .38 ) 600322 10 .31 5 1 . 7 2 ( . 5 2 ) 1 N48.7101 2) • 123 .1701 2) 1 1 7 ) 4 / 7 1 . 3 6 ) 600327 01 :39 16.411 .58 ) 1 N48.6841 8) • 123 .2041 2) 54 (14 ) 3 / 51 .04 ) 600728 09 :10 11.281 .58 ) 1 N47.693I 5) • 1 2 1 . 9 0 8 1 5) 9 ( 5 1 3/ 5 I . 1 9 ) 600910 15 :06 33.641 . 5 8 ) 1 N47 .677 I 5) • 1 2 3 . 1 7 6 1 8) 60 ( 17 ) 5/ 81 .50 ) - 235 -APPENDIX 3 REVISED PARAMETERS FOR EARTHQUAKES IN THE  VANCOUVER ISLAND - PUGET SOUND REGION (1970-1978) D M E O R I G I N T I M E S E A 6 C Y L M S E 7 0 0 3 0 7 1 0 4 5 3 0 . 0 7 1 . 3 1 1 1 N 4 8 . 6 6 0 1 5 ) 7 0 0 5 1 8 0 5 2 9 5 1 . 8 0 1 . 2 9 ) 1 N 4 8 . 6 0 3 ( 4 ) 7 0 0 8 0 7 0 6 : 4 3 2 7 . 3 1 1 . 5 7 ) 1 N 4 8 . 3 9 5 I 3 ) 7 0 1 2 2 5 0 8 : 1 6 3 3 . 6 0 1 . 6 2 ) 1 N 4 8 . 1 4 4 I 4 ) 7 1 0 1 2 5 2 1 . 3 7 5 3 . 1 1 1 . 7 5 ) 1 N 4 8 . 3 2 9 ( 8 ) 7 1 0 5 1 7 1 9 4 1 1 5 . 1 9 ( 1 . 3 8 ) 1 N 4 8 . 7 4 9 I 1 3 ) 7 1 0 6 1 4 0 0 . 1 1 1 2 . 5 6 ( .44 ) 1 N 4 8 . 3 S 3 I 4 ) 7 1 0 9 0 5 0 3 3 2 2 . 3 5 ( . 4 7 ) 1 N 4 8 . 2 5 9 I 5 ) 7 1 0 9 0 b 1 8 3 1 1 2 . 0 8 ( 1 . 4 6 ) 1 N 4 8 . 3 3 7 ( 1 1 ) 7 1 0 9 2 1 0 9 1 6 1 3 . 1 8 ( . 3 7 ) 1 N 4 8 . 4 7 7 ( 4 ) 7 1 1 0 0 1 1 8 4 5 1 3 . 8 9 ( 1 . 2 0 ) 1 N 4 8 . 6 2 0 1 1 8 ) 7 1 1 0 1 2 1 2 5 4 1 8 . 5 5 ( . 1 7 ) 1 N 4 8 . 1 7 6 ( 2 ) 7 1 1 0 3 1 0 0 5 3 3 0 . 8 6 1 . 6 7 1 1 N 4 8 . 4 5 3 ( 6 ) 7 1 1 2 2 1 2 2 4 3 3 7 . 8 2 ( . 3 7 ) 1 N 4 8 . 0 2 K 2 ) 7 2 0 2 2 7 0 8 2 7 5 6 . 8 6 1 . 2 3 ) 1 N 4 8 . 0 9 0 ( 2 ) 7 2 0 3 0 1 1 5 1 0 1 1 . 7 0 ( . 5 4 ) 1 N 4 8 . 1 3 U ( 6 ) 7 2 0 1 0 1 1 2 2 0 1 1 . 5 3 ( . 3 1 ) 1 N 4 8 . 9 4 2 1 3 ) 7 2 0 1 1 9 1 0 1 6 2 . 7 5 ( . 4 2 ) 1 N 4 8 . 9 0 6 ( 4 ) 7 2 0 4 2 1 0 4 0 9 S 9 . 1 2 ( . 5 5 ) 1 N 4 8 . 7 1 3 ( 5 ) 7 2 0 1 2 9 0 8 0 6 1 6 . 9 2 ( . 4 2 ) 1 N 4 8 . 4 0 7 ( 4 ) 7 2 0 4 2 9 2 1 0 2 2 6 . 9 1 1 . 6 6 ) 1 N 4 8 . 6 4 5 ( 5 ) 7 2 0 5 1 8 0 5 0 7 3 1 . 5 8 ( . 8 1 ) 1 N 4 8 . 7 6 0 I 7 ) 7 2 0 5 2 0 1 6 3 2 1 7 . 4 9 ( . 6 3 ) 1 N 4 8 . 5 9 2 ) 5 ) 7 2 0 5 2 0 1 9 5 6 1 7 . 7 4 ( . 5 0 1 1 N 4 8 . 6 4 6 ( 4 ) 7 2 0 6 2 8 1 8 1 6 5 3 . 9 7 ( . 2 4 | 1 N 4 8 . 4 8 6 I 3 ) 7 2 0 7 0 5 1 8 4 6 1 6 . 7 8 ( 1 . 1 1 ) 1 N 4 8 . 7 4 1 ( 1 0 ) 7 2 0 7 2 9 0 1 5 8 3 . 4 2 ( . 2 2 ) 1 N 4 8 . 5 5 5 ( 2 ) 7 2 0 9 0 6 1 4 5 0 4 4 . 5 5 1 . 2 5 ) 1 N 4 8 . 1 8 6 ( 2 ) 7 2 0 9 0 7 1 7 5 5 1 5 . 8 7 1 . 7 4 ) 1 N 4 8 . 2 7 2 ( 7 ) 7 2 1 0 0 4 1 2 3 8 4 9 . 8 9 ( . 2 4 ) 1 N 4 8 . 4 4 7 ( 3 ) 7 2 1 0 1 0 1 1 2 6 5 3 . 2 6 ( . 3 7 ) 1 N 4 8 . 5 3 8 ( 4 ) 7 2 1 0 1 2 1 3 0 7 3 6 . 8 8 1 . 3 5 ) 1 N 4 8 . 3 6 3 I 3 ) 7 2 1 0 1 6 2 2 0 8 3 1 . 4 6 ( . 7 6 ) 1 N 4 8 . 9 2 1 1 1 6 ) 7 2 1 0 3 0 2 0 . 2 8 3 9 . 7 7 ( . 3 5 ) 1 N 4 8 . 7 0 7 ( 3 ) 7 2 1 1 0 9 0 1 1 9 1 8 . 7 9 ( . 6 7 ) 1 N 4 8 . 4 3 9 ( 5 ) 7 2 1 1 2 2 1 2 3 8 2 6 . 4 1 ( . 3 5 ) 1 N 4 8 . 7 5 2 ( 4 ) 7 2 1 1 2 9 0 9 1 3 5 2 . 1 1 ( . 3 9 ) 1 N 1 8 . 7 8 1 ( 5 ) 7 2 1 2 0 2 1 6 2 9 8 . 2 8 1 1 . 0 5 ) 1 N 4 8 . 4 8 7 ( 6 ) 7 2 1 2 0 9 0 1 5 5 3 5 . 3 9 1 . 2 5 ) 1 N 4 8 . 5 5 0 I 2 ) 7 2 1 2 0 9 1 3 1 4 3 7 . 1 7 ( . 3 7 ) 1 N 4 8 . 9 0 0 I 4 ) 7 2 1 2 0 9 1 3 3 0 1 7 . 8 0 ( . 4 0 I 1 N 4 8 . 8 6 9 ( 4 I 7 3 0 1 0 1 0 1 2 0 6 0 . 0 0 ( . 5 6 ) 1 N 4 8 . 3 2 0 ( 3 ) 7 3 0 1 0 1 1 1 13 3 0 . 9 6 ( . 4 1 ) 1 N 4 8 . 3 0 9 ( 2 ) 7 3 0 1 0 2 0 4 31 5 8 . 1 5 ( . 4 1 ) 1 N 4 8 . 7 7 5 ( 5 ) 7 3 0 1 2 5 1 5 3 0 3 1 . 6 1 ( . 4 1 ) 1 N 4 8 . 4 5 4 1 4 ) 7 3 0 2 0 2 1 2 3 7 5 . 8 5 ( . 1 4 ) 1 N 4 8 . 0 2 7 ( 1 ) 7 3 0 2 2 5 0 2 0 3 2 0 . 8 5 ( . 4 1 ) 1 N 4 8 . 7 9 0 ( 6 ) 7 3 0 2 2 6 2 0 1 1 7 . 6 5 ( . 6 9 ) 1 N 4 8 . 8 1 2 ( 6 ) 7 3 0 3 0 2 1 4 0 1 5 3 . 3 9 1 . 2 8 ) 1 N 4 8 . 1 6 9 ( 2 ) 7 3 0 3 0 5 0 9 : i i 1 3 . 1 8 1 . 4 2 ) 1 N 4 8 . 3 2 2 ( 3 ) 7 3 0 3 2 8 0 8 0 5 1 3 . 7 5 ( . 7 5 ) 1 N 4 9 . 0 4 0 ( 8 ) 7 3 0 3 3 1 0 7 5 1 1 2 . 0 6 ( 1 . 2 1 ) 1 N 4 8 . 6 2 0 ( 1 8 ) L O N G S E M 1 2 3 . 3 8 2 ( 5 ) U 1 2 2 . 7 0 3 1 3 ) W 1 2 2 . 6 6 R I 6 ) W 1 2 2 . 8 2 3 1 8 ) X 1 2 3 . 4 3 9 I 8 ) y l 2 2 . 7 9 9 ( 3 8 ) K 1 2 2 . 7 6 2 I 4 ) U 1 2 3 . 0 8 5 I 8 ) W 1 2 2 . 6 9 0 1 1 6 ) U 1 2 3 . 2 2 2 1 6 ) M 1 2 3 . 1 9 2 ( 9 ) W 1 2 2 . 9 S 8 I 3 ) W 1 2 2 . S 8 2 I 7 ) U 1 2 2 . 4 6 8 ( 2 ) U 1 2 3 . 0 5 8 1 4 ) y i 2 2 . 4 7 0 ( 3 1 W 1 2 2 . 2 0 9 I 2 ) U 1 2 2 . 1 3 2 ( 3 ) U 1 2 2 . 1 9 2 I 3 ) W 1 2 2 . 3 7 4 I 3 ) U 1 2 2 . 3 6 4 I 3 ) U 1 2 2 . 9 7 2 I 6 ) W 1 2 2 . 4 0 5 1 2 ) M 1 2 2 . 3 8 6 1 2 ) y l 2 2 . 5 0 7 ( 2 ) H 1 2 2 . 7 9 6 I I S ) M 1 2 2 . 6 9 6 I 2 ) W 1 2 2 . 7 7 7 1 2 ) W 1 2 2 . 0 6 6 I 5 ) W 1 2 2 . 2 4 0 1 2 ) W 1 2 2 . 2 9 4 ( 3 ) U 1 2 2 . 0 8 8 ( 2 ) W 1 2 2 . 1 2 7 1 6 ) M 1 2 2 . 3 8 0 ( 2 ) W 1 2 3 . 3 4 9 ( 6 ) W 1 2 2 . 1 5 3 1 3 ) W 1 2 2 . 1 9 7 I 3 ) W 1 2 2 . 3 0 9 I 4 ) U 1 2 3 . 0 1 2 I 2 ) y l 2 2 . 1 1 7 ( 3 ) W 1 2 2 . 1 M 0 I 3 ) W 1 2 2 . 0 9 6 1 3 ) y l 2 2 . 1 0 4 ( 3 ) U 1 2 2 . 6 9 K 3 ) y l 2 3 . 2 5 3 l 4 ) W 1 2 3 . 3 0 3 I 2 ) y l 2 2 . 7 8 3 ( 2 ) y l 2 2 . 4 4 8 ( 3 ) W 1 2 3 . 0 7 0 1 4 ) y l 2 2 . 0 7 6 ( 3 ) y l 2 2 . 7 1 3 ( 5 ) W 1 2 2 . 6 9 5 ( 7 ) P T H S E U S T P H R M S 1 9 1 9 ) 5 / 7 1 . 3 6 ) 2 4 ( 9 ) 8 / 1 0 ( . 3 3 ) 2 7 ( 1 1 ) 5 / 9 1 . 3 5 ) 2 5 ( 1 4 ) 5 / 8 1 . 5 2 ) 3 8 ( 1 6 1 5 / 5 1 . 0 0 ) 4 9 ( 9 9 1 3 / 6 1 . 3 7 ) 2 8 ( 6 ) 7 / 1 1 ( . 3 9 ) 5 0 ( 8 1 7 / 1 0 ( . 4 9 ) 1 5 ( 9 9 ) 5 / 6 ( . 4 2 ) 2 5 ( 6 ) 7 / 1 1 ( . 4 3 ) 2 3 1 8 ) 8 / 9 ( . 4 1 ) 2 9 ( 7 ) 5 / 8 1 . 2 0 ) 3 6 ( 1 6 ) 6 / 1 0 1 . 4 1 ) 2 4 ( 9 ) 7 / 1 3 ( . 4 3 ) 4 8 ( 4 ) 6 / 1 0 1 . 2 9 ) I S ( 2 0 ' ) 6 / 8 ( . 4 0 ) 1 6 ( 3 7 ) 1 0 / 1 7 ( . 4 4 ) 2 2 ( 5 ) 7 / 1 1 ( . 2 9 1 2 3 ( 1 1 ) 8 / 1 3 ( . 4 7 ) 2 7 1 6 ) 8 / 1 4 ( . 5 1 ) 1 ( 2 1 ) 6 / 9 ( . 3 8 ) 2 0 ( 3 2 ) 7 / 9 ( . 3 6 ) 2 5 ( 6 ) 7 / 9 ( . 2 7 ) 2 2 ( 5 ) 6 / 9 1 . 2 2 ) 2 5 ( 4 ) 8 / 1 3 1 . 2 8 ) 3 3 ( 4 5 ) 5 / 8 1 . 4 9 ) 6 1 ( 3 ) 7 / 1 K . 1 7 ) 5 8 ( 4 ) 6 / 1 0 ( . 2 2 ) 1 1 ( 1 6 > 4 / 7 ( . 3 8 ) 2 7 ( 4 ) 9 / 1 4 ( . 2 9 ) 2 6 ( 5 ) 9 / 1 5 ( . 1 1 ) 1 4 ( 9 ) 1 0 / 1 7 ( . 1 1 ) 6 ( 5 5 ) 8 / 1 3 1 . 5 6 ) 2 6 I 4 ) 7 / 1 3 ( . 2 9 ) 4 8 ( 1 2 ) 1 2 / 1 2 ( . 3 5 ) 2 3 ( 5 ) 9 / 1 2 ( . 3 7 ) 2 6 ( 6 ) 9 / 1 3 ( . 1 1 ) I S 1 6 0 I 6 / 8 ( . 4 7 ) 1 ( S ) 1 3 / 1 6 ( . 5 9 ) 1 9 I 5 ) 1 1 / 1 5 ( . 3 5 ) 1 3 ( 9 ) 1 0 / 1 4 ( . 3 8 ) 1 4 1 1 3 ) 8 / 1 2 ( . 4 3 ) 1 3 ( 1 0 ) 8 / 1 1 ( . 3 1 ) 2 6 ( 4 ) 8 / 1 3 ( . 3 6 1 2 3 ( 6 ) 1 0 / 1 4 1 . 5 0 ) 4 2 ( 3 ) 9 / 1 3 ( . 2 0 ) 6 0 ( 3 ) 9 / 1 2 1 . 2 2 ) 1 5 1 2 1 ) 9 / 1 2 ( . 3 9 ) 2 4 ( 7 ) 8 / 1 3 ( . 4 5 ) 1 5 ( 1 5 ) 8 / 1 1 1 . 4 0 ) 2 0 ( 7 I 5 / 8 1 . 3 4 ) 2 0 ( 1 9 ) 5 / 8 1 . 5 6 ) - 236 -730113 730531 7J0606 730612 730727 730905 73091 1 730916 730921 731107 731110 731111 731120 731205 731208 731229 710302 710120 710516 710517 710522 710606 710706 710711 710918 710925 7 H 0 0 1 711029 711101 711101 711102 711112 711115 711215 711215 750107 75011 5 750629 750825 750903 750905 750915 750916 750916 750916 750921 750925 750928 750929 751003 751003 751006 751007 751007 751013 751011 751101 751105 751119 751130 751210 75121 1 760103 760101 760112 03 ' 1 5 12 ;11 16 18 33 51 27 31 3 5 32 3 5 5 7 25 17 33 38 01 13 58 16 18 1 3 27 16 28 08 22 5 6 ;10 36 19 58 07 11 21 15 38 :07 5 6 :09 51 30 09 :00 :23 :01 ;00 05 31 3 7 05 31 3 9 51 11 28 9 7 28 20 :02 11 01 117 9 .92 ( 30 .131 5.281 27 .111 12 .56 ( 39 .761 2 0 . 6 5 ( 1 35 .781 3 3 . 5 5 1 16 .01 11 5 5 . 7 K 53 .661 8 .09 ( 38 .981 5 1 . 6 8 « 5 9 . 6 6 ( 30.39< 59 .211 36 .65 I 1.131 18 .391 33 .27 1 16 .651 51 .13C 7.91 ( 31 .191 37 .221 18 .771 59 .911 27 .631 28 .801 38 .9211 39 .771 , 8 .031 .271 52 .821 , 10 .171 23 .351 57 .181 53 .131 10 .861 05 I 12 1 251 98 I 501 111 17 1 . 67 1 , 09 1 ', 71 1 , 821 , . 63 1 .09 1 6. 17 . 3 0 . 0 9 1 13 .331 33 .11 I 13.811 32 .931 .171 7.311 15 .171 .29 I 11 .611 7.701 721 701 52 I 55 I 23) 29 ) 12) 26) 29) 61 ) 13) 37 ) 58 ) 37 ) 61 ) 15 ) 27) 67) 35 ) 29 ) 11 ) 28) 29) 19 ) 37 ) 22 ) 17) 30) 39) 60 ) 71 ) 10 ) 30 ) 39) 92 ) 22 ) 25 ) 17) 19 ) 30 ) 16 ) 11 ) 17) 26 ) 18 ) 22 ) 12 ) 11 ) 20 ) 20) 18 ) 33) 26 ) 37) 27 ) 61 ) 21 ) 00) 28) 97) 32 ) 39 I 29 ) 60 ) 17) N47.787I 3) N48.4011 1) N 1 8 . 7 7 K 6) N48 .932 I 1) N18 .50SI 3) N18.1061 2) N I B . 9 3 6 ( 11 ) N18.7731 3) N18.57S1 3) N18 .239123) N48.190I 1) N18.6271 9) N48.9491 6) N98.509I 1) N49 .203110 ) M 8 . 0 1 5 1 3) N48.7211 3) N IB.8821 5) N18.1081 9) H18.12S1 1) N IB.6181 5) N98 .649 I 3) N18.1161 2) N19.7611 7) N48 .267 I 3) N98.2061 2) N18.2251 2) N48.3061 2) N48.6091 4) N48 .577 I 6) N98 .732 I 6) N49 .130150 ) N48 .571 I 2) N IB .1991 2) N48.5171 5) N48.1201 2) N18.1181 2) NUS.6391 2) N48.050I 1) N17.959I 2) N47.972I 2) N48.306I 1) N48.309I 1) N98.289I 2) NIB.9361 2) N18 .30SI 1) N97.9611 1) M B . 2 6 1 1 1) N18.6511 2) N IB.5291 2) N48 .562 I 3) N19.3071 1) N49.3191 3) N47.9771 2) N18.6081 21 N 4 7 . 2 2 K 4) N18 .320 I 2) N47.9001 5) N48.5871 4) N49.242I 5) N49.2531 3) N49.260I 6) N48.749I 3) N49.054.1 4) N4B.307I 1) H123, W122, W123, W122, W122, W122, W122, W122, W123, W122. H122, W123, W121, W123, W124, U122, U122. N122. U123, U123, W122, W123, U122, y i 2 4 , W123. W123. W123. y 122. y i 2 3 , W122. W122. W122, U121. U122. U122. W122. U 2 2 . W123, W122. U124, U124, H122, U 122, U122, W122, WI22, U122, U122. W123, H122. W122. W122, U122. U121. W122. M124, U122. U121. W123. W123, N123. W123. H124. U123. U122. 0381 3) 4291 3) 2501 4) 0901 3) 3191 2) 7171 4) 2551 7) 4501 2) 2311 3) 525113) 6861 1) 0991 3) 9651 4) 5721 3) 0011 6) 7891 4) 44 11 1 ) 1201 3) 0071 5) 1881 3) 9801 3) 0641 2) 8401 4) 6181 8) 2061 6) 2361 4) 0281 2) 7061 2) 1941 4) 2951 3) 3901 3) 3841 7) 9881 2) 1051 2) 0031 5) 4041 2) 4371 2) 0241 1) 6141 1) 42 11 31 3911 3) 1071 1) 0991 1) 1021 3) 1221 3) 0921 1) 4351 2) 5941 1) 0271 2) 7251 2) 7121 2) 6981 3) 6691 2) 8971 4) 2031 2) 5271 9) 8761 3) 8151 3) 8711 5) 6331 3) 6541 2) 6861 S) 5831 4) 5211 4) 0731 2) 1 1 9 ) 12 /14 ( .57 1 8 199 ) 7 /11 ( .49 ) 15 1 7 ) 11/151 .48 ) 11 153) 9/141 .40 ) 22 1 6 ) 13/171 .41 ) 60 1 6 ) 12 /15 ( . 3 9 ) 22 117) 6 / 8( . 6 7 ) 31 1 5 ) 10 /15 ( . 3 0 ) 51 1 4 ) 12 /17 ( . 32 ) 25 (45 ) 7 / 1 0 ( 2 . 0 9 1 34 1 4 ) 9/1 1 ( .14 ) 19 ( 7 ) 1 3 / 1 3 * .34 ) 21 1 6 ) 11/131 . 4 2 ) 2 ( 6 ) 6 / 8 1 .46 ) 64 I 9 ) 10/121 .32 ) 58 (13) 9 / 9 1 .24 ) 9 (26 ) 6 / 61 . 0 6 ) 17 (16 ) 7 / 91 . 2 9 ) 52 1 9 ) 11/111 . 3 5 ) 26 ( 6 ) 12/151 .44 ) 22 1 7 ) 1 3 / l b l . 54 ) 57 ( 3) 9/171 . 2 7 ) 56 ( 5 ) 10/191 .41 ) 8 ( 99 ) 4 / 61 .12 ) 45 1 4 I 9 /151 .53 ) 45 ( 3 ) 7/131 . 3 7 ) 23 ( 4 I 9 / 17 ( . 36 ) 23 ( 8 ) 6/11 ( .32 ) 46 ( 6 ) 13/171 .36 ) 35 ( 8 ) 4 / 4 1 .00 I 26 ( 6 ) 4 / 81 . 3 2 ) 0 (81 ) 4 / 71 .51 ) 0 ( 5 ) 9/171 .29 ) 1 ( 5 ) 15/201 .19 ) 13 (50 ) 12/141 . 66 ) 20 1 7 ) 13/181 .38 ) 26 ( 7 ) 9 /121 - .33) 14 ( 3) 6/101 .11 ) 25 '. 4 1 8/141 . 2 1 ) 20 1 10 ) 12/221 . 50 ) 2 111 ) 10/171 . 5 3 ) 2 ( 3 ) 14/231 . 36 ) 1 ( 3 I 13/211 .31 ) 3 1 6 > 6/111 . 1 6 ) 19 (13 ) 8 /13 1 .45 ) 1 ( 3 ) 11/181 .30 ) 32 1 6 ) 18/201 .35 ) 22 ( 4 ) 15/221 . 34 ) 20 1 4 ) 13/231 . 4 3 ) 4 ( 5 ) 13/151 .31 ) 16 ( 7 ) 15/161 . 4 3 ) 3 ( 6 ) 13/201 . 5 5 ) 1 ( 7 ) 10/171 .46 ) 20 (12 ) 15/191 . 50 ) 2 ( 8 ) 8/131 .39 ) 45 128 ) 15/171 . 4 9 ) 49 ( 6 ) 1 4 / l b t .34 ) 7 (99 ) 3 / 5 1 . 16 ) 28 ( B ) 10/121 .56 ) 5 (18 ) 4 / 71 . 36 ) 7 199 ) 10/161 .45 ) 16 ( 6 ) 19/211 .62 ) 19 ( 8 ) 14/161 .45 ) 21 ( 4 > 15 /16 1 .32 1 9 (14 ) 13/201 . 3 7 ) - 237 -760112 00- 51 49 .071 . 2 3 ) 1 N 4 8 . 3 0 2 ) 1 760118 08 38 10 .651 .26 I N48 .490 I 3 760131 12: 27 14.721 . 1 6 ) N48 .343 I 2 760201 00 09 57 .301 . 2 9 ) . N49 .227 I 2 760218 00 17 8 .63 ( .21 ) N49.230I 1 760306 01 : 57 30 .491 • 21 > N49 .244 I 3 760310 0 3 . 54 53 .14 1 . 1 6 ) N49 .19b( 2 760320 12 09 1 9 . 4 1 ( .74 ) N49.276I 5 760407 0 2 . 09 1 2 . 3 3 1 . 2 4 ) N49.240I 3 760X11 20 34 1 4 . 5 8 ( . 3 7 ) N48 .3S2 I 3 760504 12 08 58.11 < .11 ) N48.629I 1 760505 01 08 19 .331 .21 > N48.13QI 1 760505 19 23 1 1 . 0 3 ( . 1 3 ) N48.605< 1 760516 08 35 15 .061 .16 ) N48.8001 2 760601 2 2 . 18 3 0 . 2 B ( . 3 8 ) 1 N48.552I 3 760621 00 21 41 .731 . 1 5 ) N48.3291 1 760725 06 56 51 .701 . 2 2 ) 1 N49.2521 3 760803 07 56 5 .28 ( .22 ) N48 .880 I 3 76081 1 05 05 31 . 90 ( . 16 > N49 .23S I 2 760813 15 04 25 .84 1 . 30 ) N48 .484 I 4 760820 00 01 3 7 . 0 7 ( .36 ) N47 .034 I 3 760830 04 17 5 2 . 5 4 ( .58 ) . N48.8171 5 760830 18 03 .57 1 .00 1 N49 .21SI 2 760902 13 36 1 1 . 2 8 ( . 09 1 1 N48.2001 1 760908 19 47 4 4 . 6 1 ( . 1 2 ) N4B.1991 1 760919 14 41 42 .301 .54 ) N48.213I 3 760926 00 19 3 8 . 9 6 ( .29 ) I N46.168I 2 761030 13 46 56 .871 .45 ) 1 N48.5201 2 761030 18 42 1 0 . 1 7 ( .59 ) N48 .850 I 5 761219 13 38 2 0 . 0 6 ( .47 ) N48.1931 4 761231 06 55 3 0 . 2 4 ( .07 ) L N48.153I 1 770203 06 23 32 .51 ( . 2 2 ) N48 .682 I 2 770215 13 03 48 .34 ( . 3 3 ) 1 N48.2601 4 770108 01 10 5 7 . 5 2 * .33 ) I N4B.668I 2 770420 13 07 28 .371 . 7 0 ) 1 N48 .386 I 4 770426 07 03 1 8 . 0 6 1 .48 ) 1 N48 .935 I 3 770617 22 43 7 .78 ( . 1 9 ) 1 N48 .267 I 2 770623 23 26 4 8 . 9 8 1 .38 ) 1 N49 .048 I 3 770627 22 00 10 .64 (2 • 19) 1 N46 .764 I 3 770627 22 01 41 .291 . 2 0 ) 1 N48 .632 I 2 770701 11 13 1 5 . 4 7 t .36 ) 1 N48 .227 I 1 770704 14 19 4 6 . 9 2 ( . 2 7 ) 1 N48.890< 2 770710 07 19 30 .861 . 7 3 ) 1 N48 .527 I 3 770711 10 36 51 .81 14 .00 ) 1 N 4 7 . 7 6 3 ( 4 4 770717 02 39 4 7 . 9 1 ( .88 ) 1 N49 .646 ( 9 770725 21 .04 3 .76 1 . 1 7 ) 1 N48 .077 ( 1 770725 22 00 1 4 . 0 9 ( .22 ) 1 N46.750I 4 770821 02 34 24 .621 . 2 2 ) 1 N48.005I 1 770828 19 19 4 .391 .23 ) 1 N48 .893 ( 3 780329 03 51 3.311 .21 ) 1 N48 .537( 2 780329 12 16 3 8 . 8 9 ( .11 ) 1 N4B.199I 1 780404 19 26 49 .151 .74 ) 1 N48 .114 I 7 780420 19 .14 20 .221 .45 ) 1 N48 .734 I 3 780505 05 .29 48 .321 .11 ) 1 N48 .475 ( 1 780517 04 43 36 .83C .34 ) 1 N48 .843 I 2 780520 14 .06 15.621 . 2 9 ) 1 N48.3351 1 780618 00 53 4 .84 1 . 1 3 ) 1 N48 .428 ( 1 780723 05 47 45 .891 . 0 0 ) I N48 .096 I 3 780816 02 .39 24 .69 ( .21 1 1 N49 .2S0 I 3 780819 01 51 1 8 . 7 6 ( . 0 9 ) 1 N48.6351 1 780823 10 37 18.641 . 1 9 ) 1 N4B.37S I 3 780926 17 34 47 .421 .41 ) 1 N47 .983 ( 2 780927 12 .34 18 .701 . 1 0 ) 1 N48 .260( 1 781118 18 .26 15 .02 1 . 1 7 ) 1 N4B . 87S( 2 y l 2 2 . 0 9 2 ( 1 ) 0 ( 3 ) y l 2 4 . 7 6 9 1 3) 33 ( 7 ) U122 .313 I 2) 21 1 5 ) H123 .634 I 1) 27 ( 9 ) K l 2 3 . 6 2 7 ( 1) 25 ( 7 ) U123 .646 ( 2) 7 111) W123.684I 2 ) 17 ( 3 ) y l 2 2 . 4 3 0 ( 1 0 ) 5 ( 5 ) W123.634I 2) 12 ( 7 ) U124 .786 I 4) 38 ( 6 I U122 .968 I 1) 20 ( 3 ) y l 2 2 . 6 8 2 ( 2) 59 1 5 ) W123.112( 1) 20 1 4 ) U 1 2 3 . 3 5 K 2) 60 1 3) M124.021( 2) 41 1 6 ) U 1 2 2 . 2 7 K 1) 3 1 4 ) U 1 2 3 . 6 4 M 2 ) 7 1 14 1 W122.733I 2) 20 1 5 ) y 123 .623 ( 1) 17 ( 9 ) «1 24 .6801 3) 33 1 8 ) y l 2 0 . 6 9 2 ( 4 ) 13 ( 8 ) y l 2 S . 9 7 5 » 6) 10 ( 9 ) b l 2 3 . 6 1 2 ( 2) 6 (99 ) y l 2 2 . 7 8 2 l 1 ) 25 ( 3 > y l 2 2 . 7 7 5 ( 1 ) 25 ( 3 ) y l 2 4 . 7 5 4 ( 7 ) 5 ( 20 1 y l 2 3 . 2 3 8 l 4 ) 46 1 6 ) y l 2 1 . 9 3 2 1 3) 9 164 ) y l 2 5 . 9 9 3 ( 2 7 ) 6 ( 99 ) V122 .626 ) 4 ) 60 110 1 y l 2 3 . 0 1 8 ( 1 ) 45 ( 1 ) y l 2 3 . 7 2 5 ( 3) 15 1 7 ) y l 2 4 . 6 2 7 ( 3) 25 ( 9 ) W122.150I 3) 16 ( 11 ) y l 2 4 . 8 5 9 l 5 ) 4 ( 22 ) y l 2 2 . 5 1 3 l 3) 14 121) y l 2 2 . 4 1 8 ( 2) 21 ( 5) y 123.3831 3) 3 ( 7 ) M122.839I 3) 1 ( 32.) y l 2 3 . 0 6 4 ( 2) 11 (13 ) y l 2 2 . 1 2 0 ( 1 ) 0 ( 4 > U122.0281 2) 13 I S ) V122 .447 I 2 ) 0 ( 9 ) t .122. 720 (58 ) 10 1 9 9 ) W124.700I 9) 31 (21 > y l 2 2 . 8 5 9 ( 2) 59 1 5 ) V122 .914 I 4 ) 26 ( 5 ) y l 2 2 . 6 4 7 l 1) 1 ( 4 ) y123 .6151 3) 39 ( 6 ) y l 2 2 . 8 6 3 l 2) 60 ( 4 ) y l 2 2 . 7 6 5 ( 2) 2b ( 4 ) y l 2 4 . 7 1 5 ( 5) 39 ( B ) W125.2451 6) 33 ( 6 ) y l 2 2 . 5 4 5 ( 2) 11 1 6 > y l 2 2 . 1 1 4 ( 2) 0 ( 4 ) V122 .199 I 2) 1 ( 4 ) y l 2 2 . 4 5 5 l 2) 25 ( 3> y l 2 2 . 7 1 9 l 4 ) 10 (99 ) y l 2 3 . 6 4 3 ( 2) 6 1 99 ) y l 2 3 . 5 7 4 l 2 ) 52 1 2 ) y l 2 3 . 2 3 4 ( 3) 25 ( 5 ) y l 2 4 . 0 6 4 l 6) 39 ) 3 ) y l 2 3 . 2 0 1 ( 2) 44 ( 2 ) h l 2 4 . 9 3 5 ( 2) 32 ( 5 ) 9 / l 4 ( . 21 /231 . 2 3 / 2 5 ( . 4 / 81 . 4 / 6 I • 15/201 . 21 /241 . 4 / 51 • 17 /22 ( . 12 /20 ( . 14/201 . 17/261 . 15 /20 ( . 20 /211 • 3 / b( . 15 /23 ( • 1 5 / 2 K . l f l / 2 6 ( . 11 /15 ( . 12 /20( . 17 /20 ( . 13 /16 ( . 4 / B( . 2 0 / 2 K . l l / 2 0 ( . 11/151 . 10 /16 ( . 13/191 . 4 / - 8 I . 14/141 , 12/181 . 13/171 . 1 0 / 1 5 ( , 1 0 / l b ( . 9 / l 4 ( , 6/111 . 13/241 , 5 / 71 , 18 /1B( , 14/181 . 13/221 , 17/291 , 1 4 / l b l . 3 / 6 (2 , 4 / 8 1 , 25 /291 , 14/141 , 13/251 , 10/141 , 17/301 , 22 /261 , 4 / BI , 8 /111 , 22/301 , 14/201 , 21/291 , 19/281 , 3 / 61 , 12/161 19/201 , 18/191 , 8 /111 , 18/261 . 4 / 81 , 23) 4 6 ) 41 ) 16 1 11) 45 ) 37 ) 03 ) 52) 58) 30 ) 37) 34 ) 27) 10 ) 33 ) 52) 54) 23) 50) 44 ) 42) 26) 29) 33) 45 ) 37 ) 62 ) 32 ) 34 ) 17) 40 ) 33 ) 47) 58) 45 ) 50) 25 ) 43) 48 ) 41 ) 45) 50 ) 05 ) 39 ) 38 ) 40 ) 41 ) 50 ) 41 ) 38 ) 33) 32 ) 45) 38) 39) 44 ) 33 I 68 ) 17) 44 ) 21 ) 37 ) 08 ) -238-APPENDIX 4 REVISED PARAMETERS FOR EARTHQUAKES IN THE  QUEEN CHARLOTTE ISLANDS REGION (1900-1979) Part A; Changes to Canadian Earthquake Data F i l e (The f i r s t l i n e contains the present Data F i l e s o l u t i o n , the second l i n e contains r e v i s e d parameters). DAY Mar. 29 TIME 05 07 53. 05 07 51.8 Rec a l c u l a t e d w i t h ISS data LAT. 50.5 50.6 LONG. 129.5 129.87 MAG. 6.4 Apr. 10 13 40 16. 53.2 13 40 16. 54. Gutenberg and R i c h t e r (1949) s o l u t i o n 133.7 134. 6.5 Nov. 16 04 15 35. 53. 131. 04 15 25. 53.5 133. P o o r l y constrained s o l u t i o n . O r i g i n time i s r e s t r i c t e d w i t h C a l i f o r n i a data. 5.0 Mar. 30 00 08 56. 50.5 00 08 56. 50. Gutenberg and R i c h t e r (1949) s o l u t i o n 129.5 130.24 6.0 Oct. 25 17 59 14.0 56.4 136.0 5.0 57.67 136.07 This aft e r s h o c k of Oct. 24 earthquake i s moved from ISS epice n t r e of mainshock to Tobin and Sykes (1968) e p i c e n t r e of mainshock. I t has not been recomputed. Nov. 12 As above, 21 56 12.0 56.4 57.67 136.0 136.07 5.0 Nov. 21 15 13 50.0 As above. 56.4 57.69 136.0 5.0 136.07 -239--240-Aug. 02 20 44 45.0 53.9 132.1 20 44 46.2 53.87 133.35 Re c a l c u l a t e d w i t h ISS data. Magnitude rounded up from Gutenberg and R i c h t e r (1949) value of 6-1/4. Oct. 29 10 54 16.0 51.6 131.2 10 54 16.7 51.59 130.98 K e l l e h e r and Savino (1975). Feb. 22 02 10 53.0 51.6 131.2 02 11 16.0 50.07 129.42 K e l l e h e r and Savino (1975). Apr. 27 11 08 41.0 56.0 140.5 11 08 55.8 55.32 137.80 K e l l e h e r and Savino (1975). Feb. 28 01 58 06.0 53.9 132.1 01 58 07.9 53.37 132.73 Reca l c u l a t e d w i t h ISS data. Dec. 30 23 49 54.0 50.9 130.7 23 49 55.7 50.99 130.32 K e l l e h e r and Savino (1975). Aug. 22 04 01 12.0 53.75 133.25 04 01 12.2 53.62 133.27 Tobin and Sykes (1968). Magnitude from Gutenberg and R i c h t e r (1949). Aug. 23 02 59 19.0 53.8 133.2 02 59 06.1 55.08 134.01 Reca l c u l a t e d w i t h ISS data. Aug. 23 19 37 30. 52.6 132.1 19 37 33.0 52.42 131.87 Re c a l c u l a t e d w i t h ISS data. Aug. 23 19 43 35. 52.6 132.1 19 43 35.0 52.64 132.10 Re c a l c u l a t e d w i t h ISS data. Magnitude estimated from number of P a r r i v a l s . -241-Aug. 24 22 37 22 37 Tobin and Sykes (1968). 13. 56.2 132.1 5.5 13.1 52.78 132.11 4.9 Magnitude c a l c u l a t e d from VIC. Aug. 26 05 25 05 25 Tobin and Sykes (1968). 58.0 56.0 135.0 4.0 57.5 56.08 135.27 4.9 Magnitude c a l c u l a t e d from VIC. Aug. 26 22 39 22 39 Tobin and Sykes (1968). 29.0 54.5 136.0 5.5 37.2 54.67 133.88 5.0 Magnitude c a l c u l a t e d from VIC. Aug. 27 21 30 21 30 Tobin and Sykes (1968). 41.0 52.6 132.1 5.0 47.0 53.05 132.74 5.3 Magnitude c a l c u l a t e d from VIC. Sept. 05 06 54 06 54 Tobin and Sykes (1968). 06.0 53.8 133.2 5.0 10.0 53.62 132.97 4.9 Magnitude c a l c u l a t e d from VIC. Sept. 12 14 14 37 37 46.0 48.6 55.8 55.16 132.0 132.57 Re c a l c u l a t e d w i t h ISS data. Magnitude c a l c u l a t e d from VIC. 5.0 5.0 Oct. 31 01 39 28.0 56.0 136.0 01 39 29.5 56.05 135.69 Tobin and Sykes (1968). Magnitude c a l c u l a t e d from VIC and Gutenberg and R i c h t e r (1949) value of 6-1/4. 6.7 6.2 Oct. 31 02 32 02 32 Tobin and Sykes (1968). 09.0 56.0 136.0 5.0 11.3 56.02 135.91 5.1 Magnitude c a l c u l a t e d from VIC. May 22 19 49 43.0 19 49 43.3 K e l l e h e r and Savino (1975). 51.5 51.56 130.5 130.51 5.7 Aug. 08 05 05 11 12 55.0 04.0 K e l l e h e r and Savino (1975), ISS a r r i v a l s . 54.5 54.92 136.0 134.58 4.5 5.0 Magnitude estimated from number of -242--243--244-June 24 10 17 24.0 51.0 130.0 10 17 02.8 52.86 131.89 Tobin and Sykes (1968). Sep. 1 22 07 28.0 51.4 129.6 22 07 34.5 Part of an of f s h o r e swarm t h i s day (see Milne and Smith, 1966). A r r i v a l s are too small f o r accurate i d e n t i f i c a t i o n . Omit ep i c e n t r e from data f i l e . Sep. 1 23 55 47.0 51.60 129.10 23 55 46.2 As on Sept. 1 at 22 07. Sep. 2 03 46 15.0 51.5 129.0 03 45 40.8 52.09 131.49 Re c a l c u l a t e d w i t h EPB data Sep. 2 14 12 48.0 52.0 129.5 14 12 52.3 As on Sept. 1 at 22 07 and 23 55. Nov. 20 01 13 44. 53.4 130.6 01 13 52.3 52.17 130.97 Relocated w i t h EPB and USCGS data. Apr. 01 12 45 43.0 51.4 129.7 12 45 42.1 51.22 130.00 Rec a l c u l a t e d w i t h EPB data May 10 13 44 02.0 51.4 129.2 13 44 06.7 51.06 129.47 Rec a l c u l a t e d w i t h EPB data Apr. 22 10 11 48.0 51.6 129.3 10 11 35.9 51.56 130.61 Rec a l c u l a t e d w i t h EPB data Mar. 05 11 11 02.0 51.2 129.5 11 11 02.4 50.83 129.79 Rec a l c u l a t e d w i t h EPB data Feb. 15 18 27 30.0 51.35 129.68 18 27 29.2 51.14 130.14 Re c a l c u l a t e d w i t h EPB data -245-1968 Jun. 18 05 37 57.0 05 37 47.6 Rec a l c u l a t e d w i t h EPB data 51.1 50.72 129.0 130.26 4.1 1970 Feb. 19 08 09 08 09 18. 16.3 53.3 52.84 132.3 132.49 Phases repicked and recomputed w i t h s t a t i o n c o r r e c t i o n s . 4.0 1970 Jun. 24 13 09 08. 51.74 131.0 6.7 13 09 11.3 51.77 130.76 7.0 ISC s o l u t i o n p r e f e r r e d . Magnitude an estimate from ISC magnitude r e p o r t s . 1970 Jun. 24 17 16 53 51.94 130.3 17 16 49.9 51.58 130.45 Phases repicked and recomputed w i t h s t a t i o n c o r r e c t i o n s 3.3 1970 June 24 19 10 15. 51.95 130.5 19 10 13.6 51.56 130.42 Phases repicked and recomputed w i t h s t a t i o n c o r r e c t i o n s 3.9 1970 Jun. 29 02 26 40. 51.99 130.3 02 26 37.6 51.68 130.44 Phases repicked and recomputed w i t h s t a t i o n c o r r e c t i o n s 3.7 1970 Aug. 11 20 56 50.0 20 56 36.9 Rec a l c u l a t e d w i t h EPB data 52.0 51.54 130.0 130.23 3.0 1971 Jun. 29 06 28 54.0 06 28 26.9 Reca l c u l a t e d w i t h EPB data 51.2 129.6 4.2 51.03 129.68 1973 Mar. 28 06 23 07.0 06 23 05.6 Reca l c u l a t e d w i t h EPB data 51.19 129.69 3.9 50.74 129.89 1973 Oct. 11 12 13 57.0 12 13 44.4 Rec a l c u l a t e d w i t h EPB data 51.22 129.48 3.1 50.76 130.58 1974 May 25 14 00 47.0 14 00 40.3 Rec a l c u l a t e d w i t h EPB data 51.53 129.46 3.1 50.70 130.45 -246-Jun. 19 11 17 55 53.86 132.15 11 17 52.8 53.97 132.02 Reca l c u l a t e d w i t h EPB data and s t a t i o n c o r r e c t i o n s Sep. 28 00 33 59 53.66 132.18 00 34 00.5 53.70 132.03 Rec a l c u l a t e d w i t h EPB data and s t a t i o n c o r r e c t i o n s . -247-APPENDIX 4 P a r t B: Earthquakes to be added to the Canadian Earthquake Data F i l e Aug 22 09 15 21.4 54.96 133.43 I Tobin and Sykes (1968). Magnitude estimated from number of P a r r i v a l s . Sept. 02 01 31 15.5 54.22 133.61 Tobin and Sykes (1968). Magnitude c a l c u l a t e d from VIC. Sept. 12 08 36 03.5 54.87 134.32 Tobin and Sykes (1968). Magnitude c a l c u l a t e d from VIC. Sept. 20 12 18 06.4 52.87 131.32 5 Reca l c u l a t e d w i t h ISS data. Magnitude c a l c u l a t e d from VIC. 

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