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Earthquake swarm on the Queen Charlotte Islands fracture zone Wetmiller, Robert Joseph 1969

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AN EARTHQUAKE SWARM ON THE QUEEN CHARLOTTE ISLANDS FRACTURE ZONE by ROBERT JOSEPH WETMILLER B .Sc . U n i v e r s i t y o f M a n i t o b a , 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department o f GEOPHYSICS We accept \his t h e s i s as conforming to the r e q u i r e d s t anda rd THE UNIVERSITY OF BRITISH COLUMBIA A u g u s t , 1969 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and S t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d b y t h e Head o f my D e p a r t m e n t o r b y h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s . i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f GEOPHYSICS The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8, Canada D a t e J u l y 31, 1969 i ABSTRACT An ear thquake swarm o c c u r r i n g on a segment o f the ocean r i s e system i n the n o r t h e a s t P a c i f i c Ocean i s c o n s i d e r e d . The da t a are i n the form of s t anda rd se ismograph r e co rds from the western p o r t i o n o f B r i t i s h C o l u m b i a , c o v e r i n g the p e r i o d August 27 to September 1, 1967. " The s tudy f o l l o w s two l i n e s o f i n v e s t i g a t i o n . F i ve se ismograph s t a t i o n s i n wes te rn B r i t i s h Co lumbia are c o n s i d e r e d as an a r r a y and 19 o f the l a r g e r magnitude events l o c a t e d w i th r e s p e c t to t h i s a r r a y . The c l o s e s t s t a t i o n to the swarm i s on the n o r t h e r n end o f Vancouver I s l a n d about 180 km from the source a r e a . T h i s s t a t i o n r e co rded 217 d i s t i n c t a r r i v a l s i n the 5-day p e r i o d . S t a t i s t i c a l p r o p e r t i e s o f the swarm p l u s the magn i tude- f requency r e l a t i o n s h i p can thus be e s t i m a t e d . The Pn and Sn a r r i v a l s i n western B r i t i s h Columbia show the f o l l o w i n g t i m e - d i s t a n c e r e l a t i o n s h i p s : T p n = ( S . 5 1 ± 0 . 3 8 ) '•+ A / ( 7 . 6 9 ± 0 . 0 3 ) T S n = ( 10 .16±1 .25 ) + A / ( 4 . 4 5 ± 0 . 0 3 ) These t r a v e l t imes are c o n s i s t e n t w i t h a c r u s t on Vancouver I s l a n d g r e a t e r than 50 km t h i c k . The e p i c e n t e r s f o r events i n the swarm are a s s o c i a t e d w i t h two d i s t i n c t t e c t o n i c f e a t u r e s i n the n o r t h e a s t P a c i f i c Ocean - the E x p l o r e r T rench and the Queen C h a r l o t t e I s l and F r a c t u r e Zone. The events i n the swarm show a non-random d i s t r i b u t i o n i n t ime and the p o l a r i t y o f the f i r s t a r r i v a l v a r i e s w i th s t a t i o n and w i t h e v e n t . These f a c t s suggest a g e n e r a t i n g p r o c e s s i n v o l v i n g s t r a i n r e l e a s e by movement on a f a u l t or f a u l t s coup l ed w i t h some s o r t o f t r i g g e r i n g mechanism. The magnitude-f requency r e l a t i o n s h i p i s de te rmined as L o g 1 Q N = (4 .07±0 .50 ) - ( 1 . 10±0 .20 )M L f o r 2.0 < M L < 4 . 0 . The v a l ue f o r the s l ope (-1.10) i s c h a r a c t e r i s t i c o f ear thquake swarms genera ted as a r e s u l t o f v o l c a n i c a c t i v i t y . The apparent paradox conce rn ing the source mechanism i s r e c o n c i l e d i n the i d e a o f "The New G l o b a l T e c t o n i c s " t ha t e x t e n s i o n ac ross r i d g e c r e s t s ( i n v o l v i n g g e n e r a t i o n of new c r u s t a l m a t e r i a l by d i a p i r i c i n t r u s i o n s ) and t r ans fo rm f a u l t i n g o f the o f f - s e t t i n g f r a c t u r e zones w i l l occuj: t o g e t h e r . ACKNOWLEDGEMENTS I w i sh t o thank P r o f e s s o r G. K. C . C l a r ke f o r h i s p a t i e n c e i n s u p e r v i s i n g the r e s e a r c h , P r o f e s s o r R. M. E l l i s f o r h i s s u g g e s t i o n s and h e l p and a l l the o the r peop le i n the Geophys ics Department who made l i f e so i n t e r e s t i n g . The t o p i c was sugges ted by Dr . W. G . M i l ne o f the Dominion A s t r o p h y s i c a l O b s e r v a t o r y , V i c t o r i a , B. C , comput ing f a c i l i t i e s were p r o v i d e d by the Computer Sc i ence Department at the U n i v e r s i t y o f B r i t i s h Co lumbia and the t y p i n g was done by Miss Judy Cope land and M iss Mar ianne M c L a u c h l i n . For p o r t i o n s o f t h i s r e s e a r c h I r e c e i v e d a s s i s t a n c e from a N a t i o n a l Research C o u n c i l Post Graduate S c h o l a r s h i p and a N a t i o n a l Research C o u n c i l Grant No. 67-4327 o f D r . G. K. C . C l a r k i v TABLE,OF CONTENTS Page 1. INTRODUCTION 1 1.1 The Ear thquake Swarm: Summary o f A v a i l a b l e Data 1 1.2 Earthquake.Swarms 6 1.3 The Source Area 7 1.4 E s t ima te s of E p i c e n t e r s 9 1.5 T r a v e l Times and C r u s t a l S t r u c t u r e o f Vancouver I s l a n d 11 1.6 Purpose o f T h e s i s 12 2. CONTINENTAL MARGIN TECTONICS 13 2.1 The New G l o b a l T e c t o n i c s 13 2.2 The No r theas t P a c i f i c Ocean 18 2.3 Western B r i t i s h Co lumbia and Vancouver I s l a n d 28 3. INTERPRETATION OF ARRIVAL TIMES 31 3.1 E s t i m a t i n g E p i c e n t e r s 31 3.2 Model Events 38 3.3 E p i c e n t e r s i n the No r theas t P a c i f i c Ocean 52 3.4 Model P e r t u r b a t i o n s 56 4. THE EARTHQUAKE SWARM AT PHC 62 4.1 Time R e l a t i o n s o f the Swarm 63 4.2 Magnitude De te rm ina t i ons 66 5. CONCLUSIONS 71 BIBLIOGRAPHY 75 APPENDIX A ' 77 O r i g i n Time from P and S A r r i v a l s 77 APPENDIX B 81 T e s t o f L o c a t i o n Procedure 81 APPENDIX C 83 The Ear thquake Swarm at PHC 83 V LIST OF FIGURES FIGURE NO. 1.1-1 1.1-2 2.1- 1 2.2- 1 2.2-2 2.2-3 2.2- 4 2.3- 1 3.1- 1 3.2- 1 3.2-2 3.2-3 3.2-4 3.2-5 Page Area o f Study and Seismograph S t a t i o n s 2 E p i c e n t e r s f o r 14 Events i n the Swarm as C a l c u l a t e d by USCGS 4 B lock Diagram I l l u s t r a t i n g some o f the Ideas o f "The New G l o b a l T e c t o n i c s " 15 S e i s m i c i t y f o r Nor theas t P a c i f i c Ocean and Western Nor th Amer ica 20 F a u l t P lane S o l u t i o n s , Major F a u l t s and Ocean Bottom Topography f o r Nor the rn C o a s t a l C a l i f o r n i a 21 Summary Diagram o f Magnet i c Anomal ies i n the Nor theas t P a c i f i c Ocean 23 Bottom Topography f o r No r theas t P a c i f i c Ocean 24 T e r t i a r y T e c t o n i c s on the I n s u l a r B e l t of B r i t i s h Columbia 30 P o r t i o n s of the PHC Ve r t i c a l -Componen t Seismogram f o r August 28 , 1967 35 T r a v e l Times f o r Pn and Sn i n Western B r i t i s h Columbia u s i n g USCGS E p i c e n t e r s 40 (a) Model f o r the C rus t of the E a r t h i n Western B r i t i s h Columbia 43 (b) S tandard C r u s t a l Model f o r Reg iona l S e i s m i c Events 43 E p i c e n t e r s f o r 19 Events o f the Swarm C a l c u l a t e d i n Terms of a R e g i o n a l Model of the E a r t h ' s C rus t 47 E p i c e n t e r s f o r 19 Events o f the Swarm as a F u n c t i o n of O r i g i n Time 49 E p i c e n t e r s f o r 19 Events of the Swarm C a l c u l a t e d i n Terms o f a Reg iona l Model of the E a r t h ' s C rus t 51 v i T r a v e l Times f o r Pn and Sn i n Western B r i t i s h Columbia Assuming A l l Events had a V o c a l Depth o f 10 km E p i c e n t e r s P lus 90% Con f i dence E l l i p s e s f o r 8 We l l De f i ned Events Us ing a S tandard C r u s t a l Model E p i c e n t e r s P lus 90% Con f idence E l l i p s e s f o r 8 We l l De f i ned Events U s i n g a Reg iona l C r u s t a l Model H i s tog ram Showing Number of Events f o r Three Hour Pe r i ods H i s tog ram Showing Number o f Three Hour Pe r i ods f o r which N Events Occur red R i c h t e r and Body Wave Magnitude De te rm ina t i ons Magni tude-Frequency R e l a t i o n s h i p E s t i m a t i o n o f O r i g i n Time from S-P I n t e r v a l v i i L I ST 'OF TABLES TABLE NO. Page 1.1-1 USCGS P r e l i m i n a r y De t e rm ina t i on o f E p i c e n t e r s 3 3.1- 1 . P and S Events 34 3.2- 1 E s t ima t e s of L a t i t u d e , Long i tude and O r i g i n Time by Leas t Squares Procedure 46 3.2- 2 E s t ima t e s of L a t i t u d e and L o n g i t u d e . b y Leas t Squares 50 3.3- 1 Es t ima te o f L a t i t u d e and Long i tude by L eas t Squares f o r 2 Models 57 A - l O r i g i n Time from S-P I n t e r v a l 80 1. INTRODUCTION 1.1 The Ear thquake Swarm: Summary o f A v a i l a b l e Data In the 6-day p e r i o d from August 27th to September 1 s t , 1967, b e g i n n i n g at 0 6 . 2 2 . 5 0 U. T . on August 2 7 , an ear thquake swarm o f at l e a s t 217 events i n the magnitude range 2.7 to 5.2 o c c u r r e d i n the e a s t e r n P a c i f i c Ocean west o f Vancouver I s l a n d and sou theas t o f the Queen C h a r l o t t e I s l ands ( F igu re 1 .1-1) . Fou r t een of the l a r g e r events were r e co rded by se ismograph s t a t i o n s o f the U n i t e d S t a t e s Coas t and Geode t i c Surveys (USCGS) and r e p o r t e d i n t h e i r P r e l i m i n a r y De t e rm ina t i on of E p i c e n t e r s f o r t h a t p e r i o d . A summary o f . t he i r r e s u l t s f o r l o c a t i o n s and magnitudes o f the events i s g i v e n i n Tab l e 1.1-1 and the e p i -c e n t e r s p l o t t e d i n F i g u r e 1.1-2. The f o c a l depths f o r a l l events are s h a l l o w ; the depths f o r the USCGS e p i c e n t r a l e s t ima te s were r e s t r a i n e d to 33 km, excep t i n 2 cases where va lues o f 24 and 18 km are g i v e n . The e p i c e n t e r s a l l f a l l i n an i r r e g u l a r a r e a , rough l y 70 km on a s i d e , w i t h average c o - o r d i n a t e s 5 0 . 2 ° N , 1 2 9 . 7 ° W . The magnitudes (based on P wave ampl i tudes ) range from 3.1 to 5 . 1 , w i th the 2 even ts w i t h l a r g e s t magnitude o c c u r r i n g near the b e g i n n i n g and end o f the swarm. T h i s s e i s m i c a c t i v i t y was a l s o r e c o r d e d by 7 Canadian se ismograph s t a t i o n s i n wes te rn and c e n t r a l B r i t i s h C o l u m b i a , PHC, ALB, V I C , VAN, FS J , PNT and MIC. Four o f these s t a t i o n s (PHC, V IC , FS J , PNT) were equ ipped w i th c o n v e n t i o n a l W i l lmore 7—-FIGURE 1.1-1: PHYSIOGRAPHY AND LAND FORMS OF BRITISH COLUMBIA, SOURCE AREA FOR EARTH-QUAKES IS OUTLINED. SEISMOGRAPH STATIONS USED IN THIS STUDY ARE MARKED BY ASTERISKS N/LANDFORM UNITS OF BRITISH COLUMBIA COASTAL SYSTEM f ) Coatal Truufh I „' ,^  Ov'et Mountain Am EASTERN SYSTEM I 1 flaky Vountamt [ 1 Roty Mountain Trench INTERIOR SVSFfcM [ I PUin* [""] Southern and Northern ^ f Ptatciu Area* Mouniaini GREAT PLAINS I 1 Plain* ttih Mtui and Cuctls TABLE 1.1-1 USCGS PRELIMINARY DETERMINATION OF EPICENTERS EVENT LATITUDE LONGITUDE ORIGIN TIME DEPTH MAGNITUDE NUMBER (DEG. N) (DEG. W) H/M/S (KM) AUGUST 27, 1967 2 50.30 129.87 12/56/36.7 33 4.0 4 50.22 129.87 13/34/52.6 24 5.1 5 50-. 15 129.83 15/18/45. 1 33 3.8 7 50.19 129.70 18/29/07.4 33 4.1 AUGUST 2 8 10 50.19 129.48 11/32/10.5 33 3.7 11 50.10 129.09 11/41/20.6 33 3.8 32 50.34 129.73 11/45/34.6 33 3.7 13 50.13 129.55 12/39/18.1 33 4.1 0 50.23 129.60 12/59/03.2 33 4.0 0 50.24 129.78 12/59/04.4 36 4.0 17 50.30 130.08 13/49/42.1 18 4.6 19 50.41 129.55 15/07/11.7 33 4.5 21 50.40 129.84 16/20/06.6 33 5.1 SEPTEMBER 1 0 50. 60 129.51 14/11/25.4 33 4.6 4 F I G U R E 1 . 1 - 2 : E P I C E N T E R S FOR 1*4 E V E N T S I N T H E SWARM AS C A L C U L A T E D BY U S C G S . S O L I D T R I A N G L E S A R E L A R G E R M A G N I T U D E E V E N T S . D E P T H I S C O N S T R A I N E D T O 33 KM 5 Se i smographs , r e c o r d i n g 3 components at l ong- and s h o r t - p e r i o d s , w h i l e the r ema in ing s t a t i o n s (ALB, VAN and MIC) were equ ipped w i t h v e r t i c a l - c o m p o n e n t s h o r t - p e r i o d ins t ruments o n l y . A l l r e c o r d s are i n the form o f cha r t r e c o r d s , s c a l e d 60 mm/min, w i t h m a g n i f i c a t i o n s i n the 20 K range at one-second p e r i o d . PHC r e c o r d e d the most even ts , . 217, w h i l e a l l o ther s t a t i o n s r e c o r d e d an o rde r o f magnitude l e s s ; ALB 37 , VIC 19 , VAN 19, FSJ 3 3 , PNT 22 , and MIC 2. The r e co rds from MIC were not used because o f t h e i r poor q u a l i t y , and the r e c o r d s from VAN f o r t h i s p e r i o d were l o s t and so were not a v a i l a b l e . T h i s l e f t an " a r r a y " o f 5 s t a t i o n s , 3 on Vancouver I s l and and 2 on the i n t e r i o r p l a t e a u o f B r i t i s h Columbia ( F igu re 1 .1-1) , which r e c o r d e d a t o t a l o f 8 common events f o r which f i r s t a r r i v a l s (Pn) and second a r r i v a l s (Sn) c o u l d be i d e n t i f i e d w i th some c o n f i d e n c e , and 11 a d d i t i o n a l common events f o r which f i r s t a r r i v a l s a lone c o u l d be p i c k e d . A l l s t a t i o n s are l o c a t e d w i t h i n 850 km o f the s o u r c e . Measurements o f a r r i v a l t i m e s , S-P t imes and P and S amp l i tudes were made from the o r i g i n a l r e co rds o f the Dominion O b s e r v a t o r y , u s i n g d i v i d e r s and m e t r i c s c a l e . The t imes f o r P are c o n s i s t e n t to ±0.5 seconds and the t imes f o r S to ±2.0 s e conds . These l i m i t s shou ld be rega rded as the 901 c o n f i d e n c e l e v e l . The r e c o r d i n g of an ear thquake swarm i n the n o r t h e a s t P a c i f i c Ocean r e p r e s e n t s an uncommon event and r a i s e d a number o f i n t e r e s t i n g q u e s t i o n s . Where d i d the a c t i v i t y take p l a c e ? What was i t s r e l a t i o n s h i p to the t e c t o n i c s o f the r eg i on ? What s p e c i a l r e l a t i o n s h i p do ear thquake swarms bear to " the New G l o b a l T e c t o n i c s " ? ' To b e g i n w i th the l a s t q u e s t i o n f i r s t , some g e n e r a l f a c t s about ear thquake swarms can be c o n s i d e r e d . A swarm d i f f e r s from a f o r e s h o c k - a f t e r s h o c k s e r i e s - bo th terms d e s -c r i b e the oc cu r r ence of a l a r g e number o f ear thquakes w i t h i n a s m a l l r e g i o n o f space and time - i n the l a c k o f a main shock . By t h i s c r i t e r i o n the term swarm w i l l be used i n t h i s p a p e r . 1.2 Ear thquake Swarms Most ear thquake swarms occu r i n areas o f r e c e n t v u l c a n i s m ; v o l c a n i c e r u p t i o n s are u s u a l l y accompanied by swarms o f g r e a t numbers o f e v e n t s , sometimes t r a c i n g the pa th o f magma from depth to the s u r f a c e . However, i n the i n v e s t i g a t i o n o f an ear thquake swarm i n the v i c i n i t y o f Mould Bay, N .W.T . , i t was conc luded tha t the ear thquakes were o f t e c t o n i c o r i g i n , but t h a t the a p p r o p r i a t e s t r a i n r e l e a s e c o u l d have been caused by move-ment o f magma at depth c o u l d not be e l i m i n a t e d . There i s no g e o l o g i c ev idence of r e cen t v u l c a n i s m i n the a r e a . ( Sm i th , Whitham, P i c h e , 1968) . The l a c k o f a main shock , and sudden i n i t i a t i o n and t e r m i n a t i o n o f the swarm are a l s o thought to be gene r a l c h a r a c t e r i s t i c s o f v o l c a n i c swarms. A l s o used as a d i a g n o s t i c i s the v a l ue b , i n the f requency-magni tude r e l a t i o n -s h i p g i v e n by R i c h t e r (1958) as L o g 1 Q N = a - b M L (1.2-1) Va lues o f b l a r g e r than 1.0 are more c h a r a c t e r i s t i c o f v o l c a n i c swarms. I t has been sugges ted tha t ear thquake swarms are 7 a s s o c i a t e d w i t h the c r e s t s of the ocean r i s e system ( I s a c k s , O l i v e r , and Sykes , 1968 ) , and c h a r a c t e r i z e d by normal f a u l t i n g and v o l c a n i c a c t i v i t y . Ear thquakes o c c u r r i n g a long the median r i d g e o f the M i d - A t l a n t i c Ridge have a s i g n i f i c a n t l y s t e e p e r magn i tude- f requency r e l a t i o n than those o c c u r r i n g a long the o f f - s e t t i n g f r a c t u r e zones (T. J . G. F r a n c i s , 1969) . T h i s i s thought to be the r e s u l t o f d i f f e r e n t p roces ses g e n e r a t i n g the s e i s m i c i t y i n the two d i f f e r e n t a r e a s . 1.3 The Source Area • The arrangement of o c e a n i c r i d g e s and t r a n s f o r m f a u l t s i n the n o r t h e a s t P a c i f i c Ocean i s p a r t o f the much l a r g e r wor ld-wide p a t t e r n o f such f e a t u r e s which has r e c e n t l y s t i m u -l a t e d i n t e r e s t i n g l o b a l t e c t o n i c s . A "New G l o b a l T e c t o n i c s " has been proposed ( I s a c k s , O l i v e r and Sykes , 1968) t o e x p l a i n the major t e c t o n i c f e a t u r e s of the e a r t h ' s c r u s t ( o cean i c r i s e s , t r enches and i s l a n d a r c s , and t r ans fo rm f a u l t s ) i n terms o f the i n t e r a c t i o n s o f major b l o c k s o f the e a r t h ' s c r u s t moving w i t h r e s p e c t to each o t h e r . Two f e a t u r e s which are o f p a r t i c u l a r i n t e r e s t i n t h i s t h e s i s are the Cobb R ise (McManus, 1967 ) , ( f o rmer l y named the Juan de Fuca Ridge ( W i l s o n , 1965b)) and the Queen C h a r l o t t e I s l a n d F r a c t u r e Zone. The Cobb R i se p a r a l l e l s the Oregon and Washington coas t s from rough l y 500 km at s e a . I ts r e l a t i v e l y a s e i s m i c n a t u r e , (at l e a s t over the l a s t h a l f cen tu ry ) and p o o r l y deve loped . r idge-type topography , suggest t ha t i t i s s p r e a d i n g a t a f a s t e r r a te than o the r p a r t s o f the ocean r i s e 8 sys tem. I t i s t e rm ina ted on the south by an a rea o f h i g h s e i s m i c a c t i v i t y , the B lanco F r a c t u r e Zone (Tob in and. Sykes , 1968) , i n t e r p r e t e d as a d e x t r a l t r ans fo rm f a u l t o f the r i d g e - r i d g e t y p e , and on the n o r t h by the Queen C h a r l o t t e I s l ands F r a c t u r e Zone, a l s o a s e i s m i c f e a t u r e (Tob in and S ykes , 1968) , i n t e r p r e t e d as a d e x t r a l t r a n s f o r m f a u l t o f the r i d g e - a r c t y p e , ( W i l s o n , 1965b) j o i n i n g the Cobb R ise t o the A l e u t i a n T r e n c h . The e p i c e n t e r s f o r events i n the swarm, as l o c a t e d by the USCGS, p l a c e the source area i n the v i c i n i t y o f 5 0 . 2 ° N , 1 2 9 . 6 ° W . On the b a t h y m e t r i c map comp i l ed by McManus (1963) , t h i s a rea i s mapped as p a r t o f a l a r g e f r a c t u r e zone t r e n d i n g no r thwes t . The Cobb R i se i s not c o n s i d e r e d to extend to l a t i t u d e 5 0 o N ; but a zone o f " p r o b a b l e c r u s t a l t opog raphy " (McManus, 1967) a s s o c i a t e d w i t h a f e a t u r e c a l l e d the E x p l o r e r T r e n c h , t r e n d i n g n o r t h e a s t , abuts on the f r a c t u r e zone at t h i s l a t i t u d e . T h i s f e a t u r e was a l s o i n t e r p r e t e d as a p o s s i b l e r i d g e c r e s t by W i l s o n , (1965b) from the d i s t r i b u t i o n o f t o t a l magnet ic f i e l d anomal ies i n the a rea as p r e s e n t e d by Ra f f and Mason (1961) . None o f the s e i s m i c i t y r e co rded f o r the p e r i o d o f 1954-1965, (Tob in and S ykes , 1968) i n t h i s a rea has been a s s o c i a t e d w i t h the E x p l o r e r T r e n c h . The s c a t t e r o f bo th the b e t t e r and poo re r l o c a t e d e p i c e n t e r s sugges ts a d d i t i o n a l c o m p l e x i t i e s i n the t e c -t o n i c s , e s p e c i a l l y i n the r e g i o n between the E x p l o r e r T rench and the Queen C h a r l o t t e I s l ands F r a c t u r e Zone. The broad, scope of t e c t o n i c s i n the n o r t h e a s t P a c i f i c Ocean , as de te rmined by the two independent s t u d i e s of s e i s m i c i t y and ear thquake mechanisms o f T o b i n and S ykes , (1968) and B o l t , Lomni tz and M c E v i l l y , (1968) , i s one of c o n s i s t e n t r i g h t l a t e r a l movement from the Eas t P a c i f i c R i se i n the south t o the Queen C h a r l o t t e I s l a n d F r a c t u r e Zone i n the n o r t h . The mot ion i s c o n s i s t e n t w i t h the i d e a o f t r a n s f o r m f a u l t i n g and ocean f l o o r s p r e a d i n g from r i d g e c r e s t s as p roposed by W i l son (1965b) . The a s s o c i a t i o n o f the s e i s m i c a c t i v i t y r e p r e s e n t e d by the swarm w i th t h i s t e c t o n i c env i ronment depends g r e a t l y on the c o n f i d e n c e w i t h which the e p i c e n t e r s can be e s t i m a t e d . 1.4 E s t i m a t e s o f E p i c e n t e r s E p i c e n t e r s f o r ear thquakes are commonly e s t ima t ed by a t e chn ique of l e a s t squares i n v o l v i n g the r e d u c t i o n o f " e r r o r s " i n r ead a r r i v a l t imes a g a i n s t s t anda rd expec ted a r r i v a l t i m e s , by a l l o w i n g f r e e v a r i a b l e s such as l a t i t u d e , l o n g i t u d e , depth and o r i g i n t ime and choos ing those va lues o f the f r e e v a r i a b l e s which min imize the sum of the squared e r r o r s . T h i s t e chn ique r e q u i r e s the s o l u t i o n o f a s e t o f s imu l taneous n o n - l i n e a r equa t i ons i n the unknown p a r a m e t e r s . There a re 2 sources o f e r r o r s i n t h i s method. One i n v o l v e s the p r o c e s s by which a r r i v a l t imes f o r s p e c i f i c events are read at v a r i o u s s t a t i o n s and c o l l e c t e d f o r pro-, c e s s i n g . The a r r i v a l o f an event i s a "random p r o c e s s " i n the sense t ha t i t must be r e c o g n i z e d and r e co rded by an o b s e r v e r . D i f f e r e n t e x p e r i e n c e d obse rve r s g i v e n the same r e c o r d w i l l not always p i c k the same t ime f o r a g i v e n e ven t . A long w i th t h i s b a s i c r e a d i n g e r r o r are e r r o r s o f o the r s o u r c e s : i n s t r u m e n t a l 10 v a r i a t i o n s , n o i s e f a c t o r s and time miscounts - a l l o f which com-b ine to g i v e the a r r i v a l t ime o f an event a p r o b a b i l i t y d i s t r i -b u t i o n i n t i m e . The second source o f b i a s i n e p i c e n t e r l o c a t i o n p r o -cedures l i e s w i t h the " s t a n d a r d " model used t o compare the a c t u a l t imes and hence a r r i v e at a " b e s t " v a l ue f o r the source p a r a m e t e r s . On a wor ld-wide b a s i s , J e f f r e y s - B u l l e n T a b l e s have been used w i t h i n t e r p o l a t i o n p r o c e d u r e s ; f o r s p e c i f i c p rob l ems , s p e c i a l models can be g e n e r a t e d , but they a l l r e p r e s e n t some s o r t o f average f o r a r r i v a l t imes over the a rea of i n t e r e s t . The source parameters c a l c u l a t e d are then on l y those which " b e s t f i t " the model assumed. Fo r areas of anomalous t r a v e l t imes an average model f o r the upper 100 km of the e a r t h i s c e r t a i n l y i n a p p l i c a b l e . S ince the advent o f n u c l e a r t e chno logy and l a r g e e x p l o s i o n s , i t has become apparent tha t the J e f f r e y s - B u i l e n T a b l e s , used s i n c e 1940, were s e v e r a l seconds l a t e over the whole e a r t h , and b i a s e d f o r ear thquakes from areas w i t h t r e n c h -type topography . T h i s problem has been p a r t i a l l y remedied i n r e c e n t yea r s by the a d o p t i o n o f r e v i s e d new s t anda rd wor ld wide t r a v e l t ime t a b l e s ( H e r r i n e t a l , 1968) but not e l i m i n a t e d ; the l o c a t i o n o f l ongsho t was p l a c e d up to 60 km too f a r n o r t h and west by s t a n d a r d ear thquake l o c a t i o n p r o c e d u r e s , ( Se i sm ic Data L a b o r a t o r y Report #133) . The p rob lem o f a model f o r the c r u s t i s e s p e c i a l l y c r i t i c a l i n wes te rn Nor th Amer i ca and the n o r t h e a s t P a c i f i c Ocean because the^ c r u s t and upper mant le i s anomalous i n these a r e a s . 11 1.5 T r a v e l Times and C r u s t a l S t r u c t u r e o f Vancouver I s l and In the p e r i o d from 1953 t o 1960 the Dominion Obse r va to r y o f Canada c a r r i e d out e x t e n s i v e s e i s m i c r e f r a c t i o n surveys i n the v i c i n i t y o f Vancouver I s l a n d . The d a t a , r e p r e s e n t i n g some 98 shots o f app rox ima te l y 300 pounds o f e x p l o s i v e s p l u s one l a r g e e x p l o s i o n o f 1,500 t o n s , were d i s t i n g u i s h e d by the l a ck o f Pn a r r i v a l s f o r d i s t a n c e s l e s s than 400 km. The c r u s t o f the e a r t h i s t h e r e f o r e i n t e r p r e t e d to be at l e a s t 50 km t h i c k nea r the cen te r o f the I s l a n d . On the i n t e r i o r p l a t e a u o f B r i t i s h C o l u m b i a , White and Savage (1965) g i ve v a lues f o r c r u s t a l t h i c k n e s s o f 30 o r 35 km and upper mant le P-wave v e l o c i t i e s o f 7.8 or 7.7 km/sec. Immediate ly t o the sou theas t o f Vancouver I s l and i n wes te rn Washington and Oregon , D e h l i n g e r , C h i b i s and C o l l i v e r (1965) , g i ve Pn v e l o c i t i e s o f 7.67 km/sec and normal c r u s t a l d e p t h s . T h i s would i n d i c a t e t ha t the upper mantle i n the v i c i n i t y o f Vancouver I s l a n d i s anomalous: r a p i d changes i n depth occu r and s e i s m i c v e l o c i t i e s can be expec ted t o be abnorma l l y low. Coup led w i t h t h i s p rob l em , the e x i s t e n c e o f a c t i v e r i d g e s and t r a n s f o r m f a u l t s , both f e a t u r e s w i t h anomalous c r u s t a l s e c t i o n s , i n the n o r t h e a s t P a c i f i c makes the use of a normal model f o r o cean i c c r u s t i n t h i s r e g i o n unw ise . I t i s u n f o r t u -nate t ha t ve ry l i t t l e p e r t i n e n t i n f o r m a t i o n conce rn i ng the c r u s t a l s t r u c t u r e o f the source a rea e x i s t s i n t h i s c a s e . T h i s i s e s p e c i a l l y c r i t i c a l s i n ce the l o c a t i o n p rocedure i s s e n s i t i v e to the model chosen f o r the source a rea c r u s t . 12 1.6 Purpose o f T h e s i s T h i s t h e s i s was under taken w i t h the f o l l o w i n g th ree aims i n mind . (1) To examine an ear thquake swarm i n the l i g h t o f the "New G l o b a l T e c t o n i c s " w i th s p e c i a l r e f e r e n c e t o : (a) The Source A r e a . (b) The Source Mechanism. (2) To o b t a i n i n f o r m a t i o n on the upper mant le i n wes te rn B r i t i s h Co lumb ia , p a r t i c u l a r l y : (a) Depth to Moho rov iC i c d i s c o n t i n u i t y under Vancouver I s l a n d . (b) Upper mantle v e l o c i t i e s f o r Pn and Sn waves i n the v i c i n i t y o f Vancouver I s l a n d . (3) To a s s e s s the p o s s i b i l i t y of l o c a t i n g s e i s m i c events o f f the c o n t i n e n t a l margin o f wes te rn North Amer i ca on a r e g i o n a l s c a l e . 2. CONTINENTAL MARGIN TECTONICS T h i s chap te r i s borrowed from many p u b l i c a t i o n s which have appeared i n the p a s t 10 yea r s d e a l i n g w i t h t e c t o n i c s and se i smo logy on a l a r g e s c a l e and p a r t i c u l a r l y those papers which conce rn wes te rn B r i t i s h Co lumb ia . The concept o f ocean f l o o r i n g s p r e a d i n g , and the r o l e s p l a y e d i n t ha t p r o c e s s by r i d g e s , t r e n c h e s , and t r a n s f o r m f a u l t s , p r o v i d e " a p o i n t o f v i ew" from which the c o m p l i c a t e d f a c e t o f a l a r g e r p a t t e r n can be more p r o p e r l y u n d e r s t o o d . The " p o i n t o f v i ew" i s t h a t l a r g e b l o c k s o f the e a r t h ' s c r u s t can and do move w i th r e s p e c t to each o t h e r . T h i s concept has been proposed i n many fo rms : the f i r s t t h e o r i e s o f c o n t i n e n -t a l d r i f t sugges ted by F. B. T a y l o r and A . Wegener, the c o n -v e c t i o n c e l l h y p o t h e s i s p roposed by Ven ing M e i n e s z , and ideas o f expans ion o r c o n t r a c t i o n o f the e a r t h through g e o l o g i c t i m e . New i n t e r e s t i n these t h e o r i e s has been s t i m u l a t e d i n the p a s t decade by the r e c o g n i t i o n of the dynamic na ture o f ocean t renches the wor ld-wide d i s t r i b u t i o n o f the ocean r i s e s y s t em, the d i s c o v e r y of the p r o p e r t i e s o f ocean f l o o r magnet ic anomaly p a t t e r n s and f i n a l l y by the na tu re o f t r a n s f o r m f a u l t i n g . 2.1 The New G l o b a l T e c t o n i c s "The New G l o b a l T e c t o n i c s " ( I s a c k s , O l i v e r , and S ykes , 1968) r e p r e s e n t s an at tempt to v iew l a r g e segments o f g e o l o g i c a l and g e o p h y s i c a l phenomena, p a r t i c u l a r l y s e i s m i c i t y and t e c t o n i c s , 14 i n a s i n g l e cohe ren t framework. I t has the remarkable a b i l i t y o f r e c o n c i l i n g ve r y many d i v e r s e problems i n geo logy and geo -p h y s i c s . The upper p o r t i o n s o f the e a r t h , perhaps as much as s e v e r a l hundreds o f km, are c o n s i d e r e d to be made o f th ree l a y e r s ; the l i t h o s p h e r e , the as thenosphere and the mesosphe re . . The l i t h o s p h e r e i s o f the o rde r o f one hundred km t h i c k , com-p r i s i n g the c r u s t and upper m a n t l e , and i s c o n s i d e r e d to possess s i g n i f i c a n t " s t r e n g t h " i n the sense tha t i t can r e s i s t shear s t r e s s e s o f l i m i t e d v a l u e s . The second l a y e r , the a s thenosphe re , i s two or th ree hundred km t h i c k , and i s c o n s i d e r e d to have no s i g n i f i c a n t " s t r e n g t h " ; i t cannot r e s i s t shear s t r e s s o f l i m i t e d v a l u e s , but w i l l accommodate the s t r e s s e s by f l o w i n g . The f i n a l l a y e r , the mesosphere , i s o f u n c e r t a i n t h i c k n e s s and aga in can r e s i s t shear s t r e s s e s . ( F i gu re 2 . 1-1 ) . The as thenosphere co r responds rough l y t o the l o w - v e l o c i t y l a y e r i n s e i s m o l o g y . I t s t r o n g l y a t t enua tes s e i s m i c waves , p a r t i c u l a r l y h i gh- f r equency shear waves. The boundar i e s between the l a y e r s are p o s s i b l y g r a d a t i o n a l ; the as thenosphere-mesospher i c boundary has perhaps r i s e n throughout g e o l o g i c t i m e . The near s u r f a c e l i t h o s p h e r e i s d i v i d e d i n t o a few l a r g e , r e l a t i v e l y s t a b l e , l a t e r a l l y - r i g i d b l o c k s which can move w i t h r e s p e c t to each o t h e r . I t i s the mot ions o f these b l o c k s and the i n t e r a c t i o n s at the edges which produces the major t e c t o n i c a l l y a c t i v e s t r u c t u r e s on the: s u r f a c e o f the e a r t h - these are the o cean i c t r e n c h e s , r i d g e s and major t r a n s f o r m f a u l t s . The d i s t r i b u t i o n 15 F I G U R E 2.1-1: B L O C K D I A G R A M I L L U S T R A T I N G S C H E M A T I C A L L Y T H E C O N F I G U R A T I O N S AND R O L E S O F T H E L I T H O S P H E R E , A S T H E N O -S P H E R E , AND M E S O S P H E R E I N A V E R S I O N O F T H E NEW G L O B A L T E C T O N I C S I N W H I C H T H E L I T H O S P H E R E , A L A Y E R O F S T R E N G T H , P L A Y S A K E Y R O L E . ARROWS ON L I T H O S P H E R E I N D I C A T E R E L A T I V E M O V E M E N T S O F A D J O I N I N G B L O C K S . ARROWS I N A S T H E N O S P H E R E R E P R E S E N T P O S S I B L E C O M P E N S A T I N G FLOW I N R E S P O N S E T O DOWNWARD M O V E M E N T O F S E G M E N T S O F L I T H O S P H E R E . O N E A R C - T O - A R C T R A N S F O R M F A U L T A P P E A R S A T L E F T B E T W E E N O P P O S I T E L Y F A C I N G Z O N E S O F C O N V E R G E N C E ( I S L A N D A R C S ) , TWO R I D G E - T O - R I D G E T R A N S F O R M F A U L T S A L O N G O C E A N R I D G E A T C E N T E R , S I M P L E A R C S T R U C T U R E A T R I G H T . ( F R O M I S A C K S , O L I V E R AND S Y K E S 1968) 16 o f these s t r u c t u r e s as p r i n c i p a l zones o f t e c t o n i c a c t i v i t y around the e a r t h and the mot ion o f the b l o c k s as deduced from magnet ic anomal ies at sea agree remarkab ly w e l l . Each o f the t e c t o n i c s t r u c t u r e s i s symptomatic o f a d i f f e r e n t s t r e s s f i e l d ; t r enches are zones o f compress ion and u n d e r t h r u s t i n g , r i s e s are zones o f e x t e n s i o n and g e n e r a t i o n o f new c r u s t a l m a t e r i a l , and t r a n s f o r m f a u l t s are zones of shear s t r e s s e s deve loped as two b l o c k s s l i d e by one a n o t h e r . The i n t e r a c t i o n s are f u r t h e r comp l i c a t ed by the na tu re o f the l i t h o s p h e r i c b l o c k i n v o l v e d . Two types o f l i t h o s p h e r e are r e c o g n i z e d , one w i th a c o n t i n e n t a l - t y p e c r u s t and one w i t h an o c e a n i c type c r u s t . The d i v i s i o n o f the e a r t h ' s s u r f a c e i n s t a b l e b l o c k s w i t h t e c t o n i c a l l y a c t i v e m a r g i n s , by no means f o l l o w s c o n t i n e n t a l o c e a n i c m a r g i n s , but the types o f l i t h o s p h e r e i n v o l v e d i n i n t e r a c t i o n s p l a y an impor tan t r o l e . Ocean i c- type c r u s t i s g e n e r a l l y u n d e r t h r u s t c o n t i n e n t a l type c r u s t i n zones o f c o m p r e s s i o n . The development o f r i s e s on c o n t i n e n t a l c r u s t s i s more c o m p l i c a t e d than on ocean i c c r u s t . The mechanism which genera tes the energy needed f o r these a c t i v i t i e s i s u n c e r t a i n . There are two l i n e s o f thought on t h i s . In one, the as thenosphere i s the a c t i v e e l ement , c o n t a i n i n g a s e t o f c o n v e c t i o n c e l l s , wh ich c a r r y heat away from the lower m a n t l e , and to which the l i t h o s p h e r i c b l o cks a d j u s t as bes t they c a n . In the second theory the l i t h o s p h e r e i s the a c t i v e e l ement , r e p r e s e n t i n g a r e l a t i v e l y h i g h e r d e n s i t y m a t e r i a l l y i n g over the r e l a t i v e l y lower d e n s i t y a s thenosphe re . G r a v i t a t i o n a l i n s t a b i l i t y r e s u l t s i n the l i t h o s p h e r e p l u n g i n g , i n c e r t a i n p l a c e s , i n t o the 17 a s t h e n o s p h e r e . The as thenosphere a d j u s t s to t h i s by f l o w i n g , and the downthrust l i t h o s p h e r e i s g r a d u a l l y conve r t ed i n t o a s t h e n o s p h e r i c m a t e r i a l so t h a t the p r o c e s s may c o n t i n u e . Both p r o c e s s e s w i l l r e s u l t i n the same p a t t e r n o f t e c t o n i c s , but d i f f e r i n the emphasis they p l a c e on the t e c t o n i c e l emen t s . In the f i r s t , t r enches and r i s e s have the same s i g n i f i c a n c e , r e p r e s e n t i n g r e s p e c t i v e l y the f a l l i n g and r i s i n g l imb of a c o n v e c t i o n c e l l . In the s e c o n d , t r enches are the p r ima r y e l emen t , which produce the s t r e s s e s , wh i l e r i s e s are a secondary f e a t u r e , r e p r e s e n t i n g zones o f s t r a i n r e l e a s e by normal f a u l t i n g , but d e v e l o p i n g no s t r e s s e s of t h e i r own. In b o t h c a s e s , t r a n s f o r m f a u l t s are secondary e l e m e n t s , r e p r e s e n t i n g zones o f s t r a i n r e l e a s e by wrench f a u l t i n g . The emphasis p l a c e d on the r o l e o f the r i s e s i s impor tan t from a t e c t o n i c p o i n t o f v i ew. I f they are a p r imary f e a t u r e , somehow c o r r e l a t e d to c o n v e c t i o n c e l l s i n the upper m a n t l e , one can then imagine them g e n e r a t i n g t h e i r own s t r e s s e s , a n d , f o r i n s t a n c e , r i p p i n g apa r t the a n c i e n t c o n t i n e n t o f South Amer i ca -A f r i c a by b e i n g b o r n underneath i t . From the second p o i n t o f v i ew , we must look to the t r enches a lone f o r the d i s r u p t i v e f o r c e s which are so p r e v a l e n t i n g l o b a l geo logy . In f a c t , from the second p o i n t o f view o c e a n i c r i d g e s l e s s e n e x t e n s i o n a l s t r a i n s p r e s e n t i n l i t h o s p h e r i c p l a t e s , a l l o w i n g e x t e n s i o n o f the e a r t h ' s s u r f a c e t o o c c u r . I f swarms are v o l c a n i c e v e n t s , o r a t l e a s t t r i g g e r e d by v o l c a n i c a c t i v i t y , then i t would be n a t u r a l to expect ear thquake swarms to o c c u r a long the ocean i c r i d g e s y s t em, s i n c e r i d g e 18 c r e s t s are zones o f d i a p i r i c i n t r u s i o n s o f magma and w idespread v u l c a n i s m . Most ear thquake swarms, as d i s t i n c t from fo r e shock -a f t e r s h o c k sequences , l a c k a "ma jo r " e v e n t ; magnitudes are g e n e r a l l y s m a l l - l e s s than 5. Such a l a c k of a main event i s commonly a s c r i b e d as a c h a r a c t e r i s t i c o f v o l c a n i c swarms. I t may a l s o be noted tha t f o r ear thquakes o c c u r r i n g on the o cean i c r i d g e s y s t e m , the l a r g e r events tend to be from the f r a c t u r e zones r a t h e r than the r i d g e c r e s t s . T h i s suggests t h a t the f r a c t u r e zones have g r e a t e r s t r e n g t h than the c r e s t s , but a l s o may be i n p a r t due to the d i f f e r e n t s t r e s s p a t t e r n s deve loped i n the two a r e a s . T rans fo rm f a u l t s are zones o f r e l e a s e o f shear s t r a i n , d e v e l o p e d , p e r h a p s , by f r i c t i o n a l f o r c e s as two l i t h o -s p h e r i c p l a t e s move pas t each o t h e r , wh i l e the r i d g e c r e s t s are zones of r e l e a s e o f e x t e n s i o n a l s t r a i n by normal f a u l t i n g and g e n e r a t i o n o f new c r u s t a l m a t e r i a l . I t may be t ha t normal o c e a n i c c r u s t a l m a t e r i a l can r e s i s t r e l a t i v e l y l a r g e r s h e a r i n g s t r a i n s w i thou t r up tu r e than d i l a t i o n a l s t r a i n s . 2.2 The No r theas t P a c i f i c Ocean The i n t e r a c t i o n s o f the P a c i f i c Ocean l i t h o s p h e r i c b l o c k and the Nor th Amer ican c o n t i n e n t l i t h o s p h e r i c b l o c k as they move r e l a t i v e to each o the r were u l t i m a t e l y r e s p o n s i b l e f o r the ear thquake swarm s t u d i e d h e r e , so i t w i l l be wise to have a good i d e a o f the o v e r a l l t e c t o n i c framework of the a r e a . The c o n t i n e n t a l margins o f the P a c i f i c Ocean, i n a l l cases except one, , are marked by the development o f a c t i v e t r ench-type f e a t u r e s . The i s l a n d a rcs and t r enches o f the western 19 P a c i f i c ( K u r i l e , B o n i n , P h i l l i p i n e , M a r i a n a s , Tonga , and Kermadec t r enches ) are by any c r i t e r i o n the most t e c t o n i c a l l y a c t i v e f e a t u r e s on the s u r f a c e o f the e a r t h ; they have the g r e a t e s t ocean d e p t h s , the h i g h e s t s e i s m i c i t y , the deepest e a r t h q u a k e s , h i g h heat f low and e x t e n s i v e v u l c a n i s m , and l a rge n e g a t i v e f r e e a i r g r a v i t y a n o m a l i e s . The development o f t r enches i s a l s o seen o f f the coas t s o f J apan , the A l e u t i a n I s l and c h a i n , and South and C e n t r a l Ame r i c a . The s o l e e x c e p t i o n to t h i s p a t t e r n i s the western coas t o f Nor th A m e r i c a , from the G u l f o f Mexico to s o u t h - c e n t r a l A l a s k a , where the u s u a l t r e n c h -type topography i s r e p l a c e d by a s e r i e s o f r i d g e c r e s t s and t r a n s f o r m f a u l t s . These f e a t u r e s were f i r s t r e c o g n i z e d i n terms o f the s e i s m i c i t y o f the a r e a ; about o n e - t h i r d o f the s e i s m i c a c t i v i t y i n or nea r the wes te rn U n i t e d S t a t es ( e x c l u s i v e o f A l a ska ) o c cu r s o f f the coas t s o f n o r t h e r n C a l i f o r n i a and Oregon ( F i gu res 2.2-1 and 2 .2-2 ) . The d i s t r i b u t i o n o f ear thquake e p i c e n t e r s d e l i n e a t e s the s t r i k e of the t e c t o n i c zones . F o c a l mechanism s o l u t i o n s g i ve some h i n t as to the na tu re o f the zones . The c o n c l u s i o n s drawn from t h i s l i n e o f i n v e s t i g a t i o n have l a t e l y been con f i rmed by the i n c r e a s e d knowledge o f the bathymetry of the a r e a , and by the s tudy o f the magnet ic anomal ies a s s o c i a t e d w i t h the r i d g e c r e s t s . Two papers , by T o b i n and Sykes ( S e i s m i c i t y and T e c t o n i c s o f the No r theas t P a c i f i c Ocean , 1968 ) , and B o l t , Lomni tz and M c E v i l l y , ( S e i s m o l o g i c a l Ev idence on the T e c t o n i c s o f C e n t r a l and No r the rn C a l i f o r n i a and the Mendocino Esca rpment , 1968 ) , 20 120° 115' F I G U R E 2.2-1: S E I S M I C I T Y F O R N O R T H E A S T P A C I F I C O C E A N AND W E S T E R N N O R T H A M E R I C A N O R T H O F 40°N FOR T H E P E R I O D 195^-1963. ( F R O M T O B I N AND S Y K E S 1967) 21 FIGURE 2.2-2: FAULT PLANE SOLUTIONS, MAJOR FAULTS AND OCEAN BOTTOM TOPOGRAPHY FOR NORTHERN COASTAL CALIFORNIA AND OFFSHORE. (FROM BOLT, LOMNITZ AND MCEVILLY 1968) 22 d i s c u s s the s e i s m o l o g i c a l ev idence a v a i l a b l e . W i l son (T rans fo rm F a u l t s , Ocean i c R idges and Magnet ic Anomal ies Southwest o f Vancouver I s l a n d , 1965b) p roposed the e x i s t e n c e o f th ree r i d g e c r e s t s and accompanying magnet ic anomal ies i n the a r ea as r e p o r t e d by R a f f and Mason (Magnet ic Survey o f f the West Coast o f North Amer i ca 40 °N L a t i t u d e to 50°N L a t i t u d e , 1961) and McManus (Phys iography o f the Cobb and Gorda R i s e s , No r theas t P a c i f i c Ocean , 1967) has i d e n t i f i e d the r i d g e c r e s t s and f r a c t u r e zones i n terms of topography . The c o n c l u s i o n s reached from these i n v e s t i g a t i o n s sugges ts t ha t a system o f o c e a n i c r i s e s and t r ans fo rm f a u l t s i s o p e r a t i n g o f f the western coas t o f Nor th A m e r i c a . The move-ment on the system ( i n c l u d i n g the San Andreas F a u l t System of south and c e n t r a l C a l i f o r n i a ) i s c o n s i s t e n t l y r i g h t - l a t e r a l -t h a t i s , the P a c i f i c Ocean b a s i n i s moving nor thwest w i th r e s p e c t to the Nor th Amer ican c o n t i n e n t . New o c e a n i c c r u s t i s b e i n g gene ra t ed a long the Cobb and Gorda R i ses by d i a p i r i c i n t r u s i o n s and d e s t r o y e d a long the A l e u t i a n T rench by u n d e r t h r u s t i n g . There i s some doubt about the r o l e p l a y e d i n t h i s p r o c e s s by the f e a t u r e named the E x p l o r e r T r e n c h . I t was i n t e r p r e t e d by W i l son (1965b) as a r i d g e c r e s t i n terms o f the a s s o c i a t e d magnet i c a n o m a l i e s , and i d e n t i f i e d by McManus (1967) as " p r o b a b l y c r e s t a l " i n terms o f t opography , but has not had any a s s o c i a t e d s e i s m i c i t y - at l e a s t f o r the p e r i o d from 1954 through 1963 (Tob in and Sykes , 1968 ) ( F igu res 2.2-3 and 2 .2-4 ) . The l a ck o f s e i s m i c i t y does not suggest t h a t the E x p l o r e r T rench i s not 23 F I G U R E 2 . 2 - 3 : S U P E R P O S E D ON R A F F AND MASON'S SUMMARY D I A G R A M O F T H E M A G N E T I C A N O M A L I E S I N T H E N O R T H E A S T P A C I F I C O C E A N A R E ARROWS W H I C H I N D I C A T E T H E A X E S O F 3 S H O R T R I D G E L E N G T H S AND S T R A I G H T L I N E S W H I C H I N D I C A T E F A U L T S O F F - S E T T I N G T H E A N O M A L Y P A T T E R N . ( A F T E R V I N E AND W I L S O N 1966) 24 %1 5TIRN1 / $ * r SEAVQONT }(/,'••'••"' \'; J PiiJL REVERE ! -OUATSINO t, CANTON .CUC'JKIUSH C-j CANYON 6^^ , _ . £ ^  CRCWTHEB A •«AYS CANYON •^SUIOE CANTON WASHINGTON "'WILL A PA CANTON •-STGNEWALL BANK A^ TCK ' DEPTH IN FATHOMS 0 SEAMCUNT, UNJTATEO DEPTH CCWPILFD BY D. A. V; MAhUS tEPAhTWEST C 7 CCEAr.OGRSPMT ="\. \ "ToGE " UNIVERSITY C* WASHt«iGT0H,' -.OI.PMOEHJX f|lCG£ ' i a s i J ^ i Jl€HJ-' . ""ESCANABA RiDGE ESCANASA TROUGH CALIFORNIA ' TRINIDAD CANTON ' EEL CANYON BEAR VALLEf ''MENDOCINO CANYON \VENOOClMO "fOG€ ( I J K3 F I G U R E 2.2-H: B O T T O M T O P O G R A P H Y F O R N O R T H E A S T P A C I F I C O C E A N F R O M MCMANUS 1 9 6 7 . S O U R C E A R E A F O R E A R T H Q U A K E SWARM O U T L I N E D I N U P P E R L E F T - H A N D C O R N E R 25 a c t i v e l y s p r e a d i n g . In the same p e r i o d o f t ime o n l y one e a r t h -quake was l o c a t e d on the c r e s t o f the Cobb R i s e , away from the o f f - s e t t i n g f r a c t u r e zones . Most o f the c r e s t o f the Eas t P a c i f i c R i se shows a s i m i l a r tendency f o r s e i s m i c i t y to be low a long the r i d g e c r e s t i t s e l f but to concen t r a t e at the o f f - s e t o f the c r e s t by a f r a c t u r e zone . In f a c t , from the p o i n t o f view o f "New G l o b a l T e c t o n i c s " , l a ck o f s e i s m i c i t y (on r i d g e c r e s t s ) sugges t s l a ck o f s t r e n g t h (or t h i n l i t h o s p h e r e ) which sugges ts h i g h s p r e a d i n g r a t e s . On the o the r hand , we l l -deve l oped c r e s t a l topography o f the E x p l o r e r T rench would suggest c r u s t a l s t r e n g t h and low sp read ing r a t e s . The occu r r ence of an ear thquake swarm i n the a rea of the i n t e r s e c t i o n o f the E x p l o r e r T rench w i th the Queen C h a r l o t t e I s l a n d F r a c t u r e Zone r a i s e s the q u e s t i o n o f the na tu re of the E x p l o r e r T r e n c h . There are th ree ways to e x p l a i n the g e n e r a l l a c k o f s e i s m i c a c t i v i t y on the E x p l o r e r T r e n c h . F i r s t ; and most o b v i o u s , i t i s , i n d e e d , an a s e i s m i c non-ac t i v e f e a t u r e , w h i c h , though p o s s i b l y a c t i v e at one t i m e , has ceased s p r e a d i n g ( s t r a i n be ing r e l e a s e d by o the r areas o f the system - n o t a b l y the sou the rn e x t e n s i o n of the Queen C h a r l o t t e I s l a n d F r a c t u r e Zone to the Cobb R i s e ) . Second , s e i s m i c i t y i s not s t a t i o n a r y i n t i m e , a t l e a s t over a ten yea r p e r i o d . T h i r d , s e i s m i c a c t i v i t y has taken p l a c e but has not been a s s o c i a t e d w i t h the E x p l o r e r T r e n c h , e i t h e r because i t was too weak to be r e c o r d e d , or was r e c o r d e d , but i n a c c u r a t e l y l o c a t e d away from the f e a t u r e . The p r e s e n t p a t t e r n of t e c t o n i c e lements i n the n o r t h -26 eas t P a c i f i c Ocean i s r e l a t i v e l y r e c e n t . McManus (1967) a s s i gns Miocene Age (25 m i l l i o n yea rs ) to the Cobb and Gorda R i s e s . T e r t i a r y Ages (10 - 70 m i l l i o n yea rs ) are commonly a s c r i b e d to the Eas t P a c i f i c R i se (Menard, 1964) . The G u l f o f C a l i f o r n i a i s thought to have deve loped i n the pas t 25 m i l l i o n years by move-ment o f the Ba ja C a l i f o r n i a P e n i n s u l a away from c e n t r a l Mexico ( I s a c k s , O l i v e r and Sykes , 1968) . T h i s movement has r e s u l t e d i n (or was the p r o d u c t o f ) some 350 m i l e s of r i g h t - l a t e r a l s t r i k e -s l i p d i sp l a cemen t on the San Andreas F a u l t Sys tem, i n the pas t 100 m i l l i o n yea r s (Ham i l t on , 1961) . A P r e-Cenozo i c p a t t e r n o f ocean f l o o r s p r e a d i n g i s r e p r e s e n t e d by the s e r i e s o f l o n g , p a r a l l e l f r a c t u r e zones t r e n d i n g rough l y east-west a c ross the P a c i f i c - the Mendoc ino , Mur ray , C l a r i o n , C l i p p e r t o n and o ther f r a c t u r e zones . These f e a t u r e s at p r e sen t are g e n e r a l l y a s e i s m i c a l ong most of t h e i r l e n g t h , except where they i n t e r s e c t p o r t i o n s of an a c t i v e r i s e sys tem. Of p a r t i c u l a r i n t e r e s t i s the eastern-most p o r t i o n o f the Mendocino F r a c t u r e Zone where i t approaches the C a l i f o r n i a c o a s t and ac t s as the sou the rn terminus of the Gorda R i s e . The s e i s m i c a c t i v i t y on the Mendocino Escarpment between the Gorda R i se and the C a l i f o r n i a coas t i s i n t e r p r e t e d (Tob in and Sykes , 1968) as an e x t e n s i o n o f a c t i v i t y on the San Andreas F a u l t Sys tem, which passes out t o sea near Cape Mendoc ino . Mot ion o f the P a c i f i c b a s i n nor thwest w i th r e s p e c t to Nor th Amer ica ( r i g h t - l a t e r a l mot ion on the San And reas , B lanco and Queen C h a r l o t t e I s l a n d F a u l t Zones and e x t e n s i o n a c ross the Eas t 27 P a c i f i c , Go rda , and Cobb R i ses ) cannot be accomp l i shed w i thou t some s o r t o f normal p r e s s u r e . a c r o s s the Mendocino F r a c t u r e Zone . The r e s u l t o f t h i s p r e s s u r e would b e , m e c h a n i c a l l y a c l o ckw i s e r o t a t i o n o f the Gorda B a s i n . A second r e s u l t o f t h i s f o r c e would be a " s h e a r i n g o f f " o f the Gorda B a s i n by an e x t e n s i o n o f the San Andreas F a u l t System nor thwest a c ross the b a s i n to j o i n w i t h the B l anco F r a c t u r e Zone ( B o l t , Lomnitz and M c E v i l l y , 1968) . T h i s may have been the h i s t o r y o f the area between the E x p l o r e r T r ench and the n o r t h e r n end o f Vancouver I s l a n d . I f the development o f r i s e s i n the T e r t i a r y was c o m p l i c a t e d by o l d e r zones o f weakness i n the o c e a n i c c r u s t so t ha t the system ( i n c l u d i n g the E x p l o r e r Trench) was u n s t a b l e w i t h r e s p e c t to the new d i r e c t i o n o f s p r e a d i n g , a p r o g r e s s i v e s i m p l i f i c a t i o n of the system would ensue . The e x t e n s i o n o f the Queen C h a r l o t t e I s l a n d F a u l t southward to the Cobb R ise would r e p r e s e n t one a s p e c t o f t h i s c o n t i n u i n g p r o c e s s . The b a s i n eas t of the E x p l o r e r T r ench would be sheared o f f and the E x p l o r e r T rench i t s e l f cease to be an a c t i v e r i d g e c r e s t . The j o i n i n g o f the Queen C h a r l o t t e I s l a n d F r a c t u r e Zone to the San Andreas F r a c t u r e Zone would r e s u l t i n a g e o m e t r i c a l l y s imp le t e c t o n i c system - a r i d g e - a r c t r a n s f o r m f a u l t ex t end ing from the Eas t P a c i f i c R i se i n the south to the A l e u t i a n T rench i n the n o r t h . The Cobb and Gorda R i s e s would then a l s o be e x t i n c t , and would move nor thwest w i th r e s p e c t to the Nor th Amer ican c o n t i n e n t . Such a p rocess may e x p l a i n the p r e s e n t l a ck o f a c t i v i t y on the E x p l o r e r T r e n c h . 28 2.3 Western B r i t i s h Columbia and Vancouver I s l and The p r e s e n t day t e c t o n i c s i n the n o r t h e a s t P a c i f i c exp re s sed i n the p a t t e r n o f r i d g e s and t r ans fo rm f a u l t s i s one o f e x t e n s i o n a long a n o r t h e a s t t r end and wrench f a u l t i n g a long a nor thwest t r e n d . L o c a l v a r i a t i o n s i n t h i s p a t t e r n e x i s t -s e i s m i c a c t i v i t y a long an east-west t r end on a p o r t i o n o f the Mendocino Scarp and a zone o f compress ion a long the Oregon Washington Coast - but these can be e x p l a i n e d as a consequence of the geometry o f the con t a c t o f the P a c i f i c Bas in B lock and the Nor th Amer ican C o n t i n e n t a l B l o c k . The p r o x i m i t y ' o f the Queen C h a r l o t t e I s l ands F r a c t u r e Zone to the coas t o f B r i t i s h Co lumbia sugges ts t h a t the i n f l u e n c e o f r i g h t - l a t e r a l movement s h o u l d be apparent i n the r e cen t g e o l o g i c a l h i s t o r y o f the r e g i o n . The g e o l o g i c h i s t o r y o f the i n s u l a r b e l t o f B r i t i s h Co lumbia ( Su ther land-Brown, 1968 ) , i n c l u d i n g Vancouver I s l a n d and the Queen C h a r l o t t e I s l a n d s , a l t e r n a t e s p e r i o d s o f d e p o s i t i o n w i t h p e r i o d s o f v u l c a n i s m and p l u t o n i s m . E s p e c i a l l y s t r o n g p l u t o n i s m o c c u r r e d du r i ng the Mid and Late J u r a s s i c , the E a r l y C re taceous and the Late T e r t i a r y . To the eas t o f t h i s b e l t l i e s the Canadian p o r t i o n o f the Nor th Amer ican C o r d i l l e r a which has undergone a long and c o m p l i c a t e d h i s t o r y of c o m p r e s s i o n , f o l d i n g , t h r u s t f a u l t i n g and e x t e n s i v e metamorphism. T h i s i s thought to be a p r o d u c t o f a mesozo i c t r e n c h system o p e r a t i n g i n what i s now the wes te rn p o r t i o n o f Nor th A m e r i c a . The Mesozo ic phys iog raphy o f t h i s a rea may have resembled the p r e s e n t day coas t o f Southeas t A s i a w i th i r r e g u l a r , deep ly embayed 29 c o n t i n e n t a l c r u s t o f low r e l i e f w i th an o f f s h o r e i s l a n d a r c system (Whi te , 1968) . The c o a s t l i n e became emergent d u r i n g the Late J u r a s s i c ( rough l y 150 m i l l i o n yea r s ago ) . The p r e s e n t day p a t t e r n of t e c t o n i c s dates from rough l y the E a r l y T e r t i a r y (McManus, 1968) . T h i s p e r i o d w i t nes sed f o l d i n g and wrench f a u l t i n g on Vancouver I s l a n d w i th e x t e n s i v e v u l c a n i s m on the sou the rn t i p o f the i s l a n d . Intense de fo rma t i on o c c u r r e d i n the Olympic Mountains o f nor thwest Wash ington . On the Queen C h a r l o t t e I s l ands s u b a r e a l e r u p t i o n s o c c u r r e d , l a r g e f a u l t s t r e n d i n g nor thwest became a c t i v e w i t h the common c h a r a c t e r i s t i c s o f r i g h t - l a t e r a l wrench and normal eas t b l o c k down mot ion ( Su ther l and-Brown, 1968) . These f a u l t s show the ' i i n f l u e n c e o f the nearby Queen C h a r l o t t e I s l ands F r a c t u r e Zone \ which deve loped contemporaneous ly ( F igu re 2 .3-1 ) . \ The c o r r e l a t i o n o f t h i s f e a t u r e w i th the na tu re o f the c o a s t l i n e i s i m p r e s s i v e . From the A l a s k a n Panhandle to the S t r a i g h t s o f Juan de Fuca where the f a u l t zone l i e s immedia te l y o f f s h o r e the c o a s t l i n e shows the development o f f j o r d i c t o p o -graphy and o f f s h o r e i s l a n d s . At the l a t i t u d e o f Juan de Fuca where the wrench mot ion on the f a u l t i s r e p l a c e d by e x t e n s i o n a c ross the Cobb R ise the coas t undergoes a t r a n s i t i o n to a con t i nuous l i n e p a r a l l e l i n g the Cobb R ise to the west . The na tu re o f the c o a s t l i n e i n each case r e f l e c t s the p r e v a i l i n g t e c t o n i c env i ronment . i3r O I X O N E N T R A N C E ! 2>C-L ' \ ^ r p ^ ? 53-52-I S L A U D S V ' ' C . . \ M , o \ \ \ % P o s t - T e c t o n i c P l u t o n s o r e c h i e f l y \'^XgbVof T e r , i o r ) ' °ie c3 O o L E G E N D T e r t i o r y p l u t o n s o n V a n c o u v e r I s l and P o s t - T e c t o n i c p l u ' o n s S y n t e c t o n i c p l u t o n s . c h i e f l y o f J u r a s s i c a g e P l u t o n s o f u n k n o w n t y p e a n d a g e V/////A F o u l t s K - A o g e in m i l l i on y e a r s o SOUND O n V a n c o u v e r I s l o n d P o s t - T e c t o n i c P l u t o n s a r e P r e - N a n a i m o G r o u p FIGURE 3.3-1: TERTIARY TECTONICS ON THE INSULAR BELT OF BRITISH COLUMBIA SHOWING MAJOR FAULTS AND PLUTONS. CFROM A. SUTHERLAND-BROWN 1966) Stole Mi'es I 126* 5 a n Juon 31 3. INTERPRETATION OF ARRIVAL TIMES The P and S a r r i v a l s were r e co rded w i t h i n r e g i o n a l d i s t a n c e s by the se ismographs o f the Dominion Obse r va to r y of Canada, r e g i o n s where the t r a v e l t imes o f s e i s m i c events are h i g h l y dependent on the l o c a l g e o l o g y . An attempt has been made to e s t ima t e e p i c e n t e r s f o r r e co rded a r r i v a l s which takes t h i s f a c t and the o c e a n i c o r i g i n of the events i n t o a c c o u n t . These e s t i -mates are independent of the USCGS e s t i m a t e s ; they use r e c o r d s which were not a v a i l a b l e to the USCGS and c o n s i d e r a d i f f e r e n t model o f the e a r t h . 3.1 E s t i m a t i n g E p i c e n t e r s An " e v e n t " r e f e r s to an a r ray o f 5 a r r i v a l t imes i n seconds gene ra ted by a s i n g l e ear thquake at the 5 s t a t i o n s u s e d . Eve ry event i n the swarm showed the same sequence o f a r r i v a l t imes from PHC, ALB , V IC , FSJ and PNT. The a r r i v a l t imes o f events are a r ranged i n ascend ing o rde r and r e f e r r e d to a t ime a x i s w i t h the Pn a r r i v a l at PHC taken as o r i g i n . The o r i g i n t ime o f an event i s then a n e g a t i v e number r e p r e s e n t i n g the t ime taken by the Pn energy to t r a v e l from the e p i c e n t e r to PHC. The e p i c e n t e r o f an event i s an a r r a y of 4 numbers; l o n g i t u d e , l a t i t u d e i n d e g r e e s , depth i n km and o r i g i n time i n sec which l o c a t e the ear thquake i n space and t i m e . An e v e n t , - T , can be v iewed as a f u n c t i o n o f the e p i c e n t e r , E. I f the type o f event can be i d e n t i f i e d , whether i t i s a P o r 32 S e v e n t , a r e f e r e n c e event o f tha t type can be genera ted and a va lue o f E chosen which f i t s . t h e r e f e r e n c e event to the observed e v e n t . T h i s r e p r e s e n t s the s o l u t i o n o f the se t o f equa t i ons i n the 4 unknowns o f E. T CT(E) (3.1-1) Fo r a s o l u t i o n to e x i s t there must be at l e a s t 4 e q u a t i o n s ; the event must i n c l u d e a r r i v a l s a t 4 d i f f e r e n t s t a t i o n s . I d e a l l y a unique s o l u t i o n shou ld e x i s t , but because a random e r r o r , R, i s i n t r o d u c e d i n t o the da t a from s e v e r a l sou rces (Freedman, 1968) , e p i c e n t r a l e s t ima tes can on l y be quoted t o a c e r t a i n degree o f c o n f i d e n c e . The e s t i m a t i o n of e p i c e n t e r s i s d i s c u s s e d by T u c k e r , H e r r i n , and Freedman (1968) . For t h i s s tudy the r e f e r e n c e event was gene ra t ed i n terms o f a p l a u s i b l e g e o l o g i c a l model assuming r e f r a c t e d a r r i v a l a long a con t inuous upper mantle w i th cons t an t v e l o c i t i e s o f p r o p a g a t i o n . A r r i v a l t imes (and d e r i v a t i v e s ) were c a l c u l a t e d d i r e c t l y i n terms o f t h i s model as a f u n c t i o n o f the e p i c e n t e r . Wi th the i n t r o d u c t i o n o f the random e r r o r v e c t o r the se t o f equa t i ons becomes T = CT(E) + R (3.1-2) We assume the d i s t r i b u t i o n o f R to be unbounded, cen te red at zero and independent of the i n d i v i d u a l s t a t i o n s i n v o l v e d . These assumpt ions are v a l i d on l y i f w e l l d e f i n e d events and good s t a t i o n s are used and depend on the method used t o c o l l e c t the a r r i v a l t i m e s . . Freedman (1968) sugges ts t h a t e r r o r s i n v o l v e d i n r ead ing 33 the a r r i v a l t imes o f P phases on s t anda rd ins t ruments a r i s i n g f r om : (1) M i scounts - m u l t i p l e s o f s e conds , m inu t e s , hours (2) M i s i d e n t i f i c a t i o n s - m i s s i ng the f i r s t a r r i v a l o f the phase (3) I n s t rumen ta l e r r o r s - v a r i a t i o n s o f paper speed , c l o c k e r r o r s , v a r i a t i o n s i n i ns t rument response (4) Reading e r r o r s - r e s i d u a l e r r o r tha t would-remain even though a l l o t he r e f f e c t s were e l i m i n a t e d can be approx imated w e l l by a m ix tu re of normal d i s t r i b u t i o n s . Fo r t h i s s tudy the g roup ing o f e p i c e n t e r s i n a sma l l a rea and the c o n s i d e r a t i o n o f the s t a t i o n s as an a r r ay l a r g e l y e l i m i n a t e s e r r o r (1 ) . P events were u s u a l l y w e l l d e f i n e d , though o f t e n not sharp and though S events were commonly sharp they had the added d i f f i c u l t y t h a t the t r a c e was a l r e ady i n m o t i o n . No ise was a p rob lem at 2 s t a t i o n s , VIC and PNT, and a r r i v a l t imes show a g r e a t e r s c a t t e r at these s t a t i o n s . The 19 P events and.8 S events (Tab le 3.1-1) chosen f o r s tudy were d i v i d e d apriori i n t o two c l a s s e s on the b a s i s o f q u a l i t y . , 8 good and 11 f a i r . E r r o r (2) s h o u l d be e l i m i n a t e d f o r the 8 good P events but may be a f a c t o r i n the rema in ing 11 P and 8 S e v e n t s . E r r o r (3) s h o u l d be reduced to a minimum by the use o f s t a n d a r d , h i g h q u a l i t y i n s t r u -ments . The f i n a l f a c t o r , e r r o r ( 4 ) , i s ha rde r to e v a l u a t e . The r e c o r d s were read th ree t imes over a p e r i o d o f three months by the same i n d i v i d u a l . The output from the se i smographs , paper-c h a r t s s c a l e d one mm/sec, i s r a t h e r u n f o r t u n a t e as i t makes r e t r i e v a l of the da ta d i f f i c u l t ( F i gu re 3 .1-1 ) . The 8 good P and S events were p i c k e d by copy ing the r e c o r d s , c u t t i n g out the T A B L E 3 . 1 - 1 P AND S E V E N T S E V E N T D A T E P A R R I V A L A R R I V A L R E F E R R E D TO PHC P E V E N T S S E V E N T S IMBER M/D/Y H/M/S PHC A L B V I C F S J P N T 2 0 8 / 2 7 / 6 7 1 2 / 5 7 / 0 2 . 2 0. 0 2 6 . 2 4 3 . 2 5 4 . 0 7 1 . 8 4 0 8 / 2 7 / 6 7 1 3 / 3 5 / 1 9 . 9 0. 0 2 6 . 2 4 2 . 2 5 4 . 2 7 3 . 4 7 0 8 / 2 7 / 6 7 1 8 / 2 9 / 3 2 . 5 0. 0 2 5 . 2 4 1 . 4 5 4 . 6 7 2 . 8 13 0 8 / 2 8 / 6 7 1 2 / 3 9 / U 2 . 0 0. 0 2 5 . 0 4 1 . 6 5 4 . 4 7 1 . 6 1 7 0 8 / 2 8 / 6 7 1 3 / 5 0 / 1 0 . 0 0, 0 2 5 . 4 4 5 . 0 5 3 . 2 7 5 . 2 1 9 0 8 / 2 8/6 7 1 5 / 0 7 / 3 6 . 0 0. 0 2 2 . 8 4 0 . 8 5 4 . 2 7 0 . 0 20 0 8 / 2 8 / 6 7 1 5 / 2 6 / 1 7 . 0 0. 0 2 5 . 2 4 2 . 2 5 4 . 0 7 3 . 2 21 0 8 / 2 8 / 6 7 1 6 / 2 0 / 3 1 . 5 0. 0 2 6 . 6 4 4 . 0 5 3 . 8 7 4 . 8 8 9 S T A N D A R D P E V E N T 0. 0 2 5 . 3 4 2 . 5 5 4 . 0 7 2 . 7 1 0 8 / 2 7 / 6 7 0 8 / 3 5 / 0 8 0. "0 2 6 . 0 4 2 . 0 5 6 . 0 6 2 . 0 6 0 8 / 2 7 / 6 7 1 7 / 4 0 / 4 1 0. 0 2 6 . 0 4 1 . 0 5 4 . 0 7 1 . 0 10 0 8 / 2 8 / 6 7 1 1 / 3 2 / 3 3 0. 0 2 7 . 0 4 2 . 0 5 5 . 0 7 6 . 0 1 1 0 8 / 2 8 / 6 7 1 1 / 4 1 / 4 0 0. 0 2 4 . 0 3 5 . 0 5 4 . 0 7 3 . 0 12 0 8 / 2 8 / 6 7 1 2 / 2 7 / 1 9 0. 0 2 8 . 0 4 2 . 0 5 4 . 0 7 8 . 0 14 0 8 / 2 7 / 6 7 1 2 / 4 4 / 1 9 0. 0 2 3 . 0 4 5 . 0 5 4 . 0 7 3 . 0 18 0 8 / 2 8 / 6 7 1 4 / 4 4 / 0 3 0. 0 2 9 . 0 4 7 . 0 5 4 . 0 7 8 . 0 22 0 8 / 2 8 / 6 7 1 7 / 2 2 / 2 1 0. 0 2 7 . 0 4 8 . 0 5 5 . 0 7 2 . 0 2h 0 8 / 2 8 / 6 7 1 9 / 0 4 / 5 2 0. 0 2 5 . 0 3 5 . 0 5 3 . 0 7 5 . 0 32 0 8 / 2 9 / 6 7 1 1 / 4 5 / 5 8 0. 0 2 7 . 0 4 3 . 0 5 5 . 0 7 8 . 0 3 3 0 8 / 2 8 / 6 7 1 2 / 2 3 / 3 3 0. 0 2 9 . 0 4 6 . 0 5 5 . 0 7 2 . 0 2 8 / 2 7 / 6 7 1 2 / 5 7 / 0 2 . 2 2 2 . 8 6 9 . 8 9 7 . 8 1 1 3 . 4 1 5 0 . 4 k 8 / 2 7 / 6 7 1 3 / 3 5 / 1 9 . 9 2 2 . 0 6 7 . 4 9 8 . 0 1 1 4 . 0 1 4 9 . 4 7 8 / 2 7 / 6 7 1 8 / 2 9 / 3 2 . 5 2 2 . 2 6 6 . 2 8 9 . 6 1 1 5 . 4 1 4 5 . 0 1 3 8 / 2 8 / 6 7 1 2 / 3 9 / 4 2 . 0 2 2 . 4 6 8 . 0 9 5 . 6 1 1 7 . 2 1 4 6 . 4 1 7 8 / 2 8 / 6 7 1 3 / 5 0 / 1 0 . 0 2 1 . 6 6 7 . 8 9 5 . 6 1 1 6 . 0 . 1 4 9 . 0 1 9 8 / 2 8 / 6 7 1 5 / 0 7 / 3 6 . 0 2 2 . 8 6 4 . 6 9 5 . 2 1 1 6 . 4 1 4 9 . 0 20 8 / 2 8 / 6 7 1 5 / 2 6 / 1 7 . 0 2 2 . 0 6 6 . 8 9 8 . 4 1 1 6 . 4 1 4 8 . 4 2 1 8 / 2 8 / 6 7 1 6 / 2 0 / 3 1 . 5 2 2 . 0 6 9 . 4 9 8 . 2 1 1 8 . 4 1 5 4 . 4 8 9 S T A N D A R D S E V E N T 2 2 . 4 6 7 . 7 9 6 . 2 1 1 6 . 1 1 4 9 . 2 35 F I G U R E 3.1-1: P O R T I O N S O F T H E P H C V E R T I C A L - C O M P O N E N T S E I S M O G R A M F O R A U G U S T 28, 1967. T I M E MARKS A R E A T O N E M I N U T E I N T E R V A L S . N O T E O P P O S I T E P O L A R I T Y FOR F I R S T B R E A K S ON TWO L A R G E S T E V E N T S ( E V E N T S 20 AND 21) 36 t r a c e s and a l i g n i n g them toge the r f o r each s t a t i o n , so t ha t the same a r r i v a l would be c o n s i s t e n t l y p i c k e d f o r each e v e n t . The r ema in ing 11 P events are somewhat l e s s c o n s i s t e n t . I t does not seem u n r e a s o n a b l e , t h e n , to assume a normal d i s t r i b u t i o n f o r R, 2 (R~N(0,a I ) ) , at l e a s t f o r the 8 good e v e n t s . The c r i t e r i o n chosen f o r bes t f i t i s the l e a s t squares p r i n c i p l e . Wi th the assumpt ion o f n o r m a l i t y on R, e s t ima te s d e r i v e d by t h i s method are e q u i v a l e n t to maximum l i k e l i h o o d e s t i m a t e s . We choose E = E so t ha t the p roduc t R'R = (T-CT) ' (T-CT) = minimum (3.1-3) where R' i s the t r anspose o f R. Thus | ^ (R'R) |g = CR'-I^R) |g = 0 (3.1-4) o r [ ( T - C T ) ' | E C T ] £ = Q (3.1-5) CT i s a q u a d r a t i c f u n c t i o n o f E^ and E^ ( l o n g i t u d e and l a t i t u d e ) . For a s i n g l e l a y e r c r u s t o f t h i c k n e s s H, assuming r e f r a c t e d a r r i v a l s , CT has the form CI\ = E 4 + ( 2 H - E 3 ) c o s e 0 / V 0 + A/Vi (3.1-6) where E^ < H and A = / C 1 2 ( E 1 - S 1 ) 2 + C 2 2 ( E 2 - S 2 ) 2 V Q = v e l o c i t y i n c r u s t km/sec = v e l o c i t y i n basement km/sec S = ( l o n g i t u d e , l a t i t u d e ) o f s t a t i o n Cj, = n o . o f km/degree l o n g i t u d e C2 = no . o f km/degree l a t i t u d e 6 0 = s i n - ^ / V ^ 37 T h i s i s j u s t the s t anda rd form f o r r e f r a c t e d a r r i v a l s ; i t i s assumed tha t f o r the d i s t a n c e s encounte red i n t h i s s tudy a f l a t e a r t h app rox ima t i on i s accu ra te enough. E q u a t i o n 3.1-5 r e p r e s e n t s a se t o f 4 n o n - l i n e a r equa t ions 1 A i n the 4 unknowns o f E. To s o l v e t h i s se t we use the t echn ique o f Gauss-Newton R e d u c t i o n . A l i n e a r approx imat ion to CT , r e p r e -sen ted by C T * , i s taken by expanding CT i n a power s e r i e s about some s u i t a b l e p o i n t E Q . Thus CT*(E) = C T ( E o ) + | ^ C T | g (E-E )• (3.1-7) o i s a good app rox ima t i on on l y i f (E-E Q ) i s s m a l l . The g^rCTlg i s a 4 x 5 m a t r i x w i th e lements o a T J j ^ l VE^2 tE^Z JEx Li4 WX LLS 9 P T l i - II II II ^ 2 1 ^ 3 1 3 P T " - <» " " ^ 4 1 e v a l u a t e d at the p o i n t E Q . I t i s r e q u i r e d tha t the f u n c t i o n s -rir CT. be con t inuous f u n c t i o n s , ^ k J S u b s t i t u t i n g CT* f o r CT i n 3.1-5 g i v e s I E C T I E IECTIE ( g " E o ' " I ^ I E RO V-1^ O O O where R Q = T - CT (E Q ) (3.1-9) 38 To s i m p l i f y the n o t a t i o n l e t X = | w C T | ' ' (3.1-10) o E q u a t i o n 3.1-8 becomes _ ( X ' X ) ( E - E q ) = X ' R Q (3.1-11) which can be s o l v e d f o r E. The p rocess i s r epea ted u s i n g E f o r E Q u n t i l convergence to a s u f f i c i e n t accuracy i s o b t a i n e d . E k = * ( X ' X ) X « R k _ 1 (3.1-12) Where the s u b s c r i p t k i s the number o f i t e r a t i o n s . For events c o n s i d e r e d i n t h i s s tudy 3 or 4 i t e r a t i o n s were no rma l l y r e q u i r e d f o r conve rgence . A maximum of 10 i t e r a t i o n s was used i n any c a s e . I f the assumpt ion R-N(0,a 2 I ) i s v a l i d then E^ has the f o l l o w i n g p r o p e r t i e s : (1) E^ - N [ E , o z ( X ' X ) ] , where E i s the t rue e p i c e n t e r (2) An u n b i a s e d es t ima te o f o 2 i s a 2 = l R ' k . 1 ( I - X ( X ' X ) - 1 ) R k _ 1 (3) P[EJ-t / a 2 X . . < EJ* < g j+t / 0 / a 2 X . . ] = 1 - a ^ • 1 k a/2 i j k a/2 i j J th where j s u p e r s c r i p t i n d i c a t e s the j e lement o f E and i s S t u d e n t ' s " t " D i s t r i b u t i o n ( G r a y b i l l , 1961) . 3.2 Model Events A s e r i o u s c o n s i d e r a t i o n i s the form o f the r e f e r e n c e e v e n t , CT . For r e g i o n a l d i s t a n c e s s t anda rd t r a v e l time r e l a t i o n s , and hence s t a n d a r d models f o r the upper hundred km o f the e a r t h , have been sugges ted ( H e r r i n e t a l , 1968) which " s h o u l d g i ve c o r r e c t t r a v e l t imes th rough the upper mantle beneath the c e n t r a l U n i t e d S t a t e s " . . . " bu t i s not i n t ended to r e p r e s e n t the a c t u a l 39 v e l o c i t y s t r u c t u r e beneath t h i s a rea or indeed any o t h e r p a r t o f the e a r t h . " ( H e r r i n , et a l , 1968) . For t h i s mode l , t r a v e l t imes f o r Pn a r r i v a l s are T P n = 7 , 4 1 + A / 8 - 0 9 (3.2-1) where A i s measured i n km ( A < 1000 km) .Th i s r e l a t i o n i s com-p a t i b l e w i th a s imp le g e o l o g i c a l model c o n s i s t i n g o f 2 l a y e r s , 15 and 25 km t h i c k w i th P wave v e l o c i t i e s 6.0 and 6.75 km/sec , r e s t i n g on a basement w i th P wave v e l o c i t y 8.049 km/sec. ( F igu re 3.2-2(b) ) . For western B r i t i s h Co lumb ia , u s i n g 10 events from the swarm (USCGS e p i c e n t e r s ) the t r a v e l t imes f o r Pn and Sn a r r i v a l s are T p n = ( 2 .58±1 .56 ) + A / ( 7 . 6 3 ± 0 . 0 3 ) (3.2-2) T g n = (9 .61±1 .97 ) A / ( 4 . 4 6 ± 0 . 0 3 ) (3.2-3) where 135 < A < 750 km ( F i gu res 3.2-1 and 1 .1-2) . There i s a g r e a t dea l o f s c a t t e r (RMS = 4 . 0 s e c ) , but a l a r g e p a r t o f t h i s i s due to the o r i g i n t imes a s s i gned to the events by the USCGS. I f we c o n s i d e r the two events marked by t r i a n g l e s i n F i g u r e 3.2-1 the d e l a y between a r r i v a l t imes i s c o n s i s t e n t l y about 8 s e conds . The e p i c e n t e r s 4 and 21 i n F i gu re 1.1-2 as r e p o r t e d by USCGS are about 25 km a p a r t , so t ha t a p a r t o f the de l a y must be a s s i g n e d as b i a s i n the USCGS e s t i m a t e s . The f a c t t ha t the 5 a r r i v a l t imes o f an i n d i v i d u a l event show a s imp le l i n e a r r e l a t i o n to d i s t a n c e w i th a sma l l degree o f s c a t t e r sugges ts t h a t the events can be thought of i n terms o f the s imp le wave r e f r a c t e d .a long a cont inuous h o r i z o n . The r e f r a c t i n g h o r i z o n i s taken to be the M o h o r o v i c i c d i s c o n t i n u i t y -+ 4 + + FIGURE 3-2-1: TRAVEL TIMES FOR Pn AND Sn IN WESTERN BRITISH COLUMBIA USING USCGS EPICENTERS. TRIANGLES INDICATE EVENTS 4 AND 21 +A t As i 1 1 1 1 1 i — ° - ° 10.0 20 .0 30 .0 40 .0 SO.O 60 .0 70 0 EPICENTRAL DISTANCE (KM] IX10 1 ) ' so.o — I — 9 0 . 0 -1 100.0 t h i s i s the f i r s t ev idence t ha t t h i s d i s c o n t i n u i t y as c l a s s i c a l l y d e f i n e d by e x p l o s i o n se i smo lbgy e x i s t s i n the v i c i n i t y o f Vancouver I s l a n d . The apparent Pn wave v e l o c i t y sugges ted by the p r o f i l e (7 .63 km/sec) i s not an unusua l va lue f o r r eg ions o f r e cen t t e c t o n i s m and compares f a v o u r a b l y to the va lue of 7.67 km/sec found f o r the a rea o f wes te rn Washington and Oregon by D e h l i n g e r e t a l (1965) . Care shou ld be taken i n i n t e r p r e t i n g the apparent v e l o c i t y i n t h i s c a s e , because the p r o f i l e i s un reve r sed and d i p p i n g i n t e r f a c e s or t h i n n i n g l a y e r s can g r e a t l y a f f e c t i t s v a l u e . A l s o the USCGS e p i c e n t e r s were e s t ima t ed assuming normal c r u s t and so can be expec ted to be b i a s e d . Once i t was r e a l i z e d tha t the t ime o f events c o u l d be unde r s t ood as s imp le r e f r a c t e d a r r i v a l s a g e o l o g i c a l model and hence a r e f e r e n c e event c o u l d be c o n s t r u c t e d . In c o n s t r u c t i n g t h i s model the f o l l o w i n g p o i n t s were taken i n t o a c coun t : (1) The v e l o c i t y p r o f i l e s under the 5 r e c e i v i n g s t a t i o n s as deduced from independent r e f r a c t i o n s u r v e y s . (2) The t r a n s i t i o n from o c e a n i c c r u s t to c o n t i n e n t a l c r u s t . (3) The na tu re o f the o c e a n i c c r u s t i n the v i c i n i t y o f the source a r e a . T h i s was accompl i shed u s i n g a f o u r - l a y e r model w i t h v a r i a b l e depth to basement. C o n t i n e n t a l c r u s t a l p r o f i l e s were taken from Tseng (1968) , and White and Savage (1965) . Depth to basement under Vancouver I s l a n d was se t at 50 km and i n the i n t e r i o r o f B r i t i s h Columbia at 35 km. Ocean i c c r u s t was taken to be 20 km t h i c k . A t r a n s i t i o n c r u s t was i n t r o d u c e d between 42 the o c e a n i c c r u s t and the Vancouver I s l a n d c r u s t . A composi te s e c t i o n i s g i v e n i n F i g u r e 3 „ 2 - 2 ( a ) . Model events obey ing S n e l l ' s Law c o u l d be genera ted from any p o i n t i n the source a r e a . The f u n c t i o n s CT. were a l s o c a l c u l a t e d d i r e c t l y i n terms of k t h i s mode l . The parameters of the model were chosen so t ha t e p i c e n t e r s were l o c a t e d on ocean i c c r u s t , but the e p i c e n t e r s c o u l d f r e e l y move onto the t r a n s i t i o n s e c t i o n i f the a r r i v a l t imes so d i c t a t e d . The l a y e r s were c o n s i d e r e d to be everywhere homogeneous and i s o -t r o p i c and the basement v e l o c i t y taken as 7.67 km/sec w i th no at tempt made to account f o r l a t e r a l v a r i a t i o n s i n t h i s f i g u r e . As s u c h , the model r ep r e sen t s on ly an attempt t o accommodate the geometry o f wes te rn B r i t i s h Co lumb ia . The c o n s t r a i n t tha t CT(E) be a smooth f u n c t i o n i n the v i c i n i t y o f the e p i c e n t e r makes a more amb i t i ous model d i f f i c u l t , -and l a c k o f r e f r a c t i o n p r o f i l e s or o the r g e o p h y s i c a l i n f o r m a t i o n f o r the ocean i c s e c t i o n i s a s e r i o u s l i m i t a t i o n . The p resence of a major t r a n s f o r m f a u l t and ocean r i s e i n t h i s a rea sugges ts t ha t the geometry i s more c o m p l i -c a t e d , so t ha t e p i c e n t e r s l o c a t e d on t h i s model can be expec ted t o be b i a s e d . T h i s i s e x a c t l y the same c r i t i c i s m made e a r l i e r o f the USCGS e s t i m a t e s , but on a d i f f e r e n t s c a l e . I t i s hoped tha t the e f f e c t o f the r e g i o n a l b i a s has been reduced from the i n t e r p r e -t a t i o n o f the a r r i v a l t i m e s . When the l o c a t i o n p rocedure was f i r s t i n i t i a t e d the f o u r v a r i a b l e s , l a t i t u d e , l o n g i t u d e , depth and o r i g i n time were f r e e to assume any v a l u e , f o r which t r a v e l t imes would be gene ra t ed FOCAL REGION PHC A ALB VIC FSJ PNT A A A A 5.0 5.0 6.0 5.9^ 6.0 6.6 6.6 7.1 7.67 ESTIMATED P VELOCITY (KM/SEC) 6.0 5.0 v4.7 6.1 5.9 7.67 50 km i — 50 km ( b ) 6.75 8.049 F I G U R E 3 .2-2 : ( a ) C O M P O S I T E S E C T I O N FOR C R U S T O F T H E E A R T H I N W E S T E R N B R I T I S H C O L U M B I A ( b ) S T A N D A R D C R U S T A L M O D E L F OR R E G I O N A L S E I S M I C E V E N T S A F T E R H E R R I N E T A L 1968 44 to the 5 s t a t i o n s . U n f o r t u n a t e l y , the use o f on l y f i r s t a r r i v a l s made i t i m p o s s i b l e to sepa ra te e f f e c t s c o r r e s p o n d i n g to changes i n depth o f focus from changes i n o r i g i n t i m e . M a t h e m a t i c a l l y the two v a r i a b l e s were not independent and t h e i r dependence caused the i t e r a t i o n p rocedure t o blow up . In a d d i t i o n the d i v i s i o n o f the model i n t o d i s t i n c t l a y e r s meant tha t the f u n c t i o n CT was not a smooth f u n c t i o n o f d e p t h . E i t h e r depth o r o r i g i n t ime had to be removed from the i t e r a t i o n p rocedure and because o f the u n d e s i r a b l e ma themat i ca l q u a l i t i e s a s s o c i a t e d w i t h d e p t h , i t was removed and se t at 10 km f o r the d u r a t i o n . The v a l ue o f 10 km (as opposed to 33 km used by the USCGS) was chosen t o be c o n s i s t e n t w i th the sha l l ow na tu re o f a l l the s e i s m i c a c t i v i t y i n the ocean r i d g e system (ear thquakes a long the San Andreas F a u l t are seldom deeper than 20 km ( B o l t , Lomnitz and M c E v i l l y , 1968)) and because o f the s u s p i c i o n tha t the swarm might be a v o l c a n i c e v e n t . T h i s r e p r e s e n t s an impor tan t s t ep i n the i n t e r p r e t a t i o n . The number o f degrees of freedom i n the model has been i n c r e a s e d from one to two by c o n s t r a i n i n g the e p i c e n t e r s t o a h o r i z o n t a l p l ane at a depth o f 10 km. The immediate r e s u l t o f t h i s i s to i n c r e a s e the p r e c i s i o n o f the e s t ima tes o f the rema in ing p a r a -m e t e r s , ( l a t i t u d e , l o n g i t u d e and o r i g i n t ime) as the RMS i s d i s t r i b u t e d among 3 v a r i a b l e s i n s t e a d o f 4, wh i l e d e c r e a s i n g the a c c u r a c y . Rea l d i f f e r e n c e s i n depths o f focus undoubted ly do e x i s t , but i n l i g h t o f the sha l l ow na tu re o f the s e i s m i c i t y a long the ocean r i d g e system i t would seem reasonab le to suppose t ha t these 45 d i f f e r e n c e s are s m a l l . E r r o r s i n f o c a l depth, coupled w i t h the poor s t a t i o n d i s t r i b u t i o n , w i l l r e s u l t i n e r r o r s i n the p o s i t i o n of e p i c e n t e r s of c o r r e s p o n d i n g magnitude so t h a t the maximum amount of e r r o r from t h i s source should be s m a l l . Shocks w i t h focus s h a l l o w e r than 10 km w i l l be moved away from the c e n t e r of the s t a t i o n d i s t r i b u t i o n ( t o the southwest). Once depth was r e s t r a i n e d the l o c a t i o n procedure was workable. E s t i m a t e s o f l a t i t u d e , l o n g i t u d e and o r i g i n time c o u l d be o b t a i n e d from a r r i v a l times i n about three i t e r a t i o n s . The f i n a l v a l u e s were independent of the i n i t i a l s t a r t i n g p o i n t and the s t a n d a r d d e v i a t i o n f o r the b e t t e r events was commonly l e s s than one second. Estimates (based on a normal d i s t r i b u t i o n ) of the v a r i a n c e o f the a r r i v a l times gave f i g u r e s around one s e c . The l o c a t i o n procedure was s u c c e s s f u l and the assumptions on which i t was based were j u s t i f i e d . ( F i g u r e 3.2-3 and Table 3.2-1). There were s e r i o u s d i f f i c u l t i e s a s s o c i a t e d w i t h the e p i -c e n t e r s however. They were p l a c e d i r r e g u l a r l y on the model and the o r i g i n times advanced about 10 s e c . T h i s was taken to be a consequence o f the poor s t a t i o n d i s t r i b u t i o n which p l a c e d a l l the r e c o r d i n g s t a t i o n s on one s i d e of the s o u r c e . Thus l o n g i t u d e and o r i g i n time, though not s t r i c t l y dependent v a r i a b l e s , were h i g h l y c o r r e l a t e d and t h i s i n t r o d u c e d a b i a s i n t o the e s t i m a t e s . To remove t h i s b i a s an independent method of e s t i m a t i n g o r i g i n time was r e q u i r e d . I t can be noted, however, that the e p i c e n t e r s o f the swarm c o n s i d e r e d as a whole, form a d e f i n i t e p a t t e r n which i s independent of the o r i g i n time assumed f o r the swarm (each event TABLE 3.2-1 ESTIMATES OF LATITUDE, LONGITUDE AND ORIGIN TIME BY LEAST SQUARES PROCEDURE EVENT LATITUDE LONGITUDE ORIGIN TIME ESTIMATE NUMBER (DEG. N) S.D. (DEG. W) S.D. . (SEC) S.D. S.D. T (SEC) 2 50. 13 0.06 130. 19 0.26 -32. 70 2. 17 0.42 4 49. 97 0.11 130. 85 0.48 -38.16 4.20 0. 68 7 49. 88 0.10 130. 85 0.43 -38.72 , 3.79 0. 60 13 49. 94 0.02 130. 55 0.08 -36.39 0.-69 0.12 17 50. 17 0.2 9 130. 68 1.30 -35.90 11.02 1.93 19 49. 86 0. 18 130. 45 0.73 -36.68 6.30 1.08 20 49. 95 0.12 130. 87 0.52 -38.61 4.55 0.73 21 50. 12 0.17 130. 70 0.77 -36.02 6.5 6 1. 14 89 50. 00 0.07 130. 66 0.31 -39.63 2.70 0.46 ON 1 50.57 0.15 126.84 0.23 -11.66 1.41 2.76 6 49. 92 0.15 130.75 0.63 -38.22 5.48 0.90 10 49. 88 0.39 131.32 1. 69 -41.42 14.99 2. 18 11 48. 67 1. 03 134.53 3.48 -77.33 34.79 2.39 12 49.80 0.67 132.25 2.91 -49.11 26. 39 3.17 14 50. 17 0. 38 130.01 1.61 -31.26 13.03 2. 63 18 50. 38 0. 31 130.37 1.44 -31.38 11.63 2.25 22 50.44 0. 15 128.92 - 0.73 -21.82 5.85 1.71 24 48.31 1.95 136.41 6.44 -95.06 66.00 3.35 32 49. 92 0.56 131.45 2.43 -41.82 21.59 3.09 33 50. 43 0.07 129.11 0.23 -27.19 0.67 1.10 S.D. = STANDARD DEVIATION 47 F I G U R E 3 . 2 - 3 : E P I C E N T E R S FOR 1 9 E V E N T S O F T H E SWARM C A L C U L A T E D I N T E R M S O F A R E G I O N A L M O D E L O F T H E E A R T H ' S C R U S T . S O L I D T R I A N G L E S I N D I C A T E B E T T E R D E F I N E D L O C A T I O N S . D E P T H I S C O N S T R A I N E D T O 1 0 KM 48 c o n s i d e r e d to take the same time to reach the f i r s t s t a t i o n ) ( F i gu re 3 .2-4 ) . The e f f e c t of t h i s assumpt ion i s to bunch the events t o g e t h e r i n space more than they can be expec ted to be i n r e a l i t y , but the assumpt ion i s not i n c o m p a t i b l e w i t h the i d e a o f an ear thquake swarm. The p a t t e r n which i s apparent i s a no r t h-west t r e n d - t h i s t r end i s a s s o c i a t e d w i th the Queen C h a r l o t t e I s l ands F r a c t u r e Zone. The e f f e c t o f e a r l i e r o r i g i n t imes i s t o move the p a t t e r n of e p i c e n t e r s to the southwest w i thou t changing the p a t t e r n a p p r e c i a b l y . An o r i g i n t ime can be c a l c u l a t e d f o r an ear thquake o c c u r -r i n g w i t h i n r e g i o n a l d i s t a n c e s i f bo th P and S a r r i v a l s are r e c o r d e d . T h i s method i m p l i e s the f o l l o w i n g assumpt ions : (1) P o i s s o n ' s Ra t i o i s cons tan t a long the ray p a t h . (2) Both P and S f o l l o w the same pa th from source to r e c e i v e r . These two c o n d i t i o n s are bo th s a t i s f i e d f o r l o c a l events i f the geometry o f the t r a v e l paths i s not too c o m p l i c a t e d . For the 8 good P and S e v e n t s , o r i g i n t imes were c a l c u l a t e d u s i n g a l l 5 s t a t i o n s . For the r ema in ing 11 on l y PHC was u s e d . (Appendix A ) . I f o r i g i n t imes were c a l c u l a t e d by t h i s method the e p i -c e n t e r s o f the swarm c o u l d be e s t ima ted a g a i n . Depth was c o n s t r a i n e d t o 10 km and o r i g i n t ime t o the v a l ue c a l c u l a t e d from the S-P t i m e s , so tha t l o n g i t u d e and l a t i t u d e were the on l y f r e e v a r i a b l e s . The p a t t e r n i n t h i s case i s l e s s d i s t i n c t (Tab le 3.2-2 and F i g u r e 3 .2-5 ) ; the nor thwest t r end i s p a r t i a l l y o b s c u r e d , and by c o n s i d e r i n g on l y the 8 good events a n o r t h e a s t t r end i s e v i d e n t . The n o r t h e a s t t r end i s a s s o c i a t e d w i t h the E x p l o r e r T r e n c h . 49 F I G U R E 3 . 2 - 4 : E P I C E N T E R S FOR 19 E V E N T S I N T H E SWARM.AS A F U N C T I O N O F O R I G I N T I M E . T H E 3 C L U S T E R S ' FROM L E F T T O R I G H T A R E FOR V A L U E S O F O R I G I N T I M E O F - 3 5 , - 3 0 AND - 2 5 S E C TABLE 3.2-2 ESTIMATES OF LATITUDE AND LONGITUDE BY LEAST SQUARES ORIGIN TIME FROM S-P INTERVAL EVENT LATITUDE LONGITUDE ORIGIN TIME ESTIMATE NUMBER (DEG. N) S. D. (DEG. W) S. D. (SEC) S. D. S.D. T (SEC) 2 50. 21 0. 05 129.81 0. 04 -29.60 4. 63 0. 62 4 50.16 0. 09 129.91 0. 07 -30.20 3. 5 8 1.24 7 50. Ok 0. 07 130.00 0. 06 -32.20 3. 48 1.01 13 50. 05 0. Ok 130.04 0. 03 -3 2.10 2. 05 0.5 7 17 50.28 0. 15 130.13 0. 11 -32.30 5. 81 2.03 19 50.05 0. 10 129.54 0. 09 -29.10 3. 40 1.50 20 50. 13 0. 09 130.00 0. 07 -31.2 0 3. 03 1.20 21 50.29 0. 11 129.76 0. 09 -28.20 1. 99 1.50 89 50. Ik 0. 07 129.96 0. 05 -30.90 1. 28 0.90 O 1 49.94 0.42 129.72 0. 34 -31.20 0.0 6.06 6 50. 11 0.10 129.82 0.08 -30.40 0.0 1.35 10 50. 13 0. 19 130.07 0. 14 -30.70 0.0 2.53 11 49. 91 0. 31 129.63 0.26 -30.20 0.0 4. 42 12 50.27 0.29 129.86 0.23 -28.50 0.0 3. 93 14 50.20 0. 19 129.84 0. 15 -29.90 0.0 2.64 18 50.40 0.17 130.22 0.13 -30.20 0.0 2.25 22 50. 31 0. 18 130.01 0.14 -29.90 0.0 2.42 24 49. 99 0. 40 129.70 0. 32 -30.20 0.0 5.67 32 50.18 0.25 130.12 0.19 -30.40 0.0 3.38 33 50. 29 0.12 129.98 0.09 -29.60 0.0 1.57 S.D. = STANDARD DEVIATION 51 FIGURE 3.2-5: EPICENTERS FOR 19 EVENTS IN THE SWARM CALCULATED IN TERMS OF A REGIONAL MODEL FOR THE CRUST OF THE EARTH. SOLID TRIANGLES ARE BETTER DEFINED•LOCATIONS. DEPTH IS CONSTRAINED TO 10 KM 52 The v a l i d i t y o f the two t r ends e v i den t i n the d i s t r i -b u t i o n o f e p i c e n t e r s may be a rgued . The nor thwest t r e n d , a long the f r a c t u r e zone , i s p r i m a r i l y based on the l o c a t i o n s f o r s m a l l e r magnitude and hence more p o o r l y d e f i n e d e v e n t s . The n o r t h e a s t t r e n d , a long the t r e n c h , i s a r e s u l t o f i n t e r p r e t a t i o n o f the o r i g i n t imes o f the l a r g e r magnitude w e l l d e f i n e d e v e n t s . However, the s t a t i o n d i s t r i b u t i o n i s such tha t the l o c a t i o n p rocedure i s more s e n s i t i v e to changes i n e p i c e n t e r s p a r a l l e l to the c o a s t l i n e (northwest t rend ) than changes p e r p e n d i c u l a r to the c o a s t l i n e ( no r theas t t r e n d ) . Moreover the c o n s t r a i n t o f depths to 10 km w i l l r e s u l t i n any d i f f e r e n c e s i n f o c a l depths o f events showing up as a nor thwest t r e n d . T h u s , i t i s e n t i r e l y p o s s i b l e t ha t the 6 events l y i n g a long the t r e n c h cou ld have o c c u r r e d at the same e p i c e n t e r but w i th d i f f e r i n g f o c a l depths ( t h i s would r e q u i r e a spread o f f o c a l depths o f 25 km). There i s no ev idence from the i n t e r p r e t a t i o n tha t the e p i c e n t e r s as a f u n c t i o n o f t ime t r a c e d any s o r t o f a c o n s i s t e n t p a t h . The t r a v e l t imes f o r Pn and Sn i n western B r i t i s h Columbia f o r the f i n a l s e t o f e p i c e n t e r s are ( F igu re 3 .2-6 ) : T p n = ( 5 .51±0 .38 ) + A / ( 7 . 6 9 ± 0 . 0 3 ) (3.2-4) T g n = (10 .16±1 .25 ) + A / ( 4 . 4 5 ± 0 . 0 3 ) (3.2-5) 3.3 E p i c e n t e r s i n the Nor theas t P a c i f i c Ocean How s i g n i f i c a n t these e p i c e n t e r s are i s open to q u e s t i o n . The f a c t t ha t no i n f o r m a t i o n on depth (except tha t a l l e a r t h -quakes are sha l low) i s a v a i l a b l e , l i m i t s the f i n d i n g s . E a r t h -quakes which are deeper than 10 km, and t h e r e f o r e take l e s s t ime + F I G U R E 3.2-6: T R A V E L T I M E S F O R P n AND S n IN W E S T E R N B R I T I S H C O L U M B I A A S S U M I N G A L L E V E N T S H A V E A F O C A L D E P T H O F 10 KM + + + ' 4 f 0.0 10.0 20 .0 3 0 . 0 0 40 .0 5 0 . 0 6 0 . 0 7 0 . 0 BO.O 100.0 EPICENTRfll DISTANCE (KM) "(X10 J ) 54 than normal t o reach the s t a t i o n s , w i l l be l o c a t e d c l o s e r to the c o a s t , w h i l e earthquakes s h a l l o w e r than 10 km w i l l be mapped f a r t h e r from the c o a s t . The simple model chosen f o r the o c e a n i c c r u s t , t h a t o f two l a y e r s with constant t h i c k n e s s and v e l o c i t i e s w i l l have an e f f e c t . Any departure from t h i s model by the r e a l c r u s t w i l l mean t h a t the estimates f o r e p i c e n t e r s are b i a s e d . There seems t o be no r e l e v a n t i n f o r m a t i o n a v a i l a b l e t o j u s t i f y a more ambitious model. The s t a t i o n d i s t r i b u t i o n w i l l a l s o have a s e r i o u s e f f e c t . The poor d i s t r i b u t i o n and s m a l l number of s t a t i o n s used here made i t im p o s s i b l e to determine o r i g i n time and l o n g i t u d e o f earthquakes t o g e t h e r by the i t e r a t i o n procedure. Even though o r i g i n times are c a l c u l a t e d i n d e p e n d e n t l y , the s t a t i o n d i s t r i b u t i o n may s t i l l have an u n d e s i r a b l e e f f e c t on the c a l c u l a t e d e p i c e n t e r s . For w e l l d e f i n e d e v e n t s , not too f a r from the s t a t i o n a r r a y , t h i s problem seems t o have been reduced to a minimum (see Appendix B ) . In view o f these d i f f i c u l t i e s i t would seem wise not to put too much confidence i n an i n d i v i d u a l e p i c e n t e r - i t may be a product of the s p e c i a l c o n d i t i o n s under which the a r r i v a l s were r e c o r d e d . T h i s r a i s e s the whole problem of e s t i m a t i n g s e i s m i c e p i c e n t e r s l o c a t e d on the ocean's f l o o r . E p i c e n t e r s at sea u s u a l l y cannot be confirmed except perhaps by a second l o c a t i o n determined by an independent study u s i n g the same data and same l o c a t i o n procedure. Such a comparison o n l y demonstrates t h a t the l e a s t squares p r i n c i p l e works. A more r e l i a b l e check, i s the r e l a t i o n o f the e p i c e n t e r t o a r i d g e or t r e n c h o r t r a n s -form f a u l t , as d e f i n e d independently of seismology. I f more p r e c i s i o n than t h i s i s a t t emp ted , the l i m i t a t i o n s o f the model must be c o n s i d e r e d . < I f the c e n t r a l U n i t e d S ta tes were a s e i s m i c area then the use o f the new s t anda rd P t a b l e s ( H e r r i n et a l , 1968) and s e i s m i c r e c o r d i n g s t a t i o n s d i s t r i b u t e d on a l l s i d e s o f the source a rea would a l low f o r ve ry a ccu ra te e s t i m a t i o n of e p i c e n t e r s . However, f o r a t y p i c a l low magnitude ear thquake o c c u r r i n g i n the eas t P a c i f i c , say on the B lanco F r a c t u r e Zone, and r e co rded by se i smo -graph s t a t i o n s i n wes te rn Nor th A m e r i c a , the f o l l o w i n g p o i n t s may be noted about the e s t ima ted e p i c e n t e r : (1) I f f i r s t a r r i v a l s on l y are u s e d , no i n f o r m a t i o n about depth o f focus can be e x p e c t e d . (2) I f l o n g i t u d e and o r i g i n t ime are c a l c u l a t e d i n the same i t e r a t i o n p r o c e d u r e , then the e p i c e n t e r w i l l be b i a s e d too f a r west by an amount depend ing on the s t a t i o n d i s t r i b u t i o n u s e d . In a d d i t i o n to these two p o i n t s the e p i c e n t e r w i l l be b i a s e d a c c o r d i n g to how w e l l the geo logy o f the source a rea compares w i t h the geo logy o f c e n t r a l U n i t e d S t a t e s (assuming the new s t a n d a r d P t a b l e s are u s e d ) . Fo r t y p i c a l t e l e s e i s m i c events t h i s may not be so c r u c i a l s i n c e on l y a s m a l l p o r t i o n o f the r ay da t a i s i n the c r u s t o f the source a r e a . However, s i g n i f i c a n t v a r i a t i o n s f o r events o r i g i n a t i n g i n o cean i c t r enches do e x i s t , even at t e l e s e i s m i c d i s t a n c e s . S ince one o f the o b j e c t i v e s o f t h i s s tudy was t o d e t e r -mine the e f f e c t o f the model used i n l o c a t i n g e p i c e n t e r s , the e s t i m a t i n g p rocedure was made f l e x i b l e enough to accommodate a wide range of mode l s . Any l a y e r e d g e o l o g i c a l model c o u l d be 5 6 t r a n s l a t e d i n t o t r a v e l t imes obey ing S n e l l ' s Law and events l o c a t e d w i t h r e s p e c t to t h i s mode l . I f s t a n d a r d t r a v e l t imes were used i n s t e a d o f the l o c a l t r a v e l t i m e s , the r e s u l t i s to move e p i c e n t e r s to the southwest and to i n f l a t e the c o n f i d e n c e e l l i p s e s , but g e n e r a l l y not change the r e l a t i o n between e p i c e n t e r s (Table 3 . 3 - 1 , F i gu r e 3.3-1 and F i g u r e 3 . 3 - 2 ) . T h i s assumes tha t the same s t a t i o n s are used i n bo th cases . T h i s example p o i n t s out the b a s i c u n c e r t a i n t y i n v o l v e d i n u s i n g e p i c e n t e r s w i thou t independent checks . The v a l u e s c a l c u l a t e d are on l y those which min imize the sum o f the e r r o r s s q u a r e d , w i t h r e s p e c t to some mode l . The worth a t t a ched to the v a l u e s depend on how much v a l i d i t y the model h a s . 3.4 Model P e r t u r b a t i o n s The b a s i c assumpt ion made about the d i s t r i b u t i o n o f r e a d i n g e r r o r s i s tha t i t i s symmetr ic and cen te red at z e r o . T h i s a s s u m p t i o n , though perhaps not s t r i c t l y t r u e , i s v a l i d enough and a l l ows us to make c e r t a i n judgments on the model u s e d . In p a r t i c u l a r the average r e s i d u a l f o r a l a rge number of a r r i v a l s a t a p a r t i c u l a r s t a t i o n i n d i c a t e s whether any s i g n i f i c a n t v a r i -a t i o n s w i t h r e s p e c t to the model are o c c u r r i n g . For i n s t a n c e , i f the r e s i d u a l s genera ted at each s t a t i o n by f i t t i n g events to the s t anda rd c r u s t a l model are c o n s i d e r e d ( F igu re 3 . 3 - 1 ) , i t i s e v i d e n t tha t a c e r t a i n b i a s i s p r e s e n t i n the mode l . The r e s i d u a l s are c o n s i s t e n t l y nega t i v e at the c l o s e r s t a t i o n s and p o s i t i v e a t the f a r t h e r s t a t i o n s . S ince the zero va lue i s chosen by the l e a s t squares p r i n c i p l e , on l y the T A B L E 3 . 3 - 1 E S T I M A T E OF L A T I T U D E AND L O N G I T U D E BY L E A S T S Q U A R E S O R I G I N T I M E FROM S - P I N T E R V A L S T A N D A R D C R U S T A L M O D E L E V E N T L A T I T U D E L O N G I T U D E O R I G I N T I M E E S T I M A T E N UMBER ( D E G . N) S. D. ( D E G . W) S. D. ( S E C ) S. D. S . D . T ( S E C ) 2 5 0 . 16 0. 0 9 1 3 0 . 0 7 0. 0 7 - 2 9 . 6 0 4. 6 3 1 . 2 6 4 5 0 . 1 2 0. 16 1 3 0 . 1 6 0. 12 - 3 0 . 2 0 3 . 5 8 2 . 1 7 7 5 0 . 0 0 0. 16 1 3 0 . 3 4 0. 12 - 3 2 . 20, 3 . 48 2 . 1 5 13 5 0 . 0 1 0. 12 1 3 0 . 2 9 0. 0 9 - 3 2 . 1 0 2. 05 1 . 5 2 17 5 0 . 2 2 0. 2 6 1 3 0 . 5 1 0. 1 9 - 3 2 . 3 0 5 . 8 1 3 . 5 6 .19 5 0 . 0 1 0. 1 1 1 2 9 . 7 9 0. 0 9 - 2 9 . 1 0 3 . 40 1 . 5 3 20 5 0 . 0 9 0. 17 1 3 0 . 2 5 0. 13 - 3 1 . 2 0 3. 0 3 2 . 2 6 2 1 5 0 . 2 6 0. 20 1 3 0 . 0 1 0. 15 - 2 8 . 2 0 1. 9 9 2 . 7 1 B R I T I S H C O L U M B I A C R U S T A L M O D E L E V E N T L A T I T U D E L O N G I T U D E O R I G I N T I M E E S T I M A T E NUMBER ( D E G . N) S . D . ( D E G . W) S . D . ( S E C ) S . D . S.D. T ( S E C ) 2 5 0 . 2 1 0 . 0 5 1 2 9 . 8 1 0 . 0 4 - 2 9 . 6 0 4 . 6 3 0 . 6 2 4 5 0 . 1 6 0 . 0 9 1 2 9 . 9 1 0 . 0 7 - 3 0 . 2 0 3 . 5 8 1 . 2 4 7 5 0 . 0 4 0 . 0 7 1 3 0 . 0 0 0 . 0 6 - 3 2 . 2 0 3 . 4 8 1 . 0 1 1 3 5 0 . 0 5 0 . 0 4 1 3 0 . 0 4 0 . 0 3 - 3 2 . 1 0 2 . 0 5 0 . 5 7 1 7 5 0 . 2 8 0 . 1 5 1 3 0 . 1 3 0 . 1 1 - 3 2 . 3 0 5 . 8 1 2 . 0 3 1 9 5 0 . 0 5 0 . 1 0 1 2 9 . 5 4 0 . 0 9 - 2 9 . 1 0 3 . 4 0 1 . 5 0 20 5 0 . 1 3 0 . 0 9 1 3 0 . 0 0 0 . 0 7 - 3 1 . 2 0 3 . 0 3 1 . 2 0 2 1 5 0 . 2 9 0 . 1 1 1 2 9 . 7 6 0 . 0 9 - 2 8 . 2 0 1 . 9 9 1 . 5 0 S . D . = S T A N D A R D D E V I A T I O N 58 P H C A L B V I C F S J P N T F I G U R E 3.3-1: E P I C E N T E R S P L U S 90% C O N F I D E N C E E L L I P S E S FOR 8 W E L L D E F I N E D E V E N T S U S I N G T H E S T A N D A R D C R U S T A L M O D E L A D O P T E D BY H E R R I N E T A L 1968. A V E R A G E R E S I D U A L S I N S E C A T T H E . 5 S T A T I O N S A R E SHOWN I N H I S T O G R A M U N D E R N E A T H 59 -2 — P H C A L B V I C F 5 J P N T F I G U R E 3 - 3 - 2 : E P I C E N T E R S P L U S 90% C O N F I D E N C E E L L I P S E S F O R 8 W E L L D E F I N E D E V E N T S U S I N G A R E G I O N A L C R U S T A L M O D E L . A V E R A G E R E S I D U A L S A T T H E 5 S T A T I O N S A R E SHOWN I N H I S T O G R A M U N D E R N E A T H 60 r e l a t i v e v a l ue o f the r e s i d u a l s between the s t a t i o n s i s s i g n i f i -c a n t . S ince p o s i t i v e r e s i d u a l s i n d i c a t e l a t e a r r i v a l s , t h i s would sugges t t h a t the basement v e l o c i t y o f the model ( i n t h i s case 8.05 km/sec) i s too h i g h or a l t e r n a t i v e l y the c r u s t i s t h i n n i n g towards the f a r t h e r s t a t i o n s . Fo r the f i n a l model adopted f o r the c r u s t i n t h i s s t u d y , the average r e s i d u a l f o r 8 events a t the 5 s t a t i o n s are g i v en i n F i g u r e 3 .3-2 . A r r i v a l s are e a r l y at VIC and ALB and l a t e at PHC w i t h r e s p e c t to the mode l , which has the c r u s t mant le boundary at 50 km f o r a l l s t a t i o n s . T h i s p a t t e r n i s independent o f the d e p t h . Because the ray paths to these 3 s t a t i o n s are i ndependen t , the az imuth from source to s t a t i o n i s d i f f e r e n t i n each c a s e , these r e s i d u a l s can be i n t e r p r e t e d as caused by l o c a l v a r i a t i o n s i n the t h i c k n e s s o f the c r u s t ( p a r t i c u l a r l y the bottom l a y e r ) . A g a i n , the a b s o l u t e va lue o f the r e s i d u a l i s not s i g n i f i c a n t , on l y the r e l a t i v e d i f f e r e n c e between s t a t i o n s . In t h i s case the one sec de l a y between both ALB and VIC and PHC i n d i c a t e s tha t the c r u s t i s t h i c k e r i n the n o r t h e r n p a r t o f the i s l a n d , the d i f f e r e n c e b e i n g rough l y 10 km. T h i s i s i n l i n e w i th T s e n g ' s (1968) s u g g e s t i o n tha t a major s t r u c t u r e d i s c o n t i n u i t y e x i s t s nea r the c e n t e r o f the i s l a n d ; the upper c r u s t on the n o r t h b e i n g t h i c k e r than the c r u s t on the s o u t h . I f t h i s i s t rue the b i a s i n the e p i c e n t e r s c a l c u l a t e d i n terms o f t h i s model w i l l be such as to move the e s t ima t ed p o i n t s away from PHC, i n t h i s case t o the southwes t . I t shou ld be s t r e s s e d here tha t the e p i c e n t e r s are f u n c t i o n s o f the model used to c a l c u l a t e them. Care shou ld be taken i n i n t e r p r e t i n g t r a v e l 61 t imes genera ted from such e p i c e n t e r s i n terms o f a new mode l . The r e a s o n i n g i s c i r c u l a r , the p rey i s the eve r e l u s i v e " w o o z l e " (A. A. M i l n e , 1947) . 62 4. THE EARTHQUAKE SWARM AT PHC The s u s p i c i o n e x i s t s f o r any ear thquake swarm tha t the source mechanism i s u l t i m a t e l y connec ted w i t h v o l c a n i c a c t i v i t y . S ince t h i s swarm occu r r ed on the ocean f l o o r no v o l c a n i c a c t i v i t y c o u l d be observed f i r s t hand , but the l o c a t i o n o f the source a r e a i n a r e g i o n o f seamounts suggests t ha t v o l c a n i c a c t i v i t y has o c cu r r ed i n the p a s t . A l l p r e v i o u s s e i s m i c i t y i n the area has been a s s o c i a t e d w i th the Queen C h a r l o t t e I s l ands F r a c t u r e Zone i n terms o f r i g h t - l a t e r a l wrench mot ion a long the nor thwest t r e n d . The two mechanisms are not i n c o m p a t i b l e , however, and i n v iew of the c o m p l i c a t e d na tu re o f the source a r e a , bo th p r o b a b l y p l a y a r o l e . The se ismograph s t a t i o n at Po r t Hardy was l o c a t e d l e s s than 200 km from the source and r eco rded a l a r g e number o f d i s t i n c t a r r i v a l s ( to be c l a s s i f i e d as an a r r i v a l e n t a i l e d t r a c e movement g r e a t e r than the t h i c k n e s s o f the t r a c e - r o u g h l y 1 mm). For each a r r i v a l the f o l l o w i n g parameters were e s t i m a t e d : (Appendix C) (1) A r r i v a l t ime Pn . (2) Sn-Pn t i m e . (3) Pn a m p l i t u d e , th ree components. (4) Sn a m p l t i d u e , th ree components . The e s t i m a t i o n o f ampl i tudes was somewhat sub jec t i ve , s i n c e the t r a c e movement was of such h i g h f requency and the a r r i v a l s were o f t e n i n d i s t i n c t . Some i d e a o f the r e l a t i v e energy i n v o l v e d i n 63 each event was hoped to be o b t a i n e d . 4.1 Time R e l a t i o n s o f the Swgrm A l l t imes r e f e r to a r r i v a l t imes at PHC. A l l magni tudes r e f e r to body wave magnitudes (except where n o t e d ) . The swarm c o n s i s t e d of a t l e a s t 217 events over a 132 hour p e r i o d , b e g i n n i n g at. 6 .22 .50 U. T . The main a c t i v i t y , c o n -s i s t i n g o f 141 a r r i v a l s , was r e s t r i c t e d to a 27-hour p e r i o d on August 28. For 5 hours b e g i n n i n g at 11:45 U. T . August 28 the t r a c e was i n v i r t u a l l y con t inuous m o t i o n . A h i s tog ram f o r the number o f events f o r 3-hour p e r i o d s i s g i ven i n F i g u r e 4 . 1 - 1 . The d iagram r e v e a l s th ree b u r s t s o f a c t i v i t y s epa ra t ed by i n a c t i v e p e r i o d s o f up to 12 h o u r s . The na tu re o f the decay o f the b u r s t s sugges ts some s o r t o f f o r e shock - a f t e r s h o c k sequence . Two shocks w i th magnitudes 4.9 and 4.7 occu r at 15:26 and 16:20 August 28 c o i n c i d i n g w i th the peak a c t i v i t y on August 28 . Nine shocks w i th magnitudes g r e a t e r than 4.0 p receed these two l a r g e r events i n a 4-hour p e r i o d , 2 shocks w i th magnitudes g r e a t e r than 4.0 o c cu r a f t e r i n an 18-hour p e r i o d . A lmost a l l of the l a r g e r magnitude events were c o n f i n e d to the f i r s t 40 hours o f a c t i v i t y . A magnitude 48 event o c c u r r e d at 13:35 August 2 7 , u n c o r r e l a t e d w i t h a l a r g e amount o f secondary a c t i v i t y . The h i s tog ram showing the number o f hours w i th a g i v e n number of events shows tha t the events o f the swarm are not randomly d i s t r i b u t e d i n time ( F i gu re 4.1-2) . The use o f on l y 5 s t a t i o n s over a l i m i t e d range o f az imu ths , p l u s the f a c t tha t the f i r s t a r r i v a l s f o r most events (13)^^(17) 4 8 -4 0 — 3 2 — 2 4 — 1 6 — UJ > UJ b_ O or UJ CD 6 12 18 0 A U G . 2 7 6 12 18 0 A U G . 2 8 6 12 18 0 A U G . 2 9 6 12 18 0 A U G . 3 0 6 12 18 0 A U G . 31 6 12 18 U J . S E P T . I F I G U R E 4 . 1 - 1 : H I S T O G R A M S H O W I N G NUMBER O F E V E N T S FOR T H R E E HOUR P E R I O D S . L A R G E R M A G N I T U D E E V E N T S A R E I N D I C A T E D BY N U M B E R S 2 0 -2 ' 6 -o r i or LU •L Li_ o I2H or a en Z D 4H 0 J POISSON DISTRIBUTION 8 12 n 2 0 2 4 2 8 3 2 3 6 4 4 4 8 N U M B E R O F E V E N T S F I G U R E 4.1-2: H I S T O G R A M S H O W I N G NUMBER OF T H R E E HOUR P E R I O D S F O R W H I C H N E V E N T S O C C U R R E D 66 were commonly i n d e f i n i t e , made f a u l t p l ane s o l u t i o n s based on the p o l a r i t y o f the f i r s t a r r i v a l s u n r e l i a b l e . The f o l l o w i n g g e n e r a l comments may be made, however. No event c o u l d be found which gene ra ted a r r i v a l s of the same sense at a l l 5 s t a t i o n s ; VIC and PHC were o f t e n o p p o s i t e p o l a r i t i e s . In a d d i t i o n , PHC r e c o r d e d bo th c o m p r e s s i o n a l and d i l a t i o n a l a r r i v a l s , once w i t h i n a p e r i o d l e s s than an hou r , but not enough a r r i v a l s were w e l l enough d e f i n e d to g i v e a p a t t e r n ( F igu re 3 .1-1 ) . The d i s t r i b u t i o n of events i n t ime p l u s the v a r i a b i l i t y o f the p o l a r i t y o f f i r s t a r r i v a l s sugges ts a t e c t o n i c mechanism f o r some o f the events i n the swarm. 4.2 Magnitude De te rm ina t i ons The s m a l l e s t magnitude r e p o r t e d by the USCGS was 3 . 7 . The use o f r e c o r d s from PHC a l lowed c a l c u l a t i o n of magnitudes down to 2 . 7 . T h i s magnitude r e p r e s e n t e d the minimum amount of t r a c e movement d e t e c t a b l e on the seismograms from PHC. S tandard R i c h t e r magn i tudes , M^, i^ere d e f i n e d from the u s u a l f o rmu la M L = l o g 1 0 A - l o g 1 0 A o (4.2-1) where A , exp res sed i n mm i s the mean o f the maximum t r a c e a m p l i -tude from z e r o , which would have been r e co rded by a p a i r o f s t anda rd h o r i z o n t a l t o r s i o n seismometers o r t h o g o n a l l y o r i e n t a t e d , and l o g n r i A i s a d i s t a n c e f a c t o r . For t h i s case l o g , n A ^ = -3.5 "10 O ° 1 0 O was used c o r r e s p o n d i n g to an e p i c e n t r a l d i s t a n c e o f 190-200 km ( F igu re 4 . 2 - 1 ) . No v a r i a t i o n i n t h i s f i g u r e i s c o n s i d e r e d . The maximum t r a c e amp l i tudes co r respond i n a l l cases t o the Sn a r r i v a l . 67 m • ( 4.1 ± 0.1) + ( I.I ± 0.3) Log | 0 A M L » 3.5 + L o g | 0 A m' » 1.8 + 0.8 M L t r * - 0.11 U N I T S 2 O MAGNITUDE FROM USCGS 68 The m a g n i f i c a t i o n on the h o r i z o n t a l se ismometers o f PHC at 0.8 sec p e r i o d i s 40 K. A body wave magn i tude , m, was a l s o d e f i n e d i n terms o f an e m p i r i c a l r e l a t i o n between the magnitude o f 10 l a r g e r events r e p o r t e d by the USCGS and l o g ^ A . The r e l a t i o n was l i n e a r . m = ( 4 . 1 ± 0 . 1 ) + (1 . l ± 0 . 3 ) L o g 1 Q A (4.2-2) An e s t i m a t e , based on normal d i s t r i b u t i o n s , o f the s t anda rd d e v i a t i o n of m g i v e n by t h i s f o rmu la was ±0.3 u n i t s ( F i gu re 4 . 2 - 1 ) . The r e l a t i o n i m p l i e d between and m was then . m = 0.17 + 1.1 M L (4.2-3) compared to the s t anda rd r e l a t i o n g i v en by R i c h t e r (1958) m' = 1.7 + 0.8 M L (4.2-4) The d i f f e r e n c e sugges ts t ha t e i t h e r the USCGS i s u n d e r e s t i m a t i n g the magnitude f o r s m a l l events or tha t the ampl i tude o f a r r i v a l s a t PHC i s abnorma l l y l a r g e . Us ing the R i c h t e r magnitudes the magn i tude-f requency r e l a t i o n s h i p ( F igu re 4.2-2) f o r the range o f magnitudes 2.0 t o 4 .0 becomes l o g 1 0 N = (4 .07±0 .50 ) - ( 1 .10±0 .20 ) (4.2-5) The v a l ue f o r the s l ope of the l i n e , i n t h i s case - 1 . 1 0 , i s s e n s i t i v e to the mechanism g e n e r a t i n g the ear thquakes i n q u e s t i o n . Fo r ear thquakes d i r e c t l y a s s o c i a t e d w i th v o l c a n i c e r u p t i o n s , v a l ues up to -4 have been found ( S u z u k i , 1959) . For ear thquakes at depths to 10 km but s t i l l a s s o c i a t e d w i th v u l c a n i s m a v a l ue o f -1.18 has been r e p o r t e d (Minakami , I960 ) . I t shou ld be noted tha t these va lues are d e r i v e d f o r m i c r o - e a r t h -quakes w i t h R i c h t e r magnitudes l e s s than the range c o n s i d e r e d h e r e , so t h a t the r e s u l t s must be e x t r a p o l a t e d . L o g | 0 N « (4.1 ± 0.5) - (I.I ± 0.2) M L 2 ,cr « 0.04 C> NOT USED IN FINAL SOLUTION RICHTER MAGNITUDE (M L ) F I G U R E 4.2-2: M A G N I T U D E - F R E Q U E N C Y R E L A T I O N S H I P F O R 217 E V E N T S I N T H E SWARM 70 For a mic ro-ear thquake swarm (magnitude range f rom -1.1 t o 2.4) r e c o r d e d at MBC in the Northwest T e r r i t o r i e s , Whitham (1968) r e p o r t s a v a l ue f o r the s l ope o f - 0 . 6 8 . He conc ludes t h a t the swarm was genera ted as a r e s u l t o f movement a long a f a u l t . For the swarm d i s c u s s e d here the v a l ue f o r the s l o p e , - 1 . 1 0 , i s s u g g e s t i v e o f v o l c a n i c a c t i v i t y r a t h e r than t e c t o n i c a c t i v i t y on a f a u l t . T h i s i s i n l i n e w i th T . J . G. F r a n c i s (1969) s u g g e s t i o n t h a t the magn i tude- f requency r e l a t i o n s h i p s f o r e a r t h -quakes a s s o c i a t e d w i th the r i d g e c r e s t s o f the M i d - A t l a n t i c Ridge have a s t e e p e r s l ope than those f o r ear thquakes a s s o c i a t e d w i t h the o f f - s e t t i n g f r a c t u r e zones . 71 5.. ; CONCLUSIONS In the pas t few yea rs i t has become e v i d e n t tha t the t e c t o n i c s o f the c o n t i n e n t a l margin o f western Nor th Amer ica from the G u l f o f C a l i f o r n i a to the A l e u t i a n I s l ands i s c o n -s i s t e n t l y exp re s sed as a m a n i f e s t a t i o n of a nor thwest t r e n d i n g r i g h t - l a t e r a l f o r c e . T h i s f o r c e i s r ep re sen t ed by 2 t e c t o n i c zones , e x t e n s i o n on a n o r t h e a s t t r e n d and wrench f a u l t i n g on a nor thwes t t r e n d , and can be thought o f i n terms o f movement o f the P a c i f i c B a s i n nor thwest w i th r e s p e c t to the c o n t i n e n t . The ear thquake swarm p re sen t ed here i s one r e a l i z a t i o n o f t h i s p a t t e r n . The d i s t r i b u t i o n o f 19 e p i c e n t e r s o f l a r g e r events a s s o c i a t e the swarm w i th two i ndependen t l y de termined t e c t o n i c zones : the Queen C h a r l o t t e I s l a n d F r a c t u r e Zone , a zone o f r i g h t - l a t e r a l s t r i k e f a u l t i n g , and the E x p l o r e r T r e n c h , a zone o f e x t e n s i o n and g e n e r a t i o n o f new c r u s t a l m a t e r i a l . The d i s t r i -b u t i o n o f events i n t ime and the p o l a r i t y o f f i r s t a r r i v a l s at se ismograph s t a t i o n s i n wes te rn B r i t i s h Co lumbia suggest tha t a t e c t o n i c mechanism, s t r a i n r e l e a s e by movement on a f a u l t , gene ra ted the swarm. The l o c a t i o n of the e p i c e n t e r s o f l a r g e r events a long the t r e n d o f the E x p l o r e r T r e n c h , and the magnitude f r equency r e l a t i o n of the 217 events r eco rded at PHC sugges t t ha t a v o l c a n i c mechanism genera ted the swarm. I t i s easy to accept t ha t both mechanisms c o u l d have p l a y e d a p a r t . In f a c t , the "New G l o b a l T e c t o n i c s " p r e d i c t s t ha t e x t e n s i o n ac ross r i d g e s and t r a n s f o r m f a u l t i n g a long f r a c t u r e 72 zones w i l l be c o n t i n u o u s . In t h i s case a c t i v i t y on the E x p l o r e r T rench o c c u r r e d f i r s t , f o l l o w e d by ad justments a long f a u l t zones on both s i d e s o f the t r e n c h . T h i s r e p r e s e n t s a case o f "The New G l o b a l T e c t o n i c s " i n a c t i o n . Some degree o f b i a s i n ear thquake e p i c e n t e r s l o c a t e d i n o cean i c r e g i o n s i s u n a v o i d a b l e . Problems a r i s e from two s o u r c e s : a poor s t a t i o n d i s t r i b u t i o n and use o f an u n r e a l i s t i c model f o r the e a r t h . Each case must be c o n s i d e r e d i n the l i g h t of i t s own p a r t i c u l a r c o n d i t i o n s . For t y p i c a l low magnitude sha l l ow s e i s m i c a c t i v i t y o c c u r r i n g i n the n o r t h e a s t P a c i f i c Ocean and r e c o r d e d i n wes te rn Nor th A m e r i c a , poor e s t ima tes of depth and o r i g i n t imes may occur when u s i n g the s t anda rd l o c a t i o n p r o c e d u r e . B i as g e n e r a l l y moves the e p i c e n t e r s f a r t h e r west than they are i n r e a l i t y . For good q u a l i t y r e s u l t s , independent methods f o r e s t i m a t i n g depth and o r i g i n t imes o f events are d e s i r a b l e , as w e l l as a model which r e a l i s t i c a l l y r e p r e s e n t s the geo logy o f the source a r e a . For the e p i c e n t e r s p r e sen t ed i n t h i s p a p e r , a degree o f b i a s i s p r e s e n t i n any i n d i v i d u a l e s t i m a t e , but the r e l a t i o n between e p i c e n t e r s r e p r e s e n t s r e a l d i f f e r e n c e s i n the a r r i v a l t imes a t the 5 s t a t i o n s used and hence i s t e c t o n i c a l l y s i g n i f i c a n t . The a r r i v a l s , Pn and Sn , gene ra ted by events i n the swarm and r e c o r d e d by 5 s t a t i o n s i n wes te rn B r i t i s h Columbia i n the e p i c e n t r a l range 175 to 750 km showed a s imp le l i n e a r r e l a t i o n w i th d i s t a n c e T p n = (5 .51±0 .38 ) + A/ (7 .69±0 .03 ) (5-1) T~ = ( 1 0 . 1 6 l l . 2 5 ) + A/(4 .4510.03) (5-2) , 7 3 where A i s e p i c e n t r a l d i s t a n c e i n km ( F igu re 3 .2-7 ) . T h i s f a c t sugges ted t h a t the a r r i v a l s c o u l d be unde r s tood as a s imp le t r a v e l l i n g wave r e f r a c t e d a long the Mohorov ic ' i c d i s c o n t i n u i t y u n d e r l y i n g the c r u s t o f western B r i t i s h Co lumb ia . A model f o r the c r u s t i n t h i s r e g i o n , combin ing ocean i c and c o n t i n e n t a l type c r u s t s , was c o n s t r u c t e d u s i n g a v a i l a b l e r e f r a c t i o n surveys f o r the upper c r u s t i n the a r e a . Reference t r a v e l t imes genera ted i n terms o f t h i s model showed good agreement to the obse rved t r a v e l t i m e s . Because the e p i c e n t e r s o f the events were c a l c u -l a t e d i n terms o f the mode l , some u n c e r t a i n t y was i n t r o d u c e d i n t o the parameters of the mode l . Thus an abso lu t e depth to the upper mant le under Vancouver I s l a n d c o u l d not be c a l c u l a t e d , but r e l a t i v e d i f f e r e n c e s were e v i d e n t ; the c r u s t appears to be 10 km t h i c k e r on the n o r t h e r n end of the I s l and than on the c e n t r a l or sou the rn p a r t . S e v e r a l recommendations f o r f u r t h e r s t u d i e s f o l l o w from these c o n c l u s i o n s . The s u p p o s i t i o n has been made tha t two d i s t i n c t mechanisms gene ra ted events i n the swarm. T h i s may be checked by f i r s t mot ion s t u d i e s o f the 8 l a r g e r magnitude e v e n t s . S i x o f these (numbers 2, 4, 7, 13 , 20 and 21) are a s s o c i a t e d w i th the E x p l o r e r T rench and shou ld be c h a r a c t e r i z e d by normal f a u l t i n g on a n o r t h e a s t t r e n d i n g p l a n e . The rema in ing (numbers 17 and 19) were a s s o c i a t e d w i th the Queen C h a r l o t t e I s l ands F r a c t u r e Zone and shou ld be c h a r a c t e r i z e d by 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 on a nor thwest t r e n d i n g p l a n e . The l o c a t i o n system se t up f o r t h i s s tudy w i l l p r o v i d e 74 ac cu r a t e d e t e r m i n a t i o n o f e p i c e n t e r s f o r sha l l ow focus s e i s m i c a c t i v i t y on a r e g i o n a l scale-. A r e c o r d i n g network i n c l u d i n g PHC, FSJ and a s i m i l a r s t a t i o n on the Queen C h a r l o t t e I s l ands would p r o v i d e adequate coverage o f a l l o f western B r i t i s h C o l u m b i a , the A l a skan Panhandle and ad jacen t o c e a n i c areas con -t a i n i n g the Queen C h a r l o t t e I s l ands F a u l t and r e l a t e d s y s t ems . T h i s area i s i n t e r e s t i n g f o r two r e a s o n s ; f i r s t , because the s e i s m i c i t y has been thus f a r p o o r l y e s t a b l i s h e d , and s e c o n d , bacause the o c e a n i c r i d g e system moves onshore b e f o r e j o i n i n g w i t h the A l e u t i a n T rench System. In g e n e r a l , any s tudy o f the g e o l o g i c a l - g e o p h y s i c a l env i ronment o f the c o n t i n e n t a l margin of wes te rn Nor th Amer i ca s h o u l d be u l t i m a t e l y r e l a t e d to e i t h e r the h i s t o r y o f the r e g i o n as a Mesozo i c t r ench system or the r e c e n t development of a segment of the ocean r i d g e system i n the ad j a cen t P a c i f i c B a s i n . 75 BIBLIOGRAPHY B o l t , B .A . , L o m n i t z , C . , and M c E v i l l y , T . V . , S e i s m o l o g i c a l Ev idence on the T e c t o n i c s of C e n t r a l and Nor the rn C a l i f o r n i a on the Mendocino Escarpment , B u l l . Se i sm. Soc . 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H a m i l t o n , R .M. , O r i g i n of the G u l f o f C a l i f o r n i a , B u l l . G e o l . Soc . Am. , 7_2, 1307 , 1961. H e r r i n , E . , T u c k e r , W., T a g g a r t , J . H . , Gordon, D.W., and L o b d e l l , J . L . , E s t i m a t i o n o f Su r f ace Focus P T r a v e l T imes , B u l l . Se i sm. Soc . Am. , 5_8, No. 4 , 1273-1292 , 1968. I s a c k s , B., O l i v e r , J . , and Sykes , L .R . , Se i smology and the New G l o b a l T e c t o n i c s , J . Geophys. R e s . , 73_, No. 18 , 5855-5900, 1968. McManus, R .A . , Phys iography of the Cobb and Gorda R i s e s , N o r t h -eas t P a c i f i c Ocean, B u l l . G e o l . Soc . A m . , 7_8_, 527 , 1967. Menard , H.W., Mar ine Geology o f the P a c i f i c , M c G r a w - H i l l , New Yo rk , 1964. M i l n e , A . A . , The World of Pooh; the Complete Winnie-the-Pooh and the House at Pooh C o r n e r , D u t t o n , New Y o r k , 1957. M inakami , T . , Fundamental Research f o r P r e d i c t i n g V o l c a n i c E r u p t i o n s . Pa r t 1. Ear thquakes and C r u s t a l Deformat ions O r i g i n a t i n g from V o l c a n i c A c t i v i t y , B u l l . Ear thquake Research I n s t . Tokyo U n i v . , 38 , 497-544, 1960. 76 Morgan, W. J . , R i s e s , T r e n c h e s , Grea t F a u l t s and C r u s t a l B l o c k s , J . Geophys. R e s . , 73 , 1959 , 1968. R a f f , A . D . , and Mason, H . G . , Magnet i c Survey o f f the West Coast o f North A m e r i c a , 40 °N L a t i t u d e to 50 °N L a t i t u d e , B u l l . G e o l . Soc . Am. , 72 , 1259, 1961. R i c h t e r , C . F . , E lementary Se i smo logy , W.H. Freeman and Company, San F r a n c i s c o , 1958. S e i s m i c Data L a b o r a t o r y Report No. 133 Long Range Se i sm i c Measurements - Long Sho t , UED E a r t h S c i ences D i v i s i o n , Te ledyne I n c . , A i r Force T e c h n i c a l A p p l i c a t i o n s C e n t e r . S m i t h , W . E . T . , Whitham, K., and P i c h e , W .T . , A M i c roea r thquake Swarm i n 1965 near Mould Bay, N .W.T . , Canada, B u l l . Se i sm. Soc . Am. 5_8, No. 6 , 1991-2012 , 1968. Su ther l and-Brown, A . , T e c t o n i c H i s t o r y o f the I n s u l a r B e l t o f B r i t i s h Co lumb ia , i n T e c t o n i c H i s t o r y and M i n e r a l Depos i t s o f the Western C o r d i l l e r a , Can . I n s t . M in ing and Met . Spec . V o l . 8_, 83 , 1968 . S u z u k i , Z . , A S t a t i s t i c a l Study on the Occur rence o f Sma l l Ear thquakes ( f o u r t h p a p e r ) , S c i ence Rept . Tohoku U n i v . S e r . 5, Geophys . 11 , 10-54, 1959. T o b i n , D . G . , and Sykes , L .R . , S e i s m i c i t y and T e c t o n i c s o f the No r theas t P a c i f i c Ocean , J . Geophys. R e s . , 73^, No. 12, 3821-3846, 1968. T s e n g , K., A New Mode l f o r the C rus t i n the V i c i n i t y o f Vancouver I s l a n d , M.Sc. T h e s i s , Department o f G e o p h y s i c s , U n i v e r s i t y o f B r i t i s h Co lumb i a , 1967. T u c k e r , W. , H e r r i n , E . , and Freedman, H.W., Some S t a t i s t i c a l Aspec t s of the E s t i m a t i o n of S e i s m i c T r a v e l T imes , B u l l . Se i sm. Soc . Am. 5_8, No. 4 , 1243-1260, 1968. V i n e , F . J . , and W i l s o n , J . T . , Magnet i c Anomal ies Over a Young Ocean Ridge o f f Vancouver I s l a n d , S c i e n c e , 150, 485, 1965. Wh i te , W .R .H . , and Savage , J . C . , A Se i sm ic R e f r a c t i o n and G r a v i t y Study o f the E a r t h ' s C rus t i n B r i t i s h Co lumb i a , B u l l . Se i sm. Soc . A m . , 5_5_, 463, 1965 . Wh i t e , W.H.R . , Summary of T e c t o n i c H i s t o r y , i n T e c t o n i c H i s t o r y and M i n e r a l Depos i t s of the Western C o r d i l l e r a , Can . I n s t . M in ing and Met . Spec . V o l . 8, 96 , 1968. W i l s o n , J . T . , T rans fo rm F a u l t s , Ocean i c Ridges and Magnet i c Anomal ies Southwest o f Vancouver I s l a n d , S c i e n c e , 150, 482, 1965. 77 APPENDIX A O r i g i n Time From P and S A r r i v a l s An e s t i m a t i o n of the o r i g i n t ime o f a s e i s m i c event can be made i f the a r r i v a l t imes f o r the P and S waves can be o b t a i n e d . The method employs the f o l l o w i n g assumpt ions : (1) Both P and S a r r i v a l s f o l l o w the same pa th from source to r e c o r d i n g s t a t i o n . (2) P o i s s o n ' s R a t i o i s cons tan t a long the ray p a t h . The f o l l o w i n g o b j e c t i o n s may be made t o these c o n d i t i o n s : (1) P and S a r r i v a l s f o r t y p i c a l paths encounte red i n t h i s s t u d y , i n c l u d i n g r e f r a c t i o n at v e l o c i t y d i s c o n t i n u i t i e s , do not f o l l o w e x a c t l y the same. p a t h . However, to the ex ten t t ha t the concept o f a h o r i z o n t a l l y - l a y e r e d model i s a p p l i c a b l e , i t i s on l y r e q u i r e d tha t Pn and Sn paths encounte r the same l a y e r s i n t r a n s i t , no t tha t the paths be i d e n t i c a l . (2) V a r i a t i o n s o f P o i s s o n ' s Ra t i o may be expec ted i n the c r u s t o f the e a r t h , e s p e c i a l l y when a l a y e r e d model i s c o n s i d e r e d . I t i s assumed tha t such v a r i a t i o n s are c o n f i n e d to a s m a l l p o r t i o n o f the t r a v e l p a t h . C o n s i d e r the s imp le case o f a h o r i z o n t a l l y - l a y e r e d c r u s t w i t h k l a y e r s hav ing s e i s m i c v e l o c i t i e s a and & and a t h i c k n e s s Hj . For a s u r f a c e focus ear thquake the. a r r i v a l t ime o f Pn and Sn i s g i v en by T p n - 0 =' 2 Z H. / l - ( a ; j / a n ) 2 /a. + A / a n (A-l ) T S n - 0 = 2 I H. / l - ( g j / e n ) 2 / 6 j + A / B n (A-2) 78 where A i s e p i c e n t r a l d i s t a n c e and 0 i s o r i g i n t i m e . P o i s s o n ' s Ra t i o (a) may be expressed i n terms o f s e i s m i c v e l o c i t i e s as o = i ( l - e 2 ) / ( l - 2 £ 2 ) (A-3) where e = B/a. The assumpt ion tha t a i s cons tan t cor responds to e, the r a t i o o f s e i s m i c v e l o c i t i e s b e i n g c o n s t a n t . Thus = e n f o r a l l k, and from A - l and A-2 T S n " T P n = ( Tp n -0 ) ( l - e ) / e (A-4) To determine o r i g i n t i m e , A-4 was used i n the form T P n * ^ S n - W + 0 < A " 5 > Adhere k = e / ( l - e ) . A l e a s t squares f i t o f a s t r a i g h t l i n e to T p n and T g n - T p n a t 5 s t a t i o n s gave a va lue f o r o r i g i n time d i r e c t l y i n terms o f the i n t e r c e p t o f the s t r a i g h t l i n e ( F igu re A - l ) . E s t ima t e s o f 0 and k f o r 8 events are g i v en i n Tab l e A - l . The va lues f o r the o r i g i n t imes are 4 to 6 sec e a r l i e r than the USCGS e s t i m a t e s . The va lue f o r k sugges ts a va lue f o r P o i s s o n ' s Ra t i o o f 0 . 2 6 . O r i g i n t imes f o r events f o r which T S n c o u l d not be read a c c u r a t e l y at a l l 5 s t a t i o n s were e s t ima ted from T p n and TQ at PHC o n l y , assuming t h i s va lue f o r P o i s s o n ' s R a t i o . 79 TABLE A - l ORIGIN TIME FROM S-P INTERVAL EVENT NUMBER ORIGIN (SEC) TIME K VARIANCE P(SEC) 2 -29.55 + 4.62 1.32 + 0 .10 + 3.46 4 -30.24 + 3.57 1.36 + 0 .09 + 2.69 7 -32.22 + 3.48 1.45 + 0 .09 + 2.57 13 -32.14 + 2.05 1.37 + 0 .09 + 1.50 17 -31.35 + 5 .81 1.41 + 0 .10 + 4.35 19 -29.12 + 3.39 1.28 + 0 .09 + 2 .60 20 -31.21 + 3.03 1.36 + 0 .09 + 2 .26 21 -28.22 + 1.98 1.29 + 0 .09 + 1.54 89 -30.88 + 1.28 1.29 + 0 .09 + 0.96 81 APPENDIX B Tes t o f the L o c a t i o n P rocedure To t e s t the r e l i a b i l i t y o f the l o c a t i o n program the ear thquake o f December 27, 1963 near P o r t l a n d , Oregon and Idaho as r e p o r t e d by D e h l i n g e r , Chibus and C o l l i v e r (1965) . S t a t i o n LON BLL TON BKR HLY The e p i c e n t e r as g i v e n by D e h l i n g e r (1965) i s : o r i g i n t ime 0 2 . 3 6 . 1 8 . 5 l a t i t u d e 4 5 . 6 3 ° N l o n g i t u d e 123 .38 °N A l l s t a t i o n s were to the eas t o f the source a r e a : major g e o l o g i c a l bounda r i e s were c r o s s e d by the ray p a t h s . The s t a n d a r d c r u s t a l model f o r r e g i o n a l d i s t a n c e s ( H e r r i n e t a l , 1968) was used except f o r a lower basement v e l o c i t y (7 .87 km/sec ) . Depth was c o n s t r a i n e d to 10 km. An e s t ima te o f the th ree e p i c e n t r a l parameters gave o r i g i n t ime (02 .36 .16 .6+2 .3 ) sec l a t i t u d e ( 4 5 . 5 3 ± 0 . 1 8 ) ° N l o n g i t u d e ( 1 2 3 . 5 H O . 2 4 ) °W (km) T p n ( s e c ) 180 27.8 348 50.6 454 63.9 493 67.4 767 101.5 82 I f o r i g i n t ime was c o n s t r a i n e d to 0 2 . 3 6 . 1 8 . 5 an es t ima te o f the rema in ing two parameters gave l a t i t u d e (45.61±0 . 0 3 ) ° N l o n g i t u d e (123 .31±0 .03 ) °W APPENDIX C THE SWARM AT PHC P ARRIVAL S-P P AMPLITUDE S AMPLITUDE MAGNITUDE H/M/S (SEC) V N E (MM) V N E RICHTER BODY AUGUST 27, 1967 06/22/50 0.0 1 1 1 1 2 2 2.5 2.9 06/23/20 21.6 1 1 1 3 3 3 2.6 3.1 06/24/00 0.0 1 1 1 1 1 2 2.3 2.8 06/34/30.0 22.8 1 1 1 2 2 2 2.5 2.9 07/04/19.0 22.2 1 1 1 2 2 2 2.5 2.9 07/07/30 0.0 1 1 1 2 2 2 2.5 2.9 08/34/12.0 22.4 1 1 1 3 4 4 2.8 3.3 08/35/08.8 23.0 4 2 26 8 6 3.0 3.5 09/29/20 0.0 1 1 1 2 2 3 2.6 3.1 11/52/40 0.0 1 1 1 1 1 1 2.2 2.6 12/01/05.4 22.4 1 1 1 3 4 4 2.8 3.3 12/32/50 0.0 1 1 1 1 2 2 2.5 2.9 12/57/02.0 22.6 1 2 1 6 7 6 3.0 3.5 13/01/30.0 23.0 1 1 1 2 2 2 2.5 2.9 13/11/20 0.0 1 1 1 1 1 1 2.2 2.6 13/12/42.0 22.2 1 1 1 3 3 4 2.7 3.2 13/33/36.0 22.2 1 1 1 5 6 4 2.9 3.4 13/35/19.8 22.0 11 5 24 80 55 4.0 4.6 14/00/50 0.0 1 1 1 2 2 2 2.5 2.9 14/08/04.6 21.4 1 1 1 2 2 2 2.5 . 2.9 15/17/50 0.0 1 1 1 1 1 2 2.3 2.8 15/19/06.8 22.0 2 2 6 7 7 3.0 3.5 16/59/30 0.0 1 1 1 2 2 2 2.5 2.9 17/40/41.0 22.4 1 1 1 20 22 15 3.4 4.0 17/44/30 0.0 1 1 1 2 2 1 2.3 2.8 17/52/23.0 22.2 2 1 30 34 20 3.6 4.2 18/29/32.4 21.4 6 3 33 60 44 3.9 4.5 18/34/40 0.0 1 1 1 2 3 3 2.6 3.1 20/44/50 0.0 1 1 1 2 2 2 2.5 2.9 P ARRIVAL H/M/S AUGUST 2 8 04/03/30 05/56/11.6 08/52/30 09/05/20 09/21/23.0 10/57/10 10/58/40 11/31/30 11/32/32.8 11/35/30 11/36/30 11/39/50 11/41/39.8 11/45/58.0 11/54/10 - 11/58/00 11/59/27.4 12/05/28.2 12/06/5.0.0 12/10/50 12/11/50 12/13/10 12/16/00 12/19/00 12/20/00 12/23/32.8 12/25/50 12/27/19.0 12/30/30 12/31/30 12/34/40 12/36/10 12/39/41.8 12/44/18.6 APPENDIX C (CONT.) S-P P AMPLITUDE S AMPLITUDE MAGNITUDE (SEC) V N E (MM) V N E RICHTER BODY WAVE 0.0 1 1 1 2 2 2 2.5 2.9 22.0 1 1 1 13 10 12 ' 3 . 2 3.8 0.0 1 1 1 1 1 1 2.2 2.6 0.0 1 1 1 1 1 1 2.2 2.6 21.0 1 1 1 6 7 3.0 3.6 0.0 1 1 1 1 1 1 2.2 2.6 0.0 1 1 1 1 1 1 2.2 2.6 0.0 1 1 1 1 1 1 2.2 2.6 22.6 1 1 1 10 14 15 3.3 3.9 0.0 1 1 1 1 1 1 2.2 2.6 0.0 1 1 1 1 1 1 2.2 2.6 0.0 1 1 1 1 1 1 2.2 2.6 22.2 2 2 1 5 10 6 3.1 3.6 22.4 2 2 1 5 9 6 3.0 3.6 0.0 1 1 1 1 1 1 2.2 2.6 0.0 1 1 1 2 2 2 2.5 2.9 21.8 1 2 1 5 • 9 13 3.2 3.8 22.8 1 1 1 3 3 3 2.6 3.1 22.4 1 2 2 2 3 2.6 3.1 0.0 1 1 1 1 1 1 2.2 2.6 0.0 1 1 1 1 1 1 2.2 2.6 0.0 1 1 1 2 2 2 2.5 2.9 0.0 1 1 1 2 2 2 2.5 2.9 0.0 1 1 1 1 1 1 2.2 2.6 0.0 1 1 1 1 1 1 2.2 2.6 21.8 2 2 1 4 4 4 2.8 3.3 0.0 1 1 1 2 2 2 2.5 2.9 21.0 1 2 1 8 10 10 3.2 3.7 0.0 1 1 1 1 1 1 2.2 2.6 0.0 1 1 1 2 2 2 2.5 2.9 0.0 1 1 1 2 2 2 2.5 2.9 0.0 1 1 1 2 2 2 2.5 2.9 22.4 2 3 2 16 15 22 3.4 4.0 22.0 1 1 1 9 12 10 3.2 3.8 APPENDIX C (CONT.) P ARRIVAL S-P P AMPLITUDE S AMPLITUDE MAGNITUDE H/M/S (SEC) V N E (MM) V N E RICHTER BODY WAVE 12/47/50 0.0 1 I 1 2 2 2 2.5 2.9 12/50/29.8 22.6 1 1 1 9 7 9 3.1 3.6 12/53/00 0.0 1 1 1 2 2 2 2.5 2.9 12/53/40 0.0 1 1 1 2 2 2 2.5 2.9 12/55/00 0.0 1 1 1 1 1 1 2.2 2.6 12/56/30 0.0 1 1 • 1 1 1 1 2.2 2.6 12/57/20 0.0 1 1 1 1 1 1 2.2 2.6 12/58/10 0.0 1 1 1 2 2 2 2.5 2.9 12/59/20 0.0 1 1 1 3 3 3 2.6 3.1 13/05/10 0.0 1 1 1 3 3 3 2.6 3.1 13/07/10 0.0 1 1 1 2 2 2 2.5 2.9 13/11/00 0.0 1 1 1 3 3 2 2.6 3.1 13/12/50 0.0 1 1 1 3 3 3 2.6 3.1 13/17/05.0 21.8 2 2 10 15 10 3.3 3.8 13/19/50 0.0 1 1 1 3 3 2 2.6 3.1 13/29/30 0.0 1 1 1 2 2 2 2.5 2.9 13/34/50 0.0 1 1 1 1 1 1 2.2 2.6 13/41/20 0.0 1 1 1 1 1 1 2.2 2.6 13/47/10 0.0 1 1 1 3 2 2 2.5 2.9 13/50/10.0 21.6 2 2 13 28 22 3.6 4.2 13/59/50 0.0 1 1 1 1 1 1 2.2 2.6 14/03/50 0.0 1 1 1 1 1 1 2.2 2.6 14/06/30 0.0 1 1 1 , 1 1 1 2.2 2.6 14/17/24.0 22.2 1 1 1 2 3 3 2.6 3.1 14/19/40 0.0 1 1 1 1 1 1 2.2 2.6 14/21/20 0.0 1 1 1 2 2 2 2.5 2.9 14/23/30 0.0 1 1 1 1 1 1 2.2 2.6 ' 14/34/20 0.0 1 1 1 2 2 2 2.5 2.9 14/43/20 0.0 1 1 1 1 1 1 2.2 2.6 14/44/03.0 22.2 1 1 11 11 11 3.2 3.8 14/59/50 0.0 1 1 1 1 1 1 2.2 2.6 15/02/10 0.0 1 1 1 1 1 1 2.2 2.6 15/06/16.0 21.4 1 1 1 3 4 4 2.8 3.3 15/07/36.0 22.2 3 4 3 15 2 4 15 3.4 4.0 APPENDIX C (CONT.) P ARRIVAL S-P P AMPLITUDE H/M/S (SEC) V N E 15/14/10 0.0 1 1 1 15/26/17.0 0.0 15 22 7 15/37/10 0.0 1 1 1 15/41/30 0.0 1 1 1 15/45/22.0 20.4 1 1 1 15/47/51.0 21.2 1 1 1 15/50/30 0.0 1 1 1 15/52/50 0.0 1 1 1 15/58/10 0.0 1 1 1 16/00/04.7 20.4 1 1 1 16/02/00 0.0 1 1 1 16/04/30 0.0 1 1 1 16/08/50 0.0 1 1 1 16/20/31.4 0.0 11 18 4 16/28/30 0.0 1 1 1 16/31/50 0.0 1 1 1 16/32/20 0.0 1 1 1 16/34/33.0 21.0 1 1 1 16/44/50 0.0 1 1 1 16/45/38.0 21.8 1 1 1 16/50/30 0.0 1 1 1 17/07/20 0.0 1 1 1 17/09/10 0.0 1 1 1 17/12/21.8 22.0 1 1 1 17/18/20 0.0 1 I 1 17/19/50 0.0 1 1 1 17/20/30 0.0 1 1 1 17/22/28.8 20.2 1 1 1 17/30/00 0.0 1 1 1 17/42/00 0.0 1 1 1 17/47/10 0.0 1 1 1 17/51/00 0.0 1 1 1 17/53/30 0.0 1 1 1 18/02/00 0.0 . 1 1 1 18/14/51.0 21.8 1 1 1 18/16/50 0.0 1 1 1 S AMPLITUDE MAGNITUDE (MM) V N E RICHTER BODY I 2 2 2 2.5 2.9 75 75 75 4.0 4.7 1 1 1 2.2 2.6 1 2 2 2.5 2.9 2 2 2 2.5 2.9 3 3 3 2.6 3.1 1 2 2 2.5 2.9 3 2 3 2.6 3.1 1 2 3 2.6 3.1 3 2 2 2.5 2.9 1 1 2 2.3 2.8 1 1 1 2.2 2.6 2 2 2 2.5 2.9 55 55 55 3.9 4.5 3 3 3 2.6 3.1 1 2 2 2.5 2.9 2 2 2 2.5 2.9 2 2 2 2.5 2.9 1 1 1 2.2 2.6 3 2 2 2.5 2.9 1 1 1 2.2 2.6 1 1 1 2.2 2.6 2 2 2 2.5 2.9 11 7 12 3.1 3.7 1 1 1 2.2 2.6 1 1 1 2.2 2.6 2 2 2 2.5 2.9 3 2 2 2.5 2.9 2 2 3 2.6 3.1 2 2 2 2.5 2.9 3 2 3 2.6 3.1 1 1 1 2.2 2.6 3 3 3 2.6 3.1 1 1 1 2.2 2.6 3 2 3 2.6 3.1 1 1 1* 2.2 2.6 APPENDIX C (CONT.) P ARRIVAL S-P P AMPLITUDE S AMPLITUDE MAGNITUDE H/M/S (SEC) V ' N E (MM) V N E RiCHTER BODY WAVE 18/18/50 0.0 1 1 1 2 2 2 2.5 2.9 18/27/05.0 22.0 1 1 1 3 2 2 2.5 2.9 18/37/50 0.0 1 1 1 2 2 3 2.6 3.1 18/46/40 0.0 1 1 1 1 1 1 2.2 2.6 18/51/50 0.0 1 1 1 1 1 1 2.2 2.6 19/12/10 0.0 1 1 1 1 1 1 2.2 2.6 19/15/10 0.0 1 1 1 1 1 1 2.2 2.6 19/36/00 0.0 1 1 1 2 2 3 2.6 3.1 19/48/50.4 21.8 1 1 1 2 2 3 2.6 3.1 19/50/50 0.0 1 1 1 1 1 1 2.2 2.6 20/25/34.4 20.8 1 1 1 2 2 2 2.5 2.9 20/28/50 0.0 1 1 1 1 1 1 2.2 2.6 20/30/50 0.0 1 1 1 2 2 2 2.5 2.9 20/33/56.0 21.6 1 1 1 2 3 3 2.6 3.1 20/44/30 0.0 1 1 1 1 2 1 2.3 2.8 20/46/00 0.0 1 1 1 2 3 2 2.6 3.1 20/48/13.8 22.0 1 1 1 14 12 12 3.2 3.8 20/56/50 0.0 1 1 1 2 3 2 2.6 3.1 21/07/40 0.0 1 1 1 2 2 2 2.5 2.9 21/14/40 0.0 1 1 1 1 1 1 2.2 2.6 21/54/40 0.0 1 1 I 1 1 1 2.2 2.6 22/11/16.2 21.0 1 1 1 2 2 2 2.5 2.9 22/15/24.8 21.4 1 1 1 2 3 2 2.6 3.1 22/18/41.0 20.6 1 1 1 3 3 3 2.6 3.1 22/41/30 0.0 l i 1 1 1 1 2.2 2.6 23/09/00 0.0 l l 1 2 2 1 2.3 2.8 23/17/09.4 21.0 l l 1 2 2 3 2.6 3.1 23/38/30 0.0 l l 1 1 2 1 2.3 2.8 23/47/50 0.0 l l 1 1 1 1 2.2 2.6 23/53/10 0.0 l l 1 2 2 2 2.5 2.9 GUST 29 00/08/40 0.0 l l 1 1 1 1 2.2 2.6 00/23/20 0.0 l i 1 1 2 1 2.3 2.8 00/23/40 0.0 l l 1 1 2 3. 2.3 2.8 00/27/10 0.0 l l 1 1 1 1 2.2 2.6 APPENDIX C (CONT.) P ARRIVAL S-P P AMPLITUDE S AMPLITUDE MAGNITUDE H/M/S (SEC) V N E (MM) V N E RICHTER BODY WAVE 00/55/04.6 22.2 1 1 1 3 2 3 2.6 3.1 01/15/40 0.0 1 1 1 1 1 1 2.2 2.6 01/33/20 0.0 1 1 1 1 1 1 2.2 2.6 02/02/00 0.0 1 1 1 1 1 1 2.2 2.6 03/21/12.6 20.0 1 1 1 2 2 2 2.5 2.9 04/06/40 0.0 1 1 1 1 1 1 2.2 2.6 05/05/12.0 21.8 1 1 • 1 4 4 4 2.8 3.3 05/10/30 0.0 1 1 1 1 1 1 .2.2 2.6 05/37/50 0.0 1 i . 1 1 1 1. 2.2 2.6 05/59/50 0.0 1 1 1 1 1 1 2.2 2.6 06/18/04.6 20.8 1 1 1 2 3 2 2.6 3.1 06/22/30 0.0 1 1 1 1 2 2 2.5 2.9 07/04/30 0.0 1 1 1 1 1 1 2.2 2.6 07/21/50.0 22.2 1 1 1 1 1 2 2.3 2.8 09/07/30 0.0 1 1 1 3 3 3 2.6 3.1 10/22/49.6 21.6 1 1 1 3 2 2 2.5 2.9 10/25/39.2 21.0 1 1 1 2 2 2 2.5 2.9 10/26/15.4 21.8 1 1 1 2 2 1 2.3 2.8 17/54/30 0.0 1 1 1 1 1 1 2.2 2.6 18/59/30 0.0 1 1 1 1 1 1 2.2 2.6 19/04/52.0 22.2 1 1 1 6 10 13 3.2 3.8 19/14/50 0.0 1 1 1 2 2 2 2.5 2.9 21/03/00 0.0 1 1 1 1 2 1 2.3 2.8 22/56/14.8 21.0 1 1 1 5 5 4 2.8 3.3 23/53/10.4 23.0 2 1 2 5 5 6 2.9 3.4 AUGUST 30 13/42/30 0.0 20/22/41.0 21.0 23/34/50 0.0 1 1 1 1 4 1 1 6 1 1 5 1 2.2 2.9 2.2 2.6 3.4 2.6 AUGUST 31 00/43/20 0.0 04/00/40.0 20.6 06/56/20 0.0 1 1 1 1 1 1 1 3 2 1 2 2 2.2 2.6 2.5 2.6 3.1 2.9 APPENDIX C (CONT.) P ARRIVAL S-P P AMPLITUDE S AMPLITUDE MAGNITUDE H/M/S (SEC) V N E (MM) V N • E RICHTER BODY WAVE 07/32/40 0.0 1 1 1 1 1 1 2.2 2.6 07/54/40 0.0 1 1 1 1 1 1 2.2 2.6 08/32/40 0.0 1 1 1 1 1 1 2.2 2.6 08/43/31.0 22.6 1 1 1 2 3 2 2.6 3.1 08/50/34.4 22.0 1 1 1 1 1 2 2.3 2.8 08/56/00 0.0 1 1 1 1 1 1 2.2 2.6 09/06/20 0.0 1 1 1 2 2 3 2.6 3.1 10/05/17.2 21.0 1 1 1 2 2 2 2.5 2.9 11/43/17.2 21.6 1 1 1 5 5 5 2.9 3.4 12/00/05.5 21.2 1 1 1 3 4 3 2.7 3.2 14/56/00 0.0 1 1 1 1 1 1 2.2 2.6 16/34/10 0.0 1 1 1 1 1 2 2.3 2.8 19/15/50 0.0 1 1 1 1 1 1 2.2 2.6 19/34/43.0 21.0 1 1 1 3 3 4 2.7 3.2 22/15/30 0.0 1 1 1 1 1 1 2.2 2.6 SEPTEMBER 1 03/04/50 0.0 1 1 1 2 2 2 2.5 2.9 12/14/50 0.0 1 1 1 1 1 1 2.2 2.6 12/33/30 0.0 1 1 1 2 2 2 2.5 2.9 13/06/40 0.0 1 1 1 1 1 1 2.2 2.6 

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