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A re-examination of the August 22, 1949 Queen Charlotte earthquake 1984

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A RE-EXAMINATION OF THE AUGUST 22, 1949 QUEEN CHARLOTTE EARTHQUAKE by TODD KENDALL BOSTWICK B.A., The U n i v e r s i t y Of C a l i f o r n i a , 1979 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF GEOPHYSICS AND ASTRONOMY We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA J u l y 1984 © Todd K e n d a l l B o s t w i c k , 1984 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 DE-6 (3/81) i i A b s t r a c t U s i n g p r e v i o u s l y u n a v a i l a b l e h o r i z o n t a l seismograms r e c o r d e d a t S i t k a , A l a s k a , t h i r t y - e i g h t new a f t e r s h o c k l o c a t i o n s were d e t e r m i n e d f o r the August 22, 1949 Queen C h a r l o t t e e a r t h q u a k e (Ms=8.1). The a f t e r s h o c k zone was found t o ext e n d from 300 km t o the n o r t h of t h e e p i c e n t e r t o 190 km s o u t h of the e p i c e n t e r , y i e l d i n g a t o t a l a f t e r s h o c k zone of 490 km. T h i s a f t e r s h o c k zone i m p l i e s t h a t a p r e v i o u s l y s u g g e s t e d s e i s m i c gap t o the n o r t h of the Ms=8.1 e a r t h q u a k e ( R o g e r s , 1983) does not e x i s t . The a f t e r s h o c k d i s t r i b u t i o n s u g g e s t s a time v a r i a t i o n of the r u p t u r e sequence, w i t h the a f t e r s h o c k s c l u s t e r i n g f i r s t t o the n o r t h , and then t o the s o u t h of the e p i c e n t e r . The d i r e c t i v i t y f u n c t i o n and d i f f e r e n t i a l phases were a n a l y z e d a t t h r e e s t a t i o n s . The r e s u l t s i m p l y a u n i l a t e r a l r u p t u r e p r o p a g a t i n g t o the .northwest f o r 265 km a t a r u p t u r e v e l o c i t y between 3.1 km/s and 3.5 km/s. The d i f f e r e n c e between t h e r a d i a t i o n l e n g t h and the a f t e r s h o c k zone i m p l i e s t h a t t h e r a d i a t i o n f a u l t l e n g t h does not r e p r e s e n t t h e f u l l r u p t u r e f a u l t l e n g t h . The n o n - e q u i v a l e n c e of the r a d i a t i o n f a u l t l e n g t h and the r u p t u r e f a u l t l e n g t h s u g g e s t s t h a t the d i s p l a c e m e n t o f f s e t a l o n g the f a u l t was uneven, w i t h the l a r g e s t d i s p l a c e m e n t o c c u r r i n g i n t h e zone i n d i c a t e d by the r a d i a t i o n f a u l t l e n g t h . An a t t e m p t was made t o d e r i v e the mechanism s o l u t i o n s f o r the two l a r g e s t a f t e r s h o c k s u s i n g t h e i r a z i m u t h a l s u r f a c e wave r a d i a t i o n p a t t e r n s . I t was c o n c l u d e d t h a t t h e use of t h i s t e c h n i q u e t o o b t a i n f o c a l mechanism s o l u t i o n s i s i n e f f e c t i v e f o r i i i t h i s a r e a and time i n h i s t o r y when s t a t i o n coverage was s p a r s e and the q u a l i t y of i n s t r u m e n t c a l i b r a t i o n s p o o r . i v T a b l e of Co n t e n t s A b s t r a c t i i L i s t of T a b l e s v i L i s t of F i g u r e s v i i Acknowledgement x Chapter I INTRODUCTION 1 1.1 THE QUEEN CHARLOTTE FAULT 1 1.1.1 T e c t o n i c S e t t i n g Of The Queen C h a r l o t t e F a u l t ....1 1.1.2 S e i s m i c i t y Of The Queen C h a r l o t t e F a u l t 6 i . E p i c e n t e r L o c a t i o n s 6 i i . F o c a l Mechanisms ..8 1.1.3 F a u l t Model 9 1.2 THE 1949, M 8.1, QUEEN CHARLOTTE EARTHQUAKE 11 1.2.1 F o c a l Mechanism 12 1.2.2 S e i s m i c Gap 14 1.3 THESIS OBJECTIVES 16 Chapter I I AFTERSHOCK ZONE OF THE 1949 QUEEN CHARLOTTE EARTHQUAKE ...19 2 . 1 PROCEDURE 19 2.2 RESULTS 27 Chapter I I I SURFACE WAVE ANALYSIS 34 3.1 POINT SOURCE MECHANISM SOLUTION 34 3.1.1 I n t r o d u c t i o n 34 3.1.2 Theory And Computer Programs 35 3.2 RUPTURE PARAMETERS 41 3.2.1 I n t r o d u c t i o n 41 3.2.2 D i r e c t i v i t y F u n c t i o n Theory 42 3.2.3 D i f f e r e n t i a l Phases Theory 44 3.3 PROCESSING 46 3.3.1 Data A c q u i s i t i o n 46 3.3.2 Data P r o c e s s i n g 48 3.3.3 A m p l i t u d e Data ; 49 3.3.4 Phase Data 51 Chapter IV RESULTS FOR THE M=8.1 EARTHQUAKE OF AUGUST 22 52 4.1 DIRECTIVITY FUNCTION 59 4.2 DIFFERENTIAL PHASES 65 4.3 ERROR ANALYSIS 68 4.4 SEISMIC MOMENT AND STRESS DROP 69 Chapter V RESULTS FOR THE EARTHQUAKES OF AUGUST 23 AND OCTOBER 31 ..74 MECHANISM SOLUTIONS 87 V Chapter VI SUMMARY AND DISCUSSION 94 6.1 DISCUSSION 94 6.2 CONCLUSION 99 BIBLIOGRAPHY 101 APPENDIX A - LIST OF SEISMOGRAPH STATIONS .106 APPENDIX B - WORLD AVERAGED PHASE VELOCITIES AND Q VALUES 108 APPENDIX C - EARTH MODEL USED FOR THE AUGUST 23 AND OCTOBER 3 1 MECHANI SM SOLUTIONS ....110 APPENDIX D - INSTRUMENT RESPONSE CURVES 111 v i L i s t of T a b l e s I . PUBLISHED FAULT PLANE SOLUTIONS FOR EARTHQUAKES ON THE QUEEN CHARLOTTE FAULT 8 I I . AFTERSHOCK DATA 22 I I I . DIFFERENTIAL PHASE FAULT LENGTHS 66 IV. RUPTURE LENGTH FROM DIRECTIVITY AND DIFFERENTIAL PHASES 95 V. ESTIMATE OF SEISMIC SOURCE PARAMETERS OF THE 1949 QUEEN CHARLOTTE EARTHQUAKE 100 v i i L i s t of F i g u r e s 1. TECTONIC SETTING OF THE QUEEN CHARLOTTE FAULT 1 2. RECENT TECTONIC HISTORY OF THE QUEEN CHARLOTTE FAULT ..3 3. DIRECTION OF RELATIVE PLATE MOTION 4 4. MODEL OF THE QUEEN CHARLOTTE FAULT ZONE 5 5. SEISMICITY OF THE QUEEN CHARLOTTE REGION 7 6. STRUCTURAL INTERPRETATION OF A REFRACTION SURVEY ACROSS THE QUEEN CHARLOTTE FAULT ZONE 10 7. P-NODAL MECHANISM SOLUTION FOR THE AUGUST 22, 1949 QUEEN CHARLOTTE EARTHQUAKE 13 8. PROPOSED SEISMIC GAPS ON THE QUEEN CHARLOTTE FAULT ...15 9. AFTERSHOCK LOCATIONS 23 10. RELATIONSHIP OF THE CHATHAM STRAIT AND FAIRWEATHER FAULTS TO THE QUEEN CHARLOTTE FAULT 26 11. TIME DISTRIBUTION OF AFTERSHOCKS 27 12. EMPIRICAL MAGNITUDE RUPTURE-LENGTH RELATIONSHIP FOR 7 REGIONS OF THE WORLD 2 9 13. MAGNITUDE RUPTURE-AREA RELATIONSHIPS 3 0 14. TIME PROGRESSION OF AFTERSHOCKS 33 15. DIGITIZED TUO SEISMOGRAMS FOR THE AUGUST 22 EARTHQUAKE (M=8.1) 54 16. DIGITIZED PAS AND HON SEISMOGRAMS FOR THE AUGUST 22 EARTHQUAKE (M= 8.1) 55 17. TYPICAL GROUP VELOCITY CURVES FOR LOVE AND RAYLEIGH WAVES 56 18. GROUP VELOCITY CURVES FOR THE AUGUST 22 DATA 57 19. GROUP VELOCITY CURVES FOR THE AUGUST 22 DATA 58 20. DIRECTIVITY FUNCTION CURVES 60 21. DECAYING SOURCE DIRECTIVITY FUNCTION CURVES 62 v i i i 22. THE EFFECT OF BILATERAL RUPTURE ON THE DIRECTIVITY FUNCTION 63 23. LEAST SQUARES SOLUTION TO THE DATA ..64 24. RESPONSE CURVE FOR THE PAS STRAIN METER 70 25. DIGITIZED SEISMOGRAMS FOR THE AUGUST 23 EARTHQUAKE (M=6.4) .75 26. DIGITIZED SEISMOGRAMS FOR THE AUGUST 23 EARTHQUAKE (M=6.4) 76 27. DIGITIZED SEISMOGRAMS FOR THE OCTOBER 31 EARTHQUAKE (M=6.2) 77 28. DIGITIZED SEISMOGRAMS FOR THE OCTOBER 31 EARTHQUAKE (M=6.2) 78 29. DIGITIZED SEISMOGRAMS FOR THE OCTOBER 31 EARTHQUAKE (M=6.2) 79 30. GROUP VELOCITY CURVES FOR THE AUGUST 23 LOVE WAVE DATA 80 31. GROUP VELOCITY CURVES FOR THE AUGUST 23 RAYLEIGH WAVE DATA 81 32. GROUP VELOCITY CURVES FOR THE AUGUST 23 SJP DATA 82 33. GROUP VELOCITY CURVES FOR THE OCTOBER 31 LOVE WAVE DATA 83 34. GROUP VELOCITY CURVES FOR THE OCTOBER 31 LOVE WAVE DATA 84 35. GROUP VELOCITY CURVES FOR THE OCTOBER 31 RAYLEIGH WAVE DATA 85 36. GROUP VELOCITY CURVES FOR THE OCTOBER 31 RAYLEIGH WAVE DATA 86 37. THEORETICAL LOVE WAVE RADIATION PATTERN FOR THE AUGUST 23 EARTHQUAKE 88 38. THEORETICAL RAYLEIGH WAVE RADIATION PATTERN FOR THE AUGUST 23 EARTHQUAKE 89 39. THEORETICAL LOVE WAVE RADIATION PATTERN FOR THE OCTOBER 31 EARTHQUAKE 90 40. THEORETICAL RAYLEIGH WAVE RADIATION PATTERN FOR THE OCTOBER 31 EARTHQUAKE 91 i.x 41. FOCAL MECHANISMS OF THE AUGUST 23 AND OCTOBER 31 EARTHQUAKES 92 42. DISPLACEMENT ALONG THE SAN ANDREAS FAULT FOR THE 1906 EARTHQUAKE 96 X Acknowledgement T h i s r e s e a r c h owes much t o Dr. G a r r y Rogers. Dr. Rogers p r o v i d e d the o r i g i n a l s u g g e s t i o n f o r the t h e s i s , and then gave encouragement and d i r e c t i o n d u r i n g i t s development. Dr. Rogers a l s o o b t a i n e d the SIT s e i s m i c r e c o r d s f o r me. My a d v i s o r , Dr. E l l i s , p r o v i d e d most of the computer programs used i n t h i s t h e s i s . He a l s o p r o v i d e d u s e f u l d i r e c t i o n i n the development of t h i s t h e s i s . Dr. Clowes p o v i d e d v a l u a b l e s u p p o r t as my s u r r o g a t e a d v i s o r d u r i n g Dr. E l l i s ' s s a b b a t i c a l l e a v e . A s p e c i a l t h a n k s goes t o a l l who read my t h e s i s and improved i t : Dr. C lowes, Dr. Rogers, Dr. E l l i s , and Dr. Armstrong. I would a l s o l i k e t o thank Don W h i t e , Dave M a c k i e , Ian J ones, and, most i m p o r t a n t l y , my w i f e K i m i k o , f o r making my t ime i n Canada so e n j o y a b l e . F i n a n c i a l s u p p o r t f o r t h i s p r o j e c t was p r o v i d e d by R e s e a r c h Agreement 288/83 from the Department of Energy, Mines and R e s o u r c e s , and by NSERC O p e r a t i n g Grant A2617. I . INTRODUCTION 1.1 THE QUEEN CHARLOTTE FAULT 1 . 1 . 1 T e c t o n i c S e t t i n g Of The Queen C h a r l o t t e F a u l t The Queen C h a r l o t t e F a u l t Zone a l o n g the west c o a s t of the Queen C h a r l o t t e I s l a n d s i s a t r a n s f o r m f a u l t making up p a r t of the boundary between the P a c i f i c and N o r t h American l i t h o s p h e r i c p l a t e s (see F i g u r e l ) . F i g u r e 1 - TECTONIC SETTING OF THE QUEEN CHARLOTTE FAULT Geo g r a p h i c f e a t u r e s , t e c t o n i c s e t t i n g and key seismograph s t a t i o n s (SIT and VIC) i n the Queen C h a r l o t t e I s l a n d s r e g i o n (adapted from Rogers, 1 9 8 3 ) . The f a u l t zone extends northwestward from a r i d g e - t r e n c h - 2 t r a n s f o r m t r i p l e j u n c t i o n s o u t h of the Queen C h a r l o t t e I s l a n d s t o s o u t h e r n A l a s k a where i t i s a l s o known as the C h i c h a g o f - Baranof f a u l t {Von Huene e t . a l . , 1979). C u r r e n t motion a l o n g the f a u l t , as d e t e r m i n e d from g l o b a l p l a t e motions of M i n s t e r and Jordan (1978) and mid-ocean r i d g e s p r e a d i n g r a t e s , i s p r i m a r i l y r i g h t - l a t e r a l s t r i k e - s l i p a t a r a t e of 5.5 cm/yr (see d i s c u s s i o n i n R i d d i h o u g h , 1977). The f a u l t zone c h a r a c t e r i s t i c a l l y shows two d i s t i n c t f a u l t escarpments. Wedged between the two escarpments l i e s a submarine t e r r a c e which i s e l e v a t e d more than 1 km above the b a s i n t o the west. R i d d i h o u g h e t a l . (1980) have proposed t h a t the p r e s e n t c o n f i g u r a t i o n of the f a u l t zone d e v e l o p e d about 1 Ma ago, when a t r i p l e j u n c t i o n , then l o c a t e d a t t h e end of the E x p l o r e r R i d g e , jumped n o r t h w e s t e r l y t o t h e D e l l w o o d K n o l l s i n response t o changing s p r e a d i n g c o n d i t i o n s of the E x p l o r e r R i d g e . T h i s l e d t o r i f t i n g of o l d e r ocean c r u s t (4.5 Ma) and the b e g i n n i n g of s p r e a d i n g a t the newly c r e a t e d D e l l w o o d k n o l l s . Hyndman and E l l i s (1981) have suggested t h a t t h i s change i n p o s i t i o n of the t r i p l e j u n c t i o n r e q u i r e d a landward jump of the Queen C h a r l o t t e f a u l t t o what i s now seen as the i n n e r escarpment, w i t h the o u t e r Queen C h a r l o t t e f a u l t s c a r p accommodating the t r a n s c u r r e n t motion between the P a c i f i c and N o r t h American p l a t e s p r i o r t o the jump 1 Ma ago (see F i g u r e 2).. 3 F i g u r e 2 - RECENT TECTONIC HISTORY OF THE QUEEN CHARLOTTE FAULT S c h e m a t i c r e c o n s t r u c t i o n o f p l a t e t e c t o n i c development r e l e v a n t t o the Queen C h a r l o t t e I s l a n d s r e g i o n f o r the p a s t 1 Ma. AM, t h e N o r t h A m e r i c a n p l a t e ; EX, t h e E x p l o r e r p l a t e ; PA, t h e P a c i f i c p l a t e . S i n g l e l i n e s w i t h o p p o s i n g a r r o w s r e p r e s e n t t r a n s f o r m m a r g i n s and the d i r e c t i o n o f s l i p ; d o u b l e l i n e 3 r e p r e s e n t s p r e a d i n g m a r g i n s ; and t o o t h e d l i n e s r e p r e s e n t c o n v e r g i n g m a r g i n s w i t h t e e t h on t h e o v e r r i d i n g p l a t e . At 1 Ma note the d i f f e r e n c e between the r e l a t i v e m o t i o n v e c t o r and t h e t r a c e o f the t r a n s f o r m f a u l t between E x p l o r e r R i d g e and Delwood K n o l l s . T h i s g i v e s r i s e t o i n s t a b i l i t y o f the t r i p l e j u n c t i o n and the t r a n s f o r m f a u l t and l e a d s t o a s y m m e t r i c s p r e a d i n g . A t 0.5-1 Ma s p r e a d i n g a t Tuzo W i l s o n K n o l l s b e g i n s . T h i s r e q u i r e s t h e t r a n s f o r m m o t i o n between PA and AM t o jump l a n d w a r d i n t o o l d e r c r u s t , w h i c h i s then pushed n o r t h w a r d w i t h PA. The i n s t a b i l i t y a t Delwood K n o l l s c a u s e s r e a d j u s t m e n t o f E x p l o r e r R i d g e and the t r a n s f o r m f a u l t j o i n i n g t h e two ( a f t e r R i d d i h o u g h e t a l . , 1980; Hyndman and E l l i s , 1 9 8 1 ) . (From Horn e t . a l . , 1984). 4 In the p r e s e n t c o n f i g u r a t i o n of the f a u l t zone, the b e a r i n g of the i n n e r f a u l t t r a c e l i e s 10° to 20° west of the c a l c u l a t e d r e l a t i v e motion v e c t o r of M i n s t e r and Jordan (1978) between the P a c i f i c and No r t h American p l a t e s f o r the Queen C h a r l o t t e r e g i o n . The a n g l e of t h i s o b l i q u e i n t e r a c t i o n i s s m a l l a t the n o r t h e r n end of the f a u l t but i n c r e a s e s towards the southern end a l o n g Moresby I s l a n d (see f i g u r e 3 ) . F i g u r e 3 - DIRECTION OF RELATIVE PLATE MOTION The r e l a t i v e motion arrows show the motion of t h e • P a c i f i c P l a t e r e l a t i v e t o the N o r t h American P l a t e (adapted from Rogers 1983). The a n g l e of p l a t e i n t e r a c t i o n suggests t h e r e i s an element of convergence i n the Queen C h a r l o t t e r e g i o n . The convergence c o u l d occur by o b l i q u e s u b d u c t i o n of the P a c i f i c p l a t e or c o m p r e s s i v e d e f o r m a t i o n of the Queen C h a r l o t t e I s l a n d s and a d j a c e n t a r e a s , or some c o m b i n a t i o n of both (Perez and J a c o b , 5 1980) . To account f o r the convergence, Hyndman and E l l i s (1981) have suggested an u n d e r t h r u s t model, s u b s e q u e n t l y r e f i n e d by Hyndman et a l . (1982), f o r the Queen C h a r l o t t e f a u l t zone. They suggest o b l i q u e s u b d u c t i o n c o u l d be a c c o m p l i s h e d f i r s t by a s e r i e s of s t r i k e - s l i p e a r t h q u a k e s , then by o b l i q u e convergence u n t i l c o u p l i n g between the o v e r r i d i n g and u n d e r l y i n g p l a t e i s so g r e a t t h a t i t exceeds the s t r e n g t h of the o c e a n i c l i t h o s p h e r e , and another s e r i e s of s t r i k e - s l i p e v e n t s o c c u r s . Thus, by t h i s model, t r a n s c u r r e n t f a u l t i n g o c c u r s i n the o c e a n i c l i t h o s p h e r e beneath the c o n t i n e n t a l margin. As the p l a t e i s subducted, at i n t e r v a l s i n time the f a u l t i n g must jump seaward t o remain near the edge of the c o n t i n e n t a l c r u s t (see f i g u r e 4 ) . Q.C.Troug! Deformation Front Terrace Coast Q.C. Islands F i g u r e 4 - MODEL OF THE QUEEN CHARLOTTE FAULT ZONE A p o s s i b l e t e c t o n i c model of the Queen C h a r l o t t e f a u l t zone. The o b l i q u e convergence i s r e s o l v e d i n t o s t r i k e - s l i p motion p a r a l l e l t o the margin on the Queen C h a r l o t t e f a u l t and a s m a l l component of u n d e r t h r u s t i n g p e r p e n d i c u l a r t o the margin (from Hyndman et a l . , 1982). 6 1.1.2 S e i s m i c i t y Of The Queen C h a r l o t t e F a u l t The Queen C h a r l o t t e r e g i o n i s one of the most s e i s m i c a l l y a c t i v e a r e a s of Canada. Indeed, the 1949 Queen C h a r l o t t e earthquake (MS=8.1) which o c c u r r e d i n the m i d d l e of the Queen C h a r l o t t e F a u l t i s the l a r g e s t r e c o r d e d earthquake t o have o c c u r r e d i n Canada. B e s i d e s the 1949 e v e n t , t h r e e Ms^7 earthquakes have o c c u r r e d s i n c e 1900, one i n 1929 (Ms=7) a t the southern, end of the Queen C h a r l o t t e F a u l t , one i n 1972 (Ms=7.6) a t the n o r t h e r n end of the f a u l t , and the o t h e r i n 1970 (Ms=7.0) near t h e s o u t h e r n end of the f a u l t (see F i g u r e 5 ) . i . E p i c e n t e r L o c a t i o n s E p i c e n t e r s o l u t i o n s of e a r t h q u a k e s i n the Queen C h a r l o t t e I s l a n d s r e g i o n have been o b t a i n e d by t h r e e p r i n c i p a l groups of i n v e s t i g a t o r s : T o b i n and Sykes (1968), K e l l e h e r and S a v i n o (1975), and Rogers (1983). In g e n e r a l , the a c c u r a c y of e p i c e n t e r l o c a t i o n s have been l i m i t e d by the s c a r c i t y of seismographs o p e r a t i n g i n the r e g i o n . E p i c e n t e r u n c e r t a i n t y f o r events of magnitude M >5.0 i s ±50 km f o r most e v e n t s b e f o r e 1965 and ±25 km f o r most ev e n t s a f t e r 1965. Rogers (1983), a f t e r a d e t a i l e d e v a l u a t i o n of the s e i s m i c i t y d a t a f o r the Queen C h a r l o t t e f a u l t zone, has c o m p i l e d a s e i s m i c i t y map of the a r e a from 1900 t o 1980 (see F i g u r e 5 ) . The s e i s m i c i t y shows a s t r o n g c o r r e l a t i o n w i t h the i n n e r Queen C h a r l o t t e f a u l t s c a r p w i t h l i t t l e , i f any, s e i s m i c i t y i n l a n d . T h i s s u g g e s t s t h a t a t p r e s e n t most, i f not a l l , of the P a c i f i c - N o r t h American p l a t e motion i n t h i s a r e a o c c u r s a l o n g the Queen 7 C h a r l o t t e f a u l t . F i g u r e 5 - SEISMICITY OF THE QUEEN CHARLOTTE REGION E p i c e n t e r l o c a t i o n s from 1900 t o 1980. Through the y e a r s the t h r e s h o l d magnitude r e q u i r e d f o r earthquake d e t e c t i o n has dropped from M >7 between 1900-1917, t o M >6 between 1917-1948, t o M >5.0 between 1948-1980 (adapted from Rogers, 1983). 8 i i . F o c a l Mechanisms A l l p u b l i s h e d f o c a l mechanisms f o r e a r t h q u a k e s i n the Queen C h a r l o t t e F a u l t Zone are l i s t e d i n T a b l e 1. Table I - PUBLISHED FAULT PLANE SOLUTIONS FOR EARTHQUAKES ON THE QUEEN CHARLOTTE FAULT 1 927 Oct 24 1 949 Aug 22 1 949 Oct 31 1958 J u l 1 0 1 970 Jun 24 1 970 Jun 24 1 972 J u l 30 1 972 Aug 04 1 972 Aug 1 5 1 973 J u l 01 1 973 J u l 03 1 976 Feb 03 Stauder,1959; Wickens and Hodgson,1967 Hodgson and M i l n e , 1 9 5 1 ; Wickens and Hodgson,1967; Rogers, 1983 Hodgson and S t o r e y , 1 9 5 4 ; Wickens and Hodgson,1967 Wickens and Hodgson,1967 Rogers, 1983 Stauder Chandra Chandra Chandra Chandra 1960; 1 974; 1 974 1 974; 1 974 Perez and Jacob,1980 Perez and Jacob,1980 Chandra,1974; Perez and Jacob,1980 Chandra,1974; Perez and Jacob,1980 W e t m i l l e r and Horner,1978; Rogers, 1983 s t r i k e - s l i p s t r i k e - s l i p t h r u s t s t r i k e - s l i p s t r i k e - s l i p s t r i k e - s l i p s t r i k e - s l i p t h r u s t s t r i k e - s l i p t h r u s t s t r i k e - s l i p s t r i k e - s l i p The l o c a t i o n and o r i e n t a t i o n of the Queen C h a r l o t t e F a u l t r e l a t i v e t o the g l o b a l d i s t r i b u t i o n of seismographs has e f f e c t i v e l y l i m i t e d the s i z e of e a r t h q u a k e s f o r which w e l l d e f i n e d P-nodal mechanism s o l u t i o n s have been o b t a i n e d . The s o u t h e r n e x t e n s i o n of the P-nodal f a u l t p l a n e i s u s u a l l y w e l l c o n s t r a i n e d because i t b i s e c t s the network of C a l i f o r n i a s t a t i o n s (see F i g u r e 7 ) . But t o a c c u r a t e l y d e f i n e the a z i m u t h and d i p , the e a r t h q u a k e s must be l a r g e enough t o r e c o r d w e l l i n Europe. Thus, i n the Queen C h a r l o t t e I s l a n d s r e g i o n the o n l y w e l l d e f i n e d f a u l t p l a n e s o l u t i o n s a r e those f o r the June 24, 1970 earthquake (Ms=7), and t h e August 22, 1949 earthquake (Ms=8.1), (Rogers, 1983). 9 The mechanism s o l u t i o n f o r the June 24, 1970 earthquake (Chandra 1974; Rogers, 1983) which o c c u r r e d a t the s o u t h e r n end of the Queen C h a r l o t t e f a u l t i s c o n s i s t e n t w i t h p r e d i c t e d p l a t e motions of M i n s t e r and J o r d a n (1978) or Chase (1978) f o r the a r e a . Perez and Jacob (1980) s t u d i e d 19 f a u l t p l a n e s o l u t i o n s i n the e a s t e r n G u l f of A l a s k a r e g i o n , f o u r of which o c c u r r e d on the n o r t h e r n end of the Queen C h a r l o t t e f a u l t . The sense of r e l a t i v e p l a t e motion c a l c u l a t e d from t h e s e e a r thquake mechanism s o l u t i o n s a l s o c o r r e s p o n d s t o the p r e d i c t e d r e l a t i v e p l a t e motion of the a r e a . These f a u l t p l a n e s o l u t i o n s imply t h a t the p r e d i c t e d p l a t e motions f o r the a r e a of M i n s t e r and J o r d a n (1978) or Chase (1978) a r e c o r r e c t . 1.1.3 F a u l t Model . An i n s i g h t i n t o the c r o s s - s e c t i o n a l s t r u c t u r e of the Queen C h a r l o t t e f a u l t zone has been p r o v i d e d by the r e f r a c t i o n s u r v e y a c r o s s the s o u t h e r n h a l f of the f a u l t zone d i s c u s s e d by Horn e t a l . (1984). T h e i r i n t e r p r e t e d s t r u c t u r e s e c t i o n shows t h r e e d i s t i n c t i v e c r u s t a l b l o c k s s e p a r a t e d by two major c r u s t a l l y p e r v a s i v e f a u l t s c o r r e s p o n d i n g t o the i n n e r and o u t e r Queen C h a r l o t t e f a u l t s . The rock u n i t s making up the westernmost b l o c k had v e l o c i t i e s c o n s i s t e n t w i t h u n d i s t u r b e d o c e a n i c c r u s t . The ro c k u n i t s composing the t e r r a c e b l o c k had lower v e l o c i t i e s a t e q u i v a l e n t depths than t h o s e of the ocean b l o c k or the c o n t i n e n t a l b l o c k t o the e a s t . The depth t o the base of the c r u s t a l s e c t i o n of the t e r r a c e b l o c k i n c r e a s e d i n depth from 12 to 18 km below sea l e v e l , i m p l y i n g an eastward d i p of 20° i n the 1 0 Moho. The i n n e r f a u l t s e p a r a t i n g the t e r r a c e b l o c k u n i t from the c o n t i n e n t a l b l o c k appeared t o be d i p p i n g westward 60°-80°. F i g u r e 6 shows the s t r u c t u r a l i n t e r p r e t a t i o n of by Horn et a l . (1984). T h i s i n t e r p r e t a t i o n i s a l s o c o n s i s t e n t w i t h a g r a v i t y p r o f i l e a c r o s s the a r e a . D i s t a n c e ( k m ) w 80 100 F i g u r e 6 - STRUCTURAL INTERPRETATION OF A REFRACTION SURVEY ACROSS THE QUEEN CHARLOTTE FAULT ZONE The f i r s t number i n t h e v e l o c i t y s t r a t i g r a p h y g i v e s t h e v e l o c i t y a t t h e t o p o f the u n i t , and t h e se c o n d number g i v e s the v e l o c i t y a t t h e bottom o f t h e same u n i t , a l i n e a r g r a d i e n t b e i n g assumed. RLF, Louscoone I n l e t f a u l t ; SCB, San C r i s t o v a l b a t h o l i t h ( t h e dashed v e r t i c a l l i n e s w i t h i n u n i t 4 show t h e s u r f a c e e x p r e s s i o n o f t h i s b a t h o l i t h ) ; QCF, Queen C h a r l o t t e f a u l t ; and EX2 shows the l o c a t i o n o f the p e r p e n d i c u l a r marine p r o f i l e . (From Horn et a l . , 1984) . 11 The r e s u l t s of Horn e t a l . (1984) a r e a l s o c o n s i s t e n t w i t h the r e s u l t s of Hyndman and E l l i s (1981). In a study of the m i c r o e a r t h q u a k e a c t i v i t y a l o n g the Queen C h a r l o t t e f a u l t zone f o r a t e n day p e r i o d , Hyndman and E l l i s (1981) found almost a l l the s e i s m i c i t y t o be l o c a t e d a l o n g what appeared t o be a near v e r t i c a l f a u l t a s s o c i a t e d w i t h the landward or i n n e r f a u l t s c a r p ; the o u t e r s c a r p showed no a c t i v i t y . The obser v e d maximum earthquake depth was 21 km. U s i n g the heat f l o w r e s u l t s of Hyndman e t a l . (1981) and t h e r m a l e l a s t i c arguments, Hyndman and E l l i s (1981) a l s o e s t i m a t e d the depth i n the c r u s t of the 600°C i s o t h e r m , which r o u g h l y c o r r e s p o n d s t o the depth i n the c r u s t t o which s e i s m i c i n s t a b i l i t y or b r i t t l e f r a c t u r e can o c c u r . They found the depth to t h i s boundary t o be about 8 km f o r the ocean b a s i n , 16 km f o r the submarine t e r r a c e and 35 km f o r the c o n t i n e n t a l c r u s t i n the a r e a . These r e s u l t s agree r e a s o n a b l y w e l l w i t h the ob s e r v e d maximum depth of earthquake a c t i v i t y and w i t h the depth of the Moho from Horn e t a l . (1984). 1.2 THE 1949, M 8.1, QUEEN CHARLOTTE EARTHQUAKE On Sunday August 22, 1949 a t a p p r o x i m a t e l y 9:00 P.M. ( l o c a l t i m e ) , the Queen C h a r l o t t e I s l a n d s were shaken by a s t r o n g e a r t h q u a k e . The e a r t h q u a k e , f e l t as f a r away as Whitehorse (A=6.0°) and J a s p e r (A=7.0°) caused c o n s i d e r a b l e e x c i t e m e n t . Newspapers r e p o r t e d t h a t "Nervous r e s i d e n t s of the Queen C h a r l o t t e s a re wondering i f t h e i r i s l a n d s a r e s l o w l y s i n k i n g i n t o the P a c i f i c ocean"; " S m a l l i s l a n d s have d i s a p p e a r e d 12 i n some s p o t s , and new i s l a n d s have appeared i n o t h e r s f o l l o w i n g the r e c e n t e a r t h q u a k e s o f f B.C. c o a s t " . The s e i s m i c hazards i n the Queen C h a r l o t t e r e g i o n a r e v i v i d l y i l l u s t r a t e d by these newspaper r e p o r t s and p r o v i d e a c o l o r f u l i n t r o d u c t i o n t o the 1949 Queen C h a r l o t t e E a r t h q u a k e . 1.2.1 F o c a l Mechanism S o l u t i o n s f o r the f o c a l mechanism of t h i s e a r t h quake u s i n g the f i r s t motion of P waves have been o b t a i n e d by Hodgson and M i l n e (1951), . Wickens and Hodgson (1967), and Rogers (1983). The n i n e l a r g e s t a f t e r s h o c k s have been l o c a t e d by Tobin and Sykes (1968), and more r e c e n t l y , r e l o c a t e d by Rogers (1983). The s t r i k e of the f o c a l mechanism f o r the August 22, 1949 earthquake c o r r e s p o n d s e x a c t l y w i t h the s t r i k e of the f a u l t a t the l a t i t u d e of the e p i c e n t e r . The d i p of the f a u l t i s v e r y s t e e p and w e l l c o n s t r a i n e d , w i t h the motion a l o n g the f a u l t p r i n c i p a l l y s t r i k e - s l i p c o u p l e d w i t h a v e r y s m a l l t h r u s t component (see F i g u r e 7 ) . However the d i r e c t i o n of the net h o r i z o n t a l motion d u r i n g t h i s earthquake i s s i g n i f i c a n t l y d i f f e r e n t (15°) from the p r e d i c t e d p l a t e motions f o r the a r e a . I f the p l a t e i n t e r a c t i o n models are c o r r e c t , as mechanism s o l u t i o n s of the J u l y 24, 1970 earthquake (Chandra, 1974; Rogers, 1983) and the s o u t h e r n A l a s k a e a r t h q u a k e s (Perez and J a c o b , 1980) i m p l y , then t h e r e i s a component of convergence, a l o n g the Queen C h a r l o t t e f a u l t a t the l a t i t u d e of the 1949 e a r t h q u a k e , not taken up by t h a t e a r t h q u a k e . The convergence might be t a k e n i n t o account i n s e v e r a l 13 ways. One p o s s i b i l i t y i s t h a t the l o n g p e r i o d motion of the ea r t h q u a k e i s d i f f e r e n t from the motion of the i n i t i a l f i r s t break as i n d i c a t e d by t h e f i r s t motion mechanism s o l u t i o n . Convergence c o u l d a l s o be p a r t i a l l y t a k e n up i n e i t h e r of t h e two l a r g e s t a f t e r s h o c k s on August 23 (M=6.4) and Oc t o b e r 31 (M=6.2). A l t e r n a t i v e l y , t h i s c o u l d be one of the l a r g e s t r i k e - s l i p e a r t h q u a k e s p r e d i c t e d by the p r e v i o u s l y mentioned Hyndman and E l l i s (1981) model. S.TH A S F i g u r e 7 - P-NODAL MECHANISM SOLUTION FOR THE AUGUST 22, 1949 QUEEN CHARLOTTE EARTHQUAKE August 22, 1949, mechanism s o l u t i o n lower hemisphere p r o j e c t i o n . P o s i t i o n of key s t a t i o n s a r e i n d i c a t e d on the f o c a l sphere (Adapted from Rogers, 1983). 1 4 1.2.2 S e i s m i c Gap K e l l e h e r and S a v i n o (1975) have p o i n t e d out the p o s s i b i l i t y of a s e i s m i c gap t o the n o r t h of the August 22, 1949 earthquake (see F i g u r e 8 ) . The u n c e r t a i n t y comes from whether t o c o n s i d e r as a f t e r s h o c k s an event l o c a t e d near 56°N on August 23, 1949 and two e a r t h q u a k e s (M=6.25 and 5.5) a l s o near 56°N t h a t o c c u r r e d on October 31, 1949, more than two months a f t e r the main shock. I f the northern-most e v e n t s are a f t e r s h o c k s , the r u p t u r e l e n g t h i s 470 km, and no s e i s m i c gap e x i s t s ; i f not the r u p t u r e l e n g t h i s about 300 km, and a s e i s m i c gap does e x i s t . A l t h o u g h no d e f i n i t e c r i t e r i a have been e s t a b l i s h e d f o r i d e n t i f i c a t i o n of a f t e r s h o c k s a t t e l e s e i s m i c d i s t a n c e s , K e l l e h e r (1972), when s t u d y i n g South American r u p t u r e zones, d i d not c o n s i d e r an event an a f t e r s h o c k i f i t o c c u r r e d more than two months a f t e r the mainshock or i f i t was i s o l a t e d by 50 km or more from o t h e r a f t e r s h o c k s . By t h e s e c r i t e r i a t h e s e e v e nts would not be c o n s i d e r e d as a f t e r s h o c k s and the s h o r t e r r u p t u r e l e n g t h of 300 km would be f a v o r e d . The r e s u l t s of Ben-Menahem (1965, 1978) su p p o r t t h i s i n t e r p r e t a t i o n . U s i n g Love waves (L2 and L3) r e c o r d e d on the EW s t r a i n - m e t e r a t Pasadena, Ben-Menahem d e r i v e d the f a u l t l e n g t h and r u p t u r e v e l o c i t y of the August 22, 1949 e a r t h quake u s i n g the d i r e c t i v i t y f u n c t i o n (Ben-Menahem and Toksoz, 1962) and d i f f e r e n t i a l phases (Ben-Menahem, 1961). He o b t a i n e d a f a u l t l e n g t h of 265 km and a u n i l a t e r a l r u p t u r e v e l o c i t y of 3.5 km/sec t o the n o r t h w e s t . Most r e c e n t l y , Rogers (1984), has suggested t h a t the s e i s m i c gap does not e x i s t . H i s c o n c l u s i o n i s based p r i m a r i l y 1 5 on an e x a m i n a t i o n of the SIT (A=4.0°) r e c o r d s f o r the two months f o l l o w i n g the 1949 earthquake. He noted s e v e r a l S-P i n t e r v a l s of 12-16 seconds which would p l a c e them i n the d i s t a n c e range of the s e i s m i c gap. F i g u r e 8 - PROPOSED SEISMIC GAPS ON THE QUEEN CHARLOTTE FAULT Major e a r t h q u a k e s a l o n g the Queen C h a r l o t t e F a u l t zone and the e x t e n t of t h e i r a f t e r s h o c k s . Shaded c i r c l e s a r e p o o r e r s o l u t i o n s (adapted from Rogers 1983). Rupture l e n g t h can a l s o be e s t i m a t e d from e m p i r i c a l m a g n i t u d e - r u p t u r e l e n g t h r e l a t i o n s h i p s . Rogers (1983) has p o i n t e d out t h a t o f t e n - q u o t e d m a g n i t u d e - r u p t u r e l e n g t h 1 6 r e l a t i o n s h i p s g i v e e s t i m a t e s which more than c o v e r the range of 300 km t o 470 km s u g g ested by the two i n t e r p r e t a t i o n s of the a f t e r s h o c k zone. The s h o r t e r i n t e r p r e t a t i o n of the a f t e r s h o c k zone l e a v e s a s e i s m i c gap of about 150 km. I f t h i s were a l l t o r u p t u r e d u r i n g one e a r t h q u a k e , the m a g n i t u d e - f a u l t l e n g t h r e l a t i o n s h i p s of I i d a ( 1 9 6 5 ) , Tocher (1958) and Acharya (1979) suggest a magnitude 7- 3/4 event would o c c u r . The amount of s t r a i n t h a t c o u l d have been s t o r e d i n the s e i s m i c gap by c o n s t a n t p l a t e motion s i n c e the e s t a b l i s h m e n t of the V i c t o r i a seismograph s t a t i o n i s a p p r o x i m a t e l y 4.5 meters (84 y e a r s X 5.5 cm/yr) (Rogers, 1983). T h i s i s e q u i v a l e n t t o a p p r o x i m a t e l y a 7-3/4 magnitude e a r t h q u a k e . Thus, the s t o r e d s t r a i n would appear t o be e q u a l t o t h a t e x p e c t e d t o be r e l e a s e d d u r i n g a complete r u p t u r e (Rogers, 1983). Hence the e x i s t e n c e , or n o t , of a s e i s m i c gap i s i m p o r t a n t f o r e v a l u a t i o n of s e i s m i c r i s k i n the r e g i o n . 1.3 THESIS OBJECTIVES As i n d i c a t e d i n t h i s i n t r o d u c t i o n , t h e August 22, 1949 Queen C h a r l o t t e earthquake p l a y s an i m p o r t a n t r o l e i n u n d e r s t a n d i n g the t e c t o n i c dynamics i n the Queen C h a r l o t t e I s l a n d s r e g i o n . W i t h t h i s i n mind, a c l o s e r e x a m i n a t i o n of the e a r t h q uake was u n d e r t a k e n . There were t h r e e p a r t s t o t h i s i n v e s t i g a t i o n . 1). The a f t e r s h o c k zone was re-examined. A d e t a i l e d i n v e s t i g a t i o n of the a f t e r s h o c k zone became p o s s i b l e when the r e c o r d s from the S i t k a s e i s m i c s t a t i o n (SIT) were found a f t e r 1 7 h a v i n g been l o s t f o r many y e a r s . SIT was the c l o s e s t s t a t i o n t o the e p i c e n t e r (A=4.0°) l y i n g h a l f a g a i n as c l o s e t o the e p i c e n t e r as the next n e a r e s t s t a t i o n i n V i c t o r i a (VIC, A=8.1°). T h i s a l l o w e d e a r t h q u a k e s as s m a l l as M >2.8 t o be used t o d e f i n e the a f t e r s h o c k zone as compared t o M >4.5 when o n l y VIC was a v a i l a b l e . 2) . To improve the r e l i a b i l i t y of the r u p t u r e l e n g t h d e t e r m i n e d from the d i r e c t i v i t y f u n c t i o n and d i f f e r e n t i a l phases,Love waves L2,L3, and R a y l e i g h waves R2,R3, r e c o r d e d a t Tucson (TUO), H o n o l u l u (HON), and Pasadena (PAS) were e v a l u a t e d t o supplement Ben-Menahem's (1967, 1978) r e s u l t s . T h i s was done because A k i (1966), i n a d e t a i l e d study of the N i i g a t a e a r t h quake of June 16, 1964 (M=7.5), showed t h a t the v a r i a b i l i t y of phase and a m p l i t u d e s p e c t r a over a narrow a z i m u t h due t o a) c o m p l e x i t y of the s o u r c e , b) i n t e r f e r e n c e between waves un d e r g o i n g r e f r a c t i o n s due t o l a t e r a l h e t e r o g e n e i t y of the c r u s t , and c) i n t e r f e r e n c e w i t h body waves and h i g h e r mode s u r f a c e waves, was s i g n i f i c a n t . A k i ' s r e s u l t s i n d i c a t e d t h a t the d i r e c t i v i t y f u n c t i o n and d i f f e r e n t i a l phase r e s u l t s d e r i v e d from a s i n g l e s t a t i o n were u n r e l i a b l e and t h a t b e t t e r r e s u l t s c o u l d be o b t a i n e d by a v e r a g i n g the r e s u l t s of s e v e r a l s t a t i o n s w i t h i n a narrow a z i m u t h . 3) . Mechanism s o l u t i o n s of the two l a r g e s t a f t e r s h o c k s were sought t o h e l p d e f i n e the o b l i q u e s u b d u c t i o n p r o c e s s . Forward modeling was used t o o b t a i n the a z i m u t h a l s u r f a c e wave r a d i a t i o n p a t t e r n s as a f u n c t i o n of earthquake mechanism. These t h e o r e t i c a l r a d i a t i o n p a t t e r n s were then compared t o the 18 o b s e r v e d r a d i a t i o n p a t t e r n u n t i l a 'best f i t ' mechanism was d e t e r m i n e d . 19 I I . AFTERSHOCK ZONE OF THE 1949 QUEEN CHARLOTTE EARTHQUAKE A n a l y s i s of the 1949 Queen C h a r l o t t e e a r t h q u a k e began w i t h a new d e t e r m i n a t i o n of the a f t e r s h o c k zone u s i n g the SIT r e c o r d s . The SIT r e c o r d s were examined f o r e v e n t s and phase a r r i v a l p i c k s were made. The l o c a t i o n and magnitude of each earthquake was e s t i m a t e d and used t o c o n s t r u c t a r e v i s e d a f t e r s h o c k zone. 2.1 PROCEDURE E v a l u a t i o n of the a f t e r s h o c k zone began w i t h c o n s t r u c t i o n of a t r a v e l t i m e t a b l e u s i n g a l a y e r over a h a l f s p a c e model. S p e c i f i c a l l y , the s t a n d a r d E a r t h P h y s i c s Branch model was used, a.i=6.2 km/s, a 2 = 8.2 km/s, h=36 km, and a=.25. A more s o p h i s t i c a t e d model d e r i v e d from r e c o r d i n g s on t h e m a i n l a n d of e x p l o s i o n s i n B i r d Lake i n the Queen C h a r l o t t e I s l a n d s ( F o r s y t h e t a l . , 1974) produced phase a r r i v a l t i m e s o n l y s l i g h t l y d i f f e r e n t from t h i s s i m p l e model. The slow drum speed, 15mm/min, of the SIT r e c o r d s p r e c l u d e s the a b i l i t y t o measure the phase a r r i v a l time a c c u r a t e l y enough t o d i s t i n g u i s h between models. T h e r e f o r e , the s i m p l e r s t a n d a r d model was s e l e c t e d . Phase a r r i v a l p i c k s (Pn,Pg,Sn,Sg) were a c h i e v e d by s e l e c t i n g the most prominent a r r i v a l and a s s i g n i n g t o i t a t r i a l phase. Assuming the phase i d e n t i f i c a t i o n was c o r r e c t the t r a v e l t i m e t a b l e was c o n s u l t e d t o determine when the o t h e r phases s h o u l d a r r i v e . The seismogram was then checked t o see i f the o t h e r phase a r r i v a l s o c c u r r e d a t the p r e d i c t e d t i m e s . I f the phase a r r i v a l s d i d not appear a t the c o r r e c t t i m e s a new 20 phase i d e n t i f i c a t i o n of the most prominent a r r i v a l was made and the p r o c e d u r e r e p e a t e d u n t i l t he p r e d i c t e d and observed phase a r r i v a l t i m e s c o i n c i d e d . A f t e r t h e phases were i d e n t i f i e d and the d i s t a n c e d e t e r m i n e d from the t r a v e l t i m e t a b l e , the l o c a l magnitude M was e s t i m a t e d from the l o g of the maximum t r a c e a m p l i t u d e and the l o g of the d i s t a n c e t o the e a r t h q u a k e . M.=log(a)+31ogA-2.92+0 Where a i s the maximum ground a m p l i t u d e i n *xm, A i s i n km, and /3 i s a s t a t i o n c o r r e c t i o n ( K a s a h a r a , 1981). The maximum t r a c e a m p l i t u d e was used i n s t e a d of the ground d i s p l a c e m e n t , w i t h the c o n s t a n t m u l t i p l i c a t i v e c o r r e c t i o n needed t o c o n v e r t t h e t r a c e a m p l i t u d e t o the t r u e ground d i s p l a c e m e n t obsorbed i n t o the e m p i r i c a l l y d e t e r m i n e d s t a t i o n c o r r e c t i o n . The s t a t i o n c o r r e c t i o n was d e t e r m i n e d by t r y i n g t o f i n d a c o r r e c t i o n which brought my magnitude e s t i m a t e s i n t o agreement w i t h Rogers' (1983) e s t i m a t e s f o r the 12 e a r t h q u a k e s we had i n common. A s t a n d a r d d e v i a t i o n of a=0.4 i s found when Rogers' magnitude e s t i m a t e s , based on the VIC r e c o r d s , a r e compared t o the s t a t i o n c o r r e c t e d SIT e s t i m a t e s (see T a b l e I I f o r a summary of the a f t e r s h o c k i n f o r m a t i o n ) . A f t e r t h e d i s t a n c e of the earthquake from SIT was de t e r m i n e d , t h e earthquake was a r c e d a t the a p p r o p r i a t e d i s t a n c e a c r o s s the Queen C h a r l o t t e f a u l t and p l o t t e d w i t h the assumption t h a t i t o c c u r r e d on the f a u l t (see F i g u r e 9 ) . The p r i m a r y j u s t i f i c a t i o n f o r l o c a t i n g the e p i c e n t e r s a l o n g the f a u l t comes 21 from the o b s e r v a t i o n s t h a t almost a l l the h i s t o r i c a l s e i s m i c i t y i n the Queen C h a r l o t t e I s l a n d s r e g i o n has o c c u r r e d on the Queen C h a r l o t t e f a u l t (see F i g u r e 5 ) . A comparison of the l o c a t i o n s of the e i g h t l a r g e s t a f t e r s h o c k s , c o n s i d e r e d w e l l l o c a t e d by Rogers (1983), t o the SIT e p i c e n t e r l o c a t i o n s found f o u r a f t e r s h o c k s l o c a t e d f u r t h e r n o r t h by as much as 50 km from Rogers' l o c a t i o n , two l o c a t e d f u r t h e r s o uth by f i v e and ten k i l o m e t e r s , and two agreed p e r f e c t l y . Rogers' e p i c e n t e r l o c a t i o n s were made u s i n g a computer program which u t i l i z e d the P a r r i v a l s r e p o r t e d i n the I n t e r n a t i o n a l S e i s m o l o g i c a l C enter (ISS) B u l l e t i n and J e f f r e y s - B u l l e n (1967) t r a v e l time t a b l e s . Rogers' e p i c e n t e r l o c a t i o n s a r e no more a c c u r a t e (±25 km) than the SIT d e t e r m i n a t i o n s (±25 km) as he l a c k e d good c o n t r o l of the o r i g i n t i m e . Thus, d i s a g r e e m e n t s of ^50 km i n e p i c e n t e r l o c a t i o n s a r e w i t h i n the combined u n c e r t a i n t i e s of b oth methods and i n d i c a t e t h a t a r c i n g the e p i c e n t e r s a c r o s s the Queen C h a r l o t t e F a u l t i s a v a l i d p r o c e d u r e . 22 SITKA IMPLIED ARRIVAL SITKA SITKA DATE TIME S-P DELTA SITKA ROGERS MAX. DELTA AMP./PER. SITKA CODA DURATION SITKA MAGNITUDE ESTIMATE ROGERS MAGNITUDE 8/22 05 53 OO 387SG-PG 325KM 4MM/2SEC 120 SEC 4 .47 8/22 06 07 05 527SG-PN 375KM 3MM/2SEC 135 SEC 4 .4? 8/22 06 16 29 127SG-PG 100KM 65MM? 420 SEC 3 . 77 8/22 06 50 07 167SG-PG 125KM 2MM7 60 SEC 2 . 8? 8/22 07 02 O l 587SG-PN 400KM 1 MM/2SEC 140 SEC 4 .07 8/22 07 21 55 TOO SMALL TO PICK 1MM7 100 SEC 8/22 07 44 09 <.57 48 SEC 8/22 07 50 23 30 SG-PG 250KM 4MM/2SEC 220 SEC 4 .0 8/22 08 14 31 20 SG-PG 175KM . 5MM/1SEC 56 SEC 3 .5 8/22 08 18 40 68 SG-PG SOOKM 4MM/2SEC 240 SEC 4 .0 8/22 09 08 35 40 SG-PG 340KM 2MM/4SEC 160 SEC 4 . 1 8/22 09 15 15 40 SG-PN 300KM 270KM -P 12MM/1 SEC 270 SEC 4 . 7 4 .5 8/22 12 05 02 20 SG-PG 175KM 2MM/3SEC 90 SEC 3 .3 8/22 12 2 1 21 15 SG-PG 125KM 41.5MM/2SEC 480 SEC 4 .2 8/22 13 40 25 27 SG-PN 225KM 12.5MM/1SEC 345 SEC 4 .4 8/22 15 27 15 20 SG-PG 175KM 3MM/2SEC 90 SEC 3 .4 8/22 17 55 20 6 SG-PN 125KM 3MM/1 SEC 120 SEC 3 .O 8/22 19 46 18 20 SG-PG 175KM 10MM/2SEC 250 SEC 4 .O 8/22 21 41 46 207SG-PG 170KM 2.5MM/1 SEC 120 SEC 3 . 37 8/23 OO 37 48 38 SG-PG 320KM 1.5MM/2SEC 75 SEC 3 .9 8/23 O l 00 09 427SG-PN 300KM 2MM/2SEC 120 SEC 4 .07 8/23 02 59 43 397SG-PN 300KM 250KM--P 47MM/1SEC GOO SEC 5 . 37 5 .0 8/23 14 15 34 28 SN-PN 275KM 7MM/4SEC 220 SEC 4 .4 8/23 19 38 45 707SG-PG 6CKDKM 590KM 4MM/4SEC ' 5 .2 5 .O 8/23 19 44 46 547SN-PN 550KM 570KM UNREADABLE 5 .0 8/23 20 25 44 50 SN-PN 500KM 545KM . 6 . 4 8/23 23 42 44 22 SG-PG 200KM 5MM/2SEC 120 SEC 3 . 8 8/24 02 39 35 7 SEC TOO EMERGENT TO GUESS 2.5MM/4SEC 450 SEC 8/24 09 21 02 46 SN-PN 4 50KM 4MM/4SEC 2GO SEC 4 8 8/24 12 42 48 23 SG-PG 200KM 10MM/2SEC 180 SEC 4 .  1 8/24 21 52 14 40 SG-PN 300KM 5MM/2SEC 200 SEC 4 . 4 8/24 22 38 24 75 SG-PN 525KM 520KM 10MM/4SEC 600 SEC 5. .4 4 .9 8/25 15 25 28 24 SG-PN 200KM 4MM/1SEC 105 SEC 3. 7 8/26 OS 26 14 407SG-PN 300KM 120KM-•P 65MM/4SEC 23 MIN. 5. 57 4 . 9 8/26 22 40 19 20 PG-PN 275KM 300KM 18MM/4SEC 20 MIN. 4 . 8 5. O 8/27 21 31 48 44 SN-PN 425KM 475KM 6MM/4SEC 720 SEC 4 . 9 5. 3 9/02 01 32 06 36 SN-PN 350KM 350KM 5MM/1 SEC 240 SEC 4 . 6 4 . 6 9/02 05 49 21 36 SG-PN 275KM 3.5MM/2SEC 180 SEC 4 . 1 9/02 07 59 52 327SG-PN 250KM 2MM/2SEC 45 SEC 3. 77 9/05 06 55 i o 427SN-PN 400KM 425KM- P 9MM/4SEC 300 SEC 5 . O? 4 . 9 9/ 1 1 23 29 04 42 SG-PN 225KM 3MM/4SEC 240 SEC 3. 8 9/12 08 36 48 307SN-PN 275KM 260KM- P 6MM/4SEC 240 SEC 4 . 37 4 . 9 9/12 14 38 31 307SN-PN 275KM 285KM- P 22MM/2SEC 330 SEC 4 . 97 5. 0 9/18 07 53 54 447SG-PN 325KM 2MM/4SEC 120 SEC 4 . 07 9/18 1 1 59 44 39 SN-PN 370KM 3MM/2SEC 120 SEC 4 . 4 T a b l e I I - AFTERSHOCK DATA T h i s t a b l e summarizes the a f t e r s h o c k d a t a r e c o r d e d a t SIT. The ? i n d i c a t e s the a d j a c e n t v a l u e o r r e a d i n g i s u n c e r t a i n . 23 F i g u r e 9 - AFTERSHOCK LOCATIONS The s o l i d d o t s i n d i c a t e the p o s i t i o n of a f t e r s h o c k s i d e n t i f i e d on the SIT r e c o r d s when p l o t t e d on the Queen C h a r l o t t e F a u l t . The l a t e r a l d i s t r i b u t i o n of e p i c e n t e r s a r r a n g e d p e r p e n d i c u l a r t o the f a u l t i n d i c a t e s t h a t more than one earthquake was l o c a t e d at the g i v e n p o s i t i o n on the f a u l t . The pr i m a r y j u s t i f i c a t i o n f o r l o c a t i n g the e p i c e n t e r s a l o n g the f a u l t i s the o b s e r v a t i o n t h a t almost a l l h i s t o r i c a l s e i s m i c i t y i n the Queen C h a r l o t t e I s l a n d s r e g i o n has o c c u r r e d a l o n g the Queen C h a r l o t t e f a u l t . 24 An attempt was made t o c o n s t r a i n the n o r t h - s o u t h p o s i t i o n of the l a r g e r e v e n t s more c l o s e l y by moving the e p i c e n t e r s s l i g h t l y n o r t h or s o u t h t o b r i n g the P a r r i v a l t i m e s a t VIC and SIT i n t o agreement. T h i s p r o c e d u r e , a p p l i e d t o s i x of the l a r g e r a f t e r s h o c k s , proved i n c o n c l u s i v e . In most c a s e s agreement between VIC and SIT was o b t a i n e d , but the n a t u r e of the seismograms p r e v e n t e d any s i g n i f i c a n t improvement i n e p i c e n t e r l o c a t i o n . In p a r t i c u l a r , the SIT r e c o r d s have a low n o i s e l e v e l and matching h o r i z o n t a l components. They a r e , however, l o n g p e r i o d w i t h a slow drum speed (I5mm/min) t h a t makes a c c u r a t e t i m i n g d i f f i c u l t . The VIC r e c o r d s c o n s i s t of a s h o r t p e r i o d v e r t i c a l which i s c o n t a m i n a t e d by c o n s i d e r a b l e n o i s e . For VIC, a c c u r a t e S-P times c o u l d be d e t e r m i n e d due t o the f a s t drum speed (60mm/min) i f the a r r i v a l s of S and P a r e c l e a r . However, the measured P a r r i v a l t i m e s f o r the M=8.1, M=6.2, and M=6.4 events do not agree w i t h the t i m e s r e p o r t e d i n the ISS B u l l e t i n , which makes a b s o l u t e a r r i v a l t i m e s s u s p e c t . Thus, i n s t e a d of comparing P a r r i v a l s a t VIC and SIT, I ended up comparing S-P t i m e s . The l e v e l of n o i s e , a l o n g w i t h the emergent c h a r a c t e r of the phase a r r i v a l s a t VIC, meant t h a t the phase a r r i v a l p i c k s were o f t e n u n c e r t a i n by s e v e r a l seconds. In g e n e r a l , the apparent phase a r r i v a l t imes were c o n s i s t e n t w i t h the expected a r r i v a l t i m e s based on the SIT e p i c e n t e r l o c a t i o n s . However, the u n c e r t a i n t y i n the p r e c i s e VIC phase a r r i v a l t i m e s means t h a t the e p i c e n t e r l o c a t i o n c o u l d v a r y by as much as 20 km and 25 s t i l l appear t o s a t i s f y the VIC phase a r r i v a l t i m e s . G e n e r a l l y , however, the 15 mm/min drum speed of SIT i s the main f a c t o r i n l i m i t i n g the a c c u r a c y of e p i c e n t e r l o c a t i o n . At b e s t , an a r r i v a l a t SIT can be i d e n t i f i e d t o w i t h i n 0.2 mm, which t r a n s l a t e s t o an u n c e r t a i n t y i n a r r i v a l time of 0.8 s. For an S-P t i m e , the combined u n c e r t a i n t y i s 1.6 s c o r r e s p o n d i n g t o a 15 km u n c e r t a i n t y i n e p i c e n t e r d i s t a n c e . Few, however, of the SIT phase a r r i v a l s can be measured t o the a c c u r a c y g i v e n above; i n s t e a d , an u n c e r t a i n t y of ±25 km i s more t y p i c a l . Thus, i f the assumption t h a t the e p i c e n t e r s l i e on the Queen C h a r l o t t e F a u l t i s v a l i d , then e p i c e n t e r l o c a t i o n s u s i n g SIT a r e p r o b a b l y u n c e r t a i n t o ±25 km. The p o s s i b l i t y t h a t the e a r t h q u a k e s might i n s t e a d l i e on the F a i r w e a t h e r or Chatham S t r a i t f a u l t s was a l s o c o n s i d e r e d (see F i g u r e 10). A l t h o u g h the Chatham S t r a i t f a u l t moved i n C e n o z o i c time (Lathram,1964; Page,1969), t h e r e i s no e v i d e n c e t h a t i t i s s t i l l a c t i v e . No major e a r t h q u a k e s have o c c u r r e d near the Chatham S t r a i t f a u l t i n h i s t o r i c t i m e s , and t h e r e was no m i c r o e a r t h q u a k e a c t i v i t y a l o n g i t d u r i n g a 1969 f i e l d s t u d y (Rogers,1976; Horner, 1983). There i s no d i r e c t e v i d e n c e a g a i n s t the o c c u r r e n c e of the e a r t h q u a k e s on the F a i r w e a t h e r F a u l t . However, the complete absence of s e i s m i c a c t i v i t y i n the SIT r e g i o n f o r s e v e r a l months i m m e d i a t e l y p r e c e d i n g the M=8.1 e v e n t , c o u p l e d w i t h the sudden onset of e v e n t s i m m e d i a t e l y f o l l o w i n g the M=8.1 e v e n t , s u g g e s t s t h a t the e a r t h q u a k e s are r e l a t e d t o the M=8.1 e v e n t , and as such, l i e on the Queen C h a r l o t t e F a u l t and not the F a i r w e a t h e r F a u l t . The s u c c e s s f u l 26 c r o s s - c h e c k w i t h VIC of s i x of t h e e a r t h q u a k e s a l s o i m p l i e s t h a t a l l the e a r t h q u a k e s l i e on the Queen C h a r l o t t e f a u l t . I50°W 14 0 a 130° 64 ° 62 60 58° 56° F i g u r e 10 - RELATIONSHIP OF THE CHATHAM STRAIT AND FAIRWEATHER FAULTS TO THE QUEEN CHARLOTTE FAULT The l o c a t i o n of SIT i s shown r e l a t i v e t o t h e Chatham S t r a i t f a u l t and the F a i r w e a t h e r f a u l t . The e a r t h q u a k e s are assumed t o have o c c u r r e d on t h e Queen C h a r l o t t e f a u l t as the Chatham S t r a i t f a u l t shows no e v i d e n c e of s t i l l b e i n g a c t i v e , and t h e time c o r r e l a t i o n between the M=8.1 event and t h e o t h e r e a r t h q u a k e s s u g g e s t s t h a t the e a r t h q u a k e s a r e a f t e r s h o c k s of the M=8.1 e v e n t , and, as s u c h , l i e on the Queen C h a r l o t t e f a u l t and not the F a i r w e a t h e r f a u l t ( a d a p t e d from P e r e z and J a c o b , 1980). 27 2.2 RESULTS A time h i s t o r y of the a f t e r s h o c k s i s shown i n F i g u r e 11. The e m p i r i c a l r u l e f o r a f t e r s h o c k frequency f i r s t p o i n t e d out by Omori, and g i v e n i n R i c h t e r (1958), i s , A=N(1+kt) A=Number of a f t e r s h o c k s i n a s p e c i f i e d time i n t e r v a l . k,N=Constants chosen t o f i t the d a t a . The a f t e r s h o c k s do appear to f o l l o w Omori's law, a l t h o u g h the s t a t i s t i c s a r e poor due t o the s m a l l number of measurable e a r t h q u a k e s . F i g u r e 11 - TIME DISTRIBUTION OF AFTERSHOCKS T h i s p l o t shows the number of e a r t h q u a k e s which o c c u r r e d on s u c c e s s i v e days f o l l o w i n g the M=8.1 event. D u r i n g the f i r s t s i x days of a f t e r s h o c k s the e a r t h q u a k e s range i n l o c a t i o n from 300 km t o the n o r t h of the M=8.1 • 28 e p i c e n t e r t o 190 km t o the s o u t h , y i e l d i n g an a f t e r s h o c k zone of 490 km (see F i g u r e 9 ) . U s i n g p r i m a r i l y a f t e r s h o c k d a t a t o d e f i n e the r u p t u r e l e n g t h , A c h a rya (1979) d e r i v e d r e g i o n a l m a g n i t u d e - r u p t u r e l e n g t h and m a g n i t u d e - r u p t u r e a r e a r e l a t i o n s h i p s f o r seven d i f f e r e n t p a r t s of t h e w o r l d (see F i g u r e s 12 and 13). He found t h a t the c o r r e l a t i o n between r u p t u r e l e n g t h and magnitude i s h i g h f o r each r e g i o n , but t h a t v a r i a t i o n s from r e g i o n t o r e g i o n a r e s i g n i f i c a n t . I t i s not c l e a r how much of the r e g i o n a l v a r i a t i o n s a r e due t o the type of p l a t e i n t e r a c t i o n , and how much a r e due t o the n a t u r e of the l i t h o s p h e r e of the i n t e r a c t i n g p l a t e s . For i n s t a n c e , Acharya's r e l a t i o n s h i p f o r the Andean s u b d u c t i o n r e g i o n •. i s s i g n i f i c a n t l y d i f f e r e n t from h i s r e l a t i o n s h i p s f o r b o t h the San Andreas s t r i k e - s l i p r e g i o n and the Japan s u b d u c t i o n r e g i o n (see F i g u r e 12). None of the seven r e g i o n s f o r which Acharya found m a g n i t u d e - r u p t u r e l e n g t h r e l a t i o n s h i p s match the na t u r e and s t y l e of t h e Queen C h a r l o t t e F a u l t Zone, and, t h e r e f o r e , the a p p l i c a b i l i t y of h i s r e l a t i o n s h i p s t o the Queen C h a r l o t t e f a u l t i s q u e s t i o n a b l e . L i k e w i s e r e g i o n a l or wor l d w i d e - a v e r a g e d r e l a t i o n s h i p s d e r i v e d by o t h e r a u t h o r s a r e of u n c e r t a i n u s e f u l n e s s . For i n s t a n c e , the western U.S. r e l a t i o n s h i p of Acharya (1979) p r e d i c t s a r u p t u r e - l e n g t h f o r the 1949 Queen C h a r l o t t e I s l a n d s earthquake of 301 km, but the A l a s k a - A l e u t i a n r e l a t i o n s h i p p r e d i c t s a r u p t u r e - l e n g t h of 494 km. 29 M A G N I T U D E F i g u r e 12 - EMPIRICAL MAGNITUDE RUPTURE-LENGTH RELATIONSHIP FOR 7 REGIONS OF THE WORLD T h i s f i g u r e shows the l a r g e v a r i a b i l i t y i n magnitude r u p t u r e - l e n g t h r e l a t i o n s h i p s o b t a i n e d by Acharya f o r d i f f e r e n t r e g i o n s of the w o r l d (from A c h a r y a , 1979). Wyss (1978) has suggested t h a t s i n c e l o n g but t h i n f a u l t s produce l e s s p o w e r f u l e a r t h q u a k e s than l o n g and wide f a u l t s , e a rthquake magnitude s h o u l d be e s t i m a t e d on the b a s i s of r u p t u r e a r e a . The r e l a t i o n s h i p s u g g ested by Wyss, Ms=logA+4.2 where A = f a u l t a r e a , c o n s i s t e n t l y p r e d i c t s a s m a l l e r r u p t u r e area than 30 Acharya's magnitude-rupture area r e l a t i o n s h i p f o r any i n d i v i d u a l r e g i o n (see F i g u r e 13). Thus, Wyss' r e l a t i o n s h i p can be used to suggest a minimum ru p t u r e area f o r a gi v e n earthquake. 0*| — / P / / / . ' ' W / f / / / / * / /// / 4 / / ' / V / ///, //// //// // / M A G N I T U O E F i g u r e 13 - MAGNITUDE RUPTURE-AREA RELATIONSHIPS T h i s f i g u r e shows the v a r i a b i l i t y of the magnitude r u p t u r e - a r e a r e l a t i o n s h i p s d e r i v e d by Acharya f o r f i v e d i f f e r e n t r e g i o n s of the world. Note that the r e l a t i o n s h i p of Wyss (1978) can be used as a lower l i m i t f o r a l l r e g i o n s (from Acharya, 1 9 7 9 ) . 31 For the 1949 Queen C h a r l o t t e I s l a n d s e a r t h q u a k e , Wyss' r e l a t i o n s h i p p r e d i c t s a r u p t u r e a r e a of 7943 km 2. I f t h i s r u p t u r e a r e a i s d i v i d e d by 16 km, the depth t o which Hyndman and E l l i s (1981) have suggested t h a t b r i t t l e f r a c t u r e can o c c u r i n the submarine t e r r a c e ( the oceanward s i d e of the presumed a c t i v e Queen C h a r l o t t e f a u l t ) , then the p r e d i c t e d r u p t u r e l e n g t h i s 496 km. I f t h e w i d t h of the f a u l t i s assumed t o be 21 km, the maximum ob s e r v e d depth of m i c r o e a r t h q u a k e s a l o n g the Queen C h a r l o t t e F a u l t (Hyndman and E l l i s , 1981), then the p r e d i c t e d r u p t u r e l e n g t h i s 378 km. B e a r i n g i n mind t h a t t h e s e numbers suggest a minimum r u p t u r e l e n g t h , they a r e i n r e a s o n a b l e agreement w i t h the SIT a f t e r s h o c k zone. Wyss (1979), i n a more r e c e n t paper i n which he f i t s a l e a s t squares l i n e t o a s e l e c t i o n of worldwide d a t a , s u g g e s t s the c o n s t a n t i n the fo r m u l a s h o u l d be 4.15 not 4.2. W i t h t h i s change, a r u p t u r e l e n g t h of 557 km i s d e r i v e d f o r a f a u l t w i d t h of 16 km, and 424 km f o r a f a u l t w i d t h of 21 km. I f the s p a t i a l and time development of the a f t e r s h o c k s are examined i n d e t a i l , a d i s t i n c t t r e n d i s o b s e r v e d . Twenty-two of the 23 e a r t h q u a k e s o c c u r r i n g i n the 34 hours i m m e d i a t e l y f o l l o w i n g the main shock a r e l o c a t e d t o the n o r t h of the M=8.1 e p i c e n t e r , w h i l e f i v e of e i g h t e a r t h q u a k e s t h a t o c c u r r e d i n the second 34 hours a r e l o c a t e d t o the s o u t h of the main shock (see F i g u r e 14). These a f t e r s h o c k s a r e caused by the slow r e w o r k i n g and r e a d j u s t m e n t of the s t r a i n a l o n g the f a u l t i n response t o the r a p i d change i n s t r e s s and s t r a i n of the main r u p t u r e . A p p a r e n t l y the r e w o r k i n g o c c u r r e d more r a p i d l y t o the n o r t h of 32 the main shock e p i c e n t e r than t o the s o u t h . The r e a d j u s t m e n t a l s o appears t o have o c c u r r e d w i t h s m a l l e r but more numerous e v e n t s t o the n o r t h than t o the s o u t h . The October 31, M=6.2 e a r t h q u a k e , though l o c a t e d w i t h i n the n o r t h e r n end of the M=8.1 a f t e r s h o c k zone, i s s e p a r a t e d from p r e v i o u s a f t e r s h o c k s by an absence of s e i s m i c a c t i v i t y f o r 1-1/2 months. T h e r e f o r e , the October 31 shock and the t h r e e e v e n t s o c c u r r i n g d u r i n g the 34 hours i m m e d i a t e l y f o l l o w i n g i t , though r e l a t e d t o the M=8.1 e v e n t , c o u l d be c o n s i d e r e d by K e l l e h e r ' s (1972) c r i t e r i a t o be a s e p a r a t e , d i s t i n c t earthquake sequence at the n o r t h e r n end of the M=8.1 r u p t u r e zone, and not n e c c e s a r i l y a f t e r s h o c k s of the M=8.1 e v e n t . However, i t i s i m p o r t a n t t o note t h a t r e g a r d l e s s of whether or not the October 31 e v e n t s a r e c o n s i d e r e d t o be a f t e r s h o c k s of the August 21 event,, the o v e r a l l a f t e r s h o c k zone of the August 21 e a r t h quake does not change i n s i z e . In summary, over 50 e a r t h q u a k e s were i d e n t i f i e d from the SIT r e c o r d s , i n c l u d i n g 38 which had not p r e v i o u s l y been r e c o g n i z e d . The e a r t h q u a k e s , when p l o t t e d a l o n g the Queen C h a r l o t t e F a u l t i n d i c a t e a r u p t u r e l e n g t h of 490 km. T h i s f a u l t l e n g t h i m p l i e s t h a t a s u g g ested s e i s m i c gap t o the n o r t h of the August 22 earthquake does not e x i s t . The a f t e r s h o c k d i s t r i b u t i o n a l s o s u g g e s t s a time v a r i a t i o n i n the r u p t u r e sequence, w i t h the a f t e r s h o c k s c l u s t e r i n g f i r s t t o the n o r t h , and then t o the south of the main shock e p i c e n t e r . FIRST 34 HOURS SECOND 34 HOURS OCT. 31 S E Q U E N C E F i g u r e 14 - TIME PROGRESSION OF AFTERSHOCKS D u r i n g the f i r s t 34 hours a f t e r the M=8.1 event, 22 of 23 e a r t h q u a k e s o c c u r r e d t o the n o r t h of the M=8.1 e p i c e n t e r . D u r i n g the second 34 h o u r s , 5 of 8 e a r t h q u a k e s o c c u r r e d t o the s o u t h of the M=8.1 e p i c e n t e r . The October 31 sequence i s w i t h i n the a f t e r s h o c k zone, though a t the v e r y n o t h e r n end of i t , and s e p a r a t e d i n time from the o t h e r earthquakes by over a month of s e i s m i c i n a c t i v i t y . 34 I I I . SURFACE WAVE ANALYSIS 3.1 POINT SOURCE MECHANISM SOLUTION 3.1.1 I n t r o d u c t i o n The a z i m u t h a l r a d i a t i o n p a t t e r n of Love waves and R a y l e i g h waves i s s e n s i t i v e t o the earthquake mechanism and f o c a l d epth. For t h o s e e a r t h q u a k e s which can be r e p r e s e n t e d by a p o i n t source the t h e o r e t i c a l s u r f a c e wave r a d i a t i o n p a t t e r n can be c a l c u l a t e d i n a s t r a i g h t f o r w a r d manner. By comparing the c a l c u l a t e d s u r f a c e wave r a d i a t i o n p a t t e r n t o t h e observed r a d i a t i o n p a t t e r n the e a r t h q u a k e mechanism and depth can be deduced. T h i s i s done by t r y i n g many d i f f e r e n t e a r t h q uake mechanisms u n t i l a mechanism and depth i s found f o r which t h e c a l c u l a t e d s u r f a c e wave r a d i a t i o n p a t t e r n matches the o b s e r v e d one. For eart h q u a k e s whose s p a t i a l d i m e n s i o n s a r e s m a l l compared t o the wavelength of the. s u r f a c e waves b e i n g a n a l y z e d , a p o i n t s o u r c e a p p r o x i m a t i o n can be used. A p p l y i n g Wyss' (1978) m a g n i t u d e - r u p t u r e a r e a r e l a t i o n s h i p t o a M=6.4 earthquake and d i v i d i n g by 16 km f o r the Queen C h a r l o t t e f a u l t , t he r e s u l t i n g f a u l t l e n g t h i s 9.9 km. Thus, i f s u r f a c e Waves of p e r i o d >20 s a r e a n a l y s e d , the p o i n t s o u r c e a p p r o x i m a t i o n i s v a l i d ( T s a i and A k i , 1970) and can be used f o r the August 23 M=6.4 ear t h q u a k e and the October 31 M=6\2 ea r t h q u a k e . 35 3.1.2 Theory And Computer Programs The computer programs used t o e v a l u a t e the s o u r c e mechanism were d e v e l o p e d by Herrmann (1978). These programs e s s e n t i a l l y f o l l o w H a r k r i d e r (1970) i n t h e i r method of s o l v i n g the d i f f e r e n t i a l e q u a t i o n s which a r i s e when t r y i n g t o e v a l u a t e the t h e o r e t i c a l s u r f a c e wave a m p l i t u d e spectrum r e c o r d e d a t a s t a t i o n of a r b i t r a r y , a z i m u t h and d i s t a n c e from a g i v e n earthquake i n a l a y e r e d e a r t h . The t h e o r y i s o u t l i n e d as f o l l o w s : The b a s i c e q u a t i o n t o be s o l v e d i s Newton's second law a p p l i e d w i t h i n an e l a s t i c body, p i s the d e n s i t y U i s the i t h component of ground a c c e l e r a t i o n f i s the i - t h component of the a p p l i e d body f o r c e s t i s t h e i - t h component of t r a c t i o n a c r o s s the p l a n e w i t h a normal t o the j - t h a x i s . R a y l e i g h wave s o l u t i o n s a r e sought of the form, pU=f; +r;j (1) where, U=r , ( k , z ,w)exp( i ( kx-cot) ) V=0 W = i r 2 ( k , z , w ) e x p ( i ( k x - w t ) ) W i t h , U = t o t a l d i s p l a c e m e n t i n the x d i r e c t i o n V = t o t a l d i s p l a c e m e n t i n the y d i r e c t i o n W=total d i s p l a c e m e n t i n the z d i r e c t i o n k=wave number 36 w=frequency r , ( k , z , c j ) = h o r i z o n t a l d i s p l a c e m e n t r 2 ( k, z ,a>) = v e r t i c a l d i s p l a c e m e n t T h i s c o r r e s p o n d s , f o r p o s i t i v e r e a l v a l u e s of r , and r 2 , t o prograde motion i n the x-z p l a n e . The boundary c o n d i t i o n s a r e , i . T r a c t i o n = 0 a t the f r e e s u r f a c e (z=0). i i . No so u r c e e x i s t s a t z=°°. i i i . T r a c t i o n and d i s p l a c e m e n t must be c o n t i n u o u s a c r o s s i n t e r f a c e s where medium p r o p e r t i e s have d i s c o n t i n u i t i e s . W i t h these boundary c o n d i t i o n s and the assumption t h a t each l a y e r i s homogeneous and i s o t r o p i c , the s t r e s s components become r a j = i r f l (k,z,o>)exp( ikx-cot) ) rtrt = i ( X ( d r 2/dz)+kX+2/ir , )exp( i ( k x - u t ) ) 7̂, = i ( X ( d r 2 / d z ) + k X r , ) e x p ( i ( kx-wt)) t = i r 3 ( k , z , u ) e x p ( i ( k x - w t ) ) w i t h r 3 (k , z ,CJ) = shear (or h o r i z o n t a l ) s t r e s s rn(k,z,w)=normal (or v e r t i c a l ) s t r e s s X and n=Latae c o n s t a n t s . 37 S u b s t i t u t i o n of (2) and (3) i n t o ( l ) p r o v i d e s a set of d i f f e r e n t i a l e q u a t i o n s f o r the s t r e s s - m o t i o n v e c t o r ( r i , r 2 , r 3 , r , ) which can be w r i t t e n i n m a t r i x form a s , o o • AW *) o 6 (Mt)*2/*(t)] O G O The boundary c o n d i t i o n s of v a n i s h i n g t r a c t i o n a t the f r e e s u r f a c e z = 0 and no motion a t z=<*> i . e . r , , r 2 * 0 as z •*=> r 3,r„ •O as z=z o=0 mean t h a t f o r a g i v e n a n o n v a n i s h i n g s o l u t i o n s e x i s t o n l y f o r c e r t a i n K=Kn(cj) ( A k i and R i c h a r d s , 1980). Phase v e l o c i t y Cn=cj/Kn(cj) i s a l s o d i s c r e t i z e d . T h i s i s an e i g e n v a l u e - e i g e n f u n c t i o n problem. The f i r s t computer program, SURFACE, t a k e s as i n p u t an e a r t h model and a s e l e c t e d range of f r e q u e n c i e s f o r which the e i g e n v a l u e s or phase v e l o c i t i e s a r e to be d e t e r m i n e d . To f i n d the s o l u t i o n t o t h i s e i g e n v a l u e problem i n a m u l t i l a y e r e d medium, a Thompson-Haskell m a t r i x f o r m u l a t i o n ( H a s k e l l , 1953) i s used. In t h i s method i t i s assumed t h a t the medium parameters remain c o n s t a n t w i t h i n a g i v e n l a y e r so t h a t a m a t r i x o p e r a t o r can be d e r i v e d which r e l a t e s the s t r e s s - m o t i o n v e c t o r a t one s i d e of an e l a s t i c s o l i d l a y e r t o the v e c t o r at the o t h e r s i d e of the same l a y e r . Repeated a p p l i c a t i o n of t h i s method a l l o w s one to s t a r t at the 38 h a l f s p a c e a t the bottom and work up t o the s u r f a c e l a y e r - b y - l a y e r . B r i e f l y , the computation proceeds by a p p l y i n g a s e r i e s of m a t r i x t r a n s f o r m a t i o n s t o E q u a t i o n (4) t o o b t a i n a homogeneous m a t r i x e q u a t i o n known as the f r e q u e n c y or p e r i o d f u n c t i o n . The f r e q u e n c y f u n c t i o n i s s o l v e d by s e e k i n g the z e r o s of the e q u a t i o n as a f u n c t i o n of phase v e l o c i t y , wave number, and the e l a s t i c c o n s t a n t s of the l a y e r s . .This i s done by i n i t i a l l y s p e c i f y i n g the wave number, Kn, and a t r i a l phase v e l o c i t y , Cn. The elements of the Thompson-Haskell m a t r i x a r e then formed f o r each l a y e r and m u l t i p l i e d by the m a t r i x f o r the l a y e r above i t , s t a r t i n g w i t h the h a l f space. The n u m e r i c a l v a l u e of the f r e q u e n c y f u n c t i o n i s c a l c u l a t e d from elements of the f i n a l p r o d u c t m a t r i x and s t o r e d . The f r e q u e n c y f u n c t i o n h a s . t h e i m p o r t a n t f e a t u r e t h a t , i t s v a l u e s are p o s i t i v e f o r Cn's l a r g e r than the r o o t i n the l o w e s t mode. A new t r i a l v a l u e of Cn i s chosen ( l a r g e r or s m a l l e r depending on whether the s i g n of the f r e q u e n c y e q u a t i o n i s p o s i t i v e or n e g a t i v e ) u n t i l a change i n s i g n of the f r e q u e n c y e q u a t i o n i s d e t e c t e d . A f t e r the r o o t i s b r a c k e t e d an i n t e r v a l h a l v i n g p r o c e d u r e i s used and then f i n a l l y a l i n e a r i n t e r p o l a t i o n scheme i s f o l l o w e d t o produce Cn t o the r e q u i r e d a c c u r a c y . The r e s u l t i n g e i g e n v a l u e , Cn, i s passed t o a second program, REIGEN, which n u m e r i c a l l y i n t e g r a t e s E q u a t i o n s (4) from the bottom up t o f i n d the e i g e n f u n c t i o n ( r , , r 2 , r 3 , r „) as a f u n c t i o n of d e p t h . A s i m i l i a r s e t of e q u a t i o n s and p r o c e d u r e i s f o l l o w e d f o r Love waves. 39 The e i g e n v a l u e s and e i g e n f u n c t i o n s a r e e a r t h model dependent but not s o u r c e o r s t a t i o n dependent. Once the a p p r o p r i a t e e a r t h model has been s e l e c t e d b o t h programs a r e r u n , and t h e r e s u l t i n g e i g e n v a l u e s and e i g e n f u n c t i o n s a r e s t o r e d . The t h i r d computer program, WIGGLE, t a k e s t h e e i g e n v a l u e s and e i g e n f u n c t i o n s s t o r e d on tape and computes the s u r f a c e d i s p l a c e m e n t a t an a r b i t r a r y p o i n t f o r a p o i n t s o u r c e a t a s e l e c t e d d e p t h . U s i n g the e i g e n v a l u e s and e i g e n f u n c t i o n s t h e i m p u l s e r e sponse of t h e medium, the Green's f u n c t i o n s , can be e v a l u a t e d . The R a y l e i g h Green's f u n c t i o n f o r a p o i n t f o r c e F(w) a c t i n g a t i s ( A k i and R i c h a r d s , 1980), where we have t r a n s f o r m e d i n t o a more a p p r o p r i a t e c y l i n d r i c a l p o l a r c o o r d i n a t e s y s t e m . I i n t h i s e q u a t i o n i s g i v e n by, 1=1/2 p ( r , + r 2 ) d z and t h e group v e l o c i t y , Ug, i s d e t e r m i n e d by d i f f e r e n t i a t i n g Cn. The computer program i s w r i t t e n i n terms of e q u i v a l e n t body f o r c e s ; however, f o r t h i s o u t l i n e t h e r e s u l t s can be o b t a i n e d 40 f a s t e r by u s i n g the moment t e n s o r f o r m u l a t i o n (see A k i and R i c h a r d s , 1980). where U = i - t h component of r e c e i v e r d i s p l a c e m e n t X=the v e c t o r p o s i t i o n of the r e c e i v e r Mpq=elements of the moment t e n s o r Gip=elements of the Green's f u n c t i o n ( i - t h component of d i s p l a c e m e n t due t o an im p u l s e f o r c e i n the in ' t he p - t h d i r e c t i o n ) . In d i f f e r e n t i a t i n g t he Green's f u n c t i o n o n l y the l a r g e s t or f a r . f i e l d terms a r e r e t a i n e d . The moment t e n s o r components can be computed from the ea r t h q u a k e f a u l t parameters ( e g . , Box 4.4 i n A k i and R i c h a r d s , 1980). The program QUESTION a c c e p t s the ob s e r v e d a z i m u t h a l v a r i a t i o n Love and R a y l e i g h wave a m p l i t u d e s p e c t r a f o r an ea r t h q u a k e . The program then s e a r c h e s through a parameter space of f o c a l mechanism o r i e n t a t i o n s and f o c a l d e p t h s , and computes s e v e r a l g o o d n e s s - o f - f i t c h a r a c t e r i s t i c s which a r e used t o det e r m i n e t h e b e s t f o c a l mechanism and f o c a l depth c o m b i n a t i o n which s a t i s f i e s t h e o b s e r v a t i o n s . These goodness of f i t c h a r a c t e r i s t i c s i n c l u d e : a) The c o r r e l a t i o n c o e f f i c i e n t between the observed and t h e o r e t i c a l R a y l e i g h or Love wave a m p l i t u d e s p e c t r a f o r the t o t a l i t y of d a t a from a l l a z i m u t h s and p e r i o d s . 41 b) S e i s m i c moment e s t i m a t e from Love waves. c) S e i s m i c moment e s t i m a t e from R a y l e i g h waves. d) Sum of square r e s i d u a l s between o b s e r v e d and t h e o r e t i c a l R a y l e i g h wave a m p l i t u d e s p e c t r a u s i n g the average s e i s m i c moment e s t imate. e) Square r o o t of the sum of Love and R a y l e i g h wave square residuals. The b e s t f o c a l mechanism e s t i m a t e i s u s u a l l y the one w i t h the l a r g e s t c o r r e l a t i o n c o e f f i c e n t s and f o r which the two independent s e i s m i c moment e s t i m a t e s a r e as e q u a l as p o s s i b l e . Other programs a r e a v a i l a b l e t o p l o t and d i s p l a y the r e s u l t s from t h i s program. 3.2 RUPTURE PARAMETERS 3.2.1 I n t r o d u c t i o n For l a r g e e a r t h q u a k e s the p o i n t s o u r c e a p p r o x i m a t i o n i s no l o n g e r v a l i d as the s u r f a c e wave r a d i a t i o n p a t t e r n i s a f f e c t e d by the l a r g e s p a t i a l e x t e n t of the ea r t h q u a k e . The f i n i t e d i m e n s i o n s of an earthquake i n t r o d u c e an asymmetry i n t o the s u r f a c e wave r a d i a t i o n p a t t e r n . T h i s asymmetry c o r r e s p o n d s t o the d o p p l e r e f f e c t from a moving p o i n t s o u r c e . By a n a l y s i s of the asymmetry, the earthquake r u p t u r e l e n g t h and r u p t u r e v e l o c i t y can be d e r i v e d . Both r u p t u r e l e n g t h and r u p t u r e v e l o c i t y a r e parameters of i n t e r e s t f o r the M=8.1 earthquake and can be d e r i v e d f o r t h i s e a r t h q uake as i t i s s u f f i c i e n t l y l a r g e t h a t the asymmetery i n the s u r f a c e wave r a d i a t i o n p a t t e r n i s l a r g e and e a s i l y measured. 42 Ben-Menahem (1961) was the f i r s t t o d e s c r i b e the e f f e c t on the f a r - f i e l d s u r f a c e wave a m p l i t u d e spectrum of r u p t u r e p r o p a g a t i o n a l o n g a f a u l t . He d i d t h i s by examining the f a r - f i e l d d i s p l a c e m e n t caused by a moving h o r i z o n t a l d i p o l e w i t h harmonic time dependence i n a l a y e r e d e l a s t i c h a l f s p a c e . For a v e r t i c a l s t r i k e - s l i p f a u l t of v e r t i c a l e x t e n t d, which p r o p a g a t e s a l o n g a f a u l t of l e n g t h b w i t h v e l o c i t y v, he found the f a r - f i e l d v e r t i c a l R a y l e i g h wave d i s p l a c e m e n t U t o be g i v e n by, U(o)) = ( s i n ( 2 e ) / y r ) ( g ( c j ) k ( s i n ( X ) / X ) e x p { i (^+3TT/4) } where X= (irb/X) ( c / v - c o s f i ) ( 5 J >=w(t-r/c)-X and where c i s the R a y l e i g h wave phase v e l o c i t y , r the e p i c e n t r a l d i s t a n c e , 8 the azi m u t h t o the s t a t i o n measured c o u n t e r c l o c k w i s e from the s t r i k e of the f a u l t and k t h e wave number of shear waves. g(w) i s a f u n c t i o n which depends on the source time f u n c t i o n , t h e w i d t h of the f a u l t , depth and f r e q u e n c y . For our p u r p o s e s , two components of t h i s e x p r e s s i o n a re u s e f u l f o r e x a m i n a t i o n of the 1949 e a r t h q u a k e . 3.2.2 D i r e c t i v i t y F u n c t i o n Theory The e f f e c t of the f i n i t e d i m e n s i o n s i s c o n t a i n e d i n the term X. The e f f e c t of the f i n i t e n e s s of the f a u l t on the a m p l i t u d e spectrum i s e x p r e s s e d by the f a c t o r s i n ( X ) / X which has nodes a t X=7r, 2ir, 3TT, . . . 43 To i s o l a t e the e f f e c t of the f i n i t e d i m e n s i o n s , Ben-Menahem (1961) d e f i n e d the d i r e c t i v i t y f u n c t i o n as the r a t i o of the s p e c t r a l a m p l i t u d e s l e a v i n g the source a t azi m u t h 0 t o those l e a v i n g the sour c e w i t h a z i m u t h 9+ir. d i r e c t i v i t y f u n c t i o n = | U ( u ) [ 0 ) 3 | / |U(o>)[0 +TT] | ( c / v + c o s 0 ) s i n { ( w b / 2 c ) ( c / v - c o s 0 ) } ( c / v - c o s 0 ) s i n { (a)b/2c) (c/v+cos0) j For a pure s t r i k e - s l i p f a u l t t h i s f u n c t i o n i s independent of d i p a n g l e , s o u r c e time f u n c t i o n , or the l a y e r i n g of the e l a s t i c h a l f space model. For a pure d i p - s l i p f a u l t t h i s f u n c t i o n i s u n d e f i n e d . When o n l y one s t a t i o n i s a v a i l a b l e , a t r i a l and e r r o r approach i s used t o g e n e r a t e t h e o r e t i c a l d i r e c t i v i t y c u r v e s u n t i l a c o m b i n a t i o n of r u p t u r e v e l o c i t y and r u p t u r e l e n g t h i s found which produces a d i r e c t i v i t y c u r v e t h a t best f i t s the o b s e r v e d c u r v e . W i t h more than one s t a t i o n , a l e a s t squares t e c h n i q u e can be used. The f i r s t ext.remum. w i l l o c cur when K=n. X=(irb/\) ( C / V - C O S 0 ) = T T T h i s can be r e w r i t t e n as X/c=(b/c)(c/v-cos0)=T(max) which c o r r e s p o n d s t o the maximum p e r i o d a t which an extremum w i l l o c c u r . T h i s can a g a i n be r e w r i t t e n a s , b=T(max)c/(c/v-cos0)=T(max)/(1/v-cos0/c) I f the f o l l o w i n g s u b s t i t u t i o n s a r e made, x=T(max), y=cos0/c, a=b, B=1/V t h e n , a=x/(B-y) o r , 44 y=-(l/a)x+B = Ax+B A=-l/a=-l/b. Each s t a t i o n can be p l o t t e d as a p o i n t w i t h an x v a l u e e q u a l t o the maximum p e r i o d a t which an extremum o c c u r s , and the y v a l u e e q u a l t o c o s 0 / c . A l e a s t squares l i n e through the p o i n t s can be de t e r m i n e d w i t h the n e g a t i v e r e c i p r o c a l of the s l o p e y i e l d i n g the r u p t u r e l e n g t h , and the r e c i p r o c a l of the y - i n t e r c e p t g i v i n g the r u p t u r e v e l o c i t y . Thus, t o use t h i s method, one forms the ob s e r v e d d i r e c t i v i t y f u n c t i o n c u r v e by t a k i n g the s p e c t r a l a m p l i t u d e r a t i o of R2 and R3. A graph i s then c o n s t r u c t e d w i t h one p o i n t p l o t t e d f o r each s t a t i o n . The y v a l u e f o r a g i v e n p o i n t i s the maximum p e r i o d a t which an extremum o c c u r s i n the ob s e r v e d d i r e c t i v i t y f u n c t i o n (Tmax). The x v a l u e i s the c o s i n e of the azi m u t h t o the p a r t i c u l a r s t a t i o n d i v i d e d by the phase v e l o c i t y c o r r e s p o n d i n g t o Tmax. A p o i n t i s p l o t t e d f o r each s t a t i o n and a l e a s t squares l i n e drawn th r o u g h the r e s u l t i n g a r r a y of p o i n t s . The n e g a t i v e r e c i p r o c a l of the s l o p e of the l e a s t s quares l i n e w i l l g i v e the r u p t u r e l e n g t h . The r e c i p r o c a l of the y - i n t e r c e p t w i l l g i v e the r u p t u r e v e l o c i t y . 3.2.3 D i f f e r e n t i a l Phases Theory The e f f e c t of the f i n i t e f a u l t d i m e n s i o n s on the phase spectrum can a l s o be u t i l i z e d . The d i f f e r e n c e i n phase of waves r a d i a t i n g i n t o o p p o s i t e a zimuths i s g i v e n from (5) as A4>=0(0)-</>(0 + 7r) = {w(t , - r ,/c)-X,}-{ | o > ( t 2 - r 2 / c ) - X 2 } where t , = time f o r the wave t o t r a v e l from the f o c u s t o the s t a t i o n a t 6 45 t 2 = time f o r the wave t o t r a v e l from f o c u s t o the s t a t i o n a t 6+ir r , = the d i s t a n c e t o s t a t i o n 0 r 2 = the d i s t a n c e t o s t a t i o n 8+ir X, = U b / X ) ( c / v - c o s f l ) X 2 = (wb/X) (c/v-cos0+ 7 r ) . T h i s can be r e w r i t t e n a s , A0=(1/X)(4O,OOO-2r 1+bcos0)+m+l/4. [0 The 40,000-2r 1 and m+1/4 terms a r e p r o p a g a t i o n terms which a r i s e . from t h e f a c t t h a t the two waves must t r a v e l d i f f e r e n t d i s t a n c e s t o r e a c h the r e c o r d i n g s t a t i o n . By c o r r e c t i n g the phase of each wave t r a i n f o r p r o p a g a t i o n e f f e c t s we o b t a i n the i n i t i a l f o c a l phase, <j>{ f o c a l ) =<t> (FFT) -<p ( d i s p e r s i o n )-<p( d i g i t i z a t i o n ) -#( i n s t r u m e n t ) . In t h i s p r o c e s s , we s u b t r a c t from the phase p a r t of the F o u r i e r t r a n s f o r m e d seismogram 0(FFT) a l l phase advances ( t a k e n as p o s i t i v e ) . The ^ ( d i s p e r s i o n ) term i s the compensation f o r d i s p e r s i o n which a wave e x p e r i e n c e s when i t t r a v e l s a d i s t a n c e A km w i t h a phase v e l o c i t y C. (/•(dispersion) = f A/C where f i s f r e q u e n c y , C i s phase v e l o c i t y and A i s measured i n km. The ^ ( d i g i t i z a t i o n ) term a c c o u n t s f o r the d i f f e r e n c e between the o r i g i n time of the earthquake and the f i d u c i a l time a t which the b e g i n n i n g of the d i g i t i z i n g window was chosen. <£(digitization) = f T 0 where T 0 i s the time d e l a y from the earthquake o r i g i n time t o the s t a r t of d i g i t i z a t i o n . 46 The 0 ( i n s t r u m e n t ) term i s the c o r r e c t i o n f o r the phase response of the seismograph. Once the f o c a l phase f o r each wave i s o b t a i n e d the d i f f e r e n c e i s t a k e n . (#>1 ( f o c a l ) - 0 2 ( f o c a l ) = A 0 ( f o c a l ) The p r o p a g a t i o n terms i n E q u a t i o n (6) have been s u b t r a c t e d out i n o b t a i n i n g the f o c a l phases. T h e r e f o r e the d i f f e r e n t i a l f o c a l phases w i l l be from E q u a t i o n ( 6 ) , A 0 ( f o c a l ) = b c o s 0 / X from which the r u p t u r e l e n g t h , b, can be d e r i v e d . b=XA</»( f o c a l )/cos0 U n l i k e the d i r e c t i v i t y f u n c t i o n , t h i s method p r o v i d e s an e s t i m a t e of the r u p t u r e l e n g t h which i s independent of the r u p t u r e v e l o c i t y . 3.3 PROCESSING 3.3.1 Data A c q u i s i t i o n The d a t a were o b t a i n e d by w r i t i n g t o s t a t i o n s and networks known t o be o p e r a t i n g r e l a t i v e l y l o n g p e r i o d i n s t r u m e n t s i n 1949, and t o the World Data C e n t e r A f o r c o p i e s of r e l e v a n t r e c o r d s from l o n g p e r i o d s t a t i o n s i n t h e i r f i l m l i b r a r y . There was v a r i e t y i n the type of seismogram c o p i e s sent by the r e s p o n d i n g s t a t i o n s . In some c a s e s f u l l s i z e d c o p i e s were s e n t , but i n most c a s e s p h o t o g r a p h i c c o p i e s on m i c r o f i l m were r e c e i v e d . U n f o r t u n a t e l y , not a l l of the s t a t i o n s responded, 47 and, of thos e t h a t d i d respond, not a l l sent seismogram c o p i e s . O f t e n . i m p o r t a n t i n s t r u m e n t parameters such as seismometer p e r i o d and s t a t i c m a g n i f i c a t i o n were not i n c l u d e d w i t h the s t a t i o n r e s p o n s e s . Even when t h i s i n f o r m a t i o n was i n c l u d e d the r e l i a b i l i t y of the quoted parameter v a l u e s was f r e q u e n t l y q u e s t i o n a b l e . Without t h i s i n f o r m a t i o n the seismograms were of l i t t l e use ( f o r a complete l i s t of s t a t i o n s r e q u e s t e d and t h e i r r e p l i e s , see Appendix A ) . In t o t a l , the d a t a c o l l e c t e d r e p r e s e n t a menagerie of seismograph i n s t r u m e n t t y p e s , i n s t r u m e n t p e r i o d s , and hard copy r e c o r d s t y l e s . In a l l c a s e s , o r i g i n a l r e c o r d s p r o v i d e d the most i n f o r m a t i o n . The f u l l s i z e d p h o t o c o p i e s , though g e n e r a l l y u s e f u l , tended t o have two problems. In some ca s e s s i m p l y poor p h o t o c o p y i n g was the problem, too l i g h t and smudged t o be u s e f u l . A more s u b t l e problem w i t h them was t h a t .an edge or end of the seismogram would sometimes be c l i p p e d o f f . M i c r o f i l m p r e s e n t e d problems t o o . Because of the l i m i t a t i o n s of the department dark room f a c i l i t y , i t was not p o s s i b l e t o make f u l l s i z e d seismogram c o p i e s on o n l y one p r i n t . I n s t e a d , a s e r i e s of p r i n t s had t o be made, each of o n l y one s e c t i o n of the seismogram. These s e c t i o n s were then o v e r l a p p e d and t a p e d t o g e t h e r t o reassemble the f u l l s i z e d seismogram. D e s p i t e e f f o r t s t o a v o i d i t , skews c o u l d e a s i l y d e v e l o p i n the s e i s m i c t r a c e i n the p r o c e s s of r e c o n s t r u c t i n g the f u l l s i z e d seismogram. An o c c a s i o n a l problem w i t h the m i c r o f i l m was t h a t some of the seismograms had t h e i r edges c u r l e d under when m i c r o f i l m e d . 48 Of more than 30 s t a t i o n s o r i g i n a l l y r e q u e s t e d o n l y e i g h t s t a t i o n s were e v e n t u a l l y used. Only t h r e e s t a t i o n s were s u i t a b l e f o r use i n f i n d i n g the d i r e c t i v i t y f u n c t i o n and d i f f e r e n t i a l phases, and o n l y f i v e and e i g h t s t a i o n s c o u l d be used i n d e t e r m i n i n g the s u r f a c e wave r a d i a t i o n p a t t e r n of the August 23 and October 31 e a r t h q u a k e s , r e s p e c t i v e l y . A problem w i t h a l l s t a t i o n s was the l a c k of t r u e l o n g p e r i o d i n s t r u m e n t s (the response c u r v e s f o r the e i g h t s t a t i o n s used i n t h i s t h e s i s a r e shown i n Appendix D). The low power f o r l o n g p e r i o d s meant t h a t the u s e a b l e s i g n a l was e f f e c t i v e l y bandpassed f o r s h o r t p e r i o d s by s c a t t e r i n g due t o i n h o m o g e n e i t i e s i n the c r u s t and f o r l o n g p e r i o d s by a t t e n u a t i o n due t o i n s t r u m e n t r e s p o n s e . 3.3.2 Data P r o c e s s i n g The d a t a p r o c e s s i n g began w i t h d i g i t i z i n g the s e i s m i c r e c o r d s w i t h i n group v e l o c i t y windows. The windows were s e l e c t e d a f t e r c o n s u l t i n g w o r l d averaged group v e l o c i t y t a b l e s f o r Love and R a y l e i g h waves (Kovach, 1965). The windows were chosen t o c o n t a i n Love and R a y l e i g h wave a r r i v a l s f o r p e r i o d s between 10 s and 300 s. The d i g i t i z i n g window was t y p i c a l l y between 4.5 km/s and 2.5 km/s f o r the August 23 (M=6.4) and October 31 (M=6.2), e a r t h q u a k e s , and between 3.9 km/s and 3.5 km/s f o r R a y l e i g h waves and between 4.5 km/s and 4.2 km/s f o r Love waves of the August 21 (M=8.1) e a r t h q u a k e . F u l l s i z e p l o t s were made and p l a c e d over the o r i g i n a l t o check the f a i t h f u l n e s s of the d i g i t i z a t i o n . A f t e r d i g i t i z a t i o n , the time s e r i e s was i n t e r p o l a t e d t o a 49 s a m p l i n g i n t e r v a l of 2 s, and the t r e n d and the d.c. o f f s e t removed. The time s e r i e s was then t r a n s f o r m e d i n t o the f r e q u e n c y domain where i n s t r u m e n t c o r r e c t i o n s were a p p l i e d and the NS and EW components r o t a t e d i n t o r a d i a l and t a n g e n t i a l p a r t s . In a l l c a s e s the i n s t r u m e n t response was c a l c u l a t e d w i t h c r i t i c a l damping assumed. For e l e c t r o m a g n e t i c seismometers, t h e r e was assumed t o be no feedback c o u p l i n g between the galvanometer and the seismometer. These p r o c e s s e s were performed by Herrmann's (1978) program EXSPEC. 3.3.3 A m p l i t u d e Data The F o u r i e r t r a n s f o r m e d ground motion which r e s u l t e d from the use of EXSPEC was next passed t o Herrmann's program FILTER. T h i s program a p p l i e d a narrow bandpass G a u s s i a n f i l t e r t o the r e a l f r e q u e n c i e s of the F o u r i e r t r a n s f o r m of the ground m o t i o n . The i n v e r s e F o u r i e r t r a n s f o r m was then taken and the envelope of the f i l t e r e d s i g n a l found by u s i n g b oth the r e a l and i m a g i n a r y components of the i n v e r s e t r a n s f o r m . T h i s envelope was p l o t t e d as a f u n c t i o n of the group v e l o c i t y , and the c e n t e r p e r i o d , To, of the f i l t e r passband. The envelope was s e a r c h e d f o r l o c a l maximums, wh i c h , as l o n g as two or more modes do not a r r i v e too c l o s e t o g e t h e r , w i l l r e f l e c t the s p e c t r a l a m p l i t u d e s of the v a r i o u s modes making up the s i g n a l . The peak of the envelope was m u l t i p l i e d by a f a c t o r of 4To, so the r e s u l t was d i m e n s i o n a l l y a s p e c t r a l a m p l i t u d e . T h i s p r o c e d u r e was r e p e a t e d f o r a range of c e n t e r p e r i o d s so t h a t the r e s u l t i n g p l o t r e p r e s e n t e d a form of the group v e l o c i t y d i s p e r s i o n c u r v e . 50 E s s e n t i a l l y the p l o t shows the amount of energy a r r i v i n g a t a g i v e n group v e l o c i t y v e r s u s the energy's p e r i o d . The r e s u l t of t h i s f i l t e r i n g i s the a b i l i t y t o i d e n t i f y the v a r i o u s mode a r r i v a l s and o b t a i n t h e i r s p e c t r a l a m p l i t u d e s as a f u n c t i o n of p e r i o d . FILTER as i t was o r i g i n a l l y w r i t t e n by Herrmann (1978) d i s p l a y e d the f i l t e r i n g r e s u l t s by t a b u l a t i n g a l i s t of a l l the l o c a l s p e c t r a l a m p l i t u d e maximums. The s p e c t r a l a m p l i t u d e maximum was l i s t e d a l o n g w i t h the p e r i o d and group v e l o c i t y f o r which the l o c a l maximum o c c u r r e d . W h i l e t h i s method of d i s p l a y i n g the r e s u l t s gave a l l the needed f a c t s , i t was d i f f i c u l t t o i d e n t i f y p a t t e r n s i n the l o c a l maximums and t o d i s t i n g u i s h the a r r i v a l s of d i f f e r e n t modes. The output was t h e r e f o r e m o d i f i e d t o i n c l u d e the p l o t s mentioned above. These p l o t s made i t e a s i e r t o a s s o c i a t e a g i v e n l o c a l s p e c t r a l a m p l i t u d e maximum w i t h a s p e c i f i c mode a r r i v a l . Once i d e n t i f i e d and o b t a i n e d , the s p e c t r a l a m p l i t u d e s were c o r r e c t e d f o r a m p l i t u d e d e c r e a s e due t o a n e l a s t i c a t t e n u a t i o n and g e o m e t r i c a l s p r e a d i n g t o a s e l e c t e d r e f e r e n c e d i s t a n c e . A m p ( c o r r e c t e d ) = A m p ( s i n ( A 1 / A 2 ) a 5 e x p ( - ( A , - A 2 ) y ) where A 2 i s the r e f e r e n c e d i s t a n c e and 7 i s the a t t e n u a t i o n f a c t o r g i v e n by 7 = T T / Q T U w i t h Q = q u a l i t y f a c t o r T=the p e r i o d U=the group v e l o c i t y . For the August 23 and October 31 d a t a , the r e f e r e n c e d i s t a n c e was 1000 km as o n l y R1 and L1 d a t a were used. For the August 22 51 d a t a , the r e f e r e n c e d i s t a n c e was 39960 km, which i s a p p r o x i m a t e l y h a l f way between the d i s t a n c e the R2 and R3 or L2 and L3 waves must t r a v e l . The c o r r e c t e d a m p l i t u d e s were then used i n r a t i o s t o form the d i r e c t i v i t y f u n c t i o n or p l o t t e d a z i m u t h a l l y t o d e r i v e the r a d i a t i o n p a t t e r n . 3.3.4 Phase Data A f t e r c i r c l i n g the E a r t h once or t w i c e the wave t r a i n s of the August 22 d a t a c o n s i s t almost e x c l u s i v e l y of the fundamental mode, the h i g h e r modes h a v i n g been a t t e n u a t e d . W i t h t h i s i n mind, the phase p a r t of the F o u r i e r t r a n s f o r m of the ground motion was used w i t h o u t bandpass f i l t e r i n g . To o b t a i n the f o c a l phase the ob s e r v e d phase was c o r r e c t e d f o r the time d e l a y due t o the s t a r t of d i g i t i z a t i o n and the phase d e l a y due t o p r o p a g a t i o n from the sour c e t o the r e c e i v e r . C o r r e c t i o n s f o r the phase d e l a y caused by p o l a r c r o s s i n g s ( A k i , 1966) were a l s o made. 0( f o c a l ) = <£(FFT) -#( d i s p e r s i o n ) -<p( i n s t r u m e n t )-<£( d i g i t i z a t i o n ) By t a k i n g the d i f f e r e n c e of the f o c a l phase between waves r a d i a t e d i n t o o p p o s i t e a z i m u t h s , the r u p t u r e l e n g t h of the g e n e r a t i n g earthquake was d e r i v e d u s i n g e q u a t i o n ( 6 ) . 52 IV. RESULTS FOR THE M 8.1 EARTHQUAKE OF AUGUST 22 The methods of d i f f e r e n t i a l phases and d i r e c t i v i t y r e q u i r e a wave p a i r of o p p o s i t e l y t r a v e l i n g waves. U n f o r t u n a t e l y , f o r the August 22 M=8.1 earthquake almost a l l of the s t a t i o n s had L1 and R1 a r r i v a l s o f f s c a l e and L3 and R3 a r r i v a l s e i t h e r i n d i s c e r n i b l e or t o o s m a l l t o be u s e f u l . Only t h r e e s t a t i o n s had the r e q u i r e d wave p a i r s c l e a r and on s c a l e . At Tucson (TUO), b o t h R a y l e i g h wave (R2,R3) and Love wave (L2,L3) p a i r s were o b t a i n e d . For Pasadena (PAS), Love waves (L2,L3) were o b t a i n e d on the Wood-Anderson i n s t r u m e n t s , but the R3 a r r i v a l s were too s m a l l f o r R a y l e i g h waves t o be used. At H o n o l u l u (HON), o n l y R a y l e i g h waves (R2,R3) were used as the L2 waves were o f f s c a l e . The d i g i t i z e d seismograms a r e shown i n F i g u r e 15 and 16. The group v e l o c i t y c u r v e s a re shown i n F i g u r e s 18 and 19 r e s p e c t i v e l y . Over the p e r i o d range 50-210 seconds the d i s p e r s i o n c u r v e s h o u l d be r e l a t i v e l y f l a t w i t h a g e n t l e v a l l e y around 200 seconds (see F i g u r e 17). The fundamental mode was i d e n t i f i e d by l o o k i n g f o r a s e r i e s of a r r i v a l s t h a t smoothly v a r i e d and which a r r i v e d w i t h a group v e l o c i t y c l o s e t o the ex p e c t e d group v e l o c i t y . I t t u r n e d out t h a t the l a r g e s t of the l o c a l energy maximums f o r a g i v e n p e r i o d was u s u a l l y chosen as the fundamental mode a r r i v a l . The d i s p e r s i o n c u r v e d a t a appears t o be a f f e c t e d by s c a t t e r i n g w i t h l a r g e h o l e s of energy and a g e n e r a l ragged appearance. The d i s p e r s i o n c u r v e s do not c l o s e l y resemble those shown i n F i g u r e 17. There a r e s e v e r a l reasons f o r t h i s . F i r s t , the 5 3 c u r v e s shown i n F i g u r e 17 a r e smoothed w i t h the d a t a p o i n t s s c a t t e r i n g about the smoothed c u r v e s . Second, s c a t t e r i n g of the s h o r t e r p e r i o d waves by c r u s t a l i n h o m o g e n e i t i e s a f f e c t s the s h o r t p e r i o d p a r t of the d i s p e r s i o n c u r v e s . T h i r d , the low power a t l o n g p e r i o d s of the seismographs means t h a t the a m p l i t u d e of t h e s e i s m i c s i g n a l tends t o be down a t the normal seismogram n o i s e l e v e l , and hence the l o n g p e r i o d p a r t of the d i s p e r s i o n c u r v e i s i n f e c t e d w i t h n o i s e . These c u r v e s g i v e the be s t i n d i c a t i o n of the q u a l i t y of the d a t a and i n d i c a t e how not h a v i n g t r u e l o n g p e r i o d seismographs l i m i t s the amount of l o n g p e r i o d s u r f a c e wave a n a l y s i s which can be done. 54 L2 EW NS EW L3 i 1 r i ' i i i i i i 1 1 1 1 1 1 1 1 1 0 06 S3 139 105 232 278 325 371 4|7 4S4 SID 5S7 603 649 696 742 708 83S 881 928 T/ME IN SECONDS NS EW R2 flf^Vv/^ N S R3 vr-\\s •̂v-Vt EW ~i 1 r i 1 1 i l i 1 1—'—i 1 1 1 1 i i i 0 90 181 271 361 452 542 633 723 8)3 904 994 1084 1175 1265 J356 1445 1536 1627 I 717 160< T I M E J N S E C O N D S F i g u r e 15 - DIGITIZED TUO SEISMOGRAMS FOR THE AUGUST 22 EARTHQUAKE (M=8.1) There i s a 4.5:1 v e r t i c a l e x a g g e r a t i o n as the time s c a l e has been d i v i d e d by 4.5 t o a l l o w p l o t t i n g of the whole seismogram on one page. The seismograms have been reduced i n s c a l e as w e l l . The amount of r e d u c t i o n can be d e t e r m i n e d by n o t i n g t h a t b e f o r e r e d u c t i o n the time s c a l e marks were 1 cm a p a r t . R2 R3 i 1 1 1 1 1 1 i 1 1 1 — n 1 1 i i i i i i i 0 95 130 285 379 474 559 664 759 854 9<8 1043 1130 1233 1326 1423 1S16 1612 1707 ie02 !697 T I M E I N S E C O N D S NS EW L2 L3 I I I I I I I 1 I T 1 1 I 1 1 1 1 1 1 1—=—I 1 0 93 186 279 372 465 558 652 735 838 931 1024 I 1 17 1210 1303 I 335 14.89 1582 1675 1768 10G1 T I M E I N S E C O N D S F i g u r e 16 - DIGITIZED PAS AND HON SEISMOGRAMS FOR THE AUGUST 22 EARTHQUAKE (M=8.1) The upper s e t of seismograms were r e c o r d e d a t HON, the lower s e t on the PAS Wood-Andersons. In a l l c a s e s t h e r e i s a 4.5:1 v e r t i c a l e x a g g e r a t i o n as the time s c a l e has been d i v i d e d by 4.5 t o a l l o w p l o t t i n g of the whole seismogram on one page. The seismograms have been reduced i n s c a l e as w e l l . The amount of r e d u c t i o n can be d e t e r m i n e d by n o t i n g t h a t b e f o r e reducti.on the time s c a l e marks were 1 cm a p a r t . 56 5.5 t 5 0 o o _j UJ > 5 4.5 O (C o * 4.0 X 3-5. RAYLEIGH WAVES 0 MONGOL IA • ASSAM o CUTENBERC-BULLEN A CONTI CUTENBERC-BULLEN B CONTIS LEHMANNBULLEN A CONTINENT) LEHUANN-BULLEN A OCEANIC JEFF RE rS-BULLENA MENTAL y> ENTAL " PERIOD (SEC.} F i g u r e 17 - TYPICAL GROUP VELOCITY CURVES FOR LOVE AND RAYLEIGH WAVES These p l o t s show s u r f a c e wave group v e l o c i t y c u r v e s (U) o b t a i n e d by v a r i o u s i n v e s t i g a t o r s . These c u r v e s p r o v i d e a s t a n d a r d a g a i n s t which the group v e l o c i t y c u r v e s f o r the August 22 d a t a can be compared (adapted from Kovach, 1965). 57 The upper p a i r of p l o t s a r e of the R a y l e i g h d a t a r e c o r d e d a t HON. The lower p a i r of p l o t s a r e of t h e R a y l e i g h d a t a r e c o r d e d a t TUO. The d o t s a r e the p o s i t i o n s of l o c a l maximums i n t h e s p e c t r a l a m p l i t u d e . The s o l i d l i n e i n d i c a t e s the energy a r r i v a l s s e l e c t e d as c o r r e s p o n d i n g t o the f u n d a m e n t a l mode. The c u r v e s do not c l o s e l y resemble the s t a n d a r d ones shown i n F i g u r e 17, i n d i c a t i n g how s t r o n g l y t h e d a t a a r e a f f e c t e d by c r u s t a l s c a t t e r i n g a t s h o r t p e r i o d s and i n s t r u m e n t r e s p o n s e a t l o n g p e r i o d s . 58 F i g u r e 19 - GROUP VELOCITY CURVES FOR THE AUGUST 22 DATA The upper p a i r of p l o t s a r e of the Love d a t a r e c o r d e d at PAS. The lower p a i r of p l o t s a r e of the Love d a t a r e c o r d e d a t TUO. The d o t s a r e the p o s i t i o n s of l o c a l maximums i n the s p e c t r a l a m p l i t u d e . The s o l i d l i n e i n d i c a t e s the energy a r r i v a l s s e l e c t e d as c o r r e s p o n d i n g t o the fundamental mode. The c u r v e s do not c l o s e l y resemble the s t a n d a r d ones shown i n F i g u r e s 17, i n d i c a t i n g how s t r o n g l y the d a t a a r e a f f e c t e d by c r u s t a l s c a t t e r i n g a t s h o r t p e r i o d s and i n s t r u m e n t response a t l o n g p e r i o d s . 59 4.1 DIRECTIVITY FUNCTION The d i r e c t i v i t y f u n c t i o n was d e r i v e d by t a k i n g the r a t i o of s p e c t r a l a m p l i t u d e s of o p p o s i t e l y t r a v e l i n g waves, as measured on the same i n s t r u m e n t . T h i s meant t h a t i n a c c u r a c i e s i n c o r r e c t i n g f o r i n s t r u m e n t parameters and c a l i b r a t i o n were c a n c e l e d o u t , a s i g n i f i c a n t advantage when i n s t r u m e n t m a g n i f i c a t i o n s a r e not a c c u r a t e l y known. Phase v e l o c i t i e s and Q v a l u e s were taken from averaged whole w o r l d p a t h s (see Appendix B ) . T h e o r e t i c a l d i r e c t i v i t y f u n c t i o n c u r v e s were computed and compared t o t h e o b s e r v e d d i r e c t i v i t y f u n c t i o n u n t i l the c o m b i n a t i o n of r u p t u r e l e n g t h and r u p t u r e v e l o c i t y was found which gave the best match between the o b s e r v e d and t h e o r e t i c a l c u r v e s . I t was found t h a t i n c r e a s i n g the s i z e of the r u p t u r e l e n g t h , b, moved the l o c a t i o n of the Tmax extremum t o l o n g e r p e r i o d s and i n t r o d u c e d new extremes i n the s h o r t p e r i o d range. I n c r e a s i n g the r u p t u r e v e l o c i t y , V r , moved the Tmax extreme t o s h o r t e r p e r i o d s and widened the s p a c i n g between extremes. The b e s t f i t was o b t a i n e d w i t h a r u p t u r e l e n g t h of 265 km and a r u p t u r e v e l o c i t y between 3.1 km/s and 3.5 km/s (see F i g u r e 2 0 ) . The key f e a t u r e s of the b e s t f i t t h e o r e t i c a l c u r v e s a r e s i m i l i a r t o the observed c u r v e s . In p a r t i c u l a r , the l o c a l maximums and minimums of the observed and t h e o r e t i c a l c u r v e s appear a t a p p r o x i m a t e l y the same p l a c e w i t h the e x c e p t i o n of the l o c a l maximum i n the o b s e r v e d c u r v e s around 75 seconds which i s not seen i n the t h e o r e c t i c a l c u r v e s of PAS and TUO ( R a y l e i g h ) , or a s i m i l i a r d i s c r e p a n c y around 100 seconds f o r HON. The b i g g e s t d i f f e r e n c e between the observed and t h e o r e t i c a l c u r v e s o 3 H' i n DIRECTIVITY - _ o 8 8 c o n ) r D ' •-3 h - ' T j c n w r r 0) tt> CO i£> o »—< o B» i - 1 r r a tt> O a 3 3 C 03 l O «-t »—• C r r < 3* c r o 3 r o t n r o r r i n o II 0) to >-i o i cn C O r o O r r >-t r r »—i C n> 3- yr n c n 0) 3 tt> "O n O tr -9 < 3 r o •-« n> Qj c n < f - • r r -3 o -3 D - o m •< CD r r r r O r r a O r r z 3" o ^ < f O -3 o n O 1—4 t — II C >-t o CD z c n • r r CJI t — ' M * o 0) o a O i r r n p r CD < rt> 3 i \0> tn r r c n l — O c r o o r - h < r o 1=1 0) 0) C c n r r 3 I— • Q) O-i r r o DIRECTIVITY _ _ o - 5 8 8 -3 2 o a < er o "a II M II >• cn go ^ . "* «• fc p DIRECTIVITY _ _ o 8 8 DIRECTIVITY 5 o o c o 61 i s the s i z e of the extremes. The t h e o r e t i c a l extremes a r e much l a r g e r than the observed extremes which appear t o be s i g n i f i c a n t l y smoothed. M o d i f i c a t i o n of the t h e o r e t i c a l model was t r i e d t o b e t t e r f i t t he d a t a . Ben-Menahem and Toksoz (1962) have g i v e n t h e r e l a t i o n s h i p s t o c a l c u l a t e the d i r e c t i v i t y f o r a d e c a y i n g s o u r c e of v a r i a b l e s t r e n g t h g i v e n by L=L 0exp(-2£/b 0), w i t h 0<£<b, and f o r b i l a t e r a l f a u l t i n g . A v a r i a b l e s t r e n g t h source seems p h y s i c a l l y r e a s o n a b l e , and the t h e o r e t i c a l c u r v e s g e n e r a t e d w i t h a decay c o n s t a n t £/b o = /3=0.75 f i t the d a t a s i g n i f i c a n t l y b e t t e r . The e f f e c t of the d e c a y i n g s o u r c e was t o f l a t t e n a l l the extremes (see F i g u r e 2 1 ) . t - h O CD >-3 0 O X 3 " C D " 0 <t c H i n O f t r t 3 f D 0 ) n> w t — • D D O r o ( t r t r f H — l Q TS> 0 J Cu 1 r r II H D " O H a • * < » — m cr >-"• n II O l D > r O - DJ f D K rx> ro IN t — i oi i n Z C QJ o 3 r t H - 1 C O . C 3 f) o i - t iQ a n> w r t o < w V n m o m C O a o •-» <~T T-T O O f D w H - ( D r t ra r t o O -a f - h CD »-» < C H * < II O w r t O . M- C m o i 3 < a ro z ?r S t o o 3 M- •-3 \ r t t—4 w rr c o f D w z fl 0 ) H-O o CLiO G f D 0 ) o < 3 0) 0> C O DIRECTIVITY - _ • o _ o 8 o Pi o o DIRECTIVITY o o o il H II c <- " - ° - <- «. p ^ v o — i| II II o <-. ~ ~ 2 DIRECTIVITY - _ 5 8 o w >—< O DIRECTIVITY - 5 8 8 "3 o o w *• - o - o < «r O O y-^ — O A. 30 29 63 An a t t e m p t was made t o b r i n g the 490 km a f t e r s h o c k zone i n t o agreement w i t h the d i r e c t i v i t y f u n c t i o n f a u l t l e n g t h by u s i n g Ben-Menahem and Toksoz's (1962) d i r e c t i v i t y r e l a t i o n s h i p s f o r b i l a t e r a l f a u l t i n g . The e f f e c t on the d i r e c t i v i t y f u n c t i o n o f b i l a t e r a l r u p t u r e was t o smooth and d i s p l a c e t h e l o n g p e r i o d e xtremes and damp out or e l i m i n a t e the s h o r t e r p e r i o d extremes (see F i g u r e 2 2 ) . No s u i t a b l e b i l a t e r a l r u p t u r e c o u l d be found t o f i t t h e d a t a . F i g u r e 22 - THE EFFECT OF BILATERAL RUPTURE ON THE DIRECTIVITY FUNCTION The e f f e c t of b i l a t e r a l r u p t u r e on t h e d i r e c t i v i t y f u n c t i o n i s t o smooth and s l i g h t l y d i s p l a c e t h e l o n g - p e r i o d e x t r e m e s , and e l i m i n a t e the s h o r t p e r i o d e x t r e m e s . S i n c e t h e d i r e c t i v i t y was o b t a i n e d from more than one s t a t i o n , an a l t e r n a t e approach t o f i n d i n g t h e r u p t u r e p a r a m e t e r s u s i n g the l e a s t s q u a r e s t e c h n i q u e d e s c r i b e d i n S e c t i o n 2.2 c o u l d be a p p l i e d . U s i n g t h i s l e a s t s q u a r e s method a r u p t u r e l e n g t h of 64 170 km and a r u p t u r e v e l o c i t y 1 . 9 km/s was found . These v a l u e s gave a good f i t of t h e maximum p e r i o d extreme, but i n t r o d u c e d new extremes w h i c h d i d not e x i s t i n the d a t a . When p l o t t e d (see F i g u r e 23) i t was found t h a t t h e parameter v a l u e s were b e i n g d e t e r m i n e d p r i m a r i l y by the v a l u e s of HON and were not r e l i a b l e v a l u e s f o r the ensemble. O 16 13 3D «• S» (I !Q «0 ^0 |H 110 II* tJO |4D Tmax (Seconds) F i g u r e 23 - LEAST SQUARES SOLUTION TO THE DATA The s o l i d l i n e i s the l e a s t s q u a r e s l i n e . The dashed l i n e c o r r e s p o n d s t o the b e s t f i t s o l u t i o n shown i n F i g u r e 20. 65 For the d i r e c t i v i t y f u n c t i o n the a z i m u t h a l a n g l e between the f a u l t and the s e i s m i c s t a t i o n , 0, i s measured c l o c k w i s e from the d i r e c t i o n of r u p t u r e p r o p a g a t i o n . T h i s means t h a t even though the s t r i k e of the f a u l t p l a n e i s known f o r a g i v e n earthquake t h e r e a r e two p o s s i b l e 0's f o r a g i v e n s t a t i o n depending upon which d i r e c t i o n t h e r u p t u r e p r o g r e s s e d . The t h e o r e t i c a l c u r v e s s h o u l d match the d a t a f o r o n l y one of the two 0's. By n o t i n g which of the two p o s s i b l e 0's f i t s the d a t a , the d i r e c t i o n of r u p t u r e p r o p a g a t i o n i s d e t e r m i n e d . In o r d e r t o f i t the d a t a f o r the August 22 M=8.1 e a r t h q u a k e , a r u p t u r e d i r e c t i o n t o the n o r t h w e s t had t o be used. T h i s i s i n agreement w i t h Ben- Menahem' s (1967) r e s u l t s . 4.2 DIFFERENTIAL PHASES The d i f f e r e n t i a l phase a t a g i v e n p e r i o d between o p p o s i t e t r a v e l i n g wave p a i r s was o b t a i n e d a t a v a r i e t y of p e r i o d s f o r each s t a t i o n (see T a b l e I I I ) . The f a u l t l e n g t h s o b t a i n e d from the d i f f e r e n t i a l phase method show c o n s i d e r a b l e s c a t t e r w i t h i n a g i v e n s t a t i o n f o r a l l the s t a t i o n s except PAS. T h i s s c a t t e r i s not s u p r i s i n g i n view of the ragged appearance of the d i s p e r s i o n c u r v e s . Because of the r e l a t i v e coherence of t h e . PAS r e s u l t s t h e s e r e s u l t s were s e l e c t e d as the most r e l i a b l e . 66 TUO (LOVE) DIFFERENTIAL FAULT PERIOD PHASE LAMBDA LENGTH 90. 3S 0. 2336 417. 92 114. 24 93. 00 0. 1984 430. 78 91. 41 99. 10 0. 8330 460. 34 410. 33 102. 40 0. 3796 476. 88 293. 63 103. 93 0. 2751 494. 37 143. 33 113. 78 0. 2122 334. 11 121. 20 118. 13 0. 2383 333. 63 141. 66 122. 88 O. 2128 379. OS 131. 79 128. 00 0. 1781 604. 48 113. 13 139. 64 0. 2783 663. 76 197. 74 146. 39 0. 3016 698 08 223. 18 1 53. 60 0. 2693 736. 18 212. 08 161 68 0. 2940 778. 64 244. 83 170. 67 0. 3813 826 42 337. 24 192. 00 0. 3898 941 76 394. 09 AVERAGE FAULT L E N G T H - 225. 21 PAS (LOVE) DIFFERENTIAL FAULT PERIOD PHASE LAMBDA LENGTH 90. 33 0. 7330 417. 92 346. 39 96. 00 0. 4383 443. 40 230. 96 99. 10 0. 4107 460 34 214. 01 102. 40 O. 6174 476. 88 333. 14 103. 93 0. 6072 494. 57 339. 78 109. 71 0. 4801 313. 60 279. OO 113. 78 0. 3482 334. 11 331. 28 118. 13 0. 2207 355. 63 138. 73 122. 88 0. 1353 579. 03 101. 77 128 00 0. 2763 604. 48 189 11 133. 37 0. 7233 632. 69 317. 92 139. 64 0. 2314 663 . 76 188 82 170. 67 0. 1203 826. 42 112 65 192, 00 0. 2193 941. 76 233 67 AVERAGE FAULT LENGTH" 234. 10 TUO (RAYLEIGH) HON (RAYLEIGH) DIFFERENTIAL FAULT PERIOD PHASE LAMBDA LENGTH 90 35 0. 8717 368. 63 692. 80 93. 09 1. 0122 380 43 830 24 96. 00 0. 3641 393. 28 308. 70 99. 10 0. 7663 407 03 672. 43 102 40 0. 7548 421. 73 686. 33 105. 93 0. 7158 437 36 673. 30 109. 71 0. 6666 434 60 653. 31 113. 78 1. 1638 473 05 1186 98 118. 15 0. 3775 492. 99 401. 20 122 88 0. 7416 514 71 822 94 133 57 0 9765 365 04 1189 54 139 64 0. 3719 394 11 732 51 1 33 60 1. 2902 660 40 1837. 02 170. 67 0. 8729 736. 58 . 13S6. 29 192. 00 o. 2948 862 73 348. 33 AVERAGE FAULT LENGTH** 841 . 59 DIFFERENTIAL FAULT PER 100 PHASE LAMBDA LENGTH 90 33 0. 9164 368. 63 351. 49 93. 09 1. 3652 380. 43 972. 03 96. 00 - 0. 9997 393 28 641. 85 99. 10 1. 2905 407. 03 837. 33 103. 93 0. 5259 437. 36 373. 66 113. 78 0. 8785 473. 05 678. 42 118. 13 0. 7201 492 99 379. 49 122 88 O. 4773 514. 71 401. 08 128 00 1. 1336 538. 62 996. 74 133. 37 0. 9651 363. 04 690. 23 139. 64 O. 1379 394. 11 133. 17. 161. 6B O. 4668 696. 39 330. 66 180. 71 0. 2663 791. 19 344. 24 192. 00 o. 6498 862. 73 913. 18 AVERAGE FAULT LENGTH- 634. 84 T a b l e III - DIFFERENTIAL PHASE FAULT LENGTHS The poor c o n s i s t e n c y of the r e s u l t s w i t h i n a g i v e n s t a t i o n f o r the d i f f e r e n t i a l phases i s not s u p r i s i n g . In the l i t e r a t u r e ( e . g . , Kanamori, 1970) i t i s f r e q u e n t l y recommended t h a t d i f f e r e n t i a l phases o n l y be a p p l i e d t o p e r i o d s g r e a t e r than 200 67 seconds. These recommendations, based on r e s u l t s d e r i v e d from l o n g p e r i o d i n s t r u m e n t s of the w o r l d wide s e i s m i c network, i m p l y t h a t c r u s t a l s c a t t e r i n g s t r o n g l y a f f e c t s the coherency of waves w i t h p e r i o d s s h o r t e r than 200 seconds. The l a c k of coherence i s l i k e l y t o be f u r t h e r a c c e n t u a t e d i f one does not have the l u x u r y of a t r u e l o n g p e r i o d insrument but must r e l y on i n s t r u m e n t s w i t h a peak response around 12 seconds. At p e r i o d s ^ 100 seconds c r u s t a l s c a t t e r i n g s t r o n g l y a f f e c t s the coherence, w h i l e f o r p e r i o d s l o n g e r than 100 seconds the response of the s h o r t p e r i o d seismographs a r e so poor t h a t the s i g n a l becomes b u r i e d i n n o i s e and, hence, i s i n c o h e r e n t a t l o n g p e r i o d s as w e l l as s h o r t . The r e s u l t s from PAS show the b e s t i n t e r n a l c o n s i s t e n c y and agree w i t h Ben-Menahem (1967, 1978). F o l l o w i n g the s u g g e s t i o n of A k i ( l 9 6 6 ) , a w e i g h t e d average of the d i f f e r e n t i a l phases f a u l t l e n g t h s from a l l f o u r s t a t i o n s was t a k e n . Though t h e r e was no s p e c i f i c f o r m u l a f o r d e t e r m i n i n g the w e i g h t i n g , I t r i e d t o make the w e i g h t i n g an i n d i v i d u a l s t a t i o n r e c e i v e d c o r r e s p o n d t o the i n t e r n a l c o n s i s t e n c y of the r e s u l t s of t h a t s t a t i o n . W ith the w e i g h t i n g of PAS=1.0, HON=0.5, TUC (Love)=0.25, and TUC ( R a y l e i g h ) = 0 . 2 5 an average r u p t u r e l e n g t h , from the w e i g h t e d average of the d i f f e r e n t i a l phases, of 358 km was d e r i v e d . 68 4.3 ERROR ANALYSIS The a t t e m p t s i n t h i s t h e s i s t o e x t r a c t i n f o r m a t i o n from s u r f a c e waves perhaps demonstrate one t h i n g more than any o t h e r : q u a l i t y seismographs w i t h a p p r o p r i a t e response c u r v e s a r e e s s e n t i a l f o r o b t a i n i n g d e t a i l e d i n f o r m a t i o n about earthquake source p a r a m e t e r s . O l d low power seismographs i n t r o d u c e too many u n c e r t a i n t i e s i n t o the seismogram and unduly i n f l u e n c e the subsequent a n a l y s i s . I f the fundamental mode a r r i v a l i s c o r r e c t l y i d e n t i f i e d w i t h i t s a m p l i t u d e and phase u n a f f e c t e d by s e i s m i c n o i s e and c r u s t a l s c a t t e r i n g , then the e r r o r i n t h e c a l c u l a t e d d i f f e r e n t i a l phase would come from t h r e e p o s s i b l e s o u r c e s : 1) e r r o r i n the i n s t r u m e n t phase response c o r r e c t i o n 2) improper T 0 3) wrong phase v e l o c i t i e s used. The e f f e c t on the r e s u l t s of (1) would depend on how f a r o f f the t r u e i n s t r u m e n t response was from the c o r r e c t i o n . An e s t i m a t e of 20% i n s t r u m e n t response u n c e r t a i n t y seems r e a s o n a b l e f o r these o l d e r i n s t r u m e n t s . T h i s i m p l i e s a p o s s i b l e 20% e r r o r i n r a d i a n s of t h e d i f f e r e n t i a l .phase r e s u l t s . An e r r o r At i n T 0 would l e a d t o a 5f/»=ajAt = 27rfAt e r r o r i n r a d i a n s . An e r r o r i n the phase v e l o c i t y , c, would l e a d t o , 60=6 (cor/c ) =ur 3 c / c 2 e r r o r i n r a d i a n s where -3C = e r r o r i n c 69 c = phase v e l o c i t y r = l e n g t h of p a t h over which the phase v e l o c i t y i s i n e r r o r . The f a c t i s , however, t h a t the d a t a a r e g r e a t l y i n f l u e n c e d by c r u s t a l s c a t t e r i n g and seismograph n o i s e , the e f f e c t s of which are d i f f i c u l t t o a s s e s s , and which as a r e s u l t make an o v e r a l l e r r o r e s t i m a t e i m p r a c t i c a l . E s t i m a t e s of the d i r e c t i v i t y u n c e r t a i n t y a r e s u b j e c t t o the same d i f f i c u l t i e s as above. For both the d i f f e r e n t i a l phase f a u l t l e n g t h and the d i r e c t i v i t y f u n c t i o n , the s i z e of the s c a t t e r of the r e s u l t s among the t h r e e s t a t i o n s i s p r o b a b l y the best i n d i c a t i o n of the u n c e r t a i n t y of the r e s u l t s . 4.4 SEISMIC MOMENT AND STRESS DROP The s e i s m i c moment f o r the 1949 Queen C h a r l o t t e earthquake was c a l c u l a t e d u s i n g the r e s u l t s of Ben-Menahem (1978). Ben- Menahem d i d not e x p l i c i t l y d e termine the s e i s m i c moment f o r t h i s e a r t h q u a k e . He d e r i v e d the d i r e c t i v i t y and d i f f e r e n t i a l phases, and u s i n g t h e s e r e s u l t s o b t a i n e d what he c a l l e d the e q u a l i z e d p o t e n c y . T h i s potency i s r e l a t e d t o the s e i s m i c moment by the r i g i d i t y n. I chose t o use h i s d a t a over mine as he was u s i n g a t r u e l o n g p e r i o d i n s t r u m e n t which a l l o w e d him t o measure the s u r f a c e wave a m p l i t u d e a t a p e r i o d of 200 seconds where the e f f e c t of c r u s t a l i n h o m o g e n e i t i e s i s m i n i m i z e d . I c o u l d not get a good measure of the s u r f a c e wave a m p l i t u d e a t t h i s l o n g p e r i o d s i n c e the response c u r v e s f o r a l l of my i n s t r u m e n t s meant t h a t the a m p l i t u d e of the s e i s m i c s i g n a l was a t the n o i s e l e v e l (see F i g u r e 24 and Appendix D t o compare the response of the PAS 70 4 0 0 ZOO 3 0 0 1 0 0 - 4 0 6 . 0 8 . 0 10 .0 1 2 0 sic 1 0 0 F i g u r e 24 - RESPONSE CURVE FOR THE PAS STRAIN METER (Adapted from Ben-Menahem, 1978) s t r a i n meter w i t h the response of the i n s t r u m e n t s used i n t h i s t h e s i s ) . The s e i s m i c moment i s d e r i v e d from s p e c t r a l a m p l i t u d e s by c o r r e c t i n g f o r the f i n i t e n e s s of the r u p t u r e p r o c e s s ( d e r i v e d from t h e d i r e c t i v i t y r e s u l t s ) , r a d i a t i o n p a t t e r n , and the Love or R a y l e i g h c r u s t a l t r a n s f e r f u n c t i o n . A l l of t h i s was done by Ben-Menahem i n o b t a i n i n g the e q u a l i z e d p o t e n c y . U s i n g h i s r e s u l t s and ^=3.2x10'' dyne/cm 2, a s e i s m i c moment Mo= 1 . 15 x 1 0 Z 8 dyne cm was found. T h i s compares w e l l t o the Mo found u s i n g G e l l e r and Kanamori's (1977) r e l a t i o n , M o = 1 . 2 3 x l 0 2 2 x S i s where S i s the f a u l t s u r f a c e a r e a i n km 2. 71 W i t h S=(495)x(20) an Mo of 1.2x1fJ 2 8 i s o b t a i n e d . U s i n g Gutenberg and R i c h t e r ' s ( R i c h t e r , 1958) fo r m u l a f o r s e i s m i c energy E , LogE =11.8+1.5Ms For Ms>6.5 For Ms=8.1 E =8.9 x 1 0 2 3 e r g s , and A k i ' s (1966) r e l a t i o n f o r apparent s t r e s s drop a , a = r}d=u.E /M0 where / i = r i g i d i t y E y = s e i s m i c energy r ? = e f f i c i e n c y c o e f f i c i e n t of s t r a i n energy r e l e a s e d i n t o s e i s m i c energy. a=average s t r e s s , the apparent s t r e s s drop was c a l c u l a t e d t o be an =25 b a r s . The average d i s p l a c e m e n t a l o n g t h e f a u l t can be d e r i v e d u s i n g the the s t a n d a r d e x p r e s s i o n f o r the s e i s m i c moment (Kanamori and A n d e r s o n , 1 9 7 5 ) . M0=MLWD L = f a u l t l e n g t h W=fault w i d t h D=average d i s p l a c e m e n t a l o n g the f a u l t S o l v i n g t h i s e q u a t i o n f o r the average d i s p l a c e m e n t u s i n g a f a u l t w i d t h of 21 km one o b t a i n s , D=6.5 m f o r L=265 km D=3.5 m f o r L=495 km f o r an o v e r a l l average d i s p l a c e m e n t of D=5.0 m. 72 I f we f o l l o w Ben-Menahem (1978) and break the s e i s m i c moment i n t o t h e sum of the moments g e n e r a t e d by the d i s p l a c e m e n t a l o n g two d i f f e r e n t p a r t s of the f a u l t , namely the p a r t f o r which the d i s p l a c e m e n t i s l a r g e s t (the d i r e c t i v i t y f a u l t l e n g t h ) and the p a r t f o r which the d i s p l a c e m e n t i s the l e a s t ( t h a t p a r t of the r u p t u r e which i s beyond the d i r e c t i v i t y l e n g t h ; see the d i s c u s s i o n s e c t i o n of the Summary and D i s c u s s i o n c h a p t e r f o r an e x p l a n a t i o n of t h i s ) , then we can w r i t e the r e l a t i o n f o r the s e i s m i c moment a s , M 0=MW(L,D 1+L 2D 2). I f we assume D 1=4.96 m, L.^265 km, and L 2=(495km-265km)=230 km, then D 2=1.7 m. T h e r e f o r e , the average d i s p l a c e m e n t a l o n g the whole f a u l t i s 4.96 m, but a l o n g the ends of the f a u l t o n l y 1.7 m. The s t r e s s drop was c a l c u l a t e d u s i n g the r e l a t i o n s h i p between s t r e s s drop and the s e i s m i c moment f o r a s t r i k e - s l i p f a u l t (Kanamori and Anderson, 1975), M0=(TT/2)W2LAO-. The s t r e s s d r o p , Aa, was found t o be Aa=34 b a r s . The s t r a i n energy was a l s o c a l c u l a t e d u s i n g Kamamori and Anderson's (1975) r e l a t i o n s h i p between the s t r a i n energy, AW, and the apparent and average s t r e s s , AW=(7rW2L/2M)Aad. The s t r a i n energy was found t o be AW=9.5 x 1 0 2 3 e r g s . T h i s a l l o w e d the e f f i c i e n c y c o n s t a n t TJ t o be de t e r m i n e d : . T?=ES/AW=0.93 . The average s t r e s s was c a l c u l a t e d : 73 0=0^/77=26 . 9 b a r s . And f i n a l l y t h e f r i c t i o n a l s t r e s s , a + was c a l c u l a t e d , 0̂  =a-o a = 1 .9 b a r s (Kamamori and Anderson, 1975). Ben-Menahem (1978) d e r i v e s a f a u l t depth of 40 km f o r t h i s e a r t h q u a k e , i n s t a r k c o n t r a s t t o the r e s u l t s of Horn e t . a l . (1984), and Hyndman and E l l i s (1981) who suggest a c r u s t a l d e p t h i n the a r e a of o n l y 16-21 km. Ben-Menahem's v a l u e f o r the depth was d e r i v e d by assuming the f a u l t l e n g t h t o be 265 km w i t h an average d i s p l a c e m e n t a l o n g t h e f a u l t a r b i t r a r i l y chosen t o be 3.5 m. I f an average d i s p l a c e m e n t of 4.96 m and a f a u l t l e n g t h of 495 km i s used, h i s c a l c u l a t i o n s w i l l y i e l d a d e p t h c o n s i s t e n t w i t h the e s t i m a t e d c r u s t a l d e p t h . 74 V. RESULTS FOR THE EARTHQUAKES OF AUGUST 23 AND OCTOBER 31 As mentioned e a r l i e r , out of more than 30 s t a t i o n s o r i g i n a l l y r e q u e s t e d , o n l y f i v e c o u l d be used i n d e t e r m i n i n g the s u r f a c e wave r a d i a t i o n p a t t e r n f o r the August 23 (M=6.4) earthquake and o n l y e i g h t f o r the October 31 (M=6.2) ea r t h q u a k e . The g e n e r a l problem was the l a c k of t r u e l o n g p e r i o d i n s t r u m e n t s c o u p l e d w i t h an u n c e r t a i n t y of both i n s t r u m e n t parameters and c a l i b r a t i o n a c c u r a c y . I n g e n e r a l , the i n s t r u m e n t m a g n i f i c a t i o n was e s t i m a t e d as known o n l y t o w i t h i n twenty p e r c e n t . The d i g i t i z e d seismograms f o r the August 23 and October 31 ear t h q u a k e s a r e shown i n F i g u r e s 25-29. The group v e l o c i t y d i s p e r s i o n c u r v e s a r e shown i n F i g u r e s 30-36. These d i s p e r s i o n c u r v e s g i v e an i n d i c a t i o n of the q u a l i t y of the d a t a . For i n s t a n c e , the fundamental mode a r r i v a l was c l e a r l y d i s c e r n i b l e out t o 60 seconds f o r the August 23 DBN da t a ( F i g u r e s 30, 31). In c o n t r a s t , the da t a from August 23 OTT ( F i g u r e 31) does not d i s p l a y any c l e a r mode a r r i v a l . The d i f f e r e n c e between DBN and OTT i s p a r t l y e x p l a i n e d by n o t i n g t h a t the waves a r r i v i n g a t DBN have had t w i c e as l o n g t o d i s p e r s e and s e p a r a t e as those a r r i v i n g a t OTT. T h e r e f o r e , the d i s p e r s i o n c u r v e used t o i d e n t i f y mode a r r i v a l i s more c l e a r l y seen by t h i s f i l t e r i n g t e c h n i q u e a t DBN than a t OTT. The r a p i d and st e a d y r i s e f o r OTT i n a m p l i t u d e f o r almost a l l group v e l o c i t i e s s t a r t i n g around 55 seconds i n d i c a t e s t h a t beyond 55 seconds i n s t r u m e n t response has dropped o f f so much t h a t i n c o r r e c t i n g f o r i n s t r u m e n t response o n l y n o i s e i s b e i n g a m p l i f i e d . FRE I I I I — I - II I I I 1 I I I III III I X l « III I K III II —r— I I H I T i n t I I I 1)1 7 0 I I S H I III III )0« l i t III IH S tCOKOS 1 1 1 i l l 1 1 1 1 1 1 at < I I «ti u i « M SJP I I I • ' • » • '1 P i l l I • I I » » » • » • I I. I T I M E IN SEIOIV/dj 1500 DBN 1 • • ; • • • • • • 0 T i n t . IN SECOND;, ( 7 2 5 - F i g u r e 25 - DIGITIZED SEISMOGRAMS FOR THE AUGUST 23 EARTHQUAKE (M=6.4) The upper t r a c e f o r FRE and DBN was r e c o r d e d on the v e r t i c a l seismograph, the m i d d l e t r a c e on the n o r t h - s o u th seismograph, and the lower t r a c e on the e a s t - west seismograph. The SJP s e t of seismograms has no v e r t i c a l , and, hence, the upper t r a c e c o r r e s p o n d s t o the n o r t h - s o u t h seismogram, and the lower t r a c e t o the east-west seismogram. 76 OTT 31fc <W> SiH W W * i 110 TIME. jN secoA/05 TUO I — i r T — I — i — T " r - r i i • i i » » » ' • * ' ' ' ' o SI "OS" IS! 2iD z«2 30' 3tT g'1 V?2 S"l</ 577 T I M E <IJ i ZC°t"> i 42.1 F i g u r e 26 - DIGITIZED SEISMOGRAMS FOR THE AUGUST 23 EARTHQUAKE (M=6.4) The upper t r a c e f o r OTT c o r r e s p o n d s t o the n o r t h - s o u t h seismogram, and t h e lower t r a c e the e a s t - w e s t seismogram. The o r d e r of the seismograms f o r TUO i s the same as OTT e x c e p t t h a t the t o p seismogram i s a v e r t i c a l seismogram. HON • • • A ^ A T v V W v W ^ / ^ ^ A, i r t i i i i I i i i I I I' 1 i i 1 i i i i i i i i i i i i l ' t i l i l t i i i i 6 11* T i n t IN StCOKOS DBN TIME Ikl 5 EloUOf , SJP F i g u r e 27 - DIGITIZED SEISMOGRAMS FOR THE OCTOBER 31 EARTHQUAKE (M=6.2) The upper t r a c e f o r DBN i s the v e r t i c a l seismogram, the m i d d l e t r a c e the n o r t h - s o u t h seismogram, and the lower t r a c e the east-west seismogram. The o r d e r i n g of the t r a c e s f o r HON and SJP i s the same as f o r DBN except t h a t t h e r e i s no v e r t i c a l seismogram. 78 PAS i — i — i — i — i — i — i — i i — i — i i — i — i t i i i i i i i i i i i 0 2 8 5 6 8 3 111 1 3 9 1 6 7 1 9 4 2 2 2 2 S 0 2 7 8 3 0 5 33 3 3 6 1 3 8 9 4 ! 6 44" . 4 7 2 5 0 0 5 2 7 5 5 S 5 8 3 6 1 1 6 3 8 6 6 6 6 9 4 TIME JN 5 E X 0 N 0 S BOZ — i — i — i — \ — i — i — i — 5 2 1 0 5 1 5 7 2 J 0 2 6 2 J 1 5 3 6 7 ~ I I I I I I I I I I ! 5 2 5 5 7 7 6 3 0 6 8 ? 7 3 5 7 8 7 8 4 0 8 9 2 9 4 5 3 9 7 1 0 5 0 T JME^JN SECONDS HAL I I I I I I I I I I I I I I I I I 1 1 1 6 4 1 2 8 191 2 5 5 3 1 9 3 8 3 4 4 7 5 1 ] 5 7 4 6 3 8 7 0 2 7 G 6 8 3 0 8 3 3 9 5 7 1021 1 0 8 5 1 ) 4 9 12J2 1 2 7 6 TJME JN SECONDS F i g u r e 28 - DIGITIZED SEISMOGRAMS FOR THE OCTOBER 31 EARTHQUAKE (M=6.2) The upper t r a c e f o r PAS i s the v e r t i c a l seismogram, the m i d d l e t r a c e the n o r t h - s o u t h seismogram, and the lower t r a c e the east-west seismogram. The o r d e r i n g of the t r a c e s f o r BOZ and HAL i s the same as f o r PAS except t h a t t h e r e i s no v e r t i c a l seismogram. 79 SLT — r — i 1 1 r - — i 1 1 1 1 1 1 1 1 35 Ift JM 237 285 332 380 427 475 522 563 «JT TJME JN SECONDS OTT 48 9 6 1 4 4 132 2 4 0 2 8 f l 3 3 6 3 8 4 4 3 2 4 8 0 5 2 8 S 7 G 6 2 4 6 7 2 7 2 0 "768 8 1 6 8 6 4 3 1 2 9 6 0 TJME JN S E C O N D S F i g u r e 29 - DIGITIZED SEISMOGRAMS FOR THE OCTOBER 31 EARTHQUAKE (M=6.2) For both SLT and OTT the upper t r a c e i s the n o r t h - s o u t h seismogram, and the lower t r a c e i s the ea s t - w e s t seismogram. 80 P E R I O D 6 13 21 28 36 "3 50 S8 6S 73 80 PER IOD 0 13 21 28 36 13 SO 58 6S 73 60 TUC UT F i g u r e 30 - GROUP VELOCITY CURVES FOR THE AUGUST 23 LOVE WAVE DATA The d o t s i n d i c a t e the p o s i t i o n s of l o c a l maximums of the s p e c t r a l a m p l i t u d e . The s o l i d l i n e i n d i c a t e s the energy a r r i v a l s s e l e c t e d as c o r r e s p o n d i n g t o the fundamental mode. UT means o n l y the t a n g e n t i a l component of the seismograms i s a n a l y z e d . 81 F i g u r e 31 - GROUP VELOCITY CURVES FOR THE AUGUST 23 RAYLEIGH WAVE DATA The d o t s i n d i c a t e the p o s i t i o n s of l o c a l maximums of the s p e c t r a l a m p l i t u d e . The s o l i d l i n e i n d i c a t e s the energy a r r i v a l s s e l e c t e d as c o r r e s p o n d i n g t o the fundamental mode. UR means o n l y the r a d i a l (UR) component of the seismograms i s a n a l y z e d . Z means o n l y v e r t i c a l d a t a i s be a n a l y z e d . 82 0 13 21 2 8 3 G ^ « 3 ' ^ P o 5 8 6 5 ' 7 3 80 PER IOD 0 | 1 21 2 8 3G <n SO 58 6 5 73 8 0 SJP UT SJP UR F i g u r e 32 - GROUP VELOCITY CURVES FOR THE AUGUST 23 SJP DATA The d o t s i n d i c a t e the p o s i t i o n s of l o c a l maximums of the s p e c t r a l a m p l i t u d e . The s o l i d l i n e i n d i c a t e s the energy a r r i v a l s s e l e c t e d as c o r r e s p o n d i n g t o the fundamental mode. 83 HAL UR PAS Z F i g u r e 33 - GROUP VELOCITY CURVES FOR THE OCTOBER 31 LOVE WAVE DATA The d o t s i n d i c a t e the p o s i t i o n s of l o c a l maximums of the s p e c t r a l a m p l i t u d e . The s o l i d l i n e i n d i c a t e s the energy a r r i v a l s s e l e c t e d as c o r r e s p o n d i n g t o the fundamental mode. 84 F i g u r e 34 - GROUP VELOCITY CURVES FOR THE OCTOBER 31 LOVE WAVE DATA The d o t s i n d i c a t e the p o s i t i o n s of l o c a l maximums of the s p e c t r a l a m p l i t u d e . The s o l i d l i n e i n d i c a t e s the energy a r r i v a l s s e l e c t e d as c o r r e s p o n d i n g t o the fundamental mode. 85 PER IOD 3 6 13 SO C I I 21 2B  0   58 6 S 73 8 0 '-i ' 1 1 « 1 1 ' ' ' ' , , PER/00 C I I 21 in 3G 0 3 50 Sfl 6S 7.1 8 0 H VT-L • 1 , 1 1 L _ J _ HAL UT PER IOD IG 4 3 50 p E R ! O D C 13 21 26 3 6 4 3 5 0 S 8 6 5 7 3 8 0 - J I I i i i i i i DBN UT BOZ UT F i g u r e 35 - GROUP VELOCITY CURVES FOR THE OCTOBER 31 RAYLEIGH WAVE DATA The d o t s i n d i c a t e the p o s i t i o n s of l o c a l maximums of the s p e c t r a l a m p l i t u d e . The s o l i d l i n e i n d i c a t e s the energy a r r i v a l s s e l e c t e d as c o r r e s p o n d i n g t o the fundamental mode. 86 PAS UT OTT UT F i g u r e 36 - GROUP VELOCITY CURVES FOR THE OCTOBER 31 RAYLEIGH WAVE DATA The d o t s i n d i c a t e the p o s i t i o n s of l o c a l maximums of the s p e c t r a l a m p l i t u d e . The s o l i d l i n e i n d i c a t e s the energy a r r i v a l s s e l e c t e d as c o r r e s p o n d i n g t o the fundamental mode. 87 MECHANISM SOLUTIONS Depending on the s t a t i o n , s p e c t r a l a m p l i t u d e s f o r p e r i o d s from 16 seconds t o 65 seconds were used. For each p e r i o d , the o b s e r v e d a z i m u t h a l v a r i a t i o n i n a m p l i t u d e was p l o t t e d . U s i n g a s i x l a y e r E a r t h model of N o r t h America (see Appendix C) and the Herrmann (1978) program QUESTION, the o b s e r v e d a z i m u t h a l a m p l i t u d e s were compared t o t h e o r e t i c a l a m p l i t u d e s u n t i l a 'best f i t ' mechanism was found. The s e i s m i c moment was c a l c u l a t e d s e p a r a t e l y from Love waves and then from R a y l e i g h waves. The best f i t mechanism was chosen as the mechanism f o r which the two s e p a r a t e moment e s t i m a t e s from Love and R a y l e i g h waves were i n c l o s e s t agreement. Due t o the s m a l l number of s t a t i o n s c o u p l e d w i t h the u n c e r t a i n t y i n the i n s t r u m e n t m a g n i f i c a t i o n s the s o l u t i o n s o b t a i n e d from t h i s comparison p r o c e s s a r e not m e a n i n g f u l . The 'best f i t ' mechanism was not a 'good f i t ' mechanism f o r e i t h e r the August 23 or the October 31 e a r t h q u a k e . The f i t of t h e o b s e r v e d s u r f a c e wave r a d i a t i o n p a t t e r n t o the b e s t f i t model i s shown f o r the August 23 (M=6.4) earthquake i n F i g u r e s 37 and 38, and f o r the October 31 (M=6.2) earthquake i n F i g u r e s 39 and 40. F o c a l sphere p l o t s of the b e s t f i t mechanisms a r e shown i n F i g u r e 41. The Wickens and Hodgson (1967) f i r s t motion s o l u t i o n f o r the October 31 earthquake i s a l s o shown i n F i g u r e 41 a l o n g w i t h the few f i r s t motions a v a i l a b l e f o r the August 23 e a r t h q u a k e . 88 N 2 X 10 1 F i g u r e 37 " THEORETICAL LOVE WAVE RADIATION PATTERN FOR THE AUGUST 23 EARTHQUAKE The r a d i a t i o n p a t t e r n s a r e s c a l e d t o ( i t i n a s q u a r e 2-3/4 i n c h e s on a s i d e . Beneath each r a d i a t i o n p a t t e r n t he p e r i o d i s w r i t t e n a l o n g w i t h a s c a l e r e l a t i n g t he p l o t s i z e t o u n i t s of dyne-cm. The r a d i a t i o n p a t t e r n shown i s of the b e s t ( i t mechanism s o l u t i o n . The d o t s i n d i c a t e the o b s e r v e d d a t a . Each dot r e p r e s e n t s a s t a t i o n , w i t h t he a z i m u t h of t h e d o t c o r r e s p o n d i n g t o the s t a t i o n a z i m u t h , and t h e d i s t a n c e o( the dot from t h e c * n t e r o( t h e r a d i a t i o n p a t t e r n . 8 9 N I 4 ? • 2 X JO F i g u r e 3 8 - THEORETICAL RAYLEIGH WAVE RADIATION PATTERN FOR THE AUGUST 23 EARTHQUAKE The r a d i a t i o n p a t t e r n s a r e s c a l e d t o f i t i n a square 2-3/4 i n c h e s on a s i d e . Beneath each r a d i a t i o n p a t t e r n the p e r i o d i s w r i t t e n a l o n g w i t h a s c a l e r e l a t i n g t he p l o t s i r e t o u n i t s of dyne-cm. The r a d i a t i o n p a t t e r n shown i s of the b e s t t i t mechanism s o l u t i o n . The d o t s I n d i c a t e the o b s e r v e d d a t a . Each dot r e p r e s e n t s a s t a t i o n , w i t h the a z i m u t h of the d o t c o r r e s p o n d i n g t o the s t a t i o n a z i m u t h , and the d i s t a n c e of t he d o t fro« t h e c e n t e r of the r a d i a t i o n p a t t e r n . 90 2 X 10 ~1 T = 30.0 N I 2 X 30° 2 X 30° T = 40.0 T = 50.0 N N I I 2 X 30° 2 X J0° T = "70.0 T = 80.0 F i g u r e 39 - THEORETICAL LOVE WAVE RADIATION PATTERN FOR THE OCTOBER 31 EARTHQUAKE The r a d i a t i o n p a t t e r n s a r c s c a l e d t o ( i t i n a square 2-3/4 i n c h e s on a s i d e . Beneath each r a d i a t i o n p a t t e r n the p e r i o d i s w r i t t e n a l o n g w i t h a s c a l e r e l a t i n g the p l o t a i r e t o u n i t s of dyne-cm. The r a d i a t i o n p a t t e r n shown i t of the b e l t ( i t mechanism s o l u t i o n . The d o t s i n d i c a t e the o b s e r v e d d a t a . Each dot r e p r e s e n t s a s t a t i o n , w i t h the a z i m u t h of the d o t c o r r e s p o n d i n g t o the s t a t i o n a z i m u t h , and the d i s t a n c e o( the dot ( r o a t h e c e n t e r of the r a d i a t i o n p a t t e r n . -91 a 2 X JO T = 20.0 2 X ] 0 — I T = 34.0 N I 2 X JO — I T = 36.0 2 X JO I — L T = 50.0 2 X JO ~ J I — { T - 60.0 F i g u r e 40 - THEORETICAL RAYLEIGH WAVE RADIATION PATTERN FOR THE OCTOBER 31 EARTHQUAKE The r a d i a t i o n p a t t e r n s a r e s c a l e d t o f i t i n a square 2-3/4 i n c h e s on a s i d e . Beneath each r a d i a t i o n p a t t e r n the p e r i o d i s w r i t t e n a l o n g w i t h a s c a l e r e l a t i n g the p l o t s i t e t o u n i t s of dyne-cm. The r a d i a t i o n p a t t e r n shown i s of the best f i t mechanism s o l u t i o n . The d o t s i n d i c a t e t he o b s e r v e d d a t a . Each dot r e p r e s e n t s a s t a t i o n , w i t h the a z i m u t h of the d o t c o r r e s p o n d i n g t o the s t a t i o n a z i m u t h , and the d i s t a n c e of the dot from the c e n t e r of the r a d i a t i o n p a t t e r n . F i g u r e 41 - FOCAL MECHANISMS OF THE AUGUST 23 AND OCTOBER 31 EARTHQUAKES D i p i c t e d i n t h i s f i g u r e a r e l o w e r h e m i s p h e r e f o c a l s p h e r e p r o j e c t i o n s o f : I ) T h e p o o r l y d e f i n e d P - n o d a l s o l u t i o n o f W i c k e n s a n d H o d g s o n ( 1 9 6 7 ) f o r t h e O c t o b e r 3 ) e a r t h q u a k e ( M - 6 . 2 ) , 2 ) T h e v e r y p o o r l y c o n s t r a i n e d b e s t f i t s u r f a c e w ave m e c h a n i s m s o l u t i o n f o r t h e O c t o b e r 31 e a r t h q u a k e , 3 ) T h e w e l l d e f i n e d P - n o d a l s o l u t i o n o f R o g e r s ( 1 9 8 3 ) f o r t h e A u g u s t 22 ( M - 8 . 1 ) e a r t h q u a k e . 4) T h e v e r y p o o r l y c o n s t r a i n e d b e s t f i t s u r f a c e wave m e c h a n i s m s o l u t i o n f o r t h e A u g u s t 23 e a r t h q u a k e ( M - 6 . 4 ) , 5) S u g g e s t e d p o s s i b l e n o d a l p l a n e s f r o m t h e few f i r s t m o t i o n r e a d i n g s a v a i l a b l e f o r t h e A u g u s t 23 e a r t h q u a k e . N o t e t h a t t h e p o s i t i o n o f t h e p r e s s u r e a n d t e n s i o n a x e s f o r b e s t f i t s u r f a c e wave m e c h a n i s m s c a n be r e v e r s e d , a s r i g h t - l a t e r a l o r l e f t - l a t e r a l s t r i k e - s l i p m o t i o n c a n n o t be d i s t i n g u i s h e d f r o m a m p l i t u d e d a t a a l o n e ( H e r r m a n n , 1 9 7 8 ) . 93 The c o r r e c t s u r f a c e wave s o l u t i o n must be c o m p a t i b l e w i t h the f i r s t motion data u n l e s s the f o c a l mechanism changed r a d i c a l l y as the r u p t u r e p r o g r e s s e d . I f the f i r s t motion d a t a i s used as a c o n s t r a i n t , the s u r f a c e wave mechanisms o b t a i n e d f o r the August 23 earthquake must be wrong. The c o n c l u s i o n drawn from t h i s study i s t h a t the method of u s i n g the a z i m u t h a l s u r f a c e wave r a d i a t i o n p a t t e r n t o o b t a i n the f o c a l mechanism i s i n e f f e c t i v e f o r t h i s a r e a and p e r i o d i n h i s t o r y when s t a t i o n coverage was s p a r s e and the q u a l i t y of i n s t r u m e n t c a l i b r a t i o n s poor. 94 V I . SUMMARY AND DISCUSSION 6.1 DISCUSSION The d i r e c t i v i t y f u n c t i o n and d i f f e r e n t i a l phases were a n a l y z e d a t t h r e e s t a t i o n s . The r e s u l t s i m p l y a u n i l a t e r a l r u p t u r e p r o p a g a t i n g t o t h e n o r t h w e s t "for a p p r o x i m a t e l y 265 km a t a r u p t u r e v e l o c i t y between 3.1 km/s and 3.5 km/s. These r u p t u r e v e l o c i t y r e s u l t s a r e i n good agreeement w i t h o t h e r i n v e s t i g a t o r s who have found t h a t t h e t h e r u p t u r e v e l o c i t y f o r o t h e r e a r t h q u a k e s i s i n t h e range of 3.1 km/s t o 3.5 km/s. I n g e n e r a l , i t a p p e a r s t h a t t h e r u p t u r e v e l o c i t y i s u s u a l l y s l i g h t l y l e s s t h a n t h e s h e a r wave v e l o c i t y i n the c r u s t . The r u p t u r e l e n g t h of t h e 1949 Queen C h a r l o t t e e a r t h q u a k e d e r i v e d from t h e d i r e c t i v i t y and d i f f e r e n t i a l phase f u n c t i o n s does not agree w i t h t h e f a u l t l e n g t h o f 490 km d e r i v e d from th e a f t e r s h o c k zone. I t i s t e m p t i n g t o r e s o l v e t h i s a p p a r e n t c o n f l i c t by d i s m i s s i n g t h e s u r f a c e wave r e s u l t s as b e i n g t o o l i m i t e d and i n f l u e n c e d by t h e p o o r q u a l i t y of t h e d a t a . To some e x t e n t t h i s may be t r u e , as t h e p r e c i s e v a l u e s f o r r u p t u r e l e n g t h and r u p t u r e v e l o c i t y a r e u n c e r t a i n ; n e v e r t h e l e s s , t h e d a t a a r e good enough t o i n d i c a t e t h a t t h e y a r e i n c o m p a t i b l e w i t h a s u r f a c e wave f a u l t l e n g t h o f 490 km. In f a c t , w h i l e my d a t a were l i m i t e d t o s h o r t p e r i o d i n s t r u m e n t s , Ben-Menahem's a n a l y s i s o f t h i s e a r t h q u a k e was made u s i n g t h e Pasadena EW s t r a i n - m e t e r (Tg=70 s ), a t r u e l o n g p e r i o d i n s t r u m e n t . H i s r e s u l t s of 265 km f o r the d i f f e r e n t i a l phases f a u l t l e n g t h i n the p e r i o d range 125-277 seconds show e x c e l l e n t c o h e r e n c e and cannot be d i s m i s s e d 9 5 as a f f e c t e d by n o i s e . The r e l a t i o n s h i p between a f t e r s h o c k zone and f a u l t r u p t u r e l e n g t h can be seen more c l e a r l y by l o o k i n g a t the r e s u l t s o t h e r i n v e s t i g a t e r s have o b t a i n e d . Other examples of source parameter d e t e r m i n a t i o n from d i r e c t i v i t y and d i f f e r e n t i a l phase f u n c t i o n s a r e l i s t e d i n T a b l e IV. DIRECTIVITY DIFFERENTIAL AFTERSHOCK FAULT LENGTH FAULT LENGTH FAULT LENGTH (KM) (KM) (KM) REFERENCE C h i l e 725 May 22, 1960 M=8.5 Kamchatka 700 Nov. 4, 1952 M=8.25 Mo n g o l i a 560 Dec. 4, 1957 M=8.0 San F r a n c i s c o 240 June 6, 1906 M=8.0 A l a s k a 350 J u l y 10, 1958 M=8.0 K u r i l e I s l a n d s Oct. 13, 1963 M=8.0 S a n r i k u 270 March 2, 1933 M=8.0 426 279 250 1000 675 500 400 380 300 270 P r e s s e t a l . (1961) Ben-Menahem and Toksoz (1963a) Ben-Menahem and Toksoz (1962) Ben-Menahem (1978) Ben-Menahem and Toksoz (1963b) Furumoto (1979) Ben-Menahem (1967) T a b l e IV - RUPTURE LENGTH FROM DIRECTIVITY AND DIFFERENTIAL PHASES Examples from the l i t e r a t u r e of f a u l t l e n g t h s d e r i v e d from the d i f f e r e n t i a l phases and or the d i r e c t i v i t y f u n c t i o n . The examples a r e l i s t e d i n d e c r e a s i n g o r d e r of f a u l t l e n g t h . C l e a r l y t h e r e can be a d i s c r e p a n c y of 40% between the s u r f a c e wave r u p t u r e l e n g t h and the r u p t u r e l e n g t h d e f i n e d from 96 a f t e r s h o c k s . Ben-Menahem (1 9 7 8 ) , i n s e e k i n g t o e x p l a i n the d i f f e r e n c e between the d i r e c t i v i t y f a u l t l e n g t h and the known s u r f a c e r u p t u r e of the 1906 San F r a n c i s c o e a r t h q u a k e , suggested t h a t t h e r e i s an e f f e c t i v e r a d i a t i o n f a u l t l e n g t h which may not c o r r e s p o n d t o t h e f u l l f a u l t r u p t u r e . He p o i n t s out t h a t the o b s e r v e d d i s p l a c e m e n t a l o n g t h e San Andreas f a u l t f o r the 1906 ear t h q u a k e ranged from s l i g h t l y more than 6 meters f o r the a r e a around P t . Reyes t o under 0.5 meters f o r the a r e a s o u t h of the Santa Cruz m o u n t a i n s . He s u g g e s t s t h a t once the d i s p l a c e m e n t a l o n g the f a u l t becomes l e s s than a g i v e n average v a l u e , 0.6 meters i n t h e 1906 c a s e , the a f f e c t of the r e m a i n i n g r u p t u r e on the r a d i a t i o n a m p l i t u d e and phase i s s m a l l and does not i n f l u e n c e t h e d i r e c t i v i t y and d i f f e r e n t i a l phase f u n c t i o n (see FlgUTe 42) • b e g i n ' r o d . o t i o n . l o u l t ' E - end r o d i a n o n . f a u l t L - end rupture m e l e r i JuOn Out Ml» IOIEMAI BOutilto JOO I P i ArenO 4 0 0 k m I O i l P I OclQotfrj DISTANCE FROM SE END OF THE FAULT i t m i F i g u r e 42 - DISPLACEMENT ALONG THE SAN ANDREAS FAULT FOR THE 1906 EARTHQUAKE T h i s f i g u r e shows the r e l a t i o n s h i p of t h e d i s p l a c e m e n t a l o n g t h e San Andreas f a u l t t o the d i r e c t i v i t y f a u l t l e n g t h . The d o t s r e p r e s e n t the measured o f f s e t d i s p l a c e m e n t a l o n g the f a u l t (from Ben-Menaham, 1978). 97 I f a s i m i l a r e x p l a n a t i o n i s used t o e x p l a i n the d i s c r e p a n c y between the s u r f a c e wave f a u l t l e n g t h and the a f t e r s h o c k zone f a u l t l e n g t h f o r the 1949 Queen C h a r l o t t e e a r t h q u a k e , then i t i m p l i e s t h a t d i s p l a c e m e n t a l o n g the f a u l t was uneven. The northwestward p r o p a g a t i o n of the e f f e c t i v e r a d i a t i o n f a u l t r u p t u r e s u g g e s t s t h a t the l a r g e s t d i s p l a c e m e n t a l o n g the f a u l t o c c u r r e d f o r 265 km t o the n o r t h of the e p i c e n t e r w i t h the r u p t u r e d i s p l a c e m e n t c o n t i n u i n g , but w i t h s m a l l e r o f f s e t , f o r another 35 km f u r t h e r n o r t h and 190 km t o the south of the e p i c e n t e r . I n f o r m a t i o n can be o b t a i n e d by f u r t h e r a n a l y s i s of the M=8.1 August 22 e a r t h q u a k e . In p a r t i c u l a r , t h e r e i s enough i n f o r m a t i o n g i v e n by Ben-Menahem (1967) t o det e r m i n e the i n i t i a l phase, and, hence, the time f u n c t i o n of the 1949 Queen C h a r l o t t e e a r t h q u a k e . The time f u n c t i o n would be of p a r t i c u l a r i n t e r e s t as i t can be used t o d e r i v e the r i s e time of the r u p t u r e , which can i n t u r n y i e l d i n f o r m a t i o n about the i n i t i a l s t r e s s l e v e l s ( K a s a h a r a , 1981). R i s e - t i m e a n a l y s i s a l s o p l a y s an im p o r t a n t r o l e i n e a r t h q u a k e e n g i n e e r i n g i n which p r e d i c t i o n of s e i s m i c v i b r a t i o n , a c c e l e r a t i o n and v e l o c i t y a r e d e s i r e d . The use of s u r f a c e waves t o a n a l y s e the r u p t u r e parameters proved d i f f i c u l t due t o the l a c k of l o n g p e r i o d i n s t r u m e n t s o p e r a t i n g i n 1949. Thus,- f u r t h e r study of the source parameters t h r o u g h the use of body waves would be more s u i t e d t o the i n s t r u m e n t r e s p o n s e s . Body wave d i r e c t i v i t y and a z i m u t h a l r a d i a t i o n p a t t e r n s c o u l d be o b t a i n e d t o supplement the f i r s t motion and s u r f a c e wave r e s u l t s ( e . g . , L a n g s t o n , 1978). 98 As w e l l as s t u d y i n g the main 8.1 shock an attempt was made to d e r i v e the mechanism s o l u t i o n s f o r the two l a r g e s t a f t e r s h o c k s u s i n g t h e i r a z i m u t h a l s u r f a c e wave r a d i a t i o n p a t t e r n s . I t was c o n c l u d e d t h a t the use of t h i s t e c h n i q u e t o o b t a i n f o c a l mechanism s o l u t i o n s i s i n e f f e c t i v e f o r t h i s a r e a and time i n h i s t o r y when s t a t i o n coverage was s p a r s e and the q u a l i t y of i n s t r u m e n t c a l i b r a t i o n s poor. In view of the l i m i t a t i o n s i n o b t a i n i n g the o v e r a l l s u r f a c e wave r a d i a t i o n p a t t e r n , a more p r o d u c t i v e approach t o d e t e r m i n i n g the s u r f a c e wave f o c a l mechanism might be t o s e l e c t one s t a t i o n f o r which the i n s t r u m e n t parameters and c a l i b r a t i o n a r e w e l l known and then t r y t o model the e n t i r e s u r f a c e wave t r a i n r e c o r d e d a t t h a t s t a t i o n ( e . g . , Kanamori, 1970). The d i s p e r s i o n c u r v e s f o r t h e October 31 earthquake suggest t h a t the s u r f a c e waves have not been w e l l r e c o r d e d as no c l e a r mode a r r i v a l s can be seen, and, t h u s , t h i s method might not be s u c c e s s f u l when a p p l i e d t o t h a t earthquake u s i n g the d a t a I've c o l l e c t e d (see F i g u r e s 32-35) . On the o t h e r hand, the d i s p e r s i o n c u r v e s f o r the August 23 earthquake do d i s p l a y c l e a r mode a r r i v a l s and suggest t h a t s y n t h e t i c seismogram mo d e l i n g of the s u r f a c e wave forms r e c o r d e d a t DBN might be s u c c e s s f u l (see F i g u r e s 29-31) . 99 6.2 CONCLUSION U s i n g h o r i z o n t a l seismograms r e c o r d e d a t S i t k a , A l a s k a , t h i r t y - e i g h t new a f t e r s h o c k l o c a t i o n s were d e t e r m i n e d f o r t h e 1949 Queen C h a r l o t t e e a r t h q u a k e . T h e . a f t e r s h o c k zone was found t o e x t e n d from 300 km t o t h e n o r t h o f t h e e p i c e n t e r t o 190 km t o the south of t h e e p i c e n t e r , y i e l d i n g a t o t a l a f t e r s h o c k zone of 490 km. T h i s a f t e r s h o c k zone i m p l i e s t h a t a p r e v i o u s l y s uggested s e i s m i c gap t o t h e n o r t h o f t h e 1949 e p i c e n t e r ( K e l l e h e r and S a v i n o , 1975) does n ot e x i s t . The a f t e r s h o c k d i s t r i b u t i o n a l s o s u g g e s t s a t i m e v a r i a t i o n i n t h e r u p t u r e sequence, w i t h t h e a f t e r s h o c k s c o n c e n t r a t i n g f i r s t t o t h e n o r t h and l a t e r t o t h e s o u t h o f t h e main s h o c k ' s e p i c e n t e r . The d i r e c t i v i t y f u n c t i o n and d i f f e r e n t i a l phases were a n a l y z e d a t t h r e e s t a t i o n s . The r e s u l t s i m p l y a u n i l a t e r a l r u p t u r e p r o p a g a t i n g t o t h e n o r t h w e s t f o r a p p r o x i m a t e l y 265 km a t a r u p t u r e v e l o c i t y between 3.1 km/s - and 3.5 km/s. These r e s u l t s agree w i t h t h e r e s u l t s o f Ben-Menahem (1967,1978). The d i f f e r e n c e between t h e a f t e r s h o c k zone,and t h e r u p t u r e l e n g t h d e r i v e d from t h e d i r e c t i v i t y f u n c t i o n and d i f f e r e n t i a l p h a s e s , s u g g e s t s t h a t t h e r e may be a d i f f e r e n c e between t h e f r a c t u r e l e n g t h and t h e e f f e c t i v e r a d i a t i o n l e n g t h . T h i s d i f f e r e n c e between the f r a c t u r e l e n g t h and r a d i a t i o n l e n g t h was f i r s t n oted by Ben-Menahem (1978) f o r t h e 1906 San F r a n c i s c o e a r t h q u a k e , w h i c h , l i k e t h e Queen C h a r l o t t e e a r t h q u a k e , o c c u r r e d a l o n g a s h a l l o w v e r t i c a l s t r i k e - s l i p f a u l t . T h i s d i f f e r e n c e between the r a d i a t i o n f a u l t l e n g t h and t h e f r a c t u r e f a u l t l e n g t h s u g g e s t s t h a t the d i s p l a c e m e n t a l o n g t h e f a u l t was uneven, w i t h 100 the l a r g e s t d i s p l a c e m e n t o c c u r r i n g i n the a r e a t h a t c o r r e s p o n d s t o the r a d i a t i o n f a u l t l e n g t h . The s e i s m i c moment, average f a u l t d i s p l a c e m e n t , s e i s m i c energy, apparent s t r e s s drop, s t r e s s d r o p , average s t r e s s , s t r a i n energy, and f r a c t i o n a l s t r e s s were a l l e s t i m a t e d f o r the M=8.1 event and a r e summarized i n T a b l e V. As w e l l as s t u d y i n g the main shock an attempt was made t o d e r i v e the mechanism s o l u t i o n s f o r the two l a r g e s t a f t e r s h o c k s u s i n g t h e i r a z i m u t h a l s u r f a c e wave r a d i a t i o n p a t t e r n s . I t was c o n c l u d e d t h a t the use of t h i s t e c h n i q u e t o o b t a i n f o c a l mechanism s o l u t i o n s i s i n e f f e c t i v e f o r t h i s a r e a and time i n h i s t o r y when s t a t i o n coverage was s p a r s e and the q u a l i t y of i n s t r u m e n t c a l i b r a t i o n s poor. F a u l t L e n g t h 495 km F a u l t Depth 20 km F a u l t Area 9900 km 2 Rupture V e l o c i t y 3.1 km/sec Average F a u l t D i s p l a c e m e n t 4.96 m S e i s m i c Moment 1.15 X 1 0 2 8 dyne cm S t r a i n Energy 9.5 X 1 0 2 3 e r g s S e i s m i c Energy 8.9 X 1 0 2 3 e r g s Apparent S t r e s s Drop 25 b a r s S t r e s s Drop 34 b a r s Average S t r e s s 26.9 b a r s F r i c t i o n a l S t r e s s 1.9 b a r s T a b l e V - ESTIMATE OF SEISMIC SOURCE PARAMETERS OF THE 1949 QUEEN CHARLOTTE EARTHQUAKE 101 BIBLIOGRAPHY 1. A c h a r y a , H. K. 1979. R e g i o n a l v a r i a t i o n s i n the r u p t u r e - l e n g t h magnitude r e l a t i o n s h i p s and t h e i r d y n a m i c a l s i g n i f i c a n c e . B u l l e t i n of the S e i s m o l o g i c a l S o c i e t y of Am e r i c a , 69, pp. 2063-2084. 2. A k i , K. 1966. G e n e r a t i o n and p r o p a g a t i o n of G waves from the N i i g a t a Earthquake of June 16, 1964. B u l l e t i n of the Earthquake Research I n s t i t u t e , 44, pp. 23-72. 3. A k i , K., and R i c h a r d s , P.G. 1980. Q u a n t i t a t i v e S e i s m o l o g y , V o l . 1. W. H. Freeman Co., San F r a n c i s c o , CA., 557 pp. 4. Ben-Menahem, A. 1961. R a d i a t i o n of s e i s m i c surface-waves from f i n i t e moving s o u r c e s . B u l l e t i n of the S e i s m o l o g i c a l S o c i e t y of Am e r i c a , 55, pp. 401-435. 5. Ben-Menahem, A. 1967. Source s t u d i e s from i s o l a t e d s e i s m i c s i g n a l s . I n : VESIAC c o n f e r e n c e . on the c u r r e n t s t a t u s and f u t u r e p r o g n o s i s f o r u n d e r s t a n d i n g the source mechanism of s h a l l o w s e i s m i c e v e n t s . U n i v e r s i t y of M i c h i g a n , Geophys. Lab., pp. 85-108. 6. Ben-Menahem, A. 1978. Source mechanism of the 1906 San F r a n c i s c o e a r t h q u a k e . 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B u l l e t i n of the S e i s m o l o g i c a l S o c i e t y of A m e r i c a , 73, pp. 1117- 1 1 38. 22. Hyndman, R. D., and E l l i s , R. M. 1981, Queen C h a r l o t t e F a u l t Zone: m i c r o e a r t h q u a k e s from a temporary a r r a y of l a n d s t a t i o n s and ocean bottom seismographs. Canadian J o u r n a l of E a r t h S c i e n c e s , 18, pp. 776-788. 23. Hyndman, R. D., L e w i s , T. J . , W r i g h t , J . A., B u r g e s s , M., Chapman, D. S., and Yamano, M. 1982. Queen 1 03 C h a r l o t t e f a u l t zone: Heat f l o w measurements. Canadian J o u r n a l of E a r t h S c i e n c e s , 19, pp. 1657-1669. 24. Hyndman, R. D., and W e i c h e r t , D. H. 1983. S e i s m i c i t y and r a t e s of r e l a t i v e motion of p l a t e b o u n d a r i e s of western N o r t h A merica. G e o p h y s i c a l J o u r n a l of the R o y a l A s t r o n o m i c a l S o c i e t y , 72, pp. 53-69. 25. I i d a , K. 1965. 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Kasahara, K. 1981. Earthquake M e c h a n i c s , Cambridge U n i v e r s i t y P r e s s , Cambridge, UK., 284 pp. 30. K e l l e h e r , J . A. 1972. Rupture zones of l a r g e South American e a r t h q u a k e s and some p r e d i c t i o n s . J o u r n a l of G e o p h y s i c a l R e s e a r c h , 79, pp. 2087-2103. 31. K e l l e h e r , J . , and S a v i n o , J . 1975. D i s t r i b u t i o n of s e i s m i c i t y b e f o r e l a r g e s t r i k e - s l i p and t h r u s t type e a r t h q u a k e s . J o u r n a l of G e o p h y s i c a l R e s e a r c h , 80, pp.260- 271. 32. Kovach, R. L. 1965. S e i s m i c s u r f a c e waves: some o b s e r v a t i o n s and r e c e n t developments. P h y s i c s and C h e m i s t r y of the E a r t h , 6, pp. 251-314. 33. L a n g s t o n , C. A. 1978. The F e b r u a r y 9, 1971 San Fernando E a r t h q u a k e : A study of sour c e f i n i t e n e s s i n t e l e s e i s m i c body waves. B u l l e t i n of the S e i s m o l o g i c a l S o c i e t y of A m e r i c a , 68, pp. 1.-29. 34. Lathram, E. H. 1964. Apparent r i g h t l a t e r a l s e p a r a t i o n of the Chatham S t r a i t f a u l t , s o u t h e a s t A l a s k a . B u l l e t i n of the S e i s m o l o g i c a l S o c i e t y of America, 75, pp. 249-252. 35. M i n s t e r , J . B., and J o r d a n , T. H. 1978. P r e s e n t day p l a t e m o t i o n s . J o u r n a l of G e o p h y s i c a l R e s e a r c h , 83, pp. 104 5331-5354. 36. Page, R. 1969. La t e Cenozoic movement on the F a i r w e a t h e r f a u l t i n s o u t h e a s t e r n A l a s k a . B u l l e t i n of the S e i s m o l o g i c a l S o c i e t y of A m e r i c a , 80, pp. 1873-1877. 37. P e r e z , 0. J . , and Jacob, K. H. 1980. T e c t o n i c model and s e i s m i c p o t e n t i a l of the e a s t e r n G u l f of A l a s k a and Yakataga s e i s m i c gap. J o u r n a l of G e o p h y s i c a l R e s e a r c h , 85, pp. 7132-7150. 38. P r e s s , F., Ben-Menahem, A., and Toksoz, N. M. 1961. E x p e r i m e n t a l d e t e r m i n a t i o n of earthquake f a u l t l e n g t h and r u p t u r e v e l o c i t y . J o u r n a l of G e o p h y s i c a l R e s e a r c h , 66, pp. 3471-3485. 39. R i c h t e r , C. F. 1958. Ele m e n t a r y S e i s m o l o g y . W. H. Freeman Co., San F r a n c i s c o , CA., p. 69. 40. R i d d i h o u g h , R. P. 1977. A model f o r r e c e n t p l a t e i n t e r a c t i o n s o f f Canada's west c o a s t . Canadian J o u r n a l of E a r t h S c i e n c e s , 14, pp. 384-396. 41. R i d d i h o u g h , R. P., C u r r i e , R. G., and Hyndman, R. D. 1980. The Delwood K n o l l s and t h e i r r o l e i n t r i p l e j u n c t i o n t e c t o n i c s o f f n o r t h e r n Vancouver I s l a n d . Canadian J o u r n a l of E a r t h S c i e n c e s , 17, pp. 577-593. 42. Rogers, G. 1976. A m i c r o e a r t h q u a k e s t u d y i n northwest B r i t i s h Columbia and s o u t h e a s t A l a s k a . B u l l e t i n of the S e i s m o l o g i c a l S o c i e t y of A m e r i c a , 66, pp. 1643-1655. 43. Rogers, G. 1983. S e i s m o t e c t o n i c s of B r i t i s h Columbia. Ph.D. t h e s i s , U n i v e r s i t y of B r i t i s h C o l u m b i a , 247 pp. 44. S t a u d e r , W. 1959. A mechanism s t u d y : The earthquake of October 24, 1927. G e o f i s i c a Pura e A p p l i c a t a , 44, p. 135. 45. S t a u d e r , W. 1960. The A l a s k a earthquake of J u l y 10, 1958: S e i s m i c s t u d i e s . B u l l e t i n of the S e i s m o l o g i c a l S o c i e t y of Am e r i c a , 50, pp. 293-322. 46. T o b i n , P. G., and Sykes, L. R. 1968. S e i s m i c i t y and t e c t o n i c s of the n o r t h - e a s t P a c i f i c Ocean. J o u r n a l of G e o p h y s i c a l R e s e arch, 73, pp. 3821-3846. 47. Tocher, D. 1958. Earthquake energy and ground breakage. B u l l e t i n of the S e i s m o l o g i c a l S o c i e t y of A m e r i c a , 48, pp. 147-152. 48. T s a i , Y., and A k i , K. 1970. P r e c i s e f o c a l depth d e t e r m i n a t i o n from a m p l i t u d e s p e c t r a of s u r f a c e waves. J o u r n a l of G e o p h y s i c a l R e s e a r c h , 75, pp. 5729-5741. 105 49. Von Heune, R., Shor, G., and Wageman, J . 1979. C o n t i n e n t a l margins of the e a s t e r n G u l f of A l a s k a and b o u n d a r i e s of t e c t o n i c p l a t e s . I n : G e o l o g i c a l and G e o p h y s i c a l I n v e s t i g a t i o n s of C o n t i n e n t a l M a r g i n s , e d i t e d by J . S. W a t k i n s , L. Montadent, and P. W. 50. W e t m i l l e r , R. J . , and Horner, R. B. 1978. Canadian e a r t h q u a k e s 1976. S e i s m o l o g i c a l S e r i e s of the E a r t h P h y s i c s B r a n c h , 79, p. 75. 51. Wickens, A. J . , and Hodgson, J . H. 1967. Computer r e - e v a l u a t i o n of earthquake mechanism s o l u t i o n s 1922-1962. P u b l i c a t i o n s of the Dominion O b s e r v a t o r y , Ottawa, 33, pp. 1-560. 52. Wyss, M. 1978. E s t i m a t i n g maximum e x p e c t a b l e magnitude of e a r t h quake from f a u l t l e n g t h ( a b s t r a c t ) . EOS, T r a n s a c t i o n s of the American G e o p h y s i c a l U n i o n , 59, p. 1 125. 53. Wyss, M. 1979. E s t i m a t i n g maximum e x p e c t a b l e magnitude of e a r t h q u a k e s from f a u l t d i m e n s i o n s . Geology, 7, pp 336- 340. 106 APPENDIX A - LIST OF SEISMOGRAPH STATIONS Type of Records I n s t r u m e n t s R e c e i v e d Source B e r k e l e y (BRK) B e n i o f f NS,EW,Z photocopy Berkeley-Network Aug.22,23 and Oct.31-messy t r a c e s , o f f s c a l e . (Berk-Net) Bermuda (BEC) Milne-Shaw NE,NW m i c r o f i l m World Data C e n t e r A Aug.22-off s c a l e , Aug.23-o.k., O c t . 3 1 - a m p l i t u e s t o o small.(WDC-A) . B o u l d e r C i t y (BCN) B e n i o f f NS,EW,Z WDC-A No r e c o r d s r e c e i v e d Bozeman (BOZ) McComb-Romberg N,E m i c r o f i l m WDC-A Aug.22-off s c a l e , Aug.23 and Oct.31-are good. C h i c a g o (CHK) McComb-Romberg N,E .... WDC-A no r e c o r d s r e c e i v e d C o l l e g e (COL) B e n i o f f N,E m i c r o f i l m WDC-A Aug.22,23 and O c t . 3 1 - a l l o f f s c a l e . Columbia (CSC) McComb-Romberg N,E .... WDC-A no r e c o r d s r e c e i v e d De B i l t (DBN) G a l i t z i n Z,N,E photocopy De B i l t Aug.22,23 and O c t . 3 1 - a l l e x c e l l e n t . F r e s n o (FRE) Sprengnether Z,N,E photocopy Berk-Net Aug.23 o n l y - e x c e l l e n t . H a l i f a x (HAL) Bosch-Omori N.,E o r i g i n a l Canadian-Network Aug.22-L2,R2 a m p l i t u d e s too small,Aug.23-good,Oct.31-poor.(Can-Net) H o n o l u l u (HON) Milne-Shaw N,E m i c r o f i l m WDC-A Aug.22,23 and Oct.31-good. Melbourne (MEL) Milne-Shaw E photocopy Melbourne Aug.22-L2,R2 a m p l i t u d e s t o s m a l l , Aug.23-missing, Oct.31-o.k. Mt. H a m i l t o n (MHC) Wood-Anderson N,E, B e n i o f f Z m i c r o f i l m i n s t r u m e n t t o o s h o r t p e r i o d t o be u s e f u l . Berk-Net Ottawa (OTT) Milne-Shaw N,E, B e n i o f f z o r i g i n a l Can-Net Aug;22-L2,R2 a m p l i t u d e s too s m a l l , Aug.23 and Oct.31-good. Pasadena (PAS) B e n i o f f Z,N,E m i c r o f i l m Pasadena Aug.22,23 and O c t . 3 l - a l l good. P e r t h (PER) Milne-Shaw N photocopy P e r t h Aug.22 o n l y - u n r e a d a b l e P h i l a d e l p h i a (PHI) Wenner N,E WDC-A 1 07 No r e c o r d s r e c e i v e d P i e r c e F e r r y (PFA) B e n i o f f Z,N,E .... WDC-A No r e c o r d s r e c e i v e d R a p i d C i t y (RCD) Wood-Anderson E m i c r o f i l m WDC-A in s t r u m e n t too s h o r t p e r i o d t o be u s e f u l . R i v e r v i e w (RIV) G a l i t z i n Z,N,E photocopy R i v e r v i e w Aug.22-off s c a l e , Aug.23 and Oct . 3 1 - c o n f u s e d t i m i n g , r e c o r d s suspecr S a l t Lake C i t y (SLC) Bosh-Omori-McComb-Romberg N,E m i c r o f i l m Aug.22,23-off s c a l e , O c t . 3 l - g o o d . WDC-A Saskatoon (SAS) Milne-Shaw NE,NW o r i g i n a l Can-Net Oct.31 o n l y - o . k . San Juan (SJP) Wenner N,E m i c r o f i l m WDC-A Aug.22-off s c a l e , Aug.23 and Oct.31-good. Seven F a l l s (SFA) Milne-Shaw E o r i g i n a l Can-Net Aug.22-L2,R2 a m p l i t u d e s too s m a l l , Aug.23 and Oct.31-o.k. S h a s t a (SHS) B e n i o f f Z,N,E WDC-A No r e c o r d s r e c e i v e d Shawingan F a l l s (SHF) Wood-Anderson N o r i g i n a l Can-Net Aug.22-L2,R2 a m p l i t u d e s too s m a l l , Aug.23 and Oct.31-o.k. S i t k a (SIT) Wenner N,E o r i g i n a l S i t k a Aug.22,23 and O c t . 3 1 - a l l o f f s c a l e . S t u t t g a r t (STU) G a l i t z i n - W i l i p Z,N,E S t u t t g a r t R e c e i v e d a r e p l y , but r e f u s e d t o send r e c o r d s . Tokyo (TOK) W i e c h e r t , Omori, G a l i t z i n .... Tokyo No r e s p o n s e . Tucson (TUO) Wood-Anderson NS,EW, B e n i o f f LPZ m i c r o f i l m Aug.22,23-o.k., O c t . 3 1 - r i p s i n r e c o r d s make t h i s u s e l e s s , U p p s a l a (UPP) Wi e c h e r t N,E photocopy U p p s a l a Aug.22-L2,R2 a m p l i t u d e s t o o small,Aug.23 and Oct.31-amp.too s m a l l Utsunomiya (UTS) Wie c h e r t Z,N,E .... Utsunomiya No response V i c t o r i a (VIC) B e n i o f f Z o r i g i n a l Can-Net i n s t r u m e n t too s h o r t p e r i o d t o be u s e f u l . 108 APPENDIX B ~ WORLD AVERAGED PHASE VELOCITIES AND Q VALUES LOVE PHASE VELOCITIES Q VALUES PERIOD (s) C (km/s) PERIOD (s) Q U (km/s) \) xio k 48. 4.49 (1) 44.68 1 97 5.519 6.46 ( 2 86.24 4.616 (1) 53.94 187 5.265 5.91 < 2 90.55 4.626 (1) 72.36 190 5.413 4.22 ( 2 100.61 4.651 (1) 102.59 139 4.498 4.89 ( 2 113.16 4.693 (1) 125.92 1 19 4.385 4.78 < 1 129.26 4.725 (1) 139.46 1 1 6 4.385 4.42 1 1 143.54 4.764 (1) 161.78 121 4.384 3.66 < 1 161.35 4.815 (1) 181.04 119 4.382 3.32 < L1 180.52 4.871 (1) 200.95 1 1 1 4.382 3.21 ( 1 196.05 4.917 (1) 210.21 1 1 1 4.382 3.07 I ; 1 209.57 4.959 (1) 225.70 1 1 3 4.382 2.81 < 230.79 5.025 (1) 250.66 1 1 2 4.386 2.55 I ; 1 249.79 5.085 (1) 281.55 1 1 5 4.397 2.20 < 1 256.84 5. 107 (1) 300.37 1 10 4.407 2.15 I ; 1 264.31 5.131 (1) 299.19 5.244 (1) RAYLEIGH PHASE VELOCITIES Q VALUES PERIOD (s) C (km/s) PERIOD (s) Q U (km/s) Vxio 5~ 10.0 3.39 (3) 10.0 33.0 (5) 20.0 3.62 (3) 12.0 30.0 (5) 25.0 3.53 (3) 14.0 26.0 (5) 30.0 3.85 (3) 16.0 23.0 (5) 35.0 3.90 (4) 18.0 20.0 (5) 40.0 3.93 (4) 20.0 16.0 (5) 46.0 4.0 (4) 30.0 13.0 (5) 92.0 4.083 (4) 40.0 10.0 (5) 125.0 4. 196 (1 ) 50.0 10.0 (5) 1 50.0 4.296 (1 ) 61 .08 134 3.837 10.0 (2) 1 75.0 4.432 (1 ) 73.92 1 12 3.812 9.13 (2) 200.0 4.575 (1 ) 97.73 1 18 3.758 7.24 (2) 225.0 4.739 (1 ) 125.04 127 3.707 5.33 (6) 250.0 4.918 (1) 160. 1 1 131 3.649 4. 10 (6) 275.0 5. 1 06 (1 ) 181.16 136 3.615 3.52 (6) 300.0 5.292 (1) 2.00.90 154 3.588 2.83 (6) 212.35 159 3.577 2.60 (6) 225.17 167 3.569 2.34 (6) 250.29 180 3.578 1 .94 (6) 282.21 203 3.652 1 .50 (6) 306.20 219 3.758 1 .24 (6) 109 The number i n the p a r e n t h e s i s i n d i c a t e s the source t h a t the v a l u e s were taken from. (1) Kanamori, H. 1970. V e l o c i t y and Q v a l u e s of mantle waves. P h y s i c s of the E a r t h and P l a n e t a r y I n t e r i o r s , 2, pp. 259- 275. (2) D z i e n w o n s k i , A. M., and Anderson, D .L. 1981. P r e l i m i n a r y r e f e r e n c e E a r t h model. P h y s i c s of the E a r t h and P l a n e t a r y I n t e r i o r s , 25, pp. 297-356. (3) A k i , K., and R i c h a r d s , P. G. 1980. Q u a n t i t a t i v e S e i s m o l o g y , V o l . 1. W. H. Freman Co., p.284. (4) Kovach, R. L. 1965, S e i s m i c s u r f a c e waves: Some o b s e r v a t i o n s and r e c e n t developments, P h y s i c s and C h e m i s t r y of the E a r t h , 6, pp. 251-314. (5) Herrmann, R. B., ed., 1978. Input t o the computer program QU I n : Computer programs i n earthquake s e i s m o l o g y , V o l . 2, Dept. of E a r t h and Atmospheric S c i e n c e s , S a i n t L o u i s U n i v e r s i t y , pp. (XI -1 5) - (XI--28) (6) C h a e l , E. P., and Anderson, D. L. 1982. G l o b a l Q e s t i m a t e s from a n t i p o d a l R a y l e i g h waves, J o u r n a l of G e o p h y s i c a l R e s e a r c h , 87, pp. 2840-2850. 1 10 APPENDIX C - EARTH MODEL USED FOR THE AUGUST 23 AND OCTOBER 31 MECHANISM SOLUTIONS The f o l l o w i n g model was used as the e a r t h model i n d e t e r m i n i n g the t h e o r e t i c a l s u r f a c e wave r a d i a t i o n p a t t e r n . C o n t i n e n t a l U.S.A. C r u s t a l and Upper Ma n t l e S t r u c t u r e (H=layer t h i c k n e s s ) . H a P P M,10" X f 10" km km/s km/s g/cm dyne/cnf dyne/cm 1 28.0 6.15 3.55 2.74 3.453 3.457 12.0 6.70 3.80 3.00 4.332 4.803 13.0 7.96 4.60 3.37 7.131 7.091 25.0 7.85 4.50 3.39 6.864 7.161 50.0 7.85 4.41 3.42 6.651 7.772 75.0 7.85 4.41 3.45 6.710 7.841 50.0 8.20 4.50 3.47 7.027 9.279 8.40 4.60 3.50 7.406 9.884 (From Ben-Menahem, A., and S i n g h , S. J . 1981. and S o u r c e s . S p r i n g e r - V e r l a g Co., pp. 316.) S e i s m i c Waves APPENDIX D - INSTRUMENT RESPONSE niRVF.c I O O O T \00-d o H <x UOd C C 5 1 I 10 100 '000 P E R J 0 D 1000̂ 3 l O O d I — cn <10 LD cn DBN NS.EW TS:25 S TG:25 S V:290 •H I • I • " ' I 1 I ' I ""I >—| ' | "") 1—) i | 11 r i ] I 10 100 1000 P E R I O D GALITIZIN DBN I i I i I 11 i i | 1—i I 111 i i | 1—i i i i • I>i 1—i i 11 rrt] .1 1 10 100 1000 P E R I O D BOSCH-OMORI S L T BOSCH-OMORI H A L MRGNIFICRTION - c — o § ° e n  X o 2 m m ¥ 5° > o-o 8^ o * III IIIIL MAGNIFICATION - 5 8 ' ' ' I m i l | o o o J — ' ' < o o « 2 ro ro m 0) m cd o z m H X m 30 33 - _ O O u O' o —I I I I m i l MAGNIFICATION - 5 8 —I—1,1 • I 1 I I 1111 o o o I I nu) H < CD • • • • ro ro o o ro m W B / 8^ o N S I T

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