Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

A new model for the crust in the vicinity of Vancouver Island Tseng, Kuang-Hsing 1968

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1968_A6_7 T84.pdf [ 4.04MB ]
Metadata
JSON: 831-1.0053393.json
JSON-LD: 831-1.0053393-ld.json
RDF/XML (Pretty): 831-1.0053393-rdf.xml
RDF/JSON: 831-1.0053393-rdf.json
Turtle: 831-1.0053393-turtle.txt
N-Triples: 831-1.0053393-rdf-ntriples.txt
Original Record: 831-1.0053393-source.json
Full Text
831-1.0053393-fulltext.txt
Citation
831-1.0053393.ris

Full Text

A NEW MODEL FOR THE CRUST I N THE V I C I N I T Y OF VANCOUVER I S L A N D by K u a n g - H s i n g T s e n g T a i w a n N o r m a l U n i v e r s i t y , 1 9 6 4 C e n t r a l U n i v e r s i t y , T a i w a n , 1 9 6 6 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L M E N T OF THE R E Q U I R E M E N T S FOR THE DEGREE OF M A S T E R OF S C I E N C E i n t h e D e p a r t m e n t o f G E O P H Y S I C S We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d B . S c . , N a t i o n a l M . S c . , N a t i o n a l T H E U N I V E R S I T Y OF B R I T I S H A u g u s t , 1 9 6 8 C O L U M B I A In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r a n . a d v a n c e d d e g r e e at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t the L i b r a r y s h a l l make i t f r e e l y . a v a i l a b l e f o r r e f e r e n c e and S t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department or by hiis r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g or pub 1 i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l no t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department The U n i v e r s i t y o f B r i t i s h Co lumbia Vancouver 8, Canada i A B S T R A C T S e i s m i c e x p l o s i o n d a t a o b t a i n e d b y t h e D o m i n i o n O b s e r v a t o r y i n t h e V a n c o u v e r I s l a n d r e g i o n f r o m 1 9 5 3 t o 1 9 6 3 h a v e b e e n r e s t u d i e d b y b o t h t h e t r a v e l - t i m e a n d t h e t i m e -t e r m m e t h o d s . F r o m t h e f i r s t m e t h o d , a new f o u r l a y e r m o d e l c r u s t w a s c o n s t r u c t e d : I n a d d i t i o n t o t h e s e d i m e n t a r y , a g r a n i t i c , a n d b a s a l t i c l a y e r s , s u g g e s t e d b y W h i t e ( 1 9 6 2 ) , a n u l t r a b a s i c l a y e r o f m o r e t h a n 22 km i n t h i c k n e s s a n d V p = 7.1 k m / s e c w a s r e c o g n i z e d . F r o m t h e t i m e - t e r m m e t h o d , t h i s l a y e r s t r u c t u r e w a s s u p p o r t e d a n d t h e p o s s i b i l i t y o f a m a j o r s t r u c t u r a l f e a t u r e r u n n i n g a c r o s s t h e i s l a n d i s s u g -g e s t e d b y t h e f a u l t s o n e a c h s i d e a n d t h e g r a v i t y a n o m a l y e x t e n d i n g b e t w e e n t h e m . T h e M o h o r o v i c i c d i s c o n t i n u i t y w a s n o t o b s e r v e d . A p o s s i b i l i t y i n t e r p r e t a t i o n o f t h e a b n o r m a l c h a r a c t e r i s g i v e n , a n d t h e c o m p l e m e n t a r y g r a v i t y a n d g e o m a g n e t i c d e p t h s o u n d i n g e v i d e n c e p r e s e n t e d . i i A CKNOWLEDGEMENTS W i t h s i n c e r e a p p r e c i a t i o n , t h e w r i t e r w i s h e s t o t h a n k D r . R. M. E l l i s who s u g g e s t e d t h e t o p i c , s u p e r v i s e d t h e r e s e a r c h > a n d c r i t i c a l l y r e a d t h e m a n u s c r i p t . T h e d a t a f o r t h i s p r o j e c t h a s b e e n s u p p l i e d b y t h e S e i s m o l o g y D i v i s i o n , O b s e r v a t o r i e s B r a n c h , D e p a r t m e n t o f E n e r g y , M i n e s , a n d R e s o u r c e s . I n p a r t i c u l a r , t h e w r i t e r w o u l d l i k e t o t h a n k D r . W. G. M i l n e o f t h e D o m i n i o n O b s e r v a -t o r y f o r s t i m u l a t i n g d i s c u s s i o n s a n d a d v i c e d u r i n g t h e p r e p a r a t i o n o f t h i s t h e s i s . T h a n k s a r e a l s o d u e t o D r . W. F. S l a w s o n f o r a d d i -t i o n a l a d v i c e o n c e r t a i n a s p e c t s o f t h e g e o l o g y , D r . R. A. S t a c e y f o r a l l o w i n g t h e u s e o f t h e u n p u b l i s h e d g r a v i t y a n o m a l y map, a n d M r . P. M a d d e r o m f o r a s s i s t a n c e w i t h p a r t o f p r o g r a m m i n g w o r k . F i n a l l y , t h e w r i t e r w o u l d l i k e t o t h a n k D r . J . A. J a c o b s f o r t h e a d m i s s i o n t o s t u d y h e r e a n d t h e U.B.C. C o m p u t i n g C e n t r e a n d o t h e r m e m b e r s i n t h e D e p a r t m e n t o f G e o p h y s i c s f o r t h e i n v a l u a b l e a s s i s t a n c e a n d e n c o u r a g e m e n t . M i s s J . K a l m a k o f f who t y p e d t h e t h e s i s w a s o f g r e a t a s s i s t a n c e d u r i n g t h e f i n a l p r e p a r a t i o n o f t h e t h e s i s . T h i s r e s e a r c h h a s b e e n s p o n s o r e d b y t h e N a t i o n a l R e s e a r c h C o u n c i l a n d t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a . i i i CONTENTS Chapter Page 1. INTRODUCTION 1 1-1. The E a r t h ' s C r u s t 1 1-2. N o r t h America's C r u s t 2 1- 3. Vancouver I s l a n d Region 5 2. DATA 11 2- 1. Data C o l l e c t i o n 11 2- 2. P r e c i s i o n 16 3. TRAVEL-TIME ANALYSIS 19 3- 1. I n t r o d u c t i o n 19 3-2. T r a v e l - T i m e P l o t s 19 3-3. D e t a i l e d T r a v e l - T i r a e A n a l y s i s 37 3-4. S t r a i t o f G e o r g i a I n t e r p r e t a t i o n 38 3-5. West Coast I n t e r p r e t a t i o n 40 3-6. N o r t h Coast I n t e r p r e t a t i o n 42 3-7. Juan de Fuca S t r a i t S h o r t Range Programs 43 3-8. Cross I s l a n d P r o f i l e s 43 3-9. S t a t i o n A r r a y P l o t s 43 3- 10. Summary 4 3 4. TIME-TERM ANALYSIS 48 4- 1. I n t r o d u c t i o n 48 4-2. Theory 48 4-3. P r o c e d u r e s o f A n a l y s i s 49 4-4. P r o f i l e E a s t o f Vancouver I s l a n d 52 4-5. P r o f i l e West o f Vancouver I s l a n d 59 4-6. Summary 60 Chapter Page 5. DISCUSSION 64 5-1. C r u s t a l Model 64 5-2. L i m i t a t i o n s o f the C r u s t a l Model 67 5-3. M o h o r o v i c i c D i s c o n t i n u i t y 70 5-4. G r a v i t y A n a l y s i s 72 5-5. Geomagnetic Depth Sounding and Magneto-t e l l u r i c s 74 5-6. O f f - S h o r e M a g n e t i c Anomaly 75 5-7. F u r t h e r Work 7 8 BIBLIOGRAPHY - 80 1 C H A P T E R 1 I N T R O D U C T I O N 1 - 1 . T h e E a r t h ' s C r u s t T h e d e f i n i t i o n o f t h e e a r t h ' s c r u s t a s b e i n g t h e o u t e r s h e l l o f t h e e a r t h l y i n g a b o v e t h e l e v e l a t w h i c h t h e c o m p r e s s i o n a l w a v e (P) v e l o c i t y i n c r e a s e s r a p i d l y o r u n d e r -g o e s a d i s c o n t i n u i t y t o v a l u e s o f a b o u t 8.0 k m / s e c h a s b e e n u n i v e r s a l l y a c c e p t e d s i n c e M o h o r o v i c i c d i s c o v e r e d t h e d i s -c o n t i n u i t y (M) i n 1 9 0 9 . I n t h e p a s t f e w d e c a d e s , t h e m o r e p r e c i s e e x p l o s i v e t e c h n i q u e h a s r e p l a c e d t h e n a t u r a l e a r t h -q u a k e d a t a a n d c r u s t a l s t u d i e s h a v e b e e n c a r r i e d o u t a r o u n d t h e w o r l d . F r o m t h i s d a t a a d i v i s i o n o f t h e e a r t h c r u s t i n t o t h e o c e a n i c a n d t h i c k e r c o n t i n e n t a l p a r t s h a s b e e n r e c o g n i z e d . I n 1 9 2 5 , a n o t h e r m a j o r d i s c o n t i n u i t y , t h e " C o n r a d " , w a s d i s c o v e r e d w i t h i n t h e c r u s t . I t s a p p a r e n t s u r f a c e s p e e d w a s 6.3 k m / s e c . A l t h o u g h i t h a s b e e n a s s u m e d t h a t t h e M d i s c o n t i n u i t y i s w o r l d - w i d e i n e x t e n t , d o u b t h a s b e e n e x p r e s s e d w h e t h e r t h i s i s t r u e f o r t h e C o n r a d d i s c o n -t i n u i t y . T h e s u r f a c e w a v e , r e f r a c t i o n a n d r e f l e c t i o n m e t h o d s h a v e b e e n u s e d e x t e n s i v e l y i n c r u s t a l s t u d i e s . T h e s u r f a c e w a v e m e t h o d u s i n g e a r t h q u a k e s i s m u c h c h e a p e r b u t d o e s n o t h a v e t h e r e s o l u t i o n o f t h e o t h e r t w o t e c h n i q u e s . T h e t r a d i -t i o n a l r e f r a c t i o n m e t h o d t e n d s t o s m o o t h t h e d a t a . H o w e v e r s i g n i f i c a n t t o p o g r a p h y o n d i s c o n t i n u i t i e s c a n b e d e t e r m i n e d u s i n g t h e t i m e - t e r m i n t e r p r e t a t i o n p r o c e d u r e w h i c h h a s b e e n 2 developed in the past decade. The vertical reflection method which has been undergoing rapid development in recent years relies on the refraction method for velocities but yields more detail than the other techniques. Summaries of the crustal study data (e.g.,.Lee and Taylor, 1966) indicate that the average thickness of the continental crust is about 35 km but ranging from 20 to 70 km. Velocities for compressional and shear waves are found to be about 6.2 km/sec and 3.6 km/sec in the upper crustal layer (granitic or sialic layer) and increasing with depth to about 7.0 km/sec and 3.8 km/sec in the lower crustal layer (basaltic or simaic layer). The oceanic crust is about 5 km in thickness, with a compressional velocity of 6.4 to 6.9 km/sec. The velocities for the underlying mantle rocks are about 8.1 and 4.7 km/sec respectively. 1-2. North America's Crust In North America explosion seismology studies were begun in the early 1930's, by Wood, Richter, Byerly and Gutenberg in southern California and Ewing, Leet, Slichter in the East to measure the crustal thickness and mean velo-cities. After World War II, an extensive program was carried out by Tuve and his co-workers in the Carnegie Institution of Washington. Since 1960, due to the influence of the Vela Uniform Project and the Upper Mantle Project, there has been a substantial increase in knowledge of the earth's crust. 3 T h e c o n t o u r m a p s o f m e a n . c r u s t a l v e l o c i t y , v a r i a t i o n s i n c r u s t a l t h i c k n e s s a n d v e l o c i t y b e l o w t h e M d i s c o n t i n u i t y i n t h e U n i t e d S t a t e s h a v e b e e n p r e s e n t e d b y P a k i s e r a n d S t e i n h a r t ( 1 9 6 4 ) a n d H e r r i n ( 1 9 6 6 ) ( F i g . l a , b ) . E a s t o f t l j e R o c k y M o u n t a i n s , P w a v e s b e l o w t h e M d i s c o n t i n u i t y a r e e v e r y w h e r e g r e a t e r t h a n 8.0 k m / s e c , a n d t h e m e a n c r u s t a l v e l o c i t y i s g e n e r a l l y g r e a t e r t h a n 6.4 k m / s e c ; t h e c r u s t i s g e n e r a l l y t h i c k e r t h a n 40 km, s t r o n g l y m a g n e t i c , a n d p r e d o m i n a n t l y m a f i c a n d t h e e v i d e n c e f o r c r u s t a l l a y e r i n g i s w e a k . T h i s a r e a i s r e l a t i v e l y s t a b l e a n d m a t u r e . I n t h e \ \ r e s t e r n r e g i o n e x c e p t f o r t h e c o a s t a l a r e a , t h e P w a v e s a r e e v e r y w h e r e l e s s t h a n 8.0 k m / s e c , t h e m e a n c r u s t a l . v e l o c i t y i s g e n e r a l l y l e s s t h a n 6.4 k m / s e c . I n a l a r g e p a r t o f t h a t a r e a , t h e c r u s t i s t h i n n e r t h a n 40 k m / s e c a n d i s w e a k l y m a g n e t i c . T h i s c r u s t i s a l s o p r e d o m i n a n t l y s i l i c i c a n d i s s e p a r a t e d i n t o a d i s t i n c t s i l i c i c u p p e r l a y e r a n d a m a f i c l o w e r l a y e r . M a g n e t i c o b s e r v a t i o n s , c o m b i n e d w i t h t h e h i g h h e a t f l o w a n d l o w P v e l o c i t y , i n d i c a t e a n a r e a t h a t i s r e l a t -i v e l y m o b i l e a n d h o t . T h e a h n o m a l c h a r a c t e r o f t h e c r u s t i n t h e w e s t w a s e m p h a s i z e d b y T u v e a n d T a t e l ( 1 9 5 5 ) . T h e i n a b i l i t y t o i d e n -t i f y c r i t i c a l r e f l e c t i o n s f r o m t h e M d i s c o n t i n u i t y a t P u g e t S o u n d , s o u t h e r n C a l i f o r n i a a n d t h e A n d e s l e d t h e m t o c o n -c l u d e t h a t " t h e n a t u r e o f t h e c r u s t a l v e l o c i t y t r a n s i t i o n i s d i f f e r e n t a l o n g t h e w e s t c o a s t - p e r h a p s r o u g h e r o r m o r e b r o k e n , o r p o s s i b l y e n t i r e l y d i f f e r e n t i n n a t u r e " . T h e s t u d y o f t h e B a s i n a n d R a n g e P r o v i n c e c a s t new e v i d e n c e f o r t h e 4 F i g . l a . V a r i a t i o n s i n c r u s t a l t h i c k n e s s and mean c r u s t a l v e l o c i t y i n the U n i t e d S t a t e s ( a f t e r P a k i s e r and S t e i n h a r t , 1964) Mi«ntr»«!tt*ffacilr>t.5l«/MC l.lt*A«c< crwital *il»cllr Minx cntiiat <«l<Klt, (• I I M / I M Cwlow »f • trutal tNctnm 5 a n o m a l o u s c r u s t a l m i x t u r e m a t e r i a l h a v i n g a v e l o c i t y o f 7 . 4 t o 7 . 6 k m / s e c w i t h a d i s c o n t i n u i t y a t a d e p t h 24 km ( P r e s s , 1 9 6 0 ) . T h i s r e s u l t e d i n a c o n t r o v e r s y o v e r w h e t h e r o r n o t m a t e r i a l w i t h P w a v e v e l o c i t y l e s s t h a n 7 . 9 k m / s e c s h o u l d b e c l a s s i f i e d a s a n o m a l o u s l o w e r c r u s t o r a s a n o m a l o u s u p p e r m a n t l e ( J a m e s a n d S t e i n h a r t , 1 9 6 6 ) . R e c e n t l y , f r o m t h e e v i d e n c e o f s i m i l a r g e o l o g i c a l , g e o m a g n e t i c a n d s e i s m i c r e s u l t s i n A l a s k a a n d C a n a d a , i t i s r e v e a l -e d t h a t t h e c r u s t i n o t h e r p a r t s o f w e s t e r n N o r t h A m e r i c a h a v e t h e s a m e c h a r a c t e r ( P a k i s e r a n d R o b i n s o n , 1 9 6 6 ; K a n a s e w i c h , 1 9 6 5 ) . 1 - 3 . V a n c o u v e r I s l a n d R e g i o n V a n c o u v e r I s l a n d l i e s i n t h e n o r t h e a s t e r n P a c i f i c , s o u t h w e s t o f m a i n l a n d B r i t i s h C o l u m b i a . T h e r e g i o n i s a t r a n s i t i o n z o n e b e t w e e n t h e c o n t i n e n t a n d o c e a n . T h e t e c -t o n i c f e a t u r e s a n d g e o l o g y o f t h e r e g i o n h a v e b e e n c o m p r e -h e n s i v e l y r e p o r t e d b y D u f f e l l a n d S o u t h e r ( 1 9 6 4 ) , M i s c h ( 1 9 6 6 ) , S u t h e r l a n d - B r o w n ( 1 9 6 6 ) , R o d d i c k ( 1 9 6 7 ) a n d W h e e l e r ( 1 9 6 7 ) . O n l y a b r i e f s u r v e y i s g i v e n h e r e : On t h e B r i t i s h C o l u m b i a m a i n l a n d t h e F r a s e r R i v e r v a l l e y s e p a r a t e s t h e C o a s t M o u n t a i n s t o t h e n o r t h f r o m t h e C a s c a d e M o u n t a i n s t o t h e s o u t h w h i c h c o n t i n u e s o u t h w a r d i n t o t h e s t a t e o f W a s h i n g t o n . T h e c o a s t a l t r o u g h , c o n s i s t i n g f r o m n o r t h t o s o u t h o f t h e H e c a t e D e p r e s s i o n , t h e S e y m o u r a r c h a n d t h e G e o r g i a D e p r e s s i o n , i s b e t w e e n t h e s e m o u n t a i n s 6 a n d t h e V a n c o u v e r I s l a n d M o u n t a i n R a n g e . T h e c o n t i n e n t a l s h e l f e x t e n d s t o a d i s t a n c e o f a p p r o x i m a t e l y 50 km s o u t h w e s t o f t h e c o a s t o f V a n c o u v e r I s l a n d . T h e c o m p l e x i t y o f t h e g e o l o g y o f V a n c o u v e r I s l a n d i s i n d i c a t e d b y t h e j u n c t i o n o f t h r e e d i f f e r e n t g e o l o g i c a l r e g i o n s : ( 1 ) C o a s t C r y s t a l l i n e B e l t t o t h e e a s t o f t h e G e o r g i a D e p r e s s i o n c o n s i s t s m a i n l y o f p l u t o n i c a n d m e t a m o r p h i c t e r r a i n , m a i n l y q u a r t z - d i o r i t e w i t h s u b o r d i n a t e g r a n o d i o r i t e , d i o r i t e a n d g a b b r o w h i c h b e g a n t o f o r m i n t h e M i d d l e J u r a s s i c a n d c o n t i n u e d i n t o t h e T e r t i a r y . ( 2 ) S o u t h w e s t e r n B r i t i s h C o l u m b i a a n d n o r t h w e s t e r n W a s h i n g t o n l i e i n t h e w e s t e r n p a r t o f t h e " P a l e o z o i c - M e s o z o i c C o r d i l l e r a n v o l c a n i c o r o g e n i c b e l t " o r P a c i f i c e u g e o s y n c l i n e . S i n c e t h e m i d d l e o f t h e P a l e o z o i c , v u l c a n i s m a n d t h e e r o s i o n a l p r o d u c t s o f v o l c a n i c r o c k s h a v e b e e n t h e g r e a t e s t c o n t r i b u t o r s t o t h e s t r a t i g r a p h i c c o l u m n . B r e c c i a s , c o n g l o m e r a t e s , g r e y w a c k e , g r e y w a c k e s i l t -s t o n e , a r g i l l i t e s a n d r i b b o n c h e r t a r e t h e common s e d i m e n t s . ( 3 ) I n s u l a r T e c t o n i c B e l t i s c h a r a c t e r i z e d b y s e d i m e n t a r y a n d v o l c a n i c r o c k s m a i n l y o f M e s o z o i c a g e , i n p a r t i c u l a r , b a s a l t f l o w s , b r e c c i a s , a n d e s i t i c a g g l o m e r a t e s a n d t u f f s i n t e r c a l a t e d a n d i n t e r f i n g e r i n g w i t h g r e y w a c k e , a r g i l l i t e a n d c h e r t a r e p r e s e n t i n g r e a t v o l u m e i n t h i s a r e a . G e o r g i a S t r a i t i s u n d e r l a i n b y c l a s t i c s e d i m e n t s s u c h a s c o n g l o m e r a t e , m a r i n e s h a l e , s a n d s t o n e a n d n o n - m a r i n e r o c k , o f l a t e M e s o z o i c a n d e a r l y C e n o z o i c a g e . O u t c r o p s o f T e r t i a r y s e d i m e n t a r y r o c k o c c u r a t v a r i o u s p l a c e s a l o n g t h e 7 w e s t c o a s t o f V a n o u v e r I s l a n d . T h e s t u d y o f c r u s t a l s t r u c t u r e - i n a d j a c e n t a r e a s i s i n d i c a t e d i n F i g . 2. E a s t i f t h e R o c k y M o u n t a i n s i n s o u t h e r n A l b e r t a , a f o u r l a y e r c r u s t a l m o d e l w a s r e p o r t e d ( C u m m i n g e t a l , 1 9 6 6 ) . F r o m t h e r e f l e c t i o n m e t h o d , t h e e x i s t e n c e o f t h e d i s c o n t i n u i t y b e t w e e n 6.5 a n d 7.2 k m / s e c w a s s h o w n a g a i n , a n d t h i s h a s b e e n p r o p o s e d a s a " R i e l " d i s c o n t i n u i t y ( C l o w e s , e t a l , 1 9 6 8 ) . I n P u g e t S o u n d t h e a b n o r m a l c r u s t a l m i x t u r e w a s d i s c u s s e d , a n d a n i n d i c a t i o n o f t h e s h a l l o w e s t c r u s t a l t r a n s i t i o n o f d e p t h 19 km t o t h e w e s t o f S e a t t l e w a s o b s e r v e d ( T a t e l a n d T u v e , 1 9 5 5 ) . M a r k e d c h a n g e s i n c r u s t a l s t r u c t u r e o n t h e c o n t i n e n t a l s h e l f b e t w e e n C a l i f o r n i a a n d O r e g o n h a v e b e e n s h o w n b y S h o r e t a l ( 1 9 6 8 ) . T h e f i r s t c r u s t a l s t u d y i n t h e V a n o u v e r I s l a n d r e g i o n w a s b y M i l n e a n d W h i t e ( 1 9 6 0 ) . U s i n g t h e t r a d i t i o n a l t r a v e l - t i m e p l o t a P w a v e v e l o c i t y o f 6.3 k m / s e c f o r b a s e -m e n t r o c k s n e a r V i c t o r i a w a s r e v e a l e d , a n d u p p e r l a y e r r o c k s s h o w a P w a v e v e l o c i t y o f 5.4 t o 6.0 k m / s e c f o r t h e M e t c h o s i n v o l c a n i c s . A t i m e - t e r m a n a l y s i s i n d i c a t e d a P w a v e v e l o c i t y o f 6.3 k m / s e c b e n e a t h t h e S t r a i t o f G e o r g i a a n d f o u n d a t h i c k u p p e r l a y e r a t t h e s o u t h e r n p a r t o f t h e S t r a i t . W h i t e ( 1 9 6 2 ) p r e s e n t e d t h e r e s u l t s o f t h r e e p r o f i l e s a l o n g t h e d e p r e s s i o n a r e a a d o p t i n g a t h r e e l a y e r m o d e l . F o r t h e i v e s t c o a s t o f V a n o u v e r I s l a n d , t h e m o d e l wa s o b t a i n e d a s i n F i g . 2 ( W h i t e a n d S a v a g e , 1 9 6 5 ) . 9 The Boug.uer anomaly map o f s o u t h e r n B r i t i s h Columbia o f W a l c o t t (1967) has been extended by St a c e y ( p r i v a t e com-m u n i c a t i o n ) n o r t h t o the Queen C h a r l o t t e I s l a n d s ( F i g . 3 ) . Second o r d e r l o c a l anomalies which i n g e n e r a l can be d i r e c t l y c o r r e l a t e d w i t h g e o l o g y are found superimposed on a s m a l l p o s i -t i v e f i r s t o r d e r anomaly w h i c h i s due t o deeper c r u s t a l m a t e r i a l . I n t h i s s t u d y o n l y r e f r a c t i o n d a t a are a n a l y s e d . A l l the t r a v e l - t i m e d a t a , some o f which were s t u d i e d by p r e -v i o u s i n v e s t i g a t o r s , are g i v e n a s y s t e m a t i c a n a l y s i s . In a d d i t i o n t o the t r a d i t i o n a l t r a v e l - t i m e method, the t i m e -term method i s used t o g i v e more i n f o r m a t i o n about the d i s c o n t i n u i t i e s . The g e n e r a l c r u s t a l p i c t u r e i n t h i s r e g i o n based on the s e i s m i c method i s d i s c u s s e d a l o n g w i t h the g e o l -o g i c a l , g r a v i t y , and magnetic e v i d e n c e . B O U G U E R A N O M A L Y M A P O F T H E S O U T H E R N BRITISH C O L U M B I A C O A S T (COURTESY OF R. A. STACEY) 11 CHAPTER 2 DATA 2-1. Data C o l l e c t i o n In 1953, the Dominion Observatory with the coopera-t i o n of the Royal Canadian Navy and P a c i f i c Naval Laboratory began a study of the earth's c r u s t i n the Vancouver I s l a n d region using e x p l o s i o n seismology. Between 1953 and 1963, more than 11 separate unreversed seismic e x p l o s i o n programs were c a r r i e d out around Vancouver I s l a n d . A summary of these programs i s shown i n Table 1. F i g . 4 shows the l o c a t i o n of the s t a t i o n s , the shots and the l i n e s of p r o f i l e . Besides the permanent s t a t i o n s at A l b e r n i , V i c t o r i a , Horseshoe Bay and Port Hardy, a number of temporary s t a t i o n s were operated during each program. A l i s t of these s t a t i o n s and t h e i r i n s t r umentation i s given i n Table 2. - -The data used from these programs were the shot-s t a t i o n s e p a r a t i o n , the t r a v e l time of the f i r s t a r r i v a l s , and i n s e v e r a l cases the t r a v e l time of second a r r i v a l s . The d e t a i l e d d e s c r i p t i o n s of the procedure of the f i e l d pro-gram to obtain the data are given by Milne and White (1960) and White (1962). A b r i e f review i s given here: Nearly a l l of the shots were exploded i n the water with the seismic energy being s u p p l i e d by the depth-charges ranging from 25 l b . to 1500 l b . These were detonated by a pressure device which was set i n most cases f o r a depth of 12 T A B L E 1. S u m m a r y o f t h e P r o g r a m s P r o g r a m D a t e P o s i t i o n No. o f s h o t R e c o r d i n p S t a t i o n s 1 1 9 5 3 - 1 9 5 4 J u a n d e F u c a S t r a i t 5 A L B * , HB C , V I C 2 1 9 5 6 J u a n de F u c a S t r a i t 12 AMD, V I C A L B , P N L , 3 1 9 5 7 , O c t . 25 G e o r g i a S t r a i t 17 A L B , HBC, V I C 4 1 9 5 7 , O c t . 30 J u a n d e F u c a S t r a i t t o w e s t o f I s l a n d 17 A L B , H B C , V I C 5 1 9 5 8 , A p r . 5 H o p e I s l a n d , D i s c o v e r y P a s s a g e ( R i p p l e R o c k ) C o n s t a n c e B a n k 3 A L B , HBC, V I C 6 1 9 5 9 S o u t h o f G e o r g i a S t r a i t 12 A L B , TONG, HBC, S A L L Y , V I C 7 1 9 6 0 , May 1 7 -19 G e o r g i a S t r a i t M a l a s p i n a S t r a i t J o h n s t o n e S t r a i t 1 3 A L B , S C D , V I C H I , HBC, U C L 1 , U C L 2 , 8 1 9 6 1 , J u n e 7 N o r t h o f G e o r g i a S t r a i t 11 A L B , CR, V I C 9 1 9 6 1 , N o v . 2 2 -24 G e o r g i a S t r a i t D i s c o v e r y P a s s a g e J o h n s t o n e S t r a i t 1 8 A L B , P A B , E F L , K E L , V I C 10 1 9 6 3 , M a y 7-8 J u a n d e F u c a S t r a i t t o w e s t o f I s l a n d 24 E S P , T O F , PHC, P R E , V I C 1 1 1 9 6 3 , May 1 2 -14 G o l e t a s C h a n n e l J o h n s t o n e S t r a i t 11 A L B , NAN, V I C , C A S , GON, P A B , PHC, WOL * r e f e r t o T a b l e 2 T A B L E 2. S u m m a r y o f R e c o r d i n g S t a t i o n s S t a t i o n C o o r d i n a t e s I n s t r u m e n t a t i o n A d e g . m i n . d e g . m i n . P e r m a n e n t A L B ( A l b e r n i ) 49 1 6 . 2 3 1 2 4 49 .3 W, T s = 1 . 0 , T g = 0 . 0 3 , P s = 1 . 0 HBC ( H o r s e s h o e B a y ) 49 2 2 . 6 5 1 2 3 16 .55 W, S p r e n g n e t h e r 3 c o mp. r e c o r d e r P_=112 mm/min PHC ( P o r t H a r d y ) 50 4 2 . 4 127 ' 28 .88 V/ V I C ( V i c t o r i a ) 48 3 1 . 1 7 1 2 3 24 .92 B e n i o f f , 3 c o m p . T s = 1 . 0 , T g = 0 . 2 , P s = 1 . 0 T e m p o r a r y AHD ( A l b e r t H e a d ) 48 2 0 . 3 1 2 3 20 .9 W z , T s = 1 . 0 , T g = 0 . 2 5 CR ( C a m p b e l l R i v e r 50 0 3 . 3 7 125 24 .47 H.S. CAS ( C a s s i d y ) 49 0 2 . 4 1 . 1 2 3 54 .18 W_, l . f . E F L ( E l k F a l l s ) 50 0 1 . 8 9 1 2 5 21 .82 W , l . f . , 3 s e i s m o m e t e r s p r e a d E S P ( E s t e v a n ) 49 2 2 . 7 1 2 6 32 .2 W , l . f . , 3 s e i s m o m e t e r s p r e a d GON ( G o n z a l e s ) 48 2 4 . 8 1 2 3 19 .42 w z , i . f . H I ( H o r n b y I s l a n d ) 49 3 1 . 8 124 36 .0 W , g e o p h o n e s p r e a d , Z T =1.0,T = 0 . 0 0 6 , P =90.0 s ' g ' s W , l . f . , 3 s e i s m o m e t e r s p r e a d K E L I ( K e l s e y B a y , C e e n n g h a m ) 50 2 2 . 0 2 1 2 5 54 .97 1 K E L 2 ( K e l s e y B a y , T o f t ) 50 2 1 . 38 1 2 5 5 5 . 2 8 W , l . f . , 3 s e i s m o m e t e r s p r e a d NAN ( N a n o o s e B a y ) . 49 1 6 . 5 124 0 9 . 1 W , 2 s e i s m o m e t e r s p r e a d , z l . f . , H.S., h . f . , g e o p h o n e s p r e a d P A B ( P a t r i c i a B a y ) 48 3 9 . 0 1 2 3 2 8 . 8 H y d r o p h o n e s P N L ( P a c i f i c N a v a l L a b o r a t o r y ) 48 2 5 . 75 1 2 3 2 5 . 8 W .T =1.0,T =0.25 z ' s ' g P R E ( P o r t R e n f r e w ) 50 4 1 . 7 1 2 4 2 1 . 2 H . S . , h . f . S A L L Y ( G a l i a n o I s l a n d ) 48 5 4 . 3 1 2 3 2 0 . 6 3 W , l . f . , 3 s e i s m o m e t e r s p r e a d SCD ( S c a n d o n Dam) 49 4 7 . 8 124 1 8 . 6 W , g e o p h o n e s p r e a d , Z T = 0 . 2 2 2 , T = 0 . 0 0 5 , P =60.0 s ' g ' s W , 2 s e i s m o m e t e r s p r e a d , l . f . , H . S . , h . f . , g e o p h o n e s p r e a d TOF ( T o f i n o ) 49 0 5 . 3 1 2 5 4 7 . 2 TONG ( G a l i a n o I s l a n d ) 49 0 0 . 58 1 2 3 3 4 . 0 8 W , l . " f . , 3 s e i s m o m e t e r s p r e a d U C L 1 ( U c l u e l e t 1) 49 0 2 . 9 1 1 2 5 3 5 . 9 W ,T =1.0,T = 0 . 2 5 , P =0.89 z ' s g s W z U C L 2 ( U c l u e l e t 2) 49 0 4 . 81 1 2 5 2 7 . 9 WOL ( W o l f e B a y ) 49 4 9 . 2 1 2 5 1 6 . 9 W , l . f . , 3 s e i s m o m e t e r s p r e a d A b b r e v i a t i o n s W: W i l l m o r e 3 c o m p o n e n t s e i s m o m e t e r s W : W i l l m o r e v e r t i c a l s e i s m o m e t e r z T s : s e i s m o m e t e r p e r i o d ( s e c ) T : g a l v a n o m e t e r p e r i o d ( s e c ) P s 1. h . H. f . : f . : S. : p a p e r s p e e d ( m m / s e c ) l o w f r e q u e n c y h i g h f r e q u e n c y H a l l S e a r s 4.5 c / s g e o p h o n e s 16 150 meters. When more seismic energy was r e q u i r e d , a package c o n t a i n i n g up to 3000 l b . of depth-charges were made up; these were lowered to the bottom and f i r e d e l e c t r i c a l l y . The a c o u s t i c s i g n a l picked up at the ship by micro-phone was t r a n s m i t t e d by rad i o to the recording s t a t i o n s . I f the s h o t - r e c e i v e r distance was too great to allow s a t i s -f a c t o r y r a d i o communication, the seismograph s t a t i o n was then equipped with a chronometer and a radi o to ob t a i n abso-l u t e time. Thus both shot time and seismic record time could be r e f e r r e d to a s i n g l e time standard. For each detonation, a record was kept of the ship's speed, the water depth and the depth at which the charge was f i r e d From these data and from the known rate of drop of a depth charge i n water, the time at which the seismic wave str u c k the water-earth boundary was computed. 2-2. P r e c i s ion For the pressure detonation e x p l o s i o n , the p o s s i b l e e r r o r i n detonation time i s 0.05 sec (White, 1960). For the e l e c t r i c a l l y detonated e x p l o s i o n , the u n c e r t a i n t y i s about 0.005 sec ( S t e i n h a r t and Meyer, 1961). Another major e r r o r i n the o r i g i n time (the time of the a r r i v a l of the shock waves at the s e a f l o o r below the charges) i s due to the u n c e r t a i n t i e s i n the c o r r e c t i o n of the time of the a r r i v a l of the shock at the ship f o r t r a v e l time i n the water which i s c a l c u l a t e d on the ba s i s of an 17 average v e l o c i t y of the sound wave i n the water, and of the depth of water as i n t e r p o l a t e d from the Hydrographic Survey c h a r t s . The t o t a l u n c e r t a i n t y i n the o r i g i n time i s about 0.05 sec. The. coordinates of the.ship were determined wher-ever p o s s i b l e by bearing on known land p o s i t i o n s . The e r r o r i n p o s i t i o n i s b e l i e v e d to be 0.2 km. This i s e q u i v a l e n t to an e r r o r i n t r a v e l time of approximately 0.03 sec, thus the distance measurements are b e l i e v e d to be more p r e c i s e than the time measurement. The t o t a l t iming e r r o r due to u n c e r t a i n t i e s i n detonation time, l o c a t i o n and the r e d u c t i o n of the o r i g i n time would be about 0.13 sec i n most of the shots. In order to assure the r e l i a n c e of the i n t e r p r e -t a t i o n , only prominent f i r s t a r r i v a l s and, f o r a few cases, second a r r i v a l s were used. In p i c k i n g a r r i v a l s , a s l i g h t u n c e r t a i n t y i s b e l i e v e d to be caused by the d i f f e r e n t phase responses between instruments. I f shear wave a r r i v a l s are observed and thence a v e l o c i t y obtained, Poisson's r a t i o of the c r u s t a l m a t e r i a l s can be determined. Although s e v e r a l of the s t a t i o n s were equipped with h o r i z o n t a l seismometers, w e l l defined a r r i v a l s were rare as most of the sources were suspended shots. The data of programs 1 and 3 have been p r e v i o u s l y analysed by Milne and White (1960) , most of the data from 18 p r o f i l e s 5, 7, 8 a n d 9 b y W h i t e ( 1 9 6 2 ) ; a n d m o s t o f t h e d a t a f r o m p r o f i l e s 5, 7, 8, 9, 10 a n d 1 1 , b y W h i t e a n d S a v a g e ( 1 9 6 5 ) . W i t h t h e a d d i t i o n a l a v a i l a b l e d a t a a n d t h e u s e o f t h e t i m e -t e r m m e t h o d , i t w a s b e l i e v e d t h a t a b e t t e r u n d e r s t a n d i n g o f t h e c r u s t a l s t r u c t u r e i n t h i s r e g i o n c o u l d b e o b t a i n e d . 1 9 C H A P T E R 3 T R A V E L - T I M E A N A L Y S I S 3 - 1 . I n t r o d u c t i o n T h e t r a d i t i o n a l t r a v e l - t i m e m e t h o d ( e . g . , S t e i n h a r t a n d M e y e r , 1 9 6 1 ) w a s f i r s t a p p l i e d t o t h e e n t i r e d a t a s e t o f t h i s r e g i o n . T h e u s e o f t h e a d d i t i o n a l d a t a h a s r e s u l t e d i n s e v e r a l c h a n g e s i n t h e i n t e r p r e t a t i o n o f t h e g e n e r a l c r u s t a l s t r u c t u r e f r o m t h a t r e p o r t e d b y p r e v i o u s a u t h o r s . 3 - 2 . T r a v e l - T i m e P l o t s A l l t h e d a t a w e r e p l o t t e d o n a c o m b i n e d t r a v e l - t i m e g r a p h ( F i g . 5 ) . R e d u c e d t r a v e l - t i m e p l o t s f o r e a c h s t a t i o n ( F i g . 5-1 t o 5 - 2 5 ) a n d f o r some s h o t s ( F i g . 5-26 t o 5 - 2 9 ) w e r e m a d e . W h e r e t h e s t a t i o n s o r s h o t s w e r e n e a r t o e a c h o t h e r a n d o n t h e same g e o l o g i c a l f o r m a t i o n , t h e y w e r e c o n -s i d e r e d a s a s i n g l e p o i n t ( e . g . , V I C , AHD, a n d P N L ) . T h e d a t a w e r e d i v i d e d i n t o f o u r p a r t s , e a c h p a r t b e i n g a s s o c i a t i o n w i t h , a p a r t i c u l a r v e l o c i t y l a y e r ( t h e t e r m s Po> P i > P2 a n d P3 w i l l b e u s e d t o s p e c i f y t h e u p p e r s e d i m e n -t a r y l a y e r a n d t h e t h r e e r e f r a c t o r s ) . B y a l e a s t s q u a r e s f i t t h e v e l o c i t i e s a s s o c i a t e d w i t h t h e s e l a y e r s a r e 5 . 0 5 , 6 . 1 7 , 6.55 a n d 7.14 k m / s e c . T h i s i n t e r p r e t a t i o n d i f f e r s f r o m e a r l i e r i n t e r p r e t a t i o n s i n t h e r e c o g n i t i o n o f l a y e r P 3 w i t h a v e l o c i t y o f 7.14 k m / s e c . T h e s t r o n g e s t e v i d e n c e f o r t h e e x i s t e n c e o f t h i s l a y e r i s f o u n d i n F i g . 5-18., 5 - 2 3 , a n d 5-26 a l t h o u g h e v i d e n c e f o r t h i s l a y e r i s n o r m a l l y p r e s e n t 20 FIG. 5-1 DISTANCE(KM) .0 40.0 80.0 120.0 DISTRNCE(KM) 1 6 0 . 0 200.0 o ID" o L U 00 V . LU z : CEo I—I o I L U zz I— I I—D. FIG. 5 - 6 EFL-CR 5 . 9 ± 2 . 3 % , 0 . 3 ± 0 . 1 6 . 7 ± 0 . 9 % , 0 . 9 ± 0 . 1 .0 r 1 1 I4D.0 8D.0 120.0 DISTANCE (KM) IBO.O 200.0 ' 2H o O L U C O o d_| o CO \ L U CJ> d o 1— • • CO*0 i—i o I LU CM UD-0 FIG-. 5 - 7 VIC-PAB 6.5*1.5%, 1.1±0.4 *»7*-8 I I I 1 1 90.0 140.0 190.0 240.0 290.0 DISTRNCE(KM) o in' L U C O CO \ L U O C E o CO*""" i — i Q I L U CM FIG. 5 - 8 5.9±4.2%, 0.5±0.3 MO.O I I I 48.0 56.0 64.0 DISTRNCE (KM) 72.0 80.0 25 CM' LU CO CO "X. LU (_> co * '—i o I LU FIG. 5-9 K E L xx CO I .0 «JD-D 80.0 120.0 DISTANCE(KM) IBO.O 200.0 -D 6.6%, 3.9±t.O 1.6±0.7 UD.O 8D.0 120.0 DISTANCE (KM) 160.0 200.0 26 FIG. 5-11 o UJ Ul d_ FIG. 5-12 PHC * 6 1 0 0 . 0 1 5 0 . 0 2 0 0 . 0 2 5 0 . 0 3 0 0 . 0 3 5 0 . 0 DISTANCE (KM) 27 2 8 29 6 . 4 * 2 . 3 4 ; 0.4*0.1 UO.O 80.0 120.0 DISTANCE(KM) IBO.O 200.0 o CJ LU CO •—'O CO \ L U C J z: d o I—I o i L U FIG. 5-18 V1C-PAB 11-1 .11-3 11-2 •7.0±2.04, 2.5+0.8 1 1 1 r~ 200.0 250.0 ' 300.0 350.0 UDO.O DISTANCE (KM) 450.0 30 o C D . o 07 o O CD UJ CJ 'ZL d o •—i a i UJ 21 £ ° 120.0 FIG. 5-19 CAS-NAN 11-2 6.9±0.8?6, 2.0±0.3 n i i I I 170.0 220 .0 270 .0 320 .0 370 .0 DISTANCE(KM) . FIG. 5-20 7.1±2.0%, 2.4±0 . 5 120.0 160.0 2D0.0 DISTANCE 2U0.0 (KM) 280 .0 320 .0 C D -CJ LU CO to O L Q ' C O C X C D 31 FIG, 5-21 ALB LU I X C J X X X co=H * * * * x Xx X o I LU I I 1 1 1 142.0 144-0 146.0 148.0 150.0 152.0 DISTANCE(KM) 32 %, 4.0±0.5 120.0 140.0 160.0 180.0 200.0 220.0 DISTANCE(KM) 33 3^ fsi-CJ LU CO O c o ' CO LU CO^ <—1 a 1 LU 2Z 1—O. .0 FIG, 5-27 1 1 - 7 , . 9 - 1 , 2 , 3 7 . 2 * 7 . 0 % , 3 . 2 * 2 . 5 6 . 5 * 0 . 8 % , 0 . 4 * 0 . 1 -SD.Q 160.0 240.0 DISTANCE(KM) ~ i 320.0 yoo.o cn CJ LU CO CO V. LU CJ 2; a: co * i—i a i LU I—=f . 8.0 FIG. 5-28 6 . 7 * 1 . 3 % , 0 . 5 ± 0 . 1 I I I 16.0 24.0 32.0 DISTANCE (KM) uo.o 48.0 36 w h e r e v e r t h e s h o t - d i s t a n c e s e p a r a t i o n i s g r e a t e r t h a n a b o u t 1 6 0 km. T h e s c a t t e r o f p o i n t s o n t h e c o m b i n e d t r a v e l - t i m e p l o t ( F i g . 5 ) l e a d s t o a c o n s i d e r a t i o n o f t h e f a c t o r s w h i c h c o n t r i b u t e t o t h i s : ( a ) s o u r c e l o c a t i o n a n d t i m i n g e r r o r s ( b ) l a t e r a l v a r i a t i o n i n t h e v e l o c i t y o f t h e r e f r a c t o r ( c ) c u r v a t u r e o f t h e r e f r a c t o r ( d ) f a u l t s t r u c t u r e s ( e ) c o n t i n u o u s v e l o c i t y i n c r e a s e s w i t h d e p t h w i t h i n l a y e r s ( f ) c o n s i s t e n t e r r o r s i n p i c k i n g t h e a r r i v a l s F a c t o r ( a ) w a s e s t i m a t e d i n t h e p r e v i o u s c h a p t e r a n d f o u n d t o b e a p p r o x i m a t e l y 0. 1 3 s e c , t h i s i s n o t s i g n i -f i c a n t o n t e r m s o f t h e s c a t t e r o b s e r v e d . S u b s u r f a c e i r r e g u l a r i t i e s , f a c t o r s ( b ) , ( c ) a n d ( d ) , w o u l d c a u s e c o n s i d e r a b l e s c a t t e r i n t h e d a t a . A l t h o u g h t h e s t u d i e s o f M i l n e a n d W h i t e ( 1 9 6 0 ) , a n d W h i t e a n d S a v a g e ( 1 9 6 5 ) a l o n g w i t h t h e g r a v i t y a n d g e o l o g i c a l e v i d e n c e i n d i c a t e a h i g h l y c o m p l e x s u b c r u s t a l s t r u c t u r e , t h e s c a t t e r o f d a t a d u e t o t h e s e f a c t o r s a r e g e n e r a l l y b e l i e v e d t o b e J e s s t h a n 2 s e c o n d s . On t h e t r a v e l - t i m e p l o t , a c o n t i n u o u s v e l o c i t y i n c r e a s e w i t h d e p t h ( f a c t o r ( e ) ) w o u l d r e s u l t i n c u r v e s r a t h e r t h a n t h e s t r a i g h t l i n e s o f t h e l a y e r e d m o d e l , a n d t h e t r e n d 37 would be traceable. Therefore the residuals of magnitude greater than 2.5 sec (open c i r c l e s , F i g . 5) are attributed to factor (f) and are rejected from the in t e r p r e t a t i o n . Using a least squares f i t , the data for shot-receiver distances greater than 200 km has an inverse slope of 7.14 km/sec. I f i t i s accepted that the upper mantle v e l o c i t y l i e s in the range 7.6 to 8.3 km/sec, then the depth to the M discontinuity cannot be determined by this study. Implications of this w i l l be discussed i n chapter 5. 3-3. Detailed Travel-Time Analysis On the basis of geographical separation, geological evidence, the location of p r o f i l e s and azimuth, the data were divided into 3 sets: (1) S t r a i t of Georgia and eastern Juan de Fuca S t r a i t (2) North of Vancouver Island (3) West of Vancouver Island and western Juan de Fuca S t r a i t An i n t e r p r e t a t i o n was then sought from the i n d i -vidual reduced t r a v e l time plots ( reduced travel time plots were used to obtain greater r e s o l u t i o n ) . In determining the f i n a l slope and time intercept using a least squares f i t , any data point which deviated markedly from the r e s u l t , were re-jected (e.g., 10-13 i n Fi g . 5-12 and 10.-3 i n Fi g . 5-10) as an average v e l o c i t y and depth i s sought. 38 T h e d e p t h c a l c u l a t i o n i s b a s e d o n t h e t i m e i n t e r -c e p t . H o w e v e r , i f a c r i t i c a l d i s t a n c e i s a v a i l a b l e , t h i s m e t h o d i s u s e d t o c h e c k t h e v a l u e o b t a i n e d b y t i m e i n t e r c e p t , a n d f i n a l v a l u e f o r d e p t h d e t e r m i n e d . T h e r e s u l t s f o r t h e s e v a r i o u s p r o f i l e s a r e t h e n s u m m a r i z e d i n T a b l e 4 a n d F i g . 6. 3 - 4 . S t r a i t o f G e o r g i a I n t e r p r e t a t i o n T h e v e l o c i t y o f t h e c o n s o l i d a t e d C r e t a c e o u s s e d i -m e n t a r y r o c k s (Po-) c a n b e d e t e r m i n e d f r o m t w o p r o f i l e s . T h e n e a r s h o t s r e c o r d e d a t H I r e v e a l a v e l o c i t y o f 4.0 k m / s e c ( F i g . 5-1) - a v a l u e w h i c h h a s b e e n u s e d b y W h i t e ( 1 9 6 2 ) t h r o u g h o u t t h e w h o l e a r e a . A v e l o c i t y o f 4.05 k m / s e c n e a r TONG a l s o h a v e b e e n r e p o r t e d f r o m t h e s h o r t b a s e l i n e m e a s -u r e m e n t s ( M i l n e a n d W h i t e , 1 9 6 0 ) . H o w e v e r , t h e r e s u l t f r o m T O N G - S A L L Y ( F i g . 5-2) i n d i c a t e s a n e x c e l l e n t f i t f o r a v e l o c i t y o f 4.5 k m / s e c u p t o 25 km s h o t - s t a t i o n d i s t a n c e a n d i m p l i e s t h a t t h e d e e p e r r o c k s o f t h e s e d i m e n t a r y s e c t i o n a r e m o r e c o m p e t e n t . T h i s v e l o c i t y i s p r o b a b l y m o r e r e p r e s e n t a t i v e o f t h i s l a y e r . T h e p e r m a n e n t s t a t i o n HBC r e c o r d e d n e a r l y a l l o f t h e s h o t s i n t h i s a r e a . T h e p l o t f o r t h e s e s h o t s ( F i g . 5 - 3 ) y i e l d s a l i n e g i v i n g a v e l o c i t y o f 6.5 k m / s e c , t h i s i n d i c a t e s a u n i f o r m P^ s t r u c t u r e s t r e t c h i n g f r o m t h e e a s t e r n J u a n d e F u c a S t r a i t t o n o r t h o f t h e G e o r g i a D e p r e s s i o n . A l o n g w i t h t h e e x i s t e n c e o f 1 9 4 6 e a r t h q u a k e (M = 7 . 3 , L a t . 49°9 N . , L o n g . 125°3 W . ) ( H o d g s o n , 1 9 4 6 ) , t h e a b n o r m a l a p p e a r a n c e o f 39 shots 7-7 and 7-8 i s evidence that the uniform layer i s terminated by a f a u l t beneath the southern part of Discovery Passage. Milne and White (1960) also came to t h i s conclusion. Differences between the north and south of the s t r a i t are shown in F i g . 5-4 and Fig. 5-5. Rather than the 6.5 km/sec v e l o c i t y detected at HBC the shots from the northern part indicate a 6.6 km/sec v e l o c i t y while to the south a velo-c i t y of 5.9 km/sec i s found, thus a thinner Pj layer at the north i s indicated. By assuming a plane layer between ALB and HBC the P 2 layer has a dip of 0.9 degrees from west to east, and the true v e l o c i t y i s 6.56 km/sec. Using the velo-c i t y of 4.5 km/sec (Fig. 5-2), the time intercept gives a thickness for the upper layer i n the southern part of the s t r a i t as 1.6 km.. At ALB the f i r s t a r r i v a l s from the northern part of the S t r a i t of Georgia are P 2. This implies that r e l a t i v e to the southern part of the s t r a i t the boundary i s shallower. The i n t e r p r e t a t i o n is strengthened by the plot of EFL-CR (Fig. 5-6) the v e l o c i t y of 6.7 km/sec appears again for station-shot separation greater than 40 km (and probably from 20 km). Apparently, the point 9-6 i n F i g . 5-6 refracted from an upper layer. Assuming the three points between the shot-receiver distance 20 and 40 km are from the same layer, the best f i t l i n e has an inverse slope of 5.9 km/sec, and the time intercept gives a thickness for this layer of 0.7 km. ' 4 0 This assumption is acceptable as this corresponds to the v e l o c i t y for the southern part of the s t r a i t . Underneath the 4.5 km/sec layer, the 5.9 km/sec layer detected from ALB station appeared again in seismograms of VIC (Fig. 5-8), however, the degree of uncertainty is . indicated by the appearance of the scattered points. At larger distances ,the data from the northern part of the s t r a i t recorded at VIC give a good f i t for a v e l o c i t y of 6.5 km/sec (Fig. 5-7), thus the P 2 layer beneath the 5.9 km/sec layer i s c l e a r l y indicated. At KEL the large scatter of points (Fig. 5-9) can-not be f i t t e d to obtain a reasonable value. This is at least p a r t i a l l y a ttributable to the f a u l t structure although some of the error i s due to timing errors. Fig. 6a summarizes the results over the length of the S t r a i t of Georgia. 3-5. West Coast Interpretation The objective of programs 10 and 4 was to determine the c r u s t a l structure o f f the west coast of Vancouver Island. Fi g . 5-10 to 5-12 show that the travel-time plots for pro-gram 10 and Fig. 5-13 shows the combined plot for programs 10 and 4. TOF is located at the centre of this p r o f i l e . A r r i v a l s from the south y i e l d a ?i v e l o c i t y of 6.14 km/sec .-ill while those from the north y i e l d a v e l o c i t y of 5.9 3 km/sec, thi s v e l o c i t y change can be caused either by a change in composition or by curvature of the boundary (about 4.3° slope would be required to explain the v e l o c i t y difference). The uniform gravity anomaly contours (Fig. 3) seems to favour the l a t t e r i n t e r p r e t a t i o n . A more detailed study w i l l be given i n chapter 4. At ESP two layers were observed with a crossover distance of 70 km. Considering ESP and TOF as ends of a reversed p r o f i l e , a 4° dip i s found again, this i s consistent with the r e s u l t obtained from TOF only. The time intercept indicates an unusual thickness for the upper layer i n this region. The exposed Ter t i a r y sedimentary and volcanic rocks have a measured v e l o c i t y of 5.0 km/sec (White and Savage, 1965). Using this value the layer has a thickness of 5.6 km i n the north and 3.5 km in the south. Assuming horizontal layering, a high v e l o c i t y of 7.2 km/sec was obtained at PHC ( f i g . 5-12). From the time intercept, the depth of this layer i s about 29 km. A few kilometers v a r i a t i o n of depth should be assigned due to the scatter of data. Program 4 plots with 10 for VIC (Fig. 5-13) presents a:complicated structure beneath the east of Juan de Fuca S t r a i t ; the inconsistency between the two p r o f i l e s , the high 42 v e l o c i t y but negative time intercept, suggests a high velo-c i t y i n t r usion exists and thi s might extend to deeper layers. The high density material i s indicated by the gravity anomaly map also (Fig. 3). 3-6. North Coast Interpretation The upper sedimentary and volcanic layer of velo-c i t y 5.5 km/sec shows in the plots of KEL, RBC (Fig. 5-14, 5-15). This layer overlies a 6.1 km/sec ve l o c i t y layer (Fig. 5-13). Both the time intercept and cross distance method give the upper layer thickness to be about 0.9 km. The seismograms from PHC give a v e l o c i t y of 6.5 km/ (Fig. 5-17) for the P 2 layer, but at WOL-EFL (Fig. 5-16) an excellent f i t t e d l i n e shows 6.7 km/sec. The considerable difference in v e l o c i t y suggests high v e l o c i t y material exists near the surface. On the other hand, a layer s t r i k i n g i n the RBC-PHC d i r e c t i o n and sloping up to WOL-EFL provides an a l t e r -native i n t e r p r e t a t i o n . A further discussion w i l l be given in the following chapter. The northern p r o f i l e recorded at VIC, ALB and CAS-NAN a l l show a v e l o c i t y about 7.0 ± .1 km/sec and large time intercept (Fig. 5-18, 19, 20). This indicates the P 3 layer exists beneath the eastern part of Vancouver Island. Three l a t e r a r r i v a l s 11-1, 2, 3 in Fig. 5-18 seem to be delayed by a thicker section of upper lower v e l o c i t y material. 43 3- 7; Juan de Fuca S t r a i t Short Range Programs The recordings from programs 1 and 2 in Juan de Fuca S t r a i t recorded at ALB,AHD,PNL and VIC y i e l d such a large scatter in the time distance plots (Fig. 5-21,22 ) that no d e f i n i t e conclusions are possible. However, the best f i t l i n e in Fig. 5-22 y i e l d s a v e l o c i t y of 6.4 km/sec which i s consistent with the properties of the outcropping volcanic and high v e l o c i t y basement rocks (Milne and White, 1960). 3-8. Cross Island P r o f i l e s F i g . 5-23 to 25 of HBC,ALB and UCL show t r a v e l -time curves in which shots and stations separated by.Vancouver Island. Again the existence of both P 2 and P 3 refractors beneath the island i s indicated. 3-9. Station Array Plots For shots in Johnstone S t r a i t the stations to the south may be considered as a spread of recorders, the t r a v e l times are plotted in Fig. 5-26 to 29. V e l o c i t i e s of approxim-ately 6.5 % .1 and 7.05 ± .05 km/sec are obtained. 3-10. Summary Table 4 summarizes the study of P wave a r r i v a l s using travel-time plots while Fig. 6a to 6c show, the cr u s t a l section for each d i v i s i o n . Stations V 0 T 0 T i V 2 T 2 v 3 >"T3 km/sec sec km/sec sec km/sec l.sec km/sec sec 1. Georgia S t r a i t HI . TONG SALLY ALB (South) ALB (North) VIC (South) VIC-PAB (North) EFL-CR HBC 4.0+0.1% 4.5+0.0% 5 .9±1. 5% 5.9+4.2% 5 . 9 ± 2 . 3 % 0.2+0.1 0.5±0.3 0.3+0.1 6.5±2.4% 6.6±1.3% 6.5+1.5% 6.7+0.9% 6.5±0.9% 0 .9±0 . 2 0.8±0.2 1.1±0.4 0.9+0.1 1.0+0.1 2. West Coast of Island TOF (South) TOF (North) ESP PHC VIC 5.4+0.3% -1.1±0.0 6.1+0.81 5.9 + 1. 2% 5.7+3.01 6.0±1.7% 0.8±0.1 1.2±0.1 0 .9±0 . 2 -0.2±0.4 6.6±4.2% 6.4+3.0% 1.6±0.7. 1.8+0.7 7.2+6.6% 7.2+2.0% 3.9+1.0 4.2±0.6 3. North Coast of Island RBC EFL-WOL PHC KEL VIC-PAB CAS-NAN ALB 5. 5±0.4% 5.4±5.9% 0.1±0.1 0.1±0.1 6 .1 ± 0 . 2 % 0.2±0 .0 6.5±3.0% 6.7-±1.5% 6 . 4 ± 2 . 3 % 0.7±0.7 0.7±0.3 0.4±0.1 7.0±2.0% 6.9±0.8% 7 .1 ± 2 . 0 % 2.5±0 . 8 2.0±0.3 2.4±0.5 4. Cross Island ALB HSB UCL1-UCL2 (UCL) 6. 7±0 .6% 6.4+0.2% 0.5±0.1 0.3±0.1 7.3±2.5% 4.0±0.5 5 . Juan de Fuca S t r a i t Short Range Programs ^Southern Tip of Island) AHD-PNL-VIC 6.4+0.2% 0.3±0.1 TABLE 4. SUMMARY OF TIME INTERCEPTS AND VE L O C I T I E S FOR THE CRUSTAL MODEL a. Georgia S t r a i t b. West Coast of Is land c. North Coast of Is land NORTH 0 SOUTH 4.5 5.9 5.9 6.6 KM 6.6 •10 --15 • -2 0 - • -2 5 -s. 7.1 % •» 7.1 NORTH SOUTH 6.6 7.1 10 - 15 - 20 - 2 5 30 O U T 6.1 6.5 . 5 10 . 15 - 20 *P wave v e l o c i t y in km/sec Fig. 6. Crustal models in the v i c i n i t y of Vancouver Island 46 In spite of the complexity which was shown i n F i g . 3 and S, a s t r i k i n g consistence between the v e l o c i t y was found i n each d i v i s i o n . The f i r s t layer (P 0) i n the three d i v i -sions shows some v a r i a t i o n : i n the S t r a i t of Georgia, the formation has v e l o c i t i e s from 4.0 to 4.5 km/sec but to the north of the i s l a n d , a higher v e l o c i t y of 5.5 km i s observed. This i s probably mainly due to the metamorphic rocks. On the west coast a thicker T e r t i a r y formation e x i s t s , with a higher v e l o c i t y (5.0 km/sec) than the Cretaceous formation i n Georgia S t r a i t . The second layer has a consistent v e l o c i t y of 6.0 ± .1 km/sec, thus the g r a n i t i c basement appears to be continuous throughout the whole area. However i n the northern region i t i s much thinner with the higher v e l o c i t y layer P 2 (basaltic) within 4.1 km of the surface. In the other two areas, the depth to the P 3 layer i s about the same as i n the S t r a i t of Georgia. Beneath and to the west of Vancouver Island, a 7.1 km/sec v e l o c i t y has been detected with the depth to this layer greater beneath the isl a n d . Shear waves were also analysed. However, due to the small amount of data and large scatter, neither an inde-pendent in t e r p r e t a t i o n of c r u s t a l structure or estimation of Poisson's r a t i o could be made. In this study no v e l o c i t i e s higher than 7.2 km/sec have been found. Hence the depth to the M d i s c o n t i n u i t y i f i t exists i n this region, cannot be determined. The minimum depth to t h i s discontinuity can be determined by two methods: 47 (1) By assuming that the upper mantle v e l o c i t y is i n the range 7.6 to 8.2 km/sec, and the maximum shot-st a t i o n separation (400 km) i s the c r i t i c a l distance for P n refracted as a f i r s t a r r i v a l , the depth to the M discon-t i n u i t y l i e s i s 57 ± 5 km. (2) By using data as far south as northern C a l i f o r n i a , White (1962) found a v e l o c i t y of 7.75 km/sec and a time intercept of 7.93 sec for P . Under the assump-ti o n of a continuous and horizontal M discontinuity between these regions, this y i e l d s a c r u s t a l thickness of 56 km. 4 8 CHAPTER 4 TIME-TERM ANALYSIS 4-1. Introduction Two facts suggest the application of the time-term technique to this data: (1) The region is geologically complex and hence the topography of the refractors i s unlikely to approximate horizontal layers as was assumed by the travel time method. The time-term method w i l l therefore be expected to y i e l d information i n addition to that provided by previous chapters. (2) Although the programs were designed to be li n e a r p r o f i l e s , they deviate considerably from t h i s . The wide areal extent of the data along with continuity of the 3 cr u s t a l layers throughout this region therefore suggests the application of the time-term technique. 4-2. Theory Time-term analysis deals with a set of observational equations i n each of which the tr a v e l time t . . from the i t h shot to the j t h receiver has been t r i p l y p a rtitioned to y i e l d (Scheidegger and Willmore, 1957) t . . = a . + b . + X . . / V I J i j i j ' where a i and b. are the time-terms of the i t h shot and 49 and j t h receiving s t a t i o n , respectively; V i s the average v e l o c i t y of the re f r a c t o r ; and X^. is the distance between the ith shot and j t h receiver. This set of l i n e a r equations i s solved for the time-terms a^ and b.. . The v e l o c i t y V is either con-strained or solved for as an additional unknown constant. The time-terms are functions of the depth to the marker horizon and the v e l o c i t y of the material between the surface and the marker layer. Willmore and Bancroft (1960) demonstrated the method for r e f r a c t i o n surveys. Berry and West (1966) and Smith et al (1966) have extended i t s application and the s t a t i s t i c a l analysis i n the Lake Superior experiments. Among the p r i n c i p a l advantages of the technique are that the equations can be solved without the requirement that the shot points and receiving stations be l a i d out i n any p a r t i c u l a r pattern and the necessity for making simplify-ing assumptions about c r u s t a l structure i s minimized (Willmore and Bancroft, 1960). The assumptions that must be made i n a time-term analysis are that a r e f r a c t i n g interface does exist and within the cone of c r i t i c a l l y refracted rays beneath shot points and receiving stations that this interface i s roughly planar. 4-3. Procedures of Analysis The time-term method used i n this study follows 50 the procedures of Berry and West (1966) used i n analysis of the Lake Superior data. As required by the theory, the data were f i r s t separated into groups which t r a v e l along the same re f r a c t o r . From the results of the t r a v e l time p l o t s , the data are consistent with apparent v e l o c i t i e s grouped in the ranges 5.9 - 6.1 km/sec ( P i ) , 6.4 - 6.6 km/sec (P 2) and 7.0 - 7.2 km/sec ( P 3 ) . Following the formulae given by Berry and West (1966) the time term analysis which includes the c a l c u l a t i o n of least squares v e l o c i t y , time-term at each s i t e , standard deviations of the solu t i o n and each time-term was performed on the IBM 7044 computer at the University of B r i t i s h Columbia. To solve the simultaneous equations, both the matrix inverse method (Berry and West, 1966) and i t e r a t i o n method (Mereu, 1967) were programmed. Since the i t e r a t i o n method was much fast e r , i t was adopted for a l l the f i n a l c a l c u l a t i o n s . According to the assumptions of the theory, d i f -f i c u l t i e s are encountered for refractors having a dip larger than 10° (Hobson, et a l , 1967), thus, sharp changes i n the time-term (depth) may be due to timing errors or to abrupt geological changes which cannot be adequately handled by the theory. By r e j e c t i n g these points, the erroneous data are eliminated and also the l o c a l i z e d e f f ects are smoothed out. However the broader scale features remain. Two measures of qua l i t y of the time-term solution were used: 51 (1) for the data points, the time residual (the difference between the observational and th e o r e t i c a l trav e l time), (2) for the t o t a l solution, the standard deviation of the solution (Berry and West, 1966). To obtain the best f i t solution, the data with abnormally large residuals (normally those greater than three standard deviations of the solution) were deleted and the program would be run again to obtain a new solution. The best time-term solutions for three layers were obtained with the v e l o c i t i e s 6.1, 6.6 and 7.2 km/sec for the , P 2 and P3 r e f r a c t o r s . The ar b i t r a r y constant which appears in the time-term solution has been determined by assuming that the time-term i s the same for an adjacent shot and sta-t i o n . For Pi i t is found by assuming the time term of ESP and shot 10-19 to be equal. This gives a = 0.77. TOF and 10-12 could have been to give a = 0.35. However, the f i r s t value i s preferable as the Bouguer anomaly at ESP and 10-19 is approximately the same whereas there is change of 20 mgal between TOF and 10-12. For the P 2 s o l u t i o n the constant of 0.64 is obtained using 3 station-shot pairs s t a t i o n shot a VIC 1-1 0.664 CR 9-6 0.64 PHC 11-2 0.632 52 For the P3 solution the value of a =1.64 i s yielded by the station-shot pair PHC and 11-2. The f i n a l adjusted time-terms are summarized i n Table 6a, b, c. . The time-term p r o f i l e s were plotted i n two sections - those east and north of Vancouver Island including Georgia S t r a i t , Discovery Passage, Johnstone S t r a i t , and Queen Charlotte S t r a i t (Fig. 7a) and secondly those on the continental shelf to the west and i n Juan de Fuca S t r a i t to the south (Fig. 7b). The time-terms of each s i t e (station and shot) and t h e i r standard deviations are shown. It should be pointed out that although these results are plotted as p r o f i l e s , the time-term locations deviate markedly from l i n e a r p r o f i l e s . 4-4. P r o f i l e East of Vancouver Island The p r o f i l e to the east and north of Vancouver Island i s nearly aligned except i n the Discovery Passage region. The Pi s o l u t i o n i n this p r o f i l e shows s i g n i f i c a n t undulation with 3 major features predominating. (1) Near the southern end of the p r o f i l e there is a marked thickening of the upper layer - a structure consistent with the travel time analysis. This feature has previously been reported by Milne and White (1960) and can be attributed to deposition of sedimentary material by the Fraser River. ' i * 0 50 KM Fig. 7a. Time-term p r o f i l e s east of Vancouver Island Fig.. 7b. Time-term p r o f i l e s west of Vancouver Island 55 TABLE 6a. Summary of Pj Time-Terms and Standard Deviation shot time term s tandard shot time term standard (sec) deviation (sec) deviation (± sec) (±sec) 3-1 .51 .184 10-13 .84 .216 3-2 1.3 132 10-14 .80 0. 3-3 1.47 .078 10-15 1.0 .037 3-4 1.13 .031 10-16 .91 .061 3-5 •1.02 .017 10-17 .43 0. 3-6 .70 .34 ID-18 .82 .271 3-7 .41 ..007 10-19 .62. .343 3-8 .76 .138 10-20 .88 0. 3-9 .67 0. 10-21 .94 .302 4-1 - .1 .226 10-22 .565 0. 4-2 - .05 . 339 10-24 .91 0. 4-4 .15 . 254 11-1 - .15 0. 4-5 .08 .055 11-3 .62 0. 4-6 .43 .107 11-4 .53 0. 4-9 .43 .037 11-5 .51 0. 4-11 .46 .168 11-6 .99 0. 6-4 1.64 .436 11-7 .95 .093 6-5 2.31 .455 11-8 1.01 .094 6-6 . 1.53 .14 11-9 1.16 0. 6-7 1.64 .149 11-10 .995 0. 6-11 1.0 0. 11-11 .997 0. 7-4 1.42 .197 7-5 .81 0. 7-6 2.36 0. 7-7 .87 0. stat i o n 7-13 .73 .218 7-14 - .028 .367 7-16 .7 . 309 ALB - .31 .047 7-17 .5 .09 ESP .62 .051 7-18 1.72 .138 GUR .36 .189 7-19 .72 .089 RBC - .83 .014 7-20 .95 .098 PRE .59 . 095 9-1 1.24 0. SCD - .12 .076 9-3 .956 •0. TOF .52 .056 9-5 1.24 0. VIC .13 .043 9-6 1. 28 0. EFL-CR -1.3 .046 10-1 .63 0. 10-2 .02 .013 10-5 .57 .08 10-8 .51 .307 10-10 .17 .152 10-11 .29 . 225 10-12 .83 .423 56 TABLE 6b. Summary of P 2 Time-Terms and Standard Deviation shot time standard shot time standard term deviation term deviation (sec) (±sec) (sec) (isec) 1-1 .3 0. 8-4 .48 .096 1-2 .55 0. 8-5 .51 . 234 1-3 .33 0. 8-6 .49 .057 3-1 .62 0. 8-7 .32 .045 3-2 .72 0. 8-8 .34 .041 3-3 .67 0. 8-9 .205 .062 3-4 .50 0. 8-10 .246 .106 3-5 .17 0. 8-11 .35 .098 3-6 .11 0. 8-12 .144 .06 3-7 .41 0. 8-13 .22 .042 3-8 .38 0. 8-14 .435 .002 3-9 .8 .032 8-15 .27 .10 3-10 .58 .107 8-16 .34 .122 3-11 .57 .102 8-17 .63 .055 3-12 .71 .061 8-18 .26 .001 3-13 .83 .043 8-19 .44 .019 3-14 .75 .08 9-7 1.0 .059 3-15 .48 .076 9-8 .49 .059 3-16 .34 .155 9-9 .39 .141 3-17 .42 .131 9-10 .30 .162 4-1 .764 0. 9-11 .61 .211 4-2 .66 0. 10-22 2. 73 0. 4-5 .73 0. 10-23 2.32 0. 4-6 .74 .047 10-24 2.67 0. 4-7 1.02 0. 11-1 - .46 .154 4-8 . 65 0. 11-2 .12 .015 4-9 .89 .121 11-3 .55 .02 4-10 1.06 0. . 11-4 .38 .196 4-11 .843 0. 11-5 .57 .07 5-1 .65 0. 11-6 .84 0. 6-2 .45 0. 11-7 .84 .041 7-1 1.84 .158 11-8 .86 .133 7-2 .72 .146 11-9 .97 .118 7-3 .76 .153 11-10 .85 .018 7-4 .94 .116 11-11 .68 .085 7-7. 1.3 .163 7-8 2. 78 0. 7-9 .6 .151 7-10 7-11 .47 .98 .088 .18 stat i o n 7-12 . 69 .119 ALB - .01 .0 23 7-13 .757 .074 VIC .38 . 38 7-14 .43 .102 HBC .7 .025 7-16 .17 0. CR-EFL .274 .031 7-18 .33 0. KEL -2.15 .064 8-1 .43 .004 PAB .69 .055 8-2 .11 .116 PHC .11 .031 8-3 . 32 .109 TOP UCL WOL UCL2 .22 .55 .67 .4 .005 .065 .035 .085 57 TABLE 6c. Summary of P 3 Time-Terms and Standard Deviation shot time term standard deviation (sec) (±sec) 4-1 1.56 0. 4-4 1.70 0. 4-5 1.69 0. 4-6 1.75 0. 4-8 1.6 0. 4-9 1.58 0. 4-11 1.61 0. 9-3 1.88 0. 9-4 1.34 0. 9-5 1.96 0. 9-6 2.16 0.144 10-1 1.08 0. 10-5 .84 0. 10-4 .06 0. 10-9 1.61 0. 10-10 1.62 0. 10-12 1.44 0. 10-14 .64 0. 10-16 1.2 0. 10-17 1.51 0. 10-19 1.16 0. 10-20 1. 28 0. 10-21 1.15 0. 10-11 2.3 0. 10-18 .68 0. 11-1 3.00 .59 11-2 2.95 .4 11-3 1.9 .273 11-4 1.65 .083 11-5 1.88 .133 11-6 1.76 . .253 11-7 1.5 .111 11-8 1.91 .424 11-9 2.02 0. 11-10 2.1 .11 11-11 2.3 . .275 stat i o n ALB 1.12 .066 HBC 2.13 .004 NAN 1. 54 .117 CAS 1.99 .397 VIC 2.38 .156 PAB 2.34 .132 PHC 2.47 .001 58 The increased thickness of the upper layer i s also evident i n the P 2 solution. However, the increasing time-term of the P 3 boundary i s not due to this as the time-terms are a l l for s i t e s on Vancouver Island. (2) In the northern part of Georgia S t r a i t an a n t i -s y n c l i n a l structure appears based on shot point 7-14. This, negative time term can be explained by the intrusion of high v e l o c i t y material i n the Pj r e f r a c t o r . This e f f e c t was noted i n the travel-time study (Fig. 5-1) where the Pi v e l o c i t y was not found. Instead the travel-time curves showed a velo-c i t y increase from 4.0 km/sec near the surface to the P 2 v e l o c i t y of 6.6 km/sec indicating that the f i r s t basement layer i s very thin or absent i n this region. (3) At the southern end of Discovery Passage the decreasing time term experiences an abrupt change to large residuals for shots 5-2, 7-8 and 7-7. This large change suggests that a f a u l t crosses the p r o f i l e . As further e v i -dence, Hodgson (1946) reported an earthquake occurred i n the region i n 1946. Time-terms 9-9 and 7-7 suggest a ver-t i c a l displacement i n excess of 3 km while the travel-time method suggests a displacement of about 0.4 km. This large discrepancy i s to be smoothing of the travel-time solution and the uncertainties of interpretations based on one point i n the time-term technique. The P 2 solution shows that the layer is more uni-form than i n Pi s o l u t i o n , this was expected from the travel time int e r p r e t a t i o n . In the Discovery Passage region some 59 evidence of an a n t i c l i n a l structure is seen. However, lack of data in this region does not allow a d e f i n i t e interpreta-t i o n . The P3 solution at the southern end of the p r o f i l e is based on the station's time-terms. The p r o f i l e indicates a large scale a n t i c l i n a l structure beneath the i s l a n d and deeper interface at the northern end. In Johnstone S t r a i t and Discovery Passage, the small time-terms indicate the high v e l o c i t y material is near the surface and the P 2 layer probably absent i n this region. The Pi and P 2 time-terms of ALB are negative, thus a high v e l o c i t y i n t r u s i o n probably e x i s t s . This correlates with a high in the gravity anomaly map as expected. It should again be emphasized that the three time-term p r o f i l e s are not colinear. Hence the p r o f i l e s are not plotted on a u n i f i e d scale as i t would appear that there i s an i n t e r s e c t i o n of the r e f r a c t i n g surfaces. This i s not implied i n this analysis but arises from the two dimensional 1 plot of three dimensional data. 4 - 5 . P r o f i l e West of Vancouver Island The Pi solution shows a considerable change in the time-term between shot points 4 -4 and 4 - 6 . The Leech River Fault which i s evident on Vancouver Island apparently extends to the west and appears as a discontinuity i n the time-term p l o t . The southern end of the Pj p r o f i l e shoivs 60 a decreasing time-term which is also evident on the eastern p r o f i l e indicating an upward dipping boundary. Geological evidence indicates exposed basement rocks in this area. The most s i g n i f i c a n t feature of this p r o f i l e i s that the interfaces are deeper at the northern end of the p r o f i l e than at the southern. On the Pj boundary, the change occurs near point 10-11. No data is available for P 2 i n this region - a sharp change ind i c a t i n g a f a u l t may be present between points 10-11 and 10-12. A sharp gravity anomaly i s also present at t h i s location. The i r r e g u l a r interface of P 3 . i s obtained using only one s t a t i o n . A s i g n i f i c a n t part of the surface features are believed to be caused by timing errors. The evidence of a r e l a t i v e l y shallower boundary i n the southern h a l f than i n the northern is probably caused by the ?i layer. 4-6. Summary On both p r o f i l e s the ?i boundary exhibits a sharp change i n thickness with the northern boundary l o c a l l y deeper than the southern. These changes have been interpreted as f a u l t s . On the eastern p r o f i l e this occurs i n Discovery Passage while on the western p r o f i l e to the west of Tofino. This leads one to speculate whether a major s t r u c t u r a l feature might cross the islan d at t h i s point. The gravity map indicates that this may be so. Starting from the south with the gravity low on 61 the continental shelf o f f Tofino we see a "pinching" e f f e c t (indicating a subsidiary low) in the broad gravity high trend running up the isl a n d , the Buttle Lake low, and sharp change of gravity from a high on the east to a low on the west in the Discovery Passage region of the isla n d . Hence we see a rather sharp gravity change across the island in th i s region, which adds some strength to this i n t e r p r e t a t i o n . Fig. 8 shows the histograms of frequencies and residuals. From the diagrams, i t shows that most of the residuals are quite small. A combined P 2-P 3 layer solution has been attempted, giving a least squares v e l o c i t y of 6.7 km/sec which seems to be consistent with the result obtained by White (1962) for basalt layer. However, the large v a r i a t i o n in the time-terms (Fig. 7c) indicates that the P 2 and P 3 boundaries layers are d i s t i n c t . The standard deviation for the combined solu-tion i s 0.34 compared to the smaller values of 0.12 for the P 2 solution and 0.21 for the P 3 solution. Although a contour map for each layer i s desirable, i n s u f f i c i e n t data i s available beneath the is l a n d . 80 6 0 4 0 2 0 L~L, - 1 . 0 0 + 1 . 0 P i a* = 0.13 ^standard deviation of solution - 1 . 0 rt 3 • H </) <D S-< O U c cr 2 5 0 2 0 0 1 5 0 1 0 0 30 0 +1.0 a = 0.12 r so 60 40 2 0 • 1 . 0 _ 0 + 1 . 0 D residual (sec) a = 0.21 Fig. 8. Histograms of frequency versus residual E a s t o f I s l a n d NORTH s 0 u T H 0 .5,0 KM W e s t o f I s l a n d NORTH SOUTH 50 KM F i g . 7 c . T i m e - t e r m p r o f i l e s c o m b i n i n g P 2 a n d P 3 64. CHAPTER 5 DISCUSSION 5-1. Crustal Model Lying in the circum-Pacific b e l t , Vancouver Island i s part of an active tectonic region. Since the c r u s t a l processes involved are not well understood, i t i s important to study this region using the geophysical techniques at . our disposal. In this study, the f i r s t a r r i v a l seismic data has been interpreted by the travel-time and time-term method, y i e l d i n g a four layer c r u s t a l model. The f i r s t layer, comprised of low v e l o c i t y sedi-mentary rocks, has a P v e l o c i t y of less than 5.5 km/sec. As the geological evidence and gravity map also show (Fig. 3 ) , the v e l o c i t y and thickness vary somewhat for each d i v i s i o n . In the northern part of the S t r a i t of Georgia, the upper layer has a P wave v e l o c i t y of 4.0 km/sec and a thickness of less than 1 km. However in the southern part a P wave v e l o c i t y of 4.5 km/sec and a thicker layer were revealed. This i s l i k e l y due to additional sediment being deposited by the Fraser River which have l a t e r been compacted. North of the i s l a n d , the f i r s t layer has about the same thickness comparable to that in the S t r a i t of Georgia. However the P wave v e l o c i t y i s approximately 5.5 km/sec, thus the rocks probably contain some metamorphic and a c i d i c material, as is present on the coast of the mainland. On the western side the is l a n d , the f i r s t layer has a thickness of more 65 than 4 km and the P v e l o c i t y i s also higher than i n the S t r a i t of Georgia. These differences i n thickness of sediment may be due either to d i f f e r e n t rates of sedimentation or to d i f -ferent rates of erosion in the geological past. The g r a n i t i c second layer has more consistent velo-c i t i e s of 5.9 to 6.1 km/sec beneath the whole region. In the north, using the l i m i t e d data of the RBC s t a t i o n , a layer thickness of less than 2 km i s revealed but i n the S t r a i t of Georgia and west of the i s l a n d the layer has a thickness of about 4 km. It is d i f f i c u l t to compare this thickness v/ith the results of Shor et a l . (1968) to the south due to a s i g n i f i c a n t l y d i f f e r e n t v e l o c i t y structure. It i s however thinner than the g r a n i t i c layer of the stable p r a i r i e region Alberta (10 km) and, about same thickness as in Japan - an isla n d arc (Research Group for Explosion Seismology, 1966). The b a s a l t i c or intermediate layer has been detected at most stations. The v e l o c i t y i s about 6.5 ± .1 km/sec, and this layer shows an unusual thickness of about 20 to 22 km beneath the S t r a i t of Georgia, and 18 km to the west of the i s l a n d . Underlying the b a s a l t i c layer, a material of uni-form P wave v e l o c i t y 7.1 ± .1 km/sec was observed from most of the more distant events. Thus the existence of a R i e l discontinuity as defined and proposed by Clowes et a l (1968) is c lear. It appears that the layer is another entity in 66 addition to the b a s a l t i c layer. This layer has a minimum thickness of 22 km as P n arrivalshave not been observed even for the most distant shots. The lower crust, which has the v e l o c i t y of 7.0 ^ 7.2 km/sec has been described as a " d i o r i t i c layer" by Caloi (see Steinhart and Meyer, 1962) while Press suggested that the material i s an anomalous c r u s t a l mixture of gabbro and ultramafic rocks beneath the b a s a l t i c layer. From the pet-r o l o g i c a l evidence, Ringwood and Green (1965) came to the conclusion that in regions where the upper crust has had a r e l a t i v e l y 'simple' geologic h i s t o r y , and has not been subjected to major orogenesis, regional metamorphism, and invasion by g r a n i t i c batholiths, a v e l o c i t y of 7.0 km/sec might be caused by basic rocks i n the amphibolite f a c i e s , perhaps interbedded with smaller quantities of a c i d i c rocks. A comparison of this structure to that of southern Alberta shows markedly s i m i l a r features. Both the v e l o c i t i e s and thickness of the crustal layers are very s i m i l a r . Although one might be lead to suggest that both crusts are the same enti t y with the same history of generation, Alberta i s part of the Churchill geological province while Vancouver Island is part of the C o r d i l l e r a n province (Kanasewich, 1966). On the other hand, the p r o f i l e s of Shor et al (1968) on the continental shelf o f f Oregon and C a l i f o r n i a show a markedly d i f f e r e n t crustal section with the Mohorovicic discontinuity at quite shallow depths (12 - 18 km). 67 5-2. Limitations of the Crustal Model Generally, the earth's crust has been divided into two layers - the g r a n i t i c material of the upper crust and b a s a l t i c material of the lower crust with the boundary be-tween sometimes being referred to as the Conrad discontinuity. It has not however been found everywhere. Further whether this i s a v e l o c i t y discontinuity or a rather large v e l o c i t y gradient i s s t i l l an open question. More recent r e f r a c t i o n studies and the r e f l e c t i o n p r o f i l e s indicate that additional layers are present and further, the r e f l e c t i o n studies show both the Mohorovicic discontinuity and the intermediate boundaries have i r r e g u l a r rough surfaces (Clowes et a l , 1968). This emphasizes that the r e f r a c t i o n technique smooths the data considerably. In applying the t r a v e l time method, the best results are to be expected from s t r i c t l y l i n e a r reversed p r o f i l e s . For an unreversed p r o f i l e , a dipping interface may be i n t e r -preted as a change in the v e l o c i t y of the r e f r a c t o r . Failure to observe along l i n e a r p r o f i l e s w i l l cause scatter in the data and hence more uncertainty i n the r e s u l t . It should also be pointed out that scatter in the data may indicate that the surface of the r e f r a c t o r has topography o£~short wavelength. Two methods may be used to obtain the thickness of the layer - the time intercept and cross distance - and 68 i n many cases may be used as a check on each other. In this study, the f i r s t method i s available for a l l calculations while the second could only be used i n a limited number of cases as at most stations data was only received from one r e f r a c t o r . The model found in t h i s study has two major d i f -ferences with White's model: In his three layer model the b a s a l t i c layer has a v e l o c i t y of 6.7 to 6.9 km/sec. However in this study this e n t i t y has been divided into 2 divisions with v e l o c i t i e s of 6.6 km/sec and 7.1 km/sec. Besides t h i s , the thickness c a l c u l a t i o n which he obtained almost always from the cross distance method gives s l i g h t l y d i f f e r e n t results from the study based on the time intercept. Because of the areal extent of the data and the expected short period topography of the r e f r a c t o r s , the time-term method has been applied. The uniform v e l o c i t y of each re f r a c t o r throughout the area allows this approach. This technique shows the r e l a t i v e depths along each refr a c t o r with the major feature observed being a f a u l t i n the upper layer and probably extending to deeper layers near TOF on the west coast and ELF i n Discovery Passage. The gravity feature l y i n g between these faults indicates that there may be major s t r u c t u r a l features across the island i n this region. A major l i m i t a t i o n of this method i s the assump-tion of uniform v e l o c i t y of each r e f r a c t o r . Local variations 69 i n v e l o c i t y w i l l contribute to the time-term and hence appear as errors i n depth to the r e f r a c t o r . The accuracy of the time-term solution is r e f l e c t e d by the residuals. Therefore the residuals may be used to eliminate data to obtain more accurate solutions. Any factor which violates the assumption of the uniform v e l o c i t y would be considered to appear as an abnormal residual i n the data. It is s i g n i f i c a n t that the residuals by which the data were sorted out on the basis of the travel-time method f a l l 901 below twice of the standard deviation of the solution. How-ever the combination of P 2 and P 3 layers yielded a r e l a t i v e l y larger standard deviation and only 56% of the residuals f a l l below twice the standard deviation of the solution. This indicates the c r u s t a l layering chosen in this study i s the most r e l i a b l e . Data were rejected i n the time-term analysis i f the residuals were large. These data have been checked with the plots of the travel-time and most of them were found to markedly deviate from the lines of the least squares. Besides the reasons that the abnormal residual was caused by the inhomogeneous material and large timing error, no explanation can be given from the theory. Fortunately, the number of these data are only a very small percent from the t o t a l data set. The method of time term allows considerable f l e x i -b i l i t y i n f i e l d procedure and eliminates many of the problems 7 0 encountered i n attempting to deploy seismometers along straight-l i n e p r o f i l e s . Ideally, data for this technique should be recorded at points of a grid over the area to be studied or at least have a wide areal d i s t r i b u t i o n . For data of li m i t e d areal d i s t r i b u t i o n , the small scale topographic features w i l l be observed, however the ambiguity between the v e l o c i t y of the r e f r a c t o r and the dip of the interface w i l l be present as i n the case of unreversed travel-time pl o t s . 5-3. Mohorovicic Discontinuity The f a i l u r e to observe P n is s t r i k i n g , as the shot-receiver distance in this study exceeded 400 km. By apply-ing a least square f i t to the data, i t appears that the maxi-mum P wave v e l o c i t y i s 7.1 ± .1 km/sec. In fact the a r r i v a l s of the most distant shots f a l l s l i g h t l y above this l i n e on the time-distance p l o t . The possible implications of this are as follows: (a) Deep Crustal Boundary: In Chapter 3, we have seen that even by assuming the upper mantle v e l o c i t y is as low as 7.6 km/sec, the Mohorovicic discontinuity must be at a depth greater than 52 km beneath Vancouver Island. This i s i n contrast to the normal crustal thickness to the east of Vancouver (M. J. Berry, personal communication) with the root beneath Vancouver Island due to the very thick P 3 layer. Other evidence for anomalously deep crustal bounda-r i e s have been reported, such as Eaton (1964) for the Nevada 71 region, Woollard (1960) for Chile. Thus, the deep cr u s t a l boundary probably i s a c h a r a c t e r i s t i c of an active tectonic b e l t . (b) Low v e l o c i t y layer: Several workers (e.g. Archaitibeau et a l (1968) have recently suggested on the basis of explosion data that a low v e l o c i t y layer may extend from 150 km to either the base of the crust or very close to the base of the crust under parts of North America. If a low v e l o c i t y region extended to the base of the crust beneath Vancouver Island, the absence of a t y p i c a l Pfi v e l o c i t y would be expected. This low v e l o c i t y material may be due to the temperature e f f e c t (Pakiser and Z i e t z , 1966) or may be due to the conversion of oceanic crust to continental crust (Menard, 1967). Pakiser and Zietz (1966) also suggested the p o s s i b i l i t y that molten basalt may flow l a t e r a l l y i n a s o l i d matrix of mantle p e r i d o t i t e from beneath parts of the oceanic basins and into the upper mantle beneath the active areas of the continents. (c) Mantle-crust Mix: From the analysis of seismic r e f r a c t i o n data and heat flow data, Cook (1962) suggested the p o s s i b i l i t y of the anomalous material of mantle-crust " mix in t e c t o n i c a l l y active belts within the continents. The average seismic v e l o c i t i e s i n the western U.S., 7.4 to 7.7 km/sec, are believed to be caused by a mixture of rocks in a t r a n s i t i o n a l phase between normal mantle and normal b a s a l t i c crust. In this case, no clear Mohorovicic discon-t i n u i t y would be observed and the v e l o c i t y would increase 72 gradually down to normal upper mantle v e l o c i t y . These mecha-nisms are also proposed by the opponents of the convection current theory. Here, the seismic v e l o c i t y of 7.2 km/sec would probably indicate the top of the mix. Due to lack of the heat flow information i n this region, no further conclusions can be drawn. 5-4. Gravity Analysis Gravity observations are available for t h i s region. A number of l o c a l second order Bouguer gravity anomalies are superimposed on the small posi t i v e f i r s t order one. The second order bouguer gravity anomaly are strongly cor-related with the surface geology (e.g. Sooke High to the volcanic i n t r u s i o n ) , while the f i r s t order anomaly is believed due to the lower c r u s t a l material (Walcott, 1967). A number of workers (e.g. Worzel et a l , 1955) have made estimates of c r u s t a l thickness by combining Bouguer gravity data with estimates of cr u s t a l densities. Several empirical plots of compressional wave velo-c i t y versus density of rock are shown i n Fig. 9. As the v e l o c i t y depends on the moduli of e l a s t i c i t y as well as density, s i g n i f i c a n t scatter in the data i s expected. The plots demonstrate that i t is impossible to assign a unique rock type to any v e l o c i t y value. However for c r y s t a l l i n e 1 73 Fig. 9. Relation between compressional wave ve l o c i t y and density of rocks (after Cook, 1962). 10 9 8 7 o ° 6 > 9} > Z 2 CL E o o I K •'V < v! V -! ? •* ft 5 E - : / ; »/ , off •'p 5 E ? I / . E / fj ; •f«l e ~ i » « 0 u / « / 1 / E / "J „ L / - : £ ® / i < i - / l> 1 K . c o u ^ / / / Noft on d Oraka , i X o * o [ w o r z t l , 1959 K o o l l o r d , 19 59 B i r c h ond other i ond • , I9<2 2 D e n s i t y 3 g m/ c c 74 rocks- i n the v e l o c i t y range 6.0 to 8.0 km/sec an e s s e n t i a l l y l i n e a r r e l a t i o n s h i p holds with minimal scatter i n the data. Walcott (1967) , White and Savage (1965) point out that the Bouguer and i s o s t a t i c anomalies are about zero in the S t r a i t of Georgia, thus l o c a l i s o s t a t i c equilibrium exists i n this area. Using Airy's theory, and assuming the compensation l e v e l at 60 km, the c r u s t a l column i n this region i s compared to the standard continental c r u s t a l column obtained by Worzel et a l (1955). The densities of the rock are chosen from Fig. 9. Then the c a l c u l a t i o n shows 2.84 x 33 + 27 x 3.27 = 1.0 x 2.6 + 4.0 x 2.8 + 24 x 2.9 + 3.1 x H + 3.27 (31 - II) This gives the thickness of the ultrabasic layer H = 16.4 km and the depth to the M discontinuity 45.4 km. However, i t should be emphasized that the value i s sensitive to the den-s i t y used, e s p e c i a l l y for the lower crust. A change of 0.1 gm/cm3 i n the b a s a l t i c or u l t r a b a s i c . layer causes the boundary to be 15 km deeper. This value i s less than the minimum value obtained by seismic method (56 km). A possible explanation is that the density of the lower crust material i s a s l i g h t larger than the curves i n F i g . 9 indicate. With the scatter of data, this is not unreasonable. 5-5. Geomagnetic Depth Sounding and Magnetotellurics Caner and Cannon (1965) , Caner et a l (1967) and others have carried out an extensive program of geomagnetic 75 depth sounding method i n western Canada. Although this tech-nique i s as yet not as precise a geophysical tool as explo-sion seismology i t provides complementary information about the crust and upper mantle. From the i r p r o f i l e s , they f i n d a high conductivity (10" emu ± 201) material to within 25 to 35 km of the surface i n the Canadian C o r d i l l e r a . They believe that the high e l e c t r i c a l conductivity in the upper mantle i s primarily a function of temperature and they sug-gest that the low P v e l o c i t y i s caused by an increase i n temperature rather than changes i n composition. These conductivity observations have been extended to the Vancouver Island using the magnetotelluric method (Caner and Auld, 1968). Again highly conductive material at shallow depth is indicated. Thus, the evidence indicates that the low P v e l o c i t y i s expected to underlie Vancouver Island. No heat flow data i s available for this area. These observations are consistent with the low seismic v e l o c i t i e s . 5 - 6 . Off-Shore Magnetic Anomaly The magnetic anomalies map of the P a c i f i c f l o o r to the south-west of Vancouver Island was published by Raff and Mason (1961). This reveals a series of tectonic structures i n the crust of this area with the astonishing p a r a l l e l i t y and r e g u l a r i t y . The d i s l o c a t i o n of the anomalies gives the evidence for Wilson (1965) and Vine (1966) to interpret the 76 10. Summary diagram of t o t a l magnetic-field anomalies southwest of Vanouver Island. Areas of po s i t i v e anomaly are shown in black. Straight lines indicate f a u l t s o f f s e t t i n g the anomalv pattern (after Vine, 1966). 77 136°W Fig. 11. Pattern of magnetic anomalies (after Pavoni , 1966) 78 Juan de Fuca Ridge as the continuation of the San Andreas Fault and the Queen Charlotte Fault. Moreover, one of the f a u l t s interpreted by Vine, i f extended, would intersect Vancouver Island near TOF (Fig. 10). However this is almost perpendicular to the apparent s t r u c t u r a l feature across the isl a n d . Another i n t e r p r e t a t i o n was given by Pavoni (1966). He showed the existence of the SoVanco Fault in the same general region (Fig. 11). The Leech Fault on Vancouver Island appears to be an extension of t h i s . More evidence is required to substantiate these c o r r e l a t i o n s . 5-7. Further Work The most int e r e s t i n g feature of this analysis has been the f a i l u r e to observe P n when a normal depth Mohorovicic discontinuity has been observed on the lower mainland of B r i t i s h Columbia. As this complex geological region may y i e l d further important information concerning the theory of continental d r i f t , spread of the ocean f l o o r , and tectonic a c t i v i t y , i t is important to determine a more complete seismic structure of this region. Ideally this can be achieved by explosion seismology with a p r o f i l e longer than the length of the island or by using the r e f l e c t i o n technique. However, s i g n i f i c a n t advances could be made more economically by using 79 body and surface waves from earthquakes. More heat flow and e l e c t r i c a l conductivity information i n th i s region i s desirable to complement a seismic program. 80 BIBLIOGRAPHY Berry, M. J. and G. F. West, (1966). An int e r p r e t a t i o n of the f i r s t - a r r i v a l data of the Lake Superior experi-ment by the time-term method. Bull. Seismol. Soc. Am., 56, 141-171. -Caner, B. and D. R. Auld, (1968). Can. J. Earth Sci. (in press). Caner, B. and W. H. Cannon, (1965) . Geomagnetic depth sounding and tcorrelation with other geophysical data in western North America. Nature, 207, 927-929. Caner, B., W. H. Cannon, and C. E. Livingstone, (1967). Geomagnetic depth sounding and upper mantle structure i n the C o r d i l l e r a region of western North America. J. Geophys. Res., 7_2, 6335-6351. Clowes, R. M., E.-R. Kanasewich, and G. L. Cumming, (1968). Deep cr u s t a l seismic r e f l e c t i o n s at near-vertical incidence. Geophysics, 3_3, 441-451. Cook, K. L., (1962). The problem of the mantle-crust mix: l a t e r a l inhomogeneity i n the uppermost part of the earth's mantle. Advan. Geophys., 9_, 295-360. Cumming, G. L. and E. R. Kanasewich, (1966). Crustal structure in'western Canada, Final Rept., Contract No. AF19(628)- 2835 , AFCRL, Bedford, Mass. D u f f e l l , S. and J. G. Souther, (1964). Geology of the Terrace map-area, B r i t i s h Columbia. Geol. Surv. Canada, Mem. 329. Eaton, J. P., (1963). Crustal structure from San Francisco, C a l i f o r n i a to Eureka, Nevada from seismic-refraction measurement. J. Geophys. Res., 6_8 , 5789-5806 . Herrin, E. T., (1966) . Travel-time anomalies and structure of the upper mantle (abstract). Trans. Am. Geophys. Union, 47, 44. Hobson, G. D., A. Overton, D. N. Clay, and W. Thatcher, (1967), Crustal structure under Hudson Bay. Can. J. Earth Sci., 4, 929-947. Hodgson, E. A., (1946). B r i t i s h Columbia Earthquake, June 23, 1946. J. Roy. Astron. Soc. Can., 40, 285-319. 8 1 James, D. E. and J . S. Steinhart, (1966). Structure beneath continents: A c r i t i c a l review of explosion studies 1960-1965, in The Earth beneath the Continents, J . S. Steinhart and T. J. Smith, ed., Geophys. Mon. 10, 293-333. Kanasewich, E. R., (1965). Seismicity and other properties of geological provinces. Nature, 208, 1275-1280. Lee, W. H . K . and P. T. Taylor, (1966). Global analysis of seismic r e f r a c t i o n measurements. Royal Astvon. Soc. Geophys. J., 11, 389-413. Lehmann, I., (1967). Low-velocity l a y e r s , ' i n The Earth's Mantle, T. F. Gaskell, ed., 41-62. Menard, H . W., (1967). T r a n s i t i o n a l types of crust under small ocean basins. J. Geophys. Res., 72, 3061-3073. Mereu, R. F., (1966). An i t e r a t i v e method for solving the time-term equations, i n The Earth Beneath the Continents, J . S. Steinhart and T. J. Smith, ed., 495-497. Milne, W. G. and W. R. H . White, (1960). A seismic survey i n the v i c i n i t y of Vancouver Island, B r i t i s h Columbia. Pub. Bom. Obs., 24_, 145-154. Misch, P., (1966). Tectonic evolution of the Northern Cascades of Washington State, i n Tectonic History and Mineral Deposits of the Western C o r d i l l e r a , Can. Inst. Mining and Met. Spec. Col. 8, 101-148. Pakiser, L. C. and I. Z i e t z , (1965). Transcontinental crustal and upper-mantle structure. Rev. Geophys., 3, 4, 505-520. Pakiser, L. C. and J. S. Steinhart, (1964). Explosion seismology i n the Western Hemisphere, i n Research i n Geophysics, 2. So l i d Earth and Interface Phenomena, 123-14 7. Pavoni, N., (1966). Tectonic interpretation of the magnetic anomalies southwest of Vancouver Island. Pure and Appl. Geophys., 6_3, 172-178 . Press, F., (1960). Crustal structure in the C a l i f o r n i a Nevada region. J. Geophys. Res., 6_5 , 1039-1051 . Raff, A. D. and R. G. Mason, (1961). Magnetic survey o f f the west coast of North America, 40° N. Latitude to 52° N. Latitude. Bull. Geol. Soc. Amer. 72, 1267-1270. — 82 Research Group for Explosion Seismology, (1966). Explosion seismological research in Jpan, i n The Earth beneath the Continents, J. S. Steinhart and T. J . Smith, ed., 334-348. Ringwood, A. E. and D. H. Green, (1966). Petrological nature of the stable continental crust, i n The Earth Beneath the Continents, J, S. Steinhart and T. J. Smith, ed., 611-619. Roddick, J. A., (1966). Coast c r y s t a l l i n e b e l t of B r i t i s h Columbia, i n Tectonic History and Mineral Deposits of Western C o r d i l l e r a . Can. Inst. Mining and Met. Spec. Vol. 8, 73-82. Scheidegger, A. E. and P. L. Willmore, (1957). The use of a least-squares method for the in t e r p r e t a t i o n of data from seismic surveys. Geophysics, 2_2_, 9-22. Shor, G. G. J r . , P. Dehlinger, H. K. Kirk, and W. 0. S. French, (1968). Seismic r e f r a c t i o n studies o f f Oregon and northern C a l i f o r n i a . J. Geophys. Res., 73, 2175-2194. Smith, T. J., J. S. Steinhart, and L. T. A l d r i c h , (1966). Lake Superior c r u s t a l structure. J. Geophys. Res., 71, 1141-1172. Steinhart, J. S. and R. P. Meyer, (1961). Explosion studies of continental structure. Carnegie Inst. Wash. Publ. 622, 409. Sutherland-Brown, A., (1960). Tectonic h i s t o r y of the insular belt of B r i t i s h Columbia, in Tectonic History and Mineral Deposits of the Western C o r d i l l e r a . Can. Inst. Mining and Met. Spec. Vol. 8, 83-100. Ta t e l , H. E. and M. A. Tuve, (1955). Seismic explosion of a continental crust, i n Crust of the Earth, A. Poldervaart, ed., 35-50. Vine, F. J . , (1966). Spreading of the ocean f l o o r : new evidence. Science, 154, 1405-1415. Wheeler, J. 0., (1967). Eastern tectonic b e l t of western C o r d i l l e r a i n B r i t i s h Columbia, i n Tectonic History and Mineral Deposits of the Western C o r d i l l e r a , Can. Inst. Mining and Met. Spec. Vol. 8, 27-45. Walcott, R. I., (1967). The Bouguer anomaly map of south-western B r i t i s h Columbia. S c i e n t i f i c Report No. 15, Inst. Earth Sciences, Univ. B r i t i s h Columbia. 83 White, W. R. H., (1962). Structure of the earth's crust o f f Vancouver Island. Ph.D. thesis, Dept. of Physics, Univ. of B r i t i s h Columbia. White, W. R. H. and J. G. Savage, (1965). A seismic refrac-t i o n and gravity study of the earth's crust i n B r i t i s h Columbia. Bull. Seismol. Soc. Am., 55, 463-468. Wilson, J. T., (1965). Transform f a u l t s , oceanic ridges, and magnetic anomalies southwest of Vancouver Island. Science,' 150 , 482-485. Worzel, L. and G. L. Shurbet, (1955). Gravity interpreta-tions from standard oceanic and continental c r u s t a l sections, i n Crust of the Earth, A. Poldervaart, ed., 87-100. 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0053393/manifest

Comment

Related Items