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Seismic structure across the active subduction zone of western Canada Spence, George Daniel 1984

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SEISMIC STRUCTURE ACROSS THE ACTIVE SUBDUCTION ZONE OF WESTERN CANADA by GEORGE DANIEL SPENCE M.Sc. The U n i v e r s i t y Of B r i t i s h C o l u m b i a , 1976 B.Sc.(Hon.) The U n i v e r s i t y Of C a l g a r y , 1971  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS  FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES D e p a r t m e n t Of G e o p h y s i c s And A s t r o n o m y  We a c c e p t t h i s to  the  t h e s i s as  required  conforming  standard  THE UNIVERSITY OF B R I T I S H COLUMBIA July  ©  George  1984  Daniel  Spence,  1984  In p r e s e n t i n g t h i s t h e s i s requirements  i n p a r t i a l f u l f i l m e n t of  f o r an a d v a n c e d d e g r e e a t  the  University  of B r i t i s h Columbia, I agree that  the L i b r a r y  s h a l l make  it  and s t u d y .  I  freely available  for reference  agree that p e r m i s s i o n f o r e x t e n s i v e for  s c h o l a r l y purposes  copying of  understood that for  financial  copying or p u b l i c a t i o n of t h i s  UfcOp"y^>  ^  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 2075 W e s b r o o k P l a c e V a n c o u v e r , Canada V6T 1W5 Date  (2/79^  this  thesis  It  is thesis  g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n  permission.  Department o f  further  may be g r a n t e d by t h e h e a d o f my  d e p a r t m e n t o r by h i s o r h e r r e p r e s e n t a t i v e s .  >E-6  the  rTLirUHOWuj  i i  For t h e i r c o n s t a n t s u p p o r t and c o n c e r n , I d e d i c a t e t h i s t o my p a r e n t s , G e o r g e D . S p e n c e a n d D i a n S p e n c e .  thesis  iii  ABSTRACT  The V a n c o u v e r I s l a n d in  1980  Fuca  to study  plate  principal offshore array  Seismic  the s t r u c t u r e  and  the  seismic  32  refraction  receivers  line  was  3 ocean  bottom seismometers  eastern  end o f  the l i n e , 100  interpretation reversed  to  ( l i n e I) the  techniques  were  located  profile.  of  the with the  along  the  for  the  Control  a n d by t h e m a r i n e r e f r a c t i o n  this  An  p r o f i l e was p r o v i d e d by a  (Waldron 1982).  are  procedure from  To a i d  complex  Island profile  i n t h e m o d e l i n g of  region, i n the  initial  explosions  model and f o r  two-dimensional  specified  Perturbations simultaneously  was an i t e r a t i v e in  r e c o r d e d on t h e same s e t  one o r more b l o c k s and  the  on  Two s h o t s were f i r e d a t  h a v e been d e v e l o p e d a n d a p p l i e d  first  locations  using  of  1983)  structure  traveltimes  the  margin.  and e x t e n d e d o f f s h o r e  17 s h o t s  The  two  practical  interpretation  I .  The  for  (OBS's).  de  was a 350 km o n s h o r e -  continental  of t h e o n s h o r e - o f f s h o r e  on t h e O B S ' s  the s e i s m i c  for  Juan  r e f r a c t i o n p r o f i l e a l o n g t h e l e n g t h of V a n c o u v e r  recorded  line  conducted  l o c a t e d on t h e A m e r i c a p l a t e  and  km  (McMechan a n d Spence  of  was  the s u b d u c t i n g oceanic  m a i n l a n d and a c r o s s Vancouver I s l a n d ,  westernmost  (VISP)  o v e r r i d i n g c o n t i n e n t a l America p l a t e .  p r o f i l e perpendicular  of  of  Project  of  subsequent  shots  receivers. iterations  r a y t r a c i n g . The m o d e l i s  the  positions  are  parameters  u s i n g a damped l e a s t  several  Traveltimes are  computed  represented  allowed are  technique  at  i n which the v e l o c i t y , the v e l o c i t y  boundary to  which  inversion  then  squares method.  by  gradient, to  vary.  determined  i v  Second, t h e o r y has  a fast,  been  seismograms scheme i s the  u s e d as  with  i n the  model  a  expressions amplitude  developed  uniform are  refractions,  surface  reflections  algorithm  were  dips  at  the  subducting  until  depth below  it  applied  as  reflector  may  lithosphere.  (4)  crust,  posi t i o n .  when t h e  and  Simple  which blocks,  analytic  calculated wide-angle  for  the head  reflections,  (1)  f e a t u r e s of The  l a n d w a r d of  dips  at  Vancouver  correspond A segment o f  slope, the  of  lithosphere  so t h e bend  foot  the  (3)  of  the  base  An  of  the  refraction  the  in  slope.  continental  t h e c o n t i n e n t a l Moho a t Island.  to  the  oceanic  14-16° beneath  beneath  seismogram  interpretation  the c o n t i n e n t a l  occurs  passes  and s y n t h e t i c  the  The m a j o r  upper the  high-velocity material  37 km mantle  subducting above  the  w i t h v e l o c i t y 7.7 km/s and d e p t h r a n g e ~ 2 0 - 2 5  km, may r e p r e s e n t a remnant detached  in  follows:  crust  western  in  multiples.  3° or l e s s beneath  The s u b d u c t i n g  tracing  b o t h t h e r a y t r a c i n g and f o r  A m p l i t u d e s may be  profile.  slab  The same r a y  gradient.  ray  synthetic  by l a r g e p o l y g o n a l  i n v e r s i o n procedure  model a r e  of  i n v e r s i o n method,  represented  pre-critical  both  onshore-offshore  downgoing  calculation  traveltime  used f o r  and  The t r a v e l t i m e  shelf  the  velocity  thus  waves,  (2)  for  is  computations.  structural  a l g o r i t h m b a s e d on a s y m p t o t i c  through two-dimensional media.  velocity  each  efficient  subduction  of  subducted  zone  lithosphere,  jumped w e s t w a r d  to  its  perhaps present  V  TABLE OF CONTENTS  Abstract  i i i  T a b l e Of C o n t e n t s  v  List  Of T a b l e s  vii  List  Of F i g u r e s  viii  Acknowledgements  xi  Chapter  1  1.  Introduction  1.1  Tectonic  Setting  1.2  Geophysical  1.3  The V a n c o u v e r  3  Studies  5  Island  Seismic Project  (VISP)  12  1.3.1  Program D e s c r i p t i o n  12  1.3.2  Interpretation  15  1.3.3  Interpretation  Of L i n e I V Of  Marine  Line  I  : OBS 1 To  OBS 5 1.4  17  O u t l i n e Of The T h e s i s  Chapter  2.  Ray  Laterally  Tracing  21  And  Traveltime  Inversion  In  V a r y i n g Media  24  2.1  Introduction  24  2.2  V e l o c i t y M o d e l And Ray T r a c i n g  29  2.3  Theory  30  2.3.1  The F o r w a r d P r o b l e m  2.3.2  Damped L e a s t S q u a r e s I n v e r s i o n  2.4 Chapter  Tests With A r t i f i c i a l 3.  Practical  .37  Data  Synthetic  V a r y i n g Media C a l c u l a t e d  31  39 Seismograms For  Laterally  By A s y m p t o t i c Ray T h e o r y  55  3.1  Introduction  55  3.2  V e l o c i t y M o d e l And Ray T r a c i n g  59  3.3  C a l c u l a t i o n Of A m p l i t u d e s And S y n t h e t i c S e i s m o g r a m s 3.3.1  R e f l e c t e d And R e f r a c t e d  3.3.2  Head Waves  3.3.3  A l t e r n a t i v e Approach For R e f l e c t e d  60  Rays  60 65 And  Direct  Rays 3.3.4  66  Seismogram  Synthesis  67  3.4  Results  68  3.5  Discussion  83  Chapter  4.  Interpretation  Of  Onshore-Offshore  Profile  Across Vancouver I s l a n d  85  4.1  Introduction  85  4.2  I n t e r p r e t a t i o n Of S h o t s J1  4.3  D e s c r i p t i o n Of P S e r i e s S h o t s  4.4  Ray T r a c i n g And T r a v e l t i m e I n v e r s i o n : S h o t s P 1 9 ,  And J 2  87 94  And P8 4.5  Final  P13 1 00  Onshore-offshore  R e f r a c t i o n Model  4.6 A l t e r n a t e Models C o n s i s t e n t  W i t h The S e i s m i c  107 Data  ..133  4.7  G r a v i t y M o d e l A c r o s s The S u b d u c t i n g M a r g i n  140  4.8  Discussion  148  References Appendices:  159 Additional  Record Sections  166  A . 1 Common S h o t R e c o r d S e c t i o n s  166  A.2 Selected  181  Common R e c e i v e r R e c o r d S e c t i o n s  vii  L I S T OF TABLES  2.1  Parameters for traveltime  two-layer  test  inversion  2.2  Parameters for  4.1  Parameters a f t e r inversion  model u s e d t o  for  ..42  subduction final  zone  t e s t model  iteration  l i n e I dataset  of  50  traveltime 113  vi i i  L I S T OF FIGURES  1.1  T e c t o n i c map of  the c o n t i n e n t a l margin  1.2  G r a v i t y model a c r o s s s o u t h e r n Vancouver I s l a n d  9  1.3  L o c a t i o n map s h o w i n g r e f r a c t i o n l i n e s  13  1.4  Velocity  1.5  Alternative velocity-depth profiles  1.6  Velocity  2.1  Ray p a t h c h a n g e s due t o p e r t u r b a t i o n of a b o u n d a r y  34  2.2  I n f i n i t e s i m a l p e r t u r b a t i o n of a b o u n d a r y e n d p o i n t  36  2.3  T w o - l a y e r model used t o t e s t  41  2.4  T w o - l a y e r model t r a v e l t i m e c u r v e s  structure  along  structure  iterations  of  line  S t a r t i n g model f o r  2.6  F i n a l model f o r  2.7  Initial  I and I V  IV  16 for  line  a l o n g marine p o r t i o n of  ray t r a c e  2.5  2  traveltime  IV  line  18 I  inversion  after  successive  i n v e r s i o n procedure  44  s u b d u c t i o n zone t e s t  47  s u b d u c t i o n zone t e s t  and f i n a l  traveltime curves  19  49 for  subduction  zone t e s t  50  2.8  Nonuniqueness  of m o d e l s  i n s u b d u c t i o n zone t e s t  3.1  Geometry of  3.2  Vertical  3.3  Synthetics  3.4  Rays a n d s m p l i t u d e s  3.5  The I m p e r i a l V a l l e y v e l o c i t y - d e p t h m o d e l  78  3.6  Rays a n d s y n t h e t i c s  for  79  3.7  Rays a n d s y n t h e t i c s  f o r a s u b d u c t i o n zone m o d e l  82  4.1  Shot J1 , s y n t h e t i c s  and d a t a  88  4.2  V e l o c i t y m o d e l and r a y t r a c i n g d i a g r a m  4.3  Observed data  the ray tube at  the  component a m p l i t u d e s for  53  i - t h interface  for a two-layer  61 model....70  t h e HILDERS v e l o c i t y - d e p t h m o d e l  for  for a simple  the  73  2D m o d e l  77  I m p e r i a l V a l l e y model  s h o t s P 1 9 , P13 a n d P8  for  shot  J1  91 97  ix  4.4  Observed data  receivers  4.5  Ray t r a c i n g d i a g r a m  4.6  Traveltime curves  X45 and X22  98  s h o t s P 1 9 , P13 and P8  102  for  after  one i t e r a t i o n of  inverse  procedure  105  4.7  Final  v e l o c i t y model f o r  4.8  Details  4.9  Ray t r a c e  4.10  Shot P 1 9 , s y n t h e t i c s  and d a t a  116  4.11  Shot P 1 3 ,  and d a t a  117  4.12  Shot P 8 ,  4.13  Shot  4.14  Receiver X45, synthetics  and d a t a  120  4.15  Receiver X34, s y n t h e t i c s  and d a t a  121  4.16  Receiver X22, synthetics  and d a t a  .122  4.17  Receiver X6, synthetics  4.18  Shadow z o n e s  of  ray t r a c e used  line  I....  110  model  111  i n inverse procedure  synthetics  for  final  model....112  synthetics  and d a t a  118  P2, synthetics  and d a t a  ...119  and d a t a  123  c a u s e d by c o r n e r where s u b d u c t i n g  slab  and c o n t i n e n t a l Moho meet 4.19  S h o t P 1 9 a n d P13 s y n t h e t i c s  129 w i t h 8.6  km/s b e l o w  reflector 4.20  1 32  P r e l i m i n a r y m o d e l of E l l i s  et  al..  (1983) w i t h a d d i t i o n a l  upper m a n t l e r e f l e c t o r  135  4.21  V e l o c i t y model w i t h k i n k  in subducting oceanic  4.22  Final  g r a v i t y model a l o n g l i n e  4.23  Final  v e l o c i t y m o d e l and s t y l i z e d t e c t o n i c m o d e l  4.24  E x t e n t of  4.25  Depths  I  high v e l o c i t y m a t e r i a l at  144  20 km d e p t h  from r e f l e c t i o n s e c t i o n compared  refraction  model  crust...137  149 152  to 157  X  A1 . 1 Shot J 2 ,  observed  record section  167  A 1 . 2 Shot  P1 , o b s e r v e d  record section  168  A 1 . 3 Shot  P3, observed  record section  169  A 1 . 4 Shot  P4, observed  record section  ...170  A 1 . 5 Shot  P5, observed  record section  171  A 1 . 6 Shot P 6 , o b s e r v e d  record section  .172  A 1 . 7 Shot  P9, observed  record section  173  A1 . 8 Shot  P10,  observed  record section  174  A1 . 9 S h o t  P12,  observed  record section  175  A 1 . 1 0 Shot A1 . 1 1 Shot A 1 . 1 2 Shot A1 . 1 3 S h o t  P14, P15, P16, P17,  A1 .1 4 S h o t P 1 8 ,  observed  record section  observed observed  record section record section  observed observed  record section record section  A2.1 R e c e i v e r X 2 , observed  record section  176 177 178 179 180 182  A2.2 Receiver X13, observed  record s e c t i o n . . .  183  A2.3 Receiver X15, observed  record section  184  A2.4 Receiver X17, observed  record section  185  A2.5 Receiver X19, observed  record section  186  A2.6 Receiver X23, observed  record section  187  A2.7 Receiver X31, observed  record section  188  A2.8 Receiver X35, observed  record section  189  A2.9 Receiver X40, observed  record section  190  A2.10 Receiver  X43, observed  record section  191  xi  ACKNOWLEDGEMENTS  First  and f o r e m o s t ,  I want t o t h a n k my w i f e M a r i a ,  n e a r l y a l l t h e t y p i n g and f i n a l thesis.  But  mostly  her  own  doctoral thesis  T h r o u g h o u t my r e s e a r c h h a v i n g two s u p e r v i s o r s ,  advice, I  to  them  so c r i t i c a l  should  overall  also  (rather  i n t h e n o t so d i s t a n t  while  Bob  was  their  enthusiastic  to  thank  in  interpretation writing  of  opportunity the  to  logistics  sabbatical,  s t a g e s of  declining  to  on be  Ron the  provided as  the s y n t h e t i c seismogram count  research. for  the  of  the  field  Bob i n p a r t i c u l a r p r o v i d e d  Ron  the t h e s i s ,  and  participate  f o r my o r g a n i z a t i o n a l e n d e a v o r s .  could almost always graciously  of o n e ' s  Ron and Bob i n g e n e r a l  organizing  I am  encouragement  i n d e f i n i n g the d i r e c t i o n  on  past,  I have h a d t h e d i s t i n c t a d v a n t a g e o f  p r o g r a m . W h i l e Ron was on s a b b a t i c a l , t h e needed g u i d a n c e  having  someone who h a d s u r v i v e d .  d e s i g n and f o r the  heavily!)  this  D r . R . M . C l o w e s and D r . R . M . E l l i s .  for  like  project  for  w h i c h meant so much t o me. And  she p r o v i d e d a s t e r l i n g e x a m p l e o f  grateful  preparation  I want t o t e n d e r my s i n c e r e t h a n k s f o r h e r  unwavering moral support, completed  diagram  who d i d  well  as  Similarly,  stimulus advice  in  routine. Finally, to  uplift  victor  my  during the  I felt  spirits  i n most o f  our  I by  darts  matches. D i s c u s s i o n s w i t h Ken W h i t t a l l , synthetic the g r e a t e r  seismogram part  t r a c i n g code, appreciate  r o u t i n e , are  of t h i s t h e s i s  especially  related  greatly appreciated.  with  David  Waldron  concerning  it. his  the  Perhaps  s h o u l d be d e d i c a t e d t o K e n ' s  s i n c e much o f my work was b a s e d upon  talks  to  I  ray also  marine  xi i  interpretation, was c o m p l e t e d thankfull  and I am i n d e e d g r a t e f u l  who I f e e l  in  of  matters  addition,  is  the t r u e c o r e of  instrumentation  Bob p r o v i d e d t h e g r e a t e r  long-standing  semi-regular  from the deadening  part  was  only  possible  the  Geoscience  Centre  privilege  to  organization discussing  was  work of  the  of  For of  the  Dr.  Roy  It  Hyndman,  the marine o p e r a t i o n  and  at  Centre  and t h e A t l a n t i c  Geoscience  i s a l s o made t o t h e D e p a r t m e n t  Oceans  for  Canada Forces  Pacific  for  Diving  Unit,  explosives  the p r o j e c t  participation  use  through  the  at  the  CFAV  Pacific sea,  is  could of  of  the  Endeavour.  Project many  phase  of  the  the  Pacific  particularly both  during  later  Command,  who  not  been  of  Centre.  a the  stages  in  Pacific Grateful  of F i s h e r i e s  and the  Establishment  The a s s i s t a n c e o f  appreciated.  members  needed  of  Research  especially have  which  t h e s h i p CSS V e c t o r a n d t o  Defense  Maritime  for  effort  from was  In  Bob.  Seismic  marine  personnel  indispensable.  with  projects.  Merci,  joint  the  to  motivation  geophysics.  acknowledgement  Canadian  field  my i n t e r p r e t a t i o n . O B S ' s were s u p p l i e d by t h e  Geoscience  to  group  but a l s o s u p p l i e d a  through  participation  goes  seismology  the Vancouver I s l a n d  i n d i v i d u a l s and o r g a n i z a t i o n s . project,  of  also  noon h o u r F r e n c h s e s s i o n s ,  effects  Data a c q u i s i t i o n f o r  the  and r e l a t e d  n o t o n l y were r e w a r d i n g i n t h e m s e l v e s  of  I am  i n t e r p r e t a t i o n to c o m p l e t i o n , i n c l u d i n g  w i t h i n a s i x week p e r i o d . My g r a t i t u d e  Bob M e l d r u m ,  (VISP)  interpretation  and c o n s t a n t l y amazed t h a t G e o r g e McMechan was a b l e  write-up,  break  his  i n t i m e t o p r o v i d e c o n t r o l f o r m i n e . As w e l l ,  b r i n g t h e m a j o r i t y of h i s  our  that  the  Fleet  detonated  the  The o n s h o r e  phase  undertaken  t h e CO-CRUST g r o u p ,  without  the  who p r o v i d e d  equipment,  scientific,  contributions. people  who  took  acknowledged. companies areas  Bloedel  particular, part  We  in  are  operation, station Ltd.,  also  location:  field  for  Personal Scholarship Council  Rayonier  Forest  Ltd.  financial  support  the  Canada  Forest  are  30  gratefully  following access  forest  to  Ltd.,  their  detailed MacMillan  Products  was p r o v i d e d by a  from the N a t u r a l S c i e n c e s and  Co. L t d . , Ltd.,  and  Postgraduate  Engineering  Research  (NSERC) and by an H . R . M a c M i l l a n F a m i l y F e l l o w s h i p f r o m  program  B r i t i s h Columbia. P r i n c i p a l  was  provided  by  Energy,  from  g r a n t s of  Ellis  to  the n e a r l y  Products L t d . , Tahsis  E a r t h P h y s i c s B r a n c h of  been  program  financial  on a c c e s s and s a f e t y a n d  Industries  t h e U n i v e r s i t y of field  of  providing  Crown Z e l l e r b a c h Canada L t d . , B . C . Western Forest  and  efforts  indebted  advice  Canadian  the  the  on V a n c o u v e r I s l a n d  of  maps f o r  In  organizational  NSERC  operating  supported  OSU80-00137  M i n e s and R e s o u r c e s  by NSERC o p e r a t i n g  and R . M . C l o w e s ,  contract  support  the p a r t i c i p a n t s .  for  the  from t h e  Canada  and  Analysis  has  g r a n t s A2617 a n d A7707 t o R . M .  respectively.  1  CHAPTER K_ INTRODUCTION  In  August  Seismic  Project  seismic  1980,  CO-CRUST  1  conducted the Vancouver  ( V I S P ) , a s e r i e s of  experiments  bottom seismographs  that  refraction  utilized  (OBS's)  with  both  and  reflection  land-based  explosive  and  Island  and  airgun  ocean energy  sources. This principal purpose  dissertation  r e f r a c t i o n p r o f i l e of was  mantle depths volcanic  arc  to  obtain  of  with other  gravity,  heat  major  plate a  role  a  the  Juan  model  First,  m o d e l t o be a p p l i e d  and i n the Coast  or  m i n e r a l d e p o s i t s was c o n s i d e r e d today in  the t e c t o n i c  the  location  setting of  is  mineral  the  to  upper inland  (Fig.  as  1.1).  seismicity,  v e l o c i t y model  plays  t h e r e g i o n . The d e v e l o p m e n t two  important  the p o t e n t i a l  for  In the p a s t , t o be  s e e n as  resources,  of  the  tectonic resources Vancouver  the d i s t r i b u t i o n of  somewhat a major  and  practical  f o r m i n e r a l r e s o u r c e s on  Range.  which  to the  i n the e x p l o r a t i o n for hydrocarbon  on t h e c o n t i n e n t a l s h e l f , Island  is  the  section  plate  such  of  the contemporary t e c t o n i c s  has  there  America  a seismic  understanding  for  de F u c a p l a t e  studies  i n t e r a c t i o n c o m p l e x i t i e s of  implications.  structural  continental  f l o w and m a g n e t i c s ,  seismic/tectonic  interpretation  the experiment,  geophysical  in  the  seismic  from the o c e a n i c  Together  a  concerns  haphazard;  but  c o n t r o l l i n g factor  a n d methods a r e  being  CO-CRUST ( C o n s o r t i u m f o r C r u s t a l R e c o n n a i s a n c e using Seismic T e c h n i q u e s ) i n c l u d e d i n 1980 p a r t i c i p a n t s f r o m t h e E a r t h P h y s i c s Branch (Ottawa), P a c i f i c Geoscience Centre, A t l a n t i c Geoscience C e n t r e , and the Universities of Alberta, British Columbia, M a n i t o b a , S a s k a t c h e w a n , and W e s t e r n O n t a r i o . 1  FIG.  1.1. Tectonic map of the southern British Columbia c o n t i n e n t a l margin showing the main l i t h o s p h e r i c boundaries and plate motions relative to North America. Quaternary v o l c a n o e s a r e i n d i c a t e d by s o l i d t r i a n g l e s and p r o f i l e 2 i s the l o c a t i o n of the gravity model cross section across southern V a n c o u v e r I s l a n d a s s h o w n i n F i g . 1 . 2 . TWs = T u z o W i l s o n seamount; PRfz = P a u l Revere f r a c t u r e zone; Sfz = Sovanco f r a c t u r e zone.  3  developed  which  exploration  use  (Rona  the  tectonic  1980;  Mitchell  setting and Garson  significant  a p p l i c a t i o n of  a seismic/tectonic  assessment  of  risk,  earthquake  high population density placed.  where  density,  but  facilities  seismicity  pattern  earthquakes.  and  As w e l l ,  specifically  for  also  thus  the  accurate  guide The  model  is  in  low  velocity  may be the  generating  model of  of  population  understanding  processes  location  the  not o n l y a r e a s  of  for  to  second  to energy development  necesssary  a realistic  the  areas  a  1981).  which a f f e c t s  related  The t e c t o n i c m o d e l i s  as  is  the  large  required earthquakes  themselves.  1.1  Tectonic Sett ing In the Vancouver I s l a n d  convergent, being (Fig.  w i t h the o c e a n i c  subducted 1.1).  Based  on  magnetic  de F u c a - A m e r i c a a n d l e s s (Riddihough  left-lateral between  the  frame  of  and  and  of a b s o l u t e  for  reference,  subducting  it  is  over-ridden  possible  i n an a b s o l u t e  by  the  that  sense  present  for  the  is  Juan  accommodated  Nootka  fault  fixed to  a  hot  to  southwestward motion of  some  al. spot  the E x p l o r e r p l a t e  are  by  zone  (Hyndman e t  (Riddihough 1981).  de F u c a p l a t e s  plate  the E x p l o r e r - A m e r i c a  difference  motions  is  plates  the  3 cm/year  J u a n de F u c a p l a t e s plate  regime  Explorer  patterns,  about  than 2 cm/year  b o t h t h e E x p l o r e r and J u a n  plate.  anomaly  The  tectonic  the c o n t i n e n t a l America  r a t e s are  1977).  Explorer  In terms  being  de F u c a  s t r i k e - s l i p movement a l o n g t h e  1979).  stopped  Juan  o b l i q u e l y beneath  p e r p e n d i c u l a r convergence  plates  region the p l a t e  has  Thus, extent  the America  4  The p r e s e n t plate  is  structure  at  c o n t r o l l e d by t h e p a s t p l a t e  margin,  i n w h i c h e p i s o d e s of  periods  of  strike-slip  modifications additional America  to  grown  lithosphere terranes  New  or  is  compared  those  paleomagnetic  Wrangellia t h e Queen Oregon  in the  block,  1977).  et  of  al.  km,  blocks  of  exotic  w h i c h have  been  of  origin.  A  unique s t r a t i g r a p h y  relative  to  craton,  by and  that  different in  some  faunas cases  t h e y h a v e been  rocks  Island  by  rotated  since  Karmutsen b a s a l t  flows,  the  results moved least  from  probably  represents  eastern  Island,  Group)  middle  which  are  minor  Karmutsen F o r m a t i o n  (Muller  the  km, and  late Triassic  and  Alaska,  and  northward  1300  dispersed  o v e r l a i n by up t o 6000 m of  pillow  Triassic  the  Island,  Vancouver  Sicker  arcs are  upper  by a t  On  is  now i n s o u t h e a s t  Vancouver  (the  island  has  terranes  which are  1977).  lavas,  the  American c r a t o n 4900  the  of  North  terranes,  some o f  craton,  Islands,  Paleomagnetic  Vancouver  addition  from t h e i r s i t e s  the  p i e c e s of  volcanic  of  western  recognized  Charlotte  pillow  sediments  that  which i n d i c a t e  best  characteristic basaltic  suggests  an  latitude.  (Jones  Paleozoic  on  results  or d i s p l a c e d Among  and  introduced  microplates,  its  Recently,  have  terranes),  by  1982a).  theory  piecemeal  kilometers  terranes to  (Riddihough  called  recognized  neighboring  with  the  allochthonous  c a r r i e d thousands of  s u b d u c t i o n may have a l t e r n a t e d  evidence  (variably  America that  tectonic  by  tectonic  the  h i s t o r y at  motion  plate  factor.  has  terrane  the western margin of  breccias  Karmutsen  show  that  relative  t o the  North  possibly  as  much  as  ( Y o l e and I r v i n g 1 9 8 0 ) .  The  rifting  related  to  the  5  commencement  of  northward  W r a n g e l l i a was a c c r e t e d has  been  It  by m i d - C r e t a c e o u s  fragmented  intraplate  movement.  by  time,  thrusting  strike-slip faults  is  and  (Coney e t  al.  proposed and  I t has  t e r r a n e s have modern  analogs  in  plateaus,  and  10% of  seamounts  the ocean  plateaus  are  floor  some  volcanic  comparable  to continents  subduction  on  margin.  likely  thrust  is  that  w i t h the c o l l i s o n p r o c e s s , thickened Perhaps  to the  1.2  Geophysical It  is  reflects  it  reviewed  generally  support  accepted  a  deformation active  relevant  the case  includes  for  with  and  a  about  of  the  density,  continental associated  w o u l d be new et  al. as  crust 1982).  part  of  such a h i s t o r y .  s u b d u c t i o n has  the l a s t  geological subduction.  sediments  andesitic  geophysical  Many  of V a n c o u v e r I s l a n d ,  that  sedimentary-filled of  oceanic  which comprise  (Jones  several  is c u r r e n t l y o c c u r r i n g . Riddihough the  exotic  large  1981).  is  Studies  Canada's western margin over that  accretion  f a u l t i n g w o u l d be  proportions  structure  the W r a n g e l l i a t e r r a n e ,  the  in thickness  collision  along  suggested that  a n d t h e end r e s u l t  continental present  al.  then  1980).  of  ridges,  (Ben-Avraham et  and would r e s i s t It  been  since  translation  The g r o w t h o f w e s t e r n N o r t h A m e r i c a by t e r r a n e c o m p l e x and p o o r l y u n d e r s t o o d .  that  evidence  along  volcanism includes  t h e J u a n de F u c a p l a t e ,  the  and  million  and  The  geological  margin  the  the c l a s s i c high-low  (1976)  data  which  compressive  slope,  and  the  mountains.  The  magnetic  pattern  and  information  trench,  Cascade  along  years,  Hyndman  geophysical  the c o n t i n e n t a l of  occurred  of  lineations the  on  gravity  6  field  at  the  continental  low v a l u e s above volcanic  t h e downgoing  arc,  geophysical presented  and  the  Island/Puget  seismotectonic  models  of  occurs  seismicity  localization  related  margin  in  the ocean (1983)  have  This  southern  may  was  has  produced  concentration  Sound  area.  This  of p h a s e  changes  i n the  o f Puget  the  bend  Sound  place  in  (Rogers beneath  Nootka  in fact fault  under V a n c o u v e r  basin  have  the l a r g e plate  n o t be zone  Island  where t h e f a u l t  is  zone  central  interaction  would  is.defined.  by t h e on t h e  be e x p e c t e d t o be  of t h e E x p l o r e r  plate  the  different  with the America  p r o d u c e upward p r e s s u r e  of  directly  earthquakes are  over-ridden  the  1983).  because  s t o p p e d s u b d u c t i n g i n an  i s being  boundary  the  f o r the  corresponding to the extension  the  and  margin  pattern  A strong  with  taken  of t h e E x p l o r e r  the E x p l o r e r  the  result  argued that  may  near  comprehensive  and  Puget  association  along  Explorer  America p l a t e .  the  zone. However, t h e y may  ( R i d d i h o u g h 1981)  plate,  near  in  near a l i n e  to the i n t e r a c t i o n  sense  region  the  of the s e i s m i c i t y  the  A  the s e i s m i c i t y  at the l a t i t u d e  motion  Rather, Rogers  Since  within  earthquakes  fault  to  that  values  from  (1979).  Sound  possibly  Island,  character  high  to account f o r i t .  lithosphere  t h e Nootka  from  is  large  Vancouver  to  i n heat flow  seismicity.  (1983) has d e s c r i b e d  Vancouver  continental  plate  local  by Keen and Hyndman  descending  t h e change  r e v i e w of t h e w e s t e r n Canada c o n t i n e n t a l  Rogers  Several  margin,  due  plate.  absolute America overlying greatest  where i t i s  thickest. The divided  seismicity, into  two  particularly distinct  i n t h e Puget  Sound r e g i o n ,  g r o u p s - a s h a l l o w one  is  with depths  7  less  t h a n 30 km, and a d e e p e r one where t h e  about  40  to  earthquake  70  the Puget  in  depth  were  1970  of  greatly  improved  t h e d e n s e n e t w o r k of  of  the earthquake  seismicity  Benioff  zone,  network  was  stations  in  was  the  Olympic  recent  upgrading  (1983),  a s i m i l a r d i s t r i b u t i o n of  southern  Vancouver  where t h e B e n i o f f  (1981),  with  Island was  the B e n i o f f  11  a  deeper classic  permanent  zone.  of  Taber  With  the  D a t a F i l e by R o g e r s  e a r t h q u a k e s was  seen  and s o u t h e r n G e o r g i a observed  in  In  the  and the r e s u l t s  the Canadian Earthquake  zone  seismographs  1 1 ° . The W a s h i n g t o n  westward  Peninsula,  the  Washington.  the c o n t i n e n t at  improved the d e l i n e a t i o n of of  of  d a t a by C r o s s o n  expanded  from  with  s e e n t o be d i s t r i b u t e d a l o n g a  d i p p i n g under recently  range  d i s t r i b u t i o n and l o c a t i o n of  Sound r e g i o n by t h e U n i v e r s i t y  c o m p i l a t i o n of  (1983)  The  hypocenters  establishment  zone  km.  depths  dipping  in  Strait  at  the  region,  12°  to  the  northeast. Geodetic subduction.  d a t a a l s o have p r o v i d e d d i r e c t In a p r e c i s e  l e v e l l i n g survey  across western Washington, a pattern coast  and  consistent  subsidence w i t h the  further  subduction  N o r t h A m e r i c a (Ando and B a l a z s northward  into  The q u e s t i o n a seismic  of  inland  of  aseismic  subduction  crustal  tilt  is  uplift is  on  implied  a n d by t h e l a c k o f  by  their  is  be  under  continues 1982b).  occurring in  (1979) a r g u e  that  down-to-the-continent  any l a r g e t h r u s t  W a s h i n g t o n and Oregon i n h i s t o r i c a l t i m e  outer to  (Riddihough  Ando and B a l a z s  period  plate  1 9 7 9 ) . The same p a t t e r n region  current  the  considered  whether the s u b d u c t i o n  o r an a s e i s m i c mode.  for  o v e r a 70 y e a r  t h e J u a n de F u c a  the Vancouver I s l a n d  then remains  support  (the past  earthquakes 140  in  years).  8  Aseismic  slip  calculations earthquake than that  is  suggested  by  the  o f Hyndman and W e i c h e r t ( 1 9 8 3 ) ,  rate  f o r Puget  expected  contrary  also  to  Sound was a t  slip  rate.  comes  from  accumulation  perpendicular  continental consistent strain  margin.  implies  of  that  gravity  field  margins.  His  (1979)  thrust  less  evidence strain  who f o u n d an to  the  accumulation plate,  but  earthquakes  is the  should  to r e c o n c i l e w i t h the evidence  carried  across  the  structural  shown i n F i g u r e  1.2.  control  In  from  he  further  hypocenters  i n the southern lie  be for  refraction,  that  Strait  and s u r f a c e  lithosphere  other a c t i v e  margins  The s e i s m i c i n t h e model o f 1  close to  (Barazangi  constraints Figure  was  interpretations  is  downgoing  1.2  placed require  determined of  Clowes  100 km, as  and I s a c k s  from and  has  wave  thermal  Sound  area  and  that  the  downgoing  been  found  for  1976).  further  Malecek  is  earthquake  plate  on M o h o r o v i c i c  the  on  - Puget  the v o l c a n i c c h a i n the depth to the top of  oceanic  1.1)  section,  deepest  of G e o r g i a  the  this  Based  the  the  Washington  2 (Fig.  of  available.  assumed  and  profile  construction  where  beneath  Columbia  along  reflection,  arguments,  d e t a i l e d m o d e l l i n g of  British  the  were u s e d  ( 5 0 - 7 0 km) c a n n o t  out  model  interpretations  point  strain  the  subduction.  Riddihough  beneath  of  (1981),  t h e J u a n de F u c a large  which i s d i f f i c u l t  aseismic  seismic  direction  with subduction  rate  expected,  The  strain  10  geodetic  al.  compressive  of  However,  m e a s u r e m e n t s i n W a s h i n g t o n by S a v a g e e t of  release  who showed t h a t  least a factor  from the c o n v e r g e n c e  aseismic  energy  (Moho)  comment.  seismic (1976)  and  depths Control  refraction Keen  and  Georgia  SW FIG.  NE 1.2. G r a v i t y model cross section across southern Vancouver I s l a n d (from R i d d i h o u g h 1979). Gravity profiles are f r e e o v e r sea and Bouguer o v e r l a n d ; s o l i d = o b s e r v e d , dash = calculated. Densities are i n grams per cubic centimetre. Note: 1 mGal = 10~5 m / s 2 . B a r s are s e i s m i c c o n t r o l p o i n t s ; numbers a r e s o u r c e s : ( l ) C l o w e s and Malecek (1976), Keen and B a r r e t t ( 1 9 7 1 ) ; (2) a n d (2 ) T s e n g ( 1 9 6 8 ) ; (3)Wickens (1977); (4)White et al. (1968); (5)Shouldice (1971 ) .  10  Barrett  (1971).  interpretations profile  of  Unfortunately,  are  l o c a t e d at  Figure  1.2  the Clowes and Malecek Explorer  Ridge  northwest, the  northwest  fault  (1976)  study  is  corner  distances  located  fracture  of  Figure  point  point to  1  4 at  (1975)  interpreted  Vancouver  Island  Forsyth  structure  associated  with of  Seismic  is  their  of  the  wave  model  the S t r a i t  1.2)  is  of  points  less  as  of  using  the  off  Clowes Nootka  assigned  and is  the  Forsyth southern in  the  generally  al.  (1968).  is  a change  there  a scattering  For  refraction  from  Moho d e p t h  2,  2*,  and  3  that  is  zone beneath  the  Depth e r r o r s  b u t an  error  estimate.  resolution  beneath  Control point  i n v e r s i o n of a r a t h e r  optimistic  lie  Georgia.  phase v e l o c i t i e s .  an  the  profile,  B e r r y and  sections  the  stringent.  d i f f i c u l t to determine,  arbitrary;  seismic  the e x i s t e n c e  b a s e d on t h e  assigned  Subsequently,  (1975) a l s o s u g g e s t e d t h a t  c o n t r o l at  (Fig.  surface  end  between V a n c o u v e r I s l a n d and t h e m a i n l a n d  eastern part  (1977)  (1968).  reversed  of  km t o  the  and the d e p t h  w i t h t h e Moho d e p t h o b t a i n e d by W h i t e e t  and  Island  near  to the B r i t i s h Columbia i n t e r i o r ,  segment  consistent  al.  complex  H o w e v e r , Au and  t h e c o n t i n e n t a l Moho was d e t e r m i n e d by t h e  of White et  Berry  the  agreement w i t h t h e i r r e s u l t s .  the n o r t h e a s t e r n  profile  eastern  in  in  located  the  environments;  (1971) p r o f i l e s  1.1.  these  from  zone r e g i o n 200  r e f r a c t i o n data  is  for  tectonic  t h e Keen a n d B a r r e t t  interpreted  control  depth  are  significant  z o n e a n d on t h e J u a n de F u c a p l a t e ,  control  in  profiles  and i n d i f f e r e n t  Sovanco  whereas  (1982) have  at  -  the  of  the  ±4  The bounds  matrix  3 of Wickens  scattered for  Vancouver  km are  obtained  set  of  boundaries could  be  somewhat in  the  11  generalized as  inverse  the d i s t a n c e  10%  drop-off  analysis  of  procedure,  off in  the d i a g o n a l amplitude.  Tseng  r e q u i r e d t o o b t a i n an  Interpretation  (1968), c o n t r o l p o i n t s  some d i f f i c u l t y . source-receiver  W i c k e n s (1977) d e f i n e d h i s  The  highest  separations  excess  gravity  data  depth,  a value  mantle  rather  this  require  which i s than the  problem  and  normally  of  suggested that  h y d r o u s e n v i r o n m e n t may  velocity In  of  has  30 3.3  presented even  km.  in  the  7.1±0.1 However,  the  R i d d i h o u g h (1979) the probable  for  g c m " 3 at of  this upper  considered  conditions  above  high pressure,  formation  f a c i e s w i t h the r e q u i r e d h i g h d e n s i t y  of  and  unusual  a n d l o w P-wave  characteristics. summary,  adequate  and c o n t i n e n t a l c r u s t s  to  g r a v i t y model i n F i g u r e  1.2.  constraints velocities  are  provided  generally  been o b s e r v e d .  models  constrain  the  between  the  exist end  of  the  by t h e s u r f a c e wave d a t a , of  oceanic  points  Beneath Vancouver I s l a n d ,  of  the  only  weak  and  P-wave  t h e u p p e r m a n t l e have  H o w e v e r , a r g u m e n t s s u c h as  mentioned above,  high density  seismic  characteristic  (1979)  made.  near  low t e m p e r a t u r e ,  result  refraction  300 km, a r e  characteristic  lower c r u s t .  the downgoing l i t h o s p h e r e  metamorphic  densities  arbitrary  observed,  km/s f o r a l a y e r w i t h t h e u p p e r b o u n d a r y n e a r the  the  2 and 2 * ,  velocities in  of  bounds  those  of  not  Riddihough  which r a t i o n a l i z e  the apparent  conflict  l o w P-wave v e l o c i t y d e t e r m i n e d  from s e i s m i c  data  r e q u i r e d by t h e g r a v i t y  interpretation,  have  and been  12  1.3  The V a n c o u v e r I s l a n d S e i s m i c In  there  l i g h t of  seems l i t t l e  lithospheric Vancouver  slab  of  the  under  Vancouver  s u b d u c t i o n zone s t r u c t u r e . of  the  existence  to  a  section,  subducting  The p u r p o s e  determine  if  Vancouver  p r o v i d e d by  Island other  interpretations S p e n c e 1983; complete  and  lines  have  Waldron  dissertation  principal  of  refraction  1982).  B e c a u s e of  1.3.1  Program d e s c r i p t i o n  including  al.  the  instrument  geometries,  and  are  of  these p r o f i l e s  of  a  the and o f  full  program r e l e v a n t  A  Island  are  the  of  brief  to t h i s  thesis  is  shown i n F i g u r e  1.3.  from the v o l c a n i c a r c  have  been  which  the  (McMechan and  significant  to  and Clowes  of  points  a in  here.  errors  given  the  line,  and r e f l e c t i o n  of  in  stations  relevance  shooting  description  an  line,  onshore-offshore  characteristics,  The r e f r a c t i o n p r o g r a m c o n s i s t e d them  their  refraction  discussion  for  elsewhere  a l s o be d e s c r i b e d  t o be f o u n d i n E l l i s  (1983).  experiment,  presented  interpretation will  Details  the  of  presents  mainland. Constraints  been  their  et  of  interpretation  description  location,  the  the  seismic  w h i c h s h o t s i n t h e d e e p o c e a n were r e c o r d e d on o n s h o r e on  of  c o u l d p r o v i d e more d e t a i l s  This  project's  of  Island.  P r o j e c t was  and r e f l e c t i o n methods  interpretation  (VISP)  o u t l i n e d i n the p r e v i o u s  doubt  Island Seismic  refraction the  the evidence  Project  in  and  program, recording  t i m i n g and  (1981)  and  site Ellis  the p o r t i o n s of  the  below. four p r o f i l e s ;  two  of  L i n e I extended a c r o s s Vancouver  t o the deep o c e a n ,  and l i n e  IV  was  13  1.3. Location map showing refraction reflection line (RL), and the gravity Bathymetry i s i n metres.  l i n e s I and I V , profile (GR)  14  shot to  along the  shelf  shot:  margin.  l i n e II  sources  has  For  i n the deep ocean,  into OBS's,  n o t y e t been  line I,  up t o  of  Georgia,  and l i n e I I I  using  on  only data  from the  OBS's  (Waldron  on t h e m i d - c o n t i n e n t a l  and a s e r i e s  designated  s l o p e and ocean Eighteen  as  additional  on t h e O B S ' s a l o n e .  crustal For the 1800  of  17 s h o t s  the  shots at  ranging  land  Finally,  sedimentary  a continuous  recorded along  and  II  along  in  the  structure,  although  have (J1  been  and  J2),  from  200  to  825  the  continental  marine  detectors.  for  recordings  profile  (CSP)  t h e m a r i n e p o r t i o n of  basement  In  t h e e a s t e r n end o f  seismic  f i r e d along p r o f i l e s  over  line  e a c h OBS,  depth,  and  to  upper  structure. line  IV,  38 l a n d s e i s m o g r a p h s were d i s t r i b u t e d  were  detonated  at  1.3  N,  A  s e i s m o g r a p h s were t h e n l o c a t e d a l o n g to obtain  kg  1.3  were f i r e d o v e r  150 km s e c t i o n N-A i n F i g u r e kg  825  Figure  7 km, were s h o t  into  a n d a 32 L a i r g u n was  determine  lines  region,  50 kg c h a r g e s were d e t o n a t e d  u s i n g a 5 L a i r g u n was I,  in  Two  the P s e r i e s ,  basin  of  on i s l a n d s  i n the o f f s h o r e  shown  1982).  s e p a r a t e d by a p p r o x i m a t e l y the p r o f i l e ,  and  the B r i t i s h Columbia m a i n l a n d .  4 O B S ' s were d e p l o y e d  kg,  explosives  32 l a n d s e i s m o g r a p h s were d e p l o y e d  addition,  interpreted  strike  completed.  and  3  parallel  Two a d d i t i o n a l l i n e s a l o n g  160 km r e c o r d i n g l i n e on V a n c o u v e r I s l a n d ,  Strait  F.  approximately  u s i n g an a i r g u n s o u r c e o n l y . I n t e r p r e t a t i o n  and I I I  a  l e n g t h of Vancouver I s l a n d  continental  were a l s o airgun  the  recordings  f r o m 1800,  and s h o t s of and  F,  900,  along  900  respectively.  the A-F s e c t i o n  of  900 a n d 900 kg s h o t s a t  line  and The IV  N , A and  15  The first in  reflection  was a  Fig.  was  composed  of  two p h a s e s .  10 km 1200% common d e p t h p o i n t e x p l o s i o n  1.3),  designed  upper mantle d e p t h s km  program  reflection  to test  whether coherent  c o u l d be o b t a i n e d .  spread  deployed  survey  In the second phase,  perpendicular  to  the  and i n t h e a d j a c e n t  detonated  10 km e n d - o n p r o f i l e t o t e s t  the  reflection  airgun  of  obtaining  Positive  deep  results  a n d have been  1.3.2  from  reported  been  of  line  structure Figure  using  i n Clowes et a l .  line  coast was  feasibility source.  were  obtained  et  of Vancouver I s l a n d  (1983),  al.  and t h e i r r e s u l t s  (1983).  The  i n t e r p r e t e d by McMechan a n d S p e n c e  has were  two-dimensional  (1983)  is  shown i n  1.4.  The most The u p p e r  significant 20 km o f  The v e l o c i t y i n c r e a s e s  features  the model i s from ~5.4  of  their  interpretation  relatively well  km/s a t  where a d i s c o n t i n u i t y i s p r e s e n t  to 6.75  and the  are:  constrained.  the s u r f a c e t o 6.4  2 km d e p t h . The v e l o c i t y t h e n i n c r e a s e s  depth,  an  5  (1983a).  IV a l o n g the a x i s  in E l l i s  a  IV  done by McMechan a n d S p e n c e  a l s o summarized  at  data  to  i n l e t a 32 L a i r g u n  the r e f l e c t i o n experiment  I n t e r p r e t a t i o n of Analysis  (1)  a  (RL  reflections  remained s t a t i o n a r y along  The  km/s a t  velocity  km/s 16 km jumps  to 7 km/s. (2) A  There i s at  ~23  mantle-type crust.  an a n o m a l y km  depth  i n the (Fig.  v e l o c i t y (7.8  McMechan  and  structure 1.4),  km/s)  Spence  is  just  s o u t h of  shotpoint  where a l o c a l i z e d r e g i o n imbedded  (1983)  within  speculated  v e l o c i t y a n o m a l y c o u l d be a remnant o f a s u b d u c t e d  the  that slab.  the  of  lower high  SOUTH  NORTH  N  X  A J  5.56.5-  ^  •5.5- 6.5-  10* X  6.75,7.057.07  20  a  F  ••  6.6  son  6.75,6.95 6.96  —  :  '"7.8-'  6.6  »«• •••  -7.6--  Q  6.2, 7.46  40-  7.5  50  i  100  r  i  i  200  1  7  300  DISTANCE (km) MODEL RELIABILITY: GOOD ( R E V E R S E D DATA)  FIG.  "  MARGINAL ( U N R E V E R S E D RAYS " B O T T O M , O R PARTIALLY R E V E R S E D )  P O O R (INFERRED, OR NO RAYS BOTTOM)  1.4. Velocity-depth s t r u c t u r e i n t e r p r e t e d from d a t a o f l i n e I V . Shot p o i n t l o c a t i o n s a r e i n d i c a t e d by N, A a n d F; intersection with line I by X. V e l o c i t i e s ( i n km/s) a r e shown a t b o u n d a r i e s ; where two v a l u e s are provided these are t h e v e l o c i t i e s above a n d below t h e b o u n d a r y ; v e l o c i t y g r a d i e n t s between b o u n d a r i e s a r e l i n e a r . R e l i a b i l i t y o f t h e contours is indicated by line types. Alternative interpretations a r e shown i n F i g . 1.5. ( A d a p t e d from McMechan and Spence 1983).  17  (3)  The s t r u c t u r e  weakly  of  the lower c r u s t  constrained.  lower c r u s t and  Spence  Fig.  1.5)  Three  and m a n t l e (1983).  and upper  mantle  l a t e r a l l y homogeneous  (Fig.  1.5)  were  Their preferred  c o n t a i n e d a low v e l o c i t y  by  interpretation  a n d an u p p e r m a n t l e v e l o c i t y of  7.5  1.3.3  I n t e r p r e t a t i o n of marine l i n e  : OBS 1 t o OBS 5  Data marine  recorded  on  OBS's  detonations  on  line  information  about  the  surface along  structure  line  deeper  by W a l d r o n  (Fig.  1.3)  oceanic  crust  as  velocity  i t begins  to  subduct  The t w o - d i m e n s i o n a l v e l o c i t y  (1982)  is  I a n d by a i r g u n d a t a was  37 km d e p t h .  yield  shown i n F i g u r e  was p r o v i d e d by c o n t i n u o u s  structure  lower  3 and 5 f r o m t h e s e q u e n c e o f  I  under the c o n t i n e n t a l s h e l f . interpreted  1,  the  McMechan  the  crust  I  of  (model 2 i n  throughout km/s a t  only  models  considered  zone  is  1.6.  The  seismic  near-  profiling  r e c o r d e d on t h e O B S ' s ,  determined  model  while  by t h e e x p l o s i o n d a t a  the  on t h e  OBS's. The  CSP  structure  beneath  thicknesses. to  be  section the  1.4°  profiles  ocean  airgun  r u n by  basin,  data  define  basin, of  towards  on  and  on  the  basement  on  sediment  thus  the c o n t i n e n t a l slope i s  Standard  to d i p beneath OBS  the s t r u c t u r e  Limited  the  continental  results slope,  from where  airgun  1, w h i c h i s  located  i n the  to the m i d - c r u s t at  the  data maximum  on  indicate  the c o n t i n e n t a l  km d e p t h , where t h e v e l o c i t y i s more t h a n 6 k m / s . T h i s with  seen  t h e c o n t i n e n t . Some m u l t i c h a n n e l  Chevron  t h a t t h e basement c o n t i n u e s The  information  ocean  The basement w e s t  d i p p i n g at  reflection  provides  OBS's velocity  rise. deep  almost  4  contrasts  3 and 5 on t h e of  3  km/s  FIG.  1.5. A l t e r n a t i v e v e l o c i t y - d e p t h models f o r l i n e IV. The preferred i n t e r p r e t a t i o n i s p r o f i l e 2, but s t r u c t u r e below 20 km i s p o o r l y c o n s t r a i n e d .  w  D I S T A N C E  (km)  E  CM FIG.  1.6. F i n a l v e l o c i t y s t r u c t u r e f o r the c o n t i n e n t a l margin along the m a r i n e p o r t i o n of l i n e I , i n t e r p r e t e d f r o m d a t a on O B S ' s 1,3 and 5 . D o t s show t h e l o c a t i o n s o f the OBS's. Velocities ( i n km/s) a r e g i v e n f o r t h e t o p o f e a c h r e g i o n , f o l l o w e d a f t e r the colon by the velocity gradient (in km/s/km) i f one was u s e d ( f r o m W a l d r o n 1 9 8 2 ) .  20  i n d i c a t e s that only sediments are being p e n e t r a t e d ; as w e l l , the sedimentary structure  velocities  are  higher  than  under  OBS  1.  i s c o n s t r a i n e d down to about 2 km depth by the  The  airgun  data on OBS 3, and down to almost 3 km depth f o r OBS 5. The  marine  explosions  recorded  on  d i a g n o s t i c dataset f o r determining the velocity  structure  down  to  1 form the most  OBS  sub-sedimentary  oceanic  the Moho. The assumption was made  that the e n t i r e c r u s t beneath the ocean i s d i p p i n g at  the  same  1.4° angle as the d i p of the basement, which was observed on the CSP data. The i n t e r p r e t a t i o n  i n c l u d e s a Moho at 9 km depth below  OBS 1, and a 5 km constant v e l o c i t y g r a d i e n t region i n the lower crust  w i t h i n which the v e l o c i t y  i n c r e a s e s from about 6.8 t o 8.0  km/s. The v e l o c i t y at the Moho was not w e l l c o n s t r a i n e d  by the  marine data,but no v e l o c i t y d i s c o n t i n u i t y was r e q u i r e d . The  velocity  structure  under  the  c o n t i n e n t a l slope and  s h e l f , as determined by data from OBS's 3 and 5, i s not as constrained  as  beneath  the  ocean  basin.  Since only shallow  sedimentary i n f o r m a t i o n was a v a i l a b l e from the OBS and  basement  was  not  observed,  a  well  airgun  data  t r a d e - o f f e x i s t e d between  sedimentary v e l o c i t y and basement d i p . Thus, an  assumption  had  to be made c o n c e r n i n g the d i p of the basement and the boundaries below  it;  but presumably the d i p beneath the slope and s h e l f i s  at l e a s t as great as the d i p beneath the ocean The e x p l o s i o n s  recorded  on  OBS  5  do  basin. provide  velocity  information f o r the m a t e r i a l above the subducting oceanic c r u s t . The  main  feature  of the d a t a s e t  i s that the apparent v e l o c i t y  remains near 5 km/s out to a d i s t a n c e of >30 km whereas  the  from  the  OBS,  apparent v e l o c i t y f o r OBS's 3 and 1 reaches 6 km/s  21  at  offsets  of  <20  layers  are  unusual  thickness  about  5  deeper  km/s.  velocity  km. T h i s in  the  The  by  multichannel  the  higher  o f OBS 5 ;  i.e.  position  with that  and  of  of  Wagner  a  by  the  an of  intermediate  mid-Miocene on  is  velocity  this  (1981)  r e f l e c t i o n data c o l l e c t e d  velocity  there  w i t h an i n t e r m e d i a t e  inferred  Snavely  that  region  of m a t e r i a l  block agrees w e l l  proposed  implies  the  melange basis  U.S.  of  Geological  Survey.  1.4  O u t l i n e of The  the  main  thesis  concern  of  this  the onshore-offshore  p o r t i o n of  major  of  of  contribution  techniques  to c a r r y  To p r o v i d e 17  shots  had  continental to  out  number  the  two-dimensional been  fired  of  find  Clowes  ray  tracing,  (1979)  efficient. Whittall scheme t o  has  Thus,  and C l o w e s  1.3).  basin,  At  the  and the m a i n l a n d .  above  to  It  fit  many  the  on  was the  itself  to  different  only  a method was d e v e l o p e d ray t r a c e r from  i n Chapter  both  practical  flexible  in a least-squares  2 of  media  refraction  the t h e s i s .  It  and and  which i n c o r p o r a t e d  seismic  up thus  scheme of W h i t t a l l be  the  large  through two-dimensional  and t h e r a y t r a c i n g  described  profile,  and were r e c o r d e d  present,  traveltimes  (1979)  a  development  locations  corresponding  proven  of  However,  a l s o been t h e  different  Island  f i n d v e l o c i t y models  T h i s method i s  (Fig.  interpretation  information along  at  combinations.  calculating  the  model w h i c h s i m u l t a n e o u s l y  traveltimes  shot/receiver  by  a  is  interpretation.  s l o p e and deep ocean  to  method of  line I  t h e s i s has  32 s t a t i o n s on V a n c o u v e r  necessary  is  the  thesis  the  inverse data. provides  22  an o b j e c t i v e means o f traveltime  data  finding  are  fit  ray t r a c e  in a least-squares  the need f o r the l a r g e  number of  usually  ray  involved  in  limitation  is  subjective  decisions  shot It  and r e c e i v e r s , is  possible  different of  a  the n a t u r e of  still  r e q u i r e d to  different  seismic  refraction  s t r u c t u r e of  oceanic  crust.  on  crust  Thus,  routine for  as  data  also  the e a r t h .  is  on  between  find a starting  model.  implies  line  place  It  described  is  I is  considerably  ray theory  two-dimensional  have  variable  forms  routine its  amplitudes  constraints  clear  that  the  thinner  than  the  structure  continental  a synthetic  seismogram  It  is  (ART), u s i n g the p r a c t i c a l , and  Clowes  routines  Cassell  on  strongly two-dimensional,  i n Chapter 3,  ART  McMechan a n d Mooney 1980;  1982),  the  based  efficient  (1979).  already  b a s e d on t h e W h i t t a l l  Although  existed  advantages  (e.g. of  a  and C l o w e s (1979)  ray  s p e e d of e x e c u t i o n and i t s m o d e l f l e x i b i l i t y .  The  tracer  are  latter  is especially  complex  based  s t a r t i n g m o d e l s may even  which thus  ray t r a c i n g a l g o r i t h m of W h i t t a l l  seismogram  major  the ray paths  l a t e r a l l y - v a r y i n g m e d i a was d e v e l o p e d .  asymptotic  other  reduces  perturbations  to the t r a v e l t i m e i n f o r m a t i o n , the  beneath the onshore-offshore since  the  model.  addition  velocity  and  procedure,  about  that  sense,  m o d e l l i n g . However, i t s  trial-and-error  is  such that  trial-and-error  trace  parameterizations,  the f i n a l In  of  that  parameters  region  important  such  as  the  when  interpreting  subduction  zone  of  a the  possibly western  Canadian m a r g i n . In C h a p t e r 4, dataset  is  the  presented.  interpretation The s e i s m i c  of  the  constraints  onshore-offshore p r o v i d e d by t h e  23  interpretation were  honored,  should  be  calculation Chapter tectonic  3.  as  of  with  techniques  inverse  (1982) a n d McMechan a n d S p e n c e  was t h e more g e n e r a l  consistent  interpretation traveltime  of Waldron  Finally,  the  i m p l i c a t i o n s are  subduction  zone  i n c l u d e d a p p l i c a t i o n of  procedure  synthetic  a  constraint  developed  in  Chapter  seismograms u s i n g interpreted discussed.  that  (1983) the  data  model.  The  the ray  trace  2,  and  t h e ART r o u t i n e  seismic  model  and  the of its  24  CHAPTER 2^ RAY TRACING AND TRAVELTIME INVERSION IN LATERALLY VARYING MEDIA  2.1  Introduction Much  i n f o r m a t i o n about the e a r t h ' s  traveltime  data  homogeneous using  inversion and al.  of  models  have been  sophisticated  (Bessonova et  Gilbert  1972)  al.  two-  1974),  and l i n e a r  subduction  or  zones,  dimensional  methods  about l a t e r a l l y along  strike  programming  may  faults in  varying structures, or  if  the  lateral  compared.  directly  But  examining  three-dimensional Ray  geological calculated  Jacob  laterally  interpretation provides  traveltimes models. by t h e  v a r y i n g the model fashion.  the  tracing  calculating  Numerous  (1970),  interesting  of  s u c h as  be u s e d t o if  the  a  is  extremal  (Garmany  spreading  infer  varying  are  in  either  a  examples  Sorrells  et  al.  are  One-  information is  uniform  l i m i t e d in extent  the p e r t u r b e d  region,  zone  and so  methods a r e  required.  relatively  simple  energy  matched  in  (1971),  two-  method  through  or  may by or  for  complex traveltimes  to the observed data  trial-and-error  exist  et  ridges,  terranes.  In f o r w a r d m o d e l l i n g p r o c e d u r e s , tracer  (Johnson  i n f o r m a t i o n may be o b t a i n e d  seismic  ray  data  regions  structure  variation s i d e of  more d e t a i l e d  as  inversion  by  Laterally  inversion  and a c c r e t e d  fact  and u n i f o r m s t r u c t u r e s on e i t h e r be  provided  from such  such  linearized  three-dimensional, transform  obtained  techniques,  1 9 7 9 ) . H o w e v e r , many g e o p h y s i c a l l y  strongly  is  f r o m b o t h e a r t h q u a k e s and e x p l o s i o n s .  velocity  a number  interior  by  systematic  the  literature,  Scott  (1973),  such  as  Gebrande  25  (1976), of  Aric  et  ray t r a c i n g  al.  (1980) and Clowes et  is also required for  in two- or t h r e e - d i m e n s i o n s . linearized  and  then e s t i m a t e d utilizing  laterally they are  of  to  teleseismic structure  array  a  heterogeneities; represented harmonics,  inversion  approach  traveltimes  inversions  ray  known.  utilizing  paths  three-  through by  the  same. to  a  assuming  examples  velocity  not l a r g e ,  the  (Aki et  are  the  al.  where  ray  has  perturbations  et  in  a n d C l a y t o n and Comer ( 1 9 8 3 ) ,  is  varying also  been  of  ISC  numbers  determine  Dziewonski  path  the  been a p p l i e d i n  It  massive  to  l o n g as  laterally  1977).  the  from i t e r a t i o n  b u t so  T h i s method has  used  model,  procedure,  determine  the a r r a y  been  symmetric  t h e m o d e l may change  inversion  studies  have  are  t h e r a y p a t h be  spherically  i n whole e a r t h s t u d i e s ,  traveltimes  v e l o c i t y model  may be c a l c u l a t e d u s i n g a  i n v e l o c i t y are  beneath  is  for a s p h e r i c a l l y symmetric v e l o c i t y model.  the  remain  model  m o d e l may be a p p r o x i m a t e d  with  variations  assumed  applied  velocity  i n a g i v e n b l o c k of  iteration  that  Some f o r m  the  C a l c u l a t i o n of  Alternatively,  earth  starting  schemes,  matrix  requires  paths  tracer.  varying  is,  lateral  ray  t h e same a s  velocity to  data, ray  a  squares c r i t e r i o n .  three-dimensional  dimensional  That  in  (1981).  traveltime modelling  to a s t a r t i n g  from the d a t a  i n t h e o r i g i n a l model s t i l l  teleseismic  inverse  these  perturbations  a least  In  In  al.  lower  al.  mantle  (1977),  terms  of  who u s e d a  who  spherical tomographic  technique. In more  velocity  difficult  dimensional  inversions to  avoid  ray t r a c e r .  from l o c a l e a r t h q u a k e s ,  multiple  The s i t u a t i o n  iterations is  of  it a  becomes three-  f u r t h e r c o m p l i c a t e d by  26  the need t o s i m u l t a n e o u s l y Crosson models  (1976)  velocity  most  layers  the earthquake  t a c k l e d the problem for  only, parameterizing  constant the  determine  layers.  the  one-dimensional  structure  A k i and Lee  i n t e r m s of  (1976),  w i d e l y u s e d 3D i n v e r s i o n r o u t i n e ,  into  rectangular  blocks,  and  for  the  t h e y u s e d a homogeneous h a l f  model  and  iterated  o n l y once  a d j u s t m e n t s . However, Hawley et the  method  dimensional thus  Aki  the  Gubbins  location;  inversion  of  subdivided a  s p a c e as  ( 1 9 8 1 ) were a b l e  and  assumed  velocity that  by a s i m p l e  by  formalism  window. N e v e r t h e l e s s ,  inversion  is  procedure refraction  parameter extend  introducing  three-  layer  Gubbins  b l o c k model  the  may be  Spencer  and  the  i n the  space  by Chou a n d require  region  coordinates approach  Booker  (1980),  the e x p l i c i t  rather  assumes  requires  form of that  the  traveltime  it  smoothing  specific  ray  evaluated.  concerned m a i n l y w i t h the development for  to  hypocenter  In the B a c k u s - G i l b e r t  implementation  model.  (1976),  structure  t o be known, b u t  and  inversion technique  f u n c t i o n of  does not  which i n t e g r a l s  This chapter  initial  to  the v e l o c i t y model  i n v e r s i o n , developed  flat  velocity  an  unknown f u n c t i o n w h i c h c a n be v i e w e d t h r o u g h a  paths along  the  least-squares  (1976)  Julian  for  the v e l o c i t y s t r u c t u r e  seismic  Lee  number o f p a r a m e t e r s .  t o 3D v e l o c i t y  an  their  al.  perhaps  r e s t r i c t i o n o f a homogeneous i n i t i a l  solve  c o u l d be d e s c r i b e d  is  for  is  s i m p l i c i t y in determining  ( 1 9 8 0 ) a p p l i e d an i t e r a t i v e  they  and a s m a l l  for  through the plane  3D r a y t r a c e r  simultaneously  the  and  ray t r a c i n g  removing  Using the and  of  But  velocity flat-lying  i n what  calculated  perturbation ray p a t h ,  each b l o c k .  hypocenters.  two-dimensional data  from  of  an  interpretation  of  explosions.  In  27  experiments multiple tracing  where m u l t i p l e s h o t s a r e  receivers,  r e c o r d e d on t h e  f o r w a r d m o d e l l i n g of  becomes cumbersome.  It  is  thus  t r a v e l t i m e data for a v e l o c i t y model, perturbations calculated.  to  is  of  better  the  the  ray  simple,  (1979).  gradient  constant this  several  iteration  the  paths  must  interpretation, arrivals, as The  ray  e l i m i n a t e d , and  data  i n v e r s i o n procedure  by  Whittall  represented  and  Clowes  by l a r g e b l o c k s  each  block  o f a r b i t r a r y o r i e n t a t i o n . The  necessary are  with  velocity major  its  speed of  execution  in  inverse  modelling  usually  corresponding  the  is  r e q u i r e d and i n  to  many  each  shot/receiver  Flexibility  i n ray t r a c i n g  is  in  refraction  surveys  than  in  where o n l y  the  explosion  is the  earthquake  usually  adjustments  in  the  accommodate  velocity  first  reflections  needs  to  depth  sources,  assumed.  critical  ( o r head waves)  tracer  the  (2)  be c a l c u l a t e d .  u s u a l l y as  refractions  time-consuming  and  iterations  path  the  i n which  within  modelling involving ray  ray  invert  and  Speed i s  e v e n more i m p o r t a n t  direct  by  automatically  the  are  r a y t r a c i n g method a r e  ray  combinations  velocity  scheme p r e s e n t e d  boundaries,  flexibility.  because  are  many of  forward procedure  v e l o c i t y model i s  advantages of and i t s  model  of  sense.  efficient  is  to  using a procedure  t r a c i n g method u s e d i n t h e  The  arbitrary  desirable  trace (1)  the t r a v e l t i m e s  a s s u r a n c e of h a v i n g a m o d e l w h i c h f i t s  in a least-squares The  ray  The a d v a n t a g e s a r e :  manipulations there  the  same s e t  step  In  a  is  to  from a p a r t i c u l a r  refraction identify  the  boundary  or  i n the r e g i o n below the  be and  sufficiently shape  distributions  of  boundary.  f l e x i b l e to interfaces  allow  and  to  v a r y i n g both l a t e r a l l y  and  28  vertically. The n e e d f o r  flexibility  shortcomings  of  or  which  inverse,  identification path  utilize  of  the a  different  values  may  Unfortunately, this  to  be  a  different  in  That  set  is,  for  of  or  in  this  ray  or  even  parameters,  the  present  two-  the  model  significant  at  the  about  of  the  forward  parameterization  many  no method  of  either  decision  for a given  problem  one  t y p e and the n a t u r e  different  there e x i s t s  fundamental  tracing.  subjective  With  starting  ray  arrival  parameterization.  model  related  r e f r a c t i o n modelling procedures,  involves  final  is also  details.  to  overcome  three-dimensional  interpretations. The i n v e r s e from  previous  media p r i m a r i l y the  depth  of  the v e l o c i t y of that  of  Wesson  explosion  procedure  v e l o c i t y i n v e r s i o n methods i n the nature specified  of  the  interfaces  a region. (1971),  in  for  chapter  differs  laterally  varying  ray t r a c e r .  In  may be v a r i e d  The i n v e r s e who,  refraction data,  determining  described  routine  crustal  a l s o used s e i s m i c  particular,  in addition  to  i s most s i m i l a r  to  interpretations  of  ray computations  in  l e a s t - s q u a r e s adjustments to a set  of v e l o c i t y m o d e l  p a r a m e t e r s . H o w e v e r , t h e v e l o c i t y m o d e l was a s i m p l e  f u n c t i o n of  a  Spencer  few  parameters,  Gubbins  (1980),  s i m i l a r t o the v e l o c i t y model of  and so was  With the exception discussed  in  this  of  limited the  chapter  in its  ray t r a c e r , has  type matrix  of  parameterization  i n v e r s i o n . As w i l l  or  flexibility. the  inverse  many c h a r a c t e r i s t i c s  with other v e l o c i t y inversion techniques,  either  i n the  routine i n common general  i n t h e method u s e d t o p e r f o r m  be s h o w n ,  the  and  depth  to  a  the  specific  29  boundary  plays  hypocenter velocity using is  depth  and  i n the  t h e damped  the  role  least  procedure  Gubbins  which  is  routines  earthquake  the procedures  2.2  a  i n many ways a n a l o g o u s t o  which simultaneously  location.  Inversion  squares technique  by  Crosson  1944),  (1976) a n d  (1976)  for  accomplished  (Levenberg  f o l l o w e d by A k i a n d Lee  considered  is  invert  the  and  is  which one of  Spencer  and  (1980).  V e l o c i t y M o d e l and Ray T r a c i n g A  complete  in W h i t t a l l procedure To  d e s c r i p t i o n of  and Clowes  is  repeated  define  boundaries, boundary  the  a  straight  normal  velocity  of  gradient  to  zero,  from  and  gradient  are  circular may be  calculated  boundary, source  for  1976).  of  is  given  of  the  two t y p e s  boundaries.  arbitrary  length  a  and  region  a  A  dip,  one  region  velocity  in  and d i r e c t i o n of  The  source  may be c o n s i d e r e d . within  If a  simple may  different  the  analytical  the  velocity is  a  travelled expressions  be l o c a t e d a l o n g any m o d e l  upwards a ray  and  which  w h i c h t h e t r a v e l t i m e and d i s t a n c e very  a  assigned a  The r a y p a t h w i t h i n a g i v e n b l o c k  using  model  velocity  with a  be d e f i n e d  of  assigned  non-zero  with  adjacent  the magnitude  and r a y s t r a v e l l i n g  an a n g l e w h i c h i s  divider  B l o c k s may t h u s  arbitrary.  there are  l e n g t h . A d i v i d e r boundary,  laterally  both  arc,  (Gebrande,  its  its  v e l o c i t y and g r a d i e n t . velocity  outline  and  line  separates  a  and o n l y a b r i e f  velocity structure,  v e l o c i t y along  gradient  routine  here.  model b o u n d a r i e s is  constant  (1979),  the ray t r a c i n g  is  specified  or  downwards  from  i n c i d e n t to a boundary range  of  the  the at  critical  30  angle,  then  head  i n t e r s e c t i o n of waves  are  waves  may  the c r i t i c a l  simulated  the boundary at  be p r o d u c e d .  ray w i t h  be  regular  found  corresponding the W h i t t a l l method, are  i n t e r v a l s along  in  order  and Clowes  It  an e s t i m a t e  of  a  calculate  tracer  rays  off  length. specific a  model  employs  receiver traveltime  t r a v e l t i m e . For f i n d i n g  reflected  which  the r a t e that angle.  rays,  ray a  paths,  shooting  take-off  For  along  h e a d wave b o u n d a r y .  head  of  angles  receiver  simply  requires  range for the  waves,  to  the  an  estimate  range w i t h r e s p e c t  These  i n the  on  this  the s h o t / r e c e i v e r  t h e r a t e of c h a n g e  w i t h t h e most c u r r e n t r a y  converged  and t u r n i n g  r e q u i r e d of  2.3  to  of  head  was t h u s n e c e s s a r y t o e x t e n d t h e r a y t r a c e r  changes w i t h s t a r t i n g  the  its  the  refracted  l o c a t i o n and a range of  perform m u l t i p l e i t e r a t i o n s For  shot  (1979) r a y  i n which a shot  location.  to  to the observed  specified.  boundary,  by s h o o t i n g c r i t i c a l l y  The r a y p a t h f r o m a s p e c i f i c must  the  Beyond the p o i n t  estimates are  ray is  to  distance  then  updated  iteration.  Theory It  is  assumed t h a t  choosing  a  structure  of a r e g i o n .  nature,  based  principles  structure  particular  such  sedimentary  there are  on as  basin,  s t a r t i n g model t o d e s c r i b e Some of  the  existence  or a f a u l t  area;  from for  reasons  geological  of  a  Constraints  other  reflection  example,  a or  there  general tectonic  subduction  zone.  for  the v e l o c i t y  t h e r e a s o n s may be o f  well-established  may a l s o come the  sound g e o p h y s i c a l  zone,  a  on t h e v e l o c i t y  may  or  refraction  surveys  in  be t i e  crossing  t h e p r o f i l e t o be i n t e r p r e t e d , o r t h e r e may be  lines  smaller  31  offset  surveys  in  detail.  Finally,  the c h o i c e  be g u i d e d under  the a r e a which d e s c r i b e the upper  by a g e n e r a l  of  certain  examination  consideration;  as  features  of  the  discussed  i n the  a g i v e n a r r i v a l must be a s s o c i a t e d  boundary  or a p a r t i c u l a r  Whatever model,  the  not a l l  reasons  f e a t u r e s of  i n t r o d u c t i o n to  this  with  a  the  choice  velocity  are  well-chosen  O n l y a few  parameters,  w h i c h s h o u l d be w e l l - s a m p l e d  by m u l t i p l e  2.3.1  The f o r w a r d In  the  to  rays  be  with  vary.  problem  general,  j-th receiver  allowed to  starting  structure  procedure.  are  particular  of a  by  paths,  inverse  set  model.  guiding the  data  determined  different  the  the  in  i n t h e model may  refraction  chapter,  r e g i o n of  layers  the  traveltime  for a set  of  T^j  between  observed  data  the  i - t h shot  and  is  1)  where v ( x , z )  is  the a c t u a l  element  of  arc  length  receiver  j.  model  velocity  gradient  a given  is  and  block  is  v0  the  distance  is  given  from  ds  ray  is  blocks  boundaries.  The  the i  tracer, with  to the  constant  velocity  v  by  + k z  velocity the  (1979)  i n terms of  (2.2)  the v e l o c i t y a l o n g  vertical  and  r a y p a t h L ^ j from s h o t  and C l o w e s  arbitrary  is  structure  the  specified  v = v0 where  along  For the W h i t t a l l  velocity  within  velocity  the  gradient,  top boundary.  top boundary and  z  of  is  Boundary p o s i t i o n  the b l o c k , k the  vertical  is  specified  32  by t h e c o o r d i n a t e s  of the endpoints  Perturbations both  the  to the v e l o c i t y s t r u c t u r e  velocity  v  = T-.  produce  changes  in  order,  the  a n d t h e r a y p a t h L ; J . To f i r s t  t r a v e l t i m e c a n be a p p r o x i m a t e d  T..  of the boundary.  using a Taylor series  expansion:  + I /9T \ A v 0 ( n + / 9T\ Ak.  (2.3)  2/9T\ A h n  +  ""\3hJ n c  T^j  is  the  calculated  traveltime  through  model.  The r a y p a s s e s t h r o u g h M b l o c k s w i t h i n  allowed  to vary.  the  velocity  gradient  in  Av„^ a n d A k m a r e t h e  at  the  the  top  block.  of  N  corresponding  t o the v a r i a b l e  and  the  Ah^ i s  unknown  boundary e n d p o i n t . calculated  for  change o f t r a v e l t i m e From e q u a t i o n s traveltime  starting  which v e l o c i t y  unknown, c o r r e c t i o n s  is  the  boundaries  for  number  of  endpoints  intersected  by t h e r a y ,  c o r r e c t i o n f o r the depth of the n - t h  starting  in.equation  model, and r e p r e s e n t  2.3  value.  2.1  derivative  and  2.2  the  partial  are  the r a t e of  p e r u n i t change of parameter  with respect  is  the m-th b l o c k and the v e l o c i t y  The p a r t i a l d e r i v a t i v e s  the  the  of  t o v e l o c i t y at the t o p of the m-th b l o c k  is  (9v0)m  \  9v0(v)  ds  ds  Similarly, velocity  the p a r t i a l  gradient  derivative  i s g i v e n by  (2.4)  of t r a v e l t i m e  with respect  to  33  ds  Thus,  both  numerical  partial  dividing number  derivatives  integration  implemented  for  the  along  of  segments  a l o n g each  of  of  2.1a  dD n o r m a l t o  remains  boundary.  Thus,  (2 dD c o s a ) .  of  If  the  the  was  ray t r a c e r into  a  assuming  with  its  from  by  large  constant  respect  shot  and r e c e i v e r ,  length, but  i.e.  l e n g t h of  the  the  is  to  arguments. reflected  perturbed  r a y p a t h between  the r e f l e c t i o n  the  analytically  geometrical  When t h e b o u n d a r y  t h e same;  by  at a  the  same  on  the  point  i t moves t h r o u g h s p a c e w i t h r a y p a t h c h a n g e s by a  the v e l o c i t y i m m e d i a t e l y above the  the e x t r a  For a r e f r a c t e d  are  traveltime  derived  i s changed,  the  (2 dD c o s a / v^ ) ,  where a i s  (1979)  and  by  integration  each b l o c k  length  shows a r a y between  and r e c e i v e r  then  The  obtained  segment.  expression  boundary  v^,  ray.  simply  and C l o w e s  equal  an a n g l e a f r o m a b o u n d a r y . distance  be  t h e n - t h b o u n d a r y e n d p o i n t was c a l c u l a t e d  using a simple  shot  the  Whittall  The p a r t i a l d e r i v a t i v e  Figure  may  the c i r c u l a r ray path w i t h i n  velocity  depth  (2.5)  the  distance  boundary  t r a v e l t i m e dT t a k e n by t h e p e r t u r b e d  ray  is is  and  9T dD  = 2 cosa v  ray  (Fig.  9T 9D  = c o s a - cosff v, v-  2.1b),  incident angle,  velocities  along  (2.6)  the  the c o r r e s p o n d i n g  B is  derivative  is  (2.7)  the emergent,  and v,, and  i n c i d e n t and emergent  portions  v^ of  FIG.  2.1. (a) When a b o u n d a r y i s moved by an i n f i n i t e s i m a l d i s t a n c e d D , ' t h e r e f l e c t e d r a y p a t h between a fixed shot and receiver is a l s o c h a n g e d . The h e a v y s o l i d l i n e s show the positions of boundary and ray path before the perturbation, and t h e h e a v y d a s h e d show t h e new p o s i t i o n s . The r a y p a t h i n c r e a s e s i n l e n g t h by an amount 2 dD cosa. (b) For a r e f r a c t e d r a y , t h e r a y p a t h l e n g t h i n c r e a s e s by an amount d D ( c o s a - c o s / 3 ) . LO  35  the  raypath,  general  and  consistently simple  respectively. reduces measured  geometric  derivative  from  distance  measured  If  boundary  the  geometric  to  Note  equation  from  the  correction distance  where  7 is  and h i s of  the depth at  details  of  In  f o u n d by for  to  a  Figure  ray  tracer,  for  velocity  with  respect  since  and  earthquake  to  hypocenter  method.  ray  (i.e.  a  source),  then  the as  either  (2.9)  i n t h e v e r t i c a l d i r e c t i o n and c o s a  the v e r t i c a l d i r e c t i o n c o s i n e .  I n two  derivative  the  involves  If  becomes  the c o o r d i n a t e  with  full  known.  3T = c o s a 3z vM z is  2.2,  t h e r a y w i t h t h e b o u n d a r y were r e g a r d e d the  the  the boundary. A l l  p o i n t of  where  2.2).  from the h o r i z o n t a l  t h e s t a r t i n g model a r e  inversions  t e r m i n a t i n g p o i n t of  to  (2.8)  intersection  or 2.7  (Fig.  \  intersects  the  the p a r t i a l d e r i v a t i v e s  2.6  boundary  in  may be f o u n d by t h e same g e o m e t r i c a l  equation  the  h n and h h + 1 , t h e n u s i n g  coordinates  the  is.  B  find  which the ray  the ray path  simultaneous  locations,  to  more  convert  a boundary endpoint  the boundary measured  are  angle  to  normal  at  is  below the b o u n d a r y . A  ~ cos/3 \ c o s ? / h - h n  the d i p of  2.7  the  f o r d D / d h and d h / d h n shown  = f cosa  these q u a n t i t i e s  if  required  are  t h e c h a i n r u l e may be a p p l i e d /3T\  2.6  measured  endpoints  equation  normal  is  v e r t i c a l l y at  relations  that  respect  to  the h o r i z o n t a l d i r e c t i o n  dimensions,  the  horizontal coordinate cosine.  is  partial simply  36  FIG.  2.2. With one endpoint of a boundary fixed, an i n f i n i t e s i m a l change d h n i n the d e p t h at the o t h e r e n d p o i n t is related by s i m p l e g e o m e t r y t o t h e i n f i n i t e s i m a l c h a n g e ,dD n o r m a l t o t h e b o u n d a r y a t an a r b i t r a r y p o i n t along its length.  37  2.3.2  Damped l e a s t Equation  2.3  squares  inversion  may be w r i t t e n  in matrix notation  as  AAx = A t where A i s vector  a m a t r i x of  c o n t a i n i n g the  traveltime q  residual  observations.  t h e use  of  new  partial derivatives, (M+N) p a r a m e t e r  vector,  The  equation  iteratively. model,  (2.10)  2.3,  matrix  traced  A  and  adjustments are convergence  reached  vector The  to w i t h i n  problem  is  overdetermined.  of  overdetermined  minimizing parameter normal  the  be  observations  model to  to  of  until  solution  2.10)  with  the  involves  respect  to  the  calculation  of  the  equations,  However,  difficulties  normal e q u a t i o n to a fundamental Symptoms  of  the  the parameter used • here  (1944);  parameter  than parameters,  the  a  accuracy.  A T AAx =AT A t  damped  find  repeated  (equation  leads  starting  least-squares  residuals  the  computed  t o the  is  the r e q u i r e d  to  the  l i n e a r i z e d by  must  procedure  problem  This  and A t i s  A t , and a new s e t  The c l a s s i c a l  traveltime  variations.  solution  t h r o u g h t h e new s t a r t i n g  W i t h many more t r a v e l t i m e  the  been  s o l u t i o n Ax i s a p p l i e d  calculated.  is  solution  A t ^ j = T^j - TV- , c o r r e s p o n d i n g  a n d so t h e  data  the  adjustments,  n o n - l i n e a r p r o b l e m has  The c u r r e n t  rays are  Ax i s  to  (2.11)  arise  with  this  solution  m a t r i x A T A may be s i n g u l a r lack lack  solution  of of  i n the c o n t e x t  constraint  of  first  traveltime  certain  iteration. is  The  to apply a  described  the due  parameters.  w o u l d be l a r g e v a r i a t i o n s  this difficulty  solution,  or n e a r - s i n g u l a r ,  on  from i t e r a t i o n t o  overcome  least-squares  constraint  because  by  in  method standard  Levenberg  inversion, I closely  follow  38  t h e a p p r o a c h d e s c r i b e d by A k i and L e e Large  variations  i n the parameters  (1976) and C r d s s o n  a r e damped o u t by m i n i m i z i n g  t h e w e i g h t e d sum o f  the t r a v e l t i m e r e s i d u a l s  solution  leading  vectors,  to  a  (1976).  system  and of  the  parameter  modified  normal  equat i o n s , ( A T A + 0)  Ax = A T A t  (2.12)  0 i s a d i a g o n a l w e i g h t i n g m a t r i x , g i v e n by fo /< 2  a /o, 2  0  © = e  \  0 2  0  (2.13)  0  "I Here,  6 i s an o v e r a l l  variance  of  damping  factor,  a  2  the t r a v e l t i m e r e s i d u a l s  the v a r i a n c e of  the  i-th  Equation  has  as  2.12  component its  of  solution  i s an e s t i m a t e  and  2  the  of  i s an e s t i m a t e parameter  the of  vector.  the e s t i m a t e d c o r r e c t i o n  vector Ax = ( A T A + 0 ) - 1 A T A t  (2.14)  The r e s o l u t i o n a n d c o v a r i a n c e m a t r i c e s a r e  g i v e n by  R = (A T A + 0) " ' A T A C = o  (ATA +  2  The m a g n i t u d e o f  elements  of  inversely adjustment factor.  is 0.  0)-'RT  the c o r r e s p o n d i n g parameter increased Since  proportional with a small  This is  (2.16)  t h e d i a g o n a l e l e m e n t o f R i s a good m e a s u r e  the r e s o l u t i o n of Damping  (2.15)  by  the  1972).  i n c r e a s i n g the magnitude of  individual to  (Wiggins  the  variance  s i g n i f i c a n t since  weighting  factors  variance will  have  of  the are  parameter a  the magnitude of  large the  damping parameter  39  a d j u s t m e n t s may d i f f e r  by more t h a n a f a c t o r  velocity  may  adjustments  adjustments  may  adjustment  less  several  produces  corresponding  a  large  of  change  effect  on  that  individual order  is  resolution  trade-off have  the  maintain  maximum  Thus,  to balance  damping  factor  resolution and  8  but  the  from i t e r a t i o n t o  the  related  is  large,  purpose of effects  of  adjustment values  It  as  is  decrease  enough  estimates.  iteration  possible to  the  and  to to  achieve  Usually,  of  the  standard  desirable  s m a l l as  any  values.  e x h i b i t i n g the  large  variance  so  the  ATA  the  the c o v a r i a n c e decrease,  parameter  r e q u i r e d t o have  increased,  stability  may be d e c r e a s e d  is  r e s o l u t i o n and v a r i a n c e .  overall  reasonable  factor  depth  a s m a l l change i n  matrix  i n parameter  also  a  As w e l l ,  differences  values  between  is  example,  traveltime,  large.  element.  weighting factors  As d a m p i n g the  diagonal  of m a g n i t u d e  is  for  km/s w h e r e a s But  in  the normal equations  a n d so a r e l a t i v e l y l a r g e d a m p i n g  10;  implies that  partial derivative  element  t h a n 0.1  kilometres.  with a small variance  parameter  diagonal  be  be  of  damping  the  inversion  using  synthetic  procedure.  2.4  Tests with A r t i f i c i a l Testing  data  serves  known, will of  the  is  certainty.  (equation  of  i n v e r s i o n procedure  First,  obviously necessary  correctly  validity  ray t r a c e  two p u r p o s e s .  it  Data  reproduce  that  This  is  the  linearizing  2.3).  since  "true"  to confirm that  the  model  important  approximation  synthetic  tests  to  may  degree  confirm  to the be  is  procedure  model to w i t h i n a s p e c i f i e d  particularly  Second,  the  the  traveltime useful  in  40  assessing  the  significance  is,  i f a s y n t h e t i c model i s  to  the  of  the r e s u l t s  used w h i c h i s as  model used w i t h the a c t u a l  inversion  procedure  from r e a l d a t a .  may  prove  data,  close  as  possible  the performance  applicable  to  That  of  the  the  real  i n v e r s i o n procedure  was an  to r e c o v e r a s i m p l e t w o - l a y e r model f o r which the  depth  interpretation. The attempt of  first  test  of  the ray t r a c e  t h e b o u n d a r y between t h e  layers  varied l a t e r a l l y . Figure  shows t h e s y n t h e t i c model a n d t h e r a y synthetic  "observed"  s y n t h e t i c model are considered  the  second  layers  corresponds  at  variations  in  end of  to  layer  Parameter  Table  2.1.  is  the m a n t l e ;  velocity  t h e Moho d e p t h as  large  30 as  spaced every  km  included wide-angle  Arrivals  boundary and t u r n i n g r a y s  offset  of  effect  the  the  may  arrivals,  and  km/s,  km,  p r o d u c e d a shadow zone f o r  but  there  3 km. S h o t s a t a  reversed  are  either  profile,  reflections  either  layer.  distance  230  from the Not  because  i n the boundary at  the r e f l e c t i o n s .  the 8.0  20 km b e t w e e n 70 km a n d  than the c r i t i c a l  the c o r n e r  the or  all shot-  because  120 km, w h i c h  The t o t a l  number  traveltime observations  was 3 1 . No random n o i s e  synthetic  H o w e v e r , t h e r a y t r a c i n g scheme  traveltimes.  be  k m / s / k m . The Moho  through the g r a d i e n t  both  was l e s s of  model  v e l o c i t y 6.5  0.01  r e c o r d e d on 9 r e c e i v e r s  receiver  for  t h e boundary between  gradient  a depth of a p p r o x i m a t e l y  recorded  generate  values  The  with constant  i n a forward  receivers  to  a s i m p l i f i e d c r u s t a l m o d e l . The u p p e r  the model r e s u l t  (Fig 2.3).  used  t o t h e Moho, b e l o w w h i c h t h e v e l o c i t y i s  constant  boundary i s  in  to the c r u s t  while  with  given  analogous  layer corresponds  km/s  traveltimes.  paths  2.3  was a d d e d  to  of the  involved  41  S H O T  0  A  DISTANCE  100  (KM)  200  300  o i  DISTANCE  0  FIG.  100  (KM)  200  S H O T  B  300  2.3. Simple t w o - l a y e r model used to g e n e r a t e s y n t h e t i c t r a v e l t i m e s f o r t e s t i n g the ray t r a c e i n v e r s i o n procedure. Velocity gradients are 0 in the upper l a y e r a n d 0.01 k m / s / k m i n t h e l o w e r l a y e r . The f i v e v a r i a b l e s v , , v 2 , h,, h2 a n d h 3 , a r e t o be r e c o v e r e d i n t h e i n v e r s i o n ( s e e T a b l e 2.1).  42  iteration of  0.1  to a s p e c i f i c  l o c a t i o n to w i t h i n a  km, and so t h e t r a v e l t i m e  c o u l d be as  l a r g e as  For the were  receiver  (0.1  to  vary:  the  the t o p of  boundary  0 km, 120 km and 300  traveltime  errors  the  second  was q u i t e  to e v a l u a t e the performance guesses  were  made f o r  for  the  the  the  was t o e l i m i n a t e value of  of  0.015  the e f f e c t  km/s was c h o s e n  the v e l o c i t y v a r i a t i o n s .  the  first  model  layer, of  50 ms was scheme.  and t h e g u e s s e s were  order as  of  the the  a  2  I n any c a s e , since  A  priori  an e s t i m a t e o f  For a v e l o c i t y of  confirmed  exact  values  differences. the  8.0  rms  km/s,  this  Starting Value  6.50  km/s  6.00  km/s  6.49  km/s  0.93  0 .007  km/s  v2  8.00  km/s  7.80  km/s  7.97  km/s  0.72  0 .013  km/s  h,  33.0  km  23.0  km  32.9  km  0.96  0 .38  km  h2  30.0  km  23.0  km  2 9 . 3 km  0.92  0 .46  km  h3  31.5  km  23.0  km  3 1 . 5 km  0.99  0 .23  km  Model Res.  A  error  Synthetic Value  Value  to  t h e i r main purpose  of m a g n i t u d e  Final  of  assumed  the parameter v a r i a n c e s  required,  of  the  damping  t h e v a l u e s of  were n o t  15 ms.  and t h e d e p t h s  a value  subsequent s o l u t i o n .  variances  location  km. A l t h o u g h t h e v a r i a n c e  be u s e d i n t h e w e i g h t i n g m a t r i x , by l o o k i n g a t  of  layer,  small, of  or  in  f i v e p a r a m e t e r s of  velocity  v e l o c i t y at at  due t o e r r o r  km)/(6.5 km/s),  inversion procedure,  allowed  error  tolerance  is  o  TABLE 2 . 1 . S y n t h e t i c test using a simple two-layer model (Fig. 2.3). Synthetic model, starting model, and c h a r a c t e r i s t i c s o f f i n a l m o d e l a r e s h o w n . R e s o l u t i o n and standard error a for the final model g i v e r e l a t i v e measures of parameter certainties. Overall damping f a c t o r d f o r t h e f i n a l i t e r a t i o n was 0 . 2 5 .  43  a p r e c i s i o n of a b o u t precision  assumed  t r a v e l t i m e of estimated  as  1.0  the  for  1 part for  about ~1  i n 500,  which i s  the t r a v e l t i m e s  25  s).  For  comparable  the  (50 ms o u t o f a t y p i c a l  depths,  the  rms  km. The o v e r a l l d a m p i n g c o e f f i c i e n t  first  to  i t e r a t i o n and r e d u c e d t o 0 . 2 5  error  was  6 was s e t  for  to  subsequent  iterations. As a s t a r t i n g flat-lying  at  m o d e l , t h e Moho b o u n d a r y was  a  depth  of  23 km, w i t h t h e v e l o c i t y s e t  km/s a b o v e t h e b o u n d a r y a n d 7 . 8 (Table  2.1).  In  Figure  km/s  2.4,  traveltimes  and  rms  the  s after  in F i g . 2.4), Fig.  2.4),  The f i n a l  one i t e r a t i o n o f to 0.030 s a f t e r  and t o 0.019  rms d i f f e r e n c e  final  s after  as  shown i n T a b l e  iteration  of  the parameter determined  the covariance  certainties  parameter  second  layer,  model  velocity  traveltimes reduced  was  a l t h o u g h the d i f f e r e n c e was  only  0.03  0.015  elements  obtained  from  r e l a t i v e measures  v2  at  The  for  poorly  the t o p of  compared t o t h e  km/s.  s  synthetic  s o l u t i o n . The most  velocity  in  iteration.  The d i a g o n a l errors  matrix give  the  3  2  procedure.  2.1.  i n the f i n a l  to  (curve  (curve  t o t h e v a l u e of  t h e r e s o l u t i o n m a t r i x and t h e s t a n d a r d  the d i a g o n a l  lines)  2.4,  t h e t h i r d and f i n a l  i n the ray t r a c i n g  6.0  in Figure  i n v e r s i o n procedure  was c o m p a r a b l e  to  The  T h i s was  the second  be  model.  s t a r t i n g model s.  to  boundary  dashed  starting  s o l i d dots  1.035  the  the  i n v e r t e d model r e p r o d u c e d the o r i g i n a l  model v e r y c l o s e l y , of  was  below (long  the  between t h e  traveltimes  due t o l o c a t i o n e r r o r The  for  1  shown by t h e  difference  and the s y n t h e t i c 0.266  are  just  curve  represents the t r a v e l t i m e curve synthetic  assumed  the  synthetic  r e s o l u t i o n for  the  44  SHOT A  CJ  W GO  00-  o  crj Q I  E-" to  300  100 200 DISTANCE (KM)  o w  SHOT B  0 5  CO  ^  q  00  CO  Q I  100 200 DISTANCE (KM)  FIG.  300  2.4. Performance of the i n v e r s i o n procedure f o r the twolayer model of F i g . 2.3. S o l i d dots are the s y n t h e t i c t r a v e l t i m e s , curve 1 is the s t a r t i n g model traveltime curve, curve 2 i s a f t e r the f i r s t i t e r a t i o n and curve 3 i s a f t e r the t h i r d iteration. Rms d i f f e r e n c e between the synthetic and c u r v e 3 traveltimes is 30 ms; a f u r t h e r i t e r a t i o n r e d u c e d t h e d i f f e r e n c e t o 19 ms.  45  velocity  v 2 was 0 . 7 2 ,  parameters, for  the  compared t o v a l u e s  and t h e s t a n d a r d  v e l o c i t y v,  all  of  0.92  i n the top l a y e r .  for  the  final  was r e d u c e d t o 0.1 parameter increases change  v2  8 is  and v a r i a n c e for the f i n a l  increases  to 0.013  the  Similarly, depth  to  error  km/s.  observed.  A fundamental procedure  is  the  s t a r t i n g model,  limitation  distance model.  the  if 6  resolution  for  error  if  the  choice  starting  of  the  of  o t h e r models which a l s o  values  decreases  the  inversion  paths  model  recovered  fit  is  the f i r s t  l a y e r where a l l o f  the  to  example,  the  if  the  120 km as  i n the  That  the ray paths  the l a y e r )  a  true  i n a l a y e r had been the  is,  a very poorly constrained parameter,  t h e bottom of  depth  h a d been s p e c i f i e d a t  in a d d i t i o n to  data.  the  used  velocity  t h e n i t w o u l d have been p o s s i b l e  gradient  (i.e.  ray  the v e l o c i t y g r a d i e n t  a v a r i a b l e parameter the l a y e r ,  of  For the t w o - l a y e r t e s t  c h o s e n as top  also  c h o i c e of p a r a m e t e r i z a t i o n of  200 km f r o m s h o t A i n s t e a d o f a t Similary,  0.25  trade-off  however, parameter  i n the success  subjective  including  in  km) o f  6 was  w h i l e the standard  t r u e m o d e l w o u l d n o t have been a c c u r a t e l y h2  a  s.  c a l c u l a t e model t r a v e l t i m e s .  parameter  most  For example,  i t e r a t i o n , the  In t h i s c a s e ,  t o 0.018  as  had  (0.46  o n l y s l i g h t l y and t h e rms t r a v e l t i m e d i f f e r e n c e  insignificantly  depth  which  then the standard  is  0.86  large  the next  h2,  standard  other  reduced for a given i t e r a t i o n  further reduced,  resolution  for the  The o v e r a l l d a m p i n g f a c t o r  i t e r a t i o n . If  so t h a t d a m p i n g i s between  was  and t h e l a r g e s t  the depth parameters.  0.9  e r r o r was n e a r l y t w i c e as  p o o r l y determined parameter resolution  above  the  find  velocity  especially  penetrate  and t h e r e a r e  to  at  in  t o t h e same  no r a y s  which  46  independently  sample o n l y the t o p of  the  layer.  The m o d e l may be o v e r - p a r a m e t e r i z e d the  velocity  gradient  velocity  blocks  another  problem  uniqueness  and  arises  smoothing  is  the  done  and  correct  c h o i c e of  of  the  using the In  the  would  Project, very  second  beneath  test  in Fig.  traced the  Gaussian  the  noise  the e s t i m a t e d  as  blocks  since  of  no  boundaries  would  tend  to  procedure  is  is,  the  inverse  i n s t a b i l i t y of  the ray t r a c i n g .  was made,  j u s t as  i n a forward modelling of  the ray t r a c e  I  of  the  the  not Also,  P13 and P 8 ;  Vancouver 4.  zone  used d u r i n g the  errors  of  a  with a standard picking error  of  al.  reflector dataset,  t o the t h r e e main  real  Fig.  1.3)  traveltime  the t r a v e l t i m e s ,  (boundary rays shots  line  the  were of  same 32 I.  dataset,  75 ms,  was  (1983),  to the  r e c o r d i n g of  d e v i a t i o n of  Seismic  model  et  a  velocity  Island  The t e s t  synthetic  see  case  inversion procedure,  an u p p e r m a n t l e  corresponding  i n the  the  scheme.  subduction  i n Chapter  To g e n e r a t e t h e  (P19,  locations  approximate  paths  the p r e l i m i n a r y model of E l l i s  from l o c a t i o n s  receiver  Ray  non-  That  interpreted  2.5a).  experiment  problem of  between  to represent  line  to  case,  ray paths  m o d i f i e d by t h e a d d i t i o n o f 3-4  the  this  be d i f f i c u l t t o f i n d  parameterization  which i s  to  In  many  r o u t i n e g i v e s no a d d e d a s s u r a n c e t h a t  ray t r a c e r  similar  blocks.  the  inverse  m o d e l was c o n s t r u c t e d structure  by s p e c i f y i n g  segments.  addition  allowing  r a y t r a c i n g becomes v e r y u n s t a b l e ,  b e c a u s e of  of  also  by  t h e p a r a m e t e r s . W i t h many v a r i a b l e  between  it  t h e use  boundary in  many s h o t s a n d r e c e i v e r s . very robust  but  only  i n the ray t r a c e program e i t h e r  velocities  scatter,  vary  many  f o r most of  and b o u n d a r i e s ,  o r of  to  not  random  same  was a d d e d  To  to  as the  FIG.  2.5. (a) Ray p a t h s t h r o u g h a t e s t starting model using the same shot and receiver locations as in the i n t e r p r e t a t i o n of l i n e I of t h e Vancouver Island Seismic Project. Variables i n the i n v e r s i o n p r o c e d u r e i n c l u d e the d e p t h of t h e s u b d u c t i n g Moho a t p o i n t s 1 and 2 and the depth of an upper mantle reflector a t p o i n t s 3 and 4. (b) V e l o c i t y - d e p t h p r o f i l e a t l o c a t i o n A on t h e ray trace model. A f i x e d mantle g r a d i e n t of O.OIkm/s/km i s assumed. O n l y t h e v e l o c i t y b e l o w t h e Moho ( p o i n t 5) i s v a r i a b l e .  48  artificial  data.  For  the  receivers while  test  procedure,  were m o d e l l e d as  the  far  offset  turning  the  ray  the v a r i a b l e s  depths  t h e Moho a t  mantle  reflector  (point  5 in Fig.  F i g u r e 2.5a  inverse  positions  1 and 2,  locations  2.5b).  than  complex,  considered  standard standard  portions  of  the  e r r o r of  of  i n the  added t o  subsequently  reduced to  set  The p e r f o r m a n c e 2.6,  Figure  actually  i n t o the  first  test  of  2.7  the to  final  compares  well  w i t h the  upper  of  0.01  much  more  previously.  were  for  the  for  were  procedure.  used  km  data. 1.0  inverse  0.015  1  model  to the c r u s t  is,  the  for  the  km/s f o r  the  depths.  The  was 75 ms,  the  l e v e l of  before,  the  overall  As the  inverse procedure  and T a b l e  2.2.  first  i t e r a t i o n and  is  F i g u r e 2.6  demonstrated shows a  t h e v e l o c i t y m o d e l . The d a s h e d  c o r r e s p o n d t o t h e s t a r t i n g model w h i l e the  the  the  0.25.  c e n t r a l p o r t i o n of  outline  not  that  the t i m e measurements  was  of  t h e s u b d u c t i o n zone t e s t  the v e l o c i t y and  e r r o r of  recovered,  a n d t h e Moho v e l o c i t y  model d i s c u s s e d  weighting matrix,  8  the  test  model  included  the depths  a l l parameters r e l a t e d  as  coefficient  of  was  f i x e d and d i d not e n t e r  t h e random n o i s e  Figure  first  nearly  The same v a l u e s elements  model  from  starting  method,  3 and 4,  mantle,  A f i x e d mantle v e l o c i t y gradient  the  Although the c r u s t a l were  the  offset  reflections  shows t h e  the  at  in  k m / s / k m was a s s u m e d . T h i s complicated  through  p a t h s t h r o u g h i t . The p a r a m e t e r s t o be  which are of  rays  to the near  a r r i v a l s were m o d e l l e d as  the upper mantle boundary. and  the a r r i v a l s  model a f t e r  the  heavy  two i t e r a t i o n s .  solid  The f i n a l  " t r u e " m o d e l , shown by t h e d o t t e d  in  blowup lines lines model lines,  DISTANCE 75  100 1  4  FIG.  125 1  150 1  (KM) 175 I  2 0 0  A  v  r  225 i  250 ,  I  2.6. (a) Dashed l i n e s show t h e b o u n d a r y p o s i t i o n s of the starting model (Fig. 2 . 5 ) . Heavy s o l i d l i n e s o u t l i n e t h e final positions after two iterations of the inversion procedure. Dotted lines show t h e " t r u e " p o s i t i o n s o f t h e subducting oceanic Moho and upper mantle reflector, (b) F i n a l (solid line), true ( d o t t e d l i n e ) and s t a r t i n g ( d a s h e d l i n e ) upper m a n t l e v e l o c i t y . No c h a n g e i n velocity g r a d i e n t was p e r m i t t e d . L o c a t i o n of v e l o c i t y p r o f i l e s i s a t  50  °175  FIG.  200  225  250  275  DISTANCE (KM)  300  325  350  2.7. Performance of the ray trace inversion procedure a p p l i e d t o the s u b d u c t i o n zone t e s t m o d e l . Dashed l i n e s a r e the traveltime c u r v e s f o r the s t a r t i n g model of F i g . 2 . 5 , and s o l i d l i n e s a r e the f i n a l model t r a v e l t i m e c u r v e s a f t e r 2 i t e r a t i o n s . Crosses are the synthetic traveltimes, to which 75 ms random n o i s e has been a d d e d . Rms d i f f e r e n c e beween s y n t h e t i c and m o d e l t r a v e l t i m e s i s 405 ms for the s t a r t i n g m o d e l a n d 74 ms f o r t h e f i n a l m o d e l .  51  the for  only  significant  the endpoints  Table  2.2).  the  2.7  model,  traveltimes. traveltimes  rms  r e d u c e d t o 80 ms a f t e r  s m a l l parameter difference already  only  below  subsequent  difference  synthetic  and f i n a l  traveltimes  marginally; level  of  and  to  74  since  the  the  noise,  the final  c o u l d n o t be c o n s i d e r e d  Synthet i c Value  Start ing Value  V  8.00  7.90  h,  2 0 . 0 km  1 6 . 5 km  h2  2 9 . 0 km  h3 h.  Value  405  ms. ms  Final  the for  "observed"  starting  iterations  reduced  to  curves  synthetic  was  one i t e r a t i o n and  perturbations  the  the  is  shots,  corresponding  traveltime  and  fit  the three  between  i t e r a t i o n . Subsequent  iterations  km/s  the  1.6 km  2.6a  the t r a v e l t i m e  the t r a v e l t i m e curves  and t h e c r o s s e s a r e  The and  how  (Fig.  For each of  model, the s o l i d l i n e s are  final  second  indicates  inverse procedure.  dashed l i n e s are  starting  b e i n g an e r r o r of a b o u t  the upper mantle r e f l e c t o r  Figure  i m p r o v e d by t h e the  of  difference  model This  was  after  the  produced  only  rms  traveltime  rms  value  perturbations  was from  significant.  Model Res.  a  0.94  0 .007  km/s  1 9 . 5 km  0.87  0 .66  km  2 6 . 0 km  2 9 . 7 km  0.93  0 .48  km  3 8 . 0 km  4 2 . 0 km  3 9 . 8 km  0.73  0 .84  km  5 1 . 0 km  5 7 . 0 km  5 2 . 5 km  0.29  0 .84  km  km/s  8.02  km/s  TABLE 2 . 2 . S y n t h e t i c test using a subduction zone model ( F i g . 2 . 5 ) . V a r i a b l e p a r a m e t e r s i n c l u d e d Moho v e l o c i t y v, depths h , a n d h 2 o f t h e s u b d u c t i n g Moho, and d e p t h s h 3 a n d h „ o f t h e u p p e r m a n t l e r e f l e c t o r . O v e r a l l damping f a c t o r 6 was 0 . 2 5 .  52  From T a b l e velocity mantle  v,  2.2,  point  was  very  was  traveltimes  to the  offset  reflector  point  c h a n g e of noise  depth.  a  t y p i c a l l y ~60 feel  of  which i s  points in  depth  that  of  the  estimates.  parameter  may a r i s e  starting  example  shown i n F i g u r e  a  because  the  sensitive  depth  a  to  of  traveltime the  resolved.  the  random of  Here,  the a  1  traveltime difference  of  model;  of  covariance  Additional  because of  estimates  the dependence  in particular, chosen  is  2.8.  The h e a v y for  zone t e s t ,  b o u n d a r y e a s t of  t h e c o n t i n e n t a l Moho traveltimes  the  as  there  valid  for  the  is  are  fit  values  significant  final  m o d e l on  no a s s u r a n c e  the  is  i n the  model  to within  32  real  that  i m p l i e d by t h i s p r o b l e m  from  figure the  are  the  previous  t h e c o n t i n e n t a l Moho  225 km) was c o n s i d e r e d changed  more  A  i n w h i c h the depth of  is  and,  the  earth.  solid lines "true"  of  on  minimum  a n d p e r h a p s more  the non-uniqueness  boundary p o s i t i o n s  synthetic  the  to the  compared  t h e y s h o u l d be c o n s i d e r e d  the p a r a m e t e r i z a t i o n  (the  is  reason  n o t t o o much e m p h a s i s s h o u l d be p l a c e d  values  subduction  The  the  upon e x a m i n a t i o n of  better  causes  upper  ms.  for  simple  0.29.  with respect  3 and 4 a r e  that  the  the  were n o t v e r y  small  mantle  in particular  determined  evident  traveltime  specifically,  errors  The l o c a t i o n o f  receivers is  the  o f ~75 ms. On t h e o t h e r h a n d , t h e p o s i t i o n s  perturbation  absolute  This  o n l y ~15 ms,  level  I  poorly  was  1 km c h a n g e i n d e p t h t y p i c a l l y p r o d u c e s  Moho b o u n d a r y a t km  far  derivatives 4;  0.94.  4 where t h e r e s o l u t i o n was  position  partial  parameter  poorly resolved,  reflector  to  resolved  w i t h a r e s o l u t i o n of  reflector  depth at  the best  km  f i x e d at depth,  the s t a t i s t i c a l  37 km. I f the  same  error  by  DISTANCE  FIG.  (KM)  2.8. Three different models which f i t the synthetic t r a v e l t i m e s of the s u b d u c t i o n zone t e s t m o d e l . Heavy solid lines are the boundary p o s i t i o n s assuming a c o n t i n e n t a l Moho ( e a s t o f 2 2 5 km) f i x e d a t 37 km d e p t h . Dashed lines a s s u m e a M o h o f i x e d a t 32 km d e p t h , a n d d o t t e d l i n e s a s s u m e a M o h o f i x e d a t 42 km d e p t h .  54  the  model  indicated  Similarly,  the  traveltimes  assuming  That  is,  by  dotted  lines  calculated  for  In s e c t i o n procedure  is  assumed  4.4  model. model,  applied  Chapter  line  two a l t e r n a t i v e are  I  of  presented  with  observed  traveltimes  of  If  2.8.  fits  42  km  be  the  depth.  determined  t h e Moho d e p t h h a d  the  4,  the  the  inversion,  ray  then the  trace  Vancouver of  been the  values  the  in  inversion  recorded Island  interpreted  the observed  traveltimes  4.6.  One  of  s u b d u c t i o n zone t e s t model i t s e l f ,  the p r e v i o u s  traveltimes example.  test  the  section  of  along  Seismic  the s u b d u c t i o n zone  nonuniqueness  models which f i t  is  actual  in  is a variant  a l t e r n a t i v e models the  which  t o the t r a v e l t i m e d a t a s e t  To i l l u s t r a t e t h e b a s i c  well  model  Figure  parameters.  of  The m o d e l u s e d  e q u a l l y as  a  in  for the depth would c o n t r o l  the other  the onshore-offshore Project.  5 parameters.  a v a r i a b l e parameter  value  show  lines  t h e c o n t i n e n t a l Moho c a n n o t  uniquely w i t h the other  starting  dashed  a c o n t i n e n t a l Moho f i x e d a t  t h e d e p t h of  i n c l u d e d as  the  r e p l a c i n g the  the but  synthetic  55  CHAPTER 3^ PRACTICAL SYNTHETIC SEISMOGRAMS FOR LATERALLY VARYING MEDIA CALCULATED BY ASYMPTOTIC RAY THEORY  3.1  Introduction F o r many y e a r s ,  s e i s m o l o g i s t s h a v e made  k i n e m a t i c and d y n a m i c c h a r a c t e r i s t i c s to  better  earth.  Such  specific the  model  observed  proposed,  applied  are  into  more r e a l i s t i c  a  refraction  data  structure  calculation,  synthetic  a  compared  to  of  process are  an a c c e p t a b l e f i t both  has  traveltime  interpretation  models  the  for  seismograms  incorporating the  of  trial-and-error  made u n t i l  information  until  very  to one-dimensional  bases for  synthetic  of  the e a r t h ' s  1971;  Chapman,  which  ridges,  subduction  (e.g.  seismic structure  a  inhomogeneous  media  assumed t h a t  1968;  is  (e.g.  zones, c o r d i l l e r a n structures,  of  (e.g.  Cerveny  traveltimes et  Miiller, problems spreading  rift  basins)  a cross-sectional  following a variety  calculating  velocity  F u c h s and  applied  al.,  be  theoretical  t h e most i n t e r e s t i n g  t w o - d i m e n s i o n a l and r e q u i r e  means  could only  because the  Helmberger,  seismology  t o d e s c r i b e t h e m . Ray t r a c i n g provided  the procedure  e a r t h models  1 9 7 8 ) . B u t many o f  refraction  strongly  recently  seismogram computation  v a r i e d with depth only  are  the  derived. However,  to  the  of  enabled  both  seismograms which are  This procedure  amplitude  t o be  are  the  Through  and c o m p a r i s o n s  been a c h i e v e d .  d a t a has  theoretical  seismograms.  models  generated,  and  of  requires  of  seismic  velocity-versus-depth  interpretation  model,  different  the  of  use  model  o f methods in  1977;  has  laterally Julian  and  56  Gubbins,  1977;  However, models  Gebrande,  experience  indicates  w i t h the  that  c a l c u l a t i n g amplitudes What  is  it  is  as  well  required  by  a l g o r i t h m which combines synthetic  1976;  effective  as  the  tracing  for  to  have  with  to accept  the  Thus,  must  the d e s i r e  be  of  its  an  structures.  An  structures,  those  record sections.  against  is  of  arrivals  At the  same  be f a s t a n d e c o n o m i c a l i n p u t of m o d e l  for great generality  balanced  means  generation  a n d s h o u l d have a s i m p l e m e t h o d f o r t h e  parameters.  a  realistic  seismograms f o r  a p r a c t i c a l c o m p u t e r a l g o r i t h m must  routine  one-dimensional  two-dimensional  a l g o r i t h m must be a b l e  run,  1979).  traveltimes.  commonly i d e n t i f i e d on t h e o b s e r v e d  to  Clowes,  interpreting seismologist  and r e l i a b l y g e n e r a t e s y n t h e t i c  time,  and  i n t e r p r e t a t i o n of essential  ray  seismograms  Whittall  in a  computer  s p e e d a n d s i m p l i c i t y of  use. The most routines  general  take  into  energy.  Examples  are  1982),  the  integral  generate  account  a  full  are  obtained  calculation  of  amplitudes  at  each  receiver  using  practical,  efficient  theoretical  methods  of  from  1982),  and  to  u s e d as rays  al.,  laterally-  the  various 1981).  a common f i r s t  throughout  model.  rays  weighting  schemes.  algorithms readily  times,  step  t h e n p r o c e e d s by c o m b i n i n g many  not  arrival  the  In  the  various  ray  et  and  computer are  propagating  Haddon and B u c h e n ,  still  system  Traveltimes  the  t h e WKBJ s e i s m o g r a m  (e.g.  ray t r a c i n g i s  of  seismogram  beam method ( C e r v e n y  Drummond,  methods  synthetic  t h e wave n a t u r e  the Gaussian  (Chapman a n d  these methods, to  two-dimensional  g e n e r a l i z a t i o n of  v a r y i n g media Kirchoff  of  based  available  However, on to  these the  57  interpreting In  seismologist.  t h i s chapter,  calculated (ART).  by a d i r e c t  The  single  two-dimensional synthetic  method  ray  path  particular  application  has  is  to  determine  Although  caustics,  ray  of  signals,  the  the  a  amplitude  (1974)  and  Cerveny  equations  In  t h e method d e s c r i b e d  et  al.  involving  amplitude  s i m u l t a n e o u s l y w i t h a set t r a c i n g of  rays.  the  points  many  (1977),  of  a  diffractions  i n a wide  of  each  parameters  ray,  appeared  was  equations  was  neighboring However, al.  rays  they used  (1977),  which  computations. cumbersome model  a p p l i c a t i o n of ART t o r e a l  to  by  estimate  employs  M a r k s (1980)  the and  numerical calculations  or  analytical  the  the  end  constant  expressions  elementary  amplitudes,  Marks  tube area at  the r e c e i v e r  Cassell  are  in  what  refraction of  ray tube  two area.  i n Cerveny  (1982)  avoided  by p a r a m e t e r i z i n g  velocity be  regions  gradient, used.  so For  (1980) u s e d n e i g h b o r i n g r a y s for  of  et  time-consuming p o i n t - b y - p o i n t  or t r i a n g u l a r  could  each  points  the ray t r a c i n g code d e s c r i b e d  into small rectangular  velocity  using  the  the n u m e r i c a l s o l u t i o n s  apparently  c a l c u l a t e d amplitudes  solved  for  t i m e - c o n s u m i n g . McMechan a n d Mooney ( 1 9 8 0 ) ,  data,  al.  for  excessively  the f i r s t  range  differential  B e c a u s e a s o l u t i o n must be o b t a i n e d along  that  by C e r v e n y e t  set  differential  of  implies  s u c h as  t h e method has p r o v e d a p p l i c a b l e  literature.  theory a  o f m o d e l s a n d a number o f ART a l g o r i t h m s h a v e r e c e n t l y in  are  s i m p l i c i t y , in that  this characteristic  ART c a n n o t m o d e l c e r t a i n t y p e s and  asymptotic  t h e a d v a n t a g e of  used  arrival.  of  seismograms  these  the v e l o c i t y with that  constant simple  calculating  to estimate  r e f l e c t i o n and r e f r a c t i o n  ray  phases.  58  Cassell  (1982)  reflected, layers  refracted  and  Ravindra  plane  is  within  dipping  efficient  and  of  the  given  of  homogeneous  i n Cerveny  and  and  r o u t i n e has  Whittall  and  velocity  with arbitrary  very  Clowes  and  algorithm is  allow  interpretations, (1982),  et  al.  as  et  to  to as  amplitudes  application  in  example Clowes et al.  (1983), Green use  a  traveltimes  ray t r a c e r  and  The p r o c e d u r e  develop  are  and of  specify  ( 1 9 8 4 ) . The w i d e s p r e a d  a al. et  of  the  method  of  within  calculated  has  for  by a c o m b i n a t i o n of  the the the  by M a r k s ( 1 9 8 0 ) and C a s s e l l  (1982).  The  a d d s o n l y a modest amount  to the cost  of  so t h e a d v a n t a g e s o f  efficient  its  Ellis  well  (1979)  fast.  for  encouragement  calculation  ray t r a c i n g ,  to  boundaries,  is constant  is  chapter,  ART a p p r o a c h e s a p p l i e d  seismograms  ( 1 9 7 9 ) . The  algorithm  amplitudes In t h i s  amplitude  varying  very simple  Horn  provided  determining algorithm.  and C l o w e s  laterally  is  flexible  refraction  (1983)  input  amplitudes  with  The m o d e l  ( 1 9 8 1 ) , D e l a n d r o and Moon  the  as  r a y t r a c e method f o r  by l a r g e b l o c k s  orientation.  proved s u f f i c i e n t l y  al.  boundaries,  each b l o c k the v e l o c i t y g r a d i e n t  modify,  number  for  and h e a d waves i n m o d e l s  represented  arbitrary to  expressions  was p r e s e n t e d by W h i t t a l l  model and  simple  (1971).  A simple, media  used  computations  are  a user o r i e n t e d program in laterally  a simple,  maintained.  for  inhomogeneous  easily The  calculating  media.  modified resulting synthetic  59  3.2  V e l o c i t y M o d e l a n d Ray T r a c i n g A  brief  d e s c r i p t i o n of  tracer  is  given  to the  r o u t i n e as  the c o u r s e of  in section  to  allow  reflected the  waves.  2. However,  and C l o w e s  synthetic  2.2  have  at  At  a l l other  the  ray the  ray  is  still  off head  is desired  encountered  c o n t r o l l e d by t h e a n g l e o f  refracts  the boundary. are  which r e f l e c t i o n  i n c i d e n t angle  desired,  is  less  if  angles gives a l l wide-angle waves.  branches  The c o r r e s p o n d i n g  such that  decreases  the d i s t a n c e  monotonically  with  otherwise,  of  a l o n g each branch The  range of  and  is divided  into  increases  family  used i n the s y n t h e t i c  purposes of  take-  rays  identification routine for  from  reflections  branch i s  which i s  the  reflects  a s s o c i a t e d w i t h each t r a v e l t i m e number,  then  turning  t r a v e l t i m e curve  of  boundary.  angle,  the  or  specified.  the  it  as  For  the behavior  multiple  reflections,  distance.  so  and m u l t i p l y  need t o be  incidence at  specification  in  o n l y the boundary  by t h e r a y ,  no p r e - c r i t i c a l o r  then a s i n g l e  added  not c o n s i d e r e d .  than the c r i t i c a l  through the boundary; Thus,  extensions  (1979) a l g o r i t h m was e x t e n d e d  Converted phases are  ray  algorithm.  for p r e - c r i t i c a l l y r e f l e c t e d  boundaries  (1979)  been  seismogram  p r e - c r i t i c a l and m u l t i p l e r e f l e c t i o n s ,  boundaries  If  of C h a p t e r  in section  d e v e l o p i n g the  ray t r a c i n g  and C l o w e s  2.2  described  The W h i t t a l l  the W h i t t a l l  of  labelled with a  or rays  unique  seismogram  i n t e r p o l a t i o n w i t h i n a given ray  family.  60  3.3  C a l c u l a t i o n of  3.3.1  Reflected In  a  and R e f r a c t e d  medium  distribution, connection the  A m p l i t u d e s and s y n t h e t i c  with  Ravindra,  of  1971,  a  arbitrary  inhomogeneous  asymptotic  f o r m u l a between  amplitude  Rays  an  zero-order  seismograms  the  source  reflected  or  ray at  theory  velocity  provides  a  M 0 and any p o i n t M  refracted  wave  for  ( C e r v e n y and  p.74). N  U(M)  The  ray  = 1 MMQMMQ)) L \ v(M)p(M) J  geometry  incidence  of  quantities the  ray  density  is  the  are  p is  + 0.3788v  amplitude  reflection  are  the  velocity  is  be water  with  of  zero.  surface  The  a  1.5  a density  it  the  between  and  the  point  encounters.  interface  Primed  from w h i c h  P-wave v e l o c i t y v and  R^  for  transmission (1971,  ratio  km/s,  by Young and  Braile that  if  the  where t h e m a t e r i a l  is  assumed  surface  0.25  and  except  1.0* 10 3  of  or  p.63),  The Z o e p p r i t z a l g o r i t h m assumes  of  of  (Birch,1964)  Zoeppritz routine  reflection  of  is  routine described  (1982).  than  (3.1)  L  .  a Poisson's  less  0^  from C e r v e n y and R a v i n d r a  using  has  side  cofficients  taken  and C a s s e l l  medium  by  3.1.  interface  The r e l a t i o n s h i p  Zoeppritz  (1976)  i-th  on the  approximated  calculated  2  L  the  evaluated  p = 0.252  are  n / v ' ( 0 ; ) p ' ( 0 ; )\ " R i=]{ v'(Oi)p(0 ) J  shown i n F i g u r e  ray at  emerges.  1 / 2  kg/m3 also  and an S-wave  allows  conversion  the  P-wave to  velocity  calculation  coefficients  of if  desi red. In  equation  3.1  the  geometrical  spreading  function L  is  FIG.  3.1. Geometry o f the r a y tube a t the s o u r c e M and a t t h e i-th i n t e r f a c e , where the p o i n t o f i n t e r s e c t i o n i s . The p r o p e r t i e s of t h e medium a r e l a t e r a l l y v a r y i n g i n t h e x-z p l a n e but a r e u n i f o r m i n the y - d i r e c t i o n . 0  62  given  by C e r v e n y a n d R a v i n d r a  (1971, p.74)  as  N  ( o ( 0 )) V / da(M) da(M) V' V'2 nn ffddo(0 1 d o ( M ) j i = l U ^ ' (0 )/  L =  2  V>  2  2  (3.2)  0  Here, da denotes t h e elementary or  0^ .  The  influence 3.1). be  product  term  of i n t e r f a c e s  The change  evaluated  incidence  as  to  r a y tube a r e a  in  on  equation  geometrical  i n r a y tube a r e a the  ratio  the cosine  of  1971, p . 7 9 ) . On t h e o t h e r  equation  3.2  interfaces, commonly  represents  which  velocity  to  hand,  be  on  as  i s homogeneous, so t h a t  plane of  the  follows.  the  0  boundary c a n  the angle  first  The  sphere,  of  (Cerveny and term  i n r a y tube a r e a  the unit  M ,  (see Figure  o f emergence  t h e change  i scalculated  assumed  spreading  the cosine  Ravindra,  M,  (3.2) r e p r e s e n t s t h e  a t a given  of the angle  at points  in  between  point  M  is  0  w i t h i n which t h e  elementary  area  on t h e  sphere i s do(M ) = s i n 0 0  where  8  source:  6  0  direction, a general area  is  0  the  angle  (3)  0  a n d <j>0 i s t h e a n g l e  on t h e w a v e f r o n t  from  measured  velocity  of the r a y a t the  the  vertical  from t h e x-z p l a n e . F o r  distribution,  o  o  the source  M  (1970) o b t a i n e d  the  and  d e r i v a t i v e s of from  an e l e m e n t o f  d0 d<p  C e r v e n y e t a l . (1974) a n d Wesson  integrated  z-  3t90  where f i s t h e v e c t o r p o i n t i n g from  time  or  at point M i s  3r ^ 3? 30o  coordinates  measured  three-dimensional  do(M)  d<£  tj>0 a r e t h e a n g u l a r  and  0  dt9  o  3r/30  o  known s t a r t i n g  3r/3t9 ,  values  0  0  t o r e c e i v e r M. expressions f o r  which  were  then  t o t h e p o i n t M. However,  63  for  l e s s complex v e l o c i t y  da(M)  simplify,  distributions,  the  and a n a l y t i c e x p r e s s i o n s  expressions  c a n be u s e d  for  instead  of  time-consuming numerical i n t e g r a t i o n . With the assumption dimensional,  that  varying only  the  velocity  structure  i n the x-z p l a n e ,  is  M a r k s (1980)  twoshowed  that (3.4)  Here, f.  y is  the o u t - o f - p l a n e  The f i r s t  tube  in  factor  angular 'in  epicentral  The  was  for  with  that  constant  integrating obtain  the  ray  0  at  a  calculate  Ar/At90.  direction.  r,  For a  departure  the w i d t h of  as  discussed  complex.  With  the ray  an e x p r e s s i o n  8. 0  tube  structure is  simply  by C e r v e n y and R a v i n d r a media, the  the model i s p a r a m e t e r i z e d velocity  rays  Mooney  angles  one-dimensional  l a t e r a l l y varying  more  similar  A  0  is  small  who s p l i n e d s e v e n  successive  at  difference  3r/9e9 , and by McMechan and at  of ray  dr/d6  and u s i n g t h e  0t  i n e q u a t i o n 3.4  But f o r  becomes  assumption  of  i n the z - d i r e c t i o n , the q u a n t i t y g=3y/90o  p.78). g  the width  second  than 6 to  t h e two r a y s  factor  only  Ar  epicentral distance  (1971,  to  by s h o o t i n g a  to estimate  the o u t - o f - p l a n e  the  0  is  f o l l o w e d by M a r k s ( 1 9 8 0 ) ,  who u s e d  second  varying  8  distance  each r e c e i v e r  (1980),  in  angle  (3.4)  the magnitude  I n o u r a l g o r i t h m , we e s t i m a t e  i n c r e m e n t At9 0 g r e a t e r  procedure at  in equation  the x-z p l a n e .  each d e p a r t u r e  c o o r d i n a t e and r i s  the  second in  terms  gradient,  M a r k s (1980)  for dg/dt  from Cerveny et  expression simplifying of  solved al.  blocks f o r g by (1974)  64  N 3y = s i n t 9 0 I v 0 I 2 ( 1 "STo v0 i=l  / 2k-(t--t:.,) + tan2goi/2) e ' 2k: V 1 +  where  N  =  tan20oi,e T  number o f  v e l o c i t y on e n t e r i n g  k^  =  velocity gradient  0O;  =  ray  angle  t  i-i'ti  =  total  by  noting  the d i s t a n c e  measured  1980)  r;  = v0;  tantfoi  i n the  i - t h block  entering respect  direction  the  Whittall  perpendicular  (Marks,  i - t h block  /  the to  i n the upon  the  block, velocity  block entering  and  the  and  Clowes  r^ t r a v e l l e d  epicentral (1979)  within  2k,(ti-t-  .,)  ray  the  to the v e l o c i t y g r a d i e n t  e  i-th  upon  i - t h block  9y/3</>0 i n t e r m s o f  calculated that  the  the  traveltime  leaving We may e x p r e s s  on  with  gradient  (3.5)  blocks  =  measured  '  2k^(tx-t • . , )Vl  vi 0  -1  distance tracer,  i-th  k ,  is  by  block, given  by  - 1  , 2ki(ti-ti-.,)\-l / 1 + tan a0i. e 2  ( The a b o v e e x p r e s s i o n (3.5),  after  for  utilizing  T~  r ^ may  the  3y_= 30o  substituted  trigonometric  (2tan0o^/2 )/sint90^ . Equation N sinc90 I v 0 ; r,-, v0 i =1 sin0ol  be  (3.5)  then  identity  into  equation  1+tan 6 i/2 = 2  0  reduces to  -  (3.6)  65 Combining equations  3.3,  3.4 N  do(M) = 9r c o s g ( M ) L do(M0) 3t90 v0 i=l which  is  substituted  spreading  3.3.2  and 3 . 6 ,  we have  vn;, r2 sint90;  into  (3.7)  equation  3.2  for  the  geometrical  function L .  Head Waves The scheme o u t l i n e d  asymptotic rays, the  expansion,  including  is  used.  (1979)  rays.  The c r i t i c a l  ray t r a c e r  based for  For head  coefficient  t h e h e a d wave b o u n d a r y  is  and i s v a l i d  turning  first-order  scheme  above  on  the  zero  r e f l e c t e d and waves,  which  i n the ray expansion, angle  refracted represent  a  different  r a y p a t h t o w a r d and away  i s d e s c r i b e d by t h e W h i t t a l l  i n terms  order  of c i r c u l a r a r c s .  and  Clowes  W i t h i n each  block,  we d i v i d e t h e c i r c u l a r r a y p a t h i n t o a l a r g e  number o f  of  v e l o c i t y along  equal  segment. of  a  length,  assume  a  of  thin  homogeneous  equations  i n a model of U*(M) =  5.22  and 5 . 2 9 )  homogeneous vr  iwl3/2  k- 1 L ,  re-parameterized, layers,  segments  in  each terms  whose b o u n d a r i e s  are  w i t h i n a g i v e n b l o c k b u t may be n o n - p a r a l l e l from b l o c k  t o b l o c k . We t h e n a p p l y e x p r e s s i o n s (1971,  constant  T h u s , t h e v e l o c i t y model i s  series  parallel  and  from  = /vKV/2  n  layers  tant9(Q*J (vR/v) L ,L V  2  for  from  Cerveny  t h e a m p l i t u d e of  with plane d i p p i n g s- 1 fl  j=1 j=k  and  Ravindra head  waves  interfaces:  R*;  (3.8)  cos0(o*,-)  66  where  8(0*j)  =  angle  of  i n c i d e n c e at  the  j-th  boundary.  6'(0*)  =  angle  of  emergence  the  j-th  boundary.  velocity length  V  =  1 =  between b o u n d a r i e s of  ( j = 1 ,2 ,  at  the  . . . s)  j-th  (j-1)  segment  and of  j .  the  ray  .  velocity  just  (boundary  k).  l e n g t h of  the  b e l o w t h e h e a d wave  ray path along  the  boundary  head  wave  boundary.  The  head  wave  estimate  of  dominant  coefficient  TK  is  ( 1971 , p p .  108-109)  where  = (1-v2©2)12  P3  R*,3  is  for  the  coefficient  3.3.3  and 0 =  incident  for  head  g i v e n by C e r v e n y and R a v i n d r a  sinc9(0*)/v.  coefficient at  the emergent  the  at  t h e head  critical  wave  angle,  amplitudes, calculating  refracted  little  added  previous  rays  effort  on t h e method  of  t h e model  order  parameterization  amplitudes  (i.e.  obtained  was made i n  the  of  reflected in  reflected  calculate  is  also  rays  and  cost,  an  and d i r e c t  which  independent  of  the  of  head  thin wave  applicable of  for  direct  Thus, check  ray amplitudes  an e s t i m a t e  Rays  i n terms  to  h a v i n g no t u r n i n g p o i n t s ) . or  the  ray.  A l t h o u g h the p a r a m e t e r i z a t i o n layers  boundary  and R3 , i s  A l t e r n a t i v e A p p r o a c h f o r R e f l e c t e d and D i r e c t  homogeneous  waves.  as  the t r a n s m i s s i o n ray  frequency  ray  with  rays very  may  be  f o u n d by  the  tube a r e a  is  67  made.  F o r a model  interfaces, Ravindra,  L  the  r  equation  livA/  t h e number of j=1.  3.9  in  a ray c o n t a i n s  the  ray  thin ray  its  the  ray  as  r e f l e c t i o n , but  from  the  After and  generated a  surface  product  layers  i n t h e c a s e of  are  well  (1982).  The  a n d t h e use turning  gradient,  a stack  the  the a m p l i t u d e of  of  rays.  approximated  i s due t o c o n s t r u c t i v e layers  factor  by of  turning  the  single  interference  (McMechan and M o o n e y ,  1980;  1974).  Synthesis for  are  the d e s i r e d  calculated,  (1980).  reflections, synthetic  the d i s p l a c e m e n t s  The s e i s m o g r a m s a r e  spaced d i s t a n c e s  is a  homogeneous  by j u s t  of  by s u p e r i m p o s i n g  Mooney  of  rather  particular distance.  equally and  waves  the  However, the a m p l i t u d e of  stack  amplitudes  head  and  a r e f l e c t i o n from the base of  W i g g i n s and H e l m b e r g e r ,  Seismogram  (3.9)  1 / 2  ray segments  approximated  entire  \  (c- )]  traveltime  layers.  basal  dipping  g i v e n by ( C e r v e n y and  a t u r n i n g p o i n t due t o t h e v e l o c i t y  and  homogeneous  \~1  i * l cos20'  terms  path  i s not w e l l  3.3.4  cos[e(0.)  s h o u l d n o t be a p p l i e d  If  treating  plane  T h i s method was a p p l i e d by C a s s e l l  parameterization equation  1  n  [lj = 1 vTJ\J = 1 v,  1 for  with  2.174)  l/v;  I  layers  function L is  J~  s  s  \[ L  where s i s equals  homogeneous  spreading  1971,  =  of  u s i n g the  Associated  traveltime,  an  same  seismograms  of a l l a r r i v a l s produced at  algorithm  w i t h each ray t h a t  epicentral  refractions  distance,  as  a set  are at of  McMechan  reaches a  the  complex  a m p l i t u d e and a t r a v e l t i m e b r a n c h ID n u m b e r . F o r a g i v e n b r a n c h ,  68  amplitude  and  traveltime  desired distance. a  linear  3.4  of  The s e i s m o g r a m  w i t h an a p p a r e n t  linearly  A phase-shifted  combination  transform.  are  source  a  interpolated  impulse  unit  is  the  then c o n s t r u c t e d  impulse  synthesis  to  and  its  by  Hilbert  i s c o m p l e t e d by c o n v o l u t i o n  function.  Results As a f i r s t  amplitudes  test  are  of  the  synthetic  calculated  The f i r s t  6.4  The  km/s.  km/s,  below  layer  velocity  which  is  a  routine,  for a two-layer l a t e r a l l y  model w h i c h c o u l d r e p r e s e n t , the m a n t l e .  seismogram  for example, is  at  t h e t o p of  small  homogeneous  the e a r t h ' s  30 km t h i c k w i t h  a  crust  over  velocity  the second  velocity  ray  layer  gradient  of  of  is  8.0  0.0226  km/s/km. F i g u r e 3.2a reflected in  shows t h e v e r t i c a l component a m p l i t u d e s  f r o m t h e b a s e of  the second  layer.  the  first  As w e l l ,  l a y e r and of  amplitudes  h e a d wave t h a t  would propagate a l o n g the  layers  the assumption  under  immediately below the h e a d wave i s  may  expressions  be  ray.  of  reflected, directly  the a n a l y t i c v a l u e s  Either  to asymptotic (1)  pure  between  the  the dominant frequency  calculated  a l g o r i t h m agree very w e l l .  t y p e of  the  the v e l o c i t y g r a d i e n t  ( C e r v e n y and R a v i n d r a ,  two a p p r o x i m a t i o n s  refracted  shown f o r  interface  rays  is  zero  of  the  H z . Because the v e l o c i t y model  ART a m p l i t u d e s  also  in Figure 3.2a, our  interface;  assumed t o be 6 . 4  one-dimensional, waves  that  are  rays  of  1971,  is  refracted  and head  from s i m p l e  analytic  Figure 6.6).  and the v a l u e s  As  shown  calculated  by  I n t h i s a l g o r i t h m we make one o f ray t h e o r y ,  we a p p r o x i m a t e  9r/90o  depending  on  ,the d e r i v a t i v e  the of  69  FIG.  3.2. (a) Vertical component amplitudes of reflected, refracted and head waves f o r a t w o - l a y e r m o d e l . The f i r s t l a y e r i s 30 km t h i c k a n d has c o n s t a n t velocity 6.4 km/s; the second layer has v e l o c i t y 8 . 0 km/s a t t h e t o p a n d a l i n e a r v e l o c i t y g r a d i e n t of 0 . 0 2 2 6 k m / s / k m . The h e a d wave amplitudes, c a l c u l a t e d f o r a d o m i n a n t f r e q u e n c y of 6 . 4 H z , a r e t h o s e t h a t w o u l d be p r o d u c e d a s s u m i n g t h a t t h e v e l o c i t y g r a d i e n t i s z e r o f o r a s h o r t d i s t a n c e i m m e d i a t e l y below the i n t e r f a c e between the layers. The a n a l y t i c values were calculated using simple expressions for a laterally homogeneous t w o - l a y e r m o d e l and were t a k e n f r o m C e r v e n y a n d Ravindra (1971, Figure 6.6). No surface conversion coefficients have been included. (b) V e r t i c a l component a m p l i t u d e s f o r the t w o - l a y e r model u s i n g the a l g o r i t h m s of McMechan a n d Mooney (1980) and C a s s e l l ( 1 9 8 2 ) .  AMPLITUDE  OL  AMPLITUDE  71  range  with  respect  to s t a r t i n g  f r o m two  neighboring  velocity  gradient  rays  in  homogeneous  layers.  and  calculated  those  effect  of  3.2b  calculated Cassell  routine, (pers.  comm.,  the  reflection  greater due  as  McMechan  Mooney  1983).  Head  Cassell  Figure 3.2b.  Cassell's  by a n e a r l y c o n s t a n t because  Cassell  geometrical  for  reflections  factor  the  P a r t of  the quite  this difference  for  the  is  pre-critical  arises  paper  becoming  because  the  velocity/density (McMechan,  pers.  n o t computed by t h e McMechan a n d  of  t h e head  waves  underestimate  of about  calculated  (1971).  by  from the a n a l y t i c v a l u e s  2.5.  f u n c t i o n , and not  from Cerveny and R a v i n d r a  of  pre-critical  the a n a l y t i c  This difference  (1982) u s e s an a p p r o x i m a t i o n f o r  spreading  Cassell  agree  difference  possibly  different  values  Cassell  a n d Mooney a l g o r i t h m  the  in this  waves a r e  the  model  the  the a m p l i t u d e s  McMechan  which  two-layer  r o u t i n e uses a d i f f e r e n t  a l g o r i t h m are  thin values  were s u p p l i e d by  calculated  Mooney m e t h o d . The a m p l i t u d e s the  the  However,  values,  r e l a t i o n s h i p t h a n t h e one u s e d comm.,  analytic  values  becomes l e s s .  coefficients, and  the  values  of  o f McMechan a n d Mooney ( 1 9 8 0 )  values.  analytic  different  reflection  for  the wide-angle  for  smooth  minimal.  amplitude  analytic  the d i s t a n c e  to  t o ART i s  amplitudes  of  a  number  the  For both a l g o r i t h m s ,  and  the  large  value  a l g o r i t h m thus assures that  algorithms  amplitudes  from  a  r e c e n t l y updated,  1983).  its  approximate  between  our  The  was  waves  with  diverge  the  (1982).  which  refracted well  by  by e s t i m a t i n g  we  by  The a g r e e m e n t  shows  using  (2)  block  our a p p r o x i m a t i o n s  Figure  and  a  or  angle,  the  exact  in  values arises  the head  wave  expressions  72  For  a  additional those  more  c h e c k of  complex  model,  by  the  method p r o d u c e s  the  full  also  tests  used  Miiller  (1971). R e f l e c t i v i t y seismograms,  were  the  originally  HILDERS  calculated  ( 1 9 7 1 ) and have been u s e d as methods  model  of C e r v e n y e t  al.  for  a b a s i s of  To  determine  velocity  gradient  i n the depth range  10 homogeneous  the  to calculate  layers,  the  470 V / 8 was a p p r o x i m a t e l y In F i g u r e  3.3b  are  displayed.  CPU  time  on  reflectivity  algorithm  are  algorithms. inherently  at  amplitudes  shown i n  of F u c h s Figure  comparison  11-27  470  and the deeper  in  The  also consistent  The  by o u r  change  abruptly  ART and the  in  CPU  by the  time  The  3.3c)  agreement is  seismograms  with  the  satisfactory  for  produced  our  by  w i t h t h o s e p r o d u c e d by o t h e r ART  some d i f f e r e n c e s For example, 4.5  procedure  14 s e c o n d s o f  H o w e v e r , f o r any ART a l g o r i t h m , r a y t h e o r y  approximately  the  gradient  by 3 l a y e r s .  V/8.  (Figure  purposes.  a n d so  3.3c,  3 minutes.  Amdahl  limited,  and  km was r e p r e s e n t e d  the seismograms c a l c u l a t e d  the r e f l e c t i v i t y r e s u l t s . section  3.3a)  velocity  r e f l e c t i v i t y s e i s m o g r a m s on an Amdahl  seismograms  interpretational  the  The  The c a l c u l a t i o n t o o k a p p r o x i m a t e l y the  of  r e f l e c t i v i t y response,  d e p t h r a n g e 3 4 - 3 6 km was r e p r e s e n t e d required  the  ( 1 9 7 7 ) , McMechan a n d Mooney (1980)  (1982).  of  from  t h i s m o d e l by F u c h s and M i i l l e r  Cassell  a stack  the  validity  (Figure  with  Since  response  the  an  amplitudes  ray t h e o r y .  model  model,  method.  wave  inherent in asymptotic  is  the  reflectivity the  comparison  approximations  homogeneous  our a l g o r i t h m compares  calculated  reflectivity  laterally  s and  exist  itself  compared  to  a c u s p a p p e a r s on t h e ART  160 km ( p o i n t C ) , where  from a l a r g e  is  value  to zero at  the  greater  73  V (KM/S)  DISTANCE  FIG.  (KM)  3.3. S y n t h e t i c s e i s m o g r a m s f o r t h e HILDERS v e l o c i t y - d e p t h m o d e l ( a ) . R e s p o n s e shown i n (b) i s t h e synthetic section calculated by our ART m e t h o d , while that in (c) is c a l c u l a t e d by the reflectivity method. A i l traces are m u l t i p l i e d by a f a c t o r p r o p o r t i o n a l t o t h e i r d i s t a n c e .  74  distances. at  160 km a n d i s  occurs 27  On t h e r e f l e c t i v i t y s e c t i o n  when t h e b r a n c h o f  km d e p t h  gradient just  greater distances.  rays  reflected  ( b r a n c h CD) j o i n s  grazes the  are  interface  i n ART but n o t  present  in  in  the  wave  between t h e ART a n d r e f l e c t i v i t y amplitude velocity  branch of  about  amplitude  of  are  to  due  along  8.0  from  ART  turning distances gradient  interface  section rays  due  within  greater  27  km  That to  zone  190  km.  180 km w i t h  The  distances  less  also than  generate  the  layers  3 4 - 3 6 km d e p t h . On t h e ART  depth, there  and  a  i s no e f f e c t  only  reach  However,  head  so t h e  by  present  (1974). A l t e r n a t i v e l y , the  8.0  interpretational  purposes  velocity  i n the e n t i r e  gradient  In F i g u r e  3.4,  as  and  on since  surface  replacing  increase  Cerveny  et  km/s b r a n c h c o u l d be m o d e l l e d turning  at the  t h e ART  reflections  190 km, f o r w h i c h a m p l i t u d e s (1982)  wave  amplitude  structure, the  pre-critical  t h i s was done by C a s s e l l  the  amplitudes  z o n e by one o r more s m a l l s t e p d i s c o n t i n u i t i e s , could  small  section,  from  the deeper g r a d i e n t  this  than  is,  difference the  120 t o  been m o d e l l e d s i m p l y as  at  which  non-Fermat  A second  reflectivity  at  the  t e r m i n a t i o n of  concerns  reflections  the deeper g r a d i e n t  algorithm  distance;  sections  the  pre-critical  decreases with distance. the  zone".  at  in  ray  where  approximately  k m / s . On  t h e b r a n c h EF has  the  to a sharp  the branch i n c r e a s e s w i t h d i s t a n c e .  approximating section,  (EF)  interface  turning  theory,  "shadow  smaller  t u r n i n g rays  The  corresponds  is  The c u s p i n ART  from the  t h e b r a n c h of  z o n e a b o v e 27 km ( b r a n c h B C ) .  the branches rays  non-zero at  the amplitude  at with al. for  r a y s p r o d u c e d by a weak  r e g i o n b e l o w 27 km d e p t h .  r a y s a n d v e r t i c a l component  amplitudes  are  75  displayed (Figure next  for  3.4a)  to  a  a  simple  laterally  c o n s i s t s of  a block of  dipping  d i p p i n g boundary velocity The  gradient  model  clockwise  is  6.0  of  km/s.  0.08  actually  in  thus  algorithm,  30°.  the  The  v a l u e s have  values  determined  excellent,  program-calculated  Figure  values  by l e s s  model d i s c u s s e d  Mooney are  3.5.  (1980),  i n McMechan  lateral In  routine u t i l i z e d a given block are boundaries.  In  Figure  seismogram  as  tilted of  ray  a s p e c t s of  our  against  corresponding  the agreement  the set  Mooney  of  from  is the  r a y s shown i n  (1980),  and  vertical and  3.6a,  Clowes  in Figure  3.6b  calculated  the s y n t h e t i c  (1979)  gradients ray  tracing  the v e l o c i t y g r a d i e n t s  into either  and i n F i g u r e  velocity  t o one of  velocity gradients  we d i s p l a y  is  Valley  r o u t i n e e m p l o y e d by McMechan  u n i f o r m and p e r p e n d i c u l a r  approximated  Imperial  in  w e l l as  lateral  the  shown  i n our a l g o r i t h m ,  section  contains  and  Whittall  3 top layers  V a l l e y model,  3.6c  for  been  differing  v a r y i n g model,  tracing  the  Thus t h e  V a l l e y model are the  laterally  In the ray  allowed.  e a c h of  boundary.  3.4a.  A more c o m p l e x  Figure  to the  the  3.4b,  upper  constant  checked  for  values  t h a n 0.1%  the  calculation  been  km/s  a  two-dimensional  m o d e l . As shown i n F i g u r e  analytic  has  homogeneous b u t h a s  one-dimensional the  v e l o c i t y 5.0  The d i p p i n g l a y e r  by  exercises  analytical  constant  The model  which the v e l o c i t y at  laterally  and t h e a m p l i t u d e  ART  model.  km/s/km p e r p e n d i c u l a r  from the v e r t i c a l  amplitudes  exact  is  layer  varying  3.5  the  i n the  within block  Imperial  by c r u d e l y  dividing  2 or 3 b l o c k s .  ray paths through our the  corresponding  by o u r a l g o r i t h m i s  seismograms for  the  Imperial synthetic  shown.  Imperial  Figure Valley  76  FIG.  3.4. (a) Ray t r a c i n g d i a g r a m f o r a s i m p l e t w o - d i m e n s i o n a l m o d e l . The two d a s h e d l i n e s r e p r e s e n t t h e b o u n d a r i e s of a layer dipping at 3 0 ° , w i t h i n which the v e l o c i t y i n c r e a s e s l i n e a r l y f r o m 6 . 0 km/s a t t h e u p p e r b o u n d a r y t o 8 . 0 km/s a t t h e l o w e r b o u n d a r y . The r e g i o n a b o v e t h e d i p p i n g l a y e r has c o n s t a n t v e l o c i t y 5 . 0 k m / s . The m o d e l i s a c t u a l l y l a t e r a l l y homogeneous but tilted from the vertical by 3 0 ° . (b) V e r t i c a l component a m p l i t u d e s f o r t h e model in (a). The s o l i d l i n e r e p r e s e n t s t h e a m p l i t u d e s c a l c u l a t e d by t h e t w o dimensional synthetic s e i s m o g r a m r o u t i n e . The c r o s s e s a r e the exact ART v a l u e s determined for the equivalent laterally homogeneous model using simple analytic expressions. Amplitudes, which are relative to the a m p l i t u d e on t h e u n i t s p h e r e , s h o u l d be m u l t i p l i e d by 1 0 ~ 3 .  DISTANCE 0  0  20  20  40  40 DISTANCE  (KM) 60  60 (KM)  80  80  100  100  o  DISTANCE (KM) 20 40 60  0  1.8  ; 0.71  I  80  2.0 ; 0.775  IO t z — — ZI5T;X5IZ — 333EpI«SI ZZ _ 5.65  ; 0.035  I  5.65  ; 0.022  in _ 7.2  ; 0.05  o ca FIG.  3.5. The I m p e r i a l V a l l e y model s i m i l a r t o that used by McMechan and Mooney ( 1 9 8 0 ) . L a t e r a l v e l o c i t y g r a d i e n t s i n t h e t o p 3 l a y e r s have been a p p r o x i m a t e d by dividing each layer i n t o b l o c k s . V e l o c i t i e s (km/s) are g i v e n f o r the t o p of e a c h l a y e r f o l l o w e d a f t e r t h e s e m i c o l o n by t h e velocity gradient (km/s/km).  - J 0 0  79  DISTANCE  20  FIG.  (KM)  40 DISTANCE (KM)  60  80  3.6. (a) Ray p a t h s t h r o u g h t h e I m p e r i a l V a l l e y model of F i g u r e 3 . 5 . (b) S y n t h e t i c seismograms calculated by our algorithm. (c) Synthetic seismograms calculated by t h e r o u t i n e of McMechan and Mooney (1980) for their Imperial V a l l e y model, i n which l a t e r a l l y varying v e l o c i t y g r a d i e n t s are allowed. Amplitude scaling in (b) and (c) is p r o p o r t i o n a l t o square root of d i s t a n c e .  80  model  calculated  by t h e  r o u t i n e of McMechan a n d Mooney  The s e i s m o g r a m s d e t e r m i n e d by o u r r o u t i n e closely  (Figure  calculate  ray amplitudes  3.6c).  seconds r e s p e c t i v e l y , produce  corresponding  important  that  it  example  is  reflections,  a s p e c t of  allows  the  arrivals  times  to  each  modelled  turning rays  the  synthetic  modelling  shown i n F i g u r e  3.7.  of  in  19 to  include  5  et  al.  (1983) and  purpose  of  between  observations  presenting  layers,  interpretation, changes i n  and  but  rather  structure  interpretations,  that  especially  the  The  necessary because of continental  to can  transition  features between  to the  arise  seismic  in  in regions 3.7  thicknesses,  of  independent the  oceanic  model and  type  depicts  the  each  the  the  lateral  activity subduction  J u a n de F u c a  region  of  continental  Vancouver  oceanic  which is  thus  plate  in structure  between of  main  refraction  interpretations. are  of  where t e c t o n i c  the oceanic in  The  discuss  illustrate  the great d i f f e r e n c e  l i m i t s by o t h e r  important  or  is  t o show a g r e e m e n t  two-dimensional v a r i a t i o n s  crustal  An  discussed  (1983b).  i s not  synthetics  m a r g i n between  large  structures.  the data are  al.  example  and t h e c o n t i n e n t a l A m e r i c a p l a t e Island.  of  et  o c c u r r e d . The m o d e l i n F i g u r e across  algorithm  The d a t a upon w h i c h t h e m o d e l  Clowes  this  seismogram  complex  and p r e l i m i n a r y i n t e r p r e t a t i o n s  in E l l i s  within  CPU  (1980)  f r e e - s u r f a c e m u l t i p l e s PP a n d P P P .  An  zone  agree  on an Amdahl V / 8 were 5 s e c o n d s a n d  t h e s e i s m o g r a m s . The t y p e s o f  and t h e  has  The  w i t h an a d d i t i o n a l 8 s e c o n d s f o r  p r e - and p o s t - c r i t i c a l  based  3.6b)  w i t h t h o s e d e t e r m i n e d by t h e McMechan and Mooney  algorithm  is  (Figure  (1980).  are and  controlled The  the d e t a i l s crusts.  most of  the  These  81  FIG.  3.7. (a) S c h e m a t i c d i a g r a m of t h e s u b d u c t i o n z o n e between the o c e a n i c J u a n de F u c a p l a t e and t h e c o n t i n e n t a l A m e r i c a p l a t e i n the r e g i o n of Vancouver Island. The range of v e l o c i t i e s i n km/s i s i n d i c a t e d f o r e a c h l a y e r . The d i a g r a m does not i n c l u d e the d e t a i l s of a l l l a y e r s or b l o c k s t h a t were u s e d f o r t h e r a y t r a c i n g , (b) Ray t r a c i n g d i a g r a m for the Vancouver Island subduction model. (c) Synthetic seismograms f o r the s u b d u c t i o n m o d e l . Far-offset arrivals are reflections from t h e dipping upper m a n t l e h o r i z o n . Near-offset a r r i v a l s include turning rays in the upper mantle, and large upper m a n t l e r e f l e c t i o n s as secondary arrivals.  o w  CD -  CO  < <r -r < <r <r  in  CO \ Q  I  c. CO  180  nttt  200  220  240 260 280 300 DISTANCE FROM P19 (KM)  320  340  360  83  include: at  (1) the s h a l l o w - d i p p i n g h i g h - v e l o c i t y  20-26  km  depth over the d i s t a n c e range  finger  (7.8  150-240 km,  km/s)  and  (2)  the upper mantle v e l o c i t i e s of 8.0-8.3 km/s  below the subducting  oceanic c r u s t . The  is itself  defined, through  since  subducting oceanic c r u s t  all  Only  three types of rays - two  set of t u r n i n g rays - are t r a c e d (Figure  these  represent  the  (Figure  3.7c),  arrivals  of  of  most  model  the  because  interest  kinematic  total  reflections  3.7b)  r e s u l t a n t seismograms are thus but  the  s e t s of  and one  i n t e r p r e t a t i o n . The  well-  rays take a s i m i l a r amount of time to pass  i t and t h i s time i s only a s m a l l p o r t i o n  traveltime.  not  for  very and  the  simple dynamic  c h a r a c t e r i s t i c s of the observed data reasonably w e l l .  3 . 5 Di s c u s s i o n A fast, practical theory  amplitudes  has  method  of  calculating  asymptotic  been implemented u t i l i z i n g an  technique f o r ray t r a c i n g through l a t e r a l l y v a r y i n g The  types of a r r i v a l s which may  r e f r a c t i o n s , p r e - c r i t i c a l and reflections  be modelled  wide-angle  and m u l t i p l e s . Amplitudes  in a one-dimensional  efficient  structures.  i n c l u d e head waves,  reflections,  surface  c a l c u l a t e d by t h i s method  medium are c o n s i s t e n t with those  determined  by the r e f l e c t i v i t y method, with the e x c e p t i o n of c e r t a i n of a r r i v a l s a r i s i n g theory  method  i s overshadowed by the c a p a b i l i t y of the r o u t i n e to  The  routine  types  from wave phenomena which any asymptotic  cannot d i r e c t l y handle. However, t h i s  in two-dimensional  ray  ray  limitation be  applied  structures. is  i n t e r p r e t e r s of seismic  intended  as  refraction  a practical tool data.  In  f o r use by  comparison  with  84  other asymptotic algorithm arise  are  ray theory methods, its  because of  the major  s p e e d and e a s e of  the nature  arbitrary  of  the  polygonal  use.  by  velocity  and l i n e a r v e l o c i t y g r a d i e n t .  e.g.  wedges  or  parameterization velocity  or  fault  is  its  for  gradient  blocks,  model,  limiting  vary continuously  its  other  hand,  the  models  where  the  in a lateral  a p a s s i v e c o n t i n e n t a l margin p r o b l e m . However, such  can  be  blocks.  A  several without A  scheme  number  automatically  could  be  to  to  of  modifications  further  enhance  ART  which  of  approximations practical  critical for  basis,  its  a  models adjacent  block  into  the  into  program  it  structure of  the  s e e n w i t h i n a few  points such  the  algorithm  is  possible  for  certain  types  and h i g h  diffractions.  a primary requirement.  of  wave  Changes  in  frequency  On  a  more  interactive  to the  input  be made e v e n more q u i c k l y , a n d  c h a n g e on t h e a m p l i t u d e and t r a v e l t i m e seconds.  be  incorporate  to ray theory amplitudes  and c a u s t i c s , as  to  p r o g r a m c o u l d be a d a p t e d f o r  c o u l d thus  could  a p p l i c a b i l i t y . To c o m p e n s a t e  account  waves the  where s p e e d i s  effect  to  These i n c l u d e c o r r e c t i o n s  region  velocity  laterally  divides  incorporated  l i m i t a t i o n s i n ray t h e o r y ,  behavior.  use,  by s e v e r a l  sense,  great d i f f i c u l t y .  additions  the  which  sub-blocks  implemented for  approximated  own  features,  e.g.  adequately  is  specification  tectonic  the for  which  with  Such model  On  the  characteristics  each  some t y p e s of  blocks.  somewhat  These  velocity  defined  is p a r t i c u l a r l y appropriate  a d v a n t a g e s of  the  behavior  85  CHAPTER  INTERPRETATION OF ONSHORE-OFFSHORE PROFILE ACROSS VANCOUVER ISLAND  4.1  Introduction Seismic  refraction line  I was d i r e c t e d a c r o s s t h e  the B r i t i s h Columbia c o n t i n e n t a l  margin  onshore-offshore  the  structural  provides  basic  model of R i d d i h o u g h (1979),  de F u c a p l a t e Seventeen, slope  line  subducts  shots  (the P s e r i e s )  and ocean b a s i n ,  seismographs  beneath the  (see  Fig. data  1.3). to  continental  were f i r e d  on  Vancouver  of  and J2)  were f i r e d  at  the p r o f i l e . For the oceanic  distance  was  crust  typically  from  the  93  km  and  shot p o i n t s near shots/  of  are  expected  km  l e a s t a p o r t i o n of t h e i r  interpretation  I provides  velocity  line  and t h e l o c a t i o n of  and c r u s t a l  two-dimensional  as  common s h o t  ~7  km  the e a s t e r n  end  by  shot-receiver  or  oceanic  less,  paths.  i n f o r m a t i o n about  the contact  between  common gathers,  s h o t s and r e c e i v e r s  interpretation  zone v e l o c i t y s t r u c t u r e . either  British  rays  Thus,  upper upper  the  mantle mantle  material.  The d i s t r i b u t i o n o f a  land  t o t r a v e l t h r o u g h upper  mantle m a t e r i a l for at of  32  a n d t h e maximum was 350 km. S i n c e  shots  Juan  plate.  the  t h e minimum  10  the  over the c o n t i n e n t a l  Island  has a t h i c k n e s s o f a b o u t  oceanic  test  America  C o l u m b i a m a i n l a n d . An a d d i t i o n a l two s h o t s s e p a r a t e d ( s h o t s J1  of The  i n which the oceanic  a n d were r e c o r d e d on an a r r a y  located  strike  shot  In  this  along l i n e  t o be made o f thesis,  data  I  allows  the  subduction  are  displayed  g a t h e r s o r common r e c e i v e r g a t h e r s .  recordings  on  all  32  receivers  from  In a  86  particular  shot  receiver  receiver  to  structure  us  receivers, variation For line  I,  so  the  i n the  On common r e c e i v e r  from  structure  beneath  of  interpretation  the is  of  the oceanic  oceanic  crust,  well-determined constrained a  crust  the  continental intersection  crust  ( W a l d r o n 1982)  to about  Spence  low-velocity  melange  were  point  structure  was  m a r g i n and p e r p e n d i c u l a r  Here,  continental  of  about  20  km  while  crust  the  l o w - v e l o c i t y zone and a c r u s t a l  as  l a r g e as  52 km.  the  1983).  interpretation  interpretations  for  is  thick found.  portion For  the  relatively less  sediments For  the  at  its  roughly p a r a l l e l (Fig.  crust the  to a  to  1.3). depth  was  poorly  lower  crust  thickness  a l l o w e d the  of  known  and somewhat  lower c o n t i n e n t a l  included  alternative  about  finding  was w e l l - d e t e r m i n e d  The p r e f e r r e d  however,  shots.  and a  to l i n e I  constrained. a  shot  the  controlled  w i t h l i n e I V , w h i c h was  the c o n t i n e n t a l the upper  basin where  the  to  beneath  9 km d e p t h was  slope,  crust,  to  the  receiver  the  information  the c o n t i n e n t a l  possible  gathers,  layers.  beneath the deep o c e a n i c  under  the  the o n s h o r e - o f f s h o r e p o r t i o n  (McMechan and  structure  about  shot  and  then reduced  the uppermost l a y e r s about which much  from  information  shots  the deeper of  data  on a p a r t i c u l a r  variation  beneath  structure  i n the  information  have independent  interpretation  the c o n t i n e n t a l  and  case the  layers  the  provide  17 s h o t s r e c o r d e d  we may a l r e a d y  the  include of  to  about  uppermost  and t h e v a r i a t i o n  receivers.  in this  more  Furthermore, the  tends  from a l l  displayed;  tells  shown  beneath the  seismograms are  are  of  thickness  37 km; to  be  87  4.2  Interpretation S h o t s J1  recorded  the  the c o n t i n e n t a l  for  shot  is J1  by a f a c t o r very  was  less  obtained  shown i n F i g u r e  (see  have  replacement the  been  170  corrected 5.5  km/s  near s u r f a c e m a t e r i a l  at  the  of  shot J2  the for  the  J1  s h o t J1  crustal  seismic  section  The s e c t i o n  for  shot J2  although on  there  some  and  J2,  of  is  appears  the  more  the  shots  and  by  using  a  both the water-column  and  level  receivers. and  upper  shot-  J2  data w i l l  Because  record  of  the  sections,  a l s o be v a l i d  for  an the  data.  A  preliminary  s h o t J1  interpretation  was p r e s e n t e d  one-dimensional programming 1979).  shot  for  multiplied  sea  for  control  are  J1 to  and  amplitudes  even  shots  inlet  maximum  only The  Fig. A1.1),  clipping  the  km,  4.1b;  of  interpretation  starting  i n v e r s i o n of  The i n i t i a l  dimensional for  1,  both  velocity  similarity  for  For  additional  Since  to distance.  Appendix  traces.  receivers  than  i n a deep  from these s h o t s .  t o be some i n d i c a t i o n o f distant  provide  structure.  proportional  similar  J2  w h i c h were d e t o n a t e d  crustal  is  and  land receivers,  offset  information  s h o t s J1  and J 2 ,  on  receiver  of  ray  incorporated  tracing  The ( 1 9 8 3 ) has  west of  the  interpretation been  and t o a c c o u n t  further for  model the  model  which the p r e f e r r e d  in E l l i s  et was  first  was  of  al.  arrival (1983).  determined  arrival  then  routine  first  times  modified  of W h i t t a l l  An by  of  shot  refined  J1  point  a  using  and C l o w e s  of  linear  lines  t o model  In p a r t i c u l a r ,  et  the  al. two-  (1979),  (1983)  was  I and I V .  presented in E l l i s  i n an e f f o r t  secondary a r r i v a l s .  initial  (Garmany  m o d e l of McMechan and S p e n c e intersection  traveltimes  et  al.  amplitudes an  attempt  88  FIG.  4.1. S y n t h e t i c seismograms and o b s e r v e d d a t a f o r shot J 1 , w h i c h i s l o c a t e d on t h e r i g h t hand s i d e of the sections. The first arrival traveltime picks are i n d i c a t e d by arrowheads. The theoretical traveltime curve from the synthetic seismograms is superimposed on t h e observed r e c o r d s e c t i o n . The d a s h e d l i n e c o r r e s p o n d s t o reflections from the base o f t h e 6 . 7 km/s c o n s t a n t v e l o c i t y l a y e r i n F i g . 4 . 2 . A l l a m p l i t u d e s have been m u l t i p l i e d by a factor p r o p o r t i o n a l t o d i s t a n c e . The d i s t a n c e s c a l e i n (a) a n d t h e top scale in (b) a r e measured r e l a t i v e t o shot P 1 9 , the westernmost of the P s e r i e s s h o t s i n the deep o c e a n .  89  was made t o m o d e l  the  higher  amplitude  second  shot  greater  J1  velocity  of  of  of  the  the  arrivals,  been  first  The 4.2a.  some o f  final  West  of  interpretation portion  Figure  theoretical section  the t r a c e d Chapter displayed  of  is  Vancouver  Island  upper  receivers  are  5.3  4.1b.  constrained  IV,  The  km/s.  apparent  denoted  by a  apparent  A similar  km/s a n d 6 . 7 5  set  km/s,  had  line  IV  shots along  shot  J1  in  in  is  and  the  from the  the  superimposed  Synthetic  Figure  Spence  (1983)  except  for  the  interpretation  following  throughout  are  shown i n  model,  u s i n g the  of  section.  In  t h e m o d e l , and  the  on t h e J1  record  seismograms c o r r e s p o n d i n g routine  t h e s i s and by Spence et  km/s,  al.  the v e l o c i t y of  somewhat  described (1984),  shot  because  the near  l e s s than t h a t  by McMechan and S p e n c e is  determined  J1.  This portion  and  to in are  by of  were f a r  from  surface  d e t e r m i n e d on  ( 1 9 8 3 ) . The v e l o c i t y  of  the  at  the  first  the  arrivals  model  r a y p a t h s were n o t r e v e r s e d  and because the r e c e i v e r s line  from  4.1a.  layers near  6.9  for  traced  i n t e r p r e t e d model,  material  6.5  McMechan  considered  this  distances  km/s w h i l e t h e  about  km w h i c h o r i g i n a t e s  in Figure  In the  the  km,  r a y s were c a l c u l a t e d  3  the  1983).  incorporated  Figure  and  which i s  the Vancouver I s l a n d  model t r a v e l t i m e s  in  6.7  velocities  the  rays  arrivals  4.1b).  phase,  phase i s  240  shots  4.2b,  (Fig.  i s about  model  is  first  o c c u r r i n g at  km  velocity  b e l o w ~16  the P s e r i e s  4.1,  with  (McMechan a n d Spence  110  arrival  secondary but  found for  arrivals  than about  dashed l i n e i n F i g u r e velocity  low-amplitude  is  in this  poorly region  intersection  where some t w o - d i m e n s i o n a l c o n t r o l was a v a i l a b l e .  with For  90  FIG.  4.2. V e l o c i t y m o d e l d e t e r m i n e d f r o m s h o t J1 d a t a , a n d r a y p a t h s t r a c e d t h r o u g h the m o d e l . I n t e r p r e t a t i o n of McMechan and Spence (1983) t o a d e p t h o f 16 km i s u s e d west o f 240 km d i s t a n c e . D e t a i l s i n t h e model below t h a t depth are d e r i v e d fom i n t e r p r e t a t i o n o f t h e P s e r i e s s h o t s . D i s t a n c e s are measured r e l a t i v e t o shot P 1 9 , the westernmost shot i n t h e d e e p o c e a n b a s i n . The a r r o w h e a d s i n d i c a t e t h e w e s t and east coasts of Vancouver I s l a n d . V e l o c i t i e s (km/s) a r e g i v e n f o r the t o p of each b l o c k , f o l l o w e d a f t e r the colon by t h e v e l o c i t y g r a d i e n t ( k m / s / k m ) i f one was u s e d .  D E P T H (KM)  16  DEPTH  (KM)  92  simplicity, constant  i n the d i s t a n c e  reflections  been  range of  different  for example,  w i t h the  Below 3.5 increases  the upper  first  km d e p t h  from  6.33  distance  arrivals  range of  for  past  velocity shot  a  increases  amplitude  phase  back t o t h e the  speculative.  ray  at  km/s t o 6 . 9 5  rays.  velocity of  0.018  arrivals  the  in  secondary,  is  arrivals  interpreted Spence  in a  (1983)  modelled  km/s  as  first  turning  across which  For  s i m i l a r l y inferred, across  which  a set  the r e g i o n of 4.2b,  km/s  but w i t h a s m a l l  (Fig.  the  1.5).  reflections  region;  have  a small amplitude  16 km d e p t h ,  c o n v e n i e n c e m o d e l l e d as  the corresponding  and a r e  km/s was  t r a c i n g diagram of F i g u r e  velocity  the  first  They were  There,  i n t e r p r e t e d as  surface within  be  velocity  v e l o c i t y gradient  f r o m 6 . 4 - 6 . 5 km/s t o 6 . 7 is  the  also  to t u r n i n g  e m p l o y e d by McMechan a n d  discontinuity  corresponded  the s m a l l a m p l i t u d e p r i m a r y  from 6.75  that  the model c o u l d  form the  a  120 km.  discontinuity  increased  J1,  velocity  a  so  would  corresponding  a  as  But b e c a u s e o f  structures  region  phase w i t h v e l o c i t y 6.95  below  6 km/s,  i n t e r p r e t e d model,  l i n e I V on V a n c o u v e r I s l a n d .  arrival rays  somewhat  s i m i l a r to that  layer.  5 0 - 1 2 0 km f r o m J 1 ,  The i n t e r p r e t a t i o n of 120 km i s  this  with  in this  over  by a l a y e r w i t h c o n s t a n t  i n the  large amplitude a r r i v a l s  fashion  km was m o d e l l e d  3 o r 4 km o f  well  km/s  km/s/km. T u r n i n g rays  past  3.5  2 0 - 5 0 km f r o m J1  velocity  m o d e l l e d e q u a l l y as  gradient,  the  km t o  f r o m t h e b o t t o m of  indeterminacy, valid;  f r o m 1.0  velocity layer with velocity just  arrivals to  the r e g i o n  km/s,  and the  of a r r i v a l s 6.7  km/s  which  velocity.  these a r r i v a l s  from the bottom of gradient  t u r n i n g rays to the g r e a t e r  small  the  i n the  offset  are  turn In for 6.7  region,  distances  93  w o u l d a l s o have as  the  t o p of  t o be s h a l l o w e r east  is  point  between  the  6.7  be  km/s c o n s t a n t  (at  greater,  deeper  western  at  its  maximum  near  14 km. the 6.7  the  reflections traveltime  offset,  to  is  for  overlap.  not  direct This  contrasts  of  of  well  is,  phase  for with  reflections  were more e a s i l y  the observed  record  sections.  and  be  turning out  to  depths  modelled gradient  4.2a),  the  discontinuity reflections  which  the  been  even  to  data  be a n a r r o w  (Fig.  can  4.2b),  the evidence  Spence  ~16  in  that  are  be  existence  the d i s c o n t i n u i t y at  McMechan  an  sufficiently close  the the  If  reflections  (Fig.  are  record section  support  the e x i s t e n c e  interpretation  as  above  That  4.2).  needs  i n the  section  distinct  on t h e o b s e r v e d  discontinuity. Island  rays  occurs  need t o p e n e t r a t e t o  discontinuity  form a v e r y  identified there  record  to the t u r n i n g  waveforms  expected  the  which  the  the  crossover  Figure  km/s v e l o c i t y r e g i o n has  synthetic  from  for  in  large amplitude  present  a d i s c o n t i n u i t y , but c o u l d j u s t On  the  The d i s c o n t i n u i t y  which are  shot-receiver  The t o p o f  the  shot.  inferred  p o i n t w o u l d be a t  end b e c a u s e t h e  r a y s above t h e b o u n d a r y , the  240 km i n t h e m o d e l of  and the c r o s s o v e r  is  The d e p t h  arrivals  then the t r a v e l t i m e s  from the  their  traveltimes  v e l o c i t y zone  t h e p r i m a r y and s e c o n d a r y  greater distance  zone.  same  8 km and i s c o n t r o l l e d by t h e l o c a t i o n o f  t h e d e p t h was g r e a t e r ,  as  the  i n the east than i n the west.  km f r o m t h e s h o t  would  and a l m o s t  reflections.  The  120  small amplitudes  not  easily and  thus  of  the  on V a n c o u v e r km d e p t h  (1983),  in  where  p i c k e d on some ( t h o u g h n o t a l l )  of  94  Perhaps set  is  one o f  what i s  t h e more i n t e r e s t i n g f e a t u r e s  not  seen  perturbation  of  crossing  the  Strait  distance  range  that  in  the  traveltimes of  fault  Complex  on  Island  is  the  not o b s e r v a b l e  interpretation  of  (Fig.  the  is  4.1b).  is  terms  the  of  Georgia S t r a i t  ( W h i t e a n d C l o w e s 1984)  was no s e i s m i c  evidence  data  offset  or  observed  on  to  This  for  Thus,  Coast  a the  Plutonic  B e l t on V a n c o u v e r  velocity  contrast.  r e f r a c t i o n survey  down t o d e p t h s  An  across  also concluded that  such a f a u l t  the  implies  no e v i d e n c e  Insular  a d e t a i l e d sonobuoy  for  major  corresponds  separating  from in  J1  t h e J1  across Georgia S t r a i t .  1977)  mainland  which  14 km t h e r e  discontinuity (Muller  no  amplitudes  6 0 - 9 5 km f r o m s h o t  velocity  inferred  or  Georgia,  down t o a d e p t h o f a l m o s t  major  data:  of  of  there 3  to  bottom topography  and  4 km.  4 . 3 D e s c r i p t i o n of  P series  shots  The 17 P s h o t s were c o r r e c t e d f o r detonation depth of  depth  by  placing  a l l shots at  2600 m. I n a d d i t i o n t o a  involved  a  distance  offset  sea  time  an e q u i v a l e n t  correction,  of each shot  toward the  this  d e p t h t h r o u g h the model of Waldron  the sedimentary using  structure  along the marine  and  data.  significant  for  (shots  t o P6) where t h e w a t e r d e p t h r a n g e d  the f i r s t  m; f o r t h e r e m a i n i n g s h o t s depths  The  six  in  topographic  shots  the  on  ocean  the  to  of  line  v a r i a t i o n was continental  basin  were b e t w e e n 2425 and 2625 m. V a r i a t i o n  the  who d e t e r m i n e d  portion  CSP  P1  OBS  (1982),  also  receivers.  The c o r r e c t i o n s were f o u n d by r a y t r a c i n g f r o m e a c h s h o t datum  datum  f r o m 500 t o (P8  to  I  only slope 2100 P19),  i n topography  at  95  the r e c e i v e r s by a d j u s t i n g 5.5  receiver  i n the  e l e v a t i o n to  same manner as sea  level  for  shot  J1,  u s i n g a v e l o c i t y of  km/s. A  of  was c o r r e c t e d  c o r r e c t i o n for  sediments  basin,  basement t o p o g r a p h y  was a l s o c a l c u l a t e d  where t h e s e d i m e n t a r y  well-determined  i n the  for  the  structure  interpretation  and v a r y i n g  11 s h o t s  sediment  was p e r f o r m e d Figure  for  of  Waldron  amplitudes  shots  distance,  and  relationship  the  i n the ocean  multiplied  between  for  and  charge  weight  shot  P8 i s  i n the middle corresponds  Strait  (265-285  km f r o m s h o t  two l a n d s e i s m o g r a p h  ~15  end  km w e s t  that  the  receiver  of  The large  stations,  westernmost sections,  i.e.  the  for  size and  X45  using  are P19  shown is  in  record  825  of  kg  Georgia  gathers at  on V a n c o u v e r Figure  2 / 3  sections,  mainland  on t h e l e f t  shot/receiver  W  determined  are  location  the  true  the  amplitude  the  on  show  shot/receiver  S h o t s P19 and P13  to  three  for the  Island  4.4.  Note  of  t h e common  distance  increases  left.  most  on  shot  slope.  A l l sections  t h e p r o f i l e a n d X22 l o c a t e d  prominent  amplitude  recorded  to 1  P 1 9 ) . Common r e c e i v e r  of G e o r g i a S t r a i t ,  from r i g h t t o  thickness  sections  200 k g . On a l l t h r e e  t h e gap  eastern  The  p r o p o r t i o n a l to  charge  e x p e r i m e n t a l l y by 0 ' B r i e n ( 1 9 6 0 ) . charges,  record basin.  by a f a c t o r  corrected  was  (1982).  s h o t s P1 t o P6 on t h e c o n t i n e n t a l displays  ocean  1 . 8 k m / s . No basement c o r r e c t i o n  v e l o c i t y to  4.3  representative  the  down t o t h e basement  c o r r e c t i o n s were f o u n d by e q u a l i z i n g t h e s e d i m e n t km a n d t h e  in  thickness  the  first  phases  arrivals  stations  to  i n the e n t i r e data from  the  ocean  set  are  basin  t h e e a s t of G e o r g i a S t r a i t  the  shots (see  96  FIG.  4.3. O b s e r v e d r e c o r d s e c t i o n s f o r s h o t s P 1 9 , P13 a n d P 8 . R e c o r d s e c t i o n s a r e t r u e a m p l i t u d e , so t h a t a m p l i t u d e s may be compared from shot t o s h o t . A l l a m p l i t u d e s h a v e been m u l t i p l i e d by a f a c t o r p r o p o r t i o n a l t o d i s t a n c e . T r a v e l t i m e p i c k s a r e i n d i c a t e d by a r r o w h e a d s , and include secondary arrivals for shots P19 and P 1 3 . T i m e s and d i s t a n c e s a r e a d j u s t e d t o p l a c e the s h o t a t a d e p t h of 2.6 km, and to correct the sediment layer to a t h i c k n e s s o f 1 km and v e l o c i t y o f 1.8 k m / s . The gap near the m i d d l e of all sections i n d i c a t e s the l o c a t i o n of G e o r g i a S t r a i t (at 265285 km f r o m s h o t P i 9 ) .  98  SHOT/RECEIVER  FIG.  DISTANCE  (KM)  4.4. Observed r e c o r d s e c t i o n s showing a l l shots (three a r e i d e n t i f i e d ) r e c o r d e d on r e c e i v e r s X45 and X22, which are located t o the r i g h t of the s e c t i o n s . Record s e c t i o n s are true amplitude, with a l l amplitudes multiplied by a factor proportional to d i s t a n c e . T i m e s and d i s t a n c e s are a d j u s t e d t o p l a c e t h e s h o t s a t 2 . 6 km d e p t h , and f o r shots P 1 9 - P 8 t o c o r r e c t t h e s e d i m e n t l a y e r t o a t h i c k n e s s of 1 km and v e l o c i t y of 1.8 k m / s .  99  shots  P19 a n d P13 i n F i g u r e 4 . 3 and r e c e i v e r X45 i n F i g u r e  T h e s e a r r i v a l s have an e x c e l l e n t sharp  onset,  with  the r e c e i v e r s 8.3-8.5  an a p p a r e n t  ( F i g . 4.3)  km/s  and a  Because  as  to  pick  b u t an e s t i m a t e  i s about  are  Island of  seen Fig. is  on s h o t 4.4). that  c a n be and i t s  The  for greater  Island receivers  that  the  velocity Island  of  is  shots,  closer  is  km/s  to  poorer  having a  on  some  r a t i o . However,  of  the  Vancouver  somewhat l a r g e r of  than  that  these a r r i v a l s  are  a n d r e c e i v e r X22 ( s h o t s P13 t o P 1 9 ,  approach  of  the  7.5-7.7  arrivals  is ~8.3  it  from t r a c e - t o - t r a c e ,  is  the  arrivals  primary  arrival  offsets. f i r s t a r r i v a l s across km/s  (Fig.  4.3),  n e a r l y 8 k m / s . The  the f i r s t a r r i v a l s across the shots  station  small,  stations.  shot/receiver  velocity  second  4.4.  c h a r a c t e r i s t i c of the secondary  Vancouver of  X45 i n F i g u r e  on t h e m a i n l a n d  amplitude i s  traveltimes  apparent  the  the data q u a l i t y i s  The b e s t e x a m p l e s  A consistent  traveltimes  on  signal-to-noise  discerned  P13 ( F i g . 4 . 3 )  their  markedly w i t h  on V a n c o u v e r I s l a n d , w h i c h a r e  the primary a r r i v a l .  amplitudes  v e l o c i t i e s across these  less consistent  stations,  of  8 km/s.  more c o m p l e x wave f o r m a n d l o w e r a second a r r i v a l  a  across  velocity  P1-P6  P1-P6 are q u i t e  t h e p r o m i n e n t p h a s e s seen  signals  shots  and  km/s  The l a r g e  receiver  than the mainland s t a t i o n s ,  than t h a t of The  shots  the apparent  For the s t a t i o n s the shots  for  8.1-8.2  contrast  from the c l o s e r  the amplitudes f o r  difficult  (Fig. 4.4).  seen  ratio  apparent  ocean b a s i n shots  the d i m i n i s h e d a m p l i t u d e s slope,  v e l o c i t y of reversed  a c r o s s the shots  f r o m t h e s e more d i s t a n t  continental  signal-to-noise  4.4).  (Fig. 4.4),  for a  the  while apparent  Vancouver  which i s comparable  to  100  that  for  apparent  a  mainland  the data  is  stations  the  the  west  stations  is  feature  simply  depth-dependent  from the  major  causing  the t r a v e l t i m e o f f s e t  a depth of  form a l a r g e  secondary  ray  applied.  trace  are  In t h i s procedure,  determine  traveltimes  Adjustments  the  where  the  and i s  not  fixed than  there  beneath  no  Georgia  the s t r u c t u r a l greater  is  feature  depths.  i n v e r s i o n : s h o t s P 1 9 , PI 3 and P8 r e c o r d e d on up t o 32 combinations,  considered.  procedure  receivers especially  A model of  described  and  the  i n Chapter  partial  parameters of  t o the model are  the  To a c h i e v e  two-dimensional ray t r a c i n g  t r a v e l t i m e with respect to selected model.  to  rather  we know t h a t  f i t a l l the t r a v e l t i m e s .  inversion  all  a l o w - v e l o c i t y zone.  structure  shot-receiver  arrivals  should simultaneously  to  J1,  must be a t  the P s e r i e s  number of  fault,  s u c h as  shot  relative  location  of  on  s u g g e s t i v e of a  a  14 km. T h u s ,  Ray t r a c i n g a n d t r a v e l t i m e  when  arrival  occurring  record sections  such as  two-dimensional disruption in  The 17 s h o t s o f  s  This is  feature  i n t e r p r e t a t i o n of  to  1  The a c t u a l  i n the model,  down  the  about  offset.  Strait  4.4  secondary  e a s t of G e o r g i a S t r a i t  on s h o t / r e c e i v e r  a  of  t h e same on a l l s h o t  structural  But  the  prominent t r a v e l t i m e c h a r a c t e r i s t i c  of G e o r g i a S t r a i t .  occurs  dependent  most  a traveltime delay  for  delay  while  v e l o c i t y approaches 9 km/s.  Finally,  shots  station,  data this, 2 was  is  used  derivatives the  of  velocity  then found u s i n g the  damped  least-squares matrix inversion technique. Of t h e  17 s h o t s  i n the P s e r i e s ,  ocean b a s i n  shots P19,  and P8 were s e l e c t e d  i n the ray t r a c e  inversion procedure.  P13 None  101  of  the  continental  consistent  arrivals  sedimentary thick  and  slope were  basin  structure  were n o t as  i n the  offsets  three shots  first  shot  the  to the f i n a l the  the  arrival  example,  the  the  show  shots  no  i t was f e l t  t h e v a r i a t i o n among  upper  shots  to i n the  rays  a r r i v a l s at  the  mantle  from  a r r i v a l s at  the far six  the  horizon same  of  the  the  final  best major  that  the  all  the  development model w i l l  shows r a y p a t h s receivers.  The  marked  in Figure  form  stations,  give  reflections  the second a r r i v a l s , correspond  to  the  and sole  stations.  parameters  the  observed  t h r o u g h the upper mantle near  be  through  i n v e r s e procedure are  horizon  of  the s t r u c t u r e  are a l l o w e d to v a r y : the v e l o c i t y to  the b e h a v i o r of  F i g u r e 4.5  turning  reflections  region II  traveltimes  on t h e P 1 9 , P13 a n d P8 r e c o r d s e c t i o n s  to the f i r s t  paths  (1)  a r r i v a l s the  r e f r a c t i o n m o d e l . The  details  sections.  rise  ray  (1982).  close  In t h i s example,  Only  and t h e i r  the  i s d e m o n s t r a t e d u s i n g a model which i s  and  from  arrowheads  an  c o n s t r a i n e d as  trace  4.3.  from  well  the ray  t r a v e l t i m e s w h i c h were u s e d by  charges  onshore-offshore  in later  model  because  i n t e r p r e t a t i o n of Waldron  represented  following  model  discussed  and  shots.  inversion procedure  of  because  (including a possibly  P8 t o s h o t P 1 9 , a n d so  adequately  ocean b a s i n In  (2)  from  pick  used  P 1 9 , P13 a n d P8 were u s e d b e c a u s e  P19 and P13 were t h e l a r g e s t and  were  to  upper c r u s t a l s t r u c t u r e s  Only the t h r e e shots  recorded,  P1-P6  difficult  low v e l o c i t y m e l a n g e )  ocean  shots  near  where ray p a t h s  shown i n F i g u r e  i n r e g i o n I where t h e  r e c e i v e r s are to the far  located,  turning  the v e l o c i t y  r e c e i v e r s are  4.5  located,  in and  CD  FIG.  4.5. Ray p a t h s t h r o u g h a p r e l i m i n a r y m o d e l f o r s h o t s P 1 9 , P13 and P8. O b s e r v e d t r a v e l t i m e p i c k s shown i n F i g . 4 . 3 a r e u s e d i n t h e r a y t r a c e i n v e r s e p r o c e d u r e , by w h i c h a d j u s t m e n t s t o s p e c i f i e d r a y t r a c e p a r a m e t e r s a r e a u t o m a t i c a l l y c a l c u l a t e d so that model traveltimes fit observed traveltimes. Parameters t h a t a r e a d j u s t e d a r e upper m a n t l e v e l o c i t i e s i n r e g i o n s I a n d I I , a n d t h e d e p t h s of boundaries a t p o i n t s A , B , C and D . A r r o w h e a d s i n d i c a t e t h e c o a s t s of V a n c o u v e r Island. V e r t i c a l exaggeration is 3 : 1 .  o M  103  the depths of  two  affect  f o u r p o i n t s A , B , C and D w h i c h r e p r e s e n t  boundaries  i n the model. V a r i a t i o n s i n these  the t r a v e l t i m e s  example, the  at  if  total  traveltime in  reflecting  b o u n d a r y becomes d e e p e r ,  the  reflections  increases, becomes  then  Figure 4.5, w o u l d be  however,  the change  smaller  reflecting  points  that  Figure  iteration final  4.6. of  model  perturbations  Figure  the  4.6,  dashed solid  traveltimes.  For the  initial  traveltimes  mainly to the m i s f i t traveltime close ~75  residual  to the estimated  ms.  receivers  rays  shots, to  inverse  its  shots  shown i n receivers  because  the  receivers  all  routine  determined  from  initial  the  line  the  represents  model,the had  final  indicate  represents  an  in traveltimes for  boundary  boundary.  the c r o s s e s  line  and the  calculated  the  the  of  the  traveltimes  is  shown  a  single  model,  and  were t h e n f o u n d by r e t r a c i n g t h e  traveltimes,  and  the  If  a c r o s s the  set  from a shot  the ray trace  traveltimes  directions. total  t h e r o u t i n e were a p p l i e d t o t h e  In each p a n e l of traveltimes,  The  velocity  v e l o c i t y across the  w i t h i n a v e r y s m a l l r e g i o n on t h e of  then  velocity  across  for a l l rays  the r e g i o n s ,  across  the p a r t i c u l a r  i n apparent  For  the d i p of  velocity  apparent  for  than  The p e r f o r m a n c e in  the  If  ways.  apparent  then the  increase.  apparent  and  the  f o r w a r d and r e v e r s e  also  the  smaller  becomes l a r g e r ;  lie  the  and  increases  for  both  decreases  i n one o f  ends  parameters  t h r o u g h the model i n d i f f e r e n t  the v e l o c i t y i n c r e a s e s  the  to the  model  was  e r r o r on t h e o b s e r v e d  model  final  model  between  value far  observed  initial  the  residual rms  the  of  observed  222 ms,  stations. 79  rays.  ms,  due  The rms  which  traveltime picks  is of  1 04  FIG.  4.6. Performance of the ray t r a c e i n v e r s i o n p r o c e d u r e is indicated by the traveltime fit before and a f t e r the procedure i s a p p l i e d . Observed traveltimes are shown by crosses for first arrivals and p l u s s i g n s f o r s e c o n d a r y a r r i v a l s . The d a s h e d l i n e s f o r e a c h s h o t a r e t h e t r a v e l t i m e c u r v e s f o r the i n i t i a l model of F i g 4 . 5 ; the rms residual is 222 ms. The s o l i d l i n e s a r e t h e t r a v e l t i m e c u r v e s f o r the f i n a l model a f t e r the parameter a d j u s t m e n t s have been a p p l i e d ; t h e rms r e s i d u a l i s 79 ms.  105  DISTANCE (KM)  106  The c h a n g e s i n p a r a m e t e r s 4.6  were as  the  most  velocity  of  boundary  (i.e.  velocity the  far  larger  the  iteration  I II  region  +0.045 +0.107 +2.11 -2.99 +1.39 +0.21  Ahc Ah^  significant II  changes  and t h e d e c r e a s e  increased  in  order  counterbalance  to  were  i n d i p of  t h e d i p of  the v e l o c i t y change,  s h o u l d be if  clear  a  f i n d the proper  procedure,  calculated  trace  which  model  in  subjective and able  (2)  (1) an  the i.e.  different  the  B shallower).  to act  logic  data.  objective  choices  much  implied a  decreased  to  i n s u c h a way  as  could  become  quite  m e t h o d were u s e d  adjustments  This  has  of  either  find a  the  and n o t  i n v o l v e d i n a manual  range  of  f i x e d or v a r i a b l e  in a  ray are ray  data more  procedure,  method,  starting  to  following  t h e model a n d o b s e r v e d  least-squares-sense,  larger  to  shots.  to a s t r i c t l y t r i a l - a n d - e r r o r a  The  traveltime  reflection  least-square  assured that  manner w h i c h w o u l d be  test  the  the  reflecting  w h i c h e n a b l e us t o d i r e c t l y  fits  we a r e  compared  to  that  the  adjustments to a ray t r a c e model. U s i n g the  automatically  agree  in  total  manual o r t r i a l - a n d - e r r o r  inversion  advantages:  increase  model v e l o c i t y a l s o  to reduce the apparent v e l o c i t y a c r o s s the  convoluted  the  reduce the  Because a l a r g e r  apparent v e l o c i t y ,  trace  Figure  km/s km/s km km km km  p o i n t A became d e e p e r and p o i n t  stations.  It  shown i n  follows:  v e l o c i t y , region v e l o c i t y , region depth, point A depth, point B depth, point C depth, point D Thus,  for  we  are  models  with  parameters.  1 07  4.5  Final  4.5.1  onshore-offshore  Interpretation As d e s c r i b e d  traveltimes automated  through the ray t r a c e  fit  may a l s o be a n d so  that  traveltime  models  will  the  arises  related  is  reason  t h e c h a n g e may have  to  receiver  the  "shadow  no l o n g e r  over  the  beyond the  receiver.  Amplitudes  for  the  also i n the  exists.  that  the  or  if  through  (1984),  also a  modified  and Clowes  (1979),  amplitudes  and  zone  the c r i t i c a l  section. difficulty That  from  shot  for example,  or  to  a  p o i n t on a  critical  if  corner boundary  ray  surfaced  structures  were  Spence  in  algorithm  and so  traveltimes,  such  ray t h e o r y a l g o r i t h m of Chapter  v e r s i o n of  a l g o r i t h m . To  models  data;  ray path  T h i s would o c c u r ,  the  the  may be u s e d f o r b o t h t h e r a y t r a c e  the amplitude  Changes  starting the  to  obtain  the ray t r a c i n g .  two-dimensional  described  to  sections.  the major  the  corresponding  However,  i n order  following  of  largely  modifications  fit  the change,  result  w i t h the asymptotic  incorporates  file  which  to the s t a b i l i t y  receiver,  so t h a t  Whittall  be d e s i r e d  z o n e " due t o a l o w v e l o c i t y  was c h a n g e d  al.  obtained,  to t r y d i f f e r e n t  be d i s c u s s e d  Whatever  is  fitting  is  procedure.  on t h e o b s e r v e d  introduced i n order  models  calculated  fit  to the amplitudes  is,  moved  t h e method of  inversion  m o d e l may s t i l l  find alternate  alternate  section,  f o r m u l t i p l e s h o t s and m u l t i p l e r e c e i v e r s  corresponding  a better  model  procedure  i n the p r e v i o u s  once a s a t i s f a c t o r y the  refraction  obtain the  a two  3.  The  ray t r a c i n g identical  routine  model  which  routines  are  fits  of  input  i n v e r s i o n procedure model  et  and both  applied  in  108  conjunction.  For example,  traveltimes, improves,  which  traveltime  also  fit  procedure,  whereas  has  the  again match,  to  may be a l t e r e d  contrast  section.  same  In  side the  practice,  across  of  from  and f o r d i f f e r e n t the  possible  trace  inversion  model and  observed  the a m p l i t u d e  behavior  are  not  trace  to  i n amplitude  a block,  in a block, at  the  which a  general  different  trace.  arrivals  t r a c e s on t h e same r e c o r d amplitude  trends  The d e t a i l e d  from r e c e i v e r  characteristics.  amplitudes  between individual  trace-to-trace  uncertainties  both  For the d a t a , to receiver  from  two-  some o f be  For a g i v e n model,  the  was l i m i t e d by t h e use  unknown  in  could  of  asymptotic  w h i c h c a n n o t d i r e c t l y h a n d l e wave p h e n o m e n a , effects  very  amplitudes  t r a c e s were m o d e l l e d , a n d n o t t h e  c a u s e d by s i t e - d e p e n d e n t  ray t h e o r y ,  ray  were m o d e l l e d f o r  and i n the m o d e l l i n g p r o c e d u r e .  of  the  that  and t h e a n g l e  the  calculation  effect  On t h e o t h e r h a n d ,  a boundary,  the fit  i n the o v e r a l l v e l o c i t y of  were n o t c o n s i d e r e d b e c a u s e o f  the v a r i a t i o n s  fits  the amplitude  amplitudes  variations data  that  intersect.  trace  groups  variations  Using  naturally are.  However, o n l y  different  likely  i n t h e hope t h a t  Amplitude v a r i a t i o n s the  made s u c h t h a t  by v a r y i n g t h e v e l o c i t y g r a d i e n t  ray and a boundary  on  w i t h a model  t h e n f o u n d so t h a t  s m a l l changes  traveltimes  velocity  be  are  n o t much a f f e c t e d .  sensitive  may  deteriorates.  adjustments  traveltimes is  changes  starting  or  and  by  three-dimensional  structures. The f i n a l Figure 4.7. model a r e  onshore-offshore  In F i g u r e 4 . 8 ,  given,  including  the  r e f r a c t i o n model i s full  details  of  displayed  in  the  velocity  t h e v e l o c i t i e s and v e l o c i t y  gradients  109  of  all  model  blocks  in  the  m o d e l . The f i n a l  f o r s h o t s P 1 9 , P13 a n d P8 i s  traveltime iteration residual  residual of  is  and  (Table  parameter  depth  mantle is  the  reflector.  nearly  1  for  4.1), h A at  the  is  of  the  standard absolute shown  errors  section  should  again  given i n Table  4.6,  4.1  for  there are  parameterizations  traveltimes,  and  the  models d i f f e r  by amounts  If  poorly  of  greater  be  the  4.5)  on t h e  Chapter  the  upper error  from the 2,  where  was r e l a t e d to the  emphasized  that  also  satisfy  parameters  than  the  As  to  depth the  s h o u l d n o t be c o n s i d e r e d  models  of  determined  w i t h the r e s u l t s  alternate  equivalent  further  i t e r a t i o n of  the parameters.  which  a  rms  applied,  traveltime with respect  measures of c e r t a i n t y  in  different  It  The  and t h e s t a n d a r d  2.4  the  From t h e v a l u e s  the upper m a n t l e r e f l e c t o r  s e n s i t i v i t y of  reflector.  is  final  most  0.46  This is consistent  r e s o l u t i o n of  l a c k of  79 ms.  point A (Fig.  The r e s o l u t i o n  km.  the  the  s u b d u c t i o n zone t e s t model i n s e c t i o n the poor  is  i n v e r s i o n procedure  error  i n v e r s i o n procedure is  shots  i n s i g n i f i c a n t l y t o 78 ms.  standard  through  shown i n F i g u r e 4 . 9 .  the t h r e e  the ray t r a c e reduced  resolution  for  ray t r a c e  will  as be  with  slightly  the  observed  for  the  calculated  various standard  errors. The f i n a l  model  also  incorporates  f r o m t h e c o n t i n e n t a l s l o p e s h o t s P 1 - P 6 , as ocean  basin  from  the  inversion continental was  varied  amplitude well  as  information from the main  s h o t s P 1 9 , P13 and P 8 . O n l y t h e o b s e r v e d ocean  basin  procedure.  shots To  slope shots,  were  obtain  a  included  i n the ray  traveltime  the shallow s t r u c t u r e  i n a t r i a l - a n d - e r r o r manner,  traveltimes  fit  beneath  trace  for  the  the  shots  w h i c h h a d no e f f e c t  on  w  o  cs Y  FIG.  DISTANCE (KM) 100  Vancouver Island  300  200 T  I  Mainland  T  4.7. Final v e l o c i t y model i n t e r p r e t e d from the onshore-offshore d a t a s e t a l o n g l i n e I. Numbers i n d i c a t e t h e v e l o c i t i e s ( k m / s ) a t the shallowest and d e e p e s t p o i n t s i n e a c h b l o c k . CS i s t h e l o c a t i o n o f the foot of the continental slope. Oceanic crustal model was determined by W a l d r o n (1982). The subducting ocean crust is a r b i t r a r i l y r e p r e s e n t e d by o n l y a s i n g l e l a y e r below about 30 km depth. Continental crustal model was c o n s t r a i n e d by t h e p r e f e r r e d l o w - v e l o c i t y zone i n t e r p r e t a t i o n o f McMechan a n d Spence (1983) w h i c h a p p l i e s a t t h e i n t e r s e c t i o n of l i n e I a n d l i n e I V (230 km from shot P19) .  DISTANCE 0  100  FIG.  (KM) 200  300  4.8. Details of t h e v e l o c i t y m o d e l shown i n F i g . 4.7. V e l o c i t i e s (km/s) are g i v e n f o r the top of each block, f o l l o w e d a f t e r t h e c o l o n by t h e v e l o c i t y g r a d i e n t ( k m / s / k m ) i f one was u s e d .  co FIG.  4.9. Ray paths f r o m s h o t s P 1 9 , P13 and P8 t h r o u g h t h e f i n a l v e l o c i t y model shown on F i g . 4 . 7 and Fig. 4.8. The rms residual for the t h r e e s h o t s i s 79 ms; i f a f u r t h e r i t e r a t i o n of t h e r a y t r a c e i n v e r s i o n p r o c e d u r e i s applied, t h e r e s i d u a l i s r e d u c e d i n s i g n i f i c a n t l y t o 78 ms.  1 13 Parameter  Final  Resolution  Value  Standard  Error  7 . 97 km/s  0. 68  0. 013 km/s  8 . 1 6 km/s  0 93  0. 007 km/s  "A  27 . 5 km  0 46  0 93 km  *H  39 . 6 km  0 .83  0 67 km  hc  1 7. 3 km  0 .90  0 .54 km  18 . 9 km  0 .93  0 .45 km  i  v  TABLE 4 . 1 . P a r a m e t e r v a l u e s , r e s o l u t i o n a n d s t a n d a r d error for the final iteration of the ray trace inversion procedure, a p p l i e d to the traveltime dataset of the onshore-offshore line I. ( F i g . 4.9). R e s o l u t i o n and standard error give approximate relative measures of c e r t a i n t y f o r t h e p a r a m e t e r s . The o v e r a l l d a m p i n g f a c t o r 6 was 0 . 2 5 . the  traveltime  fit  for shots  the c o n t i n e n t a l slope automated quality  ray trace  shots  were  ocean  continental  shown  the melange better  (located at presented rays w i t h i n more  the  more  the poor  data  structure  in  (1982) was n o t as w e l l - c o n s t r a i n e d  slope  and  shelf  as  beneath the  t h e m e l a n g e h a s been m o d i f i e d s l i g h t l y  in Figure  1.6,  w h i c h was d e r i v e d by W a l d r o n  The m a j o r c h a n g e was a r e d u c t i o n i n t h e v e l o c i t y  a  in  from  deep  basin. In F i g u r e 4 . 8 ,  that  included  and because o c e a n i c c r u s t a l  t h e i n t e r p r e t a t i o n of W a l d r o n the  not  i n v e r s i o n p r o c e d u r e because of  from the s h o t s ,  beneath  P 1 9 , P13 a n d P 8 . T r a v e l t i m e s  from 0.2 fit  was  km/s/km t o 0.04 obtained  for  125 km i n F i g . 4 . 8 ) , by W a l d r o n  (1982);  the  data  is,  w i t h the d a t a .  change,  interpretation  the a m p l i t u d e s of  t h e m e l a n g e t o OBS 5 were r e d u c e d and t h u s  favorably  within  r e c o r d e d on OBS 5  compared t o the  that  (1982).  gradient  km/s/km. With t h i s  from  turning compared  An a d d i t i o n a l c o s m e t i c c h a n g e  to  11 4  the model of F i g u r e base of  1.6  was t h a t an e x t r a  t h e melange above  still  appears  effect  is  to  thin  the lower ocean c r u s t . beneath  the  r e l a t e d to the f o r m a t i o n of  response to t h e i r compressed  and  l a y e r was a d d e d a t  resistance  The o c e a n  melange  the  crust  (Fig. 4.7).  t h e melange  itself;  This as  to s u b d u c t i o n , the upper l a y e r s  deformed and t h e i r  velocity  is  reduced  a are  (Waldron  1 982) . In the f i n a l procedure  ray t r a c e  (Fig.  4.9),  it  diagram f o r the ray t r a c e s h o u l d be n o t e d t h a t  used to the m a i n l a n d r e c e i v e r s , rays  in  Section  demonstrated. almost  4.4  where  I n terms of  in  contrast  traveltimes,  the  turning respect  t h e two t y p e s  are  reflected  p r o c e d u r e was  rays  to  the  t o changes  continental  far  stations  the  ray  of  first  rays  types  i n the ray trace model, since they Moho  at  a shallower angle  are  the ray t r a c e  done  using  only  reflected  rays  receivers.  H o w e v e r , no c l a i m  reflected  rays  stations.  Either  can  be  the  i s made t h a t  separately  model  a r r i v a l s at  both the  observed  at  with  reflected  the  turning  was far and  the mainland  c o u l d be r e c o r d e d a n d w o u l d c o n t r i b u t e  to  the  arrivals.  S y n t h e t i c seismograms Figures  vertical  as  that  intersect  than the  T h u s , much o f t h e d e v e l o p m e n t o f  observed  is  a r e much more u n s t a b l e  rays.  4.5.2  rays  e q u i v a l e n t , w i t h a maximum t i m e d i f f e r e n c e o f ~150 ms f o r  s h o t P 8 . The m a j o r d i f f e r e n c e b e t w e e n  the  turning  to  the ray t r a c e  inversion  4.10  component  representative  to  4.17  for the f i n a l contain  seismograms  receivers.  The  for  model  theoretical representative  synthetic  and  observed  shots  seismograms  and were  11 5  c a l c u l a t e d u s i n g the f i n a l the  theoretical  on t h e o b s e r v e d  r e f r a c t i o n model of F i g u r e  t r a v e l t i m e s of  the s y n t h e t i c s are  record sections.  Both  s e i s m o g r a m s a r e m u l t i p l i e d by a f a c t o r All  record  sections  are  observed  4.7,  and  superimposed  and  synthetic  p r o p o r t i o n a l to  distance.  t r u e a m p l i t u d e , so a m p l i t u d e s may t h u s  be c o m p a r e d f r o m t r a c e - t o - t r a c e .  I n a d d i t i o n , a m p l i t u d e s may  compared  sections  record  between  lowcut  first  record  filter  are  presented  on t h e o b s e r v e d  filtering breaks  F i g u r e s 4.10  of  record section  for  d i d not appear t o s i g n i f i c a n t l y  for  a  1  Hz  receiver X34), enhance  either  o r c o n t i n u i t y o f a r r i v a l s f r o m t r a c e - t o - t r a c e . On to 4.17,  the  location  l o c a t i o n on t h e f i n a l s h o t P19 as  its  origin;  related  features  of  a  seismogram  may  shot/receiver distance,  be  or  to  v e l o c i t y model w h i c h has  the l o c a t i o n  both distance  presented.  The m a i n c h a r a c t e r i s t i c s their  between r e c e i v e r  u n f i l t e r e d (except  referred e i t h e r to i t s appropriate its  and  sections.  The s e c t i o n s  since  shot  be  on  of  s c a l e s are  the s y n t h e t i c  the  final  seismograms  velocity  model a r e  and as  follows: (1) first  Turning rays  a r r i v a l s at  subducting  slab  t h r o u g h the upper mantle g i v e  a l l receivers. is  The v e l o c i t y a t  approximately  g r a d i e n t p e r p e n d i c u l a r t o t h e Moho preliminary the  i n t e r p r e t a t i o n of E l l i s  gradient  comparable  to  is  not  that  8.0 of  km/s, 0.01  et a l .  well-constrained. given  by  l i t h o s p h e r e beneath c o n t i n e n t s .  t h e Moho o f with a  km/s. (1983),  The  to  As  the  velocity in  the v a l u e  the of is  al.  (1977)  for  and by F u c h s  (1979)  for  et  value  the  used  Steinmetz  l i t h o s p h e r e b e n e a t h 9 Ma o c e a n i c c r u s t  rise  1 16  T  1  1  r—  SHOT P19  CO  9  oo \ Q  to  240  200  280  DISTANCE 240  200  CVJ  180  FIG.  FROM  220  P19  SHOT/RECEIVER  360  320  380  (KM)  280  260  320  340  300 DISTANCE  (KM)  4.10. Synthetic s e i s m o g r a m s and o b s e r v e d d a t a f o r s h o t P 1 9 . M o d e l t r a v e l t i m e s from t h e s y n t h e t i c seismograms are superimposed on t h e observed record section. Primary a r r i v a l s are t u r n i n g rays through the mantle. Secondary arrivals c o r r e s p o n d t o r a y s r e f l e c t e d f r o m an u p p e r m a n t l e boundary, w i t h the v e l o c i t y below the boundary .smaller, than the v e l o c i t y a b o v e . For shot P 1 9 , the model t r a v e l t i m e s to the m a i n l a n d s t a t i o n s ( 2 9 0 - 3 5 0 km) a r e a l m o s t t h e same f o r b o t h t u r n i n g r a y s and r e f l e c t e d r a y s .  4.11. P13, 4.10.  S y n t h e t i c seismograms and o b s e r v e d data for shot located 40 km e a s t o f s h o t P 1 9 . See c a p t i o n f o r F i g  118  I—I—r—  SHOT P8 w  co -  ii " 1  Q  co  200  200  CM  125 FIG.  240 280 DISTANCE FROM P19 240 280  320  380  320  380  (KM)  165 205 245 SHOT/RECEIVER DISTANCE (KM)  4.12. S y n t h e t i c seismograms and observed P8, located 67 km e a s t o f s h o t P 1 9 . S e e 4 . 1 0 . On t h e s y n t h e t i c s e i s m o g r a m s , note a r r i v a l branch to the mainland s t a t i o n s i s a r r i v a l s a r e t u r n i n g rays through the upper which a r e b l o c k e d by t h e c o r n e r -formed a t o f c o n t i n e n t a l Moho a n d s u b d u c t i n g o c e a n i c 4.18).  285  data for shot caption for F i g . that the first truncated; these m a n t l e , some o f the i n t e r s e c t i o n c r u s t (see Fig.  119  co-J  O  1—i—r SHOT P 2  00 <  9 io eo  •r < «r  <  <•  <r*  <r  «r-  «>  4 > CO  200 200  95  240 280 DISTANCE FROM P19 240 280  320  360  320  380  (KM)  135 1?5 215 SHOT/RECEIVER DISTANCE (KM)  255  4.13. S y n t h e t i c s e i s m o g r a m s and o b s e r v e d data for shot P2, located 99 km e a s t of s h o t P 1 9 . See c a p t i o n f o r F i g . 4 . 1 0 . On t h e s y n t h e t i c s e i s m o g r a m s , t h e o n l y b r a n c h p r e s e n t at the m a i n l a n d s t a t i o n s i s the branch of r a y s r e f l e c t e d at the upper mantle boundary. A l l t u r n i n g r a y s t o the m a i n l a n d stations are blocked by the corner formed af the intersection of continental Moho and s u b d u c t i n g o c e a n i c c r u s t (see F i g . 4 . 1 8 ) .  120  i  w  r  i  i  I  i  MAINLAND RECEIVER X45  co -  9 m oo  \  CM  355 FIG.  335  25  50 DISTANCE FKOM P19  25  50  315 295 DISTANCE (KM)  75 (KM)  100  75  100  275  255  4.14. Synthetic 'seismograms and observed data for mainland receiver X45, l o c a t e d a t t h e e a s t e r n m o s t end o f t h e o n s h o r e - o f f s h o r e p r o f i l e . See c a p t i o n for Fig. 4.10. The first a r r i v a l b r a n c h of t u r n i n g r a y s i s t r u n c a t e d (no a r r i v a l s b e y o n d 99 km on t h e s y n t h e t i c s e c t i o n ) due t o the effect of t h e c o r n e r where t h e c o n t i n e n t a l Moho m e e t s t h e s u b d u c t i n g o c e a n i c c r u s t (see Fig. 4.18). The secondary arrival b r a n c h of reflected r a y s i s a l s o t r u n c a t e d ; but t h i s i s due t o a p o o r l y c o n t r o l l e d f e a t u r e o f t h e velocity model ( F i g . 4 . 7 ) , i n w h i c h t h e d i p of t h e r e f l e c t o r e a s t o f a b o u t 200 km d i s t a n c e f r o m P19 s u d d e n l y i n c r e a s e s .  121  MAINLAND RECEIVER X34 O co -  W 00  H mi co P  C3  25  o  25  CM  310  FIG.  290  100  75 DISTANCE FROM P19 (KM) 50  100  75  270 250 230 SH0T/RECEIVER DISTANCE (KM)  210  4.15. Synthetic seismograms and observed data for m a i n l a n d r e c e i v e r X 3 4 . See c a p t i o n f o r F i g . 4 . 1 0 . N o t e t h a t the f i r s t a r r i v a l b r a n c h of t u r n i n g r a y s i s t r u n c a t e d due to t h e e f f e c t o f t h e i n t e r s e c t i o n o f t h e c o n t i n e n t a l Moho and s u b d u c t i n g o c e a n i c c r u s t ( s e e F i g . 4 . 1 8 ) .  122  SHOT/RECEIVER DISTANCE  FIG.  (KM)  4.16. Synthetic seismograms and o b s e r v e d data m a i n l a n d r e c e i v e r X 2 2 . See c a p t i o n f o r F i g . 4 . 1 0 .  for  123  SHOT/RECEIVER DISTANCE (PCM)  FIG.  4.17. Synthetic seismograms and observed m a i n l a n d r e c e i v e r X 6 . See c a p t i o n f o r . F i g . 4 . 1 0 .  data  for  1 24  It  was  hoped  that  the a m p l i t u d e m o d e l l i n g would p r o v i d e  more c o n t r o l on t h e m a n t l e g r a d i e n t , values  between  regions  were s a m p l e d  diagram,  shallower  Fig.  4.9).  mantle gradient amplitudes  of  However,  is  anomalous  the  sensitive  crust  angles  for  to  c o n t r o l s the are  resultant  the  factor  in  near  zero.  4.7  at to  to  intersect  surface. shots,  This  turning  trace  the  shallow  determining  the  stations,  and  amplitudes.  v e l o c i t y m o d e l , and a  ~20  stations  km d e p t h a b o v e The s l i v e r  shots.  the  velocity  considerations, Amplitudes  are  through the a decrease  sliver,  and  ray  in velocity implies  towards  i n ray paths that  the upper boundary of effect  is  path  the  surface  are  directed  that  and  a  increase  downwards,  the s l i v e r or to  reach  most  pronounced for  the  w h i c h have t h e s t e e p e s t  downward a n g l e  through the  Since t h i s  that  the  is of  basin  shots,  the apparent  velocity  of  is approximately  8.1  t h e v e l o c i t y of m a n t l e m a t e r i a l , the  nearest  crust.  a c r o s s the mainland r e c e i v e r s  km/s.  dip  ray  i n a m p l i t u d e . On t h e o t h e r h a n d , an  For the oceanic rays  t h e two  s l i v e r v e l o c i t y because the v e l o c i t y  Thus,  mantle below the ocean (3)  since  below,  in  4.9).  offset  d i r e c t e d more s t e e p l y  increase  relative  (see  to the near  the  the  to the Vancouver I s l a n d  nearest  ray path angle  i n v e l o c i t y may r e s u l t  offset  discussed  sliver  (Figs.  the v a l u e of  ray paths are  the  paths  km/s was d e t e r m i n e d m a i n l y by a m p l i t u d e  in p a r t i c u l a r  never  as  feature  mantle-like  downgoing o c e a n i c 7.7  ray  was n o t w e l l - c o n t r o l l e d by t h e  through which a l l rays  pass,  of  different  on  mantle,  the t u r n i n g ray a r r i v a l s  An  feature  least  deeper  was n o t t h e d e c i s i v e  so t h e g r a d i e n t (2)  by  and  at  boundary  through  it  which rays  implies enter  the  1 25  continental  crust  interpreted  as  related  with  the  onshore-offshore 300  km s o u t h  near  of  t o the  profile  (4)  km/s  the Vancouver 7.4  (Figs.  That  across  4.10  to  with  4.12).  a  sliver  sliver The  on s h o t p r o f i l e s  arrivals,  d e p t h s as  rays  250found  receivers  oceanic  Moho  compared  For a given  than  receivers  crust  of  of  7.7  mantle  km/s,  the upper  ~1  through  s  the  crust.  observed  boundary  the  (Figs.  For  the  are  is  This  Island  sliver shelf  w h i c h must  37 km d e p t h .  8 . 3 - 8 . 5 km/s, is  to  advanced  the apparent v e l o c i t y of  shots  first  Vancouver  thus  mainland a r r i v a l s ,  of  4.10  inner c o n t i n e n t a l  t h r o u g h t h e Moho a t  velocity.  controlled  seen f o r  Strait  of  7.5-7.7  the h i g h - v e l o c i t y  and t h e  the  receiver,  is  is  above the downgoing  20 km. T h e s e a r r i v a l s to  velocity  ray paths to the Vancouver  Island  r a y s a c r o s s the ocean b a s i n higher  Island  across Georgia  pass  Vancouver  the c o n t i n e n t a l (6)  similar  only  mainland  to the mainland s t a t i o n s .  s h a l l o w as  traveltime  enter  a  This  (1983)  apparent  the d i p of  offset  c a u s e d by d i f f e r e n t  beneath western  in  near  traveltime  s t a t i o n s compared  the  velocity  zero.  at  Taber  a subducting  sliver  is  Island  crust. for  This apparent v e l o c i t y  the mantle  is  (1983)  profile.  the Vancouver  that  4.13)  oceanic  Taber  shots,  implies  arrivals  and  interpreted  apparent v e l o c i t y  (5)  f l a t - l y i n g c o n t i n e n t a l Moho,  Island  by t h e h i g h - v e l o c i t y  is,  is  km/s a c r o s s h i s  the ocean b a s i n  rays  primarily  boundary  of  the  ~9°.  For  turning  is,  a c r o s s s the Washington margin  d e e p o c e a n s h o t s and  d i p p i n g at  That  subducting  results  an a p p a r e n t v e l o c i t y o f from  zero.  approximately  not a boundary contrasts  is  only  consistent with a  turning slightly shallow  126  dip  of  <2°  for  continental  the  rise.  oceanic  The d i p o f  crustal  layers  west  of  the  t h e c r u s t a l l a y e r s was c o n s t r a i n e d  by t h e m a r i n e i n t e r p r e t a t i o n o f W a l d r o n  (1982)  for  the  oceanic  crust. (7)  The  first  continental  shots  pick,  appears  but  velocity  is  4.8-5.3  Assuming  that  primarily block  km/s i n oceanic  the  velocity  layers  the apparent  dip  t h e Moho b e n e a t h s h o t s  (Figs.  4.14  somewhat  to 4.17),  larger  pe, the base of probably d i p at of  Figure  point  at  4.7. which  increases  the  melange  offshore  a smaller angle  the  o b s e r v e d an i n c r e a s e  subducting  slab  of  Figure  the  may  increase a  as  the  profiles  actually  layers  be  should  slab  is  that  (1983)  for  in  from 8  an  nearly  50  km  This  onshore-  km/s  the e a s t e r l y  d i r e c t e v i d e n c e of  the  significantly  the c o n t i n e n t a l s l o p e .  velocity  km/s  point  the  so b e n e a t h s h o t s P i -  oceanic  oceanic  of Taber  i n apparent 11  4.7.  on  a c r o s s the Washington m a r g i n . Taber  to  the  t h a n shown i n t h e v e l o c i t y m o d e l  subducting  shots  at  of  labelled with  However, the important i m p l i c a t i o n  experiment  interpreted this  base  p a r a l l e l t o t h e base of  velocity  and  w i t h the r e s u l t s  westerly  P1-P6,  model  than i n the observed d a t a ; the  the  to  apparent  P 1 - P 6 . On t h e s y n t h e t i c  apparent  the  is d i f f i c u l t  thereby provides control  i n d i p must o c c u r e a s t o f  is consistent  more  velocity  are  across  km/s. T h i s  to the d i p at  beneath shots  melange, of  velocity  t o be a p p r o x i m a t e l y 8 . 0  melange  velocity  apparent  P 1 - P 6 on t h e r e c e i v e r g a t h e r s  sensitive  sub-sediment  break  (1983)  for  the  shots.  He  t h e bend i n  the  landward  the  of  b e g i n n i n g of the c o n t i n e n t a l s l o p e . (8)  The a m p l i t u d e s o f  the f i r s t  arrivals  at  the  mainland  1 27  receivers  are  large  from t h e o c e a n b a s i n s h o t s  (see  and P 1 3 ) a n d r e l a t i v e l y s m a l l f r o m t h e c o n t i n e n t a l (see  shot  possibly  P2  complexly  However,  receivers,  in  from a  the  to the c o n t i n e n t a l slope by e q u i - a n g u l a r  are  also  set  slope  could  thick  slope  and  sediments.  expected  of  rays  shot  to  P19 shots  due  to  v e l o c i t y model. F i g u r e 4.18  continental  and the r e v e r s e  from the  continental  amplitudes  features  rays  attenuation losses  deformed  decreased  structural  slope  r e c e i v e r X 4 5 ) . The r e d u c e d a m p l i t u d e s  be due t o l a r g e  probably  turning  and  shots  the  shows  mainland  from a m a i n l a n d r e c e i v e r  and o c e a n b a s i n . A l l r a y s a r e  i n c r e m e n t s , and f o r e a c h s e t  of  rays  separated  there  is  a  "shadow z o n e " due t o t h e c o r n e r where t h e c o n t i n e n t a l Moho m e e t s the  subducting  within P2  oceanic  t h e shadow z o n e ;  (Fig.4.13),  there  crust. thus,  No  turning  on t h e s y n t h e t i c s e c t i o n  i s no t u r n i n g  t r u n c a t e d on t h e s y n t h e t i c s  receiver  X45  due t o t h e wave n a t u r e o f would  be  predicted  (9) which P13  most  ( F i g . 4.11)  reflections 4.17)  also  traveltimes.  (Fig.  4.12),  the  shadow  expected w i t h i n the  energy, zone,  However,  diffractions  so t h a t  reduced  the r e g i o n .  Vancouver  Island  stations,  c l e a r l y s e e n on t h e o b s e r v e d s e c t i o n s a n d r e c e i v e r X22 ( F i g . 4 . 1 6 ) ,  f r o m an u p p e r m a n t l e r e f l e c t o r .  are  However,  i t s magnitude  is  somewhat  late  compared  Receiver  the d i f f e r e n c e not too  much  to  for  X6  than  as  (Fig.  although the the  i s not c o n s i d e r e d larger  shot  interpreted  shows e v i d e n c e o f a s e c o n d a r y a r r i v a l ,  model t r a v e l t i m e s a r e  since  the p r o p a g a t i n g  Secondary a r r i v a l s at  are  f o r s h o t P8  shot  and t h e  a n d r e c e i v e r X34 ( F i g 4 . 1 4 ) .  within  amplitude a r r i v a l s are  for  ray branch present,  branch i s  (Fig.4.13),  ray a r r i v a l s occur  the  observed serious average  128  FIG.  4.18. Ray p a t h s s h o w i n g shadow z o n e s due t o t h e c o r n e r a t 215 km d i s t a n c e and 37 km d e p t h , where t h e s u b d u c t i n g o c e a n i c c r u s t i n t e r s e c t s t h e c o n t i n e n t a l Moho. Arrowheads indicate t h e c o a s t s of V a n c o u v e r I s l a n d , (a) A c o n t i n e n t a l s l o p e s h o t a t 85 km d i s t a n c e p r o d u c e s a shadow zone over the distance range 285-335 km. (b) A r e c e i v e r a t 305 km d i s t a n c e r e c o r d s no a r r i v a l s f r o m s h o t s on t h e continental s l o p e e a s t o f 70 km d i s t a n c e .  DEPTH 60  40 i  62  I  (KM) 20 i  DEPTH 0 L n  60  40  I  1  (KM) 20 i —  0 1  :  rim—  130  picking  error  secondary X6)  for  arrival  were  used  all  times in  arrivals for  the  ray  sensitive  to  the  of  trace  the  magnitude  amplitudes  contrast  is  produced  from  positive a  for  the  model except  final  km/s i n s t e a d o f  and  observed data  7.7  r e f l e c t e d a r r i v a l s at 4.19  are  too large  stations. the  It  is  preferred  the upper m a n t l e (10)  reflected  ( B r a i l e and S m i t h  depend  on  whether  with  Figure  4.10  to  and 4 . 1 1 ,  the Vancouver I s l a n d  it  on t h i s b a s i s t h a t  the  v e l o c i t y i n the  final  is  Island  receivers  However, r e f l e c t i o n s  to  the  in  the  Figure  the mainland 7.7  mainland  are  Based  km/s  is  observed  to the far  stations  their  as  effect  d e l a y e d from t h a t emphasized  an of  that  additional  the mantle the  existence  of  data,  reflections as  distinct  are  possible  and  arrival  turning  receivers  on t h e  only  i n c l u d e d for completeness  be  seen t h a t  reflector.  a r r i v a l s a n d so t h e y a r e  should  is  model f o r t h e r e g i o n below  not necessary to the m o d e l l i n g , s i n c e  slightly  to  synthetics  a low v e l o c i t y o f  they  possible  the  identical  i n comparison to the a r r i v a l s at  i n c l u d e d on a l l s y n t h e t i c s e c t i o n s .  arrivals.  1975).  shows  stations  have been  to the Vancouver  the  the v e l o c i t y below the r e f l e c t o r  Upper mantle r e f l e c t i o n s  are  at  amplitudes  4.19  model  very  the v e l o c i t y  larger  k m / s . When c o m p a r e d  in Figures  which  not  velocity contrast  s h o t s P19 a n d P13 t h r o u g h a  8.6  are  the  contrast.  that  receiver  procedure  reflections  negative,  positive  the  fit.  of  do  or  synthetics  and b e c a u s e o n l y  inversion  wide-angle  boundary from which they are However,  ms)  s h o t s P13 and P19 (and n o t  determined the o v e r a l l t r a v e l t i m e The a m p l i t u d e s  (~75  to at  ray.  the upper  show a  But  time it  mantle  131  FIG.  4.19. Synthetic seismograms f o r s h o t s P19 and P13 f o r r a y s t h r o u g h t h e same v e l o c i t y m o d e l as the final model ( F i g . 4 . 7 ) , except t h a t the v e l o c i t y below the upper mantle reflector is 8 . 6 km/s i n s t e a d o f 7 . 7 k m / s . The a m p l i t u d e s of t h e r e f l e c t i o n s a t t h e V a n c o u v e r I s l a n d s t a t i o n s , which are the secondary a r r i v a l s f o r d i s t a n c e s 2 0 0 - 2 6 0 km f r o m shot P19, are too l a r g e i n comparison with the observed record sections (Fig. 4.3).  V - 8 . 6 KM/S SHOT  PI9  o cow to oo \ Q I  in -  co  180  (- 4 4 > > > 4> 200  220  SHOT  240  260  280  300  320  340  360  260  280  300  320  340  360  PI3  o co w CO  <-! m i oo \ Q I E-  ^  CO  180  i  200  —  ZTTi ±  220  .  240  DISTANCE  FROM  P19  (KM)  133  boundary  is  speculative  corresponds Vancouver  to  the  Island  With  reflection  station  (see  the  arrival  less  t h a n 20 ms,  times  of  the e f f e c t  of  the  (Fig.  4.11),  wavelet  of  has  main  the d i f f e r e n c e  l e n g t h 200 ms,  P8 ( F i g .  start  because  to  diagram,  subducting  the  turning  ocean  shot  than that  a l t h o u g h the at  ray  expected the  onshore-offshore certain  at  particular, its  by more g e n e r a l most very  by  For a  shot  source and  turning ray  ray  is  distance  the from  those distances Moho  is  and  the  the  reflected  w i t h the t u r n i n g ray  seismic  data  discussed  in  the  f e a t u r e s were i n t r o d u c e d w h i c h were  tectonic  f e a t u r e s of existence  and  4.13),  model  n o t c o n t r o l l e d by t h e r e f r a c t i o n s e i s m i c suggested  differ  synthetics  reflected  P2 ( F i g .  w i t h the  section,  where  the t u r n i n g ray  continental  4.6  final  4.10),  elongated  290 a n d 300 km  For shot  A l t e r n a t e models c o n s i s t e n t  of  reflected  corner.  the  that  the t u r n i n g ray a l o n e .  the  between  crust.  to  and t u r n i n g r a y s  b e i n g c o m p l e t e l y b l o c k e d by t h e  even  4.9).  opposite  P19 ( F i g .  t h e o n l y a r r i v a l on t h e s y n t h e t i c s ,  previous  Fig.  i s ~75 ms a n d , a s s u m i n g  4.12),  separate,  by t h e c o r n e r  In  easternmost  the combined a r r i v a l i s  o n l y a r r i v a l on t h e s y n t h e t i c s  wave i s  the  r e f l e c t i o n on t h e  peak a m p l i t u d e l a r g e r  For shot  wavelets  for  the r e f l e c t e d  P13  stopped  110 km, w h i c h  reflection is  Thus,  t o d e c r e a s e t h e a m p l i t u d e of  P19  than  for  ray t r a c e  is  alone.  point  t h e p h a s e of  the mantle t u r n i n g ray.  a  greater  the v e l o c i t y below the r e f l e c t i n g boundary lower than  the v e l o c i t y above, of  for distances  the in  data  or g e o l o g i c a l  downgoing the  but  model  rather  principles.  oceanic are  were  crust  not n e c e s s a r y  In and to  1 34  satisfy  the r e f r a c t i o n seismic  because  the  weight  of  l o c a l l y and w o r l d w i d e , (Keen of  a n d Hyndman 1 9 7 9 ) .  w i t h two a l t e r n a t e and  the  P8  equally  other  lends  the downgoing c r u s t  constraints,  i n the s e i s m i c  as  a m p l i t u d e b e h a v i o r . The p u r p o s e models  is  The s i m p l e s t essentially  the  preliminary  4.20).  is  continental crust, determined region  below  and of  4.7) 0.01  presenting  (Fig.  (1)  (the depth of  with  (2)  (3)  two  the  of  mantle  shown as  et  al.  (1983)  is  extrapolated  The  features  model  gradient  (8.0  the upper mantle  velocity  pathway  Vancouver  I s l a n d and t h e c o n t i n e n t a l s h e l f .  is  that  the  the  i n common w i t h t h e f i n a l  t h e c o r r e s p o n d i n g d i p s a r e ~ 8 ° a n d ~ 7 ° ) , and  crust,  as  the  the geometry of  apart  to  the  of  c o n t i n e n t a l Moho a t  of  (Fig.  Island  a flat-lying  depths  is  (1983)  reflector  (4)  high-velocity  a depth  is  a shallow  The m a j o r of  ~22  m o d e l , and  30 km and l e s s b e n e a t h  from the t e r m i n a t i o n  km/s  reflector  beneath P 1 9  km f o r t h e p r e l i m i n a r y m o d e l a n d ~25 km f o r t h e f i n a l  between t h e m o d e l s ,  data  terminating against  t h e b o u n d a r y when e x t r a p o l a t e d  at  data  models.  below Vancouver  shelf.  4.20)  similar  alternative  seismic  Ellis  t h e m a n t l e v e l o c i t y and  km/s/km),  40 km o r l e s s ,  m o d e l a n d have  p e r m i t t e d by t h e s e i s m i c  upper  continental  model are  f o r P 1 9 , P13  and the s t r u c t u r e  the  characteristics  the t r a v e l t i m e s  of  by McMechan and S p e n c e  preliminary (Fig.  crust  presence  illustrated  model  an  both  be  final  model c o n s i s t e n t  oceanic  model w i l l  its  t h o s e f e a t u r e s common t o a l l  m o d i f i e d by t h e a d d i t i o n o f The  the  included  evidence,  to  the  t o show t h e v a r i a b i l i t y  and a l s o t o emphasize  support  The u n c e r t a i n t y of  well  are  geophysical  strong  models which f i t as  but  high-  western  difference  the  pathway i n the f i n a l  oceanic model  FIG.  4.20. The p r e l i m i n a r y v e l o c i t y model of Ellis et al. (1983), modified by the addition of an upper mantle r e f l e c t o r . The o c e a n i c l a y e r s a r e shown t e r m i n a t i n g a g a i n s t the c o n t i n e n t a l c r u s t a l l a y e r s . The ray trace inversion procedure has been applied to the m o d e l , and the model traveltimes for shots P19,P13 and P8 fit the observed traveltimes with an r m s r e s i d u a l o f 73 m s , e q u i v a l e n t to that for the f i n a l model.  1 36  a p p e a r s as whereas  a sliver  in  increases central  the  mantle  material  f r o m ~29 km t o ~39 km i n a  better  satisfy  abruptly  i n t e r m e d i a t e model  tectonic  against  of  principles,  eastward,  extrapolated  w e s t w a r d as  4.20.  Because the s l a b  sided,  a l l rays  are in  the  Figure  extrapolated  major  in of  t h e two m o d e l s , crust  of  a n d t h e sudden  shown  approximately is  by  elevated  i n t e r m e d i a t e model  not  30  should  instead  crust.  oceanic  layers  model crust  affected  of  are  Figure  parallel-  equally,  and  the p r e l i m i n a r y model  boundaries  increase  the  being  of is  of  In  are  comparable  in thickness  of  crust.  The  the  intermediate  transforms  to  The  stability  Grow a n d B o w i n  occurs at  depths  velocity  of  for  the  i n c o r p o r a t e d i n the f i n a l  as  8.2  portion  km d e p t h . I t  (1975,  of  F i g . 5)  is  basaltindicates  as  30  km.  used  in  the  oceanic  s h o u l d be n o t e d t h a t r e f r a c t i o n model,  with greater  of  shallow  km/s  data  eclogite,  field  a  model  from p r e s s u r e - t e m p e r a t u r e  i n d e n s i t y and v e l o c i t y t o v a l u e s  of normal m a n t l e .  the phase change an  crust  b e n e a t h c e n t r a l V a n c o u v e r I s l a n d a p p e a r s as  increases  Thus,  are  D i p s of  associated  that  the  characteristics  in the.oceanic.crust  as  under  the c o n t i n e n t a l l a y e r s  receivers  that basalt  eclogite  continental  preliminary  incorporates considerations  than those  suddenly  the p r e l i m i n a r y  region,  subducting oceanic  i n the s u b d u c t i n g oceanic  also  of  the  not s i g n i f i c a n t l y changed.  kink  depth,  feature  the oceanic the  4.21,  instead  to a set  structural  continental  km  Island.  be c o n t i n u o u s l y s u b d u c t i n g t h r o u g h o u t  the  20-25  fault-like  m o d e l may be m o d i f i e d by r e q u i r i n g t h a t  terminating  at  p r e l i m i n a r y model the d e p t h t o mantle  Vancouver  To  of  crust  below  this  effect  since  there  W  DISTANCE (KM) 100  0  FIG.  V a n c o u v e rI s l Q n d  M d n l a n d  200  300  4.21. I n t e r m e d i a t e v e l o c i t y model i n w h i c h the oceanic layers d e t e r m i n e d by W a l d r o n ( 1 9 8 2 ) a r e e x t r a p o l a t e d u n d e r t h e c o n t i n e n t a l c r u s t . The r a y trace inversion procedure has been a p p l i e d to t h e m o d e l , a n d t h e rms r e s i d u a l f o r s h o t s P 1 9 , P 1 3 and P8 was 75 ms, e q u i v a l e n t t o t h a t f o r the f i n a l model.  E  138  is  controversy  transformation presented  regarding occurs.  more d i r e c t  oceanic  crust  the  In  particular,  seismological  remains  only  to  illustrate  onshore-offshore  are  and  still  the  is  intermediate  generally  downthrown  of  the  towards  kink  difficult there  of  in  is  (Spence 1 9 7 7 ) ,  230  southern  in  not  the  depth  with  the  oceanic  data,  there  km f r o m s h o t  shot  of  point  continental  the s l a b .  20-30  w i t h the  (1983) model f o r  Vancouver I s l a n d .  P19;  the  km,  crust, zones  it  model  lower c r u s t a l  line I  l o c a t i o n map,  are  1.3),  along  the  consistent (at the  Moho  of  and the  the  to cross l i n e I  intersection  point.  a  bend  l i n e IV causes the deeper ray paths from to the n o r t h e r n r e c e i v e r s  is  portion  intersect Fig  the  central  intermediate  IV  is  with  beneath  line  been  especially  associated  A l t h o u g h the models  see  ocean  However,  km  is  The k i n k may  normal f a u l t i n g has  Another problem w i t h the  t o t a l l y consistent  middle  in  seismicity range  d i s t a n c e s up t o 2 0 - 2 5 km west the  60  intermediate  continuous  w i t h a t h r o w o f ~10  no s u b s t a n t i a l  Island.  of  the  about  a n d i n some s u b d u c t i o n  where l i n e I V a n d t h e o n s h o r e - o f f s h o r e point  (1983)  subducting  inconsistent  a  a normal f a u l t  t h e McMechan a n d S p e n c e  length  not  the  i n the downgoing c r u s t .  accept a f a u l t  is  i.e.  it  the  w i t h the seismic  a d e s c e n t mechanism f o r  to  Vancouver that  is  phase  al.  down t o a d e p t h of  m o d e l has  the c o n t i n e n t ,  the A l e u t i a n arc  s u g g e s t e d as  fault,  it  et  that  included in  consistent  p e r h a p s be i n t e r p r e t e d a s  since  Fukao  the  some d i f f i c u l t i e s w i t h t h e m o d e l . The m a i n p r o b l e m  existence  s u c h as  which  data.  Although the crust  that  at  evidence  untransformed  km. The s h a l l o w e r p h a s e c h a n g e i s model  depth  the at  Thus,  l o w - v e l o c i t y zone a b o v e t h e Moho  139  s h o u l d e x t e n d as  far  existed  intermediate  in  the  downgoing c r u s t of  the oceanic  crust  by  crust  of  to  the  Island  stations  both  section near  t o be a d v a n c e d  models  t h e d e p t h of  on t h e  d e e p as  final  corner Moho,  where for  4.18). shadow  arrivals size  by ~1  of  the  continuity  (Fig.  which i s rather  crust the  s relative  4.7) oceanic  itself is  high-velocity  and i n the  not  implied material  on t h e  basis  shallow at  high-  the Vancouver  to the  arrivals  depth  of of  since  model  discussed f i x e d at  constrained and  by  Spence  1.5).  If  a Moho d e p t h >40  interpretation,  subducting  shadow  ocean c r u s t far  a 2D s e i s m o g r a m  the  offset  km were  due  effect to  the  continental  receivers  i m p l i c a t i o n s of  routine  shadow zone  meets the  the  Moho  then a major zone,  a  (1983),  the l i n e IV d a t a s e t a l l o w the  52 km ( F i g .  the c u r r e n t  is  McMechan  m o d e l w o u l d be a l a r g e r the  final  t h e c o n t i n e n t a l Moho was  w o u l d be r e q u i r e d . H o w e v e r , of  in  the subducting  particular,  i s d i f f i c u l t to q u a n t i f y  zone,  that  model  subducting  the t u r n i n g rays to the  It  final  requirement,  40 km. A l t h o u g h t h i s  interpretations  as  this  stations.  used i n the o n s h o r e - o f f s h o r e  the  the  a sliver  In  interpretation  alternate  If  kink  and t h e d e s i r e d  r e q u i r e d to a l l o w a r r i v a l s  alternate  4.5,  preferred  be  is  P19.  then the  r e f r a c t i o n d a t a but  constraints.  the mainland  value  to  into  principles,  pathway  In in  the  shot  lost.  With t h i s  satisfy  seismic  from  " k i n k " model,  t h e n be i n t r o d u c e d a b o v e t h e  velocity  at  km  i n v o k i n g the requirement  by more g e n e r a l must  ~205  w o u l d be evolved  be c o n t i n u o u s .  necessary  as  w o u l d become a b r e a k ,  The two m o d e l s mainly  west  a  (Fig. larger  including diffraction  it  is  my  impression  is  sufficiently  large;  that with  140  a larger  shadow  turning  ray  zone,  too  arrivals.  many  receivers  Nevertheless,  it  model t h e d a t a u s i n g o n l y r e f l e c t i o n s (although  the  stations  would  Moho,  fit  the e f f e c t  to that used  deteriorates be  on t h e r e m a i n d e r  shown i n F i g u r e  in  the subduction  implies a faster  which  is  in  at  the model w i l l  i n which d i f f e r e n t  zone t e s t m o d e l . T h a t  stations the  far  continental be  similar  Moho d e p t h s is,  were  a d e e p e r Moho for  the  margin the  gravity  i n the P a c i f i c Northwest e x h i b i t a l o w - h i g h  couple  similar  to  introductory  other  subduction  normally corresponds  t o t h e t r e n c h and  trench  in  gap,  far-offset  to  reflector.  discussed  anomaly d a t a  without  possible  w i t h a deeper  G r a v i t y model a c r o s s the s u b d u c t i n g As  still  amplitudes  Thus, of  be  m a n t l e v e l o c i t y and a l s o a s h a l l o w e r d e p t h  the upper m a n t l e  4.7  2.8,  is  to the  because  too s m a l l ) .  would  which  Vancouver  Island.  conflict",  which  the  The was  Pacific  main  zones.  the  Vancouver the  chapter,  The g r a v i t y  high  to  the  low arc-  Northwest  includes a l l  Island  "gravity-seismic  concern of R i d d i h o u g h  of  (1979),  a r i s e s b e c a u s e t h e g r a v i t y h i g h demands a r e l a t i v e l y t h i n  crust  in  30 km  which m a t e r i a l  or l e s s ,  whereas  suggest that The  seismic  the c r u s t  to  be  occurs  interpretations  at  on  the major thicker  onshore-offshore  thickness seismic  than  depths of  Vancouver  Island  i s much t h i c k e r .  i n t e r p r e t a t i o n of McMechan a n d S p e n c e  t h e minimum c r u s t a l provides  w i t h mantle d e n s i t y  that  seismic  (1983),  beneath Vancouver I s l a n d  evidence implied  m o d e l must  which c o n s t r a i n s by  the  i n which is  37 km,  the  crust  gravity data.  o b v i o u s l y be c o n s i s t e n t  The with  141  the  i n t e r p r e t a t i o n of McMechan and S p e n c e  immediately  be r e c o g n i z e d  island profile w i l l of  the  Island  of  stated  the  the h i g h - v e l o c i t y s l i v e r  that  reduced.  perhaps  the  The l i m i t a t i o n o f  conflict  is  that  eastwards s u f f i c i e n t l y The r e s o l u t i o n of which  justify  pointed  and to  required properties, could  originate  lithosphere pressure solution contrast, density  4.7.1  does  be  qualitatively  severity  of  the  conflict  model  in  will  be  resolving extend  requires  of a n o m a l o u s  the  velocity.  metamorphic the  wedge o f m a t e r i a l c o n d i t i o n s of  seismic correspond  Moho, to  material  Riddihough  rocks  with  the  anomalous  material  above the  downgoing  low t e m p e r a t u r e , the  which a  arguments  lower c r u s t a l  compressional basic  special  high  i m p l i c a t i o n of marks  a  this  velocity  d i s c o n t i n u i t y marking a  contrast.  Method The o b s e r v e d  onshore-offshore the  can  and s u g g e s t e d t h a t  not  the  Vancouver  it  the c o n f l i c t  known  the  western  range,  and h y d r o u s e n v i r o n m e n t . T h u s , that  of  far.  due t o e x p e c t e d  is  cross-  the h i g h - v e l o c i t y s l i v e r does not  low  in  can  resolution  because  beneath  the c r o s s - i s l a n d  the e x i s t e n c e  with high density (1979)  However,  it  the  a straightforward  conflict.  i n t h e 2 0 - 2 5 km d e p t h  a n d so  a g r a v i t y model a l o n g  not b r i n g about  gravity-seismic  presence  that  (1983),  constraints  concept  that  g r a v i t y p r o f i l e along a l i n e very close p r o f i l e was q u a n t i t a t i v e l y of  the  cross-island  the lower c r u s t  low-velocity material.  modelled  seismic  may c o n t a i n a n o m a l o u s  The o b s e r v e d  gravity values  model  to  the  utilizing and  the  high-density, (dashed  lines  142  in Figs. the  4 . 2 2 b and 4 . 2 2 c )  and t h e o c e a n  bottom topography  l i n e were s u p p l i e d by R i d d i h o u g h ( p e r s . The  Figure  interpreted  4.22a.  gravity  model  seismic east  The  cross-island  model  of W a l d r o n  interpretation  as  the  top  of  for  (1982),  of  b l o c k s was r e d u c e d ,  of  2.72  g  cm"3  was  used,  Waldron  base  of  the  in  of  were  (1979):  mantle  is  3.34  of  2.62  material  density  of  material  embedded  3.28  3.30  g cm"3,  of  the  density expected  of  number density latter  crust  and  (1979),  (including from  the those  density  cm"3,  is  oceanic  the  same  2.92  g  in as  cm"3,  asthenosphere  is  3.285 g c m " 3 , and  the  is assigned a  g  high-velocity  cm"3. Finally,  the s l i v e r  i n the c o n t i n e n t a l c r u s t less  oceanic  increase  the b a s a l t / e c l o g i t e  the  Riddihough  g r a v i t y model a r e  is  than normal mantle  crust  the surrounding mantle. to  the ocean  far  minor  g c m - 3 . The  of  given a density  beyond t h a t  is  assumed t o  The c r u s t a l of  of  density.  i n t e r p r e t a t i o n of R i d d i h o u g h ( 1 9 7 9 ) ,  downgoing  as  above t h e downgoing l i t h o s p h e r e  somewhat  As i n t h e  marine  few  crustal  boundaries  3.295 g c m " 3 , c o n t i n e n t a l asthenosphere i s anomalous  A  the  interpretation. Densities  crustal g  of  crust  slightly different  seismic  the c r o s s - i s l a n d  Riddihough  lithosphere  instead  t h e r e g i o n of  lithosphere)  regions  slope.  a n d an u p p e r  o f a number o f  i m p l i e d by t h e c r o s s - i s l a n d other  the ocean  (1982) u s e d t h e same m o d e l as  in which the p o s i t i o n s  shown i n  features  made t o W a l d r o n ' s g r a v i t y m o d e l ;  change a r o s e because o u t s i d e shelf  is  w h i c h was b a s e d on h i s  continental  were  1982).  model  most  t h e p o r t i o n of  modifications sedimentary  gravity  incorporates  the  comm.  along  the  density  increase  density  to  the  would  be  normal l i t h o s p h e r e  p h a s e t r a n s i t i o n . But i n t h e g r a v i t y  due  to  model,  143  FIG.  4.22. (a) D e n s i t y m o d e l , w i t h d e n s i t i e s i n g c m - 3 , based on t h e o b s e r v e d g r a v i t y and on t h e f i n a l v e l o c i t y m o d e l f o r the o n s h o r e - o f f s h o r e p r o f i l e . Arrowheads i n d i c a t e c o a s t s of Vancouver Island. The s h a d e d r e g i o n i s t h e p o r t i o n o f t h e l o w e r s e i s m i c c r u s t w i t h an anomalous low-velocity highdensity relationship. The h o r i z o n t a l dashed l i n e at the b a s e of t h i s r e g i o n i s t h e s e i s m i c Moho a t 37 km d e p t h . (b) O b s e r v e d g r a v i t y v a l u e s ( d a s h e d l i n e ) and theoretical gravity v a l u e s ( s o l i d l i n e ) f o r which the shaded r e g i o n of the d e n s i t y model i s a s s i g n e d a d e n s i t y of 3.30 g c m - 3 . (c) Observed g r a v i t y v a l u e s (dashed l i n e ) and theoretical gravity v a l u e s ( s o l i d l i n e ) f o r w h i c h t h e s h a d e d r e g i o n of t h e d e n s i t y m o d e l i s a s s i g n e d a d e n s i t y of 2 . 9 2 g c m " 3 , t h e normal d e n s i t y expected for c r u s t a l material. The misfit i l l u s t r a t e s the basic g r a v i t y - s e i s m i c c o n f l i c t .  144  1 45  as  in  the  seismic  change o c c u r s even  if  3.56  v e l o c i t y model,  crust  density  would  wavelength than that The b a s e o f  be  reflector without  is  any  part  rays t r a v e l ) .  With 2.6  km o f  lithospheric  thickness  lithosphere  (aged  the a g e - t h i c k n e s s of  the  Coast  km.  thickness Ma a t  of  Forsyth  asthenospheric  extended effect  of  1975).  densities  m a n t l e was a s s u m e d Riddihough  value  to  the  absent  outside  as  of  This  value  around  20  al.  at  the  31  km  is  slightly for  the  b a s e d on  ( 1 9 7 6 ) . The to  1976).  Moho  t h e e d g e s of km d e p t h ,  its  depth of  in the  t h e Moho d e p t h  in  and  top  100 km d e p t h  East  this  the mantle;  (Fulton  the  sediments,  km  in accord with  t o ± 3 0 0 0 km and down t o 200  of m a t e r i a l  km  33 km was u s e d f o r  1 9 7 9 ) . The s t r u c t u r e s  out  in  d e p t h of  deep  was e x t r a p o l a t e d  Below  basin  model t h r o u g h which  ~1  and I s a c k s  the  longer  ocean  the c o n t i n e n t a l m a r g i n ) ,  were u s e d f o r  be  much  the  as  et  of  to  reflector  P19 be  case,  density  the western  and  ~21.5  (Barazangi  a  any  due  a  phase  data.  the s e i s m i c  water  lithosphere  margins  Mountains,  ( B e r r y and  t o be  i t could  the v o l c a n i c c h a i n , which i s  other a c t i v e  have  shot  r e l a t i o n s h i p of Y o s h i i  subducting  beneath  beneath  of  is  6-9  and  gravity  Beneath  25 km ( a l t h o u g h  than the expected  effect  t o the upper m a n t l e  model.  altering  larger  small  lithosphere  was assumed t o c o r r e s p o n d velocity  the g r a v i t y  i n the observed  the  In  were a s s i g n e d an e l e v a t e d  g c m " 3 b e l o w 30 km d e p t h ,  seismic  which the  i s assumed t o be 60 km o r g r e a t e r .  the oceanic  increased  the depth at  region,  lithospheric  Walcott  1975;  the model so  that  t h e s e l i m i t s c o u l d be s a f e l y  were the  assumed  negligible. The  gravity  field  due  to  the  structural  model  was  1 46  calculated  using  Talwarii et order  al.  a standard  ( 1 9 5 9 ) . The m o d e l p a r a m e t e r s w h i c h were v a r i e d  t o match the o b s e r v e d  mainly:  (1)  density  g  cm"3,  the d i s t a n c e  of  the  of  which corresponds  4.7.2  sedimentary  final  shaded r e g i o n lower  anomalous  r a n g e 250-400 km, and i n the d i s t a n c e  to the sediments  (2)  of  with  continental  the  range  were  wedge  and the d e p t h t o the nearby  blocks  seismic  final  crust  values  The  along  boundaries  110-200 km, much  Tofino  shown  corresponds  Basin.  when a l l of  shaded r e g i o n ,  g c m " 3 . Thus,  gravity-seismic  lower  gravity  towards  of  The the  g cm"3  illustrates  for  the  g r a v i t y values observed the  gravity  theoretical  crust,  including  density  of  these t h e o r e t i c a l the c u r r e n t  are  2.92 values  extent  of  the  conflict.  final  g r a v i t y m o d e l , most of  (1982), the  the  seismic  basin  the  ocean  the s h e l f .  westernmost  basin  the  features  of  and c o n t i n e n t a l s h e l f  due t o r e l a t i v e l y n e a r - s u r f a c e e f f e c t s .  reflects  3.30  i s a s s i g n e d a normal c r u s t a l  p r o f i l e above the ocean  of W a l d r o n  portion  4 . 2 2 c shows  the d i s a g r e e m e n t between  observed  For the  the  of  with  Figure  4.22a.  high-density low-velocity  theoretical  together  profile.  in Figure  to that  w i t h an a n o m a l o u s  4.22b,  the  gravity values  gravity  figure  is  corresponding  in Figure  the  model  and i s a s s i g n e d a d e n s i t y  model.  displayed  gravity  i n the  relationship,  and  the  gravity profiles  in  Interpretation The  the  and c a l c u l a t e d  t h e d e p t h t o t h e t o p of  3.30  Moho o v e r  t w o - d i m e n s i o n a l a l g o r i t h m b a s e d on  As i n t h e gravity  sediments  The g r a v i t y h i g h o v e r  the outer  are  interpretation  low  increasing  the  (Fig.4.22b) in shelf  thickness is  due  147  to  decreasing  to greater  water depth a l o n g  sediment  the main p o r t i o n of in of  Tofino Basin,  the c o n t i n e n t a l s l o p e and  densities  there.  the  is  shelf  The n e g a t i v e  anomaly  r e l a t e d mainly to the  t h e b a s i n and may a l s o be l e s s d e n s e t h a n on t h e o u t e r  shelf.  increasing  crustal  lithospheric  Island terms  to of  gravity  a  gravity in this  t o bend more high,  low  That  is,  which  part  of  from  the downgoing  extends  across  Vancouver  may be e x p l a i n e d  imbedded i n t h e  the p o s i t i v e anomaly  t h e d e p t h r a n g e 2 0 - 2 5 km, a n d p a r t  density  l o w - v e l o c i t y m a t e r i a l beneath  is  due  and  part  of  zone  in Fig.  the  m a i n l a n d . The a n o m a l o u s  4.23a)  maximum t h i c k n e s s  extends  of  been r e d u c e d r e l a t i v e  the lower c r u s t an e v e n b e t t e r would  be  basis  of  seismic  that  to that  the  size  of  highIsland  (the  Moho  shaded with  a  t h e main r e s u l t  of  the anomalous  zone  will  also satisfy  match  between if  the g r a v i t y d a t a .  observed  and  the r e f r a c t i o n d a t a ,  unless  peculiar  c o n f i g u r a t i o n o u t of  the  profile  or  so  fragmented  is  as  not p e r m i t t e d  of  the  in  example, values  2 0 - 2 5 km d e p t h  the extended  plane  For  calculated  the h i g h - d e n s i t y s l i v e r at  e a s t w a r d s . However, t h i s  were  has  of R i d d i h o u g h ( 1 9 7 9 ) .  o t h e r d i s t r i b u t i o n s of h i g h - d e n s i t y m a t e r i a l  obtained  were e x t e n d e d  the  material  o n l y 9 km. T h u s , p e r h a p s  the g r a v i t y m o d e l l i n g i s  Obviously,  above  Vancouver  to  Island  i s due t o a n o m a l o u s eastern  in  lower  the s l i v e r of h i g h - d e n s i t y mantle under w e s t e r n Vancouver in  the  steeply.  of h i g h - d e n s i t y m a t e r i a l  crust.  arises  r e g i o n , as  p o i n t e a s t of G e o r g i a S t r a i t ,  pockets  (seismic)  thickness  slab begins  The m a i n  the  towards  sediments center  a c o n t r i b u t i o n to  in thickness  over  the  As w e l l ,  which increase  also  on  the  s l i v e r had a cross-island  t o be u n d e t e c t a b l e  at  the  1 48  wavelengths of  4.8  seismic  energy.  Discussion The m a j o r o b j e c t i v e o f t h e o n s h o r e - o f f s h o r e  obtain  a  study  was  t w o - d i m e n s i o n a l v e l o c i t y model t o upper m a n t l e  a c r o s s t h e m a r g i n where t h e o c e a n i c J u a n de F u c a p l a t e beneath  the  continental  America  r e s u l t a n t v e l o c i t y model t o g e t h e r  plate.  depths  subducts  F i g u r e 4.23  shows  B2  of  (1984),  to  the l o c a t i o n of  along  onshore-offshore speculative terranes currently  profile.  concept  are  the  possible  of  the  requiring datasets.  the  and  the  model  also  on  the  to older  underlain  The  al.  incorporates  are  plate.  by  is  the  b a s e d on  the  seismic  w h i c h has p r o v i d e d a r e p r e s e n t a t i o n  subducting  v e l o c i t y model i s interpretation  The v e l o c i t y  the presence of  lithospheric plate  actually of  s t r u c t u r e of  several  a relatively low-velocity  continental  shelf,  the c o n t i n e n t a l c r u s t in  the  of  including  interpretation  interpretation  was  a  composite associated  melange  was d e t e r m i n e d by W a l d r o n  was e s t a b l i s h e d  l e n g t h of V a n c o u v e r I s l a n d ;  a  the oceanic c r u s t ,  m a r i n e r e f r a c t i o n i n t e r p r e t a t i o n . The b a s i c  (1983)  et  corresponding  and  geophysics  Monger  c o m p l e x i t i e s a s s o c i a t e d w i t h the subduction zone.  The f i n a l  outer  stacked  from t h i s t h e s i s ,  model  assemblages  oceanic  other  close  The t e c t o n i c  that  descending  geometry  line  vertically  surface geology, results  a  the  w i t h a s t y l i z e d t e c t o n i c model  t a k e n from t h e t r a n s - C o r d i l l e r a T r a n s e c t located  to  of  t h e most  by  seismic McMechan  model, seismic  including  beneath  the  (1982)  in a  structure and  of  Spence  a r e f r a c t i o n l i n e along  the  important  the  p r e f e r r e d v a l u e of  feature  37 km f o r t h e  of  crustal  60-J  HO)  (OZ)  H O H , OZETTE MELANGES  WR)  CRESCENT  WRANGELLIA PACIFIC RIM  AGE in Ma  FIG.  4.23. Final velocity model for the onshore-offshore profile and s t y l i z e d t e c t o n i c c r o s s s e c t i o n m o d i f i e d from the t r a n s - C o r d i l l e r a T r a n s e c t B2 of Monger e t al. (1984) No v e r t i c a l e x a g g e r a t i o n .  vo  150  thickness, zone  which a l s o  in  the  structures  i m p l i e d the e x i s t e n c e  lower  as  crust.  known  onshore-offshore  of  a  low-velocity  With  the  o c e a n i c and c o n t i n e n t a l  quantities,  the  interpretation  profile  across  Vancouver  of  the  I s l a n d was a b l e  to  extend the c r u s t a l models t o the upper mantle r e g i o n . A l t h o u g h the s u b d u c t i n g observed, control it  the  onshore-offshore  on t h e d i p o f  bends  to  oceanic  the oceanic c r u s t  relatively  small  (3°  continental  shelf.  Thus, assuming  same  that  the d i p at or  a n g l e as  not  and the p o i n t  d i v e under the c o n t i n e n t a l c r u s t .  is  the  was  directly  d a t a s e t p r o v i d e s some  information  at  crust  The  at  under  a l l shots  the melange, of  which  35 km e a s t o f  the foot  the c o n t i n e n t a l s l o p e .  is consistent  w i t h the r e s u l t s  conclusion  easternmost  of Taber  more d i r e c t l y  o b s e r v e d t h e bend i n t h e s l a b a t  km  of  landward  the  beginning  c o n t i n e n t a l m a r g i n . Taber s e d i m e n t a t i o n caused also  resulted  depth of  in  of  the slope at  the slope  may  be  found  by  the Washington  large  of  t o move s e a w a r d s ,  and  Taber  the  under  t h e d i p of the s l a b under the c o n t i n e n t a l  determined  14-16°,  With  a p p r o x i m a t e l y known  because  the depth of  the s l a b  than  37  Moho d e p t h f o u n d by McMechan and Spence  average d i p i s  who  rates  w e s t e r n V a n c o u v e r I s l a n d c a n be no s h a l l o w e r continental  This  (1983),  t h e l a c k o f an o c e a n b o t t o m t r e n c h .  the t o p of t h e o c e a n i c c r u s t  shot,  a p o i n t n e a r l y 50  (1983) s u g g e s t e d t h a t  the foot  the c o n t i n e n t a l s l o p e , shelf  of  dip  the bend i n the  s l a b must o c c u r l a n d w a r d of  is  outer  layers  subducting is  the  unit  on t h e  the other oceanic  the base of  which  controlling  the base of the melange  less)  indirect  which i s  (1983)  from  larger  than the v a l u e  seismicity  studies  under  km,  the  (1983).  The  of  9-11°  a n d f r o m an  151  onshore-offshore However, to  r e l a t i v e l y small differences  be  expected,  to the of  r e f r a c t i o n p r o f i l e off  s o u t h of  Puget  Sound  n o r t h of  the  model, (Fig. is  (Fig.  t h e s t u d i e s of T a b e r  1.1),  4.23).  velocity  the  upper  mantle  below  the  which i s c o n s i s t e n t  of  age  and  boundary w i t h the  oceanic  crust.  dataset,  a sliver  is  zone  independently  (Fig.  Figure western  of  As  be  asthenosphere  reasonably  thickness  well  well,  with  the  relative  the  preferred  l e s s than the v e l o c i t y  above,  velocities  i n the Vancouver I s l a n d  regions  feature  inner  from  interpreted  Washington  the  the onshore-offshore  continental  showing  the by the  and t h e i r p o s s i b l e  Beneath  region  above  of  appears  downgoing refraction  velocities  shelf.  h i g h - v e l o c i t y m a t e r i a l at  A map v i e w  4.24.  the  velocity  t h e 2 0 - 2 5 km d e p t h r a n g e b e n e a t h  predicted  Island,  1.4).  velocity  at  region  Vancouver  to  i n the  of m a t e r i a l w i t h m a n t l e - t y p e  I s l a n d and the  localized  is  9 Ma, the expected  expected  On t h e b a s i s o f  present  Vancouver  latitude  asthenosphere.  subduction  is  located  i n the r e g i o n can  from o c e a n i c  reflector.  t o be c o m p l i c a t e d by an a n o m a l o u s  km/s)  perhaps  the  study  reflector  km, and t h i s c o r r e s p o n d s  the upper mantle  The  lithosphere  upper mantle  For l i t h o s p h e r e  20  lithosphere  are  ( 1 9 8 3 ) were  w h i l e the present  the subducting  d i v i d i n g oceanic  d e p t h of  in structure  bend.  with  about  of W a s h i n g t o n .  a bend i n t h e c o n t i n e n t a l m a r g i n a t  The b a s e o f associated  since  the c o a s t  the  southern  location  A  of  similar  dataset  of  these  Vancouver  and w e s t e r n O r e g o n , L a n g s t o n  (1981)  was  along  and Spence  interconnection is tip  western  20 km d e p t h  refraction McMechan  (7.7  (1983) high-  shown i n Island, found a  152  FIG.  4.24. Shaded r e g i o n shows t h e p o s s i b l e e x t e n t o f p o c k e t s of h i g h - v e l o c i t y m a t e r i a l l y i n g at 20 km d e p t h beneath Vancouver Island. The boundary of the shaded r e g i o n i s i n d i c a t e d by a s o l i d l i n e where i t s l o c a t i o n i s reasonably w e l l - c o n t r o l l e d , and by a d a s h e d l i n e where i t s l o c a t i o n i s spsculat ive.  1 53  r e g i o n of  mantle-type  low-velocity at as  zone a n d a s e c o n d  40 km d e p t h . older  v e l o c i t y at  apply  for  Vancouver  Island.  features  are  reorganization explain which  That  is,  locus  of  then  Riddihough  high v e l o c i t y  starting  shallow h i g h - v e l o c i t y l o w e r one a s  lithosphere.  mantle Similar  the s p e c u l a t i o n  related arguments  regions  beneath  c a n be made t h a t  of  a subducted  slab,  of  subduction  jumped  geometry  region  perhaps stranded westward.  has  been  the  former  trench  a  suggested  to  shelf,  (Dickinson  1976;  et  j u m p i n g of  of  an  al.  a subduction  accreted  1982).  At  terrane  the  z o n e may be r e l a t e d  at  same  continental  proportions.  several  adjacent  include  the  mainland  (Jones et  Complex  PR  extrapolated  accreted  Wrangellia  on  al.  the  time,  Wagner  thrust  faulting  1977;  (Muller  volcanics crust  (Yorath  distinct  units  Muller  1982), a detached  but  model of  crust 4.23b,  represented.  1977),  the  These and  Pacific  c o a s t of Vancouver I s l a n d  1977),  a n d C u r r i e 1980; laterally,  are  the Rim and  t h e O z e t t e a n d Hoh m e l a n g e s  continental shelf  Waldron  CR i d e n t i f i e d a s  tectonic  WR on V a n c o u v e r I s l a n d  westernmost  offshore  1981;  the  terranes  terrane  OZ and HO u n d e r t h e o u t e r and  In  to  the c o n t i n e n t a l margin  a s s o c i a t e d w i t h the c o l l i s i o n process tends to t h i c k e n the to  in  Such  of T o f i n o B a s i n on t h e c o n t i n e n t a l been  the  1979).  arrival  (Jones  b e l o w w h i c h was a  high-velocity  subduction  have  The s e a w a r d the  shallow,  the e x i s t e n c e may  ocean  the  remnants  t h e p a s t when t h e  the  m a n t l e and t h e  to the c u r r e n t l y subducting may  r e g i o n of  He i n t e r p r e t e d  indigenous  16 km d e p t h ,  and  and  slope  a fragment  slab  of  (Snavely  of  Crescent  Eocene  oceanic  Y o r a t h 1 9 8 0 ) . The t e r r a n e s  in addition  the  speculation  form has  1 54  been  made  that  the  stacking  of  beneath  Wrangellia  several  t h e t e r r a n e s as  terranes,  terranes  Compared  above  the  the (Fig.  crust  might  4.23a), to  the  and a l s o  which  is  likely  the  continental  shelf  s l a b of  details  constrained  new  from  the  as  velocity material crust.  modelling is  required  R i d d i h o u g h (1979) has  anomalous  material  low t e m p e r a t u r e  may  The s e i s m i c subducted  lower sliver  low-velocity  or  underthrust  low v e l o c i t i e s  the r e g i o n .  The  may  sliver,  b u t a l s o may c o n s i s t  may be  a  oceanic  large crust  of  fragment  beneath  refraction  based  on  the  However, a g r a v i t y seismic  is in  data  model  that some  main  the  environment  above  to  similar  conclusion  high-density  portions  suggested that  heat  interpretation  leads  special  zone  gravity,  of  the  lowlower  f o r m a t i o n of  be due t o c o n d i t i o n s of  and hydrous  i n the  high-velocity  t h o s e o f R i d d i h o u g h ( 1 9 7 9 ) . The  gravity  below  to the o r i g i n a l subduction  w h i c h was  data.  conclusions  region  4.23b).  flow and l i m i t e d s e i s m i c the  of  westerly  onshore-offshore  the onshore-offshore  Riddihough (1979),  by  the  and t h e  material,  (CR i n F i g .  The i n t e r p r e t a t i o n of  m o d e l of  nature  detached  significant  of  of  4.23b).  l o w - v e l o c i t y zone  crust.  Thus,  mixture  more  speculative  mantle m a t e r i a l  lower c r u s t a l  to  added  the  1984).  plus  (Fig.  terranes,  from the m e l a n g e - l i k e  equivalent  al.  in vertical  fragmented  Rim  includes  oceanic  a number of  most  et  a  Pacific  and O z e t t e  result  has  be  be made up of p o c k e t s  from  high-velocity  (Monger  resulted  v e l o c i t y model from the  subducting  zone may t h u s material  to  corresponds  "continental"  well  there  Crescent  profile  Wrangellia  f a u l t i n g has  including  s u c h as  refraction  thrust  the  high pressure, the  subducting  155  crust.  In  the  new s e i s m i c  m o d e l , a s l i v e r of m a n t l e  with a normal v e l o c i t y - d e n s i t y r e l a t i o n s h i p , lower  crust  beneath  the anomalous been  zone,  reduced  An  western Vancouver I s l a n d . relative  to a pocket  a maximum t h i c k n e s s  of  alternative  velocity material  to that  origin  may  for  perhaps  discussed  m a t e r i a l may n o t be due subducting  but  elsewhere  a n d been t r a n s p o r t e d  accreted  terrane,  terranes already their  modern  oceans, the  analog  which i s  of  the  manifestation involving  in  of  If  of  the  the  the the  1981).  that of  As s u g g e s t e d by McMechan a n d S p e n c e (1983),  future  studies  can a r i s e exotic  reflections  of  crustal  in processes  (1983) a n d E l l i s  seismic  be  et  profiling  refraction  to determine  t o upper mantle d e p t h s c o u l d  structure  terranes.  been c a r r i e d o u t ,  Project,  to  another  has  Seismic  related  is  A feasibility  Island  today's  region  of  Vancouver  in  have  gravity-  the d e t a i l s already  fact  of  the  d e l i n e a t i n g many of study  an  is,  should include r e f l e c t i o n the  of  oceanic  That  Island  and a c c r e t i o n  is  the c r u s t a l and  formed  the stack  in  material  the  environment  part  under  terranes  plateaus,  of  the m a t e r i a l had as  low-  speculative  the p l a t e a u s present  complexity  the generation  the  region  accreted  Vancouver  has  Moho w i t h  properties  w h i c h was t h r u s t  al.  of  (1979),  current  between c o n t i n e n t a l et  the  the s i z e  high-density  in  The  the anomalous  oceanic  (Ben-Avraham  conflict  found  rather  to  i n some of  intermediate  structure  of  in place.  then the o r i g i n  origin  seismic  some  the unusual  to  in  4.22a).  previously.  slab,  Thus,  above the s e i s m i c  be  anomalous  found  Riddihough  o n l y 8 km ( F i g .  model  the  of  of m a t e r i a l  tectonic  above  was  material,  as  part  whether  acquired  al. for  model. of  the  coherent in  this  1 56  tectonic km  regime  1200%  (Clowes et  common  al.  depth point e x p l o s i o n survey;  the r e f l e c t i o n l i n e  (RL i n F i g s .  the o n s h o r e - o f f s h o r e  line  across  section  the  record  traveltime, 9.5  depth  I.  1.3  s.  These  using  corresponding  Two c l e a r near  the  are  with  the  at  it  constrained reflector  low-velocity reflector  i n mind  in  D at  the  of  that  encouraging the  results  Canadian  of  of  least  150  km  the s i t e  of  near  converted and  on t h e  the  refraction  A correlates  well  24  km  depth  boundary The  not  of  wellpossible  the  crustal  ( 1 9 8 3 ) . The d e e p e s t  correlates  with  either  subducting oceanic  seismic  although  third  middle  and Spence  the  was  could  r e f r a c t i o n model and  Steering of a  Committee  major  i n May-June  1984.  c o p y r i g h t C o n t i n e n t a l O i l Company  led  reflection includes  the m a j o r i t y  following  the  designate  The p r o g r a m  3000%-coverage p r o f i l e s ,  which i s a p r o f i l e a c r o s s Vancouver I s l a n d  to  Vibroseis1  the  crust.  t h e r e f l e c t i o n f e a s i b i l i t y s t u d y has  p r o g r a m t o be c a r r i e d o u t at  a  C near  i n the  McMechan  Lithoprobe  Vancouver I s l a n d as  two-way  model,  model.  37 km d e p t h p o s s i b l y  availability  s  h a v e been  reflector  this  refraction  c o n t i n e n t a l Moho o r t h e t o p o f The  present  the h i g h - v e l o c i t y s l i v e r ,  31 km d e p t h l i e s zone  E at  reflector  t h e b a s e of  s h o u l d be k e p t  7.0  to  16 km d e p t h d e t e r m i n e d by McMechan a n d  ( 1 9 8 3 ) . The s e c o n d with  were  were o b s e r v e d  shown s u p e r i m p o s e d  The u p p e r m o s t  correspond  energy  velocity  in Figure 4.25. refractor  was v e r y c l o s e  and  two-way t r a v e l t i m e s  refraction  depths  s  10  the l o c a t i o n of  reflections  4.4  model  Spence  and 4.24)  a n d two b a n d s of c o h e r e n t  s and 1 0 . 8  to  1 9 8 3 a ) . The p r o g r a m i n c l u d e d a  of  essentially  Vancouver Island  FIG.  4.25. Enlargement of the final velocity model under Vancouver Island. RL i n d i c a t e s t h e l o c a t i o n o f t h e 10 km e x p l o s i o n r e f l e c t i o n l i n e , a l s o shown on t h e location map ( F i g s . 1.3 o r 4 . 2 4 ) . The e q u i v a l e n t d e p t h s o f r e f l e c t o r s A , C, D and E on the reflection record section are s u p e r i m p o s e d on t h e m o d e l .  _  1 58  the  same l i n e as  resolution (or  study  repudiation)  model.  the c r o s s - i s l a n d  refraction  may t h u s p r o v i d e  nearly  of  many  of  the  profile.  immediate  f e a t u r e s of  This  high-  confirmation the  refraction  159  REFERENCES Aki,  K. and W . H . K . Lee (1976). Determination of threedimensional v e l o c i t y a n o m a l i e s under a s e i s m i c a r r a y u s i n g first P arrival times from local earthquakes, 1. A homogeneous i n i t i a l m o d e l , J . G e o p h y s . R e s . 8J_, 4 3 8 1 - 4 3 9 9 .  Aki,  K., A. Christoffersson and E.S. Husebye (1977). D e t e r m i n a t i o n of t h e t h r e e - d i m e n s i o n a l s e i s m i c s t r u c t u r e o f the l i t h o s p h e r e , J . Geophys. R e s . 8^, 277-296.  A n d o , M . and E . I . B a l a z s ( 1 9 7 9 ) . G e o d e t i c e v i d e n c e f o r subduction of t h e J u a n de F u c a p l a t e , J . G e o p h y s . 3023-3028. Aric,  Au,  K . , R . G u t d e u t s c h and A . Sailor (1980). traveltimes and rays in a medium of v e l o c i t y d i s t r i b u t i o n , Pure A p p l . Geophys.  aseismic Res. 84,  Computation of two-dimensional 118, 7 9 6 - 8 0 6 .  D. and R . M . Clowes ( 1 9 8 2 ) . C r u s t a l structure from survey of the Nootka fault zone off western G e o p h y s . J . j_68, 2 7 - 4 8 .  an OBS Canada,  B a r a z a n g i , M . and B . L . I s a c k s ( 1 9 7 6 ) . Spatial distribution of e a r t h q u a k e s and s u b d u c t i o n of t h e N a z c a p l a t e b e n e a t h S o u t h A m e r i c a , Geology 4, 686-692. B e n - A v r a h a m , Z . , A . N u r , D . J o n e s a n d A . Cox ( 1 9 8 1 ) - C o n t i n e n t a l accretion: from o c e a n i c p l a t e a u s to a l l o c t h o n o u s terranes, S c i e n c e 213, 47-54. B e r r y , M . J . a n d D . A . F o r s y t h ( 1 9 7 5 ) . S t r u c t u r e of the Canadian Cordillera f r o m s e i s m i c r e f r a c t i o n and o t h e r d a t a , C a n . J . E a r t h S c i . J_2, 1 8 2 - 2 0 8 . Bessonova, E . N . , Y . M . F i s h m a n , V . Z . R y a b o y i and G.N. Sitnikova (1974). The tau method f o r i n v e r s i o n o f t r a v e l t i m e s - I . Deep s e i s m i c s o u n d i n g d a t a , G e o p h y s . J . 3_6, 3 7 7 - 3 9 8 . B i r c h , F . ( 1 9 6 4 ) . D e n s i t y and composition c o r e , J . Geophys. Res. 69, 4377-4387. Braile, of  L . W . and R . B . S m i t h ( 1 9 7 5 ) . crustal refraction profiles,  of  the  mantle  and  Guide to the i n t e r p r e t a t i o n Geophys. J . £ 0 , 145-176.  Cassell, B.R. (1982). A method for calculating synthetic seismograms in laterally v a r y i n g media, Geophys. J . 69, 339-354. C e r v e n y , V . and R. R a v i n d r a ( 1 9 7 1 ) . T h e o r y o f S e i s m i c Head W a v e s , U n i v e r s i t y of T o r o n t o P r e s s , T o r o n t o 312 p p . C e r v e n y , V . J . L a n g e r and I. Psencik (1974). Computation of geometrical spreading of s e i s m i c body waves i n l a t e r a l l y inhomogeneous m e d i a w i t h c u r v e d i n t e r f a c e s , G e o p h y s . J . 3 8 , 9-19.  160  C e r v e n y , V., I . M o l o t k o v , and I . P s e n c i k ( 1 9 7 7 ) . Ray Method S e i s m o l o g y , C h a r l e s U n i v e r s i t y P r e s s , P r a g u e , 214 pp.  in  Cerveny, V., M.M. Popov, and I . P s e n c i k ( 1 9 8 2 ) . C o m p u t a t i o n of s e i s m i c wave f i e l d s i n inhomogeneous media - G a u s s i a n beam a p p r o a c h , G e o p h y s . J . 7_0, 109-128. Chapman CH. and R. Drummond ( 1 9 8 2 ) . Body-wave s e i s m o g r a m s i n inhomogeneous media u s i n g M a s l o v a s y m p t o t i c theory, Bull. S e i s m . S o c . Am. 7_2, 5277-5317. Chapman, CH. (1978). A new method f o r computing s e i s m o g r a m s , Geophys. J . 54, 481-518.  synthetic  Chou, CW. and J.R. Booker ( 1 9 7 9 ) . A B a c k u s - G i l b e r t a p p r o a c h to i n v e r s i o n of t r a v e l t i m e d a t a f o r t h r e e - d i m e n s i o n a l v e l o c i t y s t r u c t u r e , G e o p h y s . J . 5_9, 325-344. Clayton, R.W. and R.P. Comer ( 1 9 8 3 ) . A t o m o g r a p h i c m a n t l e h e t e r o g e n e i t i e s from body wave travel T r a n s . AGU 64 , 776.  a n a l y s i s of times, Eos  C l o w e s , R.M. and S . J . M a l e c e k ( 1 9 7 6 ) . P r e l i m i n a r y i n t e r p r e t a t i o n of a deep marine seismic survey o f f t h e west c o a s t of Canada, Can. J . E a r t h S c i . j_6, 1265-1 280. C l o w e s , R.M., A.J. Thorleifson, and S. Lynch (1981). Winona Basin, west coast Canada: c r u s t a l s t r u c t u r e from m a r i n e s e i s m i c s t u d i e s , J . G e o p h y s . Res. 86, 225-242. Clowes, R.M., R.M. Ellis, Z. H a j n a l and I . F . J o n e s ( 1 9 8 3 a ) . S e i s m i c r e f l e c t i o n s from s u b d u c t i n g l i t h o s p h e r e ? N a t u r e 303, 668-670. C l o w e s , R.M., CD. Spence, D.A. W a l d r o n and R.M. E l l i s VISP II: Lithospheric structure across the F u c a / A m e r i c a p l a t e m a r g i n , CGU Annual Meeting, B.C., Program w i t h A b s t r a c t s 8, 13. Coney, P . J . , D.L. J o n e s and J.W.H. Monger (1980). s u s p e c t t e r r a n e s , N a t u r e 288, 239-333. Crosson, R.S. ( 1 9 7 6 ) . C r u s t a l data, 1. Simultaneous hypocentre and velocity 3036-3046.  ( 1983b). Juan de Victoria,  Cordilleran  s t r u c t u r e m o d e l l i n g of earthquake least squares estimation of p a r a m e t e r s , J . G e o p h y s . R e s . 81,  C r o s s o n , R.S. ( 1 9 8 1 ) . Review of s e i s m i c i t y i n t h e Puget Sound region from 1970 through 1978: a brief summary, in ' E a r t h q u a k e H a z a r d s o f t h e Puget Sound Region, Washington State', J.C Y o u n t , e d i t o r . Open F i l e R e p o r t , M e n l o P a r k , California. D e l a n d r o , W. and W. Moon (1982). Superior-Churchill Precambrian Res. 87, 6884-6888.  Seismic structure of the boundary zone, J . Geophys.  161  D i c k i n s o n , W.R. (1976). Sedimentary basins developed d u r i n g the evolution of M e s o z o i c - C e n o z i c a r c - t r e n c h systems i n North A m e r i c a , C a n . J . E a r t h S c i . j_3, 1 2 6 8 - 1 2 8 7 . D z i e w o n s k i , A . M . , B . H . Hager and R . J . O'Connell (1977). Large s c a l e h e t e r o g e n e i t i e s i n the lower mantle, J . Geophys. Res. 82, 239-255. Ellis, R . M . and R . M . C l o w e s (1981). A c q u i s i t i o n of c r u s t a l r e f l e c t i o n / r e f r a c t i o n data a c r o s s Vancouver Island, Earth P h y s i c s B r a n c h Open F i l e R e p t . 8 1 - 1 1 , 7 2 p p . E l l i s , R . M . , G . D . Spence, R . M . Clowes, D . A . Waldron, I . F . Jones, A.G. Green, D.A. Forsyth, J.A. Mair, M.J. Berry, R.F. Mereu, E . R . Kanasewich, G.L. Cumming, Z. Hajnal, R.D. Hyndman, G . A . McMechan, and B . D . L o n c a r e v i c ( 1 9 8 3 ) . The Vancouver Island Seismic Project: a CO-CRUST o n s h o r e o f f s h o r e s t u d y o f a c o n v e r g e n t m a r g i n , Can J . E a r t h S c i . 2 0 , 71 9-741 . Fuchs, K. (1977). Structure, p h y s i c a l p r o p e r t i e s and l a t e r a l h e t e r o g e n e i t i e s of the s u b c r u s t a l lithosphere from longrange deep s e i s m i c sounding o b s e r v a t i o n s on c o n t i n e n t s , T e c t o n o p h y s i c s 5_6, 1-15. Fuchs, K. and G. M u l l e r (1971). Computation of seismograms with the r e f l e c t i v i t y method a n d w i t h o b s e r v a t i o n s , Geophys. J . 23, 417-433. Fukao, Y., S. constraint subducting  synthetic comparison  Hori a n d M . Ukawa (1983). A seismological on t h e d e p t h of b a s a l t - e c l o g i t e t r a n s i t i o n i n oceanic c r u s t , N a t u r e , 303, 413-415.  F u l t o n , R . J . and R . I . W a l c o t t ( 1 9 7 5 ) . Lithospheric shown by d e f o r m a t i o n o f g l a c i a l l a k e s h o r e l i n e s B r i t i s h C o l u m b i a , Mem. G e o l . S o c . Am. 142. Garmany, J., J.A. Orcutt and R . L . Parker (1979). i n v e r s i o n : a geometrical approach, J. Geophys. 3615-3622.  flexure as i n southern Traveltime Res. 84,  Gebrande, H. (1976). A seismic ray-tracing method f o r t w o d i m e n s i o n a l inhomogeneous m e d i a , i n E x p l o s i o n S e i s m o l o g y i n C e n t r a l E u r o p e ; D a t a and R e s u l t s , P . G i e s e , C . P r o d e h l , and A. S t e i n , e d s . , S p r i n g e r - V e r l a g , B e r l i n , 162-167. Green, A . G . , P. M o r e l - a - l ' H u i s s i e r , and C. Pike (1983). Interpretation of COCRUST s e i s m i c r e f r a c t i o n d a t a a c r o s s the S u p e r i o r - C h u r c h i l l boundary zone and the Williston Basin, CGU A n n u a l Meeting, V i c t o r i a , B . C . , Program w i t h A b s t r a c t s 8, 28. Grow, J . A . and C O . Bowin (1975). Evidence for high-density c r u s t a n d m a n t l e b e n e a t h t h e C h i l e t r e n c h due t o d e s c e n d i n g l i t h o s p h e r e , J . Geophys. Res. 80, 1449-1458. Haddon,  R.A.W.  and  P.W.  Buchen  (1981).  Use  of  Kirchhoff's  1 62  f o r m u l a f o r body-wave J . 67, 587-598.  calculations  Hawley,B.W., G. Zandt and i n v e r s i o n for hypocenter an iterative solution Res. 86, 7073-7086.  i n the  Earth,  R.B. Smith (1981). Simultaneous and l a t e r a l velocity variations: with a l a y e r e d model, J . Geophys.  Helmberger, D.V. (1968). The c r u s t - m a n t l e transition B e r i n g S e a , B u l l . S e i s m . S o c . Am. 5 8 , 1 7 9 - 2 1 4 . Horn,  Geophys.  in  the  J.R., R . M . C l o w e s , R . M . E l l i s and D . N . B i r d ( 1 9 8 3 ) . The seismic structure across an active oceanic/continental t r a n s f o r m f a u l t z o n e , J . G e o p h y s . R e s . IB9, 3 1 0 5 - 3 1 2 0 .  Hyndman, R . D . a n d D . H . W e i c h e r t ( 1 9 8 3 ) . S e i s m i c i t y and r a t e s o f r e l a t i v e m o t i o n on t h e p l a t e b o u n d a r i e s of Western North A m e r i c a , G e o p h y s . J . 7_2, 5 9 - 8 2 . Hyndman, R . D . , R . P . R i d d i h o u g h a n d R. H e r z e r ( 1 9 7 9 ) . The N o o t k a F a u l t Zone - a new p l a t e boundary off western Canada. Geophys. J . 58, 667-683. Jacob, K . H . (1970). T h r e e - d i m e n s i o n a l seismic ray t r a c i n g l a t e r a l l y heterogeneous s p h e r i c a l e a r t h , J . Geophys. 75, 6675-6689.  in a Res.  J o h n s o n , L . E . a n d F . G i l b e r t ( 1 9 7 2 ) . I n v e r s i o n and i n f e r e n c e f o r teleseismic r a y d a t a , i n M e t h o d s of C o m p u t a t i o n a l P h y s i c s , V2, 2 3 1 - 2 6 6 . J o n e s , D . L . , A . C o x , P . Coney a n d M . Beck ( 1 9 8 2 ) . The g r o w t h w e s t e r n N o r t h A m e r i c a , S c i . Am. 2 4 7 , #5, 7 0 - 8 4 .  of  J o n e s , D . L . , N . J . S i l b e r l i n g and J . H i l l h o u s e ( 1 9 7 7 ) . W r a n g e l l i a a d i s p l a c e d terrane i n northwestern North America, Can. J . E a r t h S c i . j_4, 2 5 6 5 - 2 5 7 7 . J u l i a n , B . R . and D. G u b b i n s ( 1 9 7 7 ) . Three r a y t r a c i n g , J . G e o p h y s . 4_3, 9 5 - 1 1 3 .  dimensional  seismic  Keen,  C.E. and D.L. Barrett a n i s t r o p y i n the n o r t h e a s t 1056-1064.  ( 1 9 7 1 ) . A measurement of s e i s m i c P a c i f i c , Can. J . Earth Sci. 8,  Keen,  C.E. and R . D . Hyndman ( 1 9 7 9 ) . G e o p h y s i c a l r e v i e w o f c o n t i n e n t a l m a r g i n s o f e a s t e r n and w e s t e r n C a n a d a , C a n . E a r t h S c i . j_6, 71 2-747 .  the J.  Langston, C.A. (1981). Evidence f o r the s u b d u c t i n g l i t h o s p h e r e under s o u t h e r n Vancouver I s l a n d and w e s t e r n Oregon from teleseismic P wave c o n v e r s i o n s , J . G e o p h y s . R e s . 8 6 , 3 8 5 7 3866. L e v e n b e r g , K . ( 1 9 4 4 ) . A method n o n l i n e a r problems i n l e a s t 168.  for the solution of s q u a r e s , Q. A p p l . M a t h .  certain 2 , 164-  163  Marks, L.W. (1980). Computational topics i n ray seismology, P h . D . t h e s i s , U n i v e r s i t y o f A l b e r t a , E d m o n t o n , 170pp. McMechan,, G . A . and G . D . Spence ( 1 9 8 3 ) . P-wave v e l o c i t y of t h e E a r t h ' s c r u s t beneath Vancouver I s l a n d , E a r t h S c i . 20, 742-752.  structure Can. J.  McMechan, G . A . and W . D . Mooney ( 1 9 8 0 ) . A s y m p t o t i c r a y t h e o r y and synthetic seismograms for l a t e r a l l y varying structures: t h e o r y and a p p l i c a t i o n t o the I m p e r i a l V a l l e y , California, B u l l . S e i s m . S o c . Am. 70,, 2 0 2 1 - 2 0 3 5 . Mitchell, H . G . and M . S . Garson ( 1 9 8 1 ) . M i n e r a l D e p o s i t s and G l o b a l T e c t o n i c S e t t i n g s , A c a d e m i c P r e s s , New Y o r k , 4 0 5 p p . M o n g e r , J . W . H . and E . I r v i n g ( 1 9 8 0 ) . N o r t h w a r d displacement n o r t h - c e n t r a l B r i t i s h Columbia, Nature 285, 289-294.  of  Monger, J.W.H., R.M. Clowes, R.A. P r i c e , R . P . Riddihough, P. Simony a n d G . J . W o o d s w o r t h ( 1 9 8 4 ) . C o n t i n e n t - o c e a n T r a n s e c t B 2 : J u a n de F u c a p l a t e t o A l b e r t a p l a i n s , Geol. S o c . Am. (in preparation). Muller, J . E . (1977). E v o l u t i o n of the P a c i f i c m a r g i n , Vancouver I s l a n d and a d j a c e n t r e g i o n s , C a n . J . E a r t h S c i . j_4, 20622085. O'Brien, P.N.S. (i960). J . 3, 29-44.  Seismic energy  from e x p l o s i o n s ,  Geophys.  Pavlis, G . L . and J . R . Booker (1980). The m i x e d discretecontinuous i n v e r s e problem: A p p l i c a t i o n to the simultaneous determination of earthquake hypocenters and velocity s t r u c t u r e s , J . Geophys. Res. 85, 4801-4810. Riddihough, R.P. ( 1 9 7 7 ) . A model f o r r e c e n t p l a t e i n t e r a c t i o n s o f f C a n a d a ' s w e s t c o a s t , C a n . J . E a r t h S c i . J_4, 3 8 4 - 3 9 6 . Riddihough, R . P . (1979). G r a v i t y and structure of m a r g i n - B r i t i s h C o l u m b i a and W a s h i n g t o n , C a n . J . j_6, 3 5 0 - 3 6 3 . Riddihough, R.P. p l a t e system: 1 035.  an active Earth S c i .  (1981). A b s o l u t e m o t i o n s o f t h e J u a n de F u c a r e s i s t a n c e t o s u b d u c t i o n ? E o s T r a n s . AGU 6 2 ,  Riddihough, R.P. (1982a). One h u n d r e d m i l l i o n y e a r s t e c t o n i c s i n w e s t e r n Canada, G e o s c i e n c e Canada, 9,  of p l a t e 28-34.  R i d d i h o u g h , R . P . ( 1 9 8 2 b ) . C o n t e m p o r a r y movements and tectonics on C a n a d a ' s west c o a s t : a d i s c u s s i o n , T e c t o n o p h y s i c s , 8 6 , 319-341. Riddihough, R . P . a n d R . D . Hyndman (1976). Canada's western margin -the case for s u b d u c t i o n , Geoscience 3, 269-278.  active Canada,  164  Rogers, G . C . ( 1 9 8 3 ) . S e i s m o t e c t o n i c s of B r i t i s h C o l u m b i a , P h . D . t h e s i s , 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 , 247 p p . Rona,  P . ( 1 9 8 0 ) . G l o b a l p l a t e m o t i o n and m i n e r a l resources, in The Continental Crust and Its Mineral Deposits, D.W. Strangway ( e d i t o r ) , G e o l . A s s o c . Can. Spec. Paper 20, pp. 607-622.  Savage, J.C, M . L i s o w s k i and W . H . P r e s c o t t ( 1 9 8 1 ) . G e o d e t i c s t r a i n measurements i n W a s h i n g t o n , J. Geophys. Res. 86, 4929,4949. Scott, J.H. (1973). Seismic G e o p h y s i c s , 38, 271-284.  r e f r a c t i o n m o d e l l i n g by  computer,  Shouldice, D . H . (1971). Geology of the western Canadian c o n t i n e n t a l s h e l f , B u l l . C a n . P e t r . G e o l . J_9, 4 0 5 - 4 3 6 . Snaveley, P.D.Jr. and H . C . Wagner (1981). Geological cross s e c t i o n a c r o s s the c o n t i n e n t a l margin off Cape Flattery, Washington, and V a n c o u v e r I s l a n d , B r i t i s h C o l u m b i a , USGS O p e n - F i l e Report 81-978, 5pp. S o r r e l l s , G . G . , J . B . C r o w l e y and K . F . V e i t h ( 1 9 7 1 ) . Methods for computing ray p a t h s i n complex g e o l o g i c a l s t r u c t u r e s , Bull. S e i s m . S o c . Am. 6J_, 2 7 - 5 3 . Spence, G.D., K.P. Whittall synthetic seismograms calculated by a s y m p t o t i c (accepted).  and R . M . Clowes ( 1 9 8 4 ) . P r a c t i c a l for laterally varying media r a y t h e o r y , B u l l . S e i s m . S o c . Am.  S p e n c e , W. ( 1 9 7 7 ) . The A l e u t i a n a r c : subduction, strain diffusion Geophys. Res. 82, 213-230.  tectonic blocks, episodic a n d magma g e n e r a t i o n , J.  S p e n c e r , C. and D. Gubbins (1980). Travel-time inversion for simultaneous earthquake location and velocity structure d e t e r m i n a t i o n i n l a t e r a l l y v a r y i n g media, Geophys. J. 63, 95-116. Steinmetz, L., R.B. W h i t m a r s h and V . S . M o r e i r a ( 1 9 7 7 ) . U p p e r mantle s t r u c t u r e beneath the M i d - A t l a n t i c Ridge, north of the Azores based on o b s e r v a t i o n s o f c o m p r e s s i o n a l w a v e s , Geophys. J . 50, 353-380. T a b e r , J . J . J r . ( 1 9 8 3 ) . C r u s t a l s t r u c t u r e and s e i s m i c i t y of the Washington c o n t i n e n t a l m a r g i n , P h . D . t h e s i s , U n i v e r s i t y of W a s h i n g t o n , 159pp. T a l w a n i , M . , L . W o r z e l and M . Landisman (1959). Rapid gravity calculations for 2-dimensional bodies w i t h a p p l i c a t i o n to the Mendocino submarine f r a c t u r e z o n e , J . Geophys. R e s . 64, 49-59. T s e n g , K . H . ( 1 9 6 8 ) . A new m o d e l f o r t h e c r u s t i n t h e v i c i n i t y of Vancouver Island, M.Sc. thesis, U n i v e r s i t y of British  165  Columbia,  83pp.  W a l d r o n , D . A . ( 1 9 8 2 ) . S t r u c t u r a l c h a r a c t e r i s t i c s of a s u b d u c t i n g oceanic plate, M.Sc. thesis, U n i v e r s i t y of British C o l u m b i a , 121pp. W e s s o n , R . L . ( 1 9 7 0 ) . A t i m e i n t e g r a t i o n method for computation of the i n t e n s i t i e s o f s e i s m i c r a y s , B u l l . S e i s m . S o c . Am. 60, 307-316. Wesson, R.L. (1971). Travel-time inversion for inhomogeneous crustal v e l o c i t y models, B u l l . Am. 6j_, 7 2 9 - 7 4 6 .  laterally Seism. Soc.  W h i t e , D . J . and R . M . Clowes ( 1 9 8 4 ) . S e i s m i c i n v e s t i g a t i o n of the Coast P l u t o n i c Complex-Insular Belt boundary beneath Georgia S t r a i t , Can. J . E a r t h S c i . (accepted). White, W.R.H., M . N . Bone and W . G . M i l n e (1968). Seismic refraction surveys in British Columbia: a preliminary i n t e r p r e t a t i o n , AGU M o n o g r a p h J_2, 8 1 - 9 3 . Whittall, K . P . and R . M . Clowes (1979). A simple, e f f i c i e n t method f o r t h e c a l c u l a t i o n o f t r a v e l t i m e s and raypaths in laterally inhomogeneous m e d i a , J . C a n . S o c . E x p l . G e o p h y s . 15, 2 1 - 2 9 . Wickens, A . J . (1977). Columbia, Can. J .  The u p p e r mantle of southern E a r t h S c i . J_4, 1 1 0 0 - 1 1 1 5 .  British  Wiggins, R.A. (1972). The general linear inverse problem: I m p l i c a t i o n of s u r f a c e waves and free oscillations for e a r t h s t r u c t u r e , R e v . G e o p h y s . S p a c e P h y s . J_0, 2 5 1 - 2 8 5 . Wiggins, R . A . and D . V . Helmberger ( 1 9 7 4 ) . S y n t h e t i c seismogram c o m p u t a t i o n by e x p a n s i o n i n g e n e r a l r a y s , G e o p h y s . J. 37, 73-90. Yole,  R.W. and E. Irving (1980). D i s p l a c e m e n t of Island: paleomagnetic evidence from the F o r m a t i o n , C a n . J . E a r t h S c i . JJ7, 1210-1228.  Vancouver Karmutsen  Yorath, C.J. (1980). The A p o l l o structure in Tofino Basin, Canadian P a c i f i c c o n t i n e n t a l s h e l f , Can. J . Earth S c i . 17, 758-775. Y o r a t h , C . J . a n d R . G . C u r r i e ( 1 9 8 0 ) . Some a s p e c t s o f t h e g e o l o g y and structural style of the Vancouver I s l a n d c o n t i n e n t a l m a r g i n , G e o l o g i c a l A s s o c i a t i o n of Canada, Annual Meeting, H a l i f a x , N . S . , Program w i t h A b s t r a c t s 5, p . 8 8 . Y o s h i i , T . , Y . Kono a n d K . I t o ( 1 9 7 6 ) . T h i c k e n i n g o f l i t h o s p h e r e , AGU M o n o g r a p h J_9, 4 2 3 - 4 3 0 .  the  oceanic  Young, G.B. and L . W . B r a i l e ( 1 9 7 6 ) . A computer program f o r the a p p l i c a t i o n of Z o e p p r i t z ' s a m p l i t u d e e q u a t i o n s and Knott's e n e r g y e q u a t i o n s , B u l l . S e i s m . S o c . Am. 6 6 , 1 8 8 1 - 1 8 8 5 .  166  APPENDICES: ADDITIONAL RECORD SECTIONS  A . 1 Common S h o t R e c o r d S e c t i o n s  The sections other  following  sections  figures  f o r a l l s h o t s of  than  sections  14  shots  J1,  were  Project  P8 a n d P 2 , f o r w h i c h r e c o r d P i c k s on t h e  main  record  VISP data  set  on  t h e main s e c t i o n s  are  representative  a l l s e c t i o n s may be c o m p a r e d b e t w e e n  Times and d i s t a n c e s  a d e p t h of 2.6  km, and  thickness  1  of  km  m i d d l e of a l l s e c t i o n s  in of  shown h e r e . shots.  a m p l i t u d e s have been m u l t i p l i e d by a f a c t o r p r o p o r t i o n a l  distance.  (at  Seismic  record  made i n a manner c o n s i s t e n t w i t h t h e s e c t i o n s  Amplitudes All  P13,  i n Chapter 4.  t h i s a p p e n d i x . That i s , the f u l l  the observed  the Vancouver I s l a n d P19,  were p r e s e n t e d  represent  to  are a d j u s t e d  correct  and v e l o c i t y  the of  1.8  t o p l a c e the shot  sediment  P19).  to  k m / s . The gap n e a r  i n d i c a t e s the l o c a t i o n  2 6 5 - 2 8 5 km f r o m s h o t  layer  of G e o r g i a  to at a the  Strait  170  130  90 50 SH OT/RECEIVER DISTANCE (KM)  FIG. A l . l .  Observed r e c o r d s e c t i o n  for  shot J2.  130 170 210 SHOT/RECEIVER DISTANCE (KM) FIG. A1.2.  Observed r e c o r d s e c t i o n  for  shot  P1,  104 km f r o m s h o t  250 P19.  co  100  140 180 220 SHOT/RECEIVER DISTANCE (KM) FIG. A 1 . 3 .  Observed r e c o r d s e c t i o n  for  shot  P 3 , 94 km f r o m s h o t  260 P19.  105  145 185 225 SHOT/RECEIVER DISTANCE (KM) FIG. A1.4.  Observed r e c o r d s e c t i o n  for  shot  P 4 , 89 km f r o m s h o t  265 P19.  110  150 190 230 SHOT/RECEIVER DISTANCE (KM) FIG.  A1.5.  Observed r e c o r d s e c t i o n  for  shot  P 5 , 83 km f r o m s h o t  270 P19.  115  155 195 235 SHOT/RECEIVER DISTANCE (KM) FIG.  A1.6.  Observed r e c o r d s e c t i o n  f o r shot  P 6 , 78 km f r o m s h o t  275 P19.  ^  '  •  '  130  ——I  • — ' —  1  — ' — f '  ' • •' '  '  1  170 210 250 SHOT/RECEIVER DISTANCE (KM) FIG.  A1.7.  Observed r e c o r d s e c t i o n  for  shot  P9,  63 km from s h o t  r  f  290  P19.  OJ  135  FIG.  A1.8.  175 215 255 SHOT/RECEIVER DISTANCE (KM)  295  Observed r e c o r d s e c t i o n f o r shot P10,  P19.  57 km from s h o t  ^n  r  145  " •—"  —j  • - » — — — — f - * — i » i i i—« i * j— 1  c  1  185 225 265 SHOT/RECEIVER DISTANCE (KM) FIG.  A1.9.  305  O b s e r v e d r e c o r d s e c t i o n f o r s h o t P 1 2 , 47 km from s h o t P 1 9 .  155  195 235 275 SHOT/RECEIVER DISTANCE (KM) FIG. A L I O .  O b s e r v e d r e c o r d s e c t i o n f o r s h o t P 1 4 , 36 km f r o m  shot  200 240 280 SHOT/RECEIVER DISTANCE (KM) Observed r e c o r d s e c t i o n f o r  shot  P 1 5 , 31 km from  165  205 245 285 SHOT/RECEIVER DISTANCE (KM) FIG.  Al.l2.  Observed r e c o r d s e c t i o n for  s h o t P 1 6 , 26 km f r o m s  175  215 255 295 SHOT/RECEIVER DISTANCE (KM) FIG. A1.13.  335  O b s e r v e d r e c o r d s e c t i o n f o r s h o t P 1 7 , 21 km f r o m s h o t P 1 9 .  VD  185  225  265  SHOT/RECEIVER  FIG. A1.14.  305 DISTANCE  Observed r e c o r d s e c t i o n f o r shot P18,  345 (KM)  10 km f r o m s h o t P 1 9 . CO  o  181  A.2 Selected  The  Common R e c e i v e r R e c o r d S e c t i o n s  1 0 figures  following  showing  receiver.  P 1 9 i s on t h e l e f t  (P7 a n d P11 were m i s f i r e s  distances for  by are  a  factor  adjusted  to place  1.8  and  recorded  missing). all  on  to  observed a  on t h e  given right  A m p l i t u d e s may be  amplitudes  the shots at  the sediment  km/s.  samples of  a n d s h o t P1 i s  proportional  shots P19-P8 t o c o r r e c t  1 km a n d v e l o c i t y o f  shots  a n d so a r e  c o m p a r e d beween a l l r e c e i v e r s , multiplied  17  selected  record sections Shot  all  are  have  distance. 2.6  km  been  T i m e s and  depth,  layer to a thickness  and of  SHOT/RECEIVER DISTANCE (KM) FIG. A2.4.  Observed  record section  for  receiver  X17. CD Ol  SHOT/RECEIVER DISTANCE (KM) FIG. A2.6.  Observed  record section  for  receiver  X23.  -  CO  SHOT/RECEIVER DISTANCE (KM) FIG. A 2 . 7 .  Observed  record section  for  receiver  X31 .  _ CO CO  SHOT/RECEIVER DISTANCE (KM) FIG.  A2.8.  Observed  record section  for  receiver  X35.  SHOT/RECEIVER DISTANCE (KM) FIG.  A2.10.  Observed  record section  for  receiver  X43.  PUBLICATIONS E l l i s , R.M. G.D. Spence, R.M. Clowes, O.A. Waldron, I.F. Jones, A.G. Green, D.A. Forsyth, J . A . Mair, M.J. Berry, R.F. Mereu, E.R. Kanasewich, G.L. Cumming, Z. Hajnal, R.D. Hyndman, G.A. McMechan, and B.D. Loncarevic (1983). The Vancouver Island Seismic Project: A CO-CRUST onshore-offshore study of a convergent margin, Can. J . Earth S c i . 20, 719-741. McMechan, G.A. and G.D. Spence (1983). P-wave velocity structure of the Earth's crust beneath Vancouver Island, Can. J . Earth S c i . 20, 742-752. Spence, G.D., R.M. Clowes and R.M. E l l i s (1977). Depth l i m i t s on the Moho discontinuity in the southern Rocky Mountain Trench, Canada, B u l l . Seism. Soc. Am. 67, 543-546. Spence, G.D., R.M. E l l i s and R.M. Clowes (1977). Gravity evidence against a high-angle f a u l t crossing the Rocky Mountain Trench near Radium, B r i t i s h Columbia, Can. J . Earth S c i . IA, 25-31. Spence, G.D., K.P. Whittall and R.M. Clowes (1984). Practical synthetic seismograms for l a t e r a l l y varying media calculated by asymptotic ray theory, B u l l . Seism. Soc. Am. (in press).  

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