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A refraction survey across the Canadian cordillera 1973

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A REFRACTION SURVEY ACROSS THE CANADIAN CORDILLERA BY DAVID A.G. FORSYTH B.Sc. Queen's U n i v e r s i t y at Kingston, 1968 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of GEOPHYSICS AND ASTRONOMY We accept t h i s t h e s i s as conforming to the required standard. THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1973 "In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representative. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission". • Department of Geophysics and Astronomy The U n i v e r s i t y of B r i t i s h Columbia Date June 1973 i i i ABSTRACT Record sections from p a r t i a l l y reversed r e f r a c t i o n l i n e s i n northern B r i t i s h Columbia show that the amplitudes of upper mantle a r r i v a l s vary smoothly with distance. The pattern of c r u s t a l a r r i v a l amplitudes i s not smooth. Normalization of the seismograms to remove the a m p l i f i c a t i o n caused by shot s i z e and instrument response show the e f f e c t s of recording s i t e s on P amplitudes are minimal. n Models derived from ray theory i n d i c a t e a crust which thins from about 40 km i n the Omineca C r y s t a l l i n e Belt to about 25 km i n the Insular Trough. The average P^ v e l o c i t y i s 8.06 km/s. The average c r u s t a l v e l o c i t y i s 6.4 km/s. The secondary energy would i n d i c a t e the models are g r e a t l y s i m p l i f i e d . A time-term p r o f i l e between the Omineca C r y s t a l l i n e Belt and the Coast Mountains suggests a Mohorovicic t r a n s i t i o n which i s characterized by two s i g n i f i c a n t topographic wavelengths. The shorter (200 km) wavelength c o r r e l a t e s roughly with the C o r d i l l e r a n s t r u c t u r a l elements of Wheeler et a l . (1972). The l a r g e r (800 km) wavelength may have t e c t o n i c s i g n i f i c a n c e . i i i TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES ACKNOWLEDGEMENTS CHAPTER 1 INTRODUCTION 1-1 Background 1-2 Previous work i n the area 1- 3 Present study CHAPTER 2 SYSTEMS 2- 1 Components and configurations 2- 2 C a l i b r a t i o n CHAPTER 3 RECORD SECTIONS 3- 1 Systems responses and shot c o r r e c t i o n 3-2 Formation of record sections 3- 3 Normalization CHAPTER 4 INTERPRETATION 4- 1 General p r i n c i p l e s and method 4-2 Least square a n a l y s i s 4-3 Travel-time curves 4-4 F i t t i n g travel-time curves 4-5 Record section from Greenbush 4-6 Record section from Bi r d Lake 4-7 The anomaly at 820 km distance 4-8 Time-term study 4-9 Discussion of time-term study 4-10 Limitations Page v v i v i i 1 1 1 2 5 5 7 9 fac t o r s 9 10 10 18 18 19 20 22 29 33 34 39 41 42 CHAPTER 5 SUMMARY 5-1 Travel-time models J 5-2 Time-term models 5-3 Conclusions , APPENDIX A REFERENCES V LIST OF TABLES Page TABLE 1 BC 69 Shot data 12 l a BC 70 Shot data 13 v i LIST OF FIGURES FIGURE Page 1 The l o c a t i o n map 4 2 The chart of the f i e l d recording systems 6 3 The response curves 8 4 The system f a c t o r s 11 5 The record section from Greenbush normalized with respect to distance 15 5a The record section from Greenbush normalized with respect to distance squared 16 6 The record section from Bird Lake normalized with respect to distance squared 17 7 The travel-time model (model 1) f o r the Greenbush section 23 7a The velocity-depth structure f o r model 1 24 8 The travel-time model (model 2) f o r the Greenbush section 25 8a The velocity-depth structure f o r model 2 26 9 The travel-time model f o r the Bi r d Lake section 27 9a The velocity-depth structure f o r the B i r d Lake model 28 10 The f i t of model 2 to the data of F i g . 5a 32 11 The f i t of the Bi r d Lake model to the data 35 12 Part of the Greenbush section expanded 37 13 The p l o t of the time-terms converted to depth 43 14 The c o r r e l a t i o n of the time-term surface with geology 47 ACKNOWLEDGEMENTS With sincere appreciation, the writer wishes to extend thanks to Dr. M.J. Berry for his patient advice and many helpful comments during both the field work and the interpretation at the Seismology Division, Earth Physics Branch, and to Dr. R.M. Ell i s for supervision during the writer's residence at the University of British Columbia and critical reading of the manuscript. In addition, the author is indebted to Mr. M.N. Bone of the Seismology Division and Mr. R.D. Meldrum of the Department of Geophysics, University of British Columbia, for assistance in understanding the re- cording systems and to Dr. R. Stacey of the Earth Physics Branch for dis- cussion of the final results. Data for this project was collected by teams from the Earth Physics Branch and the University of British Columbia and analysed in Ottawa. Basic computer programs for data handling and interpretation pre-existed at the Seismology Division and were modified by the author to cope with the present study. Mrs. Lois Turgeon who typed the thesis was of great help during the final preparation of the work. This research has been sponsored by the Seismology Division, Earth Physics Branch, Department of Energy, Mines and Resources, the University of British Columbia, and the National Research Council (Grant A-2617). 1 CHAPTER 1 INTRODUCTION 1-L Background With increasing interest in plate tectonics, the emphasis on large scale refraction studies has declined in favour of surface-wave and array analyses to study the lithosphere-asthenosphere relationship. However, refraction and reflection lines used to determine the velocity- depth structure and i t s l a t e r a l behaviour may yet provide a means of relating surface features to lithospheric plates in some detail. In particular, the study of this signature in the neighbourhood of plate boundaries may prove most interesting. The geology of the Canadian Cordilleran region, both onshore and offshore, would strongly favour i t s candidacy as a region where plates are and have been active (Souther, 1972; Monger, 1972; Berry et a l . , 1971; Johnson and Couch, 1970; Stacey, 1972). When subdivided into geological provinces, the region includes the Rocky Mountain Trench, the zone con- taining the Cassiar, Omineca and Columbia Mountains, the Intermontane Belt, the Coast Mountains and the Insular Belt (Monger, 1972). The present refraction study covers the area from Greenbush Lake in the Columbia Mountains to Bird Lake on Graham Island in the Insular Belt (Fig. 1). 1-2. Previous work in the area From a seismic viewpoint, the picture of the Canadian Cordillera has been incomplete as a result of limited surveys and rather incoherent data. The interpretation i s rendered additionally d i f f i c u l t due to the complexity of at least the upper crust as evidenced by the surface geology. 2 Work adjacent to the area presently under study has been o u t l i n e d by Shor (1962), by White et a l . (1968) and by Jacoby (1970). In the area of the Insular B e l t northwest of Prince Rupert, Shor suggested that the M occurred at a depth of 26 km. In southern B r i t i s h Columbia, studies were concentrated along p r o f i l e s from Puntchesakut Lake and from near B a r k e r v i l l e to Osoyoos and from Greenbush Lake to Hope. Mine b l a s t s at M e r r i t t en- abled p a r t i a l r e v e r s a l of the l i n e from Puntchesakut Lake. The Hope l i n e was unreversed. White et a l . proposed a l t e r n a t i v e i n t e r p r e t a t i o n s f o r t h e i r data. Both models were characterized by minor topography on the M d i s c o n t i n u i t y at a depth of from 28 to 30 kilometers. Their f i r s t model i n d i c a t e d a l a t e r a l change i n upper mantle v e l o c i t y from 8.1 km/s beneath Williams Lake to 7.8 km/s i n the south. The al t e r n a t e model depicted a v e l o c i t y of 8.0 km/s. During 1967, shots detonated at the south end of Vancouver Island were recorded along the l i n e used i n 1966, re-occupying many old s i t e s . This data, plagued with considerable noise, gave an apparent v e l o c i t y of 8.43 km/s, pertinent to the upper mantle east of M e r r i t t . A preliminary time-term a n a l y s i s i n d i c a t e d a southwesterly dipping M d i s c o n t i n u i t y with an upper mantle v e l o c i t y of 8.27 km/s (Jacoby, 1970). 1-3. Present study During the summers of 1969 and 1970, r e f r a c t i o n p r o f i l e s were recorded by Earth Physics Branch and U n i v e r s i t y of B r i t i s h Columbia (UBC) teams across the three i n t e r i o r g e o l o g i c a l provinces. In August 1969, shots detonated at Greenbush Lake were recorded by teams from the Earth 3 Physics Branch along roads running from southwest of Revelstoke to Vancouver Island, from L i t t l e Fort to McLeod Lake, from Nazko to B a r k e r v i l l e and from Williams Lake to B e l l a Coola ( F i g . 1). A monitor s t a t i o n was located at Lumby to record a l l shots. Teams from UBC recorded along roads from Prince George to Prince Rupert. During J u l y of 1970 an attempt was made by the same teams to reverse the Prince Rupert-Prince George l i n e from B i r d Lake and the B e l l a Coola-Williams Lake l i n e from Ripley Bay. The Nazko-Barkerville l i n e was reversed from Ripley Bay and an upper c r u s t a l v e l o c i t y was obtained along t h i s same l i n e from a shot i n Puntchesakut Lake. The present study includes data from L i t t l e Fort to Prince Rupert using the Greenbush shot point and from Prince Rupert to Prince George using the B i r d Lake shot point. F i g . 1 shows the l o c a t i o n of shots and s t a t i o n s . F i g . 1 The location map. Triangles show s i t e s re- corded from Bird Lake; dots show s i t e s recorded from Greenbush Lake. ( C o r d i l l e r a n s t r u c t u r a l elements a f t e r Wheeler, et a l . , 1972) 49° _L_——'48° 5 CHAPTER 2 SYSTEMS 2-1. Components and configurations The f i e l d recording systems are o u t l i n e d i n F i g . 2. The Earth Physics Branch systems had two v a r i a t i o n s w i t h i n the arrangement shown. On one of the systems, Electro-Tech SPA-10 a m p l i f i e r s were used instead of the AS-330 models. The Electro-Tech a m p l i f i e r s were modified to produce the same o v e r a l l gain as the AS-330 models and the d i f f e r e n c e between responses i s n e g l i g i b l e f o r the frequencies encountered i n the present data. The other system employed Electro-Tech EV-17 seismometers with a n a t u r a l period of one second, Texas Instrument a m p l i f i e r s and an Ampex tape recorder. The shape of the v e l o c i t y s e n s i t i v i t y curve f o r t h i s system i s not s i g n i f i c a n t l y d i f f e r e n t from the curve f o r the UBC systems (0.8 Hz-12.5 Hz) shown i n F i g . 3. A l l data presented herein from the 1969 f i e l d t r i p were recorded using a three-component seismometer co n f i g u r a t i o n . In 1970, the UBC teams continued with the three-component arrangement, while the Earth Physics Branch teams used s i x v e r t i c a l seismometers set at a spacing of 500 meters. The array nominally provided the option of stacking records or determining a phase v e l o c i t y to more confidently i d e n t i f y a r r i v a l s . I t was found, however, that the lack of freedom i n choosing l o c a t i o n s f o r the spreads compared to p i c k i n g i s o l a t e d or bedrock seismometer s i t e s f o r the three components r e s u l t e d i n s i g n i f i c a n t l y n o i s i e r seismograms and l i t t l e , i f any, advantage was gained i n discer n i n g P f i r s t a r r i v a l s . UNIVERSITY OF BRITISH COLUMBIA BC 69-70 3 COMPONENT ARRANGEMENT- WILLMORE MK II SEISMOMETERS. NATURAL FREQUENCY SET AT 1 Hz. O O O- GEOTECH AMPLIFIERS MODEL AS-330 -AT 0 db ATTENUATION LOW LEVEL 0/P GIVES 70 db GAIN HIGH LEVEL 0/P GIVES 70 db PLUS SEPARATION (18,24,30 or 30,36,42 db) -3 db POINTS OF FILTER SET AT .01,5Hz or .8,12.5Hz GEOTECH FM TAPE RECORDER -SIX DATA CHANNELS - 1 CHRONOMETER OR RADIO TIME CHANNEL -RECORDS AT I5/I60 IN/SEC -SET UP FOR FULL MODULATION AT t 1 VOLT RMS CHRONOMETER OR WWV/WWVB RECEIVER EARTH PHYSICS BRANCH * BC 69 3 COMPONENT ARRANGEMENT: WILLMORE MK H SEISMOMETERS. NATURAL FREQUENCY SET AT 1 Hz. O O O GEOTECH AMPLIFIERS MODEL AS-330 -AT 0 db ATTENUATION LOW LEVEL 0/P GIVES 92 db GAI HIGH LEVEL 0/P GIVES 92 db PLUS -SEPARATION (18,24,30 db) -3db POINTS OF FILTER SET AT 0.1,17.0 Hz EARTH PHYSICS BRANCH BC 70 SPREAD ARRANGMENT: 6 VERTICAL WILLMORE MK n SEISMOMETERS. SET AT 500 METER SPACING. NATURAL FREQUENCY SET AT 1 Hz o - a o - o - a o GEOTECH AMPLIFIERS MODEL AS-330 -HIGH LEVEL 0/P ONLY USED (92 db GAIN AT 0 db ATTENUATION) -3 db POINTS OF FILTER SET AT 0.1, 17.0Hz PRECISION INSTRUMENT FM TAPE RECORDER -SIX DATA CHANNELS - 1 CHRONOMETER CHANNEL •1 RADIO TIME SIGNAL EDGE TRACK -RECORDS AT I5/I6 IN/SEC -SET UP FOR FULL MODULATION AT t 5 VOLTS CHRONOMETER WWV/WWVB RECEIVER PRECISION INSTRUMENT FM TAPE RECOROER -SIX DATA CHANNELS - 1 CHRONOMETER CHANNEL •1 RADIO TIME SIGNAL EDGE TRACK -RECOROS AT I5/I6 IN/SEC -SET UP FOR FULL MODULATION AT± 5 VOLTS CHRONOMETER WWV/WWVB RECEIVER F i g . 2 The chart of the recording systems used i n the f i e l d . *Variations on t h i s system are described i n the text. 7 2-2. C a l i b r a t i o n The U n i v e r s i t y of B r i t i s h Columbia systems were c a l i b r a t e d before each f i e l d season using a Maxwell bridge i n the manner described by K o l l a r and R u s s e l l (1966). In the f i e l d , the systems were checked before each shot by monitoring the response to an a c c e l e r a t i o n step given to the seismometer mass. K t e s t s (Bancroft and Basham, 1967) were also performed before each shot. Earth Physics Branch systems were checked i n Ottawa before each f i e l d season, while a m p l i f i e r response, K t e s t and system response checks were made before each shot i n the f i e l d . T y p i c a l band-pass response curves f o r the various systems are shown i n F i g . 3. F i g . 3 The response curves of the systems shown in F i g . 2. 9 CHAPTER 3 RECORD SECTIONS 3-1. System response and shot c o r r e c t i o n f a c t o r s As the f i e l d u nits were c o n t i n u a l l y checked and corrected, i t was, deemed worthwhile to consider a l l the system f a c t o r s from seismometer to computer and use these, along with shot f a c t o r s , i n constructing record se c t i o n s . The records from any one shot point might then be expected to show some coherent energy pattern - notwithstanding the e f f e c t s of seismometer s i t e - although record sections would d i f f e r by r e l a t i v e shot point response. The system f a c t o r s are show i n F i g . 4. A s i n g l e number was used to account f o r the seismometer-amplifier response, since the recorded P coda frequencies are n a t u r a l l y r e s t r i c t e d to the passband from 3 to 6 Hz. The response of the systems i s e s s e n t i a l l y f l a t i n t h i s range. No records from the system with a 0.01-5 Hz f i l t e r are included i n the present study. The Greenbush Lake shots were r e l a t i v e l y rated using amplitudes from h e l i c o r d e r records obtained at a s t a t i o n near Lumby. The l a r g e s t amplitude was given the r a t i n g 1.0 and weaker shots were then normalized by a number which would make a l l shots e f f e c t i v e l y the same s i z e . This same form of r a t i n g was used f o r the 1970 B i r d Lake shots using h e l i c o r d e r records from a s t a t i o n at Sandspit. This mode of shot r a t i n g was preferred to that of using nominal charge weight, as i t gives a b e t t e r estimate of true seismic energy y i e l d . The shot factors are shown i n Table 1 and Table l a . 10 3-2. Formation of Record Sections The analog f i e l d data were d i g i t i z e d and arranged on permanent data tapes i n a manner which f a c i l i t a t e d the formation of record sections. - A l l programs, except the d i g i t i z i n g program, were f i r s t w r i t t e n to work on a CDC 3100 computer. The program used to d i g i t i z e the data operates on a DDP 124 computer at the Earth Physics Branch. The programs used to form permanent data tapes were o r i g i n a l l y w r i t t e n by K.G. Barr and M.J. Berry and were modified by the w r i t e r to cope with the present data and operate on a CDC 6400 computer. The program used to f i l t e r the seismograms was w r i t t e n by M.J. Berry. The program which constructs the record sections has evolved at the Earth Physics Branch through the e f f o r t s of M.J. Berry and the w r i t e r . The l i m i t a t i o n s of the playback and d i g i t i z i n g systems enabled a d i g i t i z i n g rate of 50 samples per second f o r tapes recorded at 15/160 inches per second. The tapes recorded at 1-7/8 or 15/16 inches per second were d i g i t i z e d at e i t h e r 100 or 200 samples per second. Records d i g i t i z e d at 200 samples per second were then f i l t e r e d to remove 60 Hz energy (from power l i n e s ) and the resultant time s e r i e s were decimated to 100 samples per second. Data d i g i t i z e d at 50 samples per second were expanded to the 100 sample per second standard upon formation of permanent data tapes. The lowest Nyquist frequency i s thus 25 Hz, s i g n i f i c a n t l y greater than the passband of recorded energy. Record sections were then constructed i n a reduced travel-time form with the system and shot f a c t o r s applied to the i n d i v i d u a l time s e r i e s . For 3-component systems, a v e r t i c a l trace was used. Where s i x v e r t i c a l seismometers were deployed, a "best tra c e " was chosen. 3-3. Normalization To d i s t i n g u i s h i n d i v i d u a l seismograms, i t was necessary to U N I V E R S I T Y OF BRIT ISH COLUM8IA - B C 69 . 70 3 C O M P O N E N T M K II C M " S E C . <fi.HVOLTSV- - \ . C M / S E C J T A S - 3 3 0 EFFECTIVE K LOW LEVEL GAIN-70 db-ATTENUATION = 1.11 V/CM/SEC HIGH LEVEL GAIN = (70 db + SEPARATION -ATTENUATION) ~xf V O L T A G E GAIN ( C O R R E S P O N D I N G ; — I.TO db S E T T I N G MAX. 0/P - ± 2.82 VOLTS < (2.50 V.")— -\ .2 .82 V./* <f2047 UNITS"!- - \ 5 VOLTSj-r DIGITAL . UNITS EARTH PHYSICS BRANCH - B C 6 9 3 COMPONENT -BC 70 6 VERTICAL M K n EFFECTIVE K =I.20V/CM/SEC. EFFECTIVE K = 0.57V/CM/SEC. CM ' SEC. <fl.20(0 57)vV— A CM/SEC. J+ • AS-330 R I. RECORDER MAX. 0/P ± 5 V. V PLAYBACK TAPE DECK MAX. 0/P 2.5 V DIGITIZER SSSA MAX. NUMBER IS ± 2 0 4 7 CORRESPONDING TO ± 5 V. LOW LEVEL GAIN = 92 db-ATTENUATION HIGH LEVEL GAIN= (112 db-ATTENUATION) *xf VOLTAGE GAIN (CORRESPONDING — LTO db SETTING - B C 70 6 VERTICAL MK n TEXAS INSTRUMENT AMPEX AMPLIFIER PRE AMP X<2047 UNITS \ 5 VOLTS, MAX. 0/P = ± 1.35 VOLTS EFFECTIVE K = 1.13 V/CM/SEC. GAIN TO RECORDER (107.5 db-ATTENUATION) CM SEC - X f1.13 VOLTS — \ CM/SEC. i -Xf VOLTAGE GAIN ^ » -X/2.50 V. V - > (CORRESPONDING) ( > :. J-r-« L TO db SETTING J-r-« U - 3 5 V.JT •Xf"2047 UNITS") — — I. 5 VOLTSJ-r- DIGITAL UNITS "DIGITAL UNITS F i g . 4 The system factors f o r conversion of ground v e l o c i t y to d i g i t a l u n i t s and back to ground v e l o c i t y . Note e f f e c t i v e K applies to passband centre only. TABLE 1 BC 69 SHOT DATA Greenbush Lake - Shot Point Coordinates 50° 46.90' N Water Depth 60 m 118° 20.66' W Number Date Time GMT Charge 1 Aug.5, '69 2 6 07:05:00.15 17,100 lbs TNT 3 7 07:05:00.14 17,100 lbs TNT 4 8 07:05:00.18 12,027 lbs MINOL 5 9 07:05:00.23 13,293 lbs MINOL 6 10 07:05:00.23 13,293 lbs MINOL 7 11 07:05:00.18 12,660 lbs MINOL 8 12 07:05:00.20 13,293 lbs MINOL 9 13 10 14 07:05:00.19 12,660 lbs MINOL 11 15 07:05:00.17 13,293 lbs MINOL 12 16 07:05:00.17 13,293 lbs MINOL 13 17 07:05:00.17 13,293 lbs MINOL 14 18 07:05:00.16 13,293 lbs MINOL 15 19 07:05:00.17 13,293 lbs MINOL 16 20 07:05:00.15 13,293 lbs MINOL 17 21 07:05:00.19 13,293 lbs MINOL 18 22 07:05:00.17 12,660 lbs MINOL 19 23 07:05:00.17 13,293 lbs MINOL 20 24 07:05:00.16 i' 17,100 lbs TNT ,Amplitude Shot Type at Lumby Factor Pattern 2 4.75 Point 1 9.5 Pattern 6 1.55 Pattern 1 1 / 2 " 6.3 Pattern 6 1.55 Pattern 2 4.75 Pattern 2 4.75 Pattern 2 1/2 3.8 Pattern - 2 4.75 Pattern 8 1.19 Pattern 9 1/2 1.0 Pattern 8 1/2 1.12 Pattern 8 1/2 1.12 Pattern 9 1.05 Pattern 9 1/2 1.0 Pattern 8 1/2 1.12 Pattern 7 1/2 1.27 Pattern 9 1/2 1.0 TABLE l a BC 70 SHOT DATA B i r d Lake - Shot Point Coordinates 53° 35.83' N Water depth 24 m 132° 23.92' W Amplitude Shot Number Date Time GMT Charge Type at Sandspit Factor 1 July 25, 1970 07:05:00.00 2,000 lbs NITRONE SM Pattern 4.7 3.0 2 26 07:05:00.01 6,000 lbs NITRONE SM Pattern 11.0 1.3 3 27 07:04:59.90 7,000 l b s NITRONE SM Pattern 10.3 1.4 4 28 07:05*00.00 8,000 l b s NITRONE SM Pattern 14,2 1.0 14 normalize trace amplitudes i n the following manner. F i r s t l y , i n order to disce r n f i r s t a r r i v a l s , i t was necessary to eliminate the l a r g e r shear energy. Hence, the sections are terminated at a reduced t r a v e l time of 20 or 25 seconds. Next, the trace containing the l a r g e s t amplitude wavelet - normally a record i n the v i c i n i t y of crossover - was chosen, i t s la r g e s t peak-to-peak amplitude set to 1 inch and the re s t of the traces i n the s e c t i o n were normalized r e l a t i v e to t h i s trace. F i n a l l y , to po s s i b l y d i s t i n g u i s h the f i r s t a r r i v a l beyond crossover as e i t h e r a headwave or a bodywave and to make di s t a n t records more l e g i b l e , an a d d i t i o n a l m u l t i p l i c a t i v e f a c t o r of distance or distance squared was applied. The r e s u l t s as shown i n Fi g s . 5 and 6 i n d i c a t e that by removing most of the shot and system amplitude e f f e c t s , a rather smoothly varying energy pattern may be obtained. Closely spaced, adjacent records r a r e l y d i f f e r by a f a c t o r of two i n amplitude. In the case of traces 40702 and 81802 (F i g . 5) which have been a r b i t r a r i l y reduced, the p o s s i b i l i t y e x i s t s of an i n c o r r e c t l y logged a m p l i f i e r s e t t i n g . Traces 81102 and 81202 are considered to be located on s i t e s s u i t a b l y "tuned" to 3-4 Hz energy. I t i s notable that only 2 out of 51 s i t e s respond i n t h i s manner. 15 •20 T-X/7=0 30 60402 S0SO2 BMOJ 11 SOI _ RV, « . W W , - o T̂ ^̂ VN̂  I -CD CD CD ^3 B3.0J . , . 1000 F i g . 5 The record section recorded at s i t e s from L i t t l e Fort to Prince Rupert from shots i n Greenbush Lake. The seismograms have been m u l t i p l i e d by distance and the following operations performed: --normalized with respect to trace 40402 --trace 40702 reduced by 4 --traces 81102, 81202, 81802 reduced by 2 —bandpass f i l t e r e d from 2 to 9 Hz. _ — t i c s on x-axis are at 40 km. i n t e r v a l s 16 20 T-X/7=0 30 ----- ̂ N A V v ^ W - * " " " - V ^ W l ' v - , , B0702 . ^ ^ / S ^ ^ J ^ ^ ^ ^ v ^ ^ - ^ ^ ^ ^ v ^ ^ |faaeo2 _ w BO90Z . „ _ SIO03 -w-VV en oz yyĵ . 8' 20? VAV^'ITVS^/VN^W^-^VVV'^W^^ o I CD I ^J,"?.! .~» t • 0 c i CD CD 1000 Fig. 5a The record section from Greenbush as shown i n F i g . 5 except that a f a c t o r of distance squared has been applied. Note the rather uniform l e v e l of the amplitude of the f i r s t a r r i v a l out to at least trace 82302. T i c s on x-axis are at 40 km. i n t e r v a l s . • 30 T " X / 7 - 0 2 5 CD C -) —] o ? 0 co-. " 3 - D [\3 '~D 03 rn o F i g . 6 The record s e c t i o n recorded from P r i n c e Rupert t o P r i n c e George from shots i n B i r d Lake. A f a c t o r of d i s t a n c e squared has been a p p l i e d along with the f o l l o w i n g operations: --normalized with respect t o t r a c e 86102 (2nd t r a c e ) —bandpass f i l t e r e d from 3 to 9 Hz — t i c s on x-axis are at 20 km. i n t e r v a l s 18 _ CHAPTER 4 INTERPRETATION 4-1. General p r i n c i p l e s and method In t h i s t h e s i s f i r s t a r r i v a l s beyond about 150 km, t r a v e l l i n g with a v e l o c i t y greater than 7.5 km/s w i l l be designated as P . This event i s n characterized by a frequency of about 4 Hz, a somewhat emergent a r r i v a l and an amplitude which i s generally l e s s than any other w e l l - c o r r e l a t e d event on the record sections. I t i s i n t e r p r e t e d as the a r r i v a l that behaves k i n e m a t i c a l l y l i k e an event which i s c r i t i c a l l y r e f r a c t e d at the Mohorovicic (M) t r a n s i t i o n and which t r a v e l s with the v e l o c i t y of the immediately underlying medium. The l a r g e r amplitude secondary a r r i v a l i s i n t e r p r e t e d as the r e f l e c t i o n from the M. I t i s assumed that t h i s r e f l e c t i o n i s preceded by a smaller amplitude, d i r e c t l y r e f r a c t e d a r r i v a l . The former behaves as a wave continuously r e f r a c t e d w i t h i n the crust above the M and appears as a f i r s t a r r i v a l from Greenbush only out to about 175 km. Within 340 km of Greenbush and beyond 800 km from Greenbush, f i r s t a r r i v a l s t r a v e l l i n g with P^ v e l o c i t i e s may i n d i c a t e energy from shallower and deeper horizons r e s p e c t i v e l y . This w i l l be elaborated upon i n the next s e c t i o n . F i r s t a r r i v a l s were picked from analogue records with the a i d of record sections. Least square v e l o c i t i e s were determined using a weighted analysis and preliminary models were computed using plane layer travel-time equations (Jakosky, 1961). In order to r e f i n e the preliminary models, travel-time curves 19 based on ray theory f o r the case of h o r i z o n t a l layers with a v e l o c i t y gradient (Bullen, 1 9 6 5 ; O f f i c e r , 1 9 5 8 ) were c a l c u l a t e d . Gradients were perturbed u n t i l f i r s t and secondary a r r i v a l branches were obtained which s u b j e c t i v e l y best f i t the observed amplitude d i s t r i b u t i o n . The program which uses a weighted least-square a n a l y s i s to de- r i v e v e l o c i t i e s from travel-times was o r i g i n a l l y written by K.G. Barr, modi- f i e d by M.J. Berry and adapted f o r the CDC 6 4 0 0 by the w r i t e r . Dr. Gerhard Mueller wrote the o r i g i n a l v e r s i o n of the travel-time program outlined i n Appendix A. This program has been a l t e r e d by M.J. Berry and the w r i t e r to ca l c u l a t e and p l o t the p-A graph and operate on the CDC 6 4 0 0 . Using the r e s u l t s of the travel-time a n a l y s i s as basic models f o r the e a s t e r l y and westerly ends of the area, a time-term approach was then used to study structure i n the i n t e r v a l between the shot p o i n t s . The program which computes the time-term p r o f i l e was written by M.J. Berry and adapted f o r the 6 4 0 0 by the w r i t e r . 4 - 2 . Least square a n a l y s i s F i r s t a r r i v a l data observed westward from Greenbush Lake to Prince Rupert - exclusive of data from s i t e s 4 0 1 and 4 0 2 where the f i r s t a r r i v a l i s considered a c r u s t a l phase - in d i c a t e a P N v e l o c i t y of 8 . 0 9 ± 0 . 0 1 km/s. F i r s t a r r i v a l s picked on records observed eastward from Bir d Lake i n d i c a t e a v e l o c i t y of 8 . 0 3 ± 0 . 0 3 km/s. Local consistent v a r i a t i o n s from "normal" P r v e l o c i t y were noted. From s i t e s 4 0 3 to 4 0 8 i n c l u s i v e , f o r example, the data i n d i c a t e a v e l o c i t y of 7 . 6 1 ± 0 . 0 6 km/s. F i r s t a r r i v a l s at s i t e s 4 0 8 to 4 1 7 i n - c l u s i v e i n d i c a t e a v e l o c i t y of 8 . 4 1 ± 0 . 0 7 km/s. Farther to the north, over the r e l a t i v e l y short ( 5 0 km) l i n e from B a r k e r v i l l e to Quesnel (Fi g . 1 ) , f i r s t a r r i v a l s from Greenbush i n d i c a t e a P v e l o c i t y of 20 10.04 ±.3 km/s, whereas from Quesnel to Nazko the observed P^ v e l o c i t y i s 8.16 ±.4 km/s. This l i n e as reversed from Ripley Bay in d i c a t e s v e l o c i t i e s of 6.94 ±.3 km/s and 8.12 ±.1 km/s over the same respective sections. Since the area from s i t e 401 to s i t e 417 i s unreversed, the problem of separating the e f f e c t s of v e r t i c a l (or l a t e r a l ) v e l o c i t y v a r i a - t i o n from M topography cannot be resolved at present. Assigning the l o c a l v e l o c i t y change from 401 to 417 to dip would require an M "surface" which dips down at about 4 degrees to the area immediately east of s i t e 408 and r i s e s with the same slope to the region immediately east of the Fraser River ( F i g . 1). The apparent v e l o c i t i e s immediately east of Quesnel also require an M surface which r i s e s at about 10 degrees towards the Fraser River. From F i g . 1 i t i s noted that, rather than a p r o f i l e , these data are a c t u a l l y "fan shot" over rather small angles. The fan angles subten- ded at Greenbush and B i r d Lake are about 20 and 24 degrees r e s p e c t i v e l y . The resultant model therefore must represent an average f o r the area covered. The a r e a l d i s t r i b u t i o n of recording s i t e s , the distance range and the reversed v e l o c i t i e s would i n d i c a t e that 8.06 km/s i s close to a true upper mantle v e l o c i t y f o r the region under study. 4-3. Travel-time curves A program, based on ray theory which uses as input simply the velocity-depth s t r u c t u r e , was used to construct travel-time curves for comparison with the record sections (Appendix A). As an approximation to the wave s o l u t i o n , the use of ray theory i s v a l i d provided the change i n v e l o c i t y gradient over a wavelength i s small compared with c/X Q , where c i s the v e l o c i t y and X i s the surface wavelength ( O f f i c e r , 1958). o For a surface wavelength of 1 km, any change i n v e l o c i t y gradient at M 21 surface depths must then be small compared with a gradient of about 8 s In fact, the derived models shown in Fig. 7 and Fig. 8 violate this restriction in the neighbourhood of each boundary. These areas are pre- cisely those which determine the nature of the cusps in the travel-time curves. From published wave solutions, i t is instructive to note some of the limitations of ray theory which should be considered when interpreting real data. For the case of spherical, longitudinal waves in a plane- layered structure, Cerveny (1961) has shown that the amplitude maximum is displaced beyond the ray theory critical point by a distance dependent mainly upon the index of refraction and the frequency of the incident wave. This distance interval increases and the shape of the amplitude maximum broadens as the refractive index approaches unity and the incident wave frequency decreases. Therefore, the amplitude maximum would be expected to appear on a record section as a region of large amplitudes displaced from the ray-theoretical critical point. Comparison of the present models with curves given by Cerveny (1966) and Fuchs (1970, p 536) suggests that the true amplitude maximum may be displaced some 40 to 50 km beyond the ray-theoretical critical point. Ray theory predicts that the reflected and head wave travel-time branches meet at the critical point. The wave solutions indicate that the travel-time branches, in fact, intersect beyond the ray-theoretical critical point in the zone of interference of the reflected and head waves. However, the difference in time between travel times predicted by ray theory and those computed using wave solutions - for a model characterized by velocities and frequencies not grossly different from 22 those i n the present study - appears too small to be detectable i n the r e a l data (Cerveny, 1966). It would therefore appear that travel-time curves might be good estimates of a r r i v a l times along both the r e f l e c t e d and r e f r a c t e d branches to w i t h i n about 40 km of c r i t i c a l . Used as such, they would give only a general i n d i c a t i o n of where the l a r g e s t amplitudes might be expected. 4-4. F i t t i n g travel-time curves It was to be expected, i n a d d i t i o n to the t h e o r e t i c a l l i m i t a t i o n s , that i n such a g e o l o g i c a l l y complex area a simple, continuous travel-time curve could not explain a l l the apparent energy c o r r e l a t i o n s . I t was therefore necessary to decide which energy c o r r e l a t i o n s must be explained and to produce a model which would have acknowledged l i m i t a t i o n s . ' Primary consideration was given to the f i r s t a r r i v a l s and the well-developed r e f l e c t e d branches. Since ray theory p r e d i c t s a c r i t i c a l point displaced from the amplitude maximum, the attempt was not d i r e c t e d at p l a c i n g the cusps coincident with the l a r g e s t energy on the se c t i o n . Rather, because the reflected' branch i s r e l a t i v e l y w e l l defined, the models were adjusted so as to give a good f i t to the curvature of t h i s branch. The curvature, of course, i s rather s e n s i t i v e to the v e l o c i t y gradients i n the region of the t r a n s i t i o n . Using the models suggested by the l e a s t square v e l o c i t i e s - with discontinuous v e l o c i t y increases - the curvature of the r e f l e c t e d branches was a poor f i t to the observed data. In order to produce a b e t t e r f i t , the gradients shown i n F i g . 7a, F i g . 8a and F i g . 9a were required. The gradients also b r i n g the cusp B w i t h i n about 50 km of 23 1000 i g . 7 The travel-time (model 1) f o r the Greenbush record section. Note the gradient necessary to produce the curvature of the r e f l e c t e d A-B branch. T i c s on x-axis are at 20 km. i n t e r v a l 24 BC69 LTFJ-PR -RPT . F i g . 7a The velocity-depth structure f o r Model 1. 25 F i g . 8 The travel-time model 2 f o r the Greenbush Lake section. Minor t r i p l i c a t i o n s , C-D and E-F, have been added to explain s i g n i f i c a n t secondary energy. T i c s on x-axis are at 20 km. i n t e r v a l s . 26 BC69 LTFT.-PR . RPT . 0 i — • — — • — • — • — 2 10B 4 _ i 1— 1 KM/S 10 F i g . 8a The velocity-depth structure f o r Model 2. 27 F i g . 9 The travel-time model f o r the Bi r d Lake record section. T i c s on x-axis are at 20 km. i n t e r v a l s . 28 Fig. 9a The velocity-depth structure for the Bird Lake Model. 29 what may be the l a r g e s t energy on the s e c t i o n ( F i g . 10) , 4-5. Record s e c t i o n from Greenbush F i t t i n g t r a v e l - t i m e curves t o data w i t h i n 360 km of Greenbush was d i f f i c u l t due t o the p o s s i b i l i t y o f an intermediate l a y e r . T his l a y e r i s suggested by the f i r s t a r r i v a l s on t r a c e s 403 t o 407, i n c l u s i v e , w i t h a v e l o c i t y of 7.6 km/s. However, the q u a s i - c o r r e l a t a b l e nature of the secondary energy which must be used t o d e f i n e or suggest i t s t r a v e l - t i m e t r i p l i c a t i o n leaves some doubt as t o the r e a l i t y of t h i s l a y e r . Assuming the f i r s t a r r i v a l on t r a c e s 403 t o 407 d e f i n e s a r e - f r a c t e d t r a v e l - t i m e branch, then the l a r g e r secondary energy immedi- a t e l y f o l l o w i n g P n on t r a c e s 411 t o 417, i n c l u s i v e , would make the branches t o the cusp at C ( F i g . 8 and F i g . 10) appear p l a u s i b l e . The energy i n t h i s r e gion i s confused by i t s p r o x i m i t y t o P n a r r i v a l s . C r i t i c a l energy along the r e f l e c t e d branch, CD, i s suggested by the l a r g e r secondary energy on t r a c e s 407 and 408, but again the r e f l e c t e d branch i s not c l e a r . The f i r s t p a r t of the r e f r a c t e d branch from the cusp at D ( F i g . 9) i s perhaps i n d i c a t e d by secondary events (on records 402 and 404) which occur between the more wi d e l y separated t r a v e l - t i m e branches producing the cusp at B. The a l t e r n a t i v e i n t e r p r e t a t i o n , i n terms of an i n t e r f a c e w i t h a reasonable d i p and the f a c t t h a t t h i s p a r t of the s e c t i o n i s unreversed, leaves the question open. S y n t h e t i c s e i s - mograms cons t r u c t e d f o r the d i f f e r e n t models might provide some other i n s i g h t . I t i s p e r t i n e n t t o note here a study, done on data a p p l i c a b l e t o the area beginning 100 km east of Greenbush Lake along l a t i t u d e 50°30', by Chandra and Cumming (1972). Part of the data was recorded from the Greenbush shot p o i n t . The r e s u l t s show a 7.2 km/s l a y e r at a depth of 30 about 30 km which pinches out towards the Trench. In c l u s i o n of an intermediate l a y e r i n the present model would thus be compatible with data to the east. However, since the two areas are some 300 km distant from one another, separated both by a zone of major g e o l o g i c a l complexity and one of the l a r g e s t Bouguer anomalies i n the C o r d i l l e r a , i t i s probably not v a l i d to r e l a t e the areas i n any d e t a i l . From F i g . 10, data on the record section from Greenbush are r e l a t i v e l y sparse i n the region about 400 km. However, the data from 360 to 480 km suggest that, while the amplitude of the P r a r r i v a l s behaves normally, the amplitude of energy along the A-B travel-time branch i n - creases abruptly. I t i s to be noted that at 450 km there e x i s t s another seismogram ( i n a d d i t i o n to trace 431 shown) recorded at s i t e 801 which shows p r e c i s e l y the same amplitude character. G e o l o g i c a l l y , the phenom- enon coincides with the f a u l t trending southeast from McLeod Lake to the area immediately east of Prince George ( F i g . 1). The f a u l t separates Upper Paleozoic rocks immediately to the east from T r i a s s i c - J u r a s s i c v o l c a n i c s to the west. Assuming the behaviour of the c r u s t a l phase i s r e l a t e d to the surface geology, i t suggests that t h i s surface d i v i s i o n continues to the depths of the M t r a n s i t i o n . P i c k i n g P becomes more uncertain with the occurrence of s i -n n i f i c a n t l y more noise i n the s i g n a l passband from s i t e s 802 to 812. The rather abrupt appearance and disappearance of the noise would suggest a regional e f f e c t . Superposition of the s i t e map on the g e o l o g i c a l map of B r i t i s h Columbia reveals that, excepting s i t e s 808 and 809, a l l s i t e s from 802 to 812 l i e on T e r t i a r y v o l c a n i c flows and p y r o c l a s t i c s . S i t e s immediately p r i o r to 802 l i e on T r i a s s i c and J u r a s s i c volcanics and 31 pyroclastics, sites 808 and 809 occur on Jurassic granites and sites immediately following 812 f a l l on Jurassic volcanics and pyroclastics. It appears that over this central region sites located on the tertiary sediments are characterized by a higher level of noise in the signal passband. The amplitude character of the first one or two seconds of energy in this range may be undergoing some interesting changes; however, the noise effectively precludes an interpretation. Beginning at about 660 km (site 814), the character of the firs t second of energy has changed notably. The fairly consistent 4 to 5 Hz P phase which characterized records to at least site 809 is now n more emergent in character and is followed at about 0.8 seconds by a larger amplitude energy band with a similar velocity arid frequency. This phase is well developed from sites 818 to 823. Here again, the nature of the f i r s t energy begins to change and by sites 825 a very emergent cycle ' of 3 Hz energy is immediately followed by significantly larger amplitude 4 to 5 Hz energy. An enlarged picture of this part of the section is shown in Fig. 12. At subsequent sites, the first amplitudes decay and the first arrival may have a velocity of about 8.4 km/s at the end of the section. A problem of phase identification arises here. Is the amplitude variation in the range about 820 km produced by a triplication of the P^ travel-time curve or is the larger secondary arrival the upper mantle P phase, seen in earthquakes studies? The P phase would then become the first arrival at about 820 km. For the present study, it is assumed that the effect is not due to a local surface effect. Alternative ex- planations to be considered are: * 32 F i g . 10 The f i t of Model 2 to the data of F i g . 5a. Tics on x-axis are at 40 km. i n t e r v a l s . 33 (a) structural focusing by M topography of either P r energy as described by Barr ( 1 9 7 1 ) , or P energy as described by Mereu ( 1 9 6 9 ) . Both effects could produce a triplication of the travel-time curve. (b) a triplication of the travel-time curve due to a discontinuous velocity increase in the upper mantle. These alternatives will be discussed after examining data from Bird Lake , 4 - 6 . Record section from Bird Lake The first arrival branches on the section recorded from Bird Lake are less clearly defined than on its reversed counterpart (Fig. 6 ) . This is in part due to fewer stations and a correspondingly larger station spacing but also to the use of spreads. Since these arrays could feasibly be set up only along linear segments of roadway, there was l i t t l e choice of seismometer site and hence background noise was relatively severe. Sites 8 4 0 to 8 4 9 f a l l on tertiary sediments and, like the reversed sites in the same area, are characterized by considerable background noise. Those recordings west of 849 where three components could be set on bedrock ( 8 5 0 , 8 5 6 , 8 5 7 , 8 5 9 ) show relatively minor background noise. It is noted that where an array was used, a l l six seismograms were used in determining a first arrival time. Thus, the confidence of many of the picks is not as poor as the noise level on the chosen records might suggest. On the Bird Lake section i t is noted that, due to a thinner crustal section near the coast ( P intercept of 5.5 seconds) than near Little Fort, the larger critical energy is effectively missing. From the coast eastwards, the main phases are identified as P ^ and a larger coherent second arrival with a velocity of 6.4 km/s. The amplitude of the secondary phase drops abruptly at about 2 9 0 km (from trace 8 5 7 to 8 5 6 ) . It is 34 succeeded there by a slower, more emergent phase. The cessation of the l a r g e r amplitude phase coincides approximately with the western edge of the Hazelton mountains. The surface topography changes from an area characterized by seven thousand foot peaks i n the west to the Bulkley ranges i n the east where peaks reach nine thousand feet. The question of phase i d e n t i f i c a t i o n a r i s e s again. Is the secondary phase, with a v e l o c i t y l e s s than 6.4 km/sec, simply the mantle r e f l e c t i o n which has been perturbed by l a t e r a l s tructure i n the region about s i t e 857, or i s i t a separate a r r i v a l ? Because of the d i s t i n c t p o s s i b i l i t y of l a t e r a l s tructure e f f e c t s and because the travel-time branches are so close at t h i s distance, the cusp at A ('Fig.. 9) i s not c a r r i e d out to greater distances. The e f f e c t of t h i s i s only to make the f i n a l gradient above the M (lower l e f t graph i n F i g . 11) s l i g h t l y l e s s . The velocity-depth structure f o r t h i s s e c t i o n i s shown i n F i g . 9a. The f i r s t a r r i v a l s show no well-developed v a r i a t i o n s i n v e l o c i t y and deviations from the l e a s t square v e l o c i t y (8.03 km/s) are p l a u s i b l y i n d i c a t i v e of topography. Thus, the presence of a s i g n i f i c a n t i n t e r - mediate l a y e r i s not suggested and M v e l o c i t y i s reached at a depth of 30 km. 4-7. The anomaly at 820 km distance In an attempt to decide which of the suggested explanations of the phenomena at 820 km i s more p l a u s i b l e , the following points are pertinent: (a) Comparison of F i g . 5 and 5a shows that the a p p l i c a t i o n of a distance squared f a c t o r appears to restore P n amplitude to an approximately constant l e v e l i n the distance range from 0 to .800 km. The 35 BC70 PR .RPT.-PR.GEO. ( 1-3/9-12 ) F i g . 1 1 . The f i t of the Bird Lake model to the record se c t i o n . T i c s on x-axis are at 2 0 km. i n t e r v a l s . 36 suggestion is that to this distance the first arrival travels as a head wave, probably connected with the M boundary. Beyond 800 km the amplitude of the first distinct arrival increases uniformly to a maximum at about 815 km and subsequently decays (Fig. 12). The length of the section precludes determination of the exact nature of the decay. However, this pattern is very similar to that calculated by Mereu (1969) using ray theory to analyse the focusing effect of M topography on P energy. Topographically, the phenomena occurs in the region characterized by the highest mountains in the coast range at this latitude. Culbert (1971) points out that a line between Douglas Channel and Nass River (at a distance of approximately 810 km on the record section) marks a prominent scarp line in the summit envelope. Geologically, ''the phenomenon occurs at the eastern edge of the Coast Plutonic Complex in the region of the Skeena Arch (Wheeler, 1970; Monger et a l . , 1972). The magnitude of the velocity discontinuity required to produce a travel-time triplication which might account for the amplitude anomaly is approximately 0.2 km/s at a depth of 95 km. However, this raises the question of upper mantle homogeneity. Assuming this model is true, the inference is that no similar velocity dis- continuity occurs between the M and 95 km, since no effect similar to that at 820 km distance is observed on the record section. Con- struction of synthetic seismograms for this model would aid in deciphering data from below the M. 37 890 F i g . 12 Part of the Greenbush record section expanded to show the amplitude increase near 820 km. T i c s on x-axis are at 4 km. i n t e r v a l s . 38 The reversed data from B i r d Lake i s rather i n c o n c l u s i v e with respect to e i t h e r a l t e r n a t i v e . Since the seismograms are only obtained out to 630 km from B i r d Lake, e f f e c t s from a depth of 95 km are not recorded. I f the amplitude behaviour at 820 km on the Greenbush s e c t i o n i s due to the focusing of P energy by M topography, a s i m i l a r e f f e c t , s u i t a b l y o f f s e t to about the area of trace 852 on the reversed sec- t i o n , might then be expected. Unfortunately, the lack of reversed data i n t h i s region leaves the question open. Part of the reason for the apparent lack of focussed energy may be because the distance range from B i r d Lake i s too small to witness s i g n i f i c a n t P energy return from below the M. However, there are two features of the B i r d Lake s e c t i o n which may i n d i c a t e that the lower crust and perhaps the M t r a n s i t i o n i s d i f f e r e n t i n the region of the Hazelton Mountains. It was noted i n de s c r i b i n g the seismic a r r i v a l s from B i r d Lake that the amplitude of the phase r e f l e c t e d from the M t r a n s i t i o n dropped abruptly at about 290 km. This could i n d i c a t e a major d i s c o n t i n u i t y extending to the M i n the area roughly coincident with the scarp l i n e i n the summit envelope reported by Culbert (1971). The other feature i s a strong secondary wavelet on traces 856 to 851 ( F i g . 11) s i m i l a r to the large amplitude secondary a r r i v a l on records 816 to 822 from Greenbush ( F i g . 10). Both wavelets are characterized by a frequency of 4-5 Hz, an amplitude approximately four times that of P and a v e l o c i t y of 7.7 km/s. Assuming the two wavelets are n r e l a t e d , i t suggests that the M t r a n s i t i o n beneath the Skeena Arch 39 has the rather unique property of producing this reverberation. 4-8. Time-term study In an effort to see what structure is implicit in interpreting arrivals as indicating depths to the M and whether the reversed travel- times are compatible, a time-term study was undertaken. Although the experiment was not designed to exploit this type of study, the small dif- ference between reversed, least-square velocities and the fact that at least one site had observed both shot points while many sites from each survey l i e within a few kilometers of each other are points in favour of its application. The rather limited areal distribution of sites means that the resulting model may only be valid in a, zone trending west-north- west, coincident with the general line of recordings. To determine the general topography on the M between Little Fort and Prince Rupert, a simplified version of the "Delay-Time-Function" method outlined by Morris (1972) was used. Conventionally, the refractor sur- face is determined by calculating a time term for every site. The delay- time method assumes that the time-term surface may be represented by a simple mathematical function of position. In doing so, the procedure eliminates the necessity of determining the arbitrary constant which may be subtracted from a l l shot time-terms and added to the recording site time-terms (Scheidegger and Willmore, 1957). In addition, the degree of smoothing of the data can be controlled somewhat by specifying the form of the function to be fitted. In the present study the time-term surface is described as a function of only one parameter - the distance from an arbitrary reference point. 40 Since the sites are restricted to a zone trending to the north- west, the present data were used to construct a time-term p r o f i l e , rather than a surface, along a line coincident with an approximate centre-line of the zone of sites. Polynomials in x - the distance from the mid-point of the line - were f i t t e d to the data to determine the profile. In detail, a line centered near the midst of the survey area with an azimuth coincident with the trend of the sites was chosen. Secondly, the distances from the mid-point of this line to the sites were projected onto the line and offset a distance equal to the step-out of the c r i t i c a l ray in the appropriate direction. The set of points determined by the offset distances and the travel-times was then least-square f i t t e d by polynomials of increasing order. I n i t i a l l y , a P^ velocity, an average crustal velocity and a single shot (recording) site time term were input to determine the offset distances. Subsequent f i t s used previous estimates of time•terms and P^ velocities to compute new offset distances and hence new profiles. An average crustal velocity of 6.4 km/s determined from the travel-time models was used throughout. Herein the travel time from shot i to station j i s given by the usual formula T = J + t + t « — i j where A.. is the horizontal shot-station distance, t. and t. are i j i j shot and station time terms and V i s the refractor velocity, assumed constant for the profile zone. It i s assumed that the time-term t at distance x can be adequately described by t(x) where n k t(x) = E a, x , k=0 fc and n i s the degree of the polynomial. Then the travel-time i s given by A H n k n k T« " r 1 \% V i +kf0 ^ Jo + + r 1 - ̂  . Applying the p r i n c i p l e of l e a s t squares, the quantity M M n , , A,, m-1 1 3 m-1 k=0 K 1 J V 1 J (where M i s the number of observations) Is minimized. This operation y i e l d s the normal equations which may be solved f o r the c o e f f i c i e n t s a^ and V. 4-9. Discussion of time-term study To examine the e f f e c t of a r b i t r a r i l y choosing the l i n e of p r o f i l e , s o l u t i o n s were obtained for the p o s i t i o n of the l i n e entered at 54° l a t i t u d e , 125° longitude (azimuth of 310°); 54° l a t i t u d e , 125° longitude (azimuth of 282°) and 53.5° l a t i t u d e , 124° longitude (azimuth 293°). The general topography of the time-term p r o f i l e changed i n s i g - n i f i c a n t l y ; however, the l a s t mentioned l i n e gave the best f i t numerically. The s t a t i s t i c a l F t e s t was used to t e s t the s i g n i f i c a n c e of each s o l u t i o n . I t was found that a polynomial i n c l u d i n g terms to the fourth order gave a s i g n i f i c a n t f i t . Subsequent so l u t i o n s with higher order terms showed l i t t l e improvement. Unexpectedly, the eleventh order polynomial gave a markedly improved RMS r e s i d u a l . Since t h i s poly- nomial i s s t i l l not an i n t e r p o l a t i v e f i t , the suggestion i s that the i n c l u s i o n of the eleventh order term, which introduces a wavelength of 42 about 200 km, i s s i g n i f i c a n t . A p l o t of the time-terms ( F i g . 13) shows that the smoothing e f f e c t of the fourth order polynomial i s an over- s i m p l i f i c a t i o n . In f a c t , the consistent deviation of the r e s i d u a l s from the fourth order p r o f i l e depicts a shorter (=200 km) wavelength which i s not matched u n t i l the eleventh order polynomial i s f i t t e d . Perhaps most notable i s the f a c t that the p r o f i l e points from B i r d Lake are compatible with the points from Greenbush. The reversed p r o f i l e i s thus character- i z e d by l a t e r a l elements with at l e a s t two s i g n i f i c a n t wavelengths. The broader structure has a wavelength of about 800 km and an amplitude of about 5 km, while the shorter structure has a wavelength of about 200 km and an amplitude of about 10 km. 4-10. L i m i t a t i o n s E l e v a t i o n corrections were not applied to the data since i t i s doubtful that any survey points d i f f e r i n e l e v a t i o n by more than 2500 fe e t . Recording s i t e s were not greatly d i s t a n t from the main access routes which follow the r i v e r v a l l e y s i n regions where corrections might be s i g n i f i c a n t . The v a l i d i t y of the time-term method i s i n d i c a t e d by the be- haviour of the r e s i d u a l s . In preliminary p r o f i l e s o l u t i o n s , data were re-examined i f r e s i d u a l s were anomalously large. Only four observations out of 62 could be rejected on the basis of large u n c e r t a i n t i e s assigned to them i n the least-square a n a l y s i s . Of the remaining 58 observations, 49 are shown to have a t o t a l estimated uncertainty of l e s s than 0.2 seconds. The p r o f i l e shown i n F i g . 13 and F i g . 14 i s i n r e a l i t y a p r o j e c t i o n of the structure on e i t h e r side of a l i n e centered at 53.5° F i g . 13 The p l o t of time-terms converted to depth. The o r i g i n of distance scale i s at 33.5°N l a t i t u d e , 124°E longitude. The p r o f i l e i s along a l i n e with an azimuth of 293°. Note the deviations from the fourth order polynomial are consistent in o u t l i n i n g a wavelength of about 200 km. 44 l a t i t u d e , 124° longitude with an azimuth of 293°. Inherent i n t h i s p r o j e c t i o n are two types of d i s t o r t i o n . The f i r s t i s the unknown e f f e c t of f o r c i n g onto the p r o f i l e s tructure which i s l a t e r a l l y displaced. How- ever, since 80 per cent of the s t a t i o n s are w i t h i n 60 km of the l i n e of p r o f i l e and the reversed data are consistent, the p r o f i l e i s probably a good average p i c t u r e . The second type of d i s t o r t i o n i s a shortening (<10%) of the p r o f i l e introduced by p r o j e c t i n g the i n t e r - s t a t i o n distance onto the l i n e with an azimuth of 293°. The e f f e c t shrinks the true i n t e r - s i t e distance by an amount dependent upon the cosine of the angle between the l i n e of s i t e s and the p r o f i l e l i n e . Examination of t h i s e f f e c t i n d e t a i l shows that s i g n i f i c a n t shortening occurs l o c a l l y i n the areas where the p r o f i l e crosses the l i n e of s i t e s . In these areas the apparent dip between l o c a l p a i r s of p r o f i l e points may be considerably i n e r r o r ; however, the general shape of the p r o f i l e remains the same. The tendency to make the topography more dramatic may not be i n c o r r e c t . I f the M topography follows the s t r i k e of the s t r u c t u r a l elements, which i s about North 30° West i n the area between 52° and 56° l a t i t u d e ( F i g . 1), then the p r o f i l e l i n e s t r i k i n g North 67° West crosses the s t r u c t u r e o b l i q u e l y . This would r e s u l t i n an apparent cr o s s - s e c t i o n with lower dips than a section normal to the s t r u c t u r a l elements. Thus, any e f f e c t which enhances the topography may present a more ac- curate p i c t u r e . The s i g n i f i c a n c e of the time-term p r o f i l e w i l l be o u t l i n e d i n Chapter 5. 45 CHAPTER 5 SUMMARY 5-1. Travel-time models Acknowledging some of the basic shortcomings of ray theory with respect to published wave s o l u t i o n s , the models shown i n F i g . 7a, F i g . 8a and F i g . 9a have been derived f o r the end regions of the survey. The earth's curvature has been taken into account i n the manner described by Mereu (1969). A l t e r n a t i v e i n t e r p r e t a t i o n s are presented for the data w i t h i n 360 km of Greenbush. The v e l o c i t y of 5.6 km/s f o r the surface l a y e r was chosen on the basis of recordings made from Nazko to B a r k e r v i l l e of a shot i n Punchesakut Lake. The v e l o c i t y of f i r s t a r r i v a l s at Sands- p i t from B i r d Lake shots and the v e l o c i t y log of a w e l l d r i l l e d near Nazko (J.A. Mair, personal communication) are i n close agreement with t h i s f i g u r e . In the simpler model ( F i g . 7a), the v e l o c i t y between 3.5 km and 24 km i s near 6.2 km/s. At t h i s depth a gradient i s introduced which produces the cusp at 550 km ( s i m i l a r to cusp A i n F i g , 8). Between 24 km and 36 km, the v e l o c i t y gradient i s v a r i e d i n the manner shown to produce the necessary curvature to the r e f l e c t e d branch and the cusp at 110 km ( s i m i l a r to cusp B i n F i g . 8). The upper mantle v e l o c i t y i s 8.0 km/s. The a l t e r n a t e model from Greenbush (Fig. 8a) introduces a layer at the base of the crust with a v e l o c i t y of 7.5 km/s and a v e l o c i t y d i s - c o n t i n u i t y at 95 km i n an e f f o r t to explain v a r i a t i o n s i n P^ v e l o c i t y and s i g n i f i c a n t energy immediately following the P a r r i v a l s . These 46 same observations may also be explained i n terms of topography on the M. On the bas i s of the present study, the two e f f e c t s cannot be separated. The B i r d Lake model i s shown i n F i g . 9a. Using 5.6 km/s as a surface l a y e r v e l o c i t y , the required thickness i s 4.5 km. Between 4.5 km and 20.5 km, the v e l o c i t y i s near 6.4 km/s. From 20.5 km to 29.5 km, the v e l o c i t y gradients shown i n F i g . 9a are required to produce the t r a v e l - time curve of F i g . 11. The upper mantle v e l o c i t y i s 8.0 km/s. 5-2. Time-term model Assuming the v e l o c i t y of the r e f r a c t i n g horizon and the c r u s t a l v e l o c i t y w i t h i n the c r i t i c a l l y r e f r a c t e d ray cone remain nearly constant and the slope and curvature of the r e f r a c t i n g surface are not too great (Berry and West, 1966), the time-term method may be applied. The wave- length of energy observed i n the present data would l i m i t v e r t i c a l r e s o l u t i o n to about 2 km. The s t a t i o n spacing would r e s t r i c t l a t e r a l s t r u c t u r e r e s o l u t i o n to features with a wavelength greater than about 20 km. Consistent with these l i m i t a t i o n s , the present study has delineated an M t r a n s i t i o n characterized by two prominent wavelengths ( F i g . 14 ). The l a r g e r feature with an apparent wavelength of about 800 km and an amplitude of about 5 km i s depicted by the fourth order polynomial. On the surface, t h i s feature has as c o r r e l a t i v e s the d i s t r i b u t i o n of metamorphic t e r r a i n s (Monger and Hutchison, 1971) and, not s u r p r i s i n g l y , the physiographic regions of the Canadian C o r d i l l e r a . The low grade metamorphic t e r r a i n o v e r l i e s the thinner c e n t r a l c r u s t a l s e c t i o n . The " h a l f wavelength" point has recently been depicted as the boundary between the P a c i f i c and Columbian Orogens (Wheeler and Gabrielse, 1972). I t i s also noted that the 800 km wavelength i s s i m i l a r to the dimension 47 F i g . 14 The c o r r e l a t i o n of the time-term structure with Geology ( C o r d i l l e r a model a f t e r Wheeler and Gabrielse, 1972). The t r i a n g l e s are data from Bird Lake, the dots are data from Greenbush. 48 of topography at the lithosphere i n t e r f a c e as depicted i n the plate tectonic model f o r the evolution of the Canadian C o r d i l l e r a (part c, Mid-Cretaceous (100 MY) to Oligocene (25 MY), i b i d . ) . Assuming that the p r o f i l e section i s a c t u a l l y a p r o j e c t i o n of the true structure onto a v e r t i c a l plane which crosses the s t r i k e of the structure at 40° to the West, then the true wavelength would be about 640 km. In terms of presently envisaged plate models (Isacks et a l . , 1968) and wavelengths f o r continental l i t h o s p h e r i c flexure (Walcott, 1970), t h i s number appears quite reasonable. The long wavelength s t r u c t u r e , of course, may also be explained by a regional v a r i a t i o n i n v e l o c i t y . The smaller feature with an apparent wavelength of about 200 km and an amplitude of about 10 km, i s o u t l i n e d by the consistent r e s i d u a l s to the fourth order polynomial. The feature suggests that the C o r d i l l e r a n structure elements and t h e i r s u b d i v i s i o n s , the t e c t o n i c elements, have a topographic expression on the M t r a n s i t i o n . I f t h i s i s so, then the M t r a n s i t i o n beneath southern B r i t i s h Columbia should d i f f e r considerably from the area presently under study. This i s already suggested by the d i f f e r e n t pattern of aeromagnetic Z component r e s i d u a l s (Haines et a l . , 1971) between the two regions and the f a c t that the M d i s c o n t i n u i t y may be at l e a s t 50 km deep beneath Vancouver Island (Tseng, 1968). 5-3. Conclusions On the b a s i s of the present study and the work of Shor (1962), the presence of t y p i c a l oceanic crust i s not suggested u n t i l west of Graham Island (Queen Charlotte f a u l t ? ) . From a depth of 26 km beneath Graham Island (Fig. 14), the M t r a n s i t i o n deepens s l i g h t l y beneath the c o a s t a l mountains and then r i s e s beneath the Hazelton Mountains i n the 49 region of the Skeena Arch (Fig. 14). This area i s characterized by peaks which reach 9000 f e e t . The c r u s t a l s e c t i o n then thickens immediately to the east of these mountains. The major aeromagnetic anomaly also occurs j u s t to the east of the major elevations at t h i s l a t i t u d e . The records from Greenbush which are characterized by anomalously large amplitude f i r s t energy could o r i g i n a t e from t h i s region between 126°30' and 127°30' longitude. The M t r a n s i t i o n then r i s e s to the area separating the P a c i f i c and Columbian Orogens ( F i g . 1). South of Prince George the s e c t i o n i s unreversed with the exception of the Nazko to B a r k e r v i l l e p r o f i l e . This short p r o f i l e agrees with the unreversed data i n suggesting an M structure which r i s e s from the east to the v i c i n i t y of the Fraser River. The present study i s i n good agreement with the r e s u l t s of White et a l . (1968). The apparent v e l o c i t i e s f o r p r o f i l e s 01, 02 and 03 of the 1968 study, r e i n f o r c e the present i n t e r p r e t a t i o n i n terms of M topography f o r the area east of the Fraser. For the Prince Rupert area, the model obtained by Johnson et a l . (1972) f o r the coast c r u s t a l section i s rather more complex than the present data would i n d i c a t e . The two studies concur i n suggesting an average c r u s t a l v e l o c i t y of approximately 6.4 km/s; however, the evidence f o r a strong r e f r a c t e d event with a v e l o c i t y of 6.7 km/s i n the Prince Rupert area i s missing i n the present data. This may, of course, be explained i n terms of the d i f f e r e n t azimuths of recording l i n e s and M topography, or the considerably greater s t a t i o n spacing i n the study by Johnson et a l . However, i t i s observed that the major geomagnetic anomaly which extends northwest from Ripley Bay terminates i n 50 the area of Prince Rupert. This may suggest a rather different crustal section i n detail for the area approximately mid-way between Ripley Bay and Prince Rupert. The analysis of gravity data over the general area of the survey has not been completed. However, a preliminary interpretation for the area between L i t t l e Fort and the Fraser River is in good agreement with the present suggestion of M topography (R. Stacey, personal communication). In addition to the structural pattern described above, the following general points are considered pertinent. (A) Careful consideration of systems' responses and the monitoring of a l l shots at one location enables construction of record sections which show a coherent energy pattern. The main effects of poor seismometer sites are, apparently, an amplification of background noise, but only rare amplification of the entire record by more than a factor of about two. (B) The passband of the recorded seismic energy is naturally restricted to the range from about 3 to 6 Hz. The monochromatic nature of the energy is not readily explained. Other puzzling features of the record sections which remain unexplained are: 1. the regional levels of background noise, 2. the anomalous variation in amplitude of the crustal phase with respect to P r energy. (C) The Bird Lake and Greenbush Lake shotpoints, although greatly separated and with different water depths, apparently dif f e r l i t t l e in response. 51 ( D ) P i s w e l l recorded as a f i r s t a r r i v a l northwest from Greenbush n Lake to at l e a s t 800 km, and east from B i r d Lake to 610 km. Res- t o r a t i o n of P ^ energy to an approximately constant l e v e l by the a p p l i c a t i o n of a distance squared f a c t o r suggests that the wave t r a v e l s as a head wave connected with the M t r a n s i t i o n and, tenta- t i v e l y , that upper mantle gradients are extremely small over t h i s region of the C o r d i l l e r a ( H i l l , 1971a). ( E ) The rather complex energy patterns which follow the major events ( P and c r u s t a l r e f l e c t i o n s ) suggest the models derived herein are s i m p l i f i e d versions of the true p i c t u r e . Construction of synthetic seismograms f o r comparison with the record sections and examination of the data recorded along l i n e s from L i t t l e Fort to McLeod Lake and from Williams Lake to B e l l a Coola may help to d i s t i n g u i s h be- <•tween the e f f e c t s of topography and v e l o c i t y depth st r u c t u r e . 52 Appendix A Travel-Time Curves The distance and travel time along a ray can be expressed in terms of the angle of inclination of the ray and depth z by the equations x = f Z tan0dz and t = / S — = f z ^ z o o c o c COS0 where s i s length along the ray and v i s velocity. Using the ray parameter p = rz sin , x = I dz ° cos eo pc'(z) 1 0 I = —, , > l sin0d0 = rr~\ (cos0 - cos0} pc'(z) 6 o pc'(z) o where c 1(z) i s the gradient. The travel time i s then t = r e *§ 6 c'(z)sin0 o i 6/2 . 1 r, tan 1 = -cPJzJ { L O G — 0 7 2 } . tan o and since tan 0/2' = sin©/ a\ yl - cosvJ 1 r, sin0 l-cos0 c W ° g l ^ o 7 0 • lHB~ oi o 1 V Q 1 -COS0 q = -cPJzJ l o g { ( _ v T } x i ^ o 7 0 O ) } '0 o using a linear gradient within each layer such that c'(z) z ( v ± _ 1 - V^/h^ 53 th where h^ i s the thickness of the i l a y e r . Then f o r the l a y e r i n which the ray bottoms P - i / v i + 1 and the time and distance are given by V..-V, . l-cos6. t . ( _ ^ r l l o g { _ , } A program based on the above equations was used to produce travel-time graphs. The program e f f e c t i v e l y sums the time and distance c o n t r i b u t i o n from a s p e c i f i e d number of rays per l a y e r , f o r as many layers as desired, to produce a travel-time p l o t . The program, i n i t s o r i g i n a l form, was w r i t t e n by Dr. Gerhard Mueller. 54 REFERENCES 1. Bancroft, A.M., and Basham, P.W. 1967. 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