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Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 465
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 466
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 467
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 468
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 469
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 470
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 471
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 472
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 473
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 474
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 475
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 476
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 477
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 478
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 479
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 481
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 482
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 483
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 484
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 486
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 491
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 492
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 494
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 495
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 496
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 497
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Page 499
Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Suggested Citation:"Seismology Program." National Research Council. 1965. Report on the U.S. Program for the International Geophysical Year: July 1, 1957 - December 31, 1958. Washington, DC: The National Academies Press. doi: 10.17226/26118.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

X I SEISMOLOGY PROGRAM Seismograph Va u l t at Lament G e o l o g i c a l Observatory

SEISMOLOGY PROGRAM Page A. Organization of T e c h n i c a l Panel on Seismology and G r a v i t y 463 B. Summary of Panel A c t i o n s 463 C. P r o j e c t O b j e c t i v e s and R e s u l t s 463 11.2 A n t a r c t i c Seismology Personnel 465 11.3 A n t a r c t i c Seismograph Equipment 466 11.4 A n t a r c t i c Seismograph Equipment 466 11.5 A n t a r c t i c Seismograph Equipment 466 11.6 A n t a r c t i c Seismograph Equipment 467 11.7 Reduction of Seismic Records on A n t a r c t i c T r a v e r s e s 469 11.8 Sea E x p l o r a t i o n , A t l a n t i c Ocean 477 11.9 Seismic Sea E x p l o r a t i o n of the Southeast P a c i f i c 481 11.10 C o n t i n e n t a l Seismic E x p l o r a t i o n 483 11.11 Seismic E x p l o r a t i o n of C o n t i n e n t a l S t r u c t u r e i n South America . . . 486 11.12, 11.19 - Study of Long-Period Seismic Waves 491 11.13 Lg and Rg Phase Study 492 11.14 C r u s t a l S t r a i n Accumulation 493 11.15, 11.18, 11.20 - A n t a r c t i c , A r c t i c & P a c i f i c Seismic O b s e r v a t o r i e s . . 496 462

X I . SEISMOLOGY A. Organization of T e c h n i c a l Panel on Seismology and G r a v i t y Information on the establishment of the Panel, composition of the Panel, and Panel meeting were given i n Chapter V, G r a v i t y . B. Summary of Panel Actions 1. Development of Program. I n a d d i t i o n to c o n s i d e r i n g task of working out an IGY program i n seismology and g r a v i t y which would be of appropriate s c i e n t i f i c v a l u e , the Panel l a i d great s t r e s s on the fundamental c r i t e r i a e s t a b l i s h e d by CSAGI, e s p e c i a l l y these: that the programs be e i t h e r s y n o p t i c or i n t e r n a t i o n a l i n scope or c h a r a c t e r . T h i s was considered e s s e n t i a l s i n c e much of the most s i g n i f i c a n t c u r r e n t r e s e a r c h i n these f i e l d s d i d not f a l l w i t h i n CSAGI d e f i n i t i o n s , e.g. s e i s m i c model s t u d i e s . The IGY seismology program e a r l y envisioned by the Panel included: a. Seismic programs on the oversnow t r a v e r s e s i n the A n t a r c t i c b. A n t a r c t i c s t a t i o n seismology c. Long period wave and Lg phase s t u d i e s d. S t r a i n accumulation s t u d i e s e. Seismic e x p l o r a t i o n of c o n t i n e n t a l s t r u c t u r e f . Seismic e x p l o r a t i o n of c o a s t a l s t r u c t u r e g. Seismic e x p l o r a t i o n a t sea h. Data reduction and p u b l i c a t i o n The o r i g i n a l program was subsequently expanded to include establishment of standard s e i s m i c s t a t i o n s i n s p e c i a l remote l o c a t i o n s (tlie A r c t i c ; A n t a r c t i c a ; and P a c i f i c Ocean i s l a n d s ) . S e v e r a l a d d i t i o n a l p r o j e c t s of great i n t e r e s t to the Panel that were proposed a f t e r the e a r l y formulation of the IGY program were not p o s s i b l e to include i n the f i n a l program, p a r t l y as a r e s u l t of l i m i t e d funds and p a r t l y because they were marginal i n r e l a t i o n to the CSAGI p r i o r i t i e s r e g a r d l e s s of t h e i r s c i e n t i f i c m e r i t . For the IGC-59 program, the Panel s p e c i f i c a l l y urged the c o n t i n u a t i o n of the s e i s m i c e x p l o r a t i o n of c o n t i n e n t a l s t r u c t u r e ; continued operation and c a l i b r a t i o n of the Lg and LP network; the s t r a i n seismometer s t u d i e s i n the Andes; and the main- tenance of s p e c i a l l y i n s t a l l e d s e i s m i c s t a t i o n s i n remote r e g i o n s . C. P r o j e c t O b j e c t i v e s and R e s u l t s 1. Stimmary of Program. The s e i s m o l o g i c a l program c o n s i s t e d of: a) earthquake s t u d i e s w i t h emphasis on s p e c i a l s t u d i e s of long period waves; b) s e i s m i c e x p l o r a t i o n of the c r u s t a l s t r u c t u r e ; c ) microseismic s t u d i e s ; d) the ext e n s i o n of earthquake s t u d i e s and s e i s m i c e x p l o r a t i o n techniques to A n t a r c t i c a . P r o j e c t s provided f o r the procurement, i n s t a l l a t i o n , and operation of s e i s m i c equipment a t s e v e r a l new s t a t i o n s and f o r s e i s m i c surveys on oversnow t r a v e r s e s . Other p r o j e c t s were e s t a b l i s h e d f o r the c o l l e c t i o n , r e d u c t i o n , a n a l y s i s , and p u b l i c a t i o n of data. 463

SEISMOLOGY 2. Order of Pro.iect D e s c r i p t i o n . The p r o j e c t s that are described m the fo l l o w i n g pages are i n numerical order to correspond w i t h the o r i g i n a l formulation of the pro- gram. For convenience, the p r o j e c t s a r e grouped below to r e f l e c t s i m i l a r i t y of pur- pose. A n t a r c t i c Seismology Personnel 11.2 A n t a r c t i c Seismograph Equipment 11.3, 11.4, 11.5, 11.6 Reduction and P u b l i c a t i o n of Data 11.7, 11.16 Sea E x p l o r a t i o n 11.8, 11.9 C r u s t a l S t r u c t u r e 11.10, 11.11, 11.14 Long Period Waves 11.12, 11.13 A n t a r c t i c Seismograph Measurements 11.15 A r c t i c Seismograph Measurements 11.18, 11.19 P a c i f i c I s l a n d Seismograph Measurements 11.20 3. Ca n c e l l e d P r o j e c t s . Former p r o j e c t s bearing the fo l l o w i n g numbers were e l i m i n - ated from the program or combined w i t h other p r o j e c t s : 11.1, 11.17. 464

SEISMOLOGY P r o j e c t 11.2 - A n t a r c t i c Seismology Personnel 1. Ob.iectives. T h i s p r o j e c t , under the s u p e r v i s i o n of the A r c t i c I n s t i t u t e of North America, provided f or the a d m i n i s t r a t i o n of personnel s t a t i o n e d m A n t a r c t i c a to take s e i s m o l o g i c a l measurements. The summer programs c o n s i s t e d of t r a v e r s e s t u d i e s i n - cludi n g a s e r i e s of observations of s h e l f and c o n t i n e n t a l i c e w i t h r e s p e c t to t h i c k n e s s , changes i n c h a r a c t e r w i t h depth and conformation of s u b g l a c i a l topography. Instrumen- t a t i o n c o n s i s t e d of HTL 7000B 24-trace portable seismographs, which were provided under P r o j e c t s 11.3 and 11,5. I n s p e c i f i e d areas i n a c c e s s i b l e to the oversnow t r a v e r s e s , a d d i t i o n a l s e i s m o l o g i c a l i n v e s t i g a t i o n s were c a r r i e d out w i t h a i r c r a f t , using an HTL 7000B 12-trace portable seismograph provided under P r o j e c t 11.4 ( A n t a r c t i c Seismographic Equipment). Wintering-over a c t i v i t i e s c o n s i s t e d of maintenance, c o n s t r u c t i o n of new equipment, preparation f o r f i e l d a c t i v i t i e s , e t c . The s e i s m i c instruments were taken care of i n part by personnel from other programs. These programs were supplemented by concurrent g r a v i m e t r i c s t u d i e s d e s c r i b e d i n P r o j e c t 5.1. 2. Operations. The s e i s m o l o g i c a l program m A n t a r c t i c a was di v i d e d i n t o two p a r t s : s t a t i o n seismology and t r a v e r s e seismology. a. S t a t i o n seismology. Standard three-component seismometers were operated a t Amundsen-Scott (Pole) and Byrd s t a t i o n s ( P r o j e c t 11,15); a three-component long-period seismograph was i n s t a l l e d a t Wilkes s t a t i o n ( c f . P r o j e c t 11,6); i n cooperation w i t h New Zealand, a three-component long-period seismograph operated a t H a l l e t t ( c f . P r o j e c t 11,3). b. T r a v e r s e seismology. Both oversnow and airborne t r a v e r s e s were conducted during IGY from v a r i o u s U.S, s t a t i o n s i n A n t a r c t i c a . The s e i s m i c measurements pro- vided the b a s i c data on t h i c k n e s s of the i c e sheet; these data were c o r r e l a t e d w i t h g r a v i t y and g l a c i o l o g i c a l observations to determine the main f e a t u r e s of the geometry of the i c e cover of A n t a r c t i c a , the c h a r a c t e r of the c r u s t beneath the i c e sheet, i s o s t a t i c adjustment of A n t a r c t i c a , d e n s i f i c a t i o n of snow, and some c l i m a t o l o g i c a l a s p e c t s p e r t a i n i n g more d i r e c t l y to g l a c i o l o g y . T r a v e r s e s accomplished during the IGY are d e s c r i b e d i n P r o j e c t 4.5. 3. Personnel. W.A. Wood, AINA, was a d m i n i s t r a t i v e p r o j e c t d i r e c t o r ; A.P. C r a r y ( G l a c . Headquarters O f f i c e ) supervised the planning and, during two seasons i n Ant- a r c t i c a , supervised the s c i e n t i f i c work ( i n s o f a r as communications permitted) as Deputy Ch i e f S c i e n t i s t , U.S. A n t a r c t i c Program. A. Data. S t a t i o n data were reduced and published f o r Amundsen-Scott and Byrd s t a t i o n s m connection w i t h p r o j e c t 11.15, for Wilkes i n connection w i t h p r o j e c t 11.6. Tr a v e r s e data were reduced a t the U n i v e r s i t y of Wisconsin under data r e d u c t i o n p r o j e c t 11.16. A l l these data have been transmitted to the IGY World Data C e n t e r s . 5. R e s u l t s and Bib l i o g r a p h y . R e s u l t s and bi b l i o g r a p h y are reported under the p r o j e c t s enumerated i n the preceding paragraph. R e l a t e d g l a c i o l o g i c a l r e s u l t s from t r a v e r s e s are reported under p r o j e c t 4.10. 465

SEISMOLOGY P r o j e c t 11.3 - A n t a r c t i c Seismograph Equipment 1. O b j e c t i v e s . T h i s p r o j e c t a t Lamont G e o l o g i c a l Observatory, Columbia U n i v e r s i t y , provided f o r the procurement of an earthquake seismograph s t a t i o n a t H a l l e t t S t a t i o n and portable equipment for use i n the e x p l o s i o n s t u d i e s of the A n t a r c t i c I c e Cap. 2. Operations. Under t h i s p r o j e c t , the f o l l o w i n g equipment was obtained and i n s t a l l e d a t H a l l e t t S t a t i o n : two Columbia long-period v e r t i c a l and one long-period h o r i z o n t a l seismometers; one Willmore sh o r t - p e r i o d seismometer; three Lehner and G r i f f i t h 75- second galvanometers; one 2-second and one quarter-second galvanometers; rec o r d i n g drums and a s s o c i a t e d equipment. One 24-trace portable seismograph system (Houston T e c h n i c a l Laboratory - 7000 B) was obtained and sent to A n t a r c t i c a f o r use on the t r a v e r s e s . 3. Personnel. Maurice Ewing d i r e c t e d the p r o j e c t ; Jack O l i v e r , Paul Pomeroy, George Sutton, Harold Smith, and Melvln Winters p a r t i c i p a t e d m v a r i o u s a s p e c t s of the p r o j e c t . The s t a t i o n seismographs were operated by New Zealand personnel a t H a l l e t t ; the portable seismograph was u t i l i z e d under the t r a v e r s e program (see p r o j e c t 11.2). 4. Data, R e s u l t s and B i b l i o g r a p h y . Data from the equipment provided under t h i s p r o j e c t were transmitted to the World Data Ce n t e r s . For r e s u l t s of LP s t a t i o n s e i s m i c obser- v a t i o n s , see p r o j e c t 11.12; f o r t r a v e r s e r e s u l t s , see p r o j e c t 11.7. P r o j e c t 11,4 - A n t a r c t i c Seismograph Equipment 1. O b j e c t i v e s . T h i s p r o j e c t a t Weston Observatory, Boston C o l l e g e , provided f o r the procurement of portable s e i s m i c equipment f o r use i n surveys of the A n t a r c t i c I c e Sheet. 2. Operations. A 12-trace (Houston T e c h n i c a l Laboratory 7000 B) portable seismograph system was obtained and shipped to D a v i s v i l l e , R . I . for shipment to A n t a r c t i c a f o r use i n the t r a v e r s e s . 3. Personnel. The purchasing and shipping of seismograph was d i r e c t e d by Rev. D a n i e l Lmehan, S.J., D i r e c t o r , Weston Observatory, Boston C o l l e g e . 4. Data. R e s u l t s and B i b l i o g r a p h y . The r e d u c t i o n and t r a n s m i s s i o n to World Data Centers of data, and p u b l i c a t i o n of r e s u l t s of the t r a v e r s e s , are reported under P r o j e c t 11.7. P r o j e c t 11.5 7 A n t a r c t i c Seismograph Equipment 1. O b j e c t i v e s . T h i s p r o j e c t a t the U n i v e r s i t y of Wisconsin provided f o r the procure- ment of portable s e i s m i c equipment f o r use m surveys of the A n t a r c t i c . 2. Operations. A Houston T e c h n i c a l Laboratory 7000 B portable seismograph system was obtained and shipped to A n t a r c t i c a f o r use i n the t r a v e r s e s . 3. Personnel. The p r o j e c t was d i r e c t e d by Dr. George P. Woollard, U n i v e r s i t y of Wis- consi n . 4. Data, R e s u l t s and B i b l i o g r a p h y . The r e d u c t i o n and t r a n s m i s s i o n to World Data Centers of data, and p u b l i c a t i o n of r e s u l t s of the t r a v e r s e s , are reported i n P r o j e c t 11.7. 466

SEISMOLOGY P r o j e c t 11.6 - A n t a r c t i c Seismograph Equipment 1. O b j e c t i v e s . T h i s p r o j e c t of the S e i s m o l o g i c a l Laboratory of the C a l i f o r n i a I n s t i - tute of Technology, provided for procurement of s p e c i a l s t a t i o n seismographs f o r Wilkes S t a t i o n m A n t a r c t i c a , and for a portable s e i s m i c system for use i n the t r a v e r s e pro- gram. The i n s t a l l a t i o n of the s e i s m i c s t a t i o n a t Wilkes was e s p e c i a l l y chosen w i t h a view to the l o c a t i o n of the s t a t i o n d i a m e t r i c a l l y a c r o s s A n t a r c t i c a from the s e i s m i c a l l y a c t i v e region near the Palmer P e n i n s u l a . 2. Operations. A three-component Press-Ewing seismograph system (15-second s e i s - mometers; 90-second galvanometers) was obtained and i n s t a l l e d a t Wilkes S t a t i o n . A portable seismograph system was obtained and shipped to A n t a r c t i c a for use i n the t r a v e r s e program. 3. Personnel. The p r o j e c t was d i r e c t e d by Frank P r e s s , C a l i f o r n i a I n s t i t u t e of Technology. A n t a r c t i c operations were conducted under p r o j e c t 11.2. The t r a v e r s e a c t i v i t i e s are reported under that p r o j e c t and p r o j e c t 11.7. The Wilkes seismograph s t a t i o n was operated by G i l b e r t Dewart i n 1957-58, and by Henry F. Birkenhauer, S.J., i n 1958-59. 4. Data. Data from the t r a v e r s e s are reported under p r o j e c t 11.7. A s t a t i o n b u l l e t i n f o r Wilkes S t a t i o n was prepared and transmitted to the World Data Cen t e r s . 5. R e s u l t s . C r u s t a l s t r u c t u r e i n A n t a r c t i c a was studied along s e v e r a l p r o f i l e s by means of group v e l o c i t y d i s p e r s i o n a n a l y s i s of Love and R a y l e i g h waves along t r a n s - a n t a r c t i c paths (See F i g u r e 1 ) . The d i s p e r s i o n curves show that a n t a r c t i c ( E a s t and West A n t a r c t i c a combined) v e l o c i t i e s are s y s t e m a t i c a l l y higher f o r given periods than f o r known c o n t i n e n t a l (North American and A f r i c a n ) paths. Sources of s y s t e m a t i c e r r o r were i n v e s t i g a t e d but were not found to be g r e a t enough to account f o r the observed discrepancy. I t was t h e r e f o r e concluded that e i t h e r the c r u s t a l t h i c k n e s s or the extent of A n t a r c t i c a must be approximately one-fourth l e s s than i n d i c a t e d by p o s i t i o n of the i c e sheet for the t r a n s - a n t a r c t i c path to Wilkes S t a t i o n . Because no c o n t i n e n t a l s t r u c t u r e i s known to be so abnormally t h i n , i t was assumed t h a t the extent was l e s s . A c o n s i d e r a b l e p o r t i o n of r e g i o n of Western A n t a r c t i c a has been r e v e a l e d to be below sea l e v e l . The r e s u l t s from Wilkes S t a t i o n suggest t h a t much of t h i s r egion may be oceanic ( i s l a n d a r c h i p e l a g o ) i n s t r u c t u r e r a t h e r than a low l y i n g continent depressed by the i c e sheet. The c r u s t a l s t r u c t u r e i n E a s t A n t a r c t i c a , as r e v e a l e d from s u r f a c e wave d i s p e r s i o n data along paths l y i n g i n the e a s t e r n h a l f of the c o n t i n e n t , i s c o n t i n e n t a l . The c r u s t i s about 42 km t h i c k ( i n c l u d i n g 3 km i c e l a y e r ) . The upper-mantle v e l o c i t i e s are low ( U = 7.85 km/sec) and s i m i l a r to that of the B a s i n and Range province of the United S t a t e s . 6. Bibliography. a. Papers Presented a t Meetings. R.L. Kovach and F. P r e s s : "Surface Wave D i s p e r s i o n and C r u s t a l S t r u c t u r e i n A n t a r c t i c a and Surrounding Oceans." S.S.A. Meeting, San Diego, 1961. G. Dewart and M.N. Toksoz: " C r u s t a l S t r u c t u r e i n E a s t A n t a r c t i c a from Surface Wave D i s p e r s i o n . " A.G.U. Meeting, Washington, D.C., 1964. b. Published Papers. Frank P r e s s and G i l b e r t Dewart: "Extent of the A n t a r c t i c Continent." Science 129, Feb. 20, 1959, pp. 462-63. 467

SEISMOLOGY Robert L. Kovach and Frank P r e s s : "Surface Wave D i s p e r s i o n and C r u s t a l S t r u c t u r e i n A n t a r c t i c a and Surrotmding Oceans." A n n a l l d i G e o f i s i c a 14, 1961, pp. 211-224. G i l b e r t Dewart and M. N a f i Toksoz: " C r u s t a l S t r u c t u r e i n E a s t A n t a r c t i c a from Surface Wave D i s p e r s i o n . " (To be published) 90'W 180' 24N0V B8 1000 fothoms' 2 Oct 57 H A L L E T SCOT INDIAN OCEA 9 Sep 57 4 Sep 5 7 9 0 ' E F i g u r e 1. Index map showing e p i c e n t e r s , seismograph s t a t i o n s , great c i r c l e paths and deduced c r u s t a l t h i c k n e s s e s f o r the A n t a r c t i c region. 468

SEISMOLOGY P r o j e c t 11.7 - Reduction of Seismic Records on A n t a r c t i c T r a v e r s e s 1. O b j e c t i v e s . T h i s p r o j e c t a t the U n i v e r s i t y of Wisconsin, Department of Geology, provided f or the re d u c t i o n and p u b l i c a t i o n of r e f l e c t i o n s e i s m i c data gathered during the a n t a r c t i c oversnow t r a v e r s e s c a r r i e d out from L i t t l e America, Byrd and E l l s w o r t h s t a t i o n s during the a n t a r c t i c summers of 1957-58 and 1958-59. The p r o j e c t included t r a n s c r i p t i o n and re d u c t i o n of data recorded m the f i e l d , measurement of a r r i v a l times, p l o t t i n g of t r a v e l times and appropriate graphs, and c o r o l l a r y determinations of v e l o - c i t y and depth of r e f l e c t i n g s u r f a c e s . The data c o l l e c t e d during i n d i v i d u a l readings had to be coordinated w i t h the e n t i r e s e r i e s , t a k i n g i n t o account i n t e r n a l c o n s i s t e n c y as w e l l as g r a v i t y values obtained. Personnel were drawn from the t r a v e r s e party p a r t i c i p a n t s who made the measurements on a part-time b a s i s and shared w i t h the g r a v i t y r e d u c t i o n p r o j e c t 5.6 (Reduction of G r a v i t y Measurements on A n t a r c t i c T r a v e r s e s ) . The data r e d u c t i o n was i n t e g r a t e d w i t h that of the g r a v i t y program s i n c e they were i n t i m a t e l y r e l a t e d and s i n c e the g r e a t e r wealth of g r a v i t y data allowed the s e i s m i c r e s u l t s to be expanded. Equipment f o r t h i s program was purchased under o p e r a t i o n a l p r o j e c t s 11.3, 11.4, 11.5 and 11.6 and personnel provided under p r o j e c t 11.2. The data r e d u c t i o n program was c l o s e l y i n t e g r a t e d w i t h the program f o r r e d u c t i o n of g r a v i t y measurements from a n t a r c t i c t r a v e r s e s . 2. Operations. Seismic observations from the t r a v e r s e s a t Byrd and E l l s w o r t h s t a t i o n s i n the 1956-57 program, and from the mam IGY t r a v e r s e program ( F i g . 2 ) have been reduced, c o r r e l a t e d w i t h g r a v i t y , magnetic, and a l t i m e t r y observations, and forwarded to the IGY World Data Centers (see p r o j e c t 11.2 for d e s c r i p t i o n of the t r a v e r s e ) . 3. Personnel. T h i s p r o j e c t was under the d i r e c t i o n of George P. Woollard. Trav e r s e personnel on t h i s p r o j e c t were. John Behrendt, Hugh Bennett, C h a r l e s Bentley, E. Bradley, Feng-Keng Chang, A.P. Crar y , L y l e McGinnis, Ned Ostenso, Perry Parks, Edwin Robinson, James Sparkman, E.G. T h i e l , and T. Tuncotta. 4. Data. The data t a b u l a t i o n s from the e n t i r e group of t r a v e r s e s were deposited i n the IGY World Data Ce n t e r s . 5. R e s u l t s . Seven major oversnow t r a v e r s e s , supplemented by s e v e r a l s h o r t e r t r i p s and one airborne t r a v e r s e , covered approximately 12,000 km of t r a c k i n A n t a r c t i c a . The primary s e i s m i c measurements a t l o c a t i o n s about 30 mi l e s apart on each t r a v e r s e were i c e t h i c k n e s s and depth of water beneath the i c e for those s t a t i o n s on i c e s h e l v e s . The t r a v e l time of the compressional echo served to l o c a t e the rock s u r f a c e , m u l t i p l e t r a v e l paths or shear waves were used on the i c e she l v e s to determine t h i c k n e s s of f l o a t i n g i c e . I n a d d i t i o n , many short r e f r a c t i o n p r o f i l e s were shot to give d e t a i l e d determinations of v e l o c i t y v a r i a t i o n s m n e a r - s u r f a c e snow and i c e . Long r e f r a c t i o n p r o f i l e s were shot to obtain information on s e i s m i c v e l o c i t i e s throughout the ic e c a p and i n the underlying rock, using both v e r t i c a l l y and h o r i z o n t a l l y o r i e n t e d geophones to record a l l p o s s i b l e phases. A l o w - v e l o c i t y i c e l a y e r was discovered a t the base of the icecap i n Mane Byrd Land. Uncertainty about the t h i c k n e s s (or even the e x i s t e n c e , i n many p l a c e s ) of t h i s l a y e r and i t s wave-propagation c h a r a c t e r i s t i c s diminishes the accuracy w i t h which echo time can be converted i n t o i c e - t h i c k n e s s v a l u e s ; the maximum e r r o r i s estimated to be 40 m. The e r r o r i n r e l a t i v e t h i c k n e s s between two neighboring s t a t i o n s would be con- s i d e r a b l y l e s s than t h i s but i s d i f f i c u l t to reckon, owing to l a c k of information concerning the v a r i a b i l i t y of the b a s a l l a y e r from place to p l a c e . 469

SEISMOLOGY lOO'W 80 'W I30*W 1 7 0 ' E 1 6 0 ' E I I B E L L I N G S H A U S E N F I L C H N E R I C E • S H E L F ELLSWORTH ST A R O S S ICE S H E L F CMURDO STA I UNITED S T A T E S T R A V E R S E ROUTES — OVERSNOW T R A V E R S E R O U T E S • AIRBORNE T R A V E R S E STATIONS S C A L E IN M I L E S 30-W 1 4 0 ' E 130 'E I 2 0 * E I I O ' E l O O ' E F i g u r e 2. T r a v e r s e routes i n West A n t a r c t i c a . 470

SEISMOLOGY eo'w 70 'W 1 7 0 ' E h l 6 0 * E h B E L L I N G S H A U S E N F I L C H N E R S H E L F SLLSWORTH I C E S H E L F XMURDO I 3 0 * E 120'e I I O ' E l O O ' E W E S T A N T A R C T I C A I C E S U R F A C E E L E V A T I O N S CONTOUR INTERVAL 2 0 0 meters S C A L E IN K I L O M E T E R S 20*W F i g u r e 3. I c e s u r f a c e e l e v a t i o n s i n West A n t a r c t i c a . 471

SEISMOLOGY 100* w 90*W 60*W 1 7 0 ' E I 4 0 * E I 3 0 ' E I Z O ' E I I O ' E l O O ^ E ELLSWORTH ST A W E S T A N T A R C T I C A ROCK S U R F A C E E L E V A T I O N S CONTOUR INTERVAL 5 0 0 M E T E R S (-750 meters contour shown under ice shelves only) S C A L E IN K I L O M E T E R S 100 o 2 0 0 4 0 0 F i g u r e 4. Rock s u r f a c e e l e v a t i o n s i n West A n t a r c t i c a . 472

SEISMOLOGY Surface heights u s u a l l y were c a l c u l a t e d by the summation of e l e v a t i o n i n t e r v a l s measured between t r a v e r s e v e h i c l e s as they t r a v e l e d s e v e r a l k i l o m e t e r s a p a r t . The normal c o r r e c t i o n s f o r temperature and humidity were a p p l i e d , and a l s o a c o r r e c t i o n for pressure g r a d i e n t between v e h i c l e s , as estimated from the wind v e l o c i t y two meters above the s u r f a c e . The estimated maximum e r r o r of e l e v a t i o n determination a t a l l t r a v e r s e s t a t i o n s i s 20 m r e l a t i v e to the base s t a t i o n . T h i s e r r o r decreases w i t h dec r e a s i n g d i s t a n c e between the f i e l d s t a t i o n and the base s t a t i o n . Another source of e r r o r i n Mane Byrd Land e l e v a t i o n s i s u n c e r t a i n t y i n the e l e v a t i o n of the base s t a t i o n , Byrd S t a t i o n . T h i s u n c e r t a i n t y i s estimated by the U.S. Weather Bureau to be 15 m. a. I c e - S u r f a c e Topography. The i c e - s u r f a c e contour map of a p o r t i o n of West A n t a r c t i c a i s shown m Fi g u r e 3. b. S u b g l a c i a l Topography. As the f i r s t step i n the c o n s t r u c t i o n of a contour map of the s u b g l a c i a l rock, topography, s e i s m i c r e f l e c t i o n , g r a v i t y , and a l t i m e t r y r e s u l t s were combined so that c r o s s s e c t i o n s of' the icecap along the v a r i o u s t r a v e r s e routes could be drawn. L i n e s of mean depth were then estimated by eye i n such a way as roughly to average out f e a t u r e s l e s s than 80 km i n extent. Ice-covered areas ex- h i b i t i n g a rock s u r f a c e below sea l e v e l are hachured; the boundary of t h i s zone was estimated where no d i r e c t evidence was a v a i l a b l e . I n c o n s i d e r i n g the rock e l e v a t i o n beneath the t h i c k , grounded i c e of Marie Byrd Land, i t should be remembered that the weight of the o v e r l y i n g i c e has depressed the land s u r f a c e on the order of 500 meters (up to 1000 meters i n the deepest part of the c e n t r a l b a s i n ) . Thus, sea l e v e l before the growth of the i c e i n Western A n t a r c t i c a should be f a i r l y w e l l represented by the 500-meter contour. The r o c k - s u r f a c e contour map for a po r t i o n of West A n t a r c t i c a i s shown m Fi g u r e 4. Long r e f r a c t i o n shooting has provided the wave v e l o c i t i e s m the rock beneath the i c e a t s e v e r a l p l a c e s . A value of 5.2 km/sec was found i n the f o o t h i l l s of the S e n t i - n e l Mountains and on the e a s t e r n edge of the b a s i n , whereas a s i g n i f i c a n t l y lower v e l o c i t y of 4.3 km/sec was recorded a t Byrd and L i t t l e America S t a t i o n s , both of which are w i t h i n the channel. S e v e r a l l i n e s of evidence a re i n agreement i n i n d i c a t i n g t h a t the channel between Byrd S t a t i o n and the S e n t i n e l Mountains marks the d i v i d i n g l i n e between geologic provinces, s e p a r a t i n g the v o l c a n i c mountains on the north from the folded, metamorphic mountains of the S e n t i n e l Group to the e a s t and the sedimentary, b l o c k - f a u l t e d H o r l i c k Group to the south. The F i l c h n e r I c e S h e l f extends south f o r 650 km from the i c e f r o n t and has an ar e a of approximately 300,000 square km ( e x c l u s i v e of the large i s l a n d ) as compared with 540,000 square km for the Ross I c e S h e l f . T h i s d i s c o v e r y , together w i t h that of the deep bedrock channel beneath Marie Byrd Land, has d i s c l o s e d that West A n t a r c t i c a con- s i s t s , i n f a c t , of a grea t southward extension of the Palmer Peninsula together w i t h a mountainous i s l a n d , or more probably, a s e r i e s of i s l a n d s , comprising the c o a s t a l ranges of Mane Byrd Land. A trough w i t h depths g r e a t e r than 1000 m below sea l e v e l can be seen to extend from the edge of the F i l c h n e r I c e S h e l f , near E l l s w o r t h S t a t i o n , to the mountains a t l o n g i - tude 85°W. A trough i s a l s o shown along the e n t i r e western and southern boundary of the Ross I c e S h e l f . I n between, there i s no c l e a r evidence of a s i m i l a r decrease i n rock e l e v a t i o n toward the H o r l i c k Mountains, although the rock s u r f a c e i s w e l l below sea l e v e l a l l along the mountain f r o n t . A tr o u g h - l i k e f e a t u r e i s shown from the c e n t r a l H o r l i c k Mountains to 82°S, 95°W, but t h i s could as e a s i l y be a t t r i b u t e d to the e x i s t e n c e of the sm a l l mountain group to the northwest as to any trough a s s o c i a t e d w i t h the H o r l i c k F r o n t . From the topographic information i t i s not p o s s i b l e a t present to determine whether the S e n t i n e l and H o r l i c k Mountains are joi n e d by a continuous mountain c h a i n , although i t i s c l e a r that there i s no broad connection between the Ross and Weddell. 473

SEISMOLOGY The g e o l o g i c a l evidence tends to support the hypothesis of a topographic low ex- p r e s s i n g the break between the folded mountains of the Palmer Peninsula type and the b l o c k - f a u l t e d mountains of the A n t a r c t i c Horst. However, even i f t h i s f e a t u r e e x i s t s , i t would be i n no way comparable to the major channel between the Ross and B e l l i n g - hausen Seas. c. Summary. The f o l l o w i n g are the major con c l u s i o n s reached concerning the s t r u c t u r e of West A n t a r c t i c a . ( i ) A major channel below sea l e v e l between the Ross Sea and the B e l l i n g - hausen Sea e x i s t s beneath the i c e of West A n t a r c t i c a . T h i s connection i s deep enough to have e x i s t e d before the land s u r f a c e was depressed by the weight of the o v e r l y i n g i c e c a p . Between Byrd S t a t i o n and the S e n t i n e l Mountains there i s a deep b a s i n w i t h i n the channel i n which a maximum depth g r e a t e r than 2500 m below sea l e v e l i s found. ( i i ) The combination of magnetic, ge o l o g i c , and s e i s m i c evidence leads to the c o n c l u s i o n that the channel re p r e s e n t s a fundamental d i v i s i o n between geologic pro- v i n c e s , s e p a r a t i n g the v o l c a n i c mountains on the north from the folded, metamorphic S e n t i n e l Mountains to the e a s t and the sedimentary, b l o c k - f a u l t e d H o r l i c k Mountains to the south. ( i i i ) The F i l c h n e r I c e S h e l f i s much g r e a t e r i n area and extends much f a r t h e r to the southwest than had p r e v i o u s l y been r e a l i z e d . T h i s d i s c o v e r y , together w i t h that of the channel i n Mane Byrd Land, has shown that the rock s u r f a c e of the major p a r t of West A n t a r c t i c a i s below sea l e v e l . ( i v ) The Palmer Peninsula s t r u c t u r e s extend a t l e a s t as f a r south as the 83rd p a r a l l e l and, although they do not i n t e r s e c t the A n t a r c t i c Horst i n the v i c i n i t y of 84°S, 80° to 90°W, there i s no broad connection below sea l e v e l between the Ross and Weddell Seas. (v) A deep trough extends i n l a n d f o r s e v e r a l hundred kil o m e t e r s beneath the e a s t e r n area of the F i l c h n e r I c e S h e l f . A trough i s a l s o found under the western and southern boundaries of the Ross I c e S h e l f . These troughs are connected by a narrow topographic low, expressing the break between the folded mountains of the Palmer Penin- s u l a e x t e n s i o n and the A n t a r c t i c Horst. ( v i ) F r e e - a i r g r a v i t y anomalies show West A n t a r c t i c a to be i n approximate i s o l a t e d e q u i l i b r i u m . ( v i i ) The c r u s t of West A n t a r c t i c a i s c o n t i n e n t a l i n c h a r a c t e r , but the Mohorovicic d i s c o n t i n u i t y has the r e l a t i v e l y high e l e v a t i o n ( e x c l u s i v e of the moun- tainous a r e a s ) of about 30 km below sea l e v e l . ( v i i i ) The Mohorovicic d i s c o n t i n u i t y deepens a t l e a s t to -36 km, forming a continuous trough beneath the S e n t i n e l , H o r l i c k , and Queen Maud Mountains and i n d i c a t i n g t h e i r g e n e r a l topographic c o n t i n u i t y w i t h the Palmer Pe n i n s u l a . ( i x ) The t h i n n e s t c r u s t a l s e c t i o n s are found beneath the Ross and F i l c h n e r I c e Shelves, but the e l e v a t i o n of the Mohorovicic d i s c o n t i n u i t y i n these areas i s not g r e a t l y d i f f e r e n t from that beneath the large channel i n Marie Byrd Land. (x) From the c o n f i g u r a t i o n of the i c e and rock surf a c e i t i s concluded t h a t the i c e sheet i n West A n t a r c t i c a o r i g i n a t e d as two separate icecaps i n the two mountainous a r e a s , one i n the v i c i n i t y of the E x e c u t i v e Committee Range, the other between the H o r l i c k and S e n t i n e l Mountains. As the caps expanded, they converged over the open water between and were probably i n i t i a l l y j o i n e d by a f l o a t i n g i c e s h e l f which then grew t h i c k enough to f i l l the trough completely and produce the present single-grounded i c e sheet. 474

SEISMOLOGY 6. Bibliography. a. Papers Presented a t Meetings. C.R. Bentley, L.D. McGinnis, E.S. Robinson: "A Study of Seismic V e l o c i t i e s and Related Physical Properties of the A n t a r c t i c Ice Cap." Paper presented at the A n t a r c t i c Symposium of Buenos A i r e s , Argentina, November 1959. b. Published Papers. J.C. Behrendf "Geophysical and G l a c i o l o g i c a l Studies m the Filchner Ice Shelf of A n t a r c t i c a . " J. Geophys. Res., v o l . 67, pp. 221-23, 1962. C.R. Bentley, A.P. Crary, N.A. Ostenso, E.G. T h i e l . "Structure of West A n t a r c t i c a . " Science, v o l . 131, no. 3394, pp. 131-36, 1960. C.R. Bentley, N.A. Ostenso. " G l a c i a l and Subglacial Topography of West A n t a r c t i c a . " J. G l a c i o l . , v o l . 3, pp. 882-911, 1961. C.R. Bentley: " G l a c i a l and Subglacial Geography of A n t a r c t i c a . " A n t a r c t i c Res., Geophys. Monogr. No. 7, AGU, Washington, D.C., pp. 11-25, 1962. C.R. Bentley "The Land Beneath the Ice." I n press. Feng-Keng Chang. "Seismic Wave Studies i n Northwest Mane Byrd Land, A n t a r c t i c a . " B u l l . Seismol. Soc. Amer., v o l . 54, no. 1, pp. 51-56, 1964. H.A. Neuburg, E.G. T h i e l , P.T. Walker, J.C. Behrendt, N.B. Aughenbaugh. "The Filchner Ice Shelf." Ann. Assoc. Am. Geographers, v o l . 49, pp. 110-19, 1959. N.A. Ostenso and C.R. Bentley: "The Problem of Elevation Control i n Antarc- t i c a . " IGY Glac. Rept. Series No. 2. Amer. Geogr. Soc, Sect. IV, pp. 1-26, 1959. E.S. Robinson: "Geophysical I n v e s t i g a t i o n s m McMurdo Sound, A n t a r c t i c a . " J. Geophys. Res., v o l . 68, no. 1, pp. 257-62, 1963. E.G. T h i e l , N.A. Ostenso, H.F. Bennett, E.S. Robinson, J.C. Behrendt: "IGY A n t a r c t i c Oversnow Traverse Program, 1957-1958." IGY Glac. Rept. Series No. 1 (American Geogr. S o c ) , 1958. E.G. T h i e l , C.R. Bentley, N.A. Ostenso, J.C. Behrendt- "Oversnow Traverse Programs Byrd and Ell s w o r t h Stations, A n t a r c t i c a , 1957-1958, Seismology, Gravity and Magnetism." IGY Glac. Rept. Series No. 2, Am. Geogr. Soc, 1959. E.G. T h i e l and N.A. Ostenso "Seismic Studies on A n t a r c t i c Ice Shelves." Geophysics, v o l . 26, no. 6, pp. 706-15, 1961. E.G. T h i e l and N.A. Ostenso: "The Contact of the Ross Ice Shelf w i t h the Continental Ice Shelf." J. G l a c i o l . . v o l . 3, no. 29, pp. 823-32, 1961. E.G. T h i e l "The Amount of Ice on Planet Earth. A n t a r c t i c Research . " Geophys. Monogr. No. 7, AGU, pp. 172-75, 1962. G.P. Woollard: "Crustal Structure i n A n t a r c t i c : A n t a r c t i c Research." Geophys. Monogr. No. 7. AGU, pp. 53-73, 1962. 475

SEISMOLOGY G.P. Woollard: "The Land i n the A n t a r c t i c . " Scient. American, v o l . 207, no. 3, pp. 151-66, 1962. G,P. Woollard. "The Geology of A n t a r c t i c a . " I n press. 476

:rn SEISMOLOGY Project 11.8 - Sea Expl o r a t i o n , A t l a n t i c Ocean 1. Ob.iectives. Under t h i s p r o j e c t a t Lamont Geological Observatory, seismic r e f r a c t i o n and r e f l e c t i o n measurements were made on a traverse along the eastern and weste^ basins of the A t l a n t i c Ocean, i n coordination w i t h vessels from the Woods Hole Oceano graphic I n s t a l l a t i o n and the National I n s t i t u t i o n of Oceanography (England) and the Argentine Navy. I n a d d i t i o n , a seismometer was designed f o r emplacement and recording on the ocean f l o o r (where the s i g n a l to noise r a t i o was expected to be g r e a t l y enhanced). 2. Operations. The Lamont Geological Observatory, working i n part w i t h the collabora- t i o n of the Woods Hole Oceanographic I n s t i t u t i o n and m part w i t h research vessels made ava i l a b l e through the IGY National Committee f o r B r a z i l , Argentina and the Republic of South A f r i c a , p a r t i c i p a t e d i n an extensive series of seismic r e f r a c t i o n measurements. The areas studied were the Caribbean Sea between Central America and the Greater A n t i l l e s of the West In d i e s , the eastern c o n t i n e n t a l border of South America from Buenos A i r e s , Argentina, to Cape Horn, the Scotian Sea area between the Falkland Islands and the Sandwich Islands, the Cape of Good Hope area, and the Gulf of Aden, Red Sea and Mediterranean Sea. I n a l l , 199 seismic r e f r a c t i o n s t a t i o n s were established. Of these, approximately o n e - f i f t h y i e l d i n formation on the thickness of the c r u s t . 3. Personnel. This p r o j e c t was under the d i r e c t i o n of Maurice Ewing. Others asso- ciat e d w i t h t h i s p r o j e c t were: A l b e r t Stockelj Walter Beckman, Lyle Bivens; Catherine Z i n a l ; Stanley Hunter, George Lehstern, Vincent P. Coyle, S.V. Shelminski; Howard Wehner; Louise Gerorgi; Bernard Campolli. Many a d d i t i o n a l s c i e n t i s t s of Lamont Geological Observatory and the cooperating i n s t i t u t i o n s were associated w i t h t h i s p r o j e c t or the r e l a t e d p r o j e c t s 9.3, 9.4, and 9.11. 4. Data. Data from the Western Caribbeans and Gulf of Mexico were presented m a published paper (see Ewing, Antoine and Ewmg, 1960). 5. Results. a. Caribbean Sea: Although most of the measurements i n the Caribbean Sea were made p r i o r to the IGY, the r e s u l t s are of p a r t i c u l a r i n t e r e s t . Geographically and i n terms of depth of water, i t has not been clear whether such areas should be regarded as part of the continent or of the oceans, or whether they represent a t r a n s i t i o n phase between the two. The outstanding points Ijrought out by a p r o f i l e section across the area from Cuba to Venezuela are: ( i ) The c r u s t a l thickness does not always conform to depth of water. ( i i ) The i n t e r n a l s t r u c t u r e of the cr u s t i s complex both s t r u c t u r a l l y and i n terms of number of layers present. ( i l l ) Both c o n t i n e n t a l type s t r u c t u r e and that associated w i t h oceanic conditions are present. ( i v ) As obtained i n the Peru-Chile trench, the B a r t l e t t Tfmtgh does not appear to involve the cr u s t as a whole, although the c o n f i g u r a t i o n of the upper surface of the crust and the o v e r l y i n g layers does conform to that of the surface. The geological s i g n i f i c a n c e of the intermediate v e l o c i t y values m the range 4 km/sec to 5.5 km/sec observed here i s not too c l e a r . Because of the geographic l o c a t i o n and values observed m surface limestones, these values could be assigned to carbonate sedimentary rocks. The values are also found associated w i t h volcanic rocks and c e r t a i n l y anything over 5 km/sec can be associated w i t h c r y s t a l l i n e g r a n i t i c rock at shallow depths. 477

SEISMOLOGY b. Continental Border Area of East South America: Approximately 70 measurements were made along the Argentine coast, mostly w i t h i n the c o n t i n e n t a l shelf area. Although none of the measurements give the t o t a l thickness of the c r u s t , a considerable amount of geologic information was obtained. I n general, the section on the c o n t i n e n t a l shelf i s characterized by a layer of bottom sediments having a v e l o c i t y of 1.6 to 2.4 km/sec and a thickness which averages approximately 2 km. However, beneath the c o n t i n e n t a l slope t h i s layer thickens to over 5 km; beyond the slope area the thickness i s only about 2 km. I n places, p a r t i c u l a r l y south of Bahia Grande and i n deep water beyond the c o n t i n e n t a l slope, the bottom sediments can be divided i n t o an upper member w i t h a v e l o c i t y of 1.6 to 2.0 km/sec and a lower member w i t h a v e l o c i t y of 2,1 to 2.4 km/sec. Beneath the bottom sediments a layer w i t h a v e l o c i t y of 2.5 to 3.8 km/sec i s found i n many places. I t i s not, however, a continuous feature and varies from zero to 2 km i n thickness. I n many areas t h i s layer i s replaced by one having a v e l o c i t y i n the range 4.2 to 4.9 km/sec and i n a few areas both layers are present. As w i t h the 2.5 to 3.8 km/sec laye r , the higher speed layer i s not continuous, and i t varie s i n thickness from zero to 5 km. Both of these layers probably represent consolidated sediments. Beneath the above sediments c r y s t a l l i n e rock i s encountered which has a v e l o c i t y that varies from 5.0 to 6.1 km/sec. I n some areas an underlying high speed horizon w i t h a v e l o c i t y of 5.9 to 6.4 km/sec i s also observed a t depths of the order of 5 to 7 km. On a section perpendicular to the coast from Comodoro Rivadavia, the v e l o c i t y of t h i s intermediate v e l o c i t y layer i s found to increase to 7 km/sec beneath the c o n t i - nental slope where i t reaches a maximum depth of 10 km, and then decreases i n v e l o c i t y again as i t r i s e s toward the surface beyond the slope. The oceanic s e c t i o n observed a t the end of t h i s traverse shows 2 km of bottom sediments represented by a s u r f i c i a l layer 0.3 km t h i c k of 1.8 km/sec m a t e r i a l and 1.5 km of m a t e r i a l of 2.3 km/sec which l i e d i r e c t l y on c r y s t a l l i n e rock m a t e r i a l w i t h a v e l o c i t y of 5.6 km/sec. Beneath t h i s v e l o c i t y horizon, which i s only about 1.5 km t h i c k , an intermediate v e l o c i t y layer w i t h a v e l o c i t y of 6.6 km/sec i s observed which extend down t o a depth of about 13 km where the Mohorovicic D i s c o n t i n u i t y i s encountered w i t h a v e l o c i t y of 8.0 km/sec. c. Scotian Sea-Falkland Islands Area Two traverses were established m the Scotian Sea-Falkland Islands ( I s l a s Malvinas) area. One extends westward to South America and the other d i r e c t l y south to about 57°S l a t i t u d e . W i t h i n the South American c o n t i n e n t a l shelf area, a 6 km section of sediments i s found which i s composed mostly of m a t e r i a l having a v e l o c i t y of 4.4 t o 4.9 km/sec. This layer r a p i d l y t h i n s eastward and i s replaced by sediments w i t h a v e l o c i t y of 2.9 km/sec i n the Argentine Basin, but i s again encountered w i t h a thickness of 2 km as the Falkland Islands are approached. South of the Falkland Islands the section crosses a 100 fathom bank about 100 km wide and 330 km long. The very marked change i n geologic s t r u c t u r e associated w i t h t h i s bank poses an i n t e r e s t i n g problem as to i t s o r i g i n . The s t r u c t u r e i s defined p r i n c i p a l l y by the consolidated sediment layer w i t h a v e l o c i t y of 4.5 km/sec which thickens abruptly from about 1 km on the North and zero thickness on the South to 8 km beneath the bank. The lower v e l o c i t y bottom sediments f i r s t thicken on the flanks from 0.5 km to about 3 km and then t h i n to about 0.5 km over the center of the bank. The underlying c r u s t a l rock surface characterized by a v e l o c i t y of 6.1 km/sec, although q u i t e i r r e g u l a r i n general, i s depressed about 5 km beneath the bank t o give an i n v e r t e d p i c t u r e of the surface topography. South of the bank a normal oceanic section i s ob- served although the depth of water i s about 3 km. The s u r f i c i a l sediments are less than 0.5 km i n thickness and l i e d i r e c t l y on c r u s t a l m a t e r i a l w i t h a v e l o c i t y of 6.3 km/sec which p e r s i s t s to a depth of 11 km where the Mohorovicic D i s c o n t i n u i t y i s found w i t h a v e l o c i t y of 8.3 km/sec. This p a t t e r n i s s i m i l a r to t h a t observed m the P a c i f i c Ocean rather than t h a t found i n the A t l a n t i c Ocean. 478

SEISMOLOGY d. South Georgia Area- The measurements i n t h i s area consist of a traverse ex- tending from about 48° to 57°S l a t i t u d e , which crosses the i s l a n d arc and i t s associated foredeep. Although the t o t a l thickness of the crust was not mapped, i t i s found t h a t the sedimentary layers and the underlying c r u s t a l layer are conformable to the surface r e l i e f and t h a t each of the sedimentary layers thickens as the depth of water increases. The p a t t e r n whereby the high v e l o c i t y c r u s t a l rocks are a t a higher e l e v a t i o n beneath the i s l a n d arc than beneath the trench agrees w i t h that observed i n the West Indian Arc area. V e l o c i t y horizons observed i n the Scotian Trench are 2.0 km/sec, 4.0 km/sec, and 5.6 km/sec. South of South Georgia, the c r u s t a l layer i s found to have a layered s t r u c t u r e w i t h the lower member having a v e l o c i t y of 7.6 km/sec, A s i m i l a r high v e l o c i t y basal c r u s t a l layer i s observed i n the Caribbean area. e. South A f r i c a n Area: 12 s t a t i o n s were occupied near the southern coast of A f r i c a by the VEMA of Lament and VRYSTAAT of the South A f r i c a n Navy. Tentative v e l o c i t i e s have been obtained f o r these s t a t i o n s . A t y p i c a l mantle v e l o c i t y has been t e n t a t i v e l y i d e n t i f i e d f o r s t a t i o n No. 161 (east of Durban). I t i s hoped th a t the f i n a l r e s u l t s from the s t a t i o n s near Durban can be r e l a t e d to the p r o f i l e obtained inl a n d from Durban by Hales of the Bernard Price I n s t i t u t e i n Johannesburg. f . Gulf of Aden-Indian Ocean Area I n t h i s area none of the measurements are believed to have been c a r r i e d out f a r enough to give a measure of t o t a l c r u s t a l t h i c k - ness although v e l o c i t i e s on reverse p r o f i l e s as high as 7.8 km/sec were recorded. At a l l s i t e s two sedimentary layers having v e l o c i t i e s of 1.85/sec and 3.9 to 4.6 km/sec are recognized. The aggregate thickness of the two varies from 2 to about 4 km. Beneath these sedimentary rock layers c r u s t a l m a t e r i a l w i t h a v e l o c i t y of 6.4 to 6.8 km/sec i s found. The very high v e l o c i t y m a t e r i a l 7.6 to 7.8 km/sec was observed a t only two st a t i o n s and the depth of t h i s i n t e r f a c e ranges from 9 to 11 km. I n view of the shallow depth of water, 1310 to 2670 meters, these values cannot be reconciled w i t h the thickness of c r u s t to be expected or w i t h the g r a v i t y anomalies i n the area, which suggest t h a t the base of the crust should be about -17 km. The occurrence of such high v e l o c i t y m a t e r i a l w i t h i n the c r u s t a l layer i s s i g n i f i c a n t i n t h a t i n other areas such v e l o c i t i e s have been i d e n t i f i e d w i t h the mantle rock beneath the c r u s t . g. Red Sea Area; I n t h i s area a l o c a l g r a v i t y and magnetic 'high' i s i d e n t i f i e d w i t h a submarine trench extending over most of the length of the Red Sea. Although the t o t a l thickness of the c r u s t was not measured, the seismic values do give an i n d i - c a t i o n of the cause of the anomalies i n the p o t e n t i a l f i e l d . I n the bordering shelf area the thickness of sediments varies between 2 to 6 km except i n one area near St. John's Island where the t o t a l thickness i s 10 km. The underlying c r u s t i s observed to have a v e l o c i t y of 5.9 km/sec. W i t h i n the range of measurement, no layer of greater v e l o c i t y was detected. I n the c e n t r a l trench area, the sedimentary rock thickness i s the same as the shelf area (2 to 6 km) but the underlying c r u s t a l m a t e r i a l i s charac- t e r i z e d by a high v e l o c i t y of 7.1 km/sec. As the magnetic and g r a v i t y anomalies are missing i n the graven area of the Gulf of Suez and Aqaba, i t appears t h a t the 7.1 km/sec m a t e r i a l found i n the Red Sea trench i s a basic rock i n t r u s i v e t h a t has invaded a c r u s t a l f r a c t u r e system over t h i s p o r t i o n of the graven. h. Mediterranean Area- Seismic measurements i n the Mediterranean Sea, Red Sea, and Gulf of Aden were made by the VEMA of Lamont and the ATLANTIS of Woods Hole. 15 seismic s t a t i o n s were run i n the Mediterranean. V e l o c i t i e s and thicknesses can be found i n the papers l i s t e d i n the b i b l i o g r a p h y . j. Other Work: I n a d d i t i o n to the seismic work discussed above, a sub-bottom depth recorder was developed f o r the study of sediment layers m shallow water. Several t r i a l p r o f i l e s were run i n Long Isla n d Sound and also i n Beagle Channel, Argentina. I t was demnnstrated t h a t a s u f f i c i e n t l y intense source could be used to give q u i t e s a t i s f a c t o r y r e f l e c t i o n s from sub-bottom struc t u r e s to modest depths. 479

SEISMOLOGY 6. Bibliography. a. Papers Presented a t Meetings. Charles L. Drake, Ronald W. G i r d l e r , and Mark Landisman: "Geophysical Measurements i n the Red Sea." (1959) I n t ' l . Oceanographic Congress, Amer. Assoc. Adv. Sci., p. 21. John Ewing and Maurice Ewing: "Seismic Refraction Measurements i n the Scotia Sea and South Sandwich Isl a n d Arc." (1959) I n t l . Oceanographic Congress, Amer. Assoc Adv. S c i , , p. 22. John E. Nafe, John F. Hennion, and George Peter: "Geophysical Measurements i n the Gulf of Aden." (1959) I n t ' l . Oceanographic Congress, Amer. Assoc Adv. S c i . , p. 42. b. Published Papers. J. Ewing, J. Antoine, and M. Ewing. "Geophysical Measurements i n the Western Caribbean Sea and i n the Gulf of Mexico." (1960) J. Geo. R.. 65, pp. 4087-4126. M. Ewing, W.J. Ludwig, and J . I Ewing: "Geophysical I n v e s t i g a t i o n s i n the Submerged Argentine Coastal P l a i n . " (1963) Geol. Soc. Amer., v o l . 74, pp. 275-291. Charles L. Drake and Ronald W. G i r d l e r . "A Geophysical Study o f the Red Sea." I n press. 480

SEISMOLOGY Project 11.9 - Seismic Sea Exploration of the Southeast P a c i f i c 1. Objectives. This p r o j e c t a t the Scripps I n s t i t u t i o n of Oceanography provided f o r two-ship seismic r e f r a c t i o n studies of areas of geological i n t e r e s t . The measuring program had as i t s objectives the e x p l o r a t i o n of the s t r u c t u r e of the earth's c r u s t down t o the Mohorovicic D i s c o n t i n u i t y . Because t h i s region has been l a r g e l y unexplored by modern geophysical methods, one of the paramount questions has been: are there any extensive regions whose crusts d i f f e r from the ' t y p i c a l ' P a c i f i c c r u s t a l s tructure? One such possible region i s the elevated area c a l l e d the Easter Isla n d r i s e on which i n d i c a t i o n s of anomalous s t r u c t u r e were found on the CAPRICORN Expedition. 2. Operations. The program of the Scripps I n s t i t u t i o n of Oceanography was concentra- ted i n the South P a c i f i c area between French Oceania and the coast of South America. Areas of p a r t i c u l a r i n t e r e s t studied were the Tuamotu Archipelago p l a t f o r m , the East P a c i f i c Rise and the Peru-Chile Trench. I n a l l , 39 seismic r e f r e a c t i o n s t a t i o n s were established. With the exception of f i v e of these, which were shot f o r the study o f the sedimentary column only, a l l were long-range measurements f o r the determination of c r u s t a l thickness. A n a r r a t i v e account and charts showing the l o c a t i o n of these mea- surements are given i n IGY General Report No. 2, June 26, 1958, World Data Center A, U.S. National Academy of Sciences, Washington, D.C. Scripps research vessels S.F. BAIRD and HORIZON p a r t i c i p a t e d i n t h i s expedition; explosives were furnished through the generous cooperation of ONR. 3. Personnel. R.W. R a i t t was p r o j e c t d i r e c t o r ; p r i n c i p a l s c i e n t i s t s associated w i t h Dr. R a i t t were G. Shor, V.H.W. Menard, Robert Fischer, R. von Herzen, and J.N. Nanda, Chief S c i e n t i s t , Navy of the Republic of I n d i a . 4. Results. I n general, the oceanic measurements corroborated e a r l i e r seismic studies i n the North and Central P a c i f i c . As a r u l e the sedimentary layer i s only a few hundred meters t h i c k except i n the e q u a t o r i a l barbonate area where the thickness i s about two times the normal oceanic value. There i s perhaps more v a r i a t i o n i n o v e r a l l c r u s t a l thickness than observed previously, and although the thickness of the basic c r u s t a l layer having a v e l o c i t y of about 6.7 km/sec i s f a i r l y constant, many s t a t i o n s showed an intervening intermediate v e l o c i t y layer which averages about 1 km i n thickness and has a v e l o c i t y varying from 4 to 6 km/sec. Unusual r e s u l t s were obtained i n four areas. a. I n a deepwater erabayment i n the Tuamotu Island p l a t f o r m , normal oceanic s t r u c t u r e i s found although the surrounding p l a t f o r m i s a shallow water area. b. Just east of the Easter I s l a n d Rise, the c r u s t a l layer appears to be only about 4 km t h i c k . Conditions are also d i f f e r e n t i n t h a t the thickness of the upper c r u s t a l l a y e r , about 2 km, equals t h a t of the lower lay e r , which has a v e l o c i t y of about 6.7 km/sec. c. Under the Nasca Ridge, beneath 2900 m of water, the cru s t appears t o be about 15 km t h i c k . d. At four locations on the Easter I s l a n d Rise, despite unusually long p r o f i l e s , no v e l o c i t y greater than 7.5 km/sec i s observed. This i s also an area where the heat flow i s observed to be about f i v e times the normal value. As other parts of the Easter I s l a n d Rise appear to be characterized by normal c r u s t a l conditions and normal heat flow, t h i s c o r r e l a t i o n s t r o n g l y suggests t h a t the mantle v e l o c i t y may be a f f e c t e d by abnormal thermal conditions. 481

SEISMOLOGY The measurements i n Peru-Chile Trench area are of p a r t i c u l a r i n t e r e s t . Off Antofagasta, C h i l e , the l i n e of measurements p a r a l l e l t o the axis of the trench and located about h a l f way between the trench axis and the top of the c o n t i n e n t a l slope shows that the base of the cru s t i s a t about -23 km. Along the axis of the trench the base of the cru s t i s a t about -18 km and west of the axis of the trench the mantle i s a t a normal depth of about -11 km. This suggests a normal p a t t e r n of thickening as the continent i s approached w i t h no observable superimposed e f f e c t r e l a t e d to the trench. On the series of measurements conducted a t s i m i l a r locations o f f Callao, Peru, although the oceanic and trench axis measurements corroborate those to the south, i t was not possible to o b t a i n a measurement on the c o n t i n e n t a l side of the trench. How- ever, measurements were not c a r r i e d beyond 160 km, and i t i s probable t h a t t h i s length of l i n e was not s u f f i c i e n t to ob t a i n f i r s t a r r i v a l s from the mantle f o r the thickness of crust present. 6. Bibliography. R.W. R a l t t : "Marine Seismic Studies." Trans. AGU. v o l . 41, no. 2, pp. 269-71, 1960. R.L. Fisher, R.M. Norijis: "Bathymetry and Geology of Sala y Gomez, Southwest P a c i f i c . " B u l l . Geol. Soc. Amer.. v o l . 71, no. 4, pp. 497-502, A p r i l 1960. H.W. Menard: "Geology of the P a c i f i c Sea Floor." Experimentla, v o l . 15, no. 6, pp. 205-44, 1959. H.W. Menard: "The East P a c i f i c Rise." Science, v o l . 132, no. 3441, pp. 1737-46, Dec. 9, 1960. 482

SEISMOLOGY P r o j e c t 11.10 - C o n t i n e n t a l Seismic E x p l o r a t i o n I 1 Ob.iectives. T h i s p r o j e c t of the U n i v e r s i t y of Wisconsin provided for s e i s m i c r e f r a c t i o n e x p l o r a t i o n s of the c r u s t a l s t r u c t u r e m regions of p a r t i c u l a r , i n t e r e s t , to attempt to shed l i g h t on the d i s p a r i t y of r e s u l t s between , c r u s t a l s t r u c t u r e s i n f e r r e d from previous s e i s m i c e x p l o r a t i o n s and s t u d i e s of g r a v i t y j a n o m a l i e s , r e c e n t work on the study of c r u s t a l s t r u c t u r e by observing the earthquake s u r f a c e s e i s m i c waves a t t h i s time did not lend unequivocal support to e i t h e r the g r a v i t y or s e i s m i c method. 2. Operations. The c o n t i n e n t a l measurements of c r u s t a l s t r u c t u r e , based on explo- s i v e b l a s t s were c a r r i e d out i n the c e n t r a l s h i e l d a r e a of exposed c r y s t a l l i n e basement rocks and Precambrian sediments i n the Great Lake region of the United S t a t e s , the buried s h i e l d area near the head of the M i s s i s s i p p i Embayment i n the southern United S t a t e s and the high p l a t e a u of c e n t r a l Mexico. Seismic measurements were conducted by the U n i v e r s i t y j o f Wisconsin i n Mexico w i t h the help and c o l l a b o r a t i o n of s c i e n t i s t s of the U n i v e r s i t y of Mexico and the Mexican IGY Committee and government, s e i s m i c measurements m the Great Lakes region were con- ducted through the kind agreement of the Canadian government. The U n i v e r s i t y of Wisconsin group recorded e s s e n t i a l l y i n l i n e along a s i n g l e azimuth l a i d out so as to minimize p o s s i b l e changes i n s t r u c t u r e , as i n d i c a t e d by the g r a v i t y anomaly p a t t e r n and the s u r f i c i a l geology. D e t a i l e d measurements were made a t c l o s e ranges m order to determine the e f f e c t of the s u r f i c i a l geology. S c i e n t i s t s of the U n i v e r s i t y of Wisconsin f i r e d t h e i r own charges m the Great Lakes area and u t i l i z e d quarry and mining operation b l a s t s m the other areas s t u d i e d . Measurements were c a r r i e d out beyond 300 km m order to o b t a i n f i r s t a r r i v a l s from the underlying mantle rock. Because of the great d i s t a n c e s involved the i n s t r u m e n t a l requirements are much more c r i t i c a l than i n marine c r u s t a j l measurements, and the r e - turn i n terms of number of measurements i s much more limi^ted f o r the amount of time and e f f o r t expended. Other f a c t o r s , such as land use, property r i g h t s , background s e i s m i c n o i s e , disturbance of i s o l a t e d seismometer i n s t a l l a t i o n s , the need for many recording s i t e s and s e t s of equipment, e t c . , make c o n t i n e n t a l c r u s t a l measurements a r a t h e r complicated undertaking r e q u i r i n g much planning as w e l l a's time. 3. Personnel. George P. Woollard was p r o j e c t d i r e c t o r , other s c i e n t i s t s a s s o c i a t e d with t h i s p r o j e c t were. Kenneth Bay, S.M. Burns, F o r r e s t Dowlmg, Walter Holthaus, Richard Haubrich, Michael McGrath, Robert P. Meyer, Paul Moors, Perry Parks, Robert Pooley, John C. Rose, Rudolf S c h a f f t e r , Davis Schlabach, David Schonke, Wayne SonnCag, James Sparkman, and John S t e i n h a r t . 4. R e s u l t s . I I a. United S t a t e s - G r e a t Lakes Region I n t h i s a r e a two p r o f i l e s were made approxi- mately a t r i g h t angles to each other. The U.S. Coast Guard c o l l a b o r a t e d on t h i s program by making a s h i p a v a i l a b l e so t h a t e x p l o s i v e s could be f i r e d under water, t h i s not only m a t e r i a l l y reduced the amount of e x p l o s i v e s required b u t j a l s o gave much b e t t e r energy coupling than could be obtained by f i r i n g on land. Charges of 750 l b s . of nitramon could be detected at d i s t a n c e s g r e a t e r than 275 km, no cliarges g r e a t e r than 1500 l b s . were f i r e d . b. Keweenaw Pe n i n s u l a r P r o f i l e : T h i s p r o f i l e i n northern Maine extends from Keweenaw Point i n t o western Wisconsin and i s a l i g n e d w i t h the geologic s t r i k e and the p a t t e r n of the g r a v i t y anomalies. The s t r u c t u r e found was approximately 1.5 km of m a t e r i a l having a v e l o c i t y of 4.7 km/sec o v e r l y i n g m a t e r i a l of 6.3 km/sec down to -36 km where the mantle was found w i t h a v e l o c i t y of 8.05 km/sec. No intermediate l a y e r i n g 483

SEISMOLOGY was detected. The surface rocks are a l l c r y s t a l l i n e , and f o r the most p a r t b a s a l t w i t h some gabbro and g r a n i t i c s c h i s t and gneiss. The mean Bouguer g r a v i t y anomaly i s +15 mgal, which might be i n t e r p r e t e d as evidence of a thinner-than-normal c r u s t . The source of t h i s anomaly, however, appears to be r e l a t e d p r i n c i p a l l y to the s u r f i c i a l rocks of basic composition. The cr u s t may also make a c o n t r i b u t i o n here as the velo- c i t y of 6.3 km/sec i s somewhat higher than the 6.1 t o 6.2 km/sec values u s u a l l y associated w i t h the c r u s t . c. Apostle Is l a n d P r o f i l e - This p r o f i l e s t r i k e s a t r i g h t angle to the Keweenaw p r o f i l e from the Apostle Islands i n Lake Superior to south-central Wisconsin. Geologically t h i s p r o f i l e i s oriented across a major s t r u c t u r a l dome composed almost e n t i r e l y a t the surface of g r a n i t i c c r y s t a l l i n e rocks, w i t h Precambrlan sedimentary c l a s t i c rocks a t the n o r t h (shot) end. The g r a v i t y anomaly p a t t e r n shows a pronounced minimum associated w i t h the area and along the p r o f i l e there I s a negative gradient toward the south. The mean anomaly along the p r o f i l e i s -40 mgals. The seismic s e c t i o n shows about 2 km of m a t e r i a l having a v e l o c i t y of 3.5 km/sec which i s probably r e l a t e d to the Precambrlan sediments; 3 km of m a t e r i a l having a v e l o c i t y of 5.2 km/sec w i t h the underlying cr u s t down to the mantle a t -37 km having a v e l o c i t y of 6.5 km/sec. The mantle v e l o c i t y was 7.9 km/sec. I t was not possible t o reverse e i t h e r of the above p r o f i l e s . d. Arkansas P r o f i l e : This p r o f i l e i s based on quarry b l a s t s i n the v i c i n i t y of L i t t l e Rock and extends over to the Cape Girardeau area of Missouri on the M i s s i s s i p p i River. The e n t i r e area i s characterized by s l i g h t l y p o s i t i v e Bouguer g r a v i t y anomalies and I t has been noted f o r many years that earthquake wave a r r i v a l s i n the area were e a r l i e r than would be predicted from normal c o n t i n e n t a l t r a v e l - t i m e tables. Although the shot point was located i n an area of c r y s t a l l i n e s y e n i t l c rocks, most of the p r o f i l e extended over an area of increasing sedimentary section composed of Paleozoic, Cretaceous and l a t e r m a t e r i a l . The seismic r e s u l t s were 2 km of 4.65 km/sec m a t e r i a l ; 10 km of 5.5 km/sec m a t e r i a l w i t h the underlying c r u s t having a v e l o c i t y of 6.9 km/sec down to a depth of -43 km where the mantle was encountered w i t h a v e l o c i t y of 8.15 km/sec. On the basis of magnetic and w e l l data, i t i s clear t h a t the 5.15 km/sec m a t e r i a l c o n s t i - tutes an upper layer i n the crust rather than carbonate m a t e r i a l such as i s observed a t the surface to the n o r t h of the p r o f i l e . This i s the deepest c r u s t a l s e c t i o n reported so f a r i n the United States and i s s i g n i f i c a n t i n that i t occurs i n an area of low e l e v a t i o n rather than beneath a mountainous or plateau region where a t h i c k c r u s t would be expected. The implied high density of around 3.0 gm/cc from the v e l o c i t y of 6.9 km/sec explains the lack of an appreciable negative g r a v i t y anomaly. The high c r u s t a l v e l o c i t y also explains the e a r l y earthquake a r r i v a l times i n the area. e. Plateau of Mexico P r o f i l e . This p r o f i l e i s based on open p i t mining b l a s t s near Durango, Mexico. The p r o f i l e extends southward from t h i s shot po i n t p a r a l l e l i n g the Sierra Madre Range and the g r a v i t y anomaly p a t t e r n . Near-surface d e t a i l was ob- tained by f i r i n g a reverse spread over the f i r s t 20 km of the p r o f i l e . As the mean anomaly value i s about -200 mgal, a t h i c k c r u s t a l section was a n t i c i p a t e d . The surface geology i s l a r g e l y volcanic t u f f s but there i s reason t o believe i t may, i n p a r t , be underlain by Jurassic or Cretaceous limestones which outcrop f u r t h e r south. The seismic r e s u l t s i n d i c a t e a surface layer having a v e l o c i t y of 3.1 km/sec th a t i s about 1 km t h i c k which i s underlain by a layer about 5 km t h i c k having a v e l o c i t y of 5.1 km/sec. Beneath t h i s the v e l o c i t y i s 6.1 km/sec w i t h no i n d i c a t i o n s of any increase i n v e l o c i t y w i t h depth u n t i l the base of the c r u s t i s reached a t a depth of about 40 km. The mantle rock v e l o c i t y i s 8.2 km/sec. A second measurement was attempted using quarry blasts a t a limestone s i t e about 40 miles north of Mexico C i t y , but had to be abandoned because of an excessively high ambient noise l e v e l . The noise l e v e l on the plateau a t Durango was also h i g h and i t i s possible t h a t intermediate lay e r i n g below the 6.1 km/sec might have been missed. Calculations show th a t a minimum of more than 14 km of 7.0 km/sec m a t e r i a l could have 484

SEISMOLOGY been present without having been recorded as a f i r s t a r r i v a l , and the presence of such a l a y e r would have given an o v e r a l l t h i c k n e s s of about 44 km for the c r u s t . A more complete r e p o r t on t h i s study i s given m IGY General Report S e r i e s , No. 3, J u l y 10, 1958, IGY World Data Center-A, National Academy of S c i e n c e s , Washington, D.C. 5. Bibliography. a. Papers Presented a t Meetings. R.P. Meyer, John S t e i n h a r t and G.P. Woollard " P r e l i m i n a r y R e s u l t s of C r u s t a l Studies i n Mexico." Ann. Meeting Soc. Explor. G e o p h y s i c i s t s , San Antonio, Tex., 1958. R.P. Meyer, John S t e i n h a r t , G.P. Woollard and W.E. B o n i n i " P r e l i m i n a r y R e s u l t s of C r u s t a l Studies i n E a s t e r n Montana." F o r t y - f i r s t Ann. Meeting, AGU, Washington, D.C, A p r i l 1960. John S t e i n h a r t and G.P. Woollard " S t a t i s t i c a l Uncertainty and I n t e r p r e t a t i o n Problems m Seismic C r u s t a l Studies w i t h A p p l i c a t i o n s . " F o r t y - f i r s t Ann. Meeting, AGU, Washington, D.C, A p r i l 1960. John S t e i n h a r t and G.P. Woollard " L i n e a r A n a l y s i s of the R e s u l t s of E x p l o s i o n Seismic Measurements on the Continents." Forty-second Ann. Meeting, AGU, Washington, D.C, A p r i l 1961. b. Published Papers. G.P. Woollard " C r u s t a l S t r u c t u r e from G r a v i t y and Seismic Measurements." J . Geophys. Res., v o l . 64, no. 10, pp. 1521-44, 1959. G.P. Woollard "Current Developments i n S e i s m o l o g i c a l Research." Trans. AGU, v o l . 41, no. 2, pp. 155-157, 1960. John S t e i n h a r t and R.P. Meyer* "E x p l o s i o n Studies of C o n t i n e n t a l S t r u c t u r e . " Dept of T e r r . Magn., Carnegie I n s t , of Wash., Publ. 622, 409 pp., 1961. 485

SEISMOLOGY Project 11.11 - Seismic Exploration of Continental Structure i n South America 1. Objectives. The use of explosion seismology by many groups, i n c l u d i n g the Depart- ment of T e r r e s t r i a l Magnetism, Carnegie I n s t i t u t i o n of Washington, who undertook t h i s Andes p r o j e c t , has made i t possible to obtain precise experimental data and make many explorations of c r u s t a l s t r u c t u r e . The r e s u l t s have shown th a t the c r u s t i s very com- plex I n s t r u c t u r e . I n eastern North America, c r u s t a l thicknesses of 30 and 40 km underlie surface structures of 0 to 800 m a l t i t u d e , r e s p e c t i v e l y . The s t r u c t u r e under the southern part of the Colorado Plateau of Arizona and New Mexico was explored f o r the purpose of obtaining b e t t e r measurements. There the mean topographic height i s 2000 m and the c r u s t a l depth about 32 km. I f these differences i n c r u s t a l thickness are t r u l y associated w i t h the d i f f e r e n c e I n topographic height, then the density d i f f e r e n c e between the lower c r u s t a l m a t e r i a l and the outer mantle i n some regions must be less than 0.5 gms/cm , perhaps as low as 0.2 gms/cm^. A corroborative study made near Great Salt Lake had roughly the same r e s u l t s . I n 1955 an e x p l o r a t i o n of Alaska and the Yukon T e r r i t o r y gave r e s u l t s which showed c r u s t a l thicknesses of 35 km underlying topographic heights of 1000 to 1500 m. Gravity measurements i n the Alaskan region have revealed some g r a v i t y anomalies which can be associated w i t h the seismic measurements and others which have not been so associated a t present. From these r e s u l t s i t can be seen t h a t the r e l a t i o n s h i p between topographic height and c r u s t a l depth i s not f i x e d . These few r e s u l t s i n d i c a t e t h a t the d i f f e r e n t regions I n a continent may have d i f f e r e n t density d i s t r i b u t i o n s I n t h e i r l o c a l crust and upper mantle s t r u c t u r e s . The Andes Mountains o f f e r e d a l o c a t i o n f o r making measurements I n extremely high mountains and neighboring low lands. There are extensive mining operations I n the Andes and along the west coast of South America. Measurements of ground motion caused by these explosions p a r a l l e l to and across the s t r i k e of these mountains was expected to y i e l d information on c r u s t a l depths under r a p i d l y varying topographic heights. Results such as these, added to previous explorations and to those being planned i n Europe, were expected to provide a much b e t t e r p i c t u r e of the true s t r u c t u r e of a con- t i n e n t than was i n existence before the IGY. 2. Operations. Seismic measurements I n South America were conducted by the Department of T e r r e s t r i a l Magnetism of the Carnegie I n s t i t u t i o n of Washington i n c o l l a b o r a t i o n w i t h s c i e n t i s t s from the Un i v e r s i t y of Chile and San Marcos U n i v e r s i t y i n Peru, and w i t h the assistance and cooperation of the IGY Committees and governments of Peru, C h i l e , and B o l i v i a . The Carnegie Andes Expedition f o r the IGY undertook a reconnaissance of the high regions of Peru, B o l i v i a and northern Chile to see I f i t i s possible to measure the roots of the Andes and determine t h e i r character by measuring the waves from the ex- plosions which are used i n the great open p i t copper mines I n Peru and Chi l e . I n several of these mines single shots of 40 t o 60 tons of dynamite were exploded almost once a day. By observing the compresslonal waves which t r a v e l i n the c r u s t , and also the f a s t e r waves which t r a v e l below the M-discontinuity i n the mantle of the e a r t h , both along l i n e s p a r a l l e l to the mountain ranges and across and beneath the mountains of the a l t l p l a n o , the Carnegie s c i e n t i s t s hoped to f i n d some clues to the character of the underlying s t r u c t u r e s which hold a l o f t these towering thousands of cubic miles of heavy rock. P r i o r to the s t a r t of the expedition, which covered the period August-November 1957, considerable time was spent i n the development of t r a n s i s t o r i z e d portable seismic r e - corders to be used i n places inaccessible to the trucks containing standard vacuum tube seismic recorders. I n the f i e l d the t r a n s i s t o r i z e d recorders produced records comparable to those received on the large vacuum tube systems. They were very convenient to use, because of t h e i r p o r t a b i l i t y , as a u x i l i a r y systems and were operated by the second man i n each truck. On approaching a s i t e area, one man w i t h a portable set would be dropped o f f and the other man would continue on w i t h the t r u c k t o another s i t e , thereby o b t a i n i n g 486

SEISMOLOGY two records r a t h e r than one. A l l the trucks and equipment went by ocean f r e i g h t from Baltimore t o Callao, Peru. From t h i s p o i n t the trucks were d r i v e n t o and throughout the areas of recording i n Peru, B o l i v i a , and C h i l e . Six trucks and e i g h t men l e f t Washington i n J u l y 1957 and assembled i n Lima on August 14 to begin the Andes expedition. During the f o l l o w i n g three months, the Carnegie observers made measurements and recordings a t more than 200 c a r e f u l l y selected locations deep i n the mountains of Peru, B o l i v i a , and C h i l e , using r a d i o timing techniques f o r observing the waves from nearly 60 large explosions during these months at the Toquepala mine of the Southern Peru Copper Corp., about 60 miles northwest of Tacna and a t the Chuquicamata Mine of the Chile Exploration Company, about 125 miles northeast of Antofagasta. Both of these mines are a t elevations above 3000 meters. 3. Personnel. M.A. Tuve was p r o j e c t d i r e c t o r and H.E. T a t e l was the p r i n c i p a l i n v e s t i g a t o r . Other s c i e n t i s t s who p a r t i c i p a t e d i n the expedition were L.T, A l d r i c h , F. R. Boyd, B.F. Burke, E.T. Ecklund, J. F i r o r , P.A. Johnson, P.A, Scherer, and G. W. W e t h e r i l l . 4. Results. The wave measurements showed th a t the Andean st r u c t u r e s are h i g h l y non- imiform, and furthermore demonstrated without question that 50-ton explosions are not large enough t o o u t l i n e the s t r u c t u r e s deep under the mountains by observations along l i n e s across the mountain ranges. Measurements of a c t u a l depths t o the M-discontinulty, by observing strong r e f l e c t e d waves, were successful only i n l i m i t e d sectors along the mountains and more or less p a r a l l e l t o them. Suitable observations along l i n e s across the mountains and under the bulk of the a l t i p l a n o were not obtained, i n s p i t e of weeks of i n t ensive e f f o r t and s t r u g g l e . The waves i n these d i r e c t i o n s were attenuated t o an extreme degree, a s i t u a t i o n never encountered i n the work i n North America. This ex- treme a t t e n u a t i o n may have some connection w i t h the volcanic s t r u c t u r e s i n the Andes; I t may i n d i c a t e t h a t the sharp i n i t i a l wave pulse i s converted t o secondary waves a t the countless i n t e r f a c e s w i t h i n the bulk of the mountain s t r u c t u r e s . Many successive wave conversions then reduce the i n i t i a l pulse to a long wave t r a i n of very low amplitude, which does not show on the instruments above the general noise l e v e l . A l t e r n a t i v e l y , i t may i n d i c a t e t h a t the wave f r o n t s dive steeply under the mountains, g i v i n g a "shadow zone." By sustained and intensive search, l o c a t i o n s were found f o r the Instruments which were very q u i e t by comparison w i t h anything previously used i n North America; i n many places the instruments would have detected v e r t i c a l wave motions of less than one Angstrom u n i t (roughly h a l f of a hundred m i l l i o n t h of an inch) i n amplitude. No waves were seen, f o r example, i n B o l i v i a a t 250-450 km from the explosion s i t e s , where the amplitude of a normal t o t a l r e f l e c t i o n would have been predicted t o reach as much as The r e f l e c t i o n s observed along the sides of the main bulk of the a l t i p l a n o , roughly p a r a l l e l i n g the mountains on the other hand, reached the expected value of 500 to 80Q& u n i t s i n amplitude, i n both Peru and C h i l e , as would have been predicted from work i n the Rocky Mountains and Alaska. a. Toquepala, Peru P r o f i l e s : These p r o f i l e s were based on mining operation b l a s t s at the Toquepala Copper Mine about 60 miles northwest of Tacna. Recording was c a r r i e d out along 10 azimuths on p r o f i l e s of varying lengths. F i r s t a r r i v a l s from the base of the c r u s t were obtained i n the f l a n k area of the a l t i p l a n o along the p r o f i l e s p a r a l l e l i n g the s t r u c t u r a l g r a i n of the mountains. The apparent v e l o c i t i e s are: 5.3 km/sec; 6.2 km/sec; a suggestion t h a t there i s a 6.7 km/sec layer; and 8.2 km/sec. On t h i s basis the computed thickness o f the c r u s t i s about 46 km. I f second a r r i v a l s are used i n the a l t i p l a n o proper, a depth of the order of 65 km i s suggested beneath the high plateau. Figure 5 shows the l i n e s along which observations were made i n Peru, B o l i v i a , and C h i l e . Figure 6 shows the r e s u l t s of Toquepala shots. b. Chuquicamata, Chile P r o f i l e s : This area, as the former one, l i e s on the high Andean a l t i p l a n o , and the shot s i t e , a mining operation, l i e s about 125 miles northeast of Antofagasta. As on the Toquepala p r o f i l e , i t was not possible to obtain f i r s t 487

SEISMOLOGY a r r i v a l s from the base of the c r u s t i n the a l t i p l a n o although f i r s t a r r i v a l s were obtained to the south and west over the f l a n k area. The v e l o c i t y s t r u c t u r e observed was about 5.5 km/sec f o r the surface m a t e r i a l , 6.0 km/sec to possibly 6.4 km/sec w i t h a d e f i n i t e i n d i c a t i o n by f i r s t a r r i v a l s of a 7.0 km/sec basal layer o v e r l y i n g the mantle w i t h a v e l o c i t y of about 8.0 km/sec. On the basis of t h i s s t r u c t u r e , an o v e r a l l t h i c k - ness of about 56 km was computed f o r the c r u s t . I f second a r r i v a l s are used to compute the c r u s t a l thickness beneath the a l t i p l a n o , a thickness of 70 km i s found. Without f i r s t a r r i v a l s , however, t h i s value as w e l l as t h a t i n the a l t i p l a n o of Peru cannot be said to be w e l l substantiated. Figure 7 shows the r e s u l t s obtained using the Chuquica- mata shots. Further observations of explosion waves toward the coast were made by the Geophysical I n s t i t u t e s of the U n i v e r s i t y of Chile and the San Marcos U n i v e r s i t y of Peru, using the Carnegie trucks and equipment, most of which were l e f t i n South America f o r t h i s cooperative extension of the IGY a c t i v i t y . I t i s of i n t e r e s t to p o i n t out t h a t an extensive program of observations on l o c a l and r e g i o n a l earthquakes has been developed i n the years since the IGY, w i t h U n i v e r s i t y groups i n four Andes countries continuing t h e i r c o l l a b o r a t i o n w i t h the Carnegie I n s t i - t u t i o n . (See Figure 8), 5. Bibliography. M.A. Tuve and others, Carnegie I n s t . Wash. Yearbook 1957, Dec. 19, 1958, pp. 106-11. Cuico Marcono TOQUEPALA La Pot ^ B O L I V I A •1 \ y—Y Maria EiMa . CHUOI^MATA ^ \ AntafaaottaVo / / \ / yl ^ - ' A R G E N T I N A ^- - - .^ ' \ OSalta " \ Figure 5- Seismic observations on the waves from explosions were nkiuc: by the Carnegie-IGY Andes Expedition, 1957, a t i n t e r v a l s out to several hun- dred kilometers along l i n e s r a d i a t i n g out from the large open-pit copper mines at Toquepala, Peru, and Chuquicamata, C h i l e , approximately as shown. The high ranges of the Andes l i e j u s t to the east and n o r t h of Arequipa and the two mining centers, and the high plateau extends eastward beyond La Paz. 488

SEISMOLOGY LJ < + 4 _ l (/> + 3 O z o + 2 o UJ + 1 0 EA -1 (/) o - 2 o o 111 - 3 L L J (/) - 4 Toquepala, Peru Sept-Oct 1957 J I H = Altiplano L = Low C = Coast .6 0 km/sec 6 23 km/sec slope -8 km/sec J - 4 0 80 120 160 2 0 0 2 4 0 2 8 0 3 2 0 KILOMETERS Figure 6. A r r i v a l times of explosion waves from the mine a t Toquepala, Peru, show no evidence of any layer of v e l o c i t y intermediate between the cru s t (6 km/sec) and the mantle (8 km/sec), and give only a rough i n d i c a t i o n of the depth to the M d i s c o n t i n - u i t y . (36 km i f no Intermediate l a y e r ) . Waves i n and under the high Andes were too stro n g l y attenuated t o be observed. UJ I -< o z o o UJ tn < UJ o z o o UJ tn + 4 + 3 + 2 + 1 0 -I - 2 - 3 - 4 6 km/sec slope 8 km/sec slope 7 km/sec Chuquicamato, Chile Oct 1957 _L J . 40 80 120 160 2 0 0 2 4 0 K ILOMETERS 280 320 8/58 Figure 7. I f the intermediate layer i n d i c a t e d (7 km/sec) i s a c t u a l l y present, the M d i s c o n t i n u i t y l i e s a t 56-km depth t o the south of Chuquicamata (tabs on points show d i r e c t i o n of wave t r a v e l ) ; f o r the simpler case (6 km/sec crust o v e r l y i n g 8 km/sec mantle) the calculated depth i s nearer 46 km. Again the waves were strongly attenu- ated across the high Andes. 489

SEISMOLOGY r LA YANQUERA QUI SALIN ANTOFSGASIA 5. LEGEND STATION INSTALLED I960 O STATION EXPECTED 1961 A OTHER SEISMIC EQUPMENT OPERATING SCALE IN KILOMETERS 100 0 200 400 75 70 65 LONGITUDF WEST OF GREENWICH Figure 8. Seismology s t a t i o n s i n Argentina, B r a z i l , Chile and Peru. 490

SEISMOLOGY Projects 11.12 & 11.19 - Study of Long-Period Seismic Waves 1. Objectives. This project at the LJimont Geological Observatory provided for the purchase of 10 three-component long-period seismometers, 30 long-period galvanometers, and 10 three-component recording drums, which were installed at observatories throughout the world on a cooperative arrangement with colleagues i n other countries. This provided good global coverage for the recording of the long-period waves, i.e., longer than about ten seconds i n period, i n order to study the propagation phenomena associated with such waves. This kind of seismic data i s useful to deduce properties of the i n t e r i o r of the earth. 2. Operations. The instruments chosen were the Columbia design seismometers with Lehner and G r i f f i t h long-period (90-second free period) galvanometers. The pendulums of the seismometers were adjusted to have a free period of 15 seconds. Cooperating observatories (Project 11.12) were: Lwiro, Congo; Uppsala, Sweden; Santiago, Chile; Honolulu, Hawaii; Hong Kong; Poona, India; Suva, F i j i ; Rio de Janeiro, Brazil; Tsukuba, Japan; and Buenos Aires, Argentina. Project 11.19 covered i n s t a l l a t i o n at Resolute Bay, Canada. This program was coordinated with pre-existing observations at Lamont, the Columbia University Geophysical Field Station at Bermuda; Ottawa, Canada; Waynesburg College, Pennsylvania; Perth, Australia; and Piertermaritzberg, Republic of South Africa. 3. Personnel. Maurice Ewing was project director; Lamont scientists associated with the program included Jack Oliver, Paul Pomeroy, George H. Sutton. 4. Data. Station bulletins were published by Lamont that included a r r i v a l times of identifiable phases, with annotations as appropriate, for those stations where this work was not carried out on a routine basis. 5. Results. This project was largely devoted to data gathering and, although during the course of the operations, events were analyzed and some s c i e n t i f i c discussion could be given here, the reader i s referred to interdisciplinary project 20.7 for a summary of studies carried out at Lamont on the IGY data, which includes data from this project. 6. Bibliography. See project 20.7 for bibliography. 491

SEISMOLOGY Project 11.13 - Lg and Rg Phase Study 1. Objectives. This project at the Lament Geological Observatory provided for two special long-period seismographs to study particular features of Lg and Rg phase waves, which are particular kinds of long-period seismic waves. 2. Operations. Two three-component long-period Columbia seismographs (15-second period) and six Leeds and Northrup galvanometers (8 seconds free period) with appro- priate recording drums, were supplied for i n s t a l l a t i o n at Huancayo, Peru, and Rio de Janeiro, Brazil, i n cooperation with colleagues at seismic observatories at these locations. 3. Personnel. Maurice Ewing was project director; scientists at Lamont associated with this project were J.E. Oliver, Paul Pomeroy, G.H. Sutton. 4. Data. A r r i v a l times from these seismographs were included i n the station bulletins published by Lamont. 5. Results. See project 20.7 for discussion of studies based on data from these instruments. 6. Bibliography. See project 20.7 for bibliography. 492

SEISMOLOGY Project 11.14 - Crustal Strain Accumulation 1. Objectives. This project at the Seismology Laboratory, California I n s t i t u t e of Technology, provided for the construction of two extensometers and their i n s t a l l a t i o n i n South America, i n order to provide information on strain accumulation, long-period seismic waves, and earth t i d a l motion. 2. Operations. In cooperation with the Peruvian IGY Committee and the Huancayo Geophysical I n s t i t u t e ( I n s t i t u t o Geofisico de Huancayo), a tunnel was excavated at Naffa, Peru, near Lima. A similar tunnel was excavated at Santiago, Chile, i n cooperation with the I n s t i t u t e of Geophysics, University of Chile. The tunnels are constructed i n the form of a Y with the two arms mutually perpendicular as shown i n Figure 9 . Each of the arms is approximately 30 m long. The extensometer i t s e l f i s basically a quartz tube, suspended horizontally on wire slings. One end of the tube i s fixed to the earth, the other end moves -- this motion represents the d i f f e r e n t i a l motion of the earth between the fixed end of the tube and a reference mark near the free end. For determination of secular strain increments, the position of an accurately ruled glass scale (0.01 mm ruling) attached to the end of each quartz tube is read by a measuring microscope mounted on a heavy stainless steel pedestal anchored to the earth. A d i f f e r e n t i a l movement of approximately 0.001 mm can be estimated which, considering the 25 meter length of the quartz tube, amounts to 4 x 10"8 least detectable strain increment A battery operated, resonant capacitance bridge is used as the transducer to pro- vide an e l e c t r i c a l signal for recording of long period waves, t i d a l motions, etc. Water seepage was encountered at the Santiago s i t e ; remedial action was taken i n the form of gunnite applied to the walls, which materially reduced the problem but did not entirely eliminate i t . The Ffatia site proved exceedingly dry and afforded one of the quietest sites ever occupied. 3. Personnel. Hugo Benioff was project director; Len Blayney installed the instru- ments; Paul Ledig and Albert Giesecke assisted i n supervision of construction at operation at {^ana, Peru; Cinna Lomnltz assisted i n supervision of construction and operation at Santiago. 4. Data. The records were returned to California I n s t i t u t e of Technology for analysis and are stored there for reference. 5. Results. Owing to rock creep following excavation, concrete aging affecting dimen- sions, and general instrumental s e t t l i n g down, measurements of secular strain accumulation cannot be made with assurance for several years after the i n s t a l l a t i o n of a sensitive extensometer. Preliminary estimates of strain accumulation show i t to be approximately 5 X 10"7 per year. The Peruvian instrument provided valuable and i n some Ways unique observations of the great Chilean earthquake of May 1960, which excited free o s c i l l a t i o n of the earth i n modes up to 38 and provided the f i r s t test of theory that had recently been worked out on the earth free vibration problem. Fortunately, data were also available from other kinds of instruments, for example, earth-tide gravimeters, which are sensitive only to spheroidal oscillations. Since the extensometer responds to torsional as well as to spheroidal oscillations, i t thus proved possible to separate the two phenomena. The analysis of the records and extraction of the various modes was accomplished by power spectral analysis. 493

SEISMOLOGY 30m Figure 9. Extensometer i n s t a l l a t i o n , Nana, Peru. 494

SEISMOLOGY The gravest spheroidal mode shows a s p l i t spectral peak with periods to 54.7 and 53.1 minutes. The theoretical prediction for the Bullen B model according to Alterman, Perkeris, and Jarosch is 53.7 min. The oscillations were observed for a l l modes up to 38 with corresponding periods as short as 3.7 min. For the higher modes, agreement in the observed period was found between the Chilean earthquake and the Kamchatka earthquake of 1952. In almost a l l cases agreement between experimental and theoretical predictions was close. Differences which occurred should make i t possible to d i s c r i - minate between the several earth models which have been proposed. From the width of the spectral peak, values of the dissipation function Q"l- for the earth were determined with an accuracy greater than was previously possible. For the spheriodal mode S3(T o 35.5 min), Q = 380, and for the mode Si8(T = 6.2 min), Q = 170. On the assump- tion that Q is independent of frequency, this implies a higher Q i n the core than i n the mantle. From analysis of phase difference between components of ground motion, a f a u l t length for the Chilean earthquake of about 1000 km and rupture velocities i n the range 3 to 4 km/sec was inferred. 6. Bibliography. a. Papers Presented at Meetings. Hugo Benioff, Frank Press, Stewart Smith: "Excitation of the Free Oscillations of the Earth by Earthquakes." F o r t y - f i r s t Annual Meeting, AGU, May 1960. Frank Press, A r i Ben-Menahem, M. Nafi Toksoz: "Experimental Determination of Earthquake Fault Length and Rupture Velocity." SSA Annual Meeting, San Diego, 1961. b. Published Papers. Hugo Benioff: "Fused-quartz Extensometer for Secular, Tidal, and Seismic Strains." Bull. Geol. Soc. Am.. 70, 1959, pp. 1019-1032. Hugo Benioff, Frank Press, Stewart Smith: "Excitation of the Free Oscillations of the Earth by Earthquakes." J. Geophys. Res., vol. 66, no. 2, Feb. 1961, pp. 605-19. Frank Press, A r i Ben-Menahem, and M. Nafi Toksoz: "Experimental Determination of Earthquake Fault Length and Rupture Velocity." J. Geophys. Res.. 66, 1961, pp. 3471-3485. James N. Brune, Hugo Benioff, and Maurice Ewing: "Long-period Surface Waves from the Chilean Earthquake of May 22, 1960, Recorded on Linear Strain Seismographs." J. Geophys. Res., 66, 1961, pp. 2895-2910. Hugo Benioff: "Source Wave Forms of Three Earthquakes." Bull. Seismol. Soc. Am.. 53, 1965, pp. 893-903. 495

SEISMOLOGY Projects 11.15, 11.18, 11.20 - Antarctic, Arctic and Pacific Seismic Observatories 1. Objectives. These projects, undertaken by the U.S. Coast & Geodetic Survey, were established m order to i n s t a l l seismographs at antarctic stations operated as part of the U.S. program for IGY, as well as at certain locations i n the Arctic and Pacific Oceans so that global coverage could be improved. L i t t l e was known heretofore about the seismicity of the antarctic continent and i t was also considered desirable to be able to record teleseisms from distant earthquakes over the long, essentially unknown propagation paths i n the Southern Hemisphere. 2. Operations. Special piers and instrument housings were designed for Amundsen- Scott (South Pole) and Byrd Stations, Antarctica (project 11.15). Personnel from the USCGS and the National Bureau of Standards, who were primarily occupied on other pro- grams, attended the instruments. Project 11.18 provided for the i n s t a l l a t i o n of Wilson Lamison seismographs at Thule, Greenland, where operation was under the supervision of personnel of other s c i e n t i f i c projects. Project 11.20 provided for intermediate- period instruments at Guam, Koror and Truk. USCGS personnel operated these instruments. Instrumentation at the Six IGY Stations Operated by Coast & Geodetic Survey Station Instrument Characteristics Pendulum Period Galvanometer Period Byrd South Pole Guam Truk Koror Thule Ben-Z W-L, N, E Ben-Z Ben-V-R, N, E W-L, Z Ben-V-R, f i l m Z Ben-V-R, f i l m N, E Ben-V-R, Z HTL-Z 2 Spg N, E W-L, Z Spg, N, E HTL (S-36) Z (disc. 9/20/58) W-L, Z Spg, N, E 1.2 1.1 1.1 1.0, 1.0 1.0 1.0 1.0, 1.0 1.0 0.5 6, 6 .98 4, 4 0.5 -I, 7 0.5 3.5 0.5 17, 18 1.5 0.2 0.2, 0.2 0.2 0.5 7.2, 6 1.7 0.5 1.55 7, 7 Note' Ben-Moving c o i l Benioff, Ben V-R-Variable Reluctance Benioff, W-L Wilson- Lamison, HTL-Houston Technical Laboratory geophone, Spg-Sprengnether 496

SEISMOLOGY Supplies including replacement galvanometers, recorders, chronometers, and ex- pendables such as photo paper, chemicals, etc. were shipped to Antarctica during the summers of 1957 and 1958. Delivery of supplies i n 1957 were made under severe handi- cap with some c r i t i c a l items such as galvanometers never getting through u n t i l the 1958 summer. Considerable d i f f i c u l t y was experimented with the Lehner-Griffith galvanometer; nearly every suspension was broken upon delivery even though special precautions were taken i n preparing the shipments. Because of this the Benioff moving-coil three-component f i l m seismograph recorder was converted to paper recorders. In the western Pacific and Thule, Greenland, supplies presented no particular problem although there were many times when the Pacific stations had to borrow photographic paper from one another to tide over because of slow deliveries. For 1958 the problems of maintaining continuous operations of a l l seismographs were reduced to a minimum. Geophysicists from the Washington office reviewed the seismographs i n operation at Guam, Koror, and Truk during February and March, Personnel from the Thule station v i s i t e d the Washington office for a few days for instruction i n adjusting the instrument types at Thule. In September a geophysicist was sent to Thule for a week to adjust the seismographs and to instruct local personnel i n operational procedure. For the antarctic stations, there was l i t t l e to report u n t i l seismograms were received. Several radio contacts were made during the year and a limited amount of correspondence was conducted. The continuity of the early recordings from Antarctica was far from encouraging, although a good number of usable records were collected. Various sources of a r t i f i - c i a l disturbances including tractor and sno-cat t r a f f i c , interference from radio transmitters, etc. contributed to the extensive loss of records. At Thule, temperature and floods contributed to the loss of some early records, particularly the horizontal. Seismograms were mailed periodically from this station. In the Pacific there was a minimum loss of records at the three stations. At Koror an experienced geophysicist was i n charge of the operations. This afforded an opportunity to experiment with short-period instrumentations on an island i n the Pacific where i t was d e f i n i t e l y d i f f i c u l t to obtain acceptable recordings. Analysis indicated the Benioff variable- reluctance seismograph gave the maximum performance for the detection of short-period waves (.2 - 1.2 sec.). 3. Personnel. E.B. Roberts was project director; the i n s t a l l a t i o n and operation of instrvmients and reduction of data were under the supervision of L.M. Murphy and D.S. Carder. 4. Data. Station bulletins have been published by the USCGS for these stations. 5. Results. Early results have been primarily i n the improvement of earthquake locations m many regions of the world. Telegraphic reports of P phases were received on a monthly average of 150 to 175 earthquakes from South Pole; 65 from Byrd, 60 to 70 from Thule; 15 to 25 east from Guam, Koror, and Truk. The great value of these stations was indicated by the fact that many reports were used to locate earthquakes for the Coast & Geodetic Survey program and correlated with data received from other south-latitude stations. The antarctic stations provided excellent stimuli for delineating earthquake belts i n the southern hemisphere. Several local earthquake recordings were found on the Byrd and Pole seismograms, respectively. The arctic station at Thule provided data on a surprisingly large number of north latitude earth- quakes especially i n the Arctic Circle area. At the western Pacific stations both local and distant earthquakes have been recorded. In addition, excellent records have been obtained from atomic explosions. Further details of studies based on these data w i l l be found under Project 20.6. 6. Bibliography. Seismological B u l l e t i n , IGY Supplement MSI 199A through 216A. 497

SEISMOLOGY Project 11.16 - Long-Period and Lg Phase Data Reduction and Publication 1. Objectives. This project at the Lamont Geological Observatory, Columbia University, provided for personnel to examine the seismograms from the instruments operated under projects 11.12, 11.13 and 11.19, i n order to determine a r r i v a l times of identifiable phases and to publish station bulletins. 2. Operations. Seismograms from the various stations were examined and reduced at the stations as time permitted, but this did not allow at a l l stations the detailed inspec- ti o n of the records for publication of a r r i v a l times of identificable phases and appropriate annotations. Therefore, records were returned to Lamont for further inspec- tion and for preparation of station bulletins for the stations where this work was not carried out on a routine basis. 3. Personnel. Maurice Ewing was project director, the data reduction and publication were under the supervision of J.E. Oliver and G.H. Sutton. 4. Results. Station bulletins for a l l stations that did not publish their results independently were published and supplied to the World Data Centers for further dis- t r i b u t i o n . 498

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