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Bulletin of the Seismological Society of America; December 1968; v. 58; no. 6; p. 1879-1897
© 1968 Seismological Society of America
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P-wave travel times from shallow earthquakes recorded in India and inferred upper mantle structure

K. L. KAILA, P. R. REDDY and HARI NARAIN

NATIONAL GEOPHYSICAL RESEARCH INSTITUTE, HYDERABAD-7 (A.P.) India

Abstract

P-wave travel times of 39 shallow earthquakes and three nuclear explosions with epicenters in the North in Himalayas, Tibet, China and USSR as recorded in Indian observatories have been analyzed statistically by the method of weighting observations. The travel times from {Delta} = 2° to 50° can be represented by four straight line segments indicating abrupt velocity changes around 19°, 22° and 33° respectively. The P-wave velocity at the top of the mantle has been found to be 8.31 ± 0.02 km/sec. Inferred upper mantle structure reveals three velocity discontinuities in the upper mantle at depths (below the crust) of 380 ± 20, 580 ± 50 and 1000 ± 120 km with velocities below the discontinuities as 9.47 ± 0.06, 10.15 ± 0.07 and 11.40 ± 0.08 km/sec respectively.

The J-B residuals up to {Delta} = 19° are mostly negative varying from 1 to 10 seconds with a dependence on {Delta} values indicating a different upper mantle velocity in the Himalayan region as compared to that used by Jeffreys-Bullen in their tables (1940). Between 19° to 33° there is a reasonably good agreement between the J-B curve and the observation points. From {Delta} = 33° to 50° the J-B residuals are mostly positive with an average excess value of about 4 sec.




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