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A body wave inversion of the Koyna, India, earthquake of December 10, 1967, and some implications for body wave focal mechanisms

Langston, Charles A. (1976) A body wave inversion of the Koyna, India, earthquake of December 10, 1967, and some implications for body wave focal mechanisms. Journal of Geophysical Research, 81 (14). pp. 2517-2529. ISSN 0148-0227. doi:10.1029/JB081i014p02517.

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With a generalized inverse technique, WWSSN (World-Wide Standard Seismograph Network) long-period P and SH wave forms are analyzed from the Koyna earthquake. The effects of local plane-layered earth structure near an imbedded point dislocation source are put in by using a modified plane-wave ray theory which includes the standard reflection and transmission coefficients plus source corrections for radiation pattern and geometrical spreading. The generalized inverse compares synthetic seismograms to the observed ones in the time domain through the use of a correlation function. By using published crustal models of the Koyna region and primarily by modelling the crustal phases P, pP, and sP, the first 25 s of the long-period wave forms is synthesized for 17 stations, and a focal mechanism is obtained for the Koyna earthquake which is significantly different from previous mechanisms. The fault orientation is 67° dip to the east, −29° rake plunging to the northeast, and N16°E strike, all angles being ±6°. This is an eastward dipping, left lateral oblique slip fault which agrees favorably with the trend of fissures in the meizoseismal area. The source time duration is estimated to be 6.5±1.5 s from a triangular time pulse which has a rise time of 2.5 s, a tail-off of 3.9 s, source depth of 4.5±1.5 km, and seismic moment of 3.2±1.4×10^(25) dyn cm. Some short-period complexity in the time function is indicated by modelling shortperiod WWSSN records but is complicated by crustal phases. The long-period P wave forms exhibit complicated behavior due to intense crustal phase interference caused by the shallow source depth and radiation pattern effects. These structure effects can explain much of the apparent multiplicity of the Koyna source. An interpretation of the Koyna dam accelerograms has yielded an S-P time which can be used along with the IMD (Indian Meteorological Department) epicenter and present depth determination to place the epicenter directly on the meizoseismal area.

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Additional Information:© 1976 American Geophysical Union. Received October 6, 1975; revised February 5, 1976; accepted February 9, 1976. I wish to extend my gratitude to Larry Burdick and George Mellman for use of their WWSSN short-period instrument response and for the many discussions along with Don Helmberger on source modelling and inversion theory. I would also like to thank Wayne Thatcher for a critical review of the manuscript. This research was supported by the Advanced Research Projects Agency of the Department of Defense and was monitored by the Air Force Office of Scientific Research under contracts F44620-72-C-0078 and F44620-72-C-0083. This is contribution 2661, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.
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Advanced Research Projects Agency (ARPA)UNSPECIFIED
Air Force Office of Scientific Research (AFOSR)F44620-72-C-0078
Air Force Office of Scientific Research (AFOSR)F44620-72-C-0083
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Caltech Division of Geological and Planetary Sciences2661
Issue or Number:14
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Official Citation:Langston, C. A. (1976), A body wave inversion of the Koyna, India, earthquake of December 10, 1967, and some implications for body wave focal mechanisms, J. Geophys. Res., 81(14), 2517–2529, doi:10.1029/JB081i014p02517
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76505
Deposited By: Tony Diaz
Deposited On:11 Apr 2017 17:24
Last Modified:15 Nov 2021 17:00

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