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

Abstract

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.