Relocation of the Koyna earthquakes with a new velocity model

Abstract

abstractHypocenter locations of 380 earthquakes of magnitude ≧ 3 in the Koyna region, recorded by the Central Water and Power Research Station Network of Seismological Observatories and the India Meteorological Department stations during the period December 1967 to December 1973, are determined after selection of an appropriate crustal model. The model is chosen from among the few available for the region on the basis of comparatively better hypocenter locations in terms of error parameters in the location of 32 events of magnitude ≧ 4 (master events). An epicenter map and hypocentral depth cross-section of the earthquakes in the region are presented. Least-squares fit through the best hypocentral locations along the line perpendicular to the strike of fault plane inferred by Tandon and Chaudhury (1968) gives a dip of 27° in the east-southeast direction and the surface manifestation of the fault 25 km to the west of the Koyna Dam. While the chosen strike direction of the fault is also in agreement with those obtained in a number of fault plane solutions and LANDSAT imagery (Lee and Rayleigh, 1969; Sykes, 1970; Banghar, 1972; Chandra, 1976; Rastogi and Talwani, 1980; Langston, 1981), the observed dip direction of the fault agrees only with the results of deep seismic sounding (Kaila et al., 1979) and the fault plane solution of Rastogi and Talwani (1980). The amount of dip is, however, much less than any of the values reported so far.