Current seismicity at the Koyna Reservoir, Maharashtra, India (1983-1984)

Current seismotectonics in the Koyna reservoir area of the Maharashtra State in India, is described through the analysis of earthquakes in this area in 1983 - 84. From the epicentral map, satellite imagery and geological evidences, a NW-SE trending fault zone is inferred south of Koyna. This fault is in addition to the NE-trending fault passing through the dam which was associated with the main earthquake of magnitude 6.3 and most other earthquakes. The NW-SE fault includes the NW-SE course of the Warna river. As the fault passes through the Warna reservoir and the dam under construction, the seismicity in the region has to be monitored carefully. Hypocenters of tremors during 1983 – 84 in this region were located by means of the HYPO 71 computer program. During 1983 – 84, more then 5000 tremors were recorded. Four earthquakes of magnitude > 4 occurred during this period. Forty shocks were of magnitude 3 to 4. Hypocenters of about 300 tremors were located. The epicenters indicate a broad NW-SE trend south of the reservoir. The depths are mostly within 4 km and not more than about 30 km. The depth sections indicate a nearly vertical dip of the inferred fault.