Abstract

The Koyna Dam site in western India has been seismically active since its impounding in 1962. The seismicity peaked in 1967 when an earthquake of magnitude 6.3 claimed 200 human lives and caused some damage to the dam itself. Subsequently, bursts of seismicity with M ≧ 5 earthquakes have occurred during 1973, 1980, and 1993 to 1995. The epicenters during the latest burst are concentrated around the newly impounded Warna reservoir, 25 km south of Koyna. Several shocks of smaller magnitude have occurred in this area since 1967 under the influence of the Koyna (Shivajisagar) reservoir, but the impoundment of the Warna reservoir to over 60 m has led to the enhanced seismic activity in the region. The space-time pattern of epicenters located for the period 28 August 1993 to 31 December 1995 clearly delineates two parallel trends in the NNE-SSW direction. The correlation between seismic activity and water levels in both the reservoirs suggests that the present activity was influenced more by the Warna than the Koyna reservoir. The discriminatory characteristics of reservoir-induced seismicity based on b-value, foreshock-aftershock pattern, and decay rate of aftershocks continue to be valid.

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