Enhanced reservoir-induced earthquakes in Koyna region, India, during 1993-95

Reservoir induced earthquakes began to occur in the vicinity of Shivajisagar Lake formed by Koyna Dam in Maharashtra state, western India, soon after its filling started in 1962. Induced earthquakes have continued to occur for the past 34 years in the vicinity of this reservoir, and so far a total of 10 earthquakes of M greater than or equal to 5.0, over 100 of M greater than or equal to 4 and about 100,000 of M greater than or equal to 0.0 have occurred. Every year, following the rainy season, the water level in the reservoir rises and induced earthquakes occur. Seismic activity during 1967-68 was most intense when globally, the largest reservoir induced earthquake occurred on 10 December, 1967. Other years of intense seismic activity are 1973 and 1980. During 1986 another reservoir, Warna, some 20 km south of Koyna, began to be filled. The recent burst of seismic activity in Koyna-Warna region began in August, 1993, and was monitored with a close network of digital and analog seismographs. During August, 1993-December, 1995, 1,272 shocks of magnitude greater than or equal to 2 were located, including two earthquakes of M 5.0 and M 5.4 on 8 December, 1993 and 1 February, 1994, respectively. Two parallel epicentral trends in NNE-SSW direction, one passing through Koyna and the other through Warna reservoir are delineated. The 1993 increase in seismicity has followed a loading of 44.15 m in Warna reservoir during 11 June, 1993 through August 4, 1993, with a maximum rate of filling being 16 m/week. The larger shocks have been found to be preceded by a precursory nucleation process.