Delineation of Active Faults, Nucleation Process and Pore Pressure Measurements at Koyna (India)

Earthquakes continue to occur in the vicinity of Shivaji Sagar Lake since its creation by the Koyna Dam in 1962. The seismicity peaked in 1967 with a M 6.3 earthquake which claimed over 200 human lives and destroyed the Koyna township. Earthquakes of M S 4 occur every year following an increase of water level in the reservoir. During 1973, 1980 and 1993–94 earthquakes exceeding magnitude 5 occurred. Most earthquakes of M S 4 are associated with pronounced foreshocks and aftershocks. Starting Sepember 1993, seismic monitoring was vastly improved with the deployment of additional close-by stations (analog and digital). The focal parameters now available have enabled delineation of the active faults and deciphering of the earthquake nucleation process. During 1995–96, 13 boreholes were drilled to depths of 130 to 250 m and measurement of water levels in these wells was initiated. A preliminary analysis of one yearʹs data from a borehole 1 km south of Koyna reveals tidal signatures, indicating connection of the well to a confined aquifer which is favorable for detection of pore pressure anomalies induced by crustal strain. We hope to improve our understanding of the genesis of reservoir-induced earthquakes at Koyna with these new measurements.