Precursory changes in source parameters for the Koyna-Warna (India) earthquakes

SUMMARY Precursory changes of stress drop and corner frequency are found for five main earthquakes of M w 4.1 to 4.7 that occurred near the Koyna and Warna reservoirs during the period of our close monitoring from 1994 onwards. Earthquakes had started to occur in the area in 1962 soon after the impoundment of the Koyna reservoir and are still continuing. The epicentres are located in a 30-km-long, N‐S seismic zone extending southwards from the Koyna dam. An ew Warna reservoir, situated 25 km south of Koyna, was filled to 60 m in 1992. Most subsequent epicentres are close to the new Warna reservoir. Most M ≥ 4 earthquakes in this region have been associated with foreshocks for 15‐30 days and aftershocks for over a month. At the beginning of various foreshock sequences of M w 1.5 to 2.4, the maximum stress drop values ranged from 0.25 to 0.65 MPa; the corner frequency ranged from 7.0 to 10.0 Hz for all the sequences. The stress drops decreased by over 50 per cent of their maximum value during the precursory period of 4‐17 days prior to main shocks and remained at that low level for a few days after the main shocks. Following an initial increase, the corner frequency decreased by 8t o4 5per cent. The decrease in corner frequency implies a 9‐54 per cent increase in fault length and is inferred to be caused by increased pore pressure as a result of dilatancy.