Rupture nucleation of five Koyna mainshocks during 1993 to 1996 (M 4.3 to 5.4) was studied using the space-time patterns of foreshocks within 8 km and 500 hr prior to the mainshock. The nucleation process occurred in two phases, viz., quasi-static and quasi-dynamic prior to the dynamic rupturing of the mainshock. The foreshock nucleation zone is observed to grow at a rate of 0.5 to 10 cm/sec until it finally attains a diameter of about 10 km before the occurrence of the mainshock. It is found that the fracture nucleates at shallow depths (<1 km) and gradually deepens to cause the mainshock near the base of the seismogenic layer, that is, at about 8 to 11 km depth. The nucleation of earthquakes at shallow depths may be attributed to the effect of increasing pore pressure caused by the reservoirs of Koyna and Warna Dams in subhydrostatic conditions, whereas the propagation of fractures toward the base of the seismogenic layer may be controlled by the local stress concentrations along the fault zone and pore-pressure diffusion at greater depths.

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