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 phas
es, 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 befo
re the occurrence of the mainshock. It is found that the fracture nucleates
at shallow depths (<1 km) and gradually deepens to cause the mainshock nea
r the base of the seismogenic layer, that is, at about 8 to 11 km depth. Th
e nucleation of earthquakes at shallow depths may be attributed to the effe
ct of increasing pore pressure caused by the reservoirs of Koyna and Warna
Dams in subhydrostatic conditions, whereas the propagation of fractures tow
ard 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.