Recent Indian earthquakes

The last decade of the 20th century has seen much progress in comprehending the source processes of Indian earthquakes. It has been possible mainly du e to the installation of digital broadband and short period stations as a n ational network and detailed aftershock studies conducted for several earth quakes of M greater than or equal to6, Investigations of the seismogenic fa ults, including drilling, helium measurements and magneto-telluric surveys over the fault zones for some earthquakes have provided valuable inputs to characterize the scismogenesis of the Indian lithosphere, We discuss the si gnificant results obtained from the seismological, geophysical and geologic al investigations for five recent moderate earthquake sequences (Uttarkashi 1991 and Chamoli 1999 in Garhwal Himalaya; Koyna 1967-1996, Latur 1993 and Jabalpur 1997 in the peninsular shield region). A better understanding of the causative faults and rupture processes involved in generating Indian ea rthquakes is obtained. Occurrence of Garhwal Himalayan earthquakes has been attributed to the significant concentration of stresses around the asperit y/ramp on the detachment plane due to northward under-thrusting of the Indi an lithosphere. The continued seismicity at the Koyna-Warna seismic zone an d its triggering by reservoirs has been explained in terms of southward mig ration of seismicity from Koyna reservoir, high filling rate, duration of l oading, Kaiser effect and nucleation process of moderate earthquakes. Seism ological observations, geological evidences, drilling results and modelling of intra-plate stresses showed that occurrence of stable continental regio n earthquakes like Latur (1993) can be explained in terms of sudden movemen t along the pre-existing faults caused by a coupled force system consisting of topography, density heterogeneity and a NNE compression due to movement of the Indian plate. Further, a highly conductive low-velocity fluid-fille d zone at 7-10 km depth beneath the focal zone of the Latur earthquake sequ ence, as suggested by magneto-tellurics study, low Bouguer gravity and obse rvation of a Pc phase, will enhance stress concentration in the uppermost p art of the crust, resulting in mechanical failure. Nevertheless, the nuclea tion of Jabalpur earthquake (1997) in the lower crust has been attributed t o the sudden movement along the south Narmada fault due to the stress conce ntration around the 'rift pillows', Progress is made in understanding the s ource processes, crustal velocity structure and Q for some important tecton ic regions of the Indian lithosphere and future directions have been identi fied.