A STUDY OF AFTERSHOCKS OF 20TH OCTOBER 1991 UTTARKASHI EARTHQUAKE

Abstract

Aftershock activity associated with Uttarkashi earthquake of 20th October 1991 has been studied from the local earthquake data available from a six station local network deployed in the Garhwal Lesser Himalaya. From the available local earthquake data a subset of about 430 aftershocks falling in the source region of Uttarkashi earthquake were extracted and located. The hypocenter parameters of aftershocks were estimated employing HYPOCENTER program. The aftershock activity associated with the Uttarkashi earthquake occurred in two zones a shallow zone and a deeper zone. The aftershocks confined to shallow zone occur at depths less than 4 km whereas the deeper zone of aftershock activity falls between 8 and 20 km depth. The space time distribution of the aftershocks reveals that zone of aftershock activity delineated in 1994 migrated 5 to 8 km southwest of the zone delineated in 1991. The aftershock activity also migrated from shallow depths less than 15 km to deeper depths between 15 to 20 km. Triggered activity seems to have occurred in the form of two earthquake swarms in the year 1995 & 1996 due to stress perturbation and redistribution of stresses on account of Uttarkashi earthquake. The gap observed between the two zones of aftershock activity delineated in 1994 was by and large filled by the swarm activity that occurred in 1995 & 1996. No aftershocks are located in the vicinity of the mainshock. The estimated p and b-values of the aftershock sequence of Uttarkashi earthquake is 1.77 and 0.89. iii