OCCURRENCE OF LOW-STRESS DROP EARTHQUAKES IN THE GARHWAL HIMALAYA REGION

The digital seismic data recorded by a telemetered seismic array has b een used to compute seismic moments and stress drops of local earthqua kes in the Garhwal Himalaya region based on Brune's model of the earth quake source. The seismic moments for 18 shallow focus earthquakes ran ge from 7 x 10(18) dyn cm to 6.23 x 10(21) dyn cm. These earthquakes a re found to have very low stress drops; less than 1 bar for seven even ts and between 1 bar and 10 bars for another seven events. The maximum stress drop is found to be 38 bars for an earthquake of magnitude 4.2 with focal depth 15 km. The shallow focus events having low stess dro ps in this region indicate that the crust has low strength to withstan d the accumulated strain energy.