Earthquake swarm activity in south Gujarat

Abstract

South Gujarat, a part of western coast of Indian Peninsula started experiencing earth tremors of mild intensity since early February 1986. The shocks were widely felt with rumbling sound in these areas. More than 23000 micro earthquakes have since been recorded tilt December 1988, with a major event, ML=4.6 which occurred on.27 April.1986: In view of the location of multi-purpose projects like Ukai, Damanganga, .Jhuj, Kflia etc the monitoring this activity was Immediately started through a network of seven temporary- microearthquake recording stations. This was followed by various other studies such as geodetic, geomagnatic, radon gas monitoring and temperature measurements 9f hot springs. The Unai and Mola-Amba hot springs situated in this area have indicated the temperature of about 57oC and 37°C respectively against the normal atmospheric temperature of 33o C.

The analysis by Hypo- 71 program on IBM computer of India Met. Dep., New Delhi, using a velocity model  Koyna region has shown a well concentrated seismic activity over area of 7x 10 km2 and focal depth of 1-15 km. Clear migration of the activity has been observed. The activity which concentrated around Kella dam m early February-April 1986 migrated up to 18km to its south and back again to the religion around Kelia reservoir, by September 1987 with depth of foci progressively becoming shallower towards north .The 'b" value of 1.04 is higher than that of a few tectonic sequences of Peninsular India. The rate of decay of the activity was 0.52 which is rather slow compared to other sequences of the region. Hence, the reactivation of the existing fracturies/lineaments might be responsible or the recent activity. The geomagnetic studies in this area have corroborated tile existence of  NW-SE to NNE-SSW trending conductive fractures. The earthquake activity during 1988 is quite low compared  to earlier years.