ESTIMATION OF SOURCE PARAMETERS OF INDIAN EARTHQUAKES FROM INTENSITY DATA

Abstract

Macroseismic epicenters, physical dimensions and fault orientations of fourteen Indian earthquakes primarily occurred in the Northeast India have been assessed from the available intensity data. The methodology developed by Gasperini et al. (1999) has been adopted for this purpose. The earthquakes studied confined to the moment magnitude (M) range from 5.0 to 8.5 and have occurred from 1918 to 1988. Intensity data of three recent Indian earthquakes, namely, the Jabalpur earthquake of 1997, the Chamoli earthquake of 1999 and the Bhuj earthquake of 2001 was used to validate the adopted methodology. In all these three earthquakes it is found that the estimated macroseismic epicenters lie within 30-40 km from the well constrained reported epicenters. A relation between M, and epicentral intensity has been developed for the purpose of estimating the M from intensity data. Out of the fourteen earthquakes studied, the reported epicenters often earthquakes fall in the different tectonic domains of the Northeast India. The epicenters of two earthquakes fall in the close proximity of Dharbhanga (Bihar) and the epicenters of remaining two earthquakes fall in Sagar Island (West Bengal) and Bengal Basin (Bangladesh). For majority of earthquakes, the estimated parameters from intensity data---macroseismic epicenters, the inferred source dimensions, and directions of rupture propagation, are by and large in agreement with the regional and local tectonic structure. The maximum shift between the estimated macroseismic epicenters and reported epicenters fall between 30-40 km. In twelve out of fourteen studied earthquakes, the inferred directions of faulting agree with the strike direction of faulting reported from fault plane solutions of earthquakes (Seismotectonic Atlas of GSI, 2000) occurred in the close proximity of estimated macroseismic epicenters. The maximum variation in the inferred strike directions and those reported from fault plane solutions lies between 100 and 200. Out often earthquakes occurred in Northeast India, the inferred strike directions of seven, approximately agree with the direction of insitu stresses.