ESTIMATION OF GROUND SHAKING HAZARD IN THE WESTERN HIMALAYA USING INTENSITY DATA

Abstract

This study is devoted to the delineation of areas of anomalous residual intensities for seismic hazard estimation using the isoseismals of four earthquakes that occurred in the Western Himalaya. The earthquake studied include the Kashmir earthquake of 1885, Anantnag earthquake of 1967, Badgam earthquake of 1963, and Kashmir earthquake of 2005. For the purpose of computing the epicentral distances macroseismic epicenters of these earthquakes are estimated and used instead of epicentres. For calculating the residual intensities, the model L = A + B A + C log A has been adopted. In this model Ic is the computed intensity and A is the average outer radius for each intensity level and A, B, C are the constants estimated from the data. The difference between the observed intensity and computed intensity is called the residual intensity. The residual intensities for 2005 Kashmir earthquake are found to range from -4 to +4, for 1967 Anantnag earthquake from +0.5 to -2.5, for 1963 Badgam earthquake from -2 to +2 and 1885 Kashmir earthquake from -0.5 to +0.5. The residual intensity maps of these earthquakes have brought out several areas of high and low residual intensities .The regions of positive residual intensities have the potential of ground shaking hazard more than that of the regions with low residual intensities. For 1885 Kashmir earthquake, it seems that the residual intensities do not vary much from that of the reported intensities. However, for 2005 Kashmir earthquake large trends of residual intensities are observed. For 1963 Badgam earthquake, a broad area of positive residual intensity is delineated in the Lesser Himalaya. To the southwest, this area terminates near the Main Boundary Thrust. For 1967 Anantnag earthquake, from the location of the macroseismic epicentre, the trend of isoseismals, and from the trend of residual intensity contours it appears that the rupture associated with this earthquake has primarily propagated towards the northwest from the macroseismic epicenter.