DEVELOPMENT OF AN ATTENUATION RELATIONSHIP FOR INDIAN REGION

Abstract

An attenuation relationship for peak horizontal ground accelerations for Himalayan region in India has been developed The data base consists of 666 recordings of peak ground accelerations (86 recordings from the Indian data and 580 recordings from western data) from 82 earthquakes(7 earthquakes from Indian data and 75 earthquakes from western data) with magnitude range 5 to 8.1 and the distance from the earthquake to the recording station ranging from as less as 1 kin to as high as 400 km.The least distance to the zone of energy release is used as the distance measure. The maximum of the two horizontal PGA'S is taken to compute the relationship. Site classification was made as Rock sites and Soil sites respectively. The present analysis uses a two step stratified regression model The attenuation relationship proposed is log(A) = 0.05974H- 0.90091og(X + e° 21 "" ) ,where A is the peak ground acceleration (g) ,M is the magnitude and X is the least distance to the zone of energy release . The relation for on average predicts acceleration values for soil sites by a factor 1:1.38 and (predicted to actual) whereas on average it is predicting rock site acceleration by a factor 1:0.96(predicted to actual). The residual sum of squares for equation is 10.35 The relationship is considered to be most applicable for earthquake range between 5.5 and 7.5. In developing the relationship further classifications of fault type and tectonic environment could not be made. The focal mechanisms of Indian earthquakes are not well defined, hence additional features of fault type and tectonic environment are to be incorporated in the relation to increase the dependability on the equation. Though the equation gives good results between magnitude 5.5 and magnitude 7.5 a larger data set is required for accurate prediction of acceleration for all magnitudes.