Probabilistic Seismic Hazard Assessment and Site Characterisation of Southwest India

Abstract

The present study is an attempt to perform region-specific seismic hazards assessment for the southwest part of India. The area of interest belongs to seismic zone III i.e susceptible to moderately sized earthquakes up to magnitude (MW) 6.0. The overall study area includes Goa, a major portion of Karnataka and North Kerala. The closely located epicenters of the past earthquakes along the western coastal stretch intrigued this investigation. The study houses a whole bunch of petrochemical industries and infrastructures of commercial and religious interest, making seismic preparedness inevitable. The local site effects are incorporated into PSHA, thereby, making the outcome of the study applicable to current seismic design practices. A regional seismic catalog spanning over 190 years with a few prehistoric events from the early 16th century has been compiled. The seismic hazard has been computed by for a reference site condition (VS > 1500ms-1). The investigation suspects mining-induced seismicity in Bellary and Raichur districts though there is no mention of this in the prior literature. The local site effect has been captured by performing 1D equivalent linear analysis using SHAKE 2000. The amplification models as a function of input ground motion for ‘sand’, ‘clay’ and ‘other soil’ have been developed for different periods. The ‘sand’ amplifies 33% more than ‘other soil’ and 29% more than the ‘clay’ for lower input acceleration. ‘Sand’ exhibits nonlinear behavior whereas ‘clay’ demonstrates sustained amplifications at longer periods with increasing plasticity index. These amplification models are incorporated into PSHA by transforming the GMPEs. The resulting uniform hazard spectrum (UHS) for all the three soil types was compared with the elastic spectrum of various codes. The codal provision underestimates the spectral values at smaller periods (T<0.5s) and overestimates at higher values. The local soil data was unavailable for the whole of the study region and hence, the digital elevation maps have been used to determine the site topography. The slope calculated from topography is correlated to shear velocity in the top 30m (Vs (30)) and the ground motion parameters are estimated. A maximum of 60% to 80% amplification has been observed in the study area.