Prediction of different depth amplifications of deep soil sites for potential scenario earthquakes

Deep soil basin is one of the geographical features which significantly alter the response to earthquakes. Around the world, there are regions where bedrock is at a substantial depth upon which are different layers of soil. Larger depths of soil alter the response toward earthquakes and have been reported in the past. Indo-Gangetic Basin (IGB) of India is one of the seismically vulnerable deep soil basins of the Asian continent. The present paper attempts to study the site amplifications in IGB at the surface and different depths to understand the amplification behavior of the deep soil basins worldwide. Sixteen different probable scenario earthquakes are identified based on past seismic gaps, history and seismic studies and simulated at 270 sites covering whole deep soil region of the IGB. Representative depths of input motion, density, shear wave velocity, location of the water table, suitable shear modulus reduction and damping curves have been used. One-dimensional nonlinear site response analysis was carried out using DEEPSOIL. Peak ground acceleration (PGA), peak spectral acceleration (PSA), amplification factors using the ratio of zero period, peak spectral acceleration, site factors Fa and Fv as per the National Earthquake Hazards Reduction Programme (NEHRP) and spectral accelerations at specific periods of 0.2 and 1 s are calculated and deliberated at the surface and also at different layers up to 100 m depth. Maps for spatial variation in average and maximum values of amplification as well as site factors have been presented. Average values of FPGA, FPSA, Fa and Fv at the surface were found in the range of 1.16–7.94, 1.13–7.93, 1.43–7.89 and 2.11–7.51, respectively. Around 14% of sites in the IGB have amplification values at subsurface levels exceeding those at corresponding surface levels. Amplifications observed at the subsurface level are less than that of the surface for a considerable number of sites.