An improved probabilistic seismic hazard mapping of peninsular shield region of India

This paper attempts to present improved probabilistic seismic hazard maps of the peninsular shield of India based on data driven seismicity and ground motion models. Combination of nine conventional area sources described by Gutenberg and Richter's (G-R) recurrence relationship and a vast tectonic province of southern peninsular shield described by a zone-free kernel smoothing approach has been used independently. Similar to that for the conventional area sources, the expected occurrence rates of earthquakes in different magnitude bins are estimated from the G-R relationships, but distributed non-uniformly among a grid of square cells in accordance with the smoothed epicentral density of the observed past seismicity. On the other hand, the occurrence rates for the tectonic province source are obtained directly by kernel smoothing of the gridded past seismicity. Other distinguishing features of the present study are (a) the use of a very comprehensive and thoroughly scrutinized earthquake catalogue, (b) data driven selection of ground motion prediction equations (GMPEs) using the limited accelerogram records available, (c) estimation of the source-to-site distances by accounting for the effects of the regional strike and dip angles of the fault rupture planes for a perfect compatibility with the distances involved in the selected GMPEs, and (d) permitting the rupture length to extend beyond the zone boundaries in the estimation of the source-to-site distances, which along with the non-uniform distribution of seismicity is able to normalize to some extent the effect of any uncertainty in deciding the zone boundaries. As the geometries of the area sources are decided mainly on the basis of the major tectonic features in the region, the hazard maps obtained are seen to depict the desired conformity with them fairly well. Also, due to the use of the data-driven selection of the highest ranking GMPEs, the hazard maps obtained are expected to be representative of the peninsular region. The rock site uniform hazard spectra for 14 important sites in different parts of the peninsular India as obtained from the presented hazard maps are compared with the corresponding design spectra based on the ASCE/SEI 7-10 and BIS codal provisions, and necessary modifications are proposed in the ASCE provisions in order to get the design spectra specific to the peninsular India. The paper also proposes empirical amplification factors to obtain uniform hazard spectra for a given V-S30 value from the computed rock site uniform hazard spectra.