Estimation of seismicity parameters for India

This paper is motivated by the desire to derive seismic hazard maps for the Indian subcontinent. The most up-to-date earthquake data has been collected to estimate the hazard parameters for quantifying seismicity. An earthquake catalog for the entire country from 1250 B.C.-A.D. 2008 has been compiled from various sources in the literature (Figures 2 and 3). The entire country and its neighboring region is divided into 930 nodes at a spacing of 1° × 1° in the N-S and E-W directions. Seismicity data in 300-km regions around each grid point is assembled to quantify the seismic activity. The earthquake catalogs for each node are first evaluated for their completeness. Based on the completeness test of Stepp (1972), the seismicity data is divided into complete and extreme parts. The maximum likelihood procedure of Kijko and Sellevol (1989), which utilizes the information in both the complete and extreme parts of the catalog, is used for estimating the seismicity parameters (λ, b, Mmax). The parameters are obtained at each node, and the final results are shown in the form of contours (Figures 9, 10, and 11). The obtained b-values for grid points in the Himalayas, northeast India, and the Gujarat regions are lower when compared with peninsular India. The λ-value, and Mmax are lower in PI when compared with rest of the country, indicating moderate seismicity. In this study, the spatial variation of seismicity parameters has been obtained by discretizing the entire region into equally spaced grid points. The derived seismicity parameters are to be interpreted as the characterization of seismic activity around each node. Alternatively, one can derive the spatial variation of seismicity by identifying probable source zones based on the seismotectonic features and association of seismicity with these sources (Gupta 2006). In regions where seismicity is low, such as PI, considerable care is necessary in defining source zones. It should be mentioned here that in the present analysis past seismicity has been used to estimate the seismicity parameters. The use of geological data such as strain rate or the rate of seismic moment release to constrain the seismicity parameters would increase confidence in using the obtained results (Field et al. 1999). However, with the existing limitations such as non-availability of geological data, the results obtained in the present study are the best possible. The results obtained from the present study have several uses in engineering construction and design. These results can be used to rank available construction sites for important projects. The estimated seismicity parameters combined with the fault map (Figure 1) can be used to prepare hazard curves for peak ground acceleration and response spectra at different return periods by PSHA.