Site amplification studies towards seismic microzonation in Jabalpur urban area, central India

Site amplifications at over 100 sites in the frequency range of 0.1-10 Hz were investigated in and around Jabalpur city of central India using the conventional Nakamura technique of computing H/V spectral ratios, to estimate the predominant frequencies and corresponding amplifications at each site. To validate these results, Multichannel Analysis of Surface Waves (MASW) technique was adopted, coupled with refraction measurements over short profiles (∼100 m) resulting in detailed images of shear wave and primary wave velocity structure of the shallow subsurface as well as the depth to basement, which are crucial for estimation of the amplification of seismic signals propagating through the medium. Synthetic seismograms generated based on these detailed structural models using a hybrid method comprising normal mode summation for the regional 1D structure and finite difference approach for site specific 2D structure of the medium are used to compare with site response results from the Nakamura approach. The local site amplifications expressed as the response spectral ratios - the ratio of 2D to 1D ground acceleration response-compare fairly well with the H/V results enabling proper interpretation of the composition and thickness of the upper soil cover and its hazard potential. The most significant amplification values are seen over the alluvial filled region in the north-western part of Jabalpur having soil thickness up to 30-50 m, with amplifications ranging from 4.0 to 6.0, predominantly in the frequency range of 4-5 Hz. Also, the areas covered with Lameta sediments in the west and northeast have thicker soil cover and indicate amplifications twice that of the adjoining Deccan trap areas. The Gondwana sediments are an exception where, in spite of thick soil covers, lower amplification values are observed which are about half of that in the alluvial regions with similar thicknesses. This is probably due to the compactness of the sedimentary formations in the former as compared to the loose, low velocity and low density alluvium in the latter. The study indicates the importance of a multi-disciplinary approach for better understanding the site amplification characteristics in any seismically active region.