Seismic Microzonation of Urban areas in India - A case study from Jabalpur, Central India (Invited)
Abstract
The Indian subcontinent is seismically one of the most active regions in the world, having experienced four earthquakes of magnitude greater than 8.0 during the last century. This has necessitated detailed Seismic Microzonation approaches for assessment of seismic hazard and risk potential of several highly populated Indian mega-cities that face the wrath of future earthquakes. A multi-disciplinary and multi-Institutional pilot study was launched in the Jabalpur urban area in Central India for a realistic assessment of seismic hazard and the associated risk to the region. Ground amplification at local sites in the frequency range of 0.1 - 10 Hz was obtained at more than 100 sites in and around Jabalpur using the conventional Nakamura technique to estimate the predominant frequencies and the site amplification at those frequencies for each site. A strong correlation is seen between the predominant frequencies and the ground conditions, like the local geology, the composition of the top soil cover, its thickness and impedance contrast with respect to the basement, etc. To validate these results, Multichannel Analysis of Surface Waves (MASW) technique was adopted, coupled with refraction measurements over short profiles (~100m). This has resulted in detailed shear velocity images of the shallow subsurface, which are crucial for estimation of the amplification of seismic signals propagating through the medium. Based on these results synthetic seismograms were generated using a hybrid method comprising normal mode summation for the regional 1D structure and the Finite difference approach for site specific 2D structure of the medium. Local site amplifications expressed as the Response Spectral Ratios (RSR) - the ratio of 2D to 1D ground acceleration response - were estimated. In general, the RSR values correlated well with the composition and thickness of the upper soil cover. The alluvial filled region in the northwestern part of Jabalpur having soil thickness up to 30 - 50m, showed the highest amplification, with RSR ranging from 4.0 to 6.0, predominantly in the frequency range of 4-5 Hz, and can be regarded as the most vulnerable area with reference to a future moderate sized earthquake in this region. 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 RSR 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 lessons learnt from this pilot study of microzonation in Jabalpur call for standardization of approaches to minimize subjectivity and a need to evolve strategies to convolve the site specific responses with the hazard values obtained from other studies in future.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMNS22A..02R
- Keywords:
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- 0935 EXPLORATION GEOPHYSICS / Seismic methods;
- 7212 SEISMOLOGY / Earthquake ground motions and engineering seismology;
- 7230 SEISMOLOGY / Seismicity and tectonics