Probabilistic seismic hazard analysis of Gujarat and its implication on seismic design

Seismic hazard assessment is the key tool for rational planning, safety and design in seismically vulnerable regions. The Gujarat state of India is the only state in peninsular India with the maximum seismic hazard of large shallow earthquakes originating from intra-plate seismicity. In the present study, seismic hazard assessment for the Gujarat is performed by using state-of-art probabilistic seismic hazard assessment procedures. Regional seismicity parameters are established for each of three regions of Gujarat state, namely, Kutch, Saurashtra and Mainland Gujarat. It is done by reviewing the seismotectonic setting of the region and then by performing catalogue completeness based on available earthquake records. A novel procedure is developed for assigning the maximum magnitude to each fault considered in the analysis, given the limitation in the available seismicity data for the region. Probabilistic seismic hazard assessment in terms of the horizontal component of peak ground acceleration for the rock sites is carried out using a suitably selected ground motion prediction equation (GMPE). Seismic hazard curves are developed for some of the major cities of Gujarat and hazard map showing the variation and distribution of seismic hazard in the state of Gujarat is produced. The results show the need for revision of the Seismic zoning of Gujarat as per IS 1893 Part 1 (2002) due to the observation of increased seismic hazard in some parts of Saurashtra. The output of the seismic hazard computations is also used to develop uniform hazard spectra (UHS) for the earthquake return periods (i.e. 475-years and 2475-years). The developed 2475 year uniform hazard spectra are compared with the spectra specified in the Indian seismic code IS 1893 Part 1 (2002) considering rock, medium and soft soil sites. While the codal spectra are found to be on the safer side, certain recommendations are put forth for modifications in the design response spectrum for Gujarat region. A practically reliable and more accurate definition of Importance factor assigned to buildings while designing is presented based on the obtained design spectra from hazard assessment. This is followed by assessing the effect of incorporating time dependent magnitude frequency model in the conventional seismic hazard analysis. While mostly the effect is not much, however in certain cases, the effect of using a time dependent model is found to be appreciable and hence cannot be neglected. The results of the probabilistic seismic hazard assessment of different cities of Gujarat are finally used for selection and scaling of ground motions for the selected cities.by Asim BashirM.Tech