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Generation of seismic hazard maps for Assam region and incorporation of the site effects

Bandyopadhyay Srijit, Parulekar Yogita M., Sengupta Aniruddha

Acta Geophysica

2022

Vol. 70, no 5

1957--1977

Probabilistic seismic hazard assessment (PSHA), including a site-specific amplification study, is crucial to evaluate site-specific spectra of soil sites to better understand the behavioural patterns of the soils under earthquake excitation. This paper represents the results of the PSHA for Assam state, located in the highest seismic zone of India ever delineated to date. In that sense, the study area is divided into ten areal zones concerning seismicity source modelling to represent the seismo-genesis of the Assam state in detail. The earthquake recurrence parameters of each zone are obtained from Gutenberg–Richter (G–R) recurrence relation with updated homogenized and de-clustered earthquake catalogue from 1735 up to 2021. Earthquakes with magnitude greater than 4 (M>4) are considered using eight attenuation relationships for continental active shallow crust region, subduction zone and intraplate region. Hazard curves are obtained using a logic tree structure thus minimizing the epistemic uncertainty. The peak ground acceleration (PGA) value obtained at the rock outcrop of the Assam state for 10, 5, 2, and 0.5% probability of exceedance in 50 years with return periods such as 475, 975, 2475, and 9975 years lies between 0.24 and 0.34 g, 0.3 and 0.44 g, 0.42 and 0.59 g, and 0.56 and 0.91 g, respectively. The estimated PGA value at rock outcrop level is comparatively higher than that reported in the codal provisions. Site-specific response spectra at bedrock level (Vs=1100 m/s) for major cities (Jorhat, Tezpur, Silchar, Dibrugarh, Guwahati, Nagaon) of Assam state have been proposed for different earthquake return periods of 475,2475, and 9975 years. Finally, site amplification study is performed for Guwahati city and surface level 5% damped response spectra with PGA of 0.696 and 0.924 g are obtained for earthquake a return period of 2475 and 9975 years, respectively.