Design Response Spectra and Site Coefficients for Various Seismic Site Classes of Guwahati, India, Based on Extensive Ground Response Analyses

Seismicity of northeast India and its surroundings is governed by complex tectonic setting. Guwahati, the largest city of northeast India, had suffered minor to major damages during past earthquakes (EQs). Known seismic source information also suggests that Guwahati is susceptible for future EQs both from nearby as well as distance source. Taking into account the subsoil information of Guwahati, present work attempts to understand the role of Guwahati soil during probable seismic excitation. In the absence of regional ground motion records, covering a possible variation in ground motion characteristics, which can occur during potential future EQ, 30 globally recorded ground motions are used as input motions in this work. Each input motion is applied to each of the 225 boreholes resulting in 225 x 30 = 6750 ground response analyses using equivalent linear approach. Based on the response of subsoil to each input motion, site coefficients as well as average response spectra for various seismic site classes are proposed. It has to be mentioned here that in comparison to previous works which attempted to understand the subsoil of Guwahati based on specific location-based subsoil information and specific scenario EQ, present work provides a broader picture of soil response which can be applied to any level of ground shaking. Further, based on predominant period, suggestion of maximum building storeys are suggested for Guwahati. In addition, based on the work, characteristics of EQ scenario, which can trigger maximum response from local soil of Guwahati, are assessed. Based on the present analyses, design response spectra of Guwahati are also proposed. Keeping in mind Government of India's policy towards the development of northeast India and further to develop Guwahati as "Smart city", the present findings will be very useful in city planning, estimation of induced effects as well as for seismic design of probable infrastructure in Guwahati.