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Site response study of an area is an important step to assess the possible expected response such as PGA, amplification, and spectral response on the ground surface or free field response due to the local soil effects. There are several examples of past earthquakes which lessons us about earthquake damages due to local soil condition such as Niigata (1964), Mexico (1985), San-Fransico (1989), and Bhuj earthquake (2001) in India, and as a result it is necessary to incorporate site effects for the assessment of ground motion hazards; thick deposits of unconsolidated sediments generally amplify/de-amplify earthquake vibration during earthquake. Agartala is a typical example of a city located in N-W part of Tripura which constituted a thick alluvial-fluvial deposit in a plain bounded by two rivers (Haora and Kathakal). Tripura is one of the fastest growing states (infrastructure) in North East India having latitude 22.900 N-24.520 N and longitude 91.150E-92.400E shown in Figure 1. The current population of Tripura is about 36 lakhs (DES-2011 Population census report). The Capital city of Agartala has densely populated. As per City development plan, there is a high priority to build modern infrastructures from Govt. as well as private sector (Public private partnership, PPP-model) such as heath, Insurance, industry and education. It is gradually building international collaboration of various aspects such as transport (land and waterway) through Bangladesh with the South East Asia and going to become a North East Corridor. Keeping the growth of national and foreign investment about the infrastructure development, earthquake resistant design is the prime concern. The earthquake is one of the most destructive events in nature. In India, more than 60% of the area is highly vulnerable due to earthquake. In almost all other regions of the country, the North East India is one of the highly seismically active regions (zone V as per IS-1893-2002, Indian seismic code) and sixth position on the globe. Based on the present study, using the available earthquake catalogue collected from 1731 to 2011 (281years), the mean annual rate of occurrence is much higher (8-10/year) of magnitude (Mw) 4 and above. In this region, more than Mw>8 has occurred five times and Mw>7 has occurred fifteen times in the last 100 years. Out of which, upper Assam earthquake (Aug15, 1950) was the biggest one having the magnitude (Mw 8.6) in the 20th century, which caused about 1500 deaths and huge destructions (infrastructures, Road, slope failure, topographic changes and diverted the directions of stream/river) throughout the North East India and also severely affected the Agartala city. This region suffers both interplate (Eastern Himalaya), intraplate (Shillong Plateau) activities along with the subduction (Indo Burma Region) events. The complex tectonic regime produces differential movement and finally makes this area more vulnerable for future events. There is a seismic gap since 1950 of this region, scientists named it as Assam gap or aseismic corridor (Khattri & Wyss 1978) and expecting a biggest size earthquake in any time which can devastate severely the entire North East Region.
Figure 1. Map of the study area (Agartala city)