Performance-Based Design in Earthquake Geotechnical Engineering – Kokusho, Tsukamoto &Yoshimine (eds) © 2009Taylor & Francis Group, London, ISBN 978-0-415-55614-9 Site response study for urban areas of Chennai city, India—a geotechnical approach G.P. Ganapathy Centre for Disaster Mitigation and Management, VIT University, Vellore, Tamil Nadu, India S. Rajarathnam Centre for Disaster Mitigation and Management, Anna University, Chennai, Tamil Nadu, India ABSTRACT: Chennai city with population of 6 million experienced three earthquakes of M ≥ 5.0 in the years 1807, 1816 and 1823. Chennai city falls under Seismic Zone III and such being the case, Chennai is liable for site-specific amplification of seismic motion. The objectives of this paper to bring out the seismic hazard map of Chennai in terms of intensity increment for a known earthquake magnitude. The Seismic Hazard of Chennai city evaluated based upon local ground motion response or local site effects with the geotechnical data from 450 boreholes by correlation, integration and interpretation of parameters viz., SPT-N values, average shear wave velocity, relative amplification factors and intensity of increment of earthquake. The combined seismic hazard prone areas at different depths for Chennai represent the percentage of medium, high and very high hazard is 31.3%, 39.2% and 2.7% respectively. In the present study the data gap is 26.8%. 1 INTRODUCTION A United Nation Development Programme (UNDP) report says about 130 million people were found to be exposed on average every year to earthquake risk. (UNDP, 2004). Earthquake hazard zonation for urban areas, mostly referred to as seismic microzonation, is the first and most important step towards a seismic risk analysis and mitigation strategy in densely pop- ulated regions (Siefko Slob et al., 2002). The recent earthquakes have raised the question of the safety of major cities of India with its dense population and enormous infrastructures and economic investment. As a result, even relatively minor events can be the source of huge socio-economic disasters. For exam- ple, the earthquake of 12th October 1992 with M 5.4 in Egypt, where the buildings were not earthquake resis- tant, resulted in 554 deaths, 20,000 people injured with a reported loss of one million US$ (El-Sayed et al., 2001). The most severe impact of disasters is felt in urban areas, and they take the maximum time to recover from a disaster (Sinha et al., 2001). From last 50 years urban areas, particularly in developing countries have experienced unprecedented growth. Urban areas there- fore require special consideration during the eval- uation of earthquake related risk in terms of peak ground acceleration, local site effects or local ground motion response through the parameters of shear wave velocity, relative amplification factor and intensity increment. The actions we take today towards being seismi- cally resistant will increase safety and reduce losses from earthquakes that will occur tomorrow. It takes time to enhance building codes, to establish poli- cies that promote seismic resistance, and for people to be motivated to prepare. What we can initiate now is better understanding of the earthquake haz- ards of the state and how to evade them (ERM, 2001). The study area Chennai city has multi-dimensional growth in terms of development of infrastructures and population. In 1991 the population of city was 38.41 lakhs and increased to 60 lakhs in 2001 (Census, 2001). Safety of this city is extremely important for safeguarding human lives and property. To understand the seismic risk for the structures and population of Chennai, the basic input is from seismic hazard assess- ment study involving local ground response or local site effects using geotechnical approach apart from the seismic source and Peak Ground Acceleration (PGA). The present study is the first attempt to assess the site response of Chennai urban areas using geotechnical approach. 365