Site Response Study of Deep Soil Column in Lucknow, India

Earthquake activities in the many parts of world had shown the importance of local soil condition in the propagated wave motion. In this paper an attempt has been made to estimate site effects of deep soil column in Lucknow, Indo-Gangetic basin for scenario earthquakes at Himalayan plate boundary. Based on previous study, the synthetic ground motion has been generated using Stochastic Finite Fault model (FINSIM) for two scenario earthquakes at seismic gaps. One seismic gap called as western location/seismic gap is located about 307.88km from site and gives the peak ground acceleration of 0.11g at site. Another one is the Central seismic gap/location is located in central seismic gap at 229.77km from the site and gives the Peak Ground Acceleration (PGA) of 0.218g. The local soil layer details with standard penetration test N value have been collected for the main location in Lucknow from literates. The general soil found for this site is silty sand and silty clays having SPT N value up to above 100 up to a depth of 30m. The soil details extrapolated up to 100m by assuming linear variation from 30 m. The site response analysis has been carried out using equivalent linear and non linear approaches by employing SHAKE 2000 and Deep soil program. Input has been assigned at 30m 50, 75 and 100m to find effective depth of input motion. This study shows that the ground motions are being amplified within a depth of 50 to 80 m, but these results need to be further confirmed with large number of data. The input ground motions are amplified 1.06 to 2.5 times due to the soil condition. INTRODUCTION Indian sub-continent is one of the most seismic prone regions in world. Collision of Indian and Eurasian plate causes built – up of strain along the plate boundary and within the boundary. The main seismogenic regions include Kirthar, Sulaiman and Himalayan and Arakanyoma mountain range. During past two centuries many earthquakes of magnitude 8 and above have been reported in the region. These caused devastating damages and casualties to human life and infrastructure as well. Owing to the filled-up soil (Indo-Gangetic basin) in the form of loose sediments further can make this scenario more destructive. The major tectonic features in the Himalayan mountain range (Gansser, 1964; Molnar and Chen, 1982) include, from south to north, the Main Boundary thrust (MBT), the Main Central Thrust (MCT) and Indus-Tsangpo Suture (ITS) all along the entire length of the Himalayas from west to east. All these features have been shown in Figure 1. The collision rate between Indian and Eurasian boundaries has increased from 5mm-21 mm per year (Khattri, 1987, Bilham et al., 2001). In the light of built-up strains and possible seismic gap presence (Seeber and Armbruster, 1981; Khattri, 1999; and Bilham and Gaur, 2000) can cause large earthquake in the region. During the past earthquakes along the boundary, a 750 km long segment which lie between the eastern edge of the 1905 Kangra rupture zone and the western edge of 1934 Bihar Nepal earthquake remains unbroken. Earthquakes of 1803 and 1833 had caused severe damage but none of it was of larger magnitude so as to fill this seismic gap possibility of future earthquake. In this study it is assumed that the scenario earthquakes at these two seismic gaps. The seismic gaps and site in Indo-Gangetic basin is shown in Fig 1. Figure 1 also shows the seismotectonic of area close Indo-Gangetic basin. In the absence of actual ground motion to be used for site response analysis, an attempt has been made to generate 1934 Bihar-Nepal earthquake using FINSIM (Atkinson and Beresnev, 1997) and use this motion at bed rock level to study the site effects using equivalent linear and non linear approaches. Any great earthquakes anywhere in the seismic gap can cause sever loss of lives and property in nearby locations as the population density is quite high. In this study the site effects of local soil was assessed considering geotechnical properties