Topographic Amplification around New Tehri Town in Garhwal Himalaya

Abstract

The local geology and soil also play very important roles in damage to the structure during earthquake. Soft soils overlying hard bedrock tend to amplify the ground motions this is known as ground-motion amplification. Site amplification, essential for seismic zonation, is defined here as a peak value of spectrum ratio between ground surface and a base layer. Site amplification is also considered as one of the important factor to determine seismic‐wave amplitude in addition to source and propagation factors. There are various analytical and empirical methods to determine the site amplification. Analytical method requires a lot of knowledge of Geotechnical parameters. Generally, we use the empirical methods for estimation of site amplification which based on referential and non- referential site condition these empirical methods are elaborate in this dissertation project report. A 12 station local seismological network is under operation around Tehri dam in the Garhwal Himalaya. The network is installed and operated by Department of Earthquake Engineering, IIT Roorkee. Site response estimated using horizontal to vertical component ratio method and S wave spectral ratio method. Firstly, I have applied H/V ratio method on the noise data (before P wave arrival and after event) to estimate site response. Resonance frequency of all sites varies from range 0.79 Hz to 6.77 Hz and site amplification factor is highest for Khurmola site having 6.02. After that H/V ratio method also applied on the data of the local event12/03/2010 considering two 5 second windows, one window contains the portion of S wave and other window contains Coda wave. The site response obtained by using earthquake data is better agreement with resonance frequency obtained by the noise data. H/V ratio method gives a reliable estimate of resonance frequency of site. Site response is also calculated for radial and transverse component by using S wave spectral ratio of average of two local events to eliminate the source effects with reference site taken as New Tehri Town.