SITE AMPLIFICATION CHARACTERISTICS OF GARHWAL LESSESR HIMALAYA USING S AND CODA WAVES

Abstract

Site amplification characteristics from S-wave and coda waves are computed using high quality, three component digital data obtained from a 12-station seismological network deployed in the Garhwal Lesser Himalaya. The digital data in the form of 206 seismograms from six events that occured from during 2009 have been analyzed. The events span a magnitude range from MD 2.0 to M. 3.6, epicentral distance 10 km to 80 km and focal depths from 9.0 km to 51.7 km. The site amplification factors, centered at frequencies of 1, 2, 3, 4, 6, 8, 10, & 12 Hz, have been estimated from the S-wave and coda wave using the direct spectral ratio method. The estimated site amplification factors show the non-linear variation with frequency at all the sites. The amplification factors have been consistent with both type of waves. It seems that the site amplification at stations are systematically related to the sub-surface geological structure below the station sites. The site amplification factors amplify for younger soft sediments relative to older hardrock. At low frequencies up to about 4 Hz, the amplitudes of the two horizontal components match more closely, while the vertical component shows consistently lower amplification than the horizontal components. The shapes of S-wave and coda wave average spectral ratios with respect to the reference station SRK are similar, however the coda amplification factors are greater than the S-wave amplification factors by a factor of about two. The site amplification factors for the radial components are almost similar to the transverse components for the coda waves. This similarity is expected because the coda waves are consisting of backscattered S-wave coming from various directions towards the station. This results in a directionally averaged site response. The near agreement of site response computed using coda waves with that obtained using S-wave demonstrates that coda waves can be used to determine the site-specific amplification for S-wave.