SITE AMPLIFICATION ESTIMATION OF GARHWAL HIMALAYA USING HVSR AND S-WAVE METHOD

Abstract

Using high quality three-component digital data recorded on 12-stations, site amplifications have been estimated from spectral ratios of horizontal to vertical (H/V) recordings and S wave spectral ratio method. This study analyses data of 7 earthquakes ranging from M 3.0 to M 5.1, and depths from 4 to 38 km. Digital data recorded by 12 stations of seismological network around Tehri region operated by Department of Earthquake Engineering, IIT Roorkee and sponsored by THDC India Ltd. have been utilized. The noise part of the seismograms has been used for estimating site response and to extract the predominant peaks by the H/V method and the S-wave portion of seismogram is used for S-wave spectral ratio method. The amplifications determined from the H/V method are, however, different from the amplifications determined from the direct spectral ratio method. H/V spectral ratios are computed from 201 waveforms in the frequency range 1 Hz to 12 Hz and S-wave spectral ratios are computed from 114 waveforms in the frequency range 1 Hz to 12 Hz considering PRAT located on the Quartzite rock as a reference. Estimates of H/V spectral ratios has given dominant peaks between 2 Hz and 5 Hz at all sites, and beyond 5 Hz, no sharp amplification is observed. From S-wave spectral ratios, almost similar frequency dependent trends in the radial and transverse components has been revealed at the majority of the sites relative to PRAT site as a reference site and the dominant peaks are observed between 2 Hz and 6 Hz High amplification at higher frequencies at CHAN and GIYA sites seems to be due to topographical effect, as the sites are located at the mountain peak. By and large at all the sites, the vertical components show consistently low site amplifications as compared to horizontal component.