ESTIMATION OF SITE AMPLIFICATION USING S-WAVE AND HVSR METHODS

Abstract

Using high quality three-component digital data, site amplifications have been estimated from S-waves of eight local earthquakes and microtremors recorded before the four local earthquakes in the Garhwal and Kumaun Himalaya. This data was collected employing a 12-station seismological network under operation in the environs of Tehri dam. All the local earthquakes (ML 2.0 to ML 3.2) have occurred at epicentral distances between about 100 km to 200 km from the center of the network with focal depths ranging from 1 km to 23 km. Microtremor data recorded at seismological observatory located in IIT Roorkee campus is also utilized. S-wave spectral ratios were computed from 273 waveforms in the frequency range 1 Hz to 12 Hz considering PRT located on the Quartzite rock as a reference site, and also relative to a reference site which- is an average of all sites. H/V spectral ratios in the .frequency range from 1 Hz to 41 Hz have been estimated from 117 samples of microtremors with duration ranging from 30 seconds to 280 seconds recorded before the four local earthquakes. From S—wave spectral ratios, higher amplifications have been brought out at sites located - nearest to the epicenters of the earthquakes as compared to sites located away from the epicenters. Almost similar frequency dependent trends in the S-wave site amplifications in the radial and transverse components have been revealed at the majority of sites relative to PRT as a reference site. Three sites located closer to the Main Central Thrust have broad amplification peaks around 4 Hz to 5 Hz. High amplification around 10 Hz at CHN site seems to be due to topographic effect as the site is located on 2244 m high mountain peak. By and large at all the sites, the vertical components show consistently low site amplifications as compared to horizontal components particularly at frequencies above 4 Hz to 5 Hz. Estimates of H/V spectral ratios from the microtremors recorded in the early - _ morning hours have brought out dominant broad peaks between 3 Hz and 4 Hz at CNT, PRT, NTT and RAJ sites, whereas for all sites microtremors recorded in the afternoon show maximum peaks between 2 Hz and 4 Hz, and beyond 4 Hz the amplifications go on decreasing. An interesting feature of the H/ V spectral ratios at NTT and AYR sites, is a sharp peak embedded in a broad peak around 4 Hz. This response seems to be due to continuous vibrations generated by the operation of turbines which are within the radius of 2 km to 3 km from the sites.