1 ISET GOLDEN JUBILEE SYMPOSIUM Indian Society of Earthquake Technology Department of Earthquake Engineering Building IIT Roorkee, Roorkee October 20-21, 2012 Paper No. A021 ESTIMATION OF SITE AMPLIFICATION IN THE GARHWAL HIMALAYA USING S-WAVE AND HVSR METHODS Gaulkar Amol Sudhakar, Ashwani Kumar, S.C. Gupta and Arjun Kumar ABSTRACT Using three-component digital data, site amplification factors have been estimated from recordings of S-waves of eight local earthquakes and microtremors data in the Garhwal Himalaya. The data from a 12-station seismological network in the Garhwal Himalaya have been used. All local earthquakes (M L 2.0 to M L 2.92) have occurred at epicentral distances between 100 km to 200 km from the center of the network with focal depths ranging from 2.5 km to 22.7 km. S-wave spectral ratios were computed from 273 waveforms in frequency range from 1 to 12 Hz considering PRT as a reference site. H/V spectral ratios in frequency range from 1 Hz to 25 Hz have been estimated from 117 samples of microtremors with duration ranging from 30 seconds to 280 seconds. 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 brought out at the majority of sites. 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. At most of the sites, the vertical components show consistently low site amplifications as compared to horizontal components particularly at frequencies above 4 Hz to 5 Hz. H/V spectral ratios from the microtremors of morning hours have shown dominant broad peaks between 3 Hz and 4 Hz at CNT, PRT, NTT and RAJ sites, whereas at all sites microtremors recorded in the afternoon show maximum peaks between 2 Hz and 4 Hz, and beyond 4 Hz the amplifications decreases. An interesting feature at NTT and AYR sites, is a sharp peak embedded in a broad peak around 4 Hz. This response seems to be due to the vibrations generated by the operation of turbines which are within the radius of 2 km to 3 km from the sites. Keywords: S-wave method, microtremor, amplification. Introduction The estimation of amplification of ground motion from seismological data has been the subject of many studies using different approaches (e.g. Borcherdt, 1970; Phillips and Aki, 1986; Su et al., 1996; Bonilla et al., 1997). It has been observed that ground motion in the frequency range from 1 to 12 Hz is of most engineering interest and is strongly affected by local site conditions such as local topography and complex surface geology (Su and Aki, 1995). For microzonation of a given region, the site response characteristics form one of the most important inputs. Site response studies are used to quantify the amplification of ground motion and to determine natural resonance frequencies. Therefore, the frequency dependent amplification forms an important component of the seismic hazard analysis. Aim of most of the site response studies has been to identify regions having high