Determination of S-wave site response in the Garhwal Himalaya from the aftershock sequence of the 1999 Chamoli earthquake

Site response in the Garhwal Himalaya is studied using, digital seismograms recorded by a five-station 24-bit digital microearthquake network established to monitor the aftershocks of the 28 March 1999 Chamoli earthquake (m(b) 6.3). Fifteen aftershocks (M-d greater than or equal to2.0) are chosen for the site response estimation using horizontal-to-vertical spectral ratio and generalized inversion techniques. Site response curves at all the five sites show station-to-station variation of the site factor reflecting the changes in geologic/geotectonic/soil conditions. A comparison of the site response values obtained by the inversion with those computed using receiver function technique show a large scatter even though the pattern of the curves remains more or less similar. The peaks yielded by both the methods have been observed to occur at the same frequencies. The variation of site response corroborates the abrupt changes in intensity from one location to another due to local site condition.