SITE RESPONSE CHARACTERISTICS OF PARTS OF GARHWAL HIMALAYAS USING LOCAL EARTHQUAKES

Abstract

Site response characteristics are very important in defining the damageability associated with an earthquake. The local topography or geology shapes the extent of effects faced during any ground motion occurrence. Garhwal Himalayas, being one of the most seismically active zone in India are prone to severe earthquakes. This report contains 6 events taken from 18 station seismological network for the study. All the local events were recorded within the epicentral distance range of 44 – 155 kilometre, and magnitude from ML 3.0 to ML 4.3 and total of 282 seismograms are analysed. Two methodologies are used for computing site amplification factors i.e. S wave direct spectral ratio method for the centered frequency values of 0.125, 1, 2, 3, 4, 5, 7, 9 & 12 Hz, and horizontal to vertical spectral ratio method (H/V) and the results are compared in this report. A non-linear variation with frequency is seen at all the sites. It is evident that the site amplification at stations is related to the sub-surface geological structure below the station sites. Spectral ratios obtained using both the methods have shown satisfactory consistency with each other. Most of the stations are having maximum response near the frequency range of 2-4 Hz using both the methods. TRIY and CHAN station have shown the peak response at the higher values of frequency i.e. near 7-9 Hz frequency values. In S wave direct spectral ratio method, radial and transverse components are seen to be in good agreement with each other at a station. GIYA station is found to have the maximum site response using both the methods.