Estimation of site effects in Delhi using standard spectral ratio

Delhi, a city of more than 10 million inhabitants, lies approximately 200. km from Main Boundary Thrust (MBT) and 300. km from Main Central Thrust (MCT), the two most active thrusts of the Himalayas. The city has a varying soil cover and some parts have thick sediments that can potentially amplify the earthquake shaking. Estimates of site effect in Delhi using the standard spectral ratio technique are available only at few locations and significantly differ from those computed from shear-wave profiles inferred from bore-hole penetration tests. Site effects have also been estimated in Delhi theoretically as well as using microtremor studies, yet their knowledge is significantly inadequate to assess true nature of ground amplification.In the present work, site amplification is estimated at 55 different sites in Delhi using data from 13 different earthquakes using the standard spectral ratio method. The site IMD Ridge (NDI) is taken as the reference site on the basis of local geology. The results show significant variations in amplification factor from one place to another. Higher amplification is observed at both banks of Yamuna river. There are some sites, where less or no amplification is observed. The predominant frequency is in general found to be lower near Yamuna banks than the other sites. Based on the observations, we also present an amplification map of the city.