ESTIMATION OF GROUND MOTION IN DELHI

Abstract

Delhi-The National Capital of India with its long history, rich culture and strategic importance, is of prime consideration for earthquake risk reduction and hazard mitigation planning. Delhi is a fast growing megacity that influences the economic and industrial developments of much of the country which emphasizes the need for realistic estimation of strong ground motion from future earthquakes in Delhi. Earthquake hazard estimation in Delhi, due to its vicinity to earthquakes at regional (250-300kms) and local distances is almost an essentiality for its 18 million populations. One of the foremost elements in estimating the seismic hazard is the availability of ground motion records at various sites in Delhi from earthquakes of varying magnitude and distances. Unfortunately, hardly any such data is available in Delhi except at the India Meteorological Department (IMD) station NDI. Given this scenario, the best possibility is to estimate (either through synthesis or otherwise) the ground motion at NDI and through the transfer functions (to be obtained) at different sites, one could estimate ground motion at all these sites in Delhi. Since the local geological and top soil conditions vary quite drastically in Delhi, one has to have a lot of transfer functions spatially as close as possible. In this thesis, transfer functions at 55 sites from 13 earthquakes are obtained using standard spectral ratio (SSR) technique. Some of the transfer functions were taken from earlier studies. Most of the data are from a portable broadband network operated by Wadia institute of Himalayan geology (WIHG) and some are from recently installed strong motion instruments. The transfer functions are found to be significantly different than earlier theoretical estimations (Iyengar and Ghosh, 2004). The ground motion at NDI was synthesized for four scenario earthquakes (three regional and one local) using empirical Green's function (EGF) technique. Then through the computed transfer functions, the ground motion is estimated at all the sites. The 5% damping horizontal acceleration response spectra are estimated at different sites in Delhi from a local A/w = 5.5 earthquake as well as from Mw= 7.5, 8.0, and 8.5 earthquakes in Himalayas using random vibration theory (RVT) approach. The estimated horizontal response spectra are compared with the IS 1893-2002 response spectra of 5% damping, with zero period acceleration (ZPA) of 0.24g (zone factor for zone IV). Estimated response spectra at all hard sites are grouped together and compared with IS 1893-2002 spectra for hard rock (Type I), while response spectra of all other Delhi sites are compared with the IS 1893-2002 spectra for soft soil (Type III). With respect to the postulated local and Himalayan-arc earthquakes, the rocky or hardsoil site, zone IV response spectra of IS 1893-2002 are conservative while at soft soil sites it is deficient for 8.0 and 8.5 magnitude earthquakes from Himalayas. However for 7.5 magnitude earthquake from Himalayas and 5.5 magnitude local earthquake, the soft-soil site, zone IV response spectra of IS 1893-2002 are in agreement. Finally the spectral acceleration maps at four different natural periods are produced for these scenario earthquakes. The spectral acceleration map is strongly influenced by the shallow geological and soil conditions as indicated in the contour maps. Three pockets of high acceleration values are seen due to both regional and local earthquakes. These pockets seem to coincide with the junctions of (a) Aravalli quartzite and recent Yamuna alluvium (towards the East), (b) Aravalli quartzite and older quaternary alluvium (towards the south), and (c) older quaternary alluvium and recent Yamuna alluvium (towards the North). Ml The results of this thesis may very well be utilized for preparation of near real-time shake maps. In case of an inevitable earthquake, use of these transfer functions will provide the city planners and disaster management personnel a reasonable idea of the level of shaking in different areas of the city, once the exact ground motion from NDI is available.