Site classification and estimation of surface level seismic hazard using geophysical data and probabilistic approach

Abstract

This paper presents the site classification of Bangalore Mahanagar Palike (BMP) area using geophysical data and the evaluation of spectral acceleration at ground level using probabilistic approach. Site classification has been carried out using experimental data from the shallow geophysical method of Multichannel Analysis of Surface wave (MASW). One-dimensional (1-D) MASW survey has been carried out at 58 locations and respective velocity profiles are obtained. The average shear wave velocity for 30 m depth (Vs 30) has been calculated and is used for the site classification of the BMP area as per NEHRP (National Earthquake Hazards Reduction Program). Based on the Vs 30 values major part of the BMP area can be classified as “site class D”, and “site class C”. A smaller portion of the study area, in and around Lalbagh Park, is classified as “site class B”. Further, probabilistic seismic hazard analysis has been carried out to map the seismic hazard in terms spectral acceleration (S a) at rock and the ground level considering the site classes and six seismogenic sources identified. The mean annual rate of exceedance and cumulative probability hazard curve for S a have been generated. The quantified hazard values in terms of spectral acceleration for short period and long period are mapped for rock, site class C and D with 10% probability of exceedance in 50 years on a grid size of 0.5 km. In addition to this, the Uniform Hazard Response Spectrum (UHRS) at surface level has been developed for the 5% damping and 10% probability of exceedance in 50 years for rock, site class C and D. These spectral acceleration and uniform hazard spectrums can be used to assess the design force for important structures and also to develop the design spectrum.