Seismic microzonation of Dhaka city based on site amplification and liquefaction

Abstract

Microzonation is a systematic way for the management of earthquake vulnerability, integrating for application the earth and engineering sciences. It can be achieved by inputting local site characteristics, ground shaking,. liquefaction, slope stability, high ground water, fault rapture, economic involvement such as land cost, transportation costs, seismic damage etc. In the present study local site conditions and' liquefaction potential of sites were used for microzonation of Dhaka city. Dhaka was first divided into small grids. At the grid points shear wave velocities were estimated by using SPT'test results. More than two hundred bore hole data were collected and converted into shear wave velocities using empirical relations. All these data were used to estimate vibration characteristics at different grid points of the city employing one dimensional wave propagation program SHAKE. The computations were made in the frequency range of 0 to 20 Hz. at frequencies every 0.05 Hz. interval. The loss of I energy of seismic waves in the soil layers was also considered. The vibration characteristics of each site such as predominant frequency and amplification amplitudes were found. The predominant frequency (inverse of period) at the site is an important information in estimating the input force to a structure due to the earthquake. The amplification amplitude is the relative amplification of a seismic wave at ground surface level coming from baserock. In this study, at each site of Dhaka city, liquefaction potential was analysed using maximum acceleration of 150 cm/s2. This maximum acceleration is based on 200 years Peak ground surface acceleration contour estimated from earthquake hazard analysis for Bangladesh based on National Oceanic and Atmospheric Administration (NOAA) earthquake data from 1900 to 1977. The liquefaction potential of different locations of Dhaka is estimated by two methods: The Seed et al. method (1983) where SPT data was used and Iwasaki et al. method (1982) based on topographical information. The results of the amplification and liquefaction analysis were transformed into microzonation maps depicting: (i) zones showing quantitative estimates of site amplification; (ii) zones showing the natural frequency of the soils; (iii) zones showing qualitative estimates of liquefaction potential; (iv) zones showing quantitative estimates of liquefaction potential. These maps will be useful for preliminary selection of a project site, land use planning, zoning ordinances, pre-disaster planning, capital investment planning etc.