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
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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.
