A SITE AMPLIFICATION STUDY USING OBSERVED RECORDS AT BMD, DHAKA FOR SEISMIC MICROZONATION

Dhaka city is currently seismically vulnerable both in natural and social condition, therefore seismic disaster management microzonation is indispensable. In this process, local site amplification plays an important role. For this purpose, due to shortage of soil data, a number of horizontal-to-vertical spectral ratio (H/V) for microtremor was already applied to estimate predominant period and amplification factor for geomorphological types, but without verification using real earthquake records. However, there are earthquake records obtained by the vertical array at the Bangladesh Meteorological Department (BMD), Dhaka.

In this study, in order to investigate an appropriate way to estimate site amplification due to subsurface soil for seismic microzonation of Dhaka, actual amplifications calculated using six actual small earthquake records are compared with the existing results of H/V for microtremor. This study provides the conclusions: (a) The predominant period estimated using H/V for micrtremor is in a good agreement with those estimated using the real small earthquake records, (b) The amplification factor of PGV and the amplitude spectra show some differences between the real small earthquake records and the existing results of H/V for microtremor, (c) the predominant periods of the existing H/V for microtremor can be used for microzonation of Dhaka city, whereas the amplification factors estimated from them cannot be used directly, but more careful investigations are required. Finally we expect the results of this paper can be a clue for preparation of practical seismic microzonation for Dhaka city.