Earthquake hazard analysis for Bangladesh

Abstract

As a first step in earthquake disaster mitigation, this study aims to identify and evaluate the seismic hazards in Bangladesh. The seismic environment of Bangladesh is first studied. This includes geological and s,eismotectonic setup in Bangladesh, probable major , . seismic sources, existing zoning maps/etc. I Due to the lack of a proper seismotectonic map in the region under consideration, the / insufficiency of data and high unceftainties in seismic source parameters, a simple model for earthquake occurrence using a r,tewly developed earthquake catalogue is used. Initially a .detailed earthquake catalogue lanalysis is carried out to understand the nature of seismicity. For the development:, of the catalogue, earthquake data are collected from various sources, such as ISC, ISS, USGS, RRLJ and other independent researchers. I Different correlations are developed using the collected data to homogenize earthquake , magnitudes. The data complet~ness of the earthquake catalogue is examined and the time I periods in which the data' are complete are identified for each magnitude. For the maximum use of the available data, the data of the large earthquakes are taken from the • long time period while )he data of small earthquakes are taken from the shorter time period in which they aretonlplete. Using the simple catalogue based model, the seismic hazard at 42 points in Bangladesh is estimated. The seismic hazard maps are presented as contour maps in terms of horizontal Peak Ground Acceleration (PGA) based on 50, 100, 200 years return period and 10% probability of exceedance in a design life of 50 years. A return period seismic hazard map for PGA2: 150 cm/s2 is also presented. In addition expected Ms values for 34 points are estimated for 50, 100 and 200 years return period By comparing the seismic base shear coefficients with the values of the hazard maps, a new seismic zoning map for short period structures are proposed based on the 200 year PGA. Similar to the BNBC 1993 seismic zoning map, the proposed map has three seismic zones. It assigns higher seismicity to Bandarban, Faridpur, Kustia, Natore, Nilphamari, Panchogarh, Rajshahi, Sandwip and significant part of Bogra, Cox'sbazar, Khagrachari, Noakhali, Rangamati and Rangpur. These areas have been assigned low seismicity in the seismic zoning map of BNBC 1993. The findings of this study shows that considerable seismic hazard exists for major parts of the country. Based on the 1991 Census data and proposed seismic zoning map, about 42 million people, representing one third of the total population live in Zone 3 (38% area), i.e, areas which may be classified as "liable to severe damage", another 52 million (i.e. approx. 46% of the pOPulatioh) live in Zone 2 (39% area), i.e. areas "liable to i moderate damage" and the rest 19 million population live in Zone I (23% area), i.e. areas , "liable to slight damage". The outcome of this study, coupled with vulnerability studies, must guide, stimulate and facilitate the efforts of the respective government, the earthquake engineering and the disaster mitigation planning communities to take specific practical preventive measures t~ reduce seismic risk of Bangladesh .