ABSTRACT

            Decay of earthquake ground motion with distance in Bangladesh is studied from published information and isoseismal maps. A total of seven earthquakes with magnitude varying from 5.1 to 8.1 are considered. Variation of attenuation of earthquake intensity in different directions is studied. A directional concept is used for developing an attenuation law for Bangladesh. Several publications have been examined for identifying appropriate isoseismal and related intensities. This field data is used to develop two equations representing attenuation of intensity: (i) in terms of magnitude and epicentral distance and (ii) in terms of magnitude and hypocentral distance. The second equation is somewhat questionable since it requires focal depth data which has uncertainty. The developed equations are compared with the field data. Finally comparisons are also made with other attenuation laws developed for different regions including NE India.

Introduction

In the last 150 years, Bangladesh has been affected by five earthquakes with magnitude 7 or greater. Two of them had their epicenters within Bangladesh, while others were not far from Bangladesh border. However, due to lack of seismic instrumentation inside Bangladesh, measured strong motion data for these earthquakes are not available. Alternatively, earthquake intensity data observed within the country may be used for the determination of attenuation relations for these major earthquakes. These historical earthquakes occurring during 1869 to 1930 are documented in some reports such as that of Oldham (1899) for 1897 Great Indian Earthquake (M=8.7) in Assam for the then Geological Survey of India. Such reports also give isoseismals in Oldham Intensity scale which is very different from other standard intensity scales such as Modified Mercalli Intensity (MMI) scale. Some later publications also describe these historical earthquakes. The older reports have been critically re-examined and changes in intensity levels have been suggested by some authors. Recently, Sabri (2001) developed intensity attenuation laws for NE India including Bangladesh as a function of hypocentral distance and magnitude. He considered 18 earthquakes; some of them were quite distant from Bangladesh. He considered average distance of isoseismals all around the source. Ambraseys and Bilham (2003) have reassessed 1897 Great Indian Earthquake effects, re-estimating its magnitude as well as intensity levels.

Probabilistic seismic hazard assessment studies require appropriate attenuation law for the region concerned. For the current study, attenuation of earthquake intensity for four of these historical major earthquakes and three additional significant earthquakes have been used in developing the intensity attenuation law for Bangladesh. Isoseismal maps and intensity descriptions for these earthquakes have been collected from various publications, critically examined and intensity level modified if deemed necessary. These isoseismal maps and intensity levels have been used for the determination of intensity based attenuation law. This paper presents attenuation laws representing decay of earthquake intensity with distance. Finally, some published intensity versus PGA (peak ground acceleration) relationships have been used to obtain the attenuation law in terms of PGA, for comparison with attenuation laws based on PGA.

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