Seismic microzonation of Mymensingh town

Abstract

Seismic risk map at any location depends on the seIsmiC hazard as well as the vulnerability of its structures. The seismic hazard evaluation considers the likelihood of earthquake of a particular magnitude or intensity affecting a site. The seismic vulnerability, on the other hand, depends on the construction practice in an area and is related to the quality of the building stock. For old town like Mymensingh, a larger portion of buildings is very old and consequently vulnerable. The local construction practice also has a very strong bearing on the seismic vulnerability since the use of inherently strong building materials will result in structures showing better resistance to earthquakes. Mymensingh town has a current population of about 364,000. II has 21 wards. Most of the wards are located in the older part of Ihe town, which has a population density exceeding 16,800 per square kilometers. For the town, a building inventory of 1,250 buildings from eight wards was compiled in MS ACCESS for this study. Together with the data of Bangladesh Bureau of Statistics (BBS) and Mymensingh Municipality, this building inventory is used in different analysis purpose. A soil database of eighty-seven boreholes is developed in Microsoft EXCELL for the Mymensingh town. The data compiled in this database is used 10 estimate site amplification and soil liquefaction potential of the town. Both of these site effects are integrated in Geographical Information System (GIS) platform for combined hazard assessment. Three past historical earthquakes are used as scenano events namely 1885 Bengal earthquake, 1897 Great Indian earthquake and 1918 Srimangal earthquake. Intensity values obtained for these events are calibrated against attenuation laws to check the applicability of the laws for this study. Using these laws, bedrock Peak Ground Acceleration (PGA) values are obtained for Mymensingh town. Finally, a bedrock PGA value for the scenario event is selected. Seismic hazards due to local site effects such as soil amplification and liquefaction can be estimated by combining the available soil parameter data with the current hazard models. Regional structural inventories, often stored in external database management systems, are combined with the seismic hazards to produce damage distributions for the region analyzed. Earthquake damage of buildings of different typologies is estimated by using fragility cUNes proposed by Arya. PGA values are also converted into intensity values to integrate the effect of site amplification as well as liquefaction. Finally intensity based combined hazard map is proposed.