Seismic damage scenario for Dhaka city

Abstract

Earthquake risk at any location depends on the seismic hazard as well as the vulnerability of its structures. The seismic hazards 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 the city and is related to quality of the building stock. For old cities like Dhaka, 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. A comprehensive regional seismic hazard and risk analysis is a fairly standard procedure that requires combining the effects of many factors. Each of these factors usually involves the modeling and the analysis of both spatial and temporal data. The amount of required information can often be overwhelming, even for a small region. Recent advances in geographic information system (GIS) technology have created new opportunities for managing large amount of data, for interfacing with external analysis programs, and for prcsenting the results in a manner that may be useful for disaster planning, hazard and risk mitigation, and rehabilitation strategy comparison. 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 or by making use of existing maps showing estimated models of levels of these collateral hazards. Regional structural inventories, often stored in external database management systems, are combined with the seismic hazards to produce damage and loss distributions for the region analyzed. Due to recent improvement in the availability and quality of GIS technology, tabular database software, as well as computer hardware, a significant amount of current research is devoted incorporating GIS technology in seismic damage estimation for Dhaka city. Dhaka City Corporation, which currently has a population of about 6 million. It has 90 wards. Most of the wards are in older part of the city, which has a population density exceeding 60,000 per square kilometers. For the city, a building inventory of 3668 buildings from eight wards was compiled in MS ACCESS for this study. Together with BBS data, data from Geological Survey of Bangladesh (GSB) and Dhaka City Corporation, this building inventory is used in different analysis purpose. A soil database of 253 boreholes is developed in MS EXCEL. The soil data are used to develop site amplification and soil liquefaction potential assessment of the city. 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 value 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 Dhaka City. Finally, a bedrock PGA value for the scenario events is selected. 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. Earthquake damage of buildings of different typologies was estimated by using fragility curves proposed by Arya. Human casualty (death and injury) was estimated by a morbidity model proposed by Coburn. Finally, economic losses due to building damage and human casualty were estimated for each ward.