Seismic loss estimation for Sylhet city

Abstract

For accomplishing comprehensive regional seismic loss estimation, geographic information system (GIS) provides a perfect environment. A GIS has the capability to store, manipulate, analyze, and display the large amount of required spatial and tabular data. By using geographic information system to carry out regional multi-hazard seismic loss estimation for study area where reflection of ground shaking and the secondary site attributes of soil amplification, liquefaction and landslide arc the salient features. The method to combine the different hazards is based on a weighted average approach. This dissertation also covers the development of a comprehensive inventory of building for the Sylhet City. Sylhet City Corporation has twenty-seven Wards in its jurisdiction. Initially a GIS based Ward map is developed. 3611 buildings (14% of the total) arc surveyed in the study area and water pipelines data arc collected from City Corporation office. For mierozonation purpose 167 boreholes with SPT data are collected from different relevant organizations. To verify those data nine boreholes up to a depth of 30m arc carried out. Three past historical earthquakes 1885 Bengal earthquake, 1918 Srimangal Earthquake and 1897 Great Indian earthquake arc used as scenario events for this study. Intensity value obtained for these events is calibrated against attenuation laws to check the applicability of the laws for the study area. Using these laws, bedrock Peak Ground Acceleration (PGA) values arc obtained for study area for different earthquakes. Finally, a bedrock PGA value for the scenario events is selected. PGA in the bedrock level in the study area and boreholes SPT data are used to develop a regional combined seismic hazard map based on site amplification as well as liquefaction and landslide potential hazard. This hazard map is then overlaid and combined with structural inventory maps to produce maps of regional damage distributions. Combining the map of damage distributions with a map of population distributions for the area results in linal regional estimates of direct monetary loss such as building and water line distribution and non-monetary loss such as fatalities and mutinies. The investigations arc carried out for two cases such as based on combined seismic hazard considering local site effects and based on hazard due to ground shaking alone. The expected damage factor for all building decreases from about 59% to about 51% when the effects of liquefaction and landslide arc ignored. The total direct monetary loss decreases by about 9'%. However, there is a tremendous effect of liquefaction and landslide hazards over direct non-monetary loss as Fatalistic and injuries increases by 49% and 43% respectively during night. The final seismic loss estimation appears to be reasonable when compared to recent large earthquakes in urban areas of the world. The GIS-based analysis is useful to engineers, planners, emergency personnel, government officials, and anyone else who may be concerned with the potential consequences of seismic activity in a given region.