UNDERSTANDING OF SEISMIC HAZARD AND RISK ASSESSMENT IN A COMPLEX GEOLOGICAL ENVIRONMENT LIKE BANGLADESH

Seismic hazard assessment of an earthquake-prone region involves a wide range of analysis of seismological, tectono-structural, and geo-scientific data including earthquake-engineering characterization of geological materials. Bangladesh is seismically active for its juxtaposition to the subducting Indo-Burma plates (≈400km E of Dhaka) and Dauki Fault (≈200km NE of Dhaka). Due to rapid and unplanned urbanization, dense population and poorly engineered construction practices, the country is at its optimum earthquake hazard risk and disaster. The plain-land topography of Bangladesh masks the subsurface depositional facies and all geological structures of the Bengal basin, except the folded Tertiary hills in the east. Because of enormously thick fluvio-deltaic sedimentation (≈500m to 25km) in the Bengal basin the seismic hazard assessment has become difficult and dependent on complex geophysical methods. In the present investigation an attempt is made to understand the relationships among tectonic setting, structural and crustal configuration, and deformational pattern of Bengal basin to classify the earthquake source areas and examine the conditions for rational seismic hazard assessments. The study identifies multiple surface faults affected by crustal flexures and fractures, designated as in-basin earthquake source areas: a. Intra-plate source area, b. Transitional source area and c. Inter-plate source area. The seismic behaviors of these source areas are deeply related to the tectonic structure and basement or crustal configuration of the Bengal basin. A detailed geological investigation indicates the presence of systematic folding and faulting in the basin. The epicenter locations of historical and recent earthquakes in and around Bangladesh indicate the existence of active faults in the source areas. For detailed analysis, the geometry, and mechanics of the potential seismogenic faults have been studied to determine relationships among the geo-technical behavior of materials, local geological constraints including seismic site-class, liquefaction susceptibility, and the ground responses. The peak ground acceleration (PGA), peak ground velocity (PGV), spectral accelerations (SA), site amplification factors and the average shear wave velocity to a depth of 30m (Vs30) of geological formations of Bengal basin also support our viewpoint.