Geophysical characterization and earthquake hazard vulnerability of Dhaka Mega City, Bangladesh, vis-à-vis impact of scenario earthquakes

Geophysical characterization of the subsurface by shallow seismic and electrical resistivity in conjunction with the geological conditions can provide more realistic information on earthquake hazard vulnerability assessment. Earthquake hazard vulnerability assessment requires not only the knowledge of earthquake source and propagation of seismic waves, but also local geological conditions which regulate ground shaking. Geophysical methods such as shallow seismic, electrical tomography, borehole geophysics and microtremor response are the most effective tools those contribute substantially in the earthquake hazard microzoning program. Strong-motion ground shaking character of a site due to an earthquake depends firstly on the magnitude of the source (moment magnitude, Mw) and the focal depth. However, fault reactivation can develop strong ground shaking at a site. Similarly, ground liquefaction can occur due to wave attenuation and amplification and can develop long residence time for ground shaking. Hence, earthquake hazard vulnerability of a site can be addressed more realistically taking into consideration of the source response, path response and site response. Groundwater condition and lithological variation of the subsurface zone can play an important role on the seismic response of a site. Geophysical parameters such as electrical resistivity, shear-wave velocity, dominant period and amplification factor in conjunction with geological information provide earthquake hazard vulnerability status. Earthquake hazard zoning is the first and the most important required step toward seismic risk analysis and mitigation strategy in the densely populated urban areas. Major earthquakes such as Bengal Earthquake in 1885 (also known as Manikganj Earthquake), 1897 Great Indian Earthquake and 1918 Srimangal Earthquake affected Dhaka Mega City of Bangladesh significantly. These earthquakes have been considered as the scenario earthquakes for hazard vulnerability assessment of Dhaka Mega City.