SUBSURFACE GEOLOGY AND EARTHQUAKE HAZARD ASSESSMENT OF DHAKA, BANGLADESH FROM SHALLOW GEOPHYSICAL INVESTIGATIONS

Dhaka, the capital city of Bangladesh, is located mostly on Holocene river deposits at the southern fringe of the Madhupur Tract, a Pleistocene inlier. According to the Earthquake Disaster Risk Index, Dhaka's high population density, inherent weakness in building infrastructure, poor emergency preparation and recovery capability make it vulnerable to earthquakes. This vulnerability is exacerbated by neighboring faults (the Buriganga fault to the southwest, the Tongi fault to the north) and the underlying rock types that are dominated by weak mud and swampy deposits.

Vertical Electrical Sounding provides a snapshot of sediment distribution in Dhaka down to a depth of 241 m. Clay is dominant in the central and northern part of the city, whereas sand sequences outweigh the southwestern part. Sand percentages increase with depth toward northeast.

2–D Resistivity Imaging reveals lithological discontinuities, buried channels, regional tilting, and saturated–unsaturated fills. A generalized stratigraphic model exhibits three distinct units, from the surface down: i) Madhupur clay, ii) Dupi Tila sand, and iii) clay (of Girujan Formation). The lithologic data indicate the presence of a north–south trending fault between Tejgaon and Banani to the north, and around Dhaka University campus and Phulbaria Rail Station at the south, which eventually merges with the Buriganga fault.

Simulated 3–D models using borehole resistivity log data reveal a distribution pattern similar to that shown by the resistivity data from the Vertical Electrical Sounding survey. These patterns closely resemble the P–wave velocity distribution in the area.

An integrated approach has been adopted to prepare an earthquake hazard map, which divides Dhaka into four zones based on distribution of faults, subsurface lithologic changes and velocity variations.