TECTONIC AND GEOTECHNICAL REVIEW OF BENGAL BASIN FOR SEISMIC RISK ASSESSMENT IN BANGLADESH

The seismic risk in densely populated areas is one of the major socio-economic concerns that warrant serious attention. Bengal basin covers most of Bangladesh and part of India. The basin is the world’s youngest and largest delta, nearly twice the size of the Mississippi delta. It is seismically active due to a complex intraplate tectonic setting and junctions between the Indian shield and a systematically folded Indo-Burma range. The northeast motion of the Indian plate crushed the continental and oceanic crusts, pushed and yielded a deeper ocean basin (15-18 km) over undulated Cretaceous basement, and filled up with Himalayan sediments.

Four major geotectonic provinces of the basin are recognized: 1) the continental slope to the west of the Hinge Zone, 2) the stable shelf, 3) the deep central trough (Sylhet-Hatiya) and 4) the Chittagong-Tripura fold belt to the east. The ~300 km long Dauki Fault demarcates the elevated Shillong Plateau, part of the Indian Shield to the north and the deep basin to the south. The basin experienced three strong to major intraplate earthquakes: a) 1885 Bengal earthquake (rev. Mw 6.8) close to the Hinge Zone, b) 1918 Srimangal earthquake (rev. Mw 7.1) on the Sylhet (trough) fault and c) 1923 Mymensingh earthquake (rev. Mw 7.0) at the northern end of the Hinge Zone where it intersects the Dauki Fault. In addition, Bangladesh also experienced similar tectonic energy release from the Shillong Plateau earthquakes like Great Assam Earthquake (1897, rev. Mw 8.1) and moderately active Indo-Burma subduction zone in the east.

Rational assessment for seismic threats is determined by: hypocenter locations, the intensity of local/regional seismicity, differential tectonic stress condition, the geometry of discontinuities, and states of highest energy release. Site effects are evaluated from geophysical and geotechnical investigations. A 3D site-specific seismic hazard characterization of the capital megacity Dhaka is made to assess the seismic risk. It is observed that lateral and vertical discontinuities are subjected to multiple segmentations that facilitate tectonic movements. 1918 Srimangal event of Mw 7.1 is the largest recorded intraplate earthquake. Generation of enough tectonic stress in Bengal basin is very unlikely which might cause megathrust induced earthquakes (Mw 8-9) in Bangladesh.