Bangladesh: Active Faults in the Chittagong-Tripura Fold Belt
To the west of the Intraplate Deformation zone, across the Indo-Burma Ranges, lies another large contractional zone: the Chittagong-Tripura fold belt. Thirty-five percent of the country of Bangladesh sits on top of this actively deforming region, including the capital of Bangladesh, Dhaka, with 16 million people living in its metropolitan area. In addition, this fold belt extends into western Myanmar and northeastern India.
A sample seismic line across this fold belt shows excellent imaging of faults and their related folding. This region contains sediments that range from ~24 million years ago to the present, and thus provides an opportunity to examine not only fault geometry but also the history of deformation and activity of the faults. Sediments that are deposited while faults are slipping exhibit typical changes in thickness that allow us to interpret the timing and rate of slip, which will provide a basis for pinpointing the most dangerous faults in the region. These conditions should allow the acquisition of excellent quality high-resolution seismic reflection data. In addition, the extensive exploitation of hydrocarbon reserves indicates that large amounts of industry data must be available as well.
In addition to the thrust faults that are typically considered as seismic hazards (faults that dip ~20-40°), this region exhibits another important hazard: that of detachments. Detachments are faults that lie parallel to bedding and transfer shortening horizontally. Thrust faults rise from detachments to the surface; this is typically how detachments are identified, because otherwise they are impossible to distinguish from the general stratigraphy (see e.g. the dashed line in figure below. Detachments generally localize within weak sediments – salt or overpressured shale. As a consequence, many scientists believe that they are too weak to accumulate large stresses, and will instead fail progressively as stress is applied, slipping aseismically.
However, recent earthquakes have provided evidence that detachments may indeed slip seismically. The 1999 M7.6 Chi-Chi earthquake, Taiwan, ruptured several detachments together with a shallowly rooted thrust ramp in a multi-segment event (e.g., Yue et al., 2005). The 2008 M7.9 Wenchuan earthquake, China, appears to exhibit 3-6 m of coseismic slip on a deep detachment at the base of the main thrust fault (Qi et al., 2011). These observations raise the possibility that detachments may be capable of producing earthquakes with severe groundshaking. This is particularly worrisome because detachments form some of the largest faults on Earth, underlying many populated basins, including 35% of Bangladesh.
The Chittagong-Tripura fold belt poses a further concern because it extends offshore. A large thrust event occurring offshore could potentially produce a regionally important tsunami with very little warning time for coastal residents.