Geometry of the Décollement Below Eastern Bangladesh and Implications for Seismic Hazard.

Eastern Bangladesh sits on the seismically active Chittagong‐Myanmar fold and thrust belt (CMFB), a north‐trending accretionary wedge on the eastern side of the India‐Eurasia collision. Earthquakes on the basal décollement and associated thrusts within the CMFB present a hazard to this densely populated region. In this study, we interpret 28 seismic reflection profiles from both published and unpublished sources to constrain the depth of the basal décollement. To convert profiles from the time domain to the depth domain, we integrate sonic log and seismic stacking velocity data to generate time‐velocity relationships for different parts of the CMFB. Our analysis reveals that the décollement is ∼9 km deep in northeast and southeast Bangladesh, but shallows to ∼5 km in east‐central Bangladesh. The décollement has an area of 7.25 × 104 km2 (∼150 × 450 km), making it capable of an Mw 8.5 earthquake. However, the warped geometry of this fault might act as a rupture barrier were a large earthquake to occur on the décollement. Our combined velocity and fault model lay the groundwork for future studies to address seismic segmentation, ground shaking, and rupture modeling in the CMFB. Finally, we use our compiled data set to analyze the evolution of fold kinematics in the CMFB. We observe that folding style and failure mode varies, from mainly ductile deformation in the foreland to mainly brittle in the hinterland. The dual‐failure modes within the CMFB support the hypothesis that a region with ductile deformation may still be capable of seismic behavior. Plain Language Summary: In Bangladesh, the interface between two tectonic plates has created a large, nearly flat, earthquake‐producing fault called a décollement. Above the décollement, sedimentary layers are compressed to create a series of north‐south trending folds and thrust faults that extend for hundreds of kilometers. This study uses 28 seismic reflection data sets (a technology similar to ultrasounds), originally collected by the oil and gas industry, and reinterprets the subsurface structure of the folds as a way to determine the décollement depth. We find that the décollement has a curved shape, with a depth of ∼9 km in northeast and southeast Bangladesh, and ∼5 km in east‐central Bangladesh. We hypothesize that the greater amount of sediment deposited in north and south Bangladesh has weighed down the surface of the earth and warped the décollement. This is the first study to constrain the geometry of this décollement, and we find that it has the potential to host a magnitude 8.5+ earthquake. The fault model presented here can be incorporated into studies of the rupture patterns of earthquakes on this décollement to better understand the earthquake hazard in a region inhabited by over 160 million people. Key Points: We present a data‐based seismic velocity model for Bangladesh to 10 km depthThe décollement underlying the Chittagong‐Myanmar fold and thrust belt forms a broad arch at depths of 5–9 kmThis morphology is likely caused by lithospheric flexure due to sediment deposition and may impact rupture propagation [ABSTRACT FROM AUTHOR]