The shallow portion of a megathrust represents the zone of first contact between two colliding plates, and its rheological properties control the seismic and tsunami hazards generated by the fault. Unfortunately, underwater geodetic observations are sparse due to the high cost of obtaining geodetic data, meaning limited information is available on the interseismic behavior of this part of most megathrusts. The Rakhine-Bangladesh megathrust offers a unique opportunity to probe the behavior of the shallow megathrust as it is the only ocean-continent subduction zone where the near-trench region is fully accessible on land. Here, we use observations from ALOS-2 wide-swath imagery spanning 2015 to 2022 to conduct an InSAR timeseries analysis of the overriding plate within Bangladesh and the Indo-Myanmar Ranges. We identify a narrow pattern of alternating uplift and subsidence associated with mapped anticlines but show that it cannot be explained by plausible rates of slip on the megathrust or other fault structures. Instead, we argue that the deformation is likely caused by active aseismic folding within the wedge above a shallow decollement. We show that estimates of the decollement depth derived from a viscous folding model and the observed anticline spacing are in agreement with previous seismic observations of the decollement depth across the fold belt. We suggest that the role of ductile deformation in the overriding plate in subduction zones may be more important than previously recognized.

Bangladesh, a small and over populated country in Southeast Asia occupies most of the Bengal Basin that results from sediments derived from the collision of India with Asia. The basin is filled with a 19 km thick sequence of Cenozoic sediments deposited by the mighty rivers Ganges and Brahmaputra. Unconsolidated Holocene sediments susceptible to seismic amplification characterize the upper part of the Cenozoic sequence. Bangladesh sits a top on three tectonic plates; India, Tibet and Burma. The India plate is colliding with the Tibet subplate to the north, which gives rise to great Himalayas, while to the east it is subducting beneath Burma and Sunda slivers, which gave rise to Indo-Burma arc. The Surma basin of NE Bangladesh is being underthrust under the Shillong massif producing the 2-km high plateau. The Indo-Burma fold and thrust belt results from the oblique subduction of the thick sediments of the Bengal Basin on the India plate that has deformed into a series of north-south trending en-echelon folds and thrust faults. The faults rooting these folds and the underlying megathrust are capable of generating devastating earthquakes in and around Bangladesh. Past earthquakes have brought changes to the landscape, avulsion of rivers Brahmaputra and Meghna, migration of human settlements, and widespread sand liquefactions and sand and/or mud eruptions. Our GPS study demonstrated that the landward extension of Andaman-Sumatra subduction zone into Indo-Burma subduction in deltaic Bangladesh is active. The present day India-Burma oblique convergence rate is 17 mm/y and that the décollement beneath the fold-thrust belt is locked (Steckler et. al., 2016). The western part of the subduction zone over a shallow décollement shows little seismicity whereas the eastern part shows moderate seismicity of magnitude 4 to 6. Based on the GPS velocity across the fold belt and seismicity the Indo-Burma subduction zone can be potentially be divided into locked western segment and slipping eastern segment, analogous to Cascadia subduction zone. Fold belt parallel shortening across Dauki Fault in Shillong is 7 mm/yr, which is another potential source of a large earthquake. The huge population might be severely ravaged by devastating earthquakes from both these sources.

We hypothesize that onshore saline groundwater in delta systems may have resulted from rapid shoreline progradation during the Holocene. To explore this hypothesis, we develop a model for the transport of saline groundwater in a shore-normal longitudinal cross-section of an evolving ocean margin. The transport model uses a control volume finite element model (CVFEM), where the mesh of node points evolves with the changing aquifer geometry while enforcing local mass balance around each node. The progradation of the shoreline and evolution of the aquifer geometry is represented by assuming the shoreline advances at a prescribed speed with fixed top and foreset slopes. The combined model of transport and progradation, is used to predict the transient trapping of saline water under an advancing shore-line across a range of realistic settings for shoreline velocity and aquifer hydraulic properties. For homogeneous aquifers, results indicate that the distance behind the shoreline, over which saline water can be detected, is controlled by the ratio of the shoreline prorogation rate to the aquifer velocity and the Peclet number. The presence of confining units probably had the greatest impact in sequestering onshore seawater behind an advancing shoreline. Further support for the validity of the proposed model is provided by fitting model predictions to data from two field sites (Mississippi River and Bengal Deltas); the results illustrate consistent agreement between predicted and observed locations of fossil seawater.