Coseismic Deformation of the Great 1762 Arakan Subduction Earthquake Along South-Eastern Coast of Bangladesh

Large subduction earthquakes and tsunamis along the India-Burma (Sunda) plate boundary were brought to the world's attention after the catastrophic 2004 Sumatra earthquake. Subduction and thus possible megathrust earthquakes are thought to continue north of the Bay of Bengal into the Ganges-Brahmaputra Delta. Here the delta is being subducted resulting in ongoing accretion that is manifested by a wide active fold belt. Bangladesh, with a population of ~160 million is crossed by this plate boundary. Yet, little is known about the hazard there from megathrust earthquakes. The one in 1762 ruptured the >600 km long Arakan segment of the northern Sunda megathrust, well into the shelf area of the delta, including SE Bangladesh. The event was estimated as Mw8.8 by Cummins (2007) based on observed uplift and subsidence. To begin documenting coseismic uplift and the consequences of the 1762 earthquake in southern Bangladesh, we conducted paleoseismological studies on Saint Martin's Island and the Teknaf coast, along the outer portion of the fold belt in southeastern Bangladesh. Our structural and age data indicate that the island of Saint Martin overlies an active anticline, thus uplift can be caused by both anticline growth and the megathrust. Present water depths on the shelf near Saint Martin are ~30 m, but the island was marine during mid- to late-Holocene, and during Oxygen Isotope Stage 5. Shoreline erosion along the southern tip of the island has uncovered the tilted limbs of the anticline. Evidence for the 1762 earthquake is manifested in uplifted coquina terraces dated through detailed radiocarbon studies at 1590-1680 cal yrs AD. Similar ages were obtained for uplifted terraces along the Teknaf coast. Dead corals of the massive species "porites" are exposed along the eastern and western parts of Saint Martin. Coastal emergence was measured by a Fast Static GPS relative to mean sea level using EGM96 geoid model to be 6.6 m for the coquina terraces and 2 m for the corals. U-Th ages of coral samples are being conducted to improve age resolution.