Evidence for the 1762 Tsunamigenic Earthquake in an Extremely Sedimented Segment of the Sunda Subduction/Collision Boundary, SE Bangladesh
Abstract:After the catastrophic 2004 Sumatra earthquake and tsunami, attention focussed on the segment of the Sunda subduction system along the heavily populated region of the Bay of Bengal. Here the Indian plate is converging obliquely with the Burma arc at GPS rates of 14mm/yr. The extremely thick sediments (~20km) of the Ganges-Brahmaputra Delta (GBD) are accreted into a very wide thrust-fold belt. Despite some likely aseismic deformation, the great 1762 earthquake proves the seismogenic potential of this boundary. Evidence is mounting that the earthquake ruptured 700km of the boundary from the Bengal Fan to the GBD shelf along Myanmar and Bangladesh. Recent U-Th ages and GPS measurements in Saint Martin Island, an anticline in the outer part of the accretion belt on the shelf, showed that corals died and were uplifted 2.5m coseismically during the 1762 earthquake. Historic accounts and modelling predicted a 1762 tsunami, but geologic evidence for it was not previously reported.
We found in both Saint Martin and ~20km north on the Teknaf coast strong evidence for tsunami emplacement. In Saint Martin we discovered a shelly sand layer that extends ~4km in the center of the island. The layer contains shallow water foraminifers, coral fragments and molluscs. Twenty C-14 ages from the shell layer range 1440-1753 cal yrs AD and average 1600 cal yrs AD. We also studied a shell layer that extends for 16,500 m2 on top of a terrace that is now 2m above present sea level and pre-1762 earthquake elevation. Ten C14 ages obtained from the shell bed range 1689-1875 cal yrs AD and average 1753 cal yrs AD. This bed is 1.2m thick and composed of molluscs and foraminifers of intertidal and subtidal origin currently found in shelf depths of 0-100 m. The layer is composed of cobble to mud size sediment and although chaotic, we identified at least three pulses of coarse sediment within an overall fining-upwards unit.
The studied 1762 shoreline uplift is likely to partly include coseiesmic elastic deformation associated with the 1762 rupture, following slow elastic subsidence, and partly related to the growth uplift due to thickening of the accretion wedge. To produce that large 2m coastal uplift and tsunami deposit with offshore fossil assemblages, in the study area, the megathrust, likely blind and nearly horizontal, must have ruptured near the deformation front.