Evidence for a tsunami generated by the 1762 Great Arakan earthquake, Southeastern Bangladesh

Abstract:

The devastating 2004 Sumatra earthquake and tsunami brought to the world’s attention the possibility of large megathrust ruptures occurring along the heavily populated coast of Bangladesh. The Sunda-Andaman subduction arc continues into Bangladesh where oblique convergence of the Indian plate with the Burma segment of the arc was measured at GPS rates of 14 mm/yr. A long section of the megathurst ruptured during the 1762 Great Arakan earthquake along the Myanmar coast. A 1762 tsunami was reported from historic accounts and predicted from modelling, but geologic evidence for it was not previously reported.

St Martin’s Island and the Teknaf peninsula in SE Bangladesh are expressions of anticlines in the outer part of the accretionary prism of the Arakan subduction boundary. Our lithology, biostratigraphy and C-14 ages in this region provide evidence for a possible tsunami associated with the 1762 earthquake and validate the continuity of the rupture zone for as much as 700 km, from the Myanmar coast, Bay of Bengal shelf to the Sitakund anticline inland. In St. Martin’s, ≈70 km north of the shelf break, U-Th ages from ~2 m uplifted dead corals provide evidence for the 1762 rupture. The associated tsunami was dated from benthic foraminifers and marine molluscs obtained from a shell bed in a trench, and from the top of marine terraces possibly uplifted during 1762 or prior earthquakes. In Teknaf a chaotic bed containing benthic foraminifers, marine shells and cobble size rocks, extends for 100's of meters on top of the 2 m uplifted terrace. The marine molluscs were dated at 1695-1791 AD.

The large ≈2m uplift found in the region is likely to be deformation associated with the 1762 rupture and could also be partly related to anelastic growth of the accretion wedge and anticlines. It is likely now slowly subsiding due to continuing elastic loading of the megathrust. To produce that large coastal, presumably coseismic, uplift, the megathrust rupture must have reached at least that far updip (west). That uplift can be achieved by a large displacement on the megathrust below that coastline and/or by displacement along the steeper imbricate thrust faults responsible for the anticlines. Because the thrust front lies in shallow water a large tsunami would not be expected consistent with geological observations that point to low height tsunami waves.