Paleoseismic Records of 1762 and Similar Prior Earthquakes Along the South-Eastern Coast of Bangladesh

Abstract:

The great 1762 Arakan earthquake caused subsidence and uplift along 700km of the Arakan coast, and is thought to derive from a huge megathrust rupture reaching northward onto the southeastern coast of Bangladesh. Paleoseismic investigations were conducted in that area to document effects of that and prior earthquakes. U/Th ages obtained from isochron analysis of uplifted dead coral heads of the Poritesspecies, collected along a south to north transect from the islands east coast reveal at least three growth interruptions caused by abrupt relative sea-level changes within the past 1300 years that we interpret to be associated with megathrust ruptures. The ages show distinct events approximately 250, 900 and 1300 years ago. The youngest of these events corresponds to the 1762 Great Arakan earthquake. The two prior events at ~1100 and 700 AD, suggest an average recurrence interval of 400-600 years.

Along the coast of Teknaf, we mapped a ~2m uplifted terrace. Marine shells on top of the terrace dated with C-14 at 1695-1791 AD link the uplift to the 1762 Great Arakan earthquake. Based on this evidence and previous work (Wang et al., 2013 and Aung et al., 2008), we estimated the 1762 rupture to be at least 700 km long, from Chebuda Island to the Sitakund anticline encompassing the Teknaf Peninsula. Considering 14 mm/yr convergence rate and 400-600 yrs recurrence interval, this rupture zone has now accumulated elastic deformation to generate a M~8.4 earthquake, close to the M8.8 estimated by Cummins (2007) for the 1762 earthquake. Published recurrence intervals based on C-14 ages along the Myanmar coast ~90 km south of Bangladesh reveal three ruptures within the last 3400 years with an average recurrence interval of 1000-2000 years (Aung et al., 2008). While the 1762 rupture reached across both areas, some of the prior ruptures may be confined to one or the other of these areas, with a smaller magnitude.

Our precise U-Th ages provide evidence of recurrence intervals of great megathrust earthquakes along the Arakan subduction zone, which is first to be proposed for historic and prehistoric earthquakes along the coast of Bangladesh. It is critical to constrain the recurrence interval and size of these earthquakes for better hazard assessment, given that three densely populated and structurally unplanned cities are near the rupture zone.