Microatolls document the 1762 and prior earthquakes along the southeast coast of Bangladesh

In order to understand the seismic hazard associated with the Sunda megathrust along SE Bangladesh, it is necessary to document geologic evidence for the 1762 Arakan earthquake (M8.5-8.8) and prior events. Historical records indicated that the 1762 earthquake caused extensive damage along the Arakan segment of the Sunda subduction system, but geologic evidence for the earthquake farther north is necessary to better understand its associated seismic hazard to the heavily populated nation of Bangladesh. To document the paleoseismic history and understand ongoing tectonic deformation, we conducted detailed analyses of corals and microatolls in Saint Martin's Island, SE Bangladesh. The coral growth bands were studied, dated using U/Th systematics and their elevation was measured using high-precision GPS with RTX™ capability. The U/Th dating and >2 m of elevation differences between dead and living corals provide strong evidence of the coseismic uplift of 1762 Arakan earthquake. Based on annual banding patterns and reconstruction of the highest level of survival, coral microatolls indicate a tectonic subsidence rate of 2.2 ± 0.8 mm/yr prior to their uplift, consistent with elastic strain accumulation before the earthquake. In contrast, annual growth bands of a living coral microatoll indicate that Saint Martin's Island is presently experiencing uplift, at a rate of 1.8 ± 0.1 mm/yr. This suggests that the anticline underlying Saint Martin's Island is actively deforming following the earthquake. Our study also provides evidence for two additional earthquakes taking place in ~700 and ~1140 CE. These findings suggest a preliminary earthquake recurrence interval of ~500 years. Based on our results, the 1762 rupture can be extended from Faul Island (18°N) to Saint Martin's Island in SE Bangladesh (20.6°N). The motions documented from the corals suggest that this segment of the Arakan collision zone is storing sufficient energy to cause a future earthquake of M > 8.