Coastal deformation and paleo-earthquake records inferred from uplifted corals on Cheduba Island, western Myanmar

Cheduba Island is a major offshore island located off the western coast of Myanmar, on the hanging-wall block of the Sunda megathrust. Several previous studies have pointed out that the island and its eastern neighbor, Ramree Island, are both the geomorphic manifestation of two major upper-plate faults that splay off from the megathrust. Not only did these splay faults rupture together with the megathrust during the 1762 Arakan earthquake, the last giant earthquake event along this plate boundary, but the one beneath Ramree has also ruptured alone at least once, creating locally destructive earthquakes. Information from Cheduba, however, is much more limited, largely due to its restricted accessibility. In this study, we report additional coastal deformation data that are likely produced by both the giant 1762 and some local earthquakes, from surveys of marine terraces and 234U-230Th dating of uplifted coral colonies of Cheduba Island.

Based on our new data, the co-seismic uplift of 1762 is likely present along the entire coast of Cheduba. The amount of uplift, however, varies in different parts of the island, reflecting the subsurface geometry of the splay fault beneath the island and consistent with previous models. Except for corals that were likely uplifted and killed by the 1762 Arakan earthquake, we also found a group of corals that appeared to be killed about a century before 1762. Although current evidence is not sufficient to confirm that these corals were indeed killed by co-seismic uplift, a splay fault earthquake in the 17th century that produced local coastal uplift would be consistent with historical documents. Therefore, we propose that the splay fault beneath Cheduba is also capable of producing local destructive earthquakes, similar to the one beneath Ramree to the east. Our results would provide additional constraints for earthquake and tsunami hazard assessments for the rapidly developing western Myanmar coastal area.