CO-SEISMIC COASTAL UPLIFT AND LONG-TERM GEOMORPHIC DEVELOPMENT ALONG THE SOUTHWESTERN COAST OF MYANMAR (BURMA)

Myanmar is located at one of the major plate boundaries of the world, between the Indian-Australian and the Eurasian plates. Along this major convergent plate boundary, strain partitioning has produced two loci of active deformation, including the subducting interface and the Sagaing fault, and both are among the largest seismic sources in Southeast Asia. For the Sagaing fault, information of past earthquakes can be obtained from the very detailed historical records of Buddhist pagodas. The western coastal area of Myanmar, on the other hand, has experienced one large historical earthquake in 1762. However, detailed information about this event is quite limited.

For the past two years, we have investigated coastal geomorphic features of two major islands in southwestern Myanmar, the Ramree Island and the Cheduba (Manaung) Island. By analyzing uplifted corals and oyster reefs, we were able to determine the amounts and patterns of co-seismic uplift in 1762 along the coast of these islands. The results are consistent with historical documents of the earthquake, and show that along the western Cheduba coast, co-seismic uplift was about 3-4 meters. This uplift decreased toward the east, and became only about 0.5 meter at the eastern coast of the island. However, further to the east, across the strait at the western Ramree coast, co-seismic uplift in 1762 was much higher, up to 5-6 meters. The uplift again decreased toward the east and became less than 1 meter near the eastern Ramree coast.

This co-seismic uplift pattern indicates that the earthquake source of 1762 was an upper-plate reverse fault system, instead of the subducting interface itself. This pattern is also consistent with the general topographic patterns of these two large islands, where the western coasts are steeper with higher and narrower marine terraces than the western coasts. Therefore, the long-term topographic development of the southwestern Myanmar coast, at least near the two major coastal islands, may be strongly related with co-seismic deformation of earthquakes produced by upper-plate structures.