| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 101 === Myanmar is located at the convergent boundary between the Indian-Australian and Eurasian plates. Along the northernmost part of the Sunda megathrust, the Indian-Australian plate subducts northeastward underneath the Burma micro-plate, and produces a series of deformation belts with many seismic activities. The last major earthquake along this belt occurred in 1762, with an estimated magnitude of about 8.5. From ages of uplifted marine terraces, it has been proposed that the earthquake recurrence interval in this area is about 900 years.
We surveyed four locations along the coast in northern Ramree Island, from north-eastern to south-western, Kyauk-Pyu (KPU), Tang-She (TS), Nga-Pyi-Taung (NPT) and Leik-ka-maw (LKM). Near the town of KPU, we found several levels of sea-notches on a sandstone ridge next to the coast. Up to four levels of uplifted sea-notches are present above the lowest notch, which has been shown to represent the co-seismic uplift during the 1762 earthquake. Since each notch has a ~1 m elevation difference, we suggest that there have been several paleo-earthquake events prior to 1762, and those events had similar magnitude to the 1762 Arakan earthquake. Along the coastline of northern Ramree Island to the west, there are 2 – 4 steps of marine terraces. In the site of TS, two steps of terraces be measured and also sampled several datable shell fragments in a trench. To the westernmost of northern Ramree Island, there is a small village named LKM. We found three groups of uplifted coral colonies with different elevations in the coastline. Th-230 ages of the corals indicate that the second group was killed by co-seismic uplift during the 1762 earthquake. The other two groups may therefore represent at least one event before and after the 1762 earthquake, respectively. The possible event after 1762 has not been reported anywhere else in western Myanmar, thus it may represent a local event. Furthermore, since all of these three uplifted coral groups are lower than the lowest marine terrace, a step of marine terrace may require more than one earthquake event to form in this area. Therefore, in the previous studies, using only the ages of local marine terraces to determine the earthquake recurrence intervals in western Myanmar may have overestimated.
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