緬甸西南部蘭裏島之古地震事件研究

Abstract

位於東南亞的緬甸，在大地構造上，是夾在歐亞板塊與印澳板塊之間的一微板塊。在緬甸板塊的西側，印澳板塊向東隱沒入緬甸板塊之下，形成一巨大的碰撞隱沒邊界。由於此一隱沒作用，造成了緬甸板塊上活躍的構造活動。在此板塊邊界上，已知唯一的大規模歷史地震活動紀錄是在發生西元1762年的若開地震，該地震的活動紀錄在緬甸西部沿海地區都可以找到，且估計的地震規模高達8.8。先前在緬甸的海階研究中指出，此種規模的大地震活動周期約為900年。 本研究前往緬甸西南外海的蘭裏島，針對蘭裏島北邊四處地點的地形特徵進行研究，藉以瞭解此地區的地震周期。除了藉由DEM的影像判研當地的地形地貌之外，亦在野外中進行詳細調查記錄，採集可供定年之材料。利用地形的抬升記錄以及可以指示古海水面的生物特徵，瞭解過去的構造活動性。過去在島嶼的東北邊，靠近皎漂鎮的海岸上，有研究報導過這地方附著在岩壁上的牡蠣化石，是因為1762年的地震抬升而死亡，並得知1762年在此處的抬升約為1 m。本研究重新調查該地點，並發現在當地的海蝕凹壁同樣也記錄到1762年的地震抬升位移。在詳細的測量後，得知該地點有多次的地震活動記錄，且每次的地震規模均相似於1762年的地震事件。在蘭裏島的西北角，靠近雷卡蒙村落的海岸上，本研究在該地點的海蝕平台上，發現數段不同高程的珊瑚礁。在經過詳細的230Th定年後，得知1762年的地震事件在該地點所造成的抬升量約略與島嶼東側皎漂鎮所測得的抬升量相同。而在1762年的事件前後，都分別存在著另一次的抬升記錄，說明在蘭裏島上有更為複雜的構造抬升活動。根據珊瑚礁的分析結果，得知這些構造活動的抬升量，都與1762年的地震事件所造成的抬升量相近，顯示即使這些抬升事件只是地區性的構造現象，也可能成為極大的地震災害。雖然先前的研究報告指出在此地區具有約900年的地震周期，但仍然不得不謹慎思考地域性地震活動的可能性。

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.