利用抬升之珊瑚探討緬甸西部基督島的海岸變形與古地震事件

Abstract

緬甸西部外海的基督島（Cheduba Island）位於巽他大型逆衝帶（Sunda megathrust）變形前緣的東側，前人研究指出基督島與其鄰近的蘭裏島（Ramree Island）長期的變形行為，分別受控於底下的兩條隱沒帶分支斷層。在1762年阿拉干大地震中，巽他大型逆衝帶連同上部的分支斷層一起破裂，在這兩座島沿海留下了連續的海岸抬升紀錄。除了1762年的地震事件外，在蘭裏島北岸的珊瑚微型環礁（coral microatoll）也記錄下由單一條分支斷層破裂所造成的地區型地震事件。然而，現今仍未有研究討論基督島是否也經歷過類似的地區型地震事件。因此，本研究藉由基督島上抬升之珊瑚，重建1762年阿拉干大地震以及其他古地震事件造成的海岸變形，並探討基督島釋放應力的地震模式。 本研究利用鈾釷定年以及 RTK-GNSS 取得抬升之珊瑚的年代和高程資料，用以重建古地震事件的抬升量。根據抬升珊瑚的分布，1762年地震時平行基督島西部海岸線的抬升量由北至南遞減；在垂直西部海岸線的剖面中，抬升量分布與前人研究相似，符合基督島的背斜狀構造特徵。然而抬升量在全島的數值皆高於前人報導的結果，可能指示1762年大地震的滑移量在過去可能被低估。此外，本研究首次找到可能用來指示十七世紀地震的地質證據，這項發現與當地的口述傳說相符。根據珊瑚死亡事件群的時空分布，本研究推測這次地區型地震可能是由基督島北方的分支斷層獨自破裂所引發。基於這項新的地震紀錄，本研究認為基督島的地震模型為一雙峰模式，顯示分支斷層也具有獨自產生地震的孕震能力。然而，目前對於基督島的震間變形行為的暸解仍十分有限，這使得準確估算地區型地震事件的再現週期變得十分困難。未來應對於巽他大型逆衝帶投入更詳盡的古地震以及古測地研究，以有效預防未來地震發生時的災害。

Cheduba Island is a major offshore island off the western coast of Myanmar and lies on the hanging-wall block of the Sunda megathrust. Previous studies indicate the long-term deformation behavior of Cheduba Island and its neighboring Ramree Island is controlled by two major upper-plate faults that splay off from the megathrust. During the 1762 Arakan earthquake, the Sunda megathrust ruptured along with two upper-plate splay faults, resulting in continuous coastal uplift along western Myanmar. Apart from the 1762 earthquake event, coral microatolls on the northern coast of Ramree Island have also recorded local events produced by the rupture of an upper-plate splay fault. However, whether Cheduba Island has experienced similar local earthquake events remains unknown. Therefore, in this study, we conducted paleoseismic investigations on Cheduba Island, aiming to reconstruct a more detailed earthquake history using uplifted corals. This study utilizes uranium-thorium dating and RTK-GNSS to obtain the ages and elevation data of the uplifted corals to reconstruct coastal deformations of paleo-earthquake events. According to the distribution of uplifted corals, the uplift caused by the 1762 earthquake along the western coast of Cheduba Island decreases from north to south. Along the profile perpendicular to the western coast, the pattern of uplift closely resembles findings in previous studies, and is consistent with a major anticline structure beneath Cheduba Island. However, the amounts of uplift are higher than those previously reported across the island, indicating that the slip during the 1762 earthquake may have been underestimated. Furthermore, we found geological evidence for an earthquake in the early seventeenth century, consistent with oral historical records. Based on the spatiotemporal distribution of uplifted coral groups, this earthquake may have been a local event produced by the rupture of an upper-plate splay fault located offshore northwestern Cheduba Island. Based on this new earthquake record, this study proposes a bi-modal earthquake model for Cheduba Island. This model suggests that the upper-plate splay fault offshore western Cheduba Island is also capable of producing local earthquakes. However, since information about the interseismic deformation behavior on Cheduba Island is limited, it is difficult to accurately estimate the recurrence interval of these local earthquakes. In the future, it is necessary to conduct more comprehensive paleoseismic and paleogeodetic studies along the Sunda megathrust to effectively mitigate future earthquake disasters.