- 學位論文
利用珊瑚微型環礁探討緬甸西部蘭裏島之震間變形與地震週期
Inter-seismic behavior and earthquake recurrence interval indicated by coral microatolls in Ramree Island, western Myanmar
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摘要
西元2004年的南亞大海嘯為巽它大型逆衝帶引發之地震所致，而位於此逆衝帶向北延伸的緬甸西海岸區域，同樣也面臨著地震與海嘯災害的潛在風險。在該區域歷史記錄中唯一一次的大規模地震是西元1762年的阿拉干地震，前人於位在隱沒帶上盤的蘭裏島與基督島，利用抬升的海岸地形已對該地震的規模與該地區的上盤有所認識。然而，若想估算此地可能的地震平均再發週期，尚需了解現地震間沉降速率、地震同震抬升量、與長期抬升速率等資訊。 本研究利用珊瑚微型環礁作為古海水面指標，以了解隱沒帶上盤震間時期的變形特性。珊瑚的生長高度大致受到海水面低潮線的限制，此高度稱為最高生存面。一旦珊瑚的生長高度與最高生存面達成平衡，珊瑚微型環礁的外形與其年生長紋即可記錄相對海水面的變化。本研究將珊瑚板狀樣本切片後拍攝X光影像以辨識年生長紋高程，並利用鈾釷定年技術確認珊瑚的年代及建立年代模型，以重建該地的相對海水面歷史，同時結合氧同位素分析的結果，協助辨認伴隨氣候事件而發生的海水面震盪。另一方面，利用海階海岸線角的高度與海階沉積物的年代，則可以估計該地的長期抬升速率。 本研究於距海溝80公里遠的蘭裏島海岸邊一處名為雷卡蒙的漁村進行野外工作，對一抬升的化石珊瑚微型環礁群進行測量，並採集板狀樣本分析。此群珊瑚死於西元1848年所發生的地區性地震抬升，記錄到該地震前約50年的相對海水面歷史。本研究亦調查雷卡蒙地區海階，共可分為四階，其高程介於1.0到11.4公尺之間。結果顯示珊瑚最高生存面的平均上升速率為每年5.3到5.8公釐，此速率可代表該地區地殼於震間沉降的速率。期間也記錄到五次相對海水面短暫下降的事件，配合珊瑚生長型態與氧同位素分析結果，推測其事件為印度洋偶極震盪事件所造成。而第四階海階形成的年代介於7.5至8.2 ka之間，估計現地長期抬升速率為每年1.0到1.7公釐。 結合前人對雷卡蒙地區古地震事件的了解與本研究的結果，估計蘭裏島西北部區域性構造的地震再發週期應落在300到360年之間，若考慮隱沒帶地震與區域性構造地震的綜合貢獻，地震平均再發週期會介在220到280年之間，此結果較過去認為的週期更短，故應對該地區可能的地震災害做好準備。
並列摘要
The Sunda megathrust is one of the major plate boundaries in South Asia, along which the Indian-Australian plate subducts northeastward beneath the Burma micro-plate offshore western Myanmar. In the past several centuries, the 1762 Arakan earthquake was the only giant event along this plate boundary, and caused significant co-seismic uplift along a large stretch of the western Myanmar coast. In previous studies, the magnitude of the event and the potential seismogenic structures were analyzed based on uplifted coastal features on the Ramree and Cheduba Islands, two of the largest coastal islands of western Myanmar. However, in order to estimate average earthquake recurrence interval, inter-seismic deformation rate, co-seismic uplift, and long-term uplift rate are all necessary. We utilized coral microatolls as natural tide gauge to analyze relative sea-level history and to obtain information of land-level change during the inter-seismic period, and utilized the elevations of marine terrace shoreline angles and the age of marine terrace deposits to determine long-term uplift rate. For some coral genus such as Porites, the highest level of survival (HLS) is constrained within a few centimeters of the lowest tide level. Therefore, once the relationship between HLS and the sea level is established, the morphology of the microatolls can provide us the relative sea-level history based on the patterns of their annual growth bands under x-radiographs. U-Th dating technique can constrain the age of the coral, and we can also identify sea level anomalies caused by climatic events through oxygen isotope analysis. We collected two slabs of coral microatolls from the intertidal zone near Leik-Ka-Maw, a small village in northwestern Ramree Island, approximately 80 km away from the trench. The microatolls were uplifted and killed during a local earthquake event in 1848, and preserved HLS records of about 50 years prior to the 1848 event. We also surveyed four marine terraces with elevations between 1.0 and 11.4 meters. Our results show that the coral recorded a HLS rise at a rate about 5.3 to 5.8 mm/yr, which represents land-level subsidence during the inter-seismic period. Several temporary HLS Diedown events are also present, likely produced by Indian Ocean Dipole events based on the microatoll morphology and results of oxygen isotopic analysis. The marine terrace level between 10.8 and 11.4 meters high formed during 7.5 ka and 8.2 ka, constraining a long-term uplift rate between 1.0 and 1.7 mm/yr. The recurrence interval of upper-plate splay fault earthquakes calculated using these results is between 300 to 360 years, and the average recurrence interval of both types of earthquakes (megathrust and splay fault) is between 220 to 280 years, both are shorter than the number obtained in previous studies. Therefore, it is important to further assess and prepare for earthquake hazards in this area.
參考文獻
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