在台灣西南部，馬尼拉海溝往北延伸逐漸與歐亞大陸邊緣斜向聚合，更往北則因呂宋島弧劇烈碰撞形成複雜的台灣造山帶。此隱沒-碰撞系統具有複雜的板塊構造組成，主控著台灣西南海域的大地構造和地震活動。並且，一些主要構造，例如古呂宋琉球轉型板塊邊界(LRTPB)，大陸海洋邊界(COB)等，與馬尼拉海溝交會處都有顯著的海溝形狀和地震分佈形態的改變；存在於增積岩體內的脫序斷層(OOST)更被視為最容易引發大海嘯的地質構造。一直以來，由於缺乏大型的歷史地震記錄，台灣西南外海區域一直被認為是台灣周遭海域具有較低地震潛勢地區域，但2006年12月屏東外海規模7.2的強震和2012年10月於高雄市外海發生的Mw5.5地震引起國人關切。並且，不但美國地質調查局(USGS)發布的海嘯報告中指出馬尼拉海溝為高危險區域，新加坡南洋理工大學地球觀測研究所的研究團隊最新研究更顯示，在18世紀晚期一場地震引發的海嘯曾重創高雄和台南沿海村鎮。因此，對台灣西南外海地震活動和孕震構造特性的了解是非常重要的。但由於這些地震位於海域，陸上測站所收集的資料並無法提供足夠的資訊。因此為了解決以上問題，本計畫希望以台灣西南外海海域為研究區域，藉由臨時海底地震網的設置對於此區域地震的特性來進行觀察和研究。本計畫前期研究成果顯示，海底地震儀網絡在約20天中所記錄到的地震數量與世界網數十年所記錄到地震個數相近，相信藉由本計畫能夠收集到的資料結果必定能夠提供許多珍貴的資料。本計畫預定在2018年能把前期計畫中地震網位置往南延伸至靠近北緯20度的區域。馬尼拉海溝在該區域有最大曲率，若能在該區域施測，應更能了解海溝改變方向的機制和其對隱沒帶地震活動可能造成的影響。 ;In southwestern Taiwan, the South China Sea Plate subducts eastward beneath the Philippine Plate. The northern prolongation of Manila Trench, however, is colliding obliquely with the Eurasia Continental Shelf with a northwestern direction, creating the Taiwan Orogen. This subduction-collision system has a very complex tectonic environment which is characterized by variable seismic activities. Moreover, many different tectonic units are also observed in the vicinity, such as Luzon and Ryukyu transform plate boundary (LRTPB) and ocean and continental boundary (COB), where visible changes of bathymetry and earthquake distribution occurs. The presence of the Out of Sequence Fault (OOST) in the Manila accretionary wedge is even considered to be the most tsunamigenic structure. Generally, the southeastern Taiwan offshore area is considered as a low seismic potential area. Meanwhile, the occurrence of the 2006 December Mw 7.2 Pingtung earthquake and the 2012 October Mw 5.5 Kaohsiung earthquake have obtained the attention from publics. Furthermore, a recent report of USGS has pointed out that the Manila Trench could have high potential for generating both local and Pacific-wide tsunamis. Thus, it is important to clarify the role and nature of different geological features in the area. In this project, we plan to deploy a temporal ocean bottom seismic network in the vicinity of the southwestern Taiwan offshore area. We except to characterize the seismicity in these specific areas in order to better understand the influence of several tectonic structures in the area and its seismogenic characteristics.
