歐亞板塊向東南沿著馬尼拉隱沒帶隱沒至菲律賓海板塊之下，馬尼拉隱沒帶系統北段座落於台灣島與呂宋島之間。自從2004年的蘇門答臘─安德曼大地震之後，馬尼拉隱沒帶的海嘯潛勢與災害風險開始被大量關注。馬尼拉隱沒帶的地震活動相對於琉球海溝的地震活動度較低，雖然如此，台灣西南外海的海床坡度較淺，且台灣西南部的人口分佈密集，因此對於馬尼拉隱沒帶附近地震震源機制解分佈圖與地震活動特性的了解就顯得相當重要。然而，即使馬尼拉隱沒帶部分的大型構造已經透過許多地球物理及地質研究資料加以描繪，但由於缺乏地震測站的覆蓋造成不完整的地震記錄依舊限制我們對於發生於海域區域地震特性的了解。 為了得到更多關於北馬尼拉隱沒帶的地震地體構造的資料，我們於2016年6月22日至7月13日及2017年6月27日至7月20日這兩個時段各佈放了8個海底地震儀所組成的臨時地震網，在這44天的記錄時間之中，共有1437筆地震事件被記錄且定位出來。在同一個記錄期間內，北馬尼拉隱沒帶在世界地震網的資料當中只記錄到15個地震，且皆未位於海底地震儀地震網內，本研究定位出來的地震位置分布與世界地震網的資料也相差甚遠，顯示在海域佈放海底地震儀的臨時地震網是有效且有必要性的。本研究重新定位的結果可將馬尼拉海溝北部之地震分為三群，除了海溝後的地震群之外，地震事件大多分布於海溝前的隱沒板塊之中，呈現西北─東南走向，深度大約分布在25公里到40公里之間。另外，本研究的地震事件規模幾乎都小於2.5，正說明了陸上測站無法記錄發生於海域規模小地震的缺點可以被海底地震儀補足。本研究的地震定位結果也顯示，鮮少地震發生於隱沒板塊與上覆板塊的板塊交界面，許多淺層沉積層中已被辦識出的斷層上也沒有記錄到地震。從海溝前地震的傾沒方向與走向，推測海溝前地震形成機制可能與隱沒板塊的撓曲有關，而其中地震型態沿著南北方向的差異似乎受到地殼厚度與性質所影響，除此之外，位於海溝前地震群的複合震源機制解為走向滑移斷層，其錯動型態與呂宋─琉球轉形邊界走向平行，與前人研究中在其構造南部末端的所推論的剪切應力環境相符。而海溝後的地震群位置相當集中，應為區域效應所導致，我們推測是被埋沒的海底山隱沒時造成的擠壓應力與荷重效應，進而影響隱沒板塊地殼底部的板塊撓曲所造成，而逆推之三維速度模型的速度高區與此地震群位置相符，我們推測與海底山底部的上部地函物質有關。 ;The Eurasian Plate (EP) subducts beneath the Philippine Sea Plate (PSP) along the Manila subduction zone. The northern portion of this subduction system is located between the Taiwan and Luzon Island. Since the 2004 great Sumatra-Andaman earthquake, the tsunamigenic potential and the hazard risk of the Manila subduction zone has been underlined. Although some main tectonic structures of the area have been imaged based on geophysical approaches, the incomplete earthquake records caused by the poor seismic station coverage still limits our understanding for the seismogenic characteristics of the area. In order to obtain more information about the seismotectonics of the northern Manila subduction zone, two temporal seismic arrays composed of 8 ocean bottom seismometers (OBSs) were deployed along the northern Manila trench from June 22 to July 12, 2016 and from June 27 to July 20, 2017, respectively. During this recording period, 1437 earthquakes were determined and relocated. Less than 15 earthquakes have been reported in the international or local catalogue for the same period, showing the efficiency and necessity of OBSs networks for the seismictoctonic studies for the marine area. The result shows that the earthquakes appear to be segmented and divided into different groups. Furthermore, the hypocenters distribution reveals that most of the events occurred in the subducting plate to the west of the trench with depth shallower than 40 km and magnitude lower than 2, which infers the seismogenic behavior of the northern Manila Subduction system should be mainly controlled by the plate bending or/and the reactivation of tectonic structures in the subducting plate. Relatively less earthquakes are located to the east of the trench. Instead of spreading along the plate interface, they demonstrate a spatial concentration distribution. From the earthquakes relocation results, the earthquake events separate to three swarms. We suggest that the earthquake swarms in front of the trench are influenced by the plate bending in the subducting plate which could be observed by the seismic pattern. Furthermore, the composite focal mechanism inferred that these earthquakes are facilitated by strike-slip fault. The direction of the strike is parallel to Luzon Ryukyu Transition Plate Boundary, which is consistent to with the shearing feature in the southern end of Luzon Ryukyu Transition Plate Boundary. In the other hand, the earthquake swarm behind the trench is caused by local effect. We suggest that it is caused by the plate bending in the bottom of the subducting crust influence by compression stress or the loading of the buried seamount.
