台灣西南外海的變形前緣接續馬尼拉海溝的北端，是南海海盆的最北端，其 東側是活動大陸邊緣，而西側是被動大陸邊緣。因此，此轉換帶在地質構造上介於 東邊逆衝斷層與褶皺構造帶及西邊正斷層構造間，顯得特別複雜。事實上，其真正 的空間位置一直沒有被仔細確定。因為變形前緣是活動大陸邊緣的西界，意味著最 西界的前緣逆衝斷層可能的發生處，如同台灣陸上的車籠埔斷層在1999 年突然發生 規模7.3 的地震，該前緣逆衝斷層也可能產生規模7 以上的地震，如此的地震會產 生海嘯影響到台灣南部。另一方面，被動大陸邊緣因有產生正斷層及向東南滑動的 海底山崩潛勢，而大規模山崩也會威脅到台灣南部的安全。即使如此，我們對此被 動大陸邊緣的地質構造也不甚清楚。因此被動大陸邊緣位於南海的最北端，地質構 造的特性與南海的初始張裂活動有關。在此三年的計畫，希望利用海洋地球物理的 方法去了解從淺部水深地形的變化到深部地殼的地質構造，除了可以了解被動大陸 邊緣與活動大陸邊緣的地質構造及其交互作用，進而可以評估此地體構造上可能造 成的災害性地震及海嘯的可能性。 ;The deformation front (DF) off SW Taiwan is the northward extension of the Manila Trench and is also located in the nethermost portion of the South China Sea (SCS). To the east of the DF, it is an active continental margin where the SCS lithosphere subducts beneath the Luzon Arc, while to the west of the DF it the passive continental margin of the northern SCS. Thus, the transition zone is complicated and separates the thrusts and folds to the east from the normal faults to the west. In fact, the exact trace of the DF was never been well defined. However, the frontal thrust abutting the DF is similar to the Chelungpu Fault in middle Taiwan that occurred an earthquake of magnitude 7.3. In the case of frontal thrust, it may generate large earthquake as well as large tsunami and jeopardize the south Taiwan. On the other hand, a long and SE dipping normal fault in the passive continental margin may cause a large submarine landslide and tsunami. In that case, a large tsunami also threatens the south Taiwan. Although the geology of continental margin in southernmost SCS involves the initial continental breakup, it was never been studied in details. Thus, we expect to collect and analyze marine geophysical data to better understand the geological structures of both the passive and the active continental margin of the northernmost SCS in 3 years. Especially we will use multi-beam bathymetry to study the morphological features and use seismic reflection profiles to understand the deep structures. Our results can also help us estimate potential large earthquakes and tsunamis in the northernmost SCS.
