由於南中國海盆地隱沒到呂宋島（菲律賓）西邊之下，而形成活動聚合板塊邊緣的馬尼拉海溝。本研究區域和其他隱沒區域類似，是可能產生大地震、海底山崩和海嘯的高風險區域，而這些自然災害會對附近區域的人口和基礎建設造成極大的威脅。因此，研究現今馬尼拉隱沒系統的地殼構造以及變形顯得非常重要。本研究主要利用由海研五號（R/V Ocean Researcher 5）所收集的多頻道反射震測資料以及高解析水深資料所作研究的結果。 本研究區域隱沒系統在聚合界面以及弧前區域有著不同的變形機制。透過海上震測資料以及水深資料的分析顯示地形和變形機制上大致以北緯17度為一界線來區分，海底地形的變化是因海洋岩石圈中的差異造成，使得馬尼拉海溝和其弧前區域在此界線上呈現著不同的變形機制，如因南海海盆擴張導致海山、海脊、沉積物供給量、重新活動的構造和斷層等。馬尼拉海溝北段主要被歸類為增積型的聚合邊界（寬廣且平緩的增積前緣），而南段則主要是侵蝕型的聚合邊界（狹窄且陡峭的增積前緣），故馬尼拉海溝南段陡峭的坡度使得此區域較容易發生海底崩塌（slope failures）和塊體運動（mass wasting）。此一界線（17°N）也將弧前盆地分隔成北呂宋海槽（北段弧前盆地）和西呂宋海槽（南段弧前盆地），相較於沉積物較少卻有大型斷層的北段弧前盆地，南段弧前盆地因擁有大量的沉積物而產生較快速的應力累積。進一步的構造影像分析顯示，馬尼拉海溝南段侵蝕型的聚合邊界因過去海山的隱沒曾造成大量的海底崩塌現象，因此推斷這個古海底崩塌事件可能在鄰近的區域造成海嘯。資料顯示對比過去數值研究模型，停止張裂的中洋脊(Scarborough Seamount Chain)在北緯16度附近似乎尚未開始隱沒。 海底山隱沒至上覆板塊下時，可能會隨著向下的隱沒作用而造成弧前地區的破裂，而隱沒接觸區域附近的破裂網絡(network)可能因此限制未來地震的破裂區域。雖然也有可能發生一些較小規模的地震，但是此現象使得馬尼拉海溝南段發生大地震(Mw≥9) 的機率變得較低。馬尼拉南部地殼受應力形成壓縮變形並與菲律賓斷層帶走向滑動的斷層重疊，在此區域的西北東南走向的海脊和海盆與剪力(shearing)以及較陡峭的增積岩體前緣有關係，且有助於造成此馬尼拉海溝區的海底邊坡不穩定。 ;Crustal structure and deformation in the Manila subduction zone
Leo Armada Taiwan International Graduate Program- Earth System Science National Central University
ABSTRACT
The Manila trench is the active convergent margin where the South China Sea (SCS) basin is being subducted beneath the western portion of Luzon Island (Philippines). The deformation in the study area, as in other subduction zones, is associated with natural hazards like large mega-thrust earthquakes, submarine landslides and tsunamis. These processes pose great risks to populations and infrastructures in surrounding regions. In this regard, it is important to study the present crustal structure and ongoing deformation in the Manila subduction zone. This dissertation presents research results derived from new multi-channel seismic reflection (MCS) data (collected using the Ocean Researcher 5) and high quality bathymetry.
The subduction zone is characterized by varying deformation patterns in the subduction interface and the overlying fore-arc region. Analyses of the marine MCS and bathymetric datasets indicate a distinct morphological and deformational boundary near 17°N latitude. Differences in the nature of the subducting oceanic lithosphere (i.e. seafloor relief related to seamounts and ridges, sediment supply, reactivated features and faults associated with the SCS opening) cause variations in the deformation observed across this boundary in the Manila trench and fore-arc. The northern segment is classified as an accretionary margin (with a wide frontal wedge with gentle slope) while the southern segment is mainly an erosive margin (with a narrow and steep frontal wedge). Consequently, the steep frontal wedge in the southern segment is prone to submarine slope failures and mass wasting. The boundary also separates the fore-arc basin into the North Luzon Trough (northern fore-arc basin) and the West Luzon Trough (southern fore-arc basin). Abundant sediments in the southern fore-arc basin and less sediments in the northern fore-arc basin imply faster strain loading in the southern segment compared to the northern segment of the subduction mega-thrust. Furthermore, detailed images of the structure indicate that the erosive margin in the southern segment has a history of seamount subduction with associated large submarine slope failures. The inferred ancient submarine mass wasting events may have caused tsunamis in adjacent areas. The data also show that the Scarborough Seamount Chain (extinct mid-ocean ridge) is not yet subducted near 16°N latitude, contrary to previous models.
Seamounts in the subducting oceanic lithosphere may induce fracturing of the overlying fore-arc crust as it progresses downward. The fracture networks near the subduction interface may then limit the rupture area of future mega-thrust earthquakes. This will lead to the low probability of great subduction earthquakes (Mw≥9) in the southern segment of the Manila trench, although intermediate sized earthquakes should not be discounted. The upper crustal structure of the southern end of the Manila trench is characterized by compressive deformation. It is also overlapping with strike-slip (shear) motion related to the Philippine Fault Zone. The NW-SE oriented ridge and basin morphologies related to shearing and the steep frontal wedge are conducive to submarine slope failure in this portion of the Manila trench.
