隱沒帶依照沉積物的供應量，可分為沉積物較多的增積型（accretion type）隱沒帶及幾無沉積物分布隱沒的侵蝕型（erosion type）隱沒帶，侵蝕型隱沒帶由於缺乏沉積物隱沒，常導致隱沒岩石圈與上覆岩石圈地殼直接刮擦而崩塌，甚而這些侵蝕下來的塊體部分會跟著隱沒岩石圈隱沒下去。而增積型隱沒帶隱沒沉積物（under-thrusting sediments）的物理性質，包括孔隙率、含水量、膠結狀態（consolidation state）及引伸出的隱沒沉積物孔隙液壓及有效應力等參數，深深影響周邊逆衝斷層的活動，進而發生災害性大地震。最新研究顯示，在加拿大外海的卡斯卡迪亞（Cascadia）隱沒帶亦為增積型隱沒帶，根據地震及相關地球物理觀測建議的強耦合地帶，可以觀察到該區隱沒沉積物的膠結呈現過度膠結狀態（over consolidation），經推算之地層孔隙液壓過高，可能導致大地震的發生。另外，若隱沒沉積物為濁流性質，因快速沉積通常呈現高孔隙液壓狀態，這使得源自於深部的大型逆衝斷層得以破裂至地表而引發海嘯型大地震。本計畫的第一年期間我們已分析數條由過去TAIGER計畫所蒐集之長支距震測資料，但缺乏靠近台灣西南外海的資料，因此本年度計畫預定將資料蒐集重點放在台灣西南外海變形前緣區域進行資料蒐集，整合分析所有資料後，預期可建立馬尼拉海溝北段隱沒沉積物的流體狀態模型。 ;The subduction zones can be classified as two categories: accretion type and erosive type based on sediments supply. Due to lack of sediments subduction, the subduction crust can be “scratched” overlying crust and result in crustal blocks erosion. Those debris or crustal blocks are even under-thrusted between accretionary prism. In contrast to erosive type margin, physical properties of under-thrusting sediments which include porosity, consolidation status as well as pore pressure and effective stress can affect the activities of surrounding thrust faults. The resent study in the Cascadia margin offshore Canada shows that an over consolidation under-thrusting sediments is identified and suggests a strong coupling origin strongly based on earthquake studies. In the other hand, an under consolidation under-thrusting sediments regime located offshore Orogen State USA is associated with gas hydrate distribution on top of high fluid content under-thrusting sediments layer. This indicates that Orogen State region belongs to weak coupling area. Furthermore, a case study in the Alaska margin pointed out when subduction sediments are turbidity origin, it tends to be over pressure and reveal high potential to generate tsunamigenic earthquake. In first year project, we have analyzed several seismic profiles collected by TAIGER project. However, due to lack of seismic profiles southwestern offshore Taiwan and middle section of the northern Manila subduction system. In this project, we will collect new seismic data in those two area and combine analyze TAIGER data and new data to propose a physical property model of the under-thrusting sediments. We will also discuss the relationship between fluid within under-thrusting sediments and catastrophic earthquake.
