臺灣南部海域增積岩體流體滯留深度與力學強度之探討

根據歷史海嘯資料，大多的海嘯事件皆由海底地震引發，且發生於隱沒帶的海底地震具有較高的威脅性。增積岩體為隱沒帶重要的地質特徵之一，但影響其參數中的滑脫面強度(F)以及增積岩體強度(W)卻尚未被研究清楚。前人研究發現臺灣南部海域存在增積岩體的構造，為釐清增積岩體的力學機制，故此，本研究利用TAIGER計畫在臺灣南部馬尼拉海溝隱沒帶收集的四條長支距震測資料進行分析，以期瞭解臺灣南部海域増積岩體由南到北的力學機制之演變與影響因素。在震測剖面處理上，本研究使用特徵向量濾波(eigenvector filter)及Radon濾波消除剖面的複反射能量，並利用層間速度(interval velocity)進行剖面時深轉換以獲得地層中正確的構造深度。藉由速度資料可獲得地層中的流體滯留深度(fluid retention depth, FRD)，同時量測海床坡度(α)及滑脫面傾角(β)。最後將測得數值經由臨界不均質増積楔型體理論進行計算，並利用航次OR1-1032於小琉球外海取得岩芯之平均抗剪摩擦角(φ)推算影響增積岩體強度的基底滑脫面上的孔隙水壓比(λb)及基底滑脫面上的平均摩擦係數(μb)。彙整四條震測剖面的結果，發現流體滯留深度在斷層發生前會出現較深的深度，也許可以藉由此深度位置指示未來斷層發育的方向。本研究計算結果與前人於其他地區計算結果比較，發現臺灣南部海域的増積岩體有較強的滑脫面斷層強度及較弱的增積岩體強度，岩體強度約為斷層強度的2到4倍。北邊兩條測線(MGL0905-25A及MGL0908-4)因有海底山隱沒，故推算後之基底滑脫面上的孔隙水壓比較低，基底滑脫面上的平均摩擦係數較高。且測線MGL0905-25A底滑脫面上的平均摩擦係數較其他測線高出許多，可能造成能量在此累積，尚未釋放，未來有規模較大地震發生的可能。

關鍵字

不均質増積楔型體理論；流體滯留深度；滑脫面斷層強度；增積岩體強度；基底滑脫面之Hubbert-Rubey孔隙水壓比；基底滑脫面上的平均摩擦係數

並列摘要

According to historical tsunami data, most of the tsunami events are triggered by submarine earthquakes, and the submarine earthquakes that occur in the subduction zone are highly threatening. The accretionary wedge is one of the important geological features of the subduction zone, but the strengths of decollement and accretionary wedge are difﬁcult to constrain. In the Manila subduction zone south of Taiwan, previous studies have reported that a major decollement is present in the accretionary wedge. This study analyzed four large-offset seismic profile collected during the TAIGER survey in 2009, for the purpose to understanding the evolution and influencing factors of the mechanical mechanism of the accretionary wedge in southern Taiwan. In seismic process, Radon filter and eigenvector filter are applied to eliminate the multiples on the seismic profiles and uese interval velocity to time-depth correlation. The fluid retention depth (FRD) is constrained by the velocity structure, and measure the seafloor slope (α) and the decollement dip(β). Controlling factor of shape of Manlia subduction zone is investigated via the critically heterogeneous Coulomb wedge theory. Also, the average shear-friction angle (φ) of the core obtained from the OR1-1032 survey at Xiao Liuqiu is used to calculate the pore-water pressure ratio (λb) and the average coefficient of friction (μb) of the decollement. Integrating the results from previous studies and this study, found that the fluid retention depth (FRD) deeper before the fault occurs and may indicate the direction of future fault development. Compare to previous studying, wdge in the southern of Taiwan relatively sronger decollement strenght and weaker wege strength. The wege strnght is about two to four times the decollement strenght. Because of the seamount subduction in lines MGL0905-25A and MGL0908-4, the pore-water pressure is relatively low and the average coefficient of friction is relatively high. And the average coefficient of friction of line MGL0905-25A is much higher than other lines, there may be a high potential to produce large earthquakes in the future.

並列關鍵字

critically heterogeneous Coulomb wedge theory ； fluid retention depth (FRD) ； strengths of decollement ； strengths of wedge ； Hubbert-Rubey pore-water pressure ratio ； average coefficient of friction