琉球隱沒帶位在台灣東部海域，由菲律賓海板塊隱沒到歐亞板塊之下所形成。過去的研究認為隱沒板塊粗糙體(Asperity)是影響隱沒帶地震規模大小的主要原因。而近幾年的研究表示增積岩體深部的隱沒沉積物(Under-Thrusting Sediment)物理性質也扮演了重要的角色。過去在琉球隱沒帶最南段已有許多地震與地體構造的相關研究，然而對隱沒沉積物物理性質卻還不甚清楚，為了探討琉球隱沒帶最南段的隱沒沉積物物理性質，本研究處理了5條台灣大地動力學計畫(TAIwan GEodynamics Research program, TAIGER)長支距多頻道反射震測線，搭配重合前深度移位(Pre-Stack Depth Migration, PSDM)技術得到地層速度模型，經過與正常膠結狀態地層速度模型修正後，得到代表沉積物膠結狀態(Consolidation State)的殘差速度模型。為了進一步探討影響沉積物膠結狀態的可能原因與其物理意義，本研究利用日本南海海槽(Nankai Trough)的大洋鑽探計畫(Ocean Drilling Program, ODP)鑽井資料進行地層孔隙率(Porosity)推估，並計算地層平均有效應力(Mean Effective Stress)分佈。 其結果顯示在加瓜海脊西側，靠近台灣的區域增積岩體深部大致上呈現不足膠結狀態(Under-Consolidation)，並隨著遠離台灣逐漸轉變為過度膠結狀態(Over-Consolidation)，在加瓜海脊東側則呈現不足膠結狀態為主。經過比對地層孔隙率分布與前人研究，可以推測不足膠結狀態的沉積物與孔隙流體有關。從平均有效應力分布中可以觀察到在加瓜海脊西側，隱沒沉積物在靠近台灣的區域有較大範圍的低有效應力區，並隨著遠離台灣逐步縮小，表示靠近台灣的區域增積岩體內的逆衝斷層較容易發生滑動，屬於低耦合狀態，相反的在遠離台灣的區域逆衝斷層較不易滑動，屬於強耦合狀態。 ;Ryukyu Subduction Zone is located in the eastern offshore of Taiwan, where the Philippine Sea Plate is subducting beneath the Eurasian Plate. The previous studies suggest that the existence of asperity on the downgoing plate may be accumulated the strain on the plate interface and also could be linked to occurrence of the catastrophic earthquake. However, recent studies (i.e. 2011 East Japan Trench earthquake) have shown that the physical properties of the Under-Thrusting Sediment may also play an important role. In the last decade, there have been studies on earthquake related or surface structures in the southernmost Ryukyu subduction zone. However, the physical properties of under-thrusting sediments in the study area are still not clear. In order to estimate the physical properties (such as velocity structures, porosity as well as stress field) of the under-thrusting sediments in the southernmost Ryukyu Subduction Zone, we have re-processed five large-offset multi-channel seismic sections from the TAIwan GEodynamics Research program (TAIGER). The P-wave velocity model was derived by pre-stack depth migration (PSDM) technique. The status of consolidation residual velocity model in the study area was furthered corrected by using normal consolidation velocity model. In addition, distributions of porosity and mean effective stress field were estimated by using the physical parameters derived by Ocean Drilling Program (ODP) data in the Nankai Trough. The results show that an under consolidation, higher porosity and low effective stress field was founded near Taiwan. This high fluid regime becomes narrow gradually far from Taiwan. This indicates that the source of the fluid comes from Taiwan island. The thrust faults in the accretionary prism are easier to slip. East of 122°15 ′E, the under-thrusting sediments become stronger coupling. East of the Guaga Ridge, the under-thrusting sediments are fluid rich as evidenced by occurrence of low frequency earthquakes constrained by previous studies.
