臺灣南部海域馬尼拉隱沒系統的構造特徵

Abstract

一般認為隱沒帶是容易引發大地震的區域，主要是因為此地區的大地震多是由兩板塊間的巨型逆斷層(megathrust)所引起，此斷層常是大地震的孕震帶，板塊間聚合的應力容易在此累積，當應力累積到一定程度，地震發生時，斷層的錯動造成海床的破裂，也可能同時產生大海嘯。臺灣南部外海的馬尼拉隱沒帶，也被認為有類似的巨型逆斷層的構造。本研究重新處理以及分析四條切過臺灣南部增積岩體的長支距震測剖面資料，來探討此地區巨型逆斷層的構造活動。 馬尼拉隱沒系統是由歐亞板塊的南中國海洋岩石圈向東隱沒至菲律賓海板塊之下形成，由西向東分別有馬尼拉海溝、增積岩體、北呂宋海槽及呂宋火山島弧。 馬尼拉隱沒系統的增積岩體可分為上部斜坡以及下部斜坡，上下斜坡間可能存在著一條主要的脫序逆衝斷層(out-of-sequence thrust)。增積岩體上部斜坡的下方可能存在著底板作用(underplating)所形成的雙軌構造(duplex)，造成上部斜坡被抬昇，代表著沉積物在隱沒過程中被帶入較深處，在上部斜坡下方形成此構造。本研究在東經120度，北緯19度的地區，觀察到可能是雙軌構造的頂衝逆斷層的反射，另在下部斜坡的下方觀察到滑脫面。滑脫面以上的沉積物被擠壓成為增積岩體，滑脫面以下屬於伸張環境，並且沉積物被帶往更深處。震測剖面分析亦顯示上下部斜坡交界處存在著數條從巨型逆斷層分支出來的逆斷層，沒有和滑脫面相連，且斷層面可向上連接至海床。這些斷層就是巨型分枝斷層(megasplay fault)，其存在代表著這地區是大地震以及產生海嘯的高風險地區。由南到北，滑脫面的分布有逐漸變寬變廣的趨勢，此分布和增積岩體的範圍大小呈正相關，推測越往北部，大陸地殼物質開始被帶入隱沒帶，板塊隱沒角度變小，沉積物量變大，造成增積岩體由南到北變廣，滑脫面的發育也隨著增積岩體發育而變廣。巨型分枝斷層的分岔點以及出露在海床上的位置和海溝的距離由南到北並無明顯的變化，代表著巨型分枝斷層的發育位置尚未受到弧-陸碰撞顯著的影響。

Historical record of significant disastrous earthquakes (Mw>8) has mostly been regarded as megathrust earthquakes that slipped along the interface of underthrusting plate and overriding plate. This kind of thrust is thought to be the seismogenic zone of the great earthquakes. Stresses between two plates can be easily accumulated in this zone, and sea floor may rupture during the co-seismic slip of the megathrust, generating great tsunami. Previous studies have reported that a major out-of-sequence thrust is present in the accretionary wedge south of Taiwan, thus megathrust earthquakes could be generated in the Manila subduction zone south of Taiwan. This research analyzes 4 large-offset seismic profile data collected during the TAIGER survey in 2009 for the purpose of investigating the deep structures of the Manila subduction zone. Advanced processing techniques have been employed to eliminate the multiple energy on the seismic profile that improves the quality of the deep crust images. Manila subduction system is located between the Philippine Sea Plate and the Eurasian Plate. This subduction system can be divided into four major tectonic features from west to east: Manila Trench, accretionary wedge, North Luzon Trough and Luzon volcanic arc. The accretionary wedge can be further divided into a lower slope domain and an upper slope domain by a major out-of-sequence thrust. Duplex structure produced by underplating processes may exist below the upper slope domain, as inferred by the roof thrust reflection. Decollement has clearly been observed below the lower slope domain of the accretionary wedge. The range of the decollement increases from south to north, inferring that the continental materials were carrying into the subduction zone and the subduction angle is decreasing from south to north. This study has identified two or three thrusts branching out from the megathrust and not connected to the decollement. These thrusts may extend to the sea floor, and they are so-called megasplay faults. There is no obvious change on the branching point and megasplay fault position from subduction to incipient collision. The existence of the gigantic thrust represents this area have high potential of seismic and tsunami threat.