台灣西南海域非活動型大陸邊緣海底塊體運動之探討

Abstract

海底塊體運動(submarine mass movement)為影響大陸邊緣形貌的營力之ㄧ，也是大陸邊緣沉積物傳輸的重要機制，除此之外海底塊體運動可能引發海嘯，對沿海地區的人與建築物帶來了潛在的地質災害威脅。因此近代研究學者開始著手研究海底塊體運動分佈、塊體物質來源、發生機制、演化的模式，藉此提防海底塊體運動所帶來的危害。 台灣西南海域非活動型大陸邊緣，為具有廣大的大陸棚、坡的大陸邊緣，由於受到強烈海底塊體運動的侵蝕，在大陸邊緣上分佈了密集的海底峽谷與海脊，造成地形坡度變化甚大。另外從1990年代開始陸續從上萬公里長的多頻道反射震測資料中辨識出 「海床仿擬反射」 (bottom simulating reflection，簡稱BSR)，證明顯示本研究區域富存著天然氣水合物，然而天然氣水合物的解離可能因此造成地層的不穩定，而引發海底塊體運動。上述因素，提供了本研究區域海底塊體運動的驅使機制。 本研究結果顯示從過去到現在，發生過多次的海底塊體運動。根據不同回聲型態的辨識，顯示塊體運動的種類分佈會根據不同的水深、地形環境，呈現不同的型態，並且各塊體運動可能因為水力參數 (hydrodynamic parameters)的改變而彼此轉換，如碎屑流轉換成濁流、轉動滑移轉變為碎屑流。另外由多頻道反射震測剖面顯示，海底塊體運動的發生可能會造成天然氣水合物穩定帶的變化。

Submarine mass movement is one of the major forces which affect the morphology of continental margin. It is also one of the major mechanisms of sediment transport, and may cause geohazard. Therefore it is important to investigate the distribution, source, mechanisms and evolution model of submarine mass movement. The passive margin offshore of Southwestern Taiwan receives considerable quantities of terrigenous sediment that deposited on the continental shelf and slope. Due to the effect of strong submarine erosion, it could produce steep ridges and submarine canyons on the continental slope. From the multi-channel seismic reflection profile data, we can indentify bottom simulating reflector (BSR) which suggests that gas hydrate is widely distributed in this region. The dissociation of gas hydrate could result in filling of free gas in a layer, and change the steady state of strata. Those factors above are the major mechanisms which trigger submarine mass movement. Numerous submarine mass movements have been observed in the study area, and the distribution of the submarine mass movement has a tendency of depending on water depth and topography. The characteristics of submarine mass movement can transform from one type to another, according to changes of hydrodynamic parameters, and submarine mass movement may contribute to the migration of gas hydrate stability zone (GHSZ).