臺灣西南海域重力流引發海底地質災害事件之研究

Abstract

海底塊體運動為海床上沈積物之抗剪強度無法承受環境中外應力作用時發生變形破壞，並且由於自身重力影響所產生向下或向外搬運的現象。由於其形成快速搬運的過程具有潛在破壞海床與水下設施之能力，因此可能引發海底地質災害。在台灣西南海域分別因2006年屏東地震、2009年莫拉克颱風及2010年甲仙地震等事件，造成多處海底電纜斷裂的紀錄。研究調查結果顯示，這些海底電纜斷裂的發生，皆與地震或颱風誘發海底塊體運動而以重力流形式往深海傳輸有關。觀察海底電纜斷裂點位置，大致沿著高屏海底峽谷由上游至馬尼拉海溝區域呈有時序的分布，但並非所有位於峽谷內之海底電纜皆發生斷裂。因此本研究推測高屏海底峽谷地形在海底電纜斷裂事件中應具有特定影響，且重力流在傳輸過程中，應存在影響其對海底電纜所形成之拉力大小的因素。而在屏東地震發生後1分鐘內，枋寮海底峽谷區域即發生2處海底電纜斷裂的紀錄，因此枋寮海底峽谷區域應為地震直接誘發海底塊體運動的區域。本研究利用每100公尺見方為一網格點的水深資料結合海底電纜斷裂資訊，探討高屏海底峽谷區域內海底電纜斷裂與地形之間的關係，以及使用連續變頻聲納剖面資料，搭配每50公尺見方為一網格點的水深資料，分析枋寮海底峽谷區海底塊體運動活動之紀錄及形成原因。 本研究分析結果顯示高屏海底峽谷區域內，在2006年屏東地震及2009年莫拉克颱風兩事件中，分別產生兩個主要重力流活動而造成海底電纜一系列斷裂。但比較峽谷內重力流流經海底電纜未造成斷裂及造成斷裂的點位間之坡度值，並無一明顯門檻值可區分兩者，且觀察重力流產生之流速與其流徑坡度間，及斷裂點坡度與水深間之相關係數及決定係數，皆顯示兩組因子間無明顯相關性。因此推測坡度不是最主要影響海底電纜發生斷裂的因子，地質條件或電纜當時所處狀態可能才是影響海底電纜是否斷裂的主要因素，而地形僅影響重力流流動方向。 枋寮海底峽谷區域內，地形分析結果顯示位於峽谷頭部右側海床上，從等深線200公尺處至等深線400公尺處，有平行等深線方向的長條狀特殊地形條帶分布。將此區域內連續變頻聲納資料依其回聲特徵分類成平坦狀 (type 1)、堆狀 (type 2)、透明狀 (type 3)及不規則狀 (type 4) 等四大類又十種型態後，比對回聲特徵的空間分布顯示，長條狀特殊地形條帶與顯示為海底崩移或濁流堆積之type 2-1位置相呼應。且峽谷頭部區域表現出地層發生液化(type 4-1) 及流體移棲 (type 4-3) 所造成之回聲特徵，說明此區域內沈積物孔隙間大多填充液體或氣體。基於觀察結果認為由於區域內海底地下水活動、流體移棲及頻繁地震的發生等因素，造成此區域沈積物較易變形破壞而發生海底塊體運動。

Submarine mass movements occur when the seabed could not sustain the stress applied, then the seafloor material will be transported downdip due to gravitational forces and may cause geohazards. In the area off southwest Taiwan, there were large scale submarine cable breakages after the 2006 Pingtung earthquake, the 2009 Morakot typhoon and the 2010 Jiashian earthquake. Investigation results show that both earthquake-induced submarine landslides and the flood-induced hyperpycnal flows have generated turbidity currents that carried considerable amount of sediments from the upper reach of the Gaoping Submarine Canyon (GPSC) to the Manila Trench, and damaged submarine cables lying across the GPSC. Most of the cable broken sites are along the axis of the GPSC, thus canyon morphology could be an important factor controlling transport processes of submarine mass movements and the sediment gravity flows passing through the crossing cables along their ways, and broke many of them. On the other hand, within a minute after the Pingtung Earthquake, submarine cables were broken at two places in the Fangliao Submarine Canyon (FLSC) area, suggesting that submarine mass movements were triggered on sites by the earthquake nearby. In this study, we use high-resolution bathymetry data (gridded at 100 m interval) and the cable breakage information (including the times and locations of the breakages) to investigate if the canyon morphology controls submarine cable breakages in the GPSC. We also use high-resolution bathymetry data (50-m grid spacing) and chirp sonar profile data to investigate the possible causes of submarine mass movements in the FLSC. In the GPSC, this study found that both the 2006 Pingtung earthquake and the 2009 Morakot typhoon have produced two sediment gravity flows each, many cable breakages were recorded along gravity flows 2006-1 and 2009-2, however, not all the cables crossing the GPSC were broken, and there is no obvious threshold on local slope values differentiating sites where cables were broken or not. In addition, the local slope values and the gravity flow speeds between two cable breakage sites in the gravity flow 2006-1 and 2009-2 show little correlation. We thus suggest that the local slope values are not the most important factor controlling whether the submarine cables to be broken or not. In the FLSC area, there are linear topographic features parallel to the isobaths at water depths between 200 to 400 m. Distribution map of different types of chirp sonar echo characters have been established. Four types of echo character patterns (flat, mounded, transparent and irregular) and ten sub-types have been identified in this area, and the echo characters of those topographic liner features suggest that they could be formed by the activities of the submarine landslide or turbidity current (type 2-1). Furthermore, the echo types 4-1 and 4-3 show that the pores of the sediment are filled with the liquids or gases, and there are frequent earthquakes which happened in the FLSC. Therefore, the local geological settings should be an important factor controlling the submarine mass movements in the FLSC.