2006年屏東地震引發沉積物之跨峽谷傳輸地質紀錄

Abstract

臺灣係菲律賓海板塊與歐亞板塊弧陸碰撞造山所生，面臨頻繁、嚴重的地震活動為其不可避免之宿命，地震所引發的沉積物重力流，也因此成為臺灣西南海域常見之地質災害。屏東地震發生於2006年12月26日，為規模7.0雙主震，震央位處臺灣西南海域，其所觸發一系列塊體運動與沉積物重力流於海底峽谷傳輸過程中，共破壞超過十條海底電纜，經濟損失以外，屏東地震對西南海域自然環境的影響，亦為此區近百年最鉅之地震活動。本研究利用沉積物分析及地球物理資料觀察，試圖重建屏東地震發生當時，枋寮海底峽谷頭部西側陸棚，漁民回報海面泥水湧出現象始末，並聯結峽谷兩側研究結果以通盤瞭解屏東地震引發沉積物重力流之傳輸模式。 前人研究指出，枋寮峽谷頭部東側線性凹陷區海床易液化的不穩定性，以及屏東地震造成之大規模海床崩塌，皆表示該區域受屏東地震影響輸出大量沉積物；而峽谷西側聲納剖面並無觀察到屏東地震所致明顯海床變動，卻可於岩芯表層側向追跡近一米厚之細粒事件沉積紀錄。綜合峽谷兩側研究結果，此事件紀錄可能為枋寮峽谷東側線性凹陷區的沉積物重力流，於順延西北地勢傾向朝枋寮峽谷移動過程，細粒懸浮沉積物並未以一般沉積物重力流於海床搬運，並且進入海底峽谷之傳輸模式移動，而係沿襲西北方向運動慣性跨越峽谷，最後堆積於峽谷西側陸棚海床。 枋寮峽谷頭部西側陸棚海域穩定沉積速率十年約一公分，與本研究劃定之近一米厚屏東地震事件紀錄差異甚大，兩者相較反映該區域沉積作用為屏東地震等大型極端事件活動所主導，且主要為海床原有沉積物側向搬運，即自峽谷東側線性凹陷區，跨峽谷傳輸後再沉積於峽谷西側，迥異於一般陸棚環境主要受河川輸出陸源物質影響之環境特性。

Caused by the oblique collision between the Eurasian and Philippine Sea Plates, Taiwan Island has to face the threat of frequent and severe earthquake activities inevitably. Thus, earthquake-induced sediment gravity flows also become a regular marine geo-hazard in the offshore area of Taiwan. The Pingtung earthquake doublet, occurred on December 26th, 2006, is one of the most impact earthquake events off southwestern Taiwan in the past 100 years. More than ten submarine cables were broken by the Pingtung earthquake and its aftershocks. After the earthquake, local fishermen immediately reported they were trapped in disturbed water at the west side of the Fangliao Submarine Canyon (FLSC) head. For figuring out the linkage between the earthquake and disturbed water, and further understanding the transportation process of the sediment gravity flows induced by the Pingtung earthquake, series of core and geophysical survey works were conducted in the past couple of years. On the east side of the FLSC head, the linear depressions which might be caused by earthquake induced liquefactions were a source area of gravity flows triggered by the Pingtung earthquake, and subsequently these gravity flows may transport northwestward into the FLSC by following the topography of the depressions. In this study, observation of chirp sonar profiles didn’t reveal any significant seafloor changes on the west side of the FLSC head, while a traceable event layer about one meter thick with fine grain size relative to the Pingtung earthquake was discovered in series of cores collected in this area. All results on both sides of the FLSC suggest that the event layer on the west side of the FLSC head would be the result of the cross-canyon transport of the suspended sediments in the gravity flow which was induced by the Pingtung earthquake and originated on the east side of the canyon. Furthermore, comparing with the long-term sedimentation rate at Gaoping Shelf (~ 0.1 cm/yr), our results point out the sedimentation process on the west side of the FLSC head might be dominated by extreme events like the Pingtung earthquake and largely influenced by lateral transportation, i.e., the cross-canyon movement, of inherent deposits on the shelf area instead of delivering sediments from rivers.