台灣南部海域海底崩移之分佈與特徵

The Distribution of Submarine and Characteristics Landslides Offshore Southern Taiwan

吳佳瑜(Chia-Yu Wu)

指導教授 ： 劉家瑄

國立臺灣大學/理學院/海洋研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.02509

摘要

海底崩移是大陸斜坡上沈積物往深海盆地搬運的重要機制之一；突發的大規模海底崩移事件可能會造成海嘯而帶來浩劫。此外，海底崩移也可能代表古氣候中的變異事件。然而，由於過去缺乏精密的水深資料，因此，在台灣周遭海域尚未有系統的針對海底山崩作基本的型態描繪與成因機制的探討。
海底崩移大多發生於地形上坡度較陡之處，而台灣地區是個構造活動十分旺盛的地區，許多的地震事件造成海底的地形變動；且劇烈的造山活動發生時，往往同時也伴隨著強烈的侵蝕作用，這使得大量沉積物往海中運送。另外根據調查發現，台灣西南海域富含大量的天然氣水合物（gas hydrate），這些都是誘發海底崩移的潛在原因。從上述幾點得知，台灣周圍海域實為值得細究海底崩移之所在。
本研究使用精密之台灣周圍海域水深地形與側掃聲納資料，利用GIS軟體作為分析處理的平台，以海底崩移所呈現出的地形特徵來辨識其發生的位置，前人的研究在海底崩移的地形上發現下列特徵：在坡度劇烈改變處，有弧形主要崩面(arcuate headscarp)出現；而沿著主要崩面的兩側向下坡處延伸，則會出現趨近平行的側壁(sidewall)。有時候也會發現碎石堆(rubble)或崩塌物質殘留在崩塌地的底部端緣。辨識出海底崩移之後，本研究並計算其面積、體積、崩移坡度、主要崩面的高度，以及崩移所發生的水深位置等…資訊。此研究的區域涵括東經119.3˚ 至121.5˚，北緯21.5˚ 至23˚，並以台灣為界劃分為東南海域與西南海域兩側分別討論。
辨識結果顯示出在台灣南部海域共有72處的海底崩移，面積大小由1.3 km2至57.5 km2不等，而崩移的總體積為85.01 km3，平均每個崩移的體積是為1.18 km3。從此可看出本區域中的崩移量並不大，這可能代表了崩移作用在台灣南部海域中，可能並不是沉積物搬運的主要方式。
而從地形特徵上的不同，我們區分出三種不同型態的海底崩移，此三種不同型態的海底崩移應是受到沉積物流變性與後期沉積作用的不同，所型塑出來不同的海底崩移形貌。第一類崩移多發生在具有較高黏滯性(cohesive)的崩移物之處，崩落的物質會在破壞面的下緣部分形成趾部(toe)的堆積。第二類的崩移則多發生於黏滯性較低的沉積物區；或是具有沉積物崩移之後，因時間增長使得後期的沉積或侵蝕作用造成崩移物的地形特徵消失。第三類則為另一種有黏滯性質崩移的型態，然而本類型之崩移物質僅在滑移面上滾動，因此地形上並未看到如第一類的趾部堆積。本研究發現不論東南海域或西南海域，海底崩移型態皆以第二類型居多，顯示此處的崩移物質之黏滯性較低。
在台灣東南外海，海底崩移多分佈在大陸坡之上，從崩移的分佈推測上海底崩移的穩定與否是受到大陸坡的坡度大小所影響，而使沉積物本身過度的荷重所造成崩移作用；而誘發崩移的原因則可能肇因於頻繁的地震。在台灣西南海域，海底崩移的分佈多在海底峽谷之谷壁以及大陸邊緣上部陸坡接近棚坡交界處，其分佈受到構造走向的影響；而在誘發崩移的原因則可能是峽谷的下切作用造成。

關鍵字

海底崩移 ； 台灣南部海域 ； 地形特徵 ； 流變性

並列摘要

Submarine landslide is one of the mechanisms for sediment transport, and may cause geohazard. However, due to lack of high-resolution bathymetric data, few studies have been conducted on submarine landslides offshore Taiwan.
Taiwan is located in a tectonically highly active area, large quantities of sediment have been rapidly accumulated on the seafloor, and large amount of gas hydrates have been found in the offshore area of southern Taiwan. Thus, the geological condition is suitable for the development of submarine landslide.
In this study, we use GIS (Geographic Information System) software to identify submarine landslides in an area between 21.5˚N and 23˚N, and from 119.3˚ E to 121.5˚E, offshore SE and SW Taiwan, based on seafloor morphology derived from multi-beam bathymetric and SeaMARC II side-scan sonar data. Typical geomorphologic features of a submarine landslide consist of an arcuate headscarp, two sub-parallel sidewalls, and sometimes with slumped material appear at the base of the landslide scar. We also categorize landslides into three different types based on their morphology and the rheology of the sediments. Landslide types 1 and 3 are both consisted of cohesive material, and type 2 might represent less cohesive material comparing to types 1 and 3.
Based on our investigation, the total number of submarine landslides identified is 72, and the sizes of the identified landslides range from 1.3 km2 to 57.5 km2. The total volume of the landslides is 85.01 km3, and the average volume of the landslides is 1.18 km3. These results seem to suggest that landslides may not play an important role for sediment transport in the study area.
The statistic data also show that most frequently observed submarine landslide belong to type 2 submarine landslide, that is, the non-cohesive submarine landslide type, which suggests that submarine landslides in southern offshore Taiwan might occur more easily in less cohesive sedimentary strata.
We also observe that most of the submarine landslides in SW offshore Taiwan are located on the sidewalls of submarine canyons, the arrangements appear to be highly controlled by the structural trends. The triggering mechanisms for submarine landslide might be periodic slope-clearing landslides and gas hydrate dissociation.
In SE offshore Taiwan, the distribution of submarine landslides is controlled by the slope gradients, the steepen slope area present more submarine landslides. The triggering mechanism in this area could be frequent earthquakes or tremors.

並列關鍵字

submarine landslide ； offshore southern Taiwan ； geomorphologic features ； rheology

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