臺灣西南海域沈積物重力流引發之海底電纜斷裂事件

Sediment gravity flow induced submarine cable failures off southwestern Taiwan

鄭屹雅(Yi-Ya Cheng)

指導教授 ： 蘇志杰

國立臺灣大學/理學院/海洋研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.03210

摘要

臺灣西南海域，包含高屏陸棚、高屏陸坡至馬尼拉海溝北端，自2006年以來經歷多次颱風及地震事件（2006屏東地震、2009莫拉克颱風和2010甲仙地震），並引發海底電纜斷裂事件。本研究利用海研一號於2006至2012年間收集的岩芯和變頻聲納剖面分析結果，配合中華電信海底電纜斷裂資料及經濟部水利署測站記錄，討論海底地質災害事件與海纜斷裂事件間的可能關係。
高屏海底峽谷頭部接續高屏溪，當高屏溪發生洪氾事件時，異重流沿著高屏海底峽谷往外海傳輸。而起始於高屏陸棚的枋寮海底峽谷，沈積物可能源自於中央山脈南端的低度變質區，其後板岩碎屑直接進入狹窄的高屏陸棚，經枋寮海底峽谷往深海傳遞。
高屏陸棚之海底地層剖面皆有混亂狀及液化構造，顯示高屏陸棚為地質不穩定帶，地震或颱風容易引發海底山崩。2006年屏東地震引發之斷纜事件，將斷纜位置之距離及間隔時間估算出濁流速度，計算出的濁流流速差異甚大，因此認為屏東地震為多次沈積物崩移短距離傳輸所致。2009年8月8日莫拉克颱風侵臺，南臺灣降下豪大雨，根據高屏溪流域里嶺大橋的記錄，顯示8月9日之河川懸砂量高達兩千萬噸，這些懸浮沈積物由高屏溪入海所形成的異重流可能破壞位於高屏海底峽谷內的海底電纜。此外，莫拉克颱風期間臺灣西南沿岸地下水井因大雨補注，地下水水位明顯增高，顯示區域內之地下水補注通道發達。
根據海底斷纜的資料，這些斷纜位置大多沿著高屏海底峽谷或枋寮海底峽谷軸部分布，顯示這些地區可能為沈積物侵蝕及傳輸的重要通道。從高屏海底峽谷及枋寮海底峽谷中採集之岩芯中，皆可以觀察到濁流沈積物、木屑、破碎的生物殼體，顯示沈積物在短時間內傳輸至峽谷中下段。高屏陸坡盆地Ⅲ的岩芯內觀察到210
Pb活度值呈指數型遞減，顯示在近期未受到濁流之影響。

關鍵字

海底峽谷 ； 異重流 ； 濁流 ； 沈積物液化 ； 海底電纜 ； 屏東地震 ； 莫拉克颱風

並列摘要

Since 2006, southern Taiwan has been through several big typhoons and earthquakes which triggered submarine landslides and turbidity currents and damaged many submarine cables off southwestern Taiwan. In this study, we integrate the results of sub-bottom profiles, sedimentological analysis, hydrological data, and time series of cable breakages to find how the natural hazards (e.g. typhoon and earthquake) caused submarine cable failures.
Off southwestern Taiwan, two major submarine canyons, the Gaoping Submarine Canyon (GPSC) and Fangliao Submarine Canyon (FLSC), are incising from continental shelf to deep sea floor and both of them transport considerable amounts of sediment to the South China Sea. GPSC is directly connected to the Gaoping River on land. During the flooding event, hyperpycnal flows following the GPSC down to deep sea. In contrast, the FLSC, which is smaller, younger and confined to the slope, does not associate with any river on land. The source of sediments might directly from the Central Mountain Range. Torrential rains induced landslides delivered large amount of slate fragments through narrow Gaoping shelf into the deep sea.
From sub-bottom profiles, the liquefaction structures and chaotic deposits are widely spread off southwestern Taiwan. In such unstable geological and hydrological setting, the submarine landslides can be easily triggered by earthquake or extreme flooding. According to time series of cable breakages in 2006 Pingtung Earthquake, we suggests that a series of aftershocks or sea floor sediment liquefactions might induced submarine landslides and sediment gravity flows. In 2009 typhoon Morakot, torrential rain induced hyperpycnal flow in GPSC may broke submarine cables at the junction of
iv
upper and middle reaches. During typhoon period, the groundwater levels raised in Pingtung Plain. The increased groundwater pressure might lead to high flux of submarine groundwater discharge and induced liquefaction on seafloor which could triggered submarine landslides and damaged submarine cables.
Most of the broken cable sites are along the GPSC and FLSC axis, indicating that canyon might be the major transport path of sediment. Cores collected from the GPSC and FLSC have observed turbidites、wood fragments and broken shells. It shows that sediments are transported rapidly through the submarine canyons. The total 210
Pb activity profiles in cores collected from Gaoping intraslope basin III show an exponential decrease with depths, which suggests there is no severe sediment-laden flow pass through in the past 10 decades. Coarse slate fragments are widely distributed along the FLSC. In contrast, we do not find any coarse grain sediment in the middle reach of GPSC. Our finding suggests that the deep sea coarse slates might be generated from the FLSC.

並列關鍵字

submarine canyon ； hyperpycnal flow ； turbidity current ； liquefaction ； submarine cable ； Pingtung earthquake ； Morakot

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