台灣苗栗外海淺層構造初探

Abstract

臺灣位於歐亞大陸板塊與菲律賓海板塊的交界，菲律賓海板塊以每年約八公分的速度向西北方移動，長期下來，使臺灣形成一系列東北西南走向的褶皺逆衝帶。碰撞始於東部，愈往西部，斷層愈年輕活動力也愈強，學者將最西邊的逆衝斷層稱之為變形前緣。認為變形前緣為彰化斷層向南延伸至梅山、新化斷層，向北受到西北-東南走向的三義－埔里地震帶截斷、出海，座落在苗栗、新竹外海。 前人研究發現新竹斷層會向海域延伸，成為一條變形前緣線，入海後往西至35公尺深的海床，海床表面的沉積層受力產生褶皺或斷裂，顯示其活動是很年輕的。接續前人研究，本研究區域緊鄰其西南側的苗栗縣竹南後龍外海，使用多音束測深系統和側掃聲納系統得到高精度水下地形和辨認海床底質，將地形進行坡度分析後，把觀察到的線性構造在震測剖面上檢視，分析是否與構造活動有關。最後，利用淺層震測系統，去定位從剖面上所觀察到的斷層位置，了解斷層走向的趨勢。 研究結果顯示在研究區域發現的斷層與陸地上的新竹斷層的走向非常接近，新竹斷層為高傾角的逆衝斷層，而研究區域內的斷層傾角約30度向東南與新竹斷層傾角接近，因此我們認為該斷層是新竹斷層外海延伸的部分。

Taiwan is located in the collision zone between the Eurasia plate and the Philippine Sea plate. The Philippine Sea plate moves to northwest at a rate of 7 cm per year relative to the Eurasian plate resulting in forming a series of fold-and-thrust belts aligned in northeast-southwest direction. Westward compressive forces gave younger or more active faults are located in the west. Scholars refer the west edge of fold and thrust belts as the location of so-called deformation front. To the north of the Sanyi-Puli Seismic Zone, it is believed the front goes to ocean floor and is located offshore of Miaoli and Hsinchu County. Previous studies indicated that the deformation front follows the trace of Hsinchu fault extending offshore to the place where water depth at 35m. Beneath water layer, sediment strata were deformed compressively to imply its young occurrence. Accordingly, this study was proposed, we therefore believed that the trace of deformation front shall be turned southward and aligned offshore along the coastal line of Chunan-Houlong. The tools to trace the deformation front offshore Chunan and Houlong are reflection seismic system, multi-beam echo sounder and side-scan sonar system in this study. Survey results indicate offshore fault appears along a reverse fault with 30° dipping angle southeastwards, and its trace extending from Hsinchu Fault to the deeper area over 50 meters depth.