台灣位處歐亞大陸板塊與菲律賓海板塊的交界，顯著的板塊碰撞力在西部麓山帶西緣產生一連串近南北走向的逆衝斷層，以及近東西走向的平移斷層。在大地構造上，以北港高區為分隔，其南北的相對移動速度有明顯差異，使得附近地塊產生旋轉，造成走向滑移斷層的出現，屯子腳斷層即是近東西向的走向滑移斷層。 屯子腳斷層位在后里台地的東南側，以N60°E 的方位往西南延伸，跨越大甲溪至大肚台地北側，全長約12 公里。該斷層是因1935 年新竹-台中地震所引發的地震斷層，水平位移1.5 公尺，垂直位移最多60 公分，錯動量不大卻引起嚴重災情，是台灣近百年來災情最為慘重的一次地震。 屯子腳斷層以東有三義斷層，西是彰化斷層，西南側遇大肚背斜，地下深部則有泰安斷層，使斷層活動會受附近構造影響，因此本研究以高解析度淺層反射震測為主，配合深部震測剖面圖，針對屯子腳斷層沿線及其附近構造進行探測，藉以瞭解在區域構造的作用下，斷層演化及地質架構。結果如下： 1.屯子腳斷層的形貌類似壓縮型花狀構造，受到擠壓應力使得地層拱起並平移錯動，花狀構造僅出現在2 公里以上，構造起伏約50?80 公尺。 2.屯子腳斷層的應力中心在斷層中段，地層被抬升至距地表260 公尺處，錯動幅度約60 公尺；而斷層東端則潛伏到三義斷層之下，且受到阻擋而停止；斷層的西端則與大肚背斜相交，在兩者的互相擠壓之下，使得此處的地層被抬升到較淺處，深度約200 公尺，而地層的變位也較大。 3. 屯子腳斷層受控於深部的泰安斷層，在近地表1 公里附近產生錯動，其錯動的弧度最大僅80 公尺。屯子腳斷層的錯動僅分佈在斷層沿線，寬度約1 公里，並無往北或往南延伸，是個小規模的花狀構造。 4. 根據深部的震測剖面發現在后里台地的北側也有一列西北—東南走向的花狀構造，代表以前在后里台地北側就曾經發生過走向滑移斷層，不過在漸移(propagation)環境之下，在后里台地的南側再次因構造應力而開始活動，而屯子腳斷層應該是現代構造作用時再次錯動所造成的走向滑移斷層。 The Tuntzuchiao fault is a right lateral strike-slip fault trending N60°E with the maximum horizontal displacement 2m and the vertical displacement 60cm. The fault was activated by the Hsinchu-Taichung Earthquake (M=7.1) in 1935. The total length of the Tuntzuchiao fault is about 12 km. The east of the Tuntzuchiao fault is bounded by the Sanyi fault; the west is the Changhua fault; the south lies the Dadu anticline. A large boundary fault, named the Ta-an fault by CPC, has been found at deep places, which may control the development of the Tuntzuchiao fault above it. By using the reflection seismic method, we depicted special features of the Tuntzuchiao fault and obtained some interesting findings: 1. The feature of the Tuntzuchiao fault is like a compressed flower structure. The structural layers are bended with limited breakages. The depth range of the flower structure is less than two kilometer. The layer uplift is about 50~100 meter. An apparent near-vertical breakage is visible at the southern side of the flower structure, which extends to the surface. 2. The stress of the Tuntzuchiao fault is centered at the middle and the amplitude of deformation is about 60m. The Sanyi fault blocked the Tuntzuchiao fault at its eastern end, where the Tuntzuchiao fault is buried under the Sanyi thrust. On the western side, the Dadu anticline significantly interrupts the Tuntzuchiao fault. 3. The deformation of Tuntzuchiao fault is apparently influenced by the underneath Ta-an fault. The Tuntzuchiao fault is found spreading within 1 km range of the Ta-an fault at shallow depths, which appears as a small flower pattern above large base trunk. 4. After checking several deep reflection seismic profiles, we also find a series of flower structures in the northwestern Holi terrace, which trends in the NW-SE direction conjugating with the Tuntzuchiao fault. Thus, the area must be in a translational environment on the northern slope of the Peikang High, which develops strike-slip faults with flower structures.
