台灣西北部苗栗—台中地區地質構造分佈上，三義斷層是一條極為特殊的低角度逆衝斷層，該斷層從豐原往北，以幾近正北方向穿過大甲溪、后里台地、大安溪後，進入苗栗丘陵，但在三義附近突然轉為朝東，直到大湖附近才停止。前人研究（主要為中油探勘團隊）指出三義斷層為低角度（小於30度）之逆掩斷層，主要沿著中新世之東坑層（即河排層）底部，逆衝滑移至少10公里，將其上之老地層覆蓋於年輕地層（例如頭嵙山層）之上，此往上爬昇超覆之斷層滑動，右移錯開出磺坑背斜之南端，形成舌形突出，造成三義附近之斷層轉折。一般以三義斷層在后里台地上為紅土礫石所掩覆，認為係屬老斷層，現今已不再活動。但中央地質調查所以一大甲溪北岸之露頭，及斷層兩側水系分佈，認為下盤之紅土台地堆積層仍有受到攪動，因而將之定位為第二類活動斷層（十萬年內或錯移台地堆積層）。 為探討三義斷層是否為活動斷層，本研究採用淺層反射震測法，在斷層可能經過之位置佈線施測，除了尋找斷層在地表淺處的正確位置，並推論斷層錯動的形式及機制，以進一步瞭解該斷層之特性。本研究由北而南共安排了十條測線，所獲資料品質尚稱良好（深度達600公尺），可以清楚看出剖面東半邊為向東傾斜之地層，應為斷層上盤之東坑層以上之地層，剖面西半邊則為水平之頭嵙山層。 本研究獲得以下幾點結論： 1.三義斷層上盤為東坑層，地層傾角為20度至25度之間，下盤為頭嵙山層，地層維持水平狀。東坑層沿著其底部之三義斷層逆衝到頭嵙山層之上。 2.上盤東坑層內有一些次生破裂，但規模不大，若干可能延伸到地表（如大甲溪北岸露頭），下盤之頭嵙山層則未受到東坑層逆衝太大影響，故在頭嵙山層內並沒有發現任何破裂痕跡。 3.三義斷層之位態及運動行為適用於「薄皮逆衝理論」，此斷層面沿著東坑層底部滑動，逆衝到頭嵙山層上。在大安溪以北，逆衝爬昇至銅鑼向斜及出磺坑背斜南端之上，在大安溪以南，則擠壓上衝至厚層之頭嵙山層之上。 4.由地物資料及鑽井資料可知東坑層底部近地表處已受到侵蝕移除，並由台地堆積層或現代沖積層所取代，此長期之侵蝕沈積，代表此斷層形成年代已經很久。 5.由地形上可發現近地表之斷層線受到河流的侵蝕沈積後，已向東移動了數百公尺，明顯指出三義斷層已經有一段很長的時間沒有活動，故現今活動可能性應屬不高。 The Sani fault is a low angle thrust fault distributed in the Maioli-Taichung area, northwestern Taiwan. The fault trends in a northward direction from Fengyuan, across the Tachiahsi stream, the Houli terrace, the TaAnhsi stream into the Miaoli hills. After this, the fault trace changes to the east near the town of Sani and stops near the town of Tahu. The previous studies point out that the Sani fault is a low angle overthrust fault which slides along the bottom of the Miocene Tungkeng formation by more than 10 km. The old structures thrust above the young structures which forms a tongue-shape intrusion, cutting through the southern flank of the Chuhuangkeng anticline. The Sani fault is covered by a thick laterite and gravel layer on the Houli terrace. Hence, it is recognized as an old fault, and may not be active now. However recently, the Central Geological Survey (CGS) bases on an outcrop along the Tachiahsi stream, claims that the fault may be quite young. The problem of the activity of the Sani fault seems still unsolved. The purpose of this study is to re-examine the Sani fault and to determine the degree of its activity. The method of the shallow seismic reflection is used for the fault detection. Besides finding accurate surface positions of the fault, we also carefully examine the patterns of faulting in order to understand its characteristics. More than 10 seismic lines are deployed form the north to the south across the fault. The quality of the obtained seismic profiles is considerably good. We can easily identify the structures dipping to the east with the Tungkeng Formation thrusting over the young Toukoshan Formation. The following conclusions can be drawn: 1.The hanging wall of the Sani fault is the Tungkeng formation which dips to the east by 20o-25o. The footwall is the Toukoshan formation which is relatively flat. 2.There exist several secondary fractures in the Tungkeng formation, which sometimes are large and may extend to the surface. However, the thrusting dose not affect the Toukoshan formation much. No obvious fractures in the Toukoshan formation are found. 3.The behaviors of the Sani fault can be explained by the theory of thin-skinned thrust. The sliding fault plane is along the bottom of the Tungkeng formation. 4.According to the drilling data and the geophysical results, we find that some front parts of the Tungkeng formation have been eroded and replaced by terrace deposits. This strongly implies that the fault may have certain ages and could not be active at the present time. 5.The fault line is found to move to the east by several hundred meters on some deep deposited river bank. It also indicates that the SanI fault has no activity for a long time. The possibility of fault’s reactivity could be quite low.
