觸口斷層位於中台灣內麓山帶與外麓山帶的分界上，一直被認為「深具活動性」的重要斷層，且觸口斷層週遭構造複雜，西邊接近北港高區，南邊有著關仔嶺背斜，其斷層線上也存在幾個小背斜，這些構造都直接或間接影響到觸口斷層的發育方式。根據地調所的主張，觸口斷層從桶頭南方往南延伸到關子嶺東方，長約40公里，其中並以水社寮斷層（或稱竹崎斷層）及獺頭斷層分隔成北中南三段。北段為南莊層蓋在頭嵙山層之上，中段為桂竹林層下段蓋在卓蘭層之上，南段則只在鳥嘴層（相當於桂竹林層上段）內錯動，斷層斷距由北往南漸減。地形上，中段（觸口村附近）下陷並發育出八掌溪，南北二段則為高聳山地。本研究利用淺層反射震測法，配合深部震測剖面、地形及地表地質，探討觸口斷層及其與鄰近區域之地下構造，並建立該地區之基本斷層構造架構，發現觸口斷層不僅依照斷層線形式分為北中南三段，依照其活動方式之不同也可分為三段，觸口斷層之南北分段性極為明顯；北段活動方式為沿著一階梯狀滑脫面低角度逆衝，中段進入嘉義平緩區，斷層活動方式為薄皮逆衝而上其上盤擠壓力有限造成下盤影響較小，南段進入關仔嶺雙衝構造以及白河古脊之阻擋，斷層傾角增加。 觸口斷層之北中南三段各自座落在不同的背景地質構造區內，有不同的斷層型態，其活動的方式也不相同，在活動斷層特性上，宜分開為不同之三段來看待。 The Chukou fault is located along the boundary of the inner and the outer foothill zones of western Taiwan and has been recognized as an important active fault with ‘high activity potential’. The Chinese Geologic Survey (CGS) ranked the Chukou fault as a ‘first class’ fault. The fault extends from Tongtou southward to Kuantzuling, about 40km long, which can be divided into three parts (north, middle, and south) separated by the Shueidiliao fault (also called the Chuchi fault) and the Tatou fault. The northern part has the Nanchuang formation thrusting over the Toukoshan formation with significant thrust distance, approximately 8 km. The middle part has the Kueichuling bed covering on the Cholan bed, thrust by 4 km. The southern part only thrusts within the Niaotzuei formation. Thus, the fault slip decreases from north to south. This research uses the shallow seismic reflection method cooperating with deep seismic profiles to investigate the Chukou fault and surrounding underground structures. The results can be summarized as follows: 1. The northern part possesses a low angle thrust about 30o, and the fault plane parallels to the layer of the Nanchuang formation. The fault line has a curve shape concaving toward the west. A ‘central line’ was proposed to explain such apparent structure variations. This central line represents the highest point where the Peikang base high entering Taiwan island. This makes the Chukou fault as well as the Chiuchiungkeng fault become sequential imbricated thrust structures. 2. At the middle part, the foot wall side has flat structures with a few anticlines developed close to the fault. The flat region is named the ‘Chiayi flat region’. The Chukou fault plane is dipping 35 o eastward and cut the Kueichulin layers. There is a Lunhou fault developing to the east of the Chukou fault. 3. The southern part is dominated with the Naiotzuei formation with the fault dipping 40o cutting through layers. This part is largely affected by the Kuantzuling duplex structure, and has high-angle thrust fault over the eastern limb with larger displacement. The Chukou fault and the Lunhou fault gradually merge southward. 4. The three parts are located in different geological structure areas. They have different fault types, different ways to be active, thus should be considered as three distinct sections of an active fault.
