湖口斷層位於湖口台地北緣，為東北東—西南西走向，全長約23公里，大致沿高速公路南側向西南西延伸。由現今地形特徵看來似為斷層崖，故一般認為其為活動斷層。在地質構造上，湖口斷層位於湖口台地北緣，南側湖口台地已知為湖口背斜，北邊依序為楊梅地塹、平鎮背斜及雙連坡向斜（Hsiao，1967），經此後一直向北方外海之觀音高區抬升，讓人不禁懷疑其中與弧陸碰撞所造成之碰撞前緣的關連性。 湖口斷層由於鮮少露頭被發現，斷層位置多為台地堆積層所覆蓋，前人所作之震測研究於淺層構造因解析度不足較為模糊不清，故本研究施以淺層反射震測調查，期能明瞭地下淺層構造以及地層錯動情形。 本研究設計了十一條測線橫跨可能斷層位置，並儘可能將斷層位置包含於測線內加以偵測。另外設計了一組五條短測線之廣域測線，用以觀察湖口背斜淺部構造及其地層分布趨勢。經資料分析結果發現，在淺部地層構造中並未發現斷層的存在，並推測斷層可能存在於地下深處，以斷層彎曲折皺的方式造成湖口背斜。 本研究獲得以下結論： （1）從震測剖面看來，「湖口斷層」並無錯動現象，在淺部構造上此斷層僅代表地層彎曲的邊界，但本研究傾向於在深部存在有盲逆衝斷層的可能，並造成上覆地層之斷層彎曲褶皺，因此本研究依舊稱之為「湖口斷層」。 （2）「湖口斷層」沿著湖口台地北界地形線延伸發展，正確位置為向台地內收縮約50公尺，且為店子湖層所覆蓋。 （3）由構造變形分析，本研究認為應力來自南方，以北偏西14度方向推擠，最大衝擊點位於楊梅交流道附近，自此向東西兩側減弱其效應。 （4）湖口背斜為一不對稱背斜，北面陡急南面舒緩，其背斜軸向東延伸至平鎮東南，但抬升量已不多。在構造區東邊，背斜軸與斷層漸漸靠近。 （5）由上覆紅土台地堆積層亦受到擾動的情形來看， 此構造發生的年代相當年輕，故仍有其一定的危險性存在，值得注意。 The Hukou fault is located at the northern border of the Hukou terrace. It runs almost in the east-western direction. Physiographically, the fault looks like a scarp along the terrace’s boundary and is generally attributed as an active fault. The purpose of this study is to detect the Hukou fault and the structures in its vicinity using the shallow seismic reflection method. Several long-range seismic lines (400 m long) are distributed perpendicular to the fault trace with the visible seismic images down to the depth of 700 meters. Besides this, we also used five short-range seismic lines across the Hukou anticline, from the edge of the terrace to Tienzuhu, to investigate the structure of this fault-related anticline. After carefully reserving the characteristics of each reflector, we are able to correlate different layers on different seismic profiles. On the seismic profiles, we find that all structure layers vary quite smoothly. The layers bend upward, but not broken at the suspected place of the Hokou fault. It seems that the scarp represents a boundary of curved stratum (i.e. part of an anticline), not a presentation of an actual fault. Furthermore, the positions where the structures bend are shifted to the east from the cliff of the terrace by about 50 m. The young Tienzuhu Formation has covered this shifted area. Thus, we tend to propose that a blind thrust fault might hide at the bottom and it makes the ‘fault-bend’ fold structure. We also found that the Hukou anticline is an asymmetric anticline. It is steeper in the north flank than in the south flank. The axis is extended toward the southeast of Pingchen and becomes closer to the fault with decreasing uplift. One important seismic images are obtained on the plane area where the young lateritic terrace deposits is obviously disturbed by the tectonic stress. Because of this, we intend to propose that the potential risk of the fault activity might not be ignored.
