桃園臺地受由東南向西北壓應力之影響，地形呈現南、北兩組拱升地壘包夾一處下陷地溝的變位模式。其中南地壘的前緣斷層為湖口斷層，是臺地上規模最大且變位最明顯之斷層，其截切過店子湖面（LH）、湖口面（LT1）、楊梅面（LT2）、中壢面（LT4）等。而斷層活動後，斷層崖後的地盤抬升所引起的河流回春，會在斷層後發育構造性階地（tectonic terraces）。本文於上述地形面上，分別選擇老坑溪、波羅汶溪、茄苳溪、頭重溪、大坑缺溪及老街溪等河流作為示例，進行調查各構造河階與湖口斷層活動之關係，並據此推測湖口斷層的活動特性。 湖口臺地上的切割溪流老坑溪、波羅汶溪及矮仔坪的頭重溪均發育有4段構造性階地，鳳山崎臺地的茄苳溪發育2段構造階地；平鎮臺地的大坑缺溪、老街溪則發育3段構造河階。6條河流的最低位階地則均為近期河流平時流水量和輸沙變化調整下形成的自律性階地（self-regulation terrace）。由構造階地的階數及前緣斷層崖坡上肩狀面分布的分析，推斷湖口斷層於桃園面（LT5）形成之前，可能至少有過七次變位活動。而中壢面（LT4）形成後，湖口斷層再活動所引發的逢安橋、大庄兩後背斷層，僅截切中壢面，未於大坑缺溪、老街溪的新形成構造階地面活動，且逢安橋斷層活動時間可能早於大庄斷層。 斷層崖、河階崖高差及階數均由店子湖面向東西兩側年輕地形面減少，證明湖口斷層的活動具累積性，且斷層活動量由店子湖面向東西兩側遞減。根據圓山、員山的穹窿構造及茄苳溪構造階面的拖曳曲隆變形，可推斷湖口臺地前緣的湖口斷層應延伸至鳳山崎臺地北緣。 本文為國科會專題研究經費輔助，編號NSC90-2116-M-133-001

　　　　　Because the effect of compressive stress toward northwest on Taoyuan tableland, the landform is formed two sets uplift horst displacement model. The Kokou frontal fault of south horst is the largest scale and displacement on tableland, it cut off four river terraces surfaces including Dianzihu surface (LH), Hokou surface(LT1), Yangmei surface (LT2) and Chungli surface (LT4). Therefore, the uplifted basement would cause river rejuvenation and developed tectonic terraces on back of fault scarp. Base of the landform characteristics, we would infer the activity of Hokou fault by the relationship between tectonic terraces and Hokou active fault. According to the numbers of tectonic terraces, we can find the Hokou active fault movements have been reacted seven times, since the Taoyuan surface (LT5) formed before at least. The accumulation of the Hokou fault has been prove by the fault scarps, height and numbers of river terraces on Dianzihu surface (LH). The fault movement for the east and west side of the Dianzihu surface (LH) were reducting. Owning to the dome structure of Yuanshan and drag displacement of Qiedung river tectonic terraces surface, we can draw a conclusion that Hokou frontal fault should be extended from Hokou tableland to the northern of Fengshangi tableland.