桃園台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爲湖口斷層,是台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,會在斷層崖後發育構造性階地(tectonic terraces)。本文於湖口面(LT1)上,選擇波羅汶溪、茄苳溪作爲示例,進行調查二溪之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。
湖口面(LT1)上的切割溪流波羅汶溪、茄苳溪含原地形而分別發育有5段及3段構造性階地。而二溪的最低位階地均爲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介布的分析,推斷湖口斷層於湖口面(LT1)形成之後至今,可能至少有過七次變位動。
Different altitudes of river terraces existed in Taoyuan Tableland that can be recognized as six surfaces including Tientzuhu Terrace (LH), Hukou Terrace (LT1), Yangmei Terrace (LT2), Talun Terrace (LT3), Chungli Terrace (LT4) and Taoyuan Terrace (LT5) Because the Hukou Terrace (LT1) is cut by Hukou active fault, therefore, five and three levels of river terraces were developed on Hnkou Tableland cutting by Pu Lo Wen stream and Chia Tung stream. According to the evidence of river terraces, the Hukou active fault has reacted seven times after the formation of Hukou Terrace (LT1).