Abstract: Geological, geomorphic and geophysical methods, including drilling and chronological dating, are applied to determine whether the suggestively buried Taoyuan fault exists. Stratum units at two sides of the fault are continuous. The “normal fault”, found at east side of Yuanjiang River, Yaohe town, is actually a fissure caused by un-loading. Weak cementation of the Eogene breccia is also favorable to formation of the extensive fissure. One meter of offset along the fissure is not the result of tectonic movement, but comes from up-plate falling under the gravitational action. Quaternary thicknesses at the two sides of Yuanjiang River show no abrupt change, but a normal sedimentation. According to the geomorphic feature and the fault location, suggested by previous work, three lines of shallow seismic survey are arranged at the two sides of Yuanjiang River. The sections of shallow seismic survey show that boundaries at different layers are clear and can be pursued continuously. There are no evidences to demonstrate that the suggestively buried Taoyuan fault may exist. Two drilling bores are arranged at Xunyangping Quaternary-covered area and they are located at two sides, 20m away from the suggestively buried fault. The drilling results show that the Quaternary layers are crossed almost at the same depths and the lower basal rocks are revealed, which are red clay-sand rocks of the Eogene. Thermoluminescence dating samples are collected respectively at depths of 4.5m, 9.8m and 16.8m. The experiment results are (2.2±1.8)×10⁴a, (2.9±2.5)×10⁴a and (13.1±11.0)×10⁴a, which shows that the Quaternary strata, formed at the end of mid-Pleistocene, are continuous at two sides of the suggestively buried fault. There is no neo-activity. In conclusion, there is no evidence to support the existence of the suggested Taoyuan fault. The Taiyangshan fault zone, on which the 1631 M6³/₄ earthquake occurred, is bounded at the north of Changde-Yeyang-Changsha fault zone. It provides a reasonable geological background for division of seismic belts and potential seismic sources and is also important for better understanding of structural conditions of the 1631, M6³/₄ Changde, Hunan, earthquake.

Key words: buried fault, un-loading fissure, shallow seismic survey, the 1631 M6³/₄ Changde earthquake

摘要： 通过对湖南常德地区桃源推测隐伏断裂沿线详细的地质地貌调查,以及配套的物探、钻探工作和不同深度钻孔沉积物的新年代测试,表明前人推测的桃源隐伏断裂两侧地层单元连续分布,物探剖面界面清楚,可连续追踪,没有显示断错现象;钻孔中的3个热释光年代样品也表明(13.1±11.0)×10⁴a以来的第四纪地层在断层可能通过的位置两侧可以完全对应,下伏的基岩岩性相同,不存在新活动性。在尧河乡沅江东岸发现的“正断层”实际上为卸荷张裂缝。桃源推测隐伏断层没有存在的证据,发生1631年常德6³/₄级地震的太阳山断裂带被严格地限制在常德-益阳-长沙断裂带以北。这为地震带以及潜在震源区的划分提供了一个合理的地质背景,对于认识1631年湖南常德634级地震的构造条件也具有重要意义。

关键词: 隐伏断层, 卸荷张裂缝, 浅层物探, 1631年常德6³/₄级地震