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

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