关键词: 桃川－户县断裂, 渭河盆地, 浅层地震探测, 隐伏断裂定位, 断裂带结构与展布

Abstract: The Weihe basin has experienced strong faulting and sedimentation since early in the Cenozoic era, with many buried active faults having developed. The nearly E-W-trending Taochuan-Huxian fault is one of these faults. To exploring and know the location and structural characteristics of the Taochuan-Huxian fault segment that is hidden in the Weihe basin, and its activity in the Late Quaternary, are of important significance for the researches of seismo-tectonic structure and seismic hazard of strong earthquakes in the studied region. For this purpose, we deployed 7 profile lines for shallow seismic reflection surveys, relied on the "Xingping Active Fault Project". Based on these surveys, we determined the existences and hidden positions of the Taochuan-Huxian fault and its branches in the Weihe basin by combining with data from some existing seismic reflection profiles of shallow-depths and middle- to deep-depths. Our research suggests that the Taochuan-Huxian fault (F8) is connected to the southern margin fault of the Taibai basin in the west, and eastward, passes through the northern margin of the Qinling Mountains and enters into the Weihe basin at the town of Tangyu, Zhouzhi county, and then is concealed under the loose sediment in the Weihe basin. The strike direction of this fault is the northeast when crossing obliquely through the town of Zhouzhi county, then gradually turns to a nearly east-west direction between Zhouzhi and Huxian, showing a northward convex bend in the fault trace buried in the basin. Further eastward, the Taochuan-Huxian fault (F8) is connected to the Tieluzi fault near the town of Yinzhen, Huxian county. In addition, a buried antithetic fault (DF3) (also a secondary branch) of the buried Taochuan-Huxian fault (F8) was found to be present between the north of Zhouzhi and the north of Huxian, and it extends roughly parallel to F8 under the loose sediment. This research also reveals that in the central portion of the Weihe basin, the northern margin fault of the Qinling Mountains, the Weihe fault and the Taochuan-Huxian fault, together with their branch faults, constitute a large-scale fault zone with the tectonic feature of negative flower structure, as known from the interpreted cross-sections; among them, the F8 and DF3 faults and their secondary strands consist of a relatively small-scale negative flower structure. By combining with relevant information such as that from a composed cross-section using geological logs of multiple boreholes, and so on, we concluded that, within the studied region of this research, the fault zone that has the buried F8 fault as its principle fault was active at least in the late Pleistocene, and hence is an active fault zone. Finally, the reason that those faults, mentioned above, in the Weihe basin, show the tectonic pattern of negative flower structure, instead of that of stair-stepping or ladder structure, is discussed in this article, and one possible interpretation is proposed that the dominant motion of these active faults are not normal faulting, but sinistral strike-slip faulting.

Key words: Taochuan-Huxian fault, Weihe Basin, Shallow seismic surveying, Locations of buried faults, Structure and extension of fault zones