The Weinan Tableland Piedmont fault is an important near-EW-trending Holocene active fault in the southeastern margin of the Weihe Basin, which is closely related to the occurrence of the 1556 Huaxian M8 earthquake. The northern branch of the fault, the northern branch fault in front of the Weinan tableland, passes through the urban area of Weinan. Therefore, finding out the distribution, shallow structure, late Quaternary activity, and seismic capacities of the northern branch fault are of great significance for local earthquake prevention and reduction. The Weihua fault zone, which is composed of F₁ and F₂ faults, generally strikes near east-west and has a gentle wave shape on the plane. It is a group of active normal faults rising in the south and descending in the north belt one. The Wei-Hua fault zone can be divided into two segments, east and west, and according to its spatial location and geometric distribution, strike change and the difference in geology and landforms on both sides. The eastern section is distributed in front of Huashan Mountain and is called Huashan Piedmont Fault(F₂); the western section is distributed in Piedmont of Weinan tableland and is called Weinan Piedmont Fault(F₁). There is a large sub-parallel branch fault about 2km to the north of the Piedmont Weinan tableland fault(F₁)in the west section, which is called the branch fault on the north side of the Piedmont Weinan tableland. It is also the boundary fault between the Weinan tabland and the Gushi Sag. The Weinan tableland Piedmont Fault(F₁)starts from the Weinan Xihekou in the west and extends eastwards through the Fenghe River to Mayukou, Huaxian County, with a length of about 54km; it strikes NWW from the Mayukou to Chishui River, and nearly EW from the Chishui River to the Fenghe River, the west of the Minhe River is NE to NEE, and it is mostly distributed in the form of broken lines or oblique rows. The fault plane dips northward with a dip angle of 60°~70°. The latest activity of the fault is manifested in the latest terraces and alluvial-pluvial fans faulting the Holocene strata, river valleys, and gullies; along the main fault, and a series of stepped normal faults on the north and south sides, a Holocene steep ridge belt with a width of between tens of meters and hundreds of meters, the Holocene strata are vertically faulted by 6~7m, and the vertical slip rate since the Late Pleistocene is about 0.29mm/a. In this paper, the shallow location and structural characteristics of the branch fault on the north side of the front of the Weinan tableland are determined through the combined profile detection of shallow seismic exploration and drilling, and evidence of the new activity of the fault is provided. The shallow seismic exploration results of the four survey lines all reveal the existence of a branch fault on the northern side of the front of the Weinan tableland, as well as the distribution location and cross-sectional structural characteristics of the fault new understanding. The results show that the branch fault on the north side of the Weinan Tableland Piedmont fault is a parallel branch of the main fault in front of the Weinan tabland. The branch fault on the north side of the front of the Weinan tableland is located at the front edge of the second-level terrace of the Weihe River in front of the Weinan tableland. The south end of the road, the mouth of the river, Zhangbaozi, and the outside of the north gate, have a length of at least 22km. The main section of the fault is inclined to the north, with a dip angle of about 70°~80° and a break distance of 6~20m at the upper breaking point, so it is a normal fault. Mainly concealed active faults, which have at least faulted the strata from the Middle Pleistocene to the late Pleistocene in the upward direction. In the four seismic sections, it appears as a normal fault zone with a width of 200~1 800m, including the main and secondary normal faults. Stepped structures and small grabens; secondary faults also fault up at least the Late Pleistocene strata. The combined geological profile of the Chongye Road borehole revealed that the main fault on the north side of the Weinan tableland had been faulted with many landmark strata of the Late Quaternary, and the latest fault occurred after 19ka; the average vertical activity rate since the middle of the Late Pleistocene between 0.07~0.26mm/a. Combined with phenomena such as fault ridges developed along the surface of the fault, it is judged that the fault was active in the Holocene. The branch fault on the north side of the front of the Weinan tableland has had strong activity since the late Quaternary, which means that the fault, as one of the branches of the southeastern boundary zone of the Weihe fault basin-the Weihua fault zone-obviously bears part of the deformation of the belt At the same time, the fault is located in the historically strong earthquake-prone area of the southeastern boundary of the Weihe fault basin, and it cannot be ruled out that it once participated in the rupture of the 1556 Huaxian M8 earthquake. Considering that the branch fault on the north side of the Weinan tableland passes through the urban area of Weinan, its potential seismic hazard and hazard are urgent research topics.

Weihe Basin, which is wide in the east and narrow in the west, deep in the south and shallow in the north, is one of the typical Cenozoic grabens in Asia continent, connecting the Ordos block in the north, Qinling fold belt in the south, adjacent to the arcuate fault belt in the northeast margin of Tibet Plateau in the west and the Shanxi rift zone in the east. The Weihe Basin has experienced strong faulting and sedimentation since early Cenozoic, with many buried active faults developed. The nearly E-W-trending Taochuan-Huxian Fault is one of these faults. The middle-deep depth seismic profiling shows that the buried segment of Taochuan-Huxian Fault in Weihe Basin is located between the Qinling north margin fault and the Weihe Fault and it is a fundamental fault that cuts through the Palaeozoic stratum and divides the Xi'an depression into two parts. To explore and know the location and structural characteristics of the Taochuan-Huxian fault segment hidden in the Weihe Basin and its activity in the Late Quaternary is of important significance for the researches of seismo-tectonic structure and seismic hazard of strong earthquakes in the study region. For this purpose, we deployed 7 profiles for shallow seismic reflection surveys, relied on the “Xingping Active Fault Project”. Based on these surveys, we determined the existence and hidden positions of the Taochuan-Huxian Fault and its branches in the Weihe Basin by combining with the data from some existing seismic reflection profiles of shallow-depths and middle-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 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)connects 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)is found 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 study region of this research, the fault zone with the buried F₈ fault as its principal fault was active at least in the late Pleistocene, and hence is an active fault zone. Finally, the reason is discussed in this article for the faults, mentioned above, in the Weihe Basin that show the tectonic pattern of negative flower structure, instead of that of stair-stepping or ladder structure, and one possible interpretation is proposed that the dominant motion of these active faults are not normal faulting, but sinistral strike-slip faulting. Since the Cenozoic, the subduction of the Indian plate to the Eurasian plate caused the Tibet Plateau to be pushed out to the northeast and blocked by the Ordos block. Because of obstruction in the north, the material flows eastward along Qinling Mountains in the south, resulting in the extrusion shearing effect on the Weihe Basin in the middle. In addition, recent seismic and geological studies have discovered that many active faults in Weihe Basin and its edges are obviously of sinistral strike-slip, which also proves that the movement of these active faults in the basin is not dominated by normal faulting, but sinistral strike-slipping.

The Kouzhen-Guanshan Fault trends in near E-W direction and obliquely cuts the active NEE-striking northern boundary fault zone of the Weihe Graben Basin, a fault zone that constitutes the boundary between Weihe Graben Basin and the Ordos block. Medium to small earthquakes occur frequently along the fault. Since the 1980s, a series of researches have been carried out on this fault, and certain cognition has been gained on its geometry, kinematics, tectonic evolution, recent activity and seismogenic capacity. However, most of the eastern segment of the fault is concealed in the Quaternary sediments of Weihe Graben Basin, and the corresponding research and attention are less. By conducting new field geological surveys and combining data from fault-crossing leveling and creepmeter observation, we studied the activities of the Kouzhen-Guanshan Fault during the late Quaternary and in the recent decades, supplemented the geological evidence of fault activity in the late Quaternary, and analyzed the characteristics and differences of tectonic activities on the western and eastern segments of the fault. Our research provides new insights as follows: 1)For the Kouzhen-Guanshan Fault, previous geological surveys were mainly carried out in the western segment with a focus on studying the vertical movement. It is considered that the fault activity has been stronger in the western segment and weaker in the eastern segment since the late Pleistocene. Our field investigation of three geologic cross-sections on the eastern bank of the Shichuan River in the eastern segment provides the understanding of the geological activity on the eastern segment. It reveals that the eastern segment of the Kouzhen-Guanshan Fault has a vertical motion component since the late Pleistocene, where the late Pleistocene stratum has been vertically offset by 8.8m, yielding a vertical slip rate of >0.13mm/a. At places between the central and western segments of the fault, the offset gullies were gradually cut down after the accumulation of loess layer L₁, and the age of S₁ at the bottom of L₁ can represent the lower limit of the left-lateral dislocation age of these gullies. The horizontally-faulted geomorphic features produced in the late Pleistocene have an average left-lateral displacement of 34m, which yields a left-lateral strike-slip rate of >0.49mm/a. These suggest that the Kouzhen-Guanshan Fault is a normal-sinistral oblique-slip one dipping steeply to the south; it would also be a growing transfer fault to adjust the non-uniform horizontal extension between segments of the Weihe Basin by obliquely cutting the northern boundary fault zone of the Basin. 2)Creeping movement is found to occur continuously on two connecting segments of the Kouzhen-Guanshan Fault at least in the last more than 30 years. Fault-crossing leveling observation for more than 30 years has been carried out on the Kouzhen and Jingyang sites on the western segment of the fault, respectively, and fault-crossing creepmeter observation has been carried out for nearly 7 years at Jingyang site, both of which have detected the present activity characteristics of the western segment of the fault. Among them, the two fault-crossing leveling observation time series show that the trends of vertical creep movement are basically the same since 1986. The creepmeter observation at Jingyang site shows that the fault has experienced continuously normal-sinistral creeping, and the horizontal-transverse stretching alternates with sinistral creeping since 2012. At Kangcun site on the western segment of the fault, fault-crossing leveling observation has been carried out for nearly 20 years. For the western segment, the fault creep is relatively stable with time and shows normal-sinistral oblique-creep faulting with the rates of 0.16~0.76mm/a for the vertical component, 0.42~0.78mm/a for the sinistral-creep component, and 0.15~0.26mm/a for the horizontal-transverse stretching component, respectively. Although technical means to observe or detect horizontal deformation are absent on the eastern segment of the fault, the campaign leveling surveys suggest that the fault creep on this segment has an average rate of 1.59mm/a for the vertical component(relative decline in the southern part of the fault)and shows a time series pattern of “step-like” or “episodic” creep, and the fault creep here with a rate as high as 13mm/a during the “step-like” period(2011 to 2014)may represent one slow slip event. 3)The present vertical creeping velocity of the eastern and western segments of the fault is different. The creep rate of the eastern segment is higher than that in the west, which may reflect the eastern segment of the fault is closer to the core of Weihe Graben Basin in space. This inference can be derived from the evidence that the new activity of the fault zone in the northern margin of Weihe Graben Basin, the development of ground fissures belt and seismicity along the Kouzhen-Guanshan Fault are all stronger in the eastern segment. 4)Both the seismicity and the cause of ground fissures belt along the Kouzhen-Guanshan Fault are closely related to the motion of normal-sinistral oblique-creep on this fault, which is controlled by the fault activity and should be the reflection of the surface macroscopic deformation of creeping. 5)The observed creeping movement on the Kouzhen-Guanshan Fault, especially, the phenomenon of “episodic” creep(rarely reported in China)in the vertical motion component on the eastern segment of the fault, proves that slow slip or creep may also occur on faults in tectonically active tensional environments of mainland China. There is obvious difference of normal creep faulting in the eastern and western segments of the fault. It is further necessary to study the differences in the friction properties of the fault segments reflected by the differences in the creep characteristics of these two segments, as well as seismic tectonic and seismic precursory implications of creeping with different characteristics. We therefore suggest strengthening the monitoring of the fault motion and the study of potential seismic hazards. 6)Regarding the “step-like” or “episodic” creep of the fault, the existing research mainly comes from the strike-slip fault. It is found that the present vertical motion component of the Kouzhen-Guanshan Fault shows obvious “step-like” or “episodic” creep characteristics. Therefore, it is necessary to study the relationship between the creeping effect and the phenomenon of seismicity and ground fissures alone the fault. In the future, we intend to combine the microseismic activity and fault friction theory to study the possible mechanism of the “episodic” creep, as well as the tectonic and seismic precursory implications of slow slip events similar to those observed at Kangcun site during 2012—2014.