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.

Study of historical earthquake is one of the important methods to understand the seismic activities and analyze the seismogenic faults. On the May 25^th, 1568 AD, a destructive earthquake occurred to the northeast of the present-day city of Xi'an, Shaanxi Province. Because this earthquake happened shortly after the 1556 M8 earthquake and was regarded as an aftershock, it has received little attention in previous studies. Previous earthquake catalogue agreed in assigning a magnitude 6 3/4 to this earthquake but had different epicentral locations and seismic intensity, and the seismogenic structure remains ambiguous.
Based on textual research of historical earthquake and field investigation, the Jingyang County, Gaoling County, and Xianning County, were the worst hit area by the earthquake, and the areas, including Yongle Town, Gaozhuang Town at southeastern Jingyang County to Gaoling County and its southeastern present-day Jijia and Zhangbu, should be the mesoseismal area of this earthquake. The epicenter intensity of this earthquake is Ⅸ⁺(9~10 degrees), and the magnitude is estimated to be 7. The isoseismal lines were drawn to exhibit the various intensities of the areas damaged during the event, with its major axis directed NWW. Intensities reached Ⅸ⁺ in the zone extending west-northwest parallel to the Weinan-Jingyang Fault. This fault, characterized by a normal fault that developed during the Cenozoic extensional history of the Weihe Basin, dipping to the north at an angle of 60°~80°, is one part of the southern boundary faults in Weihe graben. There are geomorphological and geological evidences of recent activity of the fault during (180±30)a BP to (1 600±30)a BP. At T₁-T₂ fluvial terraces on the north bank of Weihe River, the scarps were faulted during Ming Dynasty, and sandy soil liquefaction, dense structural tensional fissures and faulted strata are noted in stratigraphic profiles and trenches. Thus, we suggest that this fault can reliably be regarded as being active during Holocene, and re-name the earthquake as the Shaanxi Gaoling earthquake.

Based on the survey and study of active faults at three sites,i.e.Yaodian,Shiheyang and Dujiapu on the north bank of Weihe River in Xianyang,Shaanxi,this paper probes into the methodology of survey of the loess-covered active faults coincident with terrace scarps,and presents the displacement amount of the Weihe Fault zone at Shiheyang in late Pleistocene.At Shiheyang,exploration of the Weihe Fault zone was carried out by means of shallow seismic prospecting,drilling,topographic analysis and age dating.The initial survey result showed a displacement of 17.94m of the stratum S1 on the Weihe Fault zone.The causes leading to this false result were mainly due to incorrect judgment on geomorphic unit,and followed by the so big spacing of drill holes that the subtle change of strata tilting due to erosion couldn't be seen.The drop of the same stratum at the profile detected at two drill holes far away from each other was mistaken for fault displacement.With the scarp caused by erosion added to the fault displacement,the fault throw was magnified.By densifying the drill holes to a spacing of 1.9m between holes,we get the displacement of the top of S₁ to be only about 1.2m.At Yaodian,data are available,including the 200m deep drilling section data,the densified mid-deep drilling data and shallow seismic prospecting data.Drilling data with borehole spacing of 30m revealed an offset of 4.8m on the top of S1 by the Weihe Fault.Since the two holes were located at scarp change zone,the 4.8m height difference of the top of S1 might be the elevation difference of tilted terrain superimposed possibly with certain amount of faulting.The 30m hole spacing is too large to affirm that S1 has been faulted.The drilling section at Dujiapu was implemented at last,in which deficiencies in dealing with the first two ones were avoided.At this site,the shallow seismic methods couldn't be performed,therefore the fault was located by combining the deep drilling with shallow drilling at a hole spacing as small as possible(2～3m).In spite of the small borehole spacing,it was difficult to identify the displacement amount of the fault according to the paleosol layer S1,which is probably due to too small fault throw.All the explorations of fault at the above three sites have a certain deficiency in methodology,mainly in the depth and spacing of drill holes.The common shortcoming is that no deep trenches were excavated.If allowable,it would be better to verify the fault location and activity by trenching.The above results show that the exploration of loess-covered active faults coincident with terrace scarps shall be carried out with comprehensive method combining topographic analysis,shallow seismic survey,drilling and trenching.Particularly for drilling exploration,deep,medium and shallow holes shall be combined in use with the medium and deep holes drilled to determine the location of faults at depth,and the shallow holes used to identify the location and activity of faults near surface.Due to river erosion,the fluvial deposition layer in terrace scarp zone is tilted.Aeolian paleosol layer draping over the tilted layer is tilted too.As a result,the spacing between holes must be small(2～3m preferable)when such strata are used to identify the location and movement of faults.Excessive spacing may lead to the addition of the height of erosion-formed scarp to the fault offset,thus greatly overstating the later.It is highly recommended to make verification by trenching in the end.The above exploration results show that the Weihe Fault zone coincides with the scarps of the third terrace at Yaodian,Shiheyang and Dujiapu.The displacements associated with faulting only have a small proportion of the terrace scarp and the 1～2m offset of the first late Pleistocene paleosol layer by faulting is much less than the difference in elevation of terrace surface.The previously thought 4.8m and 17.94m displacements are incorrect.

The Longmenshan Fault zone is an important thrust belt on the eastern margin of the Qinghai-Tibet Plateau,consisting of the back-range,the central and the front-range faults,which differ from each other in size and activity.The rupture zone of the Wenchuan earthquake of 12 May 2008 occurred over a length of～270km along the Yingxiu-Beichuan Fault(a segment of the Central Fault)and a length of～70km along the Guanxian-Anxian Fault(a segment of the Front-Range Fault).The northern end of the fracture zone is at the Nanba region in Central Fault.In this work,we make a detailed field investigation on the northeast segment of the Longmenshan Fault zone.Qingchuan Fault is the northeast segment of the Longmenshan Back-range Fault,and the Chaba-Lin'ansi Fault is the northeast segment of the Longmenshan Central Fault.Along the above two faults,we make geological and geomorphologic mapping of Tuguanpu,Da'an and Hujiaba regions,where the Qingchuan Fault runs through the Tuguanpu and Da'an area,and Chaba-Lin'ansi Fault runs through the Hujiaba area.Based on the field investigation,there are five terraces in the northeast Longmenshan area along the major rivers.The height above the river of T1 terrace is about 3～5m,and the formation time is Holocene.The heights of T2 and T3 terraces are 10m and 30～35m above the river,and the deposition time of alluvium and diluvium is Late Pleistocence.The remnant of T4 terrace's sediment covers on some hills,with the height above the river of about 60～70m.In the remnant,granite cobble and sandstone cobbles have been air slaked,these gravels have the shapes only.T5 terrace's height is about 90m,the sediment on it has been eroded.Qingchuan Fault and Chaba-Lin'ansi Fault were strongly active faults in the times before T3 and after T4 formed.Some fault grooves were formed on T4 or T5 terrace,they have 30～180m in width,and 8～20m in depth.The vertical displacement of T4 terrace's gravels is 10～15m.Fault groove didn't form on T3 terrace,or the terrace height on a fault wall is consistent with other fault wall.At some places,T3 terrace's gravels overlie the fault zone.

The Weihe Fault is an important blind fault in Weihe Basin and controls the formation,evolution and seismicity of Weihe Basin. The deep seismic reflection survey results show that the fault is not a deep crustal fault; it is located right below the C layer at about 15km depth and cuts through the crystalline basement and the C layer,causing a throw of about 4km between the two sides of crystalline basement. The dip angle at the shallow part of the fault(depth<5km)is big and flattens with depth,and the fault turns to be a listric fault.Shallow seismic survey results show that the dip angle of the Weihe Fault in the middle and deep parts is about 85°; the attitude is different on the two walls of the fault,the footwall is horizontal and the hanging wall is tilting to the south direction; and its dip angle increases quickly.Drilling survey results show that the fault at the shallow part is obviously manifested. The lithology,thickness and attitudes of strata are quite different between the two sides of fault. The attitude on the footwall is horizontal and that on the hanging wall tilts a bit to the fault side. The late Pleistocene displacement is about 4～6m.Trenching results show that the Weihe Fault near ground is still active. Since Holocene epoch it has undergone 3 paleoearthquakes and 1 history earthquake,so it is a Holocene active fault.

Weihe Fault is an important buried fault in Weihe basin.The predecessors have investigated the location and activity of the fault from various points of view,but up to now,the level of researches on the precise location and activity for the fault is still very low.There are few strata profiles of late Pleistocene which are found to be offset by the fault zone.Especially,it is still unknown whether the Weihe Fault was active in Holocene and there were paleoseismic events occurring on it.It is indicated from exploratory trench excavated at Bili village in the west section of Weihe Fault that over the past 9110a,the Yaodian—Zhangjiawan segment of Weihe Fault zone has experienced a historical earthquake and 3 paleoearthquake events.The historical earthquake is manifested by soil liquefaction.According to the study on historical and cultural relics,stratigraphic chronology and seismogenic tectonics,we propose the occurrence time of the historical earthquake is between 1487 and 1568;the age of paleoseismic event I is(9110±90)a,but there is no answer for the age of event II and event Ⅲ.The coseismic vertical displacement of event I,II and Ⅲ is 0.5m,0.5m and 0.2m respectively.The exploratory trench excavation also indicates that the Yaodian-Zhangjiawan segment of the Weihe Fault is a Holocene active fault.

The Lintong-Chang'an Fault zone locates in the middle part of Cenozoic Weihe depression.It is the boundary fault controlling the Lishan diamond block and Xi'an sag.The landforms are obviously different between the sides of the fault,and the geomorphic forms are stepped fault scraps and loess scraps.In the paper,by field geological survey to the Zhongdi Village,Wangjiabian Village and Qiaogou profiles on the Lintong-Chang'an Fault,and in combination with the dating data of regional loess and paleosol profile(An Zhi-sheng and Sun Jian-zhong),the fault is studied in order to explore the times of its latest activity and the characteristic of its late Quaternary movement.The fault strikes NE as a whole and is characterized with tensile vertical movement.The fault obviously offset the first paleosol layer S₁ in loess stratum,indicating that it is still active since late Pleistocene epoch.But most fault displacements are less than 2m,the slip rate is low,and the activity level is higher in the northern and central segments than that in the southern segment of the fault.Regarding that the Lintong-Chang'an Fault consists of several secondary faults,its whole activity should be much higher than the local slip rate of the fault we have derived.The fault displacements show an increasing trend with depth and the slip rates calculated using the dating data of different strata are almost the same.So perhaps,the fault is mainly dominated by vertical creep-slip since the late of middle Pleistocene epoch.

Lintong-Chang'an Fault is an important boundary fault between Lishan uplift and Zhouzhi-Huxian depression in Weihe basin.By the field survey to the natural gullies,the earth fetching areas,and the excavated slope and chasm for road foundation,we discovered 40 outcrops of the Lintong-Chang'an Fault.According to the measurements of dislocation of the various periods' paleosoil horizons,we get the Quaternary dislocation distribution of the fault,which shows that the fault dislocation in the middle segment is the biggest,so is its activity along this segment.