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.

The northern Qinling fault zone is an important active structure in the southern margin of the Weihe Graben Basin, containing many branch faults, of which the near EW striking Taochuan-Huxian Fault is located on the northern side of the fault zone, and the eastern segment is buried in the Weihe Graben Basin. Shallow seismic exploration has been carried out on the middle part of the buried segment of this fault, and the fault inferred to be a late Pleistocene fault with normal strike-slip movement, but the age and rate of the latest activity have not been determined. By conducting new shallow seismic and drilling joint exploration, we further study the shallow structure, the geometric distribution, the latest activity era and the slip rate in the Quaternary in the two segments of the Taochuan-Huxian Fault. The profile of shallow seismic exploration line TB1 reveals that the west segment of the Taochuan-Huxian Fault with NEE trend can extend at least 20km westward from Taochuan Town. The main fault plane dips to N, and the normal-slip movement has faulted the Quaternary bottom boundary and the underlying crystalline basement in the Taibai Basin. The vertical offset of the Quaternary bottom boundary is about 300m, and the remnants of the old thrust structure are still preserved in the fault zone. The shallow seismic reflection lines ZZ1 and YX1-2 reveal the location of the eastern Taochuan-Huxian Fault with the EW striking buried in the Quaternary of the Weihe Graben Basin in Zhouzhi and Huxian. The main fault plane dips to N, and the fault zone is represented by a fault depression zone of about 6km wide and a stepped structure of about 4km wide respectively. The fault up-breakpoints on both profiles offset the bottom boundary of the Holocene in the Weihe Graben Basin. The drilling joint profile exploration applied at Tanjiazhai in Zhouzhi County and Xiashimasi in Meixian County show that the Taochuan-Huxian Fault is distributed in the junction of the southern Weihe Granben Basin and the Qinling Mountains, where the Holocene marker layer S₀ has been vertically offset by 4~5m, yielding an average vertical slip rate of 0.4~1.3mm/a. Combined with the results of shallow seismic surveys, it is well demonstrated that the eastern segment of the Taochuan-Huxian Fault(buried in the Weihe Graben Basin)shows Holocene activity, and it is significantly more active than the western segment(the Taibai Basin segment). This may be due to the fact that the eastern segment has been incorporated into the Weihe Graben Basin and has become part of the primary active tectonic zone on the block boundary, while the western segment has not been incorporated. Spatially, the eastern segment of the Taochuan-Huixian Fault is subparallel to the middle-eastern segment of the North Qinling Fault, which is capable of generating strong earthquakes of magnitude 7 or higher. As an important branch of the North Qinling Fault, the Taochuan-Huixian Fault may also be under the same strong seismic background. These two faults probably jointly control the important active boundary of the southern margin of the Weihe Graben Basin. Future research in seismology and geology of these two faults should be strengthened, including their interrelationships at depth, their roles in vertical and horizontal movement distribution, and their seismogenic capacity and potential seismic hazard. In particular, the activity of the Taochuan-Huoxian Fault since the late Quaternary has only recently received attention, and the level of seismo-geological research on the fault is generally low. In this paper, we conducted preliminary studies on the location, shallow tectonic structure, activity segmentation, latest activity and Holocene vertical slip rate of this fault. Future research on the seismogenic structure of the Taochuan-Huoxian Fault needs to be strengthened in order to deepen and improve the understanding of the fault activity and to provide a basis for analyzing the seismic hazard of this fault.

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.

Earthquake disaster reduction approach in China is essentially top-down, which is highly effective in mobilizing large-scale disaster reduction activities. However, the overall resilience of a society to earthquake also heavily depends on actions from various bottom-up components/actors(e.g., family, community), pointing to the strong need for a governance model that integrates the existing top-down approach with broad bottom-up engagement of grass-roots and the public. To accumulate research evidences for developing that governance, the overall objective of the work of creating a seismic scenario for Weinan City, Shaanxi Province, China(the Weinan scenario work, in short), was thus planned to address in particular the following two major gaps in earthquake disaster risk reduction in China: (i)between top-down and bottom-up earthquake disaster risk reduction(DRR)approaches, with a particular emphasis on the weak bottom-up aspect, and(ii)between science and earthquake DRR policies and practices, especially the insufficiency in the research and associated applications relevant to various bottom-up components/actors.
Using the paradigm of trans-disciplinary, participatory action research, the Weinan scenario work delivered this objective through direct interactions and close collaborations between two different groups of people: multi-disciplinary UK-USA-China collaboration research team and various local DRR practitioners and other stakeholders. The overall progresses include: 1)Using pan-participatory methodology, the two groups worked closely together to co-identify earthquake risk, co-explore pathways to risk reduction and resilience building, and so on, which ensured the reliability of the scenario results and the local context-appropriateness and then applicability of the scenario work's DRR recommendations; meanwhile, with action research process, the two groups realized synchronous interactions and seamless connections between the three large aspects in risk science of risk assessment, risk communication, and risk reduction practice improvement, which have often been conducted separately, thereby resulted in a kind of direct, immediate, and in-situ/on-site “science research into policies and practices”; 2)By serving both governments and bottom-up actors, and by looking at earthquake DRR issues from multi-scale point of view, the two groups co-addressed how to improve both top-down and bottom-up earthquake DRR policies and practices. Especially, zooming in on community-based disaster risk reduction(CBDRR), school-based DRR, family-based DRR, and broad disaster reduction education-the broadest and most sustainable linkages between top-down approach and bottom-up pathway to earthquake DRR, a large scale of specialized surveys and other relevant investigations were conducted, a series of current baselines and future improvement directions were identified; 3)Focusing on bettering disaster reduction education and improving long-term risk communication, the two groups co-created two versions of storytelling-led and latest science-grounded scenario narratives that are different in both contents and presentation styles/formats: one for government officials, the other for the general public. By constructing the plot of the story to properly highlight the key earthquake risk problems facing Weinan, we hope non-technical readers can easily understand research findings and better follow DRR recommendations provided, further facilitating “science into policies and practices”; by unfolding and illustrating the disaster-amplification or superimposing effects produced by a distinct local vulnerable social element-poor rural family with left-behind children, we hope readers can understand earthquake risk as deeply and comprehensively as possible from multi-perspectives; by incorporating elements of sensibility, emotion, humanity, and artistic appeal into rational but often “dry” sciences, we hope to help intensify resonance, build consensus, inspire emotion, improve DRR attitudes, foster DRR values, and then motivate DRR action and participation; and most importantly to inspire long-lasting learning, reflection, and action improvement among local population-the most direct, fundamental, and broad actors for reducing local earthquake disaster risk.
The participatory action research-guided Weinan scenario work has the utility of “throwing out a brick to attract a jade” for China's earthquake DRR field, it also provides the international similar studies with valuable experience and implications from China context.

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.

Coseismic displacement plays a role in earthquake surface rupture, which not only reflects the magnitude scale but also has effect on estimates of fault slip rate and earthquake recurrence intervals. A great historical earthquake occurred in Huaxian County on the 23^rd January 1556, however, there was lack of surface rupture records and precise coseismic vertical displacements. It's known that the 1556 Huaxian earthquake was caused by Huashan front fault and Weinan plateau front fault, which are large normal faults in the east part of the southern boundary faults in Weihe Basin controlling the development of the basin in Quaternary. Here, we made a study on three drilling sites in order to unveil the coseismic vertical displacements.
It is for the first time to get the accurate coseismic vertical displacements, which is 6m at Lijiapo site of Huashan front fault, 7m at Caiguocun site, and 6m at Guadicun site of Weinan plateau front fault. These coseismic displacements measured based on same layers of drilling profiles both at footwall and hanging wall are different from the results measured by former geomorphological fault scarps. It's estimated that some scarps are related with the nature reformation and the human beings' activities, for example, fluviation or terracing field, instead of earthquake acticity, which leads to some misjudgment on earthquake displacements. Moreover, the vertical displacements from the measurement of geomorphological scarps alone do not always agree with the virtual ones. Hence, we assume that the inconsistency between the results from drilling profiles and geomorphological scarps in this case demonstrates that the fault scarp surface may have been demolished and rebuilt by erosion or human activities.

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.

Field investigations show that the M_S8.0 Wenchuan earthquake of 12^th May 2008 ruptured two NW-dipping imbricate reverse faults along the Longmenshan Fault zone at the eastern margin of the Tibetan Plateau.This earthquake generated a 240km long surface rupture along the Beichuan-Yingxiu Fault characterized by right-lateral oblique faulting and a 90km long surface rupture along the Guanxian-Jiangyou Fault characterized by dip-slip reverse faulting.Maximum vertical and horizontal dispacements of 6.2m and 4.9m,respectively,were observed along the Beichuan-Yingxiu Fault,whereas a maximum vertical displacement of 3.5m occurred along the Guanxian-jiangyou Fault.This co-seismic surface rupture pattern,involving multiple structures,is among the most complicated of recent great earthquakes.Its surface rupture length is the longest among the co-seismic surface rupture zones for reverse faulting events ever reported.Aftershocks recorded by local network clearly outline the hanging wall of the Beichuan-Yingxiu Fault and indicate that the fault dips about 47? to the west.Industry seismic lines,in addition to surface ruptures and aftershocks,allow us to build a 3D model for the rupture geometry that shows crustal shortening is the dominant process along the Longmen Shan to accommodate long-term deformation.Oblique thrusting accomplished by the earthquake indicates that the east-southeastward extrusion of Tibet Plateau accommodates,in part,the continuing penetration of the Indian plate into the Eurasian plate,and this extrusion is transformed at the eastern margin of the Tibetan Plateau into crustal thickening and shortening along the Longmenshan Fault zone that is responsible for the growth of high topography in the region.

The late Pleistocene aeolian loess distributes widely in the loess tableland area.It has obvious features and is directly related with faulting.By the observation,measurement and dating to three typical sections at Xiaobaopo,Qiaogou and Zhongdicun,this paper obtained the activity parameters of the Lintong-Chang an Fault since the late Pleistocene and the age stratigraphic sequence of the tablelands of Bailuyuan,Shaolingyuan and Henglingyuan.Research results show that the Bailuyuan tableland has experienced

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.

The earthquake occurred in Jiezhou,Gansu Province on July 1,1879 (May 12,Guangxu 5,Qing Dynasty) is one of the largest earthquake events in Chinese history. The effect of this event spread over more than ten provinces. According to historical data,the magnitude of this earthquake has been estimated to be 8 or 71/2,and the macroscopic epicenter has been located at Mianshanheba (104.7E,33.2N),south of Wudu,Gansu Province,with type II precision of estimation (≤25km error). As the earthquake occurred in the areas of high mountain ridges difficult to be approached,it has not been studied in detail so far. It is one of the 3M=8 earthquakes in China that their relation to active faults had not been revealed so far. Recently,we have found that in satellite images a NNE-trending lineament can be clearly observed along the northern foot of the Shuikeng Mountains to the northeast of Wenxian County,Gansu Province. It extends eastward from Shifang Village to the northwest of Wenxian County,passing through Liangjiaba,Malian River,Fanjiaba,Miaobeihou,Guanjiagou,Jianzuishan Mountains,Shuikengya,northern slope of the Fangmashan Mountains,Songjia Mountains and Shenjiana'an,and then gradually becomes unclear after crossing through the Bailongjiang River and turning northward at 500m downstream from the Majiaba. The lineament has a total length of about 30km,among which about 21km can be identified to be surface fracture zone (from the bank of the Bailongjiang River to the top of the western hills of Guanjiagou). The lineament offsets left-laterally a series of ridges and gullies. In comparison with geologic data of this region,this lineament image coincides well with the NEE-trending Fanjiaba-Linjiang Fault. The interpretation of satellite images for the whole region and re-collation of historical records about this earthquake have confirmed that the Fanjiaba-Linjiang Fault bears a close relation to the southern Wudu earthquake,and is most probably the causative fault of this earthquake. This study,therefore,provides new insights into the following aspects,such as the occurrence and mode of motion of the causative fault,the length of surface ruptures,the amount and variation of horizontal displacement along the fault,the location of macroscopic epicenter,the probable magnitude of the event and the distribution of seismic hazards etc. All these results have led to the satisfactory solution of outstanding issue about the causative fault of the 1879 southern Wudu earthquake. They are also of great importance to seismic risk assessment in the border areas of Gansu,Qinghai and Sichuan Provinces as well as in southeast Gansu Province.

Recognition of major tectonic events and tectonic cycles in Quaternary is an important topic that attracts more and more attention of many geologists. Recently, the research in this aspect has focused mainly on Quaternary sedimentation and geomorphologic evolution, while little attention has been paid to the study of fault evolution, which is directly related to tectonic cycle or tectonic event. This is mainly because of the lacking of geological evidence and the limitation of dating technique. Nevertheless, significant progress has been made in loess study, especially the characteristics and dating of loess-paleosoil sequences, which provide a time scale for regional correlation and timing of loess deposits. In this paper, an attempt has been made to discuss the migration and variation of Quaternary activity of fault zone through the analysis of loess deposition along the zone by using this time scale. The main purpose of this study is to provide direct evidence for the division of tectonic cycles in Quaternary. The present study deals mainly with the Lintong-Chang'an Fault and the Lishan mountain front fault on the southern margin of Weihe basin, as well as the Kouzhen-Guanshan Fault on the northern margin of the basin. The loess deposits along these fault zones have been studied in detail, while the main unconformities in loess sequences were identified in the geological sections across the fault zones and dated by using the loess-paleosoil time scale. The results show that tectonic unconformity presents broadly along the Lintong-Chang'an Fault and the Lishan mountain front fault, appearing as discordant contact of the S 8 paleosoil layer with the underlying strata. The underlying strata are offset significantly by the fault, but the overlying strata of S 8 are offset inconsiderably. Along the Kouzhen-Guanshan Fault zone, the S 1 paleosoil layer discordantly contacts with the underlying strata, which are significantly offset by the fault, but the overlying strata of S 1 layer are inconsiderably offset. In term of the loess-paleosoil time scale, the following conclusion can be drawn from the result of this study: The activity of the Lintong-Chan'an Fault zone on the southern margin of the Weihe basin was markedly changed at 800～900ka B.P. At the same time, the migration of activity occurred along the Lishan mountain front, while strong activity started to occur along the Weinan Yuan front Fault and the whole Weinan Yuan began to be uplifted. At 120ka B.P., the activity of the Kouzhen-Guanshan Fault zone began weakened. Thus, the variation of fault activity in this area may indicate two major tectonic events in mid-late Quaternary. This result may provide basic material for regional correlation of Quaternary tectonic events and for the research of the tectonic manifestation of tectonic events.