The most significant feature of active faults on remote sensing images is fault lineament. How to identify and extract fault lineament is an important content of active fault research. The rapid development of remote sensing technology has provided people with extremely rich remote sensing data, and has also created the problem of how to choose suitable data for fault interpretation. In the traditional fault interpretation, people pay more attention to high-resolution optical images and high-resolution DEM, but optical remote sensing images are greatly affected by factors such as weather condition, vegetation and human impacts, and the time and economic costs for obtaining high-resolution DEM are relatively high. Due to the low resolution, the medium-resolution DEM(such as Aster GDEM, SRTM1, SRTM3, etc.)is generally used to automatically extract structural lineament, and then analyze the overall regional structural features, but it is rarely used to visually interpret active faults. ALOS-PALSAR DEM is generated from SAR images acquired by the phased array L-band synthetic aperture radar mission sensor of the Japanese ALOS satellite. It is currently a free DEM with the highest resolution(resolution of 12.5m)and the widest coverage. Based on ALOS-PALSAR DEM and ArcGIS 10.4 software, this paper generates a hillshade map and visually interprets the fault lineaments in the West Qinling Mountains. When generating a hillshade map, we set the light azimuths to be oblique or orthogonal to the overall trend of the linear structures, the light azimuths to be consistent with the slope direction of the hillslope, and the light dips to be a medium incident angle. Based on the hillshade map generated from ALOS-PALSAR DEM, this paper summarizes the typical performance and interpretation markers of fault lineaments on the hillshade map(generated by DEM), and visually interprets the V-shaped fault system in West Qinling Mountains where the research on fault geometry is limited based on the interpretation markers. The results of the research are as follows: First, this study discovers a number of fault lineament zones, including the fault lineament located between the Lintan-Dangchang Fault and the Guanggaishan-Dieshan Fault, the NE-directed fault lineament zone between the Lixian-Luojiapu Fault and the Liangdang-Jiangluo Fault, and the arc-shaped dense fault lineament zones distributed south of the Hanan-Daoqizi Fault and the Wudu-Kangxian Fault; Second, this study completes the geometric distribution images of the known active faults, such as the western and eastern sections of the Lintan-Dangchang Fault, the western and eastern sections of the Liangdang-Jiangluo Fault; Third, fault lineaments in the West Qinling Mountains exhibit a “V” shape, with two groups of fault lineaments trending NW and NE, whose tectonic transformation mainly consists of two kinds: mutual cutting and arc transition. The Lintan-Dangchang Fault cuts the Lixian-Luojiapu Fault, the Lintan-Dangchang Fault and the Guanggaishan-Dieshan Fault are connected with the Liangdang-Jiangluo Fault in arc shape, and the Tazang Fault is connected with the Hanan-Daoqizi Fault in arc shape. The research results show that ALOS-PALSAR DEM has an outstanding capability to display fault lineaments due to its topographic attributes and strong surface penetration. In circumstances when the surface is artificially modified strongly, the spectrum of ground objects is complex and the vegetation is dense, the ALOS-PALSAR DEM can display fault lineament that cannot be displayed on optical remote sensing images, indicating that the medium-resolution DEM is an effective supplement to high-resolution optical remote sensing images in the fault lineament interpretation. The research results are of great significance for improving the geometric image of the V-shaped fault system in the West Qinling Mountains. It is also the basis for further research on fault geometry, kinematics, regional geodynamics and seismic hazard.

Located in the intervening zone between Tibetan plateau and surrounding blocks, the Lintan-Dangchang Fault(LDF)is characterized by north-protruding arc-shape, complex structures and intense fault activity. Quantitative studies on its new activity play a key role in searching the seismogenic mechanism, building regional tectonic model and understanding the tectonic interaction between Tibetan plateau and surrounding blocks. The LDF has strong neotectonic activities, and moderate-strong earthquakes occur frequently(three M6~7 earthquakes occurred in the past 500 years, including the July 22nd, 2013, Minxian-Zhangxian M_S6.6 earthquake), but the new activity of the fault is poorly known, the geological and geomorphological evidence of the Holocene activity has not been reported yet. Based on remote sensing interpretation and macro-landform analysis, this paper studies the long-term performance of LDF. Based on the study of fault activity, unmanned aircraft vehicle photogrammetry and differential GPS, radiocarbon dating, etc., the latest activity of LDF is quantitatively studied. Then the research results, historical strong earthquakes and small earthquake distribution are comprehensively analyzed for studying the seismogenic mechanism and constructing regional tectonic models. The results are as follows: Firstly, the fault geometry is complex and there are many branch faults. According to the convergence degree of the fault trace and the fault-controlled macroscopic topography, the LDF is divided into three segments: the west, the middle and the east. The west segment contains two fault branches(the south and the north)and the south Hezuo Fault. The south branch of the west segment mainly dominates the Jicang Neogene Basin, and the south Hezuo Fault controls the south boundary of Hezuo Basin. The middle segment has more convergent and stable trace, consisting of the main fault and south Hezuo Fault, and these faults separate the main planation surface of the Tibetan plateau and Lintan Basin surface geologically and geomorphologically. The fault traces in the east segment are sparsely distributed, and the terrain is characterized by hundreds of meters of uplifts. The branch faults include the main fault, Hetuo Fault, Muzhailing Fault and Bolinkou Fault, each controlling differential topography. Secondly, the motion property of the LDF is mainly left-lateral strike-slip, with a relative smaller portion of vertical slip. The left-lateral strike-slip offset the Taohe River and its tributaries, gullies and ridges synchronously, and the maximum left-lateral displacement of the tributary of Taohe River can reach 3km. Meanwhile, the pull-apart basins and push-up ridges associated with the left-lateral fault slip are also developed in the fault zone. The performance of vertical slip includes tilting of the main planation surface, vertical offsets of the boundary and interior of Neogene basin and hundred meter-scale differential topography. The vertical offset of the Neogene is 300~500m. Thirdly, one fault profile was newly discovered in Gongqia Village, revealing a complete sequence of pre-earthquake-coseismic-postseismic deposition, and this event was constrained by the radiocarbon ages of pre-earthquake and post-earthquake deposition. The event was constrained to be 2090~7745aBP(confidence 2σ), which for the first time confirmed the Holocene activity of the fault. Fourthly, a gully with two terraces at least on the west side of Zhuangzi Village in the east segment of the main fault retains a typical faulted landform. The T2/T1 terrace riser of the gully has a left-handed dislocation of 6.3~11.8m, and the scarp height on terrace T2 is 0.4~0.7m, the radiocarbon age of the terrace T2 is7170~7310aBP, so the derived left-lateral strike-slip rate since the early Holocene in the east segment of the main fault is 0.86~1.65mm/a, and the vertical slip rate is 0.05~0.10mm/a. The derived slip rates are in line with the regional tectonic model proposed by the predecessors, so the LDF plays an important role in the internal deformation of the West Qinling. The clockwise rotation of the middle to east segments of the LDF acts as an obstacle to the left-lateral strike-slip motion, which inevitably leads to the redistribution and rapid release of stress, so earthquakes in the middle-east segment of the LDF are unusually frequent.

The Hongtong earthquake occurring on 25 September 1303 in both Linfen Basin (LFB)and Taiyuan Basin (TYB)in Shanxi Graben is the first M8.0 earthquake based on the Chinese literature in China mainland, 392 years later, the Linfen M7.5 earthquake occurred on 18 May 1695 in Linfen Basin with its macro-epicenter distance of only 40km south of the Hongtong earthquake. Due to their close macro-epicenter distance and shortly interval of 392a, it attracted continuous attention to the geoscientists around Southern Shanxi Graben, southeastern Orods Plate. This paper combines the historical documents and interpreting the coseismic triggered disasters in study area. The results show that:1)the number of building damaged in the southern TYB and Lingshi Uplift (LSU)during 1303 Hongtong earthquake is similar to that of the LFB, indicating that the TYB and LSU maybe suffered the same or even worse earthquake disaster losses during the 1303 Hongtong earthquake. While the 1695 Linfen earthquake is confined within the LFB and south of Hongtong County; 2)More than 11 000 loess landslides were triggered by the 1303 Hongtong earthquake event between LFB and TYB, which is consistent with the literature records. We suggested the macro-epicenter of the 1303 Hongtong earthquake should move about 60km northward from the present location (36.3°N, 111.7°E)near Hongtong County to the new location (36.8°N, 111.7°E) between Huozhou City and Lingshi County, the new macro-epicenter location can reasonably explain the large-scale centralized earthquake-triggered landslides during the event. The landslides had aggravated the severity of the loss; 3)Our result helps to understand the spatial distribution of the two strong earthquakes and the relationship between them, especially the distribution map of earthquake-induced loess landslides by 1303 Hongtong earthquake extracted using the Google Earth images, which supports the amendment of the macro-epicenter.

A complete understanding to the disasters triggered by giant earthquakes is not only crucial to effectively evaluating the reliability of existing earthquake magnitude, but also supporting the seismic hazard assessment. The great historical earthquake with estimated magnitude of M8.5 in Huaxian County on the 23^rd January 1556, which caused a death toll of more than 830 000, is the most serious earthquake on the global record. But for a long time, the knowledge about the hazards of this earthquake has been limited to areas along the causative Huashan piedmont fault(HSPF) and within the Weihe Basin. In this paper, we made a study on earthquake triggered landslides of the 1556 event along but not limited to the HSPF.
Using the high-resolution satellite imagery of Google Earth for earthquake-triggered landslide interpretation, we obtained two dense loess landslides areas generated by the 1556 earthquake, which are located at the east end and west end of the HSPF. The number of the interpreted landslides is 1 515 in the west area(WA), which is near to the macro-epicentre, and 2 049 in the east area(EA), respectively. Based on the empirical relationship between the landslide volume and area, we get the estimated landslide volume of 2.85~6.40km³ of WA and EA, which is equivalent or bigger than the value of ~2.8km³ caused by Wenchuan earthquake of M_W7.9 on 12^th May 2008. These earthquake triggered landslides are the main cause for the death of inhabitants living in houses or loess house caves located outside of the basin, such as Weinan, Lintong, Lantian(affected by WA) and Lingbao(affected by EA). Our results can help deeply understand the distribution characteristics of coseismic disaster of the 1556 Huaxian earthquake to the south of Weihe Basin, and also provide important reference for the modification of the isoseismals.

The Riyue Mt. Fault is a secondary fault controlled by the major regional boundary faults (East Kunlun Fault and Qilian-Haiyuan Fault). It lies in the interior of Qaidam-Qilianshan block and between the major regional boundary faults. The Riyue Mt. fault zone locates in the special tectonic setting which can provide some evidences for recent activity of outward extension of NE Tibetan plateau, so it is of significance to determine the activity of Riyue Mt. Fault since late Pleistocene to Holocene. In this paper, we have obtained some findings along the Dezhou segment of Riyue Mt. Fault by interpreting the piedmont alluvial fans, measuring fault scarps, and excavating trenches across the fault scarp. The findings are as follows:(1) Since the late Pleistocene, there are an alluvial fan f_p and three river terraces T₁-T₃ formed on the Dezhou segment. The abandonment age of f_p is approximately (21.2±0.6) ka, and that of the river terrace T₂ is (12.4±0.11) ka. (2) Since the late Pleistocene, the dextral strike-slip rate of the Riyue Mt. Fault is (2.41±0.25) mm/a. In the Holocene, the dextral strike-slip rate of the fault is (2.18±0.40) mm/a, and its vertical displacement rate is (0.24±0.16) mm/a. This result indicates that the dextral strike-slip rate of the Riyue Mt. Fault has not changed since the late Pleistocene. It is believed that, as one of the dextral strikeslip faults, sandwiched between the the regional big left-lateral strike-slip faults, the Riyue Mt. Fault didn't cut the boundary zone of the large block. What's more, the dextral strike-slip faults play an important role in the coordination of deformation between the sub-blocks during the long term growth and expansion of the northeast Tibetan plateau.

On 20 April 2013, a destructive earthquake, the Lushan M_S7.0 earthquake, occurred in the southern segment of the Longmenshan Fault zone, the eastern margin of the Tibetan plateau in Sichuan, China. This earthquake did not produce surface rupture zone, and its seismogenic structure is not clear. Due to the lack of Quaternary sediment in the southern segment of the Longmenshan fault zone and the fact that fault outcrops are not obvious, there is a shortage of data concerning the tectonic activity of this region. This paper takes the upper reaches of the Qingyijiang River as the research target, which runs through the Yanjing-Wulong Fault, Dachuan-Shuangshi Fault and Lushan Basin, with an attempt to improve the understanding of the tectonic activity of the southern segment of the Longmenshan fault zone and explore the seismogenic structure of Lushan earthquake.
In the paper, the important morphological features and tectonic evolution of this area were reviewed. Then, field sites were selected to provide profiles of different parts of the Qingyijiang River terraces, and the longitudinal profile of the terraces of the Qingyijiang River in the south segment of the Longmenshan fault zone was reconstructed based on geological interpretation of high-resolution remote sensing images, continuous differential GPS surveying along the terrace surfaces, geomorphic field evidence, and correlation of the fluvial terraces.
The deformed longitudinal profile reveals that the most active tectonics during the late Quaternary in the south segment of the Longmenshan Fault zone are the Yanjing-Wulong Fault and the Longmenshan range front anticline. The vertical thrust rate of the Yanjing-Wulong Fault is nearly 0.6~1.2mm/a in the late Quaternary. The tectonic activity of the Longmenshan range front anticline may be higher than the Yanjing-Wulong Fault. Combined with the relocations of aftershocks and other geophysical data about the Lushan earthquake, we found that the seismogenic structure of the Lushan earthquake is the range front blind thrust and the back thrust fault, and the pop-up structure between the two faults controls the surface deformation of the range front anticline.

Yuxian-Guangling Basin is a half-graben basin unit belonging to the basin-ridge structure zone in northwest Beijing area.The southern boundary of this basin is controlled by a normal fault belt called the Yuguang Basin South Margin Fault(YBSMF).The YBSMF is about 120km long,with a general strike of N70°E,and is an active fault zone.The YBSMF was evolved from the propagation,interaction or linkage of existing isolated segments and the forming of new fault segments,and there are actually many segments and places along the YBSMF where the faults propagate and grow.However,except the study on the fault growth at the Jiugongkou segment by Cheng Shaoping in 1998,which indicated that the fault has propagated several kilometers westwardly in the late Late Pleistocene alluvial fans,the research about the propagation and growth of the faults at other places and segments is quite limited.At these segments and places,in what ways or patterns does the fault propagate,grow,link and evolve？What on earth controls and affects the propagation and growth of the faults？All these questions still remain unanswered yet and deserve further analysis and study.Based on high-resolution remote sensing image interpretation,DEM 3D analysis,field geological investigation,trenching and so on,we made a research on the fault growth of the YBSMF.According to the fault geometry,fault activity and the difference of the faulted landforms,the YBSMF belt can be divided into five segments: Shangbaiyang segment,Tangshankou segment,Beikou segment,Songzhikou segment and Shanghupen segment.The faults grow and evolve both between adjacent segments and within each segment.Besides,some new faults also form in the proluvial fans in front of mountains.After a detailed comparison and analysis of all the sites of fault growth along the YBSMF,we find out several characteristics and rules about the growth of the fault.First,the faults often grow or evolve where the fault geometry is irregular,and the irregularity of fault geometry is a primary factor which determines whether the faults propagate and grow or not.The irregular segments where the faults propagate and grow can be divided into two categories.The first type mainly includes the uneven or unsmooth segments,such as the segments with convex or concave arcs,edges or corners,and so on; the second type mainly consists of two nearly parallel faults with a gap between them,which causes the discontinuity of the fault geometry along the strike.Second,fault growth leads to the "cut off" and elimination of the irregularity of fault geometry,such as cutting off the uneven or unsmooth segments,and linking the discontinuous segments along the strike.The elimination of the irregularity makes the fault geometry smooth and continuous,and reduces the roughness on the sliding surface,which contributes to the downward slip of the half-graben block inside the basin along the sliding surface.Third,the degree of "cut off" or elimination may be affected by the spatial scale of the irregular shape.As the scale of the irregularity increases,the fault will propagate a larger distance to overcome the hindrance of the roughness,so it will take more time for the irregular segments to be completely "cut off" or eliminated,and vice versa.Therefore,after the same period of time,the irregularity with a small scale has been completely "cut off" or eliminated,while the irregularity with a large scale may be still in the process of segment linkage or cutting off,so the degree of "cutting off" or elimination is lagging behind and relatively lower.

The May 12,2008 M_S 8.0 Wenchuan earthquake in Sichuan Province,China produced a 240km long co-seismic surface rupture zone along the Longmenshan Central Fault.Our investigation focuses mainly on the three regions along the co-seismic surface ruptures,i.e.the Nanba Town and the Fenghuang village on the northern segment of the rupture zone,and the Yingxiu town on the southern segment.We studied the river terraces in these regions offset by the active fault,measured the surface ruptures and the fault scarps on the multilevel terraces and obtained the height of the fault scarps on each terrace,namely,the cumulative vertical slip of the active fault recorded by terrace,through data calculation and analysis.If we use the vertical slip of this earthquake as the average vertical slip of the paleoearthquakes,then the ratio of the cumulative vertical slip of each terrace to this average vertical slip is the cumulative times of paleoearthquakes recorded by each terrace.The research results show that T₁ of every study area has undergone only one paleoearthquake event ever since its formation,T₂ about 5 paleoearthquake events,T₃ about 9～11 paleoearthquakes and T₄ about 20 paleoearthquakes.Based on the research result of this paper,and combined with the previously dating ages of the terraces,we can obtain some reliable data about the recurrence intervals of the paleoearthquakes.

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 investigation and study of fault activities are a basic work for urban earthquake prevention and disasters reduction.In order to find out the location,characteristics and activities of the Laoyachen Fault in Zhengzhou,the high-resolution shallow seismic P and S wave survey profiling across the Laoyachen Fault was carried out at the end of 2006,and different seismic sources along with combinations of diverse observation geometries with different parameters were used.The fine structures in different depths beneath the profile were obtained and the patterns as well as characteristics of the Laoyachen Fault were determined.The results show that the Laoyachen Fault,running in NW and dipping in NE,is a normal fault and its dip angle is about 60°～70°,which incises strata of Eocene,Permian,Carboniferous or Ordovician epoch and goes up to the top boundary of Eocene stratum at the 800～850m depth.There is no any reflector of offset stratum found in Q+N strata.The borehole geological sections across steep slopes of earth surface present that the layers inferred from reflected seismic wave groups of shallow seismic profile are well correspondent with boring geological layers.The borehole results reveal that the three reference laminas,i.e.the boundary between Malan loess and silt with clay soil at about 21m in depth,the calcareous gravel clay layer of 53.9m deep,and the calcareous silt layer of 61.9m deep,all have not depth variations at the two sides of surface steep slopes and are situated almost at the same ground surface elevations,which suggests that the steep slopes at the earth's surface should not result from the activities of Laoyachen Fault.In this study,through shallow seismic P wave and S wave exploration as well as combination of joint borehole geological sections,not only the location and characteristics of Laoyachen Fault was determined,but the geological and seismological evidences for the fault activity estimations were provided.

There exist some uncertainties in dating paleoearthquakes.To improve the credibility of determination of the earthquakes,comprehensive analysis of movement patterns,sedimentary environment and micro-topography has been made to investigate the paleoseismic events along the Zemuhe active fault zone.Combined trenches were excavated near Daqingliangzi,which reveal three palaeoearthquakes with ages of 160a,3100a and 5500～8900a,respectively,and a recurrence interva1of about 3000 years.Sedimentary processes related to strike-slip fault type earthquakes are discussed and a sedimentary model is put forward for strike-slip fault on condition that bulges and reverse scarps were formed at hillside area.

The active faults in Altai Mountains are mainly in NW—NNW direction and can be divided into three belts:the western margin of Altai Mountains active fault zone, the central Altai Mountains active fault zone and the eastern margin of Altai Mountains active fault zone. The eastern marginal active fault zone consists of two right-lateral strike-slip faults,the Hovd active fault and the Har-Us active fault,and the compressional basins between them. Several segments of earthquake ruptures have been reported along the strike-slip faults in former studies. We also found two new segments of earthquake rupture on the middle to south parts of the fault zone in recent field investigation. One segment is on the Jargalant Fault in the middle part of the Har-Us Fault zone,the other segment is on the Tugen Gol Fault in the south part of the Hovd Fault zone. The earthquake surface rupture on the Jargalant Fault has been studied in more detail in this paper. The rupture zone is 50km long with about 4～5m dextral displacement,implying a large and recent earthquake. Earthquake ruptures have been observed on each segment of the eastern margin of Altai Mountains active fault zone. So,the eastern margin of Altai Mountains is a strongly active tectonic zone with large earthquake.

Datong Fault belt is a northwest trending fault in the north of Qinghai-Tibet plateau which controls the boundary of Xining Basin and Datong Basin.It consists of the Maziying-Miaogou(F₁)Fault and the Laoye mountain-Nanmenxia Fault(F₂).There is obvious displacement in vertical direction along the fault belt.The field investigation results show that this belt has long-term activity.There are several meters-long crushed zone and veins along the fault side in the basement rock.In the visible profile of fault,the Cambria system thrusts to the red brick Quaternary gravel,and there are several centimeters-thick fault gouges along the fault side.ESR dating of the fault gouge in the fault profile shows an age of(610?61)ka.The covering deluvial loess is not offset,and the OSL result is(14.6?1.5)ka.So it can be concluded that the fault belt was active in middle Pleistocene but not in later Pleistocene according to the age data and geomorphologic feature.Interior stratum of the Datong Basin is mainly featured with fold with the major axis in northwest direction.According to the relation of fault and fold deformation,Datong Fault is a transversal tear,which is due to uneven compression of the folds in different parts and the NNE-oriented regional compressional stress.It is common among the NE-trending faults in northeastern Qinghai-Tibet plateau.These NE-trending faults aren't large,and most are located in the active plate.They are all nearly vertical to the axis of the folds and compressive basins.

The newest active time and segmentation of the fault are of special significance in the safety evaluation of major engineering projects.This paper discusses the active times and segmentation characteristics of the Guanguanling Fault zone through interpreting aerial photos,field investigation,topographic and geomorphic surveys and analysis of trench logs on paleoearthquake in connection with the study of the geological and seismologic problems in Heishanxia project of the Huanghe River.The Guanguanling active fault zone lies on the northeastern margin of the Qinghai-Tibet block.It is part of the Zhongwei-Tongxin arc active fault zone,striking near EW generally,with a total length of about 60km.It consists of 5 discontinuous secondary faults arranged in left step en echelon,namely,Jingtaixiaohongshan(F_1-1),Guanguanling(F_1-2),Shajing(F_1-3),Zhongweixiaohongshan(F_1-5)and Qingshan-Gushanzi(F_1-4),respectively.Since Late Quaternary,the fault is characterized with intense sinistral strike-slip and compressional thrust and has offset a series of ridges and small gullies and terraces.At the same time,fault scarps were developed along the fault zone.The study reveals that the latest earthquake occurred 700～1200a BP,the largest displacement took place in Guanguanling,and the maximum horizontal sinistral displacement reaches 6m since Holocene.

The Xining basin is a Cenozoic basin bounded by the Riyueshan Fault and the Lajishan Fault on the south,the Dabanshan Fault on the north.Controlled by these active boundary faults and under the near NE-oriented compressional stress,fold deformation occurred inside the basin.Several active faults of NE or EW trending were detected.Nevertheless,all of them are small and weakly active.By the study on the relationship between fault and folding,we think that the Huangshuihe Fault and the NW-strike north bank of Huangshuihe Fault are tensional faults developed on the apex of an anticline;the Nanchuanhe Fault is a transverse tear fault resulting from differential folding on the two sides of the fault;and the east bank of Beichuanhe Fault is a compressional fault developed on the core or limb of an syncline.By the balanced cross-section analysis of fold deformation and inversion of gravity anomaly data,we obtained that the depth of detachment plane is about 4～5km.Thus,based on the above data,the seismotectonic model of the Xining urban area is built.We conclude that the faults of the area are the secondary faults related to folding of overburdens and have no potential of producing earthquakes larger than magnitude 6 for their small scale and shallow depth.

The interaction zone between southern Tianshan and northern Tarim locates at northeast side of Pamir. It is a region with high seismicity. We built a seismotectonic model for the west part of this zone from data of geological profiles, deep crust seismic detection and earthquake focal mechanisms in this paper. Geological profiles show different structure styles between the north of Artux anticline and the south of it. There are southward thrust blocks with basement rocks and strongly deformed Mesozoic and Cenozoic folds at the north, but northward reverse faults and flat Neogene - Quaternary shallow folds at the south. A fault named Qiligaike at the north side of the Artux anticline can be identified from its linear feature on satellite image. It cuts many folds at the north, and controls the twisting deformation of Artux anticline at the south. These deformation features on two sides of the fault imply the fault is a high angle strike-slip fault. Deep crust seismic detection also shows different velocity structures between Tianshan and Tarim Basin. Depth of the crystalline basement is 4km in Tianshan and 10km in Tarim; Depth of crust in Tianshan is 55km and 50km in Tarim. Steep slope of the crust exists at the northwest of Artux. These indicate the existence of high angle faults. Based on the synthesized geological features, deep crust structure, and earthquake focal mechanisms, we think that the main regional tectonic is featured as that the Tianshan tecto-lithostratigraphic unit overthrusts on the Tarim block. The Tianshan tectonic system includes the Maidan fault and thrust sheets in front of the fault; The Tarim tectonic system includes the underground northern Tarim margin fault, conjugate fractures in basement and overthrust faults in shallow. The northern Tarim margin fault is a high angle fault existing in deep of the Tarim crust, adjusting different trending deformation between Tianshan and Tarim. It is the major active fault that can generate large earthquakes. The other faults as the Tianshan overthrust system and the Tarim basement faults in this area may generate moderately strong earthquakes with different styles.

The Kunlunshan M_s 8.1 earthquake of 2001 is a large earthquake produced by a large-scale intraplate strike slip faulting. Field observation shows that the surface rupture of this earthquake is about 426km long,and the maximum sinistral displacement is about 6m. As compared with the other similar intraplate large earthquakes,its rupture length is extraordinarily longer but its horizontal displacement is relatively small. The distribution of horizontal displacement along the surface ruptures is markedly controlled by fault structure. At the section where the horizontal displacement is small,the vertical displacement is also smaller,and no transformation of horizontal and vertical displacements is observed. This feature is also different from that of the other earthquakes. The relationship among rupture length,displacement and seismic magnitude follows a certain empirical equation,in which the ratio between the average displacement amount and rupture length is called "Ultimate linear strain",and is constantly within the range of 10^-5 for the rupture in the earth's crust. This feature can be used as criterion for testing the independency and integrality of the rupture segments. In this paper,we calculated the "ultimate linear strain" of the entire rupture zones and sub-segments of several large earthquakes,including the 1920 Haiyuan earthquake,1951 Bong Co earthquake,1932 Changma earthquake and 2001 Kunlunshan earthquake. The results show that average values of ultimate linear strain of the Haiyuan,Bong Co and Changma earthquakes are approximately close to the statistic value,but the values of ultimate linear strain of their sub-segments are significantly higher. In contrast,the value of ultimate linear strain for the entire surface rupture of the Kunlunshan earthquake is much lower than the statistic value,but the values of ultimate linear strain of its four sub-segments,except the west Taiyanghu segments,are close to the statistic value. Therefore,the Kunlunshan earthquake (M_s 8.1) should be produced by four relatively independent faulting events,instead of a uniform faulting. These characters provide geological evidence supporting the deduction that the Kunlunshan earthquake is successively triggered multiple earthquake events,rather than a single earthquake.

The 14 November,2001 M_S8.1 West Kunlun Pass Earthquake is the largest event associated with the longest surface rupture that has occurred in the Tibetan Plateau since 1951. We made 291 surficial left lateral slip measurements and 111 net vertical slip measurements along the main fault zone. The displacement on the main fault strand is dominated by left lateral strike slip of 2.7m in average,with vertical slip component of mostly less than 1m. The maximum left lateral slip is 6.4m,with as much as 5.1m of vertical slip component. Sinistral surficial slip is quite variable along the main strand of the rupture at distance scales ranging from a few tens of meters to a few hundreds of kilometers,with slip gradient ranging between 10^-1～10^-4. The slip variations over short length scales (tens of meters to a few kilometers) might be caused by variations in thickness of unconsolidated sediments,fault strike and slip of the previous earthquake,distributed non brittle deformation and secondary fractures,complexities in fault geometry,and perhaps by measurement error. Despite this short wavelength variability,there is fairly regular long wavelength (tens to hundreds of kilometers) behavior to the east of the Buka Daban Peak. One notable characteristic of slip distribution along the faults is that very large surficial slips (as large as 5～6 meters) were observed at 5～6 sites located at different surface rupture segments in asymmetry to their left lateral slip functions. Slip on each of these rupture segments diminishes away from the highest slip site to its terminations with different slip gradients. This asymmetric distribution of slips may indicate the propagation direction of the rupture along the faults. This long wavelength variation in slip might be influenced by fault geometry,while the segmentation of the surface rupture zone might play a key role. It should be pointed out that the surficial slip (at both short and long length scales) is only a near field slip measured in the field by using tape measure. Therefore,it should be considered as a minimum value,and may represent the real variations in the amount of brittle slip on visible fractures at the surface,but it potentially underestimates the actual slip produced by the earthquake and slip distribution over the whole surface rupture due to the difficulty in identifying distributed non brittle deformation. This calls for caution in discriminating between one or multiple discrete events and in estimating the size of past and future earthquakes by using displaced deposits in trenches or offset geomorphologic features along strike slip prehistoric fault ruptures.

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.