On January 8th, 2022, an M_S6.9 earthquake occurred around Menyuan County(37.77°N, 101.26°E), Qinghai Province. The epicenter is located in the northeastern part of the Tibetan plateau, where the western section of the Lenglongling Fault meets the eastern section of the Tolaishan Fault. In order to know the spatial distribution of coseismic surface rupture zone as soon as possible, and determine the seismogenic structure, the post-earthquake GF-7 remote sensing images of the Menyuan M_S6.9 earthquake were analyzed. Moreover, combining the interpretation of the GF-7 images and the field investigation, the distribution of the co-seismic surface rupture was determined and the typical coseismic landforms, and the image recognition features of various co-seismic landforms are interpreted and summarized. The results show that the earthquake produced two major surface rupture zones with a left-stepped oblique spatial arrangement. The main northern branch rupture distributes on the west side of the Lenglongling Fault, with a length of about 22km and a strike of 100°N~120°E, the secondary rupture of the southern branch distributes along the eastern section of the Tuolaishan Fault, with a length of about 4km and a strike of N90°E. The total length of the two rupture zones is about 26km.

Along the rupture zones, a series of typical left-lateral strike-slip coseismic landforms were formed, such as tensional fractures, tensional-shear fractures, pressure ridges, pressure bulges, left-lateral strike-slip gullies, as well as left-lateral strike-slip roadbeds, etc. We divided the surface rupture into six segments to conduct detailed observation and analysis, that is, the west of Daohe segment, Liuhuanggou segment, Honggou segment, Yongan River segment and Yikeshugou segment, from west to east among the main rupture zone of the north branch, as well as the secondary rupture zone of the south branch. In general, each co-seismic landform has its distinctive image characteristics, and we obtained them from the interpretation and summarization of the GF-7 images. The shear fractures located at the two ends of the main rupture and in the areas where the surface rupture is weak are zigzaggy on the remote sensing images, while the shear fractures located in the areas where the surface rupture is intense are shown as dark, wide and continuously smooth stripes; thrust scarps are represented on remote sensing images as shaded, narrow and slightly curved strips; the pressure ridges and pressure bulges exhibit black elliptical feature on the images that are parallel or at a smaller angle to the main rupture; tensional-shear fractures are displayed as black strips arranged in en echelon with a 30°~45° intersection angle with the main shear rupture, and their linear features are not as straight as those of shear ruptures yet are still distinct; the coseismic scarps formed on the ice are manifested in the images as traction bend and texture change. Based on the GF-7 images, the cumulative dislocations of typical sinistral landforms along the co-seismic surface rupture on Lenglongling Fault are interpreted and futher compared with the previous study. This is the first time of application of GF-7 to the strong earthquake geohazards monitoring since it was officially launched in August 2020. From this study, it can be seen that with its high resolution, GF-7 can be used to accurately identify faulted features. Not only it could provide information of the geometric roughness, complexity and segmentation of the fracture, but also can record clear dislocations of the landforms. The study of the GF-7 images in the 2022 Menyuan earthquake has showed that the GF-7 images can provide strong data support for the geology and geological hazard studies.

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.

The Sanweishan Fault is located in the front of the northwest growth of the northern margin of Tibetan plateau, a branch fault of the Altyn Tagh Fault which extends to the northwest. The latest seismic activity of the Sanweishan Fault reflects the tectonic deformation characteristics of the northern plateau. Meanwhile, it is of great significance for the seismic risk assessment of Dunhuang and its adjacent areas to understand the characteristics of earthquake recurrence. The Sanweishan Fault runs along the western piedmont of the Sanwei Shan, with a total length of 175km. The fault is characterized by left-lateral strike-slip and reverse faulting, with local normal fault features. Based on the geometry, the fault can be divided into three segments, i.e. the Shuangta-Shigongkouzi, the Shigongkouzi-Shugouzi and the Shugouzi-Xishuigou segment from east to west. Previous studies about the paleoearthquakes on the Sanweishan Fault mainly focus on the middle and east segments of the fault, while the west segment of the fault has been less studied. Meanwhile, the available research does not involve the recurrence characteristics and possible magnitude of the paleoearthquakes. Based on high-resolution satellite images, we found that the main fault has grown toward the basin and formed fault scarps in the western segment of the Sanweishan Fault. We have carried out a detailed study on these fault scarps. Based on trench excavation and chronological study on the latest fault scarps, this paper determines the sequence of the paleoseismic events on the fault and discusses the recurrence characteristics and possible magnitude of earthquake for the Sanweishan Fault.
In the western segment of the fault, through satellite image interpretation and field investigation, we found new fault scarps distributed on the alluvial fan in front of the mountain near Gedajing. We called it Dunhuang segment of the Sanweishan Fault. The activity characteristics of the fault scarps may reflect the latest seismic events in the western part of the Sanweishan Fault. Different from the sinistral strike slip of the main Sanweishan Fault, this fault segment shows the characteristics of thrust with low angle. According to the differential GPS survey, the height of the fault scarp is approximately 2.2m. The paleoseismic trench was excavated across the fault scarp. Based on the analysis of paleoseismological trenching and optical stimulated luminescence dating, two paleoseismic events are determined. Event E1 occurred at approximately(35.1±3.7)~(36.7±4.1)ka; event E2 occurred at approximately(76.5±8.8)~(76.7±8.3)ka. According to the strata offset and corresponding age, the vertical slip rate of the Sanweishan Fault is determined to be(0.03±0.01)mm/a, with a corresponding shortening rate of(0.09±0.01)mm/a. Together with the previous results, we consider that the Sanweishan Fault is characterized by segmentation. The middle and east segments may have the ability of independent rupture, and also the characteristics of cascading rupture with the Dunhuang segment. According to the existing results, we conclude that the recurrence interval for cascading rupture behavior on the Sanweishan Fault is approximately 40ka, which shows a characteristic of low slip rate and long-term recurrence. The best estimated magnitude is inferred to be in the range between M_W7.1 and M_W7.5 based on the empirical relationships between moment magnitude and rupture length.

The Laohushan fault zone is located in the northeast margin of the uplift area of the Tibetan plateau. It belongs to the eastern segment of the Laohushan-Maomaoshan-Jinqianghe Fault in the eastern segment of the North Qilian fault system. It was manifested as compressive thrust in early stage, but its mechanical properties changed into left-lateral strike-slip movement after middle Pleistocene. There occurred the Jingtai M_S6.8 earthquake in 1888, Tianzhu M_S6.2 earthquake in 1990 and Jingtai M_S5.9 earthquake in 2000 along the fault in history.
With the construction of the national important projects in earthquake industry-“Digital seismic network project of the 10th Five Year Plan” and “Chinese seismic background field detection project”, a number of modern seismological stations were built in Gansu Province and its adjacent areas. Contrast with seismographic network, the mobile broadband seismic array has the advantages of relatively dense stations, small spacing, uniform distribution, and high data integrity rate. Combining the two observational methods has gradually become the main development direction at home and abroad.
Based on the data of small earthquakes in the Laohushan zone recorded by 20 stations of the digital seismic network in Gansu and its adjacent seismic network during the years of 2008 to 2019, and 18 portable seismographic stations from the 2^nd-phase project of China Seismic Detection Array during the years of 2014—2015, we relocated the dense earthquakes by double-difference method and obtained the source parameters for 700 earthquakes. The accurately located small earthquakes distribute along both sides of the Laohushan Fault, which is NW-trending obviously. Most earthquakes distribute at the depth range of 0~10km of the earth's surface after the relocation, and the result shows that the focal depths are more concentrated.
Generally, the earthquakes are closely related to active tectonics, large earthquakes usually occur on fault zones with obvious activity, but the distribution of small earthquakes is related to the complex stress state underground and the complex structures of fault zones. We can inverse the shapes and positions of the fault planes using spatial distribution of hypocenters of small earthquakes according to the principle that clustered earthquakes occur near the faults. We obtained the parameters of the Laohushan Fault, which has a strike of 103°and a dip angle of 89°, by using the simulated annealing algorithm and the Gauss-Newton algorithm. On this condition, rake angle of the fault plane is further inferred from regional tectonic stress parameters. These inversion results of the fault parameters indicate that it's a left-lateral slip fault with a high dip angle and a length of 38km. It extends from Xijishui county town of Jingtai in the southeast to Songshan of Tianzh county town in the northwest. Comparing the inversion fault plane parameters and the focal mechanism solutions of the 1990 Tianzhu M_S6.2 and 2000 Jingtai M_S5.9 earthquakes, both of the results are identical. Besides, the spatial distribution of inverted fault plane and the location of Laohushan Fault by the previous studies are basically parallel.
In the past, the studies of active faults mainly focused on the qualitative researches of macroscopic survey. With the technological development of earthquake location and inversion method in recent years, many quantitative researches have gradually been carried out on the determination of active fault parameters. The inversion results of Laohushan fault plane and the previous studies on the geometric characteristics of the fault are verified each other. It is proved by facts that it's an important research means of active faults. It can provide more evidences for determining fault parameters by inversion.

The Ebomiao Fault is a newly discovered active fault near the block boundary between the Tibetan plateau and the Alashan Block. This fault locates in the southern margin of the Beishan Mountain, which is generally considered to be a tectonically inactive zone, and active fault and earthquake are never expected to emerge, so the discovery of this active fault challenges the traditional thoughts. As a result, studying the new activity of this fault would shed new light on the neotectonic evolution of the Beishan Mountain and tectonic interaction effects between the Tibetan plateau and the Alashan Block. Based on some mature and traditional research methods of active tectonics such as satellite image interpretation, trenches excavation, differential GPS measurement, Unmanned Aircraft Vehicle Photogrammetry(UAVP), and Optical Stimulated Luminescence(OSL)dating, we quantitatively study the new activity features of the Ebomiao Fault.
    Through this study, we complete the fault geometry of the Ebomiao Fault and extend the fault eastward by 25km on the basis of the 20km-fault trace identified previously, the total length of the fault is extened to 45km, which is capable of generating magnitude 7 earthquake calculated from the empirical relationships between earthquake magnitude and fault length. The Ebomiao Fault is manifested as several segments of linear scarps on the land surface, the scarps are characterized by poor continuity because of seasonal flood erosion. Linear scarps are either north- or south-facing scarps that emerge intermittently. Fourteen differential GPS profiles show that the height of the north-facing scarps ranges from (0.22±0.02)m to (1.32±0.1)m, and seven differential GPS profiles show the height of south-facing scarps ranging from (0.33±0.1)m to (0.64±0.1)m. To clarify the causes of the linear scarps with opposite-facing directions, we dug seven trenches across these scarps, the trench profiles show that the south-dipping reverse faults dominate the north-facing scarps, the dipping angles range from 23° to 86°. However, the south-facing scarps are controlled by south-dipping normal faults with dipping angles spanning from 60° to 81°.
    The Ebomiao Fault is dominated by left-lateral strike-slip activity, with a small amount of vertical-slip component. From the submeter-resolution digital elevation models(DEM)constructed by UAVP, the measured left-lateral displacement of 19 gullies in the western segment of the Ebomiao Fault are(3.8±0.5)~(105±25)m, while the height of the north-facing scarps on this segment are(0.22±0.02)~(1.32±0.10)m(L₃-L₇), the left-lateral displacement is much larger than the scarp height. In this segment, there are three gullies preserving typical left-lateral offsets, one gully among them preserves two levels of alluvial terraces, the terrace riser between the upper terrace and the lower terrace is clear and shows horizontal offset. Based on high-resolution DEM interpretation and displacement restoration by LaDiCaoz software, the left-lateral displacement of the terrace riser is measured to be(16.7±0.5)m. The formation time of the terrace riser is approximated by the OSL age of the upper terrace, which is (11.2±1.5)ka BP at (0.68±0.03)m beneath the surface, and(11.4±0.6)ka at (0.89±0.03)m beneath the surface, the OSL age (11.2±1.5)ka BP at (0.68±0.03)m beneath the surface is more close to the formation time of the upper terrace because of a nearer distance to sediment contact between alluvial fan and eolian sand silt. Taking the (16.7±0.5)m left-lateral displacement of the terrace riser and the upper terrace age (11.2±1.5)ka, we calculate a left-lateral strike-slip rate of(1.52±0.25)mm/a for the Ebomiao Fault. The main source for the slip rate error is that the terrace risers on both walls of the fault are not definitely corresponded. The north wall of the fault is covered by eolian sand, we can only presume the location of terrace riser by geomorphic analysis. In addition, the samples used to calculate slip rate before were collected from the aeolian sand deposits on the north side of the fault, they are not sediments of the fan terraces, so they could not accurately define the formation age of the upper terrace. This study dates the upper terrace directly on the south wall of the fault.
    Since the late Cenozoic, the new activity of the Ebomiao Fault may have responded to the shear component of the relative movement between the Tibetan plateau and the Alashan Block under the macroscopic geological background of the northeastern-expanding of the Tibetan plateau. The north-facing fault scarps are dominated by south-dipping low-angle reverse faults, the emergence of this kind of faults(faults overthrusting from the Jinta Basin to the Beishan Mountain)suggests the far-field effect of block convergence between Tibetan plateau and Alashan Block, which results in the relative compression and crustal shortening. As for whether the Ebomiao Fault and Qilianshan thrust system are connected in the deep, more work is needed.

In this study, we described a 14km-long paleoearthquakes surface rupture across the salt flats of western Qaidam Basin, 10km south of the Xorkol segment of the central Altyn Tagh Fault, with satellite images interpretation and field investigation methods. The surface rupture strikes on average about N80°E sub-parallel to the main Altyn Tagh Fault, but is composed of several stepping segments with markedly different strike ranging from 68°N~87°E. The surface rupture is marked by pressure ridges, sub-fault strands, tension-gashes, pull-apart and faulted basins, likely caused by left-lateral strike-slip faulting. More than 30 pressure ridges can be distinguished with various rectangular, elliptical or elongated shapes. Most long axis of the ridges are oblique(90°N~140°E)to, but a few are nearly parallel to the surface rupture strike. The ridge sizes vary also, with heights from 1 to 15m, widths from several to 60m, and lengths from 10 to 100m. The overall size of these pressure ridges is similar to those found along the Altyn Tagh Fault, for instance, south of Pingding Shan or across Xorkol. Right-stepping 0.5~1m-deep gashes or sub-faults, with lengths from a few meters to several hundred meters, are distributed obliquely between ridges at an angle reaching 30°. The sub-faults are characterized with SE or NW facing 0.5~1m-high scarps. Several pull-apart and faulted basins are bounded by faults along the eastern part of the surface rupture. One large pull-apart basins are 6~7m deep and 400m wide. A faulted basin, 80m wide, 500m long and 3m deep, is bounded by 2 left-stepping left-lateral faults and 4 right-stepping normal faults. Two to three m-wide gashes are often seen on pressure ridges, and some ridges are left-laterally faulted and cut into several parts, probably owing to the occurrence of repetitive earthquakes. The OSL dating indicates that the most recent rupture might occur during Holocene.
    Southwestwards the rupture trace disappears a few hundred meters north of a south dipping thrust scarp bounding uplifted and folded Plio-Quaternary sediments to the south. Thrust scarps can be followed southwestward for another 12km and suggest a connection with the south Pingding Shan Fault, a left-lateral splay of the main Altyn Tagh Fault. To the northeast the rupture trace progressively veers to the east and is seen cross-cutting the bajada south of Datonggou Nanshan and merging with active thrusts clearly outlined by south facing cumulative scarps across the fans. The geometry of this strike-slip fault trace and the clear young seismic geomorphology typifies the present and tectonically active link between left-lateral strike-slip faulting and thrusting along the eastern termination of the Altyn Tagh Fault, a process responsible for the growth of the Tibetan plateau at its northeastern margin. The discrete relation between thrusting and strike-slip faulting suggests discontinuous transfer of strain from strike-slip faulting to thrusting and thus stepwise northeastward slip-rate decrease along the Altyn Tagh Fault after each strike-slip/thrust junction.

On the basis of summarizing the circulation characteristics and mechanism of earthquakes with magnitude 7 or above in continental China, the spatial-temporal migration characteristics, mechanism and future development trend of earthquakes with magnitude above 7 in Tibetan block area are analyzed comprehensively. The results show that there are temporal clustering and spatial zoning of regional strong earthquakes and large earthquakes in continental China, and they show the characteristics of migration and circulation in time and space. In the past 100a, there are four major earthquake cluster areas that have migrated from west to east and from south to north, i.e. 1)Himalayan seismic belt and Tianshan-Baikal seismic belt; 2)Mid-north to north-south seismic belt in Tibetan block area; 3)North-south seismic belt-periphery of Assam cape; and 4)North China and Sichuan-Yunnan area. The cluster time of each area is about 20a, and a complete cycle time is about 80a. The temporal and spatial images of the migration and circulation of strong earthquakes are consistent with the motion velocity field images obtained through GPS observations in continental China. The mechanism is related to the latest tectonic activity in continental China, which is mainly affected by the continuous compression of the Indian plate to the north on the Eurasian plate, the rotation of the Tibetan plateau around the eastern Himalayan syntaxis, and the additional stress field caused by the change of the earth's rotation speed.
    Since 1900AD, the Tibetan block area has experienced three periods of high tides of earthquake activity clusters(also known as earthquake series), among which the Haiyuan-Gulang earthquake series from 1920 to 1937 mainly occurred around the active block boundary structural belt on the periphery of the Tibetan block region, with the largest earthquake occurring on the large active fault zone in the northeastern boundary belt. The Chayu-Dangxiong earthquake series from 1947 to 1976 mainly occurred around the large-scale boundary active faults of Qiangtang block, Bayankala block and eastern Himalayan syntaxis within the Tibetan block area. In the 1995-present Kunlun-Wenchuan earthquake series, 8 earthquakes with M_S7.0 or above have occurred on the boundary fault zones of the Bayankala block. Therefore, the Bayankala block has become the main area of large earthquake activity on the Tibetan plateau in the past 20a. The clustering characteristic of this kind of seismic activity shows that in a certain period of time, strong earthquake activity can occur on the boundary fault zone of the same block or closely related blocks driven by a unified dynamic mechanism, reflecting the overall movement characteristics of the block. The migration images of the main active areas of the three earthquake series reflect the current tectonic deformation process of the Tibetan block region, where the tectonic activity is gradually converging inward from the boundary tectonic belt around the block, and the compression uplift and extrusion to the south and east occurs in the plateau. This mechanism of gradual migration and repeated activities from the periphery to the middle can be explained by coupled block movement and continuous deformation model, which conforms to the dynamic model of the active tectonic block hypothesis.
    A comprehensive analysis shows that the Kunlun-Wenchuan earthquake series, which has lasted for more than 20a, is likely to come to an end. In the next 20a, the main active area of the major earthquakes with magnitude 7 on the continental China may migrate to the peripheral boundary zone of the Tibetan block. The focus is on the eastern boundary structural zone, i.e. the generalized north-south seismic belt. At the same time, attention should be paid to the earthquake-prone favorable regions such as the seismic empty sections of the major active faults in the northern Qaidam block boundary zone and other regions. For the northern region of the Tibetan block, the areas where the earthquakes of magnitude 7 or above are most likely to occur in the future will be the boundary structural zones of Qaidam active tectonic block, including Qilian-Haiyuan fault zone, the northern margin fault zone of western Qinling, the eastern Kunlun fault zone and the Altyn Tagh fault zone, etc., as well as the empty zones or empty fault segments with long elapse time of paleo-earthquake or no large historical earthquake rupture in their structural transformation zones. In future work, in-depth research on the seismogenic tectonic environment in the above areas should be strengthened, including fracture geometry, physical properties of media, fracture activity behavior, earthquake recurrence rule, strain accumulation degree, etc., and then targeted strengthening tracking monitoring and earthquake disaster prevention should be carried out.

The hypothesis that strong earthquakes in China mainland are controlled by the movement and interaction of active-tectonic blocks was advanced by Chinese scientists, with the remarkable ability to encompass geological and geophysical observations. Application of the active-tectonic block concept can illustrate 6 active-tectonic block regions and 22 active-tectonic blocks in mainland China and its neighboring regions. Systems of active-tectonic block boundaries are characterized by a zone of decades or hundreds of strong earthquakes. One of the greatest strengths of the modern active-tectonic block hypothesis is its ability to explain the origin of virtually all the M8 and 80% M7 earthquakes on the main continent in eastern Asia. In other words, active-tectonic block boundary stands in strong causal interrelation with recurrence behaviors of strong earthquakes and thus, it is possible to predict an earthquake occurrence in principle. After nearly two decades of development and improvement, the active-tectonic block hypothesis has established its theoretical foundation for the active tectonics and earthquake prediction, and is promoting the transition from probabilistic prediction to physical prediction of strong earthquakes. The active-tectonic block concept was tested by application to a well-documented, high-frequent earthquake area, and was found to be an effective way of describing and interpreting the focal mechanism and seismogenic environment, but there are still many problems existing in the active-tectonic block hypothesis, which confronts with rigorous challenges. Future progress will continue to be heavily dependent on the high-precision synthetic seismogram, especially of critical poorly documented settings. It is well known that strong earthquakes occur anywhere in the interactions among the active-tectonic block boundaries where there is sufficient stored elastic strain energy driving fault propagation, and then releasing the stored energy. Therefore, future studies will focus on the mechanism and forecast of the strong earthquake activity in the active-tectonic block boundary zone, with fault activity within the active-tectonic block boundary zone, quantifying current crustal strain status, upper crust and deep lithosphere coupling relation, strong earthquake-generating process and its precursory variation mechanism in seismic geophysical model as the main research contents, which are the key issues regarding deepening the theory of active-tectonic block and developing continental tectonics and dynamics in the modern earth science.

The river system is very sensitive to landscape fluctuations and the pattern of drainage contains the past and present tectonic information and can record the latest even tiny change along the orogenic belt system. Therefore, fluvial geomorphology is always used to describe the shapes of river channels and recognize the different segments of active faults. Qualitative and quantitative geomorphic analyses can provide useful information on detecting active tectonic features and the influence of landscape change and evolution. Quantitative analysis such as analysis of river longitudinal profile and geomorphic indices can help researchers evaluate the relative level of tectonic activity and characterize the geomorphic features of landscape quantitatively.
Our study focuses on the geomorphic analysis of Shiyanghe River Basin which is located in the eastern part of Qilianshan Mountains. The tectonic deformation is very strong since late Cenozoic, and Quaternary active thrust faults, strike-slip faults and active folds are distributed all over the region, indicating that the whole region is suffering from crustal shortening and sinistral shear. In this region, the latest tectonic deformation and tectonic activities have been recorded by its fluvial system.
Based on GIS spatial analysis technology, the longitudinal profiles of seven tributaries(including Gulanghe River, Huangyanghe River, Jintahe River, Zamuhe River, Xiyinghe River, Dongdahe River and Xidahe River)in the Shiyanghe River Basin are extracted by using digital elevation models(DEM)and Matlab script. In channel longitudinal profiles, most tributaries in Shiyanghe River Basin exhibit an increased channel gradient in their midstream and downstream area. This pattern is consistent with the models of transient channel profile which suggests an increase in rock uplift rate or base level fall. The longitudinal profiles of seven tributaries are analyzed synthetically by using the method of bedrock channel erosion model, and the concavity(θ), steepness index(k_sn), as well as the knickpoints information(including distribution, elevation, distance from mouth and drainage area)of seven tributaries are obtained. The result shows that each of the tributaries in the Shiyanghe River Basin at least has one major knickpoint. The comprehensive study of the longitudinal profiles, knickpoints and the lithology of the river basin show that the Gulanghe River, Jintahe River, Zamuhe River, Xiyinghe River, Dongdahe River and Xidahe River all have ‘slope-break’ knickpoints, which suggest that they are in a transient state. The knickpoints represent a transient response to the dynamic surface uplift since late Cenozoic. Therefore, we can conclude that the evolution of fluvial geomorphology in eastern Qilian Mountains is mainly related to tectonic activities. Channel segments upstream of knickpoints exhibit lower concavities(mean θ is 0.458±0.053)and higher channel steepness indices(mean k_sn is 129.09±1.82). In contrast, lower channel segments are more complanate(mean k_sn is 68.162±0.821)and exhibit a higher concavity(mean θ is 0.831±0.147). The distribution of concavity is related to the erosion rate, thence, we can infer that the higher value of concavity in downstream area indicates the higher erosion rate. Because the different steepness index(k_s)and the concavity(θ)below and above the reach of knickpoints indicate that they have different development trends in different channel segments, and the distribution of knickpoints represent the evolution process of the longitudinal section of the tributaries. Using the concavity value of the knickpoint, each lower reach longitudinal profile of tributary is fitted. According to the fitted result, the calculated approximate average erosion volume of the Shiyanghe River Basin is 488m since it formed, and the average erosion volume of the six tributaries, which originated in Gulang nappe, is 508.5m. The total amount of erosion is positively correlated with rock uplift when a river is in transient state. Thence, it concludes that the Gulang nappe has experienced a strong uplift. Furthermore, we obtained the spatial distribution of k_sn values of the whole fluvial system in the Shiyanghe River Basin from calculating and interpolating the k_sn values, and combined the geomorphic parameters results to analyze the tectonic significance of the Shiyanghe River Basin synthetically. The spatial distribution of k_sn values of the Shiyanghe River Basin represent the accommodation of geomorphic landscape to the tectonic force and the manner of channels responding to tectonic forces. In this study, most of the channel gradients obtained from midstream are higher than upstream and downstream and k_sn values in downstream reaches is less than 60m^0.9, the high k_sn values are in the Gulang nappe, reaching over 1 400m^0.9, which indicates that the Gulang nappe has experienced uplift since the Quaternary. Therefore, we conclude that the regional difference of the k_sn is mainly controlled by the uplift rate of bedrocks.
Based on the comprehensive analysis of geomorphic parameters and tectonic background, we conclude that the geomorphic evolution of the Shiyanghe River Basin is in a non-equilibrium state, and the tectonic deformation is the main factor affecting the geomorphic evolution of the eastern Qilianshan Mountains and controlling the present tectonic pattern, geomorphic development and evolution history of the study area.
According to the river longitudinal profiles and modeling analysis, this study indicates that the quantitative geomorphic analyses can provide useful and effective information on detecting active tectonic features and the influence of landscape change and evolution, and the geomorphic indices are useful and appropriate tools to analyze the coupling of tectonic and geomorphy.

With the development of photogrammetry technology and the popularity of unmanned aerial vehicles (UAVs)technology in recent years, using UAV photogrammetry technology to rapidly acquire high precision and high resolution topographic and geomorphic data on the fault zone has gradually become an important technical means. This paper first summarizes the basic principle and workflow of a new digital photogrammetry technology, SfM (Structure from Motion), which is simple, efficient and low cost. Using this technology, we conducted aerial image acquisition and data processing for a typical fault landform on the northern of Caka Basin in Qinghai. The digital elevation model (DEM)with 6.1cm/pix resolution is generated and the density of point cloud is as high as 273 points/m². The coverage area is 0.463km². Further, the terrain and slope data parallel to the fault direction are extracted by topographic analysis method, and combined with the contour map and the slope diagram generated by the DEM, a fine interpretation and quantitative study of complex multilevel geomorphic surfaces is carried out. Finally, based on the results of sophisticated interpretation of geomorphology, we got the vertical displacements of the T₁ terrace to the T₃ terrace as (1.01±0.06)m, (1.37±0.13)m and (3.10±0.11)m, and the minimum vertical displacements of the T₄ terrace and the T₅ terrace as (3.77±0.14)m and (5.46±0.26)m, respectively, through the topographic profile data extracted by DEM. Such vertical displacement parameters are difficult to obtain directly by traditional remote sensing images, which shows the great application prospect of UAV photogrammetry technology in the quantitative study of active tectonics.

The NE margin of Tibetan plateau outspreads northeastward in late Cenozoic. The west Qinling locates at intervening zone among Tibetan plateau, Sichuan Basin and Ordos block, and is bounded by East Kunlun Fault in the southwest, the north margin of West Qinling Fault in the northeast, and the Longmen Shan Fault in the southeast. The west Qinling has been experiencing intense tectonic deformation since late Cenozoic, accompanying by uplift of mountains, downward incision of rivers, frequent moderate-strong earthquakes, vertical and horizontal motion of secondary faults, and so on. A series of "V-shape" faults are developed in the transfer zone between East Kunlun Fault and north margin of West Qinling Fault. The NWW-NW striking faults include Tazang Fault, Bailongjiang Fault, Guanggai Shan-Die Shan Fault, and Lintan-Dangchang Fault; EW-NEE-NE striking faults include Ha'nan-Qingshanwan-Daoqizi Fault, Wudu-Kangxian Fault, Liangdang-Jiangluo Fault, and Lixian-Luojiapu Fault. Among them, the Southern Guanggai Shan-Die Shan Fault (SGDF)is one of the principle branch which accommodates strain partitioning between the East Kunlun Fault and the north margin of west Qinling Fault. Although some works have been done and published, the geometry of SGDF is still obscure due to forest cover, bad traffic, natural and manmade reworks. In this paper, we collected remote sensing images with various resolutions, categories, imaging time. The selected images include composite map of Landsat image (resolution is 28.5m among 1984-1997, and 14.5m among 1999-2003), Landsat-8 OLI image (15/30m), Gaofen-1 (2m/8m), Pleiades (0.5m/2m), DEM (~25m)and Google Earth image (submeter resolution). After that, we reinforced tectonic information of those images by Envi5.2 software, then we interpreted SGDF from those images. As indoor interpretation fulfilled, we testified indoor interpretation results through geomorphological and geological investigation. Finally, we got fault distribution of SGDF. Conclusions are as follows:First, remote sensing image selection and management is crucial to indoor interpretation, and image resolution is the only factor we commonly consider before, however, things have changed in places where there is complex weather and dense vegetation. Image categories, imaging time and bands selected for compositing in pretreatment and etc. should all be taken into consideration for better interpretation. Second, SGDF distributes from Lazikou town in the west, extending through Pingding town, Zhou County, Huama town, then terminating at Majie town of Wudu district in the east, the striking direction is mainly NWW, and it could be roughly divided into 3 segments:Lazikou-Heiyusi segment, Pingding-Huama segment, and Huama-Majie segment, with their length amounting to 47km, 32.5km, 47km, respectively. The arrangement pattern between Lazikou-Heiyusi segment and Pingding-Huama segment is right-stepping, and the arrangement pattern is left-stepping bending between Pingding-Huama segment and Huama-Majie segment. Third, SGDF controlled magnificent macro-topography, such as fault cliff, fault facet, which often constitute the boundary of intermontane basins or erosional surfaces to west of Minjiang River. Micro-geomorphic expressions were severely eroded and less preserved, including fault scarps, fault troughs, sinistral offset gullies and geomorphic surfaces. Finally, SGDF mainly expresses left-lateral dominated motion, only some short branch faults with diverting striking direction exhibit vertical dominated motion. The left-lateral dominated component with little vertical motion of SGDF is consistent with regional NWW-striking faults as Tazang Fault, Bailongjiang Fault and Lintan-Dangchang Fault, also in coincidence with regional boundary faults such as east Kunlun Fault and north margin of west Qinling Fault, illustrating regional deformation field is successive in west Qinling, and NWW striking faults show good inheritance and transitivity on differential slip rate between east Kunlun Fault and west Qinling Fault. The geometry of SGDF makes quantitative studies possible, and also provides scientific basis for keeping construction away from fault traces.

The Sanweishan fault is located in the northern margin of the Tibetan plateau. It is a branch of the Altyn Tagh fault zone which extends to the northwest. A detailed study on Late Quaternary activity characteristics of the Sanwei Shan Fault can help understanding the strain distribution of the Altyn Tagh fault zone and regional seismic activity and northward growth of the Tibetan plateau. Previous research on this fault is insufficient and its activity is a controversial issue. Based on satellite images interpretation, field investigations and geological mapping, this study attempts to characterize this feature, especially its activity during Late Quaternary. Trench excavation and sample dating permit to address this issue, including determination of paleoseismic events along this fault.
The results show that the Sanweishan fault is a large-scale active structure. It starts from the Shuangta reservoir in the east, extending southward by Shigongkouzi, Lucaogou, and Shugouzi, terminates south of Xishuigou, with a length of 175km. The fault trends in NEE, dipping SE at angles 50°~70°. It is characterized by left-lateral strike-slip with a component of thrust and local normal faulting. According to the geometry, the fault can be divided into three segments, i.e. Shuangta-Shigongkouzi, Shigongkouzi-Shugouzi and Shugouzi-Xishuigou from east to west, looking like a left-or right-step pattern. Plenty of offset fault landforms appear along the Sanweishan Fault, including ridges, left-lateral strike-slip gullies, fault scarps, and fault grooves. The trench study at the middle and eastern segments of the fault shows its activity during Late Pleistocene, evidenced by displaced strata of this epoch. Identification marks of the paleoearthquakes and sample dating reveal one paleoearthquake that occurred at(40.3±5.2)~(42.1±3.9) ka.

Qilian Shan-Hexi Corridor is located in the northeastern margin of the Tibetan plateau, which hosts many active strike-slip and thrust faults as well as folds. Previous study on this area was mostly concerned with large faults at the boundary of the corridor, while rare work on active tectonics in the interior of the corridor. On 25 October 2003, the Minle-Shandan M_S6.1 earthquake occurred in this area, which is related with the Minle-Yongchang fault hidden beneath the south piedmont of the Dahuangshan Mountains. As there is no obvious rupture trace on the surface, the quantitative study of this fault has never been reported so far.
In order to obtain quantitative parameters of this active structure efficiently, the software of ERDAS was used to generate pointscloud data from SPOT6 stereo-pair. Two-meter resolution DEM imagery from point cloud has the line accuracy of height about 1m. Three swath profiles were extracted from the DEM data, which show that high geomorphic surfaces are all uplifted and folded. By differential GPS measurement, the vertical uplift of the thrust-related fold is estimated to be about 2.0m on the T₂, and the strike of the fold deformation is nearly 311°, which is close to the result of the fault parameter determined by aftershocks, and also in agreement with the focal mechanism solutions. Furthermore, the location of fold axial zone is consistent with the actual investigation data. These indicate that there is obvious tectonic deformation in the west part of the Minle-Yongchang fault. It supports the view that this fault is the seismogenic structure of the 2003 Minle-Shandan earthquake. Stereo-pair is of high importance in active tectonics research, which can provide significant guidance for field geologic investigations and determining the location of tectonic deformation, according to this research.

Three-dimensional scanning with LiDAR has been widely used in geological surveys. The LiDAR with high accuracy is promoting geoscience quantification. And it will be much more convenient, efficient and useful when combining it with the Unmanned Aerial Vehicle (UAV). This study focuses on UAV-based Laser Scanning (UAVLS)geological field mapping, taking two examples to present advantages of the UAVLS in contrast with other mapping methods. For its usage in active fault mapping, we scanned the Nanpo village site on the Zhangxian segment of the West Qinling north-edge fault. It effectively removed the effects of buildings and vegetation, and uncovered the fault trace. We measured vertical offset of 1.3m on the terrace T₁ at the Zhang river. Moreover, we also scanned landslide features at the geological hazard observatory of Lanzhou University in the loess area. The scanning data can help understand how micro-topography affects activation of loess landslides. The UAVLS is time saving in the field, only spending about half an hour to scan each site. The amount of average points per meter is about 600, which can offer topography data with resolution of centimeter. The results of this study show that the UAVLS is expected to become a common, efficient and economic mapping tool.

On the day Wu-wu of the second month in the second year of Chuyuan period in the reign of Emperor Yuan of the western Han Dynasty, that is, April 17, 47 BC, an earthquake occurred near the county seat of Huandao County, Longxi Prefecture. This earthquake caused serious damages to the city wall, government office buildings and civilian houses in the Huandao county seat, many people died, landslides, ground fracturing and spring gushing, etc. occurred. On the basis of textual research on the historical earthquake data and the field investigation, we affirm that the ancient Huandao county seat locates at the new Wangjia Village of Santai, southeast of Wenfeng Town in Longxi County at present. The ancient Huandao county seat is the most seriously damaged area according to the historical earthquake data, so it should be in the meizoseismal area of this earthquake. The epicenter intensity of this earthquake is about 9~10 degrees, and the magnitude of this earthquake is estimated to be about 7. Considering the intensities of other towns damaged during this earthquake, we draw the isoseismal lines of this earthquake, with its major axis directed NNW. The direction of the major axis of the isoseismal lines and the location of the epicenter are approximately consistent with the strike of the western segment of the Gangu-Wushan secondary fault on the northern margin of western Qinling fault zone. This fault segment has clear evidences of new activity during late Holocene, which are characterized by left-lateral strike-slip faulting with normal components; the fault dips NE and faulted the T₁-T₂ pluvial and alluvial terraces. Up to now, there are some surface deformation traces, such as deeply-incised seismic fault grooves, densely developed structural tensional fractures in the loess stratum, landslides and a series of beaded dolines etc. Combining the results of relocation of small to moderate earthquakes in the research area and comprehensive analysis on their distribution features in plane and profile, we get the results that the causative structure of the 47 BC Longxi earthquake is the western segment of Gangu-Wushan secondary fault on the northern margin of western Qinling fault zone. This fault zone is an important active block boundary fault in the eastern margin of Tibeten block, and has the tectonic condition to generate M ≥ 7 large earthquakes in the past and in the future.

Qilian Shan-Hexi Corridor is located at the northeastern margin of Tibetan plateau. Series of late Quaternary active faults are developed in this region. A number of strong earthquakes even large earthquakes occurred in history and present-day. In the past, the study of active faults in the area was mostly concentrated in the northern margin fault zone of the Qilian Shan on the south side of the corridor, while the research on the interior and the north side of the corridor basin was relatively rare. We found a new fault scarp in the northern part of the Baiyanghe anticline in Jiuxi Basin in 2010. It is an earthquake surface rupture zone which has never been reported before. In this paper, we carried out palaeoearthquake trench analysis on the newly found earthquake surface rupture zone and textual research of relevant historical earthquakes data.
According to the interpretation of aerial photo and satellite image and field investigation, we found the surface rupture has the length of about 5km. The rupture shows as an arc-shaped line and is preserved intact comparably. The lower terrace and the latest flood alluvial fan are offset in addition to modern gullies. By differential GPS measurement, the height of the scarp is about 0.5~0.7m in the latest alluvial fan and about 1.5m in the T₁ terrace. From the residual ruins along the earthquake rupture zone, we believe the surface rupture might be produced by an earthquake event occurring not long ago. In addition, the rupture zone locates in the area where the climate is dry and rainless and there are no human activities induced damages. These all provide an objective condition for the preservation of the rupture zone. The trench along the fault reveals that the surface rupture was formed about 1500 years ago, and another earthquake event might have happened before it. Based on the textural research on the historical earthquake data and the research degree in the area at present, we believe that the surface rupture is related to the Yumen earthquake in 365, Yumen Huihuipu earthquake in 1785 or another unrecorded historical earthquake event.

Using quantitative geomorphic factors for regional active tectonic evolution is becoming more and more popular. Qilian Mountains-Hexi Corridor which locates in the northern edge of Qinghai-Tibet plateau is the most leading edge of the plateau's northward extension. The uplift rate of all segments and the intensity of influence from tectonic activity are the important evidences to understand the uplift and extension of the plateau. Heihe River Basin is located at the northern piedmont of the western segment of Qilian Mountains, the development of the rivers is influenced by the tectonic activity of the Qilian Mountains, and the unique river morphology is important carriers of the regional tectonic uplift.
Geomorphologic indexes such as hypsometric integral, geomorphologic comentropy and river longitudinal profiles were extracted by GIS tools with free access to the Shuttle Radar Topography Mission(SRTM)DEMs, and according to the different expression of the geomorphological indexes in the Heihe River Basin, we divided the drainage basin into two parts and further compared them to each other.
Recent studies reveal that obvious differences exist in the landscape factors(hypsometric integral, geomorphology entropy and river profiles)in the east and west part of the Heihe Basin. The structural intensity of the west part is stronger than that of the east, for example, in areas above the main planation surface on the western part, the average HI value is 0.337 8, and on the eastern part the HI value is 0.355. Accordingly, areas under the main planation surface are just on the contrary, indicating different structural strength on both sides. Similar phenomenon exists in the whole drainage basins. Furthermore, by comparing the fitting river profiles with the real river profiles, differential uplift is derived, which indicates a difference between west and east(with 754m on the western part and 219m on the east). Comprehensive comparison and analysis show that the lithologic factors and precipitation conditions are less influencing on the geomorphic factors of the study area, and the tectonic activities, indicated by field investigation and GPS inversion, are the most important element for geomorphic evolution and development. The variation of the geomorphologic indexes indicates different tectonic strength derived from regional structures of the Qilian Shan.

Because of the strong uplift of the Qilian Shan since late Cenozoic,the drainage basins that are derived from the mountains have undergone strong tectonic deformation.So the typical geomorphology characteristics of these drainage basins may indicate the strong tectonic movement in the region.For example,the Shule River drainage basin,which originates from the western part of the Qilian Shan owns unique geomorphology characteristics which may indicate the neotectonic movement.
Stream networks of the Shule drainage basin extracted from the DEM data based on GIS spatial analysis technology are graded into five levels using Strahler classification method.Four sub-catchments,numbered 1,2,3 and 4 are chosen for detailed analysis.Furthermore,the four sub-catchments,the hypsometric integral curves,Hack profiles,SL index and average slope of the Shule drainage basin are determined by GIS tools.In addition,we analyzed the slope spectrum of the Shule drainage basin.
The average elevation of the Shule drainage basin is very high,however,the slope of the drainage basin is very low,the gentle slope occupies so large area proportion that the slope spectrum shows a unimodal pattern and a peak value is in low slope region (0°~5°),so tectonic movement has a strong influence on the drainage basin.Under the intensive impact of the tectonic movement of the active fault and regional uplift,the hypsometric integral curve is sigmoid,revealing that the Shule drainage basin is in the mature stage.The Hack profile is on a convex,the longitudinal profile is best fitted by linear fitting and the abnormal data of the SL index of the Shule River has a good fit with the section through which the active fault traverses,that means the tectonic movement of the active fault has strong influence on the river's SL index.It is worth noting that lithologic factors also have great impact on the river geomorphology in some sections.
According to the above analysis,we recognize that in the interior of active orogen,the evolution of river geomorphology usually is influenced by tectonic movement and reveals the regional neotectonics in turn.

It is well known that the slip rate of Kunlun Fault descends at the east segment, but little known about the Awancang Fault and its role in strain partitioning with Kunlun Fault. Whether the sub-strand(Awancang Fault) can rupture simultaneously with Kunlun Fault remains unknown. Based on field investigations, aerial-photo morphological analysis, topographic surveys and ¹⁴C dating of alluvial surfaces, we used displaced terrace risers to estimate geological slip rates along the Awancang Fault, which lies on the western margin of the Ruoergai Basin and the eastern edge of the Tibetan plateau, the results indicate that the slip rate is 3mm/a in the middle Holocene, similar to the reduced value of the Kunlun Fault. The fault consists of two segments with strike N50° W, located at distance about 16km, and converged to single stand to the SE direction. Our results demonstrate that the Awancang fault zone is predominantly left-lateral with a small amount of northeast-verging thrust component. The slip rates decrease sharply about 4mm/a from west to east between the intersection zone of the Awancang Fault and Kunlun Fault. Together with our previous trenching results on the Kunlun Fault, the comparison with slip rates at the Kunlun fault zone suggests that the Awancang fault zone has an important role in strain partitioning for east extension of Kunlun Fault in eastern Tibet. At the same time, the 15km long surface rupture zone of the southeast segment was found at the Awancang Fault. By dating the latest faulted geomorphologic surface, the last event may be since the 1766±54 Cal a BP. Through analysis of the trench, there are four paleoearthquake events identified recurring in situ on the Awancang Fault and the latest event is since (850±30)a BP. The slip rate of the Awancang Fault is almost equivalent to the descending value of the eastern part of the east Kunlun Fault, which can well explain the slip rate decreasing of the eastern part of the east Kunlun Fault(the Maqin-Maqu segment)and the characteristics of the structure dynamics of the eastern edge of the Tibet Plateau. The falling slip rate gradient of the eastern Kunlun Fault corresponds to the geometric characteristic. It is the Awancang Fault, the strand of the East Kunlun Fault that accommodates the strain distribution of the eastward extension of the east Kunlun Fault. This study is helpful to seismic hazard assessment and understanding the deformation mechanism in eastern Tibet.

Jinta Nanshan Fault is an important fault in northeast front of Qing-Zang Plateau, and it is crucial for determining the eastern end of Altyn Tagh Fault. However, there is still debate on its significant strike-slip movement.
In this paper, we study the Late Quaternary activity of Jinta Nanshan Fault and its geological and geomorphic expressions by interpreting aerial photographs and high-resolution remote sensing images, surveying and mapping of geological and geomorphic appearances, digging and clarifying fault profiles and mapping deformation characteristics of micro-topographies, then we analyze whether strike-slip activity exists on Jinta Nanshan Fault.
We get a more complete fault geometry than previous studies from most recent remote sensing images. Active fault traces of Jinta Nanshan mainly include 2 nearly parallel, striking 100°~90° fault scarps, and can be divided into 3 segments. West segment and middle segment form a left stepover with 2~2.5km width, and another stepover with 1.2km width separates the middle and east segment.
We summarize geomorphic and geologic evidence relating to strike slip activity of Jinta Nanshan Fault. Geomorphic expressions are as follows:First, fault scarps with alternating facing directions; second, sinistral offset of stream channels and micro-topographies; third, pull-apart basins and compressive-ridges at discontinuous part of Jinta Nanshan Fault. Geologic expressions are as follows:First, fault plane characteristics, including extremely high fault plane angle, unstable dip directions and coexistence of normal fault and reverse fault; second, flower structures.
Strike-slip rate was estimated by using geomorphic surface age of Zheng et al.(2013)and left-lateral offset with differential GPS measurements of the same geomorphic surface at field site in Fig. 4e. We calculated a strike-slip rate of (0.19±0.05)mm/a, which is slightly larger than or almost the same with vertical slip rate of (0.11±0.03)mm/a from Zheng et al.(2013).
When we confirm the strike-slip activity of Jinta Nanshan, we discuss its potential dynamic sources:First, eastern extension of Altyn Tagh Fault and second, strain partitioning of northeastward extension of Qilian Shan thrust belt. The first one is explainable when it came to geometric pattern of several E-W striking fault and eastward decreasing strike slip rate, but the former cannot explain why the Heishan Fault, which locates between the the Altyn Tagh Fault and Jinta Nanshan Fault, is a pure high angle reverse fault. The latter seems more explainable, because oblique vectors may indeed partition onto a fault and manifest strike-slip activity.

The Gulang M8.0 great earthquake occurred in 1927, many places in Gulang and adjacent areas had suffered destruction in various degrees. So far, divergences exist in the former studies on its seismogenic structure. It is known that clustered small earthquakes often occur in vicinities of fault plane of large earthquake. In this study, the precisely relocated earthquakes which occurred near the earthquake rupture zone between 1985 and 2012 are used, and two strip-shaped zones of clustered small earthquakes are chosen according to the previous studies of the causative structure of this earthquake. Based on the simulated annealing and Gauss-Newtonian nonlinear inversion algorithms, we obtained fault plane parameters of the earthquake such as strike, dip, and location using data of densely concentrated small events. On this condition, rake angles of the fault plane are further inferred from regional tectonic stress parameter. Then we discussed briefly the seismogenic environment and causal mechanism of the earthquake, combined with the results of deep tectonics and surface investigations. The focal fault we inverted locates within the meizoseismal area (intensity Ⅺ)of the Gulang M8.0 in 1927, suggesting that the focal fault obtained by inversion is possibly the causative structure of this earthquake. Besides, we found a clustered zone of small earthquakes near the south-central part of the main fault, and a fault plane could be derived from them, which we think might be a tensional seismic fault developed on the main fault when the whole earthquake-hit region rotated counterclockwise during the big earthquake.

An earthquake with M_S 6.6 occurred near the border between Minxian and Zhangxian counties in southeastern Gansu Province on July 22, 2013. This earthquake caused serious personnel casualties and property damages. According to the field investigation, the intensity of the epicenter area is about Ⅷ, the causative structure is a branch fault of the eastern segment of Lintan-Dangchang active fault.The southeastern region of Gansu Province is located at the eastern boundary of Tibetan active block. A series of strike-slip faults with thrust components are developed and their combination is complicated and a series of strong or even large earthquakes occurred in this area in the history and present-days, and one of them is the Luqu earthquake occurring in 842 AD at the boundary of Han and Tibet area(now the southeastern area of Gansu Province). The earthquake caused seismic rupture, spring gushing, landslip in the Minshan Mountains and countercurrent of the Taohe River for three days. According to the detail textual research of historical references and field investigation, the epicenter area of this earthquake locates at the Guanggaishan-Dieshan mountain area, at the border area between Luqu County, Zhuoni County and Diebu County in the Gannan Tibetan Autonomous Prefecture. The date of the Luqu earthquake is possibly on the 24^th day of the twelfth month of the second year of Huichang Reign in Tang Dynasty, that is, on January 31 or 27, 843 AD, and the magnitude of this earthquake is about 7～7 1/2 , the intensity near the epicenter area is about nine to ten. There are three late Quaternary active fault zones of thrust with left-lateral components, namely, Lintan-Dangcang Fault, Guanggaishan-Dieshan Fault and Diebu-Bailongjiang Fault. According to the comparative analysis of the field investigation of active faults in recent years and present seismic activity, we think that Luqu earthquake is the result of new activity of Guanggaishan-Dieshan Fault, the causative fault of this earthquake. This fault is an important branch fault of the eastern segment of northern boundary faults of Bayan Har block(Eastern Kunlun Fault zone), a main activity area of large earthquakes with magnitude larger than 7 in Chinese continent in the recent 10 years, and has the tectonic condition to generate M≥7 large earthquakes.

The Hexi Corridor-Qilian Fault systems,the Altyn Tagh Fault and the Haiyuan Fault jointly control the north boundary and deformation of the Qinghai-Tibet plateau. The Changma Fault,as one of the Hexi Corridor-Qilian Fault systems,is a highly active strike-slip fault,and connects the Altyn Tagh Fault and the Haiyuan Fault. Based on the active characteristics and geometrical distribution,this fault is divided into four segments. We obtain the left-lateral strike-slip rates of three segments,which are 1.33±0.39mm/yr at the west segment,3.11±0.31mm/yr at the middle-west segment,and 3.68±0.41mm/ya at the middle-east segment,respectively,and the shortening rate at the west segment(0.70±0.20mm/yr).The result shows that the sinistral slip rate of the fault is significantly increased from west segment to east segment. The activity of Changma Fault accommodates～30%reduction of Altyn Tagh Fault slip rate. The studies in this paper confirm that the sinistral slip and shortening of Changma Fault and other secondary faults in accompany with deformation inside the basin absorb most of displacement of east segment of Altyn Tagh Fault,and this structural change mode supports the hypotheses that the northeastern margin of Qinghai-Tibet plateau has a continuous crust thickening mode with lateral displacement.

The southern Wudu M8.0 great earthquake occurred in 1879,many places in Wudu and adjacent areas had suffered destruction in various degrees. So far,the research results on the causative structure of this earthquake are less and inconsistent. Because it occurred in lofty mountains and the traffic is inconvenient,it is hard to make detailed field study on the earthquake site. Based on the assumption that clustered small earthquakes often occur in the vicinity of fault plane of large earthquake,and referring to the morphology of the major axis of the meizoseismal area obtained by the predecessors,we selected a strip-shaped zone from the relocated earthquake catalog which occurred near the earthquake rupture zone in the period from 1985 to 2009 to calculate fault plane parameters of the earthquake,such as strike and dip,with the simulated annealing and Gauss-Newtonian nonlinear inversion algorithms. On this condition,the rake angles of the fault plane are further inferred from regional tectonic stress parameters. We discussed the causal mechanism of the earthquake and finally identified the length and location of the seismogenic fault. In addition,clustered small earthquakes occurred frequently in the Xionghuangshan area west of the mezoseismal area,but we didn't find clear fault planes in field investigation,so,they should not have relation with the 1879 M8 southern Wudu earthquake.

In recent years,many big earthquakes(M≥7) struck China and other nations.These big earthquakes may indicate that the earth is in a globally seismic active period.Therefore,in order to mitigate future earthquake disasters,the assessment of future big earthquake risk for major active boundary faults has been done as an important approach for mitigation.In this paper,our focus area is Northern Qilianshan-Hexi Corridor locating in northeastern of Qinghai-Tibet plateau.We collected and summarized the active faults' data sets systematically,e.g.geometrical characteristic,slip rate,rupture segmentation,latest rupture event and paleo-earthqakes.And based on these data sets,we use the methods of seismic gap identification and b value mapping to analyze the characteristics of historical earthquakes and b value.And then,high risk zones or faults of big earthquakes were identified synthetically.We think the Northern Yumushan Fault has the most probability of generating big earthquake in the future.Because the elapse time from the last event is long and b value along it is remarkably low,which betokens high stress.Meanwhile, attention should be paid too to the Jiayuguan Fault,where seismic gap and low b value zone exist too.

The northern Yumushan Fault located on the northern flank of Qilian fold system is an active fault in Holocene.The fault is about 60km long,trending NWW.It is a trust fault with left-lateral strike-slip component.The activity of the fault produced a series of scarps along the mountain front.The fault zone is divided into three segments,and the middle part is the most active.In this paper,palaeo-earthquake events on the fault are studied.With the study of trench profiles,two palaeo-earthquake events are determined.Event I occurred at 4.066±0.086ka BP,and event II is between 6.852±0.102ka BP and 6.107±0.082ka BP.The last palaeo-earthquake event on this fault occurred in 4.066±0.086ka BP. So,the northern Yumushan Fault is not the seismogenic fault of the M 7 1/2 Biaoshi earthquake of 180 AD.The elapse time from the latest event has been 4000yr,so the possibility of generating destructive earthquake in future should be recognized sufficiently.

We divide the north margin of Western Qingling Fault zone into six segments on the basis of new geology data,namely,Baoji,Tianshui,Wushan,Zhangxian,Huangxianggou and Guomatan segment from east to west.Each segment not only can rupture independently,but also can rupture together with others.The probability of seismic potential on these six segments and two combination segments is computed with the time-dependent seismic potential probability estimate method.We find from the result that,both the Huangxianggou and Zhangxian segments have the biggest probability of rupture in the future; and Tianshui segment is the second.If there will be a combined rupture,it is most likely to happen in Huangxianggou and Zhangxian segments,both of which have higher earthquake risk.We also compute b value along the fault zone.The image of b value indicates a high accumulated stress on the Huangxianggou and Tianshui segments.So we suppose that the two areas are the main locations where strong earthquakes may occur in the future.

According to the new investigation in the northern Hexi corridor,remains of two surface rupture zones are discovered on the southern margin fault of Helishan.One rupture has the length of about 7km and the other about 10km.The two surface rupture zones might be produced by the nearest earthquake event.On the surface rupture zones,there are continuous scarp and free face caused by rupture.The scarp is about 1～1.5m high and on some site is up to 2m nearly.According to the OSL result,the nearest T₁ terrace and higher flood plain forming 3000a BP are dislocated by the fault.All above reveal that the rupture age should be later than that of T₁ terrace.But in the historical data and earthquake catalogue,we didn't find related information about the fault and surface rupture in this area.The 180 AD M 8 Biaoshi earthquake and 756 AD M 7 Zhangye-Jiuquan earthquake are documented in historical data.It is inferred by textual research that the two earthquakes are related with the northern marginal fault of Yumushan in the south of basin.Due to lack of reliable evidence,there still exist many arguments on this inferred conclusion.So we hold that the two surface rupture zones were produced by one of the two large earthquakes or another unrecorded historical event.The research on the activity and surface rupture of this fault can offer valuable information for the tectonic study and strong earthquake risk estimate of this region in the future.

Shallow seismic reflection method is a commonly used technique in urban active fault detection,however,special geotectonic environment may sometimes make reflection survey inapplicable.In such cases,high-resolution seismic refraction could be a feasible option.In this study,we use the finite difference method as the main technique and the conventional methods of refraction data interpretation as auxiliary means in the interpretation of high-resolution shallow refraction data for active fault detection in Lanzhou area.After a comprehensive analysis of first-break refraction travel-time characteristics,the velocity structure and interface structure along each profile have been obtained.A detailed description of the detection results from SS04-1 and SS11-2 seismic profiles is presented in this paper.The main stratigraphic interfaces and tectonic features identified by the two profiles are quite consistent with the results from drilling surveys along the profiles.Our results indicate that high-resolution seismic refraction is an effective replacement in areas where reflection seismic survey is hard to carry out.

This paper makes a comprehensive analysis of the recent progress of the seismic active fault prospecting in Lanzhou city. Based on the satellite and aerial photos interpretation,geological and geomorphic investigation,geochemistry prospecting,shallow seismic investigation,resistivity imaging,drilling,especially large-scale trenching along the 7 active fault zones in Lanzhou city,we have achieved very important progress and gained new knowledge about the recent activity of main active faults and deformation features in Lanzhou Basin. The main conclusions are summarized bellow: (1) The Jinchengguan Fault is a thrust fault,constituting the northern boundary of the Lanzhou Tertiary Basin. It is revealed by geophysical prospecting and drilling that the newest strata offset by the Jinchengguan Fault are the early-Pleistocene sandstone and conglomerate,and that the overlying second and third terraces of the Yellow River remain intact. So,it's an early and middle Pleistocene active fault.(2) The Liujiabu Fault and Shengouqiao Fault constitute the northern and western boundaries of the Qilihe Subsidence,respectively. Revealed by geophysical prospecting,drilling and large trenching,they are not faults but lithologic boundaries of different rocks between Pliocene and early Pleistocene.(3) The Leitanhe Fault is the eastern boundary of Qilihe Subsidence,a boundary fault separating the Tertiary Lanzhou Basin into the east and west basins. According to the geophysical prospecting and drilling,the Leitanhe Fault is a thrust fault and its newest activity age is early and middle Pleistocene. It is not active since late Quaternary and does not cut the third terrace of the Yellow River.(4) The Siergou Fault is the southwestern boundary of Lanzhou Basin,a thrust fault too. It's an early and middle Pleistocene active fault and does not offset the forth terrace of Yellow River. While the Xijincun Fault is much nearer to the south margin of Lanzhou Basin and forms the southern boundary of the Tertiary Lanzhou Basin. It's an early Pleistocene fault.(5) The northern margin of Maxianshan Mountains fault is a major seismic fault on the southern margin of Lanzhou Basin,and its movement is characterized by segmentation. The east segment,the Neiguanying sub-fault,is a late Pleistocene fault. The middle segment,the Maxianshan and Qidaoliang faults,are active during late Pleistocene and early Holocene. The west segment,the Wusushan sub fault,is active during late Pleistocene and Holocene,and it's also the seismic fault of the M7 Lanzhou earthquake.On the whole,we correct the previous recognitions about the activity times of 4 faults,i.e. the Jinchengguan Fault,Leitanhe Fault,Siergou Fault and Xijicun Fault. They are all early and middle Pleistocene instead of late Pleistocene active faults. Especially,we find that the Liujiabu Fault and Shengouqiao Fault directly across Lanzhou city are not late Pleistocene or Holocene active faults but lithologic boundaries between Pliocene mudstone and early Pleistocene conglomerate. The results are very important for the urban planning and engineering construction,and will produce obvious economical and social benefits.