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.

At 01:45 on January 8, 2022, Beijing Time, an M_S6.9 earthquake occurred in Menyuan County, Haibei Prefecture, Qinghai Province, with a focal depth of 10km. The microscopic(instrument)epicenter is located at 37.77°N latitude and 101.26°E longitude in the intersection between the Toleshan fault zone and the Lenglongling fault zone in the northern Qilian-Qaidam block. The epicenter is 54km away from Menyuan County in Qinghai, 99km away from Qilian County, 100km away from Haiyan County, 83km away from Minle County in Gansu Province, 83km away from Yongchang County, and 141km away from Xining City. When the earthquake occurred, Menyuan County and Xining City, the capital of Qinghai Province, were strongly felt, and Yinchuan, Lanzhou, Xi'an and many other places were felt. At the same time, affected by the earthquake, the Lanxin high-speed rail line, an important railway transportation hub of the Belt and Road, was suspended. This earthquake is the largest earthquake in the world since 2022. It is also another earthquake of magnitude 6.0 or above in Qinghai Province following the Maduo M_S7.4 earthquake on May 22, 2021. Besides, this earthquake is the event with the highest magnitude and the longest surface rupture in the region after the two M6.4 Menyuan earthquakes of August 26, 1986 and January 21, 2016. Therefore, this earthquake has attracted much attention from the society. The coseismic surface rupture distribution, combination characteristics, development properties and coseismic displacement of this earthquake were identified in time to help to have a correct understanding of the earthquake seismogenic structure, rupture process, and assessment of short-term earthquake hazards. It is also of great significance for major project route selection, earthquake fortification and rescue and disaster relief. On the basis of the on-site seismic geological investigation, based on the interpretation and analysis of high-resolution satellite remote sensing images, and combined with the low-altitude photogrammetry of unmanned aerial vehicles(DJI PHANTOM 4RTK), the author obtained the coseismic rupture data of five typical sites along the surface rupture zone generated by the earthquake. Using Agisoft Metashape Professional software to process the aerial photos of each section indoors, a high-resolution orthophoto map(DOM)was generated. At the same time, the five typical earthquake surface rupture sections were described in detail in ArcGIS Pro software based on the orthophoto map. Preliminary research shows that the surface rupture zone of the Menyuan M_S6.9 earthquake is more than 22km long and consists of the main rupture of the northern branch and the secondary rupture of the southern branch. The north branch main rupture zone is distributed in the middle-western segment of the Lenglongling Fault of central Haiyuan fault zone, with a length of more than 18km and an overall strike of 295°. The maximum co-seismic horizontal displacement is located in the middle of the rupture zone at Liuhuangou(37.799°N, 101.2607°E), which is about 3.1m and gradually decays towards both ends. The secondary rupture of the southern branch is distributed on the local segments of the eastern Toleshan Fault in the central-western Haiyuan fault zone, with a length of about 4km and a strike of 275°, constituting a secondary branch rupture zone arranged in a left-stepped en-echelon pattern to the western segment of the main rupture zone. There are en-echelon extensional stepovers between the two rupture zones of the north and south branches. The whole surface rupture zone is mainly composed of linear shear cracks, oblique tension cracks, tension-shear cracks, compressional bulges and other structural types. The coseismic surface rupture has the characteristic of typical left-lateral strike-slip motion with a thrust component, and the maximum vertical dislocation is 0.8m.

Due to the collision between the Indian plate and the Eurasian plate, the Tibetan plateau has experienced violent uplift and strong intraplate deformation inside the plateau, which has a great impact on the tectonic evolution of the surrounding areas. The northeastern edge of the Tibetan plateau is the forefront of the northeastward expansion of the Tibetan plateau, which is the ideal place to study the deformation of the plateau as well as the far-field deformation associated with continental collision between the Eurasia and India plates. In recent years, scholars have gained a certain understanding of the characteristics of late Quaternary tectonic activity in the northeast margin of Tibetan plateau. Within the northeastern margin of Tibetan plateau, there are two major fault systems: One is the near EW-trending left-lateral strike-slip fault system, including the Kunlun, Haiyuan and western Qinling faults, the other one is the NNW-trending right-lateral strike-slip fault system, including the Elashan and Riyueshan faults. They are sub-parallel to each other. Since the Riyueshan Fault is one of the major right-lateral strike-slip faults in the northeastern margin of Tibetan plateau, its activity is of great significance for understanding the plateau expansion. Previous studies mainly focused on its northern part which is believed to be active during Holocene. However, its southern part is believed to be active during late Pleistocene, but not active since Holocene. Therefore, there are little studies focusing on the late Quaternary activities of the southern part of the Riyueshan Fault. Hence, our understanding about the characteristics of the late Quaternary activity is insufficient. During our preliminary field survey along the southern Riyueshan Fault, we found distinct deformation of Holocene landforms, such as the young alluvial fan, terrace risers and channels, which indicate its late Quaternary activity. In this study, we firstly analyze the fault geometry of the southern Riyueshan Fault based on high-resolution Superview-1 remote sensing images and carry out field verification. Based on fault geometry characteristics, fault strike orientation etc., we divided the southern Riyueshan Fault into two segments from north to south. One is the Guide segment(generally trending in NW 20°)and the other is the Duohelmao segment(generally striking in NS). During our field investigation, we found two typical sites for slip rate studies, the Rixiaolongwa site on the Guide segment and the Niemari site on the Duohemao segment, respectively. We collected high-resolution images using UAV, and then generated high-resolution DEM of these two sites. By measuring the offsets and corresponding dating results of multi-level terrace risers, we obtained the displacements of the three-level and two-level terraces at Rixiaolongwa and Niemari site, respectively. Then we collected the OSL and ¹⁴C samples on different terrace risers to constrain the age of each terrace. In the Rixiaolongwa area, the corresponding offsets of T1, T2 and T3 terraces are(26.3±3.1)m, (32.7±7.1)m and(38.6±8)m, and the age sequence is(7840±30)a BP, (9 350~10 700)a BP and(11.9±1.3)ka BP, respectively. In the Nimari area, the corresponding offsets of T1 and T2 terraces are(6.3±0.7)m and(9.7±1.7)m, and the ages are(2 860±30)a BP and(3 460±30)a BP, respectively. By applying Monte Carlo method, we obtained the corresponding slip rates of(3.37+0.55/-0.68)mm/a and(2.69+0.41/-0.38)mm/a for the Guide and Duohemao segment, which is comparable to the previously suggested slip rate of northern Riyueshan Fault. Finally, we discussed the role of the Riyueshan Fault in the tectonic deformation of northeastern Tibetan plateau.

At 02:04 a.m. on May 22, 2021, a M_S7.4 earthquake occurred in the Maduo County, Qinghai Province, China. Its epicenter is located within the Bayan Har block in the north-central Tibetan plateau, approximately 70km south of the eastern Kunlun fault system that defines the northern boundary of the block. In order to constrain the seismogenic fault and characterize the co-seismic surface ruptures of this earthquake, field investigations were conducted immediately after the earthquake, combined with analyses of the focal parameters, aftershock distribution, and InSAR inversion of this earthquake.
This preliminary study finds that the seismogenic fault of the Maduo M_S7.4 earthquake is the Jiangcuo segment of the Kunlunshankou-Jiangcuo Fault, which is an active NW-striking and left-lateral strike-slip fault. The total length of the co-seismic surface ruptures is approximately 160km. Multiple rupture patterns exist, mainly including linear shear fractures, obliquely distributed tensional and tensional-shear fractures, pressure ridges, and pull-apart basins. The earthquake also induced a large number of liquefaction structures and landslides in valleys and marshlands.
Based on strike variation and along-strike discontinuity due to the development of step-overs, the coseismic surface rupture zone can be subdivided into four segments, namely the Elinghu South, Huanghexiang, Dongcaoarlong, and Changmahexiang segments. The surface ruptures are quite continuous and prominent along the Elinghu south segment, western portion of the Huanghexiang segment, central portion of the Dongcaoarlong segment, and the Huanghexiang segment. Comparatively, coseismic surface ruptures of other portions are discontinuous. The coseismic strike-slip displacement is roughly determined to be 1~2m based on the displaced gullies, trails, and the width of cracks at releasing step-overs.

The deformation pattern in the northeastern margin of Tibetan plateau is characterized by NE compression, clockwise rotation and eastward extrusion, forming the NNE trending dextral strike-slip faults which further divide the region into several sub-blocks. The deformation of Qaidam secondary block is dominant by northwestward extrusion and rotation, which is controlled by the Elashan and East Kunlun faults. However, the deformation style of Dulan area, the junction of these two faults, remains unclear. We discovered a new active fault zone with a length of 60~70km west to Elashan Fault during our recent field geological survey around Dulan area, named Xiariha fault zone(XFZ), which is a dextral strike-slip fault zone trending NW, consisting of the Xiariha and Yingdeerkang faults. According to the remote sensing interpretation and field investigation, it is found that the Xiariha fault zone showed distinct linear characteristics, reverse scarp, sag pond and ridge dislocation on the satellite images and displaced multi-levels of alluvial fans and river terraces. According to previous studies, the exposed age of T1 terraces is Holocene in the Elashan area, which is located at east of Dulan. During the field investigation, we used the unmanned aerial vehicle(UAV)to get the fine geomorphology features along the XFZ. Also, to define the active era, we tried to find the fault section of the XFZ that could provide the information of the contact between the fault and late Quaternary strata. Based on the high-resolution DEM obtained by UAV, the offset of T1 is about 2.5m, indicating its activity in Holocene compared with the Elashan area. Along the XFZ, the fault displaced late Quaternary strata revealed on the section. The geomorphic features and fault section show that the XFZ is a late Pleistocene to Holocene active fault. The Dulan area is located at the convergence of East Kunlun Fault and Elashan Fault, the southeastern end of Qaidam secondary block, which is affected by the regional NE and SW principal compressive stress and shear stress. Under this circumstance, the Qaidam block is experiencing extrusion and rotation and there are a series of NW-trending dextral strike-slip faults parallel to the Elashan Fault and EW-trending sinistral strike-slip faults parallel to the East Kunlun Fault, such as Reshui-Taosituo River Fault, developed in the Dulan area. Therefore, we suggest that the Xiariha Fault and the nearly EW trending, Holocene sinistral Reshui-Taosituo River Fault adjust the extrusion rotation deformation jointly at the southeast end of the Qaidam block under the control of the Elashan Fault and the East Kunlun Fault, respectively. Meanwhile, the new discovery of Xiariha Fault and its activity in Holocene is not only of great significance to understand the regional tectonic deformation model, but also leads to a great change in the understanding of regional seismic risk because of its capabliliby of generating strong earthquakes. Therefore, it is urgent to carry out further research work in this area, improve the understanding of regional strain distribution mode, and provide reference for regional seismic safety issues.

The 40km-long, NEE trending Reshui-Taostuo River Fault was found in the southern Dulan-Chaka highland by recent field investigation, which is a strike-slip fault with some normal component. DEM data was generated by small unmanned aerial vehicle(UAV)on key geomorphic units with resolution<0.05m. Based on the interpretation and field investigation, we get two conclusions: 1)It is the first time to define the Reshui-Taostuo River Fault, and the fault is 40km long with a 6km-long surface rupture; 2)There are left-handed dislocations in the gullies and terraces cut by the fault. On the high-resolution DEM image obtained by UAV, the offsets are(9.3±0.5)m, (17.9±1.5)m, and(36.8±2)m, measured by topographic profile recovery of gullies. The recovery measurements of two terraces present that the horizontal offset of T₁/T₀ is(18.2±1.5)m and the T₂/T₁ is (35.8±2)m, which is consistent with the offsets from gullies. According to the historical earthquake records, a M53/4 earthquake on April 10, 1938 and a M_S5.0 earthquake on March 21, 1952 occurred at the eastern end of the surface rupture, which may be related to the activity of the fault. By checking the county records of Dulan and other relevant data, we find that there are no literature records about the two earthquakes, which is possibly due to the far distance to the epicenter at that time, the scarcity of population in Dulan, or that the earthquake occurred too long ago that led to losing its records. The southernmost ends of the Eastern Kunlun Fault and the Elashan Fault converge to form a wedge-shaped extruded fault block toward the northwest. The Dulan Basin, located at the end of the wedge-shaped fault block, is affected by regional NE and SW principal compressive stress and the shear stress of the two boundary faults. The Dulan Basin experienced a complex deformation process of compression accompanying with extension. In the process of extrusion, the specific form of extension is the strike-slip faults at each side of the wedge, and there is indeed a north-east and south-west compression between the two controlling wedge-shaped fault block boundary faults, the Eastern Kunlun and Elashan Faults. The inferred mechanism of triangular wedge extrusion deformation in this area is quite different from the pure rigid extrusion model. Therefore, Dulan Basin is a wedge-shaped block sandwiched between the two large-scale strike-slip faults. Due to the compression of the northeast and southwest directions of the region, the peripheral faults of the Dulan Basin form a series of southeast converging plume thrust faults on the northeast edge of the basin near the Elashan Fault, which are parallel to the Elashan Fault in morphology and may converge with the Elashan Fault in subsurface. The southern marginal fault of the Dulan Basin(Reshui-Taostuo River Fault)near the Eastern Kunlun fault zone is jointly affected by the left-lateral strike-slip Eastern Kunlun Fault and the right-lateral strike-slip Elashan Fault, presenting a left-lateral strike-slip characteristic. Meanwhile, the wedge-shaped fault block extrudes to the northwest, causing local extension at the southeast end, and the fault shows the extensional deformation. These faults absorb or transform the shear stress in the northeastern margin of the Tibet Plateau. Therefore, our discovery of the Dulan Reshui-Taostuo River Fault provides important constraints for better understanding of the internal deformation mode and mechanism of the fault block in the northeastern Tibetan plateau.
The strike of Reshui-Taostuo River Fault is different from the southern marginal fault of the Qaidam Basin. The Qaidam south marginal burial fault is the boundary fault between the Qaidam Basin and the East Kunlun structural belt, with a total length of ~500km. The geophysical data show that Qaidam south marginal burial fault forms at the boundary between the positive gravity anomaly of the southern East Kunlun structural belt and the negative gravity anomaly gradient zone of the northern Qaidam Basin, showing as a thrust fault towards the basin. The western segment of the fault was active at late Pleistocene, and the eastern segment near Dulan County was active at early-middle Pleistocene. The Reshui-Taostuo River Fault is characterized by sinistral strike-slip with a normal component. The field evidence indicates that the latest active period of this fault was Holocene, with a total length of only 40km. Neither remote sensing image interpretation nor field investigation indicate the fault extends further westward and intersects with the Qaidam south marginal burial fault. Moreover, it shows that its strike is relatively consistent with the East Kunlun fault zone in spatial distribution and has a certain angle with the burial fault in the southern margin of Qaidam Basin. Therefore, there is no structural connection between the Reshui-Taostuo River Fault and the Qaidam south marginal burial fault.

The northeastern margin of Tibetan plateau is an active block controlled by the eastern Kunlun fault zone, the Qilian Shan-Haiyuan fault zone, and the Altyn Tagh fault zone. It is the frontier and the sensitive area of neotectonic activity since the Cenozoic. There are widespread folds, thrust faults and stike-slip faults in the northeastern Tibetan plateau produced by the intensive tectonic deformation, indicating that this area is suffering the crustal shortening, left-lateral shear and vertical uplift. The Riyueshan Fault is one of the major faults in the dextral strike-slip faults systems, which lies between the two major large-scale left-lateral strike-slip faults, the Qilian-Haiyuan Fault and the eastern Kunlun Fault. In the process of growing and expanding of the entire Tibetan plateau, the dextral strike-slip faults play an important role in regulating the deformation and transformation between the secondary blocks. In the early Quaternary, because of the northeastward expansion of the northeastern Tibetan plateau, tectonic deformations such as NE-direction extrusion shortening, clockwise rotation, and SEE-direction extrusion occurred in the northeastern margin of the Tibetan plateau, which lead to the left-lateral slip movement of the NWW-trending major regional boundary faults. As the result, the NNW-trending faults which lie between these NWW direction faults are developed. The main geomorphic units developed within the research area are controlled by the Riyueshan Fault, formed due to the northeastward motion of the Tibet block. These geomorphic units could be classified as:Qinghai Lake Basin, Haiyan Basin, Datonghe Basin, Dezhou Basin, and the mountains developed between the basins such as the Datongshan and the Riyueshan. Paleo basins, alluvial fans, multiple levels of terraces are developed at mountain fronts. The climate variation caused the formation of the geomorphic units during the expansion period of the lakes within the northeastern Tibetan plateau. There are two levels of alluvial fans and three levels of fluvial terrace developed in the study area, the sediments of the alluvial fans and fluvial terraces formed by different sources are developed in the same period. The Riyueshan Fault connects with the NNW-trending left-lateral strike-slip north marginal Tuoleshan fault in the north, and obliquely connects with the Lajishan thrust fault in the south. The fault extends for about 180km from north to south, passing through Datonghe, Reshui coal mine, Chaka River, Tuole, Ketu and Xicha, and connecting with the Lajishan thrusts near the Kesuer Basin. The Riyueshan Fault consists of five discontinuous right-step en-echelon sub-fault segments, with a spacing of 2~3km, and pull-apart basins are formed in the stepovers.
The Riyueshan Fault is a secondary fault located in the Qaidam-Qilian active block which is controlled by the major boundary faults, such as the East Kunlun Fault and the Qilian-Haiyuan Fault. Its activity characteristics provide information of the outward expansion of the northeastern margin of Tibet. Tectonic landforms are developed along the Riyueshan Fault. Focusing on the distinct geomorphic deformation since late Pleistocene, the paper obtains the vertical displacement along the fault strike by RTK measurement method. Based on the fault growth-linkage theory, the evolution of the Riyueshan Fault and the related kinetic background are discussed. The following three conclusions are obtained:1)According to the characteristics of development of the three-stage 200km-long steep fault scarp developed in the landforms of the late Pleistocene alluvial fans and terraces, the Riyueshan Fault is divided into five segments, with the most important segment located in the third stepover(CD-3); 2)The three-stage displacement distribution pattern of the Riyueshan Fault reveals that the fault was formed by the growths and connections of multiple secondary faults and is in the second stage of fault growth and connection. With CD-3 as the boundary, the faults on the NW side continue to grow and connect; the fault activity time on the SE side is shorter, and the activity intensity is weaker; 3)The extreme value of the fault displacement distribution curve indicates the location of strain concentration and stress accumulation. With the stepover CD-3 as the boundary, the stress and strain on NW side are mainly concentrated in the middle and fault stepovers. The long-term accumulation range of stress on the SE side is relatively dispersed. The stress state may be related to the counterclockwise rotation inside the block under the compression of regional tectonic stress.

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.

The M_S 7.1 Yushu,Qinghai earthquake on April 14th,2010 generated a surface rupture zone about 65km long.The west of Ganda Village,D1,and Guo Qing Yi Rong Song Duo,D2,are the two most representative places of surface rupture characteristics of the M_S 7.1 earthquake in Yushu,Qinghai Province.Survey results on co-seismic surface rupture characteristics at the two locations show that:(1)Surface rupture extends along the remains of late Quaternary activities on the pre-existing fault,and at the west of Ganda Village,the surface rupture is composed of tensile shear fcractures arranged en echelon,with mole tracks.Most part of the surface rupture is distributed along the paleoseismology trench,and through measuring an offset wall,we get the offset of 1.4m.(2)At Guo Qing Yi Rong Song Duo,the surface rupture zone consists of a series of right-step en echelon sub-cracks with a spacing about 30m each other,and the sub-crack is formed by the right step cracks,with a spacing of 3～5m.The mole track-tension crack and crack belt compose the single rupture,which are along the slope wash in front of the mountain.The surface rupture across the river valley appears as compressional ridge and sag pond in the valley.The offset of a fence is measured to be 1.4m.(3)The surface rupture shows left-lateral strike-slip characteristics,with no significant vertical component.The surface rupture mode shows a typical strike slip character.The surface rupture distributed along the pre-existing offset landforms reflects that the Ganzi-Yushu Fault,which is an active fault of Late Quaternary,is the seismogenic fault of this earthquake.The fault has the character of in situ recurrence of large earthquakes.

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 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.