The Hexi Corridor in northwest China has obvious structural deformation and complex fracture image. With the development of several NW thrust fault zones accompanied by a large number of ancient earthquakes and historical seismic events, the earthquake disaster is relatively serious. The eastern margin of Yumushan fault is one of them. The fault is mainly developed in the east site of Yumushan Mountain, with the exposed fault plane striking NW330° and dipping about 41°~85° to the southwest as a whole. Previous research data show that the Eastern Margin of the Yumushan fault is an important part of the Qilian Mountain active thrust fault system in the northeast margin of the Tibet Plateau. It also constitutes the boundary structure between Hexi Corridor and Yumushan Uplift. Its late Quaternary tectonic deformation and recent activity characteristics reflect the northward extension process of the Qilian Mountains and the remote collision effect of the northward extrusion of the Indian Plate. However, there are still some controversies in the study of the latest activity age and deformation characteristics of the eastern Margin of the Yumushan fault zone, which directly affect the seismic risk assessment along the fault line and the Hexi Corridor, as well as the in-depth understanding of the active structural characteristics of the northeast margin of the Tibetan plateau.

Combined with remote sensing image interpretation, paleoseismologic excavations, aerial photogrammetry of unmanned aerial vehicles and late Quaternary dating, this study carried out field investigations and newly discovered the surface rupture zone of The Eastern Margin of Yumushan Fault and its activity characteristics. The results show that The Eastern Margin of the Yumushan Fault strikes NW330° combined with obvious thrust movement, which is manifested as a fault scarp landform. That’s revealing than the kinematics property of The eastern margin of Yumushan fault is dominated by thrust. The fault forms the dividing line between the Yumushan uplift and Zhangye Basin, and also the dividing line between pre-quaternary strata and Quaternary strata. The southwest side of the fault is dominated by pre-quaternary bedrock which constitutes a mountain landform. Late Quaternary sediments are exposed on the northeast side, and the Holocene strata are widely distributed around the Heihe River. The results show that there are obvious differences in the activity habits of the faults. With the Heihe River as the boundary, the fault activity difference is obvious on the south and north sides of the Heihe River. The latest surface fracture zone in the late Holocene was found along the Heiheokou segment(F_1-1). And the Hongshahesegment(F_1-2)showed pre-quaternary fault. It can be seen that the Miocene fine sandstone is in fault contact with the early Pleistocene glutenite and late Ordovician metamorphic andesite, and the fault gouge develops near the fault, which is gray-green and yellow-green with moderate hardness and easy to be wet when encountering water.

The Heihekou segment(F_1-1)starts from Daciyaohe River in the north, passes Xiaociyaokou, and reaches Heihekou in the south. The fracture zone moves towards NW330° and tends to SW, with a length of about 10km and a width of 3~10m. For river terraces, gullies, and platforms with young surface faults, the maximum height of the surface scarp is based on the DEM data generated by UAVs. The height of the T1 terrace fault scarp measured by two profile lines is(1.7±0.1)m to(3.3±0.2)m. In the excavating trenches, obvious evidence of fault activity such as traction bending of strata and directional arrangement of gravel can be seen. The strata consist of late Quaternary alluvial sand, gravel layer, loess layer, and silty layer. The optically stimulated luminescence dating results show that the latest surface rupture event occurred at(0.6±0.07)ka BP.

According to the empirical formula between maximum vertical displacement(D_max)and magnitude(M), the magnitude of the latest seismic event is estimated. The magnitude and potential seismic risk of the latest rupture event are evaluated. The results reveal that the maximum vertical displacement of the latest surface rupture event is(3.3±0.2)m. Based on the empirical relationship between magnitude and vertical displacement, it is concluded that a large earthquake rupture occurred in the eastern margin of the Yumushan fault in the late Holocene and the corresponding magnitude is estimated to be M7.5.

Derived from the analysis of existing data, the fault in the eastern margin of the Yumushan fault may conform to the quasiperiodic earthquake recurrence behavior. And the recurrence interval of strong earthquakes may exceed 1 600a. The time interval between the latest event revealed in this paper and its last seismic event is about 1 800a, which is consistent with the time interval under the fault quasiperiodic earthquake recurrence model.

The results show that the eastern margin of the Yumushan fault has intensive tectonic deformation in the late Quaternary and a large seismic background of M7 or above. The current kinematic mode of the fault is compressive shortening. Its geodynamic process may be mainly controlled by the northward extension of the Qilian Mountains and the remote collision effect of the northward extrusion of the Indian Plate. The deformation process of the fault may be in line with pre-spreading imbricate thrust deformation and the latest deformation has gradually extended from the basin-mountain boundary to the interior of Zhangye Basin, which provides new data to support the seismic risk assessment of the interior of the basin. At the same time, the latest deformation achievement of the eastern margin of the Yumushan fault has important scientific significance for improving the active tectonic image of the northeastern margin of the Qingzang plateau and discussing the kinematics model of the Qingzang plateau.

Active faults refer to faults that have been active since the late Quaternary(100000~12 0^{}000 years)which are the culprits of large earthquakes. They can be divided into Holocene faults and Late Pleistocene faults. The Holocene fault is the active fault that has displaced on or near the surface in the past 10000 years. The Active faults may cause seismic surface dislocation in the future, which will damage the project crossing the active fault. It is necessary to take measures to avoid or resist the fault. Therefore, finding out the distributions of active faults are the prerequisite for reducing earthquake disaster losses and disaster risks.

We undertook the compilation of the 1︰1000000 seismotectonic map of Tibet in the first national comprehensive risk survey of natural disasters. The preparation of a seismotectonic map is to conduct detailed investigation and research on active faults within the research scope, including large-scale active faults with a strong earthquake-generating capacity, as well as small-scale and highly active faults. The Qinghai-Tibetan plateau is a typical strong earthquake-prone area with wide distribution, high frequency, high intensity and shallow source of seismicity. This study introduces the Holocene active faults in the modified scale(I45)of 1︰1000000 international standard topographic map.

We use Satellite remote sensing images to determine the locations of the faults, identify their characteristics, and assess the ages of their latest activity and quantitative parameters such as intensity. Satellite remote sensing interpretation is the most important method to study active faults. This is especially true in the Qinghai-Tibetan plateau region, where active fault traces are clear and lack overlying Quaternary layers. High-resolution satellite remote sensing images can capture various tectonic and geomorphological phenomena formed by fault activity.

In the study area, we interpreted Six Holocene active faults by using high-resolution satellite images, including the MargaiCaka fault, the Riganpeicuo fault, the Yibuchaka graben, the Qingwahu fault, the Dongcha fault, and the central part of Qixiangcuo fault. When analyzing each fault, typical images with evidence of active faults are intercepted, and the typical remote-sensing image features of active faults are summarized. It is clear that the typical remote sensing images of active faults are the remote sensing images which can reflect the dislocation of late Quaternary strata, geological bodies and geomorphic surfaces(unit).

The latest active age, slipping senses and active intensity of above active faults in the area, as well as the overall tectonic pattern and seismic capacity of active structures in the area are discussed. The MargaiCaka fault in the north of the study area and the Riganpeicuo fault, the Qixiangcuo fault in the south are large-scale left-lateral strike-slip faults of NEE trending and have the capability of generating earthquakes of about magnitude 7.5. The NEE-trending Yibuchaka graben, the Qingwahu fault, and the NW-trending Dongcha fault in the central of the map unit have the capability of generating earthquakes of about magnitude 7. The above-mentioned faults reflect a special dynamic environment in which the area is squeezed in the north-south direction, and a V-shaped conjugate fault formed, making the plateau squeezed out to the east.

Beijing plain is a strong earthquake tectonic area in China, where the Sanhe-Pinggu earthquake with M8 occurred in 1679.The seismogenic fault of this earthquake is the Xiadian Fault. An about 10km-long earthquake surface fault is developed, striking northeast. Deep seismic exploration reveals that this surface fault is a direct exposure of a deep fault cutting through the whole crust, and it is concealed in the Quaternary layers to both ends. Previous studies have not yet revealed how the deep fault with M8 earthquake extended to the southwest and northeast. In the study of Xiadian Fault, it is found that there is another fault with similar strike and opposite dip in the west of Xiadian Fault, which is called the West Xiadian Fault in this paper. In this study, six shallow seismic profiles data are used to determine the location of this fault in Sanhe city, and the late Quaternary activity of the fault is studied by using the method of combined drilling, magnetic susceptibility logging and luminescence dating.

The results of shallow seismic exploration profiles show that the fault is zigzag with a general strike of NE and dip NW. In vertical profile, it is generally of normal fault. It shows the flower structure in one profile, which indicates that the fault may have a certain strike-slip property. On two long seismic reflection profiles, it can be seen that the northwest side of the fault is a half graben structure. This half graben-like depression, which has not been introduced by predecessors, is called Yanjiao fault depression in this paper. The maximum Quaternary thickness of the graben is 300m. The West Xiadian Fault is the main controlling fault in the southern margin of the sag.

The Xiadian Fault, which is opposite to the West Xiadian Fault in dips, controls the Dachang depression, which is a large-scale depression with a Quaternary thickness of more than 600m. The West Xiadian Fault is opposite to the Xiadian Fault, and there is a horst between the West Xiadian Fault and the Xiadian Fault. The width of the horst varies greatly, and the narrowest part is less than 1km. The West Xiadian Fault may form an echelon structure with Xiadian Fault in plane, and they are closely related in depth.

According to the core histogram and logging curves of ten boreholes and eight effective dating data, the buried depth of the upper breakpoint of the concealed fault is about 12m, which dislocates the late Pleistocene strata. The effective dating result of this set of strata is(36.52±5.39)ka. There is no evidence of Holocene activity of the fault, but it is certain that the fault is an active fault in the late Pleistocene in Sanhe region. The vertical slip rate is about 0.075mm/a since late Pleistocene, and about 0.03mm/a since the late period of late Pleistocene. These slip rates are less than those of the Xiadian Fault in the same period. According to our study, the vertical slip rate of Xiadian Fault since late Pleistocene is about 0.25mm/a.

Although the latest active age, the total movement amplitude since Quaternary and the sliding rate since late Pleistocene of West Xiadian Fault are less than those of Xiadian Fault, its movement characteristics is very similar to that of Xiadian Fault, and the two faults are close to each other in space, and closely related in deep structure. It can be inferred that the fault is probably a part of the seismogenic structure of the 1679 Sanhe-Pinggu M8 earthquake. In a broad sense, the Xiadian fault zone is likely to extend to the southwest along the West Xiadian Fault.

The time-dependent probabilistic seismic hazard assessment of the active faults based on the quantitative study of seismo-geology has the vital practical significance for the earthquake prevention and disaster management because it describes the seismic risk of active faults by the probability of an earthquake that increases with time and the predicted magnitude. The Poisson model used in the traditional probabilistic method contradicts with the activity characteristics of the fault, so it cannot be used directly to the potential earthquake risk evaluation of the active fault where the time elapsing from the last great earthquake is relatively short. That is to say, the present Poisson model might overestimate the potential earthquake risk of the Xiadian active fault zone in North China because the elapsed time after the historical M8 earthquake that occurred in 1679 is only 341a. Thus, based on paleoearthquake study and geomorphology survey in the field, as well as integrating the data provided by the previous scientists, this paper reveals two paleo-events occurring on the Xiadian active fault zone. The first event E₁ occurred in 1679 with magnitude M8 and ruptured the surface from Sanhe City of Hebei Province to Pinggu District of Beijing at about 341a BP, and the other happened in (4.89±0.68)ka BP(E₂). Our research also found that the average co-seismic displacement is ~(1.4±0.1)m, and the predicted maximum magnitude of the potential earthquake is 8.0. In addition, the probabilistic seismic hazard analysis of great earthquakes for Xiadian active fault zone in the forthcoming 30a is performed based on Poisson model, Brownian time passage model(BPT), stochastic characteristic-slip model(SCS)and NB model to describe time-dependent features of the fault rupture source and its characteristic behavior. The research shows that the probability of strong earthquake in the forthcoming 30a along the Xiadian active fault zone is lower than previously thought, and the seismic hazard level estimated by Poisson model might be overestimated. This result is also helpful for the scientific earthquake potential estimation and earthquake disaster protection of the Xiadian active fault zone, and for the discussion on how to better apply the time-dependent probabilistic methods to the earthquake potential evaluation of active faults in eastern China.

The wedge-shaped deposit formed in front of fault scarp is called colluvial wedge. Repeated faulting by faults may produce multiple colluvial wedges, each of which represents a paleoseismic event. When there are two or more colluvial wedges, the new colluvial wedge is in sedimentary contact with the fault, while the old ones are in fault contact with the fault. The shape of colluvial wedge is usually in the form of horizontal triangle, and the sedimentary facies is usually of binary structure. The overall grain size decreases gradually from bottom to top. Soil layer generally develops on the top, and different types of soil are developed under different climate or soil environments. Another deposit in front of fault scarp is the sag pond graben. The graben in front of sag pond is generally a set of sedimentary assemblages of colluvial facies, alluvial diluvial facies and swamp facies. The area close to the fault, especially the main fault, is of colluvial facies, while the area away from the fault is of alluvial and pluvial facies and marshy facies. In an accumulative cycle, the size of the deposit decreases from bottom to top, and soil layers develop on the top or surface. Multiple pile-ups may be a marker for identifying multiple faulting events. The pile-up strata such as colluvial wedge and fault sag pond can be used as identification markers for paleoseismic events. Colluvial wedge and sag pond, as the identification markers for paleoearthquake, have been well applied to practical research. However, there is still lack of detailed research on the lithological structure and genetic evolution in the interior of colluvial wedge and sag pond sediment, meanwhile, there is still a deficiency in the analysis of the completeness and the regional characteristics of paleoearthquake by using colluvial wedge and sag pond sediment. This paper discusses the method of identifying paleoearthquake by using sag pond sediments and colluvial wedge. We discuss the lithologic combination and sedimentary evolution of sag pond and choose the surface rupture zone of the 1679 M8.0 earthquake on the Xiadian Fault as the research area. In this paper, the distribution range and filling sequence of sag pond are analyzed, using borehole exploration. Four paleoearthquake events are identified since 25ka to 12ka, based on the sag pond sediments and colluvial wedge. The in situ recurrence interval of these seismic events is 480a, 510a, 7 630a and 2 830a, respectively. The lithologic combination and sedimentary evolution law of the sag pond sediments caused by an ancient earthquake are discussed. The sag pond distribution range and filling sequence are determined by the surface elevation survey and drilling exploration. The exploratory trench exposes the sag pond filling strata sequence and lithologic combination. Based on this, we analyze the three sedimentation stages of sag pond sediments formed by a paleoearthquake event near the earthquake fault. It is believed that the filling sequence is composed from bottom to top of the colluvial wedge, the erosion surface or unconformity surface, the fine detrital sediments(containing biological debris)and paleosols. For the fault-sag ponds formed by active faults, the paleoearthquakes occurred near the unconformity or erosion surface of the sediments of the fault-plug ponds. An ancient earthquake event includes the combination of organic deposits such as sediments, clastic deposits, bioclasts, burrow, plant roots and other organic deposits on the vertical scour surface or unconformity. The time interval between two paleoseismic events is defined by two adjacent unconformities(or scour surfaces). According to the vertical facies association and chronological test results of the sediments in the Pangezhuang trough of the Xiatan Fault, four paleo-seismic events are identified since the late Pleistocene period of 25~12ka BP, with recurrence intervals of 480a, 510a, 7 630a and 2 830a, respectively.

Because of the frequent seismic activity in Songyuan in recent years, the modes of tectonic movement in this area since the Quaternary have attracted increasing consideration. This paper selects the Gudian Fault which locates between the southeast uplift and central depression of Songliao Basin as the research object. We discussed the Quaternary structural characteristics of the Gudian Fault using growth strata. Using the data of deep seismic reflection prospecting for oil, we determined the location, geometry and kinematics characteristics of the Gudian Fault. And using the shallow seismic reflection prospecting data, the combined drilling exploration data and TL data, we determined precisely the inversion tectonics feature of the fault since late Cenozoic. Based on the above data, we believe that the Gudian Fault is dominated mainly by thrust-folding since Quaternary. A set of growth strata is recognized by shallow seismic reflection exploration data. According to the overlap of growth strata and the relationship between deposition rate and uplift rate, we confirm that the uplift rate of Gudian Fault in the early of Early Pleistocene is less than 0.15mm/a. And according to the offlap of growth strata and the relationship between deposition rate and uplift rate, the uplift rate of the Gudian Fault is more than 0.091mm/a in the late of Early Pleistocene and more than 0.052mm/a in middle Pleistocene. According to the chronological data, it is determined that the uplift rate of the Gudian Fault is 0.046mm/y since 205ka.

In the study of active faults, obtaining the exact age of the strata is an extremely important step. The optically stimulated luminescence (OSL) dating method, a technique closely related to thermoluminescence (TL), is developing extensively on dating for Quaternary sediments in recent years. Fukang Faults, located in the eastern Tianshan arc nappe tectonic zone, are typical arc thrusting faults. The dating samples collected from Dahuangshan trench of Fukang Fault zone are used to determine the activity of the fault. 23 OSL samples were obtained from the trench. We selected 4～11μm fine-grained quartz through pre-treatment process and analysed them by using sensitivity-corrected multiple aliquot regenerative-dose (SMAR) protocol. Equivalent dose (De) preheat plateau test is an often used approach to determine the appropriate preheat temperature in OSL dating. The preheat plateau test of sample LED12-297 shows that 220～260℃ are the appropriate preheat plateau temperature regions to get fundamental De. The dating results show that the OSL stratigraphic ages of the samples are consistent with stratigraphic sequence and that Fukang Fault is a Holocene active fault. It is found that the last event of Fukang Fault occurred (1.90±0.14) ka to (3.47±0.17) ka ago. The OSL ages and their related stratigraphic vertical displacement are used to calculate the vertical slip rate of the fault, which is 0.17mm/a.

The late-Quaternary deformation characteristics of the boundary fault zones are critical to understanding the crustal deformation of the Tianshan Mountains. Based on remote sensing image interpretation, field surveys, trenching and optically stimulated luminescence dating methods, we obtain the reliable activity evidences of the Maidan Fault in late-Quaternary.
The Maidan Fault is the boundary fault of the Tianshan Mountains and Tarim Basin. The fault, with a total length of 400km and the maximum width about 15～17km, comprises a series of secondary faults. During the late Quaternary, the fault was still very active. The fault dislocated the late-Quaternary landform surfaces, forming obvious scarps on the surfaces. The height of the scarps range several to hundred meters. Trench excavation shows that paleoearthquakes occurred on the faults during late Holocene. The vertical displacement caused by the last paleoearthquake event is above 2m. The different late Quaternary landforms with different vertical displacement heights indicate that several strong earthquake events have occurred on the Maidan Fault since the late Quaternary.
The discovery of activity on the Maidan Fault shows that the deformation does not focus solely on the newly born reverse fault and fold belt. Faults at the root of Kalpin nappe system have also participated in absorbing and partitioning some of the tectonic deformation. This phenomenon may explain why the shortening rate got by geology method of the Kalpin nappe structure is much less than that obtained by GPS. This deformation mode of the Tianshan orogenic belt is obviously different from the piggyback propagation as considered previously. The activities of the Tianshan root faults migrated to the frontal faults of the piedmont nappe, and the root fault activity weakened gradually. But the activity in Kalpin nappe structure does not accord with this mode. The root faults and the frontal faults of the Kalpin nappe structure are all obviously active, which indicates the nappe structure in the southwestern Tianshan is an out-of-sequence, or a non-sequence thrust system. This kind of structure mode brings new challenges to us in constructing seismogenic tectonic models and assessing seismic risk.