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 order to reveal the deformation and cumulative stress state in Longmenshan and its adjacent faults before Wenchuan earthquake,a 3D viscoelastic finite element model,which includes Longmenshan,Longriba,Minjiang and Huya faults is built in this paper.Using the GPS measurement results of 1999-2004 as the boundary constraints,the deformation and movement of Longmenshan fault zone and its adjacent zones before Wenchuan earthquake are simulated.The conclusions are drawn in this paper as follows:First,velocity component parallel to Longmenshan Fault is mainly absorbed by Longriba Fault and velocity component perpendicular to the Longmenshan Fault is mainly absorbed by itself.Because of the barrier effect of Minjiang and Huya faults on the north section of Longmenshan Fault,the compression rate in the northern part of Longmenshan Fault is lower than that in the southern part.Second,extending from SW to NE direction along Longmenshan Fault,the angle between the main compressive stress and the direction of the fault changes gradually from the nearly vertical to 45 degrees. Compressive stress and shear stress accumulation rate is high in southwest segment of Longmenshan Fault and compressive stress is greater;the stress accumulation rate is low and the compressive stress is close to shear stress in the northeast segment of the fault.This is coincident with the fact that small and medium-sized earthquakes occurred frequently and seismic activity is strong in the southwest of the fault,and that there are only occasional small earthquakes and the seismic activity is weak in the northeast of the fault.It is also coincident with the rupture type of thrust and right-lateral strike-slip of the Wenchuan earthquake and thrust of the Lushan earthquake.Third,assuming that the same type and magnitude of earthquake requires the same amount of stress accumulation,the rupture of Minjiang Fault,the southern segment of Longmenshan Fault and the Huya Fault are mainly of thrust movement and the earthquake recurrence period of the three faults increases gradually.In the northern segment of Longriba Fault and Longmenshan Fault,earthquake rupture is of thrusting and right-lateral strike-slip. The earthquake recurrence period of former is shorter than the latter.In the southern segment of Longriba Fault,earthquake rupture is purely of right-lateral strike-slip,it is possible that the earthquake recurrence period on the fault is the shortest in the study region.

The Gudian Fault in the southwest of Songyuan is an important fault in the central depression of the Songliao Basin. It was recognized from the petroleum exploration data. Based on the data, we conducted shallow seismic exploration, drilling exploration, age determination(OSL) and topography measurement. The fault features and its motion characteristics are analyzed with the results of shallow seismic exploration. With stratigraphic correlation and optical stimulated luminescence dating, the latest active age of the fault is determined. The surface relief of the region to the southeast of the drilling site is relatively larger than surrounding places. An 800m long section across the fault was measured by GPSRTK, and the deformation amount across the zone was calculated. Four conclusions are drawn in this paper:(1) The Gudian Fault is arcuate in shape and shows a property of inverse fault with a length of about 66km in the reflection interface T1(bottom of the upper Cretaceous Nenjiang Group). (2) The middle part of the fault rupture is wider than the ends, narrowing or dying out outwards. According to this feature and the rupture of the bottom of the fourth segment of the upper Cretaceous Nenjiang Group, the fault can be divided into three segments, e.g. Daliba Village-Gaizijing-Guyang segment, Guyang-Shenjingzi-Julongshan Village segment and Julongshan Village-Caiyuanzi segment. (3) The yellow silt layer at the base of the upper Pleistocene series ((33.66±3.27) ka BP~50ka BP) is offset by the Gudian Fault, while the upper tawny silt layer is not influenced by the fault. Thus, the fault belongs to late Pleistocene active fault. (4) The amount of geomorphic deformation around Shenjingzi is 9m. The depth of the bottom of the upper Pleistocene series is 11m and the Huangshan Group of the mid Pleistocene series exposes to the southeast of the deformation zone. Therefore, the throw of the bottom of the upper Pleistocene series is about 20m at the sides of the deformation zone. In addition, the Qianguo M6(3/4) earthquake occurred in Songyuan area in 1119 AD. Though some studies have been done, arguments still exist on the seismogenic structure of the Qianguo M6(3/4) earthquake. Combined with others studies, Gudian Fault is considered as the seismogenic structure of the Qianguo M6(3/4) earthquake.

Located in the south of the Songliao Basin, Songyuan City is one of the few high seismic intensity regions (Ⅷ degree regions) in Northeast China, where a magnitude 6(3/4) earthquake took place in 1119. Since 2013, many earthquakes of magnitude above 5 have occurred in Chaganhua Town which is 100km southwest of Songyuan. The faults in the study region are almost all in a concealed state and covered by the Quaternary system, therefore, geophysical investigation, drilling and other similar means are required to determine their distribution, occurrence, nature and active period. Many seismic explorations in this region aiming at surveying the oil bearing structure have been conducted by Jilin Oilfield, which provides detailed seismic exploration information for preliminary detection of active faults. In this paper, the main features of petroleum-related seismic data and major methods for extracting tectonic information are presented; on the plain, the trace information of the main structure is extracted by the t0 interface contour map which allows direct reflection of rises and falls of stratal interfaces and the tectonic characteristics of the corresponding geologic period; on the section, the "extending upwards" characteristics of faults are captured by tracing and marking geological phenomena in the reflective standard layer, faults, the surface of unconformity and so on. Under the comprehensive use of the "3D" structure in the interpretation of the results, accurate spatial distribution information of main faults are obtained in the study region, this offers an effective approach to preliminary judgment of the activity of faults in this region. Meanwhile, the active age of the target faults is identified by superimposing the deep and shallow seismic data and integrating with the drilling detection.

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.

High-resolution satellite image interpretation and field investigation indicate that the surface rupture zone produced by the Yutian M_S7.3 earthquake is～25km long along a NS-trending fault at the western piedmont of a snow-covered range at the upper reach of the Yurungongkash River,about 20km south of the Ashikule Volcanoes.The surface rupture zone consists of different striking ruptures with both normal and left-lateral faulting components.The maximum left-lateral and vertical co-seismic slips measured in the field are～1.8m and～2.0m,respectively.Its seismogenic NS-trending fault belongs to the secondary structure at the NE-trending tensile area of the southwestern end of the Altyn Tagh Fault,which conforms to the eastward escape of the Kunlun-Qaidamu-Qilian block,relative to the Western Kunlun block.

The Wangjiagou Fault set,a set of Holocene active faults,is located at western suburbs of Urumqi City.The faults dislocated the gravel platform of the mid Pleistocene and the third level terrace of the Wangjiagou east bank,generating apparent fault scarps of opposite-slope direction on the surface with clear geomorphic traces.There are a series of deformation indications on landform,such as seismic fault,scarp and upheaval.In the field,thirty-nine groups of data were measured by using line tape along the fault.Among them,six were measured on the third level terrace of the Wangjiagou,and the others on the mid Pleistocene platform.Based on the data measured across the fault,we obtain that the height of the scarps is 0.4～1.6m and the width of the fault deformation is about 50m on the third level terrace.And on the mid Pleistocene platform,the height of scarps is 1.5～5.0m and the width of the deformation is about 90m.After comparing the profile of strong topographic deformation zone with the trench section,we primarily recognize that the ratio of hanging wall to foot wall deformation width is 2: 1approximately.The widths of strong surface deformation belt on the mid Pleistocene platform and the third level terrace on the two walls are 60m,30m and 33m,17m,respectively.

As the most important fault of Late Pleistocene in the Lhasa area,the Nalinlaka Fault is a left-lateral thrust fault,striking NWW,dipping SSW with a high dip angle,and extending over 33km.According to the studies on the latest strata on the Nalinlaka Fault zone,this fault zone has been obviously active since Late Pleistocene and the movement left behind some geomorphologic phenomena on the earth's surface,especially at the sites of the gully west of Cijiaolin and around Xiecun village.For example,some rivers,ridges and terraces are dislocated,forming beheaded gullies,fault escarps and so on.The horizontal displacements since Late Pleistocene at the above two places are 54～87m and 20～67m,respectively.Based on the studies on the 4 trenches along the fault using progressive constraining method,we conclude that there might have occurred 5 paleoearthquake events along the Nalinlaka Fault since 70ka BP,the ages of each paleoearthquake are 8.53,54.40,<41.23,21.96,and 9.86 ka BP,and the average recurrence interval is 14.67ka.Because of the limits of trenches and earthquake events exposed by each trench,no single trench revealed completely all the 5 events.So,there may be some errors in determining the upper and lower limits of some events in this article.

The Wanyaogou Fault dislocates the Jurassic sandstone.The tilted bedrock becomes a natural barrier to the groundwater,and a water-rich stratum formed in the footwall which caused obvious resistance difference between the two walls of the fault.The electrical imaging is an effective way to detect the fault on this condition.The experimental electrical resistivity tomography survey was conducted to detect the Wanyaogao Fault.The results of 2-D resistivity inversion indicate that the electrical structures on both sides of the fault present obvious difference,the resistivity of the hanging wall is high,while that of the foot wall is low.And the interface of the high and low resistance regions inclines to the low resistance region.The electrical resistivity tomography survey was also conducted to detect other faults in Urmuqi which have similar tectonic characteristics with Wanyaogou Fault.And the electrical structures appear the similar abnormality.So the abnormal characteristic is an important indicator and basis for identification of fault in the Urumqi region.The faults in Urmuqi are almost all high-obliquity reverse faults.After comparing the electrical imaging with the trench section,we find the fault is not coincident with the borderline between the high and low resistance,but lies in the high resistance region.The fault inclination is reverse to the gradient direction of isolines.The fault location is near to the inflexion of the upper isolines.

The predominant results of the project "Active fault detecting and seismic risk assessment in Urumqi City" are introduced in this paper. The active faults detecting target region covers the Urumqi City and its urban planning area. Deep seismogenic tectonics of the faults was detected systematically during this project. The seismic risk and damage of the active faults were estimated preliminarily. Two groups of Holocene active faults,namely,Wangjiagou Fault group and Jiujiawan Fault group,were identified in the target region. The Wangjiagou Fault group is composed of four north-dipping thrust faults,while the Jiujiawan Fault group is composed of four north-dipping normal faults. Four late Pleistocene active faults,namely,Bagang-Shihua buried fault,Xishan Fault,Wanyaogou Fault and Bayangnangou Fault,were detected in the target region. A clear image of the thin-skinned thrust system under the city was made by deep seismic profile carried out by this project. Combined with the observation of mobile seismic station array and precise location of micro quakes in the target region,as well as other deep structure detecting results,such as the petroleum seismic profile,two seismic models of the target region were established. The seismic risk of the Holocene active faults and late Pleistocene active fault were estimated by paleo-earthquake trenching and seismicity analysis along the Holocene active faults,as well as the microstructure study along the late Pleistocene active faults. Meanwhile,the strong ground motion generated by potential strong earthquakes directly under the target region was predicted by numerical modeling. The surface displacement and deformation zones of the future strong earthquake along the Holocene and late Pleistocene active faults were estimated.

In recent years,the project of active fault prospecting in major cities has been widely carried out in China. However,active faults are usually developed in soft sediments,and sometimes do not leave any macroscopically observable trace. Therefore,even though fault-crossing trench is excavated,the key problems concerning the upward or downward propagation and the property of these obscured faults,as well as the mode of faulting and the time of the last faulting event can not be solved by macroscopic observation alone. In this paper,a microscopic observation method for resolving these problems is introduced through a case study of the active fault prospecting in Urumqi,Xinjiang Uygur Autonomous Region. Macroscopic observations have been carried out on 13 trenches and natural outcrops,and 53 samples have been systematically collected. Detailed micro-structural investigations have been carried out on 106 thin sections cut from the samples,while the grain-size analysis,particle size distribution(PSD)analysis and the statistic measurement of the angularity of the clastic grains from the samples have also been carried out. The problems concerning the mode of faulting,the occurrence of obscured fault,and whether the fault has cut through the overlying strata are resolved through the discovery of microscopic appearances of the faults,the finding of microstructures indicative of seismites,as well as the variation of angularity,grain-size and PSD of the clastic grains of the samples collected on and outside the fault. The results show that microscopic observation is an effective approach to the identification of obscured active faults in soft sediments,and it is widely applicable to the project of active fault prospecting in major cities currently carried out in China.

Xishan Fault group is distributed in the transition zone between the fold-reverse fault system along the front of the north Tianshan Mountains to the west and the thrust tectonics of Bogeda to the east.It is a tectonics that thrusts from the basin in the north to the mountains in the south,consisting of 4～5 faults that are more than 10 to about 30km long,showing low angles near surface and converging on the detachment surface at about 11km deep.We discovered that the activity of Xishan Fault group is distinct during late Quaternary by doing field investigation of geology and geomorphology,excavating trenches along faults and analyzing deep structure of the fault group.The faults offset the second and above terraces of Wanjiagou creek and created fault scarps of 0.5～5.4m high on the terraces.And traces of paleoearthquakes can be found easily.The younger two events on F₁,F₂ and F₃ are confined in(22.7±5.2)ka and 40ka BP by OSL samples dating,respectively and the traces of the youngest event on F₄ and the front fault of Xishan are covered by deposits whose ages of OSL samples are about(31.1±3.2)ka and(37.9±3.8)ka BP,respectively.It means that there was grouped faulting in late Quaternary in the Xishan Fault group.F₁,F₂ and F₃ or F₄ and the front fault of Xishan might rupture in a same event on near surface.Event traces on the Xishan Fault group and other reverse faults of low angle show that the deposits along the front of fault scarp,the offset relation between fault and deposit bed,and the abrupt increase and diminution of displacement on difference markers or unconformable surfaces on both sides of fault are important identification marks of paleoearthquakes along surface rupture-type reverse fault.The deposits along the front of fault scarp on reverse faults of low angle are much more different from those on normal fault.For ideal mode,the deposit in front of fault scarp of reverse fault of low angle is characterized with that the original structure of the collapsing thrust sheet front is not broken entirely on the lower part and the sloping deposit on the upper part may exist covering on both sides of the fault.We think that it is very important for reducing uncertainty of paleoearthquake identification to seek for evidences as many as possible and analyze the different influencing factors,such as tectonics,climate,environment and anthropic activities.

The Kuqa depression is located in the middle segment of the southern Tianshan Mountains. There are four E-W extending rows of reverse fault and anticline zones in the depression. From the south Tianshan Mountains towards the Tarim basin, they are the mountain piedmont, the Kasangtuokai, the Qiulitag and the Yaken reverse fault and fold zones. After a month field working, we find the crustal shortening of the Kuqa depression in late Quaternary is almost caused by the Kasangtuokai, the Qiulitag and the Yaken reverse fault and fold zones. The reverse fault and anticline zones in the Kuqa depression are very different in tectonic feature. We accurately surveyed these tectonics with total station and differential GPS in order to get a new cognition of the deformation characteristic and the slip rate. Based on the deformation characteristics of conceptual fault-propagation fold and field investigation, we think the deformation of the fault-propagation fold in the Kuqa depression is caused by faulting rather than folding. The crustal shortening rate caused by the fault is approximately near to the actual rate. So we only surveyed the deformation near the fault. The Kasangtuokai anticline is a fault-propagation fold. From late Quaternary, the deformation of Kasangtuokai anticline is mainly caused by total-uplift of the hanging wall. The deformation rate is about 1.0～2.0mm/a. The deformation feature of the Dongqiulitag anticline is similar to that of the Kasangtuokai, while the crustal shortening rate is little more than that of Kasangtuokai, about 2.5mm/a. The Qiulitag anticline is a very complicated tectonic. It is a fault-bend fold. There are two reverse faults on the core and the north limb of the Qiulitag anticline. Its tectonic deformation includes two parts: the fold rise and the uplift of the hanging wall of the fault. By surveying and dating, we get the crustal shortening rate of the Qiulitag anticline limb of about 1.06～2.0mm/a. Considering the shortening of the core fault and southern limb, the total rate is possibly more than 3.0mm/a. The Yaken anticline is a blind thrust fault-anticline fold. Its shortening rate is 1.5～2.0mm/a. So the total crustal shortening rate of the Kuqa depression is more than 5.0～7.0mm/a from late Quaternary.

The distribution, tectonic style and new displacement and other features of the main active tectonics in Kuqa depression in the front of southern Tianshan were introduced in this paper. This depression is an “eye-shaped” tectonics in plane. It is composed of two fold zones in the south and north respectively. The northern one close to the main southern Tianshan Range is a southward thrusting fault-folding system. The most recent active fold in this system is the Kasangtuokai fold belt. The southern one close to the Tarim Basin is a northward thrust fault-fold system. The recent active folds in this system are the Qiulitage fault-fold belt and other young folds in its south, such as the Yaken fold. These two folding systems embrace the Baicheng Basin which likes an eyeball in the eyelids. The Kasangtuokai Fault with a length of 60km in the north and the Qiulitag Fault with the length over 200km in the south are the most important active faults in Kuqa depression. The younger and smaller folds in the south of Qiulitag anticline belt indicate the southward propagation of the thrust fault in Kuqa depression. The petroleum seismic profiles show that the folding and faulting processes are controlled by the detachment fault between the sediment cover and the basement of the basin. The depth of the detachment fault is around 10km and possibly defines the main seismogenic zone in the depression area.

The M_S 6.8 Bachu-Jiashi earthquake of Feb.24, 2003 occurred in the western Tarim Basin and is possibly the continuation of the Jiashi strong earthquake swarms in 1997-1998. However, its focal mechanism and rupture process are different from that of the Jiashi strong earthquake swarms, according to our preliminary study on its seismic tectonics with geomorphologic information from satellite images, the deep structures from the petroleum seismic exploration, the macro damage and isoseismic features from field investigation, the relocation of the epicenters of the aftershocks, and the regional seismic tectonics from both deep and surface tectonics. The occurrence of the M_S 6.8 Bachu-Jiashi earthquake is closely related with the revealed reverse fault on the Maigaiti slope belt between the Bachu uplift and Kashi depression in western Tarim Basin. The sites of the ground fissures found in the field fit with the revealed reverse fault. Isoseismal features are also corresponding to the rupture direction of the fault. These evidences indicate that the M_S 6.8 Bachu-Jiashi earthquake is the result of the southward rupturing from deep to shallow along a north-dipping reverse fault in Tarim Basin. This reverse fault is possibly the result of the propagation of the thrust fault-fold system named Kalpintag thrust belt in the front of Tianshan.

Owing to strong and permanent Cenozoic re-orogenic processing, a lot of EW-striking active thrusts and folds have been developed in Tian Shan, resulting in crustal shortening in NS direction. There also exist NW-striking transform-like strike-slip faults that cut the Tian Shan and accommodate uneven crustal shortening larger in the west and smaller in the east. The seismogenic structures in and around the Tian Shan mainly include EW-striking thrust ramps or blind thrusts and NW-striking transform-like strike-slip faults. The 2003 AD Bachu-Jiashi earthquake is located at south of the Kalpintag nappe. A NE-trending deep seismic reflection profile about 50km long across the epicenter has been conducted after the earthquake. From this reflection profile four blind faults are identified. Together with earthquake relocation, these identified blind faults are used in the paper to interpret the seismogenic structures of the 1997 AD Jiashi strong earthquake swarm and 2003 AD Bachu-Jiashi earthquake. The 1997 AD Jiashi strong earthquakes were generated mainly by a NW-striking buried transform-like strike-slip fault, while the 2003 AD Bachu-jiashi earthquake by blind thrusts in front of the Kalpintag nappe.

The Bolokenu-Aqikekuduke Fault extends over 700km and obliquely cuts the North Tian Shan. The fault is quantitatively studied in this paper based on remote sensing data, field observation, and the analyses of the relationship between the morphology and the climate change. The fault is composed of two portions, the NW-striking western portion and the NWW-striking eastern portion. The western portion is right lateral strike-slip with a rate of 5mm/a, having a length of about 250km and extending northwestward into the Kazakhstan. Along the fault 3～4 rupture zones produced by Paleoearthquakes or historic earthquakes were found. It has the potential for generating strong earthquakes of Magnitude around 7.5. The NWW-striking fault of east portions is dominated by right lateral slip with a rate of 1～1.4mm/a. Relatively small rupture zones, no more than 10km long, were found along this fault. It has the potential for generating strong earthquake of magnitude around 7. The western section of the Bolokenu Aqikekuduke Fault is one of the major NW-striking right lateral strike-slip faults in central Asia, and it is the result of the regional clockwise shearing. It accommodates the different deformations between western and eastern Tianshan and transforms the tectonic deformation among different compressive area. The NWW-striking fault of eastern section was developed from the NWW-striking suture zone in Northern Tianshan. It is accompanied by the E-W-striking thrust system in the front of the mountains and makes up a typical strain partitioning style of the oblique compressive belt. The strain in the oblique compressive belt is partitioned into pediment thrust belt and intermontane strike-slip fault. They transform the deformation laterally along the mountains and directly toward the foreland basin.

The Tuotegongbaizi-Aerpaleike fault is located in the epicentral area of the 1902 Artux,Xinjiang earthquake of M S 81/4. It is a gently dipping reverse fault, merging downward into the decollement of South Tianshan Foreland Thrust belt at about 3km depth below the Earth surface. To the north of the epicentral area, lies the Muziduke thin-skinned arcuate nappe of the South Tianshan Foreland Thrust belt, while to the south lies the Mushi-Kashi-Artux arcuate recoil fold-thrust belt of northwest Tarim. The epicentral area is just located at the triangle zone of frontal structure of the juncture between these two major tectonic units. In the epicentral area a deep fault exists under the decollement of the thin-skinned nappe, extending downward to the Moho. It is located at the sharp gradient zone of the crust-mantle boundary between the South Tianshan and the Tarim basin. The projection of epicenter falls at a zone of abrupt change of crustal thickness. It seems that this great earthquake was the result of rapid faulting at the gradient zone of the crust-mantle boundary due to the action of N-S trending regional tectonic stress field. Because the focal depth is great, no earthquake faulting is observed on the surface. The deformation of the magistoseismic area is characterized mainly by physicogeologic phenomena such as collapse, landslides, and ground-fissures, accompanied by coseismic ruptures and folding of the active thin-skinned structures above the decollement.

This paper summarizes the basic characteristics of the arcuate structures in northeast Pamir, and analyzes the relationship between the E-W-trending thrust-anticline belts and the NNW-trending buried strike slip faults in northwest Tarim basin. In the northwest of Tarim Basin lie two thrust fault-anticline belts, which are called the Artush-Talanghe and the Kashi-Mingyaole anticline belts, respectively. Even though these two anticline belts are located far away from Pamir but closer to southwest Tianshan, their geometries and histories are similar to those of the external Pamir arcuate structures. By comparing the structures of the anticlines in Tarim basin with those of the southwest Tianshan and northeast Pamir, we find that these arcuate structures are propagating northeastward from Pamir toward the Tarim basin. The deformation in the area between the Pamir and southwest Tianshan is the result of the India-Eurasia collision. The northward indentation of the west Himalayan syntaxis has induced the development of the arcuate structures in Pamir and its fore deep depression basin. The stiff basement of the Tarim basin has also controlled the propagation of deformation. It transferred the tectonic force from Pamir to the area between its north margin and Tian Shan, resulting in intensive deformation of the area. The arcuate structures of the cover layers are thin-skinned structures resulted from crustal shortening of the Tarim basin. The focal depth and geophysical sounding data indicate that the basement of Tarim basin is under-thrusting beneath the south Pamir and south Tianshan. This movement has given rise to the propagation of deformation from the margin toward the inland of the basin. As the deformation is constrained by the orientation of tectonic force, the style of deformation in northwest Tarim basin is therefore similar to that in northeast Pamir. The NNW-trending strike slip faults in the northeast Pamir have also propagated eastward toward the Tarim basin. Most of the faults are buried, along which some strong earthquakes had occurred. It is more likely that these faults were developed in the basement of the Tarim basin. Along with the E-W-trending thrust faults, these strike slip faults have dissected the Tarim basin into several rhombic blocks, the north margins of which is dominated by compressive deformations while the east margin by dextral shearing deformations. Both the north and east margins of the block are seismogenic tectonics. The Jiashi strong earthquake swarms of 1997 and 1998 would be the result of the present-day movement of the arcuate structures in northwest Tarim basin.

The Jiali fault is one of the major faults in the southeastern Tibetan Plateau. A field investigation of this fault has been made recently. From Naqu to Jiali the fault extends roughly in N60癢 direction and consists of three segments arranged in an en echelon way. From Luoermano to Esukongma (about 40km) where the fault is the northern boundary of the Sangdi basin that extends north-south, late Quaternary surface ruptures have been found. Within this segment the creeks and gullies that cross the fault were offset and the displacements range from several meters to about 5km. The average slip rate during late Quaternary is about 15mm/a for this segment. An interesting phenomenon is that the large displacement can only be found at those places where the fault is related to the basins that extend north-south. Outside the basins, no convincing evidence has been found for late Quaternary surface ruptures and the average slip rate for the whole fault is only about 4mm/a during Quaternary. It seems that these strike-slip faults behave like a transform fault and the strike-slip motion along them are a consequence of east-west extension that creates the north-south graben systems rather than the vice versa.

The Boluokenu fault is the boundary between the north and middle Tianshan, and has many periods of activity. Microgeomorphology related to fault activity, gully right-lateral displacement, fault scarp, pull apart basin occur along the fault in the southeast of Jinghe county town to Alashankou. Large gullies on the alluvial-pluvial fan are cut by the Boluokenu fault, the largest right-lateral displacement is about 500m, and its average is 400m. Some small gullies which only developed on the fault scarp have been cut by the fault. Its right lateral displacement can be divided into four groups in displacement values, i.e. 2.4～4.0m, 5.7m, 8.3m and 15.3m. The height of the fault scarp can be divided into three groups of which the value is 1.2m, 2.51m and 3.63m, respectively. In the southwest of Alashankou, right-lateral displacement for small gullies is 1～1.7m. The heights of the fault scarp on deferent geomorphic surface are 0.55m and 1.2～1.5m. All these didplacement values may indicate that the horizontal and vertical displacement of characteristic earthquakes on the segment is 3m and 1.2m, respectively. The age of the alluvial-pluvial fan in southeast Jinghe is about 70 ka old by geomorphic dating, and the right-lateral slip rate is 4.7mm/a.

Along the southwest bank of the Second Songhuajiang river, from Xiwangjia to Hadashan, there are two patterns of stratigraphic variation. One is the faults and fractures of small scales of Quaternary. Another is the landslides of larger scales along the bank of the river. Detailed studies show that those stratigraphic alteration, faults and geofractures of Quaternary are not tectonism-genetic. They are mostly related to landslides. On the basis of analysing the geological characteristics, making use of the drilling, seismic reflection data and geophysical exploration results, it is suggested that the Second Songhuajiang fault had been inactive since Tertiary.

Seven samples of wind blow,glaciofluvial and fluvial sediments are collected from northern Tianshan Mountain Each sample is dated for fine grain using the the luminescence techinque to get three paleodose of TL,GLSL and IRSL Comparison of the paleodoses shows that they are in agreement for win blow samples,but different for the the samples that were not totally bleached before sedimented The paleodose of TL is larger than those of GLSl,IRSL

The displacement of all earthquakes with all ranges of magnitude except to the characteristic earthquakes was termed as“relative creep”in this paper. Affection of the relative creep to the recurrence interval of the characteristic earthquakes is analysed. A function is deduced to calculate the relative creep rate. More reliable estimations of seismic interval can be made by comparing the calculated results with the paleoearthquake data.

Along the middle segment of the Xiaojinag fault zone 21 basins of different scales have developed since Cenozoic. They have 4 different stages of evolution,i.e. the Eocene Oligocene,the Pliocene-early Pleistocene,the middle Pleistocene and the late Pleistocene Holocene stages. Among them,the Pliocene-early Pleistocene stage was the period of great prosperity of the basins. According to the duration time of the development of the basins,they can be classified into inherited basin,staged basin,rejuvenated basin and new born basin. The basins can be assigned to depressed and non-depressed origin. The Eocene Oligocene stage was dominated by non-depressed basins,the Pliocene-early Pleistocene stage was dominated by both depressed and non-depressed basins,while the stage after the middle Pleistocene was dominated by depressed basins. The depressed basins can further be divided into thrust type,graben-semigraben type and pull-apart type basins. Based on the analysis of the formation mechanism,plane distribtuion and the type of deposition of the basins,the authors have discussed the characteristics of the regional crustal movement in the different periods of Cenozoic along the southeastern margin of Qinghai Tibet Plateau and in Sichuan-Yunnan region.

The middle section of the Xiaojiang fault zone located in Yunnan Province is branched into east and west two faults. Between these faults there are many NE trending smaller faults and blocks cuted by nearly SN and EW trending faults. They make up the network shape structure with rectangular geometry of the middle section of the Xiaojiang fault zone. These NE trending faults inherited those of Tertiay and were innovated in Quartary with left-lateral slip. Some of them have been active till Holocene. Their activities belong to the integrated left-lateral slip motion of the Xiaojiang fault zone with much smaller magnitude and rate comparing with the main faults. Because of their motion the main faults have bending or step areas which are barries for stress and strain concentration. The blocks between the east and west main faults are cut by the NE trending faults into many secondary blocks of diamond and shuttle shape. Relative movements occur between these blocks which have important effect on the segmentation of faults as well as earthquake occurrence.