An M_S6.4 earthquake occurred in Yangbi county, Dali Prefecture, Yunnan on May 21, 2021. It is the biggest earthquake in the region during past 40 years, and its epicenter is located in the southwest boundary of the Sichuan-Yunnan rhomboid block. The type of this earthquake is of a typical “fore-main-residual” type, and cause no surface rupture, its aftershock sequence was not distributed along any known fault in the vicinity. There have been several research results which are on the seismogenic structure of this earthquake that occurred in Yangbi county, but it is also necessary to use a different type and source of data, methods and perspectives thinking angles to verify these results and supply new understandings. In this paper, based on the Yangbi sequence(M_L≥2.0)digital waveform recording and its earthquake phase data recorded by Yunnan Seismic Network between May 18, 2021 and June 13, 2021, the Yangbi sequence is relocated by HypoDD double-difference method and the spatiotemporal Yangbi sequence is also analyzed. The focal mechanism solution and centroid depth of the larger earthquakes in the sequence is obtained by the Cut & Paste(CAP)method. The results indicate that the Yangbi earthquake is distributed along the NW-SE direction as a whole, and its extension length is about 34km. The foreshock sequence has an obvious spatiotemporal migration and has round-trip activity characteristics, while the aftershock sequence has irregular spatiotemporal migration characteristics. The depth range of the aftershocks is mainly between 4km and 13km, and there were a few aftershocks whose depth are below 4km, which is reflecting that this series of earthquakes occurred in the shallow layer of the upper crust, and the rupture of the main earthquake may not extend to the surface. The trend of the belt of the aftershock is generally from the direction NW to SE, which has the obvious spatial segmentation: the aftershocks, which are located in the northwest of the main earthquake epicenter, are rare and relatively concentrated, while the aftershocks, which are located in the southeast, are dense and the width of the aftershock zone becomes larger; The foreshock sequence occurred in the southeast side of the epicenter of the main earthquake, which basically overlapped with the location of the dense segment of aftershocks, indicating that the sparse aftershocks in the northwest side of the main earthquake should belong to the triggering type, while the main earthquake rupture may belong to the unilateral rupture type extending from the epicenter to the SE direction. Besides, its fracture length is about 37km and its downdip width is about 16km. The depth cross-section of the foreshock sequence indicates that the focal depth of the sequence earthquake is generally deep in the southwest and shallow in the northeast, and the fault rupture surface is inclined to SW, with a large dip angle. While the depth cross-section of the aftershock zone shows that the main earthquake rupture is obviously segmented: the NW segment of the sequence has a simple structure, which is there existed one earthquake cluster, while the SE segment is relatively complex, which is there probably composed of two high-dip faults with SW inclination. The centroid depth of the 29 M_S≥3.0 events in the Yangbi sequence, mainly range from 3km to 13km, and their focal mechanism solutions are mostly of right-handed strike-slip type with a nodal plane of high dip Angle in NW-SE direction, and possess a certain normal fault component. In the NW segment of the sequence, the focal properties are mainly dextral strike-slip, and a few earthquakes which have positive fault components shows that there is a NW trending earthquake cluster with a SW inclination. Although the SE segment is still dominated by strike-slip faults, there are more positive faults, of which are two NW trending faults with the SW inclination. This difference reflects that the SE segment is likely to bifurcate and develop into two faults. The main shock is a right-handed strike-slip rupture, the source parameters of fault plane Ⅰ are strike 139°, dip 78° and slip angle -¹64°, and the source parameters of fault plane Ⅱ are strike 45°, dip 74°, and slip angle -12°. The centroid depth of this main shock is 5.2km, which is close to the predominant focal depth of 8.9km obtained by repositioning, indicating that the earthquake occurred in the upper crust, and the depth of seismic activity in the earthquake area is shallow. According to the spatial and temporal distribution characteristics of relocated sequence, combined with the focal mechanism solutions of theYangbi series in Yunnan in May 2021, it is indicated that both the Yangbi earthquake sequence and the source fault plane Ⅰ of main shock are NW-SE trending, which is in good agreement with the middle section of the Weixi-Qiaohou-Weishan fault(the closest to the epicentre). In addition, the focal mechanism solution of the sequence earthquakes is consistent with the properties of the Weixi-Qiaohou-Weishan fault, both of which are right-lateral strike-slip type. We conclude that the seismogenic structure of the Yangbi earthquake may be correlated with the Weixi-Qiaohou-Weishan fault, but the epicentre distribution of the sequence earthquakes is different from that of the Weixi-Qiaohou-Weishan fault. It is confirmed that in this fault, the seismogenic structure of this earthquake is a right-lateral strike-slip secondary fault with a steep dip toward SW on the west side of the southern section. Besides, in this fault, there is another NW trending branch fault in the SE section. In addition, combined with the results of the existing regional tectonic stress field in the focal area, it is believed that the earthquake should be caused by a right-handed strike-slip activity in the focal area which is under the force of NNW-SSE direction.

Coseismic surface rupture length is one of the critical parameters for estimating the moment magnitude based on the empirical relationships and later used in assessing the potential seismic risk of a region. On 22 May 2021, the M_W7.4 Madoi earthquake occurred in the northeastern part of the Tibetan plateau(Madoi County in Qinghai Province, China)and ruptured the poorly known Jiangcuo Fault along the extension line of the southeastern branch of the Kunlun Fault. We began our data acquisition using aerial photogrammetry by UAV three days after the earthquake. Between 24 May and 15 June 2021, more than 40000 high-resolution low-altitude aerial photos were acquired covering a total length of 180km along the surface rupture. Based on detailed field investigations, combined with a fine interpretation of sUAV-derived orthophotos and high-resolution DEMs, we determined a total length of~158km of the coseismic surface rupture extending to the eastern end at 99.270°E, which is basically consistent with the position given by previous geophysical methods. Although the extending segment is located beyond the end of the continuous surface rupture trace near Xuema Township, it should be included in the calculation of the length of the surface rupture as part of the tectonic surface rupture. The surface rupture is segmented into four sections, named from west to east: the Eling Lake, Yematan, Yellow River, Jiangcuo branch sections. Additionally, to the east of Dongcaoa’long Lake, we mapped semi-circular arc-shaped continuous tension-shear fractures in the dune area with a short gap(~3km)connecting to the east of the Jiangcuo branch. The surface ruptures along the southeastern Youyunxiang segment also sporadically appear in several sites, locally relatively continuous, covered by the sand dune with vertical displacements of up to 30cm. After passing through the dunes, the surface rupture of the Youyunxiang segment began to spread widely, extending continuously with a strike of nearly east-west. However, it should be noted that the rupture lengths of the Youyunxiang segment and other branches are not counted in the total earthquake rupture length. By comparing the current research results, we believe that the critical factors causing the significant differences of the measured length of coseismic surface ruptures would depend on: 1)more extensive and detailed field investigations combined with a fine interpretation of high-resolution images; 2)avoidance of repeated calculation of superimposed sections on both sides of the complex geometrical area. In this study, combined with the fine interpretation of high-precision image data, many surface rupture traces in the dunes of the Youyunxiang segment were identified(verified and confirmed by field inspection)and more continuous surface rupture segments on the F1 fault, which is difficult to reach by human beings, were discovered, also highlights the important role of digital photogrammetry in the study of active tectonics. The studies of the strong historical earthquakes around the Bayan Har block show that the coseismic surface rupture length is larger than that estimated by the empirical relationships. Further research thus is highly necessary to uncover its mechanism and indicative significance.

Detailed mapping of coseismic surface rupture can provide valuable information for understanding the geometrical complexities, dynamic rupture processes and fault mechanisms. Fault geometrical complexities, such as bends, branches, and stepovers are common in strike-slip fault systems and can control the coseismic surface rupture characteristics to a certain extent. Observational studies of surface ruptures in past earthquakes suggested that special rupture characteristics would form distributed ruptures and rupture gaps. The detailed mapping has become an important way to study the surface rupture. According to the China Earthquake Networks Center(CENC), the M_W7.4 earthquake occurred at 2:04 PM on May 22, 2021, in Madoi County, Qinghai Province. The epicenter is about 70km south of the eastern Kunlun Fault on the northern boundary of the Bayan Kera block. It is the largest earthquake that hit the Chinese mainland since the Wenchuan M_S8.0 earthquake in 2008. After field investigation and rupture mapping on the computer, Yao et al.(2022)estimated that the length of surface rupture of this earthquake is 158km. Surface ruptures of the M_W7.4 Madoi earthquake broke through the geometric discontinuities such as step-overs and bends, and formed various coseismic surface fractures, especially in the middle segment. In the survey of the Madoi earthquake, we rapidly acquired aerial image data using UAV aerial photogrammetry and obtained high-resolution digital orthograph models(DOMs)and digital elevation models(DEMs)using PhotoScan software based on the SfM algorithm processing. Those data provide an opportunity for detailed mapping of seismic rupture and also provide an important reference for fieldwork. Based on high-resolution topographic data, we carried out detailed surface rupture mapping, classification, geometric structure and strike analysis for the ~30km section of the epicenter segment. At the same time, we conducted field work to supplement and proofread the maps.
According to the characteristics of surface ruptures in the epicenter area, we divided the ruptures into six segments. The surface ruptures along segment S₁ and segment S₆ are concentrated near the main fault, while the surface ruptures in the stepover(segment S₃, S₄, and S₅)are distributed dispersively, and the secondary ruptures along the segment S₂ are also distributed scatteredly, with the main rupture missing. To reveal the distribution characteristics of surface fractures more clearly, the surface ruptures are divided into the main rupture, secondary rupture, surface rupture and collapse rupture, among which the genesis of the surface rupture is uncertain. There are a lot of typical tensile ruptures with left-lateral component in segment S₁, the strike of the ruptures is consistent with the strike of the main fault or intersects the main fault with a small angle. The maximum width of the main rupture in segment S₁ is ~50m. The main ruptures in segment S₆ are developed along with the preexisting tectonic topography and the offset of the left-lateral displaced gully is up to tens of meters in magnitude. The surface ruptures are distributed in an echelon pattern, and all intersected with the strike of the main fault at a large angle. The location and size of the step-over are determined according to the topography and rupture morphology of faults in segment S₁ and segment S₆. The surface ruptures on the floodplain and banks of the Yellow River are in various forms and difficult to classify accurately. Therefore, only the typical seismic ruptures developed along the accumulated tectonic topography are labeled as main ruptures, and other typical seismic ruptures inconsistent with the location of the main fault are labeled as secondary ruptures. The typically collapse ruptures distributed along the river bank or lake bank are labeled as collapse ruptures, while the rest are labeled as surface ruptures. Surface ruptures in segment S₃ are distributed on the planar graph, but they have a dominant strike in the NE direction that can be seen from the diagram map. In the floodplain of the Yellow River, there are typical “grid” cracks, “explosive” cracks, and tensile cracks, and many cracks are accompanied by sand liquefaction which is beadlike, single, and distributed along the cracks. After the earthquake, the geodesic and geophysical data obtained quickly from the InSAR co-seismic deformation map and precise positioning of aftershocks revealed the basic morphological characteristics of earthquake rupture and provided valuable information such as earthquake rupture length, which provided an important reference for the design of UAV aerial photography and fieldwork. Compared with the rupture trace in field investigation by Pan et al.(2021), the surface rupture coverage obtained by mapping based on UAV aerial photogrammetry technology in this study is more extensive and accurate.
In general, surface ruptures of the Madoi earthquake are widely distributed, and we have classified those ruptures into the main seismic ruptures, secondary seismic ruptures, collapse cracks, and other surface ruptures. In addition to the seismic rupture with the same strike, there are also a variety of distributed surface ruptures with different strikes from the main fault. In these distributed surface ruptures, there are also many surface ruptures whose cause is not clear and they may be affected by tectonics or strong quake. For example, the “grid” and “explosive” surface ruptures on the Yellow River floodplain may be related to the strong quake near the epicenter or may also be related to the three-dimensional dynamic ruptures process in the initial stage. In this study, the characteristics of earthquake surface rupture in the step-over and adjacent sections near the epicenter has been described in detail, which provides a deeper understanding of the distributed coseismic surface rupture in the strike-slip fault.

The May 21, 2021, Maduo M_S7.4 earthquake in Qinghai Province caused serious disasters in Maduo County and its surrounding areas. The GNSS co-seismic displacement field data can play a key role in quickly determining the influence range of the earthquake and serving for the rapid investigation. After the earthquake, we immediately collected the data of 18 GNSS stations surrounding the epicenter, including 7 stations that recorded 1Hz high-frequency observation data. Various data were used to rapidly obtain the GNSS co-seismic displacements, such as, the 15-minute high-frequency data, 5 hours after earthquake and multi-day displacement data. In this paper, we used three methods to obtain the co-seismic displacement, including the dynamic difference method for 1Hz frequency data by GAMIT/GLOBK Track module, and the static difference method for the post-seismic 5-hour data and for the pre- and post-seismic multi-day data by GAMIT/GLOBK. The results are shown as follows:
(1)The dynamic difference method for 1Hz frequency data by GAMIT/GLOBK Track module has ability to quickly process the data and acquire the co-seismic displacement. When using the high-frequency data to obtain co-seismic displacement by Track module, it is suitable for the near field stations which have a large value of co-seismic deformation. However, in the far field, the accuracy of the solution is at cm level restricted by the distance of stations. In addition, the result of the Track is influenced by the stability of reference station. Although the results obtained by Track are not accurate, it can be used as a method to quickly judge the characteristics and amount of coseismic surface motion.
(2)Comparing the results obtained from the post-seismic 5-hour data and the pre- and post-seismic multi-day data, the GNSS stations’ displacements have good consistency in the magnitude, direction and influence range, especially in the near field. The difference of the results by the two methods is from 1mm to 4mm. Considering the processing accuracy of the GAMIT/GLOBK, the value of the difference is not unreasonably high. When the displacement value is small, it is difficult to obtain accurate results. In addition, the direction of the pre- and post-seismic multi-day result is consistent with that from the post-seismic 5-hour data, and the value increased. If we regard the result of the pre- and post-seismic multi-day data as the result of one day data after the earthquake which is included in the post-seismic displacement, this phenomenon coincides with the afterslip deformation, and the difference may be caused by the afterslip, especially in the near field. Although the difference exists, taking into account the timeliness and the overall consistency, we believe that using the postseismic 5-hour data to quickly obtain the co-seismic displacement is credible in an emergency.
(3)Based on the analysis of various results, it is preliminarily judged that the Maduo earthquake is dominated by left-handed strike-slip. The maximum displacement at the station QHMD, which is about 40km from the epicenter, is about 24cm to the west and 8cm to the north. The earthquake affected the area around epicenter including Maduo, Xining, Dulan, Delingha in the north, and Zebra and Ganzi areas in the south. From the comparison of the results of the static difference method for the 5 hours and multi-day data, it is believed that the post-seismic deformation taking place in the near field is significant, and continuous attention is required in the later stages.

On January 19, 2020, an M_S6.4 earthquake occurred in Jiashi county. This earthquake located in the intersection of the three tectonic systems of South Tianshan, Tarim Basin and West Kunlun-Pamir. From 1997~2003 a group of strong earthquake swarms with M_S≥6.0 occurred in this area, which constitute an extremely rare Jiashi strong earthquake swarm in mainland China. Based on the digital waveforms of Xinjiang Seismic Network, the best double-couple focal mechanisms of the main shock, foreshock and some aftershocks with M_S≥3.6 were determined by CAP method, the Jiashi M_S6.4 earthquake sequence was relocated by multi-step locating method. We analyzed the characteristics of focal depth, focal mechanisms and source rupture to determine the seismogenic structure. The nodal plane parameters of the best double-couple focal mechanism by CAP method are: strike 190°, dip 32° and rake 31° for nodal plane Ⅰ, and strike 74°, dip 73° and rake 118° for nodal plane Ⅱ; the centroid depth is 12.1km. The focal mechanism of main shock is thrust type, but the M_S5.4 foreshock is a strike-slip event with a focal depth of 17.1km, and the focal mechanism parameters are: strike 83°, dip 78°, rake 173° for nodal plane I and strike 174°, dip 83°, rake 12° for nodal plane Ⅱ. The foreshock and mainshock are very close in space, but the rupture types are quite different, which shows the complexity of the seismogenic structure. The relocated sequence shows two dominant distribution directions, namely, the near EW direction and the near SN direction. Most of the aftershocks in the sequence are distributed in the EW direction, parallel to the strike of the Kepingtage nappe structure. The M_S5.4 foreshock and the M_S6.4 mainshock are both located near the dominant distribution in the near NS direction, and have a certain spatial distance from the distribution of aftershocks in the near EW direction. This feature may reflect that the mainshock and subsequent aftershocks are located on different fault zones. Combined with the geological structural background near the source area, it is inferred that the seismogenic structure of the M_S5.4 foreshock is a strike-slip fault L₀ with a high dip angle in NNW direction, and the basic information of the seismogenic fault L₀ may be: strike NNW(about 175°), the fault plane is nearly upright, and the fault depth can reach about 15km. L₀ may be a branch fault of the NNW-directed seismogenic structural system of the Jiashi earthquake swarm from 1997 to 1998. Since most of the aftershocks distributed on the east side of the Fault L₀, we judge that L₀ and related faults may have a certain control effect on the distribution of aftershocks. According to the location of the main shock, the spatial distribution of aftershocks and the occurrence characteristics of the fault in the source area, it is inferred that the seismogenic structure of the Jiashi M_S6.4 mainshock is a NS-directed gentle-dipping fracture. The main shock caused the simultaneous activity of the Kepingtage nappe structure, resulting in a dense distribution of aftershocks with a certain distance from itself.

Late Cenozoic and modern tectonic deformation in mainland China is mainly characterized by active block movement, and the average slip rate of faults in the fault zone at the block boundary is an important indicator for quantitatively measuring the intensity of fault activity. The Tianshan Mountains, as the largest revival orogenic belt within Eurasia, with crustal movement basically manifesting as near north-south deformation and a large number of strong seismic surface ruptures, is one of the regions with strong tectonic movement and one of the key seismic hazard zones in China. Many experts have conducted relevant studies on the Tianshan region using GPS technology and have obtained some useful conclusions. These studies have not divided and analyzed the fault zone in detail, but only divided the Tianshan seismic zone into several major fault zones, such as the eastern and western sections of the northern Tianshan, and the eastern and western sections of the southern Tianshan. In order to analyze the activity characteristics of the major faults in the Tianshan region more clearly, this paper refines the major faults and selects 14 major active faults in combination with the distribution of active faults in China proposed by Xu Xi-wei et al. 18 blocks are divided into secondary blocks in Tianshan region, with the major active blocks in the Tianshan region taken as the boundary; The GNSS data of the surrounding areas of 1999—2015 in the Tianshan seismic zone are collected in this paper and used to calculate the velocity field results, and the block locking depth and the slip rate of major faults are calculated using the elastic block model to quantify the seismogenic capacity of major faults. Because the fault closure will produce obvious elastic deformation gradient around the fault, the greater the depth of fault closure is, the greater the influence will be. The fault locking depth can be constrained by the method of GPS data fitting of this model, and the influence of fault locking depth is verified by the method of GPS minimum residual RMS in this paper. According to the optimal locking depth obtained in this paper, the velocity field in Tianshan area is simulated and calculated. The residual mean value of the velocity field simulated by the elastic block model is small, and the average velocity error in the east-west direction is 1.57mm/a, the average velocity error in the north-south direction is 1.72mm/a. At the same time, the slip rate of major faults is obtained. The results show that: the horizontal shortening of the whole Tianshan region is significant, which is consistent with the tectonic background of the region, and the shortening value in the southern Tianshan region is higher than that in the northern Tianshan region; the shortening tensile rate is significantly larger than the slip rate, which shows that the fault zone at basin mountain junction in the Xinjiang Tianshan region is dominated by backwash activity; the extrusion rate in the western section of the southern Tianshan fault zone is in a high value state, reaching(-6.3±1.9)mm/a, which is higher than that in the eastern part of the southern Tianshan; the extrusion rate in the western part of the northern Tianshan is also higher than that in the eastern part. All the strong earthquakes of magnitude 8 and more than 80% of the strong earthquakes of magnitude 7 and above in China occurred in the boundary zones of active blocks according to the historical records, the motion characteristics of the boundary zone of active blocks play an important role in controlling the generation and occurrence of earthquakes, and the seismicity of faults may be quantitatively calculated by the loss of seismic moment. In this paper, we collected a list of strong earthquakes of magnitude 6 and above in the Tianshan area since 1900, estimated the seismic moment release of the main faults in the Tianshan seismic zone based on the above list, and compared it with the calculated seismic moment accumulation to obtain the seismic moment loss of the corresponding fault. Among them, the maximum release of seismic moment of the Beiluntai Fault reached 8.69×10¹⁹N·m; due to the release of several moderate and strong earthquakes, the seismic moment of middle of Bo-A Fault and Keping Fault have not reached the deficit state at present, the surplus is -1.85×10¹⁹N·m and -3.06×10¹⁹N·m, respectively; The smallest area of earthquake release is the northern Tianshan mountain front fault, which is only 0.11×10¹⁹N·m, because there was only one earthquake with a magnitude of 6 in 1907, and the earthquake accumulation reached 11.53×10¹⁹N·m, generating an earthquake deficit of 11.42×10¹⁹N·m, which could produce a magnitude of 7.3 earthquake. The results show that front margins of the northern Tianshan Fault, the Maidan Fault, the north section of Ertix Fault and the west of Kashihe Fault have a large seismic moment loss and have the potential to generate earthquakes of magnitude 7 and above, while Beiluntai Fault and the middle section of the Keping Fault show a surplus state, and there is no possibility of a strong earthquake in a certain period of time in the future.

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.

Running across the urban areas of Changzhou, Wuxi and Suzhou, the NW-trending Su-Xi-Chang Fault is an important buried fault in Yangtze River Delta. In the respect of structural geomorphology, hilly landform is developed along the southwest side of the Su-Xi-Chang Fault, and a series of lakes and relatively low-lying depressions are developed on its northeast side, which is an important landform and neotectonic boundary line. The fault controlled the Jurassic and Cretaceous stratigraphic sedimentary and Cenozoic volcanic activities, and also has obvious control effects on the modern geomorphology and Quaternary stratigraphic distribution.
Su-Xi-Chang Fault is one of the target faults of the project "Urban active fault exploration and seismic risk assessment in Changzhou City" and "Urban active fault exploration and seismic risk assessment in Suzhou City". Hidden in the ground with thick cover layer, few researches have been done on this fault in the past. The study on the activity characteristics and the latest activity era of the Su-Xi-Chang Fault is of great significance for the prevention and reduction of earthquake disaster losses caused by the destructive earthquakes to the cities of Changzhou, Wuxi and Suzhou.
Based on shallow seismic exploration and drilling joint profiling method, Quaternary activities and distribution characteristics of the Su-Xi-Chang Fault are analyzed systematically. Shallow seismic exploration results show that the south branch of the Su-Xi-Chang Fault in Suzhou area is dominated by normal faulting, dipping to the north-east, with a dip angle of about 60° and a displacement of 3~5m on the bedrock surface. The north branch of the Su-Xi-Chang Fault in Changzhou area is dominated by normal faulting, dipping to the south, with a dip angle of about 55°~70° and a displacement of 4~12m on the bedrock surface. All breakpoints of Su-Xi-Chang Fault on the seismic exploration profiles show that only the bedrock surface was dislocated, not the interior strata of the Quaternary.
On the drilling joint profile in the Dongqiao site of Suzhou, the latest activity of the south branch of Su-Xi-Chang Fault is manifested as reverse faulting, with maximum displacement of 2.9m in the upper part of Lower Pleistocene, and the Middle Pleistocene has not been dislocated by the fault. The fault acts as normal fault in the Pre-Quaternary strata, with a displacement of 3.7m in the Neogene stratum. On the drilling joint profile in the Chaoyang Road site of Changzhou, the latest activity of the north branch of Su-Xi-Chang Fault is manifested as reverse faulting too, with maximum displacement of 2.8m in the bottom layer of the Middle Pleistocene. The fault acts as normal fault in the Pre-Quaternary strata, with a displacement of 10.2m in the bedrock surface.
Combining the above results, we conclude that the latest activity era of Su-Xi-Chang Fault is early Middle Pleistocene. The Su-Xi-Chang Fault was dominated by the sinistral normal faulting in the pre-Quaternary period, and turned into sinistral reverse faulting after the early Pleistocene, with displacement of about 3m in the Quaternary strata. The maximum magnitude of potential earthquake on the Su-Xi-Chang Fault is estimated to be 6.0.

The migrating and enriching of fault gas during dynamic load-unload process are important indexes to evaluate the stress state and tectonic activity of underground medium. The Hutubi underground gas storage provides a natural experiment site for the analysis of the relationship between the gas geochemistry and the stress-strain status. In this paper, the soil gas concentrations of Rn, CO₂, Hg and H₂ during the gas injection in the Hutubi underground gas storage were analyzed. The results show that the soil gas contents and changing trend are close to the background value in the non-reservoir area and fault zone, which may reveal the weak activity of the fault. Significantly higher concentrations of soil gas H₂ and Hg are observed in the gas storage area, where H₂ maximum reaches 5.551×10^-4 and Hg maximum reaches 53ng/m³. Moreover, the abnormal soil gas H₂ and Hg measurement locations are more consistent. The variation trends of soil gas Hg, H₂, Rn, and CO₂may be related to the different gas generation and response mechanisms. The concentrations of soil gas H₂ and Hg are sensitive to the variation of pressure and the development of cracks in the underground gas storage, and they can reveal gas injection's effect on fault activity. This study provides a new basis for analyzing the influence of gas injection and withdrawal in Hutubi underground gas storage on fault activity.

Based on the digital waveforms of Xinjiang Seismic Network, the Hutubi M_S6.2 earthquake sequence (M_L ≥ 1.0) was relocated precisely by HypoDD.The best double-couple focal mechanisms of the main shock and aftershocks of M_L ≥ 4.0 were determined by the CAP method. We analyzed the characteristics of spatial distribution, focal mechanisms and the seismogenic structure of earthquake sequence. The results show that the main shock is located at 43.775 9°N, 86.363 4°E; the depth of the initial rupture and centriod is about 15.388km and 17km. The earthquake sequence extends unilaterally along NWW direction with an extension length of about 15km and a depth ranging 5~15km. The characteristics of the depth profiles show that the seismogenic fault plane dips northward and the faulting is dominated by thrusting. The nodal planes parameters of the best double-couple focal mechanisms are:strike 292°, dip 62° and rake 80° for nodal plane I, and strike 132°, dip 30° and rake 108° for nodal plane Ⅱ, indicating that the main shock is of thrust faulting. The dip of nodal planeⅠis consistent with the dip of the depth profile, which is inferred to be the fault plane of seismogenic fault of this earthquake. According to the comprehensive analysis of the relocation results, the focal mechanism and geological structure in the source region, it is preliminarily inferred that the seismogenic structure of the Hutubi M_S6.2 earthquake may be a backthrust on the deeper concealed thrust slope at the south of Qigu anticline. The earthquake is a "folding" earthquake taking place under the stress field of Tianshan expanding towards the Junggar Basin.

The Pishan M_S6.5 earthquake occurred in the west Kunlun piedmont area. According to the surface deformation data obtained by the Pishan M_S6.5 earthquake emergency field investigation team, combined with the positioning accuracy of spatial distribution of aftershocks information, the focal mechanism solutions and deep oil profile data, we think the Pishan M_S6.5 earthquake is a typical thrust faulting event, and the seismogenic structure is the Pishan reverse fault-anticline, which did not produced obvious surface fault zone on the surface. In the vicinity of the core of the Pishan anticline, we found some tensional ground fissures whose strikes are all basically consistent with the anticline. We propose that the surface deformation is caused by the folding and uplift of the anticline. The Pishan earthquake is a typical folding earthquake. The tectonic deformation of the west Kunlun piedmont is dominated by the thickening and shortening of the upper crust which is the typical thin-skinned nappe tectonic. The Pishan earthquake occurred in the frontal tectonic belt, the root fault of the nappe structure has not been broken, and we should pay attention to the seismic risk of the Tekilik Fault.

An M_L4.7 earthquake occurred on February 2,2012 in Liaoning Gaizhou (40.56°N,122.36°E),since then,small earthquakes are frequent in this area,and until now the seismic activity does not stop,several earthquakes with magnitude larger than 4.0 have occurred.As of October 30,2014,1223 earthquakes have happened in the Gaizhou area,including 934 earthquakes with the magnitude M_L1.0~1.9,247 with the magnitude M_L2.0~2.9 and 45 with the magnitude M_L3.0~3.9.Meanwhile,earthquakes are continuously active in Haicheng area where the M_S7.3 earthquake happened in 1975,and there are over 1100 earthquakes (M_L ≥ 1.0) having occurred since the Gaizhou earthquake swarm activity.Because the polarization direction of the fast shear wave is very sensitive to the variation of the principal stress environment,the shear wave splitting parameter can reflect the regional stress state and the local structural features,especially effective for the analysis of small-scale stress environment characteristics.So based on the seismic activities of the two earthquake clusters,this study analyzes the characteristics of shear-wave splitting in Gaizhou-Haicheng area.Preliminary results show that predominant polarization direction of fast shear-waves in the old earthquake region of Haicheng is stable,consistent with the direction of regional stress field.Due to presence of active fault below the Gaixian station (GAX),the predominant polarization direction of fast shear-waves is more complicated.There are two predominant polarizations,consistent respectively with Jinzhou Fault strike which is below the station and the maximum principal stress direction in this area.In addition,Gaizhou earthquake swarm activity increased after December 22,2013,and after the time node,the predominant polarization direction of fast shear-waves in Gaixian station is SEE,which is close to the predominant polarization direction of fast shear-waves in Yingkou station,at the same time consistent with the maximum principal stress direction of this region.Thus it can be inferred,the enhanced activity of Gaizhou earthquake swarm since December 22,2014 may be related to local enhancement of regional stress.In addition,the average time-delays of slow waves in station YKO and GAX show that there are no obvious changes before and after the time point of December 22,2013,which is different greatly with the previous related researches on the variation of slow wave time-delays,and there is no possibility that the Gaizhou earthquake swarm evolved into foreshock sequences from current preliminary results.We should do more work to study the details of the time delay variation of shear wave splitting parameter.

On 16^th September 2013, an M5.1 earthquake occurred in Badong County, Hubei Province, which is the biggest one since the first water impounding in 2003 in the head region of the Three Gorges Reservoir area. The crustal velocity information is needed to determine the earthquake location and focal mechanism. By comparison, the 1-D velocity structure model from Zhao was adopted in this study. Double difference location method was applied to determine the precise locations of the M5.1 earthquake sequence. Relocation results show that the dominant distribution of this sequence is along NEE direction. In order to understand its seismogenic structure, focal depth profiles were made. Profile AA' was along the sequence distribution, and the earthquake sequence extended about 12km. Focal depth of mainshock is deeper than that of aftershocks, and earthquake rupture propagated laterally southwestward. The seismic profile BB' and CC' were perpendicular to profile AA', which represent the dip direction. Both profiles show that the focal depth becomes deeper toward southeast, and dip angle is about 50°. It means that the possible seismogenic fault strikes NEE and dips southeast. Focal mechanism could provide more information for judging the seismogenic structures. Many methods could obtain the focal mechanism, such as P-wave first motion method, CAP method, and some other moment tensor methods. In this paper, moment tensor inversion program made by Yagi Y is adopted. 12 regional seismic stations ranging from 100~400km are picked up, and before the inversion, we removed the mean and trend. The seismic waveforms were band pass filtered between 0.05 and 0.2Hz, and then integrated into displacement. Green's functions were calculated using the discrete wavenumber method developed by Kohketsu. The focal mechanism of the M5.1 mainshock manifests that the NEE-striking fault plane probably is the possible seismogenic fault, which is consistent with the analysis of focal depth profiles. The focal mechanisms of the M_L≥2.0 aftershocks are retrieved by P-wave first motion method, and the nodal plane I is in accordance with the earthquake sequence distribution and the fault plane of the mainshock. FMSI program was adopted to inverse the stress field in the earthquake area, and the results show that the earthquake sequence is under the control of the regional stress field. The earthquake sequence occurred on the stage of slow water unloading, and ETAS model was introduced to testify the influences of water level fluctuations on earthquakes. The results denote that the reservoir played a triggering role in the earthquake, however, the NEE-striking seismogenic fault is the controlling factor.

On 27^th and 30^th March 2014, an M4.2 and M4.5 earthquake sequence occurred in Zigui County, Hubei Province, and the earthquake sequence type is double seismic type. The two earthquake sequences occurred at the water unloading stage of the 175m trial impounding, and G-R relations showed the similar characteristics with that of the tectonic earthquakes. In order to verify the influences of dam reservoir on earthquake triggering, ETAS model was introduced, the results showed that the slow water level changes had little impact on the occurrence of earthquake. Double difference precision relocation results indicated that the two earthquake sequences occurred at the intersection part of a NE-striking fault and the NNW-striking Xiannvshan Fault, and the preferred direction of aftershock distribution was separately NE and NNW. Moment tensor inversion method and P wave initial motion method were used to determine the focal mechanisms of the two earthquakes, and the results indicated that the two earthquakes were controlled by the regional tectonic stress field and were of reverse-slip type. Comprehensive analysis showed that the M4.2 earthquake was caused by a small-scale fault striking NE with a big dip angle. From the hypocenter profile, it can be seen that the M4.2 earthquake sequence was restrained by an east-dip fault, and the M4.5 earthquake sequence was the product under the conjugate action of the NE-striking fault and the NNW-striking Xiannvshan fault.

Based on 306 focal mechanism solutions of M_S≥3.5 earthquakes from 2003 to 2014 in Tianshan seismic zone, we divide the Tianshan seismic zone into grids of certain size and use multiple focal mechanism solutions around each grid node to do the stress tensor inversion, and then calculate the temporal and spatial distribution of the focal mechanism consistency parameter for each grid node. On this basis the stress state of the Tianshan seismic zone is analyzed and the relationship between temporal and spatial distribution of focal mechanism consistency parameter and strong earthquakes is discussed. The result shows that the principal compressive stress of Tianshan is quite similar to the former research result; the P axis of the whole Tianshan is mainly in NS direction and in NNE and NNW in localized areas. There is a corresponding relationship between the temporal-spatial distribution of the focal mechanism consistency parameter and the strong earthquakes; strong earthquakes in the middle-east area of Tianshan Mountains often occur in the low value zones of focal mechanism consistency parameter or near the edge of the zones. Since the second half of 2011, the focal mechanism consistency parameter in the middle-east area of Tianshan Mountains presents a process from disorder to consistency, which corresponds to the group activity of strong earthquakes in Xinjiang in this period; the focal mechanism consistency parameter in the southern section of Tianshan also decreased before 2008 Wuqia M_S6.9 earthquake. In addition, we can see from the b-value image of Tianshan that earthquakes with magnitude larger than 5 occur mostly in the relatively low b value area.

Running diagonally across the urban area of Xuzhou, the Feihuanghe(the abandoned Yellow River)Fault starts from Jiahezhai in the northwest, extending southeastwards through Sushantou, Xuzhou City and Liangtang along the abandoned Yellow River till the north of Wangji Town of Suining County, striking NWW, dipping SW, with a total length of about 70 kilometers. It is a buried fault, crosscutting Xuzhou-arc structure. There are significant topographic features indicating the existence of the fault on the earth's surface, which are clearly displayed in remote sensing images. There have been no devastating earthquakes occurring along the fault since the recorded history.
Feihuanghe Fault is one of the target faults of the project "Urban active fault exploration and seismic risk assessment in Xuzhou City". Few researches have been done on this fault in the past. The previous analysis assumes that the fault is a sinistral transtensional fault with extensional faulting in the Xuzhou-Suzhou arcuate structure at first and transtensional faulting of the Neocathaysian system later.
Based on field geological survey, shallow seismic exploration and composite drilling section method, Quaternary activities of Feihuanghe Fault are analyzed. Shallow seismic exploration results show that the Feihuanghe Fault is composed of a NE-trending south branch and a SW-trending north branch, forming a graben structure with the width of 1～2km. All breakpoints of the Feihuanghe Fault on the seismic exploration profiles show that only the bedrock surface was dislocated, not the interior strata of the Quaternary. The composite drilling profiling results show that Feihuanghe Fault has dislocated the strata of Mid Pleistocene, but not the top surface of Mid Pleistocene. Furthermore, we discovered a secondary fault of Feihuanghe Fault exposed at Fengshan Hill, and its latest activity date is the mid period of Mid-Pleistocene inferred from the cementation degree of gouge, dating results and geomorphic features. Combining the above results, we conclude that Feihuanghe Fault is of sinistral strike-slip in the early stage, and extensional faulting since the Quaternary, and the latest activity date is the middle period of Mid Pleistocene. Controlled by the tectonic setting, the activities of the NW-trending faults in Xuzhou area are significantly weaker than that of the NW-trending fault in adjacent southwest Shandong.

After a large earthquake, more seismic activities are observed in the focal region and its adjacent area. The obvious increased earthquakes are called the aftershocks. Generally speaking, aftershock sequence gradually weakens and sometimes has ups and downs. The time when the aftershock activity begins to be confused with background seismic activity is known as the aftershock activity duration. Aftershock sequence is one of the enduring research fields in seismology. Aftershocks accord with two important statistical relationships, one is the G-R relationship describing the relation between the magnitude and frequency, the other is the modified Omori formula describing the characteristics of aftershock decay with time. On this basis, a number of studies from different angles explain the mechanism of aftershock activity. From the perspective of the medium heterogeneity, it is universally accepted that aftershock is a result of further rupture of residual asperities. From the perspective of stress, these models, e.g. rate-state dependence, subcritical crack growth, creep or afterslip and so on, think that the fault stress change caused by mainshock is the main cause for aftershock. But other researchers, by studying real aftershock observations, think that the fault stress change caused by mainshock is not the main cause or has very weak control over the aftershocks. Pore pressure diffusion caused by mainshock fault slip is also considered as an important incentive for aftershocks. There is a relationship between the frequency of aftershocks and pore pressure changes. Dry rock pressurized in physical experiment can produce acoustic emission sequence similar to mainshock-aftershock sequence type earthquake. Though fluid plays an important role in aftershock activities, it is not the essential element for aftershock. Overall, there is no single model which can fully explain the phenomenon of aftershock activity.
Assuming the rupture of the residual asperities inside the mainshock rupture plane randomly leads to the aftershocks, the size of the residual asperities conforms to fractal distribution, and the rupture or instability strength of the residual asperities accords with the lognormal distribution. Taking the postseismic stress relaxation as the mechanical load, the loading stress attenuates according to negative exponential law. Taking the Coulomb failure as the judgment criterion of the instability, combining the mechanical interactions among the residual asperities, the artificial aftershock sequence, including occurring time, location and magnitude, is simulated under different conditions. The agreement between output and the actual statistical characteristics of aftershock activities is detected by G-R relationship and modified Omori formula as a basis for further adjustments to the model parameters. On this basis, the influences of the medium viscosity properties on aftershock activities have been discussed.
The results show that viscosity coefficient of rheological properties of the lower part of the lithosphere has an important effect on the duration of aftershock activity. The viscosity coefficient of the lower part of the lithosphere controls the duration of the aftershock activity, the lower the viscosity coefficient, the sooner the stress relaxation of the lower lithosphere, and the faster the loading rate to the upper part of the lithosphere, the shorter the duration of the aftershock activity. On the contrary, the higher the viscosity coefficient, the slower the loading rate to the upper part of the lithosphere, and the longer the duration of the aftershock activity. This simulation conclusion is consistent with the observed result. The viscosity coefficient as one of the important lithosphere physical parameters controls the decay rate of aftershock activity. Under this model conditions, p value, the decay rate of modified Omori law, changes with the viscosity coefficients in a negative exponential function. The relationship that the viscosity coefficient is lower and the decay of aftershock sequence is faster provides a reference for the study of the main influence factors of aftershock decay. The relationship corresponds to the observation that the decay rate of the aftershock sequence shows a good positive correlation. The b value of the G-R relationship of aftershock sequence characterizes the ratio relationship of large to small earthquakes. The modeling studies suggest that the G-R relationship of the aftershock sequence is irrelevant with the viscosity coefficient, but mainly controlled by the size distribution of the residual asperities. In another word, it is mostly correlative to the heterogeneity of tectonics and medium.

On 13, April, 2010, a great earthquake of M_W7.0 occurred in Yushu County, Qinghai Province, which is another big one in China since 2008 Wenchuan earthquake. And with seismic wave data, InSAR data and field investigations, many researchers studied the focal mechanism and source rupture process of this earthquake and many valuable results were obtained. However, there are some arguments on the rupture velocity. Some think that this earthquake is a super-shear rupture event, and some insist on opposite opinion. In order to explore whether it is a super-shear rupture event or not, this study chooses the teleseismic wave data recorded by 33 seismic stations with epicentral distances between 30～90 degrees, good azimuth coverage and high signal-noise ratio to reexamine the rupture process using Yagi's program. By comparison of different given rupture velocities in the range of 2.5～5.5km/s, it is found that rupture velocity of 4.7km/s yields the smallest normalized misfit between the observed and synthetic waveforms. And the inversion result is more in accordance with field observation. The relationship between subfault dimension, rise time and rupture velocity is discussed, which shows that the rupture velocity is not so dependent on the two parameters. And by teleseisemic analyses using an envelope deconvolution method with an empirical Green's function, the location and timing of the high-frequency event also show a rupture velocity of 4.7 to 5.8km/s, which is apparently greater than the shear wave velocity in this region. By comprehensive analyses, it can be concluded that the super-shear rupture exists in this earthquake. According to our inversion result, the strike, dip, and rake angle of this earthquake separately is 300, 88 and 4. Beach ball shows the seismogenic fault is of strike-slip type, which is consistent with the Ganzi-Yushu Fault. And the rupture extended to the surface on the northwest and southeast segments of the Yushu Fault with the length of 19km and 31km. Due to the existence of pull-apart Longbao Basin, the central part where the epicenter is did not rupture. By comprehensive analysis, super shear rupture is one of the main reasons that caused serious damage to Yushu County.

Large amounts of seismic records from Zhejiang digital seismic network are collected,and by double difference seismic tomography,fine velocity structure and local heterogeneity of upper crust in Zhejiang Province and its surrounding areas are studied,and the relationship of the crustal velocity structure to the local active faults,geological and geographical structure is discussed. Research shows that there is a large-area low velocity zone developed in northern Zhejiang at depths between 3.5km and 6.5km,and also some low velocity zones distributed sporadically in southern Zhejiang,mostly around Shanxi Reservoir in Wenzhou. The research indicates that most earthquakes happened along active faults or active fault segments,and are concentrated in zones. These zones are almost all in the transitions between low to high velocity zones,and are significantly related to the regional topographic features. The correlation relation derived by this paper between earthquake,active fault,and geological and geographical structure will be of theoretical value and practical significance to the study of seismogenesis and the prediction of future strong earthquakes.

Based on field investigation in the reservoir head area of the Yangtze Three Gorges,combining with its seismogeological background and past research achievements,the reservoir head area is divided into 31 predictive units,and together with 8 impact factors,the possibility and magnitude of reservoir induced seismicity(RIS)are predicted using statistical forecasting model.The results show as follows:(1)it is quite possible that M_L=3.0～4.5 earthquakes will be triggered along the Jiuwanxi-Lukouzi Fault and Xiannvshan Fault in the reservoir area;(2)According to the analysis of earthquakes at home and abroad,the RIS takes place mainly in carbonatite and igneous rocks,and concentrates in karst developing segment,yet it is little possible that earthquake happens in clastic rock area.From the predictive results,it is possible to trigger M_L 4.5～6.0 earthquake in two places,the limestone area of southern Badong and the limestone area on the Gaoqiao Fault;(3)Around the Gaoqiao Fault,tectonic reservoir-induced earthquake is quite likely to occur.Geological investigation shows that the M_L 5.1 Longhuiguan earthquake in 1979 was possibly related to the Gaoqiao Fault which had a certain activity during the early period of reservoir impounding.The maximum magnitude of earthquake happening around the Gaoqiao Fault reaches to M3.3.The bigger earthquakes might be induced near the reservoir segment of Gaoqiao Fault along with the water storing to the design level.

The late Pleistocene aeolian loess distributes widely in the loess tableland area.It has obvious features and is directly related with faulting.By the observation,measurement and dating to three typical sections at Xiaobaopo,Qiaogou and Zhongdicun,this paper obtained the activity parameters of the Lintong-Chang an Fault since the late Pleistocene and the age stratigraphic sequence of the tablelands of Bailuyuan,Shaolingyuan and Henglingyuan.Research results show that the Bailuyuan tableland has experienced

Weihe Fault is an important buried fault in Weihe basin.The predecessors have investigated the location and activity of the fault from various points of view,but up to now,the level of researches on the precise location and activity for the fault is still very low.There are few strata profiles of late Pleistocene which are found to be offset by the fault zone.Especially,it is still unknown whether the Weihe Fault was active in Holocene and there were paleoseismic events occurring on it.It is indicated from exploratory trench excavated at Bili village in the west section of Weihe Fault that over the past 9110a,the Yaodian—Zhangjiawan segment of Weihe Fault zone has experienced a historical earthquake and 3 paleoearthquake events.The historical earthquake is manifested by soil liquefaction.According to the study on historical and cultural relics,stratigraphic chronology and seismogenic tectonics,we propose the occurrence time of the historical earthquake is between 1487 and 1568;the age of paleoseismic event I is(9110±90)a,but there is no answer for the age of event II and event Ⅲ.The coseismic vertical displacement of event I,II and Ⅲ is 0.5m,0.5m and 0.2m respectively.The exploratory trench excavation also indicates that the Yaodian-Zhangjiawan segment of the Weihe Fault is a Holocene active fault.

Lintong-Chang'an Fault is an important boundary fault between Lishan uplift and Zhouzhi-Huxian depression in Weihe basin.By the field survey to the natural gullies,the earth fetching areas,and the excavated slope and chasm for road foundation,we discovered 40 outcrops of the Lintong-Chang'an Fault.According to the measurements of dislocation of the various periods' paleosoil horizons,we get the Quaternary dislocation distribution of the fault,which shows that the fault dislocation in the middle segment is the biggest,so is its activity along this segment.

In order to investigate the present-day movement characteristics of Haiyuan active fault zone and Xiangshan-Tianjingshan active fault zone on the northeast margin of Tibetan Plateau in detail, we established a GPS profile across the fault zones. The profile, extending from Lanzhou, Gansu to Zhongwei, Ningxia, is composed of 12 stations and locally reinforced the existing regional GPS network of Crustal Movement Observation Network of China (CMONOC). These new stations, together with the existing GPS stations, constructed a spatially dense profile whose average interval of the stauions is ~22km. Considering that there were two continuous GPS stations of CMONOC (i.e. XNIN and YANG) around the region, we tried a “Flexible Observation Method” in GPS observations. The method allows non-synchronal observations for all the GPS observation teams and makes the observation schedule rather flexible. In data processing, we used the advanced strategy of “Precise Point Positioning” of GIPSY software. Our result shows that with the support of CMONOC, especially the continuously observed fiducial GPS network of CMONOC, we can use the “Flexible Observation Method” and “Precise Point Positioning” data processing strategy to effectively observe local GPS networks to monitor crustal deformation with satisfying accuracy.

The earthquake occurred in Jiezhou,Gansu Province on July 1,1879 (May 12,Guangxu 5,Qing Dynasty) is one of the largest earthquake events in Chinese history. The effect of this event spread over more than ten provinces. According to historical data,the magnitude of this earthquake has been estimated to be 8 or 71/2,and the macroscopic epicenter has been located at Mianshanheba (104.7E,33.2N),south of Wudu,Gansu Province,with type II precision of estimation (≤25km error). As the earthquake occurred in the areas of high mountain ridges difficult to be approached,it has not been studied in detail so far. It is one of the 3M=8 earthquakes in China that their relation to active faults had not been revealed so far. Recently,we have found that in satellite images a NNE-trending lineament can be clearly observed along the northern foot of the Shuikeng Mountains to the northeast of Wenxian County,Gansu Province. It extends eastward from Shifang Village to the northwest of Wenxian County,passing through Liangjiaba,Malian River,Fanjiaba,Miaobeihou,Guanjiagou,Jianzuishan Mountains,Shuikengya,northern slope of the Fangmashan Mountains,Songjia Mountains and Shenjiana'an,and then gradually becomes unclear after crossing through the Bailongjiang River and turning northward at 500m downstream from the Majiaba. The lineament has a total length of about 30km,among which about 21km can be identified to be surface fracture zone (from the bank of the Bailongjiang River to the top of the western hills of Guanjiagou). The lineament offsets left-laterally a series of ridges and gullies. In comparison with geologic data of this region,this lineament image coincides well with the NEE-trending Fanjiaba-Linjiang Fault. The interpretation of satellite images for the whole region and re-collation of historical records about this earthquake have confirmed that the Fanjiaba-Linjiang Fault bears a close relation to the southern Wudu earthquake,and is most probably the causative fault of this earthquake. This study,therefore,provides new insights into the following aspects,such as the occurrence and mode of motion of the causative fault,the length of surface ruptures,the amount and variation of horizontal displacement along the fault,the location of macroscopic epicenter,the probable magnitude of the event and the distribution of seismic hazards etc. All these results have led to the satisfactory solution of outstanding issue about the causative fault of the 1879 southern Wudu earthquake. They are also of great importance to seismic risk assessment in the border areas of Gansu,Qinghai and Sichuan Provinces as well as in southeast Gansu Province.

We have conducted an integrated survey and investigation on the Quaternary activity of the Liaocheng-Lankao buried fault. The used methods include geochemical exploration, shallow seismic exploration, drilling geological profile and neo-strata dating. The object is to determine the accurate location of the fault, dislocation amount of each time period since Quaternary and the offset age of the last time of dislocation. The results show that the dislocation of the fault extends upward to the depth 20m or so below the surface. This fault has been active in early Holocene time. The average slip rate of the fault is 0.12mm/a.

The Nujiang fault zone can be divided into two sections of SN and NE strike and is arc in plane. Its deformation is mainly dextral shear of Himalayan age which could be divided into transpression deformation in early stage and transtension deformation in late stage and has different characteristics in two sections. The transpression structure in early stage has developed mylonite zone and its plunge angle of lineation was different in two sections. In the section of NE strike,both doxtral shear and thrusting are quite clear. The transtension structure in late stage developed mainly in southern end of two sections and shaved progressive trend from south to north since Miocene epoch according to distribution characteristic of the oldest stratum in basin. The complexity of Nujiang fault zone in Himalayan age was caused by the inhomogeneity of deformation in time and space. These deformation features are described and analysed in detail in the paper.