The NW-trending Cangshan-Nishan(Cang-Ni) Fault, extending 170km, exhibits significant tectonic deformation and seismic activity in the late Quaternary, attracting considerable attention. This study investigates the relationship between residential building damage in Guanzhuang village, Baiyan town, Pingyi county, Shandong Province, and the activity of the Cang-Ni Fault. Based on field investigations, geomorphological measurements, fault activity identification, surface structure analysis of quartz fragments in fault gouge(SEM), particle size distribution analysis, InSAR time-series analysis, historical research on the 179BC M7 Qichu earthquake, and previous earthquake damage studies, the following conclusions were drawn: 1)Severe structural damage with left-lateral displacement of 0.5-2cm was observed in buildings along the main Cang-Ni Fault, with cracks originating at foundations and decreasing in scale upward. In contrast, buildings along secondary faults experienced relatively minor damage. Foundation cracks in the main fault zone were wide and numerous, propagating upwards to narrower wall fissures, indicative of structural failure. 2)Damaged buildings in Guanzhuang village align with the left-lateral strike-slip zone of the Cang-Ni Fault, coinciding with the epicenter of the 179BC M7 Qichu earthquake. This area represents the hub of late Quaternary fault activity, characterized by significant displacement and recent faulting strata. 3)SEM analysis of quartz fragments suggests that the Cang-Ni Fault's latest activity occurred in the early Holocene, with a combination of stick-slip and creep-slip mechanisms. Particle size analysis revealed a high proportion(36%-43%)of ultra-fine particles(<0.001mm)and fractal dimension values of 2.817-2.857, favoring fault creep. 4)InSAR data confirm ongoing sinistral strike-slip movement of the Cang-Ni Fault, indicating surface creep deformation. 5)Damage to residential buildings is attributed to the sinistral strike-slip and reverse-thrust creep deformation of the fault, stratigraphic and structural differences across fault walls, uneven stress distribution, and swelling effects of fault gouge. This research provides a case study of building damage caused by active fault creep, emphasizing the importance of precise identification and spatial mapping of active faults for seismic design and site selection in engineering projects. The findings offer valuable insights for mitigating seismic hazards in fault zones.

As one of the most serious geological disasters, earthquake is of sudden and destructive characteristic. Therefore, it is of great significance to earthquake monitoring and early warning. The phenomenon of surface thermal infrared radiation enhancement is a common precursor of moderate and strong earthquakes and has been used as an important reference information for early warning and short term prediction. A variety of explanations have been given to understand internal mechanism of the above phenomenon, in which the stress-induced heating hypothesis is widely accepted and has been confirmed in the laboratory rock mechanical loading experiments, that is, under ideal conditions in the laboratory, the rock heats up when it is pressed and cools down when it is stretched. Under field conditions in practice, however, weak seismic precursors of thermal anomalies are often interfered by various environmental factors(solar radiation, atmospheric movement and human activities, etc.), and it has not been investigated whether the corresponding relationship between the above crustal compression-extension motions and thermal radiation anomalies can be observed under field conditions. The earth's gravity field, as one of the basic physical fields of the earth, contains the density distribution of crustal structure, which can be served to study the migration of the earth's material, the deformation of the crust and the change of the stress field. In this paper, we use GRACE gravity and MODIS thermal infrared remote sensing data to verify the stress-induced heat hypothesis in the field with Wenchuan earthquake as the time node. Firstly, the crustal mass density obtained by GRACE satellite was compared with thermal infrared radiation. Then, the gravity anomalies extraction method based on maximum shear strain and in-situ temperature method were used to obtain the gravity anomalies and thermal anomalies respectively. Furthermore, the correlation between the two anomalies before the earthquake was detected from the time scale and space scale respectively, and the consistency analysis between the above anomalies and the spatial distribution of the tectonic fault zone was carried out. For this purpose, two important indicators i.e., anomaly intensity and anomaly distribution, were established in time domain and space domain, respectively. The following conclusions could be drawn: 1)The stress-induced heating hypothesis can be verified by remote sensing in field conditions. The warming zone of the crust(positive thermal offset index)corresponds to the compression zone, and the cooling zone(negative thermal offset index)corresponds to the stretching zone. The consistency of positive and negative variation between the crustal mass density and thermal offset index is 88.9%, which provides field observation evidence for the stress-induced heating hypothesis. 2)The spatio-temporal variation of gravity anomalies and thermal anomalies before earthquake has strong correlation. In the time domain, there is a strong correlation between the gravity anomalies and the thermal anomalies, which shows that the intensity of the two anomalies suddenly increases synchronously and reaches the maximum simultaneously three months before the earthquake. In the spatial domain, gravity anomalies mostly occur at the junction of positive and negative values of thermal offset index, which indicates that the spatial distribution of gravity anomalies and thermal anomalies also has a certain correlation. In addition, the two anomalies appear to be distributed along the fault zone for many times, which shows that they are closely related to tectonic activities.

The terrain in southeastern Tibet is steep and the valleys are crisscrossed. Since the Quaternary, glacial ice and debris have blocked the course of the Yarlung Tsangpo River and its tributary river valleys to form giant dammed lakes, and the huge flood deposits formed by the dammed lake outburst floods are often associated with moraines, ice water deposits, lacustrine deposits, aeolian sand or other running water sediments to form complex river valley accumulation landforms. Different types of sediments in alpine and canyon areas are similar in morphology, structure and fabric, and are difficult to distinguish. Grain size and morphological characteristics are the most important structural characteristics of sediment, and the distribution rules are controlled by many factors such as sedimentary environment, physical properties of detrital material, transporting medium and transporting mode, etc., which is an important proxy index for restoring paleoclimate and inverting paleoenvironment. However, the relevant research on identifying sediment types in alpine valley area of southeast Tibet by grain size and morphology index is still in the exploratory stage. In order to understand the particle size characteristics and spatial differentiation laws of outburst flood sediments and the micromorphological characteristics of particle surfaces, we collected 33 samples of Holocene flood retention sediments preserved along the river within about 350km from the outlet of the Jiacha Gorge in the middle reaches of the Yarlung Tsangpo River to Pai Town, and measured them with Malvern 3000 laser diffraction particle size meter and Zeiss Signma scanning electron microscope, combined with digital geomorphology(DEM)data extracted river channel width and steepness coefficient. The features of spatial distribution law of particle size are analyzed, and the following understanding is obtained. The particle size of outburst flood retention deposits is characterized on the whole by fine-silty sand(2.57~5.18Φ)with poor sorting, positive skew and narrow peak state. Two end element models are obtained: The main peak of EM1 terminal element is 3.16Φ, with an average percentage content of 42.7%, which may represent the alluvial characteristics of higher energy of outburst floods in alpine valley areas, and the main peak of EM2 terminal elements is 2.06Φ with an average percentage content of 55.6%, which can be used to indicate the accumulation process of the outburst flood lag deposits. Affected by the width of the river, the EM1 content has a tendency to increase downstream, while EM2 has the opposite trend. The surface microstructure of quartz particles in the outburst flood lag deposits is mainly characterized by mechanical scratches, shell-like fractures, upturn cleavage and cleavage steps, with low structural maturity, mostly angular shape, and rare denudation pores of chemical origin. As a typical representative of climbing sand dunes in the valley area of the semi-humid monsoon area, the genesis of the dunes is of great guiding significance for revealing the source of sand dunes in the valley area of the alpine valley area, identifying paleoflood deposit and aeolian deposit, distinguishing aeolian deposit and paleoflood slackwater deposits on both sides of the riverbank, and windbreak and sand fixation engineering in the Yarlung Tsangpo River. By comparing the particle size and surface micromorphology characteristics of the known outburst flood deposits of the Yarlung Tsangpo River, we believe that the sand source of the Fozhang dunes is mainly from the outburst flood deposits and was transformed later by wind forces.

The occurrence of earthquakes is closely related to the crustal tectonic movement and the migration of earth mass, which consequently cause the changes of the earth‘s gravitational field. Global time-varying gravity field data obtained by GRACE gravity satellite can be used to detect pre-seismic gravity anomalies. For example, gravity signals caused by several large earthquakes, such as the 2005 M_W8.6 Indonesia earthquake, the 2010 M_W8.8 Chile earthquake and the 2011 M_W9.0 Japan earthquake, have been successfully extracted using GRACE data. However, previous studies on GRACE satellite-based seismic gravity changes focused more on the dynamics of the co-seismic gravity field than on the pre-seismic gravity anomalies which are of great significance for the early warning of earthquakes. Moreover, the commonly adopted difference disposal of the gravity field with the gravity field of adjacent months or the average gravity field of many years when obtaining gravity anomalies cannot effectively remove the inherent north-south stripe noise in GRACE data. On the contrary, it is more likely to cause the annihilation of the medium-high order information in GRACE gravity field model, which results in the loss of some gravity information related to tectonic activities. To explore the pre-seismic gravity anomalies in a more refined way, this study proposes a method of characterizing gravity variation based on the maximum shear strain of gravity, inspired by the concept of crustal strain. In other words, the gravity strain tensor is obtained by further calculating the second-order gradient of the increment of disturbance potential after the removal of hydrological disturbance, and then the maximum shear strain of gravity is ultimately generated to characterize the pre-earthquake tectonic activities. Then, to better understand the seismogenic process of the fault zone by further extracting the pre-earthquake anomalous changes, the data of the maximum shear strain time series are analyzed in this study by means of the offset index K to describe the gravity anomaly. Because the maximum shear strain is calculated by the second-order gradient of GRACE gravity field, this method can suppress the stripe noise better than the difference disposal, thus effectively improving the sensitivity of gravity anomaly detection. The exploratory experiments are carried out in the Tibetan plateau and its surrounding area, which locates among the Pacific Ocean, the Indian Ocean and Eurasia, with the highest altitude, most complex topography and frequent strong earthquakes. Ultimately, the Wenchuan earthquake and Nepal earthquake were used as an example to complete the extraction of pre-earthquake gravity anomaly information by the above method, and the pre-earthquake tectonic activity of the fault zones was analyzed. The results show that a large area of gravity anomalies consistent with the spatial distribution of the fault zone appeared on the Longmenshan fault zone during the half a year before the earthquake, and the maximum anomalous value appeared within 50km from the epicenter, while no anomalies appeared during the non-earthquake period. In addition, compared with the traditional methods, the proposed method has a better ability to extract anomaly information of gravity field, which provides a new idea for understanding the dynamic mechanism of large earthquakes using GRACE data.

The Daliangshan fault zone(DF)constitutes an important part of the large-scale strike-slip Xianshuihe-Xiaojiang fault system(XXFS). Affected by the channel flow of the middle-lower crust in the western Sichuan region, the XXFS is strongly active, and large earthquakes occur frequently. On average, there is an earthquake of magnitude 7 or more every 34 years. However, the DF, as an important part of the middle segment of the XXFS, has only recorded several earthquakes with magnitude 5-6, and no earthquakes with magnitude over 6 have been recorded. The reason for the lack of strong earthquake records may be related to the lack of historical records in remote mountainous areas, but the main reason may be attributed to the active behavior of the faults. He et al.(2008)hold that the DF is a new fault, resulting from straightening of the middle section of the XXFS, and its activity gradually changes from weak to strong, and will probably replace the Anninghe-Zemuhe Fault. However, this view lacks evidence of strong earthquakes. In recent years, some scholars have studied the paleoearthquakes on the DF, and found the signs of strong earthquake activity, and considered that the fault has the seismogenic capacity of earthquakes with magnitude more than 7. These studies are mainly concentrated in the middle and southern segments of the DF. Although there are scattered activity data and individual trench profiles, direct evidence of Holocene activity and paleoearthquake data are very scarce in the northern part of DF. On the basis of the previous studies, combined with our detailed field geomorphological surveys, we excavated a set of two trenches at Lianhe village in Shimian Fault to reveal the direct evidence of fault activity in Holocene. From paleoseismic analysis and radiocarbon samples accelerated mass spectrometry(AMS)dating, four paleoseismic events are identified, which are E1 between 20925—16850BC, E2 between 15265—1785BC, E3 between 360—1475AD, and E4 between 1655—1815AD. The results of the latest two events should be relatively reliable, and the latest event may be related to the Moxi earthquake of magnitude 7^3/4 on June 1, 1786 or the Dalu earthquake of magnitude ≥7 on June 10, 1786. Among the four events revealed, three are since the Holocene, and the recurrence interval of the latest two events is about 800 years. Compared with other active faults at the triple junction, the recurrence interval is slightly longer than that at the northern segment of the Anninghe fault zone, but close to that at the Moxi segment of the Xianshuihe fault zone. Compared with the western segment of Xianshuihe Fault and the northern segment of Anninghe Fault, the Shimian Fault also has a higher seismic risk, which needs further attention.

The Xijiang Fault is an important NW-trending fault with a length of~200km, located in the western part of the Pearl River Delta. A M4 3/4 earthquake occurred at the northern end of the fault(Sihui)in 1445 and a magnitude 5 earthquake occurred at the southern end of the fault(Modaomen Waters)in 1905. Heshan is the boundary between the southern and the northern segments of this fault. The southern segment which is called Heshan-Modaomen segment is mainly hidden faults. The activity of Heshan-Modaomen segment remains controversial due to the lack of systematic studies for the deep and shallow exploration, which affects the assessment and prevention of earthquake disaster risk. In this paper, we concentrate particularly on the distribution and activity of Heshan-Modaomen segment using seismic geological surveys, shallow seismic exploration, joint borehole profile detection, and Quaternary geochronology.
Field geological surveys show that the fault zone is prominently normal sinistral strike-slip faults, striking about N310°~330°W, with a width of 10~20m. Most of them dip northeast at angles of 60°~80°. Observations on typical outcrop show that cataclasite, breccias and siliceous rocks are developed on the faults. Fault planes often have smooth and polished surfaces and no fault geomorphology has been developed along the fault zone. The overlying eluvial weathered soil materials have not been disturbed or cut. We carried out shallow cross-fault sounding of 7 profiles in the hidden section of the fault zone using longitudinal wave reflection method of multifold coverage observation system. As a result, we obtained the reflection time sections of the target stratum and the main structure. A total of 13 breaking points to be investigated were explained. We also performed cross-fault drilling at the location of the seismic data interpretation profile and catalogued drilling cores. ¹⁴C and OSL samples were collected systematically. The ¹⁴C dating was performed by the BETA Laboratory in the United States and 16 valid age data were obtained. OSL dating was performed by the OSL Laboratory of China University of Geosciences(Wuhan)and 6 age data were obtained.
This paper presents the study results of two representative cross-fault profiles. The shallow exploration survey line XJ1 and the row drill profile P1 are located in the southern section of the fault where six boreholes are arranged. We find the existence of bedrock faults on the joint borehole profile. The grooves developed thereupon are filled with the late Pleistocene paleochannel deposits with no obvious faults observed. The overlying Holocene strata are horizontal and continuous, without cutting and disturbance. Combined with the stratigraphic age, we infer that the fault has been inactive for at least about 11 000 years. The shallow exploration survey line XJ2 and row drill profile P3 are located in the northern section of the fault, where a total of seven boreholes are arranged. The borehole sections reveal the existence of fault crushed zone in the underlying bedrock(Cambrian hornstone). The tectonites are mainly fault breccias and cataclastic rocks with chlorite alteration. Groove landforms are formed along the fault zone with strong erosion at the later stage, and filling and accumulation occurred since the Holocene transgression with no fracture cutting or stratum disturbance. According to the landform, the occurrence of faults and the development of transverse active faults, the Heshan-Modaomen segment of Xijiang Fault can be further divided into two segments with the boundary of Zhupai Island. Both of them have been inactive since the Holocene.

Surface rupture zone of historical earthquake is the most intuitive geomorphological response to fault activity. The rupture pattern, coseismic displacement and its geometric spatial distribution are important for determining segmentation and long-term movement behaviors of active fault. In the Barkol Basin of Xinjiang, according to the comprehensive result from remote sensing image interpretation, field surgery, high-resolution small unmanned aerial vehicles photography, terrain deformation measurements and trench excavation on geomorphological points, not only the new surface ruptures of the two M71/2 historical earthquakes in Barkol in 1842 and 1914 were found and defined between Xiongkuer and the southwest of Barkol County in southwestern part of the basin, but also the latest deformation evidence of the EW fold-up faults in the eastern part of the Basin was identified.
Combined with the ancient document analysis of the two historical earthquakes, we finally conclude that the surface rupture zone in the western segment on the southern margin of the Barkol Basin is the seismogenic structure of the M71/2 earthquake in 1842. The surface rupture zone is mainly characterized by left-lateral strike-slip, roughly with en echelon arrangement spreading from Xiongkuer to the south of Barkol County. The length of the surface rupture zone determined by field investigation is at least about 65km, and the maximum horizontal displacement appears around the Xiongkuer Village. At the same time, the surface rupture zone gradually shows more significant thrust extrusion from west to east, and has a tendency of extension towards the central of the Barkol Basin. The average observed displacement of the entire surface rupture obtained by counting the coseismic offsets of multiple faulted gullies is(4.1±1.0)m, with the coseismic characteristic displacement of ~4m. The epicenter position should appear at the place with the largest horizontal dislocation amount near Xiongkuer Village.
In addition, the length of the fold-blind fault zone in the vicinity of the Kuisu Town and the eastward extension to the Yanchi Township of the Yiwu Basin, which was discovered in the center of the Barkol Basin, is about 90km. The folded blind fault causes significant fold deformation in the latest sedimentary strata such as floodplain, and in addition, as shown on many outcrop sections, the bending-moment faults associated with the coseismic fold deformation have ruptured the surface. Therefore, the location of the epicenter should be located at the maximum fold deformation, which is near the Kuisu Town. The new research results not only further improve the understanding of the epicenter location and seismogenic faults of the two historical earthquakes in the Barkol Basin, but also provide an important reference for analyzing regional seismic hazards.

The Yarlung Tsangbo fault zone, one of the most important geological interfaces in the Yarlung Tsangbo suture zone which is a huge geotectonic boundary with nearly east-west-trending in southern Tibet Plateau, has undergone a long-term tectonic evolution. Studying this fault zone can help us understand the development and evolution history of the suture zone and the tectonic mechanism of subduction-collision about the Tibet Plateau, so it has always been a hot topic in the field of geology. Most of existing data suggest that the current tectonic activity in southern Tibet is given priority to the rift system with nearly north-south-trending, and the Yarlung Tsangbo fault zone with nearly east-west-trending has relatively weaker activity since late Quaternary. There are only some evidences of Holocene activity found in the Lulang town section near eastern Himalayan syntaxis, and there are few reports about the reliable geological evidences of late Quaternary activity of the section on the west of Milin County of the fault zone.
Based on image interpretation, field investigation and chronological method, we found several fault profiles along the Yarlung Tsangbo fault zone near the Angren Lake in this study. These profiles reveal that loose fault gouge has been developed on the fault plane which nearly extends to the surface and offsets the loess sediments and its overlying alluvial-proluvial gravels. The loess is characterized by coarser grains, higher content of fine sand and tiny small gravels. The results of the two OSL dating samples collected in the loess are(94.68±6.51)ka and(103.84±5.14)ka respectively, showing that the loess revealed at the Angren site should be the middle-late Pleistocene sand loess distributed on the high-terraces along the Yarlung Tsangpo River. Consequently, the Angren segment of the Yarlung Tsangpo fault zone is active since the late Quaternary. In addition, synchronous left-lateral offsets of a series of small gullies and beheaded gullies can be seen near the profiles along the fault, which are the supporting evidence for the late Quaternary activity of the fault.
However, the segment with obvious geomorphology remains is relatively short, and no evidence of late Quaternary activity have been found in other sections on the west of Milin County of the Yarlung Tsangpo fault zone. Existing data show that, in the southern Tibet, a series of near NS-trending rift systems are strongly active since the late Quaternary, cutting almost all of the near east-west-trending tectonic belts including the Yarlung Tsangpo fault zone. In addition, majority of the earthquakes occurring in southern Tibet are related to the NS-trending rift systems. Tectonic images show that the Angren segment locates between the Shenzha-Dingjie rift and the Dangreyong Lake-Gu Lake rift. These two adjacent rifts are special in the rift system in southern Tibet:Firstly, the two rifts are located in the conversion position of the trend of the whole rift system; Secondly, the size of the two rifts varies significantly between the north side and the south side of the Yarlung Tsangbo fault zone. Thirdly, the Shenzha-Dingjie rift seems to be of right-lateral bending, while the Dangreyong Lake-Gu Lake rift shows left-lateral bending. These characteristics may lead to the fact that the amount of absorption and accommodation of the rift activities in the north side of the Yarlung Tsangbo fault zone is larger than that in the south side during the migration of the plateau materials, leading to the differential movement of the block between the two sides of the fault zone. Therefore, the Yarlung Tsangbo fault zone possesses the accommodating tectonic activity, of course, the intensity of this accommodating activity is limited and relatively weaker, which may be the reason why it is difficult to find large-scale tectonic remains characterizing the late Quaternary activity along the fault zone. The scale of the rift system in southern Tibet is systematically different between the two sides of the Yarlung Tsangbo fault zone, so it cannot be ruled out that there are also weak activities similar to the Angren segment in other sections of the fault zone.

Small earthquakes have been recorded in Yibin area, Sichuan Province since 1970, the frequency and intensity of seismicity have shown an increasing trend in recent ten years, and the earthquakes are distributed mainly in Changning, Gongxian and Junlian areas. Based on the seismic data from January 2008 to May 2015 recorded by Sichuan and Yunnan regional networks and Yibin local network, seismicity analysis, precise location and velocity structure inversion for earthquakes in Yibin area are carried out, the three-dimensional spatial distribution of seismic activity and the velocity structure at different depths in this region are investigated, trying to analyze the seismic activity law and seismogenic mechanism in Yibin area.
The earthquake relocation result shows that the spatial cluster distribution of earthquakes is more obvious in Yinbin area, the earthquakes are concentrated in Changning-Gongxian and Gongxian-Junlian regions. The seismic activity presents two dominant directions of NW and NE in Changning-Gongxian region, and shows asymmetric conjugate distribution, the long axes of NW-trending and NE-trending seismic concentration area are about 30km and 12km respectively, and the short axes are about 5km. There is a seismic sparse segment near Gongxian, the frequency and intensity of seismicity in the southeast side are obviously higher than that in the northwest side, and the earthquakes with larger magnitude are relatively deep, the focal depth is gradually shallower with the distance away from Gongxian. Seismic activity is sparse in the west and dense in the east in Gongxian-Junlian region, the predominant direction of earthquakes in the seismic dense area of the eastern segment is NE. Seismic activity extends in opposite direction in the easternmost part of the two earthquake concentrated area.
The P-wave velocity structure at different depths in the study area is obtained using joint inversion method of source and velocity structure. In view of the predominant focal depth in this region, this paper mainly analyzes the velocity structure of the upper crust within 10km. Within this study area, the P-wave velocity of earthquake concentration areas is relatively high within 10km of the predominant focal depth, especially in the northwest of Gongxian and eastern Junlian area, the P-wave velocity on the southeast of Gongxian increases gradually with depth, especially at 6km depth. These high-velocity zones are generally related to brittle and hard rocks, where the stress is often concentrated.
Comparing earthquake distribution and velocity structure, seismic activity in this area mainly occurs in high-low velocity transition areas, the inhomogeneity of velocity structure may be one of the factors controlling earthquake distribution. The transition zone of high and low velocity anomalies is not only the place where stress concentrates, but also the place where the medium is relatively fragile, such environment has the medium condition of accumulating a large amount of strain energy and is prone to fracture and release stress.

Analysis of stress state of faults is helpful to understand crustal mechanical properties and seismicity. In the paper, we invert the horizontal crustal stress field in the southeastern Tibetan plateau using focal mechanism solutions of small and medium-size earthquakes, and apply them to estimate the stability of regional major faults.
Firstly, we collect focal mechanism solutions of small and medium-sized earthquakes in the southeastern Tibetan plateau. The dataset includes more than 1 000 focal mechanism solutions in the past twenty years. Magnitudes of these earthquakes vary from M3.0 to M6.0. Most of the focal mechanism solutions were determined using waveform inversion technique. Although most of focal mechanism solutions in the southeastern Tibetan plateau are strike-slip faulting, their spatial pattern is different in sub-regions. Normal faulting earthquakes mainly occurred in the western Sichuan region, reverse faulting earthquakes mainly occurred in the boundary zone between the Tibetan plateau and the South China craton, and strike-slip faulting earthquakes mainly occurred in the central and southern Yunnan region.
Next, we settle on a mesh with grid spacing of 0.5° in longitude and latitude in the region and invert the horizontal crustal stress field at each grid point. Spatial variation of the maximum principal stress axis in the southeastern Tibetan plateau shows a clockwise rotation around the eastern Himalaya syntax. The azimuth of maximum compressional stress axis is about 88.1° in the western Sichuan region, about 124.6° in the South China craton, and about 21.6° in the western and southern Yunnan region. The azimuth of regional maximum compressional stress is nearly parallel to the direction of terrain elevation gradient, and that of the minimum compressional stress is nearly parallel to the tangential direction of the topographic elevation contours. The spatial pattern reflects the control role of gravity spreading of the Tibetan plateau on the regional horizontal stress field.
Finally, we analyzed regional fault stability based on these collected focal mechanism solutions. The fault instability parameter (I) is defined based on the Mohr-Coulomb criterion and indicates the degree of fault approximating to rupture. The instability parameters on fourteen major faults in the southeastern Tibetan plateau were calculated. Our results show that the stability of the Lianfeng-Zhaotong Fault is the lowest before 2014 in the region, which indicates the fault zone is close to rupture at that time. Our results provide a new useful tool to assess regional seismic potential using dense focal mechanism solutions.

The Anninghe Fault has been suggested as an important segment of the fault system along the eastern boundary of the Sichuan-Yunnan faulted block in the southeastern region of the Tibetan plateau. Reliable determination of the Late Quaternary slip rate on the Anninghe Fault is very helpful and significant for revealing deformation mechanism and kinematic characteristics of the Sichuan-Yunnan faulted block, which further helps us understand fault activity and seismic potential of the region. However, previous studies were focused mainly on the northern segment of the Anninghe Fault, while slip rate on its southern segment has been less studied. Therefore, in this paper, we chose two sites at Dashuigou and Maoheshan on the southern segment of the Anninghe Fault, and used high-resolution images of unmanned aerial vehicle (UAV)photogrammetry technology, detailed field survey, multiple paleoseismic trenching and radiocarbon dating methods to constrain slip rate on the southern fault segment of the Anninghe Fault. Specifically, we suggest that the slip rate at the Dashuigouo site is narrowly constrained to be~4.4mm/a since about 3^￣￣300a^￣￣BP based on a linear regression calculation method, and speculate that a slip rate of 2.6~5.2mm/a at the Maoheshan site would be highly possible, although we poorly constrained the whole deformation amount of the two branch faults at the Maoheshan site from multiple paleoseismic trenching. The data at the two sites on the southern segment show a consistent slip rate compared with that of the northern segment of the Anninghe Fault. Moreover, considering a similar paleoseismic recurrence interval on the two segments of the Anninghe Fault from previous studies, we further suggest that the fault activity and deformation pattern on the two segments of the Annignhe Fault appears to be well consistent, which is also in agreement with the regional tectonic deformation.

A lot of seismic volcanic rocks and strong earthquake-induced thixotropic deformation structures in soft mud-sandy sediments(seismites)were identified from the Upper Cretaceous Shijiatun Member of the Hongtuya Formation for the first time in Jiaozhou City of the Zhucheng Sag, eastern China. Seismic volcanic rocks are volcanic rocks with co-seismic deformation structures which were produced by major earthquakes destroying volcano ejecta. Seismites are sediment layers with soft-sediment deformation structures formed by strong earthquake triggering saturated or semi-consolidated soft sediments to produce liquefaction, thixotropy, faults, cracks and filling and so forth. The Shijiatun Member of the Hongtuya Formation mainly consists of basaltic volcano rocks interbedded with mud-sandy(muddy sand and sandy mud)deposition layers of the river-lake facies. In the Shijiatun Member, main types of seismic volcanic rocks are shattered basalts with co-seismic fissures and seismic basaltic breccias. The thixotropic deformations of soft mud-sandy sediments mainly include thixotropic mud-sandy veins and thixotropic mud-sandy layers with tortuous boundaries. Under the strong earthquake action, saturated mud-sandy sediments could not be liquefied, instead resulting in thixotropy, i.e. their texture can be damaged and their flow-ability or rheology becomes strong. Because basaltic volcano rocks were damaged(shattered, seismic broken), a major earthquake can lead to thixotropic mud-sandy sediments flowing along seismic fissures in basalts, resulting in the formation of deformation structure of thixotropic veins, and boundaries between volcano rock and mud-sand layer became quite winding. Under the koinonia of gravity and vibration force, seismic breccia blocks sunk into thixotropic mud-sandy layers, resulting in the formation of inclusions of thixotropic mud-sandy sediments. Seismic intensity reflected by these strong earthquake records during the end stage of the Late Cretaceous was about Ⅶ to more than X degrees. The Shijiatun Member is mainly distributed in the south of the Baichihe fault in the northern Zhucheng Sag, and the fault has generated many strong tectonic and earthquake activities at the end of the late Cretaceous, also provided the channel for intrusion and eruption of basaltic magma then. At the end of the late Cretaceous, intermittent intrusion and eruption of basaltic magma took place along the Baichihe fault, meanwhile the volcano earthquakes took place or tectonic earthquakes were generated by the Baichihe fault which caused the deformation of the volcano lava and underlying strata of red saturated muddy-sand, resulting in the formation of various seismo-genesis deformations of volcanic rocks interbedded with mud-sandy sediment layers. Therefore, strong seismic events recorded by them should be responses to strong tectonic taphrogenesis of the Zhucheng Sag and intense activity of the Baichihe fault in the end of Late Cretaceous. In addition, these seismogenic deformation structures of rock-soil layers provide new data for the analysis of the failure effect produced by seismic force in similar rock-soil foundations.

Alluvial fans that are in the process of development always show complex geomorphic features due to natural modification. Accordingly, analyzing these fans whether to be influenced by tectonic deformation is one of the technique difficulties in active tectonic studies. Complex alluvial fans are the focus of the study of active tectonics such as fracture mapping and activity behavior analysis, for they have often retained important structural information. Traditional measurement methods, such as satellite remote sensing, RTK GPS and Lidar, are difficult to meet the demand for the study of micro tectonic deformation because of the reason of accuracy or cost performance. The recent UAV photogrammetry technology, due to its many advantages such as low cost, high resolution, and efficiency of exporting DEM and DOM data, has been widely used in three-dimensional modeling, ground mapping and other fields. In the quantitative study of active tectonics, this technology fills up the deficiency in the research of the micro structure of the traditional measurement. Through detailed field investigations and paleoseismic trenching, we further used this technology to obtain the topographic data of a complex alluvial fan located at the southern marginal fault of Barkol Basin, Xinjiang. Pointing at the alluvial fans that are in the process of development, and on the basis of topographic analysis and image processing for DEM, we take the research method of secondary partitions of the geomorphic surface and cut the alluvial fans longitudinally according to the difference of its age. Through the establishment of profile cluster within each partition, separate analysis and data contrast with the adjacent partitions, we acquired the tectonic activity information during the development of alluvial fan. The tectonic vertical deformation of this alluvial fan is about 2.5m.

The Xianshuihe Fault, the boundary of Bayan Har active tectonic block and Sichuan-Yunnan active tectonic block, is one of the most active fault zones in the world. In the past nearly 300 years, 9 historical earthquakes of magnitude ≥ 7 have been recorded. Since 2008, several catastrophic earthquakes, such as Wenchuan M_S8 earthquake, Yushu M_S7.1 earthquake and Lushan M_S7 earthquake, have occurred on the other Bayan Har block boundary fault zones. However, only the Kangding M_S6.3 earthquake in 2014 was documented on the Xianshuihe Fault. Thus, the study of surface deformation and rupture behavior of large earthquakes in the late Quaternary on the Xianshuihe Fault is of fundamental importance for understanding the future seismic risk of this fault, and even the entire western Sichuan region. On the basis of the former work, combined with our detailed geomorphic and geological survey, we excavated a combined trench on the Qianning segment of Xianshuihe fault zone which has a long elapse time. Charcoal and woods in the trench are abundant. 30 samples were dated to constrain the ages of the paleoseismic events. Five events were identified in the past 9  000 years, whose ages are:8070-6395 BC, 5445-5125 BC, 4355-4180 BC, 625-1240 AD and the Qianning earthquake in 1893. The large earthquake recurrence behavior on this segment does not follow the characteristic earthquake recurrence model. The recurrence interval is 1000~2000 years in early period and in turn there is a quiet period of about 5 000 years after 4355-4180 BC event. Then it enters the active period again. Two earthquakes with surface rupture occurred in the past 1000 years and the latest two earthquakes may have lower magnitude. The left-lateral coseismic displacement of the 1893 Qianning earthquake is about 2.9m.

Cascade rupture events often occur along large strike-slip fault zone.The 1920 AD M 8¹/₂ earthquake ruptured all 3 segments of the Haiyuan Fault,and the Salt Lake pull-apart basin is the boundary between the west and middle segment of the fault.The data of trenching and drilling reveal 7 events occurring since last stage of late Pleistocene,and the two youngest events are associated with the historical records of 1092 AD (possibly) and 1920 AD respectively.These events are all large earthquakes with magnitude M>8,and the recurrence of them is characterized by earthquake clusters alternating with a single event.Now it is in the latest cluster which may last about 1000 years.Comparison of the paleoseismic sequence of this study and previous results reveals that the cross-basin fault in the Salt Lake pull-apart basin does not always rupture when cascade rupture events occur along the Haiyuan Fault,and likely ruptures only when the magnitude of the events is large (maybe M>8).Though there are many advantages in paleoseismic study in pull-apart basin,we should avoid getting the paleoseismic history of major strike-slip fault zones only depending on the rupture records of inner faults in pull-apart basins with large scale (maybe a width more than 3km).

Two earthquakes with magnitude larger than 7.0 occurred in 2008 and 2014 on the southwestern end of the Altyn Tagh Fault, which is located in the northwestern borderland of Tibetan plateau. Occurrences of these two earthquakes provide important insights into regional geodynamics and potential seismic risk. Layered viscoelastic model is employed in the paper to study the interaction between these two events. We find that most of aftershocks were triggered by coseismic stress produced by the 2008 Yutian earthquake, and the effect of this earthquake is insignificant on the occurrence of the 2014 Yutian earthquake. However, stress transfer by viscoelastic relaxation of postseismic deformation is in favor of occurrence of the 2014 Yutian earthquake. The coseismic and postseismic stress transfer produced by the 2014 Yutian earthquake leads to stress increasing on the western segment of the Altyn Tagh Fault. Since the occurrence time of the last major earthquake on the western segment of the Altyn Tagh Fault is tens of years ago, it should have accumulated large moment deficit on the fault segment. The Altyn Tagh Fault should be considered as a fault with high potential seismic risk.

Using double-difference precise location results of seismic S wave data from 2000 to 2012, distribution and variation of dynamic moving average Q_s value are imaged in Hainan Island and adjacent regions through seismic attenuation tomography. We explore regional crustal S-wave attenuation characteristics of temporal and spatial variation combined with seismic activity, volcanoes and heat distribution, and GPS baseline changes. The results show that:(1)Attenuation imaging shows that there is a significant lateral heterogeneity in Q_s distribution in the study area and the Q_s values are high in the central region and low in the north and south regions of Hainan Island. Low-Q_s areas are baseically converged to the north of Wangwu-Wenjiao Fault, to the south of Jianfeng-Diaoluo Fault, and to the east of Puqian-Qinglan Fault. Earthquakes are basically converged to the edge of the transitional regions of high and low Q_s values. Heat flow sites and volcanoes zones are located in low-Q_s area in the region. (2)There is a strong correlation between dynamic moving average Q_s value, seismic activity and Luzhou-Qiongzhong GPS baseline. From 2000 to 2008, the average Q_s value of the study area is relatively high, the seismic activity is strong, and Luzhou-Qiongzhong GPS baseline is decreasing. From 2008 to 2012, the average Q_s value of the study area shows a downward trend, the seismic activity is weak, and the Luzhou-Qiongzhong GPS baseline displays an elongation trend.

Nine earthquakes with M≥6 have stricken the northern segment of the Red River fault zone since the historical records, including the 1652 Midu M7 earthquake and the 1925 Dali M7 earthquake. However, there have been no earthquake records of M≥6 on the middle and southern segments of the Red River Fault, since 886 AD. Is the Red River fault zone, as a boundary fault, a fault zone where there will be not big earthquake in the future or a seismogenic structure for large earthquake with long recurrence intervals?This problem puzzles the geologists for a long time. Through indoor careful interpretation of high resolution remote sensing images, and in combination with detailed field geological and geomorphic survey, we found a series of fault troughs along the section of Gasha-Yaojie on the southern segment of the Red River fault zone, the length of the Gasha-Yaojie section is over ten kilometers. At the same time, paleoseismic information and radiocarbon dating result analysis on the multiple trenches show that there exists geological evidence of seismic activity during the Holocene in the southern segment of the Red River fault zone.

The 2008 Wenchuan earthquake occurred along the Longmen Shan fault zone, only five years later, another M7 Lushan earthquake struck the southern segment where its seismic risk has been highly focused by multiple geoscientists since this event. Through geological investigations and paleoseismic trenching, we suggest that the segment along the Shuangshi-Dachuan Fault at south of the seismogenic structure of the Lushan earthquake is active during Holocene. Along the fault, some discontinuous fault trough valleys developed and the fault dislocated the late Quaternary strata as the trench exposed. Based on analysis of historical records of earthquakes, we suggest that the epicenter of the 1327 Tianquan earthquake should be located near Tianquan and associated with the Shuangshi-Dachuan Fault. Furthermore, we compared the ranges of felt earthquakes(the 2013 M7 Lushan earthquake and the 1970 M_S6.2 Dayi earthquake)and suggest that the magnitude of the 1327 Tianquan earthquake is more possible between 6&frac12; and 7. The southern segment of the Longmen Shan fault zone behaves as a thrust fault system consisting of several sub-paralleled faults and its deep structure shows multiple layers of decollement, which might disperse strain accumulation effectively and make the thrust system propagate forward into the foreland basin, creating a new decollement on a gypsum-salt bed. The soft bed is thick and does not facilitate to constrain fault deformation and accumulate strain, which produces a weak surface tectonic expression and seismic activity along the southern segment, this is quite different from that of the middle and northern segments of the Longmen Shan fault zone.

Based on the finite element models, in this paper, the impact of Japan 3·11 earthquake on the south-central section of Tanlu Fault zone is discussed in the view of displacement field. From the displacement magnitude acquired by the numerical simulation of Japan 3·11 earthquake, this earthquake has a certain impact on Tanlu Fault zone in mainland China area. The simulation results show that the coseismic horizontal displacement near the Tanlu Fault zone caused by Japan 3·11 earthquake has reached millimeter to centimeter level. On the east side of the fault zone, the maximum displacement value in Northeast China reaches to 30 millimeters, and the maximum displacement value in Yantai, Weihai and other places of North China also reaches to 12 millimeters. On the west side of the fault zone, the maximum displacement value in Northeast China reaches to 20 millimeters, and the maximum displacement value in North China is only 8 millimeters. The displacement value difference is very obvious in both sides of Tanlu Fault zone. In addition, the simulation results also show that the displacement values in the east-west direction is much bigger than the one in the south-north direction of the Tanlu Fault zone and nearby areas; especially the displacement direction is basically near to east-west direction in North China. Meanwhile, bigger NS-direction displacement appeared in Northeast China, and the maximum displacement value even reaches to 8 millimeters. Therefore, the impact of Japan 3·11 earthquake on this area is more complex. Considering the actual delay effect of crustal medium to stress transfer, the impact of Japan 3·11 earthquake on the Tanlu Fault zone will have a delay, so the follow-up effect will stay for a certain time. Although the displacement value in eastern North China(Tanlu Fault zone and its nearby area)is smaller than the one in Northeast China, the impact of Japan 3·11 earthquake still cannot be ignored. Considering that many magnitude 4 or 5 earthquakes have occurred in Kenli, Changdao, Rongcheng, Laizhou in Shandong Province and in the Rudong, Gaoyou in Jiangsu Province since 2011, it cannot be excluded the possibility that a larger earthquake will happen in this area in the future. Thus, due attention has to be paid to this.

Nonvisibility(dieout)of fault strands occurs primarily in stratigraphic units associated with young paleoseismic events, which may cause misidentification of the young events and bring more uncertainties for seismic risk assessment. Based on previous related studies, this paper integrates case studies in mainland China to discuss the nonvisibility of fault strands and identification of young paleoseismic events. Nonvisibility of fault strands is prevailing in sandy, soil, silty, loess, and clay-sandy units, and is more possibly associated with strike-slip faults comparing with normal and reverse faults. Case studies on several trenches across surface ruptures produced by the Wenchuan earthquake and others located at different regions suggest that trench siting, excavation, and comprehensive analysis are key technical points to identify young paleoseismic events in the stratigraphic units where nonvisibility of fault strands is prone to occur. Stratigraphic units with more sequences have been suggested to be good sites for trenching to avoid misidentification produced by nonvisibility of fault strands. Multiple trenching is facilitated to lower the influence of local nonvisibility. Assumed extending of upper and lower units, grain sizes, color, and soil horizon are the basic methods to identify nonvisibility. Analysis of microstructures, grain sizes and magnetic susceptibility is one of the future studies related to identification of nonvisibility of fault strands.

Lowering uncertainties of paleoseismic data is very important, which facilitates medium and long-term earthquake prediction and seismic risk assessment in paleoseismic studies. Among these uncertainties, paleoseismic timing is highly focused and its constraint is one of the most key factors in lowering uncertainties. To get the age as true as possible, choosing right dating, sampling methods and techniques on event ages are essential. Among the several primary dating techniques, radiocarbon dating is prior to the other methods. We should choose material that is breached completely during transportation for OSL dating, and samples that are hardly affected by kinds of factors for ¹⁰Be dating. Sampling at appropriate sites based on analysis of tectonic background and using sequential ages to constrain paleoseismic events are the primary keys of techniques on accurate event dating. Interval value is suggested for age constraint when using stratigraphic chronology. When there are multiple ages for different samples within a layer, we should choose the youngest and reliable age. These rules are recommended when constraining paleoseismic ages at a single site. Temporal and spatial correlation such as successive limit method, Z statistics, overlapping distributions likelihood approach, event window and ad hoc weighted overlap methods are the primary analysis approaches for event dating at multiple sites along faults or fault segments.

The Anninghe and Zemuhe Fault systems show characteristics of a left-lateral strike-slip movement since late Quaternary and they are located along the eastern boundary of the Sichuan-Yunnan Fault block in the southeastern region of the Tibetan plateau. The N-S striking Anninghe Fault is divided into the northern and southern segment around Mianning. The northern segment has an average recurrence interval of large earthquakes of about 500～700 years and a left-lateral slip rate of 4mm/a since Holocene. However paleoseismic behavior along the southern segment has been less focused. We excavated several trenches at Yuehua along the southern segment and used multiple radiocarbon dating to constrain the average recurrence interval of large earthquakes of this segment, which is about 600～800 years. The Zemuhe Fault has an average recurrence interval of paleoearthquakes of about 2300 years with a left-lateral slip rate of 2.4～3.6mm/a since Holocene. Comparing with the fault behavior between the Anninghe Fault and Zemuhe Fault, we find that the recurrence interval of the Anninghe Fault is shorter than that of the Zemuhe Fault and has a relatively larger left-lateral slip rate, indicating an inconsistent paleoseismic behavior. We suggest this inconsistence may be related to different strikes of the two faults, the uplift of the Luoji Shan and the distribution of the N-S trending strike-slip fault system on the south of the Anninghe Fault.

Normal faults, developed within extensional environment, are widely found in North China. Given the varieties in surface ruptures of different earthquakes and their depositional environment, some issues are needed to be paid much attention to in exposing the actual and complete history of paleoseismic events occurring along normal faults. In this paper, based on the existing knowledge about surface rupture characteristics of large earthquakes and indicators of normal fault, combining the cases study in China and the factors of geological, geomorphologic and climatic environment, some key techniques and methods in paleoseismic study on normal faults in mainland China are recommended as follows: (1)Choosing appropriate trenching sites according to local conditions. In the area where the faulted surface deposits are mainly alluvial-fluvial materials of piedmont or river and lake sediments, the trenching sites should try to meet following conditions: the geomorphy can reveal multiple fault events with not too large single displacement, the erosion(or denudation)of external force and the accumulation processes maintain relative balance, the sediments are medium-fine grained, and the samples for dating are easy to be collected. The sites where the faulted sediments are mainly composed of loess or secondary loess or sandy loam should be avoided to excavate trenches for paleoseismic study, however, if it cannot be avoided, the areas with weaker erosion and accumulation near small gullies are the choices to be considered, because these areas may have different deposits from upstream of the gullies, and some supplemental information such as tectonic landform are needed to substantiate the paleoseismic analysis.(2)Recording and analyzing the trench profiles in detail in the field. For the deposits(e.g. loess)with no stratification, the key observation point is the slight change in the color, grain and orientation, which may indicate the stratigraphic boundary. Indicating the scarp-derived deposits units such as colluvial wedge is the key to analyzing paleoseismic events, and the indicated elements conclude the messy configuration and nodules in the collapse facies, and the soil developed in the upper of the erosion facies. When the scarp-derived deposits are difficult to distinguish from normal strata, we should, by "brushing", "jabbing" or "microscopic analysis", try to analyze the color, grain, non-loess materials(e.g. small gravel, plant roots, etc.)and the enrichment degree of calcareous materials(e.g. calcium-mod, calcium-nodule, calcium-dot, calcium-filament, etc.), to identify the stratigraphic boundary.(3)Synthetically analyzing and checking the paleoseismic results combining other information. The appearances of the scarp-derived deposits revealed by trench are often obscure, so supplemental information from geomorphology and multiple trenches are necessary. Some techniques and methods, such as progressive constraining method of paleoseismic events, fault displacement constraining method, correlating method between multiple trenches, inversion and reconstruction of fault events, etc., are helpful for judging whether the paleoseismic results are actual and complete.

An M_S 7.0 earthquake attacked southern segment of the Longmenshan Fault zone on 20 April 2013, in Lushan County, Sichuan Province, southwest of China. The seismic intensity of the meizoseismal area of the Lushan event is Ⅰ Ⅹ(the Chinese Seismic Intensity Scale). The meizoseismal area strikes NE, and is approximately 24km long and 11km wide. In the post-earthquake emergency scientific survey, some members found a series of co-seismic surface rupture signs at Longmen Township, one of the most damaged areas by this quake, north of Lushan County. The reported typical surface rupture signs include intensive shear-fissures along the channel at Zhanghuo Group, rotation of bricks near a white tower at Wanghuo Group, and lots of extensive cracks. On the basis of analyzing such surface rupture signs, Han et al., (2013)deduced that there might be a blind fault along Lushan County and Longmen Township(named the Lushan-Longmen presumed blind fault), and this fault probably was the seismogenic fault. Therefore, to confirm whether there is a potential seismogenic fault along Lushan and Longmen is very important not only to research of seismogenic fault for this earthquake but also to the reconstruction in the disaster-hit areas. Through surface ruptures surveying and trench excavating, we conclude that: there are no co-seismic surface ruptures at Longmen. Meanwhile, artificial seismic prospecting outcome, which shows nonexistence of the Lushan-Longmen presumed blind fault at least 800m below the ground surface, also supports our idea. Consequently, the reported shear-fissures and extensive cracks are not produced by the seismogenic fault, but most likely by ground shaking during the earthquake.

On April 20,2013,a strong earthquake of M_S 7.0 struck the Lushan County,Sichuan Province of China. In this paper,basic information of the April 20,2013 Lushan earthquake,historical earthquakes in the Lushan earthquake struck area and associated historical earthquake-triggered landslides were introduced firstly. We delineated the probable spatial distribution boundary of landslides triggered by the Lushan earthquake based on correlations between the 2008 Wenchuan earthquake-triggered landslides and associated peak ground acceleration(PGA).According to earthquake-triggered landslides classification principles,landslides triggered by the earthquake are divided into three main categories: disrupted landslides,coherent landslides,and flow landslides. The first main category includes five types: rock falls,disrupted rock slides,rock avalanches,soil falls,and disrupted soil slides. The second main category includes two types of soil slumps and slow earth flows. The type of flow landslides is mainly rapid flow slides. Three disrupted landslides,including rock falls,disrupted rock slides,and soil falls are the most common types of landslides triggered by the earthquake. We preliminary mapped 3883 landslides based on available high-resolution aerial photographs taken soon after the earthquake. In addition,the effect of aftershocks on the landslides,comparisons of landslides triggered by the Lushan earthquake with landslides triggered by other earthquake events,and guidance for subsequent landslides detailed interpretation based on high-resolution remote sensing images were discussed respectively. In conclusion,based on quick field investigations to the Lushan earthquake,the classifications,morphology of source area,motion and accumulation area of many earthquake-triggered landslides were recorded before the landslide might be reconstructed by human factors,aftershocks,and rainfall etc. It has important significance to earthquake-triggered landslide hazard mitigation in earthquake struck area and the scientific research of subsequent landslides related to the Lushan earthquake.

In general,the displacement produced by a magnitude 6～7 earthquake is relatively small,even does not reach the surface,so it is difficult to be preserved in geological records. On the other hand,the seismogenic fault of such earthquakes is easy to be considered incorrectly as a non-active fault since Holocene,consequently overlooking the real seismic hazard in the future. To solve this problem,we propose a type of faults that are capable of generating M6～7 earthquakes,but with weak surface activity and cannot produce conspicuous surface displacement. To recognize such faults from geological records,which have no visible evidence of activity since middle-late Pleistocene,is the key to intermediate-and long-term earthquake prediction. The specific procedures of the technology are as follows: First,we determine the seismotectonic setting of the tectonic system in which the target fault lies. Second,we establish the relation between the target fault and other active faults in the same tectonic system,which have records of historical earthquakes or paleoearthquakes. Then we compare varied seismogenic units in the same-order structure,same tectonic system,and varied stages in the same tectonic process. The case studies demonstrate that this is an effective method for intermediate-and long-term earthquake prediction. The cases studied include the Puduhe-Xishan Fault in Kunming City,Hanzhong Basin in the north section of the Longmen Shan Fault zone,Dachuan-Shuangshi Fault in the south section of the Longmen Shan Fault zone,and the Guguan-Guoshun Fault of the Longxian-Baoji Fault zone. These faults all show weak activities on the surface and have potential for earthquakes with estimated magnitude 6.5～7.0.In addition,by estimation using this method,the Taoyuan-Guichuan Fault of the Longxian-Baoji Fault zone has a seismic risk of M6.0～6.5 earthquake,and the Longxian-Qima-Mazhao Fault is capable of producing an earthquake about M7.5.

The co-seismic surface rupture signs of the "4.20" Lushan M_S7.0 earthquake are found at Longmen Township,Lushan County. The sites of rupture signs have a linear distribution with a 2～3km length and N40°～50°N strike. The maximum shortening of the rupture is about 8cm,uplifting is about 1～2cm. Strike-slip component is not observed,but the dynamic process of the earthquake is characterized by compression from northwest to southeast. The observed co-seismic surface ruptures can be oblique shear-fissure,or thrusting crack,however most of them are extensive fissures,which can be explained by the local extensive stress-field on the top of the thrust bending. Although these ruptures have different geometric shapes or variant mechanic features,they similarly reflect the northwest-southeast compression and the surface lift-bending on the top of a thrusting seismogenic structure. Comparing with Dachuan-Shuangshi Fault(frontal fault)and Dayi-Mingshan Fault(piedmont fault),Lushan-Longmen presumed blind fault is more likely the seismogenic fault,which is also consistent with the results of the Lushan earthquake sequence relocations and the seismic intensity contours. As the seismogenic fault of the Lushan earthquake has surpassed the frontal fault of Longmen Shan,it may be a new-generated tectonics,which implies that it is important to re-evaluate the seismic risk at the piedmont area of the Longmen Shan. However,the conclusions are still very primary and geophysical survey is needed to demonstrate the existence of the Lushan-Longmen presumed blind fault.

In the paper,we use 296334 P_n rays from 27777 earthquakes recorded by 1354 seismic stations in the eastern Asia region,an area spanning from 15癗-60癗 and 60癊-145癊,to invert the P_n velocity structure of uppermost mantle and its lateral variations. We also discuss the tectonic implication of the lateral variation based on the P_n velocity structure. Inversion results show that the mean velocity of P_n is 8.03km/s in the eastern Asia region. The lateral variation spans from -0.42km/s to 0.41km/s. Two different tectonic regions can be identified according to distributions of P_n velocity and its lateral variation. In the region from the boundary of the Pacific plate to the east of 108癊,i.e.the eastern China,P_n velocity is relatively low. In the region located at west of 108癊,P_n velocity is high. The different distribution of P_n velocity is consistent to the regional geodynamic background. The dominant geodynamic effect is subduction of the Pacific plate in the eastern China and collision of the Indian Ocean plate in the western China. At the same time,P_n velocity is relatively low in active tectonic regions and high in the stable tectonic regions. The P_n velocity also shows negative correlation to the regional seismicity. The higher of the seismicity,the lower of the P_n velocity,and vice versa. The negative correlation indicates the relationship of lateral variation of P_n velocity and tectonic deformation with seismicity level. The region with low P_n velocity usually is a region with high tectonic deformation and intensive seismicity.

Because b-value can be used as an indicator to measure the level of crustal stress,the seismicity pattern recognition based on b-value is considered as one of the most intuitive methods for earthquake forecast research and experiments. The intense seismicity in the Sichuan-Yunnan region makes the area to be an ideal field for earthquake forecast experiment. We investigated the spatial and temporal variations of b-values before nineteen earthquakes with M≥6.5 in the Sichuan-Yunnan region from 1981 to 2008 using regional small earthquake data. The results show that spatial b-value variations portending M≥6.5 earthquakes in the Sichuan-Yunnan region are insignificant. Less than 50%of the major earthquakes occurred in the regions with low b-values relative to the entire study area. In terms of temporal evolutions,the b-values in seismic source regions showed a systematic drop before the occurrence of earthquake. They showed an obvious decreasing trend during the period one year prior to strong earthquakes in 58%of the seismic source regions. Combined with other geological and seismological data,the short-term seismicity data can provide certain enlightenment for the estimation of potential seismic hazard in specific regions.