With the acceleration of urbanization process, solving the earthquake and its associated disasters caused by buried active fault in urban areas has been a difficult issue in the construction of urban public security system. It is difficult to deal with the anti-seismic issues of cross-fault buildings using the existing techniques, therefore, reasonable setback distance for buried active fault in urban area is the only method for the planning and construction at the beginning. At present, theoretical research about setback for active fault is becoming more and more mature, and the mandatory national standard “Setback distance for active fault” will be enacted soon. As a result, how to work on the basis of these theories and national standards is in urgent. In recent years, the exploration of urban active faults was successively completed. However, there are no typical cases of how to make full use of the achievements of urban active fault projects in the follow-up work, and how to guide urban construction based on the project conclusions, so as to ensure urban safety and rational development of urban economy.

In this paper, taking a site along the Anqiu-Juxian Fault in the Tanlu fault zone in Xinyi city as an example, based on the results of 1︰10 000 active fault distribution map, and referring to the stipulation of national standard “Setback distance for active fault”, 12 shallow seismic survey lines with a spacing of less than 50m were laid out firstly, and the results of shallow seismic exploration show the existence of two high-dip faults in the site. Secondly, considering the shallow seismic survey results and the geologic site conditions, five rows of borehole joint profiles were selected along five of the shallow seismic survey lines. Based on the location of the faults and stratigraphy in the site revealed by the borehole joint profiles, and considering the latest research results of Quaternary stratigraphy and the conclusion of urban active faults detection, the west branch fault is constrained to be a Holocene active fault and the east branch fault is an early Quaternary fault. As a result, we precisely mapped the trace, dip and upper breakpoint of the fault in the site based on the shallow seismic exploration and joint borehole profile. The accurate positioning of the plane position of the active fault differs by about 200m from the 1:1000 strip distribution map.

According to the relevant national standards and scientific research results, active faults in the site shall be avoided. Based on the surface traces of active faults revealed by the accurate detection in the site, the active fault deformation zone was delineated, and the range of setback distance for active fault was defined outside the deformation zone. The detection results accurately determined the plane distribution of the active fault in the site, which meets the accuracy of the development and utilization of the site. Based on the accurately located active fault trace, and complying with the forthcoming national standard “Setback distance from active fault”, this study not only scientifically determines the setback distance for active fault in the site, but also releases the scarce land resources in the city. This result achieves the goal of scientifically avoiding potential dangerous urban hidden active fault and making full use of land.

The case detection process confirms that the results of urban active fault detection are still difficult to meet the fault positioning accuracy required for specific site development, and the range of active fault deformation zone within the site must be determined based on the precise positioning method for hidden active faults as stipulated in the national standard “Setback distance for active fault”. The national standard “Code for seismic design of buildings” only specifies the setback distance for active faults under different seismic intensity, but does not provide any clear definition of the accuracy of active fault positioning, so it is difficult to define the required active fault positioning degree and boundary range of the deformation zone of active fault in practice. The national standard “Setback distance for active fault” clearly defines various types of active fault detection and positioning methods, determines the scope of active fault deformation zone and the accurate setback distance for active fault in different cases. The specific case proves that before developing and utilizing specific sites along urban concealed active faults, relevant work shall be carried out according to the national standard “Setback distance for active fault” to effectively resolve the issue about the relations between urban development and urban safety, so the promulgation and implementation of national standard should speed up.

The eastern marginal fault of Daxing Uplift is located in the southeast of the Beijing Plain, which is a boundary fault that controls the Daxing Uplift and the Langgu Sag. It intersects obliquely with the NNE-trending Xiadian Fault in the north where a magnitude 8 earthquake occurred in 1679. The overall strike of the fault is northeast, dipping southeast. Previous studies have suggested that the youngest stratum of the fault is the Mid Pleistocene of the Quaternary and it is not an active fault since the Late Quaternary. Based on high-precision shallow seismic exploration data, this study carried out high-density composite drilling geological section surveys and obtained evidence of obvious activity of the fault since the Late Quaternary. The fault is shown as an active normal fault in the composite drilling geological section. The top of the footwall of the fault is the 7m-thick silty clay marker layer buried at the depth of 74m and the top of the hanging wall is 102m deep, the amount of dislocation is about 28.0m. Fault slip surfaces were found in the cores of two of the boreholes, with depths of 54.2m and 39.4m, respectively. The buried depths of the top surface of the marker layer in the two boreholes with a horizontal distance of 2m are 8m and 10m, respectively, the dislocation amount is 2m. Combined with the observation of core deformation characteristics of the two boreholes, it is believed that the buried depth of the upper breakpoint of the fault may be shallower. This research has changed the understanding that the fault zone on the eastern margin of the Daxing Uplift is not active. This new discovery not only has great application value for understanding the risk of large earthquakes of this fault zone and the risk of earthquake disasters in Beijing, but also has scientific significance for the study of fault development and evolution and the deep-shallow coupling process in North China since the late Cenozoic. The main knowledge gained is as follows: 1)Through high-precision shallow seismic exploration, it is found that the Neogene and above strata in the study area generally show an inclined morphology which is deep in the south and shallow in the north. The strata below the Neogene are in angular unconformity contact with the bottom interface of the Neogene, and the depth of the shallowest upper breakpoint is about 38~43m. 2)The combined drilling geological section exploration reveals rich dislocation information of stratigraphic markers and further confirms the existence of active faults by borehole stratigraphic correlation. In the drill cores, fault slip surfaces were observed in the late Pleistocene strata at the depth of 39.4m, 51.5m and 54.2m, respectively. The stratigraphic comparison of the boreholes 5^# and 9^# with a hole spacing of 2m further reveals a fault throw of about 2m in the stratum at the buried depth of 8~10m, thus, it is inferred that the depth of the upper breakpoint on the fault may be 8m or shallower. According to the stratigraphic age data of adjacent boreholes in this area, it is considered that the fault is a Holocene active fault. The specific age of the latest activity and its activity parameters will be further studied through the subsequent borehole chronological tests and large-scale trench excavation.

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.

Pull-apart basin and push-up structure are the two most common and important structures formed within a strike-slip fault system. The term of pull-apart basin was firstly introduced when researchers discussing the formation of Central Death Valley, California. A pull-apart basin typically forms and develops between en echelon surficial strands or along a releasing bend of the transform fault. The diagonal cross-basin fault, formed diagonally within a pull-apart basin, connects the two en echelon strands bounding the basin. This type of fault not only plays an important role in the extinction of pull-part basin, but also controls the occurrence of large earthquakes. Therefore, it is of great significance to study the formation and evolution of cross-basin faults. However, compared with extensively studied pull-apart basins, fewer studies have been conducted on cross-basin fault, which greatly hampers our understanding of the structural evolution process of pull-apart basin and strike-slip fault. In this study, we take the Ganyanchi(Salt Lake)basin, the largest pull-apart basin located in the central part of the Haiyuan Fault, northeastern corner of the Tibetan plateau, as an example to investigate the character and general mechanism of the cross-basin fault. Geomorphological investigation, shallow artificial seismic exploration, and composite drilling geological survey are carried out along the cross-basin fault in Ganyanchi Basin. The main conclusions are listed as below: 1)The cross-basin fault in Ganyanchi Basin is a reverse strike-slip fault dipping to SW, rather than the previously claimed a strike-slip fault with significant normal component; 2)Although with a classic rhombic shape, the Ganyanchi Basin is actually an asymmetric pull-apart basin, which is mainly controlled by the northern boundary fault, i.e., the Nan-Xi Hua Shan Fault. Under the control of the Nan-Xi Hua Shan Fault, more than 680m thick growth strata were accumulated in the basin, and a rollover anticline was formed by the growth strata; 3)The example of the Ganyanchi asymmetric pull-apart basin suggests that a possible mechanism for the reverse cross-basin fault is probably the “straightening of strike-slip fault”, that is, the earlier formed antithetical normal fault adopts reverse component of straightened basin boundary fault, and then undergoes rotation and finally becomes a synthetical reverse fault with great strike-slip component. The rollover anticline, mainly controlled by the master boundary fault of an asymmetric pull-apart basin, may also partly contribute to the rotation of the cross-basin fault. As more natural pull-part basins needed to be investigated, caution should be taken when this suggested model is applied elsewhere.

Complex geometrical structures on strike-slip faults would likely affect fault behavior such as strain accumulation and distribution, seismic rupture process, etc. The Xianshuihe Fault has been considered to be a Holocene active strike-slip fault with a high horizontal slip rate along the eastern margin of the Tibetan plateau. During the past 300 years, the Xianshuihe Fault produced 8 earthquakes with magnitude≥7 along the whole fault and showed strong activities of large earthquakes. Taking the Huiyuansi Basin as a structure boundary, the northwestern and southeastern segments of the Xianshuihe Fault show different characteristics. The northwestern segment, consisting of the Luhuo, Daofu and Qianning sections, shows a left-stepping en echelon pattern by simple fault strands. However, the southeastern segment(Huiyuansi-Kangding segment)has a complex structure and is divided into three sub-faults: the Yalahe, Selaha and Zheduotang Faults. To the south of Kangding County, the Moxi segment of the Xianshuihe Fault shows a simple structure. The previous studies suggest that the three sub-faults(the Yalahe, Selaha and Zheduotang Faults of the Huiyuansi-Kangding segment)unevenly distribute the strain of the northwestern segment of the Xianshuihe Fault. However, the disagreement of the new activity of the Yalahe Fault limits the understanding of the strain distribution model of the Huiyuansi-Kangding segment. Most scholars believed that the Yalahe Fault is a Holocene active fault. However, Zhang et al.(2017)used low-temperature thermochronology to study the cooling history of the Gongga rock mass, and suggested that the Yalahe Fault is now inactive and the latest activity of the Xianshuihe Fault has moved westward over the Selaha Fault. The Yalahe Fault is the only segment of the Xianshuihe Fault that lacks records of the strong historical earthquakes. Moreover, the Yalahe Fault is located in the alpine valley area, and the previous traffic conditions were very bad. Thus, the previous research on fault activity of the fault relied mainly on the interpretation of remote sensing, and the uncertainty was relatively large. Through remote sensing and field investigation, we found the geological and geomorphological evidence for Holocene activity of the Yalahe Fault. Moreover, we found a well-preserved seismic surface rupture zone with a length of about 10km near the Yariacuo and the co-seismic offsets of the earthquake are about 2.5~3.5m. In addition, we also advance the new active fault track of the Yalahe Fault to Yala Town near Kangding County. In Wangmu and Yala Town, we found the geological evidence for the latest fault activity that the Holocene alluvial fans were dislocated by the fault. These evidences suggest that the Yalahe Fault is a Holocene active fault, and has the seismogenic tectonic condition to produce a large earthquake, just like the Selaha and Zheduotang Faults. These also provide seismic geological evidence for the strain distribution model of the Kangding-Huiyuansi segment of the Xianshuihe Fault.

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.

Ganyanchi (Salt Lake)basin, located in the central part of the Haiyuan Fault, northeastern corner of the Tibetan plateau, is the largest pull-apart basin along this fault. Due to its location in northeastern Tibet, the Ganyanchi Basin preserves an important sedimentary record of tectonism and climate change associated with progressive growth of the Tibetan plateau. The sediments of this basin also contain abundant information regarding the deformational history of the bounding strike-slip fault, i.e., the Haiyuan Fault. Therefore, a detailed study on the depository history of the Ganyanchi Basin is of great importance. Earlier studies only focused on regional geological mapping and paleoseismic research, however, no sedimentologic or chronological work has been done in the Ganyanchi pull-apart basin. To address this problem, we drilled a 328m-deep borehole, named HY-C8, at the south of the cross-basin fault and near the active depocenter, and employ magnetostratigraphic analyses and seismic reflection data to constrain the age and to deduce the evolving history of the basin. The deep borehole profile shows that the stratigraphy of the basin can be divided into three main units (Unit Ⅰ, Ⅱ and Ⅲ), which began to deposit at about 2.76, 2.33 and 1.78Ma, respectively. The grain size of the deposits manifests an upward thinning trend, which probably implies the profile is a characteristic retrogradational sequence. The magnetic susceptibility results indicate that the playa lake probably was formed at about 1.78Ma ago, the corresponding playa-lake deposits recorded more than eight high susceptibility sections, which are most likely due to the iron sulfides (such as melnikovite, pyrrhotine etc.)that were usually produced in high-lake-level and reduction conditions. A combination of boreholes and shallow seismic reflection data indicates that the Ganyanchi Basin is mainly controlled by the cross-basin fault and its northern boundary fault, and the depocenter, probably deeper than 550m, lies in between these two faults. Finally, the sedimentary facies of the Ganyanchi Basin experienced a four-stage evolving history:eluvial facies (before~2.76Ma)to alluvial fan facies (about 2.76~2.33Ma)to distal alluvial fan facies (2.33~1.78Ma)to playa lake facies (1.78Ma~present). Based on accumulation rates, the stage of playa lake can be divided into two subchrons, and the depositional rates of subchrons 2 (about 0.78Ma~present)is as high as 232.5m/Ma, which probably was caused by the activity along the cross-basin fault in the Ganyanchi Basin.

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.

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).

Active fault is one of potential geohazards in cities. Locating and dating buried active faults in urban areas have been a difficult issue in active fault exploration. In this paper, we take the detection of the buried active fault performed at Hehuan Road in the north of Suqian city as an example. We preliminarily mapped the fault through field investigation and shallow seismic reflection survey technique. Furthermore, based on the principle of doubling section method, we conducted multiple drilling to constrain the upper faulted point which is located in a range of 5m in horizon and 4.4~6.1m in depth. Finally, we determined the exact location and latest activity of the fault by trenching. Obviously, good results have been acquired on the accurate location and activity of the Suqian segment of Anqiu-Juxian Fault using multi-level and multi-means detection method. Besides, we observed from the detection at the Hehuan Road site that at least four paleoseismic events occurred during the past 80000 yrs, and the result indicates that the latest faulting event on the fault is younger than(5.9±0.3)ka BP and the buried active fault at the Hehuan Road is a Holocene active fault. The result of buried active fault detection at the Hehuan Road site provides quantitative parameters for evaluation of seismic hazards and planning the width of safety distance in Suqian City.

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.

Hetao fault-depression zone, the largest one of 4 fault-depression zones around the Ordos block, is characterized by intense tectonic activities. According to historical records, 2 large earthquakes, occurring in 849AD and 7BC respectively, were recognized to be located at this zone. However, there is still some dispute about the seismogenic structure of the 849AD earthquake, and there is no tangible geological evidence to support the view that the 7BC event occurred in Hetao fault-depression. In this paper, based on the image interpretation(from Google Earth), field investigation, trench excavation, and ¹⁴C and single grain OSL dating, we analyzed the tectonic landform and paleoseismic events on the Daqingshan piedmont fault, Wulashan piedmont fault and Langshan piedmont fault in the Hetao fault-depression zone. Furthermore, a comparative study of the latest rupture events on the 3 active faults was carried out. In order to lower the uncertainty of paleoseismic event dating, several effective measures, such as sampling according to the stratigraphic sequence, collecting multi samples in important strata, were adopted. Combining the previous achievements, the seismogenic structures of the 849AD earthquake and the 7BC earthquake were discussed. The results support that the Daqingshan piedmont fault is the seismogenic structure of the 849AD earthquake, and the latest surface rupture event of the Langshan piedmont fault may be related to the 7BC earthquake.

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.

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.

Lingqiu Basin is located in the northeast of the Shanxi graben system,where a M_S 7.0 earthquake occurred in 1626.The achievement of active fault research in this basin could contribute not only to the study of the seismogenic structure of the earthquake in 1626,but also to the research of the types of large earthquakes in Shanxi graben system. Much work has been conducted here,laying the foundation for the active fault study in this area. However,the spatial distribution and activity of several major faults,and the seismogenic structure of the earthquake in 1626 are still in discussion. This paper analyzes the geomorphologic characteristics in the whole basin via interpreting SPOT5 images,SRTM3 and fieldwork,and acquires some new knowledge of the major faults in combination with trenching. The activity of the main segment of the piedmont fault of Taibaiwei Mountains is limited to the late Pleistocene; The NEE-striking Shuijian-Luoshuihe Fault has obvious geomorphic features to the west of Lingqiu County,and the geomorphic feature of the fault is not remarkable to the east of the county. Its latest event left a 1m-high fault scarp on the surface. The NW-striking Huashanhe Fault behaves as a hinge fault. In the northern basin,the fault dips west,producing a height difference of about 10m in terrace T1 of the Huashanhe River. In the southern basin,the fault dips east. Profiles and geomorphic features show the south segment of the fault is an active strike-slip fault with a high angle. Thus,we consider the earthquake in 1626 resulted from the conjugated action of the NEE-striking Shuijian-Luoshuihe Fault and the NW-striking Huashanhe Fault.

The main purpose of paleoseismic study is to distinguish or reveal deformation evidence of large earthquakes recorded by geologic and geomorphic features,and obtain corresponding seismic parameters such as timing,recurrence behavior,and coseismic displacement of large earthquakes. To achieve the aforementioned scientific aim,whether a trenching site preserves evidence of a complete paleoseismic sequence since late Quaternary and contains multiple measurable samples or not,and whether it can accurately identify paleoseismic events and collect well-constrained samples on events or not,all of these problems are directly responsible for reliabilities on assessment of future large earthquake hazard. Due to special displacement styles on strike-slip faults,good trenching sites are not widespread. Through comprehensive analysis on characteristics of coseismic surface ruptures and influencing factors on several trenching cases,we suggest micro-landforms such as depressions,basins,troughs,sag ponds,successive-offset channels,continuous scarp-derived deposit and multiple geomorphic surfaces are likely to be good trenching sites for paleoseismic studies on strike-slip faults. Multiple trenching or three-dimensional trenching should be the primary layout on strike-slip faults. Offsets of micro-landform across a fault,young stratigraphic units overlying on faulted units,locally distributed scarp-derived colluvial deposits,filled fissures,abrupt increases or decreases in displacement of different stratigraphic units on a fault,warping in different degrees,and multiple periodic paleo-sag ponds accumulation,all of these deformation evidences are good indicators for identifying paleoseismic events. To narrow uncertainties of paleoseismic studies,we should base on an organized research process and make a technical proposal and sophisticatedly conduct trenching work. Conclusions need to be repeatedly checked and widespread discussed,and also we should pay much attention on details and use more evidence to support or supplement analysis.

Slip rate of active faults is deterministic to compare active earthquake behaviors among different faults or different segments along a fault,and also it is a key parameter for seismic hazard assessment.Geologically reliable estimation on slip rate is subject to two active tectonic parameters,the cumulative displacement produced by multiple surface-rupturing seismic events and the corresponding true ages, respectively.Generally, for strike-slip faults,we carefully measure geomorphic expressions,such as deformation or offset produced by multiple faulting on river terrace,alluvial-fluvial fans or gullies,and then integrate geochronological constraint from dating on these geomorphic expressions.Based on the above two crucial parameters,we further determine slip rates along faults.However,this paper is attempted to use another deformation of geomorphic expression,a growth model for a small triangular pull-apart basin(sag pond),to constrain fault slip rate at the Daqingliangzi section of Zemuhe Fault on the southeastern margin of the Tibetan Plateau.Based on several three-dimension trench excavations,reliable radiocarbon dating at the bottom of stratigraphic unit in the triangular pull-apart basin(sag pond),detailed field investigation along the Daqingliangzi section of Zemuhe Fault and accurate RTK(GPS)survey,we suggest that Holocene average left-lateral slip rate of the Zemuhe Fault is constrained between 2.4±0.2mm/a and 3.6mm/a,which is a little smaller than those estimated by other geoscientists,however this strike-slip rate is much more accordant with paleoseismic recurrence behaviors and present velocity field obtained from GPS measurement across the Zemuhe Fault.

There are several thrust-fold belts developed in the Kalpin nappe system of the southwestern Tianshan Mountains.Not only deformation rates of these thrust-fold belts are inconsistent,but also the paleoearthquakes recurrence laws on these thrust-fold belts in the nappe system are different.The Beichuan-Yingxiu Fault and the Pengguan Fault ruptured simultaneously in the M_w 7.9 Wenchuan earthquake.Therefore,it is worth discussing the question of how to determine the cascade-rupturing of a paleoearthquake on two or three thrust faults.We measured the scraps of different heights on the geomorphic surfaces(alluvial-proluvial fans)of different stages in eastern Kalpintage and Saergantage and analyzed the paleoearthquake events revealed by trenches in Shanchakou and Saergantage.Using the ¹⁰Be exposure age,we obtained the ages of the geomorphic surfaces.Then we got the upper and lower limit time of each paleoearthquake from the age of adjacent geomorphic surfaces.Finally,we got the recurrence intervals of different paleoearthquakes,the vertical dislocation of a single event,and the time range of the respective events.The results show that since 20ka BP,the average recurrence interval of paleoearthquake in the piedmont of east Kalpintage is 6.7±0.84ka,the vertical dislocation of a single event is 1m; the average recurrence interval of paleoearthquake in the piedmont of Saergantage is 5.4±0.50ka,and the vertical dislocation of a single event is 0.8～1.2m.The intensity of paleoseismicity is basically identical and the recurrence interval in Saergantage is slightly shorter.In the end,we discussed the possibility of cascade-rupturing accompanying these paleoearthquake events and found that the second and the third paleoearthquake events revealed by the trenches overlap in their occurrence time ranges,indicating the possibility of cascade-rupturing during the earthquake.