Terrestrial in-situ cosmogenic nuclide dating(TCND)is one of the most important geochronological techniques for the paleoseismic study of bedrock fault scarps, landslides, and rock avalanches. With many target minerals, due to its uncomplicated composition, widespread occurrence, and simple chemical treatment, Quartz has emerged as an ideal dating material for terrestrial in-situ cosmogenic nuclides dating methods, such as ¹⁴C, ¹⁰Be, ²¹Ne, and ²⁶A1. Prior to accelerator mass spectrometry measurement, the separation of pure quartz from field-collected rock samples was a pivotal step in TCND. However, the elevated aluminum content in quartz samples undermines the reliability of TCND results. Generally, most of the aluminum content in samples originates from impurities like feldspar. To ensure accurate dating outcomes, the content of Al in samples should be reduced to less than 200 ppm. Therefore, effective separation of feldspar and quartz in samples and obtaining pure quartz is the first step in TCN dating. The conventional HF/HNO₃ etching method to separate and purify quartz is widely utilized, but it is time-consuming and low-efficiency. Particularly during the HF/HNO₃ etching stage when dealing with granitic samples containing abundant feldspars and mica impurity minerals necessitates repeated treatments to eliminate feldspars completely; this not only increases etching cycles but also leads to sample loss significantly. It has a great impact on the application of in-situ cosmogenic nuclide dating in active tectonics. Consequently, physically separating quartz from samples before chemical purification can effectively shorten the chemical etching duration while the flotation separation method can effectively remove most gangue minerals in quartz and achieve preliminary purification of quartz.

This article presents a laboratory-integrated flotation purification device and proposes enhancements to the conventional quartz etching process in order to improve its purification efficiency. The purification device uses dodecylamine as the collector, hydrofluoric acid as the feldspar activator, nitric acid as the regulator, and eucalyptus oleanol as the foaming agent. The bubbling component within the device provides sufficient carbon dioxide bubbles to float out feldspar and other minerals in the sample reversely. To evaluate its efficacy in flotation separation, enrichment, and purification, this study conducted tests on two commonly encountered bedrock samples of granitic gneiss and quartzite.

Observation results under a stereomicroscope show that the quartz content in the quartz component after floating is more than 90%. The etching results of the whole rock and the floated quartz components show that after etching 2-3 times with HF/HNO₃, the Al concentration can be reduced to less than 200ppm, which fully meets the requirement of cosmogenic nuclide dating. The quartz separated by flotation from cryptocrystalline quartzite samples can also reach the dating requirements after etching 7-8 times.

Compared to direct etching following bulk-rock sample crushing, this approach reduces etching time by over a half, significantly minimizing reagent consumption for HF/HNO₃ etching and thereby enhancing TCND efficiency. The bubbling power section of our flotation device directly introduces carbon dioxide gas into the flotation liquid to increase the bubble content in the slurry. Consequently, there is improved collision and contact between quartz and feldspar particles with bubbles, resulting in enhanced flotation effectiveness. This system can be effectively employed for separating feldspar from other impurity minerals present in gneiss samples. The proposed flotation process in this study is straightforward and user-friendly while allowing flexibility in adjusting sample quantities ranging from tens to hundreds of grams as required. Furthermore, this high-efficiency flotation separation system may offer insights into processing zircon, apatite, and other dating samples.

Using the Monte Carlo random sampling method, a set of probabilistic seismic hazard analysis calculation programs that integrates our country’s traditional planar potential seismic source zone and three-dimensional fault sources is developed. The program is not only suitable for our country’s traditional regional area sources, but also considers the rupture scale of earthquakes and is compatible with the probabilistic seismic hazard calculation of three-dimensional fault sources. The algorithm developed in this paper efficiently realizes the three-dimensional simulation of the seismic event set of the fault source and introduces the earthquake rupture scale into the probabilistic seismic hazard analysis calculation in China, which significantly improves the rationality of the seismic hazard calculation in the near-fault area. In order to improve the execution efficiency of the program, the algorithm adopts the method of filling grid points in the planar potential seismic source zone in advance and randomly simulating the uniform distribution of seismic events in the planar potential seismic source zone. For the seismic hazard calculation of elliptical attenuation relationship, the algorithm uses pre-constructed three-dimensional matrices of the distance of the ellipse minor axis under different magnitudes, distances, and different angles between sites and the ellipse long axis direction of potential seismic source zone, and directly obtains the corresponding distance of ellipse minor axis through table look-up and interpolation. The algorithm developed in this paper avoids the problem of low computational efficiency in the iterative approximation of the distance of the ellipse minor axis. The mathematical expression of the three-dimensional fault source is based on the Frankel fault plane form of the 2002 edition of the National Seismic Hazard Map of the United States. The surface track and average dip Angle of the fault are used to create the rectangular fault plane, in which the dip direction of each rectangle is always perpendicular to the strike of its local fault segment. To maintain the coordination between the rupture area and the magnitude, the rupture of the earthquake occurring on the fault plane should not exceed the fault plane or the combination of fault planes. If the boundary of the rupture plane is outside the fault boundary, the entire rupture plane will move so that the boundary of the entire rupture plane matches the boundary of the fault plane. Using the probabilistic seismic hazard program of the Seismic ground motion parameters zonation map of China(2015)and the algorithm developed in this paper, the regional seismic hazard of the study area including Changsha-Zhuzhou-Xiangtan of the urban agglomeration in Hunan Province with moderate to strong seismic activity are calculated. Seismic hazard at different probability levels(return periods of 50.8, 475 and 2 475 years, respectively)for the Changde near-fault sources and Zhuzhou sites are also computed. The comparative study shows that the procedure of the Seismic ground motion parameters zonation map of China(2015)underestimates the seismic hazard near the three-dimensional fault source, and the degree of underestimation becomes more significant as the probability level decreases. Considering the influence of the earthquake rupture scale at the low exceedance probability level, the decomposition results of the seismic hazard for sites near fault show that the contribution of the seismic hazard is different from that of the traditional method of the Seismic ground motion parameters zonation map of China(2015), which mainly focuses on the earthquake of high magnitude. However, earthquakes of all magnitudes on the fault source can contribute to the seismic hazard, but the proportion of high magnitudes is the largest. Finally, an example verifying the probabilistic seismic hazard program(data set 1 case 10)from the Pacific Earthquake Engineering Research Center(PEER)is used to verify the reliability of the algorithm developed in this paper.

It is difficult to use traditional trenching and field geological investigation to yield the age of paleoseismic events along active fault in western mountainous areas of China where the geomorphic trace mark and sediments are often eroded or altered by human activities. The recurrence interval of paleoearthquake possesses greater uncertainty. It is necessary to yield ages of paleoearthquake event from different ways and examine the reliability of paleoearthquake results. In these regions, an earthquake with magnitude greater than 7 can produce rock avalanches around 200~400km away from the epicenter, such as the Wenchuan earthquake in 2008, due to their structure setting of strong neotectonic activity and the higher topographic relief. Therefore, the seismic bedrock landslide and rock avalanche can record the occurrence time, intensity and damage of strong earthquake in the mountainous area. This provides a new way to assess the frequency and intensity of paleoearthquake occurring in the intraplate continental areas(such as the north-south seismic zone)where strong earthquakes recurred for hundreds to thousands of years based on the seismic landslide records. Identifying ancient earthquake bedrock collapse relics in Quaternary deposits and accurately determining their ages will not only help broaden the study on the recurrence history of active fault, but also assess the earthquake risk in mountainous area.
As shown by previous studies, the Schmidt-hammer exposure-age dating(SHD)method is a relatively simple, rapid, cheap and non-destructive in-situ exposure age dating method. In this study, ancient earthquake bedrock landslides and rock avalanches with known historical records distributed on the Qinling northern piedmont fault and the Huashan piedmont fault are used to preliminarily establish the rock weathering factor with age calibration curve. The rebound values of rock surface at dozens of sampling sites of each rock avalanche and landslide are measured by Schmidt hammer and analyzed statistically. The weathering factor of the exposed rock of each rock avalanche and landslide is calculated and the solution of SHD method is discussed. The reliability of SHD is evaluated according to the measured data and the records of historical age. The main conclusions are as follows:
(1)The Schmidt hammer rebound value of rock surface at three ancient earthquake bedrock landslides and rock avalanches is negatively correlated with their historical ages. The older the historical record age, the lower the average rebound value of the rock, and vice versa. Based on the statistical analysis of weathering factors of rocks of bedrock landslides and rock avalanches, a preliminary age calibration curve is obtained as T=(19 723±888)×f_w-(2 145±166). This curve can be used to infer the bedrock landslides and rock avalanches of more than 5×10² a BP, and it provides a new relative dating method for the ancient bedrock landslide and rock avalanches within the age of 3 000a BP in the northern margin of the Qinling Mountains.
(2)Under the climatic and lithological conditions of the northern margin of the Qinling Mountains, the relative ages of bedrock landslide and rock avalanches can discriminate the interval of millennium scale according to the rock rebound value measured by Schmidt hammer. However, it cannot distinguish the difference in weathering degree of the bedrock landslide and rock avalanches with the interval of less than 500 years.
(3)The Schmidt hammer rebound value measured repeatedly on fresh rocks shows that the fluctuation range of the rebound values is small, within the value of 0-3, which is helpful to rapidly select qualified sampling sites for terrestrial in-situ cosmogenic nuclide dating(TCND). Thus, the Schmidt hammer value can be used to evaluate whether the rock samples have the problem of nuclide inheritance induced by complex exposure history such as post-exposure and secondary transportation. This would introduce greater objectivity to the sample selection and possibly require less samples, thus reducing the costs; meanwhile, it will improve the dating efficiency and ensure the reliability of TCND. Therefore, SHD method is a valuable complementary method to TCND.
(4)Under the climatic and lithological conditions in the northern margin of the Qinling Mountains, the rebound value decreases by (25%±1%) for rocks after weathering for 2ka, by (16%±1%) for 1ka, and by (15%±1%) for 0.5ka.

The Karakoram Fault is located in the west of the Qinghai-Tibet Plateau and crosses Kashmir, Xinjiang and Tibet in China. It is a large normal dextral strike-slip fault in the middle of the Asian continent. As a boundary fault dividing the Qinghai-Tibet Plateau and the Pamir Plateau-Karakoram Mountains, the Karakoram Fault plays a role in accommodating the collision deformation between the Indian plate and the Eurasian plate and in the tectonic evolution of the western Qinghai-Tibet Plateau. The fault trace in Ngari area is clear and the faulted landforms are obvious, which show strong activity characteristics in late Quaternary. As a large active fault, only one earthquake of magnitude 7 has been recorded on the Karakoram Fault since the recorded history, namely, the Tashkurgan earthquake of 1895 at its north end. There are no records of strong earthquakes of magnitude≥7 along the rest of the fault, and no paleo-seismic research has been carried out. Ages of recent strong earthquake activity and earthquake recurrence intervals are not clear, which greatly limit the accuracy of seismic risk assessment. In this study, we investigated the fault geometry and faulted landforms in Ngari area, collected OSL samples of the faulted landforms and sag ponds in Zhaxigang, Menshi and Baga towns and preliminarily discussed the ages of recent strong earthquake activity.

Study shows that the fault can be divided into three sections by Zhaxigang town and Suoduo village, and the structure and properties of each section are significantly different. In west Zhaxigang town section, the fault is dominated by dextral strike-slip with certain vertical movement, it is almost straight on the surface, with river terraces, alluvial-proluvial fans and water system faulted ranging from tens to hundreds of meters. In Zhaxigang town to Suoduo village section, the normal faulting is remarkable, the main fault constitutes the boundary fault between Ayilari Mountain and Gar Basin; fault facets and fault scarps are common along the fault line, there are also secondary faults with the same or opposite dip as the main fault developed near the piedmont basin. In east Suoduo village section, the main part of the fault is located at the south foot of Gangdise Mountain, and in addition to the piedmont fault, several approximately parallel faults are also developed on the southern alluvial-proluvial fans and moraine fans which are mainly dextrally faulted with certain vertical component.

According to the analysis of the faulted landforms and dating of the OSL samples collected from the sag ponds and faulted landforms in the west of Zhaxigang town, the east of Menshi town and the east of Baga town, the ages of recent strong earthquake activity on the fault are analyzed as follows. In the west of Zhaxigang town, the age of recent strong earthquake activity of the fault is constrained to be close to 2.34kaBP according to the average OSL dating results of KKF-3 and KKF-4. In the east of Menshi town, the recent earthquake activity age of fault f₂ is 4.67~3.01kaBP, but closer to 3.01kaBP according to the OSL dating results of KKF-11 of the youngest faulted geomorphic surface and average OSL dating results of KKF-6 and KKF-13 collected from sag ponds. In the area near Angwang village, Baga town, it is inferred that the recent strong earthquake activity age of the fault is close to 2.54kaBP according to the OSL dating results of KKF-2 collected from sag pond. If the faults of above three places are active at the same time, the age of recent strong earthquake activity of the fault is close to 2.63kaBP. The Karakorum Fault in Ngari area has obvious segment boundaries, and the activity of each segment and in its internal branch faults is most likely to be independent.

The earthquake recurrence interval on the fault is estimated to be 2.8ka according to the slip rate and the amount of displacement. From the above analysis, it can be seen the time since the last strong earthquake activity of Karakorum Fault may have been very close to the interval of earthquake recurrence. If the fault is characterized by a quasi-periodic in-situ recurrence, the energy accumulation in the fault may have reached a very high degree and the risk of recurrence of strong earthquake events of the fault may be very high, so more attention should be paid and more detailed research on the paleo-earthquake events and recurrence intervals should be carried out as quickly as possible.

At 02:04 a.m. on May 22, 2021, a M_S7.4 earthquake occurred in the Maduo County, Qinghai Province, China. Its epicenter is located within the Bayan Har block in the north-central Tibetan plateau, approximately 70km south of the eastern Kunlun fault system that defines the northern boundary of the block. In order to constrain the seismogenic fault and characterize the co-seismic surface ruptures of this earthquake, field investigations were conducted immediately after the earthquake, combined with analyses of the focal parameters, aftershock distribution, and InSAR inversion of this earthquake.
This preliminary study finds that the seismogenic fault of the Maduo M_S7.4 earthquake is the Jiangcuo segment of the Kunlunshankou-Jiangcuo Fault, which is an active NW-striking and left-lateral strike-slip fault. The total length of the co-seismic surface ruptures is approximately 160km. Multiple rupture patterns exist, mainly including linear shear fractures, obliquely distributed tensional and tensional-shear fractures, pressure ridges, and pull-apart basins. The earthquake also induced a large number of liquefaction structures and landslides in valleys and marshlands.
Based on strike variation and along-strike discontinuity due to the development of step-overs, the coseismic surface rupture zone can be subdivided into four segments, namely the Elinghu South, Huanghexiang, Dongcaoarlong, and Changmahexiang segments. The surface ruptures are quite continuous and prominent along the Elinghu south segment, western portion of the Huanghexiang segment, central portion of the Dongcaoarlong segment, and the Huanghexiang segment. Comparatively, coseismic surface ruptures of other portions are discontinuous. The coseismic strike-slip displacement is roughly determined to be 1~2m based on the displaced gullies, trails, and the width of cracks at releasing step-overs.

China’s seas and adjacent regions are affected by interactions among the Eurasian plate, the western Pacific plate, and the Philippine Sea plate. Both intraplate and plate-edge earthquakes have occurred in these regions and the seismic activities are frequent. The coastal areas of China are economically developed and densely populated. With the development and utilization of marine energy and resources along with the development of national economy, the types and quantity of construction projects in the marine and coastal areas have increased, once an earthquake happens, it will cause huge damage and loss to these areas, therefore, the earthquake-related research for these sea areas cannot be ignored and the need for study on these areas is increasingly urgent. One type of essential basic data for marine seismic research is a complete, unified earthquake catalog, which is an important database for seismotectonics, seismic zoning, earthquake prediction, earthquake prevention, and disaster reduction. Completeness and reliability analysis of an earthquake catalog is one of the fundamental research topics in seismology.
At present, four editions of earthquake catalogs have been officially published in China, as well as the earthquake catalogue compiled in the national fifth-generation earthquake parameter zoning map, these catalogs are based on historical data, seismic survey investigations, and various instrumental observations. However, these catalogs have earlier data deadlines and contain the earthquake records for only the offshore regions of China, which are extensions of coastal land. Distant sea regions, subduction zones, and adjacent sea regions have not been included in these catalogs. Secondly, there were no cross-border areas involved in the compilation of earthquake catalogs in the past. It was not required to use magnitudes measured by other countries’ seismic networks and observation agencies to develop an earthquake catalog with a uniform magnitude scale, moreover, there was no formula suitable for the conversion of magnitude scale in China’s seas areas and adjacent regions. Little research has been conducted to compile and analyze the completeness of a unified earthquake catalog for China’s seas and adjacent regions. Therefore, in this study, we compiled earthquake data from the seismic networks of China and other countries for China’s seas and adjacent regions. The earthquake-monitoring capabilities of different sea areas at different time periods were evaluated, and the temporal and spatial distribution characteristics of epicentral location accuracy for China’s seas and adjacent regions were analyzed. We used the orthogonal regression method to obtain conversion relationships between the surface wave magnitude, body wave magnitude, and moment magnitude for China’s seas and adjacent regions, and established magnitude conversion formulae between the China Seismic Network and the M_L magnitude of the Taiwan Seismic Network and the M_S magnitude of the Philippine Seismic Network. Finally, we developed an earthquake catalog with uniform magnitude scales for China’s seas and adjacent regions.
On the basis of the frequency-magnitude distribution obtained from the magnitude-cumulative frequency relationship (N-T) and the Gutenberg-Richter(GR)law, we conducted a completeness analysis of the unified earthquake catalog for China’s seas and adjacent regions, Then, we identified the beginning years of each magnitude interval at different focal depth ranges and different seismic zones in the earthquake catalog.
This study marks the first time that a unified earthquake catalog has been compiled for China’s seas and adjacent regions, based on the characteristics of seismicity in the surrounding sea regions, which fills the gap in the compilation of the earthquake catalogue of China’s seas and adjacent areas. The resulting earthquake catalog provides a basis for seismotectonics, seismicity study, and seismic hazard analysis for China’s seas and adjacent regions. The catalog also provides technical support for the preparation of seismic zoning maps as well as for earthquake prevention and disaster reduction in project planning and engineering construction in the sea regions. In addition, by evaluating the earthquake-monitoring capability of the seismic networks in China’s seas and adjacent regions and analyzing the completeness of the compiled unified earthquake catalog, this study provides a scientific reference to improve the earthquake-monitoring capability and optimizing the distribution of the seismic networks in these regions.

The location of the buried faults, the fault broken layers and the depth of breakpoints in the Tangshan-Hejian-Cixian seismotectonic zone are not clear. We implemented 4 shallow seismic exploration profiles on the Daming Fault, Cangxi Fault, and Dachengdong Fault. Line DZ1 is located on the Daming Fault in the southeast of Daming County. Five breakpoints were dectectd, which are all normal faults, with depths of 95~125m and displacements about 6~12m, offsetting late Pleistocene but not the Holocene. Line DZ2 is located in the east of Xianxian County to dectect the Cangxi Fault. Three breakpoints were detected, all are normal faults, with depths of 170~190m and displacements about 7~10m. The upper breakpoints of the three faults cut the middle Pleistocene. The lines DZ3 and DZ4 are located in the west of Litan Town, Dacheng County. Four breakpoints were detected, with the upper breakpoint depth of 120~130m and displacements about 5~15m. They are all normal faults, and the upper breakpoints of the faults cut the Pleistocene strata.
The result of the exploration of Cixian-Daming Fault is not consistent with the buried depth 1 200m proposed by XU Hua-ming. It is proved that the activity of the fault is also consistent with the overall activity of the Cixian-Daming Fault, which is an active fault since late Pleistocene.
The Dachengdong Fault and Cangxi Fault offset the middle Pleistocene strata. Although the late Pleistocene active faults are generally defined as active faults in the practice of active tectonics research in China, strong earthquakes in eastern China have shorter recurrence period, and earthquakes of magnitude 6 or so may also occur in some middle Pleistocene active faults.
During the compilation of GB18306-2015 “Seismic ground motion parameter zonation map of China”, there were no late Pleistocene active faults in the M6~6.5 potential source areas in eastern China. Therefore, we believe that the Dachengdong and Cangxi faults still have the ability to generate earthquake of magnitude 6 or so, and the faults have some similarities with the seismogenic structures of Xingtai earthquake swarm. Under the action of the latest tectonic stress field, the “deep faults” tearing ruptured successively and expanded upwards, resulting in stress migration and loading between two neighbouring en-echolon concealed faults, so, the Dachengdong and Cangxi faults are the product of this three-dimensional rupture process. The Dachengdong Fault is a “newly-generated” fault resulting from the tearing rupturing and upward expanding of the pre-existing concealed “deept faults” in the middle and lower curst.

The Hetao depression zone, located to the north of Ordos block, is a complex depression basin that consists of two sub-uplifts and three sub-depressions. The depression zone is subject to the regional extensional stress field driven by the Indo-Asian continental collision and the westward subduction of the Pacific Plate. The Baotou uplift that separates the Baiyanhua sub-depression and Huhe sub-depression is mainly composed of Archean gneiss and is overlaid by Quaternary sedimentary strata. The two sub-depressions are bordered by the Wula Mountains and Daqing Mountains to the north, respectively. The bedrock exhumed in Wula Mountains and Daqing Mountains consists mostly of Precambrian granitic gneiss, and the piedmont depressions are infilled by thick Cenozoic strata. The Wulashan piedmont fault and Daqingshan piedmont fault extend along the range front of Wula Mountains and Daqing Mountains, respectively. The subsidence is controlled by the two boundary faults. Previous studies have preliminarily documented the characteristics of the northwest boundary fault of Baotou uplift. Combining shallow seismic exploration, active fault mapping, and geological drilling, this paper presents a detailed study on the tectonic characteristics of the Baotou uplift.
The shallow seismic exploration reveals that the Baotou uplift is an asymmetrical wedge with a steep southeast wing and a gentle dipping northwest wing. The Baotou uplift is wider in the northeastern part and narrows down towards the southwest. In seismic profiles, the Baiyanhua sub-depression and the Huhe sub-depression manifest as asymmetric dustpan-like depressions with south-dipping controlling faults. Baotou uplift is bounded by the Xishawan-Xingsheng Fault to the northwest and Daqingshan piedmont fault to the southeast. The two faults exhibit significant difference in many aspects, such as fault geometry, fault displacement, the latest active time, and so on. The southeast boundary fault of Baotou uplift is the Baotou section of the Daqingshan piedmont fault which is a Holocene active fault and the major boundary fault of Huhe sub-depression. East of Wanshuiquan, the fault strikes EW-NEE; west of Wanshuiquan, the strike changes to NW. The Daqingshan piedmont fault appears as a south-dipping listric fault in seismic profiles whose dip decreases with depth and cuts through all the sedimentary strata in Huhe sub-depression; the fault extends along the late Pleistocene lacustrine platform at surface with prominent geomorphological evidences. The Xishawan-Xingsheng Fault is a buried high-angle normal fault that mainly dips to the northwest and strikes NE. The fault strike changes to NNE at the eastern tip. Based on the results of seismic exploration and geological drilling, the Xishawan-Xingsheng buried fault is an early to middle Pleistocene Fault capped by late Pleistocene lacustrine strata. We reckon that the Xishawan-Xingsheng Fault is one of the synthetic faults that dip towards the main boundary fault of Baiyanhua sub-depression.
Similarities in lithology, geometry, and structural characteristics of south boundary faults all indicate that Baotou uplift is the western extension of Daqing Mountains. Multiple factors may contribute to the formation of Baotou uplift, such as tectonic subsidence and the development of large-scale river system and mega-lake. We suggest that the upwelling of asthenosphere may play a primary role in the evolution of Wulanshan piedmont fault and Daqingshan piedmont fault. Separated by the Baotou uplift, the Wulashan piedmont fault and Daqingshan piedmont fault can be regarded as independent seismogenic faults. The Hetao depression zone is featured by complex inner structures, and many scientific issues are subject to further researches. Thus, more attention should be paid to the secondary structures within the depression zone for a better understanding on the formation and evolution of Hetao depression zone.

Based on DEM data and ArcGIS software, we extract the geomorphic parameters of drainage basins and rivers that flow through the Huashan piedmont, which include stream length-gradient index (SL), stream-power incision model normalized channel steepness index (k_sn), hypsometric integral (HI), valley floor width to valley height ratio (Vf)and mountain front sinuosity (Smf). Study shows that all parameter indexes have obviously different distributions roughly bounded by Huaxian and Huayin. In the Huaxian to Huayin section, the stream length-gradient index has extremely high abnormal values near the fault, the values of river mean SL, mean k_sn, HI, Vf and Smf are concentrated in 500~700, 120~140, 0.5~0.6, 0~0.1 and 1.0~1.1, respectively. Between Lantian and Huaxian and between Huayin and Lingbao, the parameter indexes distributional characteristics are largely the same, with the values in 300~500, 100~120, 0.4~0.5, 0.2~0.6 and 1.2~1.5, respectively. Comprehensive analysis suggests that tectonic activity is the primary factor responsible for these differences. We divide each geomorphic parameter into three classes (strong, medium, and low)and calculate the relative active tectonics (Iat)of the Huashan piedmont. The results show that the Iat values in Huaxian to Huayin section are in 1.0~1.5, those at other places are in 1.5~3.0, indicating that the tectonic activity from Huaxian to Huayin is most intense, while that of other places are relatively weak. Field geological investigations show that the Huashan piedmont fault can be divided into Lantian to Huaxian section, Huaxian to Huayin section and Huayin to Lingbao section. In Huaxian to Huayin section the fault has been active several times since Holocene indicative of strongest activity, while in Lantian to Huaxian section and Huayin to Lingbao section the fault was active only in the late Pleistocene and its activity was weaker as a whole. Tectonic activity of the Huashan piedmont derived from river geomorphic parameters is consistent with field geological investigations, indicating that geomorphic parameters of rivers can be used to characterize activity of faults on a regional scale.

Based on the 1︰50000 active fault geological mapping, combining with high-precision remote imaging, field geological investigation and dating technique, the paper investigates the stratum, topography and faulted landforms of the Huashan Piedmont Fault. Research shows that the Huashan Piedmont Fault can be divided into Lantian to Huaxian section (the west section), Huaxian to Huayin section (the middle section) and Huayin to Lingbao section (the east section) according to the respective different fault activity.
The fault in Lantian to Huaxian section is mainly contacted by loess and bedrock. Bedrock fault plane has already become unsmooth and mirror surfaces or striations can not be seen due to the erosion of running water and wind. 10~20m high fault scarps can be seen ahead of mountain in the north section near Mayu gully and Qiaoyu gully, and we can see Malan loess faulted profiles in some gully walls. In this section terraces are mainly composed of T₁ and T₂ which formed in the early stage of Holocene and late Pleistocene respectively. Field investigation shows that T₁ is continuous and T₂ is dislocated across the fault. These indicate that in this section the fault has been active in the late Pleistocene and its activity becomes weaker or no longer active after that.
In the section between Huaxian and Huayin, neotectonics is very obvious, fault triangular facets are clearly visible and fault scarps are in linear distribution. Terrace T₁, T₂ and T₃ develop well on both sides of most gullies. Dating data shows that T₁ forms in 2~3ka BP, T₂ forms in 6~7ka BP, and T₃ forms in 60~70ka BP. All terraces are faulted in this section, combing with average ages and scarp heights of terraces, we calculate the average vertical slip rates during the period of T₃ to T₂, T₂ to T₁ and since the formation of T₁, which are 0.4mm/a, 1.1mm/a and 1.6mm/a, and among them, 1.1mm/a can roughly represent as the average vertical slip rate since the middle stage of Holocene. Fault has been active several times since the late period of late Pleistocene according to fault profiles, in addition, Tanyu west trench also reveals the dislocation of the culture layer of(0.31~0.27)a BP. 1~2m high scarps of floodplains which formed in(400~600)a BP can be seen at Shidiyu gully and Gouyu gully. In contrast with historical earthquake data, we consider that the faulted culture layer exposed by Tanyu west trench and the scarps of floodplains are the remains of Huanxian M_S8½ earthquake.
The fault in Huayin to Lingbao section is also mainly contacted by loess and mountain bedrock. Malan loess faulted profiles can be seen at many river outlets of mountains. Terrace geomorphic feature is similar with that in the west section, T₁ is covered by thin incompact Holocene sand loam, and T₂ is covered by Malan loess. OSL dating shows that T₂ formed in the early to middle stage of late Pleistocene. Field investigation shows that T₁ is continuous and T₂ is dislocated across the fault. These also indicate that in this section fault was active in the late Pleistocene and its activity becomes weaker or no longer active since Holocene.
According to this study combined with former researches, we incline to the view that the seismogenic structure of Huanxian M_S8½ earthquake is the Huashan Piedmont Fault and the Northern Margin Fault of Weinan Loess, as for whether there are other faults or not awaits further study.

Based on the 6 campaigns relative gravity observation data of the Xichang gravity survey network from 2012 to 2014, we analyze the gravity change patterns between two adjacent observation campaigns, the dynamic patterns of cumulated gravity change relative to the first campaign and the gravity time-variation at some stations near the epicenter of Ludian earthquake in the study area. We find that there was no obvious cumulative trend anomaly before the Ludian M_S6.5 earthquake 2 years ago, and the possible precursor signals are related to the gravity difference changes of the stations located at the two sides of the fault slip surface during the period of 2014-03—2014-06. Using equivalent source model to inverse these gravity anomalies, we deduce the density changes in 10^-5g/cm³ orders of magnitude to the depth of epicenter. It is inferred that the change of the mass source in the short time may be related with the filling or migration of pore fluid in the crust medium. Under the equal dynamic condition of short-time tectonic movement, fault slip in the seismogenic zone may be triggered due to fluid migration and filling, thus, resulting in generation of earthquake.

Based on the 1: 50 000 geological mapping of active fault, the paper investigates the stratum, topography and faulted landforms of the northern marginal fault of Emei Platform, and preliminarily divides the northern marginal fault of Emei Platform into three sections by two stepovers near Tanjiazhuang Village and Nanliu Village according to different fault activity of each section.
At west of Tanjiazhuang Village is a loess platform, and the high terrain scarp can be seen from the northern margin. The height of scarp decreases progressively and the slope becomes gentle westwards at the place between Nanchi Village and Xikang Village, and to the place near Xiaoliang town, we cannot see obvious terrain scarps. The faulted sections can only be seen in the gullies which cross the terrain scarp at the south of Guozhuang Village and Tanjiazhuang Village. The fault dislocates the Pliocene red clay and the middle Pleistocene Lishi loess and covered by Malan loess; continuous paleosoil can be seen across the terrain scarp in some gullies. These indicate that in this section the fault was active in the early middle Pleistocene and its activity becomes weaker or no longer active after that.
The fault in the section between Tanjiazhuang Village and Nanliu Village can be divided into three parts by Shidian Village and Jinming Village, which are named, from west to east in sequence according to each faulted landform, the northern marginal fault of lacustrine terrace, the piedmont fault of Zijin Mountain and the northern marginal fault of loess platform. The fault transition area between each part is continuous and the fault is in linear distribution, so we see the whole fault section as having the same activity. In this section the Holocene diluvial fan is faulted. At least two plaeoearthquake events happened since Holocene, and the latest activity is in (2.00～1.29) ka BP according to Renzhuang trench and Jinsha trench, which can be well compared with former researches. The fault slip rate is over 0.33mm/a in the section south of Maguduo Village and is more than 0.36mm/a according to Renzhuang trench since the later period of the late Pleistocene.
In the section between Nanliu Village and Xizhangpo Village, the fault distributes along the frontal edge of the diluvial platform and is covered by thick loess. A 50～200m high linear terrain scarp formed due to the activity of fault can be seen along the frontal edge especially in the part between Xunwang Village and Xulu Village. At north of Wuzhai Village, the height of scarp decreases progressively and to the place near Xizhangpo Village, the terrain scarp cannot be seen clearly. In this section, Malan loess is faulted, which indicates that this fault section has been active since the late Pleistocene, but the evidence of Holocene fault activity has not been obtained yet due to the non-development of Holocene stratum. The fault slip rate is no less than 0.1mm/a since the late Pleistocene according to the faulted section at south of Xunwang Village.

Kouquan Fault is located in the north part of Shanxi graben which controls the west edge of Datong Basin.Two M6 1/2 earthquakes happened in the west side of basin in historical time,and there has been a concern about the future hazard of the fault.However,previous researches on Kouquan Fault were limited only in several points,especially,there was lack of measurements and dating data.Based on the 1 :50000 geological mapping of Kouquan Fault,the paper investigates the late Quaternary faulted landforms of its middle part(the part between Shangshenquan village and Yangjiayao village),combining with remote sensing interpretation of Spot image and field validating of the study area,and finally obtains the late Quaternary dip-slip rates of this fault.Five stratiform landforms can be found from piedmont to riverbed.The topmost part(the fifth geomorphic surface)is piedmont erosion surface which might be the planation surface of Tangxian period; the fourth geomorphic surface,which formed in the end of the middle Pleistocene to the early stage of late Pleistocene,consists of T₃ terrace of big rivers and diluvium mesas developed on piedmont; the third and the second geomorphic surface can be found in valleys and are represented as T₂ and T₁ terrace,respectively.Diluvium mesas of the same period formed in the end of the late Pleistocene and the middle stage of Holocene are distributed in different parts in front of mountains.The first geomorphic surface is flood plain and modern alluvial fans at mountain front.According to OSL dating and radiocarbon dating of different terraces,we obtained the ages below: T₃,no less than 70ka; T₂,about 33ka; T₁,4～8ka.The characteristic of faulted landform of the research area is different due to the fault activity of different parts.In the segment with intense faulting,the fault trace is obvious,and we could see fault scarps and triangular facets in the field,low river terraces such as T₁ and T₂ had been faulted; In the segments with less activity,the fault trace is unclear,the older fault scarps have gentle slope due to river erosion and reverse slope,and there is no evidence of faulted low terraces.Based on faulted landforms of the different terraces,we divide the middle part of Kouquan Fault into three sections by Baipo village and Chanfang village.At south of Baipo village,the diluvium mesas corresponding to the period of T₃ were faulted,but there is no evidence found dislocating the younger geomorphic surface.This indicates that this part has not been active since Holocene; T₂ and the older terraces were faulted between Baipo village and Chanfang village; the evidence of offset of T₁ terrace could be seen at the north of Chanfang village,especially in the part between Xiaoyukou village and Louzikou village.In the section north of Chanfang village,the fault throw of T₁ terrace is 50cm in Dayukou village,over 3m in the part between Xiaoyukou village and Louzikou village,and 25～30cm in the part between Shijing village and Ermaokou village; the fault throw of T₂ terrace is 5.7m in the part between Chanfang village and Dayukou village,over 17.5m in the part between Xiaoyukou village and Louzikou village,and 13m in the part between Shijing village and Ermaokou village. We calculated the slip rates combining with the fault throw of T₂ terrace at different sites,and the results are as follows: >0.53mm/a between Xiaoyukou village and Louzikou village,0.4mm/a between Shijing village and Emaokou village and 0.17mm/a between Chanfang village and Dayukou village.These maybe indicate that the late Quaternary activity of the fault was centered on the part between Xiaoyukou village and Louzikou village,and became weaker towards both sides.