At 4:00am on October 11, 2018, under the influence of heavy and continuous rainfall, a large-scale rocky landslide occurred in the Baige village of Bolo Town, Jiangda County, Tibet Autonomous Region, which is located at the upper reach of the Jinsha River. During its sliding, the landslide body is cut out from the upper part of the high and steep slope and falls rapidly, and the lower rock mass is continuously scraped, which increases the volume remarkably. With the disintegration of the landslide mass, the landslide mass is transformed into a fast and remote debris flow sliding. The massive debris flow materials rapidly flowed down to block the Jinsha River, forming a barrier dam. Then the lake rose and flooded many roads. At 5:00pm on the October 12^th, the barrier dam was overtopped and gradually washed by the river to form a drainage channel. At 9:00am on the 13^th, the dam was completely flushed open, accomplishing the flood discharge and relieving the danger caused by the landslide. At 5:00pm on November 3, 2018, the trailing edge of the Baige landslide experienced a sliding rupture, which led to the debris flow, at a high speed, piled up the dam from the first landslide, and blocked the Jinsha River again. The height of the second barrier dam was 50m higher than the first one, forming a larger barrier lake. After the landslide occurred, the water level of the upper reaches of the barrier lake continued to rise, and Jiangda County, Boro Town, Baiyu County Jinsha Town and other towns on the upper reaches of the Jinsha River were flooded. After the second floodwater released, a large scale flood occurred in Jinsha River, which caused the flooding of cities and towns in the middle and lower reaches in Sichuan, Yunnan and other riverside areas, and destructed roads and bridges, posing a great threat to the lives and property of people and the safety of infrastructure such as hydropower stations. The water level of the dammed lake was lowered by artificially constructing a diversion channel to eliminate the danger of dam break and avoid the occurrence of greater flood hazards. On the basis of field investigation on the landslide site, it is found that after the first landslide, three potential unstable rock masses were found at the trailing edge and both sides of the landslide. According to radar monitoring, three potential unstable rock masses at the trailing edge of the landslide are still continuously deformed, with obvious activity, and there is a risk of blocking the Jinsha River again. The author was monitoring constantly the unstable rock of the trailing edge of the Baige landslide for 7.5 days adopting D-InSAR. The surveillance results indicate that there is a slight sliding on the upper side of the landslide and there are four major deformation regions on the upper edge of the landslide. Besides, four measuring data points, selected within the four major deformation areas, show that the deformation value is 200mm and the deformation rate on the landslide top reaches 300mm/day, which suggests that the current landslide is still not stable and there is the risk of blocking the Jinsha River by the landslide. This paper, using PFC2D, simulates the stability of unstable rock on the trailing edge of landslide under the influence of gravity, torrential rain, and earthquake and analyzes the landslide’s stability scientifically in terms of simulation results. The simulation results show that the slope only deforms slowly under static action, without obvious destabilizing sliding. The initial deformation of the slope is basically consistent with the results of radar monitoring displacement, indicating that the sliding body of the slope still has a sliding trend under static action, and is not stable. Under the action of heavy rainfall, with the increase of time step, the deformation and displacement of slope is also increasing. In the process of operation, tensile cracks gradually appear in the slope, and continue to develop until it is cut through, and instability failure occurs. The ground motion is input from the bottom of the slope model in the form of velocity. When the model is running, tensile cracks first occur at the back edge of the slope on the right side. As the shear failure occurs in the middle of the slope and the tensile crack at the back edge goes through, the whole slope becomes unstable and fails. But on the whole, it’s basically stable. The simulation results show that the unstable rock in the trailing edge of the landslide will still lose stability under the inducing factors such as heavy rainfall and earthquake. It’s necessary to take appropriate engineering measures such as slope cutting to control the unstable rock, and the real-time monitoring and early warning system should be set up to eliminate the hidden danger caused by the slide of unstable rock blocking the Jinsha River again in time. At the same time, this paper also provides reference significance for further understanding the development and evolution process, as well as the deformation failure mechanism of landslide and debris flow in alpine regions. It also provides theoretical guidance for emergency measures and disaster prevention and mitigation after a disaster happens.

On May 21, 2021, a strong earthquake of magnitude 6.4 occurred in Yangbi County, Dali Prefecture, Yunnan Province. The focal depth of this earthquake is 8km. The earthquake broke the calm of magnitude 6 earthquake that had lasted for more than 6 years in Yunnan, and is a significant strong earthquake in the northwestern Yunnan region. Before the M_S6.4 Yangbi earthquake, the foreshock activity near the epicenter was frequent, and the maximum magnitude of foreshock is 5.6. After the M_S6.4 earthquake, another M_S5.2 earthquake, and many aftershocks of magnitude 3 and 4 occurred. The earthquake sequence was very rich. In order to further study the spatio-temporal distribution, source characteristics and seismogenic structure of the magnitude 6.4 earthquake sequence in Yangbi, in this paper more than 2 800 seismic events of the Yangbi earthquake sequence were relocated using the double-difference relative positioning method based on the seismic phase data from the Seismic Cataloging System of China Earthquake Networks Center, and finally 2 116 precise location results were obtained. At the same time, based on the broadband digital waveform data provided by the China Earthquake Networks Center, focal mechanism solutions 31 earthquakes of the sequence were obtained by MTINV program.

The results of the moment tensor inversion show that the moment magnitude of the Yangbi M_S6.4 earthquake is M_W6.0, the centroid depth is 10km, and the optimal double-couple solution is strike 135°, dip 81° and rake 176° for nodal plane I, and strike 226°, dip 86° and rake 9° for nodal plane Ⅱ. It is a strike-slip earthquake. Combining the strike of the fault in the earthquake source area and the distribution of aftershocks, it is inferred that the seismogenic fault is the nodal plane Ⅰ which strikes NW. Focal mechanism solutions of other 30 earthquakes of the sequence are mainly strike-slip type, which are consistent with the main shock. There are also a few events with mixed types. The focal mechanisms of several earthquakes close to the occurrence time of the M_S6.4 main earthquake are in good agreement with the main earthquake. The relocation results show obvious linear distribution characteristics of the sequence. The overall strike is in the NW direction and the dip to the SW direction. The depth profile sequence is horizontally linear along the strike. The dip angles of the fault planes in the south and north sections are different. The dip angles of the northern section are approximately vertical, and that of the southern section is about 45° or so. However, the sequence of the northern section is more concentrated along the fault plane than southern section. The dominant strike of the Yangbi earthquake sequence is NW-SE, the dip angles are concentrated between 70° and 90°, and the rakes are distributed around 180°, indicating that the Yangbi earthquake sequence is mainly characterized by strike-slip faulting. The dominant azimuth of the P-axis is SN and that of the T-axis is EW. The plunge of P-axis and T-axis are near horizontal. This indicates that the activities of the Yangbi earthquake sequence are mainly controlled by the regional SN-direction horizontal compression stress field. The dominant directions of the sequence’s fault planes and P-axis parameters are single, indicating that it is less likely that complex fault activity and large-scale stress adjustment will occur in the source area of this earthquake.

Integrating the results of relocations and focal mechanisms, it suggests that the seismogenic fault of Yangbi earthquake is a right-handed strike-slip active fault, striking northwest and dipping to the southwest, and the dip distribution is segmented. The dip angle of the northern segment is nearly vertical, and the dip angle of the southern segment is lower than that of the northern segment. There may exist rupture segmentation in the fault in the earthquake source area, and the structure morphology of local small areas may be more complicated.

On May 22, 2021, an M_S7.4 earthquake occurred in Maduo County, Guoluo Prefecture, Qinghai Province, which is the biggest earthquake in mainland China since the 2008 Wenchuan M_S8.0 earthquake. It occurred in the Bayan Har block in the northern part of the Qinghai-Tibet Plateau, indicating that the Bayan Har block is still the main area for strong earthquakes activity in mainland China. In order to study the source characteristics and seismogenic structure of the Maduo earthquake, we used the double-difference location method to analyze the spatial distribution of earthquake sequences within 15 days after the mainshock. At the same time, the focal mechanism solutions of 15 aftershocks with M_S≥4.0 are also obtained by full-waveform moment tensor inversion. We hope to provide seismological evidences with reference value for the study of the dynamic process of the Madao M_S7.4 earthquake and the geological tectonic activities on the northern side of the Bayan Hala block.

The results of moment tensor inversion show that the moment magnitude of the Maduo earthquake is about 7.24, the centroid depth is 13km, and the best double-couple solution is strike 283°, dip 59° and slip -4° for the nodal plane I, and strike 15°, dip 86° and slip -149° for the nodal plane Ⅱ, which indicates a strike-slip earthquake event. According to the strike of the fault and the distribution of aftershocks in the source area, we infer that the nodal plane I, which strikes NWW, is the seismogenic fault plane. The focal mechanism results of 15 aftershocks show that the aftershock sequence is mainly strike-slip type, which is consistent with the main shock. Meanwhile, there are also some other types reflecting the local complex structure. The differences in the direction and type of focal mechanism may reveal changes in the direction and characteristic of the fault from north to south. The azimuth of the P-axis is NE-SWW, and the azimuth of the T-axis is NNW-SSE. Both plunge angles are within 30° and close to horizontal, which shows that the activities of the Maduo earthquake sequence are mainly controlled by the horizontal compression stress field in the northeast-southwest direction. From NWW to SEE, the dip angle of fault plane increases gradually from 77° to 88°, and the northern segment dips to SW.

Based on the results of relocation, moment tensor inversion and geological structure, preliminary conclusion can be drawn that the seismogenic fault of the Maduo earthquake may be the Kunlun Mountain Pass-Jiangcu Fault, which is a left-handed strike-slip fault. At the same time, there are certain segmental differences along the fault. The strike of the northern section is mainly NW, that of the middle section is NWW, and the southern section is near E-W, and the fault plane dips to the southwest with the dip angle increasing gradually from NWW to SEE.

The neotectonic movement in the middle reaches of the Jinsha River is active and the earthquakes occur frequently. Lacustrine sediments are commonly distributed on both sides of the river with stable sedimentary environment, good horizontal continuity and relatively developed stratification, which are good carriers for recording paleo-seismic events. In this study, a large number of soft sedimentary deformation structures are found in the riverside lacustrine sediments in the Taoyuan Town area in the middle reaches of Jinsha River, with strong deformation and large scale. We focus on the comprehensive analysis of four soft-sedimentary deformation profiles. In which the profiled strata are mainly medium-fine sand and clay. And the soft sedimentary deformation structures mainly include sand liquefaction, rootless faults, clay lumps and folds.
Causes analysis: In the profiles of soft sedimentary deformation structures, there are medium and fine sand layers whose thickness is from thick to super thick. Sedimentary bedding has not been observed in the sand layer; and a large number of clay debris or lumps are involved in the sand layer, which are often filled between the adjacent clay lumps; and there are quicksand channels in the sand layer. All the features indicate that the sand layer in the study profiles has been liquefied. In the study profile, we found that the soft sedimentary deformation structure has the following characteristics: The faults found in the study profile extend downward and terminate in the lower liquefied sand layer and a large number of clay lumps. There are clay lumps in the place where the clay fold structure develops, and a large number of liquefied sand bodies are filled between the fold structures. The deformation structures in the profiles are not contrastive in terms of extension, chaotic deformation characteristics and obvious stress direction. Based on the characteristics of sand liquefaction and clay deformation in the above profile, it is inferred that the deformation structure in the profile is mainly due to sand liquefaction. The liquefaction strength of sand layer determines the deformation degree of clay layer.
Trigger factors analysis: There are many factors that can trigger the liquefaction deformation of the unconsolidated sediment, such as flood, freeze-thaw, collapse and earthquake, which can cause the liquefaction deformation of the sediment under certain conditions. In this paper, the possible trigger factors are analyzed based on the combination of the structural characteristics of soft sedimentary deformation, sedimentary environment and geological background of the area. First the stratigraphic characteristics also reflect the hydrostatic sedimentary environment at that time. The soft sedimentary deformation on such a large scale could not be mainly caused by the disturbance of lake waves. The research profiles are located at a sheltered bay with weak hydrodynamics, and no alluvial strata have been found in the upper part of the soft sedimentary deformation stratum. Moreover, the soft sedimentary deformation structure caused by flooding is often a small-scale curly layered structure, which has a large difference with the deformation structure and scale in the study profiles. This suggests that alluvial and diluvial events are not the main triggering factors of the deformation. Although the landslide is likely to occur near the study area, no trace of bedrock landslide is found near the study profiles. Therefore, the invasion of bedrock landslide into the sedimentary layer cannot be the triggering factor. Moreover, the occurrence of lacustrine sedimentary layer is nearly horizontal, which is a relatively stable sedimentary state, and it is impossible to form such a large-scale slump structure due to its own gravity effect. And we don't find any sliding surface in the profiles. Therefore, the collapse is ruled out. According to the geological background and geological survey of the study area, this area does not have the conditions triggered by volcanism, glaciation and freeze-thaw. Because of the active neotectonic movement and frequent earthquakes in the study area, and seismic actions are the main trigger factors for liquefaction. So it is considered that seismic action may be the main trigger factor for the strong liquefaction deformation in the study area. According to the previous studies, the relationship between the soft sedimentary deformation structure, the liquefaction thickness and the seismic strength is discussed, the magnitude of this ancient seismic event probably reached 7 or higher.
There are sand layers in the section of “soft sedimentary deformation structure” caused by earthquake, the lower stratum is sand layer and the upper stratum is clay layer. The thickness and deformation strength of the lower sand layer determine the strength of the deformation structure of the overlying clay layer. The upper and lower surface of the sand layer are undulating, and there are clay lumps in the sand layer. The deformation structure of clay layer is complex and there is no obvious deformation rule.

Zircon is one of the most commonly used accessory minerals rich in U and Th for(U-Th)/He dating system. Compared with apatite, zircon has a higher He closure temperature (~190℃), which gives it more advantages in solving the problem of source material and thermal history reconstruction in sedimentary basins. However, the crystals of zircons often have U and Th zoning development, with obvious differences in concentration. Even the standard sample of FCT(Fish Canyon Tuff)zircon which is widely used in (U-Th)/He dating has an average age dispersion of about 10%. In this study, the Alphachron He isotope mass spectrometer is used for laser melting of a batch of single grains of FCT zircon(11 grains)to determine their ⁴He content. The contents of U and Th of parent isotopes are accurately determined by automatic injection of Agilent 7900 ICP-MS and isotope diluent method. The Th/U ratios of the 10 FCT zircons calculated with (U-Th)/He average age in this paper range from 0.52 to 0.67, which are consistent with the Th/U ratios of 186 reported so far. According to the Th/U ratios of 189 FCT zircons published in the statistical literature, we found that only three of them had high Th/U ratios, namely, 1.12, 1.16 and 1.5, the other 186 FCT zircons(occupy>98%) had a Th/U ratio less than 1. Based on previous results and the 10 Th/U ratios measured in this paper, 196 FCT zircons have a normal Th/U ratio ranging from 0.27 to 1.00, with an average ratio of 0.56(n=196). Excluding one abnormally old age, the(U-Th)/He ages of the remaining FCT zircons in this study range from 26.61 to 31.91Ma, with a weighted mean age of (28.8±3.1)Ma (2SD, n=10), which is consistent with the mean age ((28.3±3.1)Ma, 2σ, n=127) or (28.29±2.6)Ma(2σ external error, 9.3%, n=114)obtained by several other international laboratories. This indicates that the zircon single particle(U-Th)/He dating process established by our laboratory is reliable. For the zircon samples with U, Th banding and concentration differences prevailing, determining the distribution of U, Th elements in the crystal prior to the (U-Th)/He experiment is essential for understanding effects of geometry and elemental zoning on nuclear recoil and diffusion and the interpretation of (U-Th)/He age data.

On July 31th, 2016, a magnitude 5.4 earthquake struck Cangwu Country, Guangxi Zhuang Autonomous Region, it was the largest earthquake recorded by Guangxi Seismological Network since it set up. The number of people affected by the earthquake had reached 20 000, and the direct economic losses caused by the earthquake were nearly 100 million Yuan.
After the earthquake, USGS provided a global earthquake catalog showing that the focal depth of Cangwu earthquake was about 24.5km. However, the result given by the Global Centroid Moment Tensor showed the focal depth of this earthquake was 15.6km. However, the result obtained by Xu Xiaofeng et al. using CAP method was 5.1km. It was clear that the focal depths of Cangwu earthquake given by different institutions were quite different from each other. However, accurate focal depth of the earthquake has important significance for exploring the tectonic mechanism near the epicenter, so it is necessary to further determine the more accurate depth of the Cangwu earthquake.
In order to further accurately determine the focal depth of Cangwu earthquake, we used the global search method for travel-time residual to calculate the focal depth of this earthquake and its error range, based on the regional velocity model, which is a one-dimensional velocity model of the Xianggui tectonic belt produced by the comprehensive geophysical profile. Then, we inverted the focal mechanism of this earthquake with the CAP method. Based on this, the focal depth of Cangwu M_S5.4 earthquake was further determined by the method of the Rayleigh surface wave amplitude spectrum and the sPL phase, respectively.
Computed results reveal that the focal depth of this earthquake and its error range from the travel-time residual global search method is about(13±3)km, the focal depth inverted by CAP method is about 10km, the focal depth from sPL phase is about 10km, and the focal depth from Rayleigh surface wave amplitude spectrum is about 9~10km. Finally, we confirmed that the focal depth of Cangwu M_S5.4 earthquake is about 10km, which indicates that this earthquake still occurred in the upper crust. In the case of low network density, the sPL phase and Rayleigh wave amplitude spectrum recorded by only 1 or 2 broadband stations could be used to obtain more accurate focal depth.
The focal depth's accuracy of Cangwu M_S5.4 earthquake in the USGS global earthquake catalog has yet to be improved. In the future, we should consider the error of the source parameters when using the USGS global earthquake catalog for other related research.

LA-ICP-MS(laser ablation-inductively coupled-mass spectrometry)has been recently used for rapid, accurate and precise U-Pb geochronology on zircon grains. In this paper, we adopted an Agilent 7900 quadrupole ICP-MS coupled with a Resolution M50-LR 193nm excimer laser system to establish integrated measurement procedures. Before analysis, the system is tuned to achieve sensitivities better than 30000 cps/s for ²³⁸ U with a 40μm spot size, at~3.5J/cm². Detailed parameters for laser system and ICP-MS are presented here. Then, we analyzed five reference zircons(91500, GJ-1, Plesovice, FCT, Penglai)with a wide range in age from~1064 to~4.4Ma. Two standard zircons, 91500 and GJ-1, are employed as external reference standards. Generally, second zircon standard is analyzed in an effort to ensure accuracy and evaluate reproducibility. A typical analysis sequence includes one international glass standard(NIST610), two external reference standards, five grains of unknown zircon with every eight ablations. Laser induced time-dependent elemental fractionation is corrected using the intercept method, whereas instrument drift, mass bias and elemental fractional caused by ionization differences are corrected by external reference standard 91500 or GJ-1. Compared with 91500 and GJ-1, common Pb content of Plesovice, FCT, Penglai can't be ignored. Thus, we did common Pb correction for the above three standard zircons. The performance of the established procedure was assessed by analyzing zircon range in age from~1 064 to~4Ma. The results show that the ages of these five references are consistent with the ages of published studies with accuracy for three international references(91500, GJ-1, Plesovice)better than 3% and two young secondary references(FCT, Penglai)lower than 7% at the 2 sigma level, which indicates that our analytical procedure is reliable. For individual laser analysis, the uncertainties are mainly from three sources:Measurement error of isotope ratio, error of correction factors for instrument drift and element fractionation, and error of recommended age of external references. Compared to three international references, there are three extra uncertainties for young reference zircons, including:1)little radioactive isotopes closing to blank level increase the measurement error of isotope ratio; 2) effect of common lead becomes more significant;3) the nonhomogeneous samples couldn't match references well. Therefore, accuracy and precision of measurement depend on absolute age, content of common lead and matching degree between references and samples. In summary, the accuracy and precision obtained using the technique presented in this study are similar to those of other LA-ICP-MS laboratories.

(U-Th)/He isotopic dating has been developed very quickly in recent years, due to the recognition that the thermal history of rock at low temperature can be effective revealed by such dating method. In particular, He closure temperature in apatite (40~80℃) is very low, so apatite (U-Th)/He ages can reflect the thermal history information of the low-temperature stage, and have a good application prospect in the field of low-temperature thermal chronology. However, because of many influence factors and complicated measurement procedures, the development of apatite He dating in China remains in its early stage. In this study, a measurement procedure was established at the (U-Th)/He dating laboratory of Institute of Geology, China Earthquake Administration. We measured the daughter isotopic helium by diode laser heating four batches of a total seventy-five grains of Durango apatite in an Alphachron helium mass spectrometry system. Then the apatite grains were dissolved to precisely measure the concentration of parent nuclides (U, Th)using the solution isotope dilution method through an automatic sampling ICP-MS (Agilent 7900). Results show that the Th/U values of Durango apatite grains were in the range of 17.23 to 23.60, while all the 75ages were in the range of 28.61 to 34.51Ma with an average of (31.71±1.55)Ma (1σ), which are consistent with the international calibrated ages.

The 1739 M8.0 Pingluo earthquake is the largest destructive earthquake occurring on the Yinchuan plain in history. However, there are different understandings about the seismogenic structure of this earthquake. In this paper, we re-evaluate the seismogenic structure of the 1739 M8.0 Pingluo earthquake after our investigation and detailed measurement of the seismic dislocations on the Great Wall and the surrounding tableland, and also the latest results of trenching, drilling, and shallow seismic exploration are considered as well. The results show that the latest rupture event of the Helanshan piedmont fault occurred after 600~700a BP, the Great Wall built in Ming Dynasty about 500 years ago was faulted by Helanshan piedmont fault. Although the distribution of Yinchuan buried fault coincides much with the distribution of the meizoseismal area, the fault's northward extending stopped at Yaofu town, and its Holocene active segment is less than 36km in length. The latest surface rupture occurred shortly before 3400a BP. The 1739 Pingluo earthquake did not rupture the ground surface along the Yinchuan buried fault. The presence of growth strata and the non-synchronous deformation of strata near the fault demonstrate that Yinchuan buried fault did not rupture at all or there was rupture but absorbed by the loose layers in the 1739 Pingluo earthquake. Therefore, the Helanshan piedmont fault is the seismogenic structure of the 1739 M8 Pingluo earthquake, rather than the Yinchuan buried fault, and there is no synchronous rupture between two faults. The difference of location between the seismogenic structures and the meizoseismal area of the Pingluo M8 earthquake may be caused by the factors, such as fault dip, groundwater depth, basin structure, loose formations, the degree of residents gathering, so on. The phenomenon that the meizoseismal area shifts to the center of the basin of earthquake generated by faulting of a listric fault on the boundary of the basin should be paid more attention to in seismic fortification in similar areas.

Based on the discussions on the basic ideas, methods and procedures for detecting buried faults and taking the example of Luhuatai buried faults in Yinchuan Basin, the paper introduces in detail the multi-means, multi-level detection methods for gradually determining the accurate location of faults. Multi-means refer to the technical methods such as shallow seismic exploration, composite drilling section, trenching, dating of sedimentary strata samples and calculation of upward continuation of fault's upper breakpoints, etc. Multi-levels refer to gradually determining accurate location of fault at different levels with the above means.
Results of shallow seismic exploration reveal that the Luhuatai buried fault has a strike of NNE in general, dip SEE, with the dip angle between 73° to 78°. Geometrically, the fault consists of a main fault and a small north-segment fault in plane. The main fault runs along the NNE direction from Xixia District of Yinchuan City, passing through Jinshan Township to Chonggang Township, and there is a 4km or so intermittent zone between the main fault and the small north-segment fault. The small north-segment fault is 9km long, distributed between the north of Chonggang Township to the south of Shizuishan City. According to dating of sediments sampled from drill holes, the main fault can be further divided into the southern segment and the northern segment. The southern segment of Luhuatai buried fault is active in Pleistocene, while the northern segment is active in Holocene.
Shallow seismic exploration can detect the upper breakpoint of fault deeper than drilling or trenching does. If simply connecting the vertical projections of these breakpoints on the surface, there is a certain bias of fault strike. To this end, we did accurate location for the Holocene active northern segment of Luhuatai buried fault, in which upward continuation calculation is done based on the fault dip to match the upper breakpoint of fault obtained from shallow seismic exploration with the depth of the upper breakpoints obtained from drilling. Through the accurate location of the fault, we get the geometric distribution, occurrence and segmentation of activity of Luhuatai buried fault at the near-surface. Our results provide reliable basis for the safety distance from active faults for engineering construction projects in the Luhuatai buried fault area of Shizuishan City. The methods discussed in this paper for accurate location of buried active faults are of reference value for buried fault exploration in other similar cities or regions.

Yellow River Fault is the longest, deepest fault in the Yinchuan Basin, also is the eastern boundary of the basin. Because its north section is buried, its activity and slip rate remains unknown, which made a negative impact on understanding the evolution and seismic hazard of the Yinchuan Basin. In this study, a composite drilling section with a row of drillholes were laid out along the northern section of the Yellow River Fault based on the results of shallow seismic exploration near the Taole Town, where oil seismic exploration data are available. Fault activity and slip rate are obtained by measuring the age of samples of holes. The results show that the northern section of the Yellow River Fault is a late Pleistocene or Holocene Fault, its accumulative displacement is 0.96m since (28.16±0.12)ka BP, with an average slip rate of 0.04mm/a, which is significantly lower than the southern section. The activity intensity of the northern section of the Yellow River Fault is significantly lower than the southern section since Late Quaternary. In the Yinchuan Basin, the Helanshan eastern piedmont fault is the most active fault since late Quaternary, next is the Yellow River Fault, then, the Yinchuan buried fault and Luhuatai buried fault. Although the Yellow River Fault is the deepest and the longest fault, its maximum potential earthquake is magnitude 7, this seismogenic capability is weaker than the relatively shallower Helanshan eastern piedmont fault, on which occurred the Pingluo M8 earthquake in 1739 AD. Yinchuan Basin is the result of long-term activities of the four major faults, which shaped the special structure of the different parts of Yinchuan Basin. The Yellow River Fault controlled the evolution of the south part of Yinchuan Basin. The two-layer crustal stretching model can help us understand the structural deformation between the upper crust and the lower crust beneath Yinchuan Basin. Deformation of the upper crust is controlled by several brittle normal faults, while the deformation of the lower crust is controlled by two ductile shear zones. The shear sliding on Conrad discontinuity coordinates the extensional deformation of different mechanical properties between the upper and the lower crust. Yellow River Fault might have cut deeply into the Moho in Mesozoic, the tectonic activity in Yinchuan Basin began to migrate and was partitioned into several faults since the beginning of the Cenozoic, mainly in the Helanshan eastern piedmont fault. This may be the reason why the Yellow River Fault has lower seismogenic capability than the shallower Helanshan eastern piedmont fault.

The goal of the exploration to active fault is to understand its activities,relative parameters,the spatial distribution characteristics and its deep structures. There're three major stages for active fault exploration,namely, preparation stage,exploration stage and analysis stage. The construction of the database for each of these stages has different focus.
Based on the review of other articles about the development of active Fault database from international sources,the paper introduces the construction of databases based on the Technological System of China Earthquake Active fault Exploration project. And along with the development and implementation of the projects such as ‘Seismic Risk Assessment of Active fault in Key Earthquake Monitoring Areas in China’,‘China Earthquake Active Fault Exploration’, so on,database templates corresponding to each of the stages of active fault exploration are worked out according to the design idea,architecture and implementation of ArcGIS-based active fault database and the work procedure for active fault exploration. The main functions of the bulk storage software and data quality monitoring software developed for the construction of the database are introduced.
Due to the numerous data and the extensive sources as well as the complexity of the data acquisition during the building of database,there are chances to have either manual or systemic errors,and moreover,the data quality might be impacted,resulting in a database failing to represent the real activities of the active faults. On the other hand,the data stored in the database lacks consistency and integrity,thus,the database is ineffective and opposite to the original intention of its construction. The paper analyzes the main data sources used to establish the active fault database and the causes for generating low quality data,and discusses the advantages for building the active fault database simultaneously along with the implementation of active fault exploration.

China Earthquake Urban Active Fault Surveying Project is a national important scientific and engineering project in recent years.Its map achievement,which includes 1:250,000 regional seismotectonic map and 1:50,000active fault distribution map of twenty cities,is an important integrated document and will be utilized in seismological and geologic research,protecting against earthquake,and relief of disaster.However,these maps are not drawn in uniform standards.As a result,there is lack of normalization in stratigraphic division,map information expression and map layout.The lack of standardization will lead to further problems when publishing and utilizing these documents because of the diverse information expression.This paper discusses the design philosophy,data scheme and expression,cartographic generalization,illustration standard and mapping procedure of the 1:250,000 regional seismotectonic maps and 1:50,000 urban active fault distribution maps.The map information is from urban active fault databases,which are based on ArcGIS Geodatabase technique,and the mapping procedure is based on ArcGIS mapping template technique.Therefore,the paper also introduces the mapping procedure in ArcGIS software and references the information organization in urban active fault database.Since cooperation among industries,universities and geological research institutions becomes increasingly prominent,the mapping achievement of active fault surveying is in urge of standardization and normalization.The work in this paper is based on years of work of active fault survey project.We have collected suggestions and advices from first-line technological staff to scientific experts,and then revised our work in many details.It is expected that this work can promote the standardization and normalization of the active fault map achievements.

Luhuatai Fault is one of the important buried tectonics in Yinchuan Basin.Based on the results of shallow seismic exploration,we conducted composite drilling section exploration and dating of the samples of borehole.Some useful data of the fault were obtained,such as the depth of upper breaking point,the latest activity age,displacement in late Quaternary,and slip rates,etc.This study shows that the activity is different between the north and south segment along Luhuatai Fault.The north segment is a Holocene fault,while the south segment is a late mid-Pleistocene fault. From north to south along the north segment of Luhuatai Fault,the activity has enhanced,and the faulting is stronger in late Pleistocene than Holocene.

In this paper,an optimized drilling exploration method,the doubling section method,was summarized after many composite drilling section explorations of buried active fault in urban areas.Operation steps of this method are as follows: Firstly,drill a borehole at each of the two ends of the drilling section to make sure that fault is between the two boreholes,then,drill the third borehole at the middle of the two holes; and secondly,confirm again the segment where the fault is and drill the next borehole in the middle of it.By repeating the similar practice,the accurate location of fault can be constrained progressively.Meanwhile,this paper also uses a quantitative indicator,the key horizon gradient between two boreholes,instead of stratigraphic throw,to determine the location of buried fault and puts forward two criterions: 1)the fault is located between two boreholes if the key horizon gradients between these two boreholes are positive and increase with depth; and 2)the fault is located where the key horizon gradients between two boreholes increase obviously relative to the previous values and that of adjacent segments,besides the increase with depth.While in contrast,the key horizon gradient in a normal fault segment decreases obviously.Application cases show that the method can determine precisely the location of buried active fault.

The eastern piedmont fault of Helanshan Mountains is an important tectonic controlling the west boundary of Yinchuan graben.The south segment of the fault locates right in the west of Yinchuan city,which has a length of about 13.2km,strikes NNE-NE and dips south-east at an angle of 50°～80°.The main part of the fault lies between the Ordovician and Quaternary systems,forming the borderline between the hills and diluvium.Parts of the segment of the fault appear in alluvial fans and are displayed as geomorphic scarps.The paper selects the region on the two banks of Dashitou channel to excavate two trenches along the fault based on 1:10,000 geological mapping of the fault area.The result reveals three events since 14ka BP with the ages of 13.8,7.9 and 3.0ka BP and the recurrence intervals of 6 and 5ka,respectively.

As the requirement of study on paleontology,paleoclimate,magma process,volcanic disasters and paleomagnetism,scientists pay more attention to the young volcanic rocks dating in recent years.In this paper we review the achievements of the ⁴⁰Ar/³⁹Ar dating on young volcanic rocks during the last 10 years,and compare the limitations and strengths of K-Ar,conventional ⁴⁰Ar/³⁹Ar and laser ⁴⁰Ar/³⁹Ar dating methods.As the development of ⁴⁰Ar/³⁹Ar dating relies on the technique improvement,we discuss the influence of the new noble gas mass spectrometer and the full automation of the dating system on the young volcanic dating.Neutron irradiation and flux monitor standard sample are important to get accurate age,so we talk about the neutron flux gradient of Beijing 49-2 reactor which we usually use for sample irradiation and choose the suitable standard for young volcanic sample.We also discuss the analytical error and its sources in detail according to the experience in young volcanic rocks dating.The analytical precision of ³⁶Ar,system blank and mass discrimination are the 3 main internal factors influencing the precise dating of young volcanic rocks.We give two graphs to illustrate the fact and some suggestions to solve this problem.At last,we talk about the problems of ⁴⁰Ar/³⁹Ar dating on young volcanic rocks and what we should do to solve these problems in the future.The improvement of accuracy and precision of ⁴⁰Ar/³⁹Ar dating of young volcanic rocks will make it a more widely used technique and play a more important role in Quaternary geosciences.

This paper introduces the result of exploration of the Yinchuan buried fault using the composite drilling section method. As one of the main buried faults in Yinchuan plain,the Yinchuan buried fault has restricted seriously the development of Yinchuan City for a long time due to its indistinct location and unclear activity property. So the Yinchuan buried fault was taken as one of main tasks of active fault exploration in Yinchuan City. Most of shallow seismic explorations had been done before the drilling. However,due to the limited precision of shallow seismic exploration,the actual location of the Yinchuan buried fault can't be explored. For obtaining the information about the location and the depth of the upper break point, the active time and slip rate of the Yinchuan buried fault,three composite drilling sections,Xinqushao, Manchun and Banqiao,were laid out along the Yinchuan Fault based on the result of shallow seismic exploration. After comparing with the marker horizons disclosed by drilling,the position,scale and the depth of the upper break point of Yinchuan buried fault were found,and the buried active fault was located precisely. From the exploration result we get the apparent dip of the Yinchuan buried fault as 71 degrees at Xinqushao,71 dgrees at Manchun and 66 degrees at Banqiao,and the depth of the upper break points as 5.18～8.30m,5.01～6.50m and from 10.0～13.59m,respectively. Therefore,the latest active date of the Yinchuan buried fault is determined and the question whether the fault is active or not is answered by dating. The Yinchuan buried fault at Xinqushao and Manchun sections is manifested as a Holocene active fault, and at Banqiao,it is shown as a late Pleistocene active fault. The slip rate of the Yinchuan buried fault since late Pleistocene is 0.14mm/a at Xinqushao,0.05mm/a at Manchun and 0mm/a at Banqiao. Based on the result obtained from seismic exploration and the spatial positions of the three composite drilling sections,we draw the following conclusions:the Yinchuan buried fault can be divided into two segments with Yingu Road as the boundary; the northern segment was active in Holocene and the southern one was active in late Pleistocene; the activity of the northern segment is more recent than that of the southern one.

1:250,000 regional seismotectonic map is one of graphic documents achieved in the urban active fault survey project.At present,the 1:250,000 regional seismotectonic maps of various cities are drawn mostly basing on the existing 1:200,000 geological maps.The available paper-made 1:200,000 geological maps are the graphic documents under Beijing 54 Coordinates Systems.However,according to Stipulation on Technical System for China Earthquake Active Fault Surveying issued by China Earthquake Administration,the 1:250,000 regional seismotectonic maps are the graphic documents under Xi'an 80 Coordinate Systems.This article introduces the basic concepts of coordinates systems and the map projection and proposes explicitly that when scanning and digitizing paper geological map,the coordinate's projection parameters indicated in the paper geological map shall be taken for setting up projection information.The conversion of screen coordinates to map coordinates is realized through space coordinate registration,and by projection conversion,the Beijing 54 Coordinates System is converted to Xi'an 80 Coordinates System.Finally,with the aid of ArcGIS software,the 1:250,000 regional seismotectonic maps are compiled based on 1:200,000 geologic maps.

The project of "Yinchuan active fault exploration and earthquake risk assessment" is aimed at strengthening the fundamental work of engineering construction for earthquake resistance and prevention in Yinchuan City.In order to achieve this general goal,a joint multi-disciplinary exploration of the Yinchuan buried fault and the Luhuatai fault has been carried out on the basis of collection and analysis of predecessors'data.This paper discusses the methods of constructing three-dimensional subsurface structure model of the project area of Yinchuan active fault exploration by using the achievements of the Yinchuan active fault exploration,and puts forward main work steps and suggestions for the model construction.The steps for 3D modeling of subsurface structure using active fault survey data are as follows:To collect and pre-process the data of seismic exploration of active fault;Using seismic interpretation software to interpret the fault and the bed position,draw bed plane structure map,and output the fault and bed position data,meanwhile,to further complete the 3D visual modeling with this software;To load fundamental data,bed plane and fault data into the 3D modeling software GOCAD,carry on the fault plane and the bed plane revision and reconstruct 3D model,construct three-dimensional cutting section,and demonstrate the three-dimensional model achievements.Since the seismic survey data are not standard,it increases the difficulty in earthquake data explanation and affects the explanation precision.Therefore,we propose to standardize the archiving of primary seismic survey data,add a cross line in the survey area of each active fault,and if possible,use the regional time-depth conversion used by petroleum sector to enhance the explanation precision of seismic section.

Yinchuan Basin is a graben-like downfaulted Cenozoic era basin located on the west edge of Ordos Massif.Its activity is violent and deposition is very thick.Yinchuan City is located in the middle of Yinchuan Basin.The seismic petroleum exploration shows that a buried active fault lies in the east of Yinchuan City,named as the Yinchuan buried fault,which strikes NNE and dips west,with a total length of more than 80km.Because the seismic petroleum exploration did not gain any explained signals at the depth ranging from 0 to 400m,so whether the Yinchuan buried fault is active or not in the late Quaternary and its exact surface projective location hasn't been known yet.It has been a “worry” in the urban planning and development of Yinchuan for a long time.Under the financial support of the national and local governments,we launched the project entitled “The prospecting of active fault and earthquake risk assessment in Yinchuan City”.In order to facilitate the exploration,we selected Xinqushao village in the southeast suburb of Yinchuan City to be the site for the integrated test exploration of the Yinchuan buried fault before the exploration,based on the information obtained from the seismic petroleum exploration.Considering that the thick Quaternary sediment in Yinchuan reaches to 1609m,and that the depositional environment is the Yellow River flood plain and the lateral change of lithology is complex,we adopted in the test exploration the train of thoughts of “inferring an unknown fact from a known fact,and from deep to shallow and directly to the top”.The experimentation has been developed step by step according the working order of multilevel seismic exploration→composite geological profile drilling→trenching.Along the same measuring line at Xinqushao,first,we adopted the seismic reflection exploration of primary wave in three levels with the group interval of 10m→5m→1m to catch the master fault of the Yinchuan buried fault,and by tracing upward layer by layer in the order of the three exploration ranges,i.e.1400～400m→600～80m→150～20m,the position of the master fault at ±20m depth under the ground and its offset trace were primarily identified.And then,along the master fault and within the range of 100m at its both sides,9 boreholes of 20.5～100m were arranged for the composite geological profile drilling.The resulting information about the throws of the master fault was obtained,they are 20.34m,9.66m and 2.25m respectively at the depth of 43.75m,20.33m and 13.04m from the ground,and the buried depth of the upper offset point ≤8.34m.At the same time,using the intact core specimen from the fault plane of the borehole No.7,we calculated the dip angle of the fault as 71°at the depth of 55.27m and figured out the exact position of its extension to the earth's surface.Finally,a large-scale trial trench,which is 40 meters long,8～12 meters wide and 6 meters deep,was arranged across the master fault.The trenching revealed that the actual buried depth of the upper offset point of the master fault is 1.5m and there are seismic remains,such as offsets of 5 stages,sand liquefaction and surface rupture,etc.Among the 5 stages offsets,4 events occurred prior to 3170±80 a BP,belonging to the mid to late Holocene paleo-earthquakes.The age of the last event cannot be determined and it is inferred to be the result of the M8.0 Yinchuan-Pingluo earthquake in 1737.In a word,through the comprehensive test exploration,we find that the Yinchuan buried fault is a Holocene active fault,which lays solid base for the next exploration.