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.

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.