Abstract:In this paper, the Hong Kong-Zhuhai-Macao long-span continuous isolated bridge was studied. The fiber-plastic hinge element was used to simulate the nonlinear state of the reinforced concrete pier, and a 3D finite element model of the full bridge was established. The time history analysis of the isolated and non-isolated bridges were carried out. Seismic performance of the isolated and non-isolated bridges under the action of rare and extremely rare earthquakes was quantitatively evaluated by using the curvature ductility ratio of bridge pier and the ultimate allowable displacement of bearing as the indexes of bridge damage and failure. The failure modes of isolated and non-isolated bridges were discussed, and the influence of material nonlinearity on the seismic response of bridge structure was also studied. The results showed that, whether material nonlinearity is considered or not has a great impact on the seismic response of non-isolated bridge and a small impact on the isolated bridge. Under strong earthquakes, the seismic performance of isolated bridges was significantly higher than that of non-isolated bridges, and their failure mode are also different. The pier of isolated bridge is not damaged, for the pier components are well protected. The pier of non-isolated bridge is seriously damaged under extremely rare earthquake, and the pier components are damaged before the pot bearing.

Abstract:In recent years, destructive earthquakes occurred frequently in Songyuan area, Jilin Province, so the seismic resilience of group structures in this area is necessary to be analyzed. The earthquake response and seismic resilience of group structures in cities and towns of Songyuan area were analyzed and compared in this paper. The evaluation process of seismic resilience of group structures was determined according to the Standard for Seismic Resilience Assessment of Buildings (GBT 38591-2020), and a quantitative analysis of the seismic resilience of group structures was conducted based on the resilience index method and the resilience grade method. The evaluation results of the seismic resilience of urban and rural buildings in the Songyuan area can provide a theoretical basis for local earthquake prevention and disaster mitigation.

Abstract:Seismic landslides will bring great damage to the natural environment and people's lives. To evaluate and judge the extent of seismic impact on slopes in a large area, the risk assessment method of potential earthquake-induced landslides with predictive properties is mainly adopted. It is of great significance for engineering construction and disaster prevention to deeply study the basic theory of seismic slope hazard analysis and to draw the actual hazard distribution map. The research progress of data for seismic hazard analysis of slopes with predictive properties is reviewed. The data for evaluating slope seismic risk with permanent displacement method can be classified into three types: (1) The judgement basis of slope failure degree under the influence of earthquake; (2) Regional ground motion parameters, such as peak acceleration and Arias intensity; (3) The basic parameters of slope, such as cohesive force, friction angle, gravity, slider thickness, and slope angle. The accuracy of slope seismic risk assessment is related to the depth and accuracy of the three kinds of data. The research status and application of the three types of data are described in detail. Finally, some problems in the theory and the future research directions are analyzed.

Abstract:As a kind of destructive geological disaster, earthquake and rainfall are two key factors to induce landslide. From the perspective of earthquakes occurred during rainfall, this paper improved the Newmark model based on the Green-Ampt rainfall infiltration model, and derived the slope safety factor FS under the coupled action of two factors. The seismic landslide risk prediction and the influence of slope and infiltration depth factor on the displacement in an area of Ludian County, Yunnan Province were carried out under three conditions, i.e., no rainfall, rainfall with ponding, and rainfall without ponding. Then the Newmark cumulative displacement distribution and risk zoning in the study area were obtained through a comparison between the three cases. The results show that: compared with the case without rainfall, the area proportions of high-risk area of earthquake landslide in the latter two cases increase from 1% to 9% and 12% with the increase of rainfall time, respectively, and the area proportions of low-risk landslides reduce from 51% to 35% and 33%, respectively. With the increase of slope and infiltration depth, the displacement and the risk of landslide also increase. The improved Newmark model fully considers the promotion effect of rainfall on the occurrence of earthquake landslide and can better reflect the relative landslide risk of each site in the study area, thus having certain guiding significance for the landslide risk prediction.

Abstract:The seismic response analysis of site is an important guarantee for the smooth implementation and long-term safe operation of many offshore projects, and it is very important to study the small strain dynamic properties of seabed soil. In this paper, the small-strain dynamic properties of offshore seabed soil in the Liaodong Bay were tested and analyzed by using the in-situ borehole sampling and indoor resonant column instrument. The applicability of the existing empirical formula for predicting the initial shear modulus G_max of seabed soil was discussed. The test results in this study were compared with the existing shear wave velocity test results of seabed soil in the surrounding sea area. The attenuation characteristics of dynamic shear modulus and the development trend of damping ratio of the seabed soil in small strain range were analyzed, and a quantitative simulation was carried out to evaluate the normalization characteristics of dynamic shear modulus' attenuation curve of seabed soil. The results showed that the shear wave velocity of undisturbed seabed soil obtained by resonance column test is in good agreement with the existing test results in the surrounding sea area. Compared with land soil, the normalized G/G_max of marine soil decays more slowly and the damping ratio increases more slowly in the small strain range. By introducing the critical shear strain, the attenuation curves of small strain dynamic shear modulus of seabed soil corresponding to different soils and different buried depths can be normalized. These research findings can provide some guidance in reasonably evaluating the site response of offshore projects.

Abstract:To reveal the seismic response characteristics of pebble soil sites, a series of shaking table tests were conducted on a typical pebble soil site in Chengdu, Sichuan Province. The amplification coefficient of peak acceleration, the acceleration spectrum response, and the dynamic earth pressure response of the pebble soil site under different seismic waves and excitation intensities were studied, and the nonlinear effect of site seismic response and the relationship between dynamic shear stress and dynamic shear strain of soil were analyzed. Results showed that the surface layer of the pebble soil site has obvious amplification effect on the seismic wave, and the amplification coefficient of peak acceleration is between 1 and 1.4; the amplification effect of the lower layer is small, and the amplification coefficient is between 0.9 and 1.2. The pebble soil site has the function of low-frequency amplification and high-frequency filtering; the upper and lower limits of filtering frequency band gradually move to the low-frequency direction with the increase of excitation intensity. The soil is not damaged when the excitation intensity is small, and the dynamic earth pressure increases gradually during the earthquake. With the increase of excitation intensity, the dynamic earth pressure response decreases sharply and then increases gradually. Under the low-intensity seismic excitation (SN1), the middle soil first enters the nonlinear response stage, and the energy loss of seismic wave in the middle soil layer is the largest. When the excitation intensity is large (EL3), the soil has a large deformation, and the maximum dynamic shear strain of the soil is 1.7%. At this time, the amplification effect of pebble soil site on the seismic wave is obviously weakened.

Abstract:As an earthquake-prone country, the earthquake damage to infrastructure in China is particularly serious. To protect the normal operation of infrastructure and the safety of people's property, the bad performance of ordinary bolt supporting structures under earthquake and the defect that the anchoring force can not be formed quickly in rescue and disaster relief were concerned in this paper. Combined with the advantages of recycled bolt and enlarged head bolt, this paper put forward a novel anchoring structure. The proposed structure can form the anchoring force quickly, can be reused and has high bearing capacity. The structure composition and working mechanism were discussed, and the bearing capacity of the structure was analyzed and verified in combination with the in-situ test. The results show that the structure has good bearing capacity, which is conducive to maintaining the safety and stability of the slope. The study can provide a reliable basis for the application of the structure.

Abstract:To explore the characteristics and development law of sand liquefaction in Tai'an County, Liaoning Province, a series of undrained dynamic triaxial tests were carried out to study the variation of dynamic pore pressure and dynamic strain of Tai'an sand under dynamic load. The main factors affecting the sand liquefaction were analyzed, and the dynamic response characteristics of sandy soil under dynamic loads were revealed. The results showed that the ud curves of samples with different dynamic stress include two types: steep increase in the late stage and uniform increase. When the dynamic stress amplitude is large, the u_d increases at a fast rate, showing the characteristics of uniform increase; when the dynamic stress amplitude is small, the u_d curve slowly increases in the initial stage and steep increases in the later stage. Relative density Dr and dynamic stress amplitude CSR are the main factors that determine the liquefaction of sand soil. Under the equivalent dynamic load, the CSR gradually accelerates with the continuous development of Dr. When the Dr is large enough, the sand soil is basically in a dense state and liquefaction is difficult to occur. Under the action of cyclic load, the dynamic response of Tai'an sand include three stages, i.e., overall compression, tension-pressure balance and tension in the initial, middle and later stages, respectively. At the later stage, the tensile strain of the specimen increases rapidly, which is prone to tension failure. The research results provide a theoretical support for the seismic design of buildings in Tai'an area, Liaoning Province.

Abstract:The problem of uncertainty and fuzziness of the mapping relationship between influencing factors of sand liquefaction discrimination and sand state was discussed in this paper. Based on the factor reduction of neighborhood rough set (NRS), a support vector machine (SVM) discriminant model of sand liquefaction was constructed by using the improved sparrow search algorithm (ISSA) with multi-strategy integration to optimize the parameters C and g. 42 groups of examples in Songyuan area, Jilin Province were taken as the overall sample set, including 35 groups as the training set and the other 7 groups as the test set, and nine influencing factors were reduced to four factors by using the NRS. The ISSA-SVM model was then used for prediction, and the sensitivity analysis of the four factors was carried out. The results show that factor reduction can eliminate redundant attributes and reduce the complexity of the model; ISSA algorithm has strong exploration, convergence, and local escape ability. As compared to other models, the NRS-ISSA-SVM discriminant model for sand liquefaction has higher accuracy and stronger gene-ralization ability. To accurately distinguish the liquefaction state of sand, the four factors, i.e., the water level buried depth, the seismic intensity, the standard penetration number, and the thickness of non-liquefied soil, need to be accurately identified, especially the first three factors. The NRS-ISSA-SVM discriminant model for sand liquefaction proposed in this paper can not only accurately judge the sand state of other unknown points in the area, but also provide reference for other similar problems.

Abstract:Based on the detailed results of field survey, section mapping, and Quaternary dating data, the Late Quaternary activity characteristics of four major faults, i.e., the new Taoyuan fault, Daoguo fault, Hejiangcun fault, and the known Longpan-Qiaohou fault in the Taoyuan segment of Longpan-Qiaohou fault zone were studied. The results show that the activity characteristics of the Taoyuan section of Longpan-Qiaohou fault zone in Late Quaternary are obvious and the activity intensity is medium. The Longpan-Qiaohou fault and Hejiangcun fault are Holocene active faults, while the Taoyuan fault and Daoguo fault are Late Pleistocene faults. Since the Late Pleistocene, the Longpan-Qiaohou fault and Daoguo fault are dominated by left-lateral strike-slip, while the Taoyuan fault and Hejiangcun fault are dominated by normal strike-slip. The activity of these faults affects the seismic risk and tectonic stability of the study area and its vicinity to varying degrees. The research results in this paper provide the new data for further understanding the Late Quaternary activity of the Taoyuan segment of Longpan-Qiaohou fault zone, and can also provide basic geological data for further understanding the seismic geological characteristics and seismic safety evaluation of engineering construction in the study area.

Abstract:Huahai fault, an active fault developed in Huahai basin, is located on the north side of Altyn Tagh fault, western end of Hexi Corridor. The understanding of the fault activity is helpful to evaluate the seismic risk in this area, and also has important scientific significance for in-depth understanding of the interaction between the Qinghai-Tibet Plateau and surrounding blocks during the northward expansion. The results of remote sensing interpretation and seismic geological survey show that the Huahai fault is mainly developed in the Huahai basin, with a length of about 25 km and a fault strike of NNW. From south to north, the Huahai fault starts from the south of Huahai Town, then passes through Xiaoquan, Daquan, Shuangquanzi, and Shanshui river, and gradually disappears in front of the alluvial fans at Beishan Mountain. In geomorphology, the southern part of Huahai fault shows a linear fault scarp, and the northern part forms the boundary between aeolian sand mound and alluvial diluvial fan. Three trenches were excavated across the fault scarp in the north section of the fault. The OSL dating results suggest that the latest paleoseismic event along the fault occurred about 50 000 years ago, and no distinct evidence of fault activity was found since Holocene. Therefore, the Huahai fault is a Pleistocene active fault, and the vertical slip rate since Late Quaternary is less than 0.03 mm/a. The analysis results of regional geodynamic background show that the Huahai fault is a secondary active fault formed in Huahai basin under the northward expansion of the Qinghai-Tibet Plateau.

Abstract:The characteristics of crustal deformation movement around the epicenter before the Aksai M5.5 earthquake in 2021 were studied in this paper. According to the GPS observation data provided by CMONOC, the characteristics of crustal deformation before the M5.5 earthquake were discussed by the GPS velocity field analysis, the area strain rate field analysis, and the baseline time series between stations. The results show that the epicenter of Aksai M5.5 earthquake is located in the southwest side of the seismogenic fault, where the far-field velocity is large and the near-field velocity is small, and the velocity field directions are obviously different on both sides of the seismogenic fault. It is also located in the high-value area of the compressive area strain rate field consistent with the type of seismogenic structure, namely the compressive strain accumulation area. The turning change of baseline length across the seismogenic fault more than 3 years before the earthquake indicates that the left-lateral strike-slip rate on both sides of the seismogenic fault decreased significantly before the earthquake. The acceleration of the compression rate on both sides of the fault reflects that the strain energy accumulated before the earthquake. The results can provide basic research data for understanding the seismogenic process of the Aksai M5.5 earthquake. Besides, it can provide a reference for the prediction of strong earthquakes in areas with similar structural types.

Abstract:The dynamic characteristics of water temperature at different observation points are various due to the difference in observation environment, wellbore conditions, structural conditions of observation site, medium conditions, and ground hydrodynamic conditions. The water temperature at different layers of the same observation well often show different co-seismic responses due to the difference in the installation depth of water temperature sensor and the distribution of wellbore heat source. In this paper, the wavelet transform algorithm was used to analyze the co-seismic response changes of water temperature at different depths of the well in Sanhai Village, Fangxian County during three earthquakes. Combined with the well temperature gradient, the surrounding rock characteristics, and the aquifers distribution, a simple well-aquifer model was proposed. Furthermore, the dynamic mechanism of different changes in water temperature at different layers of the same well was discussed, and it is preliminarily believed that the dynamic mechanism is derived from the change caused by thermal convection from water flow.

Abstract:The analog water radon observation is an important measurement item of precursory observation in earthquake monitoring and prediction of China.During the process,a series of ma-nual observation methods,i.e.,regular sampling,bubbling,measured value calculation,and data warehousing are adopted.These observation methods,which are backward and have a large degree of human operation error,will affect the quality of observation data to a certain extent.As a key link in the process of analog water radon observation,the bubbling operation directly affects the accuracy of water radon measurement.In this paper,the traditional analog radon bubbling device was improved,and a set of automatic bubbling device suitable for all types of analog radon instruments was developed,which can solve the problem of large interference to the measured value of water radon caused by the non-standard operation during the process of artificial bubbling.The automatic bubbling device can reduce the difficulty of bubbling operation,reduce man-made ope-ration error,so as to improve the quality of water radon observation data.

Abstract:To analyze the influence of control points on the adjustment accuracy of regional mobile gravity network, based on the mobile gravity observation data of Jiangsu area, the influence of control points on the adjustment accuracy of gravity network was analyzed from the perspectives of the number and spatial distribution of control points used in the adjustment calculation, and the monitoring ability of Jiangsu gravity network was further analyzed. The results show that when using a single datum point for adjustment calculation, the datum point should be as close as possible to the geometric center of the network or multiple closed loop nodes. When the control points with uniform spatial distribution are adopted, there is no obvious difference in the point value accuracy between the measuring points around the control point and the measuring points in the middle network. Combined with the spatial resolution of Jiangsu regional gravity network and the adjustment accuracy of observation data, it is considered that Jiangsu gravity network has a good monitoring effect on M≥5 earthquakes. The results of this paper can provide a reference basis for reasonably using datum points for adjustment and improving the accuracy of gravity data processing.

Abstract:In this paper, the annual variation characteristics of each channel at 404 stations were analyzed by using the observation data of ZD8MI multi-separation array observation system from June 2018 to June 2020. It was found that the annual variation characteristics of each channel were different with the difference of observation pole distance. The experimental observation at different polar distances and the simulation analysis of annual change showed that the change of shallow resistivity is relatively large, while the change of deep resistivity is relatively small. The annual variation of 404 station is caused by the seasonal variation of shallow medium resistivity with temperature. The phase difference of annual variation form of different channels is caused by the variation of shallow medium resistivity at different depths. Therefore, the observation of earth resistivity at different depths by using the multi-separation array observation system is particularly important to quantitatively identify the surface non-seismic interference.

Abstract:This paper reviewed and analyzed the modulation ratio of the earth tides during 13 strong earthquakes above M6 in Chinese Mainland since 2009. The conclusions are as follows: (1) The strong earthquakes with M≥ 6 in Chinese Mainland occur frequently during the lunar phase, that is, they are obviously modulated by the solid earth tide; (2) The effect of tidal stress during the lunation is beneficial to the activities of strike slip and normal earthquakes, and the triggering effect on strike slip earthquakes is more significant. The effects of solid tide on seismic faults with different distribution modes and movement properties are obviously different; (2) Before the strong earthquakes with M≥6, the modulation ratio anomalies of small earthquakes near the focal area are widespread, including some small earthquakes occurring in the non-lunar phase period. Special attention should be paid to the phenomenon that the abnormal area of small earthquake's modulation ratio gradually shrinks to the epicenter and the abnormal area gradually decreases before the earthquake.

Abstract:In this paper, the GNSS data from Crustal Movement Observation Network of China (CMONOC) were used to analyze the changes of velocity field, principal strain rate field, maximum shear strain rate field, and surface strain field, as well as the changes of active fault slip rate and cross-fault baseline in the northeastern Qinghai-Tibet Plateau from 1998 to 2018 after excluding the effect of co-seismic response and other disturbances. The secondary blocks in the study area were subdivided to obtain the change of strain rate in each secondary block. The trend and dynamic characteristics of crustal movement field in the study area were also obtained. The results show that the thrust movement along the middle and eastern segment of Altyn Tagh fault, the junction of Qilian block and Qaidam block, the junction of Bayankala block and Qiangtang block, the middle section of the southern boundary of Qilian block, the Haiyuan-Liupanshan fault, and the western segment of the northern West Qinling fault, the left-lateral strike-slip movement along the western section of northern boundary of Qilian block and the Zhuanglanghe fault, and the sinistral strike-slip movement along the eastern section of northern boundary of Qilian block and the eastern section of northern edge of West Qinling Mountains all belong to the continuous local strain enhancement movement with a certain degree of crustal deformation. The eastern section of Altyn Tagh fault zone, the middle and western section of East Kunlun fault zone, the northern boundary of Qilian block, the northern section of Zhuanglanghe fault, the southern section of Haiyuan fault, the northern section of Liupanshan fault, and the eastern section of northern edge fault of West Qinling Mountains may be locked, and M_S＞6.0 earthquakes may occur in these areas in the next decade.

Abstract:The strong fault dislocation caused by earthquake is an important reason for the serious damage on underground structures such as tunnels. Based on the investigation of seismic damage on tunnels caused by the dislocation of left-lateral strike-slip reverse fault during the Menyuan M6.9 earthquake in Qinghai Province, this paper studied the characteristics and causes of the seismic damage, and mainly obtained the following conclusions: (1) The left-lateral strike-slip reverse fault causes serious dislocation of Daliang tunnel, with the maximum horizontal offset of about 1.78 m and the maximum vertical uplift of about 0.68 m. (2) The earthquake damages are mainly concentrated within the influence range of the fault. The severely damaged section of the tunnel is about 350 m, accounting for 5.33% of the total length of tunnel; the less seriously damaged section is located within 402 m at the large mileage side and 646 m at the small mileage side, accounting for 15.96% of the total length of tunnel; the earthquake damage in other sections is generally slight. (3) Construction joints and invert filling layers are more sensitive to strong earthquakes, whose seismic damage is prominent. The beneficial enlightenment obtained from the analysis of seismic damage characteristics in this study can provide reference and guidance for the seismic design of similar projects.

Abstract:To study the seismogenic structure and tectonic stress field characteristics of the 2022 Menyuan M_S6.9 earthquake sequence, this paper collected the P-wave first motion data of M≥1.9 earthquakes from January 8 to January 30, and the focal mechanism solutions of 66 earthquakes were obtained based on the inversion method of P-wave first motion polarity. The focal mechanisms were classified according to the areal strain of horizontal strain rosette. Although the focal mechanisms were widely distributed, the strike-slip types were in majority, indicating that the earthquake sequence was dominated by strike-slip. Then, based on the focal mechanism solutions, the tectonic stress field was inverted by the grid search method, and the NEE-SWW compression and NNW-SSE tension were shown in the study area. The P and T axes of the focal mechanism of main shock were close to the stress direction of the local stress filed. Finally, the relationship between the focal mechanism of the main shock and the stress regime was simulated. It is concluded that the earthquake ruptures along the optimal nodal plane under the action of tectonic stress field, which can be regarded as a normal release after the accumulation of strain energy in the study region. This study has reference significance for analysis of subsequent seismogenic mechanism and geodynamic.

Abstract:In this paper,we used the maximum entropy spectrum method to study the geoelectric field data observed by the Songshan,Shandan,Gaotai,and Guazhou stations around the epicenter of Menyuan M_S6.9 earthquake on January 8,2022,and analyzed the power spectral density (PSD) variation of geoelectric field before and after the earthquake.The results show that:(1) The increased PSD of low frequency components was recorded before the Menyuan M_S6.9 earthquake by the four stations;(2) The changes of PSD before the Menyuan earthquake were basically synchronous in the four stations;(3) Combined with the microfracture mechanism and the viewpoint of "multi-point field",we drew the conclusion that the electromagnetic anomalies before the Menyuan M_S6.9 earthquake were mainly caused by the electromagnetic radiation generated by the focal development.

Abstract:Based on the electron density data observed by Langmuir probe and the Oxygen ion density data observed by plasma analyzer of Zhangheng-1 electromagnetic satellite, the characte-ristics of electron density and oxygen ion density anomalies before the Menyuan, Qinghai M_S6.9 earthquake on January 8, 2022 were analyzed, and the anomalies before previous earthquakes were also summarized. The results show that high values of electron density and oxygen ion density appear 11 days before the Menyuan M_S6.9 earthquake. Through the statistical analysis of ionospheric anomalies before 16 earthquakes, it is found that anomalies are generally observed within 6 days prior to earthquakes, and most of them occur before strike-slip and thrust earthquakes. The acoustic gravity wave mechanism and electric field mechanism were considered as the possible generation mechanism of anomalies.

Abstract:The Maduo, Qinghai M_S7.4 earthquake on May 22, 2021 broke the long-term M7 earthquake quiescence in Chinese Mainland, followed by the Menyuan, Qinghai M_S6.9 earthquake on January 8, 2022. Understanding the precursory anomaly evolution before the two earthquakes is of great significance for the seismogenic process and short impending tracking of strong earthquakes in the Tibetan Plateau. In this paper, the anomaly characteristics, coseismic response, and post-earthquake effect of underground fluid at Zuoshu station in Xining City, Qinghai Province before the two earthquakes were analyzed, and the results show that the sudden drop of dynamic water level in Zoshu on February 25 and August 25, 2021 corresponds to the Maduo M_S7.4 and Menyuan M_S6.9 earthquakes, respectively. Furthermore, the trend turning anomaly of the dynamic water level and water temperature, which simultaneously appeared at Zuoshu station on July 10, 2021, has a certain spatio-temporal significance for the subsequent Menyuan M_S6.9 earthquake. Through a discussion on the mechanism of water level and water temperature precursory anomalies at Zuoshu seismic station, it is found that the abnormal changes of underground fluid have a good short-term indication for strong earthquakes in the secondary block of Qinghai Province and its surrounding areas.

Abstract:Based on the analysis of various seismological indexes of the Menyuan MS6.9 earthquake on January 8, 2022, it was found that the load/unload response ratio (LURR) presented obvious anomalies before and after the earthquake, with a short-term abnormal trend. The anomaly characteristics of M≥6 earthquakes along the Lenglongling fault zone of Qilian Mountain structural belt in northeastern Qinghai-Tibet Plateau were analyzed by the analogical induction method. It was considered that the LURR usually fluctuated around 1, and gradually rose to the peak value two months before the strong earthquake. Then the LURR fell back to about 1, and after a period of lag time (usually 10 days, or even longer), the strong earthquake occurred, and the greater the magnitude, the longer the lag time. Strong aftershocks in the study area have similar characteristics. The characteristics of LURR can provide reference for earthquake prediction and post-earthquake trend judgment in areas around the epicenter and areas with similar structural belt.

Abstract:On January 8, 2022, an M_S6.9 earthquake occurred in Menyuan County, Qinghai Province. A local collapse of the Ming Dynasty Great Wall occurred in Shandan County, a low intensity area. As a key national heritage site, the collapsed wall was 114 km away from the epicenter, and was subjected to relatively weak ground vibration, thus its seismic failure mechanism deserves in-depth study. After the earthquake, an engineering geological investigation and disease survey were carried out on the collapsed walls, and then the theoretical analysis and numerical simulation were performed to study the seismic failure mechanism of the collapsed walls in combination with the survey results. The results reveal that: in the Shandan section of the Ming Great Wall, the wall erosion disease is developed, and the structural cracks with no protective measures are exposed. The collapsed wall is near the national road and the village road, hence the environmental vibration effect is apparent. The weathering difference between the north and south sides of the wall is manifest; the moisture of the north side is higher than the south side, thus the strength of the rammed-earth is lower. The existing diseases of the wall aggravate the effect of seismic inertia force, resulting in the local collapse of the Great Wall. The M_S6.9 earthquake has a warning significance for the protection of the Ming Dynasty Great Wall sites in the Hexi region of Gansu. The research results can provide a certain scientific basis for the seismic protection of the Ming Dynasty Great Wall in future.

Abstract:On January 8th, 2022, an M_S6.9 earthquake occurred in Menyuan Country, Qinghai Province. To study the co-seismic surface displacement caused by the Menyuan earthquake, we proposed four different dislocation models based on the data of seismogenic fault, and four co-seismic surface displacement fields induced by the Menyuan earthquake were calculated with the analytical solution of surface displacement proposed by Okada. Combined with the field observation data, the sliding form of the seismogenic fault and the associated effect on the surrounding surface were discussed. The results show that the the west side of Lenglongling fault extending to the Tolaishan fault is preliminarily judged as the seismogenic fault of the earthquake. The fault is dominated by left-lateral strike-slip, and the maximum dislocation on the fault reaches about 4 m. Southwest surface of the epicenter moves in the NE direction, and the southeast side moves in the SE direction; the northwest and northeast sides move in the direction of NW and SW, respectively. A small area near the epicenter has a horizontal surface displacement of more than 1.5 m, and there is a vertical surface displacement of more than 0.5 m along the rupture zone. In-situ monitoring results show that the maximum horizontal dislocation is about 2.1~2.3 m, and the maximum vertical dislocation in some areas reaches 0.7 m. Taking the epicenter as the center, the influence range of surface displacement caused by fault is about 30 km × 36 km, and the surface displacement within this range is greater than 0.1 m. The study can provide a reference for the post-earthquake recovery and the subsequent project fortification in this area.

Abstract:Based on the observation data of mobile geomagnetic survey points in some areas of Qinghai and Gansu from 2020 to 2021, the abnormal changes of lithospheric magnetic field in the study area were obtained, and the anomalies characteristics of lithospheric magnetic field before the Menyuan, Qinghai M_S6.9 earthquake on January 8, 2022 were systematically analyzed. The results show that: (1) Before the Menyuan earthquake, each component of the lithospheric magnetic field in the epicentral area showed obvious anomalies. The horizontal vector of the lithospheric magnetic field showed abnormal change of direction turning and amplitude weakening near the epicenter; the vertical vector direction showed obvious anomaly of south-north offset. In addition, the magnetic declination had a zero-variation line near the epicenter, showing a high gradient belt distribution. The total intensity and vertical component had a zero-variation line distribution and bending near the epicenter. (2) According to the piezomagnetic effect, the lithosphere magnetic field in the north of Lenglongling fault increased, and the stress mainly released at a low level. The lithosphere magnetic field in the south of Lenglongling fault decreased, and the stress mainly accumulated at a high level. Before the Menyuan earthquake, the piezomagnetic effect caused by the tectonic stress adjustment of Lenglongling fault might be the main reason for the abnormal variation of lithosphere magnetic field in the epicentral region.

Abstract:Based on the 1-second sampling data of 14 geomagnetic stations in Gansu, Qinghai, and Sichuan areas, we analyzed the temporal and spatial variation characteristics of the geomagnetic vertical intensity polarization anomalies before Menyuan, Qinghai M_S6.9 earthquake on January 8, 2022. The results show that there were high value anomalies of geomagnetic vertical intensity polarization at the end of October 2021. The anomalous stations are mainly distributed in Qinghai, Gansu, and Sichuan areas. The Menyuan earthquake occurred 73 days after the occurrence of anomalies, and the epicenter was near the anomaly threshold line of high polarization. It is also found that the geomagnetic polarization anomalies have certain temporal and spatial variation characteristics. In terms of time, the polarization anomalies of different stations have good time synchronization; spatially, the anomalous high-value area of polarization shows the characteristics of appearing near the epicenter, then expanding continuously, and finally shrinking toward the epicenter. In addition, the normalized zero polarization value, the anomaly duration, and the epicentral distance of anomalous stations show a negative correlation. As the epicenter distance of abnormal stations decreases, the normalized zero polarization value becomes higher and the anomaly duration becomes longer. It is consistent with the attenuation characteristic of the seismic electromagnetic signal. According to the spatial-temporal relationship between polarization anomalies and earthquakes, the high-value anomaly of geomagnetic polarization corresponds to the subsequent Menyuan M_S6.9 earthquake.