Abstract:Under earthquake action, deep-water cable-stayed bridges located in high-seismic-intensity areas are affected by the strong earthquake and the nearby water body. Therefore, choosing a reasonable seismic system is important for the structure. Here with the Shuanghe super-large bridge of Geqiao Expressway in Yunnan Province taken as an example, the influence of hydrodynamic effect on the seismic response of cable-stayed bridge and its relationship with seismic intensity were analyzed. On this basis, the longitudinal and transverse seismic systems of cable-stayed bridges were studied. Results show that the internal force at the tower bottom and the structural displacement response will be increased by the hydrodynamic action, and the greatest effect appears in shear force. The amplification effects of hydrodynamic action on different responses of the structure vary with the increase in earthquake intensity. Thus, hydrodynamic action under various earthquakes should be considered in the seismic design. The longitudinal cooperative seismic system with viscous dampers set at the cable tower, auxiliary pier, and abutment was recommended for the cable-stayed bridge because it can most effectively reduce the longitudinal internal force and longitudinal displacement of the pier and tower bottom. The combined restraint system with fixed constraints at the cable tower and elastic-plastic constraints such as steel damper at the pier and abutment was recommended for the cable-stayed bridge. The transverse internal forces at the bottom of the pier and tower could be reduced at the same time, and the overall transverse displacement response of the structure could be effectively controlled by using the system.

Abstract:To study the influence of the height of dense longitudinal reinforcement on the seismic performance of railway gravity bridge piers, the principle of determining the height of longitudinal reinforcement at the bottom of bridge pier was proposed and the formula of reinforcement height was deduced. Through numerical analysis, the hysteretic curve and skeleton curve of the pier were studied, and the seismic performance of pier after adding longitudinal reinforcement at the pier bottom was compared and analyzed. The results show that the horizontal bearing capacity of the pier can be effectively improved by increasing the amount of longitudinal reinforcement at the pier bottom. The reinforcement ratio of section after increasing the longitudinal reinforcement at the pier bottom can be adopted when checking the seismic design of pier. The rationality of the formula was also verified by the numerical analysis results. The research results can provide a valuable reference for the seismic design of railway gravity piers.

Abstract:In view of the freeze-thaw spalling disease for loess cutting slopes in seasonal frozen regions, the effect of loess tensile strength cannot be ignored. Remolded loess taken from an artificial cutting slope in Heping Town, Lanzhou City, was analyzed in this paper to discuss the freeze-thaw attenuation characteristics of loess tensile strength. First, the tensile strength of loess samples under different water contents (11%, 13%, 15%, 17%, and 19%), different dry densities (1.55, 1.60, 1.65, 1.70, and 1.75 g/cm³), and different freeze-thaw cycles (0, 1, 3, 5, 7, 11, and 15) were measured with the axial splitting method. Then, a single-factor freeze-thaw attenuation model for loess tensile strength was proposed based on the freeze-thaw attenuation law of loess tensile strength. Finally, the multivariate freeze-thaw attenuation model for loess tensile strength was proposed based on a multivariate nonlinear regression analysis of water content, dry density, and tensile strength. Results show that the tensile strength presents an exponential decay with the increase of freeze-thaw cycles and reaches a stable value after three to five freeze-thaw cycles; the initial water content has a great influence on the freezing-thawing attenuation of loess tensile strength. The multivariate freeze-thaw attenuation model of loess tensile strength can forecast the tensile strength of Lanzhou loess with water content, dry density, and freeze-thaw cycles as variables.

Abstract:To study the failure mode of reinforced concrete (RC) frame columns under different shear-span ratios, on the basis of the RC columns in Xuankou middle school that collapsed during the Wenchuan earthquake in 2008, a 1:4 RC column model was designed and constructed, and the plane frame quasi-static test was conducted. Three sets of RC frame column models with different shear-span ratios (λ=3, λ=4.25, andλ=6) were designed. The hysteresis curves, skeleton curves, and different failure modes of the RC frame column were obtained through a quasi-static test, and the ratio of shear bearing capacity to axial force of the RC frame column was defined as the seismic performance coefficient α. With the use of three sets of test data, the failure rule of the RC frame column based on the seismic performance coefficient α was summarized: When α ＜ 0.16, flexure failure happens on the RC frame column; when α≥0.16, shear failure happens on the RC frame column. If the value of α is well grasped in the seismic design of the RC frame column, then the shear and brittle damage of the column can be avoided. In addition, three methods for calculating the shear force from strain were compared, and an effective shear force calculation method in the linear case was obtained; that is, the actual stress state at the end of the RC frame column was retrieved by the measured strain information.

Abstract:One important purpose of the structural health monitoring is to detect and localize the structural damage. A new structural damage detection and localization method is proposed by combining the time series model of monitoring data with kernel ridge regression. First, a theory was proposed that the difference between the autoregressive coefficient vectors of damaged and undamaged structures is related to the damage coefficient vector, and the relation was included in the structural damage detection matrix. The damage detection matrix of structure can be obtained by the kernel ridge regression algorithm in machine learning. Then, the method was verified by using the numerical model of a concrete frame. The results show that compared with other regression algorithms, the proposed method can greatly improve the fitting and generalization performance of the model, thus it can be better applied to structural damage detection; the method can effectively detect the single damage scenario and multiple damage scenario, and the accuracy is high.

Abstract:Research on the dynamic response characteristics and deformation and instability mechanism of loess slopes under earthquake action has important theoretical and practical significance. However, relatively few studies on this topic have been conducted from the perspective of dynamic response spectrum characteristics. The peak ground acceleration data of a loess slope obtained from large-scale shaking table model tests were used in this paper. First, the dynamic instability mechanism of the loess slope was discussed by analyzing its variation law, especially from the perspective of its frequency spectrum characteristics. Then, the analysis of the absolute acceleration response spectrum was performed on the acceleration time histories of measuring points at different elevations of the slope and the points at the vertical and horizontal directions inside the slope. Finally, the dynamic instability mechanism of the loess slope was proposed from the perspective of frequency spectrum changes. Results show that the response process of loess slope under dynamic seismic action can be divided into three stages: elastic stage, plastic stage, and failure stage. When the loess slope enters the failure stage, it is accompanied by an increase in the peak value of the response spectrum or the change of the dominant period. In the elastic phase, the increase in the peak acceleration of the response spectrum is consistent with that of the input ground motion amplitude; in the plastic stage, the increase in the peak acceleration of the response spectrum is smaller than that of the input ground motion amplitude. This study suggests that the highlight of the first peak of the response spectrum can be used as one of the bases for judging the damage degree of the slope.

Abstract:To study the dynamic response characteristics of structures in ground fissure sites under earthquake, a ground fissure site in Xi’an City was chosen as the research background in this paper, and different foundation forms of the frame structure were considered, i.e., the independent foundation, the raft foundation, the pile foundation, and the pile-raft foundation. Dynamic response characteristics, such as peak acceleration and story drift ratio of the structure, as well as their influencing factors and laws, were studied systematically. Results show the following: (1) The acceleration response and story drift ratio response of the structure in the ground fissure site have an obvious amplification effect. With increasing distance from the ground fissure, the peak value of dynamic response gradually declines and finally tends to be stable. The influence range of the amplification effect is about 24 m, and the structure within this range needs to improve its seismic fortification level. (2) With the increase in the story height, the peak acceleration of the structure increases, showing an “S” shape as a whole, and the story drift ratio increases first and then decreases. Both of them show an obvious “hanging wall effect”, that is, the structural response of the hanging wall is stronger than that of the footwall. With the increase in the input ground motion intensity, the difference in dynamic responses of the structure on the hanging wall and footwall is further expanded, and the hanging wall effect is increasingly significant. (3) The acceleration responses of the structure on the raft foundation, pile foundation, and pile-raft foundation are similar, while the acceleration response of the structure on the independent foundation is obviously different from the first three. However, with the increase in floors, the influence of foundation form on the story drift ratio gradually weakens. (4) A certain difference exists in the peak acceleration, peak displacement, and peak appearance time of structures on the hanging wall and footwall, because when a seismic wave passes through the ground fissure, complex reflection and refraction occur, and the structure is subject to non-uniform excitation.

Abstract:The results of the strong motion observation, experimental analysis, and numerical simulation show that ridge terrains can substantially influence seismic ground motion. In this study, the three-component ground motion acceleration time histories of the mainshock and aftershocks of the Wenchuan earthquake recorded by three topographic effect observation arrays were selected. The peak ground acceleration, peak acceleration ratio, and response spectrum ratio of each observation point were obtained after a baseline correction, and the influence of the ridge terrain on earthquake ground motion was analyzed. The results show that the ridge terrain has a remarkable effect on the seismic ground motion, and the topographic effect is directional, varying with the ridge terrain. The topographic effect in the horizontal direction is more significant than that in the vertical direction. The spectral ratio, which is related to the period, is not always greater than 1.0 in the analyzed period.

Abstract:Based on the waveform data of Ningxia regional seismic network and three different velocity models, the focal mechanism solution of Guyuan, Ningxia M_S4.6 earthquake on September 2, 2017 was calculated by using three methods, i.e., the Hash method, the Snoke method, and the gCAP method. Then the central focal mechanism solution of the earthquake was given according to the results. The results show that the central focal mechanism solution is that: nodal plane I: strike 41°, dip 79°, slip angle -175°; the nodal plane II: strike 310°, dip 85°, slip angle -11°. The minimum 3-D rotation angle between the results of Hash method and the central solution based on velocity model 3 is the lowest, and their focal mechanism solution parameters are the closest. The fine layered velocity structure model can help to improve the precision of the results, and the inversion results of Hash method and Snoke method are obviously influenced by the velocity structure model. Compared with the Snoke method, Hash method has lower requirements for the station distribution, but higher requirements for the waveform quality, and it requires that the whole waveform and the shear wave band both have a high signal-to-noise ratio in a certain range. In reality, for regional seismic networks with poor azimuth coverage of stations, or for the epicentral area with relatively few stations, the Hash method can be used to enrich the calculation results when the parameters of the fine velocity model and the threshold of waveform signal-to-noise ratio are set accurately.

Abstract:The waveform characteristics of natural earthquakes and artificial blasting are similar but difficult to distinguish. Combined with gray wolf optimization (GWO) and support vector machine (SVM), a new method for identifying the nature of seismic events is proposed in this paper. The signals of seismic events during the Yushu M7.0 earthquake in Qinghai Province and some artificial blasting events were analyzed by the Mel frequency cepstrum coefficient (MFCC) method. Through the pre-emphasis, fast Fourier transform, Mel filter, and discrete cosine transform, the sample entropies of the static coefficient and first-order and second-order differential coefficients were extracted as the sample feature set. GWO was used to optimize the penalty coefficient and kernel radius in the radial basis kernel function of SVM to form a new GWO-SVM classifier. Then, the GWO-SVM classifier was used to identify events. The results show that the recognition effect of the GWO-SVM classifier is obviously better than that of other classifiers, i.e., SVM, RobustBoost ensemble learning, linear discriminant analysis (LDA), and probabilistic LDA. Under 1 000 cycles of recognition experiments, the average accuracy of the GWO-SVM classifier increased by 9.2% compared with that of SVM, and the standard deviation was reduced by more than 3.2. The t-test proves that the MFCC sample entropy has a reliable earthquake event classification effect, and the GWO-SVM and MFCC sample entropy can be used as identification methods and classification criteria for natural earthquake events and artificial blasting events.

Abstract:Before the Guye M_S5.1 earthquake in Tangshan on July 12, 2020, the short-electrode-spacing deep-well geoelectrical resistivity showed a change rapid decrease-return phenomenon. The variation pattern and anomaly duration conform to the seismogenic mechanism of geoelectrical resistivity, but the decreasing amplitude is far smaller than that of the seismogenic process of large polar distance geoelectrical resistivity. Therefore, based on the underground electrical structure and device system of Tongzhou station, this study used the numerical analysis method to analyze the detection depths of ground resistivity and short-electrode-spacing deep-well geoelectrical resistivity. The results show that when the resistivity of the medium in the bottom layer decreases, the observed value of the ground resistivity and short-electrode-spacing deep-well geoelectrical resistivity at Tongzhou station will also decrease, and the decreasing range increases with the upward expansion of the upper interface of the deep medium resistivity change area. Compared with the observation of surface resistivity, the short-electrode-spacing deep-well geoelectrical resistivity observation can receive deep resistivity change signals more significantly, thus having a stronger ability to reflect earthquake preparation.

Abstract:The analysis of foreshock sequences is of great significance in the study of the nucleation process of the mainshock. To explore the seismogenic mechanism of the Yingjiang M_S5.8 earthquake on March 10, 2011, the match and locate (M&L) method for small events was used to analyze the continuous seismic records of five stations around the mainshock. A total of 92 seismic events with M_S≥2.0 were selected to perform the foreshock detection and enrich the foreshock catalog. Then, combining the distribution characteristics of foreshocks and aftershocks with the b value change trend before and after the mainshock, a study on the seismogenic mechanism of the Yingjiang M_S5.8 earthquake was conducted. The results show that the foreshocks of the Yingjiang M_S5.8 earthquake mainly occurred on the west side of the first branch of the Dayingjiang fault, and the aftershocks spread in two dominant directions of ENE and SSE. Based on inferences, the Yingjiang earthquake was caused by the rupture of a conjugate fault system consisting of the ENE-trending Dayingjiang fault and an SSE-trending blind fault due to the continuous eastward extrusion of the Indian plate. The change in the b value before and after the mainshock suggests that the fluids widely existing under the Tengchong volcanic area play an important role in inducing and promoting the occurrence of the earthquake.

Abstract:To accurately distinguish the time-frequency response characteristics of interference signals in fixed-point tidal deformation observation data, we propose a frequency division time-frequency analysis method based on wavelet decomposition and the synchrosqueezing transform. First, the original data were decomposed and reconstructed by selecting the appropriate wavelet basis and decomposition levels, and the target interference component signal was extracted. Second, based on the frequency division strategy, the high-frequency interference component was processed using the synchrosqueezing generalized S-transform with a good high-frequency resolution, and the medium- and low-frequency interference components were processed using the synchrosqueezing wavelet transform with a good low-frequency resolution. The synchrosqueezing transform can further “squeeze” the instantaneous frequency band on the basis of the results of the traditional method and obtained time-frequency spectrum results with higher precision. The proposed method was used to process and analyze typical interference data affected by many factors in the deformation observation of Xiaomiao station in Xichang from 2016 to 2020, such as zeroing and calibration, cavern entrance, road construction and traffic, pumping, rainfall, and seismic waves. The time-frequency response characteristics of various deformation observation methods under different interference factors were obtained, thus providing a reference for the accurate identification of interference and abnormal characteristics.

Abstract:This paper focuses on the hydrogen concentration in the soil gas of 380 measuring points in the southern Tangyin graben and reveals the relationship between the spatial distribution characteristics of hydrogen concentration and tectonic activities in this area. The results show that the hydrogen concentration in soil gas in the southern Tangyin graben ranges from 0.26×10^-6to 175.50×10^-6. The background mean value and lower threshold are (17.25±11.19)×10^-6and 57.30×10^-6, respectively. The hydrogen concentration in soil gas shows the spatial distribution characteristics, i.e., low in the west and high in the east. The points with a hydrogen concentration below the background value are mainly scattered in the western part of the study area, which resulted from the overburden thickness and surface sedimentary characteristics, whereas the high-value abnormal points are mainly concentrated in the east part of the study area and distributed along the Tangzhong and Tangdong fault zones. The line connecting the high-value points of the hydrogen concentration isoline is basically consistent with the fault strike. Although there is no high-value anomaly in the Tangxi fault zone, the relatively high hydrogen concentration value is still along the fault zone and its adjacent areas, reflecting the control of the fault structure on the hydrogen release. Furthermore, the spatial distribution of the hydrogen concentration isoline shows a high-value abnormal band with the NE-SW trending in the study area. Thus, we speculated that there is a large-scale hidden fault. The hydrogen concentration in the Tangxi fault zone is significantly lower than that in the Tangzhong and Tangdong fault zones. Based on the geophysical exploration data in the study area, the hydrogen concentration and abnormal contrast are considered the reflection of the tectonic activity background. Some deep-source hydrogen diffuses and migrates to the surface through the deep fault and favorable deep-shallow tectonic association. It mixes with the shallow-source hydrogen in the shallow part of the crust and forms a high concentration abnormal band in the fault zone. Therefore, the application of hydrogen in the monitoring of tectonic activities in the south of Tangyin graben should be further explored.

Abstract:Tectonic activities freely occurred in the Bailong River Basin, which is one of the most significant controlling factors for frequent natural disasters in the area. Many scholars have performed significant research on the tectonic activities and associated influence in the study area. However, few of the explorations were made on buried active faults. In this paper, based on three methods, i.e., the high-density resistivity method, seismic reflection method, and seismic refraction tomography method, we propose a comprehensive geophysical exploration method for buried active faults in the Bailong River Basin. Through the method, we can not only identify the features of faults but also confirm their activities. The water-bearing fault and non-water-bearing fault were taken as examples, and similarities and differences in the geophysical characteristics of the two types of faults were found. The comprehensive geophysical prospecting method proposed in this paper has a good exploration effect on the two types of faults. Hence, it can be widely applied to the exploration of buried active faults in the Bailong River Basin and has certain reference significance for explorations of other areas.

Abstract:It is of great practical significance for earthquake prevention and disaster reduction to explore the spatiotemporal regularity of regional earthquakes and evaluate the future earthquake situation. In this study, we selected M_S≥6.8 earthquakes in Iran from 1890 to 2020. The space-time symmetry of regional earthquakes was analyzed using the commensurability method, triplet method, and butterfly structure diagram, and the physical mechanism of earthquakes was discussed in combination with the changes in the sunspot number and Earth’s rotation speed. The results show the following: (1) The strong earthquakes in Iran have good commensurability. According to the temporal symmetry trend, the signal of the next M_S≥6.8 earthquake in Iran is strongest in 2022, followed by that in 2024. Moreover, it is more likely to happen at the end of 2022 via the triplet method. (2) In terms of spatial symmetry, the next earthquake is more likely to occur in the southeast region of 32°N, 54°E according to the migration rule of the longitude and latitude and the epicentral triangle rule of historical earthquakes. (3) The M_S≥6.8 earthquakes in Iran mostly occurred in the extreme years and the descending period of sunspots. At the same time, they mostly happened during the deceleration period of the Earth’s rotation and the extreme years of the day length, which proves the reliability of the space-time symmetry method to a certain extent. This study further deepens the understanding of the regularity of earthquakes and has a certain explorative significance for the regional application of the triplet method.

Abstract:The rapid and accurate identification of natural earthquakes and artificial blasting events is one of the most important tasks of seismic monitoring networks and is also an important basis for improving the quality of seismic observation records and conducting seismic research. In view of the shortcomings of the mainstream classification and recognition methods, such as the back propagation (BP) neural network and support vector machine (SVM), a classification and recognition algorithm for earthquake events based on the improved empirical wavelet transform (EWT) and LogitBoost ensemble classifier was proposed. First, the traditional EWT was improved based on the S-transform spectrum energy curve, and the signal was adaptively decomposed into several intrinsic mode functions. Then, the maximum amplitude ratio of the P-wave to S-wave, the Shannon entropy and logarithmic energy entropy of the first four intrinsic mode functions, and the main frequency of the reconstructed signal after de-noising were extracted. Finally, a decision tree ensemble classifier based on LogitBoost was designed to classify earthquake events. Experimental results show that the proposed algorithm has good robustness and can effectively solve the problem of insufficient samples, and the classification accuracy rate is over 93.1%. Compared with the ensemble learning AdaBoost, BP neural network, and SVM, the classification accuracy of the proposed algorithm is improved by more than 1% and has an excellent classification ability.

Abstract:The study on the space-time symmetry of earthquakes is of great significance to disaster prevention and reduction. Combined with the commensurability theory, we used the butterfly structure diagram and commensurability structure system to study the space-time symmetry of M_S≥7.1 earthquakes in the Iranian Plateau and its surrounding areas. Then, we conducted a secondary prediction to test the first prediction results. The results show that the space-time symmetry is significant in the study area, with the strongest signal of the M_S≥7.1 strong earthquakes in 2023, which passes the secondary prediction test. The epicenters in the study area greatly migrated from east to west before 1960 and from north to south after 1960. Three or four strong earthquakes can form a gyration, and most of them show a closed-ring shape. The epicenter migration takes the form of a double symmetry axis in the latitude. The symmetry axis changed from 43°E to 53°E before and after 1960, and the epicenter symmetrically migrated along 35.5°N after 1960. Future earthquakes may occur near 44°E, 37°N.