Abstract:The strength reduction finite element method(SRFEM)is an effective approach for evaluating the seismic safety factor of slopes.The key to this method lies in selecting a rational criterion for slope failure.At present,three types of criteria are commonly used,namely the penetration of plastic zones(equivalent plastic strain),non-convergence of numerical simula-tion calculations,and abrupt displacement changes at characteristic points.Most scholars be-lieve that the penetration of equivalent plastic strain is a sufficient but not a necessary condition for slope failure.Moreover,the selection standard for the magnitude of plastic strain has cer-tain non-deterministic and other subjective factors.The problem with using non-convergence of finite element calculations as a criterion is that the convergence standard is limited by the soft-ware,and its physical meaning is not clear.The abrupt displacement change or non-conver-gence of characteristic points usually involves selecting characteristic points at the toe of the slope,the midpoint of the slope face,and the crest of the slope.This situation can only reflect the local state of the slope.The slope may be locally unstable rather than globally unstable.Therefore,these three types of criteria have certain differences and subjectivity in practical applications. In this study,through the SRFEM,the concept of a rainbow-like displacement contour map(RLDCM)during slope instability was proposed.A rainbow-like displacement contour map was established as a criterion for the seismic stability of slopes.That is,when the slope fails globally,the displacement contour map should meet the following three conditions:① The soil elements within the landslide mass undergo relative sliding along the circular arc slip surface compared with the stable area inside the slope,presenting a clear circular-arc-shaped boundary.② The displacement of the landslide mass shows a circular-arc-shaped belt distribu-tion along the slope direction,with distinct layers resembling a rainbow.③ The displacement of each layer of the landslide mass first increases and then suddenly decreases from the slope surface to the inside.The layer with the maximum displacement is the slip surface of the slope. The main conclusions drawn from this study are as follows: 1)The appearance of the RLDCM can serve as a criterion for the overall failure of soil slopes under seismic action.It has a clear physical meaning and is easy to operate.Moreover,it can also be combined with other criteria,such as the penetration of plastic zones,to confirm the accuracy of the safety factor. 2)The safety factors obtained using four criteria,i.e.,penetration of plastic zones,ab-rupt displacement of characteristic points,displacement time-history curves,and RLDCM,are not significantly different.Among them,the safety factor obtained using the abrupt displace-ment criterion has subjective differences and a range of acceptable values. 3)When using the abrupt displacement criterion of characteristic points,several points should be selected on the slope surface.Monitoring points at the toe of the slope may yield inac-curate results because they may be located above the shear exit.

Abstract:The seismic ground motion topographic effect,as an important research content in the field of seismic engineering,the study on the mechanism of the effect of complex terrain on the ground motion characteristics can provide basis for engineering seismic defense.A large number of post-earthquake site investigations have shown that the complexity of local terrain has a significant impact on the distribution of seismic damage,especially the irregular terrain can change the intensity and spectral characteristics of ground motion.As a type of locally irregular topography widely existing in nature,the unique geometric shape of river terraces can cause complex scattering and diffraction of seismic waves,resulting in variation of ground mo-tion in its local areas,and then affecting the seismic damage degree of surrounding buildings. Based on on-site seismic damage investigation data of the river terraces during Wenchuan MS8.0 earthquake in 2008,buildings in the area with thicker alluvial deposits at the front edge of the terrace suffered severe damage,while those in the area with thinner slope deposits at the rear edge of the terrace suffered relatively mild damage.In general,the seismic damage at the front edge was significantly greater than that at the rear edge.At the same time,in order to study the mechanism of this seismic damage feature,the river terraces were selected as the re-search object,and the finite difference software FLAC3D is used to establish three-dimensional river terrace analysis models with different thicknesses of overburden soil layers,and then sim-ulate and calculate the ground motion response under impulse loading,thus further revealing the influence law and internal mechanism of river terrace topography on ground motion characterist-ics and the distribution of seismic damage to buildings. For the same order terrace,the peak values of the horizontal and vertical acceleration and the 90％energy duration all show an upward trend with the increase of the overlying soil thick-ness,reaching the maximum value at the front edge of the terrace and the transition point with the steep slope.Meanwhile,the ground motion level at the front edge of each terrace is signific-antly higher than that at the rear edge.As the terrace order decreases,the peak values of the ho-rizontal and vertical acceleration and the 90％energy duration at the corresponding area also gradually decrease.Similarly,the trend of the Fourier spectrum amplitude and ratio at different monitoring points on the same order terrace is basically consistent,but the amplitude and ratio increase gradually as the monitoring point approaches the edge of the terrace facing the steep slope and the front edge of the area with increased soil thickness.As the terrace order increases(the terrain height rises),the natural frequency of the structure increases accordingly,thereby significantly enhancing the amplification effect of low-frequency ground motion.The character-istic period Tg,platform value,and platform amplification coefficient β in the standard re-sponse spectrum of seismic acceleration for different monitoring points are all affected by the terrace order and overlying soil thickness.The Tg value decreases gradually as the terrace order decreases;the platform value increases as the overlying soil thickness at the front edge of the terrace increases;however,the platform amplification coefficient β decreases as the terrace order increases,and the amplification coefficient at the front edge of the terrace is significantly larger than that at the rear edge. River terraces have a significant impact on the propagation of ground motion and the seis-mic damage degree to buildings.The change in the thickness of the overlying soil layer leads to different distributions of building damage by affecting the amplification of ground motion,while the terrace orders exacerbate or mitigate the seismic damage by affecting the spectral char-acteristics and overall level of ground motion.Therefore,the terrace orders and overlying soil thickness are the key factors affecting ground motion response,and the amplification effect of ground motion is stronger in high terrace and thick covering soil layer,which leads to severe damage of buildings.

Abstract:The recognition and classification of seismic events hold significant importance in seismic monitoring and earthquake disaster mitigation.This research primarily focuses on 1935 seismic event data with low magnitude(ML≤3.0)in the North China region,encompassing three distinct types of events:natural earthquakes,artificial explosions,and mining collapses.Preliminary analysis of the earthquake catalog involved the geographical distribution examina-tion,annual trends,and magnitude distribution of these events.Preprocessing of raw seismic data included amplitude normalization,detrending,mean removal,and band-pass filtering(0.5-20 Hz).Additionally,short-time Fourier transform analysis was utilized to visualize waveform and spectrogram characteristics,facilitating the observation and analysis of both time and frequency domain features.Based on the analysis results,62 features across 10 categories,including time,P/S amplitude ratio,frequency,zero-crossing rate,peak amplitude,peak ground acceleration,energy,signal characteristics,angle,and other ratios,were extracted as the foundation for classification. This research employed K-Nearest Neighbors(KNN),Adaptive Boosting(AdaBoost),and Light Gradient Boosting Machine(LGBM)algorithms to classify the seismic events.The models were trained on the extracted 62 features for binary and ternary classification tasks of natural earthquakes,artificial explosions,and mining collapses.The basic principles of KNN,AdaBoost,and LGBM algorithms were initially introduced,followed by a description of the training process for the classification models.To ensure balanced sample distribution for each event type,data were selected based on uniform distribution of time and geographical location.Ultimately,545 events for each event type,totaling 1635 seismic events,were chosen as the sample data for building the classification models.The dataset was divided into training and testing sets using a holdout method,with 75%of the data used for model construction and validation,and 25%for evaluating model performance.The training data covered the main geo-graphical range of the North China region(109.3°E-123.5°E,34.1°N-43.7°N),ensuring the models could capture the region's diversity and complexity.The testing data covered a slightly different geographical range(110.8°E-124.1°E,34.9°N-42.7°N). The 62 features were used to train classification models by KNN,AdaBoost,and LGBM algorithms.Models were trained with number 0 representing natural earthquakes,number 1 representing artificial explosions,and number 2 representing mining collapses.Various classi-fication models were evaluated using KNN,AdaBoost,and LGBM,with each model trained and tested 100 times for 0-1,0-2,1-2,and 0-1-2 classification tasks.AdaBoost and LGBM demonstrated superior performance compared to KNN across all classification tasks,espe-cially in 0-1 and 0-1-2 classification tasks.LGBM consistently exhibited the best overall performance,maintaining an accuracy of over 95%and showing high stability.In different classification tasks,0-2 classification yielded the most outstanding results,followed by 1-2 classification. Following the training of classification models,the focus shifted to comprehensive evalu-ation of these models using testing data.Each model was used to identify the event types in the testing data,yielding performance results for each model across different classification tasks.Confusion matrices were generated based on identification results,demonstrating excellent per-formance for each classification task,particularly in the 0-2 classification using three different classification algorithms. Based on confusion matrices,performance evaluation metrics,including accuracy,preci-sion,recall,and F1 score,were calculated.In the 0-1 classification task,AdaBoost per-formed the best,achieving an accuracy of 96.69%.In the 0-2 classification task,all three algorithms performed well,with evaluation metrics exceeding 99.26%.In the 1-2 and 0-1-2 classifications,LGBM exhibited the best performance.Overall,each classification model demonstrated excellent performance,with accuracy,precision,recall,and F1 score all exceed-ing 89.71%. LGBM exhibited superior overall performance,maintaining an accuracy of over 96.32%and demonstrating high stability.KNN still has significant room for improvement,possibly due to its sensitivity to data,resulting in relatively weaker performance compared to AdaBoost and LGBM.AdaBoost's overall performance lies between LGBM and KNN. Finally,ROC curves were plotted to visualize the recognition of the testing dataset using three different classification algorithms(KNN,AdaBoost,LGBM).While KNN algorithm performance for 0-1 and 1-2 classifications requires optimization,all other models performed exceptionally well in the ternary classification scenario.Confusion matrices and evaluation met-rics indicate that the constructed classification models perform well on testing data,with ROC curve analysis further confirming the excellent performance of the classification models in vari-ous tasks,revealing the applicability of different algorithms in their respective tasks,and providing strong support for the practical application of the models.

Abstract:On January 8,2022,the MS6.9 earthquake in Menyuan County,Qinghai Province stands as the largest and most destructive seismic event in the region in recent years.The earth-quake originated at the junction of the Lenglongling and Tuolaishan faults,an area with com-plex tectonic structures.It occurred in a zone within a 200 km radius of the epicenter where both population and monitoring stations are sparse.This sparsity meant previous research has largely relied on earthquake catalogs derived from existing stations,leading to a lack of comprehensive and in-depth understanding of the region's fine-grained tectonic features and the intricate mech-anisms underlying its seismic activity,thus necessitating further intensive research. This study compiled high-density monitoring network data within a 200 km radius of the MS6.9 Menyuan mainshock,spanning 129 days before to 235 days after the main event.Employing deep learning models,it conducted microseismic detection and location analysis to further dissect the earthquake.Using the DiTingPicker deep learning picking model,the study automatically detected seismic events and picked seismic phases from continuous raw seismic data.Subsequent phase association was performed via REAL,followed by absolute and rela-tive location calculations using Hypoinverse and HypoDD,respectively.This process yielded an earthquake catalog with 3 637 entries covering September 1,2021 to August 31,2022,detecting twice as many earthquakes as the official monitoring network catalog. Depth projections of the earthquakes revealed that the Lenglongling and Tuolaishan faults have nearly vertical dips,with most aftershocks occurring at depths of 10-20 km.Analysis of the sequence using the epidemic type aftershock sequence(ETAS)model indicated no distinct foreshocks preceding the Menyuan earthquake.Additionally,the temporal and spatial distribu-tion of b-values within the sequence suggested that the Menyuan area is under heightened stress conditions;stress remains incompletely released on the eastern side of the Lenglongling fault and the western side of the Tuolaishan fault,pointing to a certain potential for future strong earthquakes.

Abstract:According to the China Earthquake Networks Center,a MS6.2 earthquake occurred in Jishishan County,Gansu Province,at 23:59 on December 18,2023,with an epicenter at(35.70°N,102.79°E)and a focal depth of 10 km.Studies indicate that the causative fault of this earthquake is a buried branch fault of the southern margin fault of the Laqi Ridge,active in the Late Pleistocene,with a strike of N311°W,a dip of N54°E,and a length of approximately 13.9 km.As of 8:00 on December 22,2023,the earthquake had resulted in 148 fatalities and three missing persons.According to seismic damage investigations,the Jishishan earthquake in Gansu Province exhibited the phenomenon of"moderate earthquake,severe disaster".To analyze this phenomenon in the event,this paper examines the spatial distribution characte-ristics,rupture directivity effect,hanging wall and footwall effect,response spectrum characte-ristics,and attenuation of peak ground acceleration(PGA),aiming to provide references for site selection in post-earthquake reconstruction and seismic design of engineering structures. In the Jishishan MS6.2 earthquake,a wealth of high-quality strong motion acceleration records were captured by China's National Strong Motion Observation Network and National Earthquake Early Warning Network,including 305 sets of three-component strong ground motion acceleration records(116 from bedrock stations and 189 from soil stations)captured by the Early Warning Network and 37 sets of three-component strong ground motion acceleration records captured by soil stations of the Strong Motion Observation Network. Firstly,the original acceleration time histories were processed.Due to factors such as instrument installation tilt and temperature effects,these original acceleration time histories exhibited zero drift.Therefore,zero offset correction was performed on the original accelera-tion time histories.After zero offset correction,acceleration time histories without zero drift for each station were obtained,totaling 342 sets.After excluding data from stations where the three-component PGAs are less than 10 cm/s2,170 sets remained.Using these data,spatial dis-tribution maps of three-component PGAs were drawn,and the spatial distribution characte-ristics of peak acceleration were reflected by comparing and analyzing them with the trend of the causative fault. Next,the directional effect of the Jishishan MS6.2 earthquake was investigated.Two sta-tions located in the forward and backward rupture zones of the causative fault,with the same site conditions and similar distances to the epicenter,were selected.The duration,peak values,and response spectra of the acceleration and velocity time histories of the two stations were com-pared and analyzed to reflect the influence of the fault rupture directional effect on the spatial distribution of ground motion.For the analysis of the hanging wall and footwall effect,two sta-tions located on the hanging wall and footwall,with the same site conditions and similar dis-tances to the epicenter,were selected.The duration,peak values,and response spectra of the acceleration time histories of the two stations were compared and analyzed to examine the influ-ence of the hanging wall and footwall effect on the spatial distribution of ground motion. Then,two nearby stations:one founded on soil and the other on bedrock were selected to compare their acceleration response spectra.By analyzing the spectral values and the length of the platform section of the acceleration time histories,the impact of site conditions on ground motion was elucidated.Finally,the ground motion of stations near the epicenter was analyzed.Stations within 20 km of the epicenter were selected,and the acceleration zoning of each sta-tion was obtained by consulting the China Ground Motion Parameter Zoning Map(GB 18306-2015).The design response spectrum parameters of the site where each station was located were obtained according to the Code for Seismic Design of Buildings(GB 50011-2010).A comparison diagram of the acceleration response spectra and design spectra of near-field stations was drawn,and the actual ground motion intensity of the near-fault area was analyzed by comparing the spectral values and the length of the platform section. The results show that:①Stations with any component PGAs greater than 10 cm/s2 are dis-tributed in the northwest direction,consistent with the distribution of the causative fault,indic-ating that the distribution of the causative fault significantly controls the spatial distribution of PGAs.②The rupture directional effect significantly influences the distribution characteristics of PGAs and PGVs near the fault.The PGVs recorded by stations located in front of the rupture are greater than those recorded by stations behind the rupture for all three components,espe-cially in the EW and NS components,reaching up to 3-4 times higher.③The hanging wall and footwall effect significantly affects the distribution of PGAs;the three-component accelera-tion response spectra recorded by hanging wall stations are greater than those recorded by foot-wall stations in the high-frequency range,indicating more pronounced high-frequency and short-period characteristics.④In the horizontal direction,different frequency bands exhibit different amplification effects on acceleration for rock and soil sites:rock sites exhibit signific-ant amplification in the high-frequency range,whereas soil sites exhibit significant amplifica-tion in the lower-frequency range.⑤The acceleration spectra observed at the epicenter signifi-cantly exceed the levels of rare and extremely rare earthquakes specified in current seismic design codes.This indicates that the actual seismic forces experienced by buildings may sur-pass their structural capacity to resist damage or prevent collapse,explaining the phenomenon of"moderate earthquake,severe disaster"in this event.

Abstract:The Changning region of Sichuan Province is located at the junction of the Sichuan Basin and the Yunnan-Guizhou Plateau in southern Sichuan,Since 2015,seismic activity has significantly increased,prompting scholars to conduct in-depth investigations into the funda-mental causes of this surge in seismic activity.The study of magnetic material distribution is of paramount importance for elucidating the geological evolution and underground medium struc-ture of the Changning region.Magnetic material localization is a key aspect of the investiga-tion,relying on the inversion of magnetic data obtained from detailed magnetic sensor measure-ments.This process helps to precisely determine the spatial coordinates of magnetic targets. Analysis of the spatial distribution of lithospheric magnetic field,acquired from 952 mea-surement points with an average spacing of 2 km in Changning,Sichuan,reveals pronounced disparities in magnetic field distribution between the eastern and western sectors of the Chang-ning backslope.Notably,these variations appear to correlate with areas of heightened seismic activity within the region.For instance,the inner segment of the backslope predominantly exhibits positive lithospheric magnetic field values,juxtaposed against negative values along the periphery.The most significant concentrations of negative magnetic fields are observed along the northwestern boundary of the backslope,while the northeastern sector predominantly manifests positive lithospheric magnetic fields,with negative values dominating the southern flank. This paper adopts the magnetic dipole theory as a foundational framework to explore the distribution characteristics of magnetic dipole structures within the Changning area.By inver-ting the magnetic moment information derived from equivalent magnetic dipole sources,the study aims to delineate the spatial distribution and properties of underground magnetic dipole structures. Based on the lithospheric magnetic field data,a technical approach for regional magnetic dipole inversion was established.The inversion technique used was as follows:the lithospheric magnetic field of the Changning Anticline after polarization was subjected to a fast Fourier transform to obtain the lithospheric magnetic field distribution corresponding to rock materials at different depths.On this basis,the magnetic moment distribution of magnetic dipoles at diffe-rent depths underground was inverted,and the magnetic dipole structure underground in the study area was reconstructed. The results indicate that magnetic materials in the shallow region of Changning are scattered and relatively weak.Magnetic materials are concentrated at depths below 6 km,mainly in the northwestern end of the Changning anticline,which is also a seismically active area,coinciding with the area where magnetic materials are concentrated.

Abstract:The propagation of seismic waves in the saturated two-phase media and the dynamic response of two-phase media under seismic waves are important branches of dynamic research,which is of great significance in fields such as soil dynamics,geotechnical engineering,seis-mic engineering,geophysics,etc.The finite element method is usually used to solve the dynamic response of a saturated two-phase medium,especially for large,complex,strongly nonlinear,and low-frequency seismic dynamic response problems,and the key to the accuracy of finite element dynamic response calculation is the time-domain numerical integration format.Many researchers have proposed a series of fully explicit time-domain calculation methods for different saturated two-phase media models,such as the central difference combined with the Newmark constant average acceleration method,the central difference combined with the time-domain finite element method,and the central difference combined with the precise time-integration method,etc. Throughout the above research,this paper will further investigate the integral format that considers the acceleration input for solving the dynamic equations of saturated two-phase me-dia.The specific method is as follows.Firstly,the dynamic equations of one-dimensional com-pression and shear waves are established based on the soil mechanics model of a saturated two-phase medium.Secondly,for these dynamic equations,the Galerkin weighted residual method is used for spatial discretization to get the weak form of the discrete Galerkin.After that,the time domain step-by-step integration method that combines central difference method with the Newmark linear acceleration method(γ=1/2,β=1/6)is used for time discretization.The cent-ral difference method is used to solve the solid-fluid phase displacement,of which any node i at a discrete time p+1 can be represented by the velocities and displacements at the time p.The Newmark linear acceleration method is used to calculate the velocities,of which any node i ata discrete time p+1 can be calculated from the acceleration,velocity,and displacement at the time p and the displacement at the time p+1.Then the dynamic balance equation method calcu-lates the corresponding accelerations from the displacements and velocities at the time p+1.To this extent,the time-domain recursive formulas for the dynamic response of internal nodes have been established.Thirdly,the multi-transmitting artificial boundary is introduced to limit the finite element research area.The dynamic response of artificial boundary nodes in the two-phase media is obtained by interpolating the dynamic response of internal nodes.So far the near-field wave motion problem in the saturated two-phase media can be solved by combining the time domain recurrence formulas with the multi-transmitting artificial boundary.Finally the rationality of the algorithm is verified by using two numerical examples,one is internal wave,and the other is external scattering.The former compares the numerical solution of one-dimen-sional soil column under step load and sinusoidal load with the corresponding analytical solu-tion,and the calculation results of the two are consistent.The latter uses the numerical algorithm in this paper to calculate the seismic response of a saturated half-space.Namely the elastic dynamic response of the two-phase media at the vertical incidence of the Wolong wave is calculated and analyzed by using this method.The time history curves of displacement,velo-city,and acceleration of four points at different depths(e.g.,A,B,C,and D points)are drawn.The calculated results are in accordance with the elastic wave theory.This is sufficient to prove the correctness and effectiveness of the explicit finite element method derived in this paper.The following conclusions are obtained:①The time-history curves of displacement,velocity,and acceleration in solid and liquid phases are the same when the seismic wave is incident vertically in the form of a shear wave.The waveforms of the displacement,velocity,and acceleration are the same as the corresponding waveforms of the input seismic wave,but the peak values differ;②The peak displacement of the point A at the free surface of the two-phase medium is 15.558 cm and-19.34 cm,which is close to twice that of the incident seis-mic wave such as 7.958 cm and-9.754 cm.This conforms to the theory of elastic waves in two-phase media and proves the applicability of the explicit finite element method in solving the elastic seismic response of saturated two-phase media in this paper;③The elastic dynamic response of the two-phase media solved in this paper decreases with the decrease in the intensity of input seismic wave field because the solid phase constitutive relationship is linear elastic.This provides an effective method for solving the structural dynamic and wave equations,and also lays a foundation for future research on the numerical simulation of nonlinear waves in two-phase media.

Abstract:The origin of the Changbaishan volcanic province(CBSVP)has been considered to mainly relate to the Pacific Plate subduction.Therefore,it is necessary to dynamically recon-struct the Pacific Plate subduction processes towards and beneath the CBSVP since the Cenozoic in order to investigate its origin.This reconstruction requires consideration of the seismic dis-continuities in the upper mantle.There are two main global seismic discontinuities in the upper mantle:the 410-km and 660-km discontinuities,which have been considered as phase trans-ition boundaries.The 410-km discontinuity represents the exothermic boundary of the α-olivine to β-spinel transition,featuring a positive Clapeyron slope.Consequently,negative buoyancy is generated and attaches to a subducting slab near this discontinuity,accelerating the slab's subduction into the the mantle transition zone(MTZ)and slightly enhancing the transportation of upper mantle materials and the penetration of the slab through the 660-km discontinuity.However,some investigators hold opposing views regarding the effects of the olivine-spinel transition on the penetration of the 660-km discontinuity.The Clapeyron slope(γ410)and thick-ness(δh410)of the 410-km discontinuity range from 1.0 to 3.8 MPa/K and from 2 to 40 km.The 660-km discontinuity is the endothermic boundary of the transition from ringwoodite to bridgmanite and magnesiowüstite.This boundary,generally speaking,exhibits a negative Clapeyron slope.Consequently,positive buoyancy is generated and attaches to a subducting slab near the discontinuity,hindering the slab's sinking into the lower mantle.Some investi-gators have considered this to be the primary cause of stagnant slabs in the MTZ,while others do not believe that the buoyancy resulting from the phase transition at this boundary is sufficient to compensate for the density of a cold subducting slab.The Clapeyron slope(γ660)and thick-ness(δh660)of the 660-km discontinuity range from-0.4 to-6 MPa/K and from approximately 2 km to 70 km.The δh660 is between 35 km and 70 km in Northeast China.These results indi-cate that the two discontinuities have the wide ranges of the Clapeyron slopes and thicknesses.It favors predicting a stagnant slab in the MTZ mapped by seismic tomography,using a geody-namic model that incorporates a γ660 ranging from-1.5 to-8 MPa/K,without considering the 410-km discontinuity,while when considering the 410-km discontinuity,the γ410 generally takes 3 to 4 MPa/K and the γ660-2 to-6 MPa/K.It may well reproduce the stagnant Pacific slab in the MTZ beneath East Asia using geodynamic models that consider either γ660=-1.5—-3 MPa/K and δh660=40 km or γ660=-2 MPa/K and δh660=40 km in the absence of the 410-km discontinuity.Alternatively,when considering the 410-km discontinuity,it may use models with either γ410=4 MPa/K,γ660=-2 MPa/K and δh410=δh660=64 km or γ410=3 MPa/K and γ660=-3 MPa/K in the presence of the 410-km discontinuity.These studies have provided us with basic knowledge of the effects of the two discontinuities on slab subduction dynamics,but in the meantime,we find that previous researches have presented a wider range of Clapeyron slopes and thicknesses for the 410-km and 660-km discontinuities,aiming to rep-licate the stagnant slab structure in the MTZ.Consequently,it is essential to clarify the impacts of these two discontinuities on the dynamic process of Pacific Plate subduction,as well as the slab structure,westward movement distance,and sinking depth within the mantle.Therefore,in this study,we set up a series of data-assimilation three-dimensional geodynamic models that incorporate varying values of γ410,γ660,δh410 and δh660 to predict the westernmost positions,bottom depths and subduction processes of the Pacific Plate since the Cenozoic.It is found that:① The γ410,to a certain degree,affects the westward movement distance of the Pacific slab in the mantle,while it slightly influences the sinking depth of the slab.As the γ410 de-creases,the average movement distance increases gradually,and the sinking depth becomes slightly shallower.The maximum differences are about 80 km and 50 km for the distance and sinking depth,respectively,over a range of γ410 from 0 to 6 MPa/K;② With the increase of|γ660|,the effect of γ410 gradually weakens.When|γ660|≥4 MPa/K,the effect of γ410 can be almost ignored;③ The γ660 has significant effects on both the westward movement distance and sinking depth of the Pacific slab in the mantle.When|γ660|≤5 MPa/K,the average slab westward movement distance increases as|γ660|increases.However,when|γ660|＞5 MPa/K,the situation reverses,that is,the larger the|γ660|,the smaller the average slab westward move-ment distance.The sinking depth consistently becomes shallower with the increase of|γ660|.The maximum differences exceed 300 km and 600 km for the distance and sinking depth,respec-tively,over a range of γ660 from-1 MPa/K to-6 MPa/K;④ Both δh410 and δh660 have slight effects on the dynamic processes of the Pacific slab and the slab structure within the mantle;⑤ A geodynamic model that merely considers the effects of both 410-km and 660-km discon-tinuities may predict the westernmost position of the Pacific slab in the modern mantle reason-ably,but it struggles to reproduce the slab's sinking depth reasonably.

Abstract:In recent years,distributed acoustic sensing(DAS)has experienced rapid develop-ment and extensive applications in seismology and various other fields,owing to its advantages of low cost,high spatial resolution,and real-time monitoring capabilities.As a critical sensing component in DAS observational systems,optical fibers can directly affect the quality of ac-quired data and,consequently,the results.Meanwhile,the physical properties of optical fibers are also closely related to the cost and difficulty of field deployment.However,a quantitative guideline for selecting suitable optical fibers for geophysical surveys is still lacking.In this study,to fill this gap,we first propose a comprehensive set of criteria based on the key attrib-utes of seismic data to evaluate the quality of DAS records quantitatively,including relative sensi-tivity,signal-to-noise ratio,fidelity compared to data recorded by nodal stations,and semb-lance across multiple channels.Then,we conduct rigorous evaluations of DAS data recorded by two diverse seismic observational systems:A DAS cable deployed in a surface trench and a DAS cable deployed in a vertical borehole. In the former experiment,a trench approximately 20-30 cm deep and 90 m long was ex-cavated on a lawn adjacent to two main urban roads.Five regular optical communication cables were deployed in this trench to ensure identical coupling conditions.The five optical cables were then fused end-to-end into a single continuous cable,which was connected to a DAS inter-rogator for data collection.Along the trench,21 short-period nodal seismometers were deployed at 4-m intervals to assess the fidelity of the DAS records.Active signals from ham-mering were generated,and spatiotemporal 2D cross-correlation template matching was used to accurately determine the location of the DAS channels.Then,the signals recorded by different optical cables were compared trace-by-trace.The analyzed data include active signals from hammering,surface waves generated by traffic,and cross-correlation functions of ambient noise.For the active-source signals,we calculated the relative sensitivity,signal-to-noise ratio in both time and frequency domains,and data fidelity.Since DAS records strains or strain rates rather than particle velocities(as seismometers do),unit conversion is required before calculat-ing fidelity.For the traffic-generated surface waves,fidelity and semblance were computed.The fidelity of the surface waves was notably higher than that of the signals generated by ham-mering.For the cross-correlation functions from seismic ambient noise,only the signal-to-noise ratio was compared.Systematic evaluations with various sources revealed distinct differences between the evaluated cables,with the overall performance of the optical cables in vibration reception ranked as follows:YZ,GYFTY,GYTA,ADSS,and GJFJV. The best-performing YZ cable retained for the borehole experiment,in which three addi-tional armored optical cables(YPTPU,SCJKBH,and SCTX3Y)were also selected.These four cables were installed in a 200-meter-deep borehole.Below 80 m depth,expansive clay pel-lets were used as infill,while silica sand was used above this depth.Given the limited space in the borehole,which made fusing challenging,each cable was connected to an equivalent inter-rogator for data acquisition.Several active sources were used near the well to generate signals.Due to significant differences in coupling between the shallow and deep sections of the bore-hole,only data recorded by channels shallower than 80 m were considered for evaluation.The relative sensitivity,signal-to-noise ratio,and semblance of data from the four cables were eval-uatedand considerable differences are observed.Overall,the YPTPU cable is found to have the best performance for DAS data collection in the borehole. The results of this study indicate that distinct variations in data quality can be found among different optical cables,particularly within the borehole applications.Therefore,proper selec-tion of cables is essential for collecting high-quality DAS data,which is crucial for monitoring weak signals and achieving reliable subsurface imaging.

Abstract:The Moho discontinuity represents a critical boundary within the Earth's lithosphere and plays a significant role in the processes of lithospheric formation and evolution.As a key structure for describing both the thickness and structural characteristics of the Earth's crust,variations in the depth and thickness of the Moho discontinuity directly manifest the long-term dynamic processes that the crust has experienced.Therefore,accurately determining the depth of the Moho is of great importance for understanding the global processes of crustal formation and evolution.One of the most distinct characteristics of the Moho is the pronounced change in seismic wave velocities as seismic waves cross this boundary,making seismological methods become the most widely used approach in Moho-related researches.Over the years,numerous researchers have employed a range of seismological techniques to carry out extensive studies on the global distribution of Moho depths.Although the Moho is universally recognized as a con-tinuous first-order discontinuity at the global scale,its structure in tectonically complex regions remains highly intricate and variable.Since the early 21st century,the focus of scientific inquiry has increasingly expanded beyond seismic characteristics to the electrical properties of the crust-mantle boundary.With the ongoing accumulation of long-period magnetotelluric(MT)data and the deepening of global research on the Moho,particularly following the intro-duction of the concept of the electrical Moho(eMoho)and breakthroughs in its study,there has been growing recognition of the potential of magnetotelluric methods for probing the electrical properties of the Moho.The application of MT methods in this context has demonstrated signi-ficant scientific potential and feasibility for revealing the electrical characteristics of the crust-mantle interface.This study,based on long-period broadband MT data from four stations in the North China region,investigates the depth distribution of the electrical Moho in the region.It aims to provide a solid foundation of electrical evidence for gaining a comprehensive under-standing of the depth distribution of the Moho across North China,elucidating the mechanisms behind the destruction of the North China Craton,and contributing to the understanding of the seismogenic processes in the Capital Circle region.In this research,long-period MT data from several stations within the electromagnetic network of the Capital Circle region were utilized to invert the deep electrical structure of the area.The results exhibit a high degree of consistency with previous studies that used seismic,electromagnetic,and gravity data to determine subsur-face structures at the same stations.The findings indicate that significant resistivity variations are present near the seismologically determined Moho within the Capital Circle region.To fur-ther investigate this,this study developed three stratigraphic models,validating the feasibility of employing the first derivative of the inverted resistivity curve as a method for delineating the depth of the electrical Moho beneath individual measurement points.After applying this meth-od to the inversion results,the distribution of the electrical Moho became clearly evident.The results demonstrated that the derived electrical Moho is generally consistent with the Moho depth obtained from seismic,gravity,and other geophysical data,indicating a robust agree-ment across multiple methods.By integrating findings from previous studies,the research hy-pothesizes that the high-conductivity thin layer observed in some parts of North China may res-ult from the partial melting and subsequent recrystallization of lower crustal materials,leading to the accumulation of sulfides.In particular,the resistivity variations observed beneath the Dalian and Wudi stations deviate from theoretical expectations,likely due to the unique litho-spheric structure associated with the coastal regions.Specifically,the low-resistivity anomaly detected beneath the Wudi station may suggest the presence of an asthenospheric upwelling channel,which could provide a pathway for the movement of deeper mantle materials toward the surface.The findings of this study not only enhance our understanding of the distribution and characteristics of the electrical Moho in the Capital Circle region,but also provide a valu-able reference for the study of Moho-related processes in other regions with similar geological contexts.The research contributes important insights into the geodynamic mechanisms govern-ing lithospheric evolution,particularly in tectonically active regions,and underscores the com-plementary nature of seismic and magnetotelluric methods for investigating complex subsurface structures.Through continued accumulation of MT data and further development of interpre-tative models,magnetotelluric methods are poised to offer increasingly refined insights into the nature of the crust-mantle boundary and its role in broader geophysical processes.These results are not only significant for understanding regional tectonics,but also for advancing global studies of the Earth's lithospheric dynamics.

Abstract:On December 18,2023,a MW6.0 earthquake struck Jishishan County,Gansu Province,China.This earthquake occurred in southeastern Gansu,within the northern seg-ment of the North-South Seismic Belt,a critical area forming part of the northeastern margin of the Qinghai-Xizang Plateau.The region is intersected by several active fault zones,including the East Kunlun fault,Qilian-Haiyuan fault,Altyn Tagh fault,and West Qinling fault.Jishishan County lies on sedimentary strata at the junction of the Loess Plateau and the Qinghai-Xizang Plateau,where significant site amplification effects on the western edge of the Loess Plateau can exacerbate surface damage.The Jishishan Mountains,located in the easternmost segment of the Qilian Mountains,form part of the northern margin of the Qinghai-Xizang Plateau.This region has undergone significant continental crustal thickening due to the ongoing collision between the Indian and Eurasian plates.As the plateau continues to push northward and expand laterally,large-scale left-lateral strike-slip faults and thrust fault zones have developed.The Jishishan Mountains,trending northwest-southeast(NNW-SSE),are bounded by faults on both sides,exhibiting evidence of thrusting and strike-slip motion.Consequently,investigating the coseismic surface deformation field,geometric parameters of the seismogenic fault,and stress field disturbances induced by fault rupture is of paramount importance. In this study,Interferometric Synthetic Aperture Radar(InSAR)technology was employed to process Sentinel-1A SAR images covering the epicentral region.Shuttle radar topography mission(SRTM)data of National Aeronautics and Space Administration(NASA),with a 30-meter DEM resolution,were utilized for image coregistration and removing topographic phase contributions.Precise Orbit Ephemerides(POE)data from the European Space Agency(ESA)were used to correct orbital errors,while atmospheric corrections were performed using the Generic Atmospheric Correction Online Service for InSAR(GACOS)to minimize atmospheric noise.The final step involved geocoding the output,which generated coseismic surface defor-mation fields in the line of sight(LOS)direction for both ascending and descending tracks of the Jishishan earthquake. The spatial distribution of aftershock hypocenters,based on clustering patterns near the main fault,allowed for clear delineation of the fault plane's geometric structure.A single plane was used to model the seismogenic fault in this study.By using a dislocation model in an elastic half-space,coseismic surface displacements and the double-difference relocation algorithm for the earthquake sequence were then inverted to determine fault geometry parameters and non-uniform fault slip distributions. The results indicate that the 2023 Jishishan earthquake was a reverse fault event on a buried fault,with maximum surface displacements of 7.0 cm and 6.8 cm observed in the ascending and descending tracks,respectively.Joint fitting of small-earthquake fault planes and inversion of fault geometry parameters show that the seismic source depth is primarily concentrated between 6 and 15 km.The fault trends northeast,strikes at 323°,and dips at 50°.Rupture is predominantly concentrated at depth of 4-11 km,with a maximum slip of 0.2 m occurring at 5.7 km depth.The earthquake likely occurred on the southern edge of the Lajishan fault,which is characterized as a NNW-trending,east-dipping thrust fault. The distributed slip model suggests that primary rupture occurred at depth between 0 and 8 km,with significantly reduced slip observed at 10 km,implying that coseismic rupture may have triggered deeper aftershocks.The seismogenic fault mechanism is predominantly thrust-dominated.Additionally,deep fault structures inferred from aftershocks show slight variations in dip angle between deeper and shallower sections,as observed in InSAR surface deformation.Near the surface,the fault dip angle becomes shallower,indicating a decrease in dip from depth to the surface-consistent with the potential bending geometry of deep thrust faults.Con-sequently,a depth of 10 km was selected for calculating Coulomb stress disturbances. Analysis of Coulomb failure stress changes(ΔCFS)in the surrounding area induced by the 2023 Jishishan earthquake reveals positive ΔCFS values at both ends of the seismogenic fault.The distribution of Coulomb stress changes indicates that several fault segments are in a stress-loaded state(with ΔCFS exceeding 10 kPa),including the entire southern segment of the Lajishan south margin fault;the WNW-trending segment of the Lajishan north margin fault and its NNW-trending segment south of the epicenter;the North margin fault of the West Qinling;and the segment of the Daotanghe-Linxia fault east of the epicenter.These areas exhibit rela-tively high seismic hazard and warrant continuous monitoring.