Table of Content

20 August 2026, Volume 48 Issue 4

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Research paper

APPLICATION OF HVSR INVERSION METHOD BASED ON COVARIANCE MATRIX ADAPTIVE EVOLUTION STRATEGY (CMA-ES) ON TAILINGS DAM STRUCTURE DETECTION

LI Rui-jing, XIE Fan, DONG Li-na, LI De-kang, JIAO Cheng-li

2026, 48(4):  921-940.  DOI: 10.3969/j.issn.0253-4967.20250006

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China has the largest number of tailings ponds in the world. Failure of tailings dams can cause severe ecological damage, economic losses, and casualties in downstream areas. Conventional monitoring approaches, including surface deformation measurements such as GPS and InSAR, as well as borehole-based instruments such as pore-pressure sensors and deep displacement meters, are limited by high costs, sensor vulnerability, and their limited ability to directly characterize the internal mechanical properties of dam structures. Therefore, there is an increasing need for damage-sensitive and cost-effective monitoring techniques that can directly evaluate the mechanical properties of tailings dams during operation. In recent years, seismic ambient-noise-based methods have attracted growing attention in subsurface structure investigation. Among these methods, the characteristic resonant frequency derived from the horizontal-to-vertical spectral ratio(HVSR) of seismic ambient noise is sensitive to variations in subsurface velocity and impedance, making it particularly suitable for structural health monitoring of tailings dams.

In this study, we deployed three seismometers at different elevations of the Xiaodonggou tailings dam in Chengde, China, to analyze HVSR characteristics and infer the internal layering of the dam. Full-day seismic records were divided into 72 non-overlapping 20 min windows, and the Konno-Ohmachi algorithm was applied to smooth the HVSR curves. Steady-state data were then selected to obtain representative HVSR curves for each station. Furthermore, an evolutionary inversion algorithm based on covariance matrix adaptation evolution strategy (CMA-ES) was employed to invert the HVSR curves and characterize the layered structure of the tailings dam. The reliability of CMA-ES was evaluated by comparing its inversion results with those obtained using particle swarm optimization(PSO), grid search, and the open-source software HVSRInv.

The results show that the peak frequencies of the HVSR curves at the three stations increase from 1.4 to 3.7Hz from the crest to the toe of the tailings dam, indicating the presence of a well-defined stratified interface with substantial lateral variation in depth. The HVSR curve at the crest exhibits the largest standard deviation, likely due to interference from discharge activities, highlighting the influence of external disturbances on monitoring results. The CMA-ES inversion results indicate a strong S-wave velocity impedance contrast between unsaturated and saturated tailings sand. Moreover, the depth of the upper unsaturated tailings layer inferred from the inversion agrees best with the phreatic surface measured in the borehole profile at the dam crest. Compared with PSO and grid search, CMA-ES produces the smallest interface-depth error, with the interface depth corresponding to the HVSR peak frequency being closest to the phreatic-line depth observed in borehole data. In addition, compared with the open-source HVSRInv software, the CMA-ES inversion yields a better fit to the observed HVSR curves, further supporting its accuracy and reliability for structural inversion of tailings dams.

These findings demonstrate the strong potential of HVSR-based methods for structural health monitoring of tailings dams and confirm the reliability of CMA-ES for HVSR inversion. This study provides a scientific basis for assessing tailings-dam stability and mitigating potential geohazards. Future work should focus on improving the anti-interference capability of the HVSR method and conducting long-term observations to evaluate its applicability and stability under varying environmental conditions.

THE DEVELOPMENT AND STABILITY EVALUATION OF LOCAL N-S TRENDING STRIKE-SLIP FAULTS IN SOUTHERN SICHUAN BASIN

WANG Xi-you, YAN Yi-xi, NIU Wei-tao, YI Xin-zheng, LI Lin, TIAN He-feng, HE Ye, LIANG Shu-jun, YUAN Yuan

2026, 48(4):  941-958.  DOI: 10.3969/j.issn.0253-4967.20250150

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As global energy demand continues to increase, shale gas development through large-scale hydraulic fracturing, involving subsurface fluid injection and extraction, has been increasingly practiced worldwide. In this context, identifying pre-existing geological structures and evaluating their stability in shale gas production areas are essential for ensuring safe shale gas development. This study focuses on a shale gas production block in the southern Sichuan Basin, China, where a prominent N-S trending strike-slip fault system is developed. By integrating tectonic geomorphological analysis, 3D seismic reflection interpretation, microseismic monitoring data, and mechanical analysis based on the Mohr-Coulomb failure criterion, we systematically delineate the spatial geometry and kinematic characteristics of pre-existing faults in the study area and assess the stability of the representative Fault F₂ and its secondary faults, providing a reference for subsequent shale gas development and production safety.

A joint surface-subsurface structural analysis of the fault system was conducted using high-resolution surface DEM data and seismic reflection data. The results show that the fault system consists of dozens of subparallel, nearly N-S-trending strike-slip faults with right-lateral kinematics. These faults extend to the surface in the core of an NEE-SWW-trending anticline, forming distinct linear fault valleys, and widely produce right-lateral offsets of several hundred meters, approximately 300-1 000m, in drainage systems and ridges.

Furthermore, the stability of a representative fault, F₂, and its impact on shale gas production were assessed by combining real-time microseismic monitoring data acquired during hydraulic fracturing with quantitative mechanical analysis based on the Mohr-Coulomb failure criterion. Microseismic event locations indicate that, although most events are concentrated near the main Fault F₂, they are not distributed along F₂ in the N-S direction. In addition, focal mechanism solutions suggest that these microseismic events are dominated by left-lateral strike-slip motion, opposite to the kinematic characteristics of F₂. These observations indicate that the large N-S-trending main fault, which exceeds 10km in length, is unlikely to be the seismogenic fault. Instead, NE-trending secondary left-lateral strike-slip faults derived from F₂ and located within approximately 300m of the main fault may be more susceptible to stress-perturbation-induced seismicity during shale gas development.

Mechanical analysis based on the stress field inverted from focal mechanism solutions and the Mohr-Coulomb failure criterion further supports this interpretation. The results indicate that the present-day maximum principal stress is oriented approximately N17°E. Under this stress regime, the NE-trending secondary faults are more favorably oriented for slip and require a smaller fluid-pressure perturbation to reach failure than the main N-S trending fault. Consequently, these secondary faults exhibit significantly higher instability potential, whereas the main fault remains comparatively stable owing to its less favorable orientation relative to the contemporary stress field and its larger scale, which may facilitate faster fluid-pressure diffusion.

These findings highlight that not all pre-existing faults pose high risks to shale gas development and that fault stability must be evaluated in relation to the in situ stress state. In subsequent development of this block, attention should be paid to the high-risk NE-trending secondary faults, and appropriate avoidance or mitigation measures should be adopted. The results provide support for optimizing engineering design, mitigating induced-seismicity risk, and promoting safe and efficient shale gas development.

THREE-DIMENSIONAL GEOLOGICAL MODELING AND MECHANISM ANALYSIS OF WATER INJECTION-INDUCED SEISMICITY IN THE HUAI'AN SALT MINING AREA

CHENG Jia, WANG Qi-xin, WANG Zi-yue, XU Xi-wei, CAI Jun-tao, XU Zhi-ping, YANG Yan-bing, CHEN Xiao-bin, LIU Qiao-xia

2026, 48(4):  959-980.  DOI: 10.3969/j.issn.0253-4967.20250054

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Human-induced seismicity associated with large-scale industrial resource extraction has attracted increasing public and scientific attention because of its potential to damage infrastructure. Previous studies have mainly focused on the spatiotemporal evolution of seismicity, surface deformation monitoring, and geomechanical stress perturbations induced by fluid injection. However, the integrated effects of complex geological settings, localized fault structures, and lithological heterogeneity on induced seismic responses remain insufficiently constrained.

In this study, we investigated recent seismic swarms associated with water injection and subsequent brine extraction at the Huai'an anhydrous glauberite salt mine in the Subei Depression, eastern China. Solution mining in such seismically sensitive settings involves the continuous circulation of fluids that dissolve target evaporite formations, thereby perturbing the local stress-strain equilibrium. To characterize the geological framework of the mining area, we integrated lithological and structural stratigraphic data from the Zhaoji sub-depression with regional seismic velocity profiles and electrical resistivity structures to construct a detailed three-dimensional geological model. High-resolution DEM data, drilling records, and three-dimensional electrical and velocity structures were used to extract key geophysical parameters, including resistivity and S-wave velocity, and to characterize the continuous distribution of strata. These data were then processed using discrete smooth interpolation in GOCAD. Drilling data provided vertical stratigraphic constraints, whereas microseismicity and three-dimensional electrical structures constrained the lateral and deep extent of the model.

The resulting three-dimensional structural model clearly delineates the depression geometry of the mining area. Cross-sections show that Member 4 of the Funing Formation reaches its maximum thickness in the depression center and thins outward, defining a pronounced depression structure extending from the Quaternary strata to the pre-Cretaceous basement. Using a high-precision microseismic catalog relocated with the double-difference method since 2019, we further analyzed the spatial distribution of earthquake swarms across different sedimentary units and delineated the three-dimensional extent of water inflow within the salt mining area. We then examined the distinct causal mechanisms of these swarms and their relationship with dynamic groundwater migration.

The relocated earthquake catalog reveals two distinct seismic clusters with different seismogenic mechanisms. Cluster 1 is located within the Funing Formation beneath the mining zone and consists of diffusely distributed, low-magnitude microseismic events concentrated in Member 4, where the target salt and anhydrous glauberite layers are extensively developed. These localized events likely reflect distributed, mechanically weak fracturing caused by local strain perturbations and immediate stress relaxation associated with repeated water injection and salt dissolution. In contrast, events within the underlying Members 1-3 of the Funing Formation have relatively larger magnitudes and are dominated by thrust-type focal mechanisms. This pattern indicates localized stress concentration caused by downward fluid infiltration and partial dissolution of salt layers, with mudstone barriers locally trapping elevated pore pressure.

Cluster 2, located to the north near Gaojiayan Town within the Taizhou Formation, is associated with natural groundwater inflow. The microseismic events in this cluster are aligned with pre-existing secondary faults and localized tectonic fractures characterized by relatively weak mechanical cementation. This spatial correspondence indicates that fluid migration pathways and the mechanical properties of fault zones exert primary control on the propagation of induced seismicity.

Overall, the systematically observed seismicity in the Huai'an region is dominated by small-to moderate-magnitude events confined to specific rheological stratigraphic units or localized fracture zones, suggesting a relatively low likelihood of future large-scale destructive earthquakes. Nevertheless, our results demonstrate that transient pore-pressure variations, fluid migration, lithological heterogeneity, and localized reactivation of secondary faults jointly control the spatial distribution of induced seismicity in actively exploited salt mines. This study provides a physically informed framework for assessing seismic hazards related to industrial fluid injection in the complex Subei Depression and extends the application of three-dimensional geological modeling to the analysis and prediction of engineering-induced geological hazards.

EXPERIMENTAL STUDY ON THE EFFECT OF STRESS PERTURBATION ON ACOUSTIC EMISSION ACTIVITY DURING FAULT SLIP

HUANG Yuan-min, GUO Yan-shuang, MA Sheng-li, LI Xiao-hui, ZHANG Hui

2026, 48(4):  981-994.  DOI: 10.3969/j.issn.0253-4967.20250058

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The stress-triggering earthquake, particularly the mechanisms and conditions of stress-induced earthquakes, is of great theoretical and practical significance for understanding seismic mechanisms and earthquake prediction. Monitoring changes of seismic activities caused by stress perturbations makes it possible to predict regional seismic hazards, which has become one of the primary methods in earthquake forecasting. Acoustic Emission(AE)is the laboratory signal analogous to seismic events. Studying the evolution of AE activities under stress perturbations in laboratory experiments helps identify the sub-instability stage of fault slip and captures the precursors of fault stick-slip, providing crucial reference for the studies of earthquake forecasting.

To investigate the effects of stress perturbations on fault stick-slip and to identify and analyze precursors of fault stick-slip, this study conducted frictional experiments using a servo-controlled biaxial loading machine with a direct shear structure of three granite blocks. Small-amplitude sine or square waves with different amplitudes and periods were modulated to normal and shear loading to investigate the effects of stress perturbations on fault slip instability, acoustic emission(AE) activityand fault strain.

The findings reveal that normal stress perturbations can significantly reduce the failure stress at the fault stick-slip, and the difference in failure stress before and after stress perturbation increases as the stress perturbation grows. In contrast, shear stress perturbations lead to failure stresses that were sometimes higher and sometimes lower than those under unperturbed conditions, with the differences between the high and low shear stress at the fault stick-slip increasing with the perturbation stress. The perturbation period has little influence on fault slip, and the square-wave stress perturbations exhibit greater effects on fault slip than the sinusoidal ones. AE activity shows a rapid increase before fault stick-slip. Normal stress perturbations suppress this AE activity, increasing before stick-slip, whereas shear stress perturbations enhance this increase. Normal stress perturbations do not alter the macroscopic distribution of fault stress along the fault strike, but shear stress perturbations increase spatial heterogeneity in fault stress. The amplitude, period, and loading rate of stress perturbations, along with fault normal stress, influence only the failure stress of stick-slip and the number of AE events, not the AE evolution.

Based on the experimental results, it is concluded that normal stress perturbations can change the contact between asperities on the fault, thereby affecting the fault frictional strength, without altering the spatial distribution of fault stress. Conversely, shear stress perturbations modify the shear stress only at certain fault sections, increasing the heterogeneity of the fault stress. This indicates fundamentally different mechanisms between normal and shear stress perturbations on triggering fault stick-slip. Furthermore, normal stress perturbations weaken the increasing of AE activities before fault stick-slip, thereby increasing the difficulty of predicting fault stick-slip according to the AE activities. In contrast, shear stress perturbations enhance this phenomenon, thereby increasing the possibility of predicting fault stick-slip based on AE activities.

QUANTIFYING THE ROLE OF NORMAL-FAULTING EARTHQUAKES IN CRUSTAL THINNING WITHIN THE QINGHAI-XIZANG PLATEAU

WANG Yue-xiang, WANG Teng

2026, 48(4):  995-1010.  DOI: 10.3969/j.issn.0253-4967.20250005

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The Qinghai-Xizang Plateau, known as the “Roof of the World,” is characterized by complex deformation mechanisms driven by the ongoing India-Eurasia collision. The deformation characteristics and mechanisms of the Qinghai-Xizang Plateau have long been a focus of attention in the field of geophysics, and many studies have been conducted both domestically and internationally to deeply explore its internal deformation structural features. Additionally, the frequent occurrence of normal-faulting earthquakes within the plateau has a significant impact on crustal deformation. However, previous research has not extensively explored the relationship between these earthquakes and the overall crustal deformation of the Tibetan plateau, particularly in terms of their quantitative contribution to regional crustal thinning. Previous studies have primarily focused on horizontal strain rates, leaving a gap in understanding how seismic activity vertically impacts crustal evolution. In this study, we aim to quantify the role of normal-faulting earthquakes in crustal thinning by integrating seismic catalogs, geodetic observations, and theoretical models. Our objectives include: 1)evaluating the spatiotemporal distribution of normal-faulting earthquakes and their cumulative seismic moment release; 2)developing refined methodologies to estimate crustal volume loss caused by these earthquakes; and 3)reconciling theoretical calculations with geodetic measurements to clarify their relative contributions to observed crustal thinning.

We combined a comprehensive earthquake catalog(2000-2024, M_W>5.0) from the Global Centroid Moment Tensor(GCMT) project with GPS-derived horizontal strain rate fields. Earthquakes were filtered using T-axis and P-axis dip angles (<50°) and manually verified to ensure normal-faulting mechanisms. Geodetic data from CMONOC and InSAR were utilized to derive horizontal strain rates, thus linking horizontal extension to vertical thinning. Two theoretical approaches were employed: 1)Seismic Moment Tensor Analysis: We calculated vertical strain rates using the cumulative seismic moment tensor's M_zz component, accounting for slip angles and excluding strike-slip contributions. 2)Coseismic Vertical Deformation Integration: We integrated vertical surface displacements over infinite and finite domains to estimate crustal volume loss.

Both methodologies yielded consistent results. Normal-faulting earthquakes contributed approximately 2.5×10^-9/a to the crustal thinning rate, accounting for ~30% of the GPS observation result(~7.5×10^-9/a). Including smaller-magnitude earthquakes(M_W<5.0) could increase this contribution to ~35%. The cumulative volume loss from 2000-2024 totaled 1.9×10⁹m³, driven predominantly by high-elevation(>4 200m) events. Earthquake depth showed no influence on crustal thinning due to compensating far-field uplift and near-field subsidence in an infinite domain. At the same time, the error is permissible in a finite domain. Spatial heterogeneity in strain rates highlighted stronger extension in southern Tibet, aligning with eastward crustal extrusion.

This study quantitatively demonstrates that normal-faulting earthquakes are a significant driver of crustal thinning in the Qinghai-Xizang Plateau, contributing ~30% to observed rates. The consistency between seismic moment and deformation integral methods underscores the robustness of our approach. Our findings bridge seismic activity with long-term tectonic processes, offering insights into plateau evolution and earthquake hazard assessment. Future work should incorporate smaller earthquakes, post-seismic relaxation, and erosional effects to refine these estimates.

NORTH LIMB FAULT OF THE KASHI ACTIVE ANTICLINE, SOUTHWESTERN TIANSHAN

HUANG Jie, GAN Jun-feng, YUAN Zhao-de, CHEN Jie, LI Tao, LIU Qi, DI Ning, YANG Hui-li

2026, 48(4):  1011-1034.  DOI: 10.3969/j.issn.0253-4967.20250067

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Active anticlines, as first-order structural elements within foreland thrust systems, play a fundamental role in seismic hazard assessment. Surface fault expression and deformation style directly constrain estimates of seismic potential. The Kash anticline thrust system, located within the Pamir-South Tianshan convergent zone, hosts Holocene-active deformation. Its intense and persistent tectonic activity poses a significant seismic threat to Kashgar City. However, fault-related studies remain limited, and key issues—including fault trace distribution on both limbs, structural segmentation, and slip rates—remain unresolved.

This study applies a multi-scalar methodological framework to systematically investigate the fault-related deformation of the southwestern segment of the Kashgar anticline: 1)High-resolution satellite imagery, UAV-derived orthophoto mosaics, and digital elevation models(DEMs), combined with detailed field investigations, were employed to identify and interpret fault traces; 2)Paleoseismic trenching was conducted at representative geomorphic sites, resulting in the excavation of three trenches(TC1-TC3)at two key locations(Shuijing and Haitang), directly revealing fault geometries and deformational features; 3)At trench TC4, optically stimulated luminescence(OSL)dating was performed on eolian sand lenses located within the overlying(XJ24-49)and underlying(XJ24-50)strata across the fault tip to constrain the timing of fault activity; 4)UAV-based high-resolution DEMs were used to systematically measure fault scarp heights, and cumulative offset probability density(COPD)curves were generated and compared with trenching results.

The integrated interpretation of satellite image analysis, UAV photogrammetry, field surveys, trench excavations, and scarp morphology analysis delineates the spatial distribution, segmentation characteristics, and paleoseismic deformation history of the northern limb of the Kashgar anticline. A nearly east-west-trending fault scarp extends for ~20km along the northern limb. Multiple Holocene alluvial fan surfaces are displaced by the fault, with cumulative vertical displacements ranging from 0.13m to 3m. The fault system can be subdivided into western, central, and eastern segments based on geomorphic and structural features, demarcated by the Chakmak River and Ake Expressway. In the western segment(~15.7km), located along the mountain front, north-directed thrust scarps dominate. Trenching at TC1 and TC2 reveals north-vergent reverse faults reaching the ground surface, with no overlying sediments, indicating that the most recent surface-rupturing event is geologically recent. The central segment(~4.7km), situated across T2 and T3 terrace surfaces, shows well-developed scarps produced by both north-verging thrusts and south-verging backthrusts. Trench TC3 exposes such a fault pair, and the most recent earthquake event is capped by only ~0.3m of sediment, suggesting an occurrence within the past century. In the eastern segment, low-angle north-dipping faults are observed in a large quarry exposure, but historical imagery shows no surface faulting. TC4 reveals a buried fault with no surface rupture. OSL dating of sediments overlying the fault tip yields an age of(1.0±0.9)ka, while the deformed underlying unit dates to(8.6±2.9)ka, suggesting a Holocene-active blind fault. The COPD curves derived from cumulative offset measurements in the western and central segments were compared with deformation revealed in trenches TC1 and TC3. The first prominent peak in the COPD likely represents cumulative effects from multiple seismic events rather than a single coseismic displacement.

By integrating tectonic geomorphology, trench observations, chronological constraints, statistical analysis of fault scarps, and historical seismicity—especially the 1902 Artux earthquake rupture zone—the following conclusions are drawn: 1)The northern limb of the Kashgar anticline exhibits clear segmentation: the western segment is dominated by north-verging thrust faults; the central segment is characterized by both forethrust and backthrust structures; and the eastern segment is defined by a buried Holocene-active fault; 2)It is inferred that the northern limb of the Kashgar anticline likely participated in the 1902 Artux earthquake, generating a series of fault scarps at the surface; 3)COPD curves alone may be insufficient to reliably resolve individual paleoearthquake events, especially when the first major peak reflects cumulative displacements from multiple events. Additional constraints, including trenching and chronometric data, are essential to refine paleoearthquake interpretations.

SURFACE DEFORMATION CHARACTERISTICS OF THE 2025 DINGRI M_W7.1 EARTHQUAKE DERIVED FROM GF-2 IMAGERY USING OPTICAL IMAGE CORRELATION TECHNIQUE

SU Qiang, PAN Jia-wei, LI Hai-bing, LIU Fu-cai, Marie-luce Chevalier, YANG Shao-hua, WU Kun-gang, JIANG Chen-yi

2026, 48(4):  1035-1051.  DOI: 10.3969/j.issn.0253-4967.20250066

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Accurate quantification of near-surface coseismic fault-slip distribution is critical for understanding earthquake rupture propagation and fault structural evolution. Traditional field surveys and geodetic methods have inherent limitations in resolving spatially distributed coseismic surface deformation with high precision. In recent years, rapid advances in sub-pixel correlation techniques applied to optical satellite imagery have provided an effective approach for comprehensive and high-resolution quantification of coseismic surface displacements. On 7 January 2025, an M_W7.1 earthquake struck Dingri County, Tibet, at a focal depth of 10km, causing significant casualties and economic losses and generating a coseismic surface rupture zone of approximately 35km along the Dingmu Co Fault. Following the event, numerous studies investigated coseismic surface deformation using high-resolution remote sensing interpretation, field surveys, GNSS, and Interferometric Synthetic Aperture Radar(InSAR). However, owing to methodological limitations, the distributed surface deformation associated with the Dingri earthquake has not yet been fully resolved. To further constrain the coseismic surface deformation characteristics of this event, this study uses pre- and post-event Chinese Gaofen-2 (GF-2) optical satellite images with a spatial resolution of 0.81m. Coseismic displacements are quantitatively derived using sub-pixel image correlation implemented in the COSI-Corr software package.

The image correlation results demonstrate that GF-2 imagery has strong capability for quantifying coseismic displacement and clearly reveals an approximately 6km-long coseismic surface rupture segment northeast of Nixia Co. Consistent with this result, field investigations indicate that the area northeast of Nixia Co experienced the largest coseismic surface rupture, with a maximum vertical offset approaching 3m. The rupture traces extracted from image correlation show good overall agreement with those mapped through field surveys and high-resolution unmanned aerial vehicle(UAV)imagery. The north-south displacement field derived from image correlation indicates northward motion of the eastern footwall and southward motion of the western hanging wall, consistent with field observations of coseismic rupture characteristics. In addition, the coseismic displacement vector field clearly reveals oblique slip, with the western hanging wall moving toward the SWW and the eastern footwall moving toward the NEE. Integrated analysis of multi-source datasets indicates that the Dingri earthquake was dominated by normal faulting with a significant sinistral, or left-lateral, strike-slip component.

Based on the image correlation results, horizontal displacement components in the east-west and north-south directions were extracted along the rupture trace using swath profiles. The results indicate that east-west extensional displacement across the Nixia Co segment predominantly ranges from 0.8 to 2.8m, with an average of approximately 2.0m and a maximum of 3.9m. The sinistral strike-slip displacement mainly ranges from 0.4 to 1.6m, with an average of approximately 0.9m and a maximum of 1.9m. Comparative analysis shows that, although the deformation trends derived from image correlation are consistent with field measurements, most field-measured coseismic displacements are systematically smaller than those obtained from image correlation. This discrepancy suggests that the 2025 Dingri earthquake likely produced a certain degree of off-fault deformation. Furthermore, the image correlation results indicate that the ratio of extensional to strike-slip displacement is approximately 2:1 for both average and maximum values. Based on this kinematic relationship, together with fault dip angles derived from inversion results reported in previous studies, the fault slip rake angle of the Dingri earthquake is estimated to range from -69° to -73°.

EARTHQUAKE RELOCATION AND b-VALUE SPATIOTEMPORAL VARIATION IN WEIYUAN, SICHUAN BASIN, CHINA

DENG Zi-yu, CHEN Cheng-feng, ZHANG Rui-qing, QIANG Zheng-yang, TIAN Wen

2026, 48(4):  1052-1067.  DOI: 10.3969/j.issn.0253-4967.20250063

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Seismic activity across the Sichuan Basin has historically remained low. Since 2014, however, seismicity has increased dramatically in the Weiyuan region of the southern Sichuan Basin, where large-scale shale gas hydraulic fracturing(HF)operations have been conducted. In particular, three successive earthquakes with M≥4.0 struck Rongxian during February 24-25, 2019. Just six months later, a moderate M5.4 earthquake occurred in the vicinity of Weiyuan County, the largest event recorded in the study area to date.

To enhance local seismic monitoring capabilities, a temporary seismic network was deployed across the Weiyuan region by the Institute of Geophysics, China Earthquake Administration(CEA) in 2019. The network consists of 40 broadband stations equipped with CMG-3ESPC/40T sensors with an average interstation spacing of ~8km. Using waveform data recorded by this network, we first constructed a high-precision earthquake catalog using a machine-learning workflow. The b-value for the entire catalog was then estimated with the maximum-likelihood method. Robust and timely estimates of b-values are essential for quantitative constraints on the evolution of earthquake sequences.

In this study, we detected and relocated a total of 49 982 events using the machine-learning-based LOC-FLOW from continuous waveforms recorded by the temporary network between October 2019 and December 2020. Cross-correlation differential travel times of these events were incorporated to constrain the relative location. We further applied a local magnitude calibration to calculate magnitudes for small-to-moderate earthquakes with hypocentral distances of ≤50km. The resulting high-resolution earthquake catalog has a completeness magnitude(M_c) of 0.4, and the largest event corresponds to the M_L5.2 Zizhong mainshock. HypoDD relocation results show that the majority of earthquakes are confined to depths less than 7km, with an average depth of 3.5km, and generally align with the burial depth of the shale gas reservoir in the study area. Specifically, the hypocenter of the Zizhong M_L5.2 earthquake lies at a focal depth of 4.43km. In map view, seismicity in the Weiyuan region delineates linear clusters adjacent to known faults, predominantly striking NW or nearly NS to NNE.

The b-value from the Gutenberg-Richter frequency-magnitude law is a critical parameter for characterizing regional seismicity. Based on the enhanced earthquake catalog, we further estimate b-values with the maximum-likelihood estimation technique to investigate their spatiotemporal variations within the study region. The spatial distribution of b-values shows clear lateral heterogeneity across the region. The estimated b-values are 0.92, 0.93, and 0.81 for southwestern(Region I), southeastern (Region Ⅱ), and northeastern(Region Ⅲ) Weiyuan regions, respectively, with corresponding completeness magnitudes of 0.6, 0.4, and 0.5. Particularly, Region III(northeastern Weiyuan) has a lower b-value than that estimated from the entire catalog(0.93), suggesting a locally elevated crustal stress within this zone.

Moreover, we detect a systematic decrease in b-values with increasing depth, likely reflecting the increase in rock confining pressure. An exception appears at the southern segment of the Molin fault, where the events at shallow depth generally correspond to low b-values, indicating concentrated elastic stress accumulating near the fault termination. Additionally, we identify a near-vertical seismic cluster characterized by high b-values. Some local earthquakes with relatively high b-values are also concentrated around the burial depth of the shale gas reservoir, which are likely triggered by shear failure via high-pressure fluid diffusion.

Time-dependent b-values reveal that decreases in b-values generally coincide with elevated seismicity, implying that small-to-moderate earthquakes mainly release stress accumulation within the shallow crust. Before the 2019 Zizhong M5.2 earthquake, the b-values decreased rapidly over a short period, followed by gradual recovery after the mainshock. Such temporal evolution is likely associated with the deformation process within the source region, including preseismic stress accumulation, coseismic stress relaxation, and post-seismic stress adjustment. The high-completeness catalog and detailed spatiotemporal b-value distributions in our study provide a fundamental dataset for traffic light monitoring frameworks and assessing seismic hazard in the Weiyuan region.

GEOLOGICAL TECTONICS AND CONSTRUCTION OF A HIGH-PRECISION EARTHQUAKE CATALOG AROUND HAINAN ISLAND BY USING MACHINE LEARNING

LI Yun-hui, LEI Jian-she, GUAN Hui-min, HU Xiao-hui, HAO Jiang-bo

2026, 48(4):  1068-1092.  DOI: 10.3969/j.issn.0253-4967.20260024

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Located at the southernmost margin of the South China Block, Hainan Island is influenced by both the spreading of the South China Sea and the upwelling of the Hainan mantle plume, resulting in regional relatively higher seismicity. To better investigate the characteristics of microseismic activity and its relationship with deep crustal structures in this region, we applied the LOC-FLOW earthquake-location workflow to continuous waveform data recorded from March 2023 to August 2024. The dataset comprises observations from 40 HAVESArray broadband portable stations and 28 provincial permanent stations. These portable and permanent stations complement each other and provide good azimuthal coverage, thereby enabling the construction of a high-resolution earthquake catalogue. A total of 2,671 high-quality earthquake events were obtained, with local magnitudes M_L ranging from -1.2 to 5.1. Most events with magnitudes greater than 3.0 occurred in the Beibu Gulf, north of the Leiqiong Strait. After relocation, the mean travel-time residual is 0.11s, and the average location uncertainties in the north-south, east-west, and depth directions are 0.50km, 0.48km, and 0.66km, respectively. The number of events identified in this catalogue is 7.5 times that reported by the China Earthquake Networks Center catalogue for the same period, and the magnitude of completeness Mc is reduced from 1.3 to 0.4. Our results reveal a distinct circum-island coastal distribution of microseismicity around Hainan Island. Four major seismic swarms are identified in northeastern Hainan, Wanning-Wenchang, Ledong, and Dongfang. The northeastern Hainan swarm forms a nearly north-south-trending seismic belt, which spatial distribution is inconsistent with mapped faults in the region, suggesting a complex local geological structure. The newly identified Wanning-Wenchang swarm exhibits a segmented NNE-SSW distribution and can be divided into northern, central, and southern segments, forming a bowl-shaped hypocentral geometry. The central segment reaches the greatest depth, extending to approximately 18km. The Ledong swarm generally follows a NW-SE trend, which is consistent with the principal compressive stress axis and indicates the structural control of the regional stress field on swarm development. The Dongfang swarm is characterized by clustered seismicity, steep faulting, and clear segmentation. By integrating the relocated catalogue with existing velocity models, we further discuss the roles of crustal heterogeneity and deep structures in controlling these seismic swarms. Under the influence of deep fluids associated with the Hainan mantle plume, crustal heterogeneities such as buried and deep-seated faults appear to exert a primary control on microseismicity. In addition, deep-fluid migration may contribute to earthquake generation, while the potential influence of seawater infiltration cannot be excluded. These results provide new constraints on the seismogenic mechanisms of Hainan Island and offer scientific support for regional seismic hazard assessment in and around the island.

RETROSPECTIVE EVALUATION OF ANNUAL EARTHQUAKE DISASTER LOSS PRE-ASSESSMENT FOR THE 2024 WUSHI M7.1 EARTHQUAKE AND SOME OF THE RECENT DESTRUCTIVE EARTHQUAKES IN XINJIANG

WEN He-ping, LI Shuai, Alimujiang·Yalikun, ZHAO Jiang-tao, LI Jin-xiang, LUO Ju

2026, 48(4):  1093-1106.  DOI: 10.3969/j.issn.0253-4967.20250053

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Earthquake disaster loss pre-assessment refers to the evaluation of potential disaster losses under scenario earthquake in key seismic hazard areas and the assessment of existing earthquake emergency preparedness capabilities. This work is typically conducted annually. In this study, we compare the actual earthquake damage, the relief supplies and rescue forces which invested in disaster relief of the Wushi M7.1 earthquake in 2024 Xijnjiang during the emergency rescue period, with the results of the annual earthquake disaster loss pre-assessment, It can be considered that the pre-assessment results of the disaster area size, casualties, the number of evacuated and relocated persons, and disaster relief needs are generally good.

Due to the potential for destructive earthquakes to cause significant casualties, casualty estimation is prioritized in annual pre-assessment work. So we conduct a retrospective evaluation of the annual pre-assessment of casualties for seven destructive earthquake which cause casualties since 2015 in Xinjiang. The pre-assessment results of three earthquakes with a magnitude of 6 are basically the same as the actual number of casualties; the other four earthquakes(including two magnitude 5 and two magnitude 6 earthquakes)did not occur within the assessment area and its 50-kilometer buffer zone, resulting in no valid evaluation outcomes.

The primary cause of earthquake casualties in Xinjiang in recent years(from 2015)is the damage of old buildings or ancillary facilities and the coseismic rupture of earthquakes. The sequence type of the earthquake and the occurrence time of the earthquake are objective factors for casualties. Foreshock may reduce casualties caused by the main shock under typical conditions. Earthquakes occurring during late-night and early-morning hours(People's vigilance is at its lowest during this period) often result in higher fatalities. The elderly and children are identified as vulnerable groups during earthquakes.

The implementation of annual earthquake disaster loss pre-assessment in Xinjiang demonstrates that estimating disaster scope, casualties, economic losses, and rescue demands requires localized models and must integrate region-specific critical factors. Therefore, following an analysis of key factors including seismic fortification standards for buildings, population distribution, earthquake-induced geological hazards, and scenario earthquakes that are involved in Xinjiang's annual earthquake disaster loss pre-assessment work.

We can conclude that in the future, the annual pre-assessment work in Xinjiang should be done better:

(1)According to the progress of Xinjiang's earthquake-safe housing projects, continuously optimize the structural ratio of regional residential buildings;

(2)According to the change of urban ground motion parameters, seismic zonation, and building fortification standards, the parameters of the building vulnerability model should be adjusted;

(3)We should pay great attention to the spatial variation of population migration with the seasonal alternation, and the difference in the rate of personnel in the room varies with different earthquake occurrence times;

(4)Under the characteristics of large dispersion and small aggregation of Xinjiang's population distribution, the scenario earthquake can be set directly below the densely populated area to constrain the upper limit of pre-assessment of casualties and rescue requirements;

(5)Sufficiently estimate potential casualties caused by earthquake geological disasters.

CHARACTERISTICS OF DISASTER OF DINGRI M6.8 EARTHQUAKE IN XIZANG

JIANG Li-xin, LIU Qin, DONG Xiang, XI Nan, XIAO Ben-fu, YANG Li-chen, YANG Zhi-gao, LIANG Ming-jian, DAI You-lin, TANG Cai-cheng, YE Zhao-heng, TAN Xin

2026, 48(4):  1107-1121.  DOI: 10.3969/j.issn.0253-4967.20250056

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This study presents a field investigation and damage analysis of the Dingri M6.8 earthquake in Xizang, which occurred at 09:05 on January 7, 2025. Combined with emergency products derived from seismic monitoring data, this paper provides a comprehensive overview of casualties, focal mechanism, estimated intensity, and surface rupture characteristics. Differences between rapid intensity assessment results and field-surveyed intensity results are compared, and the damage characteristics of different building types within each intensity zone are summarized. The results indicate that the earthquake occurred in a region with strong fault activity and produced severe damage, with the maximum intensity reaching Ⅸ in the epicentral area. The surface rupture extended for approximately 26km, with a maximum vertical displacement of about 3m. The rupture zone passed through Changsuo township, Dingri county, causing severe damage to buildings near the fault and resulting in substantial casualties. Although Changsuo township is located approximately 16km from the epicenter, its death toll and mortality rate were higher than those of Cuoguo township, which is only about 3km from the epicenter, indicating the complexity of the rupture process of major earthquakes and the possible discrepancy between the instrumental epicenter and the area of strongest macroseismic effects. The earthquake occurred within the Shenzha-Dingjie tectonic rift zone at a shallow focal depth of approximately 10km. The shallow source depth, together with the relatively soft soil conditions of the plateau river floodplain, amplified the destructive effects of strong ground shaking. In addition, the seismic fortification intensity of buildings in the severely affected area was lower than the actual seismic intensity. More than 98% of the houses in the earthquake-stricken area were earth-wood structures. Traditional Tibetan dwellings in the epicentral area were mostly unconstrained stone-wood or earth-wood structures, which were highly vulnerable to collapse and disintegration under strong shaking. The overall collapse rate in the intensity Ⅸ zone exceeded 60%. The limited indoor survival space after collapse, combined with heavy roofs and walls, was the primary cause of the high casualty rate. The earthquake occurred in the early morning, when most residents were asleep, leaving little time for emergency response or evacuation. Fatalities were therefore concentrated among sleeping residents, with elderly people and children accounting for a relatively high proportion. Rescue operations were further constrained by the high-altitude plateau environment and severe winter conditions, with temperatures reaching approximately -18℃, which accelerated energy loss among buried victims and increased mortality among trapped and injured individuals. Based on these damage characteristics, this study proposes countermeasures including strengthening the management of rural residential construction, advancing investigations of the seismotectonic environment in the disaster area, avoiding known active faults in post-earthquake reconstruction site selection, and improving the accuracy of seismic intensity zoning maps.

AN IMPROVED METHOD FOR ESTIMATING ENGINEERING SITE V_S30 BASED ON TERRAIN FEATURES AND SLOPE——A CASE STUDY OF XINJIANG REGION

JIAO Feng, LIU Ye, REN Ye-fei, WANG Hong-wei, WEN Rui-zhi

2026, 48(4):  1122-1137.  DOI: 10.3969/j.issn.0253-4967.20250009

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Time-averaged shear-wave velocity to 30m(V_S30)is one of the most widely used parameter for characterizing local site conditions, classifying engineering sites, evaluating site amplification, and developing ground motion prediction models. Direct determination of V_S30 generally requires borehole investigation and in situ shear-wave velocity testing to a depth of at least 30m. Such measurements are reliable but costly and spatially sparse, making them difficult to obtain over large regions. Consequently, regional V_S30 maps are commonly developed from proxy variables such as geology, geomorphology, and topographic slope. Existing slope-based models are simple and broadly applicable, but a single slope parameter cannot fully represent regional differences in terrain and near-surface materials. Terrain-classification-based models incorporate multiple terrain descriptors, yet they usually assign one representative V_S30 value to all locations within the same terrain class, thereby neglecting within-class variability and producing spatially discontinuous estimates at class boundaries.

To address these limitations, this study proposes an improved V_S30 estimation method that combines terrain features with slope information. Xinjiang, China, was selected as the study area. Three terrain attributes—slope, surface texture, and local convexity—were derived from a 30-arc-second digital elevation model. Following the Iwahashi-Pike terrain-classification scheme, the study area was divided into 16 terrain classes through a nested decision-tree procedure. A total of 4 115 engineering boreholes with measured shear-wave velocity profiles were compiled to develop the model.

Model development involved three steps. First, for each terrain class, 65% of the boreholes were randomly selected as the training subset and the remaining 35% were used for validation. This procedure was repeated 1 000 times, and the class-specific representative V_S30 value associated with the minimum mean squared prediction error was retained. Second, because several terrain classes contained relatively few boreholes, F-tests at a significance level of 0.05 were used to evaluate whether sparsely sampled classes with similar terrain characteristics could be combined. The results supported merging classes 3 and 4, classes 13 and 14, and classes 15 and 16, whereas classes 1 and 2 were retained separately. Third, within each final terrain group, a logarithmic regression between V_S30 and slope was examined. When the slope coefficient was statistically significant, a class-specific regression equation was used to produce continuously varying V_S30 estimates; otherwise, the cross-validated representative value was assigned to that terrain group. Statistically significant V_S30-slope relationships were identified for seven terrain groups.

The resulting terrain-slope hybrid model was used to generate a regional V_S30 map for Xinjiang and was evaluated against two widely used proxy approaches: the WA07 slope-only model and the Yong12 terrain-only model. Compared with the two reference models, the proposed method better represented within-class spatial variability and reduced abrupt changes caused by assigning a single constant value to an entire terrain class. For logarithmic V_S30 residuals in the validation dataset, the hybrid model produced a mean residual of 0.018 8, a standard deviation of 0.264 8, a mean absolute error of 0.174 9, and a root-mean-square error of 0.265 2. The corresponding standard deviation, mean absolute error, and root-mean-square error were 0.278 9, 0.183 0, and 0.280 7 for Yong12 and 0.328 4, 0.260 9, and 0.335 7 for WA07, respectively. An additional assessment based on locally smoothed means and standard deviations yielded a typical standard deviation of 77.42m/s for the hybrid model, lower than 79.06m/s for Yong12 and 83.40m/s for WA07. Site-classification performance was further examined using the NEHRP B, C, D, and E categories. The terrain-slope hybrid model achieved an overall classification accuracy of 80.85%, compared with 75.56%for Yong12 and 69.79%for WA07.hese results demonstrate that incorporating slope-dependent variation within terrain classes can improve both the accuracy and spatial continuity of regional V_S30 estimation. The proposed method provides a practical approach for regional seismic hazard and risk studies, rapid site-conditioning mapping, and engineering applications in areas where borehole measurements are limited. Nevertheless, its applicability should be further examined in extensive plains, deep basins, and densely urbanized areas, where the relationship between topography and shallow subsurface conditions may be weak or where buildings and vegetation may bias DEM-derived slope estimates.

THE INFLUENCE OF M_S7.0 EARTHQUAKES OCCURRING WITHIN THE QINGHAI-XIZANG PLATEAU TECTONIC BLOCK ON SEIS-MIC ACTIVITIES IN THE SICHUAN-YUNNAN REGION: BASED ON THE ANALYSIS OF CO-SEISMIC STEP-CHANGING OF THE GROUNDWATER LEVELS

MIAO A-li, ZHOU Zhi-hua, HU Xiao-jing, ZHANG Yi, WANG Wei, YANG Yun, YANG Yao, YE Bi-wen, ZHU Tao, YUAN Gui-ping, WANG Kai

2026, 48(4):  1138-1157.  DOI: 10.3969/j.issn.0253-4967.20250046

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The Sichuan-Yunnan rhombic block is located at the southeastern margin of the Qinghai-Xizang Plateau. Its current activities revolve around the eastern Himalayan tectonic in a clockwise direction, playing a role in regulating the eastward extrusion of materials in the Qinghai-Xizang Plateau region, or in adjusting the strong dextral shear between the Indian plate and the Yangtze block. This block is characterized by large-magnitude and high-frequency seismic activities. Seismic activities are mainly the result of crustal deformation. The linkage effect of great seismic activities can reflect the holistic and coordinated deformation of the tectonic system. This relationship of the integrity and coordination of crustal deformation is of certain significance for analyzing and predicting the trend of future great seismic activities in the region. The co-seismic response of underground fluids is one of the most effective and direct ways to reveal the response of crustal media to the stress-strain process. The spatial clustering areas of co-seismic response anomalies have certain indicative significance for future seismic danger zones.

This article analyzes the impact characteristics of major earthquakes with magnitudes 7 or above occurring within the Qinghai-Xizang Plateau block on the groundwater levels of fluid wells in the Sichuan-Yunnan region. It studies the changes in well-aquifer hydrological parameters before and after the coseismic response of fluid well levels, analyzes the genetic mechanism of coseismic step changes in water levels, and examines the indicative significance of coseismic water level step changes for future earthquake locations. The results show that: 1)In terms of morphology, after a major earthquake, some of the water levels of underground fluid wells in the Sichuan-Yunan region experienced step-up changes, while others experienced step-down changes. Both the step-up and step-down changes last for a period of time, and the duration of the co-seismic step change does not seem to be related to the well seismic distance directly. The co-seismic response patterns of the same well after different earthquakes are not constant, which is related to the changes in the permeability of the aquifer before and after major earthquakes. In terms of spatial distribution, the water level of underground fluid wells in the Sichuan-Yunan region exhibits a relatively concentrated regional pattern of synchronous rise and fall. 2)The change in permeability of aquifer rock caused by seismic waves may be the reason for the change in the seismic response pattern of well water level caused by far-field earthquakes. The “permeability enhancement” model may be the most reasonable explanation for the coseismic rise and fall of well water levels in the Sichuan-Yunan region after a major earthquake. After a major earthquake, the phase lag of the aquifer increases, the permeability decreases, and the surrounding pore water is compressed and squeezed into the well, which causes the well water level to rise. On the contrary, the phase lag of the aquifer decreases, the permeability increases, and the water in the wellbore flows out, which causes a decrease in the well water level. 3)Based on comprehensive analysis of the coseismic changes in well water levels in the Sichuan-Yunan region and GPS results, it's believed that the coseismic response changes can reflect the regional stress in a certain. It has certain indicative significance for the determination of seismic activity in the subsequent region on a medium to long-term scale. Based on the data since 2010, after 5 major earthquakes, there were 3 times causing changes in the water level of underground fluid wells, and subsequent earthquakes with magnitude of 6 occurred every time, with a probability of 60% and an interval of 15-22 months. The occurrence occurred in areas with concentrated changes in the water level of wells. In the future prediction of earthquake locations in the Sichuan-Yunan region, attention should be paid to the concentrated areas of coseismic changes. This study enhances the predictive ability of fluid observations for earthquake locations.

Academic Debate

REVISION OF THE BOUNDARIES OF THE SEISMIC ZONE IN NORTHEASTERN CHINA

BAO Ren-kai, ZHOU Qing, WU Guo

2026, 48(4):  1158-1175.  DOI: 10.3969/j.issn.0253-4967.20250055

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The delineation of seismic zones and belts forms the basis for seismic zonation and seismicity analysis, among which the definition of seismic-zone boundaries is particularly critical. Accurate boundary delineation helps distinguish regions with different levels of seismic hazard and provides a scientific basis for earthquake-resistant engineering, urban planning, and infrastructure development. It also supports the rational allocation of disaster-mitigation resources, strengthens earthquake monitoring in key areas, and reduces potential casualties and economic losses. Ultimately, improved seismic-zone delineation contributes to enhanced seismic resilience and societal safety.

Seismicity in China exhibits pronounced regional variations. In eastern China, seismic activity is relatively frequent in North China, whereas the Northeast seismic zone is characterized by lower earthquake frequency and magnitude. In previous revisions of the Seismic Ground Motion Parameter Zonation Map of China, the southern boundary of the Northeast seismic zone was defined along the Chifeng-Kaiyuan Fault. However, its northern, eastern, and western boundaries were largely aligned with national borders, which is not well justified from seismological or geological perspectives. For the next generation of seismic zonation maps, the boundaries of the Northeast seismic zone should therefore be revised by considering the regional tectonic setting, contemporary tectonic stress field, and spatial pattern of seismicity. This study aims to provide a more geologically consistent delineation of the Northeast seismic zone based on the structural characteristics of Northeastern China and adjacent regions.

We conducted an integrated analysis of the tectonic setting, geophysical field characteristics, seismicity, and contemporary tectonic stress field of northeastern China and its neighboring regions. The tectonic features, block composition, evolutionary history, and geomorphological characteristics on both sides of the national borders show a high degree of similarity. Geophysical evidence further indicates that the region is generally tectonically stable and may be regarded as an independent block. Analysis of the relationship between seismicity and fault structures shows that the Sikhote-Alin mountains share similar fault orientations and seismicity patterns with northeast China and can reasonably be incorporated into the same seismic zone. In terms of the contemporary stress field, both northeast China and adjacent regions are mainly affected by the subduction of the Pacific plate, resulting in a near east-west compressional stress regime that extends eastward to Sakhalin island and northward to the northeastern segment of the Outer Xing'an Range.

On the basis of these multidisciplinary constraints, we propose a revised boundary for the seismic zone in northeast China. The southern boundary remains along the Chifeng-Kaiyuan Fault. The northern boundary is extended to the southern margin of the Outer Xing'an Range, the eastern boundary reaches the eastern foothills of the Sikhote-Alin mountains near the western coast of the Sea of Japan, and the western boundary is expanded by approximately 150~200km beyond the current national border.

This revised boundary has important practical implications. First, it improves the precision of seismic-hazard delineation in northeastern China and its surrounding regions, thereby enhancing the scientific reliability and accuracy of regional hazard assessment and providing a stronger basis for seismic zonation mapping. Second, the revised boundary offers useful guidance for urban disaster-prevention planning, seismic design of major engineering projects, and optimization of regional seismic monitoring networks.