Table of Content

20 April 2026, Volume 48 Issue 2

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Review

RESEARCH ADVANCES OF THE LATE QUATERNARY ACTIVITY OF XIAOJIANG FAULT ZONE IN THE SOUTHEASTERN MARGIN OF QINGHAI-XIZANG PLATEAU

CHANG Yu-qiao, ZHANG Hui-ping, ZHAO Xu-dong

2026, 48(2):  279-313.  DOI: 10.3969/j.issn.0253-4967.20240135

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The Xiaojiang fault zone is the southernmost branch fault of the Xianshuihe-Xiaojiang left-lateral strike-slip fault system, together with the Xianshuihe Fault, the Anninghe Fault and the Zemuhe Fault, which forms the eastern boundary of the Sichuan-Yunnan block on the southeast margin of Qinghai-Xizang Plateau and plays an important role in accommodating the clockwise rotation around the eastern Himalayan syntaxis and S-SE extrusion of the Sichuan-Yunnan block. The scientific understanding of geometric structures, the slip rate, the strong earthquake recurrence and the mechanism of deformation compatibility of different segments of the Xiaojiang fault zone, especially the intersection with the Red River fault zone, remains highly debated. In order to better understand the role of the Xiaojiang fault zone in the process of material extrusion of the southeastern margin of the Qinghai-Xizang Plateau, based on the systematic summary of previous research results about the Xiaojiang fault zone, topographic relief and slope in the study area were extracted, and combined with geomorphological characteristics analysis, the geometric structure and kinematic characteristics, strike-slip rate at different time scales, seismic activity and deformation coordination with the Red River fault zone of the Xiaojiang fault zone are combed and summarized. combined with geomorphological characteristics analysis by extracting the relief and slope of the study area. The geometric segments and the late Quaternary activity characteristics of each segment of the Xiaojiang fault zone are described in detail. Then, we mainly reviewed the slip-rates at different timescales(geodetic scale, late Quaternary scale, and long time scale) of the Xiaojiang fault zone and their spatio-temporal variations. The seismicity characteristics of the Xiaojiang fault zone are summarized from three aspects: Paleoearthquakes, historical earthquakes and present earthquakes. The geometric relationship, deformation decomposition and transformation of faults at the intersection of the Xiaojiang fault zone and the Red River fault zone are analyzed. Besides, the geometric and kinematic characteristics of the deformation of the Xiaojiang fault zone and the coupling relationship between deep and shallow tectonic deformation are discussed, combined with geomorphic feature analysis. At last, he risk of future strong earthquakes in the Xiaojiang fault zone is discussed based on the results of previous studies on heterogeneous interseismic coupling along the Xiaojiang fault zone, etc. Through comprehensive analysis, it is concluded that the geometric and kinematic characteristics of the Xiaojiang fault zone are consistent with a clockwise rotation of the Sichuan-Yunnan block about the eastern Himalayan syntaxis. The deep tectonic process has an obvious controlling effect on the shallow tectonic deformation, and the Xiaojiang fault zone has a profound influence on the crustal structure, the deformation of the medium and the process of material migration. As the southeast boundary fault of the Sichuan-Yunnan block, the Xiaojiang fault zone has well restricted the S-SE movement of crustal material during the process of extrusion from the southeast margin of the Qinghai-Xizang Plateau to the east. The results show that the strike adjustment of the Xiaojiang fault zone, the change of movement characteristics, the spatio-temporal evolution of strike-slip rate, the segmentation of geomorphic parameters and the distribution of seismic activity are highly consistent with the clockwise rotation direction of the Sichuan-Yunnan block around the eastern Himalayan syntaxis. There is an obvious coupling relationship between deep tectonic action and shallow tectonic deformation in the Xiaojiang fault zone. The risk of strong earthquakes is high in the Xiaojiang fault zone, especially in the Qiaojia-Dongchuan area in the north and the confluence area of the Red River fault zone in the south. The probability of strong earthquakes in the future is increasing in each section of the Xiaojiang fault zone, especially in the northern section of the Qiaojia-Dongchuan fault zone and the intersection of the Red River fault zone and the Xiaojiang fault zone, the risk of strong earthquakes in the future is high, which should be paid great attention to.

Research paper

QUANTIFYING THERMAL DAMAGE OF QUARTZDIORITE: A COMPARISON STUDY

SHEN Feng-ming, WANG Li-feng, GUO Yan-shuang, QI Wen-bo, ZHOU Bin

2026, 48(2):  314-328.  DOI: 10.3969/j.issn.0253-4967.20240124

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Rock damage is a widespread natural phenomenon closely associated with earthquakes, landslides, and engineering practices such as geothermal energy development and nuclear-waste disposal. For instance, fault damage zones influence seismic energy release, whereas in geothermal exploitation thermal damage can modify the physical and mechanical properties of rocks, thereby affecting extraction efficiency and reservoir stability. Consequently, thermal damage in rocks has become a key topic in geomechanics with increasing engineering relevance. Previous studies show that heating-cooling cycles induce microstructural changes, including mineral thermal expansion and crack initiation/propagation, which in turn lead to macroscopic degradation of mechanical properties. Thermal damage also alters other physical parameters, such as thermal conductivity, porosity, and compressive strength. Despite growing practical demand, systematic comparisons among methods for evaluating rock damage remain limited. To address needs in geothermal engineering and earthquake-related studies, we conduct thermal-damage experiments on quartz diorite and compare several quantification approaches, including P-wave(longitudinal)velocity, quality factor(Q), total signal power, and mass-loss rate. We further evaluate the temperature ranges over which each method is most applicable.
Quartzdiorite samples were collected from Fangshan, Beijing(density: 2.88×10³kg/m³), with mineral grain sizes of 1~5mm. The major components of the samples are labradorite(53%), biotite(27%), orthoclase(10%), quartz(6%), and amphibole(4%). Cylindrical cores(ϕ50mm×100mm) were prepared, and 24 specimens with initial P-wave velocities of(5250±150)m/s were selected to ensure comparable initial properties. The specimens were divided into eight groups(three per group) and heat-treated from 25 to 800℃. Elastic-wave signals were acquired using a matrix-type ultrasonic measurement system developed by the Institute of Geology, China Earthquake Administration. From the recorded waveforms we derived P-wave velocity, Q, and total signal power. Thermal damage was quantified by comparing changes in velocity, Q, signal power, and mass before and after heating.
With increasing treatment temperature, the apparent color of the specimens shifts from gray at room temperature to light red. P-wave velocity decreases at ~300m/s per 100℃ over 25~400℃, accelerates to ~1130m/s per 100℃ over 400~600℃, and then slows to ~450m/s per 100℃ over 600~800℃. Q and total signal power exhibit similar behavior, with pronounced reductions over 200~600℃(Q decreases by 76% and signal power by 92%), followed by a more gradual decline over 600~800℃. The mass-loss rate increases rapidly to 0.15% over 25~300℃, remains nearly stable between 300℃ and 500℃, and then rises to 0.25% over 500~800℃.
Overall, thermal damage in quartz diorite increases monotonically with temperature, as indicated by the velocity-based damage factor and a temperature-dependent thermal-damage relation, with the most rapid increase occurring at 400~600℃. The different metrics show distinct temperature ranges of sensitivity. P-wave velocity provides an effective damage indicator across the full temperature interval. Q is more suitable for quantifying damage below 600℃ but becomes less sensitive at higher temperatures. Total signal power is more informative above 600℃. The mass-loss rate shows clear temperature dependence mainly over 25~400℃ and 600~800℃, making it most useful within these intervals. These results clarify the respective applicability of common damage quantification methods and may inform their use in field and engineering settings.

EXPERIMENTAL STUDY ON THE EFFECTS OF FAULT CONTACT HETEROGENEITY ON SEISMIC ACTIVITY

LI Zi-hong, ZHUO Yan-qun, CHEN Shun-yun, CHEN Hao, LU Li-li

2026, 48(2):  329-350.  DOI: 10.3969/j.issn.0253-4967.20240118

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Fault-contact heterogeneity is a common property of natural fault zones. Studying the relationship between seismic activity and fault-contact heterogeneity may provide new perspectives on the spatial relationship between small seismic events and mainshocks. In this study, granite samples were prepared by polishing, and experimental faults with heterogeneous contact characteristics were constructed by prefabricating the geometric morphology of the fault surfaces. The experimental faults were designed with a periodic, alternating structure of strong-contact and weak-contact zones of equal area, thereby achieving controllable fault-contact heterogeneity. On this basis, shear tests were performed under different average normal stresses to investigate fault-slip instability, generating stick-slip motion and simulating natural earthquakes at the laboratory scale. During the experiments, acoustic emission(AE)signals and local fault strain were synchronously monitored, and the magnitudes of AE events were calculated using the empirical Green’s function method. This method provides a unified scale for results under different experimental conditions, facilitating not only horizontal comparisons between groups but also scaling analogies between laboratory earthquakes and natural earthquakes, thus improving the extrapolation applicability of experimental results. Meanwhile, the μDA method was adopted to supplement the calculation of the mainshock magnitude. Key parameters, including the spatial-temporal distribution, magnitude characteristics, event frequency, and stress drop of foreshocks and mainshocks within the fault stick-slip cycle, were systematically analysed to reveal the coupled effects of fault contact heterogeneity and average normal stress on seismic activity. The experimental results showed that: 1) The fault contact heterogeneity has a significant influence on the distribution of small AE events and the initial locations of mainshocks. Namely, the small AE events are concentrated in areas of high normal stress or in areas with high gradients of normal stress. At the same time, the initial locations of the mainshocks are usually relative to the areas of high ratio of shear stress to normal stress, suggesting that the mechanisms of small AE events and mainshocks may be different. Additionally, the frequency of small AE events increases significantly as the mainshocks approach, while spatially, the small AE events activity is not in the same contact zone as the mainshocks. 2) The moment magnitudes of the AE events, which were obtained based on an empirical Green’s function method, are mostly between -8 and -7 for the small AE events but are between -4.6 and -4.1 for the mainshocks. Both the displacement results are used to calculate the main shock magnitude, which is between -3.4 and -3.0. The results of different methods can provide important constraints for the laboratory seismic scale system. 3) The average normal stress has significantly different effects on the magnitude and stress drop between the small AE events and main shocks. The magnitudes and stress drops of small AE events are not sensitive to the average normal stress. On the other hand, both the magnitudes and stress drops of the main shocks increase significantly as average normal stress increases. In either small AE events or the mainshocks, the logarithm of the stress drop increases with magnitude, and the relationship between the two is approximately linear, consistent with results from some other laboratory experiments. In summary, fault-contact heterogeneity significantly affects the spatiotemporal distribution of acoustic emission events. In conclusion, the heterogeneity of fault contact significantly influences the spatiotemporal distribution of acoustic emission events. Notably, the spatial inconsistency between major earthquakes and minor seismic activity highlights potential uncertainties in strong earthquake prediction using minor seismic data. The non-uniform distribution of fault-contact stress serves as a key factor controlling the spatial distribution of minor and major earthquakes, providing crucial laboratory evidence for understanding the relationship between background seismic activity and strong earthquakes in natural fault systems.

A VIEW OF STRATIFIED RESPOSE BASED ON SHORT-TERM CHANGES OF APPARENT RESISTIVITY AFTER STRONG EARTHQUAKE

XIE Tao, HAN Ying, LI Xin-yan, YU Chen, LI Xiao-fan, ZHANG Guo-ling, ZHANG Li-qiong, LIAO Xiao-feng

2026, 48(2):  351-369.  DOI: 10.3969/j.issn.0253-4967.20240136

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The relationship between pre-seismic anomalous trends in apparent resistivity(continuous decreases or increases) and the accumulation of seismogenic deformation has been confirmed by numerous earthquake cases as well as experimental and theoretical studies. After an earthquake releases the previously accumulated deformation, apparent resistivity is expected to change in the direction opposite to the pre-seismic anomaly. In practice, however, pronounced post-seismic step changes are uncommon in observational records because apparent resistivity is an indirect proxy for crustal deformation. Based on a literature survey and data analysis, we identify several stations that exhibited significant resistivity changes within days after the 1976 Tangshan M_S7.8 earthquake, the 1976 Songpan-Pingwu M_S7.2 earthquake, and the 2008 Wenchuan M_S8.0 earthquake. Although the recovery trends over months or longer are generally opposite to the pre-seismic anomalous patterns, the short-term responses during the first few days after the mainshock vary among stations. Some stations that showed a continuous pre-seismic decrease displayed an immediate post-seismic reversal. Others continued to decrease for several days after the earthquake before turning upward and entering a recovery phase opposite to the pre-seismic anomaly.
In this study, we compile electrical sounding data from five stations that recorded notable changes within days following the three earthquakes. Using apparent-resistivity sensitivity-coefficient theory, we evaluate the depth-dependent response of formation resistivity. The results suggest that, immediately after an earthquake, resistivity variations in shallow media are larger than those in the underlying strata. As stress and deformation continue to relax, changes in deeper strata increasingly control the longer-term recovery of the preceding anomaly. Thus, resistivity changes at different depths can diverge during the immediate post-seismic period, and the observed behavior cannot be primarily explained by water-level fluctuations.
We interpret these patterns in the context of post-seismic stress-strain relaxation. During rupture of a locked fault, accumulated deformation near the source is partially released. Because geological materials are not perfectly elastic, stress and deformation continue to be released after the mainshock, and aftershocks persisting for months further unload residual deformation. Surface rupture during the mainshock promotes more complete deformation release in shallow strata during the immediate post-seismic stage. In contrast, deformation release efficiency likely decreases with depth owing to higher confining pressure, producing larger resistivity increases in shallow layers than in deeper ones. As stress relaxation progresses in the source region, deformation release subsequently affects the observation-station area. Because shallow deformation has already been substantially released, resistivity variations in shallow strata become less pronounced during this phase, whereas continued deformation release at depth drives sustained resistivity increases. Consequently, resistivity changes in deeper strata dominate during the prolonged post-seismic recovery stage.
These findings help clarify the relationship between apparent-resistivity variations and regional deformation across three phases: pre-seismic deformation accumulation, short-term post-seismic adjustment, and long-term post-seismic recovery.

RECOGNITION OF HVDC INTERFERENCE EVENTS IN GEOELECTRIC FIELD BASED ON BI-LSTM

HU Hao-di, ZHANG Yu, WANG Lan-wei, KE Hao-nan

2026, 48(2):  370-385.  DOI: 10.3969/j.issn.0253-4967.20240108

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China is one of the world’s most earthquake-prone countries, and earthquake prediction is critical for mitigating disaster impacts. To detect precursors, China has built the world’s largest seismic precursor observation network, with geoelectric field observation as a key method. However, geoelectric field observations are affected by multiple factors, such as interference from high-voltage direct current(HVDC)transmission and subway operations. HVDC’s impact is growing with recent infrastructure expansion. Unbalanced operation of HVDC parallel lines severely distorts geoelectric field data, causing large-amplitude, wide-ranging interference that impairs data usability for earthquake forecasting studies. Disturbance patterns vary across regions and lines, with spikes, steps, and combinations as typical HVDC-induced anomalies. Automated recognition remains underdeveloped; current practices rely on manual monitoring to identify affected stations and periods, consuming substantial labor and time.
To enhance the efficiency of identifying disturbed data while ensuring accuracy, this study proposes a Bi-LSTM-based interference recognition method for the automatic identification of HVDC interference types and periods in geoelectric field data, drawing on existing research on HVDC-induced geoelectric interference mechanisms and typical patterns. The method aims to reduce reliance on expert knowledge and provide a reliable basis for subsequent data quality control and interference correction. Geoelectric field data present regular diurnal variations and distinct temporal characteristics, while HVDC interference signals feature long durations and complex morphological evolution, making unidirectional temporal modelling insufficient to fully capture their dynamic features. Compared with other deep learning methods, Bi-LSTM can simultaneously utilize forward and backward temporal information to capture dependencies from past and future time points, achieving higher recognition accuracy and robustness than unidirectional LSTM or other time-series models, thus effectively capturing implicit temporal features in geoelectric data and realizing efficient and accurate identification of disturbed anomalies to meet technical requirements. To ensure sufficient samples for model training, simulated interference is superimposed on normal geoelectric data to generate three main types of disturbed data. Samples are divided into training, validation and test sets: the training set is used to train the model’s internal neural parameters, the validation set for hyperparameter adjustment, and the test set to evaluate the model’s generalisation ability.
The trained model achieved 96.6% accuracy and precision on the test set. To verify its generalization ability and reliability in real scenarios, the Bi-LSTM model was applied to actual disturbed geoelectric field data. Results show its automatic interference labels are highly consistent with manual annotations. These indicate the Bi-LSTM method can model temporal dependencies in geoelectric data and recognize nonlinear interference patterns to a certain extent. Compared with conventional approaches, it performs better in interference event localization and classification, showing potential applicability for HVDC-induced disturbances.
The results of this study can greatly improve the efficiency of interference recognition, laying the foundation for the subsequent processing and effective use of interference data, while also providing new ideas and technical references for recognizing other factors that interfere with geoelectric field observations, such as power supply interference affecting geoelectric field measurements in co-located station resistivity observations.

LATE QUATERNARY ACTIVITY CHARACTERISTICS IN THE SHIBAN RIVER TO NANNIWAN SEGMENT OF THE SOUTHERN MAOMAOSHAN MARGINAL FAULT

XU Wei, LI Guang-tao, CHENG Li, ZOU Zi-nan

2026, 48(2):  386-402.  DOI: 10.3969/j.issn.0253-4967.20240092

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The Southern Maomaoshan Marginal Fault is a key structural constituent of the Qilian-Haiyuan fault zone, situated along the northeastern margin of the Qinghai-Xizang Plateau. Historically, research efforts have been predominantly concentrated on the Maomaoshan Fault along the northern flank, while the Southern Maomaoshan Marginal Fault on the southern flank has received comparatively less attention. This study seeks to investigate this understudied fault segment from the Shibangou River to Nanniwan, with three core objectives: 1) to reveal its Late Quaternary activity characteristics, 2) to assess its seismic hazard potential, and 3) to offer novel perspectives on regional tectonic evolution.
This study employed various methods, including high-precision topographic and geomorphic surveys, geomorphic surface dating, and trench excavation. Using high-precision airborne LiDAR technology, detailed 3D topographic data along the fault were obtained, and the heights of fault scarps were determined. Combined with optically stimulated luminescence(OSL)dating, the ages of geomorphic surfaces were measured. Additionally, trench excavations were conducted to reveal the near-surface structural characteristics of the fault, providing direct evidence of paleo-earthquake events.
The results indicate that the Southern Maomaoshan Marginal Fault has been significantly active during the Late Quaternary from Shibangou to Nanniwan segment, with left-lateral strike-slip movement accompanied by north-to-south thrusting. Airborne LiDAR revealed two distinct clusters of vertical displacement along the fault, indicating that the fault has experienced at least two strong paleo-earthquake events during the Late Quaternary. Based on fault scarps, trench profiles, and OSL dating, the recent strong earthquake activity has been constrained to 5.11~6.28kaBP, with vertical slip rates of 0.42~0.75mm/a since the Late Pleistocene. Trench excavations revealed that the near-surface structure of the fault displays typical flower structures characteristic of strike-slip faults.
The Southern Maomaoshan Marginal Fault is a Holocene active fault, which, together with the Maomaoshan Fault, accommodates tectonic deformation in the Maomaoshan region of the Qilian-Haiyuan fault zone. Its left-lateral strike-slip and thrust kinematics play an important role in shaping regional tectonic evolution. Given its proximity to several villages and the fact that its western segment constitutes a critical economic and transportation corridor, this fault underscores the urgency of seismic hazard assessment. Future studies ought to prioritize exploration of its slip rate and strong-motion earthquake recurrence patterns, integrating these results into regional hazard frameworks—a step that will enhance our comprehensive understanding of the northeastern Tibetan plateau’s tectonic evolution.

REVEALING THE SHALLOW SEDIMENTARY STRUCTURE OF THE WESTERN WEIHE BASIN USING THE HVSR METHOD WITH A SHORT-PERIOD DENSE ARRAY

RUAN Ming-ming, TIAN Xiao-feng, ZHENG Cheng-long, SONG Xiang-hui, WANG Liang, HE Chang-xun, WANG Shuai-jun

2026, 48(2):  403-422.  DOI: 10.3969/j.issn.0253-4967.20240113

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The Weihe Basin, located in the central segment of the North-South Seismic Belt, has experienced multiple historical strong earthquakes. Its thick sedimentary cover can produce pronounced site effects that may amplify seismic damage. At the same time, these sediments preserve key records of the basin’s structure and evolutionary history. However, a detailed shallow three-dimensional(3D)sedimentary model has been lacking for the western basin margin. In 2021, the Geophysical Exploration Center of the China Earthquake Administration deployed a dense seismic array of 288 three-component short-period EPS-2 seismometers across the western Weihe Basin, covering Fufeng, Meixian, and surrounding areas. The array had an average station spacing of ~1.5km and recorded continuously for 60 days.
Using these data, we first applied a short-term average/long-term average(STA/LTA) algorithm to automatically identify and remove nonstationary noise. We then used Konno-Omachi smoothing to obtain stable horizontal-to-vertical spectral ratio(HVSR)curves. The results show widespread multiple peaks across the study area. For each curve, we extracted the amplitude ratios and frequencies of the visible peaks. For the two most consistently observed peaks, f₀ and f₁, both amplitude ratios and resonant frequencies were interpolated by Kriging to produce two-dimensional(2D)distribution maps. In addition, adopting an estimated average shear-wave velocity, we converted peak frequencies to sediment thickness using the standard quarter-wavelength relation.
By integrating existing borehole and geological constraints, we interpret f₀ as the impedance contrast at the interface between the Neogene Youhu Formation and the Quaternary Sanmen Formation, whereas f₁ corresponds to the interface between the Quaternary Sanmen Formation and the overlying Qinchuan Group. Based on these two interfaces, we constructed a layered shallow 3D sedimentary model for the region. Depths derived from HVSR are consistent with borehole data from wells Wei-4 and Wei-5. Comparisons with two published borehole profiles further show that cross-sections extracted from our 3D model reproduce the depth-variation trends observed in the borehole records.
Spatial variations in burial depth and amplitude ratio for f₀ and f₁ are closely related to regional tectonic and sedimentary evolution. The depth to the base of the Sanmen Formation, inferred from f₀, is shallower in the west and deeper in the east, with the transition bounded by the Qishan-Mazhao Fault. The corresponding amplitude ratio(reflecting impedance contrast)is higher in the west and lower in the east. This pattern is consistent with uplift and erosion of the western block since the Pliocene, which produced thinner Sanmen sediments and a stronger impedance contrast, whereas the eastern depression underwent more continuous deposition and weaker environmental variability, yielding thicker Sanmen deposits and a smaller impedance contrast. Peak f₁ reflects the interface between the Sanmen Formation and the Qinchuan Group. From the Sanmen stage to deposition of the Qinchuan Group, paleo-Sanmen Lake in the central Weihe Basin contracted markedly and loess deposition migrated from the basin margins toward the center. The later onset of loess accumulation in the basin center resulted in thinner deposits and a larger vertical impedance contrast, whereas peripheral areas experienced more continuous fluvial-aeolian sedimentation during the Quaternary, leading to a smaller impedance contrast. Pre-existing topography and faulting also influenced thickness variations; for example, along the northern segment of the Weihe Fault, uplifted terrain corresponds to thinner deposits, whereas the southern downthrown block contains thicker accumulations.
In summary, we developed a shallow 3D sedimentary model for the western margin of the Weihe Basin and produced maps of fundamental resonance frequency and site amplification factor. The model agrees with available borehole constraints and regional geological understanding. These results provide a data basis for site-effect assessment and a geophysical reference model for investigating tectonic and environmental evolution. The HVSR approach used here may also serve as a useful complementary method for future studies of shallow sedimentary structure.

P-WAVE VELOCITY STRUCTURE OF THE CRUST AND UPPER-MOST MANTLE IN THE CENTRAL AND SOUTHERN SEGMENT OF TANLU FAULT ZONE AND ITS TECTONIC IMPLICATION

LI Xi-bing, TAO Xiao-san, GU Qin-ping, PENG Xiao-bo, WANG Yu, ZHU Feng

2026, 48(2):  423-441.  DOI: 10.3969/j.issn.0253-4967.20240115

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In 1668, a major M_W8.5 earthquake struck Tancheng, located in the central-southern segment of the Tanlu fault zone. Jointly resolving P-wave velocities in the crust and uppermost mantle can improve constraints on uppermost-mantle velocity, which is essential for understanding the tectonic mechanisms of strong earthquakes in this fault system and the deep seismogenic setting. Here we perform a joint inversion for crustal and uppermost-mantle P-wave velocity beneath the central-southern Tanlu fault zone using travel times of the first-arriving Pg and Pn phases, together with secondary Pg phases recorded by the seismic network. Incorporating secondary Pg arrivals increases crustal ray coverage and substantially improves the resolution and accuracy of the uppermost-mantle velocity structure.
The resulting P-wave model shows that near-surface(0km)high-velocity anomalies broadly correspond to mountain ranges, whereas low-velocity anomalies are mainly associated with sedimentary basins. Arc-shaped, belt-like high-velocity anomalies(5.40~5.54km/s) are observed in the northern Jiaodong Peninsula, the Taihang Mountains, and the Sulu orogenic belt, particularly in the Huaibei-Jining-Tai'an region. In contrast, low-velocity anomalies occur beneath Wuhan(Jianghan Basin), Hefei-Liuan(Hefei Basin), and Juxian(Shandong), and a prominent NS-trending low-velocity belt extends along the Shangqiu-Heze-Puyang-Liaocheng corridor(4.80~4.89km/s). At depths of 10~15km, high velocities dominate beneath the Dabie orogenic belt, Huainan-Bengbu, the Luxi uplift, and the Jiaodong Peninsula, whereas pronounced low-velocity anomalies appear beneath the Yangzhou-Changzhou-Suzhou to Shanghai-Nantong sector of the Subei Basin and the southern Yellow Sea. At ~15km depth, velocity structure differs markedly across the Tanlu fault zone: The western side from Juxian southward to the Dabie orogen exhibits strong high-velocity anomalies, while the eastern Subei-southern Yellow Sea Basin and the Xinyi-Lianyungang area are characterized by low velocities.
At ~20km depth, the velocity pattern changes relative to the upper and middle crust and shows strong along-strike segmentation within the Tanlu fault zone. North of Tancheng, a low-velocity anomaly is present; from Tancheng to Sihong, a high-velocity anomaly reaches a maximum of 6.39km/s; from Sihong to Lujiang, low velocities dominate, with a minimum of 6.03km/s near Jiashan. The Dabie orogenic belt exhibits a high-velocity anomaly of ~6.43km/s. At depths of 25~30km, which mainly reflect the middle-lower crust, high-velocity anomalies dominate beneath the Dabie orogen, the Taihang Mountains, and northern Jiangsu-southern Yellow Sea, with additional localized highs near Jiaozhou(Shandong) and Suqian(Jiangsu). Low-velocity anomalies occur in the Tancheng-Anqiu-Weifang segment of the Tanlu fault zone and also along the Liaocheng-Jinan corridor and near Jiashan and Wuhu.
Pn velocities in the central-southern Tanlu fault zone are strongly heterogeneous. North of Tancheng, low Pn velocities are mainly distributed on the eastern side of the fault zone, whereas south of Hefei they occur predominantly on the western side. High Pn velocities appear on the eastern side of the fault zone in the Tancheng-Jiashan region. Such heterogeneity likely reflects lateral variations in mechanical strength at the top of the upper mantle beneath different segments of the fault zone.
Overall, crustal velocity anomalies correlate with surface geomorphology and the distribution of major faults. In particular, velocity anomalies at 10~15km depth delineate the strike of the Tanlu fault zone. Across the central-southern Tanlu fault zone, the South China Plate and the Lower Yangtze Block display a clear crustal velocity contrast. In the upper-middle crust, low velocities prevail west of the Tanlu fault zone and high velocities to the east; in the lower crust, pronounced high velocities occur east of the Tanlu fault zone and south of the Jiashan-Xiangshui Fault, whereas low velocities dominate to the west. The relative strength contrast between upper and lower crust inferred from the spatial relationship between seismicity and velocity anomalies may play an important role in controlling earthquake occurrence. By incorporating a 3D crustal model, the joint inversion enhances the resolution of uppermost-mantle structure, revealing low-velocity anomalies flanking the epicentral region of the 1668 Tancheng M_W8.5 earthquake and strong uppermost-mantle heterogeneity beneath the central-southern Tanlu fault zone. Integrating crustal and uppermost-mantle velocity patterns, we divide the central-southern Tanlu fault zone into three segments—north of Tancheng, Tancheng to Jiashan, and south of Jiashan—consistent with previous segmentation studies.

HIGH-RESOLUTION CRUSTAL VELOCITY AND ANALYSIS OF SEISMIC SPATIAL VARIATION CHARACTERISTICS IN THE CENTRAL AND SOUTHERN SEGMENTS OF THE TAN-LU FAULT ZONE

QIAN Ting, CHEN Hao, HE Yi-cheng, JIANG Hao-lin, DU Hang, NI Hao-qi

2026, 48(2):  442-459.  DOI: 10.3969/j.issn.0253-4967.20240095

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The Tan-Lu fault zone(TLFZ)is a major tectonic structure in eastern China, characterized by significant seismic activity and a complex geological setting. Understanding the spatial distribution of seismic activity and its correlation with crustal velocity structures is crucial for assessing seismic hazards and improving our knowledge of the underlying mechanisms. This study aims to provide a detailed analysis of the high-resolution crustal velocity structure and seismic activity in the middle and southern segments of the TLFZ, focusing on identifying the key factors controlling spatial variations in seismic activity.
We collected Pg and Sg arrival time data from 2009 to 2021, covering a broad region from 115°E to 122°E and from 30°N to 38°N. A total of 16,525 earthquakes were selected based on strict criteria, including a minimum of 3 recording stations per event and a maximum focal depth of 20km, to ensure high-quality data from 332 fixed seismic stations. We employed a two-dimensional tomographic imaging method with a grid size of 0.25°×0.25° to generate high-resolution velocity models for the upper crust. The method incorporates station and event corrections to minimize travel-time residuals and uses a damped least-squares inversion technique with Laplacian smoothing to stabilize the solution. The resulting velocity models are validated through resolution tests and residual analysis, demonstrating the robustness of the imaging results.
The tomographic results reveal distinct high- and low-velocity anomalies in the upper crust, which are closely correlated with regional geological structures. Based on the velocity structure and seismic activity, the TLFZ can be divided into four sub-segments: Bohai Bay-Wulian, Wulian-Tancheng, Tancheng-Jiashan, and Jiashan-Guangji. The Wulian-Tancheng segment exhibits significantly high-velocity anomalies and is associated with the highest seismic activity in the region. This segment is characterized by a small curvature and a near-vertical fault geometry, which facilitate the accumulation of elastic strain energy and promote seismic activity. Additionally, the high-velocity anomalies form a sharp velocity gradient with the underlying low-velocity zones, creating favorable conditions for strong earthquakes. The spatial distribution of seismic activity is closely related to the crustal velocity structure, with high-velocity anomalies favoring seismic events. The results suggest that seismic activity in the TLFZ is influenced by several factors, including the physical properties of the medium, external stress fields, fault geometry, and crustal fluid systems.
This study demonstrates that the spatial differences in seismic activity within the middle and southern segments of the TLFZ are controlled by the interplay of crustal velocity structure, fault geometry, and crustal fluid systems. High-resolution tomographic imaging provides valuable insights into the mechanisms underlying seismic activity in this region. The findings highlight the importance of considering multiple factors when studying seismogenic mechanisms and suggest that future research should integrate various methods and data to better understand the complex interactions between crustal structures and seismic activity. Besides, it not only contributes to understanding the TLFZ's seismic behavior but also has implications for seismic hazard assessment and mitigation in the region. The detailed analysis of the velocity structure and its correlation with seismic activity provides a foundation for further exploration of the deep processes that influence seismicity and can enhance understanding of seismogenic mechanisms in complex tectonic settings.

CONSTRUCTION OF THE LITHOSPHERIC MAGNETIC FIELD IN THE EAST DABIE REGION BASED ON MULTIPLE DATA SOURCES AND ANALYSIS OF ITS RELATIONSHIP WITH TECTONICS

CHU Fei, XIAO Wei-peng, SUN Hong-bo, LIANG Xiao, FANG Zhen, HUANG Xian-liang, SUN Bo-le, YING Yun-xiang, ZHENG Hai-gang

2026, 48(2):  460-474.  DOI: 10.3969/j.issn.0253-4967.20240137

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As a vital component of the Earth’s internal magnetic field, the lithospheric magnetic field provides essential information for revealing the material composition, structure, and tectonic evolution of the crust and upper mantle. Constructing high-precision lithospheric magnetic field models and integrating them with geological structural features to analyze how tectonics and strata control the magnetic field is crucial for understanding regional geology and deep structural processes.
To achieve this, we employed the equivalent source method to integrate ground and aeromagnetic survey data from the study area(115.3°-116.5°E, 30.96°-31.88°N). Eighteen ground total field magnetic intensity data were measured in December 2022.Following diurnal and secular variation corrections and stripping off the main magnetic field, we obtained the lithospheric magnetic field, which possesses high absolute accuracy but limited spatial resolution. In contrast, the 5km×5km-resolution aeromagnetic data cover a much broader area but lack fine-scale detail and reflect a smoothed composite magnetic field due to the flight altitude. The equivalent source method effectively reconciled these heterogeneous datasets by calculating an equivalent source layer that could predict both data types, thereby reducing systematic bias and generating a high-fidelity, unified magnetic anomaly map on a consistent datum. Subsequently, using the interpolation-cut method—a potential field separation technique suitable for regional studies—the fused magnetic anomaly field was decomposed into source components from varying depths. This process yielded a series of horizontal slices of the lithospheric magnetic field from the near-surface down to approximately 33km(essentially reaching below the regional Curie isotherm).Such depth dependent separation is critical for distinguishing shallow local magnetic sources from deep regional-scale structures.
The results show that the fused lithospheric magnetic field model exhibits significantly higher resolution and clearer geological interpretation than single datasets or global models such as EMAG3. The model clearly delineates two first-order magnetic anomaly domains: a prominent, broad NNW-SSE trending negative anomaly belt outlining the North Huaiyang fold belt, and a distinct positive anomaly block associated with the North Dabie uplift. These large-scale features are interpreted as reflecting fundamental differences in basement rock magnetism, corresponding to the metasedimentary sequences of the North Huaiyang and the magmatic-metamorphic complexes of the Dabie Block, respectively.
The depth slices obtained via potential field separation provide new insights into three-dimensional structures and the controlling roles of major boundaries. Key findings include: The Qingshan-Xiaotian and Feixi-Hanbaidu faults are clearly identified as the southern and northern boundaries of the North Huaiyang negative anomaly. Their control becomes more pronounced at greater depths(25-30km), confirming their status as major deep-seated faults. The core of the North Huaiyang negative anomaly mainly originates from mid-to-lower crustal depths(15-25km), while the Dabie positive anomaly is strongest within the 15-20km range and attenuates significantly below 25km. The Tudiling-Luo’erling fault exhibits a clear magnetic discontinuity, constraining its cutting depth to roughly 10-15km, suggesting it is a potentially active fault that does not penetrate the lower crust. The Meishan-Longhekou fault influences the upper crust but shows no significant features in the deep(>25km)magnetic structure. Based on linear anomaly features and discontinuity patterns, particularly evident in the 10-20km slices, we infer the presence of a previously unrecognized NNE-trending concealed fault in the northwestern part of the study area, with an estimated cutting depth of 10-15km.
In conclusion, by applying multi-source data fusion, this study establishes a high-precision lithospheric magnetic field model for the Eastern Dabie region. Subsequent multi-scale analysis via potential field separation effectively couples magnetic signals from different depths, providing reliable geophysical evidence for the geometry, depth extension, and tectonic roles of major faults and structural units. This work not only deepens understanding of the deep structure of the Eastern Dabie orogenic belt and provides geometric constraints for tectonic interpretation, but also demonstrates an effective methodological framework for integrated geological-geophysical interpretation in complex terranes.

AEROMAGNETIC INTERPRETATION OF BURIED FAULT SPATIAL DISTRIBUTION CHARACTERISTICS IN THE NORTH CHINA BASIN

FAN Ji-di, WANG Xin, XIANG Jian-bin

2026, 48(2):  475-496.  DOI: 10.3969/j.issn.0253-4967.20250142

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The North China Basin, with great thick sedimentary layers, is located in the eastern part of the North China Craton and is one of the important sedimentary basins in eastern China. There are a large number of buried faults in the North China Basin. Some of buried faults are strong activity and have experienced major earthquakes, such as the 1679 Sanhe-Pinggu M8 earthquake and the 1976 Tangshan M_S7.8 earthquake. The geometric distribution and depth of these buried faults remain uncertain. Thus, it is very important to study spatial distribution and the intersecting relationships of the main faults in the region, and high-resolution aeromagnetic data is key.
In the paper, the high-precision aeromagnetic field data of the North China Basin are collected and are reduced to the pole at variable latitudes. Using the vertical first-order derivative and tilt-derivative edge-enhancement methods, and with reference to existing geological data, the geometric distribution and intersection relationships of the major faults are reinterpreted and corrected. The derivative methods have inherent limitations in the sedimentary area; the correction results are compared with previous data to ensure the reliability of the interpretation results. Moreover, the Euler deconvolution method is used to estimate the apparent depth of some buried faults in the basin. Its cutting depth is inferred and verified with other geophysical results.
The result shows that the aeromagnetic field is divided into four blocks: the chaotic magnetic zone of the northern Yanshan uplift, the high magnetic zone of the western Taihang uplift, the medium-high magnetic zone of the central North China Basin, and the low magnetic zone of the southern Luxi uplift. The North China Basin is characterized by multiple secondary structural units with alternating NE-trending uplifts and depressions. The 59 major faults are interpreted and corrected using the aeromagnetic field and its derivative, with faults mainly oriented NNE and NE, supplemented by NW, NS, and near-EW orientations. In the NNE direction, among them buried faults such as the Baoding-Shijiazhuang Fault, the Cangxi Fault, the Cangdong Fault, and the Tangshan Fault exhibit beaded bands, linear anomalies, magnetic gradient strip and so on. In the NW direction, the magnetic field characteristics of the Xiadian Fault, the Baodi Fault, and the Nankou-Sunhe Fault are faint, showing magnetic conversion and fault-bent strips, suggesting they played a regulatory role in structural deformation.
The Euler results show the apparent depth and cutting scales of 13 buried faults within the basin. Among them, 6 buried faults are middle-shallow scale faults: the Huangzhuang-Gaoliying Fault, the Nankou-Sunhe Fault, the Baodi Fault, the Cangdong Fault, the Cangxi Fault and the Liaocheng-Lankao Fault; and 7 buried faults are middle-deep scale faults: The Baoding-Shijiazhuang Fault, the Shunyi-Liangxiang Fault, the Xiadian Fault, the Jiyunhe Fault, the Tangshan Fault, the Chengxi-Yangerzhuang Fault and the Handan Fault. Multi-scale aeromagnetic computation and inversion provide a new technical approach for investigating the spatial distribution of buried faults. For large areas, especially sedimentary basins with great thick sedimentary layers, this technology is used for preliminary exploration of the geometric distribution and depth of buried faults.

LANDSLIDE SUSCEPTIBILITY ASSESSMENT OF THE 2014 LUDIAN EARTHQUAKE: COMPARATIVE STUDY BASED ON CONVOLUTIONAL NEURAL NETWORK AND RANDOM FOREST

WANG Wan-tong, MA Si-yuan, YAN Wu-jian, YUAN Ren-mao

2026, 48(2):  497-519.  DOI: 10.3969/j.issn.0253-4967.20240139

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Landslides are among the most common geological hazards worldwide, causing severe environmental damage and significant socio-economic losses in affected areas. In China, geological disasters are characterized by wide spatial distribution, high frequency, strong destructiveness, and substantial losses. In particular, landslides triggered by strong earthquakes have profoundly impacted the socio-economic development of southwestern China. Ludian County in Yunnan Province is located within the Sichuan-Yunnan region, where both seismic activity and intense rainfall pose serious hazards. The region’s steep topography, fractured rock masses, abundant precipitation, and active faulting contribute to frequent landslides and debris flows. Therefore, establishing a rapid and accurate landslide susceptibility assessment system is of great importance for improving disaster prevention and risk management and for reducing losses associated with earthquake-induced landslides.
Using the 2014 Ludian earthquake as a case study, a database of coseismic landslides was established through visual interpretation of ultra-high-resolution satellite imagery from the Google Earth platform. Three-dimensional landslide and non-landslide samples were extracted based on multiple influencing factors. A rapid landslide susceptibility assessment model, AlexNetCBAM, was developed based on a convolutional neural network(CNN), and a Random Forest(RF)model was employed for comparison. Model performance was quantitatively evaluated using statistical indicators, including receiver operating characteristic(ROC)curves, area under the curve(AUC), and F1-score, to verify the reliability and applicability of the models in the Ludian area.
Ten influencing factors, including elevation, slope angle, slope aspect, seismic intensity, distance to faults, and lithology, were selected. The Gini index was used to quantify the contribution of each factor to landslide occurrence. The results indicate that distance to faults, seismic intensity, and peak ground acceleration(PGA)exhibit the strongest predictive capability, suggesting that seismic factors exert a dominant control on landslide occurrence in the Ludian earthquake area compared with topographic and geological conditions. Removing highly correlated and low-importance factors reduces redundancy and improves data quality, providing a solid foundation for model development.
A comparison of the landslide susceptibility maps(LSMs)generated by the two models shows that areas of high susceptibility closely correspond to observed landslide distributions. Extremely high and high susceptibility zones are mainly concentrated in two regions: the southeastern valley area far from the epicenter, and the zone extending from the BXF fault to the Niulan River along a west-east direction on the western side of the epicenter. These areas should be prioritized for post-earthquake monitoring, early warning, and emergency response.
Comparative analysis of the ROC curves and related statistical metrics indicates that the prediction accuracies of the AlexNetCBAM and RF models are 0.84 and 0.82, respectively, with corresponding AUC values of 0.91 and 0.90. These results demonstrate that the AlexNetCBAM model slightly outperforms the RF model in predicting coseismic landslides in the Ludian area. Correlation analysis of landslide susceptibility index(LSI)values between the two models reveals a linear relationship of LSICNN=0.82*LSIRF, indicating that the CNN-based model more effectively classifies non-landslide pixels into low-susceptibility zones and landslide pixels into high and very high susceptibility zones. Compared with the traditional RF model, the CNN model exhibits stronger predictive capability, highlighting the adaptability and advantages of deep learning methods in earthquake-induced landslide susceptibility assessment.
This study enables rapid and accurate identification of coseismic landslide-prone areas and provides a scientific basis for emergency response planning, including rescue team deployment and resource allocation, thereby helping to reduce casualties and property losses.

ASSESSING SEVERELY AFFECTED AREAS BY EARTHQUAKE USING MOBILE SIGNALLING DATA: A CASE STUDY OF THE 2023 M_S6.2 JISHISHAN EARTHQUAKE, CHINA

GUO Xin-xin, WEI Ben-yong, SU Gui-wu, QI Wen-hua, ZHANG Teng-fei

2026, 48(2):  520-539.  DOI: 10.3969/j.issn.0253-4967.20240101

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On December 18, 2023, at 23:59, a M_S6.2 earthquake struck Jishishan County in the Linxia Hui Autonomous Prefecture, Gansu Province, causing significant casualties and widespread damage to infrastructure. Rapid and accurate identification of severely affected areas in the aftermath of an earthquake is a crucial challenge for earthquake emergency management. Mobile signalling data, known for its strong responsiveness to earthquake impacts, presents a valuable tool for swiftly identifying these critical areas. By analysing abnormal variations in mobile signalling data before and after an earthquake, emergency response efforts can be better informed, providing timely and effective support for the allocation of resources and deployment of aid to the most affected regions.
This study uses the Jishishan M_S6.2 earthquake as a case study to systematically explore the effectiveness of mobile signalling data in assessing earthquake-affected areas. Through comparative analysis, correlation analysis, and spatial interpolation techniques, the findings reveal significant anomalies in mobile signalling patterns post-earthquake, with varying response intensities across different seismic zones. Following the Jishishan earthquake, mobile signalling data showed marked anomalies, characterised by an overall increase in detected devices compared to pre-earthquake levels. A strong correlation was found between changes in mobile signalling and the earthquake’s impact field. Compared to the month-on-month anomaly rate, the year-on-year anomaly rate shows a more pronounced correlation with earthquake impact variables, providing a more signalling response to the earthquake’s effects. The spatial extent of severely affected areas and actual damage can be simulated with greater accuracy by applying the Kriging interpolation method based on mobile signalling anomalies. Using the 5-minute average year-on-year anomaly rate, the interpolation simulation offers a clearer distinction between severely and moderately affected areas. Moreover, compared to the distribution of seismic intensity zones, results derived from mobile signalling data are more detailed and precise. Due to the timing of the Jishishan earthquake, the number of detected devices after the event was generally higher than before it.
Although factors such as the earthquake’s timing, terrain conditions, and the integrity of infrastructure can influence changes in mobile signalling, this data remains a highly effective tool for the rapid and accurate identification of severely affected areas. Compared to traditional assessment methods, mobile signalling data reflects real-time population distribution and offers several advantages, including spatial continuity, timely acquisition, and large data volume. These attributes make mobile signalling a promising method for quickly and precisely pinpointing severe areas affected by a disaster.

GRAVITY OBSERVATION DURING 2021—2023 AND MAGMA ACTIVITY IN CHANGBAISHAN-TIANCHI VOLCANO

HU Ya-xuan, ZHANG Guo-qing, XIONG Guo-hua, ZHUANG Wen-quan, FENG Bing, KONG Qing-jun

2026, 48(2):  540-560.  DOI: 10.3969/j.issn.0253-4967.20240134

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Tianchi volcano at Changbai Mountain is the most eruption-prone Cenozoic active volcano in China. It experienced four major Holocene eruptions, among which the eruption around AD 946 is regarded as one of the largest worldwide in the past 2000 years. In recent years, Tianchi has shown clear signs of unrest, including a strong disturbance from 2002 to 2005 and a weaker episode from December 2020 to June 2021. The volcanic area remains under a compressional stress regime, and continued attention to regional stress conditions and magmatic activity is therefore warranted.
Gravity monitoring provides an effective means to track and investigate active volcanoes by constraining subsurface mass redistribution associated with magma and hydrothermal processes. Because magma originates in the mantle and differs in density from crustal rocks, its movement within the crust can produce microgravity changes of tens to hundreds of microgals. The observed gravity field reflects the combined effects of topography, lateral density variations within the crust and surrounding materials, and even contributions from upper-mantle structure. Differences in station elevation, changes in near-surface rock density, and magma migration(intrusion and withdrawal)can all drive measurable gravity variations, which can be quantified through high-precision repeat gravity surveys.
To assess current magmatic activity, the Second Monitoring and Application Center of the China Earthquake Administration conducted three phases of mobile gravity measurements in 2021, 2022, and 2023. Observations were collected using two Burris relative gravimeters with a nominal precision of 10×10^-8m/s^-2. Each campaign was carried out during July-August with the same instruments and a consistent observer team, thereby reducing potential influences from seasonal effects and operator-related differences. The results show that gravity changes from 2021 to 2022 ranged from -161.4 to 23.4μGal and were predominantly negative. From 2022 to 2023, changes ranged from -37.8 to 135.7μGal and were mainly positive. Over the full interval from 2021 to 2023, gravity variations ranged from -90.6 to 56.0μGal, with negative changes prevailing.
Gravity anomalies are observed within ~35km of the crater, implying the approximate extent of active magma/hydrothermal influence. Stations with changes exceeding 100μGal are concentrated within ~12km of the crater, which may reflect magma-related processes in an intermediate, roughly cylindrical conduit. The largest gravity change occurs west of the crater, suggesting the presence of a nearby deep fault. In addition, gravity changes on both sides of the crater show clustered patterns, potentially indicating activity associated with distinct slab-like magma bodies.
The combined analysis of gravity and deformation observations is widely used for long-term volcanic-hazard assessment because it provides complementary constraints on subsurface structure, mass redistribution, and/or pressure changes. The gravity-height change gradient(Δg/Δh)directly reflects the balance between mass change and surface displacement and can be interpreted using established zoning based on the relationship between Δg and Δh. Data plotting in Zone Ⅰ (below the Bouguer-corrected free-air gradient, BCFAG) indicate subsidence with negative Δg, consistent with a decrease in density or mass(e.g., magma drainage, a falling water table, void generation, or vesiculation), implying a low likelihood of eruption. Zone Ⅱ (above the free-air gradient, FAG) also indicates subsidence but with positive Δg, reflecting increased density or mass(e.g., magma input, rising water table, dyke emplacement, bubble resorption, void filling, or hydrothermal cementation), likewise suggesting a low eruption probability. Zone Ⅲ (between FAG and BCFAG)corresponds to subsidence with positive Δg and is interpreted as increased density accompanied by decreased mass, which may result from magma drainage, a falling water table, or void closure and can be associated with summit-collapse potential. Gradients plotting along or close to the ordinate indicate shallow processes such as magma and/or gas fluctuations within feeder conduits, near-surface dyke emplacement, or hydrothermal activity.
Leveling and InSAR observations since 1992 indicate short-term deformation fluctuations but low long-term deformation rates during quiescent periods. Deformation is mainly concentrated near the crater, with localized contributions from fault activity. During 2021-2023, Tianchi remained in a quiescent state with minimal deformation. Based on the observed deformation behavior, Δg/Δh is inferred to fall predominantly within Zone Ⅰ for 2021-2022 and Zone Ⅲ for 2022-2023. Given the large gravity changes and the small deformation amplitudes(within several tens of millimeters), Δg/Δh is expected to plot close to the ordinate. After accounting for density constraints and excluding the influence of water bodies, the gravity variations are best explained by subsurface mass redistribution, suggesting ongoing magma and/or gas migration within transport conduits or along faults.
Integrating magnetotelluric, seismic, and gravity constraints on the crust-mantle magmatic system beneath the crater, we propose that the minor unrest in 2020-2021 increased seismicity, likely modulated by the surrounding compressional stress field. Rising magma may have partially reopened previously obstructed pathways and migrated into pre-existing reservoirs of different sizes and depths. This process could have induced micro-fracturing in the host rock, triggering earthquakes at multiple depths and producing regional gravity changes. After April 2021, seismicity returned to background levels. The presence of substantial gravity changes without pronounced deformation suggests that magma migration primarily involved intrusion into, or withdrawal from, pre-existing voids, fractures, or fault-related spaces. The negative gravity changes observed in 2021-2022 may reflect magma withdrawal and mass loss following the 2020-2021 unrest, whereas the positive changes in 2022-2023 suggest renewed ascent of melt under compressive loading, with magma filling existing voids and fault zones. Over 2021-2023, the dominant negative trend implies that the mass withdrawn in 2021-2022 exceeded the mass added in 2022-2023. Collectively, these results indicate that magmatic activity beneath Tianchi remains ongoing.

THE STRESS FIELD INVERSION AND TRIGGER RELATIONSHIPS OF THE JINGGU EARTHQUAKE SEQUENCE

LI Yan, CHEN Jun-lei, LÜ Si-yu, WANG Yu-dong, FU Lei

2026, 48(2):  561-581.  DOI: 10.3969/j.issn.0253-4967.20240120

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The Jinggu earthquake sequence exhibits two dominant spatial orientations. Immediately after the mainshock, aftershocks propagated along the fault plane in a NW-SE direction. Subsequently, following two M_W5.5 strong aftershocks, the sequence expanded primarily in a NNW-SSE direction. These observations imply relatively complex triggering processes between the mainshock and the two M_W5.5 events, as well as substantial interactions among aftershocks. Clarifying triggering relationships within the sequence therefore requires a stress-based analysis.
To investigate these relationships, we first compiled focal mechanism solutions for 698 historical earthquakes and inverted the regional tectonic stress field in southwestern Yunnan using a 1°×1° grid. We then inverted the local stress field of the seismogenic fault using focal mechanisms of M_W≥3.0 events within the Jinggu sequence. The regional and local stress fields were used to constrain static Coulomb stress calculations on the optimally oriented fault plane, thereby improving result robustness. Based on the mainshock rupture model, we used Coulomb3.3 to evaluate the sensitivity of static Coulomb stress change to different friction coefficients and centroid depths. We further constructed source-fault models for 20 M_W≥3.0 aftershocks using empirical scaling relationships and calculated Coulomb stress transfer among these events. This workflow aims to quantify stress triggering from the mainshock to aftershocks and among aftershocks themselves.
The results indicate that the regional stress field in the area 23°~24°N and 99.5°~100.5°E is broadly consistent with the local stress field inferred for the Jinggu seismogenic fault. The maximum principal stress axis indicates NNE-SSW compression, with dominant azimuths of 12°(regional) and 25°(local). The minimum principal stress axis indicates NWW-SEE extension, with dominant azimuths of 102°(regional) and -65°(local). Stress shape ratios(R) of 0.50 and 0.59 suggest overall stress regimes approximating a uniaxial extension-uniaxial compression state and a biaxial extension-uniaxial compression state, respectively.
Coulomb stress-change patterns computed using different friction coefficients show consistent spatial trends. As the friction coefficient increases, the likelihood of mainshock-triggered aftershocks increases, but the effect becomes weak once the coefficient exceeds ~0.4. Accordingly, we adopt the commonly used empirical value for subsequent calculations. Coulomb stress-change patterns are also broadly consistent across tested depths; therefore, we use a centroid depth of 5km. Under these assumptions, ~73.46% of aftershocks are located within stress-loading zones. Cross-sections along the sequence trend indicate Coulomb stress changes extending vertically to ~25km depth and laterally for ~40km along strike. The concentration of aftershocks within stress-loading areas is consistent with the inferred mainshock rupture process.
Within the M_W≥3.0 aftershock set, ~55% of events are classified as triggered. The two M_W5.5 strong aftershocks are associated with stress loadings of 0.251MPa and 0.376MPa, respectively, far exceeding the commonly cited triggering threshold of 0.01MPa. Coulomb stress calculations for the M_w≥3.0 sequence further indicate a multiphase triggering process governing its spatiotemporal evolution. Aftershocks initially expanded preferentially toward the NW and later shifted toward the SE. The first M_W5.5 event marks a key turning point: it not only directly triggered a subsequent cluster of aftershocks extending approximately NS, but also altered the overall migration pattern of the sequence.
Mutual triggering among aftershocks is already pronounced prior to the M_W5.5 event; specifically, 8 of the 13 preceding events show clear triggering effects on subsequent earthquakes. This behavior is primarily attributed to superposition of static Coulomb stress perturbations generated by multiple events, which can operate through two end-member mechanisms. When stress changes from different sources are similarly oriented, they act constructively on the receiver fault and expand the affected region beyond the initial perturbation. Conversely, when stress orientations differ, they generate complex spatial variations in stress magnitude and direction on the receiver fault, producing a heterogeneous aftershock distribution. As a result, aftershocks not directly triggered by the mainshock may still be promoted by stress transfer from other aftershocks. The M_W5.5 aftershock sequence may have occurred on a branching structure with a different orientation from the mainshock fault, or on an unmapped blind fault. The distinctive triggering behavior of the Jinggu sequence provides useful constraints for assessing post-seismic evolution and for seismic hazard analysis.

JOINT TOMOGRAPHY OF FIRST ARRIVAL AND REFLECTION WAVES AND ITS APPLICATION ON SUBSURFACE STRUCTURE DETECTION IN THE HUHHOT-BAOTOU BASIN

PAN Ji-shun, ZHANG Yun-tao, WANG Fu-yun, FENG Shao-ying, JI Ji-fa, QI Hui, TANG Yi-fan, WANG Xin-yu

2026, 48(2):  582-596.  DOI: 10.3969/j.issn.0253-4967.20240028

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This study utilizes deep seismic reflection data from the Hohhot-Baotou Basin and applies a joint tomographic imaging approach that integrates first-arrival and reflection waves to derive a high-resolution P-wave velocity structure of the shallow subsurface. The primary objective is to elucidate the geological structural characteristics of the basin, with particular emphasis on fault activity, deep fault relationships, and fine-scale lithospheric architecture. The results are of substantial scientific and practical significance for mineral resource assessment, geological exploration planning, and earthquake hazard mitigation.
The joint tomographic imaging framework employs the Fast Sweeping Method(FSM) for forward modeling, which is based on a grid equation solver and effectively circumvents the nonlinear issues of conventional rays in complex media. In the inversion stage, a memory-efficient quasi-Newton method(L-BFGS)combined with adjoint techniques is adopted. By approximating the Hessian matrix, this approach substantially reduces computational cost and is well suited for large-scale tomographic problems. The resulting velocity models reveal pronounced lateral velocity contrasts at distances of approximately 45km and 53km, extending downward to the base of the basin.
The imaging results provide a detailed depiction of the shallow P-wave velocity structure of the Hohhot-Baotou Basin. The identified velocity contrasts at 45km and 53km correlate closely with the Ordos North Marginal fault zone, thereby supporting the reliability of the tomographic results. Further validation is achieved through integration with regional geological information, previous studies, and Bouguer gravity anomaly data. The joint inversion of first-arrival and reflection waves preserves stratigraphic continuity while restoring intralayer velocity variations, resulting in improved imaging resolution and reduced noise and inversion artifacts.
Laterally, clear stratigraphic layering and refined velocity variations are observed between 28.92km and 45km. Vertically, zones of abrupt velocity change extend to the base of the Paleogene strata(T_E, approximately 2km depth), which is disrupted by fault F₆ and approaches the T_g interface. This configuration corresponds well with the fault-controlled stratigraphic block defined by Faults F6 and the Ordos North Marginal Fault(F₇). These findings enhance the understanding of the basin’s subsurface architecture and provide new constraints on regional crustal evolution.
In conclusion, the joint tomographic imaging of first-arrival and reflection waves successfully resolves the shallow P-wave velocity structure of the Hohhot-Baotou Basin, offering a robust basis for interpreting its tectonic framework and fault systems. By combining the high-frequency sensitivity of first-arrival waves with the high-energy characteristics of reflection waves, the method achieves higher-resolution subsurface imaging, facilitates the identification of small-scale geological bodies, suppresses seismic artifacts, and improves overall imaging quality. The velocity anomalies associated with the Ordos North Marginal fault zone further highlight the close relationship between subsurface velocity structure, regional tectonics, and seismic activity. Overall, this study advances the understanding of the basin’s subsurface structure and provides valuable data support for future crustal evolution research, geological exploration, and seismic hazard assessment