Table of Content

20 October 2025, Volume 47 Issue 5

Previous Issue   

CHARACTERIZATION OF ELECTRON SPIN RESONANCE DATING SIGNAL AT g=2.002 3 IN CONTINENTAL BARITE

ZHAO Lin, WEI Chuan-yi, YIN Gong-ming, JI Hao, LIU Chun-ru, YANG Gui-fang, XU Xing-shuo

2025, 47(5):  1307-1325.  DOI: 10.3969/j.issn.0253-4967.2025.05.20240033

Asbtract ( )   HTML ( )   PDF (5686KB) ( )  

Figures and Tables | References | Related Articles | Metrics

Barite, a non-metallic mineral primarily composed of barium sulfate(BaSO₄), is widely distributed in nature. Its electron spin resonance(ESR) signal( {\mathrm{SO}}_3^{-}) has been used to date geological events such as hydrothermal activity and tectonic movements. In recent years, ESR dating of barite has been extensively applied in seafloor hydrothermal systems; however, studies on continental barite remain limited. This gap highlights the need for fundamental research into the properties of ESR signals in continental settings, providing a basis for assessing the feasibility and reliability of ESR dating.

This study investigates sedimentary barite(XBD-B1, LSJ-B1) and fault-related barite(LJF-B1) from continental environments, focusing on the ESR signal at g=2.002 3. Key properties examined include microwave saturation power, photosensitivity, thermal stability, and signal saturation. Equivalent doses were calculated and compared with results obtained from the g=1.99${9}^{}$95 signal. The main findings are as follows:

(1)The microwave saturation power of the ESR signal(g=2.002 3) in continental barite ranges from 0.2 to 1mW, with 0.1mW identified as the optimal power for accurate signal measurement.

(2)The ESR signal(g=2.002 3) can be fully annealed after heating at 380℃ for 15 minutes. Its thermal decay follows a second-order kinetic model, with a thermal lifetime at room temperature(20℃) of at least 10⁶ years. Under irradiation of ~1${0}^{}$0000Gy, both g=1.99${9}^{}$95 and g=2.002 3 signals remain unsaturated, suggesting high saturation doses. Considering thermal stability, signal saturation, and a 10% decay allowance, the maximum measurable ages for the ESR signals are 152Ma(XBD-B1), 72Ma(LSJ-B1), and 1.5Ma(LJF-B1).

(3)For fault-related barite, the thermal lifetimes and equivalent doses obtained at g=1.99${9}^{}$95 and g=2.002 3 are consistent within error, indicating that results from these two measurement positions can complement and validate each other. Integrating both enhances dating reliability.

(4)The ESR signal at g=2.002 3 in continental barite exhibits photosensitivity similar to Al centers in quartz, with partial bleaching possible. Thus, fresh, unexposed samples should be collected, stored in darkness, and sealed. If only sun-exposed samples are available, non-bleachable signal components should be extracted for dating.

In summary, the ESR signal of barite(g=2.002 3) is suitable for dating the crystallization age of fault barite since the middle to late Early-Pleistocene, demonstrating notable reliability. This study identifies barite as a promising material for ESR dating of bedrock fault activity and as a robust absolute dating method for constraining the depositional ages of barite deposits.

ANALYSIS OF CRUSTAL STRUCTURE AND STRONG EARTH-QUAKE ACTIVITY OF THE IRANIAN PLATEAU BASED ON GRAVITY INVERSION IN SPHERICAL COORDINATE SYSTEM

DAI Meng-xue, XU Rong-rong, ZHAO Fei-yu, PEI Jun-ling

2025, 47(5):  1326-1342.  DOI: 10.3969/j.issn.0253-4967.2025.05.20250096

Asbtract ( )   HTML ( )   PDF (8134KB) ( )  

Figures and Tables | References | Related Articles | Metrics

The multi-stage tectonic evolution of the Iranian Plateau, as recorded in its deep lithospheric structure, provides a comprehensive geological archive of the complete transition from oceanic subduction to continental collision. This unique geological setting constitutes an ideal natural laboratory for investigating the geodynamic processes associated with incipient continental collision and plateau uplift mechanisms. The long-term convergence of the Arabian-Eurasian plate has led to the development of intricate tectonic deformation features, accompanied by significant seismic activity. Consequently, the deep crustal structure of this region, particularly the morphology of the Moho discontinuity, provides critical constraints for understanding the dynamics of continental collision, the seismogenic environment, and the processes of lithospheric evolution. Although various geophysical methods have been employed to investigate the deep structure of the Iranian Plateau in recent years, significant uncertainties remain due to the sparse distribution of seismic stations and the oversimplification of model assumptions.

To analyze the crustal structure and characteristics of strong seismic activity of the Iranian Plateau, this study calculated the Moho depth distribution using a fast nonlinear gravity inversion method in a spherical coordinate system, based on the GOCO06 static gravity field model, the CRUST1.0 model, as well as topographic and seismic data. The results demonstrate that the variable-density nonlinear gravity inversion method in a spherical coordinate system exhibits strong adaptability and effectiveness within the complex tectonic setting of the Iranian Plateau. The Moho depth distribution derived from this method shows a strong spatial correspondence with geomorphological features, tectonic structures, and seismic responses. Compared to the traditional Parker-Oldenburg frequency domain method, this method operates in the spatial domain, thereby avoiding boundary effects and truncation errors associated with the fast Fourier transform and improving the stability and physical interpretation of gravity inversion in large areas.

In addition, the Moho depth of the Iranian Plateau exhibits significant regional heterogeneity and lateral variability. The Zagros Fold-and-Thrust Belt and its adjacent regions exhibit the most profound Moho depth, generally exceeding 55km and locally reaching 60~65km, highlighting pronounced crustal thickening. In contrast, the blocks within central Iran show relatively gentle Moho depths(35~50km) and a more homogeneous crustal structure. The Makran subduction zone and the coastal areas along the Gulf of Oman have shallower Moho depths, mostly below 35km and locally less than 30km, suggesting a thinner crust. The South Caspian Basin exhibits Moho depths of 30~35km, demonstrating characteristic features of a typical thin-crust basin.

Furthermore, seismic activity analysis indicates that earthquakes with a magnitude of more than 5.0 are predominantly concentrated in regions characterized by steep Moho depth gradients and positive isostatic gravity anomalies. Areas with intense tectonic stress and insufficient isostatic compensation, such as the Zagros Fold-and-Thrust Belt, the Sanandaj-Sirjan Zone, and the Kopet Dag Mountains, are high-seismicity zones, where crustal stress is concentrated and seismic sources are densely clustered. Overall, the central region of Iran exhibits higher lithospheric rigidity, a stable crustal structure, and relatively low seismic activity levels.

This study demonstrates the effectiveness and applicability of the spherical-coordinate-based variable-density nonlinear gravity inversion method for investigating deep structures in geologically complex regions. The inversion results reveal a clear spatial correlation between the crustal structure and strong seismic activity on the Iranian Plateau, providing new geophysical evidence for understanding the region's deep tectonic framework and seismogenic environment. In addition, the results of this study indicate that this method not only effectively characterizes deep structural features under complex geological settings but also offers valuable insights into the interpretation of seismotectonic frameworks. Consequently, it provides a scientific basis for assessing regional crustal stability and informing strategies for earthquake hazard prevention and mitigation.

THREE-DIMENSIONAL WAVE VELOCITY STRUCTURE AND SEISMOGENIC STRUCTURE FOR THE JIANGDU EARTHQUAKE SWARM IN JIANGSU

LI Ting-ting, MIAO Fa-jun, SUN Ye-jun, FAN Wen-hua, GONG Jie, GU Qin-ping, DU Hang, SUN Xiao-hang, ZHANG Cen, LI Zi-ye

2025, 47(5):  1343-1363.  DOI: 10.3969/j.issn.0253-4967.2025.05.20240114

Asbtract ( )   HTML ( )   PDF (9407KB) ( )  

Figures and Tables | References | Related Articles | Metrics

Since April 23, 2023, multiple earthquakes have occurred in Jiangdu, Jiangsu Province. On April 27, 2023, at 09:39 local time, an M3.1 event struck the region, followed by a series of seismic activities that constituted the Jiangdu earthquake swarm(hereafter referred to as “Jiangdu swarm 1”). Seismicity gradually diminished by June 22, 2023. On May 28, 2024, renewed seismic activity was observed in the same area, forming another swarm(hereafter “Jiangdu swarm 2”). On July 8, 2024, at 16:07 local time, an M3.6 earthquake occurred, after which activity again subsided by July 16, 2024. To investigate the velocity structure, seismotectonic setting, and possible relationship between these two swarms, both sequences were analyzed collectively(hereafter referred to as the Jiangdu earthquake swarm).

In this study, PhaseNet, a deep learning-based phase detection method, was employed to detect earthquakes in the epicentral area. The HypoDD algorithm was then used for precise relocation, producing a high-resolution catalog of the Jiangdu swarm. Additionally, seismic reports from January 2009 to July 2024 covering Jiangsu and adjacent provinces were compiled. Using the TomoDD double-difference tomography method, we inverted the three-dimensional velocity structure of V_P, V_S, and Poisson's ratio in the epicentral area. To constrain the seismogenic fault properties, focal mechanism solutions for seven M_L≥3.0 earthquakes were obtained with the HASH algorithm. Integrating precise locations, 3D velocity structures, and focal mechanisms, we identified the seismogenic faults of the Jiangdu swarm and analyzed its seismotectonic environment.

The results show that earthquakes in the Jiangdu swarms exhibit two predominant alignments, trending NNW and NNE, with focal depths concentrated between 7~16km. For Jiangdu swarm 1, focal mechanisms of four earthquakes indicate a NW-striking plane I, consistent with the NNW alignment of the relocated sequence. This plane is interpreted as the causative fault, which is a left-lateral strike-slip structure with minor normal faulting. For Jiangdu swarm 2, focal mechanisms of three earthquakes reveal a NE-striking plane I, consistent with the NNE alignment, and interpreted as a left-lateral strike-slip fault with a minor reverse component. Overall, the seven focal mechanism solutions show good agreement with the relocation results, indicating predominantly sinistral strike-slip motion.

Near the epicentral area, remarkable velocity contrasts are observed, with the Chenjiapu-Xiaohai Fault exerting a significant segmentation effect. The NW side of the fault is marked by low velocity and low Poisson's ratio anomalies, while the SE side displays increasing high-velocity anomalies with depth. Strong stratification of velocity and Poisson's ratio is also evident. The Jiangdu swarm is situated in a low-V_P, low-V_S, and low-Poisson's ratio anomaly zone, where the drop in P-wave velocity is more pronounced than in S-wave velocity, suggesting no involvement of fluids during the sequence. The low Poisson's ratio and narrow fault zone indicate that brittle fracture of rock strata was the dominant mechanism.

The Jiangdu source region is rich in shale oil and gas. The abundant shale gas is adsorbed in the pores and fractures of the rock formation. Long-term extraction and hydraulic fracturing enlarge rock fractures, reducing the effective elastic modulus and lowering seismic wave velocities. Based on precise relocation, seismogenic fault geometry, and crustal velocity structures, we infer that the two seismogenic faults of the Jiangdu swarm are likely subsidiary branches of the Chenjiapu-Xiaohai Fault. One is a concealed left-lateral strike-slip fault trending SSE-NNW, and the other is a concealed left-lateral strike-slip with thrust component trending SSW-NNE. The brittle failure of the hard rock strata directly triggered the Jiangdu swarms, representing two concentrated episodes of stress release. These findings provide new insights into the seismogenic environment and mechanisms of earthquake swarms in the Jiangdu region.

Research paper

THE S-WAVE VELOCITY STRUCTURE OF THE TENGCHONG-BAOSHAN REGION FROM JOINT INVERSION OF RECEIVER FUNCTION AND RAYLEIGH WAVE PHASE VELOCITY

ZHANG Tian-ji, QIN Min, DANG Wen-jie, JIN Ming-pei, LI Feng-ying, YANG Li-wei

2025, 47(5):  1364-1381.  DOI: 10.3969/j.issn.0253-4967.2025.05.20240111

Asbtract ( )   HTML ( )   PDF (10397KB) ( )  

Figures and Tables | References | Related Articles | Metrics

Located in the Tengchong-Baoshan region, the Tengchong volcanic group represents one of the youngest intraplate volcanic systems in mainland China. The area is characterized by frequent seismic activity, numerous hot springs, hydrothermal eruptions, and the potential for future volcanic events. Owing to its unique geographical location and complex geological setting, the Tengchong-Baoshan region has long been a hot point of scientific research. However, controversies remain regarding the distribution and sources of magma in the volcanic area, largely due to differences in datasets and the non-uniqueness of inversion methods. Imaging the S-wave velocity structure of the crust and uppermost mantle is therefore essential for advancing our understanding of magmatic and seismic processes in the Tengchong volcanic field. Yet, the relatively low spatial resolution of large-scale models has limited the ability to resolve fine-scale structural features. Previous studies in this region were further constrained by the sparse and uneven distribution of seismic stations.

To address these limitations, we analyzed teleseismic waveform data recorded by 76 seismological stations across the Tengchong-Baoshan region. P-wave receiver functions were extracted using a time-domain iterative deconvolution technique. Employing a two-step joint inversion approach that combines receiver functions with Rayleigh wave phase velocity, supplemented by a bootstrap resampling procedure, we derived a three-dimensional S-wave velocity model of the crust and uppermost mantle down to ~150km depth. A comparative analysis with prior models was conducted, followed by an integrated interpretation using results from geothermal, electromagnetic, helium isotope, and seismic velocity ratio studies, in order to investigate the distribution and source of magma in the Tengchong volcanic area.

Our results show that, while the overall velocity structure is broadly consistent with previous studies, the low-velocity regions in our model exhibit the lower absolute velocities and more detail. Three prominent low-velocity zones (V_S<3.4km/s) are identified in the crust along the Tengchong volcanic belt from north to south, interpreted as partially molten magma chambers. These zones, designated LV1, LV2, and LV3, occur at depths of 10~30km, 10~30km, and 10~26km, respectively. LV1 and LV2 are situated along the Tengchong Fault with the Nujiang Fault forming the eastern boundary, whereas LV3 lies between the Nujiang and Longling-Ruili faults. These fault systems-the Tengchong, Longchuanjiang, Nujiang, and Longling-Ruili faults-play a key role in controlling magmatic activity. In addition, a low-velocity layer is observed at 10~30km depth within the Baoshan block to the east of the Nujiang Fault, though its velocities are higher than those beneath the Tengchong volcanic area. We infer that this anomaly may reflect the influence of high-temperature, volatile-rich magma migrating from the deeper magma reservoir beneath Tengchong, as well as contributions from fluids and fault-related fissures.

Since 1900, earthquakes of M_S≥5.0 have predominantly occurred in transitional zones between high- and low-velocity regions. Notably, the epicenters of the 1976 M_S7.4 and M_S7.3 Longling earthquakes are underlain by a low-velocity layer, suggesting that magma and fluids may have contributed to rupture initiation in fault-fractured regions. In the upper mantle(60~120km depth), widespread low-velocity anomalies are observed beneath the Tengchong-Baoshan area, which extend upward and connect to the crustal low-velocity zones beneath Tengchong. This large-scale low-velocity mantle anomaly likely serves as a magma source feeding the crustal magma chambers. Furthermore, asthenospheric upwelling and associated lithospheric thinning provide the geodynamic mechanism driving magmatism and volcanism in the Tengchong region.

COSEISMIC DEFORMATION FIELD AND SLIP MODELS OF JANUARY 23, 2024 M_S7.1 WUSHI EARTHQUAKE, XINJIANG, CHINA

YANG Jian-wen, JIN Ming-pei, LI Qing, LI Zhen-ling, YE Beng, LI Jian, ZHANG Ying-feng

2025, 47(5):  1382-1395.  DOI: 10.3969/j.issn.0253-4967.2025.05.20240027

Asbtract ( )   HTML ( )   PDF (4712KB) ( )  

Figures and Tables | References | Related Articles | Metrics

According to the official determination of the China Seismic Network, at 02:09 on January 23, 2024, a magnitude 7.1 earthquake occurred in Wushi County(41.26°N, 78.63°E), Aksu Prefecture, Xinjiang, with a focal depth of 22km. The earthquake occurred at the junction of the southern Tianshan Mountains and the Tarim Basin, located between the Keping foreland thrust belt and the Kuqa foreland thrust belt, and was caused by the northward extrusion of the Eurasian Plate by the Indian Plate. The Wushi earthquake is the largest earthquake in the Tianshan seismic belt since the Suusamyr M_S7.3 earthquake in Kyrgyzstan in 1992. It caused casualties and varying degrees of damage to buildings and infrastructure in Wushi and Akqi counties.

As a shallow-source thrust earthquake, the Wushi event has a high efficiency of seismic energy radiation, leading to stronger ground vibrations and building damage than other earthquakes of similar magnitude. In addition, the seismogenic faults of intracontinental thrust earthquakes rarely rupture the surface or produce only short surface rupture zones, complicating studies of the fault structure and rupture mechanism. Further research on the source rupture process is therefore necessary. The earthquake also alters the surrounding stress field and may affect nearby fault activity. Coulomb stress modeling can estimate the relative stress changes and triggering effects in the epicentral region, which is important for understanding seismogenesis and long-term earthquake prediction.

In this paper, using Sentinel-1A ascending and descending satellite imagery, the co-seismic deformation field of the Wushi earthquake is derived. Constrained by ascending and descending orbit deformation data, independent and joint inversions of the earthquake's source slip model are performed to investigate co-seismic deformation and rupture characteristics. Furthermore, Coulomb stress variations at different depths induced by coseismic dislocation are calculated, and relative stress changes as well as the triggering effects on major faults near the epicenter are evaluated. The main findings are as follows:

(1)Based on the coseismic deformation field of the Wushi earthquake obtained using the D-InSAR “two-track method”, the results show clear interference fringes in both ascending and descending orbits. The long axis is distributed roughly along the NE-SW direction, including two deformation zones, though the NW block exhibits stronger deformation than the SE block. The maximum LOS deformation of the ascending orbit is about 0.77m, while that of the descending orbit is about 0.48m. The positive and negative deformation within the same block are consistent between ascending and descending tracks. Combined with the imaging geometry, these results suggest that the deformation is dominated by vertical displacement, consistent with the typical features of thrust-type seismic deformation.

(2)Constrained by the coseismic deformation data of both orbits and applying the SDM layered model, the independently and jointly inverted source slip models indicate upward rupture propagation along the fault from the initial rupture point. The fault dislocation is characterized mainly as left-lateral reverse faulting. The main rupture zone extends about 45km, with primary slip concentrated along a fault plane striking between about 27~72km and dipping between about 2~25km. The largest rupture zone is biased toward the SW of the epicenter, and the local rupture on the SW side(near strike about 55km)may have broken the surface. Parameters of the slip model are broadly consistent. The moment magnitude derived from ascending and descending data is about M_W7.1, and the maximum slip is about 2.1m, located on the fault plane(41.25°N, 78.59°E) at about 10.3km depth.

(3)The coseismic Coulomb stress results reveal significant stress changes near the epicentral region. Stress loading is pronounced on the northeastern section of the Koksal Fault, the central and northeastern sections of the Tuoshigan Fault, the central and southwestern sections of the Maidan-Shayilam Fault, and the central and southwestern sections of the Wensubei(Kuqi)Fault near the epicenter. This indicates that the regional seismic risk requires close attention.

RESEARCH ON INTENSITY EVALUATION OF XINJIANG THRUST-TYPE EARTHQUAKES BASED ON INSAR COSEISMIC DEFORMATION FIELD

WANG Shun, YAO Yuan, GAO Ming-xing

2025, 47(5):  1396-1415.  DOI: 10.3969/j.issn.0253-4967.2025.05.20240005

Asbtract ( )   HTML ( )   PDF (9779KB) ( )  

Figures and Tables | References | Related Articles | Metrics

Accurately and rapidly assessing seismic intensity following an earthquake is essential for effective emergency response, targeted disaster relief, and scientifically informed post-disaster reconstruction. This need is particularly acute in seismically active and often remote regions such as Xinjiang, China. Situated in the interior of Eurasia, Xinjiang is characterized by complex geological structures, where compressional forces from the north and south dominate tectonic activity across the Tianshan, Pamir, and other mountain ranges. Such tectonic environment produces frequent strong earthquakes, most of which are thrust events. Compared with strike-slip and normal faulting, thrust earthquakes are associated with shallow fault dips and may be linked to near-horizontal detachments. Fault displacement is typically absorbed by distributed fold deformation along the fault and attenuates rapidly, often producing little or no surface rupture. These characteristics complicate the interpretation of coseismic rupture processes and the spatial distribution of earthquake damage. Combined with the region's rugged terrain and sparse infrastructure, thrust earthquakes pose a serious threat to lives and property in Xinjiang.

High-quality, rapid post-earthquake intensity assessments are therefore critical to reducing earthquake impacts. Intensity maps are a primary basis for emergency rescue, recovery, and reconstruction. Traditional field investigations of intensity, however, require considerable human and material resources, pose safety risks to investigators, and are influenced by subjective judgment in assessing building damage. Additionally, since the widespread implementation of seismic-resistant housing projects in Xinjiang after 2003, the uniformity of residential building types has further limited the effectiveness of on-site evaluations.

With the advancement of remote sensing technology, Interferometric Synthetic Aperture Radar(InSAR)has emerged as a powerful tool for surface deformation monitoring and disaster assessment. Its all-weather, all-day imaging capabilities, unaffected by conditions such as rain or snow, make Differential InSAR(D-InSAR) an important technique for monitoring earthquake-induced surface deformation. To explore the relationship between seismic intensity and coseismic deformation and to address the challenge of rapid thrust-earthquake intensity assessment in Xinjiang, this study investigates three thrust earthquakes: the 2015 Pishan M_S6.5, the 2017 Jinghe M_S6.6, and the 2020 Jiashi M_S6.4 events.

Comparisons between InSAR-derived coseismic deformation fields and field-surveyed seismic intensities reveal a strong correlation. In population centers, deformation of 0.5~1.5cm corresponds to intensity Ⅶ, while deformation exceeding 1.5cm corresponds to intensity Ⅷ. Using these relationships, a linear regression model was developed between deformation and intensity levels. Furthermore, based on both a single-factor evaluation(coseismic deformation) and a multi-factor framework that integrates InSAR deformation, coseismic stress changes, population density, source distance, and sedimentary thickness, intensity assessments were performed using the AHP-entropy weight method.

The results indicate that:

(1)D-InSAR can rapidly monitor large-scale surface deformation after an earthquake, providing comprehensive and accurate coseismic deformation patterns. Unlike traditional methods dependent on sparse seismic station data, InSAR directly reflects the spatial distribution of regional deformation and supplies valuable geological background information for seismic intensity evaluation, especially in regions with limited building-type diversity or seismic station coverage.

(2)There is a clear relationship between seismic intensity and coseismic deformation. Mapping deformation fields onto intensity scales allows for the rapid estimation of earthquake intensity levels. Using historical deformation-intensity relationships enhances early evaluations of both the intensity grade and its spatial extent in future earthquakes.

(3)Multi-factor evaluation combining InSAR deformation with stress change, population density, focal distance, and sediment thickness improves the reliability of seismic intensity assessments compared to single-factor approaches. This method integrates both natural factors(e.g. geology, topography) and socioeconomic factors(e.g. population distribution), thereby capturing the complexity and diversity of earthquake impacts.

Overall, the AHP-entropy weight-based multi-factor evaluation framework demonstrates strong potential for application in earthquake risk assessment, disaster prevention and mitigation. At the same time, this study discusses the limitations of applying InSAR for thrust-earthquake intensity evaluation, offering insights for future research. The findings support more accurate and rapid post-earthquake assessments and highlight the value of InSAR technology in evaluating strong earthquake intensity in Xinjiang.

Research paper

RESEARCH OF THE INSAR COSEISMIC DEFORMATION AND INTERSEISMIC FAULTING MECHANISM OF THE 2023 JISHISHAN M_W6.1 EARTHQUAKE

YANG Ying-hui, XIE Li-yuan, XU Qiang, WANG Yun-sheng, ZHAO Jing-jing, WANG Dao-ming, CHEN Qiang

2025, 47(5):  1416-1437.  DOI: 10.3969/j.issn.0253-4967.2025.05.20240016

Asbtract ( )   HTML ( )   PDF (13585KB) ( )  

Figures and Tables | References | Related Articles | Metrics

On December 18, 2023, a M_W6.1 earthquake struck Jishishan County in the Linxia Hui Autonomous Prefecture of Gansu Province. The strong ground shaking caused widespread building collapse and significant casualties, underscoring the severe societal impact of seismic events in this region. Because the epicenter is located within a tectonically active area characterized by frequent crustal deformation and complex fault interactions, clarifying the seismogenic mechanisms is essential for understanding regional seismic hazards.

To investigate the fault structures and rupture processes associated with the event, we applied time-series Interferometric Synthetic Aperture Radar(InSAR) analysis to descending-orbit deformation rate fields across the epicentral region, integrating these results with pre-earthquake Global Positioning System(GPS)observations. Constrained by these geodetic datasets, fault coupling behavior was estimated using a Markov Chain Monte Carlo(MCMC) inversion. The preferred fault model suggests an interseismic slip rate of ~1.9mm/a and a fault locking depth of ~11km. These results are consistent with geological field observations, providing validation for the geodetic modeling. The inversion also reveals a crustal shortening rate of ~1.0mm/a across the Lajishan Fault, highlighting the potential for reverse-faulting earthquakes in the region.

For coseismic deformation, we employed both ascending- and descending-track Sentinel-1 SAR acquisitions to extract high-resolution displacement fields. Using these InSAR constraints, the optimal fault geometry was determined from posterior probability density distributions of fault parameters. A linear inversion incorporating Laplacian smoothing was then applied to estimate the coseismic fault slip distribution. The results show that the earthquake was generated by low-angle thrust faulting, with the seismogenic fault plane striking 131° and dipping southwest at 23.3°. A prominent high-slip asperity was identified at depths of 3~8km, with a maximum slip of 0.35m at ~6.8km depth, consistent with regional reverse-faulting characteristics. Model residuals are mostly <5mm, concentrated northwest of the epicenter, where decorrelation is relatively strong.

Coulomb stress changes were further calculated based on the inverted slip model. The analysis shows that coseismic slip significantly modified stress distributions, with most aftershocks occurring in regions of increased Coulomb stress. This demonstrates the critical role of post-seismic stress redistribution in controlling aftershock occurrence and highlights the importance of stress analysis for seismic hazard assessment.

Beyond the main rupture, the earthquake also affected adjacent fault systems, including the middle-northern segment of the Jishishan Fault and the central segment of the northern margin fault of the Western Qinling Mountains. These segments now experience elevated Coulomb stress, implying heightened seismic risk. This finding underscores the necessity of continuous geodetic monitoring and regular seismic hazard reassessment in the region.

By integrating GPS and InSAR observations, this study provides a comprehensive and quantitative analysis of coseismic deformation and fault motion during the Jishishan earthquake. The results not only enhance understanding of earthquake dynamics but also contribute to improved seismic hazard assessment and long-term risk mitigation in tectonically active regions.

APPARENT RESITIVITY VARIATION AT KEPING SEISMIC STATION BEFORE THE MODERATE EARTHQUAKES IN SOUTH TIANSHAN AREA, XINJIANG

LI Xin-yan, ZENG Xian-wei, LI Meng-ya, WEI Ding-jun, CUI Jin

2025, 47(5):  1438-1455.  DOI: 10.3969/j.issn.0253-4967.2025.05.20240040

Asbtract ( )   HTML ( )   PDF (5554KB) ( )  

Figures and Tables | References | Related Articles | Metrics

The south Tianshan area is one of the main moderate earthquake preparation areas in China. Apparent resistivity monitoring has been demonstrated as an effective approach for intermediate-to short-term earthquake forecasting, as validated by numerous documented seismic cases. To improve regional seismic monitoring networks and observe the spatial and temporal variations in the apparent resistivity of shallow crust caused by the seismic preparation process, the Keping apparent resistivity continuous observation station was built in 2013 at the junction of the Southern Tianshan Mountains and Tarim Basin by the Earthquake Agency of the Xinjiang Uygur Autonomous Region. The observation system at this station has operated stably, with consistent data variations and high accuracy in both measurement channels (the NS channel and EW channel) over the past ten years. According to the sensitivity theory, our analysis demonstrates that the annual variation pattern (summer decrease/winter increase) observed in both measurement channels at Keping station is consistent with negative surface influence coefficients. Eleven earthquakes (M_S≥5.0) occurred within 400km of the Keping station after 2018(aftershocks and foreshocks excluded), including two M6 and one M7 events. For qualitative analysis of the relationship between the seismogenic processes of earthquakes and the apparent resistivity variations observed at the Keping station, soil temperature and soil water content at different depths from the ERA5 assimilation datasets (ECMWF Reanalysis v5) were used to analyze the different hydrothermal condition effects on annual variation and to identify the anomalies from the background value by the interquartile range method. Our analysis of the fault virtual fault dislocation model and apparent resistivity anisotropy revealed four apparent resistivity decreases at Keping station, temporally correlated with the nucleation phases of seven M5.0+ earthquakes within a 250km radius during 2018-2024. Specifically, the rock experiment demonstrated that the resistivity of water-bearing rocks exhibits decreased variation under compressive stress. This finding is consistent with the decline changes at Keping Station, located within a compressive stress enhancement zone associated with the seismogenic process of the Wushi M_S7.1 earthquake in 2024 and the Jiashi M_S6.4 earthquake in 2020, respectively, revealed by the fault virtual fault dislocation model. Moreover, the anisotropic variations in apparent resistivity, as evidenced by experimental results, theoretical modeling, and seismic case studies, demonstrate a consistent pattern: the observed variation amplitude is maximized perpendicular to the direction of maximum principal compressive stress(σ₁), minimized parallel to σ₁, and exhibits intermediate values at oblique orientations. The focal mechanism solutions reveal that the principal compressive stress axes (P-axes) of all seven earthquakes are approximately N-S oriented, consistent with the deformation characteristics of the Keping thrust tectonic system. Furthermore, the angle between the EW channel and the principal compressive stress direction is consistently larger than that of the NS channel. Correspondingly, apparent resistivity variations in the EW direction at the Kalpin Station were consistently more pronounced than those in the NS direction, exhibiting clear anisotropic behavior. Therefore, based on the fault virtual fault dislocation model, rock physics experiment, and apparent resistivity anisotropic variation, the apparent resistivity anomalies of Keping station were proved to be associated with the seismogenic process of seven moderate earthquakes. Furthermore, simulation was carried out with reference of the electrical structure of the EW direction in Keping station, the underground medium resistivity of the EW direction in Keping station decreases by 10%, the change of apparent resistivity compared with the background value of 1.05%can be recognized, and the upper interface of the apparent resistivity change needs to rise to at least 243m above ground. Based on the electrode spacing configuration(current electrode spacing: 1 000m), this depth can be fully detected by the existing observation devices in the Keping station. This study provides both theoretical and practical support for identifying georesistivity anomalies at the Keping Station and enhancing regional seismic monitoring capabilities.

PRELIMINARY STUDY ON THE ABNORMAL CHARACTERISTICS OF INDUCED CURRENT DISTRIBUTION IN DAILY VARIATION OF GEOMAGNETIC FIELD DURING THE YANGBI M_S6.4 EARTH-QUAKE BASED ON ABNORMAL POSITIONING OF VERTICAL COMPONENT OF STATION GEOMAGNETIC FIELD

SUN Jun-song, FENG Zhi-sheng, WU Ying-yan, LI Hong-yu, YANG Jie

2025, 47(5):  1456-1476.  DOI: 10.3969/j.issn.0253-4967.2025.05.20240042

Asbtract ( )   HTML ( )   PDF (6135KB) ( )  

Figures and Tables | References | Related Articles | Metrics

On April 24, 2021, a spatially correlated anomaly in geomagnetic diurnal variation was observed in the Sichuan-Yunnan region of China. Subsequently, on May 21, 2021, the M_S6.4 Yangbi earthquake occurred. The earthquake prediction research department suggests that this anomaly is related to the seismic event. The spatial correlation method for geomagnetic daily variation is a seismic geomagnetic field analysis technique with the same physical basis as the geomagnetic low point displacement method, geomagnetic loading-unloading response ratio method, and geomagnetic daily ratio method. This study uses spatially correlated geomagnetic anomaly data to assume the existence of subsurface currents, and inverts their possible distribution and shape using the Biot-Savart law and Monte Carlo modeling. An underground current distribution model potentially generating the observed spatial geomagnetic anomalies is established to explore its quantitative relationship with the Yangbi earthquake.

Based on the principle of spatial correlation anomalies, a “current in situ recurrence” phenomenon was detected in the region six months before the earthquake, with anomalies extending over 500km, meeting established anomaly criteria. Observation data from April 24 and 28, 2021, were used for underground current calculations. Stations near the epicenter were prioritized, supplemented by data from Sichuan, Yunnan, and Chongqing to account for the sparse station distribution west of the epicenter.

Using the Biot-Savart law and the Monte Carlo method, the three-dimensional distribution of induced currents-the abnormal source of geomagnetic phase-reversal anomalies observed within one month before the Yangbi earthquake-was inverted and located. Their spatiotemporal distribution patterns and their relationship with regional structures and seismic activity were examined. Based on these current distributions, the structural features of the middle and lower crust in the Yangbi earthquake area were outlined. It is inferred that during the short-term and imminent stage of the earthquake, long-period induced currents repeatedly appeared in the upper, middle, and lower crust within structural zones, suggesting that these sites repeatedly experienced fluid activity in the run-up to the event.

According to current-source localization of the abnormal phase distortion in the daily variation of the geomagnetic vertical component prior to the earthquake, mountain-shaped induced current surfaces repeatedly emerged in the source region and its vicinity, spanning the upper, middle, and lower crustal levels of the structure during the short-term and imminent stage of the Yangbi M_S6.4 earthquake. The current peaks were located in the middle to upper crust, with their bases at the top of the mantle. These peak-shaped induced currents persisted for several hours and fluctuated over time like ocean waves, rising and falling in succession. The same regions reappeared repeatedly, while areas on both sides of the epicenter alternated in occurrence. The peak-shaped induced current surface represents an electrical structural interface at the top of a transient, peak-shaped, high-conductivity channel. This surface, composed of multiple structural layers, shares the spatiotemporal variability of the underlying conductive channel, which rapidly assembles and disintegrates on an hourly scale. Thus, during the short-term and imminent stage of the Yangbi earthquake, the recurrent long-period induced currents in different crustal levels reflect repeated episodes of fluid activity within the structural framework.

The formation mechanism of the peak-shaped induced current surface is proposed to be linked to electrically isolated, fluid-rich conductors within the high-conductivity belt. Driven by the upwelling of deep mantle-derived thermal fluids, the high-resistance blocks flanking the conductive zone undergo outward arching. As mantle fluids penetrate and interact with fluids in the conductive belt, the previously disconnected fluid-bearing conductors become electrically connected, forming a transient peak-shaped high-conductivity channel. The long-period induced currents distributed along its upper surface constitute the peak-shaped induced current surface. Therefore, the occurrence of such peak-shaped induced currents is likely tied to deep tectonic processes involving fluids.

PRELIMINARY STUDY ON LATE QUATERNARY ACTIVITY OF THE EASTERN SEGMENT OF THE NORTHERN MARGIN FAULT OF THE HAMI BASIN

ZHAO Xue-feng, SHEN Jun, JU Guang-hong, MA Fei-peng, ZHAO Wen-gang, SONG Xu

2025, 47(5):  1477-1493.  DOI: 10.3969/j.issn.0253-4967.2025.05.20240100

Asbtract ( )   HTML ( )   PDF (13946KB) ( )  

Figures and Tables | References | Related Articles | Metrics

The fault on the northern margin of the Hami Basin is in the eastern segment of the Tianshan tectonic belt and is a deep-seated major fault that offsets the Moho discontinuity. The entire fault lies along the southern piedmont of the Barkol Mountains and the Harlik Mountains. In this study, the section of the fault along the southern piedmont of the Harlik Mountains is referred to as the eastern segment. Previous research on this fault has primarily focused on its western segment, where it has created distinct offset landforms on the surface and displaced Holocene strata, indicating activity during the Holocene. In contrast, the eastern segment of the fault is situated in the piedmont zone where the Harlik Mountains meet the Hami Basin. This area is characterized by a thin overburden, predominantly composed of coarse-grained colluvial deposits. These conditions make fault identification challenging and complicate studies of its activity. Previously, few scholars have conducted research on fault activity in this area, leading to divergent understandings regarding the precise location and activity of this fault segment. Therefore, it is necessary to employ new methods and technologies to carry out further investigation.

This study, integrated with engineering requirements, adopted a multi-technique integrated approach with mutual validation to conduct preliminary research on this fault segment. Detailed interpretation of remote sensing imagery from the Shangmiaoergou to Bamudun Reservoir area revealed that the fault has created several scarps on the surface. However, these scarps are only distributed on older geomorphic surfaces, making it uncertain whether the fault has displaced Late Quaternary landforms. Based on remote sensing interpretation and field geological surveys, microtremor surveys were carried out. The inversion results of the microtremor data reveal a significant low-velocity anomaly zone in the shear wave velocity at the location where the fault passes, exhibiting a certain width. This indicates that the fault traverses this area, and it was observed that the fault has a relatively steep dip at depth. The microtremor inversion results successfully revealed the deep structure of the fault and validated the understanding derived from remote sensing interpretation and field investigations. To address whether the fault extends to the surface and the timing of its most recent activity, two trenches were excavated east of the microtremor survey line, and aeolian loess samples were collected for geochronological analysis to study the fault's activity preliminarily. Trench profiles and geochronological results indicate that the fault has been active since the Late Pleistocene and exhibits characteristics of multiple episodes of activity.

Therefore, this study has obtained important evidence regarding the Late Quaternary activity of the eastern segment of the North Margin Fault in the Hami Basin, leading to the following conclusions: 1)Microtremor surveying offers advantages such as strong anti-interference capability, high efficiency, and minimal site constraints. In this study, the microtremor profiles provided the three-dimensional geometry and sectional characteristics of the fault at depth. This comprehensive multi-method approach, with mutual validation, can be highly effective for active fault detection in similar regions. 2)Geomorphological evidence for the fault's Late Quaternary activity includes the offset of the T3 terrace and alluvial fans formed during the Late Pleistocene. Fault movement has produced discontinuously distributed scarp landforms on the surface, with a total height ranging from 11 to 13m. Geochronological results also indicate that the fault has been active since the Holocene. 3)Microtremor profiles indicate a fault fracture zone width of 100m and a dip angle of 60°. Trenches and an adit were excavated on an alluvial fan, where microtremor surveys detected anomalies that exposed multiple fault planes. These fault planes generally dip northward with dip angles ranging from 35° to 54°; the dip angle is steeper at depth and becomes gentler near the surface. The phenomena revealed by the microtremor profiles are consistent with those observed in the trench and adit profiles. Furthermore, the width of the fault fracture zone measured at the adit entrance is 68m. This discrepancy arises because the microtremor-derived fracture zone includes not only the main boundary faults but also adjacent areas with reduced strength. Therefore, comprehensive analysis suggests that the width of the fault fracture zone is approximately 100m. 4)By sieving and testing loess particles within the colluvial deposits, the vertical slip rate since the Holocene is preliminarily estimated to be approximately 0.09mm/a. Integrated with regional geological data, the vertical slip rate since the mid-Late Pleistocene is inferred to be about 0.2~0.3mm/a.