SEISMOLOGY AND GEOLOGY

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STUDY ON SEISMOGENIC TECTONICS OF THE 2025 MYANMAR M_S7.9 EARTHQUAKE

XU Bin-bin, ZHANG Yi-peng, LU Le-jun, TIAN Qing-ying, YANG Xue, WANG Yang, ZHANG Pei-zhen

SEISMOLOGY AND GEOLOGY 2025, 47 (2): 649-670. DOI: 10.3969/j.issn.0253-4967.2025.02.20250089

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According to the China Earthquake Networks Center, an M_S7.9 earthquake(hereafter referred to as the Myanmar earthquake)struck the Mandalay region of Myanmar(21.85°N, 95.95°E)on March 28, 2025, at a focal depth of 30km. The earthquake occurred along the central segment of the Sagaing Fault and was characterized by a right-lateral strike-slip rupture, generating a ~350km-long surface rupture zone with a maximum coseismic horizontal displacement of 6 meters. The event caused extensive damage to buildings and varying degrees of destruction to infrastructure, including roads and bridges.
Situated in a critical tectonic region where the Indian Plate obliquely converges with the Eurasian Plate, the Myanmar earthquake offers valuable insights into plate boundary deformation processes. Detailed analysis of this event enhances our understanding of the deformation mechanisms along the Myanmar plate boundary and provides essential constraints for seismic hazard assessment along the southeastern margin of the Eurasian Plate. This research holds scientific significance for elucidating continental lithospheric deformation in response to oblique plate convergence. The findings contribute to regional early warning strategies and disaster mitigation efforts and offer a valuable reference for seismic risk studies in comparable tectonic settings worldwide.
This study integrates Global Navigation Satellite System(GNSS)data from across the Sagaing Fault region, establishing a comprehensive GNSS velocity field for Myanmar and addressing previous gaps in coverage along the fault’s southern segment. Using multiscale spherical wavelet analysis and GNSS velocity profiles, we examine the deformation characteristics of the region. We calculate the slip rate deficit distribution along the Sagaing Fault and assess postseismic Coulomb stress changes. Combined with historical seismicity data, we investigate the seismogenic structure and stress perturbations in surrounding areas. The key findings are as follows:
(1)The Myanmar M_S7.9 earthquake was a right-lateral strike-slip event along the central Sagaing fault. The region is affected by the northeastward oblique convergence of the Indian Plate and southeastward extrusion of crustal material from the Tibetan plateau, resulting in strong north-south shear and east-west shortening. The Sagaing fault accommodates most of this deformation, with a rapid right-lateral slip rate of approximately 21~22mm/a.
(2)High-resolution GNSS velocity profiles indicate significant fault locking at depths of 15~25km along the Sagaing Fault. The slip rate deficit analysis reveals a high locking ratio across the fault, indicating elevated seismic potential. Notably, the central segment shows lower seismic moment accumulation compared to the northern and southern segments, forming a ~300km-long seismic gap since 1900, capable of generating earthquakes exceeding magnitude 7.5.
(3)Coulomb stress modeling suggests that the earthquake significantly altered the regional stress field. Stress accumulation zones were identified at both ends of the Sagaing fault and in the central Shan Plateau to the east. These regions of increased stress transfer and loading exhibit heightened potential for future large earthquakes, underscoring the need for enhanced seismic monitoring.

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THREE-DIMENSIONAL MODEL OF SEISMOGENIC FAULT AND SEISMIC ENVIRONMENT OF XIZANG DINGRI M_S6.8 EARTHQUAKE OF JANUARY 7, 2025

GUO Zhao-wu, LU Ren-qi, ZHANG Jin-yu, FANG Li-hua, LIU Guan-shen, WU Xi-yan, SUN Xiao, QI Shi-miao

SEISMOLOGY AND GEOLOGY 2025, 47 (3): 671-688. DOI: 10.3969/j.issn.0253-4967.2025.03.20250030

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At 09:05a.m. on January 7, 2025, a magnitude M_S6.8 earthquake struck Dingri County, Xizang, China, resulting in 126 fatalities and a maximum seismic intensity of Ⅸ. Occurring within a seismically active and tectonically complex region, this event drew significant attention from both the scientific community and the public. The epicenter was located near the Dengmecuo Fault, which has been preliminarily identified as the seismogenic fault.

This study utilized publicly available geological survey data, aftershock relocations, and focal mechanism solutions to construct a detailed three-dimensional geometric model of the Dengmecuo Fault. The model was developed using the SKUA-GOCAD 3D modeling platform, enabling a comprehensive analysis of the fault’s geometry. Results reveal pronounced geometric segmentation along the fault plane, with the spatial distribution of these structural features closely correlating with observed seismicity, highlighting the influence of fault geometry on earthquake generation.

The M_S6.8 Dingri earthquake occurred near a prominent structural irregularity on the Dengmecuo Fault, at point P3, where the fault plane bends into an eastward-projecting arc. This three-dimensional structural mutation likely played a role in the nucleation of the event, underscoring the relationship between fault complexity and seismic rupture. The Dengmecuo Fault, situated in the southern Tibetan plateau, is a listric normal fault characterized by a steep upper section and a gentler lower section that terminates within a detachment layer in the upper crust. It does not extend into the deeper lithosphere, indicating that it is part of the region’s shallow normal fault system. The earthquake is interpreted as the release of accumulated stress along this shallow fault structure.

To evaluate post-earthquake stress transfer and seismic hazard, Coulomb stress modeling(Coulomb 3.4)was conducted. The analysis indicates that several regional faults are now in a state of increased Coulomb stress, including the southern segment of the Dengmecuo Fault, the middle segment of the south Xizang detachment system, the southern segment of the Shenzha-Dingjie Fault, the central Yarlung-Zangbo Fault, and the midsection of the Dajiling-Angren-Renbu Fault. These fault segments are identified as potential sites for future seismic activity and merit heightened monitoring.

This study presents a detailed characterization of the three-dimensional geometry of the seismogenic fault responsible for the Dingri M_S6.8 earthquake and offers a preliminary analysis of regional seismogenic structures. The findings provide valuable insights into the tectonic setting of southern Xizang and contribute to the assessment of regional seismic hazard.

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LATE QUATERNARY THROW RATE OF THE SEISMOGENIC FAULT(DENGMECUO FAULT)OF THE 2025 M_S6.8 DINGRI EARTHQUAKE IN SHIGATSE

GAO Yang, WU Zhong-hai, HAN Shuai, TIAN Ting-ting

SEISMOLOGY AND GEOLOGY 2025, 47 (3): 689-706. DOI: 10.3969/j.issn.0253-4967.2025.03.20250034

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On January 7, 2025, a significant earthquake occurred in Dingri, Shigatse, China. Both the China Earthquake Networks Center and the USGS provide focal mechanism solutions indicating that the earthquake was a normal fault event. The epicenter was located in the Dengme Cograben, part of the southern segment of the Dinggye-Xainza rift, with the seismogenic fault identified as the bounding normal fault of the Dengme Cograben, the Dengmecuo fault. The late Quaternary throw rate of this fault is crucial not only for regional seismic hazard assessments but also for understanding the east-west extensional deformation within the Tibetan plateau. Previous studies on the kinematics of the Dengmecuo fault report varying throw rates: (0.28±0.04)mm/a since ~56ka, (0.28±0.04)mm/a since ~50ka, and(0.09±0.03)mm/a since ~95ka. However, the observed maximum coseismic vertical displacement of ~3m during the January 2025 earthquake suggests that the maximum cumulative time for this displacement is approximately 37ka, based on the published late Quaternary throw rate. This is inconsistent with the ~5ka elapsed time since the most recent earthquake on this fault, highlighting the uncertainty in the late Quaternary throw rate and limiting our understanding of strain partitioning and seismic risk in the southern Dinggye-Xainza rift.

To resolve this uncertainty, we combine high-resolution remote sensing image interpretation with field geological and geomorphological surveys to determine the geometric distribution of the Dengmecuo fault. Two study sites were selected along the fault, each featuring clear faulted geomorphology suitable for dating. Low-altitude photogrammetry using an unmanned aerial vehicle(UAV)was combined with optically stimulated luminescence(OSL)and AMS ¹⁴C dating to refine the late Quaternary throw rate of the Dengmecuo fault and assess its seismic hazard.

Our results show that the Dengmecuo fault, which strikes NNW and dips NW or W, is approximately 58km long and can be divided into northern, central, and southern segments. The northern segment is defined by the Laangshuiku area, while the southern boundary is located at the edge of Pum Qu. In the northern study site, vertical offsets of 19.2(+3.5/-2.3)m on the T2 alluvial fan and 8.0(+0.9/-0.7)m on the T1 alluvial fan correspond to formation ages of(28.3±1.4)ka and(12.0±1.5)ka, respectively. By matching the vertical offsets with their respective formation ages, we estimate a throw rate of (0.7±0.1)mm/a. However, the throw rate for the T1 fan is uncertain, as its vertical offset is smaller than the cumulative displacement since its formation. At the southern study site, combining a vertical offset of 5.3(+0.3/-0.5)m with the formation age of the T1 terrace ((9.2±1.0)ka), we calculate a throw rate of(0.6±0.1)mm/a.

Overall, our results indicate late Quaternary throw rates of(0.7±0.1)mm/a since ~28ka and(0.6±0.1)mm/a since the Holocene. Additionally, using the relationship S=D/R_x, where S is the slip rate, D is displacement, and R_x is the recurrence interval, we estimate a slip rate of(0.5±0.1)mm/a based on the average value of the maximum displacement(2.5~3m)of 2025 Dingri M_S6.8(Shigatse)earthquake as 2.75m and the interval of paleoseismic events during the Holocene as(5500±1100)a. This result is consistent with the throw rate of(0.6±0.1)mm/a since the Holocene determined from faulted geomorphic surfaces.

Finally, combining the throw rate of (0.6±0.1)mm/a since the Holocene with the ~5ka elapsed time since the most recent earthquake, we conclude that the Dengmecuo fault had accumulated 2.5~3.5m of coseismic displacement before the 2025 Dingri M_S6.8 earthquake, corresponding to a magnitude of M_W6.9-7.0. These parameters align with the 2025 M_S6.8 earthquake, indicating that the Dengmecuo fault posed a significant seismic risk prior to the event.

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CRUSTAL DEFORMATION CHARACTERISTICS AND PROBABILITY PREDICTION OF STRONG EARTHQUAKE RISK IN XINJIANG AND ITS ADJACENT REGION

CHEN Chang-yun, YIN Hai-quan

SEISMOLOGY AND GEOLOGY 2025, 47 (2): 384-404. DOI: 10.3969/j.issn.0253-4967.2025.02.20240151

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Based on the GNSS velocity field, we analyzed the present-day crustal deformation characteristics of Xinjiang and its adjacent regions before the January 23, 2024, Wushi earthquake using the spherical least squares configuration method. Based on the fundamental concept of active blocks, Xinjiang and its neighboring areas were divided into 17 active blocks by integrating regional seismic geological data. The slip rates of the boundary fault zones of these active blocks were then calculated using a three-dimensional elastic block model. Based on this block delineation, the study area was further divided into 91 potential seismic hazard zones. We incorporated geodetic observations, including fault strike-slip rates and dilatational strain rates derived from the GNSS velocity field, into the conventional probabilistic forecasting of significant seismic hazards. The relationship between the Wushi 7.1 earthquake and the regional crustal deformation characteristics, as well as the prediction results of the probability of strong earthquakes, was comprehensively analyzed.
The direction and magnitude of GNSS velocity field motions in the study area under different dynamical backgrounds are distinctly characterized by zoning. GNSS stations east of longitude 76° move toward the north or northeast, while those on the west side move north or northwest. The velocity field difference is primarily evident on the north and south sides of the Tianshan Mountain. The velocity change of GNSS sites from the Tibetan plateau to the Tarim Basin is minor, but it significantly decreases after crossing the Tianshan Mountain, indicating that the Tianshan Mountain tectonics absorb most of the remote effects of the Indo-Eurasian plate collision. The principal strains obtained from the least-squares configuration results reveal that the extrusion characteristics are most prominent near the western section of the Southern Tianshan Mountains and the Altyn Tagh fault zone. In the western section of the Southern Tianshan Mountains, the direction of the principal compressive strains is perpendicular to the tectonic direction in the region. This suggests that the region is primarily influenced by a force perpendicular to the tectonic direction, resulting in the predominant retrograde movement of major fault zones in the area. Moreover, in addition to the western section of the Southern Tianshan Mountains, the region with a greater main compressive strain is the Altyn Tagh fault zone. The direction of the main compressive strain intersects obliquely with the Altyn Tagh fault zone, suggesting that the force background is linked to the left-lateral reverse slip movement of the Altyn Tagh fault zone.
The results derived from the inversion of the three-dimensional elastic dislocation model reveal that the motion features of the main active faults within the study area indicate a predominance of dextral slip along the northwest-trending fault zones in the Tianshan region, while sinistral slip motion is primarily observed along the northeast-trending or northeastern fault zones. Apart from the Altyn Tagh fault zone, the strike slip rate of the dextral-slip fault zones is notably higher than that of the sinistral-slip fault zones. The fault zones at the northern edge of the Junggar Basin and the major faults in the Tianshan region are primarily characterized by extrusion movements. The extrusion rate of the fracture zones in the South Tianshan Mountain is higher than that in the North Tianshan Mountain. Specifically, the west section of the Keping fault zone and the west section of the Nalati fault zone exhibit the most prominent extrusion movements.
The study area has been divided into 91 potential seismic hazard zones based on block delineation and the findings from previous research. The slip rates and regional surface strains of the major faults obtained from the inversion are utilized in classical probabilistic predictions to derive quantitative results regarding the strong earthquake hazard in the study area over the next 50 years. The results indicate that the strong earthquake hazard is primarily concentrated in the western section of the Southern Tianshan region. This includes areas such as the north-east or nearly east-west-trending Maidan fault, the Nalati fault, and the Usun Ridge Fault, as well as the north-west-trending Talas-Fergana fault zone and the north-west section of the Kyzyltau fault zone. Furthermore, the probability of strong earthquakes is elevated in the northern Luntai fault compared to the surrounding faults. In the northern Tianshan region, areas with relatively high probabilities of strong earthquakes include the Fukang fault and the western section of the Bogda fault.
The seismic mechanism solution reveals that the M7.1 Wushi earthquake was a thrust earthquake, aligning with the characteristics of the Maidan fault zone. In this zone, the seismogenic fault is primarily influenced by extrusion motion. The Wushi earthquake occurred in the Maidan fault zone, situated at the border of the high shear strain rate zone and the high probability hazard zone. This occurrence validates the effectiveness and accuracy of the probabilistic prediction method.

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GROWING HISTORY AND GEOMORPHIC RESPONSE OF THE EASTERN TERMINATION OF KASHI ANTICLINE, SOUTHWESTERN TIAN SHAN: AN INTEGRATED ANALYSIS OF GEOLOGY, GEOMORPHOLOGY, SEISMIC REFLECTION PROFILE AND MAGNETOSTRATIGRAPHY

HE Peng-yu, LI Tao, CHEN Zhu-xin, CHEN Jie

SEISMOLOGY AND GEOLOGY 2025, 47 (2): 369-383. DOI: 10.3969/j.issn.0253-4967.2025.02.20240157

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The Kashi anticline, located along the leading edge of the Kashi foreland thrust system in the southwestern Tian Shan of China, confronts the Atushi Anticline to the north and connects with the Mingyaole Anticline to the west. The Kashi anticline manifests at the surface as an elongated hillock with a nearly EW strike. The morphology of the anticline is roughly box-shaped, with the southern limb being gentler compared to the northern limb. As a significant component of the Tianshan orogenic belt’s frontal zone, the study of the tectonic evolution of the Kashi Anticline is crucial for understanding the Cenozoic tectonic deformation and crustal shortening processes of the Tian Shan. Previous studies on this fold are primarily focused on its surface-expression part, with little or no focus on its lateral termination that is not expressed on the land surface, which has limited the comprehensive understanding of the anticline’s overall evolutionary process. The study employs the depth-relief area method, combined with high-resolution seismic reflection profile data, to conduct a detailed structural analysis of the eastern termination of the Kashi anticline.
Through meticulous interpretation and quantitative analysis of the seismic profiles, the following key insights have been obtained: Firstly, along the seismic profile, the Cenozoic strata are approximately 6.8km thick, while the Mesozoic-Paleozoic strata are about 2.2km thick. The eastern termination of the anticline detaches along the Paleogene unit, with a depth of ~6.8km. This detachment layer governs the deformation patterns and magnitudes of the overlying strata. Secondly, along the seismic profile, the total shortening of the anticline is estimated to be(882±79)m, of which approximately 94% is attributed to shear shortening, and about 6% is due to curvietric shortening. During the folding process, materials with an excess area of ~3.4km² enter the cross section. Thirdly, according to the analysis of growth strata and published magnetostratigraphic data, the folding of the eastern termination initiates at the age of ~2.1Ma, which implies that the initiation age of the fold should be much older than 2.1Ma. The shortening rate remains at an approximate constant of ~0.4mm/a since the folding initiation. Fourthly, for the pre-growth strata, the uplift of the anticline gradually increases upward with depth, reaching a maximum of approximately 770m at the top boundary of the pre-growth strata. Analysis of the growth strata indicates that the uplift rate either keeps a constant of ~0.4mm/a, or increases significantly from an earlier rate of ~0.1mm/a to a later rate of 0.4 mm/a at the age point of ~1.6Ma. Notably, because the uplift rate is smaller than the sediment rate, the fold exhibits no expression on the surface of the anticline.
Our study exemplifies that an analysis of the buried lateral termination of a fold can well determine the detachment level, shortening and uplift histories, initiation age of the fold, as well as whether or not the excess area enters the cross section during the folding process. These constraints provide a completer and more reliable basis for understanding the entire growing history of the fold. Furthermore, the results demonstrate that the analysis of buried structures in the frontal zones of orogenic belts is indispensable for a comprehensive understanding of regional tectonic deformation characteristics and evolutionary history.

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INSAR COSEISMIC DEFORMATION AND SEISMOGENIC STRUCTURE OF THE 2024 M_W7.0 WUSHI EARTHQUAKE

CHEN Zi-long, LIU Gang, LI Qi, CHEN Wei, ZHAO Xin-yu, LIN Mu, TAO Long-wen, QIAO Xue-jun, NIE Zhao-sheng

SEISMOLOGY AND GEOLOGY 2025, 47 (2): 429-447. DOI: 10.3969/j.issn.0253-4967.2025.02.20240142

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On January 30, 2024, an M_W7.0 earthquake struck the Wushi region of the southern Tianshan Mountains, Xinjiang, China. This earthquake, located in a tectonically active zone dominated by intense crustal shortening and thrust faulting, providing a valuable opportunity to investigate fault geometry and rupture mechanisms in the region. We utilized Sentinel-1 InSAR data combined with advanced inversion techniques to analyze the coseismic deformation field, determine fault parameters, and explore the spatial relationship between the mainshock and aftershocks.
High-resolution coseismic deformation fields were generated using D-InSAR processing of Sentinel-1A ascending and descending orbit data. Nonlinear inversion methods were employed to calculate fault geometry and sliding distribution, with both single-fault and dual-fault models tested to accommodate the complex faulting characteristics. Residual analysis was performed to examine the relationship between the mainshock and the M_W5.7 aftershock, and geological surveys were used to validate fault models and rupture characteristics.
The maximum line-of-sight(LOS)displacement of the coseismic deformation field reached 70cm, displaying an elliptical pattern along the Maidan Fault. The dual-fault model revealed significant geometric complexity: the fault strike rotates clockwise by 20°~25° east of the epicenter, and the dip angle decreases from 60° in the west to 40° in the east. Fault slip was primarily concentrated west of the epicenter, characterized by high-angle thrusting with a left-lateral component, while slip in the eastern segment was lower in magnitude and relatively dispersed. The overall distribution exhibited shallow slip deficit. The correlation between geometric variations and sliding distribution suggests the presence of a geometric barrier east of the epicenter, acting as an obstacle to rupture propagation. The coseismic rupture was confined between the Yushanguxi River and the Wushi depression, with fault steps and structural complexities on the eastern and western boundaries limiting the rupture extent. The M_W5.7 aftershock produced a clear LOS deformation field, with the fault strike deviating by ~10° from the mainshock trace and dipping southeast. The surface trace of the aftershock fault closely aligned with mapped surface ruptures, and the shallow slip magnitude matched the observed vertical offsets from geological surveys.
We also demonstrates that fault geometry plays a significant role in controlling rupture propagation and termination. The geometric barrier east of the epicenter effectively limited eastward rupture propagation, while a wide fault step near the Yushanguxi River constrained the western rupture. The Wushi earthquake was identified as a blind rupture event, with no significant primary surface rupture. The distribution of secondary geological hazards aligned well with fault slip characteristics, and the spatial relationship between aftershock slip and the mainshock highlights fault segmentation within the Ush thrust belt.

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EARTHQUAKE SEQUENCE RELOCATION AND SEISMOGENIC STRUCTURE OF THE 2024 M_S7.1 WUSHI EARTHQUAKE ON JANUARY 23, 2024, XINJIANG

WANG Xue-zhu, WU Chuan-yong, LIU Jian-ming, ZANG Ke-zhi, YUAN Hai-yang, GAO Zhan, ZHANG Jin-shuo, MA Yun-xiao

SEISMOLOGY AND GEOLOGY 2025, 47 (2): 488-506. DOI: 10.3969/j.issn.0253-4967.2025.02.20240153

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At 2:09 on January 23, 2024(Beijing time), an earthquake of M_S7.1 occurred in Wushi County, Aksu region, Xinjiang Uyghur Autonomous Region. This earthquake is the first earthquake with a magnitude greater than 7 in the Tian Shan seismic belt after the 1992 Susamer M7.3 earthquake. This earthquake occurred near the range-front Maidan fault which is the boundary structure between the southern Tian Shan and Tarim Basin. In fault geometry, the Maidan fault shows a complex geometry. On the plane, the Maidan fault comprises multiple roughly parallel secondary faults. On the profile, the gently dipping mountain front fault controls the folding deformation of the late Cenozoic. The front and back mountain faults converge in the deep part. This causes the Tarim Plate to deeply subduct along a large subduction zone under the Tianshan. The thrust-nappe structural system is often composed of multiple fault zones that can generate strong earthquakes because these fault zones converge to the same detachment surface in the deep part. Because a fault zone triggers or suppresses strong seismic activity in adjacent faults after a strong earthquake.
Because the main earthquake did not generate obvious coseismic surface ruptures, there is still a great controversy and uncertainty on its seismogenic structure. Determination of the seismogenic structure is crucial for analyzing the potential location of the next major earthquake. It is of great significance for evaluating the future risk of strong earthquakes in the region and the stress loading and triggering relationships between different faults.
According to the analysis of Google’s high-precision satellite image interpretation results and field inspections, the geometric structure of Maidan is more complicated. The fault is roughly near the ancient stream of Yu Shangu Xi River, and it can be divided into two branches, east and west of the left steps. The faults all show clear signs of activity at the surface. In this study, we utilize the earthquake relocation results to determine the seismogenic structure of the 2024 Wushi event. Our results show that the main shock began to break in the deep and the aftershocks are extended from deep to the part. The deeper focal depth may be an important factor in preventing the coseismic sliding of the Wushi M_S7.1 earthquake from being transmitted to the surface. The seismic sequence is exhibited by the northeast-southwest. The long axis direction is about 55°, and the total length is about 85km. The aftershock sequence is divided into the northeast, middle section, and southwest section. The rupture range of the Wushi M_S7.1 earthquake is about 35km, which is the middle section of the aftershock sequence. The aftershocks in the northeast section are mainly distributed along F_1-1, the aftershocks in the southeast section are mainly distributed along F_2-2 and the aftershocks in the middle section are mainly distributed along F_1-2. The aftershocks of different sections are distributed on different branches, which means that the strong earthquake triggered the adjacent seismic activity, which belongs to a more complicated grade joint rupture earthquake. At the same time, the CAP waveform counter and method was adopted to obtain the focal mechanism. The nodal planes parameters of the best double-couple focal mechanisms are: strike 115°, dip 52° and rake 132° for nodal plane I, and strike 240°, dip 54° and rake 49° for nodal plane Ⅱ, the depth of the centroid is about 17km. Based on the inversion results of the focal mechanism and the spatial distribution characteristics of the earthquake sequence, we believe that nodal plane II is the seismogenic fault plane. Based on the spatial distribution characteristics of the earthquake sequence, the focal mechanism solution, and the geological structure data of the earthquake area, we suggest that the Biedieli-goukou fault is the piedmont branch of the Maidan fault. The upper part of the fault(4~5km)has no rupture, and it still has a strong risk of strong earthquakes in the future. The Wushi M_S7.1 earthquake triggered the Biedieli fault and the Aheqi fault. During the Late Quaternary period, these faults have repeatedly ruptured to the surface of strong earthquake incidents.

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SEDIMENTARY RECORDS AND SOURCE ANALYSIS OF A TSUNAMI EVENT ABOUT ~1 000 YEARS AGO IN THE PEARL RIVER ESTUARY ALONG THE COAST OF SOUTH CHINA

WANG Wei-tao, YANG Xiao-qiang, SHU Peng, ZHANG Yu-hao, LIANG Hao, LI Lin-lin, LI Zhi-gang, WANG Da-wei, ZHANG Pei-zhen

SEISMOLOGY AND GEOLOGY 2025, 47 (4): 999-1019. DOI: 10.3969/j.issn.0253-4967.2025.04.20240170

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Since the Cenozoic time, the South China Sea(SCS)has formed one of the largest semi-enclosed marginal basins along the East Asian continental margin through the geological processes such as South China Sea plate rifting, seafloor spreading, and plate subduction. In the South China Sea Basin and its surrounding regions, a series of active geological structures have developed, for example, the Manila subduction zones, the Littoral(Binhai) fault zone, and the Continental Slope(Lupo) fault zone. The activity of these tectonic zones is highly prone to triggering extreme natural disasters such as earthquakes and tsunamis. Along the coastal zone of the northern part of the South China Sea(the South China continental margin), there are densely populated large cities with critical infrastructure, which are also regions severely affected by extreme natural disasters like earthquakes and tsunamis. Therefore, identifying the sedimentary records of large-scale paleotsunami events in the northern part of the South China Sea and analyzing their potential triggering mechanisms are of great significance for seismic-tsunami hazard assessment.

This study focuses on the sedimentary strata of boreholes E15 and E12 from the Pearl River Estuary along the South China coast. Based on AMS ¹⁴C dating and a comparison of magnetic susceptibility data between boreholes E15 and E12, a high-precision chronological sequence was established for the core of borehole E15, spanning approximately the last 1000 years to the present. The core E15 is 5.9m long, with a progressively younger AMS ¹⁴C age sequence in the upper part of the core section from 4.24~0m. However, AMS ¹⁴C ages of the sediments in the lower part of core E15, from 5.9 to 4.24m, are sometimes reversed. The reversal ages may be attributed to the reworking or recycling of the sediments in the lower part of core E15.

To reveal the depositional processes of borehole E15, we conducted detailed analyses of sedimentary grain size, sedimentary color, and geochemical elemental composition. The lower part of the core section(5.9~4.24m) for E15, consists of dark gray to grayish-black medium-to-coarse sand layers with poor sorting and contains abundant marine biodetritus. In contrast, the upper part of the core section(4.24~0m) for E15 is composed of dark gray to grayish-brown silty mud, fine sandy silt, and delicate sand layers. The upper core section exhibits finer grain size, lighter color, faint horizontal bedding, and higher terrestrial-derived elemental content, representing typical delta-shallow marine depositional environment. The lower part core section is characterized by a coarser grain size(medium to coarse) of sands, which lack clear sedimentary structures and exhibit higher offshore marine-derived elemental content, but relatively lower terrestrial-derived elemental content.

Based on the sedimentary features and geochemical composition, the sands from 5.9~4.24m within the borehole E15 are completely different from the overlying normal, typical shallow sea-delta sediments. Considering the reversal AMS ¹⁴C ages, coarser grain size, poor sorting, darker color, higher offshore marine-derived elemental content, and lower terrestrial-derived elemental content in the lower part core of the E15, we propose that the sand layers with abundant marine biodetritus in the lower part of boreholes E15 (5.9~4.24m) may be deposits from an extreme hydrological event occurring approximately 1000 years ago. In fact, tsunami deposits dating back about 1000 years have been widely documented along the northern and western coasts of the South China Sea, the inner islands of the South China Sea, and the northwestern Philippines. Therefore, we suggest that the event deposits in the Pearl River Estuary region, along the northern part of the South China Sea, at ~1000 years ago, may also be the result of a tsunami event.

Combining sedimentary evidence and numerical simulations, we hypothesize that a strong submarine earthquake may have occurred along the Manila subduction zone in the eastern South China Sea approximately 1000 years ago, triggering a large-scale tsunami. The medium and coarse-grained sand layers in the lower part (5.9~4.24m) of the E15 borehole within the Pearl River Estuary may be the consequence of this tsunami event.

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SEISMOLOGY AND GEOLOGY 2025, 47 (3): 0-0.

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GEOLOGICAL HAZARD CHARACTERISTICS AND MACRO-SCOPIC EPICENTER OF WUSHI M_S7.1 EARTHQUAKE ON JANUARY 23, 2024, XINJIANG

CHEN Jian-bo, ZHENG Li-ming, LI Tao, CHEN Jie, YAO Yuan, QIAN Li, ZHANG Bo-xuan, HU Zong-kai, LIU Guan-shen, LI Jin

SEISMOLOGY AND GEOLOGY 2025, 47 (2): 561-576. DOI: 10.3969/j.issn.0253-4967.2025.02.20240066

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The Wushi M_S7.1 earthquake is the largest seismic event in Xinjiang in the past decade and the most significant earthquake in the Tianshan seismic belt since the Suusamyr M_S7.3 earthquake in Kyrgyzstan in 1992. The microseismic epicenter is located at the transition between the Keping nappe structure belt and the Kuqa nappe belt in the southwestern Tianshan Mountains, near the intersection of the southern Tianshan Mountains and the Wushi Depression. This region is a compressional fault depression valley formed in the context of Tianshan orogeny. The northern boundary of the valley is controlled by the Maidan reverse fault zone, while multiple rows of newly developed linear uplifts and piedmont fault scarps in the northern depression are governed by low-angle overthrust and recoil faults. Focal mechanism analysis indicates that both the main shock and aftershocks exhibit a compressional thrust-type mechanism, consistent with the structural characteristics of the Wushishan front depression. Post-earthquake investigations revealed relatively minor casualties and structural damage, but secondary geological disasters were severe. Using field surveys, large-scale satellite image analysis, and drone aerial photography conducted immediately after the earthquake, this study investigates the spatial distribution, types, and development characteristics of seismic-induced geological hazards in various gullies within the affected region. Although no large-scale surface ruptures were observed at or near the epicenter, extensive seismic geological hazards were identified, providing crucial insights for determining the macroseismic epicenter and assessing the post-disaster intensity and loss in the earthquake area.
Geological hazards induced by the Wushi earthquake were primarily concentrated in Biediligou, Qialematigou, Cocriu Musugou, and adjacent mountainous areas within the VII-degree seismic intensity zone. The most severe damage occurred in the Biediligou-Qialematigou region, where the macroseismic epicenter was located, with diverse types of secondary disasters, including rockfalls, landslides, rolling stones, bank slope collapses, subgrade settlement cracks, and ground fissures. Some ground fissures were accompanied by displacement. Seismic geological hazards were predominantly observed near the microseismic epicenter and within 10km of the north and south sides of the fault zone, with collapses being the most prominent, followed by pavement cracking and riverbank slope failures caused by ground fissures and uneven subgrade settlement. Post-earthquake investigations also identified numerous concentrated surface tension and shear cracks in the macroseismic epicenter, alongside earthquake-induced collapses, landslides, slope instabilities, subgrade settlements, and structural failures. Coseismic InSAR deformation field and image offset analysis revealed a maximum line-of-sight displacement of approximately 40cm and a minimum displacement of -16cm. The long axis of the deformation zone trends NE, with its strongest deformation occurring between Biediligou and Qialamatigou. This spatial distribution aligns well with the observed ground fissures and seismic deformation zones near the macroseismic epicenter, indicating that structural deformation plays a critical role in controlling the formation of earthquake-induced geological hazards.

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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

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

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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.

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SURFACE RUPTURE CHARACTERISTICS OF THE JISHISHAN M_S6.2 EARTHQUAKE ON DECEMBER 18, 2023

LI Lin-lin, JIANG Wen-liang, LI De-wen, JIAO Qi-song, LUO Yi, LI Yong-sheng, TIAN Yun-feng, LI Ying-ying

SEISMOLOGY AND GEOLOGY 2025, 47 (4): 1058-1074. DOI: 10.3969/j.issn.0253-4967.2025.04.20240010

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At 23:59 on December 18, 2023, an M_S6.2 earthquake struck Jishishan County, Linxia Hui Autonomous Prefecture, Gansu Province. The epicenter(35.70°N, 102.79°E) was located in the southeastern segment of the Lajishan fault zone, with a focal depth of approximately 10km. According to the Ministry of Emergency Management, the maximum intensity reached Ⅷ degree, with the NNW-striking long axis of the isoseismal zone consistent with the fault strike. Although moderate in magnitude, the earthquake has caused over 150 fatalities, primarily due to its occurrence at midnight, high population density, poor seismic resistance of housing, and secondary hazards such as debris flows.

Following the event, a comprehensive field investigation was conducted at the epicentral area. Utilizing UAV imagery, digital surface models(DSMs) derived from GF-7 satellite data, and InSAR analysis using Sentinel-1 SAR data, the characteristics of the surface ruptures and the seismogenic structure were examined.

The InSAR results from the ascending orbit of Sentinel-1 revealed that coseismic deformation was dominated by uplift, with a maximum line-of-sight(LOS) displacement of approximately 65mm. The primary deformation zone exhibited an elliptical shape, trending NNW, and extended approximately 10km along the fault strike—consistent with the expected rupture length for an event of this magnitude.

Field surveys and UAV imagery identified a ~1km-long NNW-trending surface rupture east of Yinjiashan Village, which cut across multiple geomorphic units. These ruptures exhibited dominantly thrusting motion with minor right-lateral strike-slip components and were linearly distributed, indicating a tectonic origin rather than landslides or secondary processes. The maximum observed offset reached 8cm vertically and 2cm horizontally.

Digital surface model interpretation from GF-7 imagery revealed several NNW-trending linear structures along the eastern front of the Jishishan Mountains, forming linear topographic scarps and ridges. The observed surface rupture corresponds with one of these structures(F₁₂). Additionally, two local rivers exhibit sharp deflections toward the NNW, controlled by these structures, supporting the interpretation that they are branch faults of the northern Lajishan fault.

According to empirical relationships between magnitude, rupture length, and displacement(Wells & Coppersmith, 1994), an M_S6.2 event is expected to produce a rupture length of approximately 10km. Similar surface rupture lengths(~15km and ~11km) were observed in the 2021 M_S6.1 Biru earthquake. Combined with InSAR-derived deformation extent and the lack of field coverage south of the observed rupture, it is inferred that the surface rupture may extend several kilometers southward along the same structural lineament.

In conclusion, the seismogenic fault responsible for this event is the northern Lajishan fault, comprising multiple NNW-trending branch faults along the eastern front of the Jishishan Mountains. The surface rupture identified corresponds to one of these secondary faults. Despite of the modest scale of the Lajishan fault zone compared to major structures like the Altyn Tagh, East Kunlun, and Qilian-Haiyuan fault zones, and its lack of historical large earthquakes, this event highlights the potential seismic hazard posed by smaller faults under the influence of ongoing crustal uplift and tectonic extension in the northeastern Tibetan plateau.

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STRUCTURAL SETTING AND IMPLICATIONS OF THE 2015 PISHAN M_W6.4 EARTHQUAKE IN THE FRONT OF WESTERN KUNLUN

YANG Wen-xin, LI Tao, CHEN Jie, YAO Yuan

SEISMOLOGY AND GEOLOGY 2025, 47 (2): 507-532. DOI: 10.3969/j.issn.0253-4967.2025.02.20250049

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As the dynamic hub between collisional orogenic belts and foreland basins, the formation and propagation of foreland fold-and-thrust belts are often accompanied by destructive earthquakes. In this process, the detachment, which is regarded as an incompetent layer, plays a significant controlling role in the propagation of the thrust system and the occurrence of strong earthquakes. Therefore, research focused on the structural patterns and physical property architecture of foreland fold-and-thrust belts is conducive to analyzing their activity characteristics and seismogenic patterns, thereby providing a basis for regional seismic hazard assessments.
The western Kunlun orogenic belt is situated at the Qinghai-Xizang Plateau’s northwestern margin and the Tarim Basin’s southwestern edge. Governed by the convergence of the Indian and Eurasian plates, the compressive collision between the Qinghai-Xizang plateau and the Tarim Basin has led to the formation and propagation of the Western Kunlun foreland fold-and-thrust system, triggering intense and frequent tectonic and seismic activities within the region. A M_W6.4 earthquake occurred in Pishan on July 3, 2015 in the front of western Kunlun, which was one of the most destructive seismic events that happened in Xinjiang in the recent decade. The epicentral area of this earthquake experienced an intensity of VIII, with many regions feeling strong tremors. The disaster area was vast, affecting a large population, and was accompanied by hazards such as sand liquefaction, ground fissures, and collapses. Studies on this event will provide an opportunity to better understand active structures, stress state, and future seismic hazard of the front of western Kunlun and the adjacent region.
Synthesizing geology, geomorphology and seismology studies, Slik Anticline, located on the western Kunlun foreland fold-and-thrust belt, is regarded as the seismogenic fault of this event. The Slik Anticline is located within the Guman fold belt of the Hotan thrust belt, which lies in the eastern segment of the western Kunlun foreland. The Slik ramp, situated beneath the Slik Anticline, is approximately at a depth of 10 to 15km, with a ramp angle of about 20 degrees. The Pishan earthquake ruptured blind ramp under the Slik Anticline. From north and shallow to south and deep level, the fault is revealed with the structural style of lower flat, ramp and upper flat and may merge with other thrust in a deeper part near to the hinterland. The upper and lower flats are corresponded to regional detachment of lowest Paleogene and inner Cambrian incompetent strata, respectively. And the mainshock occurred at the Slik ramp which connecting these two detachment, which implicates that the Pishan earthquake is an event caused by independent structural unit. The postseismic deformation mainly occurred on the lower flat as afterslip, and accumulated gradually over time. The moment released after a slip in 5 months corresponds to a sub-M_W6.3 event, and is equivalent to the moment released by the mainshock.
Worldwide, earthquakes exhibiting similar characteristics include the 2015 Gorkha M_W7.8 earthquake in Nepal, the 2005 Kashmir M_W7.6 earthquake, the 1999 Chi-Chi M_W7.6 earthquake in Taiwan, China, and the 1995 Colima-Jalisco M_W8.0 earthquake in Mexico, among others. Conducting detailed analyses of these seismic events provides a valuable entry point for clarifying the structural patterns, stress states, and seismogenic features of foreland fold-and-thrust fault zones. Furthermore, it aids in the in-depth analysis of regional seismogenic models and predicting future seismic trends.
Focusing on the Pishan earthquake, expect settling slip by the folding strata above the upper flat; this phenomenon may also depend on both matter properties and gravitational differences between the upper and lower flats. 1)During the fault deformation, a detachment with low competence and low friction coefficient is unable to accumulate significant stress, resulting in the absence of notable stress drops during frictional sliding and instead manifests as a state of after-slip activity. 2)When the detachment, especially the shallow one, has an undersized gravitational load, it may fail to effectively play a role in detachment, leading to a geological process where fault activity may be altered or inhibited. Occurring at the lower flat, the afterslip of the Pishan earthquake cooperated these two situations. 1)The deeper detachment, located on the Cambrian gypsum, may have a lower friction coefficient compared with the upper detachment. 2)And apparently, the deeper detachment possesses more gravitational load. 3)Plus, stress cannot accumulate to a high level at a flat part of a seismogenic fault. These points may lead to the afterslip release of accumulated stress, mostly on the lower flat, as the postseismic deformation. Therefore, the risk of a major earthquake occurring at the lower flat of the fault may be relatively low.
On the one hand, this phenomenon may cause stress loading on the lower fault ramp or the root fault at a deeper level below the fault flat, increasing the seismic risk in the deep structures. On the other hand, if an earthquake was trigged on the lower ramp, the lower flat above it, which had undergone a slip and released a significant amount of energy, would limit the extent of the rupture.
Furthermore, Cambrian gypsum layers are widely present at depth, which can serve as deep detachment surfaces and lower fault flats at the scale of the study area and its adjacent region. Therefore, whether the seismic activity and energy release pattern of the Slik anticline can represent the seismogenic patterns in the front of western Kunlun, and whether such a pattern can effectively provide a basis for regional seismic hazard assessment, deserves further attention and research.

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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

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

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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.

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CHARACTERISTICS OF SEISMICITY AND TECTONIC STRESS FIELD IN THE NORTHEASTERN REGION OF THE PAMIR PLATEAU

CUI Hua-wei, YIN Xin-zhong, CHEN Jiu-hui, GUO Biao, LI Tao, YAO Yuan, LI Shi-ying, JIA Zhen

SEISMOLOGY AND GEOLOGY 2025, 47 (2): 577-596. DOI: 10.3969/j.issn.0253-4967.2025.02.20250047

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The northeastern region of the Pamir Plateau marks the convergence of the Pamir Plateau, the Tianshan Mountains, and the Tarim Basin. This area is associated with high levels of seismic activity and has experienced significant casualties due to several major earthquakes. In this study, earthquakes are relocated using the double-difference relocation algorithm, based on seismic observation reports from the China Earthquake Networks Centre(CENC)covering the period from 2008 to 2023. A total of 916 earthquake focal mechanisms were collected from various institutions and studies, including the GCMT, WWSSN, previous research, and the Xinjiang Earthquake Agency. The magnitudes of these events range from M2.8 to M8.2, and the time span extends from 1902 to 2024. The tectonic stress field was inverted using a spatial-temporal stress inversion algorithm with a 1.0°×1.0° grid, based on these historical focal mechanisms.
The relocated earthquakes are distributed along major tectonic zones situated between the Pamir Plateau, the Tianshan Mountains, and the Tarim Basin, forming several seismic clusters within the study area. Most events have focal depths of less than 30km. Seven cross-sectional profiles were selected to analyze the spatial distribution of seismicity. The regional tectonic stress field is primarily dominated by strike-slip and reverse faulting regimes. The maximum principal stress axes are compressional in a nearly horizontal south-north(S-N)direction, while the minimum principal stress axes are extensional in the east-west(E-W)direction, with either horizontal or vertical plunges. Based on the spatial distribution of seismicity and tectonic features, the study area is divided into several subregions: the central Pamir Plateau and its eastern and western flanks; the collision zone between the Pamir Plateau and the Tianshan Mountains and its surrounding areas; the Tianshan Mountains; and the interior of the Tarim Basin.
Preliminary results of earthquake relocation show that focal depths in the southeastern and northwestern parts of the Qimugen Arc Tectonic Belt are deeper than those within the belt. This is attributed to the subduction of the eastern and northeastern Pamir beneath the Tarim Basin toward the east and northeast. Accordingly, the Moho depth is also greater in these regions. These observations suggest that the Qimugen Arc Tectonic Belt represents a key tectonic boundary, with both the Moho and earthquake depths shallower within the belt than in its southeastern and northwestern margins. Focal depths along the southern margin of the Tianshan Mountains are deeper than those in the adjacent Tarim Basin, which is consistent with a deeper Moho in the former. Similarly, both the Moho and earthquake depths within the Ferghana fault and Piqiang fault zones are greater than those outside these faults along the southern margin of the Tianshan Mountains, indicating that these faults serve as significant tectonic boundaries. An area of sparse seismicity overlaps with the source region of the 1902 Atushi(Kashgar)M_W7.7 earthquake, suggesting that accumulated crustal stress was likely fully released by this event. Seismicity in the Tarim Basin primarily consists of aftershock sequences from the 1996-1999 earthquake clusters and the 2003 Bachu-Jiashi M_S6.8 earthquake. The main contributing factors to seismicity in the Tarim Basin are thought to be its low seismic wave velocity, low crustal density, and brittle crustal properties.
The initial results of the tectonic stress inversion indicate that the central Pamir Plateau is undergoing east-west lateral extension under a strike-slip stress regime, driven by compression from the Indian Plate to the south and obstruction from the Tianshan Mountains to the north. A normal faulting regime in the central grid of the Pamir Plateau is interpreted as a center of expansion or gravitational collapse. The tectonic stress field exhibits significant variation across the flanks of the Pamir Plateau. The R-value, stress regime, and plunge of the maximum principal extension axes show gradual and, in some areas, abrupt changes from the central region to the western and eastern margins, as well as in the collision zones between the Pamir Plateau and the Tianshan Mountains. The Tianshan Mountains continue to uplift due to near S-N compression, displaying a dominant reverse faulting regime. In the Bachu uplift and depression region in the southwestern Tarim Basin, the stress field is characterized by both reverse and strike-slip regimes, likely resulting from differences in crustal velocity, density, and structural composition.

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COSEISMIC SURFACE RUPTURE OF THE M_S6.8 DINGRI EARTHQUAKE IN XIZANG, CHINA, BASED ON GF IMAGERY INTERPRETATION

QIAO Jun-xiang, SHI Feng, LI An, LI Tao, ZHANG Da, WANG Xin, Gesangdanzhen, SUN Hao-yue

SEISMOLOGY AND GEOLOGY 2025, 47 (3): 789-805. DOI: 10.3969/j.issn.0253-4967.2025.03.20250043

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On January 7, 2025, at 09:05, a M_S6.8 earthquake occurred in Dingri County, Shigatse City, Xizang, China, with a focal depth of 10km and an epicenter located at 87.45°E, 28.50°N, as per data from the China Earthquake Networks Center. Rapid identification of coseismic surface ruptures is crucial not only for determining the seismogenic structure but also for post-earthquake damage assessment and emergency response. Based on the objective geological conditions of the Dingri earthquake-affected area, preliminary interpretation of pre- and post-earthquake high-resolution remote sensing data to delineate the spatial distribution and geometric characteristics of coseismic surface rupture zones before field investigations can effectively guide emergency response teams in rapidly and accurately identifying coseismic surface ruptures and conducting subsequent field surveys. This methodology demonstrates high feasibility and necessity for optimizing field workflow efficiency and ensuring targeted structural analysis of seismogenic faults.

This study utilizes high-resolution imagery from the Gaofen -1 satellite to interpret pre- and post-earthquake images, rapidly obtaining the spatial distribution and geometric characteristics of the coseismic surface ruptures. The seismogenic fault is identified as the Dengmecuo Fault, located in the southwest segment of the Shenzha-Dingjie Rift. The coseismic surface rupture zone is primarily distributed near Gulong Village in the northern and central segments of the Dengmecuo Fault, with discontinuous extensions of approximately 15km, consistent with the location of the pre-existing fault. The coseismic surface rupture zone is tectonically partitioned into three distinct segments based on their geographic distribution: The Nixiacuo and Yangmudingcuo segments situated within the northern fault segment, and the Gurong segment developed along the eastern piedmont front of Gurong Village. These three segments exhibit marked differences in spatial scale and morphological characteristics between remote sensing observations and field investigations. The Nixiacuo segment exhibits linear and continuous coseismic surface ruptures, extending approximately 5km, with prominent linear traces visible in satellite imagery, facilitating clear identification. Field investigations reveal that this segment predominantly develops a series of large-scale tensional fractures and three categories of fault scarps with differential heights, with a maximum coseismic displacement of ~3m recorded. In contrast, both the Yangmudingcuo and Gurong segments exhibit smaller-scale coseismic surface ruptures localized along the eastern graben-bounding basin-range boundary. These secondary ruptures are characterized by minor extensional cracks with limited opening amounts(<1m vertical displacements)and fault scarps, manifesting as dark-gray linear features in remote sensing imagery that coincide with pre-existing rupture traces. Their partial obscuration in spectral signatures renders comprehensive visual interpretation impractical, necessitating field validation for complete delineation. Furthermore, this study identifies an ~10km-long associated surface deformation zone along the eastern Dengmecuo Lake shoreline, exhibiting a structural assemblage of extensional fissures proximal to the mountain front and compressional ridges adjacent to the lakeshore, with concomitant sand boil structures observed within the deformation zone. These extensional features present as dark-toned linear traces paralleling the main surface ruptures, displaying discontinuous arcuate configuration convex toward the mountain front within the mid-fan sector of alluvial fans. The compressional uplift zone west of the extensional belt appears as curvilinear bands with central whitish zones flanked by shadowed margins in imagery, demonstrating enhanced spatial continuity along distal fan margins.

The conclusions of this study exhibit high consistency with previous research based on submeter-level resolution remote sensing imagery, confirming the reliability of remote sensing interpretation. However, interpretations derived from 2m resolution imagery have inherent limitations, including difficulties in identifying small-scale surface ruptures and distinguishing surface deformations of diverse genetic origins. Based on existing research integrated with remote sensing image interpretation and field investigations, the identification capability of coseismic surface ruptures in 2m-resolution remote sensing imagery is fundamentally governed by the three-dimensional geometric parameters of the rupture zone—specifically, rupture zone width, along-strike continuity length, and fault scarp vertical displacement. From the case study of the Dingri earthquake, we hypothesize that GF-1 satellite imagery with 2m resolution is capable of resolving coseismic surface ruptures characterized by a minimum rupture width of 1m, along-strike continuity exceeding 10m, and vertical offset≥1m Furthermore, the composite deformation pattern observed along the eastern shore of Dengmecuo Lake, characterized by rear tensile fractures and frontal thrusting, is interpreted as shallow detachment sliding induced by seismic shaking, representing a secondary associated surface deformation zone rather than direct fault displacement. Therefore, the identification of coseismic surface ruptures necessitates integrating remote sensing interpretation with field investigations, requiring not only the analysis of their geometric configuration and vertical displacement but also a synthetic evaluation of genetic origin based on the geological environment to achieve comprehensive and accurate determination.

This study conducted a detailed interpretation of the coseismic surface rupture zone in the earthquake-affected area using GF-1 image data acquired within 24 hours post-earthquake, rapidly delineating the overall geometry and location of the coseismic surface rupture. This approach effectively supported subsequent field investigations and enhanced the efficiency of earthquake emergency response. Furthermore, rigorous field reconnaissance was carried out to validate the remote sensing interpretation results. The findings demonstrate a high consistency between the interpreted segments of the coseismic surface rupture and field observations, confirming the reliability of the remote sensing interpretation. This highlights the potential of domestic high-resolution satellite data for rapid coseismic surface rupture mapping and seismogenic structure identification, providing a feasible, rapid, and efficient methodology for future emergency response to major earthquakes.

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STUDY ON THE INFLUENCE OF LARGE LOW DIP THRUST FAULTS AND THICK BASINS ON STRONG GROUND MOTION: A STUDY IN KASHGAR AREA, XINJIANG, China

SHEN Zhu-yue, ZHAO Shun, SUN Yao-chong, XU Du-yuan, YU Hou-yun, ZANG Nan, WANG Chun-jing

SEISMOLOGY AND GEOLOGY 2025, 47 (2): 610-626. DOI: 10.3969/j.issn.0253-4967.2025.02.20240158

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Since the Cenozoic era, the Xinjiang region has undergone significant geological evolution, resulting in numerous large-scale low-angle faults and extremely thick sedimentary basins. These unique geological features have profoundly influenced strong ground motion within the area, particularly in regions such as Kashgar, which is situated near the Pamir Plateau. This study focuses on the effects of the Tuomuluo’an Fault and the thick sedimentary layers of the Tarim Basin on seismic activity and the amplification of strong ground motion. Additionally, the Kashgar region’s geographical position south of the Tuomuluo’an front fault further highlights its vulnerability to complex seismic dynamics.
To comprehensively understand these phenomena, researchers developed a series of low-angle thrust fault models, thick sedimentary basin models, and strong ground motion models based on detailed data from the Tuomuluo’an fault system around Kashgar and sedimentary layer characteristics in the Tarim Basin. Numerical experiments were conducted using dynamic simulations of earthquake sources and seismic wave field propagation to analyze how fault dip angle, sedimentary layer thickness and earthquake source location influence strong ground motion.
The numerical experiments revealed several critical insights into the seismic behavior of the region. First, it was found that the Tuomuluo’an fault in the Kashgar region exhibits a greater tendency for earthquake ruptures to reach the surface when occurring along the upper ramp segment. This is due to the specific geometric configuration and stress conditions of this portion of the fault system. In contrast, ruptures along the lower slope are less likely to propagate to the surface, indicating that the fault’s dip angle significantly determines the likelihood of surface-breaking earthquakes. Second, the study demonstrated that the thick sedimentary basin of the Tarim Basin significantly amplifies strong ground motion in the Kashgar region. This amplification occurs because the soft, low-velocity sediments in the basin trap seismic waves and cause prolonged shaking, increasing ground motion intensity during an earthquake. To further investigate this phenomenon, a scenario earthquake was simulated with its nucleation location on the eastern segment upper ramp of the Tuomuluo’an fault. The results generated peak ground velocity and intensity maps essential for disaster assessment and planning.
The simulation revealed that earthquake disasters in the Kashgar area are divided into two distinct regions: one near the seismogenic fault extending along its direction, where strong shaking is expected to be most intense, and another above the Tarim thick sedimentary basin, where ground motion is significantly amplified due to the geological characteristics of basin. Additionally, complex terrain changes influenced earthquake disaster zones, highlighting the importance of topographical factors in determining seismic hazard distribution.
The study concludes that several key factors significantly affect strong ground motion in the Kashgar region: the dip angle and geometric shape of the Tuomuluo’an fault, the thickness of sedimentary layers, and the regional topography. These findings enhance our understanding of seismic hazard assessment in the area and provide valuable insights for disaster preparedness and mitigation strategies. The geological conditions of the Xinjiang region, characterized by large-scale low-angle faults and thick sedimentary basins since the Cenozoic era, further underscore the importance of these findings for regional earthquake risk evaluation.
This research provides critical scientific evidence to improve seismic hazard assessment in the Kashgar region. The study offers practical recommendations for enhancing earthquake preparedness and reducing potential disaster impacts in the Xinjiang region by identifying the key factors that influence strong ground motion.

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SEIMOTECTONIC ANALYSIS OF 2023 JISHISHAN M_S6.2 EARTHQUAKE IN GANSU PROVINCE

ZHANG Bo, WANG Ai-guo, FENG Zi-wei, HE Xiao-long, ZHU Jun-wen, YAO Yun-sheng, CAI Yi-meng

SEISMOLOGY AND GEOLOGY 2025, 47 (6): 1586-1605. DOI: 10.3969/j.issn.0253-4967.2025.06.20240048

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The 2023 Jishishan M_S6.2 earthquake struck within the Linxia Basin along the eastern front of the Jishishan Mountains. This region is characterized by the Jishishan Fault thrusting over the Linxia Basin. However, the dip direction of the seismogenic fault remains debated, with arguments for both west- and east-dipping geometries. Faults near the epicenter include the East Margin Fault of the Jishishan Mountains(EJSF); in addition, the South Margin Fault(SLJF) and North Margin Fault(NLJF) of the Lajishan Mountains may extend southward toward the epicentral area. Active anticlines are also present. Consequently, determining whether the earthquake originated on a single fault(and which one)or involved rupture of multiple faults is an urgent and critical question. To address this, we investigated faults, mountain-basin geological sections, and earthquake-induced fissures near the epicenter. Integrating these observations with a more complete relocated earthquake catalog and five shallow-seismic profiles, we conducted a comprehensive analysis of the seismogenic fault and rupture mechanism of the 2023 Jishishan M_S6.2 earthquake. The results are as follows.
First, the faults near the epicenter include the EJSF, SLJF, and NLJF. The EJSF, situated along the eastern margin of the Jishishan Mountains, comprises multiple west-dipping reverse faults, with its most recent activity in the late Pleistocene to Holocene. The SLJF is an east-dipping reverse fault primarily north of the Yellow River and shows no discernible activity since the late Quaternary. The NLJF is a west-dipping reverse fault mainly developed north of the Yellow River; south of the river it is concealed beneath younger deposits. Its latest activity occurred primarily in the late Pleistocene.
Second, the Jishishan-Linxia Basin section shows early Paleozoic magmatic rocks thrusting over the Linxia Group. The Linxia Group dips overall SW at 10°~20°, locally up to 29°. Near the epicenter, an asymmetric anticline deforms the Linxia Group, with a steeper eastern limb-indicative of EJSF propagation into the basin. Overlying early-Middle Pleistocene deposits display minor folding, but deformation amplitudes are markedly weaker than within the Linxia Group.
Third, the meizoseismal zone exhibits diverse earthquake-induced fissures, including gravity, tectonic, and landslide-related fissures. Most tectonic fissures are narrow(<1cm), with maximum widths of ~5cm. They are concentrated at NWW(21%), NNW(30%), and NE(16.5%) within the EJSF's left-stepping zone, with predominant orientations matching the fault strikes. Over 50% of fissures exploit pre-existing bedrock weaknesses(faults, bedding, joints), while most others follow artificial discontinuities(e.g., road-embankment joints). Their preferential development along weak zones indicates these features result from ground shaking rather than primary fault rupture, further evidenced by mixed sinistral/dextral offsets lacking uniform sense.
By integrating fault mapping, mountain-basin sections, and shallow-seismic profiles, we infer that both the SLJF and NLJF terminate abruptly south of the Yellow River and do not extend to the epicentral area. Only the EJSF and its associated thrust system-including blind faults and folds within the Linxia Basin-are developed near the epicenter. Analysis of relocated aftershocks from the Gansu digital seismic network, early-warning stations, and temporary arrays indicates the Jishishan earthquake likely nucleated on a blind thrust or fold branching from the EJSF. The >10km hypocentral depth greater than 10km further argues against an east-dipping back-thrust as the seismogenic source. Fault geometry and slip-rate results suggest that left-lateral slip along the West Qinling Fault transfers strain via vertical uplift along the EJSF and western Jishishan margin faults, together with crustal shortening in basins flanking the Jishishan Mountains. This strain partitioning constitutes the primary driving mechanism for the 2023 M_S6.2 event.

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

WANG Shun, YAO Yuan, GAO Ming-xing

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

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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.

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RESEARCH ON ATTENUATION CHANGES OF SUBSURFACE MEDIA IN THE QILIAN MOUNTAINS REGION BASED ON ACTIVE AIRGUN SOURCE

ZOU Rui, GUO Xiao, SUN Dian-feng, WANG Ya-hong, ZHANG Yuan-sheng, QIN Man-zhong, LIU Xu-zhou, LI Shao-hua, SONG Ting, LIU An-guo

SEISMOLOGY AND GEOLOGY 2025, 47 (4): 1204-1221. DOI: 10.3969/j.issn.0253-4967.2025.04.20240071

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Accurately characterizing stress state changes along active faults in seismogenic zones remains a key challenge in geophysics. Although direct observation of stress evolution in the subsurface is difficult, such changes can be indirectly inferred by monitoring variations in seismic wave parameters—particularly wave velocity and attenuation—to track dynamic alterations in regional stress fields. However, wave velocity changes associated with stress redistribution are typically extremely subtle, requiring high-precision observation systems and highly repeatable seismic sources to be detected reliably.

In recent years, land-based seismic airgun active-source technology has emerged as a promising tool for long-term monitoring of crustal media. Airgun sources offer distinct advantages, including high signal repeatability, strong energy output, long propagation distance, and minimal environmental impact, making them ideal for detecting both static structures and temporal variations in crustal properties. In July 2015, we established the Qilian Mountain airgun active-source system at the Xiliushui Reservoir in Zhangye, Gansu Province, to investigate crustal structure and its temporal changes in the fault zones of the eastern Qilian Mountains.

Previous studies using repeatable sources have largely focused on waveform traveltime variations to detect media changes. However, relatively few studies have explored the application of seismic wave amplitude variations, especially in the context of monitoring attenuation. Non-elastic attenuation, commonly described by the quality factor(Q), captures energy losses due to inelastic behavior and heterogeneities in the medium. It is highly sensitive to factors such as microcrack formation, fluid presence, temperature fluctuations, and phase transitions, making it an important indicator of subsurface physical and mechanical states.

One reason for the limited application of attenuation monitoring is the complexity of amplitude interpretation, as amplitudes are affected by geometric spreading, reflection, refraction, scattering, and other propagation effects. Nevertheless, laboratory studies demonstrate that attenuation is more sensitive than wave velocity to stress-induced changes, and under controlled field conditions—such as fixed source-receiver geometry and waveform consistency—high-precision monitoring of attenuation is feasible.

In this study, we apply the spectral ratio method to waveforms generated by the highly repeatable Gansu Qilian Mountain airgun source to calculate the time-dependent attenuation parameter (t^*) at multiple stations in the eastern Qilian region. We then analyze the temporal variations of t^* across different seismic phases and compare these changes with traveltime variations, as well as with surface environmental variables such as barometric pressure, temperature, and precipitation in Zhangye Ganzhou.

Our results show a positive correlation between attenuation changes and traveltime shifts. At station ZDY27, located near the epicenter of the 2019 Zhangye Ganzhou M_S5.0 earthquake, a relative change in t^* of approximately 0.03 seconds was observed. These findings demonstrate that airgun-based attenuation monitoring is a robust and sensitive method for detecting subsurface stress and property changes. This approach provides an important supplement to existing monitoring methods and enhances our capability for continuous, high-resolution surveillance of crustal media in seismically active regions.

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DENSE BENDING MOMENT NORMAL FAULT SCARPS ALONG THE GUMAN ANTICLINE AT THE FOOTHILL OF THE WEST KUNLUN MOUNTAINS

XU Jian-hong, CHEN Jie, LI Tao, ZHANG Bo-xuan, DI Ning

SEISMOLOGY AND GEOLOGY 2025, 47 (2): 405-428. DOI: 10.3969/j.issn.0253-4967.2025.02.20240148

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Bending-moment fault and flexural-slip fault are two types of fold-related faults in compressional tectonic environments. Historical earthquake records suggest that both fault types may be active simultaneously, with their fault scarps providing crucial insights into strong seismic events. In the northern region of the Guman anticline, located at the foothills of the West Kunlun Mountains, numerous prominent bending-moment normal fault scarps have developed, reaching heights between 0.5m and 16.0m. This study focuses on a fault scarp segment approximately 5.4km long and 4.2km wide. A digital elevation model(DEM)with a 0.2m resolution was generated using drone photogrammetry. A total of 739 cross-fault scarp profiles were extracted, providing key parameters such as scarp height, slope, displacement continuity, and cumulative displacement trends. Data analysis yielded the following findings:
(1)In the study area, dense bending-moment normal faults align along the active anticline axis, dipping toward the axial plane at angle of 70°~80°, as observed in a trench. Among these faults, more than a dozen dip northward, whereas only 1-2 dip southward, forming asymmetric grabens. This asymmetry may be attributed to the overall northward tilt of the strata and the differing limb structures of the underlying anticline. These faults divide the terrace surfaces into multiple rectangular blocks, 380~650m wide. The blocks exhibit outward tilting relative to the fold axis, with those cut by north-dipping faults tilting southward and those cut by south-dipping faults tilting northward. The degree of tilting and fault displacement is closely related to the thickness of the underlying anticlinal strata and the extent of stratal bending.
(2)Displacement profiles along the faults reveal a step-like decrease in displacement as terrace surfaces become progressively younger, with maximum slope profiles displaying similar trends. This pattern suggests long-term fault activity. Cumulative displacement data confirm this trend, with displacement values of(54.5±3.3)m for terrace T3c and(19.5±1.1)m for terrace T1c. The total displacement of T3c is approximately 2.8 times that of T1c, and displacement ratios across different terraces range from 1.5 to 5.5. Higher ratios indicate greater displacement accumulation on older terraces, suggesting an earlier onset of fault activity. These displacement rankings imply that an initial framework of faults developed in the region, followed by subsequent fault intrusion. Notably, Fault F₈ exhibits a displacement ratio of 5.5, forming a(1.0±0.3)m high fault scarp on the young T1b terrace, indicating that even the earliest-formed faults remain active.
(3)Seismic reflection profiles reveal that the south flank of the Guman anticline dips 3°~6° northward, while the north flank dips 12°~14° northward. The underlying blind thrust exhibits a lower flat-ramp-upper flat geometry. However, bending-moment normal faults are not visible in the seismic reflection data, suggesting that they are secondary structures associated with anticline deformation. The fault zone aligns with the anticline’s fault-bend axis, indicating ongoing activity in the anticline zone. The bending-moment normal faults are rootless, meaning they are not primary seismogenic faults. Instead, they primarily develop in poorly layered strata and are largely independent of the kinematics of fold growth. Their formation is closely tied to the degree of strata bending and the thickness of overlying beds. Despite their shallow nature, the bending-moment normal faults exhibit long-term activity, providing evidence that the underlying anticline remains active. These findings support the interpretation of the Guman anticline as an active fault-bend fold.

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JOINT INVERSION OF THE 2025 DINGRI M_S6.8 EARTHQUAKE RUPTURE PROCESS BASED ON TELESEISMIC P WAVES, STRONG-MOTION AND INSAR DATA

XU Yue-yi, XU Bei-bei, XU Chen-yu, SHAO Zhi-gang, HU Chao-zhong

SEISMOLOGY AND GEOLOGY 2025, 47 (3): 734-746. DOI: 10.3969/j.issn.0253-4967.2025.03.20250033

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At 09:05 Beijing time on January 7, 2025, a M_S6.8 earthquake struck Dingri County, Shigatse City, Xizang Autonomous Region, China, with a focal depth of 10km. The earthquake caused strong ground shaking, reaching a maximum intensity of Ⅸ on the China Seismic Intensity Scale, and resulted in 126 fatalities. This seismic event occurred along the Dengmoco Fault, which is part of the north-south-trending Shenzha-Dingjie normal fault system in the southern Qinghai-Xizang Plateau. The fault extends approximately 60km, dips westward, and has maintained a vertical slip rate of(0.28±0.04)mm/a over the past 1 000 years. Geological investigations indicate that the most recent paleo-earthquake on this fault occurred 4 800-4 968 a BP, with an estimated recurrence interval of(5 500±1 100)a in the Holocene. These characteristics demonstrate that the 2025 Dingri earthquake represents a typical event within the fault’s long-term seismic cycle.

To determine the source mechanism of this event, we performed a joint inversion of the moment tensor using teleseismic P waves and W-phase. By combining the relatively high-frequency P-waveforms with the low-frequency W-phase records, this approach enables a more robust determination of the focal mechanism while providing improved constraints on the centroid location, particularly the centroid depth. The results indicate a moment magnitude of 7.02, with a centroid time offset of 9.7s and a shallow centroid depth of 6km. The optimal centroid is located at(28.6°N, 87.5°E), about 24° east of north from the epicenter. The best-fitting focal mechanism solution yields two nodal planes: Nodal Plane 1 with a strike of 344°, dip of 48°, and rake of -105°; and Nodal Plane 2 with a strike of 185°, dip of 44°, and rake of -74°. Considering the west-dipping geometry of the Dengmoco Fault, Nodal Plane 2 is interpreted as the likely fault plane responsible for the rupture. Based on Nodal Plane 2, we further conducted a joint rupture process inversion using teleseismic P waves, strong-motion waveforms, and InSAR deformation data. The combination of seismic and geodetic observations provided complementary constraints, enhancing both the spatial and temporal resolution of the dynamic rupture process. The results indicate a predominantly normal faulting mechanism with a minor left-lateral component. The rupture propagated mainly northward, with limited southward extension. The rupture lasted approximately 36 seconds, with the main slip occurring between 8 seconds and 24 seconds and concentrated within depths of 0-10km, generating a significant surface rupture approximately 20km north of Changsuo Township. To further explore the fault geometry, we conducted a grid search using a dual-fault model to identify the optimal strike for the northern segment. The analysis identified 240° as the best-fit strike, which slightly improved the overall data fitting and exhibited better consistency with the surface topography. The resulting slip model retained the main rupture characteristics observed in the single-fault scenario. The rupture process can be divided into three stages: ① 0-7s initial nucleation near Cuoguo Township with a relatively minor slip(<1.3m); ② 8-24s primary rupture occurred approximately 20km north of Changsuo, with a peak slip of 4.3m; ③ 25-36s rapid slip attenuation after propagating beyond the northern fault bend.

Our study indicates that the complex fault structure plays a critical role in rupture dynamics. The relatively minor slip observed between Cuoguo Township and Changsuo Township, together with the sparse aftershock activity, suggests the existence of an unidentified east-dipping, NW-trending fault segment in this region. Additionally, the abrupt rupture termination near the northern segment of the Dengmoco Fault is likely influenced by an unrecognized NE-trending subsidiary fault. This study underscores the importance of rupture directivity in seismic hazard assessment and reveals the structural complexity of the Dengmoco Fault system. Overall, the results enhance our understanding of the seismogenic mechanism and contribute to more accurate earthquake hazard evaluations in the region.

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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

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

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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.

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THE MAXIMUM VERTICAL DISPLACEMENT OF THE M_S6.8 EARTHQUAKE IN XIZANG AND ITS SURFACE DEFORMATION STYLE

ZHANG Da, SHI Feng, LUO Quan-xing, QIAO Jun-xiang, WANG xin, YI Wen-xing, LI Tao, LI an

SEISMOLOGY AND GEOLOGY 2025, 47 (3): 707-717. DOI: 10.3969/j.issn.0253-4967.2025.03.20250041

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On January 7, 2025, at 9:05 AM, a M_S6.8 earthquake occurred in Dingri County, Shigatse City, Xizang Autonomous Region. The epicenter was located at 87.45°E and 28.50°N, with a focal depth of 10 kilometers, as determined by the China Earthquake Networks Center. Regarding the maximum vertical displacement of this earthquake, there are differing opinions due to variations in the reference markers used during field measurements of coseismic displacement or deformation amplitude, as well as divergent understandings of the deformation style of earthquake scarps and pre-existing scarps. Additionally, factors such as the complex structure of the surface rupture zone and the short duration of field investigations contribute to the current discrepancies in understanding the maximum co-seismic displacement. Different scholars have varying perspectives on this matter. To determine the maximum co-seismic displacement and provide data for subsequent research, this study utilized GF1 satellite imagery and drone aerial surveying technology to measure the maximum co-seismic displacement following the earthquake.

The seismogenic fault of the Dingri earthquake is the Dengmecuo Fault. The surface rupture caused by this earthquake is mainly concentrated near Nixiacuo, at the northern end of the Dengmecuo Fault. The surface rupture trace in the Nixiacuo section is evident, striking northeast and arranged in a linear pattern. It develops along the existing steep scar in front of the mountain, cutting through a series of landforms, including alluvial fans and moraines, and aligns well with the original steep scar. This section of surface rupture is large in scale, developing a series of extensional fractures and fault scarps. The surface rupture phenomenon is most pronounced at a location 800 meters north of Nixiacuo, where the largest co-seismic vertical displacement also occurs. At the location of the maximum co-seismic vertical displacement, the main rupture diverges into several secondary ruptures, which later converge back onto the main rupture. Simultaneously, several extensional fractures develop, and a clear vertical displacement is observed between the hanging wall and the footwall, accompanied by rock collapse. This study employed field observations and UAV aerial survey technology to determine the maximum coseismic surface displacement resulting from the Tingri earthquake. Since the surface rupture generated not only vertical displacement but also ground fissures, leading to some horizontal displacement, the displacement could not be measured directly. To address this, we identified two points on the hanging wall and footwall of the fault, measured the distance l between them, and determined the inclination angle θ of l relative to the ground. The vertical displacement was then calculated using the trigonometric relationship l ×sinθ. Four sets of measurements were taken, yielding a result of(2.47±0.1)m. UAV aerial survey technology was used to capture orthophotos of the location with the maximum coseismic vertical displacement. Three profiles were measured, and the largest recorded coseismic vertical displacement was 0.2m.

In this study, we collected empirical formulas derived from the maximum moment magnitude of the Dingri earthquake inverted by other scholars. The calculated maximum co-seismic surface displacement ranged from 2.37m to 2.97m, which is consistent with the(2.47±0.1)m observed in the field and the(2.6±0.2)m obtained using drone aerial survey technology. The moment magnitude of the Yutian earthquake is equal to that of the Dingri earthquake, but its maximum co-seismic surface displacement is smaller than that of the Dingri earthquake. Firstly, the Yutian earthquake exhibited both left-lateral strike-slip and normal fault characteristics, whereas the Dingri earthquake was mainly characterized by normal faulting. Secondly, the Yutian earthquake produced relatively continuous surface ruptures, whereas the Dingri earthquake produced highly discontinuous surface ruptures with significant displacement differences between segments.

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SUMMARY OF PRECURSORS OF THE MENYUAN M_S6.9 EARTHQUAKE AND ITS INSPIRATION FOR SUB-INSTABILITY THEORY

GAO Shu-de, WU Xiao-ping, LUO Wei-bin, YANG Xiao-peng, WANG Wen-cai, YAO Sai-sai, CHEN Shuang-gui, LIU Jun

SEISMOLOGY AND GEOLOGY 2025, 47 (6): 1546-1565. DOI: 10.3969/j.issn.0253-4967.2025.06.20240054

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The “sub-instability” earthquake prediction theory, combined with field-to-source and source-to-field techniques, offers a method for determining the location and timing of future earthquakes in key geological structures and active fault zones. Following the 2021 Maduo M_S7.4 earthquake, seven more earthquakes with magnitudes ≥5 occurred along the eastern section of the Altyn Tagh fault zone and the Qilian Mountain seismic belt. According to active earthquake period statistics, the Qilian Mountains tectonic belt remains one of the most active seismic zones in mainland China. Among the moderate to strong earthquakes, the Menyuan M_S6.9 earthquake in Qinghai had the most significant impact.
The evolution from earthquake incubation to development is complex and multifaceted. The successful spatiotemporal prediction of the Menyuan M_S6.9 earthquake exemplifies the effectiveness of integrating long-, medium-, short-, and imminent-term earthquake forecasting by Chinese seismologists. In late 2015, the China Earthquake Administration's M7 Task Force identified high-risk zones for major earthquakes(M≥7) in mainland China for the period 2016-2025, based on geological structures, seismological data, and geophysical changes. The central section of the Qilian Mountain seismic belt(specifically the Lenglong Ridge Fault)was designated as one of these high-risk zones, representing a long-term forecast. In 2021, the China Earthquake Administration identified the region from Jiayuguan in Gansu Province to Menyuan in Qinghai Province as a seismic hazard zone for the following year(with a forecast of around magnitude 6), reflecting a medium-term prediction. Additionally, in December 2021, the Qinghai Earthquake Agency issued a short-term forecast(one month prior to the event), warning of a potential magnitude 6-7 earthquake in the Qinghai region.
An analysis of seismic activity tracking and anomalous spatiotemporal evolution leading up to the earthquake revealed several key seismological anomalies detected by the Gansu-Qinghai seismic network. Notably, the Qilian Mountain seismic belt experienced 1.95 years of quiescence for magnitude 5 earthquakes, which was subsequently interrupted by the M_S5.5 Aksai earthquake on August 26, 2021. Additionally, the M_L3.6 earthquake in Guyuan, Ningxia, on June 22, 2021, ended an 880-day period of quiescence in the central-eastern section of the Qilian Mountains. These disruptions increased concerns about the likelihood of intermediate-to strong-magnitude earthquakes in the region.
Major geophysical anomalies, including 23 instances of localized deformation, electromagnetic, and subsurface fluid anomalies, were detected along the Qilian Mountain structural belt over the past four years. Analyzing the sequence of these anomalies revealed that: 1)5 background anomalies(lasting over 1 year)were distributed 300km from the epicenter; 2)2/3 of the 8 medium-term anomalies(lasting 0.3 months to 1 year)were within 200km of the epicenter; and 3)4/5 of the 10 short-term and imminent anomalies(less than 3 months)were within 180km of the epicenter. Since July 2021, medium-to short-term anomalies have become concentrated in the central-eastern section of the Qilian Mountain seismic belt, an area with one of the strongest monitoring capabilities and densest station distributions within the Gansu-Qinghai seismic network. As these anomalies emerged in clustered, synchronous patterns, seismic authorities promptly organized field verification and tracking.
By integrating significant seismic events with geological structural features, authorities issued short-term forecasts prior to the earthquake, leading to effective disaster mitigation outcomes. This demonstrates that, under tectonic conditions in areas covered by seismic monitoring networks, the “sub-instability” theory can guide the research on anomalous changes in observational data, providing valuable support for earthquake prediction and reducing potential losses from future major earthquakes.

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GEOMETRIC STRUCTURAL FEATURE OF THE TANGDONG FAULT IN THE SOUTHEASTERN MARGIN OF TAIHANG MOUNTAIN: BASED ON SHALLOW SEISMIC EXPLORATION AND 3D MODELING

CAI Ming-gang, PENG Bai, LU Ren-qi, ZHANG Yang, LIU Guan-shen, XU Fang, TAO Wei, ZHANG Jin-yu, HAO Chong-tao

SEISMOLOGY AND GEOLOGY 2025, 47 (6): 1667-1687. DOI: 10.3969/j.issn.0253-4967.2025.06.20240015

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Accurate characterization of shallow fine-scale geometric structures in active faults is critically important for earthquake disaster prevention, mitigation efforts, and advancing our understanding of seismic mechanisms. Integrated approaches combining artificial seismic exploration with three-dimensional structural modeling provide powerful capabilities for revealing detailed spatial architectural characteristics of buried fault systems. This study investigates the Tangdong buried active fault along the seismically significant southeastern margin of the Taihang Mountains, employing advanced geophysical methods to elucidate its complex geometric configuration and kinematic behavior.
High-resolution imaging was achieved through a targeted small-spacing shallow reflection seismic exploration survey. A comprehensive array of 10 survey lines spanning a cumulative length of 28km yielded high-quality seismic reflection datasets. Advanced data processing techniques, including noise attenuation and velocity analysis, were applied to generate optimal high-resolution seismic profiles. These profiles enabled detailed structural interpretation of fault geometry and displacement characteristics. Subsequently, a geometrically constrained 3D fault model was constructed using the SKUA-GOCAD software platform, facilitating comprehensive spatial analysis of the fault system.
Key findings reveal the Tangdong Fault as a high-angle normal fault with a dominant North-Northeast(NNE)strike direction. Significant along-strike segmentation characterizes its shallow architecture: the northern segment features a bifurcated structure comprising two distinct subsidiary faults(F_3-1 and F_3-2). In contrast, south of Weixian Town, these faults converge into a single strand(F_3-2). Shallow dip angles exhibit considerable spatial variation, ranging from approximately 55° to 80°. Notably, the central segment between survey line L6(Panshitou Xincun) and line L7(north of Gangpo Village)displays a relatively gentler dip angle compared to adjacent segments, resulting in a distinctive saddle-shaped geometric configuration.
Clear spatial partitioning of recent activity is observed between the subsidiary faults. South of survey line L6, contemporary deformation is predominantly localized on Fault F_3-2, whereas north of L6, activity is exclusively manifested on F_3-1. Integration of deeper-penetration petroleum seismic profiles confirms the fault's listric geometry, characterized by a steep upper section that progressively shallows with depth. The eastern(F_3-1) and western(F_3-2)branches converge and merge into a unified fault plane at approximately 1.8km depth. The 3D structural model further validates this geometric configuration near line L4 in Weixian Town and effectively visualizes the along-strike dip variations.
These comprehensive findings provide fundamental insights into the three-dimensional geometry, segmentation patterns, and kinematic behavior of the Tangdong active fault. The integrated methodology significantly enhances our understanding of neotectonic deformation processes, offering critical scientific support for fault avoidance zoning, seismic hazard assessment, and earthquake risk mitigation strategies in this tectonically active region. This robust methodological framework establishes a transferable approach for characterizing concealed active fault systems in analogous tectonic settings globally.

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LATE QUATERNARY ACTIVITY OF THE XIETONGMEN TO DENGMECUO SEGMENT ALONG THE XAINZA-DINGGYE RIFT IN SOUTHERN QINGHAI-XIZANG PLATEAU

WANG Duo, CHEN Li-chun, LI Yan-bao, WANG Hu, JIA Yong-shun, GAO Yin-yi, XUE Ke-yi

SEISMOLOGY AND GEOLOGY 2025, 47 (3): 718-733. DOI: 10.3969/j.issn.0253-4967.2025.03.20250012

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The nearly north-trending rift system and nearly east-west strike-slip faults are the major structures accommodating the east-west extensional deformation within the Qinghai-Xizang Plateau. The rift systems are roughly separated by the nearly east-west striking Indus-Yarlung suture zone into north and south parts. The eastern rift systems are a strongly active major seismic zone in Xizang, which is characterized by faulted landforms of alluvial fans, river terraces, and moraines with large magnitude earthquakes. The Yadong-Gulu rift experienced the 1411 Damxung M8 earthquake and the 1952 Gulu M7.4 earthquake. On January 7, 2025, a M_S6.8 earthquake occurred along the southern segment of the Xainza-Dinggye rift in Dingri County, Xigaze, which caused widespread concern about the seismic and Late Quaternary active behaviors along the rift systems. However, few studies on fault activity at the junction of the north and south segments of the rift systems were conducted along both side of the Indus-Yarlung suture zone, which greatly hinders us from understanding the active deformation process and seismic activity of the rift systems in southern Qinghai-Xizang Plateau.

The Xainza-Dinggye rift can be divided into the north and south segments, named the Xainza and Dinggye rifts, by the Indus-Yarlung suture zone. The Xietongmen and Dengmecuo segments are the northern and southern adjacent segments of Xainza-Dinggye rift, respectively. The Xietongmen segment as the south end of the Xainza rift intersect with the Indus-Yarlung strike-slip fault. There are no strong earthquakes of over M6 recorded near the Xietongmen segment, and the small and medium earthquakes are significantly less than those of other segments. The 2025 Dingri earthquake has caused great social concern about whether a larger earthquake will occur between the Dengmecuo and Xietongmen segment of the Xainza-Dinggye rift, especially in the densely populated area of Xietongmen County. A recent earthquake risk survey has not yet found evidence of Holocene activity along the Xietongmen segment. Meanwhile, previous studies suggest very low activity during the late Quaternary along the Dengmecuo segment, which is in stark contrast to current strong earthquake activities. Therefore, there are still great uncertainties about the fault geometry and activity of the Xainza-Dinggye rift on both sides of the Indus-Yarlung suture zone.

To determine the Late Quaternary activity of the Xietongmen to Dengmecuo segments of the Xainza-Dinggye rift, we used remote sensing interpretation, field survey, optically stimulated luminescence, and radiocarbon dating methods on the displaced landforms. We found new evidence of the latest activity on the eastern and western branches of the Xietongmen segment at the past millennium. The western branch of the Xietongmen segment has crossed the Yarlung Zangbo River southward, then terminates at the intersection basin with the latest active branches of southern Yarlung Zangbo fault. Our results also suggest the Dengmecuo segment had strong activity with several hundred meters width since the late Quaternary. The latest faulting has extended northward into the mountainous area according to the surface ruptures and positioning aftershocks of the M_S6.8 earthquake. The nearly east-west striking Indus-Yarlung suture zone constitutes a north-trending structural gap zone between the two segments. The fault geometries, displaced landforms, and seismic activity of these two segments reveal that they are probably approaching by cutting through this structural gap, and the seismic risk cannot be ignored in the future. Due to the limitations of existing data, further detailed field investigations, geodetic observations, and geophysical deep data are needed to verify and improve our speculation. Our results of the Late Quaternary activity of the Xietongmen to Dengmecuo segments of the Xainza-Dinggye rift provide scientific support for seismic risk assessment of national engineering projects and post-disaster reconstruction of the recent M_S6.8 earthquake in the Xigaze region.

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CHARACTERISTICS OF LATE QUATERNARY ACTIVITY AND TECTONIC IMPLICATIONS OF THE MAYAXUESHAN FAULT IN THE EASTERN QILIAN SHAN

CHEN Yan-wen, YUAN Dao-yang, YAO Yun-sheng, YU Jin-chao, WEN Ya-meng, SU Rui-huan, SUN Hao

SEISMOLOGY AND GEOLOGY 2025, 47 (6): 1566-1585. DOI: 10.3969/j.issn.0253-4967.2025.06.20240052

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The Qilian Shan, Hexi Corridor, and Longzhong Basin, on the northeastern margin of the Tibetan plateau, form the leading edge of the plateau's outward advance into the mainland. They are young, critical components of the orogen, characterized by thrust faults, active folds, and strike-slip fault zones. The Mayaxueshan Fault(MYF), at the eastern end of the Qilian Shan, is a boundary thrust separating the northeastern Qilian margin from the Longzhong Basin. Constraining the late Quaternary activity and slip rate of the MYF is essential for elucidating regional deformation patterns and the mechanisms of Tibetan plateau uplift and outward growth, and it is also vital for seismic-hazard assessment.
Through remote-sensing interpretation, geological and geomorphic mapping, unmanned-aircraft photogrammetry, and optically stimulated luminescence dating, this paper examines the geometry, geomorphic expression, late Quaternary activity, and vertical slip rate of the MYF, and discusses its seismic risk and tectonic significance. Results show that the MYF is a SW-arching thrust-fault zone ~152km long, dipping SW-S at 32°~71°. The fault cuts a series of NE-SN-trending gullies, offsets landforms of multiple tiers, and forms scarps at 0.9~14.8m height. Variations in geometry and late Quaternary activity divide the MYF into three sections: Mayaxueshan(west), Baoquanshan(middle), and Hunanshan(east). Activity decreases from west to east, with respective activity epochs of late Holocene, end of late Pleistocene, and late Middle Pleistocene. From scarp heights and ages of corresponding geomorphic surfaces at Mayinggou and Shangbacigou, the vertical slip rate of the Mayaxueshan segment since the Holocene is(0.50±0.02)mm/a. This segment is thus capable of generating strong earthquakes of M6.7-7.0 in the future.
Comparative analysis suggests the western and middle-eastern sections of the MYF may belong to different fault systems. The Mayaxueshan segment shows a closer structural affinity with the Zhuanglanghe Fault; together they form arcuate thrust belts produced by regional compression. In contrast, the Baoquanshan segment appears to have evolved synergistically with the Baiyinbaiyangshugou Fault, constituting another arcuate belt in the north-central Longzhong Basin. As a whole, the MYF inherits an ancient arcuate architecture bulging toward the SW. Since the late Quaternary it has undergone progressive, west-to-east segmented reactivation, producing a mismatch between fracture geometry and the NE-oriented regional compressive stress and yielding along-strike differences in late Quaternary activity. On the northeastern Tibetan margin, NE-directed regional compression acting against stable blocks has generated foreland-propagating thrust-fold belts and curved extrusion structures, leading to crustal shortening and thickening and rapid mountain uplift. Consequently, the Tibetan plateau has risen and extended northeastward through the stepwise outward push of secondary blocks.

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THE CHARACTERISTICS OF TECTONIC STRESS FIELD AND SPATIOTEMPORAL EVOLUTION OF SEISMIC ACTIVITY IN LUXIAN-RONGCHANG

TANG Mao-yun, LI Cui-ping, HUANG Shi-yuan, DONG Lei, GAO Jian, LI Yong

SEISMOLOGY AND GEOLOGY 2025, 47 (4): 1183-1203. DOI: 10.3969/j.issn.0253-4967.2025.04.20240008

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In recent years, seismic activity in the Luxian-Rongchang area has increased significantly and garnered widespread attention from researchers. Since the M_S6.0 earthquake in Luxian County in 2021, there has been a significant increase in small seismic activity and significant spatiotemporal migration distribution characteristics in the area, which has brought new challenges to the understanding of seismic risk in the region. The genetic mechanism and prevention and control behaviors for future risk have become a common concern for the public. This article relocated earthquakes using double-difference location since July 2021 and inverted the focal mechanism solutions of M_L≥3.0 earthquakes, employing the CAP method since 2009, in the Luxian-Rongchang area, based on seismic data from the Chongqing and Sichuan Regional Seismic Networks. Furthermore, it explored the seismogenic fault and the seismogenic mechanism of earthquakes in different periods by fitting parameters of the seismogenic fault and inverting the tectonic stress fields.

The results show that seismic activity in Luxian-Rongchang has migrated from the Luoguanshan anticline to two areas within the Yujiasi and Xianglushan synclines since 2021. The relocated seismic activity exhibits two NE-NNE-oriented bands, within an overall depth range of 2~12km. The rupture of the seismic source is mainly of the thrust type, with an average focal depth of 3.8km, which is consistent with the depth of the shale layer of the Silurian Longmaxi Formation. The seismic activity within the Yujiasi syncline exhibits migration towards the NE direction over time, and the southwestern end forks into two branches. The strike direction of the seismic activity band within the Xianglushan syncline is NNE and dip towards the SE direction. There are no matching faults on the surface of the two seismic bands. Based on the determination of fault plane parameters and the focal mechanism solutions for earthquakes, it is speculated that the seismogenic fault of the seismic band in Yujiasi syncline corresponds to a hidden thrust fault with a strike of NNE and dip of NWW; The Xianglushan seismic band is a hidden thrust fault with a strike of NNE and dip of SE. The tectonic stress environment in the study area is relatively simple, which is mainly dominated by NWW-oriented horizontal compression tectonic stress fields. Significant earthquakes in these two regions are primarily controlled by regional stress fields and hidden faults.

In addition, we argue that the mechanism of seismic activities in Luxian-Rongchang resulted from rupture along pre-existing hidden faults, driven by fluid pore pressure diffusion, as was the case before. The difference is that fluid pore pressure is probably raised by different sources, including the long-term injection of wastewater in Luoguanshan anticline by the former research and hydraulic fracturing within the Yujiasi and Xianglushan synclines. Due to the lack of detailed shale gas development data in this study, there are still shortcomings in revealing its seismogenic mechanism through spatiotemporal distribution characteristics and focal mechanism solution. Further comprehensive data collection is needed in the future. Furthermore, it is worth noting that the earthquake magnitude in the region is still dominated by small and medium-sized earthquakes. Post-earthquake risks still warrant attention.

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3-D CRUSTAL S-WAVE VELOCITY STRUCTURE IN AND AROUND THE DATONG VOLCANIC GROUP: CONSTRAINTS FROM DIRECT TOMOGRAPHIC IMAGING OF AMBIENT-NOISE SURFACE WAVES

LI Ruo-hao, LEI Jian-she, SONG Xiao-yan

SEISMOLOGY AND GEOLOGY 2025, 47 (4): 1090-1112. DOI: 10.3969/j.issn.0253-4967.2025.04.20240121

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The Datong volcanic group is located in the central part of the North China Craton, and it has attracted widespread attention due to its complex geological tectonic environment with a high level of seismicity. In this study, we collect continuous seismic waveforms from January to December 2020 recorded at 56 provincial permanent seismic stations of the China Earthquake Administration. In data processing, we first conduct rigorous preprocessing of the raw waveform data, including mean removal, detrending, and bandpass filtering to ensure data quality. After performing cross-correlation of ambient noise, we manually extract Rayleigh wave dispersions in the periods of 5-30s, which effectively reflect the velocity structural characteristics of the crustal and uppermost mantle. Based on the extracted dispersion data, we apply the direct surface wave tomographic method to construct a three-dimensional S-wave velocity structure model extending to a depth of 40km with a spatial resolution of 0.75°×0.75° in the horizontal directions.

Our imaging results show that the distribution of S-wave velocities corresponds well with geological structural features in the upper crust. The Shanxi rift zone generally exhibits low-velocity anomalies, reflecting the structural characteristics of the Taiyuan Basin, Xinding Basin, and Datong Basin, which are speculated to be related to the Cenozoic sedimentary layers covering the shallow subsurface in this area. In contrast, the Lüliang Mountains and Taihang Mountains exhibit high-velocity anomalies due to their exposed bedrock. In the middle to lower crust, the low-velocity anomaly beneath the Datong volcanic group extends across the northern part of the Shanxi rift zone to the west of the zone, possibly caused by extensive magma activity in the crust due to the upwelling of hot mantle materials under the region. A discontinuous low-velocity anomaly body exists in the crust beneath the Datong volcanic area, potentially serving as a conduit for magma upwelling, but with a possibly discontinuous magma supply. Combining previous deep mantle imaging studies, we speculate that the crustal low-velocity anomalies reflecting hot materials beneath the Datong volcanoes could be jointly caused by the westward deep subduction of the Pacific slab, the extrusion of asthenospheric mantle materials by the Indo-Eurasian collision, and mantle plume activities. Our findings not only deepen our understanding of the deep structure and dynamics of Datong volcano, but also provide new insights into understanding the tectonic evolution of the North China Craton.

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RELOCATION AND FORESHOCK SEQUENCE IDENTIFICATION OF DINGRI M_S6.8 EARTHQUAKE IN XIZANG

YIN Xin-xin, ZUO Ke-zhen, ZHAO Cui-ping, CAI Run

SEISMOLOGY AND GEOLOGY 2025, 47 (3): 850-868. DOI: 10.3969/j.issn.0253-4967.2025.03.20250025

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Using seismic data from the Tibet Regional Seismic Network between January 2021 and January 2025, we relocated 7, 951 earthquakes employing the double-difference algorithm. To ensure relocation reliability, we selected phase data with minimal travel-time residuals, constraining earthquake pairs to a maximum separation of 30km and requiring at least eight common phase arrivals per pair. This yielded 4, 370 high-precision relocations, with average relative errors of 0.130km(longitude), 0.131km(latitude), and 0.199km(depth). The relocated mainshock location is(28.501°N, 87.477°E)with a focal depth of 9.3km. The aftershock sequence extends approximately 70km in a nearly north-south direction, with depths mainly concentrated between 3 and 15km.

We conducted a detailed analysis of the foreshock activity preceding the M_S6.8 Dingri earthquake. Due to sparse station coverage near the epicenter, traditional seismic monitoring methods were insufficient for detecting small events. To address this, we applied the deep learning-based PhaseNet model to continuous waveform data from the nearest station(ZHF, ~50km from the epicenter), in combination with a single-station amplitude-magnitude empirical relationship for magnitude estimation. This approach significantly improved catalog completeness. Within the 56-hour window prior to the mainshock, we identified 90 seismic events, of which 80(88.9%)were microearthquakes with magnitudes M_L<2.0. In contrast, the regional network recorded only 8 events in the same period. A reliable single-station magnitude calibration was established(log₁₀A=0.77M_L+1.36) using 293 aftershocks. For commonly detected events with M_L<2.0, the average magnitude difference between the single-station and regional network methods was just 0.09, confirming the accuracy of the single-station approach. Based on the enhanced catalog and using the maximum curvature method accounting for magnitude uncertainty, the completeness magnitude was determined to be M_L1.10. The b-value, estimated via the maximum likelihood method, was 0.58±0.07. Relocation results show that foreshocks were spatially clustered within the eventual aftershock zone, approximately 20km from the mainshock epicenter. Eight foreshocks occurred within the final hour before the mainshock, with the largest(M_L3.7)occurring approximately one hour prior.

These findings demonstrate that deep learning-based, single-station detection methods can substantially enhance earthquake monitoring in regions with sparse seismic networks. The spatial and temporal characteristics of the foreshock sequence offer critical insights into earthquake preparation processes. The single-station magnitude estimation method presented here provides a valuable reference for seismic monitoring in similarly data-limited regions.

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RESEARCH ON GROUND MOTION SIMULATION OF THE DINGRI M_S6.8 EARTHQUAKE IN XIZANG BASED ON DIFFERENT SOURCE MODELS

YIN Xiao-fei, QIANG Sheng-yin, ZHANG Wei, SHAO Zhi-gang, WANG Wu-xing, YUAN Xiao-xiang, LI Yong-sheng, LIU Hao

SEISMOLOGY AND GEOLOGY 2025, 47 (3): 897-916. DOI: 10.3969/j.issn.0253-4967.2025.03.20250038

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On January 7, 2025, a magnitude M_S6.8 earthquake occurred in Dingri County, Shigatse City, within the Xizang(Tibet)Autonomous Region. This normal-faulting earthquake struck the Lhasa Terrane in the southern Tibetan plateau, a region characterized by a series of nearly north-south trending normal faults and associated tectonic rift valleys—features indicative of the region’s ongoing extensional deformation and potential for future strong seismic events. Given the high seismic hazard in the southern Qinghai-Xizang Plateau and the area’s complex mountainous terrain, which increases the likelihood of secondary disasters such as landslides, assessing strong ground motion is crucial for linking and quantifying seismic hazard and risk. Accordingly, simulating strong ground motion for a hypothetical M_S6.8 earthquake in this region holds significant practical value. Such analysis contributes both theoretical insight and practical guidance for regional seismic disaster prevention and mitigation.

This study simulates the strong ground motion of the Dingri M_S6.8 earthquake using two source models derived from joint inversion of InSAR coseismic deformation, teleseismic waveforms, and strong motion recordings. A three-dimensional curvilinear finite-difference method with curved grid meshing is employed to model the seismic wave propagation and ground motion characteristics. By comparing the results of the two source models, the spatial distribution of seismic ground motion and the underlying causative mechanisms are analyzed. The key findings are as follows:

(1)Simulated ground velocity time histories at four near-field stations, processed with a 0.2Hz low-pass Butterworth non-causal filter, closely match the observed strong motion records, verifying the accuracy and reliability of the simulations.

(2)Due to a NNE-directed unilateral rupture, peak ground velocities(PGVs)in the forward rupture direction(NNE)are significantly higher than those in the reverse direction(SSW), demonstrating a clear rupture directivity effect.

(3)A comparison of PGV distributions across the fault shows that values on the upper plate(western side)are significantly higher than those on the lower plate(eastern side), indicating a strong upper-plate effect. Vertical surface displacements on the fault’s upper plate, as simulated by the two models, reach 2.0m and 2.1m, respectively—values that are in close agreement with field measurements from the Dingri earthquake geological survey.

(4)Both source models simulate a maximum seismic intensity of Ⅸ, with high-intensity zones extending predominantly in the NNE direction. The simulated intensity distributions are generally consistent with field observations, though discrepancies exist in two areas: From northern Dingri County to southwestern Angren County, and in central-southern Gangba County. The intensity distribution produced by source Model 2 shows better agreement with the observed data.

This study highlights the importance of using source models derived from the joint inversion of InSAR, teleseismic, and strong motion data—or a broader combination of geophysical constraints including GPS—to improve the accuracy of strong ground motion simulations. The results offer an important scientific basis for advancing our understanding of seismic wave propagation and strong ground motion characteristics associated with normal faulting earthquakes in the southern Qinghai-Xizang Plateau.

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PN WAVE VELOCITY AND ANISOTROPY TOMOGRAPHY IN THE UPPERMOST MANTLE OF PAMIR PLATEAU AND ADJACENT REGIONS

LIU Jia-xin, PEI Shun-ping, GUO Yi-cun

SEISMOLOGY AND GEOLOGY 2025, 47 (2): 547-560. DOI: 10.3969/j.issn.0253-4967.2025.02.20240155

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Since the Cenozoic era, the ongoing collision between the Indian Plate and the Eurasian Plate has formed the largest and youngest continent-continent orogenic belt on Earth. The Pamir Plateau, located at the western end of the India-Eurasia collision zone, is one of the most tectonically active and structurally complex regions globally. It is characterized by widespread folds and faults, frequent M≥7.0 earthquakes, and numerous intermediate to deep-focus earthquakes, making it an ideal natural laboratory for studying plate tectonics and orogenic processes. Investigating the velocity and anisotropic structure of the uppermost mantle beneath the Pamir Plateau and its surrounding regions is of great significance for understanding the tectonic deformation characteristics and dynamic mechanisms of continent-continent collisions. The Pn-wave travel-time tomography method is an effective approach for studying the physical properties of the uppermost mantle, it offers several advantages: 1)Pn-wave ray paths are concentrated in the uppermost mantle, and there is a relatively abundant record of arrival times; 2)this method can simultaneously obtain mutually constrained velocity and anisotropy structures of the uppermost mantle; and 3)although it only provides lateral velocity and azimuthal anisotropy information for the uppermost mantle, it offers higher resolution and accuracy, and the results are not influenced by deeper mantle structures. In this study, using 363 414 Pn-wave arrival times recorded from 103 190 events at 471 stations, we performed Pn-wave travel-time tomography to obtain higher-resolution seismic velocity and anisotropy images of the uppermost mantle beneath the Pamir Plateau and adjacent regions compared to previous studies. The tomography results exhibit a clear correlation with geological structures, with the following key findings: 1)Lower Pn-wave velocities are observed in tectonically active regions such as the Pamir Plateau, Tienshan, Hindu Kush, West Kunlun, and Altyn Tagh, while higher velocities are found in stable ancient blocks like the Indian Plate, Tarim Basin, Tajik Basin, Junggar Basin, and Fergana Basin. 2)The India-Eurasia collision has generated strong Pn-wave azimuthal anisotropy. On both sides of the Pamir collision zone, in the Indian Plate, Tarim Basin, and Tajik Basin, the anisotropy axes align with the plate motion directions. In contrast, in the central part of the collision zone, the anisotropy axes are nearly perpendicular to the maximum compressive stress and crustal motion directions. A similar anisotropy pattern is observed on both sides of the Tienshan. Pn-wave velocities primarily reflect the properties of the uppermost mantle, which is predominantly composed of peridotite. Temperature has a more significant influence on velocity than pressure, and temperature variations are closely linked to tectonic activity. Typically, stable cratonic regions exhibit higher Pn-wave velocities, while tectonically active or volcanic regions with significant fluid activity show lower velocities. The velocity differences between active and stable blocks are mainly attributed to temperature variations. Comparing the surface boundary between the Eurasian and Indian Plates with the high-velocity anomalies observed in tomography reveals that the high-velocity anomaly of the Indian Plate extends approximately 200km northward beneath the Tibetan plateau. Pn-wave azimuthal anisotropy is generally attributed to the preferred orientation of olivine crystals caused by mantle deformation. The anisotropy direction of the Indian Plate is predominantly north-south, consistent with its GPS motion and maximum compressive stress directions. From the Tajik Basin to the western Tarim Basin, the fast-axis anisotropy direction gradually shifts from NW to NE, aligning well with the maximum compressive stress and GPS directions. This is primarily due to the simple shear between the crust and upper mantle caused by crustal shortening and uplift, while the lithospheric mantle subducts during intense plate collision. Similarly, in the Fergana Basin, northern Tarim Basin, and Junggar Basin, the anisotropy directions are nearly north-south, consistent with GPS and maximum compressive stress directions. In regions of intense deformation, such as the Hindu Kush, Pamir Plateau, West Kunlun, Altyn Tagh, and Tienshan orogenic belts, the anisotropy directions are perpendicular to the maximum compressive stress directions, indicating strong pure shear deformation in the uppermost mantle. This suggests that in continent-continent collision zones, not only does the crust undergo significant shortening and uplift, but the uppermost mantle also experiences substantial compressional deformation. Finally, we propose an improved dynamic model of continent-continent collision to elucidate the collision process between the Indian Plate and the Tarim and Tajik Basins, as well as the mechanisms of anisotropy formation.

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STUDY ON THE QUATERNARY ACTIVITY CHARACTERISTICS AND TECTONIC SIGNIFICANCE OF THE WANGHU FAULT IN TAIYUAN BASIN

ZENG Jin-yan, WANG Kai, CHEN Wen, REN Rui-guo, YOU Wen-zhi, GU Bi-ying

SEISMOLOGY AND GEOLOGY 2025, 47 (4): 1167-1182. DOI: 10.3969/j.issn.0253-4967.2025.04.20240012

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The Wanghu Fault is a major geological structure in Jinzhong City, Shanxi Province, traversing the Yuci urban area. Previous studies suggested that the fault originated in the Late Pleistocene, extending approximately 17.0km from Nanguan in the south to Dongshagou in the north, with a NNW strike and SW dip. However, previous research was primarily based on hydrogeological and seismic safety assessments, lacking detailed investigation into the fault’s structural characteristics and activity history—particularly regarding definitive evidence of its active period. This uncertainty has posed challenges for disaster prevention and urban planning in Jinzhong City.

To address this gap, an integrated investigation was conducted combining shallow seismic exploration, field geological surveys, and borehole-profile studies. In the northern section—located in the Yuci urban area where morphological features are indistinct—shallow seismic imaging, borehole profiling, and chronological testing were employed to delineate the fault’s position and assess its activity. In the southern section near Donghao and Liutai villages, where topographic contrast between mountainous and basin terrain is more pronounced, field geological surveys and age-dating methods were used to trace the fault along the geomorphic boundary.

The results reveal that the Wanghu Fault is a complex fault zone composed of multiple strands, forming the eastern boundary of the southern Taiyuan Basin. It extends approximately 27.0km, strikes nearly N-NNW, dips west to southwest at 45°~60°, and is classified as a normal fault with a dextral(right-lateral)slip component. The fault has been active since the late Middle Pleistocene. It comprises two segments:

The northern segment, concealed beneath urban cover, begins at Xiaoyukou Village and connects with the eastern segment of the Tianzhuang Fault. It passes through Sucun, Niedian, Beiguan, and Dadongguan villages, intersecting the Xiaohe Fault. It trends nearly N-S, dips westward, with a minimum displacement of 4.46~4.79m, and spans 16.0km.

The southern segment is exposed in the Loess Plateau area, beginning north of Donghao Village and extending southeast through Beizhao, Yuchengping, and Liutai Villages. It strikes NNW, dips southwest at 60°~75°, with a minimum displacement of 0.7~1.3m, and is about 11.0km long.

Analysis of fault location, activity period, and gravity anomaly data indicates that the Wanghu Fault marks the Quaternary structural boundary at the northeastern margin of the Taiyuan Basin. Together with the Jiaocheng Fault to the west and the Taigu Fault to the southeast, it outlines a semi-fan-shaped fault-controlled basin, deeper in the west and shallower in the east. This finding challenges previous interpretations that the basin’s northeastern boundary was merely a depositional interface between Late Cenozoic fluvial-lacustrine and loess sediments, lacking any associated faulting. Furthermore, it revises the timing of fault activity from the previously assumed Late Pleistocene to the Middle Pleistocene. These findings are critical for earthquake hazard mitigation, urban development, and land-use planning in Jinzhong City. They also provide valuable insights into the tectonic evolution and seismic potential of the Taiyuan Basin.

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SIMULTANEOUS INVERSION OF CRUSTAL VELOCITY STRUCTURE AND EARTHQUAKE RELOCATION IN THE NORTHWEST OF THE BEIJING AREA

GONG Meng, ZOU Xian-kun, WANG Xiao-shan, LI Guang, SHENG Shu-zhong, LI Hong-xing, XU Rong-hua, LU Chang-sheng

SEISMOLOGY AND GEOLOGY 2025, 47 (4): 1132-1151. DOI: 10.3969/j.issn.0253-4967.2025.04.20240014

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The northwest Beijing area is located in the northwest of the ancient North China Craton block. Due to the long-term and frequent geological structure evolution and tectonic movement of the North China block, the complex geological structure pattern has been created in this area, with the Yanshan tectonic belt in the north, the North China rift basin in the south, the Shanxi depression belt in the west, and the Bohai Sea in the east. Due to its unique geographical location and frequent seismic activity, this area has long been a key concern for geologists and seismologists.

We collected P waves’ absolute and relative travel time of 20 442 earthquakes in the northwest of the Beijing area record by 145 seismic stations deployed in Hebei, Shanxi and Neimeng Provinces, during January 2009 to December 2020 and used double-difference seismic tomography joint inverted the Seismic source location parameters and the 3D P-wave velocity structure of the study area. To improve the uniformity of ray coverage and the accuracy of data in inversion, seismic phases were selected based on the following conditions. 1)The P-wave phases of each earthquake are required to be recorded by at least four stations; 2)Seismic phases with error greater than ±0.5s were eliminated by using the epicentral distance-travel time fitting curve of the selected earthquake; 3)The distance between each earthquake pair is required to be less than 10km, and the number of double difference data formed by each earthquake pair is required to be larger than 8.

In the inversion process, the research area is divided into three dimensional grids according to the station location and earthquake distribution. The horizontal direction is divided into 0.3°×0.3° grids, In vertical depth, nodes are set at 0km, 5km, 10km, 15km, 20km, 25km, 30km, 35km, 42km, 50km, and 60km respectively. The value of the damping coefficient is set to 600, the value of the smoothness factor is set to 40, and the number of iterations is set to 10. Thus, after 10 iterations of inversion, the distribution range of residual travel time of seismic data decreases from ±3s to±1s, and the horizontal and vertical errors of the source location after relocation are 0.1~0.9km and 0.1~1.5km, respectively. In order to ensure the accuracy of velocity structure inversion, the reliability of the results is evaluated by using Differential Weighted Sum of Nodes(DWS) and a detection board. Finally, the P-wave velocity structure at depths less than 50km underground in the study area, along with the relocation source parameters of 17613 earthquakes, are obtained.

The results show that: 1)The focal depth of earthquakes is mainly distributed in the 5~25km depth range. The relocated earthquakes are more closely clustered near the fault zone and the seismic spatial distribution can better describe the geometric morphology of deep faults. There are several NE-trending and NW-trending faults with deep development and steep dip Angle in the Zhang-Bo earthquake zone. Both the Xiadian fault and the Xinhe fault are nearly vertical deep faults with a high dip Angle. 2)The variation of the P-wave velocity had a good correlation with the topography, geomorphology and tectonic environment. Influenced by the surface sediments, the P-wave velocity in shallow crust of the Shanxi fault depression belt, Hebei plain and inter-mountain basin shows low-velocity anomalies. The P-wave velocity in the crust of the junction area of Shanxi, Hebei, and Mongolia is relatively low. The significant low-velocity anomaly of the Zhang-Bo earthquake belt at depth of 42km underground is related to the Destruction of the North China Craton and the upwelling of deep thermal materials. 3)Based on the P-wave velocity structure and seismic relocation results, the fault is developed in the earthquake-prone areas in the northwest of Beijing and its adjacent areas. Most earthquakes occur in the brittle-ductile transition zone between the brittle upper crust and the ductile middle and lower crust. In summary, the seismicity in the northwest Beijing area is closely related to the development of deep and shallow faults and the velocity structure.

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