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    RESEARCH ADVANCES OF THE LATE QUATERNARY ACTIVITY OF XIAOJIANG FAULT ZONE IN THE SOUTHEASTERN MARGIN OF QINGHAI-XIZANG PLATEAU
    CHANG Yu-qiao, ZHANG Hui-ping, ZHAO Xu-dong
    SEISMOLOGY AND GEOLOGY    2026, 48 (2): 279-313.   DOI: 10.3969/j.issn.0253-4967.20240135
    Abstract （468）   HTML （25）    PDF（pc） （9401KB）（206）       Save

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

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    LATE-QUATERNARY ACTIVITY OF THE XUSHUI SOUTH FAULT AND NIUDONG FAULT IN THE NORTH OF THE NORTH CHINA PLAIN, EVIDENCE FROM DRILLING
    HUANG Xiong-nan, YANG Xiao-ping, LIU Bao-jin, SHI Feng, ZHUANG Qi-tian, HAO Hai-jian, SHI Jin-hu, SUN Hao-yue, LU Ren-qi, HU Zong-kai, LI Kang, CAO Jun, SHU Peng, REN Guang-xue, WANG Zhen-nan
    SEISMOLOGY AND GEOLOGY    2026, 48 (1): 19-42.   DOI: 10.3969/j.issn.0253-4967.20240075
    Abstract （414）   HTML （30）    PDF（pc） （12851KB）（207）       Save

    Since the Late Pleistocene, the North China Plain, far from plate boundaries, has experienced a transformation in its geodynamic environment, with most pre-existing normal faults no longer adapting to the new tectonic stress field and thus becoming inactive. However, historical and modern earthquake catalogues indicate that the epicenters of many strong earthquakes occur near pre-existing buried normal faults on the North China Plain. There is currently a debate over whether strong earthquakes are caused by the continued activity of these pre-existing faults or by new faults.

    The Xiong'an New Area is located in the northern part of the North China Plain. Based on seismic reflection profiles and drilling studies, previous studies suggest that some pre-existing faults in this region were active into the early Quaternary but have been inactive since the Late Pleistocene. Relevant research lacked evidence from high-precision composite drilling profile investigations, making the conclusion about inactivity since the Late Pleistocene questionable. The crust-mantle structure in this area is unique, the mantle is uplifted and there is deep fluid convection along pre-existing faults, which is similar to typical intraplate seismic zones in the world, such as the New Madrid area in the United States. The epicenter of the M5¾ earthquake in 1679 was located in the study area. The determination of the latest active age of the fault is of great significance for the seismic hazard assessment of Xiong'an New Area and further research is needed. In this paper, based on shallow seismic exploration, we analyze the burial depth of the upbreakpoints of the Xushui South Fault and the Niudong Fault in the Xiong'an New Area using a composite drilling profile method, and then determine fault activity using Quaternary dating methods.

    The targeted shallow reflection seismic explorationwith survey lines laid across the Xushui South Fault and the Niudong Fault has confirmed that the upper breakpoints of the middle section of the Xushui South Fault and the eastern branch of the middle section of the Niudong Fault are both at a depth of about 80 meters or less. This result may indicate the Xushui South Fault and the Niudong Fault were active during the Late Pleistocene. Perpendicular to the projection traces of the upper breakpoints of these two faults, two composite drilling profiles were arranged with boreholes on both sides of the shallowest identified upper breakpoints. They are named as the Rongcheng drilling profile and Xiongxian drilling profile, respectively. The drilling profiles consist of 5 and 4 drill holes, respectively, and the final hole depth is approximately 150 meters for both. The average hole separations were 23.3m and 30.4m respectively.

    Based on material composition, particle size, color and consolidation degree, etc., we divided and recorded 104 and 112 units respectively for the two rows of drilling cores at the drilling site. After a comprehensive analysis, the detailed units mentioned above are reclassified and combined into 8 and 9 stratigraphic units, according to sedimentary cycles, sedimentary facies and other characteristics, and in combination with the dating results. These units cover the Holocene to the low Pleistocene series. The systematic throws of the top and bottom boundaries of the stratigraphic units and the fault zone structures preserved in the core led to the recognition of two normal faults in both the Rongcheng and Xiongxian drilling profiles. The faults in the Rongcheng profile have an up-breakpoint buried approximately 24m deep and the vertical offset of the base of the Upper Pleistocene is 15.15m. For the Xiongxian drilling line, the buried depth of the up-breakpoint of the faults is about 42m, with a vertical offset at the base of the Upper Pleistocene of 9.1m. The dating results of the samples collected near the up-breakpoints suggest that the middle segment of the Xushui South Fault and the eastern branch of the Niudong Fault were still active around approximately 20,000 years and 36,000 years ago, respectively, indicating they are Late Pleistocene faults.

    Since the Paleogene, the Xushui South Fault and the Niudong Fault have been active as bounding normal faults for the Rongcheng Uplift and the Niutuo Town Uplift, with their lower endpoints reaching depths of approximately 20 kilometers. The lithosphere beneath these faults exhibits a cold crust and a warm mantle, with a slight bulge in the upper mantle. The Xushui South Fault and the Niudong Fault serve as conduits for the ascent of deep fluids and heat. The geothermal gradient in the uplifted areas is notably higher than in the surrounding subsiding basins. Our work indicates that these faults remained active in a normal faulting mode at the end of the Late Pleistocene. Combined with previous shallow seismic exploration, it’s suggested that the Xushui South Fault and the Niudong Fault were only active in their central segments after the Late Pleistocene.

    Although the activity of these faults has diminished since the Late Quaternary, their deep-seated environment is similar to that of the New Madrid Seismic Zone in the United States, where a soft, warm upper mantle contacts a cooler, harder crust, and uneven crustal density leads to stress accumulation preferentially along pre-existing faults. Additionally, these faults act as pathways for deep fluids, which can trigger strong earthquakes.

    The continued activity of the Xushui South Fault and the Niudong Fault at the end of the Late Pleistocene is related to their deep-seated environment. Some historical strong earthquakes in the North China Plain may have been triggered by reactivation of pre-existing faults with similar characteristics.

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    SEIMOTECTONIC ANALYSIS OF 2023 JISHISHAN MS6.2 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
    Abstract （401）   HTML （31）    PDF（pc） （17791KB）（176）       Save

    The 2023 Jishishan MS6.2 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 MS6.2 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 MS6.2 event.

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    A STUDY ON THE DURATION OF AFTERSHOCK SEQUENCES FOLLOWING SIX LARGE EARTHQUAKES IN North China
    LIU Yue, LÜ Xiao-jian
    SEISMOLOGY AND GEOLOGY    2026, 48 (1): 142-161.   DOI: 10.3969/j.issn.0253-4967.20250097
    Abstract （376）   HTML （20）    PDF（pc） （5203KB）（177）       Save

    The duration of the aftershock sequence following large earthquakes is a crucial issue in seismicity research and is significant for seismic hazard estimation. Previous studies based on three large earthquakes that occurred since 1966 have provided important insights into aftershock activity and duration in North China. However, understanding the persistence of aftershock sequences following historical large earthquakes remains limited. In this study, we aim to comprehensively study the duration of long-lived aftershocks following large earthquake sequences in North China. Based on the modern earthquake catalog from 1970 to 2022, combined with historical earthquake records, we systematically investigate the aftershock characteristics of six large earthquakes with a magnitude of M≥7 in North China, including the 1668 Tancheng M8½, 1679 Sanhe-Pinggu M8, 1937 Heze M7.0, 1966 Xingtai MS7.2, 1975 Haicheng MS7.3, and 1976 Tangshan MS7.8 earthquakes.

    The determination of the aftershock zone is crucial in this study. We conducted a thorough review of existing research on earthquake rupture, including field investigations, fault slip inversions, and aftershocks distribution. By synthesizing these results with the seismic catalog data used in this study, we selected the aftershock zones. According to the analysis of the spatial distribution of earthquakes, temporal frequency variation, and data fitting based on the Omori-Utsu law, our study shows that the aftershocks of the six large earthquakes are still ongoing.

    The aftershock zones of the Xingtai MS7.2 and Haicheng MS7.3 earthquakes are dominated by events with magnitudes around ML≥3. In contrast, ML≥4 earthquakes at a rate of approximately 1.5 events/a from 2013 to 2022 are observed in the Tangshan MS7.8 aftershock zone. We fit the data of the Haicheng and Tangshan earthquake sequences according to the Omori-Utsu law. The p value is around 0.8, indicating a slow decaying rate. For historical earthquakes, the spatial-temporal distribution of events indicates that aftershocks are alive. In the aftershock zone of the 1668 Tancheng M8 earthquake, an average of 0.63 earthquakes/a with ML≥3.0 occurred from 2013 to 2022. Additionally, the 1679 Sanhe-Pinggu aftershocks with magnitudes ML≤2 are still active. Following the 1937 Heze M7 earthquake in Shandong Province, a MS5.9 strong aftershock occurred in 1983. The aftershock zone records an average of 1.8 earthquakes/a with ML≥2.0 during the last decade, which is higher than the rate in surrounding areas. Hence, the aftershock sequences of the Tancheng and Sanhe-Pinggu earthquakes have persisted for 354 and 343 years, respectively, and the Heze earthquake sequence has lasted for nearly 90 years.

    According to the rate-and-state friction law, we deduce that the aftershocks can last for more than 300 years with a fault slip rate less than 1mm/a. This also supports the ongoing long-lived sequences of the 1668 Tancheng and the 1679 Sanhe-Pinggu earthquakes. The tectonic loading rate in North China is slow, causing a long recurrence interval of large earthquakes, as well as the slow decaying of aftershocks.

    In all, our results indicate that aftershock sequences following M≥8 earthquakes in North China can persist for over 300 years, and the aftershock sequences of M>7 earthquakes such as the Xingtai, Haicheng, and Tangshan earthquakes, will continue for a long period. This study provides us with a further understanding of the aftershock decay in North China.

    The aftershock sequences of the Tancheng and Sanhe-Pinggu earthquakes have persisted for 354 and 343 years, respectively; the Heze earthquake sequence has lasted for nearly 90 years; and the Xingtai, Haicheng, and Tangshan regions remain seismically active with frequent small earthquakes. These findings indicate that aftershock sequences of M≥8 earthquakes in North China can persist for over 300 years, and thoses of modern large earthquakes, such as Xingtai, Haicheng, and Tangshan, will continue for a long period.

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    THE LATE QUATERNARY ACTIVITY CHARACTERISTICS OF THE MIDDLE-NORTHERN SECTION OF THE SHANGWUJING FAULT IN THE LUXI BLOCK
    WANG Lei, REN Zhi-kun, WU Hong-bin, WANG Zhi-cai, ZHU Xiao-xiao, FU Jun-dong, JI Hao, XUE Jun-zhao, LEI Zhao-wei, WANG Ji-qiang
    SEISMOLOGY AND GEOLOGY    2026, 48 (1): 1-18.   DOI: 10.3969/j.issn.0253-4967.20250065
    Abstract （371）   HTML （48）    PDF（pc） （17021KB）（180）       Save

    Since the Middle to Late Miocene, the tectonic regime of eastern North China has undergone a significant transition from extensional deformation to a shear-dominated system under a nearly E-W compressional stress field. This shift is attributed to the combined influence of the eastward extrusion of the Tibetan plateau and the retreat of the West Pacific Plate. The Tanlu fault zone, a major active strike-slip structure in eastern China, serves as the tectonic boundary between the Luxi and Ludong blocks. Within the Luxi Uplift, located in the east part of the North China Craton, a prominent basin-and-range system has developed, controlled by a series of NW-trending faults. Late Quaternary activity along these faults, including the Cangni Fault, Xintai-Mengyin Fault, Tongyedian-Sunzu Fault, Zhangdian-Renhe Fault, Yidu Fault, and Shuangshan-Lijiazhuang Fault, demonstrates a parallel, roughly equidistant arrangement, with a convergent pattern toward the Tanlu fault zone.

    The Shangwujing Fault is a crucial NE-trending dextral strike-slip fault, and the Shuangshan-Lijiazhuang Fault is a NW-trending sinistral strike-slip fault. Spatially, these two faults form an X-shaped conjugate structure, which governs the formation and evolution of the Linqu Basin. The shallow surface expression of the Shangwujing Fault can be segmented into northern, central-northern, central-southern, and southern sections, demarcated by Tongyugou Village, Dongliushui Village, and Yiyuan Beibudong Village. While prior studies identified Late Pleistocene activity in the central-northern segment, evidence for fault motion between Jiujie Village and Dongliushui Village remained insufficient.

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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
    Abstract （370）   HTML （19）    PDF（pc） （9407KB）（138）       Save

    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 VP, VS, and Poisson's ratio in the epicentral area. To constrain the seismogenic fault properties, focal mechanism solutions for seven ML≥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-VP, low-VS, 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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    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
    Abstract （348）   HTML （37）    PDF（pc） （13946KB）（173）       Save

    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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    COSEISMIC DEFORMATION FIELD AND SLIP MODELS OF JANUARY 23, 2024 MS7.1 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
    Abstract （336）   HTML （16）    PDF（pc） （4712KB）（136）       Save

    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 MS7.3 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 MW7.1, 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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    ANALYSIS OF CRUSTAL STRUCTURE AND STRONG EARTH-QUAKE ACTIVITY OF THE IRANIAN PLATEAU BASED ON GRAVITY INVERSION IN SPHERICAL COORDINATE SYSTEM
    DAI Meng-xue, XU Rong-rong, ZHAO Fei-yu, PEI Jun-ling
    SEISMOLOGY AND GEOLOGY    2025, 47 (5): 1326-1342.   DOI: 10.3969/j.issn.0253-4967.2025.05.20250096
    Abstract （334）   HTML （7）    PDF（pc） （8134KB）（73）       Save

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

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

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

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

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

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    INVESTIGATION OF THE SUITABILITY OF (U-TH)/HE DATING MINERALS
    YANG Li, YANG Jing, ZHANG Bin, YUAN Wan-ming, LI Xiao, YE Zhang-huang
    SEISMOLOGY AND GEOLOGY    2025, 47 (6): 1495-1525.   DOI: 10.3969/j.issn.0253-4967.2025.06.20240050
    Abstract （334）   HTML （16）    PDF（pc） （3158KB）（84）       Save

    (U-Th)/He dating is characterized by a low closure temperature(approximately 70℃) and exceptional sensitivity to low-temperature thermal events, allowing for the reconstruction of detailed thermal histories in geological bodies below 300℃. This method has significant potential for constraining the timing of ore deposit formation, documenting uplift and erosion, investigating mountain denudation processes, deciphering deep-time thermal histories, pinpointing the timing of metamorphic deformation, and tracing thermal-tectonic processes in continental rifts and passive margins. Careful consideration of mineral internal structure, closure temperature,4He diffusion and retention behavior, and radiation damage mechanisms is critical for accurate data interpretation. In addition to commonly used apatite and zircon, the method has been expanded to include minerals such as titanite, xenotime, rutile, and garnet, which are suitable for investigating ancient metamorphic thermal events(up to 3 950Ma), including the emplacement age of kimberlites and the burial-exhumation histories of cratons. Moreover, (U-Th)/He dating applied to monazite, garnet, and olivine presents promising solutions for dating young volcanic rocks(as recent as 2.0ka), offering new geological insights. Minerals such as hematite, magnetite, and fluorite allow for the direct dating of ore deposits. Studies on goethite offer new methods for determining the timing and rates of precipitation and weathering in extremely young geological events(~0.4 Million Years), which aids in continental surface reconstruction. The inclusion of minerals such as calcite, conodonts, and crinoids enriches the toolkit for studying sedimentary basin evolution, while investigations of meteorites offer unique perspectives on planetary formation and evolution. Accessory minerals such as spinel, perovskite, and epidote highlight their potential applications in future geochronological studies.
    In (U-Th)/He geochronology, different minerals demonstrate distinct advantages and limitations. Apatite (U-Th)/He ages are affected by radiation damage and α-particle ejection effects. Age precision can be enhanced through α-particle capture model corrections, multi-elemental analyses, and selection of unaltered grains. The 4He diffusion behavior in titanite is strongly influenced by crystal size, with radiation damage as a key factor. Monazite, enriched in cerium and lanthanum, exhibits high resistance to radiation damage, though its 4He diffusion is significantly affected by thorium content and lattice defects. Xenotime exhibits anisotropic 4He diffusion; high U-Th-induced radiation damage alters diffusion behavior, thereby increasing age uncertainty. Conodonts can constrain the thermal evolution of sedimentary rocks; however, their U-Th content, REE concentrations, and microstructural features influence the reliability of their ages. Zircon, rich in uranium and thorium, displays low 4He diffusivity, which is modulated by grain size, morphology, and accumulated radiation damage. An ideal zircon grain is a tetragonal prism with a 2︰1 length-to-width ratio and a size ranging from 75 to 150μm. Selecting transparent grains with minimal internal cracks or inclusions and evaluating them using multiple analytical techniques can improve age precision. The garnet (U-Th)/He method is particularly effective in determining the emplacement ages of kimberlite bodies and constraining the timing of volcanic eruptions. Olivine contains relatively low concentrations of uranium and thorium, yet it exhibits stable 4He diffusion properties. However, challenges persist, including reduced age precision resulting from low U-Th content, complexities in correcting for initial 4He, and the implantation effects of 4He from surrounding basaltic matrices. Rutile's susceptibility to radiation damage and its anisotropic 4He diffusion behavior can significantly affect dating accuracy. Nonetheless, rutile remains a promising chronometer for unraveling the thermal histories of metamorphic and igneous terrains. Hematite, characterized by multiple diffusion domains, can effectively retain its initial 4He, making it applicable for studying fault slip histories and the timing of hydrothermal fluid circulation. Major limitations include high-temperature 4He release, grain size reduction from fault slip, and surface alteration effects. Low U-Th content, intrinsic crystal defects, and hydration behavior further reduce 4He retention in hematite, resulting in closure temperatures as low as 25-60℃. Magnetite (U-Th)/He geochronology faces issues including sluggish 4He diffusion, variable sample purity, and the risk of uranium loss during high-temperature extraction. Calcite, despite being abundant and chemically stable, is limited in (U-Th)/He dating by its low 4He retention, low closure temperatures(40-80℃), and inherently low helium concentrations. Excess 4He within inclusions and complex multi-domain diffusion behavior contribute to significant variability in age results. Crinoids are capable of resolving thermal histories within the 60-110℃ range. Fluctuations influence the spatial and temporal distribution of crinoids in sea level and oceanic geochemistry. Despite technical challenges-such as low equivalent uranium content and poorly constrained 4He diffusion-crinoid (U-Th)/He geochronology holds considerable promise for paleoenvironmental and paleoclimate studies when optimized analytical protocols, improved diffusion models, and targeted fossil specimens are employed. Fluorite (U-Th)/He dating offers unique advantages, especially in the absence of traditional chronometers, making it an indispensable tool for dating low-to high-temperature hydrothermal systems. Although its closure temperature and 4He diffusion behavior remain poorly constrained, fluorite (U-Th)/He dating provides unmatched potential for deciphering ore deposit ages and associated thermal evolution. Phosphate minerals in meteorites enable the determination of formation and evolutionary stages via (U-Th)/He geochronology. Although meteorite (U-Th)/He dating is constrained by sample rarity, acquisition difficulties, and complex thermal evolution, studying He diffusion in extraterrestrial materials expands the method's applicability and accuracy. Additionally, accessory minerals such as spinel, perovskite, and titanite exhibit potential for (U-Th)/He dating of crustal processes, orogenic dynamics, and deep-earth environments. Epidote, widely distributed in sedimentary and metamorphic rocks, is a promising mineral for tracking rapid cooling episodes and reconstructing paleoclimate conditions.
    To effectively apply the (U-Th)/He technique and yield robust age determinations, it is crucial to understand the effects of grain size, compositional zoning, internal lattice damage, uranium mobility, inclusions, mineral purity, and closure temperature. This review outlines the characteristics and application scopes of diverse (U-Th)/He minerals, identifies potential sources of data divergence, and proposes corresponding mitigation strategies. The goal is to assist researchers in accurately interpreting (U-Th)/He ages and their geochronological significance, thereby promoting the refinement and broader application of the method.

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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
    Abstract （332）   HTML （21）    PDF（pc） （20162KB）（168）       Save

    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(F3-1 and F3-2). In contrast, south of Weixian Town, these faults converge into a single strand(F3-2). 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 F3-2, whereas north of L6, activity is exclusively manifested on F3-1. 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(F3-1) and western(F3-2)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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    THE TIDAL TRIGGERING CHARACTERISTICS OF MS≥4.0 AFTERSHOCKS IN JANUARY 23, 2024 WUSHI MS7.1 EARTHQUAKE SEQUENCE, XINJIANG, CHINA
    JIANG Hai-kun, SONG Jin
    SEISMOLOGY AND GEOLOGY    2025, 47 (6): 1526-1545.   DOI: 10.3969/j.issn.0253-4967.2025.06.20240055
    Abstract （322）   HTML （20）    PDF（pc） （5892KB）（142）       Save

    On January 23, 2024, an MS7.1 earthquake occurred in Wushi, Xinjiang, China. The Wushi MS7.1 earthquake was a thrust-dominated rupture with a strike-slip component, trending nearly NE, dipping NW, and exhibiting a relatively steep dip angle. Most of the MS≥4.0 aftershocks in the Wushi sequence also displayed thrust or thrust-dominated strike-slip rupture mechanisms. The aftershocks were primarily distributed along the northeastern segment of the Maidant-Shayilam Fault, generally extending in a NE-NEE direction, with a distribution scale of approximately 62km. In terms of the aftershock activity, the MS5.8 strongest aftershock on February 25 can be regarded as a significant temporal marker in the sequence's evolution: before February 25, aftershock activity was intense, with maximum seismic activity levels ranging from MS5.0 to MS6.0; afterward, aftershock activity noticeably weakened, maximum seismic activity levels dropping to the range of MS4.0 to MS5.0. Specifically, among the 50 MS≥4.0 aftershocks recorded until April 30, 2024, 45 occurred before February 25, including all 10 MS≥5.0 strong aftershocks. A similar pattern was observed in the temporal variation of focal mechanisms-before February 25, the focal mechanisms showed good consistency, whereas afterward, their consistency deteriorated. The stronger aftershocks with MS≥4.0 in Wushi sequence exhibited significant temporal clustering characteristics. In the early post-seismic stage before February 25, the vast majority of MS≥4.0 stronger aftershocks occurred during spring and neap tidal periods, displaying clear triggering features associated with the semimonthly tide. During this period, although aftershock activity gradually weakened and earthquake frequency decreased, the timing of larger aftershocks still indicated modulation by factors with a periodicity of “day” or its multiples, closely related to diurnal or semidiurnal tidal triggering.
    Focusing on the tidal triggering phenomenon of larger aftershocks in the Wushi sequence, this study calculates the tidal normal stress, tidal shear stress, and tidal Coulomb failure stress changes(ΔCFS) on both nodal planes of the focal mechanism solutions for all MS≥4.0 earthquakes in the sequence before and after the events. Based on the results of these calculations, we analyze the tidal triggering characteristics of the occurrence times of larger earthquakes in the sequence. The study primarily focuses on two aspects: one aspect examines the statistical characteristics of tidal triggering, specifically the triggering patterns of larger aftershocks in relation to spring-neap tides and semidiurnal tides. Another aspect is the mechanical relationship between tidal triggering and fault motion, investigating which tidal stress component(under different tidal force conditions)plays a dominant role in triggering aftershock activity in the Wushi sequence. In our study, ΔCFS>0 serves as the fundamental prerequisite for possible tidal triggering of seismic activity and is a necessary condition for determining tidal influence. Building upon this criterion, we further explore whether the triggering of seismic rupture is primarily due to enhanced tensional effects(increased normal stress), enhanced shear effects(increased shear stress), or a combination of both. The analysis aims to clarify the dominant tidal stress mechanism driving aftershock activity in the Wushi earthquake sequence.
    The results indicate that the more vigorous aftershock activity of Wushi sequence during spring tide periods may be related to the progressive increase in tidal ΔCFS on the mainshock rupture plane(Nodal Plane I). In contrast, the fluctuations in stronger aftershocks during neap tide periods may be associated with the peak and subsequent decline of tidal ΔCFS on the auxiliary focal mechanism plane(nodal Ⅱ). From the perspective of tidal stress components, the fluctuations of MS≥4.0 stronger aftershocks during syzygy spring tides(new/full moons) are primarily driven by the combined effects of: maximum tidal normal stress(σn) and contemporaneously increasing tidal shear stress(τn) on nodal I. This suggests that the fracture planes of the triggered earthquakes are likely closer to nodal I of the mainshock's focal mechanism. Conversely, the stronger aftershock activity during quadrature neap tides(first/third quarter moons)is mainly influenced by the combined effects of the maximum tidal shear stress(τn) and contemporaneously increasing tidal normal stress(σn) on nodal Ⅱ, implying that the rupture planes of these triggered earthquakes may be closer to nodal II of the mainshock's focal mechanism.
    The findings further reveal that 70% of MS≥4.0 earthquakes in the Wushi sequence exhibit significant semi-diurnal tidal triggering characteristics, with 64% of these events showing tidal triggering effects on both nodal planes of their focal mechanisms. This phenomenon may be attributed to the unique geometric relationship between the nodal planes of the thrust-type ruptures in the Wushi MS7.1 mainshock and most MS≥4.0 aftershocks, where the two nodal planes share similar or nearly identical strikes but dip in opposite directions. It should be noted that tidal triggering signals on both nodal planes do not imply simultaneous rupture on both planes. Further analysis indicates that the semi-diurnal tidal shear stress component, superimposed along the fault slip direction, serves as the dominant tidal triggering factor for MS≥4.0 earthquakes in the Wushi sequence, with 60% of the triggered events being influenced by tidal shear stress(τn). Regarding the temporal relationship between earthquake occurrence and tidal stress, 94% of the semi-diurnally triggered MS≥4.0 earthquakes occurred either during the peak tidal stress phase(within ±1.5 hours of the peak stress)or the adjacent stage following the peak stress.
    It is noteworthy that the relatively relaxed and fractured structural and medium environment of the aftershock zone, combined with the critically high stress state immediately after the mainshock, along with the cumulative “slip-promoting” effect resulting from the co-directional superposition of tidal stresses and tectonic stresses, may be the possible reasons for the pronounced tidal triggering characteristics displayed by the early strong aftershocks in the Wushi earthquake sequence.

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    STRESS FIELD IN SHANXI RIFT AND ITS DYNAMIC SIGNIFICANCE
    WANG Xia, SONG Mei-qing, WU Hao-yu, LIANG Xiang-jun, LÜ Rui, GUO Wen-feng, ZHANG Na, LI Jin
    SEISMOLOGY AND GEOLOGY    2026, 48 (1): 257-277.   DOI: 10.3969/j.issn.0253-4967.20240030
    Abstract （317）   HTML （18）    PDF（pc） （17863KB）（119）       Save

    To carry out the more refined structural stress field and its formation mechanism, we mainly use the focal mechanism solution of small-moderate earthquakes since the observation in the Shanxi rift. The statistical results of focal mechanism solution classification show that the main types are strike-slip and normal faulting mechanism, and the percentage of strike-slip faulting, normal faulting and normal with strike-slip component is about 77%, which conforms to the transtensional deformation characteristics in the Shanxi rift. We adopt the damping stress inversion methodand invert stress fieldof the whole domain, 5 Basins, 0.5° grid of spatial distribution. We obtain the stress field parameters and stress shape factor(R value).

    This study indicates that the Shanxi rift region is characterized by a NW-SE extensional stress field with localized strike-slip stress regimes, reflecting its heterogeneous nature The inversion result of 724 ML<3.0 focal mechanisms is normal faulting regime, and the results of 301 ML≥3.0 focal mechanisms is strike-slip faulting regime. Although there are slight differences in the results of subseismic magnitude categories, the azimuth and plunge angle of σ3 are mostly similar, which can still reflect the stable NW-SE tensional stress environment. The results also indicate that the Shanxi rift exhibits heterogeneous stress field characteristics, with the northern and southern basins(Yuncheng Basin and Datong Basin) in a strike-slip faulting regime, while the Xinding, Taiyuan, and Linfen Basins are in a normal faulting regime. Generally, this reflects that the stress environment of the Shanxi rift is dominated by tensile forces, accompanied by a shear component.

    The 0.5° grid of stress field results show that the azimuth angle of the most tension principal stress σ3 in the entire area of Shanxi rift is dominated by the NW-SE direction and the plunge of σ3 is nearly horizontal, which is generally perpendicular to the direction of the main control fault in Shanxi depression basins. While the azimuth angle of σ1 is roughly parallel to the direction of the main control fault, and the spatial change of the plungeangle is not uniform. The spatial distribution results of stress field generally reflect the stable and horizontal tension, and the local heterogeneous stress field is caused by the changes of the most compressive principalstress σ1 and the intermediate principal stress σ2. The spatial distribution of stress field results show that the value of R is mostly less than 0.5, which reflects that intermediate principal stress σ2 is compressive stress, according with the stress state of Shanxi rift that is dominated by tension accompanied of a shear component.

    In addition, the GPS velocity profile results also show that there is a extension movement of about 0.5mm/a in Shanxi rift, and there is local strike-slipping movement. The main crustal deformation of Datong Basin is the NW-SE extension with the a rate of 0.5-1mm/a, including minimal shear or strike-slip component. The main crustal deformation of Taiyuan Basin exhibits the NW-SE extension with 0.3-0.7mm/a, showing weak shear or strike-slip component. The Yuncheng Basin demonstrates both extensional and strike-slip deformation, with an extensional rate of about 0.6mm/a and a dextral strike-sliping rate of about 0.7mm/a. From a relative quantitative perspective, GPS data and other measurement techniques have shown that the Shanxi rift exhibits both extensional and shear deformation characteristics. The Shanxi rift serves as the boundary of a secondary block within the North China block, and its formation and dynamic source are related to the remote action of Pacific plate subduction and Indo-Europe collision extrusion.

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    CHARACTERISTICS OF VERTICAL SLIP RATE EVOLUTION ALONG THE JIAOCHENG FAULT ZONE, SHANXI, SINCE LATE PLEISTOCENE
    LUO Jia-xin, LI Bin, LI Zi-hong, FAN Kun
    SEISMOLOGY AND GEOLOGY    2026, 48 (1): 43-63.   DOI: 10.3969/j.issn.0253-4967.20240051
    Abstract （315）   HTML （14）    PDF（pc） （10049KB）（86）       Save

    The Jiaocheng fault zone is the largest seismogenic fault and the principal boundary-controlling structure within the Taiyuan Basin, located in the central segment of the Shanxi Graben System. The fault zone extends for approximately 125km, trends overall in a NE direction, and dips to the SE with dip angles ranging from 40° to 80°. It is characterized as an active dip-slip normal fault with a right-lateral strike-slip component. The activity of the Jiaocheng fault zone has played a decisive role in the formation and evolution of the Taiyuan Basin, as well as in strain accumulation and the occurrence of major earthquakes in the region. In recent years, ground fissures have continued to develop along the fault zone, with the Qingxu-Wenshui segment being the most active. This segment has formed a surface rupture zone up to 48km in length and 80-120m in width. Extensive ground cracking has damaged roads, bridges, and buildings, resulting in severe social impacts and economic losses. It has become one of the longest, most destructive, and socially influential ground fissure zones identified in China.

    To further clarify the vertical slip rates of the Jiaocheng fault zone since the late Pleistocene and to investigate its evolutionary characteristics, a systematic and comprehensive study was carried out. This study integrated tectonic geomorphological analysis, trench excavation, across-fault leveling, GPS observations, and InSAR measurements. Analyses of vertical slip rates derived from multiple datasets indicate that the Jiaocheng fault zone has experienced pronounced spatial segmentation and significant spatiotemporal variations in vertical slip since the Late Pleistocene. Specifically: 1) since the Late Pleistocene, the vertical slip rates are estimated to be 0.82-0.90mm/a for the Shanglan segment, 0.98-1.17mm/a for the Jinci segment, 0.51-1.66mm/a for the Qingxu-Wenshui segment, and approximately 0.43mm/a for the Fenyang segment; 2) since the Holocene, the overall average vertical slip rate of the fault zone has decreased, with rates reduced to 0.63-1.04mm/a for the Shanglan segment, 0.23-0.45mm/a for the Jinci segment, 0.44-1.54mm/a for the Qingxu-Wenshui segment, and nearly zero for the Fenyang segment; 3)although fault activity shows a general trend of northward propagation, the Qingxu-Wenshui segment has consistently remained the most active portion of the fault zone in terms of vertical slip. This observation is consistent with the widespread development of ground fissures and recent field evidence in the area.

    Modern geodetic data further indicate that the Jiaocheng fault zone remains active at present, with significant vertical slip still occurring, particularly along the Qingxu-Wenshui segment. In addition, fault activity exhibits clear interactions with the external environment. Field investigations show that ground fissures predominantly develop along the fault zone, with their planar distribution approximately parallel to the fault trace, directly reflecting the influence of fault activity on surface deformation. Monitoring data reveal similar temporal trends between cross-fault leveling measurements and groundwater level variations, indicating a modulatory effect of groundwater dynamics on fault activity. Although coal mining along the fault zone does not directly trigger fault motion, it may indirectly enhance fault activity through groundwater depletion.

    This study provides quantitative constraints on the vertical slip rates of the Jiaocheng fault zone since the late Pleistocene and further clarifies the spatial segmentation and temporal evolution of its vertical slip behavior. The results improve the understanding of the long-term spatiotemporal characteristics of fault motion and provide a scientific basis for evaluating the potential for strong earthquakes along the Jiaocheng fault zone, assessing seismic hazards in the Taiyuan Basin and surrounding areas, and deepening insight into regional tectonic processes and the mechanisms of ground-fissure-related disasters.

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    MONITORING STUDY OF INTERSEISMIC DEFORMATION OF THE HAIYUAN FAULT ZONE BASED ON TIME SERIES INSAR
    QIN You-sen, XU Xiao-bo, LI Yan-chuan, ZHANG Ying-feng, LIAN Da-jun, YANG Zhao-hui, CHEN Kai, RONG Xin-yue
    SEISMOLOGY AND GEOLOGY    2025, 47 (6): 1688-1707.   DOI: 10.3969/j.issn.0253-4967.2025.06.20240104
    Abstract （313）   HTML （23）    PDF（pc） （17058KB）（136）       Save

    Owing to the ongoing collision between the Indian and Eurasian plates, the Tibetan block manifests as the most intensely deforming intracontinental tectonic unit globally. The interseismic phase, defined as the protracted stable period between characteristic earthquakes, features persistent relative movement of fault-bounded blocks driven by plate convergence or strike-slip motion. Locked segments at depth impede shallow slip, resulting in sustained accumulation of elastic strain within the crustal medium. This strain buildup manifests as long-term, stable tectonic deformation within the surface displacement field, known as interseismic deformation. As the critical phase for seismic energy accumulation, the Haiyuan fault zone(HYFZ)has historically experienced two major earthquakes: the 1920 Haiyuan and 1927 Gulang events. As a significant active block boundary and intense seismicity zone in the northeastern Tibetan margin, and one of China's most earthquake-prone regions, monitoring the HYFZ's interseismic deformation is particularly crucial.
    This study employs Small Baseline Subset Interferometric Synthetic Aperture Radar(SBAS-InSAR)technology with Sentinel-1A T135 and T62 track data(2018-2024) to monitor the HYFZ's interseismic deformation. The resulting deformation field covering the entire HYFZ was analyzed using cross-fault techniques, with emphasis on deformation at structural step-overs. Results indicate minimal deformation-rate differences(characteristic of interseismic locking) across the central-eastern Lenglongling, Jinqianghe, Maomaoshan, and western Laohushan segments. Notably, near the 2022 Menyuan earthquake hypocenter, the Lenglongling Fault exhibited a significant differential rate reaching 4mm/a. Shallow creep was observed along the eastern Laohushan segment, while the central Haiyuan strand showed an obvious maximum differential rate of 3.7mm/a with left-lateral strike-slip characteristics.
    Time-series analysis of a near western Lenglongling cross-fault profile revealed an accelerated trend on the southern block approximately two years before the 2022 Menyuan earthquake, with notably higher acceleration compared to the northern block. This suggests the southern block acted as the driving block. Subsequent slip-rate inversion for each HYFZ segment utilized the arctangent elastic dislocation model. Integrating InSAR and GPS data, the study transformed the deformation field into the Eurasian reference frame. Tectonic blocks adjacent to the HYFZ were defined as the Lanzhou, Qilian Shan, Ordos, and Alashan blocks. A block-based negative dislocation model inversion yielded locking depths and slip deficits along the entire HYFZ. Results indicate: maximum locking depth(16km) at the Lenglongling-Jinqianghe junction, minimum depth(<1km) in western Laohushan, and overall slip deficit rates decreasing eastward within the range of 1.9-5.2 mm/a.
    The comprehensive seismic hazard assessment suggests that the Lenglongling segment(deep locking, large slip deficit)faces significant hazard potential despite small earthquakes, requiring vigilance for large events. The Jinqianghe, Maomaoshan, and western Laohushan segments(constituting the “Tianzhu Seismic Gap”)exhibit deep locking, substantial slip deficits, and no major earthquakes in several centuries, indicating high risk. The eastern Laohushan and central Haiyuan segments(shallow locking, small deficits) are likely undergoing post-seismic adjustment, primarily experiencing small earthquakes with minimal potential for large events.

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    DISTRIBUTION FEATURES OF PREFILLED LANDSLIDES IN BAIHETAN HYDROPOWER STATION RESERVOIR AREA OF LOWER JINSHA RIVER
    BIAN Mei-fang, WANG Ying, CHEN Xiao-li, YUAN Ren-mao, TIAN Ying-ying, JIN Li-zhou
    SEISMOLOGY AND GEOLOGY    2026, 48 (1): 102-126.   DOI: 10.3969/j.issn.0253-4967.20240090
    Abstract （310）   HTML （13）    PDF（pc） （14018KB）（66）       Save

    The Baihetan Hydropower Station, the second largest hydropower station in the world and in China, is located on the lower reaches of the Jinsha River. The reservoir area is characterized by complex topography, geomorphology, geological structures, and climatic conditions, resulting in frequent geological hazards such as landslides. In addition, the well-known Dongchuan debris flow gully, one of the most representative debris flow areas in China, is situated within the Xiaojiang watershed upstream of the reservoir. Therefore, investigating the spatial distribution characteristics of landslide hazards in this region is of great significance for disaster risk mitigation, engineering safety, and regional sustainable development.

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    HIGH-RESOLUTION CRUSTAL VELOCITY AND ANALYSIS OF SEISMIC SPATIAL VARIATION CHARACTERISTICS IN THE CENTRAL AND SOUTHERN SEGMENTS OF THE TAN-LU FAULT ZONE
    QIAN Ting, CHEN Hao, HE Yi-cheng, JIANG Hao-lin, DU Hang, NI Hao-qi
    SEISMOLOGY AND GEOLOGY    2026, 48 (2): 442-459.   DOI: 10.3969/j.issn.0253-4967.20240095
    Abstract （310）   HTML （18）    PDF（pc） （13063KB）（87）       Save

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

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    SUMMARY OF PRECURSORS OF THE MENYUAN MS6.9 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
    Abstract （306）   HTML （21）    PDF（pc） （9299KB）（127）       Save

    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 MS7.4 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 MS6.9 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 MS6.9 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 MS5.5 Aksai earthquake on August 26, 2021. Additionally, the ML3.6 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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    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
    Abstract （296）   HTML （17）    PDF（pc） （9779KB）（129）       Save

    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 MS6.5, the 2017 Jinghe MS6.6, and the 2020 Jiashi MS6.4 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 OF THE INSAR COSEISMIC DEFORMATION AND INTERSEISMIC FAULTING MECHANISM OF THE 2023 JISHISHAN MW6.1 EARTHQUAKE
    YANG Ying-hui, XIE Li-yuan, XU Qiang, WANG Yun-sheng, ZHAO Jing-jing, WANG Dao-ming, CHEN Qiang
    SEISMOLOGY AND GEOLOGY    2025, 47 (5): 1416-1437.   DOI: 10.3969/j.issn.0253-4967.2025.05.20240016
    Abstract （282）   HTML （16）    PDF（pc） （13585KB）（84）       Save

    On December 18, 2023, a MW6.1 earthquake struck Jishishan County in the Linxia Hui Autonomous Prefecture of Gansu Province. The strong ground shaking caused widespread building collapse and significant casualties, underscoring the severe societal impact of seismic events in this region. Because the epicenter is located within a tectonically active area characterized by frequent crustal deformation and complex fault interactions, clarifying the seismogenic mechanisms is essential for understanding regional seismic hazards.

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

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

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

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

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

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    AEROMAGNETIC INTERPRETATION OF BURIED FAULT SPATIAL DISTRIBUTION CHARACTERISTICS IN THE NORTH CHINA BASIN
    FAN Ji-di, WANG Xin, XIANG Jian-bin
    SEISMOLOGY AND GEOLOGY    2026, 48 (2): 475-496.   DOI: 10.3969/j.issn.0253-4967.20250142
    Abstract （275）   HTML （13）    PDF（pc） （9151KB）（51）       Save

    The North China Basin, with great thick sedimentary layers, is located in the eastern part of the North China Craton and is one of the important sedimentary basins in eastern China. There are a large number of buried faults in the North China Basin. Some of buried faults are strong activity and have experienced major earthquakes, such as the 1679 Sanhe-Pinggu M8 earthquake and the 1976 Tangshan MS7.8 earthquake. The geometric distribution and depth of these buried faults remain uncertain. Thus, it is very important to study spatial distribution and the intersecting relationships of the main faults in the region, and high-resolution aeromagnetic data is key.
    In the paper, the high-precision aeromagnetic field data of the North China Basin are collected and are reduced to the pole at variable latitudes. Using the vertical first-order derivative and tilt-derivative edge-enhancement methods, and with reference to existing geological data, the geometric distribution and intersection relationships of the major faults are reinterpreted and corrected. The derivative methods have inherent limitations in the sedimentary area; the correction results are compared with previous data to ensure the reliability of the interpretation results. Moreover, the Euler deconvolution method is used to estimate the apparent depth of some buried faults in the basin. Its cutting depth is inferred and verified with other geophysical results.
    The result shows that the aeromagnetic field is divided into four blocks: the chaotic magnetic zone of the northern Yanshan uplift, the high magnetic zone of the western Taihang uplift, the medium-high magnetic zone of the central North China Basin, and the low magnetic zone of the southern Luxi uplift. The North China Basin is characterized by multiple secondary structural units with alternating NE-trending uplifts and depressions. The 59 major faults are interpreted and corrected using the aeromagnetic field and its derivative, with faults mainly oriented NNE and NE, supplemented by NW, NS, and near-EW orientations. In the NNE direction, among them buried faults such as the Baoding-Shijiazhuang Fault, the Cangxi Fault, the Cangdong Fault, and the Tangshan Fault exhibit beaded bands, linear anomalies, magnetic gradient strip and so on. In the NW direction, the magnetic field characteristics of the Xiadian Fault, the Baodi Fault, and the Nankou-Sunhe Fault are faint, showing magnetic conversion and fault-bent strips, suggesting they played a regulatory role in structural deformation.
    The Euler results show the apparent depth and cutting scales of 13 buried faults within the basin. Among them, 6 buried faults are middle-shallow scale faults: the Huangzhuang-Gaoliying Fault, the Nankou-Sunhe Fault, the Baodi Fault, the Cangdong Fault, the Cangxi Fault and the Liaocheng-Lankao Fault; and 7 buried faults are middle-deep scale faults: The Baoding-Shijiazhuang Fault, the Shunyi-Liangxiang Fault, the Xiadian Fault, the Jiyunhe Fault, the Tangshan Fault, the Chengxi-Yangerzhuang Fault and the Handan Fault. Multi-scale aeromagnetic computation and inversion provide a new technical approach for investigating the spatial distribution of buried faults. For large areas, especially sedimentary basins with great thick sedimentary layers, this technology is used for preliminary exploration of the geometric distribution and depth of buried faults.

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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
    Abstract （271）   HTML （21）    PDF（pc） （10362KB）（140）       Save

    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 S-WAVE VELOCITY STRUCTURE OF THE TENGCHONG-BAOSHAN REGION FROM JOINT INVERSION OF RECEIVER FUNCTION AND RAYLEIGH WAVE PHASE VELOCITY
    ZHANG Tian-ji, QIN Min, DANG Wen-jie, JIN Ming-pei, LI Feng-ying, YANG Li-wei
    SEISMOLOGY AND GEOLOGY    2025, 47 (5): 1364-1381.   DOI: 10.3969/j.issn.0253-4967.2025.05.20240111
    Abstract （269）   HTML （16）    PDF（pc） （10397KB）（68）       Save

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

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

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

    Since 1900, earthquakes of MS≥5.0 have predominantly occurred in transitional zones between high- and low-velocity regions. Notably, the epicenters of the 1976 MS7.4 and MS7.3 Longling earthquakes are underlain by a low-velocity layer, suggesting that magma and fluids may have contributed to rupture initiation in fault-fractured regions. In the upper mantle(60~120km depth), widespread low-velocity anomalies are observed beneath the Tengchong-Baoshan area, which extend upward and connect to the crustal low-velocity zones beneath Tengchong. This large-scale low-velocity mantle anomaly likely serves as a magma source feeding the crustal magma chambers. Furthermore, asthenospheric upwelling and associated lithospheric thinning provide the geodynamic mechanism driving magmatism and volcanism in the Tengchong region.

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    PRELIMINARY STUDY ON THE ABNORMAL CHARACTERISTICS OF INDUCED CURRENT DISTRIBUTION IN DAILY VARIATION OF GEOMAGNETIC FIELD DURING THE YANGBI MS6.4 EARTH-QUAKE BASED ON ABNORMAL POSITIONING OF VERTICAL COMPONENT OF STATION GEOMAGNETIC FIELD
    SUN Jun-song, FENG Zhi-sheng, WU Ying-yan, LI Hong-yu, YANG Jie
    SEISMOLOGY AND GEOLOGY    2025, 47 (5): 1456-1476.   DOI: 10.3969/j.issn.0253-4967.2025.05.20240042
    Abstract （266）   HTML （13）    PDF（pc） （6135KB）（82）       Save

    On April 24, 2021, a spatially correlated anomaly in geomagnetic diurnal variation was observed in the Sichuan-Yunnan region of China. Subsequently, on May 21, 2021, the MS6.4 Yangbi earthquake occurred. The earthquake prediction research department suggests that this anomaly is related to the seismic event. The spatial correlation method for geomagnetic daily variation is a seismic geomagnetic field analysis technique with the same physical basis as the geomagnetic low point displacement method, geomagnetic loading-unloading response ratio method, and geomagnetic daily ratio method. This study uses spatially correlated geomagnetic anomaly data to assume the existence of subsurface currents, and inverts their possible distribution and shape using the Biot-Savart law and Monte Carlo modeling. An underground current distribution model potentially generating the observed spatial geomagnetic anomalies is established to explore its quantitative relationship with the Yangbi earthquake.

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

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

    According to current-source localization of the abnormal phase distortion in the daily variation of the geomagnetic vertical component prior to the earthquake, mountain-shaped induced current surfaces repeatedly emerged in the source region and its vicinity, spanning the upper, middle, and lower crustal levels of the structure during the short-term and imminent stage of the Yangbi MS6.4 earthquake. The current peaks were located in the middle to upper crust, with their bases at the top of the mantle. These peak-shaped induced currents persisted for several hours and fluctuated over time like ocean waves, rising and falling in succession. The same regions reappeared repeatedly, while areas on both sides of the epicenter alternated in occurrence. The peak-shaped induced current surface represents an electrical structural interface at the top of a transient, peak-shaped, high-conductivity channel. This surface, composed of multiple structural layers, shares the spatiotemporal variability of the underlying conductive channel, which rapidly assembles and disintegrates on an hourly scale. Thus, during the short-term and imminent stage of the Yangbi earthquake, the recurrent long-period induced currents in different crustal levels reflect repeated episodes of fluid activity within the structural framework.

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

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    LATE QUATERNARY ACTIVITY OF PARALLEL NORMAL FAULTS IN THE SOUTHERN MARGIN OF YUGUANG FAULTED BASIN IN SHANXI RIFT AND ITS SEISMOGEOLOGICAL SIGNIFICANCE
    ZOU Jun-jie, HE Hong-lin, SHAO Zhi-gang, WEI Zhan-yu, SHI Feng, ZHANG Bo, GENG Shuang, ZHAO Jia-hao
    SEISMOLOGY AND GEOLOGY    2026, 48 (1): 64-80.   DOI: 10.3969/j.issn.0253-4967.20230138
    Abstract （265）   HTML （29）    PDF（pc） （6730KB）（114）       Save

    The southern boundary fault zone of the Yuguang Basin, located in the Shanxi Graben System, is jointly constituted by two fault segments: the piedmont normal fault distributed in the bedrock area and the foothill normal fault developed in the sedimentary area. However, academic debates persist over two critical issues: whether these faults were active synchronously during geological evolution, and whether both function as seismogenic faults that dominate earthquake occurrence in this region. A series of key scientific questions remain to be addressed urgently: What are the specific activity characteristics of these two faults?Is the seismic hazard of the Yuguang Basin controlled by a single fault, either the piedmont or the foothill one, or do the combined effects of both faults govern it? To resolve these controversies and clarify the aforementioned questions, this study selects the Tangshankou segment of the southern Yuguang Basin fault as the research target. By integrating multiple technical approaches including small unmanned aerial vehicle(s-UAV) aerial survey, detailed geological profile interpretation, and precise Quaternary dating methods, we conduct a systematic investigation into the activity epoch, displacement amplitude, and slip rate of both the foothill normal fault and the piedmont normal fault. The research results reveal that the foothill normal fault experienced two distinct paleoseismic events: the first event occurred after 32.9-31.9kaBP, with its specific displacement amplitude remaining undetermined; the second event took place during the period of (23.4±2.1)-(20.8±1.8)kaBP, accompanied by a coseismic dip-slip displacement of 0.4-0.5m, and the Holocene extensional slip rate of this fault is calculated to be 0.8mm/a. In contrast, the piedmont normal fault has accumulated approximately 7m of vertical displacement since 10.1-8.2kaBP, corresponding to a Holocene extensional slip rate of 0.4-0.5mm/a. This finding significantly revises the previous academic view that the piedmont normal fault in this segment had essentially ceased tectonic activity since the Late Quaternary. Further analysis demonstrates that both the piedmont normal fault in the bedrock area and the foothill normal fault in the sedimentary area within the Tangshankou segment are Holocene active faults, and both have generated surface-rupturing earthquakes in history. These two faults jointly undertake the regional extensional deformation and play a crucial role in the strain partitioning process within the basin boundary zone. Therefore, when conducting regional extensional deformation calculations in the boundary zone of faulted basins, it is essential to carry out a comparative analysis and careful consideration of the two fault segments in both bedrock and sedimentary areas. Only in this way can we scientifically construct a regional extensional deformation model and accurately grasp the kinematic characteristics and dynamic mechanisms of the boundary zone of faulted basins, thereby providing a reliable geological basis for seismic hazard assessment and disaster prevention and mitigation work in this region.

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    P-WAVE VELOCITY STRUCTURE OF THE CRUST AND UPPER-MOST MANTLE IN THE CENTRAL AND SOUTHERN SEGMENT OF TANLU FAULT ZONE AND ITS TECTONIC IMPLICATION
    LI Xi-bing, TAO Xiao-san, GU Qin-ping, PENG Xiao-bo, WANG Yu, ZHU Feng
    SEISMOLOGY AND GEOLOGY    2026, 48 (2): 423-441.   DOI: 10.3969/j.issn.0253-4967.20240115
    Abstract （265）   HTML （13）    PDF（pc） （15610KB）（62）       Save

    In 1668, a major MW8.5 earthquake struck Tancheng, located in the central-southern segment of the Tanlu fault zone. Jointly resolving P-wave velocities in the crust and uppermost mantle can improve constraints on uppermost-mantle velocity, which is essential for understanding the tectonic mechanisms of strong earthquakes in this fault system and the deep seismogenic setting. Here we perform a joint inversion for crustal and uppermost-mantle P-wave velocity beneath the central-southern Tanlu fault zone using travel times of the first-arriving Pg and Pn phases, together with secondary Pg phases recorded by the seismic network. Incorporating secondary Pg arrivals increases crustal ray coverage and substantially improves the resolution and accuracy of the uppermost-mantle velocity structure.
    The resulting P-wave model shows that near-surface(0km)high-velocity anomalies broadly correspond to mountain ranges, whereas low-velocity anomalies are mainly associated with sedimentary basins. Arc-shaped, belt-like high-velocity anomalies(5.40~5.54km/s) are observed in the northern Jiaodong Peninsula, the Taihang Mountains, and the Sulu orogenic belt, particularly in the Huaibei-Jining-Tai'an region. In contrast, low-velocity anomalies occur beneath Wuhan(Jianghan Basin), Hefei-Liuan(Hefei Basin), and Juxian(Shandong), and a prominent NS-trending low-velocity belt extends along the Shangqiu-Heze-Puyang-Liaocheng corridor(4.80~4.89km/s). At depths of 10~15km, high velocities dominate beneath the Dabie orogenic belt, Huainan-Bengbu, the Luxi uplift, and the Jiaodong Peninsula, whereas pronounced low-velocity anomalies appear beneath the Yangzhou-Changzhou-Suzhou to Shanghai-Nantong sector of the Subei Basin and the southern Yellow Sea. At ~15km depth, velocity structure differs markedly across the Tanlu fault zone: The western side from Juxian southward to the Dabie orogen exhibits strong high-velocity anomalies, while the eastern Subei-southern Yellow Sea Basin and the Xinyi-Lianyungang area are characterized by low velocities.
    At ~20km depth, the velocity pattern changes relative to the upper and middle crust and shows strong along-strike segmentation within the Tanlu fault zone. North of Tancheng, a low-velocity anomaly is present; from Tancheng to Sihong, a high-velocity anomaly reaches a maximum of 6.39km/s; from Sihong to Lujiang, low velocities dominate, with a minimum of 6.03km/s near Jiashan. The Dabie orogenic belt exhibits a high-velocity anomaly of ~6.43km/s. At depths of 25~30km, which mainly reflect the middle-lower crust, high-velocity anomalies dominate beneath the Dabie orogen, the Taihang Mountains, and northern Jiangsu-southern Yellow Sea, with additional localized highs near Jiaozhou(Shandong) and Suqian(Jiangsu). Low-velocity anomalies occur in the Tancheng-Anqiu-Weifang segment of the Tanlu fault zone and also along the Liaocheng-Jinan corridor and near Jiashan and Wuhu.
    Pn velocities in the central-southern Tanlu fault zone are strongly heterogeneous. North of Tancheng, low Pn velocities are mainly distributed on the eastern side of the fault zone, whereas south of Hefei they occur predominantly on the western side. High Pn velocities appear on the eastern side of the fault zone in the Tancheng-Jiashan region. Such heterogeneity likely reflects lateral variations in mechanical strength at the top of the upper mantle beneath different segments of the fault zone.
    Overall, crustal velocity anomalies correlate with surface geomorphology and the distribution of major faults. In particular, velocity anomalies at 10~15km depth delineate the strike of the Tanlu fault zone. Across the central-southern Tanlu fault zone, the South China Plate and the Lower Yangtze Block display a clear crustal velocity contrast. In the upper-middle crust, low velocities prevail west of the Tanlu fault zone and high velocities to the east; in the lower crust, pronounced high velocities occur east of the Tanlu fault zone and south of the Jiashan-Xiangshui Fault, whereas low velocities dominate to the west. The relative strength contrast between upper and lower crust inferred from the spatial relationship between seismicity and velocity anomalies may play an important role in controlling earthquake occurrence. By incorporating a 3D crustal model, the joint inversion enhances the resolution of uppermost-mantle structure, revealing low-velocity anomalies flanking the epicentral region of the 1668 Tancheng MW8.5 earthquake and strong uppermost-mantle heterogeneity beneath the central-southern Tanlu fault zone. Integrating crustal and uppermost-mantle velocity patterns, we divide the central-southern Tanlu fault zone into three segments—north of Tancheng, Tancheng to Jiashan, and south of Jiashan—consistent with previous segmentation studies.

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    CHARACTERISTICS OF EARTHQUAKE SEQUENCE TYPES WITH M≥5.0 IN YUNNAN AND ITS NEIGHBORING AREAS
    ZHAO Xiao-yan, MENG Ling-yuan, PENG Guan-ling, WANG Guang-ming
    SEISMOLOGY AND GEOLOGY    2026, 48 (1): 162-180.   DOI: 10.3969/j.issn.0253-4967.20240068
    Abstract （261）   HTML （10）    PDF（pc） （4506KB）（117）       Save

    In recent years, both governmental agencies and the public have placed increasing emphasis on the timeliness of post-earthquake trend analysis. However, during the early stages following an earthquake—particularly within the first few hours(the so-called “zero-hour” period)—the severe lack of observational data necessitates reliance on prior statistical information, such as the historical proportions of different earthquake sequence types. Accordingly, systematic statistical analyses of the proportional distribution of seismic sequence types and the characteristics of the largest aftershocks in different regions, based on historical earthquake sequence data, can provide essential reference data for the rapid identification of early sequence types and post-earthquake trends during this critical prediction bottleneck. Owing to the combined effects of regional tectonic settings, subsurface media, and stress fields, seismic activity in Yunnan is highly complex, resulting in diverse and complicated earthquake sequence types.

    Using data from the China Earthquake Cases and monthly catalogs of the Yunnan Seismic Network, this study compiled 152 earthquake sequences with M≥5.0 that occurred between 1966 and 2023 in Yunnan and its adjacent areas(20°~29.5°N, 97°~106°E). Earthquake sequences were classified according to the magnitude difference ΔM=M0-M1 between the largest and second-largest events in each sequence into three types: multiple mainshock type(MMT, ΔM<0.6), mainshock-aftershock type(MAT, 0.6≤ΔM≤2.4), and isolated earthquake type(IET, ΔM>2.4). The influence of mainshock magnitude and rupture style on sequence type was analyzed. For forecasting the largest aftershock—a key issue in post-earthquake trend analysis—statistical investigations were conducted from three perspectives: the relationship between mainshock magnitude and the largest aftershock magnitude, the time interval between the mainshock and the largest aftershock, and the spatial distribution characteristics of magnitude differences. In addition, the spatial distribution patterns of earthquake sequence types in Yunnan and surrounding regions were examined, with particular focus on the distribution of MMT sequences and their relationship with regional tectonic structures.

    The results indicate that: 1)Earthquake sequences in the study area are dominated by the MAT type, followed by MMT, with IET being the least common. Within the same sequence type, the proportions of MAT and MMT increase with increasing mainshock magnitude, whereas the proportion of IET decreases. Among different rupture styles, IET sequences are absent in normal-fault earthquakes, while thrust-fault earthquakes exhibit relatively high proportions of MMT, mainly distributed in northeastern Yunnan. 2)Linear regression analysis between mainshock magnitude and the magnitude of the largest aftershock shows the strongest correlation for MMT sequences, followed by MAT sequences, whereas IET sequences display the greatest scatter. The occurrence time of the largest aftershock is related to both sequence type and mainshock magnitude. The spatial distribution of D1 values shows significant regional differences, with the Honghe and Xiaojiang faults acting as boundaries: D1 values are highest east of the Xiaojiang Fault and the Zhaotong-Ludian Fault, and lowest west of the Red River Fault. 3)The spatial distribution of earthquake sequences in Yunnan and its adjacent regions exhibits statistically significant regional characteristics. Overall, MMT sequences are most prevalent in western Yunnan, followed by northeastern Yunnan, with relatively high proportions also observed in northwestern and southwestern Yunnan. Central Yunnan displays the simplest composition of sequence types, whereas western Yunnan shows the greatest complexity.

    The distribution characteristics of MMT sequences can be broadly summarized as follows: 1)MMT commonly occurs in source environments characterized by complex seismogenic fault systems, such as conjugate or multiple intersecting fault sets, the interweaving of concealed structures, or the coupling of fault systems at different depths; 2)newly formed fault zones, exemplified by the Longling-Lancang fault zone, which developed after cutting through folded geological bodies of older faults and consists of numerous discontinuous, small-scale, parallel, clustered, or obliquely oriented secondary faults, are prone to generating MMT sequences with relatively large magnitudes; 3)upper-crustal low-velocity zones, as the spatial distribution of sequence types is closely related to the deep structural environment, with MAT sequences mainly occurring in high-velocity zones or transitional zones between high- and low-velocity regions, whereas MMT sequences are more frequently associated with low-velocity zones in the upper crust; 4)areas near the epicenters of historical large earthquakes, where in central and northwestern Yunnan MMT sequences tend to cluster around the epicentral regions of past major events; and 5) in recent years, earthquake sequences induced by human industrial activities, such as reservoir-induced seismicity and hydraulic fracturing, have exhibited increasingly complex sequence types. It should be noted that the first four characteristics are not mutually exclusive in the seismogenic processes of MMT sequences; in many cases, the source environments of MMT events may simultaneously involve multiple factors, particularly the first three tectonic features.

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    REVEALING THE SHALLOW SEDIMENTARY STRUCTURE OF THE WESTERN WEIHE BASIN USING THE HVSR METHOD WITH A SHORT-PERIOD DENSE ARRAY
    RUAN Ming-ming, TIAN Xiao-feng, ZHENG Cheng-long, SONG Xiang-hui, WANG Liang, HE Chang-xun, WANG Shuai-jun
    SEISMOLOGY AND GEOLOGY    2026, 48 (2): 403-422.   DOI: 10.3969/j.issn.0253-4967.20240113
    Abstract （249）   HTML （8）    PDF（pc） （11705KB）（75）       Save

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

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    APPARENT RESITIVITY VARIATION AT KEPING SEISMIC STATION BEFORE THE MODERATE EARTHQUAKES IN SOUTH TIANSHAN AREA, XINJIANG
    LI Xin-yan, ZENG Xian-wei, LI Meng-ya, WEI Ding-jun, CUI Jin
    SEISMOLOGY AND GEOLOGY    2025, 47 (5): 1438-1455.   DOI: 10.3969/j.issn.0253-4967.2025.05.20240040
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    The south Tianshan area is one of the main moderate earthquake preparation areas in China. Apparent resistivity monitoring has been demonstrated as an effective approach for intermediate-to short-term earthquake forecasting, as validated by numerous documented seismic cases. To improve regional seismic monitoring networks and observe the spatial and temporal variations in the apparent resistivity of shallow crust caused by the seismic preparation process, the Keping apparent resistivity continuous observation station was built in 2013 at the junction of the Southern Tianshan Mountains and Tarim Basin by the Earthquake Agency of the Xinjiang Uygur Autonomous Region. The observation system at this station has operated stably, with consistent data variations and high accuracy in both measurement channels (the NS channel and EW channel) over the past ten years. According to the sensitivity theory, our analysis demonstrates that the annual variation pattern (summer decrease/winter increase) observed in both measurement channels at Keping station is consistent with negative surface influence coefficients. Eleven earthquakes (MS≥5.0) occurred within 400km of the Keping station after 2018(aftershocks and foreshocks excluded), including two M6 and one M7 events. For qualitative analysis of the relationship between the seismogenic processes of earthquakes and the apparent resistivity variations observed at the Keping station, soil temperature and soil water content at different depths from the ERA5 assimilation datasets (ECMWF Reanalysis v5) were used to analyze the different hydrothermal condition effects on annual variation and to identify the anomalies from the background value by the interquartile range method. Our analysis of the fault virtual fault dislocation model and apparent resistivity anisotropy revealed four apparent resistivity decreases at Keping station, temporally correlated with the nucleation phases of seven M5.0+ earthquakes within a 250km radius during 2018-2024. Specifically, the rock experiment demonstrated that the resistivity of water-bearing rocks exhibits decreased variation under compressive stress. This finding is consistent with the decline changes at Keping Station, located within a compressive stress enhancement zone associated with the seismogenic process of the Wushi MS7.1 earthquake in 2024 and the Jiashi MS6.4 earthquake in 2020, respectively, revealed by the fault virtual fault dislocation model. Moreover, the anisotropic variations in apparent resistivity, as evidenced by experimental results, theoretical modeling, and seismic case studies, demonstrate a consistent pattern: the observed variation amplitude is maximized perpendicular to the direction of maximum principal compressive stress(σ1), minimized parallel to σ1, and exhibits intermediate values at oblique orientations. The focal mechanism solutions reveal that the principal compressive stress axes (P-axes) of all seven earthquakes are approximately N-S oriented, consistent with the deformation characteristics of the Keping thrust tectonic system. Furthermore, the angle between the EW channel and the principal compressive stress direction is consistently larger than that of the NS channel. Correspondingly, apparent resistivity variations in the EW direction at the Kalpin Station were consistently more pronounced than those in the NS direction, exhibiting clear anisotropic behavior. Therefore, based on the fault virtual fault dislocation model, rock physics experiment, and apparent resistivity anisotropic variation, the apparent resistivity anomalies of Keping station were proved to be associated with the seismogenic process of seven moderate earthquakes. Furthermore, simulation was carried out with reference of the electrical structure of the EW direction in Keping station, the underground medium resistivity of the EW direction in Keping station decreases by 10%, the change of apparent resistivity compared with the background value of 1.05%can be recognized, and the upper interface of the apparent resistivity change needs to rise to at least 243m above ground. Based on the electrode spacing configuration(current electrode spacing: 1 000m), this depth can be fully detected by the existing observation devices in the Keping station. This study provides both theoretical and practical support for identifying georesistivity anomalies at the Keping Station and enhancing regional seismic monitoring capabilities.

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    GRAVITY OBSERVATION DURING 2021—2023 AND MAGMA ACTIVITY IN CHANGBAISHAN-TIANCHI VOLCANO
    HU Ya-xuan, ZHANG Guo-qing, XIONG Guo-hua, ZHUANG Wen-quan, FENG Bing, KONG Qing-jun
    SEISMOLOGY AND GEOLOGY    2026, 48 (2): 540-560.   DOI: 10.3969/j.issn.0253-4967.20240134
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    Tianchi volcano at Changbai Mountain is the most eruption-prone Cenozoic active volcano in China. It experienced four major Holocene eruptions, among which the eruption around AD 946 is regarded as one of the largest worldwide in the past 2000 years. In recent years, Tianchi has shown clear signs of unrest, including a strong disturbance from 2002 to 2005 and a weaker episode from December 2020 to June 2021. The volcanic area remains under a compressional stress regime, and continued attention to regional stress conditions and magmatic activity is therefore warranted.
    Gravity monitoring provides an effective means to track and investigate active volcanoes by constraining subsurface mass redistribution associated with magma and hydrothermal processes. Because magma originates in the mantle and differs in density from crustal rocks, its movement within the crust can produce microgravity changes of tens to hundreds of microgals. The observed gravity field reflects the combined effects of topography, lateral density variations within the crust and surrounding materials, and even contributions from upper-mantle structure. Differences in station elevation, changes in near-surface rock density, and magma migration(intrusion and withdrawal)can all drive measurable gravity variations, which can be quantified through high-precision repeat gravity surveys.
    To assess current magmatic activity, the Second Monitoring and Application Center of the China Earthquake Administration conducted three phases of mobile gravity measurements in 2021, 2022, and 2023. Observations were collected using two Burris relative gravimeters with a nominal precision of 10×10-8m/s-2. Each campaign was carried out during July-August with the same instruments and a consistent observer team, thereby reducing potential influences from seasonal effects and operator-related differences. The results show that gravity changes from 2021 to 2022 ranged from -161.4 to 23.4μGal and were predominantly negative. From 2022 to 2023, changes ranged from -37.8 to 135.7μGal and were mainly positive. Over the full interval from 2021 to 2023, gravity variations ranged from -90.6 to 56.0μGal, with negative changes prevailing.
    Gravity anomalies are observed within ~35km of the crater, implying the approximate extent of active magma/hydrothermal influence. Stations with changes exceeding 100μGal are concentrated within ~12km of the crater, which may reflect magma-related processes in an intermediate, roughly cylindrical conduit. The largest gravity change occurs west of the crater, suggesting the presence of a nearby deep fault. In addition, gravity changes on both sides of the crater show clustered patterns, potentially indicating activity associated with distinct slab-like magma bodies.
    The combined analysis of gravity and deformation observations is widely used for long-term volcanic-hazard assessment because it provides complementary constraints on subsurface structure, mass redistribution, and/or pressure changes. The gravity-height change gradient(Δg/Δh)directly reflects the balance between mass change and surface displacement and can be interpreted using established zoning based on the relationship between Δg and Δh. Data plotting in Zone Ⅰ (below the Bouguer-corrected free-air gradient, BCFAG) indicate subsidence with negative Δg, consistent with a decrease in density or mass(e.g., magma drainage, a falling water table, void generation, or vesiculation), implying a low likelihood of eruption. Zone Ⅱ (above the free-air gradient, FAG) also indicates subsidence but with positive Δg, reflecting increased density or mass(e.g., magma input, rising water table, dyke emplacement, bubble resorption, void filling, or hydrothermal cementation), likewise suggesting a low eruption probability. Zone Ⅲ (between FAG and BCFAG)corresponds to subsidence with positive Δg and is interpreted as increased density accompanied by decreased mass, which may result from magma drainage, a falling water table, or void closure and can be associated with summit-collapse potential. Gradients plotting along or close to the ordinate indicate shallow processes such as magma and/or gas fluctuations within feeder conduits, near-surface dyke emplacement, or hydrothermal activity.
    Leveling and InSAR observations since 1992 indicate short-term deformation fluctuations but low long-term deformation rates during quiescent periods. Deformation is mainly concentrated near the crater, with localized contributions from fault activity. During 2021-2023, Tianchi remained in a quiescent state with minimal deformation. Based on the observed deformation behavior, Δg/Δh is inferred to fall predominantly within Zone Ⅰ for 2021-2022 and Zone Ⅲ for 2022-2023. Given the large gravity changes and the small deformation amplitudes(within several tens of millimeters), Δg/Δh is expected to plot close to the ordinate. After accounting for density constraints and excluding the influence of water bodies, the gravity variations are best explained by subsurface mass redistribution, suggesting ongoing magma and/or gas migration within transport conduits or along faults.
    Integrating magnetotelluric, seismic, and gravity constraints on the crust-mantle magmatic system beneath the crater, we propose that the minor unrest in 2020-2021 increased seismicity, likely modulated by the surrounding compressional stress field. Rising magma may have partially reopened previously obstructed pathways and migrated into pre-existing reservoirs of different sizes and depths. This process could have induced micro-fracturing in the host rock, triggering earthquakes at multiple depths and producing regional gravity changes. After April 2021, seismicity returned to background levels. The presence of substantial gravity changes without pronounced deformation suggests that magma migration primarily involved intrusion into, or withdrawal from, pre-existing voids, fractures, or fault-related spaces. The negative gravity changes observed in 2021-2022 may reflect magma withdrawal and mass loss following the 2020-2021 unrest, whereas the positive changes in 2022-2023 suggest renewed ascent of melt under compressive loading, with magma filling existing voids and fault zones. Over 2021-2023, the dominant negative trend implies that the mass withdrawn in 2021-2022 exceeded the mass added in 2022-2023. Collectively, these results indicate that magmatic activity beneath Tianchi remains ongoing.

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    THREE-DIMENSIONAL WAVE VELOCITY STRUCTURE AND SEISMOGENIC STRUCTURE FOR THE 2023 WEISHAN EARTHQUAKE SWARM
    DAI Zong-hui, QU Jun-hao, XU Ning, LI Cui-qin, LI Dong-mei, YIN Yu-zhen
    SEISMOLOGY AND GEOLOGY    2025, 47 (6): 1649-1666.   DOI: 10.3969/j.issn.0253-4967.2025.06.20240067
    Abstract （243）   HTML （13）    PDF（pc） （8800KB）（121）       Save

    On March 25, 2023 at 13:53 local time, an earthquake of ML3.2 struck Weishan, Shandong Province. By April 4, three aftershocks with magnitudes greater than ML0 had occurred. On April 6, another ML3.3 event struck the same epicentral area, after which seismicity intensified, forming the Weishan earthquake swarm. The swarm effectively ended on June 30 following an ML0.7 earthquake.
    In this study, we used PhaseNet, a deep learning-based detector, to identify seismic events, and then applied the HypoDD algorithm for precise relocation, yielding a high-precision catalog for the Weishan swarm. In addition, using observation reports of ML≥0 earthquakes from January 2009 to March 2024 from the Shandong seismic network and neighbouring provinces, we performed double-difference tomography to invert the three-dimensional velocity structure of the source region, providing a detailed image of the subsurface architecture. The three-dimensional Poisson's ratio was then calculated from the inverted P-and S-wave velocity models using σ = V P 2 - 2 V S 2 2 ( V P 2 - V S 2 ) . Furthermore, employing the P-wave primitive-polarity picker POI(Probability of arrival time and polarity based on Order statistics and Information theory) together with the HASH method, we inverted focal-mechanism solutions for 10 earthquakes with ML≥2.0. Integrating the precise relocation, the 3-D velocity structure, and these focal mechanisms, we identified the seismogenic faults responsible for the swarm. Finally, we undertook a comprehensive analysis of the seismotectonic setting and seismic environment of the Weishan earthquake swarm.
    The results show that the Weishan swarm defines a clear NWW-trending linear zone ~3km long and<1km wide, with focal depths tightly clustered at 4~8km. Nodal parameters from the 10 ML≥2.0 focal mechanisms are consistent with the fault geometry revealed by precise relocations. Together, these indicate that the seismogenic fault is a previously unrecognized, high-angle, left-lateral strike-slip fault trending northwest-west(NWW) and dipping gently to the southwest(SW). The rupture surface is relatively small(~3km×2km), with a narrow damage zone and a comparatively planar fault plane. During the swarm, rupture initiated at ~8km depth and propagated upward with bilateral growth along strike, but did not reach the surface.
    Near the epicentral area, seismic velocities vary markedly. Prominent high-velocity and high-Poisson's-ratio anomalies occur northwest of the Sunshidian Fault, whereas low-velocity and low-Poisson's-ratio anomalies appear below ~15km southeast of the Fushan Fault. Velocity and Poisson's ratio also show clear layering across the region. The swarm itself is situated within a high-velocity anomaly for both P and S waves, and Poisson's ratio near the epicentre is relatively low, indicating that the source rocks are relatively hard.
    Based on the inferred fault properties and regional crustal structure from precise locations and focal mechanisms, we conclude that the Weishan swarm reflects brittle failure of hard layers in the source region-i.e., a concentrated release of stress in a localized volume. These findings refine our understanding of the mechanisms and seismogenic environment governing the Weishan earthquake swarm.

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    LATE QUATERNARY ACTIVITY CHARACTERISTICS IN THE SHIBAN RIVER TO NANNIWAN SEGMENT OF THE SOUTHERN MAOMAOSHAN MARGINAL FAULT
    XU Wei, LI Guang-tao, CHENG Li, ZOU Zi-nan
    SEISMOLOGY AND GEOLOGY    2026, 48 (2): 386-402.   DOI: 10.3969/j.issn.0253-4967.20240092
    Abstract （237）   HTML （18）    PDF（pc） （9670KB）（87）       Save

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

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    DYNAMIC SIMULATION OF DEFORMATION OF ACTIVE BLOCKS IN NORTH CHINA
    LI Chen, XING Hui-lin, YAO Qi, ZHONG Zhen-xiang
    SEISMOLOGY AND GEOLOGY    2026, 48 (1): 233-256.   DOI: 10.3969/j.issn.0253-4967.20240167
    Abstract （233）   HTML （12）    PDF（pc） （7478KB）（116）       Save

    The complex stress field and distinct internal tectonic framework of North China contribute to the frequent occurrence of strong earthquakes, particularly around the Ordos Basin, the Bohai Bay region, and the North China Plain. The generation and spatial distribution of these earthquakes are closely associated with the geometric structure and dynamic behavior of active tectonic blocks. Therefore, understanding the motion and deformation of these blocks is crucial for assessing the timing, location, and intensity of seismic events in the region. Current research on the deformation characteristics and mechanisms behind strong earthquakes in North China mainly focuses on kinematic methods such as fault slip rate and GNSS velocity field inversion. However, the dynamic mechanism underlying these kinematic characteristics remain debated. Moreover, while many active blocks exist in North China, the interaction and dynamic effect among them have received limited attention.

    This study integrates active block division data, GNSS velocity fields, and the spatiotemporal distribution of strong earthquakes to construct a three-dimensional finite element model covering North China and adjacent regions. Using this model, we simulate the regional stress and strain fields under varying block division schemes(primary, secondary, and tertiary levels)and assess the influence of tectonic activity along the Haiyuan, Liupanshan, and Longmenshan fault zones on block motion and deformation. The aim is to explore how the geometric configuration and hierarchical structure of active blocks affect the tectonic evolution of North China, thereby providing insight into the mechanisms of strong earthquakes in the region.

    The main findings are as follows:

    (1)As the number and resolution of active blocks increase, the motion and rotation rates of the Ordos Block, Taihang Mountain Sub-block, Jilu-Yuwan Sub-block, and Ludong-Huanghai block all show upward trends. The simulation results under the three-level block division scheme align best with current observational data. The contribution of secondary blocks to deformation is approximately three times that of tertiary blocks. This suggests that primary and secondary blocks, along with their boundaries, play dominant roles in the current tectonic pattern of North China, while tertiary structures contribute less significantly.

    (2)The collision between the Indian and Eurasian plates, along with the subduction of the Pacific and Philippine plates, exerts shear forces on North China from the south and north, resulting in a regional counterclockwise rotational pattern. The rotation rates of the Ordos, Taihang, Jilu-Yuwan, and Ludong-Huanghai blocks(referenced to the South China Block)are estimated at 2.3, 2.2, 2.0, and 3.4 nanoradians/year, respectively. These differences arise from two main factors: the heterogeneous distribution of the regional stress field and the compressive and extensional effects at block boundaries due to block interactions. The resulting uncoordinated block deformation leads to stress concentration and strain along fault zones, both within North China and at its margins, potentially triggering strong seismic events.

    (3)Since the late Miocene, tectonic extrusion from the eastern margin of the Tibetan plateau has influenced the southwestern Ordos region and the South China block through the left-lateral strike-slip motion of the Haiyuan fault, the thrusting of the Liupanshan fault zone, and deformation along the Longmenshan fault zone. The Haiyuan and Liupanshan faults directly enhance stress accumulation along the southwestern margin of the Ordos block, promoting deformation and movement in North China. Meanwhile, the Longmenshan thrusting enhances stress within the western South China block, facilitating its eastward motion. This movement establishes a left-lateral shear zone between the South China and Amur blocks, intensifying tectonic activity in the relatively weak North China region and indirectly driving block deformation across the area.

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    CHARACTERIZATION OF ELECTRON SPIN RESONANCE DATING SIGNAL AT g=2.002 3 IN CONTINENTAL BARITE
    ZHAO Lin, WEI Chuan-yi, YIN Gong-ming, JI Hao, LIU Chun-ru, YANG Gui-fang, XU Xing-shuo
    SEISMOLOGY AND GEOLOGY    2025, 47 (5): 1307-1325.   DOI: 10.3969/j.issn.0253-4967.2025.05.20240033
    Abstract （233）   HTML （7）    PDF（pc） （5686KB）（56）       Save

    Barite, a non-metallic mineral primarily composed of barium sulfate(BaSO4), is widely distributed in nature. Its electron spin resonance(ESR) signal( SO 3 -) has been used to date geological events such as hydrothermal activity and tectonic movements. In recent years, ESR dating of barite has been extensively applied in seafloor hydrothermal systems; however, studies on continental barite remain limited. This gap highlights the need for fundamental research into the properties of ESR signals in continental settings, providing a basis for assessing the feasibility and reliability of ESR dating.

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

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

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

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

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

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

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    QUANTIFYING THERMAL DAMAGE OF QUARTZDIORITE: A COMPARISON STUDY
    SHEN Feng-ming, WANG Li-feng, GUO Yan-shuang, QI Wen-bo, ZHOU Bin
    SEISMOLOGY AND GEOLOGY    2026, 48 (2): 314-328.   DOI: 10.3969/j.issn.0253-4967.20240124
    Abstract （224）   HTML （7）    PDF（pc） （3282KB）（47）       Save

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

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    THE STRESS FIELD INVERSION AND TRIGGER RELATIONSHIPS OF THE JINGGU EARTHQUAKE SEQUENCE
    LI Yan, CHEN Jun-lei, LÜ Si-yu, WANG Yu-dong, FU Lei
    SEISMOLOGY AND GEOLOGY    2026, 48 (2): 561-581.   DOI: 10.3969/j.issn.0253-4967.20240120
    Abstract （223）   HTML （20）    PDF（pc） （11182KB）（36）       Save

    The Jinggu earthquake sequence exhibits two dominant spatial orientations. Immediately after the mainshock, aftershocks propagated along the fault plane in a NW-SE direction. Subsequently, following two MW5.5 strong aftershocks, the sequence expanded primarily in a NNW-SSE direction. These observations imply relatively complex triggering processes between the mainshock and the two MW5.5 events, as well as substantial interactions among aftershocks. Clarifying triggering relationships within the sequence therefore requires a stress-based analysis.
    To investigate these relationships, we first compiled focal mechanism solutions for 698 historical earthquakes and inverted the regional tectonic stress field in southwestern Yunnan using a 1°×1° grid. We then inverted the local stress field of the seismogenic fault using focal mechanisms of MW≥3.0 events within the Jinggu sequence. The regional and local stress fields were used to constrain static Coulomb stress calculations on the optimally oriented fault plane, thereby improving result robustness. Based on the mainshock rupture model, we used Coulomb3.3 to evaluate the sensitivity of static Coulomb stress change to different friction coefficients and centroid depths. We further constructed source-fault models for 20 MW≥3.0 aftershocks using empirical scaling relationships and calculated Coulomb stress transfer among these events. This workflow aims to quantify stress triggering from the mainshock to aftershocks and among aftershocks themselves.
    The results indicate that the regional stress field in the area 23°~24°N and 99.5°~100.5°E is broadly consistent with the local stress field inferred for the Jinggu seismogenic fault. The maximum principal stress axis indicates NNE-SSW compression, with dominant azimuths of 12°(regional) and 25°(local). The minimum principal stress axis indicates NWW-SEE extension, with dominant azimuths of 102°(regional) and -65°(local). Stress shape ratios(R) of 0.50 and 0.59 suggest overall stress regimes approximating a uniaxial extension-uniaxial compression state and a biaxial extension-uniaxial compression state, respectively.
    Coulomb stress-change patterns computed using different friction coefficients show consistent spatial trends. As the friction coefficient increases, the likelihood of mainshock-triggered aftershocks increases, but the effect becomes weak once the coefficient exceeds ~0.4. Accordingly, we adopt the commonly used empirical value for subsequent calculations. Coulomb stress-change patterns are also broadly consistent across tested depths; therefore, we use a centroid depth of 5km. Under these assumptions, ~73.46% of aftershocks are located within stress-loading zones. Cross-sections along the sequence trend indicate Coulomb stress changes extending vertically to ~25km depth and laterally for ~40km along strike. The concentration of aftershocks within stress-loading areas is consistent with the inferred mainshock rupture process.
    Within the MW≥3.0 aftershock set, ~55% of events are classified as triggered. The two MW5.5 strong aftershocks are associated with stress loadings of 0.251MPa and 0.376MPa, respectively, far exceeding the commonly cited triggering threshold of 0.01MPa. Coulomb stress calculations for the Mw≥3.0 sequence further indicate a multiphase triggering process governing its spatiotemporal evolution. Aftershocks initially expanded preferentially toward the NW and later shifted toward the SE. The first MW5.5 event marks a key turning point: it not only directly triggered a subsequent cluster of aftershocks extending approximately NS, but also altered the overall migration pattern of the sequence.
    Mutual triggering among aftershocks is already pronounced prior to the MW5.5 event; specifically, 8 of the 13 preceding events show clear triggering effects on subsequent earthquakes. This behavior is primarily attributed to superposition of static Coulomb stress perturbations generated by multiple events, which can operate through two end-member mechanisms. When stress changes from different sources are similarly oriented, they act constructively on the receiver fault and expand the affected region beyond the initial perturbation. Conversely, when stress orientations differ, they generate complex spatial variations in stress magnitude and direction on the receiver fault, producing a heterogeneous aftershock distribution. As a result, aftershocks not directly triggered by the mainshock may still be promoted by stress transfer from other aftershocks. The MW5.5 aftershock sequence may have occurred on a branching structure with a different orientation from the mainshock fault, or on an unmapped blind fault. The distinctive triggering behavior of the Jinggu sequence provides useful constraints for assessing post-seismic evolution and for seismic hazard analysis.

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    EXTRACTION METHOD OF GRAVITY ANOMALY CHARAC-TERISTICS AND ITS APPLICATION IN PREDICTION OF LOCATION OF THE JISHISHAN MS6.2 EARTHQUAKE
    HAO Hong-tao, CHANG Lu-ji, DING Zhen-sen, WU Gui-ju, WEI Jin, HU Min-zhang
    SEISMOLOGY AND GEOLOGY    2025, 47 (6): 1606-1624.   DOI: 10.3969/j.issn.0253-4967.2025.06.20240024
    Abstract （218）   HTML （10）    PDF（pc） （7592KB）（86）       Save

    Gravity variations provide an important means for studying the processes of earthquake preparation and occurrence, as well as earthquake prediction. In recent years, gravity variations have achieved good applicationin determining the locations of a series of strong earthquakes, including the 2013 Lushan 7.0 earthquake, the 2017 Jiuzhaigou 7.0 earthquake, and the 2022 Menyuan 6.9 and Luding 6.8 earthquakes. However, the current analysis of gravity variation anomalies and the determination of strong earthquake locations is mainly based on manual interpretation, and face issues of low efficiency and reliance on experience. In this paper, automated identification methods of gravity anomaly featuresuch as quadrants, gradient zones, and zero lines are firstly studied. Then the method of focusing gravity variation anomaly features on active block boundary zones is studied to further identify potential strong earthquake locations. Finally, the method is validated using the example of the December 18, 2023 Jishishan MS6.2 earthquake.
    (1)By comprehensively utilizing methods such as machine learning image recognition, total horizontal gradient calculation, and grid data search, gravity variation anomaly features such as quadrants, gradient belts, and zero linesin gravity variation images can be quickly and comprehensively identified, which facilitates the comprehensive statistical analysis of gravity variation anomaly features. Based on the close correlation between seismic activity and the boundaries of active tectonic blocksin China mainland, a method of focusing anomaly featureson the block boundaries has been proposed in order to further delineate potential strong earthquake locations.
    (2)Before the Jishishan MS6.2 earthquake, gravity variation in the northeastern margin of the Qinghai-Xizang Plateau shows a spatial distribution pattern of negative to positive from southwest to northeast. Along the southern segment of the Lajishan Fault through the epicenter of the Jishishan MS6.2 earthquake, the gravity variation showed distinct zero lines and high-gradient zone features. The maximum difference between the positive and negative centers was approximately 110μGal, with an anomaly range of 200km and a duration of 1-3 years. The gravity variations in terms of pattern, magnitudeand duration are consistent with the existing anomaly indicators for magnitude 6 earthquakes. Combined with information on surface deformation, tectonic background, and geophysics, it is inferred that the gravity observation reflects the change in the distribution of crust-mantle material caused by the NE-SW compression of the northeastern margin of the Qinghai-Xizang Plateau and the reverse thrust movement of the southern segment of the Lajishan fault zone.
    (3)By focusing gravity anomaly characteristics on active block boundaries, it is helpful to give more intuitive and precise results for strong earthquake location prediction. The focused results of the three types of gravity anomalies(quadrant, gradient belt, and zero line) in the block boundary zone all show high-value characteristic in the southeastern area of the Qilian Block. The comprehensive statistical results formed by the superposition of the three types of gravity variation anomalies further highlight that the southeastern area of the Qilian Block is the most obvious high-value anomaly zone, and the Jishishan MS6.2 earthquake is located on the edge of this high-value zone. Compared with traditional manual interpretation methods, the approach presented in this study is not only more intuitive and efficient, but also significantly reduces the size of the earthquake prediction area.
    (4)This research is a preliminary exploration of automated extraction methods of gravity variation anomalyfeatures and their application in strong earthquake location prediction combined with active tectonics. In subsequent studies, it is still necessary to refine the focused seismic fault zones, taking into account non-block boundary regions such as strong earthquake rupture gaps, locked fault segments identified by geodetic measurements, sparse small-to-medium earthquake activity zones, and segments with significant Coulomb stress enhancement from numerical simulations, etc. Additionally, a comprehensive statistical analysis of the correspondence between different anomaly features and their seismogenic probabilities should be conducted based on a large number of earthquake cases to assign more reasonable weight coefficients for different gravity variation anomaly features.

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    THE RUPTURE PROCESS INVERSION OF SEPTEMBER 2022 ML6.6 AND ML6.83 EARTHQUAKES IN TAIWAN ISLAND, CHINA
    LIU Lian, QU Chun-yan, WU Dong-lin, RONG Yi-lin, CHEN Han
    SEISMOLOGY AND GEOLOGY    2026, 48 (1): 217-232.   DOI: 10.3969/j.issn.0253-4967.20240045
    Abstract （218）   HTML （5）    PDF（pc） （8990KB）（48）       Save

    The Taiwan Island is located at the tectonic junction of the western Pacific subduction zone and the Eurasian continental margin, where complex plate subduction, collision, and strike-slip motions have collectively shaped the region’s intense tectonic activity and frequent seismic hazards. The NNE-trending Longitudinal Valley Fault(LVF) in eastern Taiwan serves as a major tectonic boundary separating the Eurasian Plate and the Philippine Sea Plate. It is also recognised as one of the world’s most renowned seismically active zones. On September 17 and 18, 2022, two destructive strong earthquakes with magnitudes of ML6.6(MW6.5)and ML6.83(MW6.9)successively occurred in the southern segment of this fault. According to high-precision seismic observations from “Taiwan’s Central Weather Bureau(CWB)”, the epicenters of the two events are only 11km apart, with a time interval of 17 hours, indicating significant spatiotemporal clustering. Notably, the focal mechanisms of the two earthquakes differ distinctly: the September 17 event was a typical left-lateral strike-slip earthquake, while the September 18 event displayed a more complex composite rupture mechanism dominated by thrust motion with a significant strike-slip component. This rapid transition in tectonic deformation modes within a short period reflects the complex stress environment of the region, providing a unique natural laboratory for revealing the fine-scale dynamic processes of the fault zone and the interaction mechanisms between consecutive earthquakes.

    To systematically analyse the source rupture processes and stress triggering relationships of these two strong earthquakes, this study collected near-field strong-motion records and precise hypocenter locations from the CWB, combined with source parameters released by the United States Geological Survey(USGS). Using the Iterative Deconvolution and Stacking(IDS)technique, we performed detailed kinematic inversions of the rupture processes of both events. Furthermore, leveraging the high temporal resolution advantage of seismic waveform data, we successfully isolated the co-seismic deformation fields for each earthquake, providing crucial evidence for understanding the interaction mechanisms within the earthquake sequence. The inversion results show that the rupture durations of the two earthquakes are 27s and 48s, respectively, and that the spatial patterns of fault slip distribution are significantly different: the September 17 earthquake exhibited a single-peak slip distribution concentrated southwest of the epicenter, with a maximum slip of 1.06m. In contrast, the September 18 earthquake displayed a double-peak slip pattern predominantly distributed northeast of the epicenter, reaching a maximum slip of 3.19m and featuring a typical asymmetric bilateral rupture. The slip depths of both earthquakes are distributed within the range of 0-30km, consistent with regional seismotectonic characteristics.

    Quantitative analysis of Coulomb failure stress changes demonstrated that the September 17 earthquake induced significant static stress perturbations in the rupture area of the September 18 event, exceeding the seismic triggering threshold. Our results strongly support the domain role of static stress transfer in this earthquake sequence. Further analysis revealed that the September 18 earthquake had a higher stress drop, which may be closely related to its complex rupture mechanism and larger slip magnitude.

    Through high-resolution source rupture process inversion and Coulomb stress change calculation, this study systematically revealed the rupture differences, spatiotemporal evolution laws, and intrinsic triggering mechanisms of the short-time sequence strong earthquakes along the LVF in Taiwan, China. It not only confirms the importance of stress transfer between earthquakes but also highlights the complexity of seismic rupture processes(e.g., the diversity of rupture modes and the inhomogeneity of slip distribution). The findings provide important observational evidence for an in-depth understanding of seismic rupture behavior along the LVF, complex tectonic deformation mechanisms at plate boundaries, and the laws of earthquake generation and occurrence. Meanwhile, this study emphasizes the core significance of inter-earthquake interactions for earthquake prediction, hazard assessment, and disaster prevention and mitigation. Additionally, it provides a methodological reference for earthquake sequence research in similar tectonic environments. Future research should further integrate numerical simulations with multidisciplinary observational data to explore the physical mechanisms of seismic rupture processes. Simultaneously, in regions with high seismic hazard, such as Taiwan, China, it is necessary to continuously strengthen the development of high-density seismic network monitoring and multi-parameter early warning technologies, thereby continuously improving comprehensive prevention and control capabilities for seismic disasters and minimizing casualties and economic losses from earthquakes.

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    DISCUSSION ON STRAIN STATE AND SEISMOGENIC MECHANISM OF THE NORTH SECTION OF YILAN-YITONG FAULT ZONE
    CHANG Jin-long, GAN Wei-jun, ZHOU Chen, ZHU Cheng-lin, LIU Chang-sheng, YAO Cheng-yue
    SEISMOLOGY AND GEOLOGY    2026, 48 (1): 200-216.   DOI: 10.3969/j.issn.0253-4967.20240074
    Abstract （215）   HTML （11）    PDF（pc） （6461KB）（107）       Save

    The Yilan-Yitong fault zone(YYFZ)is the western branch of the two major branches in the northeastern segment of the Tanlu fault zone(TLFZ). Influenced by the subduction of the western Pacific Plate, the northern section of this fault(the Tangyuan-Luobei section and the Fangzheng-Tangyuan section)is seismically active. In particular, the Tangyuan-Luobei section experiences frequent moderate and small earthquakes, while the Fangzheng-Tangyuan section has a background of strong earthquakes of magnitude 7 and above. A scientific understanding of the seismic mechanisms in this area is of significant reference value for analyzing the seismic hazards of the northern section of the Yilan-Yitong Fault.

    This study determins the source mechanism solutions, calculates the regional GPS strain field, extracts point deformation anomalies, and computes the relative movement speed changes of the blocks on both sides of the northern section of the Yilan-Yitong fault. It also considers the effects of M7 and above strong earthquakes from the Japan Trench on positive loading(superposition) of co-seismic and post-seismic Coulomb stress and the post-seismic viscoelastic relaxation effect, providing a comprehensive analysis of the strain field status and seismic mechanisms in the northern section of the Yilan-Yitong fault.

    The results indicate that:

    (1)The strain field characteristics of the northern section of the Yilan-Yitong fault are characterized by NE-directed compression and NW-directed extension. The direction of the regional strain field is consistent with that of the background stress field, indicating that the currently observable strain field is controlled by the background stress field.

    (2)The relative movement speed on both sides of the Yilan-Yitong fault in the Tangyuan-Luobei section has increased, and the corresponding seismic activity has intensified, indicating that the earthquake activity in this area is closely related to fault movement.

    (3)The long-term trend of the water pipe tilt instrument, the gravity tidal factor, and the vertical oscillation tidal factor are roughly synchronous with the abnormal time period of relative block movement, suggesting that the elastic properties of the underground medium change due to tectonic movements, essentially reflecting changes in the internal stress field of the medium. The regional point deformation, gravity tidal factor, and abnormal changes in the strain field are correlated with seismic activity.

    (4)The time series of the maximum shear strain parameter shows a continuous increase in the maximum shear strain in the region, indicating strong shearing effects in this area. Similar to the time series of the volumetric strain parameter, the amplitude of the maximum shear strain parameter during this active earthquake period significantly exceeded the limit. The over-limit of volumetric strain and maximum shear strain parameters may be anomalous phenomena related to seismic activity.

    (5)Due to the western Pacific plate subduction, differential motion occurs between the Sanjiang Basin and the Xiaoxing’anling Uplift flanking the Tangyuan-Luobei section. While the strong tectonic activity of the main fault facilitates energy release, making it less prone to large earthquakes, the NW-striking secondary faults perpendicular to it are more fragmented. These secondary faults are prone to generate slip under shear stress, producing strike-slip type shallow moderate and small earthquakes. This may explain the frequent moderate and small earthquakes in the Tangyuan-Luobei section. The seismogenic process is accompanied by changes in the physical properties of the medium, reflecting alterations in the crustal stress field, as evidenced by significant annual and trend changes in point deformation and anomalies in the M2 wave tidal factor.

    (6)The results of the co-seismic static Coulomb failure stress indicate that the northern section of the Yilan-Yitong fault is located in a region of positive Coulomb triggering stress from the subduction-type strong earthquakes offshore Honshu, Japan. The Tonghe section is the main area affected by co-seismic Coulomb failure stress, and strong earthquakes facilitate seismic events on the northern section of the Yilan-Yitong fault.

    Although the impact of single strong earthquakes from the Japan trench is limited, considering the high frequency of M7 and above earthquakes and a recurrence period of approximately one year, the long-term cumulative effects over thousands of years cannot be ignored. The strong seismic activity on the northern section of the Yilan-Yitong fault primarily originates from local tectonic stress field actions, as well as the frequent subduction-type strong earthquakes from the offshore Honshu region of Japan that trigger positive loading (superposition) and post-seismic Coulomb stress effects on the northern section of the Yilan-Yitong fault zone. Due to relatively few deep earthquakes in the Fangzheng-Tangyuan section, stress is not sufficiently released, leading to the formation of stress accumulation areas that trigger strong earthquakes; this may be one of the important factors for strong earthquakes occurring in the Fangzheng-Tangyuan section.

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    EXPERIMENTAL STUDY ON THE EFFECTS OF FAULT CONTACT HETEROGENEITY ON SEISMIC ACTIVITY
    LI Zi-hong, ZHUO Yan-qun, CHEN Shun-yun, CHEN Hao, LU Li-li
    SEISMOLOGY AND GEOLOGY    2026, 48 (2): 329-350.   DOI: 10.3969/j.issn.0253-4967.20240118
    Abstract （214）   HTML （11）    PDF（pc） （4900KB）（48）       Save

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

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