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20 February 2026, Volume 48 Issue 1

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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

2026, 48(1):  1-18.  DOI: 10.3969/j.issn.0253-4967.20250065

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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.

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

2026, 48(1):  19-42.  DOI: 10.3969/j.issn.0253-4967.20240075

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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.

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

2026, 48(1):  43-63.  DOI: 10.3969/j.issn.0253-4967.20240051

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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.

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

2026, 48(1):  64-80.  DOI: 10.3969/j.issn.0253-4967.20230138

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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.

STUDY ON CENOZOIC PROVENANCE OF THE NORTHERN QAIDAM BASIN: INSIGHTS FROM QUANTITATIVE PRO-VENANCE CONSTRAINTS BASED ON DETRITAL ZIRCON U-Pb DATA

DONG Ling-feng, WANG Wei-tao, QIAO Yu-lai

2026, 48(1):  81-101.  DOI: 10.3969/j.issn.0253-4967.20240053

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The Qaidam Basin, as the largest continental sedimentary basin on the northeastern margin of the Qinghai-Xizang Plateau, has formed thick sedimentary strata since the Cenozoic. Its complete and continuous Cenozoic sedimentary strata not only record the sedimentary evolution of the Qaidam Basin but also reflect the uplift and erosion of the adjacent orogenic belts and regional climate-environmental changes. Furthermore, they serve as a crucial resource for understanding the development of the basin-mountain system, tectonic deformation, and the mechanisms and patterns driving the northward expansion of the Qinghai-Xizang Plateau on the northeastern margin. Therefore, it is vital to reconstruct the sedimentary evolution history of the Qaidam Basin during the Cenozoic. Currently, the detrital zircon U-Pb dating method is the most widely used provenance analysis method for reconstructing the basin’s tectonic evolution and the uplift history of the surrounding mountain ranges. However, this method usually relies too heavily on qualitative comparisons of zircon U-Pb age spectra and lacks reliable quantitative analysis. This limitation directly leads to significant differences and disputes in some key understandings, such as the erosion of the source and the transport path of sediments. To address this issue, we selected three representative Cenozoic sedimentary sections on the northern margin of the Qaidam Basin for our study: Dahongou, Hongshan, and Huaitoutala. At the same time, by systematically collecting the latest published U-Pb age data of detrital zircons and basement zircons in modern river sands, we redefined three main provenance areas, including the southern segment of the Qilian Mountains, the East Kunlun Mountains, and the Mesozoic strata in the northern part of the Qaidam Basin. The results show that the Dahongou, Hongshan, and Huaitoutala sections have 6, 2, and 3 distinct shifts in provenance, respectively. Although the sedimentary provenance evolution patterns of these sections are not completely consistent, there is a quasi-synchronous provenance change at ~12-10Ma. Around ~20Ma, the southern Qilian Mountains expanded towards the Qaidam Basin, causing the uplift and erosion of the Mesozoic strata on the northern margin of the Qaidam Basin. All three sections recorded the significant contribution of the Mesozoic provenance from the northern margin of the Qaidam Basin during this period. Although the East Kunlun Mountains experienced a rapid uplift and erosion event in the middle-late Miocene, this tectonic event had a relatively limited impact on the provenance of the Hongshan section, underscoring the limitations of single-section studies for elucidating regional tectonic and sedimentary evolution. Additionally, the simulation of provenance inversion suggests that the northern margin of the Qaidam Basin experienced influences from at least three distinct provenance regions throughout the Cenozoic. The zircon age distribution characteristics of simulated source area Ⅰ closely resemble the detrital zircon age distribution found in modern river sands across the eastern Kunlun Mountains, as per our compilation. Meanwhile, the zircon age distribution in simulated source area Ⅱ closely matches that of modern river sands in the southern Qilian Mountains. Additionally, the provenance area Ⅲ revealed by the inversion simulation exhibits a notable Proterozoic double-peak zircon age signature, although it shows a higher proportion of zircon components exceeding 550Ma, closely aligning with the detrital zircon age composition from the Mesozoic strata(including Jurassic and Cretaceous) of the northern margin of the Qaidam Basin that we compiled. Overall, the inversion simulation results reveal a substantial similarity in the zircon age distribution between the simulated source area III and the Mesozoic strata in the northern margin of the Qaidam Basin. Moreover, in most Mesozoic clastic samples, a significant proportion of the parent material is derived from sources. This suggests that during the Neogene, the relationship between the Mesozoic parent material sources in the northern margin of the Qaidam Basin is more complex than simply a competition between the two end-member source areas, namely the South Qilian Mountains and the East Kunlun Mountains, as has been previously highlighted. Additionally, the Mesozoic strata uncovered by the frontal thrust deformation are likely to be significant contributors to the material supply during the Neogene. Additionally, the provenance inversion simulation in this study is consistent with the above forward simulation results, indicating the rationality and credibility of the provenance interpretation. At the same time, it also shows that, in cases where the number of potential source areas is difficult to determine and the age composition of source-area zircons is difficult to constrain, provenance-inversion simulation will be one of the most effective methods. In conclusion, we believe that the quasi-synchronous provenance change at ~12~10Ma may represent the surface response to the deep-dynamic transformation of the plateau, namely, the delamination of the thickened lithosphere triggers regional tectonic uplift and surface deformation. Besides, the East Kunlun Mountains and the southern Qilian Mountains, the Mesozoic strata distributed in the transition zone between the southern Qilian Mountains and the northern margin of the Qaidam Basin are another important provenance area for the Cenozoic sedimentary process on the northern margin of the Qaidam Basin. The zircon age distribution of these strata is often confused with the provenance of the East Kunlun Mountains in previous studies. These insights provide reliable references for a deeper understanding of the evolution of the Cenozoic Basin-mountain system on the northeastern margin of the Qinghai-Xizang Plateau. We emphasize that, in complex basin-mountain systems, simply comparing the age distributions of detrital zircons in most provenance analyses is no longer sufficient. A thorough qualitative and quantitative analysis and assessment are more conducive to obtaining reliable and comprehensive results.

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

2026, 48(1):  102-126.  DOI: 10.3969/j.issn.0253-4967.20240090

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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.

AZIMUTHAL ANISOTROPY OF THE MIDDLE-UPPER CRUST IN NORTHWESTERN YUNNAN BY DIRECT SURFACE WAVE TOMOGRAPHY METHOD

YANG Jian-wen, LI Qing, YE Beng, JIN Ming-pei, CHA Wen-jian, JIA Luo-zhao

2026, 48(1):  127-141.  DOI: 10.3969/j.issn.0253-4967.20240086

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The northwestern Yunnan region is located at the collision boundary between the Indian and Eurasian plates. Owing to its unique tectonic setting, complex geological structures, and intense crustal deformation, it represents a key area for geoscientific research. Detecting the fine crustal structure of this region not only helps to elucidate crustal motion and lithospheric deformation mechanisms, but also provides an important basis for understanding plate tectonic evolution, mineral resource formation, and geological environment protection.

Seismic anisotropy refers to the phenomenon in which the propagation velocity and polarization direction of seismic waves vary with propagation direction in anisotropic Earth media. It mainly includes body-wave anisotropy and surface-wave anisotropy, the latter of which can be further divided into azimuthal anisotropy and radial anisotropy. Azimuthal anisotropy arises from differences in surface-wave phase velocities with azimuth and is an important parameter for characterizing medium deformation. Investigating crustal azimuthal anisotropy in northwestern Yunnan can reveal variations in the stress field and crustal motion during deformation processes, thereby providing critical insights into the background of crustal evolution. As a transitional layer of the lithosphere, the middle and upper crust is also an earthquake-prone zone, and its internal structure and deformation play a key role in understanding plate tectonic evolution and crustal dynamics. In recent years, with the successive deployment of short-period dense seismic arrays through active-source detection and sub-instability experiment projects, the number of seismic stations in northwestern Yunnan has increased significantly, enabling the acquisition of short-period surface-wave signals. Although short-period, high-frequency surface waves are strongly attenuated, they are more sensitive to shallow velocity structures, making them particularly valuable for resolving three-dimensional azimuthal anisotropy of the shallow crust and for understanding shallow structural and deformation patterns.

Based on two years of continuous vertical-component waveform data recorded by 74 seismic stations in northwestern Yunnan, phase-velocity dispersion curves of fundamental-mode Rayleigh waves with periods of 1-20 s were extracted. Using a direct inversion method for three-dimensional surface-wave azimuthal anisotropy, a three-dimensional azimuthal anisotropy model of the middle and upper crust above 20km depth was constructed, and the deformation characteristics and stress field state of northwestern Yunnan were analyzed. The main conclusions are as follows.

(1)Azimuthal anisotropy in northwestern Yunnan exhibits pronounced spatial zonation, generally bounded by the Weixi-Qiaohou Fault, with significant differences between its eastern and western sides. West of the fault, azimuthal anisotropy is relatively uniform and shows little variation with depth; fast-wave directions are predominantly NNW and NW, consistent with the regional principal compressive stress direction. East of the Weixi-Qiaohou Fault, azimuthal anisotropy differs markedly above and below a depth of 10km. At depths greater than 10km, fast-wave directions generally rotate clockwise, transitioning from NNW and NW in the north to nearly NS south of 26°N, indicating that crustal deformation is mainly controlled by regional strike-slip motion. Within the 10~20km depth range, dominant fast-wave directions are NNE and NE, consistent with the strike of the Chenghai Fault. This anisotropy is likely related to strong compression or strike-slip deformation near the Chenghai fault zone, resulting in the preferred alignment of minerals such as mica and hornblende along the fault strike.

(2)At the southern termination of the Weixi-Qiaohou Fault(south of Yangbi), the dominant fast-wave directions are SWW and near EW. These deformation characteristics are preliminarily interpreted to be associated with a normal-fault dislocation mechanism driven by horizontal compression in the NNW direction and horizontal extensional tectonic stress in the SWW direction.

(3)The three-dimensional layered azimuthal anisotropy model obtained in this study provides information on anisotropic variations at different depth levels in the vertical direction. Horizontally, compared with shear-wave splitting and Pms splitting methods, which only provide anisotropy information beneath seismic stations, the surface-wave-based approach enables azimuthal anisotropy imaging over the entire study area, including regions without stations, thereby offering superior horizontal and vertical resolution. Given that the lateral resolution of surface-wave methods mainly depends on station density and wavelength, three-dimensional azimuthal anisotropy imaging based on dense seismic arrays is an effective means of resolving crustal deformation characteristics and stress field states. With the implementation of the China Earthquake Science Experimental Site project, denser seismic networks are expected to be deployed in northwestern Yunnan in the near future, which will facilitate the construction of more refined three-dimensional azimuthal anisotropy models. The results of this study provide valuable reference data for regional geophysical research and scientific assessment of future earthquake hazards, and contribute to a deeper understanding of the tectonic background and dynamic mechanisms of northwestern Yunnan.

A STUDY ON THE DURATION OF AFTERSHOCK SEQUENCES FOLLOWING SIX LARGE EARTHQUAKES IN North China

LIU Yue, LÜ Xiao-jian

2026, 48(1):  142-161.  DOI: 10.3969/j.issn.0253-4967.20250097

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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 M_S7.2, 1975 Haicheng M_S7.3, and 1976 Tangshan M_S7.8 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 M_S7.2 and Haicheng M_S7.3 earthquakes are dominated by events with magnitudes around M_L≥3. In contrast, M_L≥4 earthquakes at a rate of approximately 1.5 events/a from 2013 to 2022 are observed in the Tangshan M_S7.8 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 M_L≥3.0 occurred from 2013 to 2022. Additionally, the 1679 Sanhe-Pinggu aftershocks with magnitudes M_L≤2 are still active. Following the 1937 Heze M7 earthquake in Shandong Province, a M_S5.9 strong aftershock occurred in 1983. The aftershock zone records an average of 1.8 earthquakes/a with M_L≥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.

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

2026, 48(1):  162-180.  DOI: 10.3969/j.issn.0253-4967.20240068

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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.

SLIP CHARACTERISTICS OF THE SEISMOGENIC FAULT OF THE 2022 LUDING MS 6.6 EARTHQUAKE AND AN EXPLANA-TION FOR THE NORMAL-FAULTING EARTHQUAKE TO ITS WEST

GU Pei-yuan, WAN Yong-ge, HUANG Ji-chao, JIN Zhi-tong, SONG Ze-yao, GUAN Zhao-xuan, ZHOU Zi-yao

2026, 48(1):  181-199.  DOI: 10.3969/j.issn.0253-4967.20240080

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Based on 1, 701 precisely relocated seismic events from the 2022 Luding M_S6.8 earthquake sequence, this study systematically delineates the three-dimensional geometric characteristics of the seismogenic faults using a fuzzy clustering-based fault plane determination method, providing critical insights into the complex seismotectonic processes operating along the southeastern margin of the Tibetan plateau. The results identify three prominent aftershock clusters in the main rupture area, each exhibiting distinct spatial distributions and tectonic implications. With respect to the Xianshuihe fault zone and their relative positions, Cluster A displays a slightly elliptical distribution that deviates markedly from the mapped fault trace, making it difficult to fit a single fault plane and suggesting strong control by complex subsurface structures. In contrast, Clusters B and C show well-defined elliptical distributions with pronounced long-axis orientations and high degrees of flattening, and both exhibit excellent agreement with the fitted fault planes, indicating relatively simple and coherent fault geometries consistent with regional tectonic patterns. The B and C faults, extending along the Xianshuihe fault zone, strike NNW-SSE with orientations of 159.9° and 157.2°, respectively, and both are characterized by near-vertical, high dip angles(Fault B: 88.1°; Fault C: 87.1°), consistent with the typical geometry of left-lateral strike-slip faults that dominate the region.

Notably, the area west of the main shock(Cluster A)exhibits pronounced spatial heterogeneity in focal mechanisms used for stress field inversion. Normal fault-type earthquakes account for 21.11% of the total dataset, which is unexpected in a tectonic setting primarily governed by strike-slip and thrust faulting. Clustering of focal mechanism nodal planes reveals a dominant fault plane striking 156.99°, closely aligned with the regional fault orientation despite the contrasting faulting style. Tectonic stress field inversion using a grid-search approach indicates that the main shock area is characterized by a composite stress regime, with near-horizontal compression in the NW-SE direction(P-axis azimuth 101.5°, plunge 0.9°) and vertical extension in the NNE-SSW direction(T-axis plunge 59.0°), consistent with the stress field imposed by the ongoing India-Eurasia collision. Projection of the stress field onto the fault planes shows that Fault B experiences relatively high shear stress(0.719) and compressional normal stress(-0.701), indicating conditions favorable for shear stress accumulation and potential future rupture. Fault C exhibits slightly higher relative shear stress(0.760) but weaker compressional normal stress(-0.625), suggesting spatial variations in fault strength and stress accumulation along the fault system. Both faults have slip angles smaller than 15°, indicating predominantly strike-slip motion with minor dip-slip components; the slip angles of Faults B and C are 13.0° and 14.7°, respectively. These results confirm left-lateral strike-slip as the dominant kinematic behavior, in strong agreement with GPS-derived slip rates of 9~12mm/a and historical strong-earthquake recurrence in the region.

In contrast, the stress field within Cluster A shows an anomalous pattern, characterized by near-vertical compression (P-axis plunge 84.6°, azimuth 210.3°) and near-horizontal extension (T-axis plunge 5.0°), representing an approximately 98° rotation relative to the background regional stress field. This indicates the operation of a fundamentally different local stress regime. We propose that the occurrence of normal fault-type earthquakes in Cluster A may be associated with gravitational potential energy release driven by the high topography of the Gongga Mountain region, producing shallow crustal extension superimposed on deep left-lateral strike-slip motion of the Xianshuihe Fault, thereby generating shallow normal-fault components related to gravitational collapse. Alternatively, or additionally, the presence of concealed secondary faults, highly brittle rock properties, and possible reservoir water infiltration may promote normal faulting through localized stress concentration and pore pressure variations. The relative contributions of these mechanisms, however, require further investigation using integrated geophysical observations and numerical simulations.

Overall, this study provides important constraints on fault activity and seismotectonic mechanisms along the southeastern margin of the Tibetan plateau, highlighting the complex interplay between regional tectonic stress fields and local geological structures. The identification of anomalous normal faulting west of the main rupture zone offers new perspectives on the lateral extrusion of the Bayan Har block and the role of gravitational collapse in high-relief regions, with implications for analogous tectonic environments worldwide. The integrated analysis of fault geometry, stress field inversion, and kinematic behavior contributes to improved seismic hazard assessment in this highly active region and provides a robust scientific basis for earthquake forecasting and risk mitigation.

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

2026, 48(1):  200-216.  DOI: 10.3969/j.issn.0253-4967.20240074

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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.

THE RUPTURE PROCESS INVERSION OF SEPTEMBER 2022 M_L6.6 AND M_L6.83 EARTHQUAKES IN TAIWAN ISLAND, CHINA

LIU Lian, QU Chun-yan, WU Dong-lin, RONG Yi-lin, CHEN Han

2026, 48(1):  217-232.  DOI: 10.3969/j.issn.0253-4967.20240045

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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 M_L6.6(M_W6.5)and M_L6.83(M_W6.9)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.

DYNAMIC SIMULATION OF DEFORMATION OF ACTIVE BLOCKS IN NORTH CHINA

LI Chen, XING Hui-lin, YAO Qi, ZHONG Zhen-xiang

2026, 48(1):  233-256.  DOI: 10.3969/j.issn.0253-4967.20240167

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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.

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

2026, 48(1):  257-277.  DOI: 10.3969/j.issn.0253-4967.20240030

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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 M_L<3.0 focal mechanisms is normal faulting regime, and the results of 301 M_L≥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.