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20 December 2025, Volume 47 Issue 6

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Review

INVESTIGATION OF THE SUITABILITY OF (U-TH)/HE DATING MINERALS

YANG Li, YANG Jing, ZHANG Bin, YUAN Wan-ming, LI Xiao, YE Zhang-huang

2025, 47(6):  1495-1525.  DOI: 10.3969/j.issn.0253-4967.2025.06.20240050

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(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.

THE TIDAL TRIGGERING CHARACTERISTICS OF M_S≥4.0 AFTERSHOCKS IN JANUARY 23, 2024 WUSHI M_S7.1 EARTHQUAKE SEQUENCE, XINJIANG, CHINA

JIANG Hai-kun, SONG Jin

2025, 47(6):  1526-1545.  DOI: 10.3969/j.issn.0253-4967.2025.06.20240055

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On January 23, 2024, an M_S7.1 earthquake occurred in Wushi, Xinjiang, China. The Wushi M_S7.1 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 M_S≥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 M_S5.8 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 M_S5.0 to M_S6.0; afterward, aftershock activity noticeably weakened, maximum seismic activity levels dropping to the range of M_S4.0 to M_S5.0. Specifically, among the 50 M_S≥4.0 aftershocks recorded until April 30, 2024, 45 occurred before February 25, including all 10 M_S≥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 M_S≥4.0 in Wushi sequence exhibited significant temporal clustering characteristics. In the early post-seismic stage before February 25, the vast majority of M_S≥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 M_S≥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 M_S≥4.0 stronger aftershocks during syzygy spring tides(new/full moons) are primarily driven by the combined effects of: maximum tidal normal stress(σ_n) and contemporaneously increasing tidal shear stress(τ_n) 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(τ_n) and contemporaneously increasing tidal normal stress(σ_n) 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 M_S≥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 M_S7.1 mainshock and most M_S≥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 M_S≥4.0 earthquakes in the Wushi sequence, with 60% of the triggered events being influenced by tidal shear stress(τ_n). Regarding the temporal relationship between earthquake occurrence and tidal stress, 94% of the semi-diurnally triggered M_S≥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.

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

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

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

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

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

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

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

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

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

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

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

EXTRACTION METHOD OF GRAVITY ANOMALY CHARAC-TERISTICS AND ITS APPLICATION IN PREDICTION OF LOCATION OF THE JISHISHAN M_S6.2 EARTHQUAKE

HAO Hong-tao, CHANG Lu-ji, DING Zhen-sen, WU Gui-ju, WEI Jin, HU Min-zhang

2025, 47(6):  1606-1624.  DOI: 10.3969/j.issn.0253-4967.2025.06.20240024

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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 M_S6.2 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 M_S6.2 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 M_S6.2 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 M_S6.2 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.

CHARACTERISTICS OF GRAVITY CHANGE BEFORE THE 2021 YUNNAN YANGBI M_S6.4 EARTHQUAKE AND ITS DEEP MATERIAL MIGRATION PROCESS

LIU Zhi-hui, HAO Hong-tao, HU Min-zhang, LI Zhong-ya, WANG Jian, ZHANG Xin-lin, WANG Jia-pei

2025, 47(6):  1625-1648.  DOI: 10.3969/j.issn.0253-4967.2025.06.20240062

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This study presents a comprehensive analysis of mobile gravity observation data from the western Yunnan region, focusing on the gravity variations leading up to the Yangbi M_S6.4 earthquake in Yunnan in 2021. The analysis utilizes the GLDAS global hydrological model to examine regional hydrological gravity changes based on load theory. Additionally, GNSS observations of long-term vertical displacement rates are used to assess gravity changes resulting from crustal deformation. By isolating gravity changes due to hydrological effects, crustal deformation, and other factors, the study explores potential deep-seated material migration processes in and around the epicentral area. After removing gravity changes attributed to terrestrial water loads and crustal deformation, gravity variations linked to deep structural activities were confirmed. The Bott density interface inversion method was then applied to evaluate deep material migration before the Yangbi earthquake.
The main findings are as follows: 1)Mobile gravity observations show a steady increase in regional gravity changes since 2016, culminating in 2018 in a “four-quadrant” distribution pattern around the epicenter, with maximum positive and negative gravity differences exceeding 50μGal. This pattern extends across the entire western and southwestern Yunnan region, with a spatial range of 200~300km. After 2019, cumulative gravity changes slightly decreased, with only minor local variations remaining before the earthquake. Gravity change gradients align with block boundaries, and the earthquake occurred at the center of the gravity “four-quadrant” and within a region of high gradient. 2)Gravity changes calculated from a small number of permanent GNSS observation stations range from -13 to 1μGal, indicating significant magnitude and clear seasonal variations. When annual differential GNSS data are used, the gravity effect reduces to between -8 and 1μGal. However, the limited number of GNSS stations does not fully represent the conditions across the entire survey area. Data from 165 long-term mobile GNSS observation stations revealed that long-term gravity changes range from -0.68 to 0.37μGal, indicating minimal impact. Hydrological effects, including seasonal changes, contributed between -3.3 and 0.6μGal, suggesting a minor influence. The gravity effects from vertical deformation(based on mobile GNSS data) and hydrological influences were too small(no greater than 4μGal) to account for the observed gravity changes in the mobile gravity surveys. 3)Thus, after excluding gravity changes due to terrestrial water loads and crustal deformation, gravity variations attributable to deep structural activities were confirmed. The Bott density interface inversion method was then used to evaluate regional deep material migration prior to the Yangbi earthquake.
The results show that when the reference depth for material migration is set at 10km and the density change is 1kg/m³, there is a strong correlation between the material migration-induced column height changes and the observed gravity changes. By analyzing the cumulative mass changes since 2016, it was found that material exchange within a 300km radius of the epicenter covered the earthquake zone. Specifically, from September 2016 to September 2018, the simulated net material inflow within this 300km radius reached 5.2×10¹²kg, with stress changes causing the epicentral area and surrounding regions to exhibit a “four-quadrant” pattern. From September 2016 until just before the earthquake, the simulated peak material inflow within the 300km radius centered on the epicenter reached 1.45×10¹³kg, leading to rock layer rupture and triggering the earthquake.

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

2025, 47(6):  1649-1666.  DOI: 10.3969/j.issn.0253-4967.2025.06.20240067

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On March 25, 2023 at 13:53 local time, an earthquake of M_L3.2 struck Weishan, Shandong Province. By April 4, three aftershocks with magnitudes greater than M_L0 had occurred. On April 6, another M_L3.3 event struck the same epicentral area, after which seismicity intensified, forming the Weishan earthquake swarm. The swarm effectively ended on June 30 following an M_L0.7 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 M_L≥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 \sigma = \frac{V_{\mathrm{P}}^2-2V_{\mathrm{S}}^2}{2(V_{\mathrm{P}}^2-V_{\mathrm{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 M_L≥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 M_L≥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.

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

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

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

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

2025, 47(6):  1688-1707.  DOI: 10.3969/j.issn.0253-4967.2025.06.20240104

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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.