Table of Content

    20 June 2024, Volume 46 Issue 3
    Research paper
    JIANG Hai-kun, DENG Shi-guang, YAO Qi, SONG Jin, WANG Jin-hong
    2024, 46(3):  513-535.  DOI: 10.3969/j.issn.0253-4967.2024.03.001
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    The transition from the metastable state to the meta-instability stage indicates that the seismic fault has entered an irreversible deformation process and will lead to an inevitable instability(Ma Jin et al., 2014). Therefore, identifying the meta-instability stage is helpful for the judgment of short-term earthquake precursor anomalies. Under laboratory conditions, the meta-instability stage can be visually identified through stress-time curves, thus potentially predicting the occurrence of the laboratory earthquake. However, there are significant differences between field conditions and laboratory environments. Firstly, the underground medium and structural conditions in the real earthquake source region are unclear and far more complex than laboratory specimens. Secondly, the distribution of the sensors, sensor density, as well as measurement accuracy are limited by various conditions, making it impossible to construct an ideal observation environment covering the entire region. Thirdly, the loading stress cannot be directly measured, and the current actual stress state of the study area is unknown, which is the most difficult problem to solve. Therefore, under the guidance of meta-instability experiments and theories, it is a beneficial attempt to conduct retrospective studies on typical earthquake cases with relatively good observation conditions in the past, analyze the spatial-temporal evolution of different physical fields at different stages before the earthquakes, compare the observed phenomena with the characteristics and change processes of meta-instability stages obtained from experiments or theoretical research. Its final goal is to find possible characteristics or indirect criteria for meta-instability stages under field observation conditions.

    Therefore, taking the Ludian MS6.5 earthquake as an example and under the guidance of the meta-instability experiments and theories, the paper comprehensively analyzes the relationship between the spatial-temporal evolution of precursory anomalies and the meta-instability process based on the seismic activity and the geophysical observation data prior the earthquake, and combined with numerical simulation results of the earthquake nucleation. The Ludian MS6.5 earthquake occurred on August 3, 2014 in northeastern Yunnan Province, China. The observation conditions in this region were relatively good, with 26 seismometers within a 300-km radius of the epicenter, which were able to basically monitor earthquakes with completeness magnitude ML≥1.5 and locating accuracy of less than 20km. There were 79 fixed geophysical observation stations, including 11 within 100km, 32 between 101~200km, and 36 between 201~300km. The observation terms covered 43 deformation observations(22 tilt observations, 18 borehole strain observations, and 3 gravity observations), 187 underground fluid observations(90 water physical observations such as water level and temperature, 43 material compositions measurements including radon, mercury and so on, 26 gas measurements such as CO2, and 28 ion measurements including bicarbonate, calcium, and magnesium), and 52 electromagnetic observations(36 geomagnetic observations, 16 resistivity and electromagnetic wave observations). There were a large number of credible medium- and short-term precursor anomalies before the Ludian MS6.4 earthquake, a total of 48 precursor anomalies were identified. Among of them, there were 8 seismic anomalies and 40 geophysical anomalies, accounting for approximately 15% of all measurement items. Among these 40 geophysical anomalies, 31 were proposed before the earthquake, and most of them were investigated and verified on-site with reliable changes.(Wu, et al., 2019).

    Based on this abundant precursor abnormally data before the Ludian MS6.4 earthquake and further systematic analysis, a typical earthquake case and relevant observational facts have been provided which can support the viewpoint that during the meta-instability stage, the earthquake nucleation occurred in the epicenter region and the synergy process evolved continuously in surrounding area of the epicenter. The results show that based on large-scale strong earthquake activities and the observation data of the mobile gravity, it can be determined that the concerned area was already in a high-stress state before the Luding earthquake. At that time, the stress level in the large area including the epicenter of the Ludian earthquake was relatively high, and the northeastern Yunnan region and its nearby areas where the Ludian earthquake occurred were already in a critical stress state where strong earthquakes could occur at any time. Under the premise of determining a high-stress state, according to the precursors of seismic activities and geophysical observation precursor anomalies, it can be roughly determined that the meta-instability process of the Ludian earthquake may have begun seven or eight months before the mainshock. The most prominent phenomenon or judgment index is the transition of the fault stress state from accumulation to release, characterized by the active of small earthquakes near the epicenter, as well as the synergistic phenomenon of fault deformation characterized by the significant increase in the number of geophysical observation anomalies, which is related to the expansion process of the core weakening zone in the late period of the earthquake nucleation. After that, until the occurrence of the mainshock, two times should be paying attention to. Firstly, four to five months before the mainshock, the spatial distribution range of the geophysical observation anomalies expands significantly from the epicenter area to the periphery region, indicating accelerated synergistic deformation of the fault. Secondly, after two months before the mainshock, the small earthquake activities near the epicenter began to weaken, and the micro-earthquake activities and the geophysical anomalies showed a migration and contraction towards the epicenter, which is associated with the contraction process of the core weakening zone during the final stage of the earthquake nucleation. The concept of seismic meta-instability proposed from the perspective of stress changes in seismic fault has an explicit physical implication, and the meta-instability stage is associated with the earthquake nucleation process(He et al., 2023). The basic premise for the meta-instability theory to play a role in short-term earthquake prediction lies in how to apply laboratory research results to natural earthquakes, understand whether the regional or fault stress state tends to or enters a meta-instability state through field observations, and further utilize it for practical earthquake prediction.

    WANG Xin-ru, MA Chen-yu, PAN Mao
    2024, 46(3):  536-546.  DOI: 10.3969/j.issn.0253-4967.2024.03.002
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    A digital elevation model(DEM)is a digital representation of terrain surface morphological attributes, describing ground relief with spatial position and terrain characteristics. With advancements in technology, particularly increased satellite data acquisition capabilities, accurate high-resolution DEMs have become crucial in volcanology research, especially in remote regions. The Tengchong volcanic field, one of China’s prominent young volcanic groups, has experienced Cenozoic volcanic activity from the Pliocene to the Holocene. Recent monitoring and studies indicate that three Holocene volcanoes—Heikongshan, Dayingshan, and Maanshan—pose potential future eruption risks. The volcanic activity of these three Holocene volcanoes has garnered significant attention. This paper focuses on the Heikongshan volcano in the Tengchong volcanic field of Yunnan Province, China, using DEM visualization technology to generate rendered topographic maps and optical images of the volcanic area. We interpret and analyze the volcanic landforms, summarizing the geomorphic characteristics of different volcanic cones, lava units, and lava flow features formed during eruptions. By comparing the spatial distribution of lava units over different periods, we observe that newer lava units accumulate on older ones, exhibiting distinct morphological patterns in tomography. The distribution range of lava at different periods is clearly stratified. Our study proposes a reliable approach to mapping lava units, complementing traditional mapping methods in regions with thick forest cover. We complete the zoning map of lava flow units in the Heikongshan volcanic area using DEM maps. Compared to traditional volcanic geology mapping methods, DEM-derived boundaries of lava flow units are more accurate and less affected by challenging field observation conditions. Based on the DEM model and previous geological survey results, we classify Heikongshan’s eruptive activities since the Pleistocene into four stages, each with varying coverage areas. The early lava flows(Phase I)were primarily distributed north of the Heikongshan cone, extending eastward in a tongue shape. Middle-stage active lava flows(Phase Ⅱ)were mainly around the cone. In the late period, the activity’s scale and scope decreased, with small-scale tongue-shaped lava flows moving eastward(Phase Ⅲ)and small-scale sheet flows moving northward(Phase Ⅳ). Our findings provide volcanic geomorphic evidence for understanding the eruption history and offer insights into historical volcanic hazards. This information is valuable for volcanic disaster assessment and hazard evaluation in the future.

    LI Lu-yao, DING Rui, JIANG Da-wei, ZHANG Shi-min
    2024, 46(3):  547-569.  DOI: 10.3969/j.issn.0253-4967.2024.03.003
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    The Altyn Tagh fault zone is a sizeable sinistral strike-slip fault on the northern margin of the Qinghai-Tibet Plateau, and its eastern section is obliquely connected with the northwest Qilian Mountain thrust fault zone. The left-lateral strike-slip action of the Altyn Tagh fault zone and the Qilian Mountain thrust belt constitute a structural transformation relationship. The activity behavior of this fault, especially the amount of sinistral dislocations, segmentation, and slip rate, has always been a hot topic of discussion among scholars. At present, based on geological methods and geodetic research, the slip rates of different sections of the Altyn Tagh Fault have been obtained, with a time scale ranging from tens of thousands of years to decades. The research results generally support the gradual attenuation of the slip rate of the Altyn Tagh fault zone from about 93°E to the east, indicating that its left-lateral attenuation is absorbed by a series of NW-trending thrust faults on its eastern side, and this trend has changed little for decades. The above work provides a framework for us to study the structural transformation relationship between the Altyn Tagh fault zone and the Qilian Mountains thrust belt in time and space. However, due to the limitations of previous observation points, especially the different methods of fault geomorphology measurement and dating used by various authors, there are significant differences in the obtained fault slip rates. Currently, it is not possible to analyze the segmented characteristics of the slip rate in the east section of the Altyn Tagh fault zone further and its spatial relationship with the Qilian Mountain thrust structural zone based on this. In recent years, the application of UAV aerial survey technology has allowed image data to be obtained at the centimeter to millimeter level, making the study of tectonic geomorphology more refined. Researchers can obtain the cumulative displacement or co-seismic displacement of several seismic cycles through micro-faulted landforms and reconstruct the dislocation accumulation process of active faults.

    The transfixion terraces developed across the Altyn Tagh fault are mainly controlled by regional tectonic uplift and climate change, showing regional synchronization in time, which provides convenience for the comparison of regional landforms. Although there are differences in the grading standards of terraces at different sites and the dating methods are not completely consistent, the chronological sequence of the late Quaternary river terraces in the study area generally shows good consistency. From new to old, it is about 3-4ka BP, 6-8ka BP, 10-13ka BP, 20-21ka BP and 40-50ka BP, which provides a research basis for our subsequent comparison of the displacement amount of river terraces. Based on the high-resolution image data obtained by unmanned aerial vehicle photogrammetry(SfM), this paper carried out a detailed interpretation of the 127km section of the eastern section of the Altyn fault zone and measured and counted the dislocations of different levels of risers at 9 typical river terrace dislocation points. Based on the distribution of cumulative displacement of the same terrace, the kinematic segmentation characteristics and tectonic mechanism of the eastern section of the Altyn Tagh fault zone are discussed.

    To define the displacement more accurately, we considered the following factors: 1)Usually, the riser is not a straight line but a curve formed by the free swing of the river bed; 2)Strictly speaking, a riser is a slope with a certain width, consisting of an upper edge, a lower edge, and a middle slope zone. When measuring displacement, we cannot only measure the upper edge or lower edge of the scarp but must consider it comprehensively. Based on the above prerequisites, this paper uses two envelope lines to surround the upper and lower edges of the riser and measures the distance between the corresponding envelope lines on both sides of the fault to obtain the displacement of different levels of scarp in the river terrace. Based on the above measurement methods, four dislocation values, A1-A4 and B1-B4, were obtained from the curve envelope of the upper and lower edge of the terrace scarp on both sides of the fault, respectively. After calculating the corresponding mean value, the standard deviation of the four measured values was estimated to reflect the dispersion degree of different measured values relative to the mean value, which was used as the error range of the measurement results.

    The results show that the late Quaternary left-lateral cumulative displacement tends to decline along the Altyn fault zone eastward. The cumulative dislocations are roughly the same in the same fault segment, which may indicate the consistency of seismic ruptures within the segment. In addition, the dislocation amount of the same-level terrace scarp between adjacent fault segments shows a stepwise decrease, indicating the tectonic transformation relationship of the miter fault and the possible seismic rupture segmentation, which provides a basis for the active segmental research and potential earthquake-generating capacity evaluation of the Altyn fault zone. A horizontal dislocation of 2-3m occurred on the front scarp of the T1 terrace in the Gaoyangou area, indicating that the latest earthquake surface dislocation event may have occurred in the Hongliugou-Shaping section at the easternmost end of the Altyn Tagh fault zone.

    ZHANG Hao, HUANG Wei-liang, XIANG Wen, YANG Qian-hao, LIU Bo
    2024, 46(3):  570-588.  DOI: 10.3969/j.issn.0253-4967.2024.03.004
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    River terraces are primarily formed by the erosional action of river incision under the influence of vertical movements of the crust or changes in regional erosion base levels, resulting in layered landforms. As products of the long-term evolution of river systems, the formation, development, and evolution of terraces have always been a focal point in Quaternary research. Climate change and tectonic movements play crucial roles in the evolution of river terraces, providing important evidence for studying a region’s paleoclimate and tectonic history, while also indicating the geomorphic evolution of rivers. The ages and elevations of river terraces serve as a crucial window for understanding climate fluctuations and the intensity of tectonic uplift in a specific area. This role cannot be replaced by any other method. Therefore, accurately defining the incision and deposition ages of river terraces is essential for quantitatively reconstructing the development and evolution of rivers, making it a key data point in current research on surface processes and geomorphic evolution.

    The study area is located at the southeastern margin of the Qinghai-Xizang Plateau, positioned in the main area of the Jinsha River suture zone at the southwestern edge of the Songpan-Ganzi orogenic belt and the eastern part of the Sanjiang orogenic belt. The regional tectonic setting is complex. Since the late Quaternary, the tectonic uplift at the southeastern margin of the Qinghai-Xizang Plateau has intensified, with accelerated plateau uplift in the post-Pleistocene era accompanied by significant tectonic activity. This has led to substantial incision of rivers in the region, forming multiple layers of overlapping terrace landforms on both sides of the river valleys. These landforms are crucial for quantitatively understanding the plateau uplift process and climate change.

    The Jinsha River is one of the main large rivers in the western parts of Sichuan. The river terraces developed in the Jinsha River valley serve as an important evidence for studying the deformation of the plateau crust and climate change. However, there are few Holocene terraces developed in the valley, and their resolution is low. Therefore, current research on the Jinsha River terraces mainly focuses on the orbital time scale(from tens of thousands to millions of years)of climate change and the impact of tectonic uplift, with limited studies on the role of short-term time scales(thousands or hundreds of years)in climate change and tectonic uplift, and a lack of constraints on river incision rates since the late Quaternary. The formation and evolution of river landforms since the Holocene are currently the most important means of studying recent tectonic activities and predicting future climate fluctuations. Therefore, the Baqu River, as a major tributary of the Jinsha River, with the terraces preserved in its valley, has become crucial research material reflecting the climate change and tectonic uplift in the Jinsha River Basin since the Holocene.

    The Batang segment of the Baqu River is situated in the midstream valley of the Jinsha River, characterized by a wide valley floor and gentle riverbed slope. Through drilling and shallow seismic exploration to investigate the valley stratigraphy, it was found that the valley sediments can be divided into four layers from top to bottom. The bottom layer consists of Permian strata mainly composed of weathered crystalline limestone, with a core exposure of 22m without reaching the bottom. The third layer is composed of Middle Pleistocene sediments, 68m thick, mainly consisting of large boulders, small gravel, and calcareous clay. The second layer comprises Late Pleistocene sediments, 30m thick, primarily consisting of large gravel and clay. The first layer is mainly composed of fine-grained clay with a small amount of sand and gravel blocks, 10m thick. This indicates that the valley has experienced at least two significant aggradation stages. Using Electron Spin Resonance dating methods, it was determined that these two aggradation events began at approximately 318ka and 143ka, corresponding to Marine Isotope Stages(MIS)10-9 and MIS 6-5, respectively, during glacial melting phases.

    Four levels of river terraces are developed within the valley, with T1-T3 being aggradational terraces and T4 being a bedrock terrace. T1 has a terrace height of 5~10m, T2 ranges from 15~25m, T3 ranges from 30~40m, and T4 has a terrace height of 120m. The terrace topography is generally parallel to the longitudinal profile of the modern riverbed, with only minor fluctuations, indicating a predominant overall uplift in the area after terrace formation, with consistent tectonic uplift rates and insignificant differential uplift. Combining Optically Stimulated Luminescence dating, Carbon-14 dating, and cosmogenic nuclide dating methods, it was determined that T1-T3 formed between 1~5ka, specifically 1~2ka, (3.1±0.2)ka, and(4.5±0.4)ka, respectively, while T4 formed around 62ka. A comparison of terrace ages with paleoclimate data revealed that the incision times of T1-T3 corresponded to transitions from cold to warm climates. Calculating the incision rates of terraces based on their ages and terrace heights and comparing them with incision rates in different sections of the Jinsha River, it was found that from the Late Pleistocene to the mid-Holocene, the Baqu River incision rate was(1.5±0.3)mm/a, consistent with other sections of the Jinsha River in western Sichuan. From the mid-Holocene to the present, the incision rate increased to(5.5±0.8)mm/a, approximately four times the incision rate during the Late Pleistocene. While there is a lack of quantified results on river incision rates since the Holocene in surrounding rivers, the enhanced incision rate aligns with the current vertical crustal deformation rates, indicating that intensified crustal uplift since the Holocene may be the primary driver of rapid river incision.

    CHEN Bai-xu, YU Zhong-yuan, XIAO Peng, DAI Xun-ye, ZHANG Shi-long, ZHENG Rong-ying
    2024, 46(3):  589-607.  DOI: 10.3969/j.issn.0253-4967.2024.03.005
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    The Hexi Corridor in northwest China has obvious structural deformation and complex fracture image. With the development of several NW thrust fault zones accompanied by a large number of ancient earthquakes and historical seismic events, the earthquake disaster is relatively serious. The eastern margin of Yumushan fault is one of them. The fault is mainly developed in the east site of Yumushan Mountain, with the exposed fault plane striking NW330° and dipping about 41°~85° to the southwest as a whole. Previous research data show that the Eastern Margin of the Yumushan fault is an important part of the Qilian Mountain active thrust fault system in the northeast margin of the Tibet Plateau. It also constitutes the boundary structure between Hexi Corridor and Yumushan Uplift. Its late Quaternary tectonic deformation and recent activity characteristics reflect the northward extension process of the Qilian Mountains and the remote collision effect of the northward extrusion of the Indian Plate. However, there are still some controversies in the study of the latest activity age and deformation characteristics of the eastern Margin of the Yumushan fault zone, which directly affect the seismic risk assessment along the fault line and the Hexi Corridor, as well as the in-depth understanding of the active structural characteristics of the northeast margin of the Tibetan plateau.

    Combined with remote sensing image interpretation, paleoseismologic excavations, aerial photogrammetry of unmanned aerial vehicles and late Quaternary dating, this study carried out field investigations and newly discovered the surface rupture zone of The Eastern Margin of Yumushan Fault and its activity characteristics. The results show that The Eastern Margin of the Yumushan Fault strikes NW330° combined with obvious thrust movement, which is manifested as a fault scarp landform. That’s revealing than the kinematics property of The eastern margin of Yumushan fault is dominated by thrust. The fault forms the dividing line between the Yumushan uplift and Zhangye Basin, and also the dividing line between pre-quaternary strata and Quaternary strata. The southwest side of the fault is dominated by pre-quaternary bedrock which constitutes a mountain landform. Late Quaternary sediments are exposed on the northeast side, and the Holocene strata are widely distributed around the Heihe River. The results show that there are obvious differences in the activity habits of the faults. With the Heihe River as the boundary, the fault activity difference is obvious on the south and north sides of the Heihe River. The latest surface fracture zone in the late Holocene was found along the Heiheokou segment(F1-1). And the Hongshahesegment(F1-2)showed pre-quaternary fault. It can be seen that the Miocene fine sandstone is in fault contact with the early Pleistocene glutenite and late Ordovician metamorphic andesite, and the fault gouge develops near the fault, which is gray-green and yellow-green with moderate hardness and easy to be wet when encountering water.

    The Heihekou segment(F1-1)starts from Daciyaohe River in the north, passes Xiaociyaokou, and reaches Heihekou in the south. The fracture zone moves towards NW330° and tends to SW, with a length of about 10km and a width of 3~10m. For river terraces, gullies, and platforms with young surface faults, the maximum height of the surface scarp is based on the DEM data generated by UAVs. The height of the T1 terrace fault scarp measured by two profile lines is(1.7±0.1)m to(3.3±0.2)m. In the excavating trenches, obvious evidence of fault activity such as traction bending of strata and directional arrangement of gravel can be seen. The strata consist of late Quaternary alluvial sand, gravel layer, loess layer, and silty layer. The optically stimulated luminescence dating results show that the latest surface rupture event occurred at(0.6±0.07)ka BP.

    According to the empirical formula between maximum vertical displacement(Dmax)and magnitude(M), the magnitude of the latest seismic event is estimated. The magnitude and potential seismic risk of the latest rupture event are evaluated. The results reveal that the maximum vertical displacement of the latest surface rupture event is(3.3±0.2)m. Based on the empirical relationship between magnitude and vertical displacement, it is concluded that a large earthquake rupture occurred in the eastern margin of the Yumushan fault in the late Holocene and the corresponding magnitude is estimated to be M7.5.

    Derived from the analysis of existing data, the fault in the eastern margin of the Yumushan fault may conform to the quasiperiodic earthquake recurrence behavior. And the recurrence interval of strong earthquakes may exceed 1 600a. The time interval between the latest event revealed in this paper and its last seismic event is about 1 800a, which is consistent with the time interval under the fault quasiperiodic earthquake recurrence model.

    The results show that the eastern margin of the Yumushan fault has intensive tectonic deformation in the late Quaternary and a large seismic background of M7 or above. The current kinematic mode of the fault is compressive shortening. Its geodynamic process may be mainly controlled by the northward extension of the Qilian Mountains and the remote collision effect of the northward extrusion of the Indian Plate. The deformation process of the fault may be in line with pre-spreading imbricate thrust deformation and the latest deformation has gradually extended from the basin-mountain boundary to the interior of Zhangye Basin, which provides new data to support the seismic risk assessment of the interior of the basin. At the same time, the latest deformation achievement of the eastern margin of the Yumushan fault has important scientific significance for improving the active tectonic image of the northeastern margin of the Qingzang plateau and discussing the kinematics model of the Qingzang plateau.

    QIN Jing-jing, LIU Bao-jin, FENG Shao-ying, XU Xi-wei, TIAN Yi-ming, ZHU Guo-jun, ZUO Ying
    2024, 46(3):  608-626.  DOI: 10.3969/j.issn.0253-4967.2024.03.006
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    A comprehensive seismic profiling study was conducted across the Jizhong depression, Cangxian uplift, Huanghua depression, and Chengning uplift in the North China Plain to investigate crustal fine structure and the relationship between deep and shallow faults. Two profiles were completed: a deep seismic reflection profile spanning approximately 200km and a middle-shallow seismic reflection profile covering about 66km.

    Our results indicate a crust thickness of approximately 30 to 35km along the section, with a thin distribution in the east and thick in the west. Notably, there is a significant uplift on the Moho surface beneath the Jizhong depression, with an uplift amplitude of about 2 to 3km. The deep seismic reflection profile reveals distinct upper and lower structural characteristics of the crust. The upper crust displays typical sedimentary layer reflection characteristics, marked by alternating depressions and uplifts. Numerous large-scale faults are concealed beneath the North China Plain, playing a pivotal role in uplift and sag formation. The lower crust’s reflection structure comprises events with significant changes in reflection energy, unstable stratification, and complex occurrences, contrasting with the strong reflection energy and good horizontal continuity of the upper crust reflections. The piedmont fault of the Taihang Mountain, identified by the mid-shallow seismic profile and deep seismic reflection section, is a large shovel-shaped normal fault with a low angle, linked to the large detachment structure at the eastern margin of Taihang Mountain. The detachment structure is developed between the basement and the sedimentary cover layer, and is shown on the profile as a reflection zone consisting of 3 to 4 strong reflection phases, lasting 0.3~0.4 seconds. This detachment structure controls the formation of graben and horst structures. The Jizhong depression may have been an extensional tectonic system formed in the upper crust on the basis of the extensional detachment of the Taihang Mountain front fault. The deep seismic reflection section highlights the North China Basin’s structural features, characterized by alternating depressions and uplifts, with boundaries clearly delineated by faults such as the Cangxi, Cangdong, and Chengxi faults. These faults control the formation of graben and horst structures and are considered concealed active faults since the Quaternary period. The Cangxi fault, as the eastern boundary of the Jizhong depression, developed in the weak zone of the front thrust nappe tectonic area of the detachment slip structure. The fault plane resembles a plow shape, steep at the top and gently sloping at the bottom. It utilized or transformed the early thrust section, which is now manifested as a west-dip normal fault, controlling the basement structure and stratigraphic sedimentation on the west side of the Cangxian uplift. The Cangdong fault is the eastern boundary fault of the Cangxian uplift, which appears as an east-dipping shovel shaped normal fault on the profile, cut through the reflection waves of the Carboniferous-Permian strata, the Cambrian-Ordovician strata, the Proterozoic strata, and the crystalline basement. It terminates at the interface of the upper and lower crust at a depth of about 18km. The Chengxi fault is a west-dipping normal fault, which cuts through the Cenozoic sedimentary layer at a depth of about 600~700m in the shallow section. It terminates at the interface between the upper and lower crust in a shovel shaped normal fault downward. The deep seismic reflection section also clearly shows the coexisting structural morphology of uplift and depression. Multiple secondary faults that tilt in the same direction or opposite direction to the main fault are developed inside the depression, causing the depression to be divided into multiple secondary structural units, resulting in the complexity of the entire fault basin structure.

    In conclusion, the development of boundary faults plays a decisive role in the stratigraphic sedimentary and tectonic deformation of the strata within the depression.

    The existing deep and shallow structures and tectonic patterns in the Wuji-Yanshan section of the North China Basin are formed by the “graben-horst” structure developed in the upper crust, the complex fault combination style near the surface, the stratified reflection and the upper and lower superimposed reflection structure developed in the lower crust, and the undulating Moho surface. The findings of this study contribute to the seismological understanding of the dynamic processes occurring in the North China Basin, as well as to the analysis of the structural relationship between deep and shallow structures in the region.

    YU Yue-ying, LI Zheng-kai, YANG Yun, KANG Qing-qing, QIAN Jia-wei, WANG Jun-fei, QU Min, ZHOU Yu-chen, LI Ying-chun, XU Tian
    2024, 46(3):  627-644.  DOI: 10.3969/j.issn.0253-4967.2024.03.007
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    The seismic activity in Dongtai, Jiangsu, suddenly intensified from November to December 2022. The largest earthquake observed during this period was a magnitude MS3.0 event on 25 December, which was felt reported by many nearby residents and caused a certain degree of social impact. The Dongtai area is situated in the central part of Jiangsu Province, within the Dongtai depression in structure, which is a secondary tectonic unit in the southern part of the Subei-South Yellow Sea Basin. Multiple fault zones developed in the region. The prominent known fault zones near the epicenter include the Taizhou fault, the Chenjiabao-Xiaohai Fault, and the Benchahe Fault. Among them, the closest to the epicentral area is the Taizhou fault. Additionally, the Subei Basin has a long history of industrial activity. Its geological conditions are complex, and the resources are extremely scattered and fragmented. The scale of underground resource extraction is predominantly small to medium-sized and has entered the middle to high exploration level. Historically, Dongtai has experienced weak seismic activity with only six earthquakes of MS≥3 within 50 kilometers of the epicenter since 1970. The sudden increase in seismic activity prompts investigation into its cause. Analyzing the structural features of the Dongtai earthquake sequence can enhance understanding of seismic activity and seismogenic mechanisms in the region.

    Previous studies on regional velocity structure have primarily focused on large scales, such as the Tan-Lu fault zone, with no specific research dedicated to the Dongtai earthquake sequence. In this study, we collected earthquake arrival time data recorded by the China Earthquake Networks Center from 2008 to 2022. Employing the double-difference tomography method, we conducted a joint inversion to investigate the velocity structure and earthquake locations in the Subei Basin. The resulting outcomes include the three-dimensional P-wave velocity structure of the area and the relocation results of 22 events within the seismic sequence. Furthermore, utilizing clear P-wave initial motion data from station waveform records, we inverted the focal mechanism solutions of the earthquake sequence using a modified grid search method. By integrating these inversion results with data on fault distribution and local industrial activity, we discussed the earthquake-triggering mechanism and possible seismogenic structures.

    The results indicate that: 1)Following relocation, the seismic sequence exhibits a zonal distribution pattern. The earthquakes are predominantly situated north of the Tai-Zhou fault in a nearly north-south orientation, spanning approximately 15 kilometers in total length, with a predominant depth range of 11 to 16 kilometers. Notably, there is no apparent correlation between the earthquakes and the surrounding known fault structures. 2)The focal mechanism solution parameters for the largest earthquake in the sequence, MS3.0, suggest a strike-slip seismogenic structure with a minor normal component. The direction of the stress axis aligns closely with the current tectonic stress field of the study area. Based on the focal mechanism solution and the distribution of the sequence, it is inferred that a dextral strike-slip hidden structure trending in a NNE-SSW direction may exist beneath the sequence. 3)The velocity structure of the epicenter area exhibits significant heterogeneity. The middle crust displays relatively high velocity, while the lower crust shows relatively low velocity. Notably, a spindle-shaped high-velocity anomaly with a P-wave velocity of 6.25km/s is observed at a depth of approximately 15km. The earthquakes primarily cluster southeast of this anomaly. 4)By examining the relationship between the spatial locations of earthquakes and their occurrence times, it is observed that the epicenters exhibit a seismogenic process extending far from the edge of the anomalous body. This suggests the outward release of accumulated elastic energy within the high-velocity anomaly, indicating a potential relationship between earthquake occurrences and the velocity anomaly. 5)Through on-site investigations of the epicentral area, data regarding local industrial activities have been collected. It was observed that three new wells and multiple industrial operation points have been established in the seismic area. Remarkably, 73% of earthquakes in the seismic sequence occurred within a 4.6km radius of well H1, with the largest earthquake in magnitude located approximately 1km from the well. A notable correspondence is observed between the Wulie-Shiyan-Qindong extraction points, the seismic sequence, and the deep high-velocity anomaly. Additionally, the operational timeframe of newly developed wells in the region closely aligns with the timing of earthquakes. However, the dominant depth of seismicity does not correspond with the drilling depth. A preliminary inference suggests that the occurrence of the earthquake sequence may be linked to the deep heterogeneous velocity structure, while industrial production activities near the epicenters may induce alterations in the regional stress state, leading to stress destabilization and subsequent energy release.

    XU Jing, JI Ling-yun, LIU Chuan-jin
    2024, 46(3):  645-664.  DOI: 10.3969/j.issn.0253-4967.2024.03.008
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    Situated as the eastern boundary of the Sichuan-Yunnan block, the Xianshuihe fault system exhibits a notably high left-lateral strike-slip rate, establishing itself as one of the most active regions for seismic activity in the Chinese mainland, profoundly influencing the occurrence of large earthquakes within the region. The fault zone and its surrounding area are relatively densely populated, intersecting with the famous Sichuan-Xizang National Highway No. 317 and No. 318 and serving as a significant focal point in the design of the Sichuan-Xizang railway. Given its substantial seismogenic capacity and associated earthquake risk, notable attention is warranted. Notably, on September 5, 2022, a left-lateral strike-slip MS6.8 earthquake struck Luding County, Ganzê Prefecture, Sichuan Province, rupturing the Moxi fault of the Xianshuihe fault zone within the southeastern margin of the Qinghai-Xizang Plateau. Our study used Sentinel-1 SAR images to obtain both the interseismic deformation (2014-2020) and coseismic deformation resulting from the 2022 Luding M6.8 earthquake. Furthermore, we estimated the fault slip rate and locking depth during interseismic periods and inverted the coseismic slip distribution model. Utilizing the co-seismic dislocation model, we quantified Coulomb stress changes on surrounding fault planes induced by the Luding event. Finally, we provide an in-depth discussion on the seismogenic structure of the Luding earthquake and offer insights into the future seismic hazard implications associated with the Moxi fault and its adjacent faults.

    We collected Sentinel-1 SAR imagery data spanning from October 2014 to April 2020 for both the descending orbit T135 and ascending orbit T026, and calculated the Line-of-Sight(LOS)direction deformation during the interseismic period covering the Moxi Fault of the Xianshuihe fault zone. The InSAR-derived interseismic deformation presented in this study effectively captures the long-term slip behavior of the seismogenic fault associated with the 2022 Luding earthquake. Our analysis reveals an aestimated slip rate of(5.9±1.8)mm/yr along the Moxi Fault. Combined with the GNSS and InSAR deformation observations, we generated a fused three-dimensional deformation field characterized by high density and precision. Additionally, we calculated the strain rate field based on the three-dimensional deformation within the study area. Our findings indicate pronounced shear deformation near the Moxi Fault, with strain highly concentrated along the fault trace. Notably, the strain concentration in the southern section of the Moxi Fault surpasses that observed in the northern section before the earthquake event. Furthermore, our analysis suggests that the Moxi Fault was locked at shallow depths before the earthquake occurrence, indicating a predisposition for seismic activity. The Luding earthquake thus transpired within the context of a seismically active background associated with the Moxi Fault.

    Following the 2022 Luding 6.8 earthquake, we acquired InSAR coseismic deformation data within the seismic region, revealing predominantly horizontal surface displacements induced by the event. Employing the Most Rapid Descent Method(SDM), we conducted inversion of the fault plane slip distribution resulting from the earthquake. Our analyses indicate maximal dislocation quantities located south of the central earthquake zone, indicative of predominantly pure strike-slip movement. Dislocations are primarily observed at depths ranging between 5km to 15km, with the maximum left-lateral strike-slip dislocation measuring 1.71m and occurring at a depth of approximately 10km. In the north of the epicenter, fault slip manifests as predominantly sinistral strike-slip motion with a partial thrust component, exhibiting a progressively deepening slip pattern towards the northern region.

    Utilizing the coseismic slip distribution derived from the 2022 Luding MS6.8 earthquake, we conducted calculations to assess the Coulomb stress changes induced by the coseismic dislocation effects across the fault plane of the Moxi Fault and its surrounding major fault zones. These fault zones include the Xianshuihe fault zone(comprising the Moxi, Yalahe, Selaha, Zheduotang, and Kangding segments), the Anninghe fault zone(encompassing the Shimian-Mianning and Mianning-Xichang segments), the Zemuhe Fault zone, and the Daliangshan fault zone(comprising the Zhuma, Gongyihai, Yuexi, Puxiong, Butuo, and Jiaojihe segments).Our analysis reveals that the Luding earthquake caused a substantial decrease in Coulomb stress within its rupture section, resulting in the formation of a stress shadow area in the southern segment of the Moxi Fault. However, it significantly increased the Coulomb stress in the northern section of the Moxi Fault that was not ruptured in the earthquake. Concurrently, the Coulomb stress on the fault plane increases significantly in the southeast section of the Zheduotang fault, the northwest section of the Shimian-Mianning segment of the Anninghe fault zone, as well as the southeast section of the Zhuma segment, and the southeast section of the Gongihai segment of the Daliangshan fault zone.

    The seismogenic structure of the 2022 Luding earthquake is a part of the Moxi Fault of the Xianshuihe fault zone. However, the magnitude and rupture length of the earthquake are significantly smaller than that of the Moxi M7$\frac{3}{4}$ earthquake in 1786, resulting in a less pronounced stress unloading effect. Additionally, the Luding earthquake triggered a noteworthy increase in Coulomb stress along the northern segment of the Moxi Fault. Consequently, the Luding earthquake did not ultimately reduce the seismic hazard within the Xianshuihe fault zone. Thus, greater attention should be directed towards the unruptured section of the Moxi Fault and its adjoining rupture with the background of large earthquakes.

    JI Guo-qiang, LEI Jian-she, ZHAO Da-peng
    2024, 46(3):  665-685.  DOI: 10.3969/j.issn.0253-4967.2024.03.009
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    The Huoshan earthquake swarm is tectonically located at the junction among the North China plate, Yangtze plate, and North Dabie orogenic belt. The geological environment in the region is complex, including the Feixi-Hanbaidu Fault, Meishan-longhekou Fault, Xiaotian-Mozitan Fault and Luoerling-Tudiling Fault, as well as the North Dabie tectonic belt, North Huaiyang tectonic belt and Liu’an basin. In the study region, seismicity is intense, and 9 earthquakes with M≥5.0 occurred along the Luoerling-Tudiling Fault in the history. In recent decades, small-to-moderate earthquakes were frequent, mainly gathering at the intersection of the Xiaotian-Mozitan Fault and Luoerling-Tudiling Fault. Furthermore, the frequency of small earthquakes in the Huoshan region has a significant correspondence to the moderate-to-strong earthquakes in East China and even eastern Tibet, so studying the deep structure can shed new light on the relationship between the Huoshan earthquake swarm and moderate-to-strong earthquakes in mainland China.

    In this study, a total of 17 920 seismic arrival-time data, including 7 706 P, 394 PmP, 9 263 S and 557 SmS arrivals, are hand-picked from 1987 local earthquakes to obtain three-dimensional crustal P-wave velocity(VP), S-wave velocity(VS)and VP/VS ratio models down to 30km depth beneath the Huoshan swarm area. The checkerboard resolution test results show that the imaging spatial resolution in most parts of the regions can reach 0.33°×0.33°, and the North Huaiyang tectonic belt near the Huoshan earthquake swarm has good recovery in the entire crust, and the North Dabie tectonic belt and Lu’an basin also have good recovery at 8-30km depths. Due to the addition of PmP/SmS arrivals, the spatial resolution at 18-30km depths is significantly improved, and the pattern and amplitude of velocity anomalies are better recovered.

    Our tomography results show that a vertical continuous high VP/VS anomaly is observed around the intersection of the Xiaotian-Mozitan Fault and the Luoerling-Tudiling Fault, especially at 18km depth appear broad low-velocity and high VP /VS anomalies. At 30km depth, the areas with high VP/VS are reduced and concentrated on both sides of the Luoerling-Tudiling Fault. There are significant high VP/VS characteristics around the Huoshan earthquake swarm. The high VP/VS anomalies extend to 18~30km depths below the Xiaotian-Mozitan Fault, suggesting that fluids could have migrated upward along the fault to reduce the effective normal stress of the fault planes, triggering the activity of the Huoshan earthquake swarm at the weak intersection between the Xiaotian-Mozitan Fault and the Luoerling-Tudiling Fault.

    Combined with the low-velocity anomalies of the upper mantle revealed by the previous tomographic results, we propose that there may be a channel for upwelling of the wet and hot upper-mantle materials with fluids to the crust along the Xiaotian-Mozitan Fault. The upwelling of the wet and hot materials may be related to the dynamics of the big mantle wedge formed due to the deep subduction of the stagnant Pacific slab down to the mantle transition zone and the eastward extrusion of materials in the upper mantle from eastern Xizang along the Dabie orogenic belt. These factors may jointly affect the seismicity characteristics of the Huoshan earthquake swarm. Our results providea new piece of seismological evidence for the interactions among the tectonic activities in the Huoshan region, Tibetan plateau and East China.

    FAN Xiao-yi, QU Jun-hao, GU Qin-ping, CHEN Fei, WANG Fu-yun
    2024, 46(3):  686-698.  DOI: 10.3969/j.issn.0253-4967.2024.03.010
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    Examining the spatial and temporal distribution of seismic activity holds significant importance for seismic risk assessment, particularly in regions prone to frequent and intense earthquakes such as the Sichuan-Yunnan region in China. It is widely recognized that earthquakes exhibit non-random patterns in both spatial and temporal dimensions.

    Early scientists endeavored to predict earthquakes using statistical principles, leading to the development of various forecasting methods. Among these, the Relative Intensity(RI)and Pattern Informatics(PI)methods emerged as statistical approaches to earthquake prediction modeling. Essentially, both methods fall under the category of smoothing seismic activity models. They employ techniques to quantify temporal changes in seismic activity graphs, generating maps that highlight areas(hot spots)where earthquakes may occur during specific future periods. While the RI algorithm’s theory is straightforward, its forecasting efficacy is robust, particularly notable in predicting major earthquakes, demonstrating similar advantages to the PI algorithm. Widely adopted globally for proactive predictions across diverse tectonic systems, it has shown commendable results in seismic forecasting practices both domestically and internationally. Over years of development, its predictive performance has gained prominence. However, further research is needed to assess its suitability for predicting minor seismic events in low-seismicity zones. Additionally, its successful application hinges on background seismic activity and the selection of target magnitudes.

    To aid seismic activity prediction in the Sichuan-Yunnan region and identify potential future seismic source areas, a comprehensive parameter analysis was conducted using the Relative Intensity(RI)algorithm with the parameter traversal test(PTT). The RI algorithm operates on the premise that the predicted intensity of future earthquakes in a given region closely mirrors the intensity of past earthquakes. While it may not explicitly consider the “active” and “quiet” characteristics of seismic activity, as a fundamental prediction algorithm, it often yields improved prediction outcomes when applied to assess seismic probability in regions with high seismic activity, such as the Sichuan-Yunnan region.

    In this study, the statistical-based Relative Intensity(RI)algorithm is employed to calculate the relative intensity of earthquakes based on quantitative earthquake characteristics. The study involves gridding the investigated area and statistically analyzing historical earthquake occurrences within each grid unit under specific magnitude conditions to inform predictions of future earthquake frequencies. The research focuses on evaluating the influence of four key model parameters: grid size, length of the anomalous learning window, starting point of the prediction window, and length of the prediction window, on the algorithm’s prediction efficiency. Furthermore, the study investigates the applicability of the RI algorithm to the Sichuan-Yunnan regions in China. The results yield two significant findings:

    (1)The integration of the Relative Intensity(RI)algorithm with the Parameter Traversal Test(PTT)yielded significantly improved results compared to random guessing, primarily due to its optimized parameter selections. These parameters include the grid size, length of the anomalous learning time window, starting time of the prediction time window, and length of the prediction time window.

    (2)The parameters of the prediction model exhibit a degree of stability and demonstrate predictive capability for seismic activity in the Sichuan-Yunnan region over the next 1-5 years. The study revealed specific rules and effective parameter intervals applicable to earthquake-prone areas in Sichuan-Yunnan.

    The findings suggest that the integration of the Relative Intensity(RI)algorithm with the Parameter Traversal Test(PTT)holds promise for predicting seismic activities in the Sichuan-Yunnan region. This approach enhances the pool of references available for predicting earthquake trends in regions prone to frequent and intense earthquakes. Further research on the RI algorithm is anticipated to yield a more refined numerical model for earthquake trend prediction, contributing to enhanced forecasting accuracy and preparedness in earthquake-prone areas.

    QIN Ke-xin, HU Gui-ming, LIU-ZENG Jing, SHEN Xu-wen, GAO Yun-peng, WANG Wen-xin, WEN Xin-yu, JIANG Shuai-yu
    2024, 46(3):  699-722.  DOI: 10.3969/j.issn.0253-4967.2024.03.011
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    The accumulation of luminescence signals in mineral crystals correlates with the duration of exposure to radiation. This phenomenon has been utilized as a tool for measuring sediment age and has found extensive application in various research endeavors. While quartz and feldspar luminescence signals have been utilized for dating in recent years, their effectiveness is constrained by early saturation, limiting their dating range to less than 300ka. In contrast, calcite exhibits high sensitivity to dose responses of thermoluminescence signals and possesses a characteristic saturation dose that can reach levels of 3 000-5 000Gy, making it a promising material for thermoluminescence dating. This has the potential to extend the age range of luminescence dating to the Quaternary period and broaden the application scope of low-temperature thermochronology. Providing quantitative descriptions of bedrock exhumation history through low-temperature thermochronology can offer crucial data support for understanding the interconnected relationship between tectonic activity, climate influences, and geomorphic evolution. Low-temperature thermoluminescence thermochronology, characterized by its high resolution and low closure temperature, presents advantages over commonly used apatite U-Th/He thermochronology in elucidating the excavation history of the Earth’s crust surface(approximately 1~2km). However, traditional minerals utilized for reconstructing bedrock cooling history, such as quartz and feldspar, exhibit rapid saturation, limiting the study period to less than 200ka. In contrast, calcite boasts an exceptionally high characteristic saturation dose and lower dose rate, making it a promising new dating mineral that extends the upper limit of low-temperature thermoluminescence thermochronology beyond 0.5Ma.

    This paper begins by introducing the principle and application of thermoluminescence dating, followed by an overview of commonly used techniques for measuring dose rate and equivalent dose. The thermoluminescence dating process primarily involves equivalent dose measurement and dose rate measurement. Considerable research has been conducted on equivalent dose, and newly developed methods such as single aliquot regenerative dose, multiple aliquot regenerative dose, and multiple aliquot-additive dose have addressed issues related to sensitivity changes caused by heating, thereby enhancing the accuracy of dating results. Additionally, the paper summarizes recent advancements in calcite thermoluminescence dating and kinetic parameters. To validate the method, we performed thermoluminescence dating analysis on calcite grains in bedrock samples collected from the Tiger Leap Gorge of the Jinsha river.

    After passing through Shigu, the Jinsha river experiences a sudden change in flow direction, carving its way through the Yulong-Haba mountain range to create the renowned “Tiger Leaping Gorge.” This geographic feature is characterized by active tectonics and intense river erosion, making it an ideal site for investigating the interplay among tectonics, climate, and surface processes. However, the Tiger Leaping Gorge primarily comprises limestone and griotte, lacking minerals such as apatite and zircon necessary for traditional low-temperature thermochronology dating(only exposed in the Upper Tiger Leaping Gorge). Consequently, it presents an ideal setting for exploring calcite low-temperature thermoluminescence thermochronology. SAR-ITL can detect the 280℃ thermoluminescence peak signal of calcite at 235℃, effectively mitigating the influence of spurious thermoluminescence. Moreover, the number of calcite grains required is lower than that of the MAAD test. The findings highlight the potential of this method for estimating the exhumation rate of carbonate rock. To facilitate its more effective utilization in the field of tectonic geomorphology, we address the challenges and potential applications of calcite thermoluminescence dating.

    LI Yi-shan, LIU Hong, SUN Feng-xia, LIU Lei
    2024, 46(3):  723-738.  DOI: 10.3969/j.issn.0253-4967.2024.03.012
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    U-series dating(Uranium series disequilibrium dating)is one of the most widely used dating methods in radioisotope geochronology, mainly based on the disequilibrium relationship between radionuclide 238U and its decay daughters 235U/234U and 230Th to measure the age of rocks, minerals, and other geological bodies. U-Th /He isotope dating is based on the decay of radioactive elements such as U and Th in mineral particles to form stable 4He isotopes. By measuring the cumulative content of these radioactive element decay products, The U-Th/He dating method has a large applicable time range for many minerals(such as apatite and zircon)and most geological periods, and can be used as a thermal timer to explain the thermal history of rocks, and can also be used as a geological timer to constrain the crystallization age of minerals and different geological events. Carbonate minerals, including calcite, dolomite, magnesite and aragonite, are widely distributed in the earth’s crust and formed in the processes of sedimentation, magma, metamorphism and hydrothermal fluid metasomatism. In recent years, with the development of closing temperature theory, the recognition of He diffusion behavior, and new progress in He measurement technique, it has been found that helium can be retained in the lattice of carbonate minerals, and the diffusion activation energy and low closure temperature are close to those of apatite. Carbonate U-Th/He isotope dating technology has been greatly developed and applied in the fields of geochronology and thermochronology, which attracted wide attention in the field of geology. Due to the large particle size and extremely low closure temperature, the application of low-temperature dating of carbonate minerals has received increasing attention. Ideally, a mineral crystal dating with U-TH /He should contain all helium from the decay of the U and Th radioisotopes inside the mineral, that is, there is no inheritance of previously existing helium, and there is no loss of helium after. Any factor that breaks the closure of the U-Th/He dating system will affect the accuracy of the dating results. Helium has a small atomic mass and no charge, and when the temperature is high enough, it easily diffuses out of the mineral lattice. This article mainly analyzes the influencing factors of He diffusion behavior and the new progress of He gas extraction and measurement technology. The study of the diffusion behavior of helium in carbonates is a key theoretical link in the development and application of U-TH/He dating methods for carbonates. Research methods, diffusion domain, crystal size, alpha particle and grain boundaries have different degrees of influence on helium diffusion behavior and helium retention. Accurate activation energy and diffusion coefficient of helium diffusion are needed to understand the mineral age of carbonates under certain geological conditions. The development of extraction and measurement technology for He gas is a key technical link in carbonate U-Th/He dating. Due to the low content of He, U and Th in carbonate samples, relatively large samples and advanced He measurement equipment such as vacuum furnaces and mass spectrometers are required. In-situ laser U-Th/He isotope dating, which has the advantages of high precision and non-destructive, has made a breakthrough in measuring carbonate ages and has gradually established a standard experimental testing process. Carbonate U-Th/He isotope dating technology has broad application prospects and research value in archaeology, brittle structure, oil and gas accumulation, oceanic crust evolution, metallogenic mechanism, and ore-forming fluid tracing, and will play an important role in solving earth science problems. In this paper, the progress of methods and techniques for carbonate U-Th/He dating in the last two decades is reviewed, the methods and basic principles of U-Th/He dating are summarized, the uncertainties affecting helium dating are analyzed, and the future development direction prospects.

    Application of new technique
    SONG Dong-mei, WANG Hao, FENG Jia-xing, SHAN Xin-jian, WANG Bin
    2024, 46(3):  739-755.  DOI: 10.3969/j.issn.0253-4967.2024.03.013
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    Fracture zones are geological formations resulting from the strong movement of the Earth’s crust, often manifesting as fragile and sensitive areas. These zones are closely linked to natural disasters such as earthquakes and landslides. Accurate extraction of fracture zones is crucial for quantitative studies of earthquake faults, providing a scientific basis for risk assessment and decision-making in earthquake prevention and mitigation. Thus, an in-depth study to determine their distribution patterns and surface geometry is essential for understanding earthquake dynamics and mechanisms.

    This paper addresses the shortcomings of existing methods in extracting fracture zones from LiDAR point clouds, which often suffer from incomplete extraction, poor continuity, and high error rates. We propose a method based on a multi-scale neural network with RS-Conv to improve the automatic extraction of fault zones in complex terrain regions. Fracture zones exhibit complex morphologies and scale features; therefore, single-scale neighborhood point sets fail to reveal their intrinsic structural information fully. Our approach begins by constructing neighborhood point sets at different spatial scales to comprehensively examine geometric features at various levels within the point cloud. The RS-Conv operator effectively portrays the spatial relationship between the center point and neighboring points. We then build a multi-scale neural network model using the RS-Conv operator as the convolution module. This model captures the spatial relationships in the point cloud, efficiently extracting deep features at different scales. The extracted multi-scale features are concatenated to form a richer and more comprehensive feature representation, which is inputted into a fully connected layer to classify the centroid and solve the fracture zone extraction problem. We compared our method with the Tensor Decomposition and Deep Neural Networks(DNN)methods using the ISPRS point cloud dataset, the Sichuan-Yunnan point cloud dataset, and the Xianshuihe dataset. Results show that our method achieves the highest classification accuracy across all three datasets. Specifically, our method’s total classification error is only 0.3%, a reduction of 0.91% -2.79%compared to other methods. This significant error reduction demonstrates the accuracy, stability, and reliability of our proposed method in handling complex point cloud data. The main conclusions of this study are as follows:

    (1)The construction of neighborhood point sets at different scales reveals that the combination of these scales significantly impacts the model’s classification performance. Selecting appropriate scale combinations is crucial for optimizing the model’s classification accuracy, facilitating better distinction between fracture zone points and non-fracture zone points.

    (2)Compared to traditional and machine learning methods, the deep learning network model developed in this study shows significant advantages in extracting fracture zones from point clouds. The model can automatically learn deep features from point cloud data and process large-scale, high-dimensional point cloud datasets, thereby achieving more accurate fracture zone extraction in complex terrain conditions.

    (3)Comparative experiments on different datasets further demonstrate the proposed method’s generalization ability. It is effective not only in extracting fracture zones under single terrain conditions but also in maintaining stable performance across multiple terrain conditions. This adaptability enhances the extraction of fracture zones in various terrain scenarios.

    In conclusion, the method proposed in this paper offers a novel approach to fracture zone extraction. It achieves higher classification accuracy compared to existing traditional and machine learning methods, effectively addressing the challenge of fracture zone extraction in complex terrain areas.