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    SEISMOGENIC FAULT AND COSEISMIC SURFACE DEFORMATION OF THE DINGRI MS6.8 EARTHQUAKE IN XIZANG, CHINA
    SHI Feng, LIANG Ming-jian, LUO Quan-xing, QIAO Jun-xiang, ZHANG Da, WANG Xin, YI Wen-xing, ZHANG Jia-wei, ZHANG Ying-feng, ZHANG Hui-ping, LI Tao, LI An
    SEISMOLOGY AND GEOLOGY    2025, 47 (1): 1-15.   DOI: 10.3969/j.issn.0253-4967.2025.01.001
    Abstract1936)   HTML59)    PDF(pc) (13725KB)(1003)       Save

    At 09:05 am on January 7, 2025, a MS6.8 earthquake occurred in the Dingri, Xizang, China. The earthquake caused serious casualties and property losses. Research on the seismogenic structure and characteristics of earthquake surface rupture in this earthquake is beneficial to understanding the rupture behavior and dynamic mechanism of normal-fault earthquakes. Meanwhile, it provides a basis for predicting the future strong earthquake trend of the southern Xizang rift fault system. Its epicenter is located at 87.45°E, 28.50°N, 13km depth, the China Earthquake Networks Center measures. In order to constrain the seismogenic fault and characterize the co-seismic surface ruptures of this earthquake, field investigations were conducted immediately after the earthquake, combined with analyses of the focal parameters, aftershock distribution, and InSAR inversion of this earthquake.

    This preliminary study finds that the seismogenic fault of the Dingri MS6.8 earthquake is the Dengmocuo fault, which is an active ~60km long, NS-NE-striking and normal fault. The total length of the co-seismic surface ruptures is approximately 25km, located on the north segment of the Dengmocuo fault. Meanwhile, a dense deformation zone of ground fracture with a length of ~10km is generated on the east side of Dengmocuo Lake along the contour line of the lake shore. The earthquake also induced a large number of liquefaction structures and tensional fractures in valleys and basins.

    Based on along-strike discontinuity due to the development of step-overs, the coseismic surface rupture zone can be subdivided into three segments: the Gurong-Qiangga, Nixiacuo, and Yangmudingcuo segments. The surface ruptures are relatively continuous and prominent along the Nixiacuo segments. Comparatively, co-seismic surface ruptures of Gurong-Qiangga and Yangmudingcuo segments are discontinuous. The maximum of coseismic vertical displacement is roughly determined to be 2.5—3.0m based on the scarps. The width of the surface rupture zone of the Dingri earthquake can reach up to 450m in some areas. The location of surface rupture zones is not limited to fault scarps and hanging walls. There are also a large number of secondary scarps and cracks distributed in the footwall. Many cracks are distributed in an en echelon or grid pattern.

    Compared to the continuous surface rupture caused by strike-slip-type earthquakes in recent years, the surface rupture of the Dingri earthquake is very discontinuous, and there is an obvious difference in displacement between each segment of the surface rupture. Preliminary speculation suggests that it may be related to the characteristics of the fault movement. Unlike strike-slip faults where the dislocation direction is parallel to the strike, the dislocation direction of normal faults is perpendicular to the strike. In addition, the observed length of surface rupture and maximum displacement of the Dingri earthquake are basically consistent with the results calculated by empirical formulations.

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    GEOLOGICAL DISASTERS AND SURFACE RUPTURES OF JANUARY 23, 2024 MS7.1 WUSHI EARTHQUAKE, XINJIANG, CHINA
    ZHANG Bo-xuan, QIAN Li, LI Tao, CHEN Jie, XU Jian-hong, YAO Yuan, FANG Li-hua, XIE Chao, CHEN Jian-bo, LIU Guan-shen, HU Zong-kai, YANG Wen-xin, ZHANG Jun-long, PANG Wei
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 220-234.   DOI: 10.3969/j.issn.0253-4967.2024.01.013
    Abstract1430)   HTML77)    PDF(pc) (14676KB)(836)       Save

    The MS7.1 earthquake in Wushi, Xinjiang on January 23, 2024, represents the largest earthquake in the Tianshan seismic belt since the 1992 Suusamyr MS7.3 earthquake in Kyrgyzstan. Preliminary precise aftershock localization and initial field investigations indicate an NE-trending aftershock zone with a length of 62km that is concentrated at the mountain-basin transition area. This event produced geological hazards, including slope instability, rockfalls, rolling stones, and ground fissures, primarily within a 30-kilometer radius around the epicenter. The epicenter, located approximately 7 kilometers north of the precise positioning in this study, witnessed a rapid decrease in geological hazards such as collapses, with no discernible fresh activity observed on the steep fault scarp along the mountainfront. Consequently, it is inferred that the causative fault for this main shock may be an NW-dipping reverse fault, with potential rupture not reaching the surface.

    Moreover, a surface rupture zone with a general trend of N60°E, extending approximately 2 kilometers, and displaying a maximum vertical offset of 1m, was identified on the western side of the micro-epicenter at the Qialemati River. This rupture zone predominantly follows the pre-existing fault scarp on higher geomorphic surfaces, indicating that it is not new. Its characteristics are mainly controlled by a southeast-dipping reverse fault, opposite in dip to the causative fault of the main shock. The scale of this 2-kilometer-long surface rupture zone is notably smaller than the aftershock zone of the Wushi MS7.1 earthquake. Further investigation is warranted to elucidate whether or not the MS5.7 aftershock and the relationship between the SE-dipping reverse fault responsible for the surface rupture and the NW-dipping causative fault of the main shock produced it.

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    APPLICATION OF DEEP LEARNING IN ACTIVE TECTONICS AND GEOMORPHOLOGY
    LIU Xin, WANG Shi-rou, SHI Xu-hua, SU Cheng, LU Chen-yan, QIAN Xiao-yuan, SUN Qiao-yang, DENG Hong-dan, YANG Rong, CHENG Xiao-gan
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 277-296.   DOI: 10.3969/j.issn.0253-4967.2024.02.003
    Abstract527)   HTML34)    PDF(pc) (5210KB)(658)       Save

    The research on active tectonics and geomorphology involves extensive sub-topics, including the kinematics of crustal movements, the processes underlying the evolution of landforms, and the associated dynamic mechanisms. These sub-topics are intricately connected with the interactions between the Earth’s endogenic and exogenic processes. In the contemporary realm of the Earth system science, research in active tectonics and geomorphology has become a hot topic for interdisciplinary study. The advancement in big data research coupled with the progressive developments in deep learning technologies has furnished this field of study with a voluminous array of data sources and the requisite analytical tools for technical analysis. In recent years, the application of big data and deep learning technologies in this research field has yielded a series of outstanding results, fostering new research directions, and ushering the discipline into a new phase. In this paper we synthesize existing research to outline the data sources pertinent to the study of active tectonics and geomorphology, including field geological survey, unmanned aerial vehicle (UAV)-based photography, aerial photography, and remote sensing observations. Then, we discuss in-depth examination of the recent innovations progresses in deep learning algorithms, including but not limited to convolutional neural networks(CNNs), deep Gaussian processes, and autoencoders. This article further elaborates on innovative applications of deep learning in the study of active tectonics and geomorphology. These include the identification of changes in glacier extent, monitoring volcanic activity and deformation, recognizing river systems, precise surveillance of landslide events, as well as observations of lithospheric deformation co-seismic surface ruptures.

    Based on the summary of prior studies, this paper showcases a distinct application instance. By employing convolutional neural networks(CNNs)within the realm of deep learning image analysis and utilizing UAV-obtained high-resolution images, we undertake the automated detection of structural fractures in granite rocks in Meizhou island, in the southeast of Fujian province, China. In fault damage zones, structural rock fractures are widely developed, and the study of their orientation, system, and secondary characteristics is of great importance for determining their mechanisms of development and the multi-phase tectonic activity events in the region. Under conventional methodologies, the study of structural fractures in rocks is time-consuming and requires considerable manual effort in conducting exhaustive field surveys and detailed interpretation of cartographic representations. However, the application of deep learning can greatly enhance the efficiency of cartographic work. This application case has improved the classic deep learning framework by developing a CNN model specifically designed for the extraction of complex features and multi-scale rock fractures. This model achieved rapid identification of over 9 000 fractures with varied shapes and complex distributions within 55 minutes, attaining an accuracy of 85% and a recall rate of 89%. These findings demonstrate that deep learning significantly enhances operational efficiency in comparison to manual statistical methods for the automated identification of rock structural fractures, while also maintaining exceptional accuracy in fracture detection. Based on the results identified by deep learning, it can be clearly observed that two sets of fractures, oriented NE and NW, develop on the granite outcrops in the study area. According to previous research and the cross-cutting relationships of the fractures, it is known that NE-oriented fractures formed earlier than NW-oriented fractures, corresponding respectively to the Indosinian Movement and the expansion movement of the South China Sea in the tectonic history of South China. Through the automated extraction of deep learning models, the workload of manual mapping can be greatly reduced, yielding results consistent with actual geomorphological phenomena.

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    GEOMORPHIC DATING OF SCARPS AND ITS APPLICATION TO ACTIVE TECTONICS AND GEOMORPHOLOGY
    PANG Zhen-hui, XU Hao-ting, SHI Xu-hua, GE Jin, LI Feng
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 251-276.   DOI: 10.3969/j.issn.0253-4967.2024.02.002
    Abstract522)   HTML31)    PDF(pc) (3088KB)(557)       Save

    Scarps are typical geomorphic features of tectonics, climatic changes, and erosion processes. On one hand, interpreting geological information encoded in scarps allows for the quantitative constraint of the kinematic and dynamic mechanisms of the active structures. On the other hand, studying the evolution processes of scarps contribute to a better understanding of the couplings among tectonics, erosion, and climate during geomorphic evolution processes. In regions characterized by adverse geological conditions, limited accessibility, and logistical challenges hindering researchers from reaching certain areas, traditional dating methods such as radiocarbon dating, luminescence dating, and cosmogenic nuclide dating often face difficulties in determining the age of scarps. The geomorphic dating method of scarps, however, offers a promising avenue to address the scarcity of chronological samples in research areas where either sample availability is limited or conventional dating techniques are impractical. This paper provides a concise summary of the theoretical evolution of geomorphic dating of scarps. Emphasis is placed on elucidating the slope evolution processes, transport models, and associated computational methodologies integral to this approach. Additionally, the specific applications of these methods in active tectonics and geomorphology are highlighted, accompanied by a case study showcasing their practical implementation.

    The theoretical foundation of geomorphic dating of scarps posits that the evolution of scarps during stable erosion stages can be simulated through models describing the evolution of slope surfaces over time. In practical dating applications, it is essential to determine the theoretical models and computational methods for the evolution of scarps. This necessitates the integration of measured profiles of the scarp to establish boundary and initial conditions, facilitating the determination of the geomorphic age of the studied scarps. On one hand, the related slope evolution model mainly involves processes such as bedrock weathering, sediment transport, and tectonic uplift. Previous studies have proposed dozens of quantitative slope evolution models and geomorphic transport functions(e.g., local linear, local nonlinear, non-local, etc.)based on various slope processes, theoretical assumptions, and numerical simulations. In various transport equations, compared to earlier local linear models, later local nonlinear transport models proposed based on experimental simulations and physical derivations exhibit higher fitting accuracy for real slope evolution. In the past decade, some scientists have proposed nonlocal transport models because of the limitations of traditional transport models, and have applied them in research. This nonlocal model assumes that the distance of sediment movement within a given area follows a probability distribution, thus allowing the simulation of long-distance slope processes over short periods. Additionally, many other transport models have been derived from specific slope processes, such as biotic disturbance and dry ravel. The solution methods for the aforementioned models vary as well. For instance, the analytical solution of a local linear diffusion transport model can be relatively easily obtained, while local nonlinear models and nonlocal models can only be numerically solved through specific approaches. On the other hand, the measured topographic profiles of the studied scarps can be used to determine the practical parameters of slope evolution models, including the present-day morphology of the scarps and their ages since their initial formation. In practical applications, various methods have emerged for the geomorphic dating of scarps, generally classified into two types based on the approach to fitting model calculations with actual topographic profiles: the mid-point slope method and the full slope method. The mid-point slope method uses the mid-point gradient value as the fitting morphological feature, representing an early method for dating scarps, mostly combined with linear diffusion transport functions and requiring numerous profiles for statistical analysis. Due to its low data utilization and limited spatiotemporal precision in statistical methods, the mid-point slope method has gradually been replaced by the full slope method. The full slope method involves fitting the overall shape of actual profile curves using model solutions. With the continuous improvement of observation techniques in the field of Earth sciences and the deepening research on related theories, the application scope of scarps geomorphic dating methods is no longer limited to the study of terraces and simple fault scarp evolution processes but has expanded to more complex geological environments, providing more precise constraints on their formation and evolution history.

    For method application, we systematically present the progress in scarp geomorphic dating research across various geomorphic settings(such as river and coastal terraces, lake shorelines, alluvial fans, marine terraces, and extraterrestrial planets). It employs the geomorphic dating of the northeastern Pamir fault scarp as a case study to further explore and anticipate the developmental trajectory of geomorphic dating of scarps within the field of tectonic geomorphology.

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    SURFACE RUPTURE INTERPRETATION AND BUILDING DAMAGE ASSESSMENT OF XIZANG DINGRI MS6.8 EARTHQUAKE ON JANUARY 7, 2025
    ZOU Jun-jie, SHAO Zhi-gang, HE Hong-lin, GAO Lu, XU Yue-yi, DOU Ai-xia, LIANG Ze-yu
    SEISMOLOGY AND GEOLOGY    2025, 47 (1): 16-35.   DOI: 10.3969/j.issn.0253-4967.2025.01.002
    Abstract591)   HTML30)    PDF(pc) (18854KB)(387)       Save

    On January 7, 2025, at 9:05 AM, a 6.8-magnitude earthquake struck Dingri County, Shigatse City, Xizang, at a depth of 10km. The maximum intensity of the earthquake reached Ⅺ degrees. This study provides a comparative analysis of pre- and post-earthquake remote sensing images using GF-2 satellite data. The results identify the Dengmecuo fault as the primary seismogenic fault for the earthquake. Surface ruptures exhibit distinct geometric variations between the northern and southern segments. The northern segment, approximately 3km in length, features a relatively simple geometry with a narrow rupture width, forming a “concentrated rupture” pattern characterized by continuity. In contrast, the southern segment, approximately 12km long, displays a more complex geometry with a wider rupture width, resulting in a “diffuse rupture” pattern marked by discontinuities. Statistical analysis of building collapses and damage in 28 administrative villages near the epicenter shows that the severity of impact follows this order: Changcuo township, Cuoguo township, and Quluo township. Affected villages were classified based on their geological and geographical conditions, revealing that the earthquake's impact diminished in the following sequence: areas near the micro-epicenter, lake regions adjacent to the surface rupture zone, and bedrock mountainous areas far from the epicenter and rupture. Coseismic surface rupture analysis reveals two fault segments near Dengmecuo Lake that did not rupture. Considering the unilateral rupture pattern from south to north and the distribution of aftershocks, it is suggested that the unruptured southern segment may pose a greater seismic hazard. At a regional scale, normal faults within the fault system, including the Quluo, Dengmecuo, Guojia, and Dingjie faults, all exhibit aftershock activity. Given the recent release patterns of moderate-to-strong earthquakes, special attention should be given to the seismic risk associated with the Quluo and Dingjie faults. Finally, based on the geographical conditions, seismogenic structures, and seismic damage patterns, this study offers strategies for mitigating seismic risks in high-altitude, high-latitude regions with diverse geological and geomorphological features, diffuse fault deformation patterns, and populations of ethnic minorities.

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    INTEGRATED INTERPRETATION ON THE PRECURSORY PROCESS EVOLUTION IN THE META-INSTABILITY STAGE OF THE EARTHQUAKE: A CASE STUDY ON 2014 LUDIAN MS6.5 EARTHQUAKE
    JIANG Hai-kun, DENG Shi-guang, YAO Qi, SONG Jin, WANG Jin-hong
    SEISMOLOGY AND GEOLOGY    2024, 46 (3): 513-535.   DOI: 10.3969/j.issn.0253-4967.2024.03.001
    Abstract366)   HTML44)    PDF(pc) (5232KB)(371)       Save

    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.

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    MECHANISM DIFFERENCES BETWEEN SEVERAL TYPICAL PYROCLASTIC ROCKS AND THEIR VOLCANISM SIGNIFICANCE
    WEI Hai-quan, CHEN Zheng-quan, LIU Yong-shun, BAI Zhi-da
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 297-311.   DOI: 10.3969/j.issn.0253-4967.2024.02.004
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    Pyroclastic rock is the most direct object of physical volcanology and the most important topic of identifying the volcanic explosive fragmentation processes. Some particular species of pyroclastic rocks and equivalents can indicate key characteristics of the volcanism process, which is the basis to estimate the eruptive risks. Volcanic hazard is potential risk related to volcanic eruption, and it is one of the most important types of disasters that human beings face in nature. Volcanic disasters are directly related to the types of volcanic eruptions, among which explosive volcanic eruptions can cause the deadly intensive volcanic risks. The direct product of explosive volcanic eruption is to form various pyroclastic rocks, which represent the different types and intensities of volcanic hazards caused by the eruption process. The primary pyroclasts and secondary fragments reflect the difference of volcanic surface processes during eruptive or intermittent periods, while the distinguish of magmatic, phreatomagmatic and phreatic eruptive deposits marks the systematic development of modern volcanology, which is the leading work in the study of volcanic hazards. 1)Pyroclastic rocks are formed directly by transporting, accumulating and diagenesis of the expelled materials during the eruption. They usually consist of the primary materials such as broken magma, accidental fragments trapped by the volcanic conduit, as well as the epiclasts captured by the volcanic fluid flowing on the surface. Pyroclastic rock, as a direct product of explosive volcanism, has naturally becomes the most important research object in volcanology. The volcanic tephra laminae preserved by fine airfall volcanic ash in basins has been attracted attention because of their good isochron and environmental indication, and the associated rocks may need to be distinguished from different types of volcanic sedimentation such as bedded tuff, sedimentary tuff and tuffaceous mudstone. The autoclastic breccia produced by lava emplacement and the hyaloclatite formed by the quenching of lava under water represent fragmentation that is closely related to the lava flow, rather than those from explosive volcanism. 2)Pyroclast is mainly the product of explosive volcanism, but it can contain a certain amount of normal sedimentation and a small amount of rock fragment near the volcanic channel and the magma chamber roof. Pyroclats are generally defined as the direct products of explosive eruption behavior, while volcaniclastics are formed by volcanic degradation such as slope displacement, avalanche, lahar, and the autoclast generated by lava flowage and quenching. This classification not only emphasizes the difference in the forming process of different volcanic products, but also helps to distinguish the different mechanism in volcanological research and hazard estimation. Different types of pyroclastic rocks are formed with different fragment mechanisms and diagenetic ways, and some specific pyroclastic rocks represent various special types and scales of volcanic hazards. Although they are usually classified as primary clastics, the hazard caused by autoclastic breccia is significantly different. Cryptoexplosive breccia, although we have employed a rock name from pyroclastic rocks, is actually more concerned with its resource economics. 3)When we study the genetic types of pyroclastic rocks, the most important basis for identification is the forming mode of the materials, that is, the type of fragmentation, which include primary volcanism and secondary volcanism. Primary clasts are divided into pyroclast, which is formed by the direct action of volcanic eruption, and autoclast, which is produced by the flow process of lava flows, While secondary(exogenous)volcanism includes various kinds of exogenous clasts(epiclast)formed by volcanic surface processes. According to the proportion of magma and water content at eruptive environment, explosive eruption can be divided into three types: magmatic eruption, phreatomagmatic eruption and phreatic eruption, which represent the most basic process of explosive eruption, and are also the problems of genetic classification and identification often faced in the study of pyroclastic rocks.

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    FINE CHARACTERISTICS OF EARTHQUAKE SURFACE RUPTURE ZONE BASED ON HIGH-RESOLUTION REMOTE SENSING IMAGE: A CASE STUDY OF LITANG FAULT
    YOU Zi-cheng, BI Hai-yun, ZHENG Wen-jun, PENG Hui, LIANG Shu-min, DUAN Lei, QIN Yi-gen
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1057-1073.   DOI: 10.3969/j.issn.0253-4967.2023.05.002
    Abstract467)   HTML53)    PDF(pc) (10517KB)(357)       Save

    Strong earthquakes(magnitude>6.5)typically cause coseismic surface ruptures of several kilometers or even hundreds of kilometers long on the surface. Coseismic surface rupture is the most intuitive geomorphic representation of an earthquake on the surface, and its geometry and distribution characteristics provide important information about the fault activity. Field investigation is the most basic means for research on coseismic surface fractures, but for areas that are hard to access or have harsh climatic environments, field investigation is often greatly limited. In recent years, the increasing abundance of high-resolution remote sensing images and the rapid development of photogrammetry methods can help us quickly obtain high-resolution topographic and geomorphic data of the study area, to better identify the fine geometry of the earthquake surface rupture zone and measure the offsets of geomorphic markers along the fault. The Litang Fault is a sinistral strike-slip fault located within the Sichuan-Yunnan rhombic block on the eastern edge of the Qinghai-Tibetan plateau. Several historical earthquake events have occurred on this fault, such as the 1890 and 1948 earthquakes, and clear seismic surface ruptures still exist along the fault so far. Previous studies have conducted a series of works on the coseismic surface rupture of this fault, but most of these works were based on field investigations or relatively low-resolution remote sensing images, and there is still a lack of fine research on the coseismic surface rupture of the fault. In this paper, the coseismic surface rupture of the 1890 earthquake which occurred on the Litang Fault was selected as the study object. To obtain high-resolution topographic data of this fault, the WorldView satellite stereo images were used to generate a 0.5-m-resolution orthophoto and a 1-m-resolution Digital Elevation Model(DEM)of the Litang fault based on the photogrammetry method. With the high-resolution topographic data, the fine geometry of the 1890 earthquake surface rupture zone was mapped in detail. The mapping results show that the total length of the surface rupture is about 27km, with an overall strike of N40°W. The rupture is mainly characterized by sinistral strike-slip motion, with a certain degree of dip-slip component in local areas. Except for the interval of approximately 6km with no surface rupture at the Wuliang River floodplain in the Litang Basin, the surface ruptures are relatively continuous at other locations. In addition, various rupture styles have been identified along the fault, including en echelon tension cracks, mole tracks, sag ponds, fault scarps, and displaced gullies. Furthermore, the sinistral offsets of 90 groups of linear geomorphic markers such as gullies and ridges were measured along the fault, which range from 1m to 82.4m. We further estimated the Cumulative Offset Probability Distribution(COPD)of the offsets located on the terrace I of the Wuliang River, which are all in the range of 0-9m. The COPD plot displays four distinct peaks at 1.3m, 2.4m, 4.3m, and6.1m, respectively. Previous studies have reported that the terrace I of Wuliang River formed at about(4 620±40)a BP. Thus, it can be indicated that the Litang fault may have experienced at least four strong earthquake events since(4 620±40)a BP, and the smallest peak of 1.3m may represent the coseismic displacement of the most recent 1890 earthquake. The rupture length of the latest 1890 earthquake was about 27km, and the coseismic sinistral offset was about 1.3m, yielding an estimated moment magnitude of MW6.8-7.1. The coseismic offset of the other three earthquakes was about 1.8m, 1.9m, and 1.1m from old to new, respectively, yielding a magnitude estimate of MW7.3, MW7.3, and MW7.0, with a size comparable to the 1890 earthquake. The research results fully demonstrate the potential of high-resolution remote sensing images in the study of fine characteristics of earthquake surface rupture.

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    CHARACTERISTICS OF FOCAL MECHANISM AND STRESS FIELD IN THE EASTERN BOUNDARY OF THE SICHUAN-YUNNAN BLOCK
    GUO Xiang-yun, FANG Li-hua, HAN Li-bo, LI Zhen-yue, LI Chun-lai, SU Shan
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 371-396.   DOI: 10.3969/j.issn.0253-4967.2024.02.008
    Abstract521)   HTML25)    PDF(pc) (11846KB)(354)       Save

    It is important to study the characteristics of the tectonic stress field studies which could provide a deeper understanding of the internal stress environment of the crust. It can provide useful assistance for exploring the relationship between the tectonic stress field and earthquake development. At the same time, it plays an important role in understanding block interactions, fault movement, tectonic deformation, and revealing the dynamic mechanical processes of the continent. The focal mechanism solutions contain abundant information reflecting the stress field.

    In this paper, using the broadband records from 128 permanent and temporary regional stations from the Chinese National Seismic Network(CNSN)deployed in the Sichuan-Yunnan Province and its adjacent, we determined the focal mechanisms of 3 951 earthquakes by the cut-and-paste(CAP)method and the HASH method. The friction coefficient and stress properties of the main active fault and characteristics of the tectonic stress field in this area are analyzed by using two different methods which are the damped inversion method(STASI)and iterative joint inversion method from focal mechanisms.

    The results of the focal mechanisms show that: there are 2 512 strike-slip earthquakes in the study area, accounting for 63.58% of all earthquakes; there are 818 normal fault type and normal strike-slip type earthquakes, accounting for 20.70% of all earthquakes; there are 621 reverse strike slip and reverse thrust earthquakes, accounting for 15.72% of all earthquakes. The most of earthquakes in the study area are distributed in active fault zones, the strike of the fault plane is consistent with the orientation of active fault zones. It revealed predominantly strike-slip faulting characteristics of earthquakes in the Eastern Boundary of the Sichuan-Yunnan Block, while the reverse thrust of earthquakes is mainly concentrated in the Longmenshan fault zone, as well as the NW trending Mabian-Yanjin Fault and the NE trending of Ludian-Zhaotong and Lianfeng faults which lied on the eastern boundary of the Sichuan-Yunnan block. Overall, the characteristics of the source mechanism are consistent with the regional tectonic background.

    Results of the stress field inversion confirmed main active fault in the Eastern Boundary of the Sichuan-Yunnan Block is under a strike-slip stress regime, maximum and minimum compressional stress axes are nearly horizontal. The maximum compressional axes are primarily oriented in NW-SE and NWW-SEE direction, and they experience a clockwise rotation from north to south. Against the strike-slip background, normal faulting stress regimes and reverse faulting stress can be seen in the regional areas. The most prominent is the Daliangshan fault zone, which has obvious differences from the overall characteristics of the stress field with the eastern boundary of the Sichuan Yunnan Block. The maximum horizontal principal stress in the northern section shows a nearly EW direction, with a strike-slip type stress property, and the NW-SE direction in the southern section, with a thrust type stress property. The distribution characteristics of the stress field are consistent with the fault type of sinistral strike-slip and thrust on the eastern boundary of the Sichuan Yunnan block

    The shape ratio R-value varies significantly, the R-value in the Sanchakou area is relatively high, with obvious extrusion characteristics, the R-values of the Xianshuihe fault zone, Anninghe fault zone and Xiaojiang fault zone are all between 0.25-0.5, showing NE-SW compression and NW-SE tension, and the tensile stress may be much less than the compressive stress(strike-slip type). The R values of the northern segment of the Daliangshan fault zone, the southern segment of the Anninghe fault zone, and Zemuhe fault zone are all between 0.5-1, showing NW-SE compression and NE-SW tension, and the compressive stress is greater than the tensile stress. To sum up, the current stress characteristics of the eastern boundary of the Sichuan Yunnan rhombic block are shear strain and local compression or tension.

    There are different friction coefficients of the main faults in the study area: The Anninghe fault zone is 0.60, the Xianshuihe and Zemuhe fault zones are 0.80, the Xiaojiang fault zone is 0.75 and northern and southern sections of the Daliangshan fault zone are 0.65 and 0.85. The friction coefficients of the Xianshuihe Fault, the southern section of the Daliangshan Fault, and the Zemuhe Fault are above 0.75. The high friction coefficients of these fault zones may be because they are strike-slip faults, and the friction coefficients themselves are relatively high. The southern section of the Xiaojiang fault zone may be related to the development of fault gouges in the fault zone.

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    STUDY ON THE SEISMOGENIC STRUCTURE OF THE 2022 GUJIAO ML4.1 EARTHQUAKE IN SHANXI PROVINCE BASED ON FOCAL MECHANISM AND SEISMIC LOCATION
    DONG Chun-li, ZHANG Guang-wei, LI Xin-wei, WANG Yue-jie, DING Da-ye, GONG Zhuo-hong
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 414-432.   DOI: 10.3969/j.issn.0253-4967.2024.02.010
    Abstract390)   HTML20)    PDF(pc) (4880KB)(339)       Save

    Understanding the mechanism of earthquake sequence in the mining area is important for the time-dependent hazard assessment. An earthquake of ML4.1 occurred in Gujiao, Taiyuan, Shanxi on February 20th, 2022, which caused strong ground motion in Gujiao and surrounding counties. The epicenter of this earthquake is located in the area of Lvliang uplift, where historical earthquakes are relatively rare. In addition, the coal resources are well developed in the earthquake source area which has attracted much attention from society and local governments.

    To investigate the mechanism and the seismogenic fault of Gujiao ML4.1 earthquake, we first apply the double-difference location method to retrieve highly accurate hypocenter locations. The results show that the earthquakes mainly occur at a depth range of 3~5km, and display a dominant distribution direction nearly EW-trending, which differs significantly from the NE-trending fault distribution pattern in this region. We further collect the broad-band seismic waveforms from the regional network of Shanxi province to perform focal mechanism inversion. The inversion results show that the Gujiao earthquake is a left-slip seismic event with a moment magnitude of MW3.96. The optimal double-couple solution is characterized by a strike of 90°, dip of 80°, and a rake angle of -21° for fault plane Ⅰ, while for the fault plane Ⅱ, the strike is 184°, dip is 69°, and rake angle is -169°. The best centroid depth is estimated to be at 3km. This earthquake shows an extremely shallow focal depth. Moreover, By using cluster analysis method, we obtained the central solution for the seismogenic fault plane of the GuJiao earthquake, with a fault strike of 91°and a dip angle of 70°. The focal solutions show that the earthquake exhibit a strike-slip type, and the orientations of earthquake sequence coincide well with the focal mechanisms.

    In addition, to discuss the effect of Gujiao ML4.1 earthquake on regional stress, we calculate the stress drop of this seismic sequence. The results show that the stress drop is significantly smaller than that of the regional earthquakes, exhibiting at least one order of magnitude lower than that of the background earthquakes in the same region. This phenomenon reflects that the stress level in the focal area of the GuJiao earthquake is not high, suggesting that the background stress enhancement in the focal area is not obvious.

    Based on regional geological structure, we found that the known faults in the region are all high-angle normal faults, and the strike of these faults are inconsistent with the focal mechanism solution of Gujiao earthquake sequence, which suggests that the existing faults are not the seismogenic fault. Taking the regional mining activities into account, we speculated that mining may cause strong disturbance to the stress field, and lead to stress redistribution within the rock mass. Such coal mining activity may generate a high stress disturbance on the hidden fault plane, and then the fault become the carrier of stress transfer. So we conclude that the seismogenic mechanism of the Gujiao-seismic sequence may be related to coal mining activities near the focal area, which leads to local stress changes, thus resulting in the activation of preexisting hidden faults and triggering the occurrence of the Gujiao earthquake.

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    THE CHARACTERISTICS AND MECHANISM OF FLUID ANOMALIES IN THE DAZHAI OBSERVATION WELL OF PU’ER, YUNNAN PROVINCE BEFORE THE M5.9 MOJIANG EARTHQUAKE ON SEPTEMBER 8, 2018
    HU Xiao-jing, FU Hong, ZHANG Xiang, LI Li-bo, HUANG Jiang-pei, LI Qiong, GAO Wen-fei
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 477-491.   DOI: 10.3969/j.issn.0253-4967.2024.02.014
    Abstract308)   HTML14)    PDF(pc) (6351KB)(335)       Save

    The precursors before earthquakes are very useful to earthquake prediction, and fluid anomalies before earthquakes are very important to precursory observations. This paper reviews the characteristics of hydrochemical ions and well-aquifer permeability anomalies of the Dazhai observation Well in Pu’er, which is in the Yunnan-Southwestern region of China, for all M≥5.5 earthquakes since 2004. We find that both the chemical ions and physical parameters before the Mojiang M5.9 earthquake exhibited the largest magnitude of changes since observation, and the abnormal state was much stronger than that of previous historical earthquakes, but the magnitude of the earthquake was below 6. About 1.5-2a before the M5.9 Mojiang earthquake, the composition of hydrogen and oxygen isotopes of the water samples in the Dazhai observation Well showed a significant deviation, accompanied by a continuously increasing concentration of fluoride ions from sources at deeper depths. This might suggest that the deep material in the earthquake source area began to be active. At the same time, starting one year before the earthquake, the phase lag of the water level in the wellhole changed from negative to positive, indicating that the source and pathway of well water recharge have been changed. In addition, around half a year before the earthquake, the continuously observed water chemical ions at shallow depths in the wellhole began to show a dramatic change. Moreover, macroscopic anomalies of hot spring water volume increased sharply before the earthquake, showing a remarkable evolution process from deep to shallow, from background to short-term, and from micro anomalies to macro anomalies before the earthquake. To investigate the causes and mechanisms of this phenomenon, we attempt to discuss the abnormal evolution process before the M5.9 Mojiang earthquake from the aspects of regional deep material activity and regional stress level. The abnormal concentration of the hydrochemical ions and the change of aquifer permeability observed continuously at the Dazhai observation well before the M5.9 Mojiang earthquake were caused by the continuous increase in shear stress in the region, which caused the aquifer to be compressed, resulting in a vertical fluid recharge and ultimately the alternation and mixing of different aquifer water bodies. In addition to being controlled by the continuous increase in regional vertical shear stress, the abnormal formation process was also accompanied by the intense activity of deep-sourced chemical elements such as helium isotope and fluoride ion. The abnormal evolution process showed a remarkably coupled process of migration from deep to shallow, which may be the reason why the shallow ion anomaly before the M5.9 Mojiang earthquake was the most significant among all the observed cases. Therefore, the evolution process of fluid activity starting from the deep and continuously transmitting to the surface with the accumulation of regional stress is essential to the abnormal evolution of the hydrological phenomenon before the M5.9 Mojiang earthquake. The regional stress and the process of deep material activity are the biggest differences between the M5.9 Mojiang earthquake and other historical earthquake cases in the study area, which will be the two main factors to be considered when similar ion changes occur again in the future. Our study provides insight into a comprehensive understanding of the predictive significance of underground fluid anomalies in the Dazhai well and the coupled evolution process of deep-shallow fluid anomalies before the earthquake.

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    JOINT INVERSION OF THE RUPTURE PROCESS OF 2018 ML5.7 XINGWEN EARTHQUAKE BASED ON SEISMIC AND INSAR OBSERVATIONS
    MIAO Si-yu, ZHANG Hai-jiang, GU Ning, LI Jun-lun, TAN Yu-yang, HUA Si-bo, ZHANG Yong
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 397-413.   DOI: 10.3969/j.issn.0253-4967.2024.02.009
    Abstract445)   HTML31)    PDF(pc) (5595KB)(330)       Save

    The ML5.7 Xingwen earthquake on December 16, 2018 is very likely induced by shale gas hydraulic fracturing, which caused not only massive landslides and rock collapse, but also some casualties in the surrounding area, with the direct economic loss of about 50 million CNY. It is of great significance to determine the source rupturing process of such an induced earthquake with large magnitude.

    Finite fault inversion is one of the commonly adopted methods to determine coseismic slip displacement distribution. For finite fault inversion, various data have different sensitivities to various aspects of the rupture process. The seismic data can provide the historical information about the earthquake rupture process because it contains the Doppler effect of the space-time rupture behavior on the fault. In comparison, the near-field geodetic data(such as InSAR and GPS)can constrain the fault parameters and the static slip distribution well because they contain the surface motion information. Therefore, the reliability of the inversion for the complex seismic rupture process can greatly be improved by combined use of seismicdata and InSAR data.

    In this study, strong-motion seismic data recorded at 8 near-field stations are chosen and filtered by a band-pass of 0.15-0.60Hz. The same InSAR data used in Wang et al.(2022)is adopted in this joint study. For inversion, a sufficiently large potential fault plane of 15km long and 10km wide is chosen and divided into 15×10 subfaults. Finally, the rupture process is obtained by joint inversion of strong-motion seismic data and InSAR data. The results show that the earthquake is characterzied by a typical unilateral rupture with the rupturing direction nearly towards the north. The duration of the rupture process was 6s, and the energy release was mainly concentrated in the first 5s. The rupture process is segmented and can be divided into two stages. The first stage is distributed from 1-3s and is located in the range of 0~5km from the source; and the 2nd stage is distributed from 3-5s and is located between 6 and 8km from the source. The coseismic slip is mainly concentrated in areas shallower than 5km, with a peak slip of approximately 0.27m. This can be used to explain why the Xingwen earthquake with a magnitude of ML5.7 caused relatively serious damages.

    Combined with the distribution of foreshocks and aftershocks, it can be seen that the foreshocks were mainly concentrated to the eastern edge of the major coseismicslip zone, which are close to some hydraulic fracturing wells. This suggests that these foreshocks occuring at the edge of the main rupture zone has a certain correlation with fluids, and the presence of fluids further leads to the fault weakening of the mainshock due to the increase of pore pressure and the decrease of effective compressive stress, which plays a triggering role in the occurrence of the Xingwen earthquake. The aftershocks are mainly distributed around the main slip zone, which are caused by after slips after the mainshock. The results from seismic inversion, InSAR inversion and joint inversion of the two data types reveal that the Xingwen earthquake is a northward unilateral rupture. The rupture propagation direction and coseismic slip distribution may be related to the physical property changes along the fault plane.

    Compared with the two single inversion results, the joint inversion overcomes the influence of uneven distribution of seismic stations, improves the resolution of slip distribution, and produces results that are more consistent with the real physical process. The slip model obtained by joint inversion in this study can be helpful for further understanding the mechanisms of induced earthquake, the correlation between induced earthquake and geological structure, earthquake disaster assessment and post-earthquake disaster prevention and hazard mitigation.

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    THE LATE QUATERNARY ACTIVITY CHARACTERISTICS AND SLIP RATE OF BATANG FAULT IN SE TIBETAN PLATEAU
    HUANG Wei-liang, ZHANG Jia-le, XIANG Wen, YANG Qian-hao
    SEISMOLOGY AND GEOLOGY    2023, 45 (6): 1265-1285.   DOI: 10.3969/j.issn.0253-4967.2023.06.002
    Abstract346)   HTML37)    PDF(pc) (17828KB)(316)       Save

    The southeastern margin of the Tibetan plateau is one of the most intensely deformed regions in the continental crust. A series of active faults with varying lengths and mechanical properties have segmented the lithosphere into multiple active blocks, with the Sichuan-Yunnan block being one of the most tectonically active regions. Its eastern boundary is characterized by secondary fault zones such as the Xianshuihe-Anninghe-Zemuhe, Xiaojiang, and Daliangshan fault zone, forming a narrow and continuous strike-slip deformation zone with a total length exceeding 1 100km. The western boundary of the Sichuan-Yunnan Block is mainly composed of the Jinsha River and the Red River fault zone, with the Jinsha River fault zone consisting of more than 20 roughly parallel secondary faults, forming a complex fault zone with 30~200km width. Despite recent GNSS network observation revealing the current tectonic deformation rates in this region, there is still a lack of research on the deformation characteristics and rates of individual active faults. This limitation makes it difficult in the assessment and understanding of seismic hazards in the area, restricting the scientific understanding of the current deformation mode in the southeastern margin of the Tibetan plateau.

    The Batang Fault, located within the Jinsha River fault zone at the western boundary of the Sichuan-Yunnan block, is a NE-trending main fault that obliquely cuts across the Jinsha River Fault, dividing later into northern and central segments. Presently, the Batang Fault is characterized by dominant right-lateral strike-slip motion. The deformation characteristics and rates of this fault since the Late Quaternary are crucial for understanding the spatial distribution of strong earthquakes and deformation patterns in the Sichuan-Yunnan block.

    The Batang Fault has a total length of 115km and is a Holocene right-lateral strike-slip active fault. The fault extends along the margins of bedrock mountains on both sides of the Maqu river and Jinsha River valleys, trending NNE or NWW to SEE, with a steep dip. The fault exhibits linear distribution of topographic features such as slopes, ridges, triangular facets, and fault scarps, essentially controlling the boundaries of bedrock mountains. In view from the geomorphology, the Batang Fault appears continuous and straight without distinct segmentation, except for localized small-scale step-like features. The Batang Fault has preserved abundant Late Quaternary activity evidence in two areas, Huangcaoping village and Batang county. This study utilized unmanned aerial vehicle photogrammetry to establish sub-meter digital terrain data for Huangcaoping and Batang site, accurately measuring displaced features such as alluvial fans and gullies affected by faulting. In Huangcaoping site, the fault has cut through multiple mountain-front alluvial fans, causing varying degrees of horizontal displacement in features such as gullies and the margin of the alluvial fans. This provided a scale for quantifying fault displacement. In Huangcaoping, five large-angle gullies intersect with the fault, one of which is a large gully developed in the bedrock mountain area. The gully has a deep incision, a narrow valley, and a rapid downstream turn to the right after exiting the mountain. The left bank of the gully preserves two geomorphic surfaces, Qo(older)and Qi(younger)surface, with the fault cutting across both surfaces, forming linear steep terrain. The measured total right-lateral offset of this gully since exiting the bedrock mountain area is(46±9)m. To constrain the activity rate of the Batang Fault at this location, we used cosmogenic nuclide single clast dating to determine the exposure age of the oldest geomorphic surface, Qo, as(12.5±0.5)ka. Considering that the formation of the river predates the Qo geomorphic surface, the age-constrained slip rate of the fault at this location is considered a maximum value, estimated at(3.6±0.8)mm/a. At Batang county, the Batang Fault has preserved clear faulted topography when cutting through the Moqu alluvial fan. The southern edge of the Moqu alluvial fan has been displaced by the fault, providing a well-preserved geomorphic marker for determining the strike-slip displacement of the fault. The Batang Fault, when intersecting the steep edge of the Moqu River alluvial fan, caused an obvious right-lateral offset, determined by comparing the consistent morphology of the steep edge on both sides of the fault. The right-lateral strike-slip displacement along the southern edge of the alluvial fan is measured at (40±5)m. The cosmogenic nuclide depth profile dating was used to determine the age of the faulted alluvial fan. From a vertical profile excavated along a man-made road on the edge of the alluvial fan, four mixed samples of small pebbles were collected from bottom to top. The calculated exposure ages of the debris flow alluvial fan are (15.2+3.2/-5.4)ka (without consideration of erosion)and (16.4+3.9/-5.6)ka (with consideration of erosion). Combining the fault displacement along the southern edge of the alluvial fan and the cosmogenic nuclide depth profile ages, the slip rate of the Batang Fault at this location is estimated to be of(2.6±0.6)mm/a (without erosion)or(2.4±0.8)mm/a (considering erosion). We believe that the age results with consideration of erosion effects is closer to the true values, thus we take 2.4mm/a as the activity rate of the Batang Fault at this location. The two slip rate values of the Batang Fault obtained in the Huangcaoping and Batang county sites are similar, indicating a right-lateral strike-slip rate of 2~4mm/a since the Late Quaternary. This rate accounts for 50%~80% of the present GPS observation shear deformation across the western boundary of the Sichuan-Yunnan block, indicating that the Batang Fault is a major deformation absorption zone in the Jinsha River fault zone. However, this rate is lower than the predicted~10mm/a using block models. The discrepancy may be due to the different understanding of the deformation mode at the western boundary of the Sichuan-Yunnan Block. In the block model, block sliding mainly relies on the primary boundary fault to regulate, but the long-term and lower geological activity rate of the Batang Fault obtained in this study does not match the assumption of a higher activity rate for the boundary fault in this model. The continuous and diffuse deformation characteristics of crustal deformation in the southeastern margin of the Tibet plateau may corroborate the lower activity rate of the Batang Fault obtained in this study.

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    PRECISE RELOCATION OF SMALL-TO-MODERATE-SIZED EARTHQUAKES IN THE DATONG VOLCANIC GROUP AND SURROUNDING AREAS
    XU Yong-qiang, LEI Jian-she
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 336-356.   DOI: 10.3969/j.issn.0253-4967.2024.02.006
    Abstract587)   HTML41)    PDF(pc) (14298KB)(315)       Save

    In the present study we collect a large amount of arrival times from 3 218 earthquakes in the Datong volcanic group and surrounding areas from January 2008 to January 2023 through the China Seismic Network Center and relocated these earthquakes using double-difference location algorithm, finally obtain 2 447 relocate earthquakes. Our result shows that most earthquakes occurred above a depth of 16km, and earthquakes in the basin occurred at depths of 5-16km. There are fewer earthquakes occur near the surface at depths of 0-2km, while 6km and 11km are the dominant depths for earthquakes. The overall strike trending of these earthquake sequences is NE-SW, which is consistent with the regional active faults and controlled grabens and semi-graben-type faulting basins. In addition, these earthquakes are more concentrated near the Kouquan fault zone and in the Datong-Yanggao earthquake zone in the eastern part of the volcanic group. The average location errors of these earthquakes in the east-west, north-south, and vertical directions are about 0.21km, 0.22km, and 0.30km, respectively, with an average travel time residual of 0.14s.

    The earthquakes near the Kouquan fault zone changed from deeper and more concentrated in the south to shallower and more scattered in the north. The earthquake sequences in the northern part of the southern section and the southern part of the middle section of the Kouquan fault zone are deeper along the NE-SW direction, roughly vertically distributed on the Kouquan fault. The earthquake sequences in the northern part of the middle section of the Kouquan fault zone did not occur on the Kouquan fault, and the distribution of earthquakes is relatively scattered, and earthquakes with larger magnitude are mostly concentrated at shallow depth, which may be related to the thick sedimentary coal-bearing strata and mining activities in the area. The strike trending of these earthquakes in the northern section of the Kouquan fault zone is, along the NE-SW direction, roughly distributed on the Kouquan fault. However, there are also earthquakes in the northern part of the Kouquan fault zone, which may suggest that the activity of the Kouquan fault zone has extended there.

    The focal depth in the source areas of the Datong-Yanggao earthquake is mostly concentrated at depths of 3-16km on the hidden fault parallel to the NE-SW trending Dawangcun fault to the east. The hidden fault has a large dip angle and dips towards NW, which intersects with the Tubao fault and the Liulengshan piedmont fault, likely related to the aftershock activity of the Datong-Yanggao earthquake.

    Earthquakes occur frequently in the middle section of the Huairen fault, followed by the southern section, and there are few earthquakes in the northern section. The seismic activity of the Shuiyu fault, the east fault of the Cailiangshan mountains, and the Yanggao-Tianzhen fault is relatively weaker. There are some seismic activities in the central part of the northern margin fault of the Tianzhen-Yanggao Basin. Earthquakes in volcanic areas occurred at the boundaries of volcanic clusters, while the seismicities inside the volcanic group area were not very strong, which suggests that the boundary of volcanic clusters is more prone to stress accumulation and earthquake generation than the interior of volcanic clusters.

    Based on the new seismic results of ambient noise tomography in the area, it is found that earthquakes are not only related to faults, but more importantly, most earthquakes occur near the high-to-low-velocity anomaly boundaries. Furthermore, there are obvious low-velocity anomalies visible beneath most earthquake source areas, which may suggest that the occurrence of these earthquakes is closely related to fluids carried by the upwelling of thermal materials rising to the crust from the mantle and reducing the effective normal stress on the fault planes.

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    SEISMOGENIC FAULT OF THE TANGSHAN MS5.1 EARTHQUAKE ON JULY 12, 2020 AND ITS IMPLICATIONS FOR REGIONAL TECTONICS
    CAO Jun, ZHOU Yi, GAO Chen, LIU Shu-feng, CHEN An, ZHANG Su-xin, FENG Xiang-dong, WU Peng, CHEN Zhao-dong
    SEISMOLOGY AND GEOLOGY    2024, 46 (5): 993-1011.   DOI: 10.3969/j.issn.0253-4967.2024.05.001
    Abstract602)   HTML60)    PDF(pc) (10827KB)(306)       Save

    On July 12, 2020, a M5.1 earthquake occurred in the Guye District of Tangshan City. This earthquake is notable as the only moderate seismic event exceeding magnitude 5 in the Tangshan area over the past two decades. However, the exact seismogenic fault responsible for this earthquake remains undetermined, complicating efforts to assess future seismic risks in the region. Post-earthquake damage assessments revealed that the macroseismic damage was distributed along two primary fault zones: a long northwest(NW)trending band and a short northeast(NE)trending band. The most significant damage occurred at the intersection of these two bands. Based on the regional geological structure and stratigraphy, field surveys identified the NE-trending Tangshan-Guye fault as a Holocene-active fault, while the NW-trending Mozhouyu fault was classified as a Quaternary fault within the area of greatest damage. Analysis of Sentinel-1A InSAR time-series data revealed differential deformation along the Mozhouyu fault. Relocation results of earthquakes greater than magnitude 1.0 over the past decade in the Tangshan region showed seismic activity distributed in two primary bands. One band aligns with the NE-trending Tangshan-Guye fault, with concentrated activity at its intersection with the Mozhouyu fault. Following the M5.1 earthquake, multiple authorities determined that the focal mechanism indicated a strike-slip earthquake, with two conjugate planes oriented in the NE and NW directions. This finding is consistent with the alignments of the Tangshan-Guye and Mozhouyu faults. Through comprehensive analysis, including post-earthquake field surveys, regional deformation data, and the relocation of smaller seismic events, it was concluded that the surface damage from the Tangshan Guye earthquake followed both NE and NW orientations. Of the two intersecting faults in the damaged area, the Mozhouyu fault is a middle Pleistocene fault, while the Tangshan-Guye fault is the most significant Holocene-active fault in the region. The characteristics of these conjugate faults align with both the source parameters and relocated seismic sequences of the Tangshan Guye earthquake. The right-lateral strike-slip motion along the Tangshan fault zone, combined with regional NE—NEE-directed compressive stress, likely caused the Tangshan-Guye fault to be blocked by the Qinglongshan complex anticline during its eastward expansion. Subsurface data further indicate that the Qinglongshan complex anticline marks a boundary of regional physical property differences. Therefore, it is concluded that the Tangshan-Guye fault and the Mozhouyu fault were the conjugate seismogenic faults responsible for the M5.1 earthquake on July 12, 2020.

    The Tangshan Guye earthquake is a typical moderate-intensity strike-slip event in the North China Plain. An analysis of 705 focal mechanism solutions from 2002 to 2020 indicates that most earthquakes in the region are predominantly strike-slip in nature. Historical strong earthquakes in the North China Plain also exhibit high-angle strike-slip faults as their primary seismogenic structures, a conclusion supported by extensive seismological research. A substantial body of seismic studies suggests that the failure of the North China Craton during the early Cenozoic was driven by crustal extension, resulting in the formation of listric(shovel-shaped)normal faults. However, these faults are no longer the main seismogenic structures for present-day earthquakes. Since the late Pleistocene, tectonic activity in the North China Plain has been characterized by the development of new, steeply dipping strike-slip faults, which cut through the older listric normal faults. These steep dip strike-slip faults have become the primary seismogenic structures responsible for regional seismicity. Future seismic hazard assessments in the North China Plain should focus on the activity of these steep dip faults, as they are more likely to generate significant earthquakes. This shift in tectonic stress is attributed to a combination of factors, including the eastward expansion of the Tibetan Plateau, the rigid deformation of the Ordos Block, and the westward subduction of the Pacific and Philippine plates. Since the late Pleistocene, these forces have redefined the tectonic landscape of the region, increasing the likelihood of strike-slip faulting.

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    GEOCHEMICAL CHARACTERISTICS AND GENESIS OF SOIL GAS IN THE PINGYUAN M5.5 EARTHQUAKE
    SU Shu-juan, CHEN Qi-feng, SUN Hao, LIU Jun, FENG Liang-le, XU Ji-long, YANG Yan-ming, LUO Kun-li
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 433-448.   DOI: 10.3969/j.issn.0253-4967.2024.02.011
    Abstract346)   HTML20)    PDF(pc) (7314KB)(303)       Save

    At 2:33 am on August 6, 2023, a M5.5 earthquake occurred in Pingyuan county, Dezhou city, Shandong Province. The faults within the epicenter and adjacent areas are deeply buried by the thick Quaternary sediment cover on which human activity is intensive, which makes it difficult to determine the location of the buried active faults from the surface based on geological and geomorphological evidences. It is necessary to detect the location of the buried active faults around earthquake areas and estimate their seismic risk.

    In this study, based on the epicenter distribution direction of major earthquake and aftershocks, seismic and geological data of earthquake areas, and damage degree of local buildings, 4 survey lines with a length of 30km were arranged across the epicenters and adjacent areas, and the concentrations of Rn, CO2 and Hg in soil gas were measured on site, and the results are as follows:

    (1)There are obvious spatial differences in the concentrations of soil gas near the epicenter and its vicinities within the distance of 30km. Gas concentrations are relatively high near the epicenter areas and the east and west ends of 4 arranged survey lines, in contrast to those which are relatively low in other non-structural control regions. The spatial distribution pattern of Rn concentration in soil gas is basically consistent with that of CO2, which may be due to CO2 used as a carrier gas of Rn to migrate to the surface. At the southern end of the Lingxian-Guanxian Fault(F1), the spatial concentration patterns of Rn and CO2 gases exhibit multiple peaks or wide anomalous zones. It is speculated that the deformation zone of the fault rupture at this location is relatively wide, and there may be secondary permeable fracture zones in the west of the F1. The escape form of Rn and CO2 gas indicates that there may indeed be multiple small fault branches near the F1, and the fault structure is relatively complex.

    (2)The spatial concentration distributions of Hg, Rn and CO2 in the epicenter areas are similar to that in its eastern region. However, in the western region of the epicenter areas, the spatial concentration distributions of Hg, Rn and CO2 vary greatly, and the Rn and CO2 concentrations near the Jiucheng Fault(F3) in the west of the epicenter regions are higher than those near epicenters. It is speculated that this phenomenon may be related to the high-concentration gas migration caused by strong seismic tectonic activities and the special nasal geological structure controlled by F3.

    (3)The concentrations of Rn, CO2 and Hg in the soil show high-value anomaly zones near the F1 and F3, and the concentrations of Rn and CO2 in the west of F3 exceed those in the epicenter area. After further earthquake relocation analysis, the spatial distribution of aftershocks exhibit a trend from F1 to F3. Combined with geochemical and geophysical research results, it is inferred that Pingyuan M5.5 earthquake should be related to the deep tectonic activities of F1 and F3.

    Above research results show that the soil gas geochemical method can be applied to define the location and distribution direction of the buried faults with thick overburden, which provides an important criterion for earthquake trend tracking analysis. This study is of greatly scientific significance in determining the dynamic source and genetic mechanism of Pingyuan M5.5 earthquake, identifying potential strong earthquake hazard areas, and assessing the risk of future earthquakes in the study area.

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    STUDY ON THE RELATIONSHIP BETWEEN LITHOSPHERIC MAGNETIC FIELD AND GEOLOGICAL STRUCTURE AND SEISMIC ACTIVITY: TAKING THE 2021 MS6.4 YANGBI EARTHQUAKE AS AN EXAMPLE
    CHEN Zheng-yu, NI Zhe, ZHOU Si-yuan, JIN Yun-hua, YANG Xin-jun
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 449-461.   DOI: 10.3969/j.issn.0253-4967.2024.02.012
    Abstract417)   HTML16)    PDF(pc) (5142KB)(303)       Save

    The lithospheric magnetic field is an important part of the earth’s magnetic field, which is affected by many factors, such as rock’s magnetization environment, underground geological structure, rock mineralogical composition, thermal and pressure state, and the deep tectonic evolution process. Most earthquakes occur in the crust and uppermost mantle, known as the lithosphere. The preparation and occurrence of earthquakes are usually accompanied by long-term accumulation and sudden release of energy, which will lead to changes in the thermal and pressure state of rocks, causing magnetic susceptibility variation in the lithosphere. Previous studies show that seismic activities can cause abnormal changes in the geomagnetic field, and there is an obvious correlation between the lithospheric magnetic field and seismic activities. The MS6.4 Yangbi earthquake on May 21, 2021, provided a unique opportunity to study the dynamic evolution of seismo-magnetic anomaly.

    Five-term repeat station vector geomagnetic data observed in Yangbi and surrounding areas from 2018-2021 were used in this paper, the first four terms were observed before the earthquake, and the fifth term was observed after the earthquake. After data processing and model calculation, the lithospheric magnetic fields before and after the earthquake are obtained, lithospheric magnetic field models are established using the Surface Spline method, and annual variations are calculated. Based on the analysis of lithospheric magnetic field combined with the regional geological structure, the Northwest Sichuan Subblock shows negative magnetic anomalies, which is consistent with the geological structural characteristics in the study area, altitude and crustal thickness increase sharply from Central Yunnan Subblock to Western Sichuan Plateau. Small areas of positive and negative magnetic anomalies are alternatively distributed in the Central Yunnan Subblock, which reflects the heterogeneity of deep lithosphere structure. The negative magnetic anomaly in the western boundary of the study area is also consistent with the geological characteristics of the Qingzang Plateau. There is also a correspondence between lithospheric magnetic field anomalies and faults, especially the total intensity. Negative magnetic anomaly strips are distributed along the strike of the Honghe Fault and Lijiang-Xiaojinhe Fault, while Weixi-Qiaohou-Weishan Fault appears at the junction of positive and negative magnetic anomalies. The statistical analysis of the MS6.0 and above seismic events from 1970 to 2021 shows that there is a correlation between lithospheric magnetic anomalies and seismic activities. Most earthquakes occur in the weak magnetic anomaly area, especially near zero contour. The earthquakes tend to be distributed in anomaly gradient belts, and the number of earthquakes in the negative anomaly area is higher than that in the positive anomaly area. Analyzing the characteristics of the pre- and post-seismic changes of declination and total intensity near the epicenter of Yangbi MS6.4, it is found that the epicenter of the Yangbi earthquake is located near the zero-contour-line of declination. During the preparation of the Yangbi earthquake, the total intensity gradually changed from a balanced distribution of positive and negative anomalies to the overall negative changes, and the magnetic anomalies recovered the trend of the balanced distribution of positive and negative changes after the earthquake.

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    INVESTIGATION OF THE SEISMOGENIC STRUCTURE OF THE 2025 DINGRI MS6.8 EARTHQUAKE IN XIZANG BASED ON THE TECTONIC STRESS FIELD PERSPECTIVE
    SHENG Shu-zhong, WANG Qian-ru, LI Zhen-yue, LI Hong-xing, ZHANG Xiao-juan, GE Kun-peng, GONG Meng
    SEISMOLOGY AND GEOLOGY    2025, 47 (1): 49-63.   DOI: 10.3969/j.issn.0253-4967.2025.01.004
    Abstract628)   HTML24)    PDF(pc) (3649KB)(301)       Save

    On January 7, 2025, at 09:05 Beijing Time, an MS6.8 earthquake struck Dingri County in Shigatse City, Xizang, as reported by the China Earthquake Networks Center. The earthquake occurred at 28.50°N, 87.45°E with a hypocentral depth of 10km, resulting in significant casualties and economic losses. In the immediate aftermath, major earthquake research institutions and seismologists, both domestic and international, promptly released the focal mechanism solution, providing crucial data for understanding the earthquake's origin and its seismogenic structure. However, the two nodal planes of the focal mechanism, derived from a double-couple source model, are equivalent, necessitating additional data or methodologies to distinguish the actual seismogenic fault plane. The parameters of the seismogenic fault are fundamental for the accurate calculation of ground motion maps, and they provide key information for seismic hazard assessment and post-earthquake rapid response guidance. Therefore, it is imperative to identify the seismogenic fault plane for the given focal mechanism solution.

    This study employs the tectonic stress field in the source region of the Dingri earthquake to calculate the instability coefficients of the two nodal planes, selecting the most unstable plane as the actual seismogenic fault. This method is based on the tectonic stress field to identify the seismogenic fault plane in the two nodal planes of the focal mechanism solution. The approach is applied to identify the seismogenic fault plane of the Dingri earthquake and nearby historical seismic events.

    Using the Global Centroid Moment Tensor(GCMT)focal mechanism solution, the study inverts the shallow tectonic stress field in the source region. The results reveal the maximum principal compressive stress axis is nearly vertical, and the maximum principal tensile stress axis is nearly horizontal with a strike orientation of E-W, which is a normal faulting stress regime. The stress field result is consistent with the normal faulting characteristics of the regions main fault structures.

    The seismogenic fault for the Dingri 6.8 earthquake is the one-striking southward and dipping westward nodal plane of the focal mechanism solution, determined to be a normal fault. Thus, we can infer that the seismogenic fault is the Dengmocuo Fault. In addition, the identification of the seismogenic fault for the historical earthquakes in the Dingri area shows that the fault is characterized by a southward strike and westward dip, with dip angles ranging from 37° to 48°, and the fault type is normal faulting.

    Identifying the seismogenic fault plane in the nodal planes of the focal mechanism solution based on the tectonic stress field, this study accurately identifies the seismogenic faults associated with the Dingri earthquake and surrounding historical events. It contributes seismological evidence for understanding the seismogenic structure of the region. It offers valuable insights for future research on seismogenic structures, particularly the determination of seismogenic faults of small and medium-magnitude earthquakes.

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    FRICTIONAL PROPERTIES OF SERPENTINE MINERALS UNDER HYDROTHERMAL CONDITIONS
    LIU Shi-min, ZHANG Lei, HE Chang-rong
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 235-250.   DOI: 10.3969/j.issn.0253-4967.2024.02.001
    Abstract389)   HTML27)    PDF(pc) (2735KB)(284)       Save

    Serpentine minerals are among the minerals commonly found in the Earth’s subduction zones, and their unique physicochemical properties have a significant impact on subducting geodynamics. Friction experimental studies of serpentine minerals are essential to gain a deep understanding of the frictional sliding stability of serpentine-containing faults in subduction zones as well as explaining the complicated misalignment behavior of faults in subduction zone. Previous laboratory research has produced an abundance of results, and this work addresses two main aspects: the stable states of occurrence and interconversion relationships of serpentine minerals, and the parameters affecting the frictional strength and sliding stability of serpentine minerals. First of all, studies on the stable endowment state of serpentine minerals and the interconversion relationship show that different types of serpentines diaplay different stable phases under different conditions. Chrysotile and lizardite are stable at low temperatures, and the stability fields of both chrysotile and lizardite roughly overlap, but chrysotile is in a substable state. Antigorite is stable at high temperature conditions, such as subduction zone mantle wedges containing high pore fluid pressure conditions, and undergoes a transition from lizardite to antigorite with increasing temperature. Secondly, studies on the factors controlling the frictional strength and sliding stability of serpentine minerals have shown that temperature, pore fluid, and the effective normal stress are all critical factors, for example, an increase in temperature can significantly increase the frictional strength of lizardite and chrysotile. In addition, the friction strength of serpentine minerals shows an obvious pressure dependence, and it was found through previous experimental studies that the friction strength of chrysotile exhibits a high-pressure sensitivity, and that the friction strength of antigorite gradually increases with increasing temperature under low fluid pressure conditions, showing an obvious temperature strengthening phenomenon. In contrast, the change in frictional strength of antigorite with temperature under high-pressure fluid pressure conditions is diametrically opposed to the results of low-pore fluid pressure conditions, which shows a clear temperature weakening phenomenon. Previous studies have also found that antigorite-undergoes a dehydration reaction with increasing temperature under lower fluid pressure conditions, and then exhibits unstable velocity weakening phenomenon, while antigorite exhibits velocity weakening phenomenon under low shear deformation rate under high-pressure fluid conditions. By analyzing the variation of friction-slip stability of antigorite with the shear slip rate can help us to better explain the phenomenon of subduction-zone slow-slip. Overall, experimental studies of the friction of serpentine minerals provide a key experimental basis for a deep understanding of subduction zone geologic processes. The results of these studies are scientifically important for predicting earthquakes and explaining the evolution of the Earth’s internal tectonics and subduction zones, providing strong support for research and practice in the field of geosciences.

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    SIMULATION OF THE ROCK SURFACE LUMINESCENCE SIGNALS ON BEDROCK FAULT SCARPS BY STICK-SLIP AND CREEP MOVEMENTS
    LUO Ming, CHEN Jie, QIN Jin-tang, YIN Jin-hui, YANG Hui-li, LIU Jin-feng, GONG Zhi-jun
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 357-370.   DOI: 10.3969/j.issn.0253-4967.2024.02.007
    Abstract370)   HTML9)    PDF(pc) (3308KB)(280)       Save

    The reconstruct of the stick-slip and creep histories is essential for understanding fault activities and seismic hazard assessment. Distinguishing stick-slip and creep using geodetic technology has become a hot research area in recent years, but distinguishing and estimating seismic slip and creep on geological timescales(e.g., over hundreds of years)is challenging due to the lack of historical, geodetic and remote sensing data extending back more than a few hundred years. This study uses a newly developed dating technique(rock surface optically-stimulated-luminescence(OSL)dating)combined with the OSL decay parameters of granite samples from the Langshan fault in Inner Mongolia to simulate optically stimulated OSL-depth curves and depths of half saturation of luminescence signal under various scenarios such as fault seismic slipping, creeping, and erosion of colluvial wedge. The study compares these OSL-depth profiles, especially the depths of the half saturation, under different slipping modes, and summarizes their features.

    During fault seismic slip, samples at different heights along the fault scarp display a “step-like” distribution pattern at their depths of half saturation. While during creep, however, they exhibit a “slope-like” pattern. Such differences may lie in that the slope during accelerating creeping is steeper than the slope during constant-speed creeping. Correspondingly, the resolution of residual luminescence-depth profile and depth of half saturation is also higher during accelerating creeping. During intra-earthquake creep events between seismic slip occurrences on the bedrock fault scarp, the distribution of half-saturation depth in the samples includes segments resembling both “steps” and “slopes”, which indicate the seismic slip and creep activities of the fault respectively. If the samples at the base of the colluvial wedge have had a sufficiently long last exposure time, the luminescence-depth profile and half-saturation depth distribution due to the erosion of the colluvial wedge would be approximately the same as in the three-phase seismic slip scenario. This indicates that samples previously buried by the colluvial wedge may be considered within the seismic displacement. Conversely, if the last exposure time of the base samples at the base of the colluvial wedge is short, the bleaching depth of the luminescence signal of these base samples will be noticeably shallower than that of the other samples within the seismic displacement, indicating the observed erosion of the colluvial wedge in this case. Furthermore, the seismic displacement ideally should include the buried location of the colluvial wedge. Therefore, when the luminescence curves and half-saturation depth distributions fail to identify the presence of the colluvial wedge, it is acceptable to include the buried location of the colluvial wedge in the seismic displacement calculation. Conversely, the luminescence-depth curves and half-saturation depth distributions document the erosion caused by the colluvial wedge. The simulation results demonstrate that this method can effectively distinguish between fault slipping and creeping, obtain corresponding displacements, and potentially record the erosion of colluvial wedge.

    This study also analyzes the temporal resolution of the method for distinguishing fault activity times and the spatial resolution for quantifying displacements. The specific situation is as follows. When exposure age of the bedrock fault scarp is within a thousand years, the rock surface OSL dating method can easily distinguish types of active slips and seismic displacements for the earthquakes with a recurrence interval of hundreds of years. When exposure age of the bedrock fault scarp is in the range of 100-101ka, the method can easily distinguish types of active slips and seismic displacements for the earthquakes with a recurrence interval exceeding a thousand years. When exposure age of the bedrock fault scarp is over ten-thousand years, the resolution of this method may be significantly reduced. The spatial resolution of seismic displacements using this method depends on interval between sampling and testing samples, typically in 10~30cm.

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    SURFACE DEFORMATION CHARACTERISTICS AND CAUSES OF THE DENGMECUO SEGMENT IN THE XIZANG DINGRI MS6.8 EARTHQUAKE
    LIANG Ming-jian, DONG Yun-xi, ZUO Hong, DAI You-lin, XIAO Ben-fu, LIAO Cheng, TAN Ling, WANG Yu-wei, LI Xiang, TANG Cai-cheng, ZHANG Wei, ZHANG Hui-ping, MENG Ling-yuan, SU Jin-rong, WU Wei-wei, LI Chuan-you, YAN Mei
    SEISMOLOGY AND GEOLOGY    2025, 47 (1): 80-89.   DOI: 10.3969/j.issn.0253-4967.2025.01.006
    Abstract950)   HTML23)    PDF(pc) (5804KB)(275)       Save

    On January 7, 2025, an MS6.8 earthquake struck Dingri, Xizang, China. According to the focal mechanism solution provided by the USGS, this event was characterized as a normal faulting earthquake. The earthquake occurred in the southern segment of the Shenzha-Dingjie Rift system, which is located on the Qinghai-Tibet Plateau. This rift system is one of the seven major rift systems in the southern part of the Tibetan plateau and is a significant controlling structure for shallow-source seismic activity within the region. Moderate to major earthquakes in the study area are primarily distributed along these rift systems. Notably, the Yadong-Gulu Rift system experienced an M8.0 earthquake in 1411 near the southern part of Dangxiong.

    The seismogenic fault of the earthquake is the Dengmecuo fault, which produced a 26-km-long surface rupture and deformation zone. The Dengmecuo fault is a branch of the southern segment of the Shenzha-Dingjie fault zone and is a Holocene active fault that controls the eastern boundary of the Dengmecuo Basin. The characteristics of the surface deformation zone in this earthquake differ between its northern and southern segments. The northern segment's surface rupture is primarily characterized by normal faulting, with a vertical co-seismic displacement of 2-3 meters. In contrast, the southern segment(the Dengmecuo segment)is mainly distributed on the eastern side of Dengmecuo Lake, with a width exceeding a hundred meters. The deformation characteristics of this segment are complex, exhibiting both extensional and compressional deformations. The extensional deformation zones in the southern segment, which align with the NNE-trending fault scarp, likely represent the tectonically seismogenic surface rupture zone of this earthquake. The compressive deformation zones, however, are believed to have formed as a result of the extensional deformation during the earthquake. These zones are influenced by seismic motion, local terrain, sedimentary characteristics, and climatic conditions and are not directly related to the fault's activity during the earthquake.

    The differences in the characteristics of the northern and southern segments of the surface deformation zone highlight the complexity of the geometric structure and motion properties of the Dengmecuo fault. Moreover, the main surface deformation zone in the southern section does not align with the surface traces of the Dengmecuo fault, suggesting that the fault may be gradually developing inward into the basin.

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    SOURCE RUPTURE MECHANISM AND STRESS CHANGES TO THE ADJACENT AREA OF JANUARY 7, 2025, MS6.8 DINGRI EARTHQUAKE, XIZANG, CHINA
    YANG Jian-wen, JIN Ming-pei, YE Beng, LI Zhen-ling, LI Qing
    SEISMOLOGY AND GEOLOGY    2025, 47 (1): 36-48.   DOI: 10.3969/j.issn.0253-4967.2025.01.003
    Abstract525)   HTML37)    PDF(pc) (6115KB)(274)       Save

    According to the official determination of China Seismic Network, at 09:05 on January 7, 2025, an MS6.8 earthquake(hereinafter referred to as Dingri earthquake)occurred in Dingri County(28.50°N, 87.45°E), Shigatse City, Xizang, with a focal depth of 10km. The earthquake occurred in the southern part of the Qinghai-Xizang Plateau, which is located in the intersection area of the Shenzha-Dingjie rift and the south of Xizang detachment system. The Dengmecuo fault(about 11km)is the closest to the earthquake, and the focal mechanism is tensile rupture. The earthquake had high magnitude, high intensity and shallow source, and the towns and villages in the epicenter area were relatively concentrated. In addition, the landform type of the epicenter and the surrounding area is a river alluvial plain, and the soil is soft, which amplifies the earthquake damage effect. Due to the comprehensive superposition of various factors, the earthquake caused severecasualties and building damage.

    The Dingri earthquake is a shallow-source normal-fault earthquake. The ground vibration and building(structure)damage caused by the release process of seismic radiation energy are higher than other earthquakes of the same magnitude, and the surface rupture characteristics are more significant. Therefore, the in-depth study of the Dingri earthquake, the acquisition of the co-seismic deformation field and the source sliding model, and the understanding of the earthquake's seismogenic mechanism and dynamic process can provide scientific and technological support for seismic damage assessment and secondary disaster analysis. In addition, based on the fault slip model, the Coulomb stress change in the surrounding area caused by co-seismic dislocation can be calculated, which is of great significance for the scientific evaluation of the future seismic risk and potential seismic disaster risk in the adjacent area.

    The Dingri earthquake occurred at a high altitude area, with an average elevation of about 4471m within 10km near the epicenter. The harsh natural conditions and the surrounding GNSS and strong seismic stations are scarce. Therefore, SAR images have become an important data source for obtaining the coseismic deformation of the earthquake and inversion of fault slip distribution. In this paper, based on the ascending and descending SAR image data before and after the Dingri earthquake taken by the Sentinel-1A satellite of the European Space Agency, the co-seismic deformation field of the Dingri earthquake was obtained by D-InSAR technology. On this basis, the source sliding model of the earthquake was jointly inverted based on the coseismic deformation data of the ascending and descending orbits, and the Coulomb stress variation characteristics of the surrounding area caused by the co-seismic dislocation were calculated. The deformation characteristics of the Dingri earthquake, the source rupture mechanism and the stress adjustment effect on the adjacent area are analyzed and discussed. Form the following understanding:

    (1)The results of the coseismic deformation field of the Dingri earthquake obtained based on the D-InSAR technology ' two-track method ' show that the long axis of the coseismic deformation field of the ascending and descending orbits is nearly NS-trending. The coseismic deformation is characterized by two obvious deformation areas in the east and west and a butterfly-like stripe pattern. The LOS deformation of the ascending and descending orbits is between -0.58~0.33m and -0.80~0.66m, respectively.

    (2)Based on the coseismic deformation data of ascending and descending orbits, the moment magnitude of the Dingri earthquake obtained by joint inversion is MW7.06 by using the SDM layered model method. The rupture process of the earthquake shows a unilateral rupture characteristic from the initial rupture point to the north along the fault. The fault dislocation is a standard fault mechanism with a little strike-slip component. The length of the main rupture zone of the seismogenic fault is about 55km, and the slip distribution is concentrated in the depth range of 0~15km underground. The maximum slip is 4.25m, which occurs at a depth of 8.6km underground. The main rupture zone of the earthquake has reached the surface, located about 35~53km north of the epicenter along the strike, and the potential surface rupture length is about 18km.

    (3)The results of the change in coseismic Coulomb stress show that the Dingri earthquake led to a decrease in coseismic Coulomb stress on both sides of the seismogenic fault. The Coulomb stress at the north and south ends of the fault rupture section and its surrounding areas increases significantly, and the loading amount is much larger than the earthquake-triggering threshold of 0.01MPa. There is a possibility of further felt aftershocks in these areas in the future.

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    RELOCATION OF THE 2022 MS6.0 MAERKANG EARTHQUAKE SWARM IN SICHUAN PROVINCE AND ITS SEISMIC FAULT ANALYSIS
    XU Ying-cai, GUO Xiang-yun
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 1006-1024.   DOI: 10.3969/j.issn.0253-4967.2023.04.012
    Abstract404)   HTML35)    PDF(pc) (13832KB)(264)       Save

    The 2022 MS6.0 Maerkang earthquake swarm in Sichuan Province is the first rare strong swarm activity with high frequency, concentrated spatial and temporal distribution, strong explosive and strong magnitude in Maerkang area in the eastern segment of Bayan Har block in China seismic network records. It is also another significantly strong earthquake event in Bayan Har block after the MS7.4 Maduo earthquake on May 22, 2021. The MS6.0 Maerkang earthquake on June 10, 2022 not only broke the 33-year record without MS≥6.0 earthquakes within 100km of the epicenter, but also broke the historical record without MS≥6.0 earthquakes within 50km of the epicenter. The earthquake swarm is mainly located in the nearly “T” shaped conjugate fault structure area composed of the NW strike Maerkang fault and NE strike Longriba fault in the Bayan Har block. This area is a relatively rare region for moderate and strong earthquakes in the history. Therefore, it is of great significance to analyze and discuss the possible seismogenic faults of the Maerkang strong earthquake sequence for the study of seismogenic structures and the risk of strong earthquakes in the weak seismic region of Bayan Har block.

    The earthquake swarm was relocated by double-difference method, and focal mechanisms and centriod depths of MS≥3.6 earthquakes were calculated by using gCAP inversion method. Then the relationship between the stress system in the Malkang area and these earthquake focal mechanisms was analyzed, and fault plane was fitted by using relocation results. Maerkang earthquake swarm is mainly distributed along NW direction, and the initial rupture depth is 9.8km on average. Depth profiles show that earthquakes are mainly concentrated at depth between 0km to 15km. The most earthquakes of early-stage occurred in 48 hours. The mid-stage and late-stage earthquakes are located less than 15km in depth and move to the northwest of the epicenters. Initial rupture depth of the largest MS6.0 earthquake is 12.5km, which is almost at the bottom of the dense area. The focal mechanism of MS6.0 earthquake is 150° in strike, 79° in dip, and 7° in rake on nodal plane Ⅰ, and 59° in strike, 83° in dip, and 169° in rake on nodal plane Ⅱ, with the centroid depth of 9km. Other focal mechanisms of MS≥3.6 earthquake are strike-slip types. Dips of nodal plane of focal mechanism range from 71° to 86°, and there exist different dip directions for one strike of every nodal plane. All azimuths of P axis are in NWW direction, and the plunges are nearly horizontal. The focal mechanisms of MS≥3.6 earthquakes show that the tectonic environment is very favorable for NE or NW strike faults to generate the strike-slip movement. Centriod depths range from 5 to 9km, which are lower than the average depth of 9.8km of relocation, indicating that these earthquakes mainly ruptured from deep to shallow. The relative shear stress of the NW nodal plane are significantly greater than that of the NE nodal plane, and the normal stress of the NW nodal plane was smaller than that of the NE nodal plane, indicating more possibility of strike-slip dislocation on the NW nodal plane. The fault plane fitting results reveal that there are obviously two nearly parallel and nearly NW strike earthquake belts in the epicenter area. Fitted fault plane parameters of the belt in the north branch show the strike 333°, the dip 88°, the slide -22°, and the belt in the south branch show the strike 331°, dip 88°, and slide -23°. It is indicated that the fault properties of these two earthquake belts are basically the same, revealing that most of earthquake activities of the swarm may be controlled by at least two parallel structures near the Maerkang fault with the NW strike, dip 88° and left-lateral strike-slip. Combined with the existing regional geological structure, it is inferred that the Maerkang earthquake swarm may be induced by the NW and NE strike conjugate faults, and the NW strike faults control most of the earthquake activities.

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    OPPORTUNITIES BROUGHT BY 3D GEOSCIENCE MODELING FOR EARTHQUAKE NUMERICAL FORECASTING
    YAO Qi, LU Ren-qi, SU Peng, WANG Hui, ZHU Ya-ling, WANG Li-wei
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 1-18.   DOI: 10.3969/j.issn.0253-4967.2024.01.001
    Abstract382)   HTML33)    PDF(pc) (1599KB)(258)       Save

    Earthquake prediction and forecasting need to transform from the traditional empirical, qualitative, and semi-quantitative to quantitative. The improvement also calls for multi-disciplinary, highly integrated physical and mechanical simulations rather than only a single discipline. The global development of observation technology and the construction of observation networks have already built a data foundation for earthquake numerical prediction and forecasting to a certain extent. However, the biggest constraint is the difficulty of synthesizing a large amount of observation data and quickly establishing complex numerical models with geological significance for numerical calculation. It is a vital issue restricting experimental research and industry development of earthquake numerical prediction and forecasting. Based on a brief introduction of the concept, development, and research status of earthquake numerical prediction and forecasting, this paper analyzes the difficulties in numerical modeling, which essentially come from the disciplinary differences between active tectonics, structural geology, solid earth, seismology, and numerical simulations. The development of 3D Geoscience Modeling and its application in the earthquake industry can establish a large-scale complex earthquake tectonic model close to the real world with geological significance. It provides a significant opportunity and technical means for developing earthquake numerical prediction and forecasting by solving the problems in numerical modeling. 3D geological modeling has built a bridge for multi-disciplinary geological applications. It can multi-disciplinary data fusion, establish a 3D geological model with geological significance and characteristics in line with geomechanical characteristics, and integrate data, geological model, up to building a numerical model, which advances the efficiency of modeling and simulation. Therefore, the rapid development of 3D geological modeling provides an opportunity to solve the modeling difficulties mentioned above in earthquake numerical prediction. Then, we briefly describe the development of 3D geological modeling technology, its application in the seismic industry, and the construction and application of 3D standard fault models domestically and overseas. Here, we introduced the development and essential contents of the Community Fault Model of Southern California in the United States for the Uniform California earthquake rupture forecast, the New Zealand Community Fault Model from the Institute of Geological and Nuclear Sciences Limited, and the Community Fault Model in Sichuan and Yunnan region in China.

    The prospective future of 3D geological modeling and its potential application in earthquake numerical prediction and forecasting makes it a common concern of researchers in earthquake science. The five future modeling trends are the joint modeling of multi-source and multi-precision heterogeneous data, the integrated modeling of the geological model-attribute model-numerical model, flat fault structure modeling, 3D fault structure modeling, data-model-calculation iteration, and mutual driving. Finally, the paper describes the difficulties of applying the 3D geological modeling technique in earthquake numerical prediction and forecasting, including the industry construction, public approval of the 3D Community Fault Model, and the variations of numerical modeling and applications. 3D geological modeling technology can provide more realistic numerical and geometric models for earthquake numerical prediction, forecasting, and related numerical computing fields, reduce construction periods, create fast iterations, and solve modeling difficulties.

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    NEW PROGRESS IN PALEOEARTHQUAKE STUDIES OF THE JIANGSU SEGMENT OF THE ANQIU-JUXIAN FAULT IN THE TANLU FAULT ZONE
    ZHANG Hao, LI Li-mei, JIANG Xin, ZHANG Dong, XU Han-gang
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 880-895.   DOI: 10.3969/j.issn.0253-4967.2023.04.005
    Abstract2739)   HTML38)    PDF(pc) (17177KB)(252)       Save

    Paleoseismology is a discipline that studies prehistoric earthquakes or earthquakes that occurred before instrumental records using geological and geomorphological methods, mainly by trench excavation and Quaternary chronology. It focuses on the time and intensity distribution of large earthquakes, to reveal the recurrence characteristics of large earthquakes and provide basic data for evaluating the probability of future earthquakes. The Tanlu fault zone is the most active fault zone in eastern China. The Jiangsu section of the Tanlu fault zone is mainly composed of five branch faults, which are strongly active in the Pleistocene. Among them, the Anqiu-Juxian Fault continued to be active until the Holocene, which is the seismogenic fault of the Tancheng 8½ earthquake in 1668. The Xinyi-Sihong section is likely to generate strong earthquakes in the future in the south-central section of the Tanlu fault zone.

    The total length of the Jiangsu section of the Anqiu-Juxian Fault is about 170km, with an overall strike of 5°~15°, extending southwards from the north Maling Mountain to the Chonggang Mountain. The geomorphic features are distributed from north to south by the alternation of the bedrock mountain and the sedimentary basin. The Anqiu-Juxian Fault shows a single exposed fault on one side of the bedrock mountain, extending to the basin into two branches in the east and west, of which the east branch is the active late Pleistocene Fault and the west branch is the Holocene active fault. The Jiangsu section of the Anqiu-Juxian Fault is dominated by dextral strike-slip and has both dip and thrust components.

    Lots of research have been done on the ancient earthquakes of the Anqiu-Juxian Fault. The trenches are mostly located in Maling Mountain, Zhangshan Mountain and Chonggang Mountain, which are in the state of uplift and denudation. The Holocene is very thin, and the dating method is mostly optical luminescence. The identification of ancient earthquake events is less since the Holocene, with the accuracy of ancient earthquake time not high and the ancient earthquake sequence not complete. According to the topographic and geomorphological characteristics of the Jiangsu section of the Anqiu-Juxian Fault, three trenches were excavated along the Anqiu-Juxian Fault, of which two were in exposed areas and one in a buried area. Three trenches completely revealed the Holocene sedimentary strata in the Jiangsu section of the Anqiu-Juxian Fault, in which MLTC2 revealed the early Holocene strata, MLTC1 revealed the middle Holocene strata, and HSTC revealed the late Holocene strata. The determination of the age of earthquake events is one of the most uncertain factors in the study of paleoearthquakes and is the main indicator of the recurrence period of paleoearthquakes. At present, most of the paleoearthquake events studied have occurred since the late Pleistocene, and the accuracy of 14C dating is the highest. A total of 13 14C samples were collected from the trenches. Combined with the paleoearthquake events and time revealed by previous trenches, it is concluded that there have been three paleoearthquake events in the Jiangsu section of the Anqiu-Juxian Fault since the Holocene, with theelapsed time of ~3000aBP, ~6000aBP and ~11000aBP, and the coseismic vertical offset are all nearly 1m.

    The 1668 Tancheng M8½ earthquake showed signs of surface ruptures in the exposed area of the Xinyi section of the Anqiu-Juxian Fault, accompanied by a large amount of sandblasting and water gushing in the buried area. Dense fissures and sand veins are observed in the late Holocene strata overlying the fault, indicating the impact of the 1668 Tancheng earthquake. More representative chronological data are needed as to whether the 1668 Tancheng earthquake ruptured Suqian City.

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    3D STRUCTURAL MODELLING OF THE ANNINGHE-ZEMUHE-XIAOJIANG FAULT ZONE IN THE EASTERN BOUNDARY OF SICHUAN-YUNNAN BLOCK USING MULTI-DATA AND IMPLICIT MODELING METHODS
    WANG Mao-mao, HU Shun-yang, MA Hao-ran, LIANG Bo-yu, ZHANG Jin-yu, LU Ren-qi
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 19-34.   DOI: 10.3969/j.issn.0253-4967.2024.01.002
    Abstract400)   HTML31)    PDF(pc) (8144KB)(249)       Save

    The Anninghe-Zemuhe-Xiaojiang fault zone is located at the intersection between the Qinghai-Xizang Plateau and the Yangtze block, representing the eastern boundary of the Sichuan-Yunnan block with frequent seismic activities. Its overall kinematic characteristics involve left-lateral strike-slip motion, and the fault structures along its strike are complex, posing significant challenges in accurately characterizing the 3D structural features of deep faults. The main issues include the structural complexity of the fault surfaces, uncertainties in the intersection relationships of fault systems, spatial constraints of blind faults, and the definition of fault surfaces in regions with weak seismic activity. Traditionally, 3D structural modeling for fault geometry heavily relies on high-resolution seismic reflection profiles, 3D seismic data volumes, and borehole data. It defines the geometric shapes of objects with limited nodes in a triangular mesh, and then simulates the topological structure of objects by connecting these nodes. However, obtaining high-resolution seismic reflection data in active tectonic areas like the eastern boundary of the Sichuan-Yunnan block is challenging, and even when available, it is often sparse in space. Alternatively, a large amount of relocated earthquakes and surface fault traces are generally used to create initial three-dimensional models of active faults. However, this approach overlooks the contributions of focal mechanism solutions in constraining the modeling, with more subjectivity in the selection of relocated seismicity, and does not adopt a differentiated weight strategy for various data sources. In this study, a 3D implicit modeling approach, combining deep and shallow geological and geophysical data that are generally available in active tectonic environments, was used to construct a detailed 3D structural model of the Anninghe-Zemuhe-Xiaojiang fault zone at the eastern boundary of the Sichuan-Yunnan block. The modeling process effectively integrated the fault plane constraints provided by focal mechanism solutions with surface fault traces and relocated seismic data, using a multi-iteration process with differentiated weight to increase the accuracy of the fault models. This approach ultimately represented the 3D complex structural features of the eastern boundary of the Sichuan-Yunnan block using multiple data sources. The modeling results show that the Anninghe-Zemuhe fault zone is characterized by a steep strike-slip fault structure with along-strike geometry variations. The Anninghe Fault shows its steepest dip angle in the central segment and gradually becomes gentler to both ends. Meanwhile, the Zemuhe Fault exhibits several asperities that are perpendicular to the direction of fault slip at a depth of 5~15km. By contrast, the north-to-central segment of the Xiaojiang fault zone is more complex. The western branch of the Xiaojiang Fault, which is an east-dipping, left-lateral strike-slip fault, is characterized by a relatively gentle fault plane with an average dip angle of 76° to 78°. The west-dipping segment of the eastern Xiaojiang Fault has a steeper dip with an average angle of 85°. The detailed 3D structural model of active faults constructed through implicit modeling can be used for analyzing fault roughness and fault system studies, which are crucial for understanding the distribution of asperities on fault planes and conducting seismic rupture simulations. Implementing the implicit modeling approach allows for the development of improved fault surface representations that can contribute to Community Fault Models in active tectonic environments, and support fault system modeling, rupture simulations, and regional hazard assessments.

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    A NEW REFERENCE SCHEME FOR THE DELINEATION OF ACTIVE BLOCK BOUNDARIES IN THE SICHUAN-YUNNAN EXPERIMENTAL SITE
    SUN Xiao, LU Ren-qi, ZHANG Jin-yu, WANG Wei, SU Peng
    SEISMOLOGY AND GEOLOGY    2024, 46 (5): 1027-1047.   DOI: 10.3969/j.issn.0253-4967.2024.05.003
    Abstract309)   HTML42)    PDF(pc) (9302KB)(246)       Save

    Active block boundaries represent areas where significant crustal stress accumulates, leading to concentrated tectonic deformation and frequent seismic activity. These boundaries are crucial for understanding the patterns of strong earthquakes within mainland China. The China Seismic Experimental Site, located in the Sichuan-Yunnan region, is a key area of tectonic deformation caused by the collision and convergence of the Indian and Eurasian plates. This region plays a vital role in transferring tectonic stress between western China and adjacent plates.

    This comprehensive study analyzes the integrity, three-dimensional characteristics, hierarchy, and tectonic activity of blocks within the Sichuan-Yunnan region, following established schemes and criteria for defining active block boundaries. After detailed research, the major active fault zones in the region have been divided into three primary active block boundary zones and sixteen secondary boundary zones.

    A new reference scheme was developed by considering several factors, including the historical distribution of strong earthquakes, the hierarchical patterns of earthquake frequency and magnitude, spatial variations in present-day deformation as revealed by GNSS data, and deep crustal differences indicated by gravity data and velocity structures. The Jinshajiang-Honghe Fault, Ganzi-Yushu-Xianshuihe-Anninghe-Zemuhe-Xiaojiang Fault, and Longmenshan Fault are identified as the primary active block boundary zones, while faults such as the Lijiang-Xiaojinhe, Nantinghe, and Longriba faults are classified as secondary boundary zones.

    Through an integrated analysis of seismic activity, current deformation patterns, fault sizes, deep crustal structures, and paleoseismic data, the study estimates that the primary boundary zones have the potential to generate earthquakes of magnitude 7.5 or greater, while the secondary boundary zones could produce earthquakes of magnitude 6.5 or greater.

    The expansion of geophysical exploration, including shallow and deep earth data, has allowed for a transition in the study of active tectonics from surface-focused to depth-focused, from qualitative to quantitative, and from two-dimensional to three-dimensional analysis. By integrating multiple data sources, i.e. regional geology, geophysics, seismicity, and large-scale deformation measurements, this study presents a more refined delineation of active blocks in the Sichuan-Yunnan region.

    The new delineation scheme provides a scientific basis for future mechanical simulations of interactions between active blocks in the Sichuan-Yunnan Experimental Site. It also offers a framework for assessing the probability of strong earthquakes and evaluating seismic hazards. The purpose of this study is to re-analyze and refine the delineation of active block boundaries using high-resolution, coordinated data while building on previous research.

    In summary, the Sichuan-Yunnan region’s primary fault zones are divided into three primary and sixteen secondary active block boundary zones. The study concludes that primary boundary zones are capable of generating magnitude 7.5 or greater earthquakes, while secondary zones can produce magnitude 6.5 or greater earthquakes. While the current block delineation scheme offers a valuable foundation, further discussion and refinement of certain secondary boundary zones are needed as detection and observational data improve. This study provides an essential framework for analyzing the dynamic interactions between active blocks, identifying seismogenic environments, and assessing seismic risks in the Sichuan-Yunnan region.

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    EFFECTS OF STRIKE-SLIP FAULT GEOMETRIC COMPLEXITY ON EARTHQUAKE RUPTURES PROPAGATION: A REVIEW
    WANG Hui, CAO Jian-ling, YAO Qi, WANG Li-wei, ZHU Ya-ling
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 201-219.   DOI: 10.3969/j.issn.0253-4967.2024.01.012
    Abstract412)   HTML28)    PDF(pc) (1897KB)(245)       Save

    Active strike-slip fault usually hosts major earthquakes. Therefore, studies on fault segmentation, which is controlled by geometric complexity, are very important for assessing the maximum magnitude of potential earthquakes. Based on previous literature, we summarized the behavior of earthquakes on strike-slip faults related to fault geometry, segmentation, and cascading rupture from the aspects of field investigation and numerical modeling.

    Previous field investigations have shown that geometrically complex sections of a strike-slip fault usually play the role of barriers that can separate earthquake rupture segments and effectively stop the propagation of earthquake rupture. Statistical results based on the limited field investigations of the surface rupture illuminated semi-quantitatively the influence of geometrically complex sections on the rupture behavior of the earthquake. Since not all earthquakes can produce surface rupture zones, the case number of surface ruptures are unlikely to meet statistical requirements in the coming years. In addition, knowledge gained from field investigation is mainly statistical results based on fault geometry and kinematics. They have some consistency but vary greatly, indicating the complexity of seismic rupture modes on strike-slip faults. No simple threshold that can be used as a criterion to refine the capability of earthquake rupture propagation on strike-slip faults with complex geometry. Based on the statistical results of field surveys, geologists have applied the factor, that complex geometricity controls earthquake rupture behavior, in seismic risk assessment. Lacking support from dynamic analysis, it is necessary to develop and integrate physics-based dynamic models to help improve earthquake-rate models and probability models.

    Numerical modeling can not only present the earthquake simulation scenario but also provide insights into the fundamental physics of dynamic rupture propagation. The modeling results improve our understanding of how the geometric complexity of the fault influences the dynamic rupture propagation. Different modeling approaches focus on different aspects of this challenging scientific problem, each with unique advantages and disadvantages. 2D modeling is relatively simple. They allow modelers to consider more complex physical processes, variated parameters, and constraints from the field and laboratory observation, etc. They provide a comparative benchmark on rupture dynamics on a strike-slip fault with complex geometry. 3D modeling can provide closer approximations to realistic faults and more direct comparisons to observations. The simulations of one earthquake rupture process may focus on the influence of single/multiple parameters on the rupture process. While the multicycle earthquake simulation can predict spatiotemporal patterns of earthquake ruptures on a strike-slip fault. Both simple 2D modeling and complex 3D modeling have shown that one of the most important factors affecting rupture behavior on strike-slip faults is the fault geometric complexity. In addition, other dynamic factors, such as the initial stress, the properties of the rock medium, and nucleation location, also affect dynamic fracture propagation on strike-slip faults. It indicates that rupture behavior on a certain strike-slip fault has its unique characteristics that are controlled by dynamic factors such as the regional tectonic environment and the properties of the fault itself. The numerical modeling provides a dynamic perspective on the complexity of rupture behaviors on strike-slip faults given by field investigations.

    In the China Seismic Science Experimental Site, 3D dynamic modeling supported by fault detection, dense geophysical observations, and high-performance computation will provide new insights into the rupture behavior in the complex multi fault system. That is helpful in determining the maximum magnitude of a potential earthquake.

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    DISCUSSION ON SEISMOGENIC STRUCTURE OF THE JUNE 2022 MS6.1 EARTHQUAKE AND ITS RELATIONSHIP WITH THE APRIL 2013 MS7.0 EARTHQUAKE IN LUSHAN, SICHUAN PROVINCE
    CHEN Han-lin, WANG Qin-cai, ZHANG Jin-chuan, LIU Rui-feng
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1233-1246.   DOI: 10.3969/j.issn.0253-4967.2023.05.012
    Abstract320)   HTML41)    PDF(pc) (7565KB)(238)       Save

    In this paper, we relocated earthquakes occurred from April 2013 to July 2022 in Lushan seismic zone, inversed focal mechanism solution of the Lushan MS6.1 earthquake on June 1, 2022 and discussed the seismogenic structure of the Lushan MS6.1 earthquake and its relationship with the MS7.0 earthquake in April 2013.

    The results of the focal mechanism solution show that the Lushan MS6.1 earthquake in 2022 is a thrust earthquake. The strike, dip and azimuth of nodal plane Ⅰ are 228°, 46° and 104° and for nodal plane Ⅱ are 28°, 46° and 76° respectively. The results of earthquake relocation show that the focal depth of the Lushan MS6.1 earthquake sequence is shallow in the north and deep in the south, the fault length is about 10km. The focal depth is mainly concentrated between 10km to 19km. The fault dip is southeast with an angle of 60°. The initial rupture point of the main shock of the Lushan MS6.1 earthquake is at a depth of 20km, located at the deepest part of the fault. The fault ruptured from deep to shallow. The Lushan MS7.0 earthquake occurred on April 2013 strikes northeast and dips northwestward, but there exists a reverse fault in the aftershock sequence that has the same direction of strike but the opposite direction of dip. This reverse fault is consistent with the strike and dip of the MS6.1 earthquake occurred in June 2022. It appears as two parallel faults in the profile. In addition to the reverse fault on the west side, the embryonic of another reverse fault seems to appear on the east side of the middle of earthquake sequence. These faults are about 10km away from the surface. The distribution of earthquakes in two northwest-oriented depth profiles shows that the dip angles of the main shock and the reverse fault of the MS7.0 earthquake is different at different locations, and these faults are not simple straight planar sections. From one year after occurrence of the MS7.0 earthquake to occurrence of the MS6.1 earthquake, the seismic activity on the main fault decreased but the seismic activity on the reverse fault on the west side of the MS7.0 earthquake sequence was more active during this period, most of the seismic activity occurred near the reverse fault that is parallel to the MS6.1 earthquake fault.

    By analyzing the seismogenic structure and seismic activity characteristics of the Lushan seismic zone, we concluded the Lushan MS6.1 earthquake on June 1, 2022 is caused by a blind thrust fault with strike towards northeast and dip towards southeast, located 10km away from the surface. It has the opposite directions of strike and dip of the Longmenshan Fault. The epicenters of the Lushan MS7.0 earthquake in April 2013 and the MS6.1 earthquake in June 2022 are located near the surface exposure traces of the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault, respectively. However, according to the analysis of the relocation aftershock depth in profile, the aftershock extension to the surface does not coincide with the surface exposure positions of the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault. Therefore, the seismogenic faults of these two earthquakes are not the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault, but two blind reverse faults. The Shuangshi-Dachuan Fault near the MS6.1 earthquake sequence and the main shock fault of the 2013 MS7.0 earthquake are thrust faults dipping northwest, while the Lushan MS6.1 seismogenic fault has opposite direction of dip. The seismogenic fault of the Lushan MS6.1 earthquake and the main thrust fault of the 2013 MS7.0 earthquake, which strikes northeast and dips northwest with the reverse thrust fault of the hanging wall, which strikes northeast and dips southeast, together form a double layer Y-shaped structure. These faults are all blind thrust faults and belong to the Qianshan-Shanqian Fault system in the southern segment of the Longmenshan fault zone. The seismogenic structure in the Lushan seismic zone is a complex fault system composed of one main northeast strike fault with dipping northwest, and three faults dipping southeast.

    From one year after occurrence of the Lushan MS7.0 earthquake to the occurrence of the Lushan MS6.1 earthquake, most of earthquakes in the Lushan seismic zone occurred near a reverse fault which is parallel to the Lushan MS6.1 earthquake seismogenic fault. These earthquakes are located in the area where the coulomb stress change caused by the MS7.0 earthquake acts as loading effect. The Lushan MS6.1 earthquake sequence is mainly distributed in the area where the coulomb stress change plays an unloading role caused by the Lushan MS7.0 earthquake. The research results showed that the coulomb rupture stress caused by the Lushan MS7.0 earthquake on the seismic nodal plane of the MS6.1 earthquake has a restraining effect on the MS6.1 Lushan earthquake.

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    CRUSTAL DENSITY STRUCTURE OF THE EASTERN TIBETAN PLATEAU AND ITS GEODYNAMIC IMPLICATIONS
    LI Dan-dan, TANG Xin-gong, XIONG Zhi-tao
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 936-951.   DOI: 10.3969/j.issn.0253-4967.2023.04.008
    Abstract424)   HTML23)    PDF(pc) (6726KB)(237)       Save

    The continuous collision and convergence between the Indian and Eurasian plates have caused strong uplift and deformation within the Tibetan plateau and the surrounding areas. The eastern Tibetan plateau, as an important channel for the eastward and south-eastward expansion of plateau materials, is an critical area for understanding the interaction between the Tibetan plateau and the eastern tectonic blocks and for understanding the eastward escape of plateau deep materials, which is of great significance for studying the uplift and deformation mechanism of the Tibetan plateau. A large number of studies on the eastern Tibetan plateau have provided an important basis for revealing the uplift mechanism of this region. However, its complex geology makes it difficult in understanding the uplift mechanism from the single geophysical interpretation. The gravity field reflects the density properties of the subsurface material, and can be related to the wave velocity properties of the seismic data by certain translation relationships. In addition, gravity data can improve the crustal model of the area not adequately covered by seismic data, which can not only provide the three-dimensional crustal density structure of the area, but also reflect the relationship between the spatial distribution of earthquakes and the crustal structure from a gravity perspective. In this paper, based on the previous research results, we selected field survey gravity data of nine intersecting lines and used the deep seismic reflection as the constraint to invert the density interface depth distribution of each line by using human-computer interaction mode, and then used the kriging interpolation method to obtain the three-dimensional Moho depth and basement depth in the area, and then we obtained the sediment thickness by analyzing the difference between the topography and the basement depth. The inversion results show that the overall trend of Moho depth is deep in the west and shallow in the east, with the deepest depth in the west being 61km and the shallowest in the east being about 40km. There is a large difference between the two sides of the arc belt formed by the Longmenshan-Anninghe-Xiaojinhe fault, with the northwest side of the arc belt basically above 52km, among which the Moho depth is about 58km in the Bayankara block and the northern part of the Chuan-Dian rhombus block, and about 53km in the Chuan-Dian rhombus block and the southern part of the Indo-China block. The Moho depth is about 42km in the Sichuan Basin on the east side of the arc belt, which constitutes a gradient zone of Moho depth around the Tibetan plateau. There also exists a depressional zone of Moho in the Bayankara block, which may be related to the eastward flow of plateau material and the blockage of Sichuan Basin, so that part of the asthenosphere material accumulates and squeezes, thus forming a relatively thicker crust and the sinking of Moho. Part of the eastward overflowing asthenosphere material turns to the south and south-east direction, resulting in the thickness of the crust in the southwest of the Chuan-Dian rhombus block is greater than the east and west sides. At the same time, the late Paleozoic mantle column activity led to the uplift of the lithosphere and the intrusion of high-density material into the lithosphere, which also blocked the southward flow of material from part of the Tibetan plateau. From the inverted sediment thickness, the sediment on the eastern Tibetan plateau is relatively thicker in the center of several tectonic blocks, up to 7km thick, while the sediment at the edges of the blocks is relatively thinner, and even bedrock is exposed in some areas. Combined with the spatial distribution characteristics of earthquakes in this area, the Moho depth and sediment thickness distribution in the eastern Tibetan plateau are strongly correlated with the distribution of earthquakes in this area, which has important reference value for future earthquake prediction.

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    EVIDENCE FOR THE HOLOCENE ACTIVITY OF THE LEIBO FAULT ZONE
    ZHANG Guo-xia, SUN Hao-yue, LI Wei, SUN Wen
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 141-161.   DOI: 10.3969/j.issn.0253-4967.2024.01.009
    Abstract315)   HTML45)    PDF(pc) (18061KB)(234)       Save

    The Yingjing-Mabian-Yanjin tectonic zone(YMYTZ)is an important boundary structure between the southeastern margin of the Tibet Plateau and the Sichuan Basin. It consists of several small-scale secondary faults with different strikes and is generally characterized by the intersections of north-northwest oriented longitudinal faults and nearly east-west oriented transverse faults. The YMYTZ is seismically very active in the late Quaternary and hosted several moderate-strong earthquakes, including two M≥7 earthquakes since 1216AD, namely the 1216 Mahu earthquake and the 1974 Daguanbei earthquake. After the Daguanbei earthquake, several M≥6 earthquakes and hundreds of M≥5 earthquakes occurred along the YMYTZ to date, implying it is a newly generated seismotectonic belt. Even so, the activity of each fault is still unclear, bringing out great uncertainty in understanding the current crustal deformation pattern and in evaluating the regional seismic potential. Specifically, although several M≥6 earthquakes have occurred along the Leibo fault zone in the southern segment of the YMYTZ, the late Quaternary activity of the fault zone has not been well determined due to insufficient work as well as subsequent lack of solid evidence. The Leibo fault zone strikes NE-SW and spreads on the southeast flank of the Chenqiangyan-Shanzhagang anticline. It starts at the Huanglang township near the Mahu Lake, cuts through the Jingkou Mountain, Lianhuashi, and Leibo, and extends southwestwards to the vicinity of Lianlajue. The latest investigation shows that the Leibo fault zone consists of four subparallel right-lateral strike-slip faults named F1—F4 from the north to the south, respectively. These fault branches together constitute a 43km-long and 10km-wide structural belt. Previous paleoseismic work along the Leibo fault zone found that the faults ruptured the late Pleistocene sedimentary layers with their upward terminations covered by the undeformed Holocene deposits, implying it was active in the late Pleistocene and has not been active since the Holocene. However, the ground surface traces of the Leibo fault zone are the most obvious among the faults in the YMYTZ, and recent seismologic studies show strong seismic activity for the Leibo fault zone, bringing out a controversy about whether it is active in the Holocene or not.

    To address the late Quaternary activity of the Leibo fault zone, we conducted detailed indoor deformed geomorphic feature interpretation on remote sensing imageries like 2m-resolution GF-2 imagery and high-resolution imageries on Google Earth, and further mapped the fault traces in the field using an unmanned aerial vehicle(UAV)derived digital orthographs and digital surface models(DSM). Based on the geological and geomorphological surveys, two trenches were excavated at Pengjiashan and Luohangou along the northern(F2)and southern(F4)branches of the Leibo fault zone respectively. On the trench walls, surface-rupturing paleoearthquakes were identified for each fault according to criteria for faulting events like cut-and-cover structures, scarps, and colluvial wedges. Subsequently, we collected and dated several radiocarbon samples from the sedimentary layers immediately before and after the rupturing events, and finally carried out stratigraphic sequence calibration using the acquired ages with the OxCal 4.4 program to constrain the timings of the revealed paleoearthquakes.

    According to the identification criteria of paleoseismic events, it was revealed 3 paleoearthquakes in the Pengjiashan trench on the northern fault branch(F2)and another 7 rupturing events in the Luohangou trench along the southern fault branch(F4). Radiocarbon sample dating constrain the ages of the paleoearthquakes along F2 to be 21190—20590BC(EP1), 20550—12120BC(EP2), and after 12090BC(EP3), while the latest two paleoseismic events on F4 occurred 9270—5040BC(EL6)and after 5000BC(EL7). Our paleoseismic studies show that the LFZ has experienced several surface-rupturing earthquakes in the Holocene, verifying it is a Holocene active fault zone. Moreover, the ages of the paleoseismic events revealed on two fault branches do not overlap with each other, suggesting they are different paleoearthquakes so that the fault branches in the Leibo fault zone are independent seismogenic structures. By collecting and analyzing the magnitudes of strike-slipping earthquakes that have generated surface ruptures in western China since 1920, it is believed that the minimum magnitudes of the paleoearthquakes determined on the Leibo fault zone are 6.5. Through the empirical relationships between magnitude and surface rupture length, it is estimated that the LFZ has the capability to produce an earthquake with M≥7.

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    INTERSEISMIC SLIP RATES AND SHALLOW CREEP ALONG THE NORTHWESTERN SEGMENT OF THE XIANSHUIHE FAULT FROM INSAR DATA
    CHEN Yi, ZHAO Bin, XIONG Wei, WANG Wei, YU Peng-fei, YU Jian-sheng, WANG Dong-zhen, CHEN Wei, QIAO Xue-jun
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1074-1091.   DOI: 10.3969/j.issn.0253-4967.2023.05.003
    Abstract320)   HTML27)    PDF(pc) (10431KB)(230)       Save

    Located in the eastern boundary of the Qinghai-Tibetan plateau, the Xianshuihe fault zone is one of the most active left-lateral strike-slip faults in Chinese mainland. As the southern boundary of the Bayanhar block, the Xianshuihe Fault accommodates the southeastward transport of material toward southeastern Asia. Earthquakes have occurred frequently along this fault, especially in the northwestern segment. More than 20 earthquakes with MW>6.0 have ruptured since 1700. The most recent MW>7 earthquake was the Luhuo earthquake in 1973, and the most recent MW>6 earthquake was the MW6.6 Luding earthquake in 2022. As one of the most active faults in mainland China, the present slip pattern of the Xianshuihe Fault, especially the shallow creep characteristics along its northwestern segment, has attracted much attention.

    The primary goal of determining slip rates of active faults using geodetic data is to quantify the seismic potential of the faults. Illuminating the long-term slip rate and shallow creep distribution of faults is the basis for calculating the seismic moment rate and evaluating the seismic potential. Due to the backwardness of early measurement methods, the seismic deformation along the Xianshuihe Fault was previously based on geologic, cross-fault short baseline and leveling surveys. With the application of GPS in tectonic geodesy, more and more GPS stations are installed near active faults, which provide accurate constraints on the long-term slip rates of the fault. Subsequently, the appearance of InSAR technology has brought a beneficial supplement to GPS, providing high spatial resolution surface velocity maps, which have been widely used to measure deep and shallow creep along active faults. It is the key to accurately characterize the fault slip behavior and evaluate the seismic potential.

    In this study, 119 Sentinel-1 satellite descent data from December 2014 to December 2021 were processed to obtain the average line-of-sight(LOS)velocity field of the northwestern segment of the Xianshuihe Fault based on the small baseline InSAR method. Then the elastic screw dislocation model was used to fit the fault normal InSAR LOS velocity profiles to estimate the long-term slip rates and shallow creep rates. Combined with the viscoelastic earthquake cycle model, the effects of the earthquake recurrence period, and rheology of the lower crust and upper mantle on slip rate estimation in Luhuo segment are analyzed. The main results are as follows:

    (1)The average InSAR LOS velocity field is in the northwestern segment of the Xianshuihe Fault during 2014—2021 has been obtained with a large range and high spatial resolution. The velocity field results show an obvious velocity gradient across the surface trace of the Xianshuihe Fault, which is consistent with the left-lateral strike-slip characteristics of the Xianshuihe Fault.

    (2)To investigate the slip rate variation along the northwestern segment of the Xianshuihe Fault, we used the two-dimensional elastic screw dislocation model to fit the 14 fault-normal velocity profiles selected along the northwestern segment of the Xianshuihe Fault and estimated the long-term slip rates and shallow creep rates using the Markov Chain Monte Carlo(MCMC)method. The results show that the overall slip rates of the NW segment of the Xianshuihe Fault range from 7.2mm/a to 11.0mm/a, and gradually decrease from west to east. The shallow creep rate ranges from 0.3mm/a to 3.1mm/a. The high creep rate appears mainly at Xialatuo and the segment from Daowu to Songlinkou. The shallow creep rates in other places are close to zero, implying that the fault is completely locked.

    (3)According to historical earthquake records, the recurrence interval of the Luhuo segment is set to be 150 years, 200 years, and 400 years, and the viscosity of the lower crust and upper mantle is set to be 5.0×1018Pa·s, 1.0×1019Pa·s, and 5.0×1019Pa·s. The slip rate of the Luhuo segment is estimated to be (7.91±0.3)~(9.85±0.4)mm/a using the MCMC method, which is slightly lower than the (10.14±0.5)mm/a obtained by the pure elastic model. In addition, when the earthquake recurrence interval is 150 years and the viscosity of the lower crust and upper mantle is 5.0×1019Pa·s, we simulate the fault-normal velocity at 5 years, 20 years, 75 years, and 125 years after the 1973 Luhuo earthquake, and find that in any period of the seismic cycle, the estimation of fault slip rate will be biased to some extent if the viscoelastic contribution of the lower crust and upper mantle is ignored.

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    MORPHOLOGIC DATING OF SCARP MORPHOLOGY BASED ON DIFFUSION EQUATION: A REVIEW
    XU Jian-hong, CHEN Jie, WEI Zhan-yu, LI Tao
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 811-832.   DOI: 10.3969/j.issn.0253-4967.2023.04.001
    Abstract506)   HTML35)    PDF(pc) (5260KB)(226)       Save

    A scarp is a common step-like landform in nature, which consists of a gently sloping plane connected to the upper and lower geomorphic surfaces of differing elevations. Common scarps include fault scarps, terrace scarps, lake shoreline scarps, shoreline scarps, volcanic ash cinder cones, etc. Scarps are often used as strain markers because of their linear characteristics and are favored in the study of active tectonics. However, it is difficult to directly constrain their ages. Instead, they are usually constrained by the ages of the upper and lower geomorphic surfaces. The scarp developed in loose deposits is controlled by a long process of low-energy degradation after a short collapse. This process can be modeled by the diffusion equation because the process can be considered as a slope process under the transport-limited condition. Under this condition, the slope can provide enough loose material for transport, that is, the material transport capacity is less than the material supply capacity. If process assumptions are sufficiently valid and rate constraints can be calibrated independently, the true age of scarps can be obtained. This method is called morphologic dating. This method has been included in many textbooks published overseas, but there have very little research on this method in China. Both linear and nonlinear models have been developed to describe scarp degradation. Linear diffusion models assume that the diffusion coefficient is a constant, whereas nonlinear transport models generally define the diffusion coefficient as a nonlinear function related to the topographic gradient. Compared to the linear transport models, nonlinear transport models can better explain the phenomenon of rapidly increasing deposition flux as the gradient approaches a critical value. In this paper, we review the study history of scarp degradation analysis and the concept model of scarp degradation. We focus on the establishment of the nonlinear model, the role of the different parameters in profile evolution, determining the best-fit age using a full-scarp nonlinear modeling procedure, and so on. Furthermore, we introduce the model of the nonlinear age chart, including the effect of far-field slope on morphologic dating of scarp-like landforms and two examples of the application of the chart, which shows that this method can correctly evaluate the ages of single-event scarps. Finally, we discuss the extension of the concept and method of the scarp degradation model, the applicability of the model, and repeated fault scarp morphological analysis. For nonlinear diffusion models, in addition to n equal to 2, two parameters (critical gradient (Sc) and diffusion constant (k)) need to be constrained. The critical gradient can be obtained from the young scarps in the study area, which roughly represents the initial state of scarp evolution, typically 0.6 to 0.7(30° to 35°). The diffusion constant needs to be characterized by a known age scarp. The slopes of the upper and lower geomorphic surfaces have an obvious influence on the morphology of a degraded scarp. These discussions indicate that both linear and nonlinear models can be used for the degradation analysis of single-event scarps, but a nonlinear diffusion model is recommended for young single-event scarps. The constant slip rate nonlinear model can be used to simulate the evolution history of<10ka high-slip rate active fault scarp. The multiple-event scarp model requires careful evaluation of the fault location and the amount of displacement per event. There are several assumptions in the scarp topography diffusion modeling, which require practice to verify its reliability. With advances in surveying technology, it is now possible to rapidly obtain high-resolution terrain data over broad areas from which numerous topographic profiles can be efficiently extracted. This provides a broad application prospect for scarp degradation analysis and morphologic dating.

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    PRECISE LOCATION AND SEISMOGENIC STRUCTURE OF THE 2022 LUSHAN MS6.1 EARTHQUAKE
    FU Ying, HU Bin, ZHAO Min, LONG Feng
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 987-1005.   DOI: 10.3969/j.issn.0253-4967.2023.04.011
    Abstract410)   HTML28)    PDF(pc) (7950KB)(225)       Save

    On June 1, 2022, a MS6.1 earthquake occurred in Lushan, Sichuan Province, western China, which is approximately 10km from the Lushan MS7.0 event on April 20, 2013. To understand if the earthquake has the same seismogenic structure as the Lushan MS7.0, we relocated the event in the Lushan area using the multi-stage locating method based on the seismic phase arrival data of the Sichuan Seismic Network from April 20, 2013, to July 1, 2022. A total of 6992 ML≥1.0 earthquakes were acquired, with a relative locating error of 0.5km and 0.7km in the horizontal and vertical directions, respectively, with a travel time residual(RMS)of 0.18s. The results show that the MS6.1 event is located at 102.943°E, 30.382°N with an initial-rupture focal depth of 15.6km, lying on the NW side of the 2013 Lushan MS7.0 event. The sub-surface rupture length of the long and short axis is 10 and 8km, measured from the dense aftershock area in NE-SW and NW-SE directions, respectively. The NE-SW profile in the Lushan area shows that the depth of Lushan MS7.0 earthquake in 2013 was about 15km, similar to that of Lushan MS6.1 and MS4.5 on June 1, 2022. The MS6.1 earthquake sequence, located at the NE end of the long axis, shows no evidence to break through the rupture termination point of the Lushan MS7.0 earthquake and enters the Dayi seismic gap, which is bounded by the 2008 Wenchuan MS8.0 and 2013 Lushan MS7.0 aftershock regions. The short-axis profile shows that the MS6.1 earthquake sequence occurred on a new back-thrust fault in the pre-existing seismogenic structure of the 2013 Lushan MS7.0. The new structure dips SE and ruptures in a slight arc protruding into the NW, parallel to the northern segment of the seismogenic structure of the 2013 Lushan MS7.0 earthquake with a horizontal distance of about 5km. The new and old structures connect at the detachment base to the main segment of the 2013 Lushan MS7.0 earthquake.

    We also inverted the focal mechanism of the Lushan MS6.1 earthquake using the CAP(Cut and Paste)method. The result indicates that the centroid depth of the MW5.7 main event is 14km which is very close to the initial-ruptured depth of 15km calculated by the phase arrival times. The best double couple parameters are 221°/40°/105° for nodal plane Ⅰ and 22°/52°/78° for nodal plane Ⅱ. The parameters are in order of the strike, dip, and rake angles. Combined with the realization of the NE-striking, SE-dipping seismogenic structure characteristics determined by the accurate locating of the earthquake sequence, it can be quickly confirmed that the nodal plane Ⅱ is the fault plane.

    Based on the accurate locating results, focal mechanism solutions, and geodynamic background of the focal area, it is inferred that the seismogenic structure of the Lushan MS6.1 earthquake is induced by the thrust dislocation of a NE-SW trending and SE inclining thrust fault in the southern section of Longmenshan fault zone. Finally, we discussed the relationship between MS7.0 and MS6.1 in the Lushan area. The two could be considered a unique sequence: the mainshock and the maximum aftershock, respectively, regarding spatial relationship and tectonic correlation. However, the time interval of these two earthquakes significantly overextends the statistical relationship between the principal earthquake and the maximum aftershock. Furthermore, considering the effects of the Coulomb stress change produced by the earthquakes repeated at the end of the Dayi gap, Lushan earthquake further enhanced the stress level in the Dayi seismic gap located in its northern segment.

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    APPLICATION OF SMALL UNMANNED AERIAL VEHICLE(sUAV)IN THE SELECTION OF SUITABLE SITES IN PALEO-SEISMIC STUDY OF BEDROCK FAULT SURFACES
    ZOU Jun-jie, HE Hong-lin, ZHOU Yong-sheng, WEI Zhan-yu, SHI Feng, GENG Shuang, SU Peng, SUN Wen
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 833-846.   DOI: 10.3969/j.issn.0253-4967.2023.04.002
    Abstract331)   HTML24)    PDF(pc) (6000KB)(215)       Save

    Bedrock normal fault scarps, as classical topographic features and geomorphological markers along mountain range fronts, form in consolidated bedrock due to faulting in extensional settings. They generally preserve more complete records of paleo-earthquakes than fault scarps in unconsolidated sediments. With the development of technologies such as fault surface morphology measurement and terrestrial cosmogenic nuclide dating, bedrock fault planes have become a nice object for paleo-earthquake study in bedrock areas. The reconstruction of paleo-seismic history from a bedrock fault scarp in terms of the times, co-seismic slips and ages by a combination of quantitative morphological analysis, TCNs dating and other physical/chemical index has been proven feasible by several previous studies.

    However, this success heavily relies on a suitable site selection along the bedrock fault scarp because erosional processes can exhume the bedrock fault surface, and the sedimentary processes can bury the bedrock fault surface. Namely, non-tectonic factors such as gully erosion, sediment burial, and anthropogenic activity make bedrock fault planes difficult to record and preserve paleo-seismic information.

    Therefore, to successfully extract paleo-seismic information from the bedrock area, it is necessary to select suitable study points along the bedrock fault scarp in advance. Traditional survey and mapping methods are time-consuming and labor-intensive, and it is difficult to understand bedrock fault scarps. The resolution of satellite images cannot obtain the fine structure of bedrock fault scarps. Small unmanned aerial vehicle(sUAV), combined with Structure-from-Motion(SfM)photogrammetry has emerged over the last decade. It is used as an established workflow in acquiring topographic data by filling the spatial gap between traditional ground-based surveys and satellite remote sensing images. As a low-altitude photogrammetry technology, it can quickly obtain high-precision three-dimensional surface structures of bedrock fault scarps.

    In this paper, taking the Majiayao bedrock fault scarp at the northern foot of Liulengshan in Shanxi Rift as an example, the high-precision and three-dimensional topographic data of the bedrock fault was obtained by using sUAV combined with SfM photogrammetry technology. The high-resolution and high-precision images of tectonic landforms can be obtained conveniently and efficiently by sUAV survey. The sUAV-obtained photos can be further processed by the SfM photogrammetry for generating a digital 3D structure of the bedrock fault scarp with true or shaded color.

    The non-tectonic factors such as rock collapse, sediment burial, and gully erosion along the bedrock fault scarp are identified by interpreting the 3D model of the bedrock fault scarp. The profile shape characteristics of the erosion, burial and tectonic fault scarps are summarized through fine geomorphological interpretation and fault profile analysis. For the erosion profile, the hanging wall slope is down-concave, showing that the fault surface below the ground surface has been partially exposed. For the bury profile, the hanging wall slope shows an obvious concave-up shape, indicating that the lower part of the bedrock fault surface has been partially buried by the colluvium. For the tectonic profile, the hanging wall slope shows a smooth and stable slope, showing the exhumation of bedrock fault scarp is controlled purely by tectonics. Finally, the study sites suitable for paleo-earthquake study on bedrock fault surfaces were selected, showing the important role of sUAV aerial survey technology in the selection of paleo-earthquake study sites in bedrock areas.

    This study illustrates that based on the high-precision three-dimensional surface structure of the bedrock fault plane from sUAV aerial survey, the existence of non-tectonic factors such as gully erosion, sedimentary burial and bedrock collapse can be clearly identified. These non-tectonic sites can be excluded when selecting suitable sites for paleo-earthquake study indoors. The shape analysis of bedrock fault scarp is also helpful to determine whether the bedrock fault surface is modified by surface process and suitable for paleo-seismic study. The sUAV aerial survey can play an important role in paleoseismic research in the bedrock area, which can accurately select the study points suitable for further paleo-seismic work in the bedrock area.

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    RELIABILITY EVALUATION OF QUARTZ ESR DATING METHOD ON EARLY PLEISTOCENE SEDIMENT: A CASE STUDY OF JINGYUAN SECTION
    WEI Chuan-yi, YIN Gong-ming, WANG Xu-long, WANG Duo, JI Hao, LIU Chun-ru, LI Xin-xiu
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1041-1056.   DOI: 10.3969/j.issn.0253-4967.2023.05.001
    Abstract316)   HTML17)    PDF(pc) (4654KB)(214)       Save

    As the most recent period of the geological record, the Quaternary climate change, tectonics and river drainage evolution have been well recorded by Quaternary sediment. Establishing the timing of these geological changes, and of their effects on the earth's environment, is a key element in Quaternary research. Because of dating range limit of quartz OSL dating and 14C dating, lacking of tephra for K-Ar dating, and strict restrictions for 26Al/10Be cosmogenic nuclide dating, the samples older than 200ka were critical but difficult in Quaternary dating, while electron spin resonance(ESR)dating method could provide absolute age for late Pliocene and Pleistocene samples. Previous studies show that quartz Al center and Ti-Li center are the most suitable signals for sediment ESR dating, and have been successfully applied into middle-late Pleistocene sediment dating. However, the application of those two centers ESR chronology into early Pleistocene or pre-Quaternary sediment remains confusion.

    In this study, early Pleistocene Jingyuan gravel layer sediment deposited at Yellow river were collected for ESR dating. The results of comprehensive comparative analysis of high resolution magneto-stratigraphy and deep-sea oxygen isotope curve of loess-paleosol sequences and high credible 26Al/10Be cosmogenic nuclide dating age make the Jingyuan gravel layer as the ideal material to evaluate the dating range, especially lower dating range, of the quartz Ti-Li center and Al center, respectively. The results show that:

    (1)The quartz Ti-Li center and Al center signal intensity of Jingyuan gravel layer was not saturated within 11 000Gy and 130 00Gy additional gamma ray dose, respectively; combined with the long thermal lifetimes of the quartz Ti-Li center(8×106a)and Al center(7.4×109a), guarantee the ESR dating range for million years.

    (2)The single saturation exponential function and “EXP+LIN” functions could provide more accuracy fitting result of equivalent dose of quartz Ti-Li center and Al center, respectively, and the fitting goodness is greater than 0.98.

    (3)The average ESR dating results of quartz Ti-Li center and Al center of Jingyuan gravel layer is~(1.67±0.15)Ma and~(1.65±0.69)Ma, respectively, which is consistent with the previously well-known age within the error range.

    To better understand the lower dating limit of the quartz ESR dating method, based on the previous analysis of the ESR signal thermal stabilities, we discuss the maximum saturation of the ESR signals and ESR signals' sensitivity. Combined with the fitting goodness evaluation of various fitting functions, we propose that the quartz Ti-Li center and Al center ESR dating method could provide reliable chronological constrains on the sand lens of early Pleistocene gravel layer. The results of our study not only provide a solid theorical foundation for the application of quartz ESR dating method for late Pliocene and early Pleistocene fluvial sediments, but also demonstrate a typical practice example of the ESR method on dating late Cenozoic sediments.

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    ANALYSIS OF BUILDING DAMAGE AND CASUALTIES OF THE 2025 DINGRI MS6.8 EARTHQUAKE IN XIZANG BASED ON FIELD INVESTIGATION
    WEI Ben-yong, ZHANG Yu-man, SHI Feng, QIAO Jun-xiang, WANG Xin, ZHANG Da
    SEISMOLOGY AND GEOLOGY    2025, 47 (1): 64-79.   DOI: 10.3969/j.issn.0253-4967.2025.01.005
    Abstract565)   HTML15)    PDF(pc) (10539KB)(209)       Save

    On January 7, 2025, at 9:05 AM, a magnitude 6.8 earthquake struck Dingri County, Shigatse City, located in the southern part of the Xizang Autonomous Region(28.50°N, 87.45°E), with a focal depth of 10 kilometers. By 7:00 PM on January 9, the earthquake had resulted in 126 fatalities and 188 injuries. A total of 27, 248 buildings were damaged, including 3, 612 collapsed structures. Timely understanding and analysis of the earthquake's damage characteristics and the causes of casualties can provide valuable references for subsequent disaster loss assessments and recovery planning.

    Based on field investigations, this study provides a comprehensive overview of the earthquake damage, covering four main aspects: seismic characteristics and affected areas, seismogenic fault and aftershock distribution, building damage and influencing factors, and the distribution and causes of casualties. The study also analyzes in detail the reasons for the severe casualties in this earthquake.

    The epicenter of the Dingri earthquake is located within the Lhasa block of the Tibetan Plateau. The earthquake was triggered by the Dengmecuo fault, a normal fault characterized by crustal extension due to fault slip. The maximum intensity of this earthquake reached IX degree, and the major axis of the isoseismal line runs nearly north-south, with a length of 191 kilometers and a short axis of 152 kilometers. The area affected by intensity VI or higher is approximately 23986 square kilometers, covering six counties and 45 towns(or streets)in Shigatse City, Xizang Autonomous Region. The earthquake caused a surface rupture of approximately 26 kilometers, with a maximum vertical displacement of about 3 meters.

    Field investigations revealed that the building structures in Dingri County mainly consist of frame, masonry, and traditional civil structures. Among these, traditional civil structures sustained the most severe damage. In extremely and severely affected areas, the majority of civil-structure buildings were either destroyed or severely damaged, with complete or partial collapses occurring. The main factors contributing to the severe damage to civil-structure buildings include the lack of seismic resistance measures, poor construction techniques, and inadequate shear resistance and bond strength of construction materials.

    The majority of casualties were concentrated in Changsuo, Cuoguo, and Quluo towns, near the epicenter. Changsuo town suffered the most severe damage, with casualties accounting for 74.60% of the total fatalities. The high casualty rate can be attributed to the strong destructive power of the earthquake, the proximity of villages to the fault lines, low seismic performance of buildings, high population density, and adverse environmental conditions such as low temperatures and oxygen deficiency.

    Based on the analysis of the causes of casualties and field investigations, this study proposes targeted countermeasures and suggestions to mitigate earthquake disaster risks and minimize casualties in Xizang. These measures include enhancing active fault detection, improving earthquake early warning capabilities, reducing seismic damage risks to traditional residential buildings, strengthening emergency response measures, mitigating the risk of secondary earthquake disasters, and increasing public awareness of earthquake risks. These recommendations aim to enhance the region's earthquake prevention and mitigation capabilities and provide guidance for post-disaster recovery and reconstruction.

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    THE NEW FINDINGS OF SURFACE RUPTURE ZONES AND ITS SEISMOLOGICAL SIGNIFICANCE OF THE EASTERN MARGIN OF YUMUSHAN FAULT, NORTHEASTERN MARGIN OF QINGZANG PLATEAU
    CHEN Bai-xu, YU Zhong-yuan, XIAO Peng, DAI Xun-ye, ZHANG Shi-long, ZHENG Rong-ying
    SEISMOLOGY AND GEOLOGY    2024, 46 (3): 589-607.   DOI: 10.3969/j.issn.0253-4967.2024.03.005
    Abstract300)   HTML35)    PDF(pc) (20130KB)(208)       Save

    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.

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    ACTIVITY PERIOD, FORMATION MECHANISM AND TECTONIC SIGNIFICANCE OF THE KALAYU’ERGUN FAULT IN THE WESTERN KUQA DEPRESSION
    SHEN Bai, ZHANG Zhi-liang, REN Zhi-kun, LIU Jin-rui
    SEISMOLOGY AND GEOLOGY    2023, 45 (6): 1247-1264.   DOI: 10.3969/j.issn.0253-4967.2023.06.001
    Abstract318)   HTML43)    PDF(pc) (5216KB)(208)       Save

    As the NW-trending dextral strike-slip fault on the northern margin of the Tarim Basin, the Kalayu’ergun Fault defines the western boundary between the western Kuqa Depression and Wensu Bulge. It holds immense importance to understand the deformation occurring within the Kuqa Depression. However, there is still ongoing debate regarding the length, activity time and formation mechanism of the Kalayu’ergun Fault. In this study, a comprehensive investigation was conducted, incorporating sub-surface geophysical data, high-resolution remote sensing satellite images, and the findings of previous researchers. The results demonstrate that the Kalayu’ergun Fault cuts off the Awate anticline in the north, and to the south, it extends near the southern flank of the North Kalayu’ergun anticline but does not reach the Middle Kalayu’ergun anticline. The total extension of the fault is estimated to be approximately 40km. And the minimum of the fault strike-slip distance is estimated by the sum of the tectonic shortening of the North Kalayu’ergun anticline and the shortening absorbed by the strata on the northern flank of the Awate anticline through drag, which amounts to about 4.1-4.3km. Additionally, the Kalayu’ergun Fault has been active since its formation in the early Pliocene, but its activity intensity has been weakened obviously. The activity of the Kalayu’ergun Fault corresponds to the deformation time of the North Kalayu’ergun anticline, which is consistent with the deformation time determined using the same structural sedimentary constraints. This indicates that the North Kalayu’ergun anticline was formed under the combined action of near north-south compressional and horizontal shear stresses. The development of this transverse fault is synchronous with the overthrust structures on both sides and is developed in synchrony with the strong uplift of the southern Tian Shan orogenic belt since the late Cenozoic. The formation of the Kalayu’ergun Fault can be affected not only by the differences in the basement nature on both sides but also closely related to the difference in the thickness of the gypsum salt layer. The former resulted in variations in horizontal shortening on both sides of the fault, leading to the tearing of the Cenozoic sedimentary cover. The latter, which under the action of the extrusion stress, influenced the generation and evolution of salt-overlying beds, and then influenced the formation of the fault. In addition, the existence of prior salt structures, also known as salt diapirs, may have also played an important role in the formation of the fault. As the boundary fault in the western part of the Kuqa Depression, the Kalayu’ergun Fault is responsible for accommodating crustal shortening on both sides and even in the whole eastern and western parts of the Kuqa Depression. As a result, the shortening of the Kuqa Depression gradually decreased from east to west. Furthermore, the Kalayu’ergun Fault also had significant impacts on geomorphology, as it controls and modifies the landscape in the southern Tian Shan foreland basin. In the meanwhile, the Kalayu’ergun Fault creates favorable conditions for the transportation and accumulation of oil and gas resources.

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    DEEP STRUCTURAL CHARACTERISTICS IN THE XINGTAI SEISMIC ZONE REVEALED BY PRECISE RELOCATION OF SMALL EARTHQUAKES AND TOMOGRAPHY
    LIU Kang, YANG Ting, LI Hong-guang, FANG Li-hua, SONG Jian
    SEISMOLOGY AND GEOLOGY    2023, 45 (6): 1328-1348.   DOI: 10.3969/j.issn.0253-4967.2023.06.005
    Abstract269)   HTML32)    PDF(pc) (11306KB)(206)       Save

    From March 8th to 29th, 1966, five earthquakes(M≥6)occurred in the Xingtai area, with the MS6.8 earthquake on March 8th and the MS7.2 earthquake on March 22nd being the most severely damaged. The Xingtai earthquake resulted in over 8 000 deaths and the economic losses up to 1 billion yuan. The Xingtai earthquake has opened the scientific practice of earthquake prediction in China and is a milestone in the development of earthquake science in China.

    Based on previous research results, there is a deep fault beneath the Xingtai earthquake area, which is the energy source of earthquakes, while there is a relatively independent fault system in the shallow part, which is generally recognized by scholars. However, the divergence regarding the seismogenic structure of the Xingtai earthquake mainly focuses on the unclear coupling relationship between the deep and shallow structural systems in the seismic area. The structural relationship between deep seismic faults and the shallow Xinhe Fault system requires new evidence to determine. In addition, previous scholars have proposed the viewpoint of “Newly generated Fault”, which can better explain the rupture characteristics of the Xingtai earthquake, but it still needs to be supported by the inversion results of the seismic rupture process based on the three-dimensional crustal fine structure. There are many small earthquakes in the Xingtai area. Deep structural information can be obtained using small earthquake data. Especially after 2009, the significant improvement in earthquake positioning accuracy in North China has made it possible to obtain new insights into deep structures. By locating small earthquakes, the spatial distribution and motion characteristics of faults are characterized, and seismic travel time tomography reveals the deep crustal velocity structure characteristics of the earthquake area. Combining previous geophysical exploration results, conducting deep and shallow structural analysis is of great significance for studying the spatial distribution, motion characteristics, and coupling relationship between deep and shallow structural systems of the fault system in the study area. The continuous aftershocks after the 1966 MS7.2 earthquake in Xingtai, Hebei Province, have provided favorable conditions for conducting studies on deep tectonic structures in the region.

    In this paper, based on the observation data of the Hebei seismostation from 1991 to 2021, we obtained the precise position results of 9 644 earthquakes in Xingtai and its neighboring area using the double-difference positioning method, and depicted the spatial patterns of deep ruptures. Based on the observation data of the North China Mobile Seismic Array from 2006 to 2008, 38 578 P-wave arrivals were used to obtain high-resolution travel time tomography results in the study area. This study shows that there are strong lateral heterogeneities in the velocity structure of the crust in the study area, with obvious low-velocity anomalies in the upper crust and high-velocity anomalies in the middle and lower crusts between the Xinhe Fault and the Yuanshi Fault, and the Xingtai earthquake is located at the junction of the high- and low-velocity anomalies, which has the medium conditions for accumulating large amounts of strain energy and is prone to rupture and stress release. The general trend of the dense zone of small earthquakes in the Xingtai earthquake area is relatively consistent with that of the eastern boundary of the high- and low-velocity anomalies. It is assumed that the deep and shallow fractures spreading along the eastern boundary of the high- and low-velocity bodies have been connected up and down and that the boundary of the anomalies is also a part where velocity changes are relatively strong and easily lead to seismic rupture; the results of various seismic and geological surveys have revealed that a deep major rupture that cuts through the entire crust exists beneath the Xingtai earthquake zone, with SE tendency and the upper breakpoint located near Dongwang, and the Xingtai earthquake prompted the deep and shallow pre-existing ruptures to connect from top to bottom.

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