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    Seismogenic fault and coseismic surface deformation of the Dingri Ms 6.8 earthquake in Tibet, China
    SEISMOLOGY AND GEOLOGY    2025, 47 (1): 1-15.  
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    REGIONAL VELOCITY STRUCTURE AND RELOCATION OF THE 2022 DONGTAI EARTHQUAKE SEQUENCE
    YU Yue-ying, LI Zheng-kai, YANG Yun, KANG Qing-qing, QIAN Jia-wei, WANG Jun-fei, QU Min, ZHOU Yu-chen, LI Ying-chun, XU Tian
    SEISMOLOGY AND GEOLOGY    2024, 46 (3): 627-644.   DOI: 10.3969/j.issn.0253-4967.2024.03.007
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    The seismic activity in Dongtai, Jiangsu, suddenly intensified from November to December 2022. The largest earthquake observed during this period was a magnitude MS3.0 event on 25 December, which was felt reported by many nearby residents and caused a certain degree of social impact. The Dongtai area is situated in the central part of Jiangsu Province, within the Dongtai depression in structure, which is a secondary tectonic unit in the southern part of the Subei-South Yellow Sea Basin. Multiple fault zones developed in the region. The prominent known fault zones near the epicenter include the Taizhou fault, the Chenjiabao-Xiaohai Fault, and the Benchahe Fault. Among them, the closest to the epicentral area is the Taizhou fault. Additionally, the Subei Basin has a long history of industrial activity. Its geological conditions are complex, and the resources are extremely scattered and fragmented. The scale of underground resource extraction is predominantly small to medium-sized and has entered the middle to high exploration level. Historically, Dongtai has experienced weak seismic activity with only six earthquakes of MS≥3 within 50 kilometers of the epicenter since 1970. The sudden increase in seismic activity prompts investigation into its cause. Analyzing the structural features of the Dongtai earthquake sequence can enhance understanding of seismic activity and seismogenic mechanisms in the region.

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

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

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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
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    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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    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
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    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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    Characteristics of Pb isotopes of fluvial detrital K-feldspar in the Yellow River Basin and its geological significance as provenance tracing
    SEISMOLOGY AND EGOLOGY    0, (): 0-0.  
    Abstract419)            Save
    Tracing sediment sources in the Yellow River Basin is of great importance for understanding the coupling relationship between uplift and denudation of the Tibetan Plateau and marine sedimentation in the western Pacific margin. K-feldspar is one of the common rock-forming minerals in river sediments, and its Pb isotopic compositions are effective tools in the provenance tracing research of large rivers. However, this research has not been carried out in the Yellow River Basin. In situ Pb isotopes of 967 K-feldspar samples were obtained by laser erosion inductively coupled plasma mass spectrometer(LA-ICP-MS). The results of 206Pb/204Pb and 208Pb/204Pb ratios showed that the Pb isotopic compositions of K-feldspar grains in the Yellow River, Daxihe River and Huangshui River in Maduo-Tongde section were significantly different from those in Lanzhou section. The Pb isotopic composition of K-feldspar in Lanzhou section of the Yellow River is consistent with that in Bayannur section of the Yellow River, which are influenced by similar eolian provenance. K-feldspar grains from the Yellow River and Fen River in the Jinshan-Shaanxi Gorge are mainly from the Loess Plateau. The K-feldspar grains in the Weihe River mainly derive from the Qinling Mountains. The Pb isotopic compositions of K-feldspar grains in the Kaifeng and Lijin sections of the Yellow River are different to those in the upper reaches of the Yellow River and the North China Plate, but similar to those in the middle reaches of the Yellow River. The Loess Plateau plays a leading role in the source of K-feldspar gains in the middle and lower reaches of the Yellow River.
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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
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    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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    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
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    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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    THE DISASTER MECHANISM OF THE MS6.9 EARTHQUAKE IN MENYUAN, QINGHAI PROVINCE, 2022
    NIU Peng-fei, HAN Zhu-jun, GUO Peng, LI Ke-chang, LÜ Li-xing
    SEISMOLOGY AND GEOLOGY    2024, 46 (4): 761-782.   DOI: 10.3969/j.issn.0253-4967.2024.04.001
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    Earthquake disasters are one of the most significant natural disasters faced by human society. Understanding and mitigating earthquake disasters have always been a key focus of research for seismologists. Conducting investigations on post-earthquake seismic disasters is of great significance for the recovery and reconstruction of disaster-stricken areas, as well as for earthquake prevention and mitigation. Earthquake disasters can be classified into two types based on their mechanisms: one is the destruction caused directly by the seismic vibrations on buildings, lifelines, and other structures; the other is the damage related to geological hazards triggered by earthquakes. The former is mainly related to the density of regional economic layout; the latter seismic geological disasters typically include collapses, landslides, debris flows, ground fissures, ground subsidence, and soil liquefaction. These geological disasters often exacerbate the impact of seismic disasters, posing a more significant threat to human life and property safety. Therefore, it is of great significance to investigate the mechanisms of significant engineering disasters caused by earthquakes, as it can provide important insights for engineering recovery, reconstruction, and site selection. The Qilian-Haiyuan fault zone is an important boundary fault on the northeastern margin of the Qingzang Plateau. It plays a crucial role in absorbing and accommodating the convergence of the Indian Plate towards the Eurasian Plate in a NNE direction. With a total length of approximately 1 000km, it is primarily composed of the Tolaishan fault, the Lenglongling fault, the Jinqianghe fault, the Maomaoshan fault, the Laohushan fault, and the Haiyuan fault, from west to east. On January 8, 2022, a magnitude 6.9 earthquake occurred near the stepover of the Longling and Tuolaishan faults of the Qilian-Haiyuan fault zone. Although the earthquake occurred in uninhabited, sparsely wooded alpine grasslands and did not cause any casualties, it completely destroyed the Liuhuanggou bridge and the south-side Daliang tunnel on the Lanzhou-Xinjiang high-speed railway, a major artery of the Silk Road transportation network in China. This marks the first time that a mainline of the high-speed railway network, which is a showcase of China's economic achievements, has been entirely disrupted by earthquake damage. Based on the high-resolution orthophoto images and digital elevation models(DEMs)obtained through post-earthquake emergency scientific investigations using the unmanned aerial vehicles, this article conducted another field investigation on earthquake disasters in vehicles; this article conducted another field investigation on earthquake disasters in the isoseismal area. First, by investigating geological disasters such as collapses, landslides, and soil liquefaction in the meizoseismal area, as well as the damage to buildings and structures. Then, based on field surveys, a detailed mapping of the reverse-type surface ruptures formed by the Mengyuan earthquake was conducted, identifying the distribution patterns and geometric and kinematic characteristics of the surface ruptures and determining the distribution of coseismic vertical displacements. Additionally, the development of geological disasters caused by this earthquake was analyzed, and the disaster-causing mechanism of the Liuhuang Bridge was discussed. The research indicates that the Liuhuanggou River, located in the isoseismal area, does not exhibit large-scale earthquake landslides and collapses. Instead, only smaller-scale rockfalls and accumulations of rolling stones, as well as localized occurrences of sand liquefaction in certain riverbeds, are observed, which is clearly inconsistent with expectations. In addition to the formation of two strike-slip surface rupture zones, the earthquake also generated a reverse-type surface rupture zone approximately 7.9km long within the Liuhuanggou river on the northern side of the western section of the Lenglongling fault. The rupture zone exhibits an unstable southward trend and is primarily composed of discontinuous arc-shaped compressional ruptures, mole tracks, tensile ruptures, and seismic scarps. Along the surface rupture zone, a total of 35 vertical displacement measurements were obtained, with the minimum displacement of (8±1)cm and the maximum displacement of (49±3)cm. The average vertical displacement is approximately 24cm, and the displacement distribution along the strike is uneven. The surface rupture zone, which cuts nearly vertically across the Lanzhou-Xinjiang high-speed railway Liuhuanggou bridge, has caused extensive surface deformation and displacement. This is the direct cause of the destruction of the Liuhuanggou bridge. This finding suggests that when implementing seismic engineering design measures for major linear projects crossing fault zones, it is important to consider the extensive shear effects of reverse-type surface rupture zones.

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

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

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

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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
    Abstract301)   HTML14)    PDF(pc) (5142KB)(251)       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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    SEISMOLOGY AND EGOLOGY    0, (): 0-0.  
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    GRAVITY CHANGES BEFORE THE PINGYUAN MS5.5 EARTHQUAKE OF 2023
    LI Shu-peng, HU Min-zhang, ZHU Yi-qing, HAO Hong-tao, YIN Hai-tao, JIA Yuan, CUI Hua-wei, LU Han-peng, ZHANG Gang, WANG Feng-ji, LIU Hong-liang
    SEISMOLOGY AND GEOLOGY    2024, 46 (5): 1172-1191.   DOI: 10.3969/j.issn.0253-4967.2024.05.010
    Abstract296)   HTML15)    PDF(pc) (15055KB)(107)       Save

    On August 6, 2023, an earthquake with MS5.5 occurred in Pingyuan County, Dezhou City, Shandong Province, which is the largest earthquake in the Shandong region in the past 40 years. Before the earthquake, Shandong Earthquake Agency conducted biannual mobile gravity measurements near the epicenter, observed the spatiotemporal gravity field changes for the four years leading up to the earthquake, and made a certain degree of medium-term prediction, predicting that the epicenter location(36.00°N, 116.10°E)would be about 130km from the actual epicenter. This suggests that it is potentially feasible to carry out medium-term prediction of moderate earthquakes based on the temporal and spatial variations of the gravity field in the tectonically weak North China. Therefore, the study of the gravity changes before the 2023 Pingyuan MS5.5 earthquake can help to deepen the understanding of the relationship between the time-space variations of the gravity field and the moderate earthquakes, enrich the database of “magnitude and gravity anomalies” in North China, and improve the science and accuracy of identifying and determining the medium- and long-term anomalies of earthquakes.

    The mobile gravity data utilized in this paper were processed and calculated using the classical adjustment method in LGADJ software. This process involved corrections for earth tide, instrument height, monomial coefficient, air pressure, and zero drift, resulting in absolute gravity values for each measurement point. Eight absolute gravity points, including Jiaxiang, Tai'an, and Zibo, served as the starting reference points. The average accuracy of the observed data point values during each period ranged from 8.5 to 16.0μGal, indicating relatively high precision. Subsequently, the calculation results of the two data sets were subtracted to obtain the relative gravity change. This change was then interpolated on a continuous grid using the Surface module of GMT mapping software and subjected to 50-km low-pass filtering. Finally, the dynamic evolution image of the gravity field was generated.

    Based on these results, this study analyzes the characteristics of regional gravity field changes since September 2019. These findings are integrated with information on deformation fields, seismic source mechanisms, and dynamic environments to explore the relationship between gravity changes before the earthquake and the seismic mechanism. The results indicate the following:

    (1)Since May 2022, precursory anomalies have been detected in the gravity field changes around the epicenter. Between May 2022 and April 2023, there was a significant increase in positive gravity changes exceeding +50μGal and a spatial extent exceeding 160km in the south of the epicenter, with positive-negative differences exceeding 70μGal on both sides of the epicenter. However, the gravity changes near the epicentre remained stable and in a “locked” state. The magnitude, range, and duration of gravity changes before the earthquakes align with previously summarized indicators.

    (2)Between September 2021 and September 2022, distinct four-quadrant distribution characteristics emerged in the regional gravity field changes. And the spatial distribution of regional gravity field changes corresponds to horizontal deformation fields, seismic source mechanisms, and coseismic displacement fields. Precisely, the compression zones of the seismic source mechanism and the inflow and subsidence areas of the coseismic displacement field correspond to regions of surface compression and gravity decrease before the earthquake. Similarly, the expansion zones of the seismic source mechanism and the outflow and uplift areas of the coseismic displacement field correspond to of surface expansion and gravity increase before the earthquake.

    (3)The leading cause of the gravity changes anomaly before the Pingyuan MS5.5 earthquake was the migration of deep-seated fluid materials, with the gravity effects generated by upper crustal deformation being a secondary factor. It is believed that the subduction of the Pacific Plate caused high-speed eastward migration of the relatively weak lower crust flow, dragging the upper crust eastward. The more rigid upper crust accumulated stress and strain during this process, developing numerous micro-fractures, while tectonic heterogeneity led to an east-west compression and north-south extension pattern. The fluid migration from compressed to expanded areas caused positive and negative differential changes in the gravitational field around the epicenter, culminating in the earthquake.

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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
    Abstract293)   HTML27)    PDF(pc) (2735KB)(238)       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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    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
    Abstract285)   HTML17)    PDF(pc) (4880KB)(285)       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 DEEP SEISMIC REFLECTION PROFILE UNVEILS FINE STRUC-TURE AND TECTONIC CHARACTERISTICS OF THE CANGXIAN UPLIFT, HUANGHUA DEPRESSION, AND ADJACENT AREAS
    QIN Jing-jing, LIU Bao-jin, FENG Shao-ying, XU Xi-wei, TIAN Yi-ming, ZHU Guo-jun, ZUO Ying
    SEISMOLOGY AND GEOLOGY    2024, 46 (3): 608-626.   DOI: 10.3969/j.issn.0253-4967.2024.03.006
    Abstract280)   HTML16)    PDF(pc) (13832KB)(140)       Save

    A comprehensive seismic profiling study was conducted across the Jizhong depression, Cangxian uplift, Huanghua depression, and Chengning uplift in the North China Plain to investigate crustal fine structure and the relationship between deep and shallow faults. Two profiles were completed: a deep seismic reflection profile spanning approximately 200km and a middle-shallow seismic reflection profile covering about 66km.

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

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

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

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    STUDY ON THE EFFECT OF EXCESS TOPOGRAPHY ON LANDSLIDES INDUCED BY LUDING MS6.8 EARTHQUAKE IN 2022
    QIU Heng-zhi, MA Si-yuan, CHEN Xiao-li
    SEISMOLOGY AND GEOLOGY    2024, 46 (4): 783-801.   DOI: 10.3969/j.issn.0253-4967.2024.04.002
    Abstract274)   HTML17)    PDF(pc) (14792KB)(89)       Save

    Strong earthquakes in mountainous regions are prone to triggering severe geological disasters, such as landslides, collapses, and debris flows. These disasters are characterized by their wide distribution, large scale, and high frequency, making them among the most destructive secondary effects of earthquakes. In recent years, the central and eastern parts of the Qinghai-Tibet Plateau have experienced frequent strong earthquakes, leading to varying degrees of earthquake-induced landslide disasters.
    Landscape evolution is significantly influenced by tectonic activities and river incising, which alter the materials of hillslopes and their topographic characteristics. Earthquakes can significantly affect the spatial distribution of co-seismic landslides, particularly in areas with excess topography. In tectonically active zones, rock uplift and river erosion gradually increase slope angles and decrease slope stability. Weathering, freeze-thaw cycles, and seismic vibrations can reduce rock strength, leading to slope erosion and landslides. Once these processes occur, the slope tends to reach a critical state of stability, which is characterized by the presence of excess topography. Landslides can rapidly reduce hillside elevations, limiting terrain relief and impacting landform evolution. Excess topography, defined as rock mass inclined at angles greater than a specified threshold, is closely related to unstable slope masses. The essence of a landslide is the disruption of slope equilibrium, often reflected in the presence of excess topography. However, the influence of excess topography on the distribution of co-seismic landslides is not well understood.
    Earthquake-induced landslides occur when slopes become unstable and slide due to seismic forces. The instability arises when ground motion alters the internal friction angle and cohesion forces along rock mass defects, making them unable to resist the gravitational forces that cause sliding. The weight of the slope material plays a crucial role in this process. As a key component of landscape evolution, landslides significantly shape geomorphic forms, as indicated by the presence of excess topography. The undulating terrain of a region is the result of long-term structural and surface erosion interactions, as well as material migration and distribution. Landslide development is closely related to the local environment, particularly geomorphic conditions. Seismic landslides also play a vital role in shaping and reorganizing active orogenic belts, influencing subsequent landscape evolution.
    On September 5, 2022, a MS6.8 earthquake struck Luding county, Sichuan province, China, with the epicenter in Hailuogou Glacier Forest Park(29.59°N, 102.08°E), at a focal depth of 16km and a maximum intensity of Ⅸ degrees. The earthquake, lasting approximately 20 seconds, was strongly felt across many parts of Sichuan Province and induced numerous landslides, causing significant damage. The affected area, located at the transition between the Qinghai-Tibet Plateau and the Sichuan Basin, features a typical alpine and canyon landscape with steep terrain and river incision, providing favorable conditions for landslides. The long-term and intense tectonic activity in the eastern Qinghai-Tibet Plateau has resulted in complex topography and geomorphology, providing the material basis and external conditions for earthquake and landslide disasters.
    With advancements in science and technology, the Digital Elevation Model(DEM)has become widely used in geoscience research. As DEM accuracy improves, its applications have evolved from qualitative descriptions of geomorphic morphology to semi-quantitative and quantitative analyses of various geomorphic parameters. Geomorphic parameters reveal the structural geomorphic information within topography, essential for understanding regional characteristics and evolution mechanisms. The Luding earthquake serves as a case study for analyzing the influence of topography on the distribution of co-seismic landslides.
    In this study, through post-earthquake remote sensing image analysis we identified 1 485 landslides(covering approximately 14.83km2)and analyzed their spatial distribution. Field surveys revealed that most co-seismic landslides are shallow, with relatively small thicknesses, primarily located along the Dadu River. Excess topography calculations based on the ALOS 12.5m DEM and subsequent quantitative analysis of its correlation with co-seismic landslides indicate a strong relationship: with a 30° threshold, excess topography peaks are found along the Dadu River and its tributaries, coinciding with the majority of landslide occurrences. A total of 91.7% of co-seismic landslides are within areas of varying excess topography heights. However, the average height of excess topography in landslide areas(~80m)significantly exceeds landslide thicknesses, suggesting that the Luding earthquake only mobilized a small fraction of the total excess topography. The remaining excess topography may represent potential unstable slopes for future landslides. Furthermore, the spatial distribution of landslides induced by previous earthquakes, such as the Wenchuan earthquake in 2008, the Lushan earthquake in 2013, and the Ludian earthquake in 2014, shows a high degree of consistency. This underscores the importance of understanding the relationship between excess terrain and landslide distribution to enhance the accuracy of earthquake-induced landslide predictions.

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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
    Abstract268)   HTML9)    PDF(pc) (3308KB)(198)       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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    THE SPATIAL AND TEMPORAL CHARACTERISTICS OF PRESENT-DAY SEISMICITY IN NORTHEASTERN LONGMENSHAN FAULT ZONE
    HU Nan, LONG Feng, WANG Ying, XU Liang-xin
    SEISMOLOGY AND GEOLOGY    2024, 46 (4): 856-875.   DOI: 10.3969/j.issn.0253-4967.2024.04.006
    Abstract264)   HTML21)    PDF(pc) (6435KB)(141)       Save

    The Longmenshan fault zone, situated along the eastern margin of the Tibetan plateau, represents a significant thrust tectonic belt characterized by pronounced segmentation. It is delineated into northern and central-southern segments at Beichuan, and along its depth, it features three sub-parallel fault belts: the Houshan fault, the Central fault, and the Qianshan fault, extending from the northwest to the southeast. Geological research indicates that since the Quaternary, the central-southern segments of the Longmenshan fault zone have exhibited considerable seismic activity, whereas the northern segment has shown minimal signs of movement. However, paleo-earthquake studies have identified substantial historical seismic events in the Qingchuan fault, a component of the northern segment, dating back to the Holocene. The devastating 2008 Wenchuan earthquake(MS8.0), which occurred in the middle section of the Longmenshan fault zone, resulted in a 240-km-long surface rupture along the Central fault. A multitude of aftershocks radiated northward from the epicenter, with no discernible surface ruptures observed in the northern segment. This study aims to provide a comprehensive analysis of the kinematic features of the northern segment by re-evaluating the Wenchuan earthquake's aftershocks and employing focal mechanisms derived from previous studies.
    Seismic activity is intrinsically linked to active tectonics, and the precise localization of minor earthquakes can offer critical insights into the underlying seismogenic processes and mechanisms. In this paper, we have compiled early aftershock relocation data and further refined the relocation of small earthquakes using an integrated seismic location technique. Seismic phase data were obtained from the networks in Sichuan, Gansu, and Shaanxi over the past decade, spanning from 2010 to 2020. To mitigate the impact of crustal velocity variations, an optimal one-dimensional velocity model for the study area was initially inverted using the VELEST program. The Hypo2000 program was then utilized to adjust the initial seismic source positions, followed by the application of the double-difference method for the relocation of minor earthquakes. The reliability of the localization outcomes, determined using the LSQR method, was verified by the SVD method. Consequently, 10 653 minor earthquakes were relocated with an average travel time residual of 0.053s, a horizontal location error of 281m, and a vertical location error of 260m.
    In the southern extremity of the study area, the relocated earthquakes are predominantly aligned along the parallel faults flanking the primary rupture zone. In the south-central region, the relocated earthquakes exhibit deviations from the rupture zone, revealing multiple seismic clusters. Towards the northern end, the relocated earthquakes demonstrate a migration from the main rupture towards the Qingchuan fault. The depth profiling of seismic sources reveals that the relocated earthquakes are concentrated between 8-15km deep, all situated above the 500℃ isothermal surface. The depth profile in the southern region continues the characteristics of the main rupture surface of the Wenchuan earthquake, while the dip angle becomes increasingly steep as it progresses northward. The northern end's depth profile suggests an interaction between the rupture surface and the Qingchuan fault. Additionally, the analysis of 32 focal mechanisms exceeding ML4.0 within the study area corroborates the geometrical structures of the fault zone, as revealed by the spatial distribution of the relocated earthquakes, further validating the reliability of relocation.
    A comprehensive analysis suggests that the current seismicity in the northern section of the Longmenshan fault zone is multifaceted, with ongoing activity on the main rupture surface(afterslip)and slip on secondary new rupture surfaces triggered by the mainshock. It is hypothesized that the spatial distribution of the relocated earthquakes retains segmented characteristics. In the southern region of the study area, thrust slip induced by the main rupture continues; in the middle region, new ruptures are concurrently active with the main rupture; and in the northern region, influenced by the high velocity of the upper crust around Ningqiang-Mianxian, the rupture zone vanishes at the surface, with the deep triggering of the Qingchuan fault by stress transfer being evident. In conclusion, the complex spatial characteristics of the current seismic activity in the northern section of the Longmenshan are attributed to the interplay of pre-existing faults, new ruptures, and the main rupture, reflecting the spatially heterogeneous process of stress transfer and adjustment following the Wenchuan earthquake, potentially linked to the complex geological structure of the region.

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    GENESIS OF SONGYUAN EARTHQUAKES BASED ON 3D RESIDUAL DENSITY STRUCTURE
    LIU Wen-yu, CHENG Zheng-pu, NIAN Xiu-qing, CHEN Yan, HU Yu-ling, QIN Zu-jian, SHAO Ming-zheng
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 462-476.   DOI: 10.3969/j.issn.0253-4967.2024.02.013
    Abstract262)   HTML16)    PDF(pc) (7342KB)(158)       Save

    Since 2006, more than 10 earthquakes with the magnitude of 4 or higher have occurred continuously in Ningjiang district and Qian’an county, Songyuan city of Jilin province, with tens of thousands of aftershocks. The largest earthquake is the magnitude 5.8 earthquake on November 23, 2013 in Qiangorlos Mongolian autonomous county, Songyuan city, Jilin province. Frequent earthquake activities not only cause a large number of damaged buildings, but also trigger geological disasters such as sand liquefaction and slope instability, which have attracted widespread attention from the public and government departments. In order to comprehensively understand the causes of earthquakes in the Songyuan area, much studies on earthquake anomalies, seismogenic structures, and source depths have benn conducted recently. However, there are still some difference opinions in understanding the causes of earthquakes in the Songyuan area, for example, the major different viewpoints include: 1)it is caused by the regional stress release caused by Pacific subduction, mainly based on the consistency between the stress field in the source area and the background stress field of the Pacific plate subducting towards the edge of the east Asian continent; 2)it is related to deep melt or fluid migration, mainly based on the existence of low-speed and low resistance anomalous bodies in the deep part of the seismic area; 3)it is related to long-term oil and gas extraction, the main basis is that the seismic source spectrum exhibits characteristics of early and fast attenuation compared to typical structural earthquakes, and the time-domain and frequency-domain characteristic parameters such as waveform complexity and spectral ratio are also significantly different from typical structural earthquakes. In addition, the seismic source mechanism contains a large number of nondual force source components and shallow seismic source depth. Moreover, there is also significant controversy over the seismogenic structure, with the focus on whether the Qian’an and Ningjiang earthquake regions are controlled by the same fault zones. Some scholars infer that the seismogenic structures in both areas are the NE-trending Fuyu-Zhaodong Fault based on the source mechanism solution. Another group of scholars believe that the Qian’an earthquake area is controlled by NW-trending hidden faults, while the seismic structure in the Ningjiang earthquake area is the second NW-trending Songhuajiang Fault. The reason for the above controversy lies in the lack of necessary constraints on the deep structure of the seismic area.

    To solve above problems, in this article we studied the three-dimensional residual density structure of the Songyuan earthquake area by performing nonlinear conjugate gradient focusing inversion on regional gravity data. Combined with oil drilling and reflection seismic data, some new insights were obtained as follows: 1)The residual density anomaly in the study area shows a high-low alternating strip distribution, with the southern anomaly trending NNW and the northern anomaly transitioning to NNE, This feature reflects the different deep earthquake environments in the Qian’an earthquake area and the Ningjiang earthquake area. The former’s source is located in the high-density anomaly body and its edge of Chaganhua, while the latter’s source is located in the middle of the low-density anomaly zone in Songyuan, indicating different rock types in the two locations. The basement of the Qian’an earthquake area is composed of limestone and metamorphic volcanic rocks, while the basement of the Ningjiang earthquake area is composed of fractured granite magmatic rocks. The stability of the basement structures in both areas is poor; 2)The seismic structures of the two are different. The former is controlled by the Chaganhua Fault and Qian’an Fault, while the latter is controlled by the Songyuan Fault and the Fuyubei Fault; 3)The formation of earthquakes is related to factors such as regional stress, basement structure, deep gas/fluid migration, and long-term oil and gas extraction. Long term water injection and oil recovery have damaged geological structures and stress environments, which may be important triggering factors. 4)In terms of the seismic source mechanism, we concluded that under the sustained action of regional stress in the near EW direction in the Songyuan area, the basement fault structures and fractures in the Qian’an and Ningjiang earthquake areas are extremely developed. Deep gas/fluid migrates upwards along the basement fractures, further reducing the stability of the basement structure. Long term water injection, oil and gas recovery, as well as fracturing, greatly damage the structural integrity of the cap rock and the distribution of deep stress, which results in the compressive and torsional strike slip movements in the relevant structures.

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    3D MODELING AND MAXIMUM POTENTIAL SEISMIC ASSESS-MENT OF THE EASTERN MARGIN FAULT OF DAXING UPLIFT
    ZHANG Ya-jing, LI Zheng-fang, ZHOU Ben-gang, XIAO Hai-bo
    SEISMOLOGY AND GEOLOGY    2024, 46 (4): 802-820.   DOI: 10.3969/j.issn.0253-4967.2024.04.003
    Abstract262)   HTML31)    PDF(pc) (6731KB)(135)       Save

    The eastern margin fault of Daxing uplift is an important boundary fault in the southeast of Beijing Plain. The fault is located in the southeast of properthe Xiadian Fault and is distributed in the correct order en echelon with the Xiadian Fault, which controls the development of Langgu secondary depression under the extensional tectonic background. Recent shallow seismic reflection profiles and borehole data have found evidence of Holocene activity in the eastern margin of the Daxing Uplift, which has changed the conclusion in recent decades that it has not been active since the late Quaternary. Because the fault is a right order echelon with the Xiadian Fault, and it is similar to the Xiadian Fault in structure, and the Xiadian Fault had the Sanhe-Pinggu M8 earthquake in 1679, it is inferred that the fault has the risk of a large earthquake. It has essential crucial application value to the seismic hazard survey in Beijing. Also, it poses a new challenge to the upper limit of the maximum potential earthquake magnitude of the fault on the eastern margin of the Daxing Uplift.
    Quaternary sediments cover the fault on the east margin of Daxing Uplift and are in a hidden state, which results in its geometric features and deep and shallow coupling relationships that cannot be visually demonstrated by two-dimensional data two-dimensional data cannot visually demonstrate. It is of great significance to establish a three-dimensional model of hidden active faults for the hazard assessment of seismic active faults. In this paper, by collecting the fine location data of small earthquakes in this area and collating several shallow seismic geophysical profiles and deep seismic reflection profiles, SKUA-GOCAD 3D geological modeling software is used to build 3D models of the eastern margin fault of Daxing Uplift and the Xiadian Fault based on the section modeling method, and the distribution of the two faults in 3D space is simulated. The geometric features and the relationship between the depth and shallow structure of the two faults are revealed, including 1)a three-dimensional fault model and stratigraphic information map; 2)a three-dimensional model diagram of fault distribution according to dip Angle; 3)Three-dimensional model diagram of fault distribution according to depth and a three-dimensional map of small earthquake distribution. The 3D map shows that there are strong structural similarities between the faults on the eastern margin of the Daxing uplift and the Xiadian faults. The contrast map shown by depth shows that both faults are deep and shallow faults, the shallow faults disappear at about 15km underground, and the deep faults extend downward to cut the lower crust and the Moho surface. The contrast diagram displayed by apparentthe dip Angle clearly reflects that the two faults have obviously different dip angles in-depth and shallow. The deep fault is almost steep, and the shallow fault shows obvious differences in different sections. The distribution range of small earthquakes is 0-25km, of which the dominant distribution range is 10-20km. Therefore, it is speculated that the east margin fault of Daxing Uplift may have the seismogenic capacity similar to the Sanhe-Pinggu M8 earthquake in 1679. However, as existing studies have shown that the activity of the Xiadan fault and its southern extension section-eastern margin of the Daxing Uplift in this region gradually weakens from north to south, the maximum potential earthquake magnitude of the east margin fault of the Daxing Uplift is inferred in this paper to be less than Sanhe-Pinggu M8 earthquake in 1679.
    Finally, by using the structural analogy of the Xiadian Fault on the eastern margin of the Daxing Uplift, and based on the structural similarity of the two faults, this paper evaluates the maximum potential earthquake magnitude that may be induced by the Daxing Fault using different experiential relations of magnitude-fault rupture scale fitted by predecessors in North China. The conclusion is as follows: the distribution range of the magnitude of the earthquake is 7.3-7.4.
    Based on the structural analogy with the Xiadian Fault and the empirical relationship between magnitude and rupture scale, the maximum potential earthquake magnitude induced by the eastern margin fault of Daxing uplift is estimated to be magnitude 7.5. This conclusion has important scientific guiding significance for earthquake disaster prevention and control in the capital area, and should be paid attention to and actively take prevention and avoidance measures.

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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
    Abstract261)   HTML19)    PDF(pc) (7314KB)(209)       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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    EVIDENCE OF HOLOCENE ACTIVITY OF NALATI FAULT ZONE WITHIN THE TIANSHAN
    WANG Lei, REN Zhi-kun, HE Zhong-tai, JI Hao-min, LIU Jin-rui, GUO Long, LI Xing-ao
    SEISMOLOGY AND GEOLOGY    2024, 46 (4): 821-836.   DOI: 10.3969/j.issn.0253-4967.2024.04.004
    Abstract260)   HTML46)    PDF(pc) (18639KB)(157)       Save

    The Tianshan orogenic belt, extending across the Euro-Asian plates, is one of the most significant intracontinental orogenic belts globally. Spanning over 2 500km, it traverses China, Kazakhstan, Kyrgyzstan, and Uzbekistan from east to west. The belt has been continuously uplifted due to the collision of the India-Eurasia plate during the Cenozoic era. The Tianshan is divided into three segments: North Tianshan, Middle Tianshan, and South Tianshan. The crust in this tectonic region is being shortened in the north-south direction, and a series of NEE-trending or NWW-trending strike-slip faults have developed to accommodate the deformation. The Nalati fault zone serves as the collision suture between the Central Tianshan block and the Tarim block and marks the boundary between Central and South Tianshan. This fault zone trends NEE and extends southwest into Kyrgyzstan, connecting to the Nikolayev line. Its eastern segment is located north of the Dayouludusi basin. The north-south shortening rate is approximately 2.0mm/a, and the horizontal strike-slip rate is about 2.9mm/a. Reports indicate a north-south shortening rate of 0.8-1.1mm/a since the late Quaternary, suggesting it is a significant Holocene active fault zone. However, research on this fault zone's activity is limited, with most studies focused on its eastern segment. Research on other sections remains scarce.
    This study focuses on the middle segment of the Nalati fault zone in Tekes county, Ili Prefecture. The Tekes section trends ENE, starting from Qiongkushitai village in the east, passing through Kalatuori, Ayakeaqia, and Kalawenkeer, and reaching Burili in the west, spanning approximately 55km. Methods employed include remote sensing image interpretation, field geological investigation, UAV aerial surveys, trench excavation, Radiocarbon-14 dating, and semi-automatic horizontal dislocation measurement. The main findings are as follows: 1)The linear geomorphological features of the Tekes segment are prominent, with typical fault geomorphological signs such as fault cliffs, triangles, scarps, bulges, gate ridges, passes, guanmen mountains, and left-lateral dislocation ridges and gullies widely observed.; 2)Small Unmanned Aerial Vehicle Mapping and LaDiCaoz semi-automatic dislocation measurement and analysis indicate a minimum horizontal displacement of approximately 3.4m; 3)Faults are developed in the Proterozoic and Paleozoic strata. A trench 4m long and 1.6m wide excavated at a series of fault reverse scarps revealed a sedimentary event of the sag pond at the hillside, indicating at least four paleo-earthquake events; 4)To date the paleo-earthquake events, we collected 11 sediment samples for Radiocarbon-14 dating at the BETA Analytic laboratory. Results show that the sample at a depth of 2m is about(7.06±0.03)ka BP, and the latest colluvial wedge is about(1.67±0.03)ka BP; 5)Using OxCal age correction, the ages of the four paleo-earthquake events were determined at a 95.4%confidence level: event E1 occurred between 2757BC and 413AD, event E2 between 3581BC and 429BC, event E3 between 4702BC and 3932BC, and event E4 between 5742BC and 5230BC. In summary, we propose that the middle segment of the Nalati fault zone has been active since the Holocene.

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    SEISMOLOGY AND GEOLOGY    2024, 46 (3): 756-759.  
    Abstract253)   HTML42)    PDF(pc) (1214KB)(148)       Save
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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
    Abstract246)   HTML33)    PDF(pc) (20130KB)(186)       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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    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
    Abstract245)   HTML35)    PDF(pc) (9302KB)(201)       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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    REGIONAL DEFORMATION BACKGROUND AND COSEISMIC DEFORMATION CHARACTERISTICS OF THE 2022 LUDING MS6.8 EARTHQUAKE
    XU Jing, JI Ling-yun, LIU Chuan-jin
    SEISMOLOGY AND GEOLOGY    2024, 46 (3): 645-664.   DOI: 10.3969/j.issn.0253-4967.2024.03.008
    Abstract244)   HTML14)    PDF(pc) (12877KB)(132)       Save

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

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

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

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

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

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    THE INFLUENCE OF SEISMIC SOURCE CHARACTERISTICS ON VELOCITY PULSE DISTRIBUTION IN SCENARIOS: A TEST IN HUYA FAULT
    JI Zhi-wei, LI Zong-chao, ZHANG Yan, JU Chang-hui
    SEISMOLOGY AND GEOLOGY    2024, 46 (5): 1207-1225.   DOI: 10.3969/j.issn.0253-4967.2024.05.012
    Abstract240)   HTML15)    PDF(pc) (9895KB)(73)       Save

    In August 1976, within a week, three earthquakes with a magnitude of 6.5 or higher occurred at the border of Songpan County and Pingwu County in Sichuan Province, China. The seismogenic structure of the three earthquakes is the Huya Fault. The Huya Fault is still a strong active fault, and there is still a possibility of major future earthquakes in the Songpan and Pingwu regions. Historical earthquake records only represent earthquakes that have occurred, and there is uncertainty in estimating earthquake motion using existing records, especially in near-fault areas without strong earthquake records. Estimating near-fault ground motion has become a research hotspot in the interdisciplinary field of seismology and engineering seismology in recent years. The research and methods differ from the design of earthquake motion methods for earthquake safety evaluation in engineering. They are developed by integrating earthquake source physics, seismic wave propagation theory, and engineering seismology. Based on the geological and geomorphological characteristics of the Songpan-Pingwu area in Sichuan Province and the process framework for constructing scenario earthquake models, we have developed three scenario earthquake source models with a magnitude of MW7.0. These models include rupture models and source mechanisms related to the Huya Fault. The seismic source parameters were referenced from existing statistical models. Utilizing the three-dimensional finite difference method, we can simulate the long-term ground motion of scenario earthquakes for its facile discretization and computational efficiency. We set virtual observation stations within the calculated area, enabling the acquisition of velocity wave fields and waveforms across diverse earthquake scenarios. Besides, the velocity pulse identification method is combined to identify the seismic motion of the virtual station to study the distribution characteristics of regional velocity pulses. We use the pulse recognition method to identify velocity pulses of earthquake motion(observed or simulated earthquake motion). It can be summarized as a continuous wavelet transform of two orthogonal components of earthquake motion to determine whether it is a pulse. When the pulse index PI>0, the original record is determined to be a pulse, and the larger the PI, the stronger the pulse characteristics of the original record. When the pulse index PI<0, the original record is deemed nonpulse. This method can obtain the pulse amplitude and pulse period. Finally, the obtained results will be fitted with the probability distribution curve of velocity pulses to explore the impact of rupture mode and source mechanism on the distribution of velocity pulses. The results of this article indicate that: 1)The rupture mode is significant to the distribution of regional velocity pulses. For strike-slip faults, the velocity pulses caused by unilateral rupture mode are mainly in the E-W direction, and the peak value of the pulses does not exceed 50cm/s. The range of pulse distribution and the peak intensity of strong vibrations generated on the surface are smaller than the bilateral rupture mode. Strong velocity pulses not only appear near the projection area of faults on the surface but also trigger velocity pulses at a distance from the epicenter due to the directional effect of rupture. 2)The shape of the velocity pulse probability distribution curve is similar to the simulated velocity pulse distribution characteristics, and there are significant differences in the distribution of seismic motions under different source mechanisms. The current velocity pulse probability distribution model only considers the rupture characteristics and the relative position relationship between stations and faults without considering the influence of source parameters such as rupture velocity. There are deviations in the fitting effect for different components, such as E-W and N-S. The speed pulse period identified by virtual stations varies from 1-7s. By adding structural measures to the building structure, the natural vibration period of the structure can be changed, thereby avoiding the potential hazards of pulse-type seismic motion. More actual observation data is needed to study the distribution of velocity pulse periods in the future. This article’s simulation results are consistent with the existing understanding of earthquake motion. However, our study employs a simplified crustal structure characterized by horizontal layers, temporarily ignoring the site condition in the simulation of long-period ground motion. We do not encapsulate the complexities introduced by the site conditions. Average shear wave velocity at 30m underground (VS30) is a factor that affects the number of pulse recognition. In addition, this article does not discuss the effects of rupture speed, the number of asperities, and the position of asperities. Therefore, we will conduct more in-depth research on these factors in our subsequent work. The research in this article calculates the scenario earthquake of the Huya Fault under rupture mode and focal mechanism. The research results can be used for seismic analysis of long-period structures, providing a reference for the construction of significant projects and seismic hazard analysis near the Huya Fault. This article referred to previous research when setting parameters for scenario earthquakes. However, due to the limitations of statistical models, the set scenario earthquakes cannot fully represent the Huya Fault situation and the region’s possible earthquake scenarios.

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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
    Abstract237)   HTML14)    PDF(pc) (6351KB)(276)       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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    SOURCE RUPTURE MECHANISM AND STRESS CHANGESTO THE ADJACENT AREA OF JANUARY 7, 2025M、6.8 DINGRI EARTHOUAKE,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/i.issn.0253-4967.2025.01.003
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    COMPREHENSIVE STUDY OF THE CURRENT CONNECTION MODE OF A NORMAL FAULT STEPOVER: AN EXAMPLE OF THE CHANFANG STEPOVER ON THE KOUQUAN FAULT IN THE SHANXI RIFT SYSTEM, CHINA
    HUA Chun-yu, SU Peng, SHI Feng, XI Xi, GUO Zhao-wu
    SEISMOLOGY AND GEOLOGY    2024, 46 (4): 837-855.   DOI: 10.3969/j.issn.0253-4967.2024.04.005
    Abstract234)   HTML20)    PDF(pc) (13930KB)(106)       Save

    The overlapping area between the ends of adjacent fault segments is known as a fault stepover. The normal fault stepover has two endmember connection modes, i.e., soft-link mode and hard-link mode. The soft-link stepover's border faults are connected through a relay ramp, and the border faults' displacements are transmitted through the bending deformation of the relay ramp. The hard-link stepover's border faults are connected through a breaching fault, and the border faults' displacements are transmitted through the faulting deformation of the breaching fault. Distinguishing the current connection mode of a normal fault stepover can shed light on the evolution stage of the normal fault. It can also indicate the potential earthquake rupture pattern in the stepover, which is important for evaluating the seismic hazard of engineering sites within the stepover. The straightforward technique to distinguish the current connection mode of a normal fault stepover is to determine whether an active breaching fault exists within the stepover. However, in many cases, due to the small amount of accumulated offset and human modification of the breaching fault, it is always hard to observe fault scarps in the field even though the fault stepover is deforming under the hard-link mode.
    The Shanxi Rift System is a prominent intracontinental rift zone in East Asia. It comprises a series of left-stepping en échelon grabens bounded by high-angle normal faults. It is distributed in an S-shaped geometry with a narrow, NNE-trending zone in the middle and two broad, NEE-trending extensional zones in the north and south. The Shanxi Rift System is one of the strong earthquake-prone regions in China. Since 780 BC, the Shanxi Rift System has hosted three M8 earthquakes, five M7-7娻 earthquakes, and a series of M6-7 earthquakes. The Kouquan fault is the western border fault of the Datong Basin in the northern part of the Shanxi Rift System. A stepover is developed near the Chanfang village on the Kouquan fault, which we named the Chanfang stepover.
    In this study, we use a combination of the tectonic geomorphological investigation in the field, high-resolution topographic data analysis, and Ground Penetrating Radar(GPR)surveying to study the current connection mode of the Chanfang stepover. Three fault outcrops on the border faults of the Chanfang stepover are investigated. The outcrop D1 is in an alluvial fan covered by loess on the southwestern boundary fault of the Chanfang stepover. Two branch faults are present at this outcrop. One offsets a bedrock surface and the alluvial fan's gravel layer. The other is the boundary between a gravel layer and the loess, where imbricated gravel can be observed. The fault outcrop D2 is also on the southwestern boundary fault of the Chanfang stepover. The fault at the outcrop D2 offsets a gravel layer and the vertical offset of the top of the gravel layer is approximately 2m. The fault outcrop D3 is located on the northeastern boundary fault of the Chanfang stepover. At the outcrop D3, the fault separates the gneiss of the Archean Jining Group from the loess. Based on the Chinese GF-7 satellite stereo imagery, we obtain the high-resolution digital elevation model(DEM)covering the Chanfang stepover and identify two levels of geomorphic surfaces, i.e., T1 and T2. The surface T1 is an alluvial fan, mainly developed in the piedmont areas. The surface T2 is an erosion surface distributed in the bedrock mountain. To quantify the deformation pattern within the Chanfang stepover, we construct a series of topographic cross-sections on the surface T1 and find a gentle geomorphic scarp within the stepover. We conduct two GPR surveying lines across the Chanfang stepover. On the GPR images, we identify two known faults, F1 and F3, that previous researchers have mapped and a buried fault, F2, that has not been constrained previously.
    The Fault F2 observed by the GPR is consistent with the geomorphic scarp constrained on the DEM, suggesting that a breaching fault exists in the Chanfang stepover. The existence of the Chanfang breaching fault indicates that the connection mode of the Chanfang stepover is the hard-link mode. We thus infer that the future earthquakes on the Chanfang stepover may cause concentrated surface ruptures on the breaching fault. This study shows that the combination of the tectonic geomorphologic investigation in the field, high-resolution topographic data analysis, and the GPR survey can effectively locate near-surface, slow, active normal faults. This comprehensive technique can be used for the connection mode of a normal fault stepover.

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    CONSTRUCTION METHOD OF SEISMOTHERMAL INFRARED BACKGROUND FIELD BASED ON GPR-LSTM
    SONG Dong-mei, ZHANG Man-yu, SHAN Xin-jian, CUI Jian-yong, WANG Bin
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 492-511.   DOI: 10.3969/j.issn.0253-4967.2024.02.015
    Abstract232)   HTML4)    PDF(pc) (11203KB)(150)       Save

    Seismic monitoring is a very important and challenging task. The continuous development of remote sensing technology has strengthened our ability to monitor the Earth’s surface on a macro scale. Research shows that an abnormal rise in surface temperature usually occurs before an earthquake, so a variety of anomaly extraction algorithms have been applied to the study of seismic thermal anomalies. Among them, the extraction method based on background field is widely used because of its strong mechanism interpretation. However, the previous anomaly methods based on the background field mainly limit the background field to a certain fixed threshold value, and ignore the small range of normal LST fluctuations caused by external factors(non-seismic). Therefore, a method of constructing a seismothermal infrared background field based on GPR-LSTM is proposed in this paper. The main idea of the method is that the LST background field is obtained by adding the established annual variable reference field and the fluctuation range of normal LST. First, the LST exhibits a range of fluctuations due to non-seismic factors such as solar radiation, weather, and human activities. Therefore, it is not reasonable to take a fixed value of the LST background field but it should have a certain fluctuation range. Therefore, the dynamic fluctuation characteristics of the background field should be reflected in the construction process of the background field in this study. Secondly, the reason why this method uses the LSTM model to predict the annual variable reference field of the earthquake period based on the annual variable reference field of the non-earthquake period is that on the one hand, the LSTM model can predict the law of long time series data, so it can better learn the annual variation law of LST in the non-earthquake year. At the same time, the LST trend of increasing or decreasing year by year caused by climate change can be predicted, which is conducive to more truly describing the LST trend of the real background field in the year of the earthquake period.

    The method includes two parts: the establishment of the annual variable reference field, the calculation of the residual fluctuation range of the actual LST and the construction of the background field. Based on the MODIS surface temperature product, the precursory thermal anomaly information of the 2008 Wenchuan earthquake in Sichuan Province and the Yutian earthquake in Xinjiang Province was extracted and analyzed by using the proposed method. Based on the MODIS surface temperature data of the 2008 Wenchuan earthquake in Sichuan Province and the Yutian earthquake in Xinjiang Province, the proposed method is used to detect and analyze the thermal anomalies before the earthquake. The following conclusions are drawn:

    (1)The established LST background field on the Qinghai-Xizang Plateau is consistent with the actual variation law of LST and is relatively stable on the whole. In the time dimension, the background field of LST shows the annual variation of high summer and low winter. In terms of spatial dimension, the established LST background field is generally high in the south, low in the middle and low in the north, and the temperature value of the background field is the lowest at the highest altitude of the Qinghai-Xizang Plateau.

    (2)The thermal anomalies obtained based on the new algorithm are usually distributed along the fault zone of the Qinghai-Xizang Plateau, and the anomaly evolution law is obvious. For the Yutian earthquake in Xinjiang, the trend of thermal anomalies is consistent with that of the fault zone in most cases, and the thermal anomalies are mainly distributed in the northern margin of the fault zone. For the Wenchuan earthquake in Sichuan Province, with the onset time approaching, the thermal anomalies gradually moved southward along the Longmenshan fault zone from the north margin, filled the entire fault zone before the earthquake, and disappeared after the earthquake. This proves the validity of the proposed background field construction method.

    (3)Compared with non-seismic years, the spatial characteristics of thermal anomalies along faults are more obvious in seismic years, and the duration and amplitude of the anomalies are longer. This indicates that the tectonic activity in seismic years is more active than that in non-seismic years, resulting in more significant abnormal warming of surface temperature.

    (4)The evolution law of thermal anomalies of earthquakes with magnitude 7 or above on the Qinghai-Tibet Plateau can be summarized as incubation—disappearance—accumulation—disappearance—earthquake occurrence, and the strength of thermal anomalies appears in the way of repeated changes. This indicates that the thermal anomaly is not continuously enhanced or weakened before the earthquake but shows the characteristics of repeated fluctuations between strong and weak until the earthquake eruption thermal anomaly disappears.

    In conclusion, the proposed method can not only ensure that the established background field is not affected by tectonic factors but also fully consider the small range of normal fluctuation of surface temperature caused by non-tectonic factors, which makes the extraction of thermal infrared anomaly information more accurate. This provides new technical means and ideas for the extraction of earthquake anomaly information and is helpful for the further development of earthquake monitoring.

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    THREE-DIMENSIONAL CRUSTAL VELOCITY STRUCTURE AND SEISMOGENIC ENVIRONMENT AROUND THE HUOSHAN EARTHQUAKE SWARM
    JI Guo-qiang, LEI Jian-she, ZHAO Da-peng
    SEISMOLOGY AND GEOLOGY    2024, 46 (3): 665-685.   DOI: 10.3969/j.issn.0253-4967.2024.03.009
    Abstract226)   HTML16)    PDF(pc) (8799KB)(107)       Save

    The Huoshan earthquake swarm is tectonically located at the junction among the North China plate, Yangtze plate, and North Dabie orogenic belt. The geological environment in the region is complex, including the Feixi-Hanbaidu Fault, Meishan-longhekou Fault, Xiaotian-Mozitan Fault and Luoerling-Tudiling Fault, as well as the North Dabie tectonic belt, North Huaiyang tectonic belt and Liu’an basin. In the study region, seismicity is intense, and 9 earthquakes with M≥5.0 occurred along the Luoerling-Tudiling Fault in the history. In recent decades, small-to-moderate earthquakes were frequent, mainly gathering at the intersection of the Xiaotian-Mozitan Fault and Luoerling-Tudiling Fault. Furthermore, the frequency of small earthquakes in the Huoshan region has a significant correspondence to the moderate-to-strong earthquakes in East China and even eastern Tibet, so studying the deep structure can shed new light on the relationship between the Huoshan earthquake swarm and moderate-to-strong earthquakes in mainland China.

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

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

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

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    CUMULATIVE DISLOCATIONS OF RIVER TERRACES IN THE EASTERN SEGMENT OF THE ALTYN TAGH FAULT
    LI Lu-yao, DING Rui, JIANG Da-wei, ZHANG Shi-min
    SEISMOLOGY AND GEOLOGY    2024, 46 (3): 547-569.   DOI: 10.3969/j.issn.0253-4967.2024.03.003
    Abstract223)   HTML27)    PDF(pc) (18302KB)(156)       Save

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

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

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

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

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    STUDY ON THE MAGMATIC PROCESSES OF POST-COLLISIONAL POTASSIC VOLCANIC ROCKS FROM WEST KUNLUN: TAKING THE PULU AND KANGXIWA VOLCANIC ROCKS AS EXAMPLES
    DING Ran, LUAN Peng, YU Hong-mei, WEI Fei-xiang, ZHAO Bo, YANG Wen-jian, XU Jian-dong
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 312-335.   DOI: 10.3969/j.issn.0253-4967.2024.02.005
    Abstract213)   HTML18)    PDF(pc) (16484KB)(162)       Save

    The West Kunlun region is located in the northwest margin of the Qinghai-Xizang Plateau. Due to the subduction and collision of the Indian plate, this region has many post-collisional potassic volcanic areas of different sizes. Scholars have conducted many volcanic geology and petro-geochemistry work in the West Kunlun volcanic area, mainly focusing on the origin of deep magmas and plate dynamics. However, the magmatic processes of these potassic volcanic rocks are still unclear. To reveal the magmatic processes in the West Kunlun region and understand the mechanism of volcanic eruption, we analyzed the whole-rock major elements, structure and compositions of the phenocrysts, the crystal size distribution(CSD), and magma crystallization temperature and pressure conditions of the Pulu and Kangxiwa volcanic rocks. The results show that the magma sources of the two volcanic regions are close and their trace element characteristics are similar. Still, their rock types and mineral compositions are significantly different. The Pulu volcanic rocks are mainly trachyandesite and basaltic trachyandesite. The phenocrysts are composed of plagioclase, olivine, clinopyroxene and a small amount of orthopyroxene. The Kangxiwa volcanic rocks are mainly phonotephrite, consisting of clinopyroxene, biotite, and a small amount of olivine and plagioclase. The erosion and zonation of plagioclase, olivine and clinopyroxene were observed under a microscope. There are Nb-Ta and Ti negative anomalies in the two regions, with relative enrichment of large ion lithophile elements(LILF)and light rare earth elements(LREE), indicating that the magma source area has the characteristics of island arc magma, which is related to plate subduction. Based on the analysis of previous Sr-Nd datas, we suggest that the magmas from these two volcanic areas originated from enriched sources.

    According to the erosion characteristics, zonal composition data, and the concave CSD pattern, we suggest that the magma in Pulu mixed with acidic magma, whereas the magma in Kangxiwa may only mix with the internal magma, resulting in a large amount of melt erosion of phenocrysts. In Pulu volcanic rocks, the retention time of the smaller size(<5mm)crystal is 190-332a, and that of crystallographic size(>5mm)is 339-860a. The CSD curves of clinopyroxene phenocrysts in Kangxiwa phonotephrites kink at the size of 1.5mm. In Kangxiwa volcanic rocks, the residence time of smaller crystallographic size(<1.5mm)is 5.8-6.4a, and that of larger crystallographic size(>1.5mm)is 9.6-21.2a. The CSD curves of the volcanic rocks from Pulu and Kangxiwa volcanic regions are concave upward, indicating that magma mixing may have occurred both in the two volcanic regions. The An values of the core and the rim of the normal zoning plagioclases and the For value of the normal zoning olivines in the Pulu volcanic rocks vary widely, and the feldspars with low An values at the rim are out of balance with the melt. This indicates that the magma of the Pulu volcanic group mixed with the acidic magma. The Mg# of the normal zoning clinopyroxenes in the Kangxiwa volcanic rocks has a narrow range, and they are all in balance with the melt. The crystallization pressure at the rim was low, and the decompression caused a large number of resorbed phenocrysts to melt. This indicates that the mixing of phenocrysts with different degrees of melting and erosion may result in upward concave CSD curves of clinopyroxenes in the Kangxiwa volcanic rocks, so the Kangxiwa volcanic rocks may only have internal magma mixing.

    The mineral-melt equilibrium thermometers show that the equilibrium temperature and pressure of the Pulu volcanic rocks are 1 035-1 218℃, 5.1-9.9kbar, respectively, and the corresponding depth is 19.4~37.3km. The equilibrium temperature of Kangxiwa volcanic rocks is 1 154-1 282℃, the equilibrium pressure is 1.2-11.6kbar, and the corresponding depth is 4.3~43.7km. The variation range of equilibrium pressure in the Kangxiwa region is large, which may be related to the deep fault zone. In this study, by quantitatively studying the magmatic processes of post-collisional potassic volcanic rocks in the West Kunlun region, we provide CSD calculations of the volcanic rocks, reveal the migration and evolution processes of magmas in the crustal magma reservoir, and provide important information for the volcanic activities in the northwest margin of the Qinghai-Xizang Plateau and its surrounding regions.

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    APPLICATION OF HIGH-RESOLUTION DIGITAL ELEVATION MODEL ON HEIKONGSHAN VOLCANO OF TENGCHONG VOLCANIC FIELD IN YUNNAN PROVINCE
    WANG Xin-ru, MA Chen-yu, PAN Mao
    SEISMOLOGY AND GEOLOGY    2024, 46 (3): 536-546.   DOI: 10.3969/j.issn.0253-4967.2024.03.002
    Abstract209)   HTML15)    PDF(pc) (4938KB)(104)       Save

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

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