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20 February 2025, Volume 47 Issue 1

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Special section: The Dingri M_S6.8 earthquake in Xizang

SEISMOGENIC FAULT AND COSEISMIC SURFACE DEFORMATION OF THE DINGRI M_S6.8 EARTHQUAKE IN XIZANG, CHINA

SHI Feng, LIANG Ming-jian, LUO Quan-xing, QIAO Jun-xiang, ZHANG Da, WANG Xin, YI Wen-xing, ZHANG Jia-wei, ZHANG Ying-feng, ZHANG Hui-ping, LI Tao, LI An

2025, 47(1):  1-15.  DOI: 10.3969/j.issn.0253-4967.2025.01.001

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At 09:05 am on January 7, 2025, a M_S6.8 earthquake occurred in the Dingri, Xizang, China. The earthquake caused serious casualties and property losses. Research on the seismogenic structure and characteristics of earthquake surface rupture in this earthquake is beneficial to understanding the rupture behavior and dynamic mechanism of normal-fault earthquakes. Meanwhile, it provides a basis for predicting the future strong earthquake trend of the southern Xizang rift fault system. Its epicenter is located at 87.45°E, 28.50°N, 13km depth, the China Earthquake Networks Center measures. In order to constrain the seismogenic fault and characterize the co-seismic surface ruptures of this earthquake, field investigations were conducted immediately after the earthquake, combined with analyses of the focal parameters, aftershock distribution, and InSAR inversion of this earthquake.

This preliminary study finds that the seismogenic fault of the Dingri M_S6.8 earthquake is the Dengmocuo fault, which is an active ~60km long, NS-NE-striking and normal fault. The total length of the co-seismic surface ruptures is approximately 25km, located on the north segment of the Dengmocuo fault. Meanwhile, a dense deformation zone of ground fracture with a length of ~10km is generated on the east side of Dengmocuo Lake along the contour line of the lake shore. The earthquake also induced a large number of liquefaction structures and tensional fractures in valleys and basins.

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

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

SURFACE RUPTURE INTERPRETATION AND BUILDING DAMAGE ASSESSMENT OF XIZANG DINGRI M_S6.8 EARTHQUAKE ON JANUARY 7, 2025

ZOU Jun-jie, SHAO Zhi-gang, HE Hong-lin, GAO Lu, XU Yue-yi, DOU Ai-xia, LIANG Ze-yu

2025, 47(1):  16-35.  DOI: 10.3969/j.issn.0253-4967.2025.01.002

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

SOURCE RUPTURE MECHANISM AND STRESS CHANGES TO THE ADJACENT AREA OF JANUARY 7, 2025, M_S6.8 DINGRI EARTHQUAKE, XIZANG, CHINA

YANG Jian-wen, JIN Ming-pei, YE Beng, LI Zhen-ling, LI Qing

2025, 47(1):  36-48.  DOI: 10.3969/j.issn.0253-4967.2025.01.003

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According to the official determination of China Seismic Network, at 09:05 on January 7, 2025, an M_S6.8 earthquake(hereinafter referred to as Dingri earthquake)occurred in Dingri County(28.50°N, 87.45°E), Shigatse City, Xizang, with a focal depth of 10km. The earthquake occurred in the southern part of the Qinghai-Xizang Plateau, which is located in the intersection area of the Shenzha-Dingjie rift and the south of Xizang detachment system. The Dengmecuo fault(about 11km)is the closest to the earthquake, and the focal mechanism is tensile rupture. The earthquake had high magnitude, high intensity and shallow source, and the towns and villages in the epicenter area were relatively concentrated. In addition, the landform type of the epicenter and the surrounding area is a river alluvial plain, and the soil is soft, which amplifies the earthquake damage effect. Due to the comprehensive superposition of various factors, the earthquake caused severecasualties and building damage.

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

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

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

(2)Based on the coseismic deformation data of ascending and descending orbits, the moment magnitude of the Dingri earthquake obtained by joint inversion is M_W7.06 by using the SDM layered model method. The rupture process of the earthquake shows a unilateral rupture characteristic from the initial rupture point to the north along the fault. The fault dislocation is a standard fault mechanism with a little strike-slip component. The length of the main rupture zone of the seismogenic fault is about 55km, and the slip distribution is concentrated in the depth range of 0~15km underground. The maximum slip is 4.25m, which occurs at a depth of 8.6km underground. The main rupture zone of the earthquake has reached the surface, located about 35~53km north of the epicenter along the strike, and the potential surface rupture length is about 18km.

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

INVESTIGATION OF THE SEISMOGENIC STRUCTURE OF THE 2025 DINGRI M_S6.8 EARTHQUAKE IN XIZANG BASED ON THE TECTONIC STRESS FIELD PERSPECTIVE

SHENG Shu-zhong, WANG Qian-ru, LI Zhen-yue, LI Hong-xing, ZHANG Xiao-juan, GE Kun-peng, GONG Meng

2025, 47(1):  49-63.  DOI: 10.3969/j.issn.0253-4967.2025.01.004

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On January 7, 2025, at 09:05 Beijing Time, an M_S6.8 earthquake struck Dingri County in Shigatse City, Xizang, as reported by the China Earthquake Networks Center. The earthquake occurred at 28.50°N, 87.45°E with a hypocentral depth of 10km, resulting in significant casualties and economic losses. In the immediate aftermath, major earthquake research institutions and seismologists, both domestic and international, promptly released the focal mechanism solution, providing crucial data for understanding the earthquake's origin and its seismogenic structure. However, the two nodal planes of the focal mechanism, derived from a double-couple source model, are equivalent, necessitating additional data or methodologies to distinguish the actual seismogenic fault plane. The parameters of the seismogenic fault are fundamental for the accurate calculation of ground motion maps, and they provide key information for seismic hazard assessment and post-earthquake rapid response guidance. Therefore, it is imperative to identify the seismogenic fault plane for the given focal mechanism solution.

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

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

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

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

ANALYSIS OF BUILDING DAMAGE AND CASUALTIES OF THE 2025 DINGRI M_S6.8 EARTHQUAKE IN XIZANG BASED ON FIELD INVESTIGATION

WEI Ben-yong, ZHANG Yu-man, SHI Feng, QIAO Jun-xiang, WANG Xin, ZHANG Da

2025, 47(1):  64-79.  DOI: 10.3969/j.issn.0253-4967.2025.01.005

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

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

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

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

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

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

SURFACE DEFORMATION CHARACTERISTICS AND CAUSES OF THE DENGMECUO SEGMENT IN THE XIZANG DINGRI M_S6.8 EARTHQUAKE

LIANG Ming-jian, DONG Yun-xi, ZUO Hong, DAI You-lin, XIAO Ben-fu, LIAO Cheng, TAN Ling, WANG Yu-wei, LI Xiang, TANG Cai-cheng, ZHANG Wei, ZHANG Hui-ping, MENG Ling-yuan, SU Jin-rong, WU Wei-wei, LI Chuan-you, YAN Mei

2025, 47(1):  80-89.  DOI: 10.3969/j.issn.0253-4967.2025.01.006

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On January 7, 2025, an M_S6.8 earthquake struck Dingri, Xizang, China. According to the focal mechanism solution provided by the USGS, this event was characterized as a normal faulting earthquake. The earthquake occurred in the southern segment of the Shenzha-Dingjie Rift system, which is located on the Qinghai-Tibet Plateau. This rift system is one of the seven major rift systems in the southern part of the Tibetan plateau and is a significant controlling structure for shallow-source seismic activity within the region. Moderate to major earthquakes in the study area are primarily distributed along these rift systems. Notably, the Yadong-Gulu Rift system experienced an M8.0 earthquake in 1411 near the southern part of Dangxiong.

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

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

Review

THE DISTRIBUTION AND RESEARCH PROGRESS OF MAIN MUD VOLCANOES IN CHINA

WANG Qian, PENG Lai, JIANG Yu-han, ZHOU Qi-chao, GAO Xiao-qi

2025, 47(1):  90-116.  DOI: 10.3969/j.issn.0253-4967.2025.01.007

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This paper provides an overview of the distribution and research progress of mud volcanoes in China, and offers a comparative analysis of the physical and geochemical characteristics of mud volcano emissions from various regions. The findings are as follows:

Mud volcanoes in China are predominantly located along faults or fault zones, primarily in seismically active regions such as Xinjiang, Tibet, and Taiwan. A comparative analysis of emissions from different regions reveals that the solid components—primarily minerals like quartz and montmorillonite—are similar across locations. The liquid emissions generally exhibit high salinity. Geochemical analysis indicates regional differences in the sources of the emitted mud water: in Xinjiang, it mainly derives from a mixture of ancient sediment pore water and atmospheric precipitation; in Tibet, it originates from deep groundwater and atmospheric precipitation; and in Taiwan, it comes from marine sediment pore water mixed with atmospheric precipitation.

In terms of gas emissions, methane is the predominant gas released by most mud volcanoes in the study area, with smaller amounts of ethane, carbon dioxide, and other hydrocarbons. Notably, some mud volcanoes in Taiwan region emit higher concentrations of carbon dioxide and nitrogen, with smaller amounts of methane. Carbon isotope analysis of the emitted gases shows that the methane is of organic origin, as indicated by δ13C1 values.

In addition to the geochemical analysis, the microbial communities associated with mud volcanoes are also significant. The origin of methane suggests the presence of methane-producing microbial communities in the surrounding environments of onshore mud volcanoes. These microbes, including aerobic methane-oxidizing bacteria and anaerobic methane-oxidizing archaea, play a role in the formation of the emitted gases. In particular, these microorganisms are found in the soil environments of mud volcanoes in eastern Taiwan region, Wusu(Xinjiang), and Dushanzi(Xinjiang), where they use carbon dioxide as a carbon source to produce methane. In submarine mud volcanoes, the eruption process provides conditions for the growth of marine microorganisms, including methane-producing bacteria.

Mud volcanoes are generally triggered by four main factors: volcanic activity, tectonic activity, sedimentary processes, and anthropogenic influences. Given their prevalence in seismically active zones, the activity of mud volcanoes is often induced by seismic events. Eruptions typically result from increased pore pressure within the surrounding rock layers. Significant physical and chemical anomalies, including elevated gas emissions and changes in the height of the mud surface, are observed before and after seismic events. For example, following the establishment of a real-time monitoring system for the Wusu mud volcano in Xinjiang in 2011, multiple earthquakes in the area were associated with changes in the mud volcano's liquid level and increased emissions of methane(CH₄)and carbon dioxide(CO₂), which gradually returned to background levels post-earthquake. These geochemical changes have been observed in other regions as well, such as the hydro geochemical changes in the north Tianshan Mountains, following the Xinyuan M_S6.6 earthquake.

These observations suggest that mud volcano emissions may serve as indicators of seismic activity. Long-term monitoring of mud volcanoes could potentially offer a means of predicting seismic events.

Research paper

INFLUENCE OF MICROSTRUCTURE DIRECTIONALITY OF LAYERED SILTSTONE ON ITS TENSILE DEFORMATION CHARACTERISTICS

ZHANG Qian-qian, CHEN De-ping, ZHOU Yong-sheng, WANG Tao

2025, 47(1):  117-130.  DOI: 10.3969/j.issn.0253-4967.2025.01.008

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The stability of rock structures plays a crucial role in various mining processes, including the study of open-pit mine slopes, predictive analyses of mine pressure in roadways, and the stability of underground quarries. Consequently, research into the strength and stability of rocks has remained a prominent area of focus in mining engineering. Numerous studies in rock mechanics have shown that the tensile strength of rock is significantly lower than its compressive strength, with most rock failures resulting from cleavage damage. As such, tensile strength is a key indicator of rock integrity. Current research on the tensile strength of laminated rocks primarily investigates the relationship between tensile strength and lamina inclination, with little attention paid to the influence of lamina spacing. Additionally, studies often describe the relationship between tensile strength and lamina inclination without exploring the underlying microstructural mechanisms responsible for rock failure. This study aims to investigate the relationship between tensile deformation characteristics, lamina spacing, and loading direction. Experimental samples were prepared with lamina spacing of 2d, 23/3d, d on the cuts of S₁, S₂, S₃ surface and level S, respectively. Brazilian splitting tests were performed to determine the tensile strength of siltstone under different loading directions. Full-field strain measurements using Digital Image Correlation(DIC)were employed to observe the deformation characteristics, and microstructural analysis was conducted based on crystal space distribution theory.

The results of the experiments are as follows:

(1)When the loading direction is parallel to the lamination direction, the tensile strengths of the S₁(2d), S₂(23/3d), and S₃(d) surfaces are 29.99MPa, 26.56MPa, and 18.92MPa, respectively. When the loading direction is perpendicular to the lamination direction, the tensile strengths of the same surfaces are 32.76MPa, 30.44MPa, and 27.77MPa. These findings indicate that tensile strength decreases with decreasing lamina spacing. Thus, as the laminae become more closely spaced, the tensile strength weakens, highlighting the contact surfaces between laminae as weak planes.

(2)The tensile strength of the sample with the S surface as the layer is not primarily influenced by weak interfaces, but rather by the mineral arrangement within the layer. When the loading direction aligns with denser mineral arrangements, the sample is more prone to cracking along the direction of looser particle arrangements, resulting in lower tensile strength. This demonstrates that preferential mineral orientation significantly impacts tensile strength.

(3)For samples with the same lamina spacing, when the loading direction is parallel to the laminae, the tensile region is concentrated along the centerline of the sample. As loading progresses, the tensile region propagates, eventually causing the sample to split into two. When the loading direction is perpendicular to the laminae, the tensile region is more dispersed initially, with several strain concentration points. These points are typically located along the transverse axis, and the sample splits in a manner similar to the parallel loading case. When the loading direction is diagonal to the laminae, the distribution of tensile regions is irregular, with the weak contact positions between laminae contributing to intermittent and tilted strain distributions.

(4)As lamina spacing decreases, the concentration of strain zones becomes denser under the same loading direction, further supporting the hypothesis that laminae contact surfaces act as weak planes.

The above findings provide valuable insights into the tensile behavior of laminated rocks and the influence of lamina spacing and mineral arrangement on rock strength.

EFFECTS OF NORMAL STRESS OSCILLATION ON THE STICK-SLIP BEHAVIOR, NEAR-FAULT STRAIN FIELD AND ACOUSTIC EVENTS FOR FAULT GOUGE LAYERS

YU Bo-wen, MA Sheng-li, ZHANG Lin

2025, 47(1):  131-149.  DOI: 10.3969/j.issn.0253-4967.2025.01.009

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Industrial activities(e.g., natural gas storage and wastewater injection)and certain natural processes(e.g., solid tides and volcanic activity)can induce stress perturbations on faults, potentially influencing seismic activity in affected regions. Understanding how fault systems respond to these stress perturbations is essential for improving the accuracy of seismic event prediction and mitigation strategies. Previous laboratory studies have primarily focused on the impact of stress perturbations on the mechanical properties of fault stick-slip behavior, such as shear stress drop and the timing of periodic stick-slip cycles. Most of these studies have modeled faults using bare rock surfaces. However, obtaining accurate stress measurements on natural faults is challenging due to the limitations of commonly used seismic monitoring technologies, such as GPS, dense seismic arrays, and distributed optical fibers, which are typically deployed on the Earth's surface or in boreholes. This complicates the extrapolation of laboratory findings to field conditions. Moreover, there is a lack of research on the role of fault gouge, which is prevalent in natural fault zones.

This study aims to investigate how stress perturbations influence the physical fields(strain and acoustic emissions)adjacent to a simulated fault containing fault gouge. A double-direct shear sample configuration was employed to replicate the fault, using two types of fault gouge: a natural fault gouge(composed mainly of dolomite, bassanite, calcite, and small amounts of quartz)and pure halite gouge. During quasi-static loading, the natural fault gouge exhibited stick-slip behavior characterized by lower stress drops and self-sustained oscillations during the “stick” phase, while the halite gouge demonstrated regular stick slip with large stress drops. Strain rosettes and acoustic sensors were used to measure shear strain along the gouge layer and detect acoustic emissions(AE)events. In each experiment, a sinusoidal normal stress oscillation with varying amplitudes was applied after several regular stick-slip cycles.

The results indicate that both gouge types exhibited similar mechanical behavior: shear stress drops, fault weakening, and an increase in stick-slip velocity with higher normal stress oscillation amplitudes. However, distinct differences were observed in the strain field response and the distribution of AE events. As the oscillation amplitude increased, the fault with natural fault gouge showed a growing number of nucleation zones, with no AE events occurring during nucleation. Fault rupture eventually became more homogeneous after the stress drop. In contrast, the halite gouge maintained a constant number of nucleation zones, but both the nucleation time and area increased with higher oscillation amplitudes. AE events persisted throughout the nucleation phase, and their locations corresponded closely with the nucleation zones. The fault rupture process remained homogeneous under all conditions for the halite gouge.

Analysis of the normal strain evolution and Young's moduli for the two gouge materials revealed that the natural fault gouge exhibited significantly higher Young's modulus than that of the halite gouge. This suggests that fault heterogeneity and plastic deformation are key factors in the evolution of the strain field. Materials with higher modulus enhance strain field heterogeneity as normal stress oscillation amplitude increases, since asperities in these materials are more responsive to stress variations. In contrast, the softer halite gouge exhibits less sensitivity to stress changes, leading to fewer variations in strain. This mechanism can be well reflected by the spatial-temporal evolution of normal strain for two types of fault gouge layers. This explains why the number of nucleation zones increases with higher oscillation amplitudes in natural gouge but remains unchanged in halite gouge. These findings offer valuable insights for the development of near-surface monitoring programs aimed at assessing seismic activity.

MECHANICAL TWINNING OF CALCITE IN THE LONGMENSHAN FAULT ZONE AND ITS IMPLICATIONS FOR FAULT FRICTION COEFFICIENT

CUI Ya-qi, DANG Jia-xiang, ZHOU Yong-sheng

2025, 47(1):  150-166.  DOI: 10.3969/j.issn.0253-4967.2025.01.010

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The friction coefficient, a critical parameter governing fault sliding, plays an essential role in evaluating fault stability and assessing seismic hazards. Its accurate determination is a fundamental aspect of fault research, and traditional methods for obtaining this coefficient typically rely on invasive techniques such as scientific drilling or friction experiments. These conventional approaches are often costly, time-consuming, and limited by their ability to capture fault zone conditions in a comprehensive and non-invasive manner. In contrast, this study proposes a more straightforward and practical approach for estimating and constraining the sliding friction coefficient of the Longmenshan fault zone, utilizing statistical analysis of calcite mechanical twin crystal density as an indirect indicator of fault slip behavior.

Calcite mechanical twin crystals, which form as a result of faulting processes, are sensitive to the temperature and pressure conditions within the formation environment. The microstructural characteristics of these twin crystals record the maximum stress experienced by different parts of the fault zone, thereby providing a valuable record of fault activity. As a result, stress values near the fault sliding surface can be used to indirectly estimate the friction coefficient of the fault. To achieve this, high-definition images of calcite twin crystals were captured under an optical microscope, followed by processing to derive the twin crystal density values. These measurements were then used in conjunction with corrected stress meter results from prior research, which were obtained through triaxial compression and torsion experiments. The twin crystal density values were incorporated into calculations of the differential stress experienced by the fault zone, and, using the known relationship between differential stress and friction, the friction coefficient was derived. To ensure the accuracy of the results, stress values for the surrounding rock were provided through regional geological survey data.

The statistical analysis revealed that the twin crystal density within the sliding zone increased markedly as the distance from the sliding surface decreased. Specifically, the twin crystal density increased from (87.39±35)mm^-1 in sample DS-1 to (218.63±36)mm^-1 in the immediate vicinity of the sliding surface. The corresponding historical maximum differential stresses were calculated based on previous stress gauge corrections, yielding values of (182.28±25)MPa and (288.30±25)MPa, respectively. Additionally, the maximum burial depth of the exposed strata was estimated to be approximately 5km, which led to the determination of confining pressure (S_V-P₀) of 83.3MPa on the outcrop. Here, S_V represents the stress of the overlying rock layers, while P₀ denotes the pore pressure. Using these values, the friction coefficients of the Longmenshan fault zone during horizontal and reverse faulting were calculated to be 0.61 and 0.13, respectively. These calculated values correspond to the upper limit of the sliding friction coefficient for the fault zone.

This study focuses specifically on the analysis of type Ⅱ mechanical twin crystals of calcite found in the surface outcrops of the Longmenshan fault zone. A particular emphasis is placed on the statistical analysis of twin crystal density the calculation of differential stress and friction coefficients. By integrating previous geological survey data and research on tectonic stress and fault structure within the study area, the calculated friction coefficient was found to range from 0.13 to 0.61. These results are in close agreement with earlier experimental and observational studies, supporting the feasibility and reliability of the method used to estimate fault zone friction coefficients. Based on calcite mechanical twin density, this approach presents a viable, non-invasive, and effective means of estimating friction coefficients in fault zones, with significant implications for understanding fault behavior and seismic risk assessment.

THREE-DIMENSIONAL SURFACE COSEISMIC DISPLACEMENTS FROM DIFFERENCING PRE- AND POST-EARTHQUAKE TERRAIN POINT CLOUDS

WEI Zhan-yu, HE Hong-lin, DENG Ya-ting, XI Xi

2025, 47(1):  167-188.  DOI: 10.3969/j.issn.0253-4967.2025.01.011

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After a major earthquake, in addition to determining the location and magnitude of the earthquake, the analysis of the coseismic displacement and deformation patterns near the seismic rupture zone is equally critical, which is crucial for an in-depth understanding of the seismic rupture process, fault behavior, and the relationship between active faults and topography. Although the geodetic techniques for observing the coseismic displacement field of the earth's surface at various spatial and temporal scales are developing rapidly, obtaining a detailed seismic rupture zone and displacement field remains challenging. Current techniques for obtaining these measurements, including Global Navigation Satellite Systems(GNSS), radar or optical remote sensing, and field observations, have their respective limitations regarding observation density, coverage area, measurement dimensions, and operational efficiency. Recently, the widespread adoption and application of high-precision and high-density topographic observation technologies(such as SfM and LiDAR)have enabled geomorphologists and geologists to capture various Earth surface features with unprecedented spatiotemporal resolution and detail. This has made it possible to map detailed three-dimensional displacement fields.

This paper introduces an Iterative Closest Point(ICP)algorithm that uses pre- and post-earthquake topographic point clouds to determine near-field three-dimensional coseismic surface displacements. The main purpose of developing this algorithm is to quickly provide data on the coseismic displacement field near the seismic rupture zone after a major earthquake, compensating for the deficiencies of existing geodetic measurements or field observations. To test the applicability and workflow of the ICP algorithm for obtaining topographic data through aerial photogrammetry in the Sichuan-Yunnan region, we selected two sets of SfM topographic point cloud data from the Jiaoji River fault on the Daliangshan fault zone. By superimposing the coseismic deformation field to simulate the pre-earthquake and post-earthquake topographic point cloud sets, we explored the accuracy of this method under different grid sizes and point cloud densities. This method can accurately recover the direction and magnitude of the coseismic displacement field under a grid size exceeding 50 meters, with horizontal and vertical accuracies of approximately 20cm and 10cm, respectively, comparable to the positioning accuracy of the topographic point clouds. As the point cloud density and grid window size decrease, the accuracy of this method in recovering co-seismic displacement fields declines.

By analyzing the potential impact of terrain changes such as tree growth, house construction, and river erosion on displacement field recovery, the results show that increasing the grid size allows pre- and post-earthquake point clouds to have sufficient terrain structure for matching, reducing the impact of local terrain changes on displacement field recovery. The grid window size is a trade-off between 1)a large scale with sufficient terrain structure and 2)a smaller scale with finer resolution. The ICP method, utilizing high-precision point clouds from before and after the earthquake, can obtain detailed three-dimensional surface displacement fields near earthquake rupture zones, providing new constraints for shallow fault slip and rupture zone deformation and aiding the study of seismic rupture processes and fault growth.

CRUSTAL DEFORMATION CHARACTERISTICS AND RELEVANT SEISMIC HAZARD OF THE TAIYUAN BASIN

CHEN Qian, ZHANG Zhu-qi

2025, 47(1):  189-213.  DOI: 10.3969/j.issn.0253-4967.2025.01.012

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The Taiyuan Basin(TB) is located at the middle part of the Shanxi Graben belt(SGB), which is the tectonic boundary between the Ordos active block in the West and the North China Plain in the East. The SGB comprises several echelon left-stepping depression basins lining up in NNE direction. Previous studies of active tectonics and crustal kinematics indicate that the present deformation of the SGB is controlled by the NNE dextral shear and NW extension between the blocks on both sides of it, but how the shear and extension are accommodated by basins remains in question. To answer the question, researchers in active tectonics have made more effort to investigate the slip type and rate of faults bounding basins. By contrast, inadequate attentions were paid to the kinematic characteristics of the internal deformation within basins. Recent studies on focal mechanisms find that not only normal slip earthquakes but also strike-slip earthquakes, which account for a major proportion of the whole seismicity, occur in the depression basins of the SGB.As can be seen, faulting with dominating normal slip on basin boundary faults cannot easily and reasonably explain how the SGB deforms at present. To comprehensively understand the present deformation mechanism of the SGB, further exploration is needed, which should be based on both the deformation characteristics of basin boundary faults and the kinematic characteristics of the relevant blocks including basins. Compared with other basins in the SGB, the TB has relatively simple features in tectonics, which an explicit kinematic model may describe. Therefore, the TB is potentially proper to demonstrate how basins deform in the SGB.

Developing typical tectonics like other basins of the SGB, the TB behaves oddly at the same time in terms of seismicity. Constituting the middle segment of a large-scale active block boundary and developing active faults reaching the middle and lower crust, this tectonic basis jointly imply the TB is eligible for the occurrence of large earthquakes. However, the historical record of earthquakes with a magnitude greater than 7 is lacking in the TB.In contrast to that, several earthquakes with a magnitude of 7 or even up to 8 have occurred in the last thousand years in the Xinding Basin and the Linfen Basin adjacent to the TB.So far, it is still uncertain how possible and how urgent a big earthquake could strike the TB in the future. To address this question, a thorough evaluation of the accumulation state of seismic energy on the boundary faults of TB is necessary.

Modeling the kinematics of block and fault with constraints from GPS velocity usually gives the spatial distributions of slip rate and locking ratio on faults, and also Euler rotation velocities and strain rates of blocks. However, due to the azimuthal heterogeneity and the sparsity in the distribution of GPS stations, the result of modeling block-fault kinematics sometimes concerns trade-offs between variables, and the optimal solution may be discrepant with the given tectonic knowledge. Furthermore, for the fault with a low dip angle, the overlap of surface strain due to faulting and that due to internal deformation of the hanging wall block could be large, which may aggravate the mentioned trade-off and discrepancy. As the block-fault kinematic model related to TB involves the above issues, appropriate conditions may be carefully selected to enhance the constraints of modeling.

In this study, a three-dimensional geometric model of two boundary faults of the TB is constructed based on the data from earthquake geology and deep exploration. A kinematic model of the blocks and the boundary faults relating to the TB is constructed based on the geometric model. According to previous studies on regional tectonics, this study attempted to obtain appropriate a priori information to provide better constraints on the modeling. Several strategies of modeling were designed based on the characteristics of a priori information. Constrained by a priori conditions, the GPS velocities at 137 sites from two data sources are simulated to get the fundamental kinematic features for the blocks and the boundary faults concerning the TB, which include the vertical axis rotation velocity and translational velocity at the centroid of the TB and the blocks in its neighbor, the distribution of locking ratio on the fault planes, and additionally the crustal strain rate within the basin. An apparent optimal model is obtained considering not only its fitting to the GPS velocities but also the good correlation between the observational features of principal axes of strain rate and that from the modeling. The results of the apparent optimal model show that the TB is spinning clockwise, and bearing the internal strain with principal axis orientating in NW.The blocks on the west side and the east side of the TB are both translating toward approximately SE under the stable Eurasian reference frame, but the east block TH is moving faster with more southward component relative to the west block LL.Furthermore, the LL and TH blocks are spinning counterclockwise. As the western boundary of the TB, the Jiaocheng fault is a normal fault with a right-lateral component. In contrast, the Taigu fault, which is on the eastern side of the basin, is a right-lateral fault with a normal component. According to the above features of the apparent optimal model, it is inferred that the combination of the tensional function across the western basin boundary and the strike-slip shearing along the eastern boundary contributes to the clockwise rotation of the TB and also causes the internal NNE dextral shear and NW extension within the basin. The model results also show extensive locking areas on the Jiaocheng fault and the Taigu fault. Taking into account of the relevent research on the recurrence of paleo-earthquakes since the late Pleistocene, the locking status of the fault indicates that the Jiaocheng fault is close to the occurrence of an earthquake greater than M7.5.In contrast, the Taigu fault has accumulated enough energy for an earthquake around M6.7, which appeals to attention and further studies on the possible occurrence of strong earthquakes in the vicinity of the TB.

DISCOVERY OF A STRONG PALEOSEISMIC EVENT IN THE CHENGHAI-BINCHUAN FAULT ZONE

LUO Lin, CHANG Zu-feng, YIN Gong-ming, MAO Ze-bin, HUA Jun, CHEN Gang

2025, 47(1):  214-234.  DOI: 10.3969/j.issn.0253-4967.2025.01.013

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The Chenghai-Binchuan fault zone is an active Holocene fault zone approximately 200km in length. The region is characterized by sharply uplifted mountains and canyon landforms, intense erosion, and frequent geological hazards such as collapses, landslides, and debris flows. These factors have obscured the preservation of displaced landforms, making it difficult to identify paleoearthquake relics and hindering the study of paleoearthquakes.

The Taoyuan Basin, a tectonic basin formed along the Chenghai-Binchuan fault zone, has undergone extensive land planning and hillside improvements as part of the resettlement efforts linked to the Ludila hydropower project on the Jinsha River. As a result, many artificial slopes have been excavated in the area. Approximately 500m northeast of Chitian village, southwest of Taoyuan town, near the confluence of the Jinsha and Kumu rivers(100.4489°E; 26.1926°N), farmers excavated a large slope for land planning and slope stabilization. This slope, approximately 50m in length and over 10m in height, exposes a relatively continuous sedimentary sequence, including sand and gravel layers. The exposed strata display typical paleoearthquake features, such as fault displacements, seismic wedges, sand veins, and soft sediment deformation, all of which are well-preserved and provide valuable insights into paleoearthquake activity.

Paleoearthquake research was conducted through satellite remote sensing interpretation, field geological and geomorphological surveys, slope profile excavation and measurement, detailed stratigraphic description, and chronological testing of sediment samples.

The findings are as follows: 1)Two fault sets, striking NW and NE, were identified on the profile. The sedimentary characteristics of each stratigraphic unit, along with the relationships between stratigraphic displacement, marker layer offsets, and features such as colluvial wedges, soft sediment deformation, and sand veins, suggest that a significant earthquake occurred in the area. Radiocarbon dating(AMS ¹⁴C)indicates the paleoearthquake event occurred between(5910±30)aBP and(4100±30)aBP. 2)The maximum vertical co-seismic displacement along the fault is 4.0m. Using the empirical relationship between fault displacement and earthquake magnitude, the moment magnitude(M_W)is estimated at 7.3, similar to the historical 1515 Yongsheng earthquake(M_W7.3). These results fill a critical gap in paleoearthquake data for the Chenghai-Binchuan fault zone and extend the region's paleoearthquake record. This research holds significant practical value for earthquake hazard zoning and seismic risk analysis for major regional projects.

PRECISE JUDGMENT AND THINKING ON THE 2022 MENYUAN M_S6.9 AND LUDING M_S6.8 EPICENTERS

ZHU Yi-qing, ZHAO Yun-feng, WEI Shou-chun, ZHANG Guo-qing, LIU Fang

2025, 47(1):  235-245.  DOI: 10.3969/j.issn.0253-4967.2025.01.014

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In 2022, the Menyuan M_S6.9 and Luding M_S6.8 earthquakes occurred successively in the north-south seismic belt of China, both of which caused severe property damage. Prior to these two earthquakes, the China Earthquake Administration conducted multiple ground gravity observations in the North-South seismic zone. It successfully made mid-term predictions based on mobile gravity observation data. The actual epicenters of the two earthquakes were not more than 56km away from the center of the predicted earthquake risk zone in 2022, especially in the precise determination of the epicenter position in 2021 and the distance between the actual epicenter and the predicted epicenter was not more than 10km. This article analyzes ground gravity fields' dynamic evolution characteristics and laws at different temporal and spatial scales before the 2022 Menyuan M_S6.9 and Luding M_S6.8 earthquakes. It reviews the prediction process and basis of these two earthquakes, which is helpful for the study of medium and short-term earthquake prediction.

Overall processing and analysis of multi-source gravity observation data in the north-south seismic zone, obtaining regional gravity field changes based on absolute gravity as the unified benchmark in the measurement area and analysis and study of the dynamic changes of the regional gravity field at different spatiotemporal scales before the Menyuan M_S6.9 and Luding M_S6.8 earthquakes. Further study of the spatiotemporal distribution characteristics and precursor mechanisms of regional gravity field changes will be done by combining geological tectonic dynamic environment and strong regional seismic activity events.

1)Before the Menyuan M_S6.9 earthquake, the gravity change first appeared as a gradient zone of gravity change that was consistent with the direction of the Qilian Mountain fault structure belt and then showed certain four-quadrant distribution characteristics around the epicenter area. The earthquake occurred at the center of the four quadrants of gravity change and near the zero value line of gravity change. 2)Before the Luding M_S6.8 earthquake, the gravity change first showed a gradient zone of gravity change that was consistent with the direction of the Xianshuihe fault structural belt and then showed significant four-quadrant distribution characteristics around the epicenter area. The earthquake occurred at the center of the four quadrants of gravity change and near the zero-value line of gravity change. 3)Gravity data reflects both earthquakes well, and obvious four quadrant distribution characteristics and high gradient zones of gravity changes were observed in the epicenter area and its vicinity before the earthquake, which may be gravity precursor information observed during the earthquake preparation process.

In the past, it was very difficult to accurately determine the location of the three elements of earthquakes(time, location, and magnitude). However, suppose potential strong earthquake epicenters and possible magnitudes are delineated. In that case, the determination of the three elements of earthquakes in the future will become the determination of one element, which will greatly improve the prediction ability of earthquakes The next step is how to carry out earthquake monitoring and tracking work in the identified potential epicenter area and its vicinity, which may help with short-term and imminent prediction. Mobile gravity measurement is the main observation method for exploring earthquake prediction. By establishing a high-density atomic gravimeter absolute gravity observation network or a relative gravity observation network with reasonable absolute gravity control and conducting intensive monitoring, it is possible to reveal the distribution characteristics of underground structures near the epicenter of high-risk areas and extract absolute gravity change signals accompanying source changes during the preparation process of strong/large earthquakes, Develop its application and research in short-term earthquake prediction.

THE CHARACTERISTICS OF TERRESTRIAL TIME-VARYING GRAVITY CHANGES AND EQUIVALENT SOURCE INVERSION ON THE SOUTHEASTERN MARGIN OF THE TIBETAN PLATEAU

ZHENG Qiu-yue, CHEN Zheng-yu, WU Yu-qin, HUANG Jiang-pei, LIU Dong, WANG Qing-hua

2025, 47(1):  246-266.  DOI: 10.3969/j.issn.0253-4967.2025.01.015

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This study analyzes terrestrial time-varying gravity data from the southeastern margin of the Tibetan Plateau, covering the period from 2014 to 2022. Using the Bayesian adjustment method, we first captured temporal variations in the regional gravity field at different time scales. Then, the Tesseriod model was applied to simulate the field source medium and construct an equivalent source model. The gravity network's ability to resolve field sources was further evaluated using a checkerboard model. In regions with high source resolution, we inverted the apparent density variations of equivalent sources to examine tectonic changes and earthquake generation. The main findings are as follows:

1)The gravity network effectively reflects recent regional earthquakes(M≥5.0), with most events occurring in the centers of four quadrants of gravity variation or within high-gradient zones of positive and negative anomalies. 2)Analysis of the field source resolution capability, based on various checkerboard models, indicates that most areas of the gravity survey network have a resolution better than 1°×1°, except at the network edges. Notably, resolution improves to 0.5°×0.5° near the Dali, Anninghe-Zemuhe-Xiaojiang fault, and Xichang regions, along the boundary of the Grade I active block. 3)Long-term gravity field inversion results reveal apparent density changes ranging from -1.2kg/m³ to 1.2kg/m³ within 1km-thick equivalent layers at depths of up to 20km. This variation is approximately 0.4‰ of the average crustal density. Spatially, regions of increased apparent density correlate with active faults along the western boundary of the Sichuan-Yunnan block(near Dali-Xiangcheng)and the Xiaojiang fault zone on its eastern boundary. 4)Apparent density changes indicate energy accumulation preceding the Yangbi M_S6.4 earthquake and two M5.0 earthquakes in Tonghai. The data capture pre-earthquake tectonic activity and subsequent crustal adjustments that led to decreased apparent density post-earthquake. Energy accumulation for the Yangbi earthquake is inferred to have started in 2015, driven by southward extrusion of materials along the eastern edge of the Tibetan Plateau and deep crustal migration. The complex location of the epicenter—at the intersection of the Weixi-Qiaohou and Honghe fault zones—may explain the area's frequent seismicity. Moreover, this region is characterized by low velocity and high conductivity, suggesting that fluid-channel materials surged into the source area, reactivating previously unmapped faults and triggering the Yangbi earthquake. Apparent density changes along the southern Xiaojing fault zone similarly show stress accumulation before the Tonghai M5.0 earthquakes, followed by a notable post-earthquake decrease and a rising trend in density between 2021 and 2022. The southern Xiaojing fault zone thus warrants close monitoring as a significant earthquake-prone region. 5)The long-term increase in apparent density results from continued southward or southeastward material extrusion and deep crustal migration along the southeastern margin of the Tibetan plateau. The spatial distribution aligns with previously identified low-velocity and high-conductivity areas. 6)Apparent density fluctuations correspond to the spatiotemporal distribution of M≥5 earthquakes, with these events often occurring at the edges of regions with increased density or at transitions between positive and negative density zones, aligning with the “hypocentroid and epicentroid” principle.

In summary, this study documents gravity field and apparent density variations across the region, providing insights for quantitative interpretation in geological and geophysical research. These findings help identify deep field source signals associated with earthquake generation, reveal relationships between regional gravity field variation and tectonic movement, and offer a better understanding of dynamic process in deep crust in the Tibetan Plateau.

THE STUDY OF FINE CRUSTAL STRUCTURE OF THE SOUTHERN MARGIN OF TAIHANG MOUNTAIN BY DEEP SEISMIC REFLECTION PROFILE

FENG Shao-ying, LIU Bao-jin, ZUO Ying, JI Ji-fa, TAN Ya-li, DING Kui, WU Quan

2025, 47(1):  267-283.  DOI: 10.3969/j.issn.0253-4967.2025.01.016

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To investigate the fine crustal structure and tectonic characteristics of the southern margin of the Taihang Mountain, we conducted a deep seismic reflection survey along a 120km profile between Huixian and Changyuan.

Regarding data acquisition, we applied a geometry with a 25m group interval, 800 recording channels, and more than 60-fold coverage. The seismic wave explosion utilized a dynamite source of 30kg, with a hole depth of 25m. The shot interval was 200m. In data processing, the most critical aspect was improving the signal-to-noise ratio. The data processing workflow mainly included static correction(first-break tomography method), multi-type and multi-method denoising before stacking, amplitude compensation, surface-consistent predictive deconvolution, several iterations of velocity analysis and residual static correction, dip movement correction, and post-stack finite-difference migration.

A seismic section with a high signal-to-noise ratio was obtained, revealing the fine crustal structure along the survey line. The crustal structure in the section exhibits clear stratification, with crustal thickness ranging from approximately 33.5km to 42.7km. The crust contains distinct reflective structures from top to bottom, being thinner in the southeast and thicker in the northwest. The upper crust is about 13.3km thick in the southeast and approximately 20.1km in the northwest. The basement reflection deepens from west to east along the profile. In the southern Taihang Mountain, the depth of the basement reflection is about 1.0~1.5km in the northwest side of the Tangyin fault depression. Within the Tangyin fault depression, the basement reflection depth is approximately 3.5~4.0km. East of stake 56.0km, the basement surface tilts eastward, with its deepest point reaching approximately 8.0~10.0km. The lower crust exhibits strong reflective properties, consisting of a series of high-energy reflection waves, with inclined and arc-shaped strong reflections indicating significant heterogeneity in lower crust composition.

The Moho shows relatively strong reflected energy and good lateral continuity. It gradually deepens from southeast to northwest, with its shallowest depth at approximately 33.5km.

Numerous fault structures are observed along the deep seismic reflection profile. We identified ten faults in the upper crust and one deep fault. In the northwest section of the profile, three faults form a Y-shaped distribution. The upper part of the fault disrupts the bottom boundary of the Q+N strata and disappears downward into the upper crust. The Tangdong fault is the main controlling boundary fault of the Tangyin fault depression. It cuts through the shallow sedimentary strata and basement reflection wave groups as a shovel-shaped normal fault and disappears into the upper crust. The Tangxi fault and Tangzhong fault merge into the Tangdong fault at depths of approximately 8.0~10.0km and 5.0~6.0km, respectively. The Changcun fault and Changyuan fault, located in the eastern section of the profile, appear in the strata beneath the Neogene and terminate at the interface between the upper and lower crust, without cutting through the upper Q+N strata. The Yellow River Fault is a large-scale fault in the Dongpu depression and serves as the eastern boundary fault of the depression. Its shallow section consists of two inclined faults, with the main fault being a northwest-trending normal fault.

The deep fault appears as a nearly vertical weak-reflection energy strip on the section or as a discontinuous zone of reflection characteristics, located on the southeast side of the Tangyin fault depression. This fault zone represents a major deep crustal fault. It offsets intracrustal interface reflections, lower crustal reflection bands, and the crust-mantle interface reflection. The deep fault in the crust has become a channel, facilitating the upwelling of deep hot material. The upwelling of deep material, magma underplating, or thermal erosion leads to the tensile extension of the crust and lithospheric thinning, potentially altering crustal structure and material composition.

MOMENT TENSOR INVERSION AND SEISMOGENIC STRUCTURE OF THE 2023 M_S5.5 PINGYUAN EARTHQUAKE

XU Ying-cai, GUO Xiang-yun

2025, 47(1):  284-305.  DOI: 10.3969/j.issn.0253-4967.2025.01.017

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On August 6, 2023, an earthquake with M_S5.5 occurred in Pingyuan County, Dezhou City, in Shandong Province, North China. Around the epicenter of the M_S5.5 Pingyuan earthquake, obvious earthquake tremors were felt through the regions of Hebei, Henan, Tianjin, Beijing etc. This earthquake also broke the historical record of no earthquake with M≥5.0 within 50km of the epicenter and aroused widespread concern of the society and seismologists. It is urgent to carry out the research on strong earthquake seismogenic structures and seismogenic environments in the basin, which is of great significance to strengthen the seismic tectonic environment exploration and earthquake damage prevention in the weak and rare earthquake areas of the North China Plain. Based on the data of the Regional Seismic Networks around the epicenter, moment tensor solution and centroid depth of the M_S5.5 Pingyuan earthquake were determined using the gCAP inversion method, and the stability of moment tensor inversion of the mainshock is evaluated by Bootstrap method. The M_L≥1.0 earthquakes of the M_S5.5 Pingyuan earthquake sequence were relocated using the double-difference relocation method. By simulation method of the relationship between regional stress regime and focal mechanism, the relative shear stress and normal stress of nodal planes from focal mechanism of the M_S5.5 Pingyuan earthquake were obtained. According to the earthquake relocation, the fault plane was fitted, and the seismogenic structure of the earthquake was analyzed. The results indicate: 1)The scalar seismic moment(M₀)of the M_S5.5 Pingyuan earthquake is 1.97Í10¹⁷N·m, while the moment magnitude is M_W5.5, and the centroid depth is 16km. The moment tensor solution(M_rr/M_tt/M_pp/M_rt/M_rp/M_tp)is -0.129/1.194/-0.459/0.009/0.336/0.245, while the double-couple(DC), isotropic(ISO) and compensated linear vector dipole(CLVD) components account for 92.40%, 6.25%, and 1.35% of the full moment tensor solution. The focal mechanism of the double-couple solution shows that the M_S5.5 Pingyuan earthquake is a strike-slip type natural earthquake event with a little normal fault component, with strike/dip/rake of 222°/71°/-156° and 124°/67°/-21° for nodal planes Ⅰ and Ⅱ. The P-axis azimuth is 84° with plunge angle of 30°, indicating the principal compressive stress in the NEE direction in the focal area, basically consistent with the direction of the main compressive stress of the regional tectonic stress in the North China Plain. The best double-couple solutions of focal mechanism obtained by the Bootstrap method are: strike 220.4°±2.49°, dip 71.5°±4.34°, rake -155.9°±3.58° for the nodal plane Ⅰ, and strike 122.2°±2.83°, dip 67.3°±3.17°, rake -20.2°±4.95° for nodal plane Ⅱ, and centroid depth 16.7±1.10km, reflecting the stability and reliability of moment tensor inversion in this paper. 2)After earthquake relocation, the epicenters of the M_S5.5 Pingyuan earthquake sequence mainly show a dominant distribution in the NE-SW direction, with a length of about 15km and a width of about 5km. The epicenter of the M_S5.5 Pingyuan earthquake is located in the southwest segment of the earthquake sequence, and the epicenters of the M_L≥1.0 earthquake are relatively concentrated on the NE side of the main earthquake, while the distribution of the epicenters of the SW side is relatively sparse. The focal depths of earthquakes with M_L≥1.0 are mainly between 8 and 22km, with an average depth of 15.4km. The focal depth profiles demonstrate the fault plane with a dip to the northwest. 3)The regional stress field of North China Craton primarily exhibits NEE-SWW compression and near N-S tensile. The main stress axis direction of the regional stress field is basically consistent with the P-axis of the focal mechanism of the M_S5.5 Pingyuan earthquake. The strike-slip component of the focal mechanism of the M_S5.5 Pingyuan earthquake shows general consistency with the focal mechanism characteristics of the eastern North China Craton. It is also consistent with the E-W compressive and S-N tensile tectonic stress background of the eastern part of the North China Craton, and the small normal components may be related to the tensile movement of the central and western region of Bohai Bay Basin. The relationship between the focal mechanism and contemporary regional stress regime shows the relative shear stress of 0.860/0.689 for nodal planes I and II under the stress field of the Pingyuan region, and the rake of shear stress for the nodal plane I is close to the rake of focal mechanism. The shear stress on nodal plane I of the focal mechanism is relatively high, reflecting that nodal plane I is very close to the fault shape of the optimal release of relative shear stress of the regional stress field, presenting the source property of right-lateral strike-slip with a small amount of normal component. The fault plane fitting also exhibits the mode of fault movement with right-lateral strike-slip. 4)Combined with the previous studies, it is inferred that the nodal plane I of the focal mechanism for the M_S5.5 Pingyuan mainshock is the possible seismic source fault with a dip angle of 71°, and the main seismogenic structure of the Pingyuan earthquake sequence may be a SW-striking blind fault with steep dip to northwest. Under the long-term contemporary principal compressive stress of the tectonic stress field in North China Plain, the M_S5.5 Pingyuan earthquake occurred as a right-lateral strike-slip seismic dislocation with a little normal fault component due to the rupture on the nodal plane with optimum shear stress release. It is suggested that the seismogenic structure of the Pingyuan earthquake may be related to the Lingxian-Guanxian fault, a hidden fault system formed by the northern section of the Guanxian fault and the southern section of the Lingxian-Yangxin fault.

HIGH-RESOLUTION S-WAVE VELOCITY STRUCTURE OF BEIJING AREA USING AMBIENT NOISE TOMOGRAPHY

JI Yu, ZHANG Guang-wei, REN Jun-jie, HE Jing, WANG Xiao-wei

2025, 47(1):  306-324.  DOI: 10.3969/j.issn.0253-4967.2025.01.018

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Beijing is located at the intersection of the Yanshan Mountains, Taihang Mountains, and the northwestern part of the North China Basin. This region has experienced multiple periods of crustal movement, forming a series of Cenozoic Basins exhibiting alternating uplifts and depressions. The intense tectonic activity has resulted in a relatively complex fault system, with the main directions of the faults being NE and NW. Notably, the Xixiadian Fault in the NE direction experienced an earthquake with a magnitude of 8 in 1679. Meanwhile, the NW-oriented Nankou-Sunhe Fault offsets multiple NE-oriented faults and directly passes through urban residential areas, making it an important active fault in Beijing. Although these intersecting faults have not experienced earthquakes of magnitude 4 or higher in recent decades, small seismic events(M<4.0)still occur frequently. Furthermore, the thickness of Quaternary sedimentary layers in the Beijing area varies significantly. These relatively low-velocity sedimentary layers can cause seismic site effects such as amplitude amplification of seismic waves and increased vibration duration. Therefore, disaster detailed study of the three-dimensional velocity structure of the upper crust and depression areas in Beijing is of great significance for analyzing potential disasters and assessing seismic hazards.

This study utilized continuous waveform data from 28 permanent seismic stations. The stations are distributed across the Beijing area, with an average inter-station distance of approximately 30km. This study's continuous constant waveform data spans from April 2022 to October 2022. For the Nankou-Sunhe Fault in the Changping area, we employed 109 short-period temporary seismic stations located around the fault zone. The seismic data is recorded from July 2, 2022, to August 3, 2022, over 33 days, with an average inter-station distance of approximately 2km. By applying the ambient noise tomography method, we successfully inverted the three-dimensional S-wave velocity structures of the upper crust for the Beijing region and the near-surface for the Nankou-Sunhe fault zone.

Our results reveal that the velocity structure beneath the Beijing area exhibits significant lateral heterogeneity. The Yanshan uplift in the north, the Jingxi uplift in the west, and the Daxing uplift in the southeastern part of the study area present obvious high-velocity anomalies, reflecting a relatively dense crystalline basement. In the Dachang depression and Shunyi depression areas, the structures from shallow to 7km depth show characteristics of low-velocity anomalies, indicating a relatively deep sedimentary layer in this area. Meanwhile, the distribution of low-velocity anomalies within the study area mainly presents a NE direction, consistent with the dominant strike direction of fault zones. Based on the results of small-scale dense seismic array inversion, it shows that the dip angle of the Nankou-Sunhe Fault is quite steep, controlling the northern boundary of the Shahe depression, and the sedimentary thickness of the Shahe depression is much deeper than that of the Shangzhuang depression. Overall, the significant low-velocity anomaly in the depression reflects the direction of the fault controls the deep sedimentary layer, and the boundary of the depression. The velocity structure in the uplift area shows a high-velocity anomaly, which reflects the relatively old rock stratum.

Additionally, the inversion results based on a small-scale dense array reveal that the fault plane of the Nankou-Sunhe Fault is relatively steep, which controls the northern boundary of the Shahe depression. The thickness of the sedimentary layer in the Shahe Depression varies significantly along the fault strike, and the deepest layer is up to 1.5km, which is much deeper than that of the Shangzhuang Depression. Furthermore, the detailed three-dimensional velocity structure model in this study provides direct seismological evidence for the existence of the Dongbeiwang-Xiaotangshan buried fault in Beijing. In summary, this study shows that seismic tomography using urban ambient noise can efficiently and economically obtain the three-dimensional velocity structure, as well as has broad prospects in identifying geological structural units and hidden faults.

THE COULOMB STRESS TRIGGERING EFFECT OF 2016 M_W5.9 AND 2022 M_W6.7 EARTHQUAKES IN MENYUAN, QINGHAI AND THEIR INFLUENCE ON THE SURROUNDING SEISMOGENIC FAULTS

NIU Peng-fei, HAN Zhu-jun, GUO Peng, LIU Guan-shen, LUO Jia-hong, GUO Zhao-wu

2025, 47(1):  325-344.  DOI: 10.3969/j.issn.0253-4967.2025.01.019

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Strong earthquakes are among the most severe natural disasters on Earth. Earthquake rupture involves complex physical processes, including fault slip, stress release, and seismic wave propagation. A deep understanding of the rupture processes of major earthquakes and the potential for triggered activity within regional active fault systems is critical for assessing the risk of strong earthquakes in a given area. Analyzing the stress transfer between earthquakes helps assess and evaluate the future seismic hazard. The Qilian-Haiyuan fault zone is an important boundary fault on the northeastern margin of the Tibetan 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 1000km, it is primarily composed of the Tolaishan Fault, the Lenglongling Fault, the Jinqianghe Fault, the Maomaoshan Fault, the Laohushan Fault, and the Haiyuan Fault. The 2016 M_W5.9 Menyuan earthquake and the 2022 M_W6.7 Menyuan earthquake both occurred in the Lenglongling segment of the Qilian-Haiyuan fault zone. Studying the effects of these two earthquakes on the Coulomb stress of the “Tuolaishan seismic gap” to the west can provide the scientific basis for a deeper understanding of the regional seismic hazard. This article is based on the Coulomb stress theory, a finite fault model provided by the United States Geological Survey(USGS), and the geometric and kinematic parameters of the main seismogenic fault determined through field investigations. It analyzes and discusses the triggering mechanism of the 2016 earthquake on the 2022 earthquake, the Coulomb stress effects of the 2022 earthquake on regional strong aftershocks, and the impact of the two earthquakes on the main seismogenic faults in the unoccupied area. The results indicate that the 2016 earthquake triggered the 2022 earthquake. Specifically, the strong aftershocks of M_W5.1, M_W5.2, and M_W4.7 on January 8 and 12 in 2022 were significantly stress-triggered by the main shock of M_W6.7 on January 8, while the M_W4.8 aftershock on January 12th was located in a Coulomb stress unloading zone, possibly delaying its occurrence. The two Menyuan earthquakes in 2016 and 2022 significantly loaded Coulomb stress on the eastern segments of the Tuolaishan fault zone and the Sunan-Qilian fault zone, with the latter experiencing Coulomb stress changes of 0.3891bar and 0.1658bar, exceeding the commonly accepted seismic triggering threshold of 0.1bar. The 1920 Haiyuan earthquake, the 1927 Gulang earthquake, and the 2016 and 2022 Menyuan earthquakes show a spatial westward migration pattern, which may indicate a westward stress transfer trend. After the 2022 earthquake, the aftershock zones were mainly distributed in two directions: east-west(EW)and north-northwest(NNW). The former extends toward the eastern segment of the Tuolaishan fault zone, where stress was not fully released during the 2022 earthquake. The NNW-trending aftershock zone extends toward the east segment of the Subei-Qilian fault zone, where there is a lack of aftershocks, indicating that fault stress was not fully released during the earthquake. The eastern segments of the Tuolaishan fault zone and the Sunan-Qilian fault zone jointly absorbed and accommodated the left-lateral shear deformation of the central-western segment of the Qilian-Haiyuan fault zone. Trench excavation revealed that the most recent surface-rupturing earthquake on the eastern segment of the Tuolaishan fault zone occurred(6080±450) a^{}BP, while that on the east segment of the Sunan-Qilian fault zone occurred between 3500 and 2328 a^{}BP. Therefore, by comprehensively analyzing data on the spatial migration trends of strong earthquakes, the spatial distribution of aftershocks, strain distribution, and studies on ancient earthquakes, it was found that the stress accumulation in the eastern segment of the Tuolai Mountain fault zone and the eastern segment of the Sunan-Qilian fault zone may be at a relatively high level. With the Coulomb stress loading from the two recent Menyuan earthquakes, the future seismic hazard of the eastern segments of these two fault zones is likely to increase further. The results of this study provide a reference basis for delineating seismic hazard zones on the northeastern margin of the Qinghai-Xizang Plateau.

THE RANGE OF DEFORMED ZONE BASED BY ACTIVE FAULT OFFSET BASED ON STATISTICAL ANALYSIS OF SURFACE RUPTURE ZONE WIDTH

ZHANG Wei-heng, ZHANG Dong-sheng, CHEN Jie, TIAN Qin-jian, HE Wan-tong

2025, 47(1):  345-366.  DOI: 10.3969/j.issn.0253-4967.2025.01.020

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Analyzing the statistical data on the widths of earthquake surface rupture zones is one of the most objective methods for determining the extent of active fault offsets and deformed zones. This information is crucial for establishing the setback distances for significant engineering projects in areas affected by active faults. In this study, we collected data from 92 cases to compute the widths of surface rupture zones and subsequently analyzed the extent of active fault offsets and deformed zones. Specifically, our dataset includes 75 cases with surface rupture zone widths extracted from published sources and 49 cases with available surface rupture vector data.

When compiling the statistical data on surface rupture widths, we categorized the data based on whether the ruptures occurred along straight fault segments or along more geometrically complex segments. For the vector data, we normalized the distributed rupture length and primary rupture length for each case to determine the distribution proportion of the rupture zones relative to fault distance.

The statistical analysis of document data shows that the total width of surface rupture zones in geometrically complex sections of normal, thrust, and strike-slip faults does not exceed 8100m, 3700m, and 10100m, respectively. In contrast, the widths of surface rupture zones in straight fault sections are generally narrower, with maximum widths of 160m, 120m, and 400m for normal, thrust, and strike-slip faults, respectively. The vector data further reveal that the maximum distances between the distributed ruptures and the primary fault are as follows: 19212m for the hanging wall of normal faults, 16244m for the footwall of normal faults, 7579m for the hanging wall of thrust faults, 4216m for the footwall of thrust faults, and 120456m for strike-slip faults. Moreover, the maximum distances from the primary fault to the margin of the closely distributed rupture zones are 700m, 200m, 1000m, 400m, and 500m, respectively. It is important to note that when excluding exceptional cases, such as earthquakes associated with fold-related faults or gently dipping thrust faults, the maximum distance from the margin of the closely distributed rupture zone to the primary fault is as follows: 400m for the hanging wall of normal faults, 200m for the footwall of normal faults, 500m for the hanging wall of thrust faults, 200m for the footwall of thrust faults, and 400m for strike-slip faults. Based on this comprehensive analysis, we suggest that the range of active fault offset and deformed zones should be considered as 400~500m from the primary fault. However, further research is needed to accurately determine the extent of fault offsets and deformed zones in areas with complex structural features, such as fault bends, step zones, fault tails, and the hanging walls of foreland thrust faults.

Future studies would benefit from incorporating more extensive and detailed surface rupture data. Continued data collection is essential to improve the accuracy and robustness of the results.