SEISMOLOGY AND GEOLOGY

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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

SEISMOLOGY AND GEOLOGY 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.

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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

SEISMOLOGY AND GEOLOGY 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.

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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

SEISMOLOGY AND GEOLOGY 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.

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SEISMOGENIC FAULT OF THE TANGSHAN M_S5.1 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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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

SEISMOLOGY AND GEOLOGY 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.

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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

SEISMOLOGY AND GEOLOGY 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.

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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

SEISMOLOGY AND GEOLOGY 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.

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THE DISASTER MECHANISM OF THE M_S6.9 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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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.

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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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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

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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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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

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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 M_W7.0. 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 (V_S30) 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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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

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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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DOUBLE-DIFFERENCE RELOCATION OF YUNNAN YANGBI M_S6.4 EARTHQUAKE SEQUENCE ON MAY 21, 2021 AND TECTONIC IMPLICATIONS

XU Yong-qiang, LEI Jian-she, HU Xiao-hui

SEISMOLOGY AND GEOLOGY 2024, 46 (5): 1066-1090. DOI: 10.3969/j.issn.0253-4967.2024.05.005

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At 21:48 on May 21, 2021(Beijing time), the M_S6.4 earthquake occurred in western Town(25.700°N, 99.880°E), Yangbi County, Dali, Yunnan Province, with a focal depth of 10km(China Earthquake Networks Center). The Yangbi earthquake is a typical type of foreshock-mainshock-aftershock earthquake, which had a significant impact on the local residents and attracted great attention from society. To better understand the seismogenic structure and mechanism of this earthquake, the present study relocates the May 21, 2021 Yangbi M_S6.4 earthquake sequence, collected from the China Earthquake Networks Center from 2021 to June 18, 2022. Finally, 2681 precisely located events are obtained through the double-difference relocation algorithm. Our results show that the Yangbi earthquake sequence extended for about 32km, mainly along the NW-SE direction, and it is an overall echelon structure changing from narrow in the northwest to broad in the southeast. The dominant depth of the earthquake sequence is 5-10km. The foreshocks were mainly active in the northern section of this earthquake sequence, with the mainshock being a unilateral rupture. The aftershocks primarily extended in the southeast direction, but the southeast extension process was not simply a unilateral extension. Multiple secondary oblique activity sequences were derived on the west side of the sequence. With the continuous release of stress in the study area, only the main rupture continued to be active in the southeastern section of the sequence in the later stage of activity. Still, the secondary oblique ruptures that evolved was no longer active. The average location errors of these earthquakes are about 0.47km in the east-west direction, about 0.50km in the north-south direction, and 0.62km in the vertical direction, and the average RMS travel-time residual is 0.22s.

This study collects broadband digital seismic waveform data of earthquakes with M_S≥4.0 on the main fault of the earthquake sequence recorded by regional seismic networks in Yunnan, Sichuan, and other areas from the International Earthquake Science Data Center. The focal mechanism solutions of the major earthquake events are obtained using the gCAP full waveform inversion method. The results show that the focal mechanism solutions of earthquakes with M_S≥4.0 on the main fault all have an NW-SE oriented nodal plane I, consistent with the dominant distribution of the NW-SE oriented sequence. Except for the nodal plane I of the Yangbi M_S5.6 earthquake, which has a northeast dipping angle, all other focal mechanism solutions have a southwest dipping nodal plane I, which was consistent with the sequence orientation as shown in the vertical cross sections. According to the inclination angles of the P, B, and T axes, the inverted focal mechanism solutions all belong to a strike-slip type.

In this study, the parameters of the seismic fault plane are fitted in segments according to the distribution density of small-to-medium-sized earthquakes. The results show that the strike trending of the main fault plane varies between 126°-137° and gradually increases from north to south, dipping towards the southwest. The dip angle varies between 79°-87° gradually decreasing from north to south. There are four secondary oblique faults with variations in striking directions of 157°, 338°, 157° and 313° from north to south, corresponding to dip angles of 86°, 87°, 87°, and 86°, respectively.

Based on the above research results, combined with the background stress field and V_P/V_S tomographic results, it is inferred that the Yangbi earthquake occurred on the high-dip-angle and NW-SW strike-slip faults in the southwest mountainous areas of Yangbi County. These faults consist of a strike-slipping main fault and multiple secondary crisscrossing small faults, which may be jointly affected by regional stress and deep fluid activity.

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USING SEISMIC AMBIENT NOISE HORIZONTAL-TO-VERTICAL SPECTRAL RATIO(HVSR) METHOD TO DETECT SITE RESPONSE AND SHALLOW SEDIMENTARY STRUCTURE IN XIONG’AN AREA

RUAN Ming-ming, LIU Qiao-xia, DUAN Yong-hong, WANG Shuai-jun, ZHENG Cheng-long, WANG Liang

SEISMOLOGY AND GEOLOGY 2024, 46 (5): 1106-1122. DOI: 10.3969/j.issn.0253-4967.2024.05.007

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The construction of the Xiong’an New Area is a national strategy and a long-term plan outlined by the Chinese government. To support the urban planning and development of this area, many scholars have conducted a series of geophysical surveys aimed at understanding the detailed subsurface structure. The Horizontal-to-Vertical Spectral Ratio(HVSR)method, first introduced by Nakamura, has recently gained widespread use for investigating shallow subsurface structures, site response, and microzonation.

In this study, we utilized a large seismic array with an interstation distance ranging from 500 to 1000 meters, deployed across the Xiong’an New Area. The array consisted of over 900 short-period seismographs, covering most of the area. Using ambient-noise recordings, we removed nonrandom transient signals from the waveform data with a short-term-average over long-term-average detector automatic picking algorithm, and applied the Konno-Ohmachi algorithm to smooth the HVSR curves. For each site, we analyzed the amplitude of the peak value of the HVSR curve(A)and the corresponding frequency(f₀). Both parameters were further elaborated through the creation of contour maps using the Kriging interpolation method. Additionally, the peak frequencies from the HVSR curves were used to calculate the sedimentary thickness, based on an average shear-wave velocity and the frequency-depth formula.

The frequency map shows that the peak frequencies range between 0.6 and 1.1Hz, with an overall peak frequency of about 0.7 to 1.0Hz. The lowest frequencies were found predominantly in the vast eastern area of the study region, corresponding to geological features such as the Niubei Slope, Niutuozhen High, and Baxian Sag. According to the frequency-depth formula, a lower peak frequency indicates greater sediment depth. The variation in peak frequencies across stations highlights changes in the bedrock interface, which correspond to fault structures depicted on the geological map. Furthermore, high-amplitude areas were mainly located between the Rongxi fault and Rongdong fault, suggesting an impedance contrast between shallow and deeper layers. Stratigraphic profiles reveal that Quaternary and Tertiary sedimentary layers directly overlie the crystalline basement composed of Proterozoic metamorphic rocks. Combined analysis of peak frequency and amplitude aligns well with the available geological data. Our analysis produced 3D depth images of the Quaternary sedimentary layer interface across the study area, clearly imaging a significant seismic impedance interface at depths of 100-220m. This shallow interface corresponds to the contrast between the Tertiary rocks and the overlying Quaternary sedimentary layers. The sediment thickness progressively increases from east to west across the study area. Interfaces derived from the HVSR profiles display similar characteristics to those on the geological map and are consistent with borehole data and results from the high-density resistivity method. Moreover, we established a power-law relationship correlating the fundamental site resonance frequencies with sedimentary cover thickness obtained from borehole data in the Xiong’an New Area. The undulating characteristics of the sedimentary layers correspond closely to fault locations and geological tectonic units, confirming that faults such as the Rongxi, Rongdong, Niuxi, Niudong, and Xushui-Dacheng faults serve as boundaries for secondary geological tectonic units, influencing the structure of the near-surface sedimentary layers.

We developed a 3D shallow subsurface sedimentary model for the Xiong’an New Area and created contour maps of amplitude(A)and peak frequency(f₀). The results both support and extend previous understandings of the region’s structure. This study demonstrates that the HVSR method, in conjunction with a large seismic array, is a rapid and effective technique for investigating shallow subsurface structures and seismic site responses. The exploration of sedimentary structures and seismic site response characteristics, which are closely related to earthquake hazards, provides a critical foundation for seismic fortification and urban planning in the Xiong’an New Area.

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STUDY ON THE SLIP RATE OF THE BAISHA RIVER SEGMENT IN THE YINGXIU-BEICHUAN FAULT IN THE LONGMENSHAN FAULT ZONE

HUA Chun-yu, SHI Feng, WEI Zhan-yu

SEISMOLOGY AND GEOLOGY 2024, 46 (6): 1295-1313. DOI: 10.3969/j.issn.0253-4967.2024.06.005

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The tectonic belt stretches approximately 400km from Lushan County to Wenchuan County in an east-west direction. The Longmenshan fault zone can be geometrically divided into several sections, including the Houshan Fault, the Central Fault, the Qianshan Fault, and the Foreland Basin(Chengdu Plain)Deformation Zone. The Central Fault is the main segment of the active tectonic belt in the Longmenshan region, and the Yingxiu-Beichuan Fault is one of the most active segments within this central section. The Yingxiu-Beichuan Fault has experienced numerous moderate and strong earthquakes throughout its history, including the Wenchuan earthquake 2008. The 2008 Wenchuan earthquake was ahigh destructive natural disaster that profoundly impacted the Chinese mainland, leading to significant economic losses and casualties. This earthquake caused extensive building collapses, leading to the loss of tens of thousands of lives, and triggered severe secondary geological disasters such as landslides, rockfalls, and mudflows, severely affecting the normal operation of transportation and communication infrastructure. The Yingxiu-Beichuan Fault was one of the key surface rupture zones during this earthquake. However, there is still some uncertainty about the slip rate of this fault.

The Baisha River segment examined in this paper is located in the southern part of the Yingxiu-Beichuan Fault, measuring approximately 14km long. This area contains 14 fractures of varying lengths and complex geometric structures, forming a fracture zone that reaches a maximum width of nearly 300m. The overall orientation of the rupture zone is about 50 degrees; however, the orientation of each small secondary rupture varies, with differences ranging from 0 to 90 degrees. The coseismic displacement along the Baisha River section displays complexity and diversity. The thrust movement primarily occurs on the northern and western walls, with some local thrust faults. Additionally, the strike-slip motion is predominantly right-lateral, exhibiting a maximum horizontal displacement of approximately 4.8m, although some local areas show left-lateral displacement.

Previous studies have employed various techniques, such as geology and geodesy, resulting in a wide range of slip rate estimates from 0.07mm/a to 1.1mm/a. The slip rate of fault is a crucial factor for analyzing the characteristics of fault activity and for studying regional kinematics and dynamic mechanisms. According to river terrace longitudinal profiles estimates, the fault has a vertical slip rate of about 0.3mm/a to 0.6mm/a. Estimates based on displaced landforms indicate a vertical slip rate between 0.07mm/a and 1.1mm/a. According to GPS observations, the horizontal slip rate in the Longmenshan fault zone has a limit of 2mm/a, but the slip rate of individual faults is lower than 0.7mm/a.

In recent years, remote sensing techniques have been extensively utilized to study surface rupture zones, particularly during significant seismic events. This paper primarily employs aerial and QuickBird satellite images captured before and after the earthquake. The resolution of the aerial images is nearly 1m, while the QuickBird satellite images have a resolution of 0.6m, both of which allow for precise interpretation of tectonic landforms. River terraces consist of terraced units, including terraced surfaces, steep terraces, terrace fronts, and terrace backs. As geomorphic markers that are relatively easy to identify and measure, river terraces are among the most essential geomorphic units in the quantitative study of active tectonics. They also serve as crucial geological relics documenting Quaternary tectonic movements and climate changes. By examining river terraces and their deformations, researchers can discuss the timing and scale of tectonic activity, making this a long-term area of research.

This paper focuses on the Baisha River section, situated in the southern part of the Yingxiu-Beichuan Fault. We employed geological and geomorphological methods along with optically stimulated luminescence dating, remote sensing interpretation, field investigations, and data analysis to assess the slip rate of the Baisha River section of the Yingxiu-Beichuan Fault within the Longmenshan fault zone. Additionally, we analyze the spatio-temporal variation characteristics of this slip rate. This study constrains the slip rate of the Baishahe segment of the Yingxiu-Beichuan Fault in the Longmenshan fault zone using 10 terrace cross-sections and terrace ages. The results indicate that the Yingxiu-Beichuan Fault Baisha River segment has a vertical slip rate since the Late Pleistocene ranges from(0.10±0.02)mm/a to(0.30±0.05)mm/a. Considering that only one event, the 2008 Wenchuan earthquake, is associated with the T1 terrace, we believe the calculated rate based on the dislocation and age of the T1 terrace may significantly deviate from reality. If we exclude the sliding rate of the T1 terrace, the vertical slip rate since the late Quaternary ranges between(0.10±0.03)mm/a and(0.30±0.05)mm/a. The linear fitting results indicate that the average vertical sliding rate since the late Quaternary is approximately 0.19mm/a.

These findings provide fundamental data for understanding the seismogenic structure of the Wenchuan earthquake and the overall characteristics of the Longmenshan fault zone, as well as for assessing its long-term seismic hazard.

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THE STUDY OF CRUSTAL THICKNESS AND POISSON'S RATIO IN TENGCHONG VOLCANO AREA BY H-к-c METHOD

ZHANG Tian-ji, LI Qiu-feng, LI Feng-ying, ZHONG Yu-sheng, DUAN Hong-jie

SEISMOLOGY AND GEOLOGY 2024, 46 (5): 1048-1065. DOI: 10.3969/j.issn.0253-4967.2024.05.004

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Tengchong volcanoes are not extinct but a group of dormant volcanoes with magma underground. The Tengchong volcanic area is a unique geological condition integrating magma activity, earthquakes, and hot springs. Crustal thickness and Poisson’s ratio are two important parameters that characterize crustal structure and material composition and are crucial for accurately detecting the location and scale of magma chambers in the Tengchong volcanic area. However, previous studies on obtaining crustal thickness and Poisson’s ratio in the Tengchong volcanic area only used nine volcanic network stations, which had insufficient resolution and could not effectively constrain the position of magma chambers. The traditional H-κ method is adopted to an isotropic crust with a flat Moho. The crust maybe anisotropic and the Moho is dipping. In the presence of a complex crustal structure with azimuthal anisotropy or dipping Moho, the H-κ results may be biased. So, we extracted 4 268 P receiver functions from teleseismic wave data recorded at 23 digital seismic stations. A H-κ-c method with harmonic corrections is used to obtain crustal thickness and Poisson’s ratio in the Tengchong volcano area. Before the harmonic corrections to the P receiver functions, we perform the incident moveout corrections and back azimuthal binning of 5°. The H-κ-c method can correct the influence of crustal anisotropy and dipping interfaces on receiver functions by harmonic transformation, can acquire more stable and reliable crustal thickness and wave velocity ratios, and can obtain information on the inclination of the Moho and crustal azimuthal anisotropy. Based on previous research, we discussed the crustal deformation mechanism of the Tengchong block and revealed the corresponding relationship between crustal structure, heat flow, earthquakes, and magmatism.

Results show the fast-wave polarization directions with a dominant NW-SE orientation in the north and change to a dominant NE-SW orientation in the south, and delay times varying between 0.06 and 0.80s, with a mean of 0.40s. It is consistent with the Tengchong block undergoing clockwise rotation around the EHS. There is an inclined Moho surface and strong azimuthal anisotropy in the intersection of the Tengchong Fault, Yingjiang Fault, and Longchuan Fault. The strong azimuthal anisotropy in the Tengchong block maybe related to the strong influence of the upwelling of the deep thermal material from the upper mantle. The fast wave polarization direction parallel to the Longling-Ruili fault indicates that the observed anisotropy may be related to the fracture of the fault. The crustal thickness ranges from 32 to 39km, and the Poisson’s ratio ranges from 0.235 to 0.326. There exist three Moho-uplifting centers, one in Gudong-Qvshi-Mazhan-Tengchong, the other in Qingshui-Xinhua, another in Zhen’an-Longxin-Xiangda. The very high Poisson’s ratio(σ>0.3) is consistently located within these three locations. We speculated that the Moho-uplifting and higher Poisson’s ratio at the three sites denote the existence of three magma chambers. The horizontal scale of the three magma chambers is respectively 20km×35km, 20km×20km, 25km×25km, and separately controlled by the Tengchong Fault and Longchuanjiang Fault, Tengchong Fault and Longchuan Fault, Nujiang Fault and Longling-Ruili Fault. The locations of the magma chambers are different from that obtained by the same receiver function method, which the different seismic stations and stacking methods may cause. The locations of the magma chambers are not exactly the same as those of the geothermal anomaly areas and the mantle-derived volatile release anomaly areas measured by the surface hot springs. The reason for this difference may be the ground temperature and the mantle-derived volatile component, which are the measurement results of the surface hot spring. The Poisson’s ratio value we calculated is the average value of the entire crust. Under the four Holocene volcanic craters of HeikongShan, Dayingshan, Laoguipo, and Ma’anshan, there is an interconnected magma chamber, the most vigorous volcanic activity since the Holocene. There are almost no earthquakes occurring in the crust at the center of the Moho uplift; most earthquakes are distributed in the crust around the Moho-uplifting centers. This may be because the hot magma heated the crust, resulted in the rocks in the crust being plastic, and it is difficult to accumulate large strains. Our results have important reference value and guiding significance for earthquake and volcanic activity monitoring, earthquake prevention and disaster reduction in the Tengchong volcanic area.

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STUDY ON SEISMOGENIC TECTONICS OF THE 2025 MYANMAR M_S7.9 EARTHQUAKE

XU Bin-bin, ZHANG Yi-peng, LU Le-jun, TIAN Qing-ying, YANG Xue, WANG Yang, ZHANG Pei-zhen

SEISMOLOGY AND GEOLOGY 2025, 47 (2): 649-670. DOI: 10.3969/j.issn.0253-4967.2025.02.20250089

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According to the China Earthquake Networks Center, an M_S7.9 earthquake(hereafter referred to as the Myanmar earthquake)struck the Mandalay region of Myanmar(21.85°N, 95.95°E)on March 28, 2025, at a focal depth of 30km. The earthquake occurred along the central segment of the Sagaing Fault and was characterized by a right-lateral strike-slip rupture, generating a ~350km-long surface rupture zone with a maximum coseismic horizontal displacement of 6 meters. The event caused extensive damage to buildings and varying degrees of destruction to infrastructure, including roads and bridges.
Situated in a critical tectonic region where the Indian Plate obliquely converges with the Eurasian Plate, the Myanmar earthquake offers valuable insights into plate boundary deformation processes. Detailed analysis of this event enhances our understanding of the deformation mechanisms along the Myanmar plate boundary and provides essential constraints for seismic hazard assessment along the southeastern margin of the Eurasian Plate. This research holds scientific significance for elucidating continental lithospheric deformation in response to oblique plate convergence. The findings contribute to regional early warning strategies and disaster mitigation efforts and offer a valuable reference for seismic risk studies in comparable tectonic settings worldwide.
This study integrates Global Navigation Satellite System(GNSS)data from across the Sagaing Fault region, establishing a comprehensive GNSS velocity field for Myanmar and addressing previous gaps in coverage along the fault’s southern segment. Using multiscale spherical wavelet analysis and GNSS velocity profiles, we examine the deformation characteristics of the region. We calculate the slip rate deficit distribution along the Sagaing Fault and assess postseismic Coulomb stress changes. Combined with historical seismicity data, we investigate the seismogenic structure and stress perturbations in surrounding areas. The key findings are as follows:
(1)The Myanmar M_S7.9 earthquake was a right-lateral strike-slip event along the central Sagaing fault. The region is affected by the northeastward oblique convergence of the Indian Plate and southeastward extrusion of crustal material from the Tibetan plateau, resulting in strong north-south shear and east-west shortening. The Sagaing fault accommodates most of this deformation, with a rapid right-lateral slip rate of approximately 21~22mm/a.
(2)High-resolution GNSS velocity profiles indicate significant fault locking at depths of 15~25km along the Sagaing Fault. The slip rate deficit analysis reveals a high locking ratio across the fault, indicating elevated seismic potential. Notably, the central segment shows lower seismic moment accumulation compared to the northern and southern segments, forming a ~300km-long seismic gap since 1900, capable of generating earthquakes exceeding magnitude 7.5.
(3)Coulomb stress modeling suggests that the earthquake significantly altered the regional stress field. Stress accumulation zones were identified at both ends of the Sagaing fault and in the central Shan Plateau to the east. These regions of increased stress transfer and loading exhibit heightened potential for future large earthquakes, underscoring the need for enhanced seismic monitoring.

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STUDY ON SURFACE WAVE TOMOGRAPHY OF THE A'ERSHAN VOLCANOES

HOU Jie, WU Qing-ju, YU Da-xin, YE Qing-dong

SEISMOLOGY AND GEOLOGY 2024, 46 (4): 893-915. DOI: 10.3969/j.issn.0253-4967.2024.04.008

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Since the Cenozoic era, a series of intraplate volcanic groups have developed along the east and west sides of the Songliao Basin in the eastern part of the Central Asian orogenic belt. The A'ershan volcanic group is one of the Cenozoic intraplate volcanoes in the eastern section of the Central Asian orogenic belt. Further study of this volcanic group is of great significance for exploring and understanding the genesis of intraplate volcanoes in the eastern section of the Central Asian orogenic belt. In the past, the distribution of mobile seismic stations and some fixed stations used for imaging research on the A'ershan volcanic group was relatively sparse and did not fully cover the A'ershan volcanic group. The resolution of the crust-mantle structure obtained in the past was also slightly insufficient for exploring the genesis mechanism of the A'ershan volcanic group. This article utilizes the vertical teleseismic waveforms of 29 broadband mobile seismic stations near the A'ershan volcanic group and 8 fixed stations around them from May 2019 to December 2021. Through frequency-time analysis technology, 11 775 Rayleigh wave phase velocity dispersion between two stations is extracted. After excluding non-monotonic rise and phase velocity dispersion curves that differ significantly from most dispersion distributions, 11 010 high-quality Rayleigh wave fundamental phase velocity dispersion curves were ultimately obtained. Then, based on classical ray theory, the two-dimensional phase velocity distribution with a period of 10-80s and a grid size of 0.5°×0.5° is inverted by using the traditional dual station method. Except for areas not covered by radiation in the edge zone, the lateral spatial resolution of phase velocity in the study area is basically within 50km. The checkerboard test also showed that dividing the grid size of the study area into 0.5°×0.5° is feasible, and anomalies with a central area scale less than 0.5°×0.5° can also be identified. Afterward, the CRUST1.0 model was used as the initial crustal model, and the PREM model was used as the initial mantle model. The crustal thickness results obtained from the receiver function were used to constrain the thickness of each layer in the initial crustal model, further reconstructing the three-dimensional S-wave velocity structure of the crust and upper mantle in the study area. The results show that: within the range of the middle and lower crust, the S-wave velocity in the A'ershan volcanic area exhibits apparent low-velocity anomalies. Based on the characteristics of the high wave velocity ratio in the area, it is speculated that there may be a crustal magma chamber in the A'ershan volcanic group. There are multiple high-velocity anomaly structures within a depth range of 40-150km in the A'ershan volcanic group. The difference in the depth of high-velocity anomalies indicates the heterogeneity of the lithosphere thickness, and it is speculated that the thickness of the lithosphere in the A'ershan volcanic area does not exceed 100km. The deeper distribution of high-velocity anomalies may represent the dismantled lithosphere, while the shallower distribution of high-velocity anomalies may represent the undeveloped lithosphere or residual lithosphere after dismantling, reflecting the possibility of lithospheric detachment and subsidence in the region. There are S-wave low-velocity anomalies in the upper mantle on the north and south sides of the A'ershan volcanic group, connecting the asthenosphere and the exposed positions of the A'ershan volcanic group on the surface. The low-velocity anomalies on the north and south sides merge at a depth of 150km. Based on the high heat flux value, high V_P/V_S, and crustal thinning characteristics of the surface near the distribution area of the A'ershan volcanic group, as well as the previous conclusion based on remote seismic P-wave and S-wave travel time tomography results that there is a clear connection between the low-velocity anomaly below the A'ershan volcanic group and the southern edge of the Songliao Basin in the deep mantle, it is speculated that this low-velocity anomaly may be caused by the upwelling of asthenosphere material caused by the detachment of the lithosphere in the Songliao Basin.

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CRUSTAL DEFORMATION CHARACTERISTICS AND RELEVANT SEISMIC HAZARD OF THE TAIYUAN BASIN

CHEN Qian, ZHANG Zhu-qi

SEISMOLOGY AND GEOLOGY 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.

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EMPIRICAL EXTRAPOLATION MODEL OF SITE SHEAR WAVE VELOCITY AND ITS APPLICABILITY IN SHANDONG PROVINCE

LI Zhi-heng, XIE Jun-ju, LI Ke-wei, WEN Zeng-ping, LI Xiao-jun, WANG Zhi-cai, XU Hong-tai, ZHAO Xiao-fen, ZHANG Na

SEISMOLOGY AND GEOLOGY 2024, 46 (4): 934-954. DOI: 10.3969/j.issn.0253-4967.2024.04.010

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Site shear wave velocity is a pivotal parameter for site classification and for quantitatively assessing the site's impact on ground motion. It has extensive applications in engineering seismic design and rapid post-earthquake damage assessment. China's seismic design standard, GB50011-2010, primarily uses two indicators for site classification: the thickness of the soil layer and the equivalent shear wave velocity of the top 20m of soil. In contrast, the United States and Europe utilize the average shear wave velocity, V_S30, at a 30m depth for site classification. Studies have indicated that considering only the top 20m of soil in classification overlooks the influence of deeper low-velocity layers on long-period structures. Additionally, reliance on the top 20m's shear wave velocity can be problematic due to its sensitivity to the properties of the fill layer and the potential unreliability of measurements in this shallow depth. To address these issues, scholars in China advocate increasing the depth considered in site classification from 20m to 30m. Current standards focus on soil layers not exceeding 20m, resulting in engineering boreholes and shear wave velocity measurements that rarely exceed this depth, especially in harder sites where boreholes often extend less than 10m. The development of site shear wave velocity extrapolation models is crucial for accurate site classification and ground motion parameter determination, particularly in the absence of deep borehole data.
Various extrapolation methods have been proposed, including the constant velocity method, velocity gradient method, and conditional independence method. The constant velocity method assumes a uniform velocity below the measured depth, while the velocity gradient method fits empirical relationships in a linear or logarithmic form. The conditional independence method leverages correlations between instantaneous and average shear wave speeds at various depths. Domestic research has led to the establishment of regional shear wave velocity extrapolation models, though their applicability is often limited to specific regions. The selection of the most suitable extrapolation method for a given region requires further investigation.
This study focuses on Shandong province, a region within China's North China Seismic Zone with a significant risk of strong earthquakes. With nearly 80% of the province requiring seismic fortification of at least Ⅶ-degree intensity, research into shear wave velocity extrapolation models is of practical importance for site categorization and seismic defense. Utilizing extensive shear wave velocity profiles and borehole lithology data, this study applies constant velocity, velocity gradient, and conditional independence methods to establish regional extrapolation models. It evaluates the applicability and accuracy of these methods in Shandong and recommends an empirical model for shear wave velocity extrapolation.
Key findings include: 1)For borehole depths less than 10m, the empirical extrapolation models for V_S20 and V_S30, utilizing the three discussed methods, exhibit considerable inaccuracies. Caution is advised when applying the wave velocity predictions from this study to depths under 10m. Notably, the BCV method tends to significantly underestimate when extrapolating from shallow data. The BCV method's predictions become more reliable and exhibit reduced error only when borehole depths exceed 10m for V_S20 and 15m for V_S30; 2)The empirical extrapolation models for V_S20 and V_S30 in Shandong province, developed using the velocity gradient method, align well with actual measurements. These models' regional applicability is supported by comparative regional analyses. The V_S30 predictions for Shandong are found to be generally lower than those in Japan but closer to those in California and the Beijing plain, with a slight increase in the higher wave speed range; 3)Considering the models' accuracy and regional applicability, the study advocates for the empirical extrapolation models of V_S20 and V_S30 for Shandong Province based on the conditional independence method. These models minimize total prediction errors across various depths. While the BCV model's performance improves at greater depths, the velocity gradient extrapolation model's efficacy diminishes.
Overall, this study contributes to the advancement of seismic design practices in Shandong province by offering empirical extrapolation models for V_S20 and V_S30, enhancing the understanding of ground motion characteristics and supporting more robust seismic resilience strategies.

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THE DISTRIBUTION AND RESEARCH PROGRESS OF MAIN MUD VOLCANOES IN CHINA

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

SEISMOLOGY AND GEOLOGY 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.

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CHARACTERISTICS OF UPPER CRUSTAL SHEAR WAVE SPLITTING IN THE NORTHEASTERN TIBETAN PLATEAU

LI Shu-yu, GAO Yuan, JIN Hong-lin, LIU Tong-zhen

SEISMOLOGY AND GEOLOGY 2024, 46 (4): 916-933. DOI: 10.3969/j.issn.0253-4967.2024.04.009

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The northeastern part of the Tibetan plateau, comprising secondary blocks such as Qiangtang, Bayan Har, Qaidam and Qilian, forming a complex tectonic pattern. This region, located at the interface between the South China Block and North China Block, has been at the forefront of the Indo-Eurasian plate collision, experiencing significant tectonic deformation. Consequently, it serves as an ideal natural laboratory for the study of plate tectonics, crustal dynamics, and seismic activity. Shear wave splitting is a method used to study the anisotropy of media, based on the phenomenon where shear waves split into two sets of wave trains, fast and slow, due to the anisotropy of the medium during propagation. In the mid-to-upper crust, this splitting characteristic is often identified through the analysis of local earthquake waveforms. The fast wave direction typically aligns with the oriented arrangement of vertical cracks, governed by the regional principal horizontal compressive stress direction. In contrast, the slow wave is nearly perpendicular to the fast wave, and its time delay is closely related to the crack geometry and internal fluid state, indirectly reflecting the degree of medium anisotropy. In this study, we have collected waveforms of local small earthquakes from January 2010 to September 2021 on the northeastern Tibetan plateau and calculated two anisotropy parameters: fast-wave polarization direction and slow-wave time delay, using shear wave splitting analysis. We subsequently construct a detailed spatial distribution map of the anisotropic parameters of the upper crust. The fast-wave polarization direction is dominated by an ENE direction, roughly parallel to the regional principal compressive stress direction, indicating that the anisotropy of the upper crustal medium is mainly controlled by regional tectonic stress. Several relatively weaker secondary fast-wave polarization directions, including NNW, WNW, and near EW, vary widely across the northern and southeastern parts of the Qilian block and the northern part of the Qiangtang block. These directions are approximately parallel to the widely distributed NW-trending faults, indicating the influence of the fault system. The fast-wave polarization directions on the northeastern edge of the Qaidam block are more discrete, with the northern margin stations showing WNW direction dominance and the north-central part showing NE or weaker NW dominance, affected by the combined effects of stress, faults, rock properties, and other factors. The slow-wave delay time serves as a quantitative indicator of the anisotropy, reflecting variations in stress level within the medium. With the thrust fault system in the northern part of the Qilian block, the slow-wave time delay varies from 1.7ms/km to 6.3ms/km, averaging(3.2±2.1)ms/km. Notably, these time delays are larger in the east than in the west, reflecting differences in the stress environment. The southeastern Qilian block and the northeastern margin of the Qaidam block exhibit a relatively uniform average time delay of(5.1±2.4)ms/km, with an overall range of 2.5ms/km to 5.7ms/km. The similar distribution of time delays may be related to similar rock properties and tectonic environments. At the northern edge of the Qaidam basin, the WNW-oriented fast-wave polarization direction, coupled with a relatively consistent slow-wave time delay ranging from 3.1ms/km to 4.5ms/km, may be a response to the high-pressure metamorphism of fractures in the deep crust. The northern part of the Qiangtang block shows a stable degree of deformation, as evidenced by the slow-wave time delay averaging(4.5±0.8)ms/km with a small standard deviation. Both the northern Qiangtang block and the periphery of the Lajishan faults(encompassing the southeastern Qilian and northeastern Qaidam blocks)host volcanic arcs and reservoir formations. However, the former exhibits shorter time delays compared to the latter, potentially attributed to differences in rock physical properties and the tectonic environment. Due to the heterogeneous distribution of data, further studies are needed to gain a more comprehensive understanding of upper crustal deformation.

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THERMAL INFRARED ANOMALIES OF MODERATELY STRONG EARTHQUAKE IN XINJIANG AND SURROUNDING REGIONS

ZHANG An-he, ZHONG Mei-jiao, AISA Yisimayili, LIU Ping

SEISMOLOGY AND GEOLOGY 2024, 46 (5): 1192-1206. DOI: 10.3969/j.issn.0253-4967.2024.05.011

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Xinjiang and its surrounding areas are one of the regions with the most frequent seismic activities and the largest intensity in the Chinese mainland. Therefore, conducting relevant earthquake prediction research is crucial for disaster prevention and mitigation. However, due to the limited natural conditions and other factors in the region, the number of site observation stations is small and the their distribution density is low. It is difficult to carry out earthquake prediction only using site observation. Remote sensing technology has the advantages of being all-weather and large-scale. With the development of remote sensing technology, many scholars have found that there are different degrees of thermal anomalies before strong earthquakes. At present, a variety of thermal anomaly extraction methods have been formed. Among them, the relative power spectrum method can remove the thermal radiation changes affected by non-tectonic activity factors such as topography, ground object types, and meteorology to highlight the thermal radiation anomalies caused during earthquake preparation. This method has been applied in Xinjiang and surrounding areas for many years. However, in the past few years, the technique for studying seismic thermal anomalies in Xinjiang and its surrounding areas has primarily focused on a single seismic event, lacking systematic combing of earthquake cases and analysis of prediction efficiency.

To further summarize the characteristics of seismic thermal infrared anomalies in Xinjiang and its surrounding areas, improve its prediction indicators, and improve the scientific and accuracy of earthquake prediction in this area, based on the blackbody brightness temperature data of FY-2 geostationary meteorological satellite, we extract the thermal infrared relative power spectrum anomaly of Xinjiang and surrounding regions from 2008 to 2021 by using the relative power spectrum method and analyze the prediction efficiency of earthquakes with different magnitudes, and summarizes the relationship between thermal infrared anomalies and corresponding earthquakes. The results show that: 1)The band 1 of the thermal infrared relative power spectrum has the highest corresponding rate of 44% for earthquakes above 5 in Xinjiang and its surrounding areas, but only 6.0-6.9 earthquakes have passed the significance test. The R-value is 0.342, which is greater than R₀(0.306). The dominant occurrence time of M5.0-5.9 earthquakes is within 3 months after the beginning of the anomaly and within 0.5 months after the end of the anomaly, while that of M6.0-6.9 earthquakes is three months and 7-12 months after the end of the anomaly. The dominant seismogenic areas of each magnitude range are within 200km from the edge of the anomaly area to the surrounding area. 2)The abnormal area and duration of the 6.0-6.9 earthquakes corresponding to the thermal infrared relative power spectrum anomaly positively correlate with the magnitude, and all pass the significance test. The peak and magnitude did not pass the significance test in the two magnitude ranges. 3)This anomaly occurs most frequently in the Altun area and has a high correspondence rate. The seismic correspondence ratio in the southern Xinjiang region is higher than that in the northern Xinjiang region; The anomaly in the basin has a higher seismic correspondence ratio and higher earthquake magnitude; 4)Most anomalies occurred in spring, and the seismic correspondence ratio of anomalies in autumn was the highest; 5)The proportion of epicenter mechanism solution types of corresponding earthquakes is consistent with that of various kinds in Xinjiang, and most events were shallow earthquakes. Shallow earthquakes may be more likely to cause thermal infrared anomalies.

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LATE CENOZOIC FLOOD BASALT ERUPTION IN DALINOR, INNER MONGOLIA

CHANG Li-wen, ZHAO Yong-wei, LI Ni, SUN Jin-heng

SEISMOLOGY AND GEOLOGY 2024, 46 (4): 876-892. DOI: 10.3969/j.issn.0253-4967.2024.04.007

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Fissure eruption refers to volcanic activity where magma is expelled to the Earth's surface through cracks in the crust. During such eruptions, large volumes of low-viscosity lava flow rapidly along these fissures, forming extensive flood-like lava fields known as “flood basalts”. Fissure eruptions are characterized by their significant magma scale and devastating impact. Although there is no historical record of volcanic fissure eruptions in China, numerous late Cenozoic lava platforms have been discovered in XilinGol League and Chifeng, Inner Mongolia. These platforms likely represent overflow basalts formed by continental volcanic fissures. In this study, we focus on investigating the lava platform within the Dalinor volcano group located in XilinGol area using tools from volcanic physics and geochemistry. The XilinGol League in Inner Mongolia hosts extensive late Cenozoic lavas, encompassing an area of approximately 10 000km² and representing one of China's largest basaltic provinces. This volcanic field, formed through flood basalt volcanism, has remained active since the late Cenozoic and poses potential risks of future eruptions. To enhance our understanding of its origin and assess eruption hazards associated with intracontinental flood basalt volcanism, this study focuses on the latest lava platforms within the Beilike region of the Dalinor Volcanic Field. In this study, we investigate volcanic activity by examining its eruptive characteristics, including magma temperature, lava viscosity, and lava flow velocity. During the volcanic eruption, a significant portion of the lava engulfed the gently sloping surfaces of fluvial and lacustrine deposits, resulting in the topographic formation of expansive lava plateaus. Throughout this process, surface lava underwent condensation to give rise to pahoehoe lavas. In contrast, the underlying plastic lava continued its effusion along eruptive fissures, causing vertical compression of the solidified crust and subsequent formation of lava ridges. In the vicinity of the eruptive fissure, a series of low ridges measuring 3-8 meters in height and tens to hundreds of meters in length emerged. At a distance over 3km from the eruption fissure, the lava flow exhibits a banded distribution with an elevated central portion and inclined edges, displaying characteristic lava levees. Columnar jointing is commonly observed at the periphery of the plateau profile. The profile exhibits stacked layers of lava, indicating a laminar flow movement of the lava flow. These geological features are consistent with the characteristics typically associated with flood basalt eruptions. The whole rock geochemistry indicates that the lava in Beilike belongs to olivine tholeiitic basalt and alkali olivine basalt. The lava is characterized by phenocrysts primarily composed of olivine and clinopyroxene, particularly augite. The Mg# values of clinopyroxene vary between 58.59 and 80.69. The eruption temperature of Beilike lava is determined by applying clinopyroxene-melts geothermo-barometry inversion, yielding a range of 1 123.2-1 173.4℃. Additionally, the viscosity of the erupting lava is obtained using a previously established calculation model, resulting in values ranging from 30 to 187Pa·S. This paper investigates the flow dynamics of “fissure eruption” through an analysis based on principles of lava fluid mechanics while considering relevant physical properties. We assume that high-temperature lava behaves as a Newtonian fluid and consider zero overflow velocity near the vent during fissure eruptions. The influence on lava flow primarily stems from surface slope, gravity, and inherent fluid properties rather than temperature variations throughout the process. We assume that the surface slope is 0.5 degrees. The lava temperature is 1 120℃, and the thickness of the lava is 1m. By adopting this approach, we can calculate the maximum attainable flow velocity upon reaching a stable state. It has been calculated that during fissure eruptions in the study area, the velocity of lava flows primarily ranged from 0.4m/s to 1m/s, with occasional instances reaching as high as 2.5m/s. The present study unveils the eruptive characteristics of flood basaltic volcanism in the Belike region. It establishes the physical parameters of the lava flows, thereby providing essential data for formulating strategies to mitigate future volcanic eruption disasters. The key parameters presented here not only contribute to the understanding of volcanism in Inner Mongolia but also hold significant reference value for basaltic volcanism in other continental intraplate environments across China. This research will enhance our understanding of this unique form of volcanism while providing a scientific basis for mitigating volcanic disasters. Moreover, the calculation methods and steps employed to derive these parameters may readily apply to other volcanic fields.

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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

SEISMOLOGY AND GEOLOGY 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.

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SURFACE RUPTURE OF THE FEBRUARY 6, 2023 M_W7.5 ELBISTAN EARTHQUAKE IN TURKEY

YU Jing-xing, REN Zhi-kun, ZHANG Hui-ping, LI Chuan-you, WANG Shi-guang, GONG Zheng, ZHOU Xiao-cheng, XU Yue-ren, LIANG Peng, MA Zi-fa, LI Jun-jie

SEISMOLOGY AND GEOLOGY 2024, 46 (6): 1263-1279. DOI: 10.3969/j.issn.0253-4967.2024.06.003

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On February 6, 2023, two destructive earthquakes struck southern and central Turkey and northern and western Syria. The epicenter of the first event(M_W7.8)was 37km west-northwest of Gaziantep. The earthquake had a maximum Mercalli intensity of Ⅻ around the epicenter and in Antakya. It was followed by a M_W7.7 earthquake nine hours later. This earthquake was centered 95km north-northeast from the first one. There was widespread damage and tens of thousands of fatalities. In response to these catastrophic events, in March 2023, a seismic scientific expedition led by China Earthquake Administration(CEA)was promptly organized to investigate the surface ruptures caused by these earthquakes. Here, we focus on the surface ruptures of the second earthquake, known as the Elbistan earthquake. The post-earthquake field survey revealed that the Elbistan earthquake occurred on the East Anatolian fault zone's northern branch(the Cardak Fault). This event resulted in forming a main surface rupture zone approximately 140km long and a secondary fault rupture zone approximately 20km long, which is nearly perpendicular to the main rupture.

We combined the interpretation of high-resolution satellite imagery and geomorphic investigations along the fault to determine the fault geometry and kinematics of the second earthquake event. The Elbistan earthquake formed a main surface rupture zone approximately 140km long, which strikes in an east-west direction along the Cardak Fault. The main rupture zone starts from Göksun in the west and extends predominantly eastward until the western end of the Sürgü Fault. It then propagates northeast along the southern segment of the Malatya fault zone. The entire Cardak Fault and the Malatya fault zone's southern segment are considered seismic structures for this earthquake. The overall surface rupture zone exhibits a linear and continuous distribution. Secondary ruptures show a combination of left-lateral strike-slip or left-lateral oblique-thrust deformation. Along the rupture zone, a series of en echelon fractures, moletracks, horizontal fault striations, and numerous displaced piercing markers, such as mountain ridges, wheat fields, terraces, fences, roads, and wheel ruts, indicate the predominance of pure left-lateral strike-slip motion for most sections. The maximum measured horizontal displacement is(7.6±0.3)m. According to the empirical relationship between the seismic moment magnitude of strike-slip faulting earthquakes and the length of surface rupture(SRL), a main rupture zone of 140km in length corresponds to a moment magnitude of approximately 7.6. Based on the relationship between the seismic moment magnitude and the maximum coseismic displacement, a maximum coseismic displacement of(7.6±0.3)m corresponds to a moment magnitude of about 7.5. The magnitudes derived from the two empirical relationships are essentially consistent, and they also agree with the moment magnitude provided by the USGS. Besides the main surface rupture zone, a secondary fault rupture zone extends nearly north-south direction for approximately 20km long. Unfortunately, due to the limited time and traffic problem, we did not visit this north-south-trending secondary fault rupture zone.

According to the summary of the history of earthquakes, it is evident that the main surface rupture zone has only recorded one earthquake in history, the 1544 M_S6.8 earthquake, which indicates significantly less seismic activity compared to the main East Anatolian Fault. Moreover, the “earthquake doublet” will inevitably significantly impact the stress state and seismic hazard of other faults in the region. Seismic activity in this area remain at a relatively high level for years or even decades to come. The east-west striking fault, which has not been identified on the published active fault maps at the western end of the surface rupture zone, and the north-east striking Savrun Fault, which did not rupture this time, will experience destructive earthquakes in the future. It remains unknown why the east-west striking rupture did not propagate to the Sürgü Fault this time. More detailed paleoearthquake studies are needed to identify whether it is due to insufficient energy accumulation or because this section acts as a barrier. If the Sürgü Fault, about 40km long, was to rupture entirely in the future, the magnitude could reach 7 based on the empirical relationship.

Considering the distribution of historical earthquakes along the East Anatolian fault zone, as well as the geometric distribution of the surface ruptures from the recent “earthquake doublet” and the surrounding active faults, it is believed that the future earthquake hazards in the northeastern segment of the East Anatolian fault zone, the northern segment of the Dead Sea Fault, and the Malatya Fault deserve special attention.

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THE CHARACTERISTICS AND MECHANISM OF GRAVITY AND MAGNETIC FIELD CHANGES BEFORE AND AFTER THE 2014 HUOSHAN M_S4.3 EARTHQUAKE

LIANG Xiao, CHU Fei, XU Ru-gang, SUN Hong-bo, XIAO Wei-peng, WANG Jun

SEISMOLOGY AND GEOLOGY 2024, 46 (5): 1151-1171. DOI: 10.3969/j.issn.0253-4967.2024.05.009

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Before the Huoshan earthquake, significant anomalies were detected in both the gravity and lithospheric magnetic fields. To comprehensively analyze the variations in these fields and their underlying mechanisms before and after the Huoshan earthquake, we used mobile gravity data from 2010 to 2015 and mobile geomagnetic data from 2013 to 2014 in Anhui Province and surrounding regions. Our analysis focused on gravity field changes from two perspectives: (1)the spatial and temporal variations in the gravity field and(2)the time series of gravity point values across the seismogenic fault(Tudiling-Luoerling fault). Additionally, we corrected for diurnal variations and long-term trends in the geomagnetic field, allowing us to track changes in the three components of the lithospheric magnetic field—total intensity, magnetic inclination, and declination—before and after the earthquake. Using wavelet multi-scale decomposition, we calculated and analyzed wavelet details at different decomposition scales for the gravity and magnetic field variations in the first half year before the Huoshan earthquake. Finally, in conjunction with underground fluid data, we examined the seismogenic background and explored the underlying reasons for the precursory anomalies observed in the geophysical fields.

The research yielded the following conclusions: Prior to the Huoshan earthquake, an anomalous high-gradient zone in both the gravity field and the total strength of the lithospheric magnetic field was observed, extending approximately 100km. Notably, the total magnetic field strength in the Huoshan area significantly decreased before the earthquake. The gravity field exhibited a small initial decline, evolving into a high-gradient anomaly zone parallel to the seismogenic fault, which culminated in the earthquake occurring near the zero contour of gravity change. The alignment of the zero lines of gravity and magnetic field changes with the strike of the seismogenic fault suggests that tectonic faults play a critical role in controlling crustal deformation and underground fluid migration. When combined with a comprehensive analysis of the regional stress field, underground fluid dynamics, and variations in the gravity and magnetic fields, this information can be instrumental in identifying and assessing seismic risk zones.

The Huoshan region is highly susceptible to seismic activity due to the influence of the Bayan Har block in the Qinghai-Tibet region, which induces stress field fluctuations in the area. The M_S7.0 Lushan earthquake in Sichuan, on April 20, 2013, significantly impacted the regional stress field, resulting in the opening of the “Huoshan Seismic Window.” In the six months preceding the Huoshan earthquake, there was an increase in crustal movement, as well as a marked rise in minor seismic activity. These factors accelerated the adjustment of the regional stress state and the migration of underground fluids, leading to expanded variations in regional gravity and magnetic fields. The Huoshan M_S4.3 earthquake exhibited distinct precursory anomalies. The wavelet multi-scale decomposition of gravity and magnetic field changes suggests that the source of the Huoshan earthquake likely originated in the middle to lower crust. The deformation and material migration in the upper crust appeared to be influenced by processes in the middle and lower crust, with energy accumulation in the upper crust triggering the opening of the “Huoshan Seismic Window” and the subsequent earthquake. Additionally, the extreme point of the wavelet details in the lithospheric magnetic field change was located near the zero line of the wavelet details in the gravity field and the fault development area.

This study concludes that regional stress fluctuations, crustal deformation, and underground fluid migration are controlled by fracture structures. The migration of underground fluids and other materials results in notable changes in the gravity and magnetic fields, particularly in areas with concentrated fault activity, underscoring the potential for predicting earthquakes using geophysical precursor signals such as gravity and lithospheric magnetic field changes.

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

XU Ying-cai, GUO Xiang-yun

SEISMOLOGY AND GEOLOGY 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.

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STUDY ON THE VARIATION CHARACTERISTICS OF GRAVITY FIELD AND APPARENT DENSITY IN URUMQI AND ITS SURROUNDING AREAS

KONG Xiang-kui, LIU Dai-qin, AILIXIATI·Yushan, LI Jie, CHEN Li, LI Rui, CHEN Rong-liu

SEISMOLOGY AND GEOLOGY 2024, 46 (5): 1123-1150. DOI: 10.3969/j.issn.0253-4967.2024.05.008

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Using seasonal gravity observation data from Urumqi and its surrounding areas, collected between April 2019 and April 2022, this paper applies absolute gravity control to perform classical adjustment calculations, identifying the spatial-temporal evolution characteristics of the gravity field in the study area. The relationship between seismic activity and gravity change has long been a topic of interest. The time-varying gravity field is a fundamental physical field that reflects the migration of mass and directly represents the internal tectonic movements of the Earth and surface mass redistribution. The link between earthquakes and gravity changes is primarily related to tectonic movement and variations in mass(density)within the Earth’s interior.

By examining gravity field changes at half-year and one-year scales and analyzing gravity profile images in relation to geological structures, this paper explores the characteristics of gravity field variations in Urumqi and its surrounding areas. To effectively separate gravity anomalies at different depth levels, wavelet multi-resolution analysis is employed to decompose the gravity field anomalies, distinguishing regional from local anomalies in the study area. Specifically, the wavelet multi-scale analysis method is applied to process gravity field dynamic data from 2019-2020, 2020-2021, and 2021-2022. This method helps isolate and interpret abnormal signals in the gravity field, improving the reliability of earthquake precursor gravity anomalies.

The gravity source characteristics provide insight into the physical property changes of the crust. In this study, the “equivalent source” inversion model is used to determine the dynamic characteristics of the crust’s apparent density. The multi-period gravity point values obtained through the adjustment method serve as input data for the equivalent source apparent density change model in the study area.

The results indicate that the gravity field in the study area exhibits clear zonation, with predominant negative changes and alternating positive and negative gravity anomalies. The wavelet gravity details show that the anomaly areas align with geological structures, and the estimated source depth, as determined by the power spectrum, is consistent with the Crust1.0 model. The inversion of the flow gravity data reveals the variation characteristics of the crust’s equivalent apparent density, which correlate well with the time-varying gravity field. Multi-scale decomposition of gravity anomalies at different depth levels further illuminates the physical property changes of the crustal medium, as reflected by the equivalent source density model. These findings, when combined with the regional tectonic background and seismic activity, offer valuable insights. The research presented in this paper provides a foundational understanding of gravity field trends in Urumqi and its surrounding areas, contributing to future predictions of gravity field changes in the region.

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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

SEISMOLOGY AND GEOLOGY 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.

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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

SEISMOLOGY AND GEOLOGY 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.

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STUDY ON PS POINT SELECTION METHOD IN COMPLEX SURFACE ENVIRONMENT

CHEN Kai, XU Xiao-bo, QU Chun-yan, ZHANG Gui-fang, LIAN Da-jun, QIN You-sen

SEISMOLOGY AND GEOLOGY 2024, 46 (5): 1012-1026. DOI: 10.3969/j.issn.0253-4967.2024.05.002

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The key challenge of PSInSAR(Permanent Scatter Interferometric Synthetic Aperture Radar)lies in the quality of Permanent Scatter(PS)points, which are difficult to extract accurately in fracture zones due to the complex natural ground cover and unique geomorphological environments. In such areas, the inability to reliably extract high-quality PS points limits the application of PSInSAR for monitoring interseismic deformation. To address the problem of high-quality PS point selection in fault zones and improve the effectiveness of PSInSAR technology for monitoring interseismic deformation, this paper presents a comparative study of coherence coefficient and amplitude deviation double thresholds. The study further integrates the Kolmogorov-Smirnov(KS)test and CR homogeneous pixel method, in addition to conventional coherence coefficient point selection techniques. Sentinel-1A SAR images from March 14, 2015, to February 16, 2020, are used as the data source, with the Wushan-Gangu section of the fault zone on the northern edge of the Western Qinling Mountains, near the northeastern edge of the Qinghai-Tibet Plateau, serving as the test area for PSInSAR processing. The quality and reliability of PS point selection using various methods are compared and analyzed.

Two sets of coherence coefficient T_γ and amplitude deviation D_γ double thresholds were tested. The coherence coefficient was set at T_γ=0.5, while the amplitude deviation D_γ was set to 0.5 and 0.3, resulting in 19806 and 2485 PS points, respectively. When the amplitude deviation threshold was lowered, the number of PS points on ridgelines decreased significantly, while there was little change in Gangu County, suggesting poor pixel amplitude stability in mountainous areas. Lowering the threshold eliminated many PS points, retaining only those with high amplitude and stable time series, typically found in hard targets like urban buildings. A KS double sample test was then applied in combination with the double threshold method, with all thresholds coefficient T_γ, amplitude deviation D_γ, and KS test P_γ set at 0.5. This approach yielded 1 313 PS points, showing a significant reduction in PS points on ridgelines and in Gangu County, while urban points became more concentrated on hard targets like buildings. Although the KS test reduced PS points in vegetated areas, it did not fully eliminate noise points. Finally, based on the double threshold results of coherence coefficient T_γ=0.5 and amplitude deviation D_γ=0.3, both the KS test and CR homogeneous pixel selection methods were applied. The CR homogeneous pixel method used a phase difference threshold P_γ=0.5 and a temporal phase stability threshold N_γ=50. This yielded 2 485 PS points for the double threshold method, 133 PS points for the double threshold plus KS test, and 414 PS points for the double threshold plus CR homogeneous pixel method. The latter two methods significantly reduced PS points, with a higher concentration of points in Gangu County, consistent with the expectation that PS points predominantly correspond to hard targets like buildings.

Statistical analysis of the results demonstrated that the combination of the coherence coefficient, amplitude deviation, and CR homogeneous pixel method provided the highest quality PS points, effectively excluding noise points in vegetated areas. The combination of coherence coefficient, amplitude deviation, and KS test ranked second, improving accuracy in urban areas but failing to eliminate noise in vegetated areas. Using Sentinel-1A SAR images and the Wushan-Gangu fault as the test area for time series PSInSAR processing, the accuracy of PS point selection was further verified. A comparative analysis of deformation monitoring results from the three methods revealed that both the KS test and CR homogeneous pixel method improved the accuracy of fault deformation monitoring, with the CR homogeneous pixel method yielding superior results. Monitoring data from 2015 to 2020 showed that the deformation rate of the northern block of the Wushan-Gangu fault ranged from -2 to -0.2mm/a, with an average deformation of approximately -1.7mm/a. In contrast, the southern block exhibited a deformation rate between 0.3 and 0.5mm/a, with an average deformation of about 1.8mm/a The relative average deformation rate between the northern and southern blocks was 0.7mm/a, indicating left-lateral strike-slip movement. Among the three methods, the double threshold plus CR homogeneous pixel method produced PS points with the smallest deformation rate standard deviation, indicating more stable and reliable deformation results.

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