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    TYPICAL CASE ANALYSIS ON SETBACK DISTANCE FOR URBAN BURIED ACTIVE FAULT: AN EXAMPLE SITE ALONG THE TANLU FAULT ZONE IN XINYI CITY
    CAO Jun, LI Yan-bao, RAN Yong-kang, XU Xi-wei, MA Dong-wei, ZHANG Zhi-qiang
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 1071-1085.   DOI: 10.3969/j.issn.0253-4967.2022.04.016
    Abstract916)   HTML71)    PDF(pc) (11099KB)(504)       Save

    With the acceleration of urbanization process, solving the earthquake and its associated disasters caused by buried active fault in urban areas has been a difficult issue in the construction of urban public security system. It is difficult to deal with the anti-seismic issues of cross-fault buildings using the existing techniques, therefore, reasonable setback distance for buried active fault in urban area is the only method for the planning and construction at the beginning. At present, theoretical research about setback for active fault is becoming more and more mature, and the mandatory national standard “Setback distance for active fault” will be enacted soon. As a result, how to work on the basis of these theories and national standards is in urgent. In recent years, the exploration of urban active faults was successively completed. However, there are no typical cases of how to make full use of the achievements of urban active fault projects in the follow-up work, and how to guide urban construction based on the project conclusions, so as to ensure urban safety and rational development of urban economy.

    In this paper, taking a site along the Anqiu-Juxian Fault in the Tanlu fault zone in Xinyi city as an example, based on the results of 1︰10 000 active fault distribution map, and referring to the stipulation of national standard “Setback distance for active fault”, 12 shallow seismic survey lines with a spacing of less than 50m were laid out firstly, and the results of shallow seismic exploration show the existence of two high-dip faults in the site. Secondly, considering the shallow seismic survey results and the geologic site conditions, five rows of borehole joint profiles were selected along five of the shallow seismic survey lines. Based on the location of the faults and stratigraphy in the site revealed by the borehole joint profiles, and considering the latest research results of Quaternary stratigraphy and the conclusion of urban active faults detection, the west branch fault is constrained to be a Holocene active fault and the east branch fault is an early Quaternary fault. As a result, we precisely mapped the trace, dip and upper breakpoint of the fault in the site based on the shallow seismic exploration and joint borehole profile. The accurate positioning of the plane position of the active fault differs by about 200m from the 1:1000 strip distribution map.

    According to the relevant national standards and scientific research results, active faults in the site shall be avoided. Based on the surface traces of active faults revealed by the accurate detection in the site, the active fault deformation zone was delineated, and the range of setback distance for active fault was defined outside the deformation zone. The detection results accurately determined the plane distribution of the active fault in the site, which meets the accuracy of the development and utilization of the site. Based on the accurately located active fault trace, and complying with the forthcoming national standard “Setback distance from active fault”, this study not only scientifically determines the setback distance for active fault in the site, but also releases the scarce land resources in the city. This result achieves the goal of scientifically avoiding potential dangerous urban hidden active fault and making full use of land.

    The case detection process confirms that the results of urban active fault detection are still difficult to meet the fault positioning accuracy required for specific site development, and the range of active fault deformation zone within the site must be determined based on the precise positioning method for hidden active faults as stipulated in the national standard “Setback distance for active fault”. The national standard “Code for seismic design of buildings” only specifies the setback distance for active faults under different seismic intensity, but does not provide any clear definition of the accuracy of active fault positioning, so it is difficult to define the required active fault positioning degree and boundary range of the deformation zone of active fault in practice. The national standard “Setback distance for active fault” clearly defines various types of active fault detection and positioning methods, determines the scope of active fault deformation zone and the accurate setback distance for active fault in different cases. The specific case proves that before developing and utilizing specific sites along urban concealed active faults, relevant work shall be carried out according to the national standard “Setback distance for active fault” to effectively resolve the issue about the relations between urban development and urban safety, so the promulgation and implementation of national standard should speed up.

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    DISTRIBUTED CHARACTERISTICS OF THE SURFACE DEFORMATIONS ASSOCIATED WITH THE 2021 MW7.4 MADOI EARTHQUAKE, QINGHAI, CHINA
    LIU Xiao-li, XIA Tao, LIU-ZENG Jing, YAO Wen-qian, XU Jing, DENG De-bei-er, HAN Long-fei, JIA Zhi-ge, SHAO Yan-xiu, WANG Yan, YUE Zi-yang, GAO Tian-qi
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 461-483.   DOI: 10.3969/j.issn.0253-4967.2022.02.012
    Abstract343)   HTML14)    PDF(pc) (23227KB)(464)       Save

    Earthquake surface ruptures are the key to understand deformation pattern of continental crust and rupture behavior of tectonic earthquake, and the criteria to directly define the active fault avoidance zone. Traditionally, surface fissures away from the main rupture fault are usually regarded as the result triggered by strong ground motion. In recent years, the earth observation technology of remote sensing with centimeter accuracy provides rich necessary data for fine features of co-seismic surface fractures and fissures. More and more earthquake researches, such as the 2019 MW7.3 Ridgecrest earthquake, the 2016 MW7 Kumamoto earthquake, the 2020 MW6.5 Monte Cristo Range earthquake, suggest that we might miss off-fault fissures associated with tectonic interactions during the seismic rupture process, if they are simply attributed to effect of strong ground motion. Such distribution pattern of co-seismic surface displacement may not be isolated, it encourages us to examine the possible contribution of other similar events. The 22 May 2021 MW7.4 Madoi earthquake in Qinghai Province, China ruptured the Jiangcuo Fault which is the extension line of the southeastern branch of the Kunlun Fault, and caused the collapse of the Yematan bridge and the Cangmahe bridge in Madoi County. The surface rupture in the 2021Madoi earthquake includes dominantly ~158km of left-lateral rupture, which provides an important chance for understanding the complex rupture system.
    The high-resolution UAV images and field mapping provide valuable support to identify more detailed and tiny co-seismic surface deformation. New 3 to 7cm per pixel resolution images covering the major surface rupture zone were collected by two unmanned aerial vehicles (UAV) in the first months after the earthquake. We produced digital orthophoto maps (DOM), and digital elevation models (DEM) with the highest accuracy based on the Agisoft PhotoScanTM and ArcGIS software. Thus, the appearance of post-earthquake surface displacement was hardly damaged by rain or animals, and well preserved in our UAV images, such as fractures with small displacement or faint fissures. These DOM and DEM data with centimeter resolution fastidiously detailed rich details of surface ruptures, which have been often easily overlooked or difficult to detect in the past or on low-resolution images. In addition, two large-scale dense field investigation data were gathered respectively the first and fifth months after the earthquake. Based on a lot of firsthand materials, a comprehensive dataset of surface features associated with co-seismic displacement was built, which includes four levels: main and secondary tectonic ruptures, delphic fissures, and beaded liquefaction belts or swath subsidence due to strong ground motion. Using our novel dataset, a complex distributed pattern presents along the fault guiding the 158km co-seismic surface ruptures along its strike-direction. The cumulative length of all surface ruptures reaches 310km. Surface ruptures of the MW7.4 Madoi earthquake fully show the diversity of geometric discontinuities and geometric complexity of the Jiangcuo Fault. This is reflected in the four most conspicuous aspects: direction rotation, tail divarication, fault step, and sharp change of rupture widths.
    We noticed that the rupture zone width changed sharply along with its strike or geometric complexity. Near the east of Yematan, on-fault ruptures are arranged in ten to several hundred meters. Besides clearly defined surface ruptures on the main fault, many fractures near the Dongo section and two rupture endpoints are mainly along secondary faulting crossing the main fault or its subparallel branches. Lengths of fracture zones along two Y-shaped branches at two endpoints are about 20km. At the rupture endpoints, the fractures away from the main rupture zone are about 5km. Some authors suggested the segment between the Dongcao along lake and Zadegongma was a “rupture gap”. In our field investigation, some faint fractures and fissures were locally observed in this segment, and these co-seismic displacement traces were also faintly visible on the UAV images.
    It is also worth noting that near the epicenter, Dongo, and Huanghexiang, a certain amount of off-fault surface fissures appear locally with steady strike, good stretch, and en echelon pattern. Some fissures near meanders of the Yellow River, often appear with beaded liquefaction belts or swath subsidences. In cases like that, fissure strikes are, in the main, orthogonal to the river. Distribution pattern of these fissures is different from usual gravity fissures or collapses. But they can’t be identified as tectonic ruptures because clear displacement marks are always absent with off-fault fissures. Therefore, it is difficult to determine the mechanism of off-fault co-seismic surface fissures. Some research results suggested, that during the process of a strong earthquake, a sudden slip of the rupturing fault can trigger strain response of surrounding rocks or previous compliant faults, and result in triggering surface fractures or fissures.
    Because of regional tectonic backgrounds, deep-seated physical environments, and site conditions(such as lithology and overburden thickness), the pattern and physicalcause of co-seismic surface ruptures vary based on different events. Focal mechanisms of the mainshock and most aftershocks indicate a near east-west striking fault with a slight dip-slip, but focal mechanisms of two MS≥4.0 aftershocks show a thrust slip occurring near the east of the rupture zone. On the 1︰250000 regional geological map, the Jiangcuo Fault is oblique with the Madoi-Gande Fault and the Xizangdagou-Cangmahe Fault at wide angles, and with several branches near the epicenter and the west endpoint at small angles. Put together the surface fissure distribution pattern, source parameters of aftershocks and the regional geological map, we would like to suggest that besides triggered slip of several subparallel or oblique branches with the Jiangcuo Fault, inheritance faulting of pre-existing faults may promote the development of off-fault surface fissures of the 2021Madoi earthquake. Why there are many off-fault distributed surface fissures with patterns different from the gravity fissures still needs further investigation. The fine expression of the distributed surface fractures can contribute to fully understanding the mechanism of the seismic rupture process, and effectively address seismic resistance requirements of major construction projects in similar tectonic contexts in the world.

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    STUDY ON THE LATE QUATERNARY ACTIVITY OF THE WEST XIADIAN FAULT IN BEIJING PLAIN
    SHEN Jun, DAI Xun-ye, XIAO Chun, JIAO Xuan-kai, BAI Qilegeer, DENG Mei, LIU Ze-zhong, XIA Fang-hua, LIU Yu, LIU Ming
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 909-924.   DOI: 10.3969/j.issn.0253-4967.2022.04.006
    Abstract426)   HTML57)    PDF(pc) (12117KB)(448)       Save

    Beijing plain is a strong earthquake tectonic area in China, where the Sanhe-Pinggu earthquake with M8 occurred in 1679.The seismogenic fault of this earthquake is the Xiadian Fault. An about 10km-long earthquake surface fault is developed, striking northeast. Deep seismic exploration reveals that this surface fault is a direct exposure of a deep fault cutting through the whole crust, and it is concealed in the Quaternary layers to both ends. Previous studies have not yet revealed how the deep fault with M8 earthquake extended to the southwest and northeast. In the study of Xiadian Fault, it is found that there is another fault with similar strike and opposite dip in the west of Xiadian Fault, which is called the West Xiadian Fault in this paper. In this study, six shallow seismic profiles data are used to determine the location of this fault in Sanhe city, and the late Quaternary activity of the fault is studied by using the method of combined drilling, magnetic susceptibility logging and luminescence dating.

    The results of shallow seismic exploration profiles show that the fault is zigzag with a general strike of NE and dip NW. In vertical profile, it is generally of normal fault. It shows the flower structure in one profile, which indicates that the fault may have a certain strike-slip property. On two long seismic reflection profiles, it can be seen that the northwest side of the fault is a half graben structure. This half graben-like depression, which has not been introduced by predecessors, is called Yanjiao fault depression in this paper. The maximum Quaternary thickness of the graben is 300m. The West Xiadian Fault is the main controlling fault in the southern margin of the sag.

    The Xiadian Fault, which is opposite to the West Xiadian Fault in dips, controls the Dachang depression, which is a large-scale depression with a Quaternary thickness of more than 600m. The West Xiadian Fault is opposite to the Xiadian Fault, and there is a horst between the West Xiadian Fault and the Xiadian Fault. The width of the horst varies greatly, and the narrowest part is less than 1km. The West Xiadian Fault may form an echelon structure with Xiadian Fault in plane, and they are closely related in depth.

    According to the core histogram and logging curves of ten boreholes and eight effective dating data, the buried depth of the upper breakpoint of the concealed fault is about 12m, which dislocates the late Pleistocene strata. The effective dating result of this set of strata is(36.52±5.39)ka. There is no evidence of Holocene activity of the fault, but it is certain that the fault is an active fault in the late Pleistocene in Sanhe region. The vertical slip rate is about 0.075mm/a since late Pleistocene, and about 0.03mm/a since the late period of late Pleistocene. These slip rates are less than those of the Xiadian Fault in the same period. According to our study, the vertical slip rate of Xiadian Fault since late Pleistocene is about 0.25mm/a.

    Although the latest active age, the total movement amplitude since Quaternary and the sliding rate since late Pleistocene of West Xiadian Fault are less than those of Xiadian Fault, its movement characteristics is very similar to that of Xiadian Fault, and the two faults are close to each other in space, and closely related in deep structure. It can be inferred that the fault is probably a part of the seismogenic structure of the 1679 Sanhe-Pinggu M8 earthquake. In a broad sense, the Xiadian fault zone is likely to extend to the southwest along the West Xiadian Fault.

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    CRUSTAL VELOCITY STRUCTURE BENEATH THE SOUTHERN LIAONING PROVINCE DERIVED FROM DOUBLE DIFFERENCE TOMOGRAPHY
    WANG Liang, JIAO Ming-ruo, QIAN Rui, ZHANG Bo, YANG Shi-chao, SHAO Yuan-yuan
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 378-394.   DOI: 10.3969/j.issn.0253-4967.2022.02.007
    Abstract422)   HTML16)    PDF(pc) (14665KB)(394)       Save

    In recent years, the southern Liaoning Province is the main area of seismic activity in Liaoning Province, and the main geological structure units in this area include the Liaohe rift and Liaodong uplift in the east. As an important manifestation of modern tectonic activity, earthquakes are less distributed in Liaohe rift. Most of the seismic activities are concentrated in eastern Liaoning uplift area on the east side of Liaohe rift. The structure in this area is relatively complex. The revival of old faults during Quaternary is obvious, and there are more than 10 Quaternary faults. Among them, Haichenghe Fault and Jinzhou Fault are the faults with most earthquakes. The 1975 Haicheng MS7.3 earthquake occurred in the Haichenghe Fault and the 1999 Xiuyan MS5.4 earthquake occurred in the east of the fault.
    In this paper, the seismic phase bulletins are used for earthquakes from August 1975 to December 2017 recorded by 67 regional seismic stations of Liaoning Province. These stations were transformed during the Tenth Five-year Plan period. Using the double-difference tomography and tomoDD program, we relocated the earthquakes and inversed the velocity structures of the southern Liaoning area.
    In the study, grid method is used for model parameterization of seismic tomography, ART-PB is used for forward calculation, damped least square method is used in inversion, and checkerboard test is used for the solution evaluation. The theoretical travel time is forward calculated by taking the checkerboard velocity model of imaging meshing and plus or minus 5% of anomaly as the theoretical model. The checkerboard test results show that the checkerboard P-wave velocity model at the depths of 4km, 13km, 24km and 35km in the study area can be restored completely, and most areas at the depth of 33km can also be restored completely.
    We calculated and got the relocations of almost all of the earthquakes in southern Liaoning area and obtained a better distribution of P wave velocities at the depth of 4km, 13km, 24km and 33km. The results show that earthquakes mainly concentrated in two areas: the Haicheng aftershock area and the Gaizhou earthquake swarm activity area. The distribution of seismicity in this area is obvious in NW direction.
    The result of P-wave tomography in 4km depth indicates the consistent characteristics of shallow velocity structure with the surface geological structure in southern Liaoning Province area. The two sides of the Tanlu fault zone are characterized by different velocity structures. The high and low velocity discontinuities are located in the Tan Lu fault zone, which is in good agreement with the geological structure of the region. In Haichenghe Fault in the Haicheng aftershock area, there are high-velocity zone in the shallow layer and low-velocity zone in the depth of 4~12km, and the low-velocity zone intrudes and deepens eastward. The Xiuyan earthquake with MS5.4 in 1999 occurred on the boundary section of high and low velocity zones. At the same time, there is a gap between Xiuyan and Haicheng sequences, which is located at the junction of high and low velocities, and there is a significant low-velocity zone underground in the region. From the perspective of mechanism of the seismogenic model, this velocity structure model may generate large earthquakes.

    There are high-velocity zones at the ends of different segments of Jinzhou Fault, and the Gaizhou earthquake swarm occurred in the high-velocity area at the end of the fault. It is speculated that the activity of the Gaizhou earthquake swarm may be caused by the rise of water saturation in rocks due to the intrusion of liquid under the condition of stress accumulation.

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    THE 2022 M6.8 LUDING EARTHQUAKE: A COMPLICATED EVENT BY FAULTING OF THE MOXI SEGMENT OF THE XIANSHUIHE FAULT ZONE
    LI Chuan-you, SUN Kai, MA Jun, LI Jun-jie, LIANG Ming-jian, FANG Li-hua
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1648-1666.   DOI: 10.3969/j.issn.0253-4967.2022.06.017
    Abstract761)   HTML74)    PDF(pc) (16086KB)(325)       Save

    The September 5, 2022, M6.8 Luding earthquake occurred along the southeastern segment of the Xianshuihe fault zone. Tectonics around the epicenter area is complicated and several faults had been recognized. Focal mechanisms of the main shock and inversions from earthquake data suggest that the earthquake occurred on a northwest-trending, steeply dipping strike-slip fault, which is consistent with the strike and slip of the Xianshuihe fault zone. We conducted a field investigation along the fault sections on both sides of the epicenter immediately after the earthquake. NW-trending fractures that were recognized as surface ruptures during the earthquake, and heavy landslides along the fault section between Ertaizi-Aiguocun village were observed during the field investigations. There are no surface ruptures developed along the fault sections north of the epicenter and south of Aiguocun village. Thus it can be concluded that there is a 15.5km-long surface rupture zone developed along the Moxi Fault(the section between Ertaizi and Aiguo village). The surface rupture zone trends northwest and shows a left-lateral strike slip, which is consistent with the strike and motion constrained by the focal mechanism. The coseismic displacements were measured to 20~30cm. Field observations, focal fault plane, distribution of the aftershocks, GNSS, and InSAR observation data suggest that the seismogenic structure associated with the M6.8 Luding earthquake is the Moxi Fault that belongs to the southeastern segment of the Xianshuihe fault zone. Slip along the segment south of the epicenter generated this earthquake, and also triggered slip along a northeast-trending fault and the northwestern section of the Moxi Fault in the epicenter. So, the M6.8 Luding earthquake is an event that is nucleated on the section south of the epicenter and then triggered an activity of the whole fault segment.

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    GEOLOGICAL DISASTERS AND SURFACE RUPTURES OF JANUARY 23, 2024 MS7.1 WUSHI EARTHQUAKE, XINJIANG, CHINA
    ZHANG Bo-xuan, QIAN Li, LI Tao, CHEN Jie, XU Jian-hong, YAO Yuan, FANG Li-hua, Xie Chao, CHEN Jian-bo, LIU Guan-shen, HU Zong-kai, YANG Wen-xin, ZHANG Jun-long, PANG Wei
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 220-234.   DOI: 10.3969/j.issn.0253-4967.2024.01.013
    Abstract473)      PDF(pc) (14935KB)(289)       Save

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

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

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    DETAILED MAPPING OF THE SURFACE RUPTURE NEAR THE EPICENTER SEGMENT OF THE 2021 MADOI MW7.4 EARTHQUAKE AND DISCUSSION ON DISTRIBUTED RUPTURE IN THE STEP-OVER
    HAN Long-fei, LIU-ZENG Jing, YAO Wen-qian, WANG Wen-xin, LIU Xiao-li, GAO Yun-peng, SHAO Yan-xiu, LI Jin-yang
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 484-505.   DOI: 10.3969/j.issn.0253-4967.2022.02.013
    Abstract388)   HTML16)    PDF(pc) (12092KB)(287)       Save

    Detailed mapping of coseismic surface rupture can provide valuable information for understanding the geometrical complexities, dynamic rupture processes and fault mechanisms. Fault geometrical complexities, such as bends, branches, and stepovers are common in strike-slip fault systems and can control the coseismic surface rupture characteristics to a certain extent. Observational studies of surface ruptures in past earthquakes suggested that special rupture characteristics would form distributed ruptures and rupture gaps. The detailed mapping has become an important way to study the surface rupture. According to the China Earthquake Networks Center(CENC), the MW7.4 earthquake occurred at 2:04 PM on May 22, 2021, in Madoi County, Qinghai Province. The epicenter is about 70km south of the eastern Kunlun Fault on the northern boundary of the Bayan Kera block. It is the largest earthquake that hit the Chinese mainland since the Wenchuan MS8.0 earthquake in 2008. After field investigation and rupture mapping on the computer, Yao et al.(2022)estimated that the length of surface rupture of this earthquake is 158km. Surface ruptures of the MW7.4 Madoi earthquake broke through the geometric discontinuities such as step-overs and bends, and formed various coseismic surface fractures, especially in the middle segment. In the survey of the Madoi earthquake, we rapidly acquired aerial image data using UAV aerial photogrammetry and obtained high-resolution digital orthograph models(DOMs)and digital elevation models(DEMs)using PhotoScan software based on the SfM algorithm processing. Those data provide an opportunity for detailed mapping of seismic rupture and also provide an important reference for fieldwork. Based on high-resolution topographic data, we carried out detailed surface rupture mapping, classification, geometric structure and strike analysis for the ~30km section of the epicenter segment. At the same time, we conducted field work to supplement and proofread the maps.
    According to the characteristics of surface ruptures in the epicenter area, we divided the ruptures into six segments. The surface ruptures along segment S1 and segment S6 are concentrated near the main fault, while the surface ruptures in the stepover(segment S3, S4, and S5)are distributed dispersively, and the secondary ruptures along the segment S2 are also distributed scatteredly, with the main rupture missing. To reveal the distribution characteristics of surface fractures more clearly, the surface ruptures are divided into the main rupture, secondary rupture, surface rupture and collapse rupture, among which the genesis of the surface rupture is uncertain. There are a lot of typical tensile ruptures with left-lateral component in segment S1, the strike of the ruptures is consistent with the strike of the main fault or intersects the main fault with a small angle. The maximum width of the main rupture in segment S1 is ~50m. The main ruptures in segment S6 are developed along with the preexisting tectonic topography and the offset of the left-lateral displaced gully is up to tens of meters in magnitude. The surface ruptures are distributed in an echelon pattern, and all intersected with the strike of the main fault at a large angle. The location and size of the step-over are determined according to the topography and rupture morphology of faults in segment S1 and segment S6. The surface ruptures on the floodplain and banks of the Yellow River are in various forms and difficult to classify accurately. Therefore, only the typical seismic ruptures developed along the accumulated tectonic topography are labeled as main ruptures, and other typical seismic ruptures inconsistent with the location of the main fault are labeled as secondary ruptures. The typically collapse ruptures distributed along the river bank or lake bank are labeled as collapse ruptures, while the rest are labeled as surface ruptures. Surface ruptures in segment S3 are distributed on the planar graph, but they have a dominant strike in the NE direction that can be seen from the diagram map. In the floodplain of the Yellow River, there are typical “grid” cracks, “explosive” cracks, and tensile cracks, and many cracks are accompanied by sand liquefaction which is beadlike, single, and distributed along the cracks. After the earthquake, the geodesic and geophysical data obtained quickly from the InSAR co-seismic deformation map and precise positioning of aftershocks revealed the basic morphological characteristics of earthquake rupture and provided valuable information such as earthquake rupture length, which provided an important reference for the design of UAV aerial photography and fieldwork. Compared with the rupture trace in field investigation by Pan et al.(2021), the surface rupture coverage obtained by mapping based on UAV aerial photogrammetry technology in this study is more extensive and accurate.
    In general, surface ruptures of the Madoi earthquake are widely distributed, and we have classified those ruptures into the main seismic ruptures, secondary seismic ruptures, collapse cracks, and other surface ruptures. In addition to the seismic rupture with the same strike, there are also a variety of distributed surface ruptures with different strikes from the main fault. In these distributed surface ruptures, there are also many surface ruptures whose cause is not clear and they may be affected by tectonics or strong quake. For example, the “grid” and “explosive” surface ruptures on the Yellow River floodplain may be related to the strong quake near the epicenter or may also be related to the three-dimensional dynamic ruptures process in the initial stage. In this study, the characteristics of earthquake surface rupture in the step-over and adjacent sections near the epicenter has been described in detail, which provides a deeper understanding of the distributed coseismic surface rupture in the strike-slip fault.

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    DISCUSSION ON COSEISMIC SURFACE RUPTURE LENGTH OF THE 2021 MW7.4 MADOI EARTHQUAKE, QINGHAI, CHINA
    YAO Wen-qian, WANG Zi-jun, LIU-ZENG Jing, LIU Xiao-li, HAN Long-fei, SHAO Yan-xiu, WANG Wen-xin, XU Jing, QIN Ke-xin, GAO Yun-peng, WANG Yan, LI Jin-yang, ZENG Xian-yang
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 541-559.   DOI: 10.3969/j.issn.0253-4967.2022.02.016
    Abstract439)   HTML15)    PDF(pc) (13089KB)(285)       Save

    Coseismic surface rupture length is one of the critical parameters for estimating the moment magnitude based on the empirical relationships and later used in assessing the potential seismic risk of a region. On 22 May 2021, the MW7.4 Madoi earthquake occurred in the northeastern part of the Tibetan plateau(Madoi County in Qinghai Province, China)and ruptured the poorly known Jiangcuo Fault along the extension line of the southeastern branch of the Kunlun Fault. We began our data acquisition using aerial photogrammetry by UAV three days after the earthquake. Between 24 May and 15 June 2021, more than 40000 high-resolution low-altitude aerial photos were acquired covering a total length of 180km along the surface rupture. Based on detailed field investigations, combined with a fine interpretation of sUAV-derived orthophotos and high-resolution DEMs, we determined a total length of~158km of the coseismic surface rupture extending to the eastern end at 99.270°E, which is basically consistent with the position given by previous geophysical methods. Although the extending segment is located beyond the end of the continuous surface rupture trace near Xuema Township, it should be included in the calculation of the length of the surface rupture as part of the tectonic surface rupture. The surface rupture is segmented into four sections, named from west to east: the Eling Lake, Yematan, Yellow River, Jiangcuo branch sections. Additionally, to the east of Dongcaoa’long Lake, we mapped semi-circular arc-shaped continuous tension-shear fractures in the dune area with a short gap(~3km)connecting to the east of the Jiangcuo branch. The surface ruptures along the southeastern Youyunxiang segment also sporadically appear in several sites, locally relatively continuous, covered by the sand dune with vertical displacements of up to 30cm. After passing through the dunes, the surface rupture of the Youyunxiang segment began to spread widely, extending continuously with a strike of nearly east-west. However, it should be noted that the rupture lengths of the Youyunxiang segment and other branches are not counted in the total earthquake rupture length. By comparing the current research results, we believe that the critical factors causing the significant differences of the measured length of coseismic surface ruptures would depend on: 1)more extensive and detailed field investigations combined with a fine interpretation of high-resolution images; 2)avoidance of repeated calculation of superimposed sections on both sides of the complex geometrical area. In this study, combined with the fine interpretation of high-precision image data, many surface rupture traces in the dunes of the Youyunxiang segment were identified(verified and confirmed by field inspection)and more continuous surface rupture segments on the F1 fault, which is difficult to reach by human beings, were discovered, also highlights the important role of digital photogrammetry in the study of active tectonics. The studies of the strong historical earthquakes around the Bayan Har block show that the coseismic surface rupture length is larger than that estimated by the empirical relationships. Further research thus is highly necessary to uncover its mechanism and indicative significance.

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    GEOLOGICAL CHARACTERISTICS AND ERUPTION HAZARDS TYPES OF BINGMAJIAO: A COASTAL VOLCANO IN EMAN, HAINAN
    ZHAO Yong-wei, LI Ni, CHEN Zheng-quan, WANG Li-zhu, FENG Jing-jing, ZHAO Bo
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 281-296.   DOI: 10.3969/j.issn.0253-4967.2022.02.001
    Abstract512)   HTML32)    PDF(pc) (12803KB)(278)       Save

    Bingmajiao volcano is a coastal volcano, located in Eman Town, Leiqiong volcanic field, China. In this paper, based on satellite image and unmanned aerial vehicle(UAV)image data interpretation, as well as field investigation, typical cross sections at different locations of the coastal volcanic cone were analyzed to identify the volcanic eruption sequence and determine the physical mechanism of eruption. The origin of pyroclasts was analyzed under microscope and scanning electron microscope. There are three types of pyroclasts in Bingmajiao volcano. The first type is in the shape similar to ropes or tree root and experienced obvious plastic deformation. The micro-plastic lava droplets with different sizes and irregular shapes are agglomerated on the surface of clasts. The vesicular structure in the clasts is extremely developed. All lines of evidence support this type of pyroclasts derived from magmatic explosive eruption without significant water-involving. The second type of pyroclasts is featured by crusted and moss-like surface with superficial cracks. The rigid shell surface fragmented, forming a large number of sheet-pieces that were re-disordered cemented. Under the surface, fine-honeycomb-like vesicular structure appears. The surface cracking supports the quenching by water under high temperature, and the interior vesicular structure shows that the core part may not be affected. These features indicate moderate water-magma interaction in the pyroclasts. The third type of pyroclasts shows no distinction between the surface and the interior. Irregular vesicles account for the major volume in the pyroclasts. Thin film-like lava separates these vesicles. Some lava broke into a large number of sheet-like pieces and agglomerated, forming strongly brittle-ductile deformed pyroclasts. Abundant cracks appear on the surface of lava. These features support this type of pyroclasts formed in relatively strong water-magma interaction. The study shows that the Bingmajiao volcano erupted in littoral environment, with the characteristic of transition from submarine volcano to terrestrial volcano. In the early stage of volcanism, submarine “fire fountain” type eruption prevailed, and pyroclastic deposits dominated by the third type of pyrolcasts formed underwater. Most were composed of sharp-hornlike volcanic lapilli. The pyroclastic deposit is loose and has no bedding, and the particle size sorting is not obvious. There is a large number of black fluidal juvenile lava with highly vesicular structure. As the eruption continued, when the pyroclastic deposits rose above the water surface, the volcanism transformed into the phreatomagmatic eruption, resulting in surge current and tuff deposit, which has obvious parallel bedding and cross-bedding. The second type of pyroclasts formed in this stage. In the late period of volcanic activity, Strombolian and Hawaiian type eruption were the main types, which formed black and red welding aggregates. Finally, the eruption turned into an overflow of lava, forming a lava platform. According to the eruption physics of Bingmajiao volcano, it is speculated that the potential eruption hazards of littoral volcano in the future include underwater “fire fountains”, surging currents, ballistic falling volcanic bombs, lava fountains and lava flows. Among them, the surge current may move at a high speed close to the sea level, affecting a range of 10km around the crater, which is the most dangerous type of volcanic eruption hazard.

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    PROBING THE SUBSURFACE ELECTRIC STRUCTURE FOR CSELF NETWORK IN CAPITAL CIRCLE REGION
    DONG Ze-yi, TANG Ji, ZHAO Guo-ze, CHEN Xiao-bin, CUI Teng-fa, HAN Bing, JIANG Feng, WANG Li-feng
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 649-668.   DOI: 10.3969/j.issn.0253-4967.2022.03.006
    Abstract418)   HTML23)    PDF(pc) (13890KB)(273)       Save

    The first control source extremely low frequency(CSELF)electromagnetic observation network through the world, consisting of 30 fixed stations located in the Beijing captical circle region(15 staions)and the sourthern secton of the north-south earthquake belt(15 stations), China, has been established under the support of the wireless electromagnetic method(WEM)project, one of the national science and technology infrastructure construction projects during the 11th Five-year Plan period. As a subsystem of the WEM project, the CSELF network is mainly to study the relationship between elctromagnetic anomalies and mechanisms of earthquake, and further improve our ability to monitor and predict earthquakes by monitoring real-time dynamic changes in both electromagnetic fields and subsurface electric structure. Carrying out the detection of the underground background electric structure in the CSELF network area/station is an important part of this project and of great significance to play its role in the study of earthquake prediction and forecast. In this paper, we elaborate how to acquire the subsurface electric structure of the CSELF network in the Beijing captical circle region and make a simple explanation for the structure. Firstly, a short magnetotelluric(MT)profile, almostly perpendicular to the regional geological strike, was deployed at each station of the CSELF network in the capital circle region during the 2016 and a total of 60 broadband MT sites was collected using ADU -07e systems. Then, all the time series data were processed carefully using the robust method with remote reference technique to MT transfer functions. MT data quality was assessed using the D+algorithm. In general, data at most sites are of high quality as shown by the good consistency in the apparent resistivity and phase curves. Different impedance tensor decomposition methods including the phase tensor analysis, Groom and Bailey(GB)tensor decompositon, and statistical image method based on multi-site, multi-frequency tensor decompositon were used to analyze data dimensionality and directionality. For data inversion, on the one hand, one-dimensional(1-D)subsurface electrical resistivity structures at each station and MT site were derived from 1-D adaptive regularized MT inversion algorithm. On the other hand, we also imaged the 2-D electric structures along the short MT profile by the nonlinear conjugate gradients inversion algorithm at each station. Robustness of all 2-D structures along each short profile were verified by sensitivity tests. Although fixed stations and MT sites are limited and distributed unevenly, the 3-D inversion of 15 stations was also performed to produce a 3-D crustal electrical resistivity model for the entire network using the modular system for 3-D MT inverson: ModEM based on the nonlinear conjugate gradients algorithem. Intergrating 1-D, 2-D and 3-D inversion results, the resistivity structure beneath the CSELF network in captical circle region revealed some significant features: The crustal electrical structures are mainly characterized by high resistivity beneath the Yinshan-Yanshan orogenic belt in the northern margin of North China, the Taihangshan area in the middle, the Jiao-Liao block in the east, while the North China Plain and Shanxi depression areas have relatively lower resistivity in the crust; There are obvious electrical resistivity difference on both sides of the gravity gradient of Taihang Mountains and the Tanlu fault zone, which indicates they could be manifested as an electric structure boundary zone, respectively. Overall, the electric structure characteristics of the entire network area shows high correspondence with the regional geological structure and earthquake activity to some extent. In summary, implementing the detection of underground electrical resistivity structure in the CSELF network of the capital circle region will provide important foundations for the researches on the regional seismogenic environment, the generation mechanism of seismic electromagnetic anomaly signals, and earthquake prediction and forecast.

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    PRELIMINARY STUDY ON FAULTED LANDFORMS AND AGES OF RECENT STRONG EARTHQUAKE ACTIVITY ON THE KARAKORUM FAULT IN NGARI, TIBET
    XU Wei, LIU Zhi-cheng, WANG Ji, GAO Zhan-wu, YIN Jin-hui
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 925-943.   DOI: 10.3969/j.issn.0253-4967.2022.04.007
    Abstract492)   HTML30)    PDF(pc) (14700KB)(264)       Save

    The Karakoram Fault is located in the west of the Qinghai-Tibet Plateau and crosses Kashmir, Xinjiang and Tibet in China. It is a large normal dextral strike-slip fault in the middle of the Asian continent. As a boundary fault dividing the Qinghai-Tibet Plateau and the Pamir Plateau-Karakoram Mountains, the Karakoram Fault plays a role in accommodating the collision deformation between the Indian plate and the Eurasian plate and in the tectonic evolution of the western Qinghai-Tibet Plateau. The fault trace in Ngari area is clear and the faulted landforms are obvious, which show strong activity characteristics in late Quaternary. As a large active fault, only one earthquake of magnitude 7 has been recorded on the Karakoram Fault since the recorded history, namely, the Tashkurgan earthquake of 1895 at its north end. There are no records of strong earthquakes of magnitude≥7 along the rest of the fault, and no paleo-seismic research has been carried out. Ages of recent strong earthquake activity and earthquake recurrence intervals are not clear, which greatly limit the accuracy of seismic risk assessment. In this study, we investigated the fault geometry and faulted landforms in Ngari area, collected OSL samples of the faulted landforms and sag ponds in Zhaxigang, Menshi and Baga towns and preliminarily discussed the ages of recent strong earthquake activity.

    Study shows that the fault can be divided into three sections by Zhaxigang town and Suoduo village, and the structure and properties of each section are significantly different. In west Zhaxigang town section, the fault is dominated by dextral strike-slip with certain vertical movement, it is almost straight on the surface, with river terraces, alluvial-proluvial fans and water system faulted ranging from tens to hundreds of meters. In Zhaxigang town to Suoduo village section, the normal faulting is remarkable, the main fault constitutes the boundary fault between Ayilari Mountain and Gar Basin; fault facets and fault scarps are common along the fault line, there are also secondary faults with the same or opposite dip as the main fault developed near the piedmont basin. In east Suoduo village section, the main part of the fault is located at the south foot of Gangdise Mountain, and in addition to the piedmont fault, several approximately parallel faults are also developed on the southern alluvial-proluvial fans and moraine fans which are mainly dextrally faulted with certain vertical component.

    According to the analysis of the faulted landforms and dating of the OSL samples collected from the sag ponds and faulted landforms in the west of Zhaxigang town, the east of Menshi town and the east of Baga town, the ages of recent strong earthquake activity on the fault are analyzed as follows. In the west of Zhaxigang town, the age of recent strong earthquake activity of the fault is constrained to be close to 2.34kaBP according to the average OSL dating results of KKF-3 and KKF-4. In the east of Menshi town, the recent earthquake activity age of fault f2 is 4.67~3.01kaBP, but closer to 3.01kaBP according to the OSL dating results of KKF-11 of the youngest faulted geomorphic surface and average OSL dating results of KKF-6 and KKF-13 collected from sag ponds. In the area near Angwang village, Baga town, it is inferred that the recent strong earthquake activity age of the fault is close to 2.54kaBP according to the OSL dating results of KKF-2 collected from sag pond. If the faults of above three places are active at the same time, the age of recent strong earthquake activity of the fault is close to 2.63kaBP. The Karakorum Fault in Ngari area has obvious segment boundaries, and the activity of each segment and in its internal branch faults is most likely to be independent.

    The earthquake recurrence interval on the fault is estimated to be 2.8ka according to the slip rate and the amount of displacement. From the above analysis, it can be seen the time since the last strong earthquake activity of Karakorum Fault may have been very close to the interval of earthquake recurrence. If the fault is characterized by a quasi-periodic in-situ recurrence, the energy accumulation in the fault may have reached a very high degree and the risk of recurrence of strong earthquake events of the fault may be very high, so more attention should be paid and more detailed research on the paleo-earthquake events and recurrence intervals should be carried out as quickly as possible.

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    STUDY ON THE DENSITY VARIATION CHARACTERISTICS OF UNDERGROUND MEDIUM IN HUTUBI GAS STORAGE BASED ON HIGH-PRECISION TIME-VARYING MICROGRAVITY METHOD
    LIU Dai-qin, XUAN Song-bai, CHEN Shi, LI Jie, WANG Xiao-qiang, LI Rui
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 414-427.   DOI: 10.3969/j.issn.0253-4967.2022.02.009
    Abstract329)   HTML13)    PDF(pc) (5321KB)(247)       Save

    In this paper, based on microgravity time-varying signals, the gravity field and underground medium density change of Hutubi gas storage were simulated and calculated, and the response relationship between gravity change and injection-production pressure was analyzed. By using the 7 phases of mobile gravity data of Hutubi underground gas storage, adopting the classical adjustment method and selecting the absolute gravity points of HKPN, HKPS and Urumqi(BJ00) and Shihezi gravity point(BJ06) of CMONOC around the gas storage area as the calculation basis, the relative gravity variation of each monitoring point in the study area was obtained with the precision ranging (3~5)×10-8m/s2 for each point in each phase. Combined with the relevant data of gas storage injection-production pressure, the response relationship image between the spatial-temporal variation characteristics of gravity field and injection-production pressure in this area was acquired. The research shows that the gravity change in the entire survey area exhibits zoning characteristics. The gravity change in the outer area of the gas storage south of Hutubi Fault is relatively small, and the gravity change in the gas storage area increases and decreases alternately. Especially in the east side of the reservoir area, the gravity change shows obvious characteristics of decreasing in spring and increasing in autumn, which causes the natural gas in the gas storage to basically drop to the lowest in March, thus resulting in the minimum internal stress in the gas storage. According to the theory of crustal stress equilibrium, when the pressure inside the gas storage tends to increase or decrease, the stress outside the gas storage will be adjusted correspondingly. When the gas injected into the gas storage spreads between the rocks and their gaps in the gas storage, it will exert a certain pressure on the rocks, causing the medium density in the underground gas storage cavity to vary in different degrees, thus resulting in the changes in the gravity values of the surface measuring points in the gas storage area. Finally, based on the dynamic change data of gravity field observed on the surface of Hutubi underground gas storage, the constraint of depth weighting function was added in the calculation process to eliminate and weaken the multi-solution and skin effect, and the compact gravity inversion algorithm of spatial distribution of underground density variation anomaly body was adopted to simulate and calculate the underground material density change image of Hutubi gas storage and the morphological structure distribution characteristics inside the gas storage. In this paper, according to the structural framework of about 1km/layer in Hutubi gas storage, all slices are constructed in the vertical direction of 1km to the crust, and a total of 9 layers are cut into them. That is, they are divided from the surface to the interior of the gas storage from 0 to 9km. Based on the change amount of gas injection and production in Hutubi gas storage, combining with the density images of underground media in different periods, it can be clearly seen that the internal cavity shape distribution inside the gas storage is irregular, so the stress on each point in the gas storage will be uneven, resulting in different density changes of the medium in different depths. The density distribution of underground medium in this gas storage varies with time, and the density variation is relatively different, but it has a certain change rule. Most density variation images show four quadrant distribution characteristics, especially at the depth of about 3000~4000m of the gas storage, where the migration degree of underground medium substances is the largest, resulting in the largest density variation in this area, with the maximum density variation of about 0.7kg·m-3. At this stage, the gas storage is just at the peak points of gas injection and production, that is, the maximum and minimum peak points of stress. In addition, the density change image has showed that the internal structure of the gas storage is in NW-SE direction, which is basically consistent with the geological structure distribution characteristics of Hutubi gas storage. Therefore, using gravity data, the structural form of Hutubi underground gas storage and the whole process of medium density changing with injection-production pressure can be clearly explained.

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    CHARACTERISTICS AND PROCESSING OF MAGNETOTELLURIC DATA UNDER STRONG ELECTROMAGNETIC INTERFERENCE ENVIRONMENT
    HAN Jing, ZHAN Yan, SUN Xiang-yu, ZHAO Guo-ze, LIU Xue-hua, BAO YU-xin, SUN Jian-bao, PENG Yuan-qian
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 736-752.   DOI: 10.3969/j.issn.0253-4967.2022.03.011
    Abstract447)   HTML25)    PDF(pc) (15760KB)(245)       Save

    With the development of national economic construction, high-speed railway, wind power stations, and photovoltaic power stations, large-scale high voltage power grids are widely distributed. Under these strong electromagnetic interference environments, obtaining high-quality magnetotelluric(MT)observation data is a practical problem. We carried out MT observation in Yinchuan, Yuncheng, Hebi, and Zhangjiakou in the past two years, and based on the data acquisition and processing results of around 500 MT stations in these four survey areas, 45 typical MT stations under strong electromagnetic interference environments are selected. Based on the nearest interference source, we sorted out these stations into seven kinds of strong electromagnetic interference environment. The seven kinds of strong electromagnetic interference environment are high-speed railway(0.5~1km), electrified railway(1.3~3.7km), wind power station(0.1~3.7km), photovoltaic power station(2~9km), large-scale high voltage power grids(0.06~0.4km), colliery(0.15~1km), and city(0.05~0.8km). The apparent resistivity curve obtained from processing of the typical MT station’s original data shows that the electromagnetic interference near the high-speed railway, electrified railway, and photovoltaic power station is mainly near-field interference. The mid-band frequency apparent resistivity curve of MT stations under near-field interferences rises along an angle of 45° while the impedance phase curve tends to 0. The electromagnetic interference of wind power generation facilities on MT data is relatively small. Large-scale high voltage power grids, colliery, and urban integrated electromagnetic interference are reflected in the apparent resistivity curve as discrete “outlier” with single or multiple frequency points. The curve does not have a stable shape at all. For the 45 typical MT stations listed in this paper under the strong electromagnetic interference environment, the data collection time covers two nights. The use of remote reference, non-robust processing, and fine spectrum selection for the full-time time series data improves MT data quality. The process of obtaining effective spectrum data and the results show that to get effective magnetotelluric data in a strong electromagnetic interference environment, the MT data observation time should include at least two nights(41h). Secondly, when the seven types of strong electromagnetic interference cannot be avoided, the MT stations should be placed at a distance of no less than 0.5km from high-speed railways, 1.3km from electrified railways, 2km from photovoltaic power stations, 0.2km from large-scale high voltage power grids, and 0.3km from colliery. It is also recommended that the distance of MT station shall be no less than 0.2km from electric wires, no less than 0.3km from transformers, and no less than 0.5km from thermal power stations in the comprehensive urban disturbance. The wind power stations have little effect on magnetotelluric data. Finally, a high-quality remote reference shall be used in the data processing. The use of this data can effectively suppress the influence of electromagnetic near-field interference by performing remote reference processing and estimating the spectrum data with the non-robust method.

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    AMBIENT NOISE EIKONAL TOMOGRAPHY BASED ON MUTI-CHANNEL CROSS-CORRELATION IN THE NORTHEASTERN MARGIN OF THE TIBETAN PLATEAU
    MA Xiao-jun, WU Qing-ju, PAN Jia-tie, ZHONG Shi-jun, XU Hui
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 604-624.   DOI: 10.3969/j.issn.0253-4967.2022.03.004
    Abstract393)   HTML41)    PDF(pc) (13190KB)(237)       Save

    The traditional surface wave tomography method is a ray-theoretic travel-time tomography based on the high-frequency approximation, and adopts the regularization method with model smoothing parameters, which is likely to produce false anomalies. The current eikonal tomography is a geometrical ray theoretic method that can obtain the travel time gradient of the wave field by tracking the propagation of the wave front, and then get the slowness vector of wave field gradient. This method has the advantages of high efficiency and high resolution. But both surface wave travel-time tomography and traditional eikonal tomography need to extract dispersion curve. For example, the extraction of dispersion curve with auto frequency-time analysis method usually requires a manual extraction again, which may increase systematic error or human error. The multichannel cross-correlation surface wave eikonal tomography for earthquakes developed in recent years does not need to extract the dispersion curve, but automatically measures the relative phase delay between nearby stations based on waveform cross-correlations by using the far field condition of wave equation, and then inverts the two-dimensional surface wave phase velocity distribution with eikonal tomography method. This method can suppress the random incoherent noise and reduce bias caused by strong multipath scattering.

    In this paper, we collected the one-year three-channel continuous waveform data from 676 temporary stations under the project China Array II and calculated the surface wave empirical Green’s function of ambient noise through noise cross-correlation from January to December 2015. The multichannel cross-correlation surface wave eikonal tomography was firstly applied to ambient noise tomography. The first step was to calculate the relative phase delay using the multi-channel cross-correlation, and at the second step, we inverted the Rayleigh wave apparent phase velocity at 8~40s periods based on eikonal equation for the whole study area, with the high resolution of about 40km in the major regions. At last, we compared our results with other results and discussed the tectonic deformation and dynamic process of the study area. The results are as follows:

    (1)In contrast to traditional eikonal tomography method in which the dispersion has to be extracted based on frequency analysis, our results can reduce the bias resulting from multi-path scattering wave and low signal-to-noise ratio, and improve the stability of inversion results. Moreover, our results of long-period surface waves have higher accuracy and stability because our method reduces short-wavelength heterogeneity.

    (2)There are obvious low-velocity anomalies in the upper crust of Hetao-Jilantai Basin at 18s period, and a weak low-velocity zone in the lower crust and upper mantle, which is associated with the upwelling of hot asthenosphere mantle materials in the “big mantle wedge”.

    (3)A weak layer with low S-wave velocity exists in the middle and lower crust of the northeastern Songpan-Garzê block and the western Qilian orogenic belt. Receiver function results indicate that there is high Poisson’s ratio(0.28)and low P wave velocity(less than 6.3km/s)in the northeastern Songpan-Garzê block, which may suggest partial melting in the middle and lower crust of the northeastern Songpan-Garzê block; The radial anisotropy from ambient noise tomography in the western Qilian orogenic belt shows negative radial anisotropy characteristics, which may be associated with the crustal shortening, thickening and coupling under the compression from the north and south blocks.

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    LATE QUATERNARY ACTIVE TECTONICS OF THE NORTH ALTYN FAULT
    YE Yu-hui, WU Lei, WANG Yi-ping, LOU Qian-qian, CHEN Li-qi, GAO Shi-bao, LIN Xiu-bin, CHENG Xiao-gan, CHEN Han-lin
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 297-312.   DOI: 10.3969/j.issn.0253-4967.2022.02.002
    Abstract529)   HTML38)    PDF(pc) (7598KB)(235)       Save

    The~1600km long, left-reverse strike-slip active Altyn Tagh fault system defines the northern edge of the Tibetan plateau, and serves as an important tectonic boundary in models describing the northward expansion of the plateau. The Altyn Tagh fault system has complex geometries, and consists mainly of the left-lateral South Altyn Fault to the south, the left-reverse or reverse-dominated North Altyn Fault to the north, and the intervening Altyn Shan. Most of the existing studies focus on the more active South Altyn Tagh Fault, but few has paid attention to the North Altyn Fault, which separates the Tarim Basin to the north from the Altyn Shan to the south, and figures importantly in understanding the tectonic evolution of the entire fault system. The kinematics of the North Altyn Fault in the Cenozoic remains disputed in whether it is a left-reverse or reverse-dominated fault. Herein, we used tectonic geomorphology analysis to systematically study the characteristics of active tectonics on the North Altyn Fault in the Quaternary. There are dozens of rivers in the Altyn Shan between the South Altyn Tagh Fault and North Altyn Fault, the majority of which originate near the South Altyn Tagh Fault and flow northward across the North Altyn Fault into the Tarim Basin. These rivers contain abundant information about the Quaternary tectonic activity of the North Altyn Fault. We used SRTM DEM data to extract the geomorphic features of 18 rivers and related catchment basins flowing across the North Altyn Fault. Geomorphic index, such as river longitudinal profiles, standardized river length-gradient index(SLK), normalized river steepness index(Ksn), area-elevation curves and their integrals(HI)of catchment basins, are analyzed. The conclusions are drawn as follows.
    The geomorphological indexes show that the eastern part of the North Altyn Fault is geomorphologically more active than the western part. Along the western part of the North Altyn Fault, the river longitudinal profile and the area-elevation curves of the corresponding catchment basins are both concave upward, with many small knickpoints on the river profile and relatively low SLK, Ksn, and HI values. On the contrary, most of the river profiles in the eastern part of the fault are convex or linear, with much larger knickpoints on the hanging wall of the North Altyn Fault, coinciding with high SLK and Ksn values. The associated area-elevation curves are mainly S-shaped and convex, and the HI values are relatively large. Tectonic geomorphic index is generally affected by lithology, climate and tectonics. The lithology of the hanging wall of the North Altyn Fault is relatively simple, consisting mainly of Precambrian metamorphic rocks intruded by some granite. There is no obvious difference in rock strength between the entire eastern and western sections. In addition, since the rivers are all located in the Altyn Shan and the area involved is not large, there is also no significant climatic variation along the strike of the North Altyn Fault in the Quaternary. Therefore, the difference of geomorphological activities between the parts should not be caused by difference in lithology and climate. Instead, we found that the eastern part of the North Altyn Fault is located to the north of the Akato restraining double bend, which features intense crustal shortening due to change of the fault strike, on the active South Altyn Tagh Fault. As such, we infer that the strong geomorphic activity of the eastern part of the North Altyn Fault likely results from intense lateral contraction from the Akato restraining double bend to the south, suggesting intimate interplay between the South Altyn Tagh Fault and the North Altyn Fault.
    Our findings also imply that the North Altyn Fault likely changed from a strike-slip-dominated fault to a reverse-dominated fault in the late Cenozoic. It can be seen from the extracted river morphology that all rivers are relatively straight when passing through the North Altyn Fault, without systematic left-lateral deflection. The geomorphic indexes, such as the locations of river knickpoint, high SLK and Ksn value, which reflect where the relatively rapid tectonic uplift has occurred, all appear in the hanging wall of the North Altyn Fault. Moreover, a south-dipping frontal fault is discovered in the north of the North Altyn Fault. This fault cut and uplifted the Quaternary alluvial fan in the hanging wall, and the amount of uplift decreases gradually from middle to both sides until it vanishes, forming a bilaterally symmetric anticline approximately parallel to the fault. The rivers across through the fault are straight and undeflected systematically. All these show typical characteristics associated with a thrust fault. We thus infer that the North Altyn Fault is dominated by reverse dip-slip in the late Quaternary. Together with the Cenozoic strike-slip motion on the North Altyn Fault by the measurement of kinematic indicators, a transition from strike-slip-dominated to reverse-dominated in the late Cenozoic is thus expected.

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    PALEOSEISMOLOGIC STUDY ON THE YUEXI FAULT IN THE MIDSECTION OF THE DALIANGSHAN FAULT ZONE SINCE THE LATE QUATERNARY
    LIU Qing, LIU Shao, ZHANG Shi-min
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 321-337.   DOI: 10.3969/j.issn.0253-4967.2023.02.002
    Abstract241)   HTML44)    PDF(pc) (15181KB)(230)       Save

    The Xianshuihe-Xiaojiang fault system(XXFS)is a strongly active left-lateral strike-slip fault zone on the eastern edge of the Qinghai-Tibetan plateau. It controls the eastern boundary of the Sichuan-Yunnan block, Which is one of the most active tectonic zones in the north-south seismic belts. There have been 36 destructive earthquakes since 1327AD. The historical strong earthquakes in the middle section of the XXFS fault system are mainly distributed along Anning River faults and Zemu River faults, such as M7.0 in 814AD, M71/2 in 1536AD, M63/4 in 1732AD, M71/2 in 1850AD and M63/4 earthquakes in 1952AD. However, as an important part of the middle of XXFS, the Daliangshan fault zone only recorded a magnitude of M51/2 in 1480AD, and there was a lack of earthquake records above a magnitude of 6 which may be due to the quiet period of earthquakes, or the location of remote mountainous areas where historical records are missing. The paleoseismic study revealed that there were surface rupture events along the Butuo and Jiaojihe faults in the southern section of the Daliangshan fault zone in 970-1510AD and 1310-1660AD respectively, with a magnitude of not less than 6.5; Along the Puxiong fault in the middle section of the Daliangshan fault zone, there was a surface rupture event in 927-1360AD, with a magnitude of not less than 7.0. However, there are no corresponding historical records of the earthquakes in these three historical periods, indicating that strong historic earthquakes in the Daliangshan fault zone may be missing.

    The Yuexi fault is the only branch fault in the Daliangshan fault zone dominated by thrust slip. The fault spreads in an arc shape, with a total length of about 50km, and controls the quaternary basins such as Zhenxi, Xinmin, and Yuexi. The topographic height difference between the fault’s two sides is about 2 000m. The middle section of the fault is the eastern boundary fault of the Yuexi Basin, which cuts through the piedmont alluvial fan, forming fault scarps several meters to tens of meters high. Together with the Puxiong fault on the east side, which is dominated by left laterally slipping, a positive flower-type structure is formed in the middle section of the Daliangshan fault zone. There are previous discoveries about fault scarps of the Yuexi fault on the piedmont alluvial fans, but no paleoseismic research has been reported up to now.

    On the basis of remote sensing interpretation and field geological and geomorphological survey of the Yuexi fault, a big trench was excavated across the 12m-high fault scarp on the late quaternary alluvial fan in the Yuexi Basin, which revealed four paleoseismic events since the late quaternary and the coseismic vertical slip of the last one is ~1.2m. Based on trench analysis, 14 stratigraphic units are defined from which carbon samples are acquired for geochronological analysis. Through radioactive carbon dating and correction of the dating data by the OxCal software, and OxCal model building to limit the age of paleoearthquake events, the ages of the four events were 25260-23880BC, 23930-23500BC, 20980-1400BC, and 270-1500AD. According to historical records, a destructive earthquake occurred in Yuexi County on September 13, 1480AD, which triggered landslides, 7 earthquakes on that day, and more than 20 aftershocks as of the 27th, with a tremor range of 150km. We consider that the latest event should be the Yuexi earthquake in 1480AD according to the historical records of earthquake damages. Based on the paleoearthquake research, this event very likely led to a coseismic rupture of the Yuexi and the Puxiong faults. According to the empirical scaling laws between magnitude and rupture length, the magnitude of the surface ruptured paleoearthquake is estimated to be more than M7.0. The results provide basic data for evaluating seismic activity and analyzing seismic risk in this area.

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    FINE CHARACTERISTICS OF EARTHQUAKE SURFACE RUPTURE ZONE BASED ON HIGH-RESOLUTION REMOTE SENSING IMAGE: A CASE STUDY OF LITANG FAULT
    YOU Zi-cheng, BI Hai-yun, ZHENG Wen-jun, PENG Hui, LIANG Shu-min, DUAN Lei, QIN Yi-gen
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1057-1073.   DOI: 10.3969/j.issn.0253-4967.2023.05.002
    Abstract208)   HTML28)    PDF(pc) (10517KB)(225)       Save

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

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    A CENTENNIAL PUZZLE OF THE EVOLUTION OF THE YANGTZE RIVER: RETROSPECTION AND PROGRESSES
    GUO Ru-jun, WEI Chuan-yi, LI Chang-an, ZHANG Yu-fen, LI Ya-wei, SUN Xi-lin, ZHANG Zeng-jie, LENG Yong-hui, SU Jian-chao, LI Guo-nai, LÜ Ling-yun, CHEN Xu, DING Zhi-qiang
    SEISMOLOGY AND GEOLOGY    2023, 45 (1): 1-28.   DOI: 10.3969/j.issn.0253-4967.2023.01.001
    Abstract393)   HTML60)    PDF(pc) (9173KB)(222)       Save

    The evolution history of the great rivers is one of the most important subjects in earth science, especially, the capture events and changes of great rivers which originate from the inner area of the Qinghai-Tibetan plateau and flow into the ocean are hot problems for geomorphology and geology. The Yangtze River is a representative river link with the Qinghai-Tibetan plateau and the Pacific Ocean, formation of the Yangtze River is considered an important mark ofthe Chinese landscape formation and the establishment of the modern geomorphic pattern of the East Asia. The evolution of the Yangtze River is closely linked to the uplift of the Qinghai-Tibetan plateau and the birth of the margin seas and monsoon evolution. In this study, we concluded the main debates on the evolution of the Yangtze River for more than one century, and the progresses of provenance analysis applied to the continental and sea basins of the Yangtze River in the past two decades. We collected the provenance analysis results from typical sedimentary depositions in the Yangtze River catchment, including the Xigeda Formation in the Panzhihua-Xichang area of the upper reaches, Cenozoic sedimentary of the Jianchuan Basin which is near the First Bend of Shigu, Gravel Layers in the middle and lower reaches, borehole sediment of the Jianghan Basin and Yangtze River Delta, and sediment of the marginal sea basins(Yinggehai Basin, Taiwan Island). We conclude that: 1)the debates on the evolution of the Yangtze River are still focused on two questions: when the Three Gorges was formed and whether south flowed off the palaeo-Jinsha River in the First Bend of the Shigu, but the debates have extended to the palaeo-drainage model in East Asia during the Cenozoic period, geomorphic formation history and exhumation-deposition process of the SE Tibet, high elevation-low relief surface formation in the SE margin of the Tibet and many important issues. 2)There is no consensus regarding the formation time and process of the Three Gorges and the First Bend, the formation time, process, and mechanism of the Yangtze River are still vigorously debated. There are mainly two views on the Miocene and early-middle Pleistocene for the formation time of the Yangtze River and mainly three paleo models of the upper Yangtze, south flow, east flow, and southeast flow. The provenance of gravel layers in the middle and lower reaches of the Yangtze River and boreholes sediment in the Jianghan Basin have complex source regions. Because of the extreme stability and multiple recycle of the detrital zircons, it is difficult to distinguish the provenance signals of the upper reaches of the Yangtze River effectively from the modern and Cenozoic sediment in basins based on the detrital zircon U-Pb age, whether the “Yangtze Gravel at Nanjin” represents the age of the Yangtze River is still strongly debated. There is still no agreement on the initial signal of the sediment of the upper Yangtze River from the boreholes record in the Jianghan Basin and the Yangtze River Delta. The boreholes deposition age is also controversial. The provenance implications of the Cenozoic sediment of the Jianchuan Basin and the Xigeda Formation for the south flow(east flow)of the Jinsha River are widely debated. The marginal sea sediment provenance signals that constrain the evolution model between the Yangtze and the Red River are also controversial. 3)There is a big difference between the drainage catchment of the paleo-Yangtze and modern Yangtze, in the provenance analysis of the sedimentary basins of the Yangtze River, suggesting constrain provenance area by multi-mineral and multi-index and strengthen the comparison between the continental and marginal sea basins. The evolution history of the Yangtze River will be reconstructed more comprehensively from the perspective of geomorphology, tectonic evolution, sedimentary paleogeography and climate change.

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    A DISCUSSION ON THE RELATIONSHIP BETWEEN THE SUR-FACE RUPTURE ZONE IN FRONT OF THE AMUNIKESHAN MOUNTAIN AND THE 1962 M6.8 EARTHQUAKE
    YAO Sheng-hai, GAI Hai-long, YIN Xiang, LIU Wei, ZHANG Jia-qing, YUAN Jian-xin
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 976-991.   DOI: 10.3969/j.issn.0253-4967.2022.04.010
    Abstract330)   HTML25)    PDF(pc) (15635KB)(213)       Save

    The investigation of seismogenic structure of historical strong earthquakes and the research on the genetic link between earthquakes and active faults are a basic seismogeologic work. In particular, the investigation of seismic surface rupture zones and the study of seismogenic structures are extremely important for understanding the characteristics of their tectonic activities. The determination of the macro-epicenter provides important evidence for the site selection for post-disaster reconstruction and avoidance. Due to the diversity of the rupture process in the focal area, the macro-epicenter and the micro-epicenter may not be identical. As the magnitude increases, the larger the focal area of an earthquake is, the more significant the gap between the macro-epicenter and the micro-epicenter will be.

    The northern margin of the Qaidam Basin is an area with frequent earthquakes, where many earthquakes with magnitude above 6.0 occurred in the history. In the early and late 1990s, small earthquake swarms with long duration and high frequency occurred in this area, which caused considerable losses to the local industry. Since the Delingha earthquake of magnitude 6.6 in 2003, two earthquakes with magnitude 6.3 and 6.4 occurred in the northern margin of the Qaidam Basin in 2008 and 2009, which aroused great attention of researchers. A new research focus has emerged on this area, and many scholars conducted in-depth research on the faults of the northern margin of the Qaidam Basin.

    The author conducted a preliminary remote sensing interpretation of the Amunikeshan Mountain segment of the northern margin of the Qaidam Basin and found that there is a very straight linear feature in the image of the Amunikeshan mountain front. On the basis of remote sensing interpretation, a related study was carried out on the Amunikeshan segment of the northern margin fault of the Qaidam Basin, which was considered to be a Holocene active fault. Since the late Holocene, the horizontal movement rate of the fault is 2.50~2.75mm/a, and the vertical movement rate is(0.43±0.02)mm/a. A 30km-long earthquake surface rupture zone was found in front of Mount Amunikeshan. It is preliminarily believed that the rupture might be caused by a strong historical earthquake. According to the catalogue of historical strong earthquakes and local chronicles, there were earthquakes of magnitude 6.8 and 6.3 occurring in this area on May 21, 1962 and January 19, 1977, respectively. There has been no detailed research report on these two earthquakes.

    Through on-the-spot geological investigation, it is found that there are fault scarps, fault grooves, seismic bulges and ridges, twisted water system and other landforms developed along the line, forming a surface rupture zone with a strike of N30°-40°W, a coseismic displacement of 2.3m, and a length of about 22km. Through trenching and excavation, the trench section reveals several faults, indicating the characteristic of multi-stage activity. In the section, the faults ruptured to the surface, and the late Quaternary activity is obvious. Combining surface relics, geological dating, and micro-geomorphic measurements, it is determined that the nature of the fault is mainly strike-slip with thrust. The investigation has found many seismic geological disasters, such as landslides, rockfalls and ground fissures along the fault, which are judged to be generated in recent decades or centuries.

    Based on the empirical statistical relationship between magnitude and surface rupture, and the empirical relationship between strike-slip fault and rupture length, the average magnitude required for producing a 22km-long earthquake surface rupture is 6.79, and the average magnitude for producing a 2.3m coseismic displacement is 7.03. In combination with the surface rupture, trench profile, geological dating, seismic geological disasters, empirical formula calculation, historical earthquake catalogue, local chronicles and other documents, it is considered that the rupture zone is most likely produced by the North Huobuxun Lake M6.8 earthquake on May 21, 1962, and its seismogenic fault is the Amunikeshan Mountain segment of the northern margin fault of the Qaidam Basin.

    Since the study area has no permanent residents or buildings(structures), which are taken as the basis for inquiring and investigating the earthquake intensity, we are unable to draw the earthquake intensity map.

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    RESEARCH ON SHALLOW STRUCTURAL CHARACTERISTICS IN THE BANQUAN SEGMENT OF ANQIU-JUXIAN FAULT ZONE BASED ON SHALLOW SEISMIC REFLECTION PROFILING
    QIN Jing-jing, LIU Bao-jin, WANG Zhi-cai, FENG Shao-ying, DENG Xiao-juan, HUA Xin-sheng, LI Qian
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 349-362.   DOI: 10.3969/j.issn.0253-4967.2022.02.005
    Abstract493)   HTML20)    PDF(pc) (3676KB)(206)       Save

    The Anqiu-Juxian Fault is the latest active fault in Tanlu fault zone, which is also the seismogenic fault of Tancheng M8.5 earthquake in 1668. In order to probe the shallow structure and the characteristics of faults in the eastern graben of Tanlu fault zone, we applied the high-resolution shallow seismic reflection method with multifold overlaying and stacking. In addition, we laid out two shallow seismic reflection lines across the Anqiu-Juxian Fault and the eastern graben of Tanlu fault zone. The shallow seismic profiles clearly reveal the stratigraphic interface morphology and shallow fault characteristics. The results show that the eastern graben of Tanlu fault zone is a graben basin consisting of multiple faults, and the thickness of Quaternary strata and graben structure characteristics are obviously affected and controlled by Changyi-Dadian Fault F1 and Baifenzi-Fulaishan Fault F2. Also, the eastern and western sides of the graben are the basement uplift areas, and the sediment thickness of the Quaternary strata in uplift areas is less than 30m. There are thick Cenozoic strata deposited in the barben, the stratigraphic morphology changes greatly laterally, showing an inclined form which is shallow in the west and deep in the east, and the Cenozoic strata are in angular unconformity contact with the overlying strata. The deepest part of Quaternary strata in the graben is located near the horizontal distance of 7400m, and its depth is about 190m. The Anqiu-Juxian Fault revealed by the shallow seismic reflection profile is composed of two branch faults dipping in opposite direction, which merge into one fault in the deep section. According to the discernible buried depth of the upper breakpoints of these faults and the characteristics of the Quaternary activity, the activity of Baifenzi-Fulaishan Fault on the western boundary of the eastern graben of Tanlu fault zone is relatively weak and the discernible depth of the upper breakpoint is 53m, we infer that the Baifenzi-Fulaishan Fault is a pre-Quaternary fault. The Changyi-Dadian Fault on the eastern boundary of the eastern graben of Tanlu fault zone not only cut the bedrock’s top interface, but also revealed signs of dislocation since Quaternary. The discernible depth of the upper breakpoint of Changyi-Dadian Fault is about 26~33m. The Anqiu-Juxian Fault is the latest active fault in the study area, which possess the characteristics of large scale and large penetration depth. The fault controls the deposition of the Cenozoic strata in the graben and plays an important role in the formation of the the eastern graben of Tanlu fault zone. The discernible depth of the upper breakpoint of Anqiu-Juxian Fault is about 17~22m. Therefore, we infer that the active ages of Changyi-Dadian Fault and Anqiu-Juxian Fault are the late Pleistocene and Holocene, respectively. The research results can provide seismological evidence for further understanding of activity mode and activity age of the seismogenic fault of the 1668 Tancheng M$8\frac{1}{2}$ earthquake, as well as the near-surface characteristics and activity of the Banquan segment of the Tanlu fault zone.

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    APPLICATIONS AND ADVANCES FOR THE COSEISMIC DEFORMA-TION OBSERVATIONS, EARTHQUAKE EMERGENCY RESPONSE AND SEISMOGENIC STRUCTURE INVESTIGATION USING INSAR
    ZHAO De-zheng, QU Chun-yan, ZHANG Gui-fang, GONG Wen-yu, SHAN Xin-jian, ZHU Chuan-hua, ZHANG Guo-hong, SONG Xiao-gang
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 570-592.   DOI: 10.3969/j.issn.0253-4967.2023.02.016
    Abstract267)   HTML22)    PDF(pc) (7303KB)(203)       Save

    With the recent development of geodetic observation theory, the increasing satellite platforms and the progress of related technology, InSAR is emerging as a new data source and useful tool for remotely-based geodetic observations. More importantly, InSAR observations play an increasingly irreplaceable role in the field of coseismic deformation observations, earthquake emergency responses, earthquake hazard evaluation and seismogenic structure research. Particularly, InSAR is the most commonly used tool in coseismic deformation measurements on the Qinghai-Tibetan plateau or other global seismic zones, where GPS data are sparse or inaccessible in some cases. Specifically, InSAR measurements help us to respond in time after disastrous earthquakes and provide valuable information associated with how the surface of the crust deforms due to large earthquakes. In the area of scientific research, InSAR provides products of surface deformation observations and serves as model constraints kinematically or dynamically in identifying the buried faults, studying the characteristics of seismogenic faults, obtaining three-dimensional displacements, and investigating the relationship between earthquakes and tectonic structures. InSAR observations and its deformation products have the technical advantages of large spatial scale, high precision and in-time, compared to other geodetic measurements. Consequently, InSAR has the ability to provide scientific and technological support for earthquake emergency observations, and meeting the practical needs of earthquake disaster reduction on the Qinghai-Tibetan plateau.

    In this review, we mostly limit our focus to the application of InSAR technology in earthquake cycle deformation monitoring in different structural settings on the Qinghai-Tibetan plateau. We also summarize the InSAR-based studies on fault kinematics and seismogenic structures related to some noted earthquakes on the Qinghai-Tibetan plateau. We highlight how the applications of InSAR data can greatly promote earthquake science and can be used as routine observations in some important areas. Then proceed to discuss the cutting-edge development trend and some new challenges of InSAR technology, which are frequently discussed and investigated, but not well resolved, in recent applications. The endeavors in increasing the precision of small-magnitude deformation measurements and expanding the InSAR data volumes can make the scientific objectives of earthquake disaster reduction on the Qinghai-Tibetan plateau and its surrounding areas feasible and reliable. To better understand how InSAR observations have changed the way we study earthquakes, we summarize the development, commercialization, insights, and existing challenges associated with InSAR coseismic deformation measurements and application in recent two decades.

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    RESEARCH PROGRESS AND PROSPECT OF SEISMIC FLUID GEOCHEMISTRY IN SHORT-IMMINENT EARTHQUAKE PREDICTION
    LI Ying, FANG Zhen, ZHANG Chen-lei, LI Ji-ye, BAO Zhi-cheng, ZHANG Xiang, LIU Zhao-fei, ZHOU Xiao-cheng, CHEN Zhi, DU Jian-guo
    SEISMOLOGY AND GEOLOGY    2023, 45 (3): 593-621.   DOI: 10.3969/j.issn.0253-4967.2023.03.001
    Abstract228)   HTML39)    PDF(pc) (2594KB)(202)       Save

    Establishing the method of short-imminent earthquake prediction is the most effective way to reduce losses caused by earthquakes and is also an important scientific issue. In the 1960s and 1970s, research on earthquake prediction was carried out successively in China and other countries in the world, and after over 50 years of development, abundant precursor observation data and earthquake cases have been accumulated, and significant progress has been made in the research of formation mechanisms of precursor anomalies and prediction methods.
    Fluid is the most active component in the earth’s interior, and the fluids in various layers of the earth often carry characteristic geochemical information. The composition and variation of seismic fluid geochemistry are sensitive to changes of underground physical and chemical conditions, making them powerful indicators of seismic and tectonic activities. The formation mechanisms of fluid geochemical precursor anomalies mainly include liquid mixing, water-rock reaction, deep magma upwelling, seismic wave vibration, pore compression and pressure solubility mechanism. The fluid chemical anomalies associated with earthquakes can be attributed to the migration process of liquid mixing and the water-rock reaction mechanism caused by crustal stress changes.
    This paper systematically summarizes the empirical formulas on the duration of anomaly, earthquake magnitude and epicentral distance, as well as the seismic fluid geochemical models and methods for short-imminent prediction established both domestically and internationally. In addition, four types of seismic fluid geochemical techniques and methods currently used in earthquake situation consultation in China are described. Nine of the most widely used prediction methods are selected to inspect the twenty-seven cases of earthquakes containing water radon or gas radon anomalies in the Earthquake Cases of China from 1997 to 2020. Generally, these methods all show strong applicability. However, empirical formulas based on different regions of the world selected to inspect the above cases generally show weak applicability. It indicates that current earthquake prediction models or methods are only representative to a certain extent, and there are still great difficulties in practical application, which also directly affects the prediction efficiency of the fluid geochemical models applied to the judgment of earthquake three elements.
    Combined with our previous results, the paper puts forward the applicable theory for the precursor mechanism-based short-imminent prediction by seismic fluid geochemistry, that is, acquiring the dynamic change characteristics of the geochemical field based on the spatio-temporal dense and multi-item observation network, establishing a deep-shallow coupling anomaly genetic model based on the material cyclic reaction, and determining the temporal and spatial relationship between the evolution of regional fluid geochemical field and fluid geochemical changes at each measuring point in the fault zone. The construction of the geochemical subsystem of China Seismic Experimental Site provides a platform for capturing the short-imminent earthquake anomalies and constructing effective fluid geochemical anomaly mechanisms and models. The causes and abnormal mechanism of fluid geochemistry can be revealed and the seismic fluid geochemical short-imminent prediction method can be established in the light of the principle of seeking the source by field and combining the field and source.

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    STUDY ON THE DEEP STRUCTURAL CHARACTERISTIC OF MAIN ACTIVE FAULTS IN HENAN PROVINCE AND ITS ADJACENT AREAS
    XU Zhi-ping, ZHANG Yang, YANG Li-pu, XU Shun-qiang, JIANG Lei, TANG Lin, LIN Ji-yan
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1521-1538.   DOI: 10.3969/j.issn.0253-4967.2022.06.010
    Abstract331)   HTML22)    PDF(pc) (7110KB)(178)       Save

    There are many first-order intersecting tectonic units and different strike faults developed widely in Henan Province, and many historical earthquakes with magnitude 6 and above occurred, which have brought great losses to people’s lives and property. In order to effectively reduce the risk of earthquake disaster in Henan Province and understand the deep seismogenic environment, we have carried out a systematic study on the deep structural characteristics of these active faults. Firstly, based on the high-precision Bouguer gravity anomaly data of Henan Province and its adjacent areas, we obtained the characteristics of gravity anomaly fields at different spatial scales in the study area by using the multi-scale wavelet analysis method. Then the detailed characteristics of different orders wavelets of Bouguer gravity anomaly field in the study area and its relationship with regional structure were analyzed. We found that within 14km of the crust, the regional tectonic activity has an obvious control effect on the trend of gravity anomaly zone. The trend of gravity anomaly zones is obviously different in different tectonic units in the study area. In the north of Henan, the trend of gravity anomaly zones is NE, which is consistent with the regional tectonic trend. The horizontal density difference is obvious. In the south of North China depression and Qinling-Dabie uplift area, the trend of gravity anomaly zones is NW, NWW and EW. In the differential uplift area of western Henan, the trend of gravity anomaly zones is NE. At the 27km depth of the crust, most gravity anomalies are in a clumpy shape, and the consistency between the trend of the gravity anomaly and the regional structure decreases, indicating the differences in regional tectonic stress effect and formation process at different depths of the crust. For example, under the northward compression from Qinling-Dabie uplift, the crust structure in the south of North China depression is different, and the difference gradually decreases from shallow to deep. At the same time, with the increasing of depth, the boundary between Qinling-Dabie uplift and southern North China depression moves to the Pingdingshan and Luohe. Our results show that the regional deep faults have an obvious control over the distribution of gravity anomalies, and the linear transition zone of gravity anomalies often corresponds to the deep faults. In order to obtain the distribution characteristics of active faults in Henan Province and adjacent areas, we analyzed the wavelet multi-scale decomposition of Bouguer gravity anomaly and identified 38 faults. Based on the seismic and geological results, we interpreted the 38 faults, including10 shallow faults in the upper crust with a depth of less than 8km, 15 faults at the bottom of the upper crust with a depth of 12~14km and 13 faults in the lower crust with a depth of 27km. In the study area, the deep faults control the boundary of the first-order tectonic units, such as Liaocheng-Lankao Fault, Tangxi Fault, Xinxiang-Shangqiu Fault, etc., and many moderately strong earthquakes occurred in these faults in history. At last, we analyzed the deep tectonic environment of historical earthquakes with magnitude 6 and above in Henan Province. The results show that the historical earthquakes with magnitude 6 in Xuchang locate near the boundary zone of second-order tectonic units. Other historical earthquakes with M6.0 locate below the secondary uplift or depression controlled by deep and large faults in the crust, such as Puyang earthquake which locates in the Dongpu depression. It can be concluded that the intersections of gravity anomalies zones with different trends, the deep seated fault-controlled intra-crust low gravity anomaly areas, and the intersections of deep seated fault with different strikes are the deep tectonic background and favorable locations for generating earthquakes with magnitude 6 and above in Henan Province. The results of analysis of the characteristics of major deep active faults in Henan Province expanded our understanding of the tectonic environment of the study area and provided a geophysical basis for earthquake prevention and disaster reduction in Henan Province in the future.

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    NEW ACTIVITY PHENOMENA REVEALED BY TRENCH ON THE NORTH SIDE OF NÜSHAN LAKE IN THE TANLU FAULT ZONE AND DISCUSSION
    ZHAO Peng, LI Jun-hui, TAO Yue-chao, SHU Peng, FANG Zhen
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 338-354.   DOI: 10.3969/j.issn.0253-4967.2023.02.003
    Abstract163)   HTML18)    PDF(pc) (13253KB)(174)       Save

    The Tan-Lu fault zone is a huge seismic-tectonic belt in the eastern China. It can be generally divided into three segments: the north, the middle, and the south segment. Among them, recent activity of the middle segment has been most thoroughly studied. The junction section between Jiangsu and Anhui Province is located in the transition zone between the middle and the south segment of the fault zone. Due to the complex tectonic structure, unevenly distributed Quaternary deposits and severely transformed surface landscape, it is difficult to study the recent Quaternary activity of the fault. Research in recent years have shown that the faults in the Fushan and Ziyang areas to the south of the Huaihe River were active during late Pleistocene-early Holocene, and their activities were characterized by thrusting, normal faulting, tension and twisting. How is the fault activity extending southwards to Nüshan Lake and whether the late Quaternary activity occurred at Nüshan Lake are issues worthy of attention.

    Geomorphology of the study area is characterized by slope plains and uplands. The uplands mostly extend in near north-south direction and are obviously controlled by the faults. In the remote sensing satellite images, linearity features of the fault from Huaihe River to Nüshan Lake are distinct. Field investigations confirmed that in the farmland to the east of Liugudui Village, north of Nüshan Lake, there are scarps extending in NNE direction and distributing intermittently due to faulting. In this study, we chose relatively clear scarps and excavated trenches across the fault. The trench revealed abundant faulting phenomena. The trench wall revealed a fault deformation zone as wide as 2~4 meters, consisting of 3 fault branches. Among them, faults f1 and f3 are the boundary faults while fault f2 is developed within the deformation zone. The latest activity of fault f3 on the west side has ruptured the overlying horizon of late Pleistocene strata, and the rupture extended upwards to the surface. OSL dating samples were collected in the uppermost layer of the faulted horizons. Dating results show that the fault has been active at least in late Pleistocene. The scratches and steps developed on the fault plane indicate that the fault has experienced thrusting and dextral faulting. The deformation zone appears dark brown, which is conspicuously different from the horizons on both sides. Materials in the fault zone are compacted, crumpled and deformed, and the alignment direction is consistent with the fault. The deformation zone contains gravels and calcium tuberculosis of different sizes. Two brownish-yellow clay masses in irregular shape are deposited near the upper part of the fault plane. Among them, the clay mass tk1 on the south wall of the trench is quite clear, with the upper part connected with f1 and the middle part obliquely cut by f2. OSL dating samples were collected from clay masses from two trench walls. The dating results are consistent with the late Pleistocene horizons, indicating that the brownish-yellow clay masses were involved in the fault zone when faulting occurred in the middle-late Pleistocene, and the faulting event occurred roughly between(50.92±4.65)kaBP and(27.12±2.26)kaBP. Our research shows that late Quaternary activity of the most active fault of the eastern branch of the Tanlu fault zone extended southwards to Nüshan Lake in Mingguang, but intensity of the fault activity has weakened.

    The segment from Sihong in Jiangsu Province to Mingguang in Anhui Province is the structural node between the middle segment and the southern segment of the Tanlu fault zone. Trench exposures in Wangqian, Sunpaifang, Dahongshan in Sihong and Santang, Ziyang, Zhuliu in Mingguang and other places revealed a variety of faulting phenomena such as wedges, wedge-shaped mass, normal faulting, negative flower-shaped structure, clay mass, etc. These show that faults that were dominantly thrusting led to the local and abundant phenomena near surface in this region. The reasons for these different phenomena may be related to the influence of regional complex stresses and their changes on large-scale fault systems at different time and spaces scales.

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    GEOMETRIC STRUCTURE CHARACTERISTICS OF XINYI SEGMENT OF ANQIU-JUXIAN FAULT
    ZHANG Hao, WANG Jin-yan, XU Han-gang, LI Li-mei, JIANG Xin, ZHAO Qi-guang, GU Qin-ping
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1448-1468.   DOI: 10.3969/j.issn.0253-4967.2022.06.006
    Abstract526)   HTML37)    PDF(pc) (16789KB)(174)       Save

    The Tanlu fault zone is the most active fault zone in eastern China. It has been active mainly along the Anqiu-Juxian Fault(AJF)since the Quaternary. Predecessors have done a lot of research on the age, paleoearthquake and geometry structure of the AJF, but most of them focus on the exposed area of the fault, and relatively few studies on the buried section. Using field geological survey, shallow seismic exploration, drilling, and paleoearthquake trench, this paper focuses on the geometry structure of the Xinyi section(the buried section)of the AJF, and analyzes its geometry distribution characteristics in the plane and the structural relationship between the deep and the shallow parts, thus filling the gap of the activity characteristics of the Xinyi section of the AJF. The results show that the Xinyi section of the AJF can be divided into three sections from north to south: the Beimalingshan-Guanzhuang section, the Guanzhuang-Tangdian section and the Tangdian-Xindian section.
    The Xinyi section of the AJF, mainly manifested as strike-slip and normal faulting, has a right-handed and right-step distribution. The step-over zone with~900m in width and~16km in length is dominated by extension, leaving a length-width ratio of 18:1, much larger than the traditional pull-apart basin ratio of 3:1. According to the shallow seismic profile, the shallow seismic line in the Guanzhuang-Tangdian section revealed the extensional fault depression basin on the north side of the terrace, and the bedrock top of the basin gradually became shallower toward the north. The top of the bedrock in the shallow seismic survey line on the north side of the Nanmalingshan suddenly became deeper, and the NNE-trending compressional near-EW basins of the Nanmalingshan and Tashan developed. The two basins were formed from different origin. With the activity of the Anqiu-Juxian Fault and the erosion and deposition of the Shu River, the two basins gradually developed and merged into a composite basin, and the basin structure was consistent with the Quaternary stratigraphic isopach.
    The Xinyi section of the Anqiu-Juxian Fault presents the deformation characteristics of the same genesis and coordinated geometric structure in the deep and superficial layers, showing a single branch in the deep, cutting through the Cretaceous strata, extending and rupturing upward along the contact interface between the bedrock mountains and the Quaternary soft soil layer in the superficial layer. The fault is shown as a single branch in the north and south Maling Mountains, and ruptured to the surface in many places. In the pull-apart basin in the middle of the fault, the thickness of the Quaternary system is more than 300m. When the Anqiu-Juxian Fault ruptures to the upper part, it divides into two branches, the east and the west, which are concealed and stand opposite to each other in the shape of “Y”, forming the Anqiu-Juxian Fault. On the east-west boundary of the fault, the latest activity is along the west branch of the fault, which is a Holocene active fault. When it extends to the basement rock mass of the Maling Mountains in the north and south, the depth of the upper fault point gradually becomes shallower until it is exposed.
    The vertical movement of the Xinyi section of the AJF shows the four quadrants characteristics of uplift and subsidence. The extensional area forms a pull-apart basin, while the compressive area constitutes an uplift. The vertical bedrock offset of the Guanzhuang-Tangdian section, with the maximum vertical offset of~230m, gradually decreases to both sides. It can be concluded that the Xinyi section of the AJF presents a spiral-like pivot movement.

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    LATE QUATERNARY SHORTENING RATE OF THE SANSUCHANG ANTICLINE, SOUTHERN LONGMEN SHAN FORELAND THRUST BELT
    ZHANG Wei-heng, CHEN Jie, LI Tao, DI Ning, YAO Yuan
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1351-1364.   DOI: 10.3969/j.issn.0253-4967.2022.06.001
    Abstract367)   HTML96)    PDF(pc) (7558KB)(169)       Save

    Fold scarps, a type of geomorphic scarp developed near the active hinge of active folds due to the local compressive stress, are formed by folding mechanisms of hinge migration or limb rotation. At present, there are several proven methods, which are only based on the fold scarp geometry combined with the occurrences of underlying beds and do not use the subsurface geometry of thrust fault and fold to obtain the folding history. The use of these methods is of great significance to illuminate the seismic hazards and tectonic processes associated with blind thrust systems.
    The Sansuchang fold-thrust belt is a fault-propagation anticline controlled by the Sansuchang blind thrust fault located in the southern Longmen Shan foreland area. Previous study used the area-depth method to calculate the shortening history of the Sansuchang anticline since the late Pleistocene(73~93ka)based on the terrace deformation of Qingyijiang River. However, due to the serious erosion damage to the terrace after its formation, the shortening history obtained by incomplete terrace deformation needs to be further verified.
    A~9km long scarp was found on the Dansi paleo-alluvial fan on the eastern limb of the Sansuchang fold-thrust belt. According to the detailed field investigation and the fold geometry built by the seismic profile, we found the scarp is near the synclinal hinge, which separates beds dipping 10°~17° and 43°~57° east and parallels with the Sansuchang fold hinge. Therefore, we determined the scarp is a fold scarp formed by the forelimb hinge migration of the fault-propagation fold.
    The maximum height of the scarp, extracted by the swath topographic profile across the scarp, is about 28~35m. According to the parameters of the fold scarp height, the underlying beds dip angle near the fold scarp, and the quantitative geometric relationship between shortening and the blind Sansuchang thrust fault, it can be estimated that, after the deposition of the Dansi paleo-pluvial fan((185±19)ka), the anticline forelimb horizontal shortening rate is~0.1mm/a, the fault tip propagation rate of the Sansuchang blind fault is(0.5+0.3/-0.1)mm/a, and the total shortening rate of the Sansuchang anticline is(0.3+0.2/-0.1)mm/a.
    The folding rates of the Sansuchang fold-thrust belt since the late middle Pleistocene has been obtained by the local deformation characteristics of the fold scarp in this study. The result is basically consistent with the shortening rate since late Pleistocene obtained by complete terrace deformation across the anticline, which proves that the shortening rate of the Sansuchang anticline is relatively stable at~0.3mm/a. It provides a new idea for studying the activity characteristics of fold-thrust belts in the southern Longmen Shan foreland thrust belt area with a fast denudation rate and discontinuous geomorphic surface.

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    QUANTITATIVE ANALYSES OF GEOMORPHOLOGIC FEATURES IN RESPONSE TO LATE QUATERNARY TECTONIC ACTI-VITIES ALONG THE MAQIN-MAQU SEGMENT, EAST KUNLUN FAULT ZONE
    LI Zhao, FU Bi-hong
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1421-1447.   DOI: 10.3969/j.issn.0253-4967.2022.06.005
    Abstract368)   HTML23)    PDF(pc) (28982KB)(169)       Save

    The Maqin-Maqu segment(MMS)of the East Kunlun fault zone(EKLF)is located in the seismic gap with a high seismic risk. Study on the geometric characteristics and late Quaternary differential tectonic activity of MMS is critical for carrying out the seismic risk assessment of the cities and towns with relatively high population like the Maqin and Maqu County in the eastern part of EKLF. Previous studies indicated that the late Quaternary left-lateral slip rate along MMS shows an eastward gradient decreasing. However, the geodynamic mechanism to explain this gradient decreasing of slip rate remains controversial. Therefore, accurately identifying the geometric and kinematic characteristics of the major fault zone of MMS and its branch faults can provide important clues for understanding the tectonic transformation mechanism and its seismic risk assessment along the eastern part of EKLF. The geomorphic index can quantitatively describe the geomorphologic characteristics, and effectively extract the active tectonic deformation from surface landscapes. The hypsometric integral index(HI)can well reveal the spatial distribution of the regional tectonic activity intensity by calculating the current three-dimensional volume residual rate of drainage basins. The stream-length gradient index(SL)can effectively reflect the regional tectonic deformation by identifying the geomorphic anomalies of river longitudinal profiles. And the topographic relief(TR)can directly evaluate the geomorphologic erosion in response to the regional tectonic activity. These geomorphic indices have been widely used to differentiate active tectonic deformation regionally.
    In this study, the geological and geomorphic interpretation of high-resolution remote sensing images are employed to determine the spatial distribution and geometrical features of the major fault zone and branch faults of MMS. The 30m AW3D30 data is used to extract systematically 69 drainage basins along the MMS and adjacent area by GIS spatial analysis technology. Our results indicate that the HI indices along the major fault zone of MMS are much higher in the western segment(0.77~0.89)than in the eastern one(0.15~0.36), and its branch faults like the Awancang Fault(AWCF)and Gahai Fault(GHF)have similar variations. Along the major fault zone of MMS, the TR indices of the Maqin-Oulasuma fault intersection area reach about 400m, and the erosion amounts decrease eastward gradually(middle: 150~180m, east: 50~72m). The TR indices along AWCF also show a trend of decreasing from west(280~350m)to east(18~65m), and the eastern segment(25~100m)of GHF account for~10%~40% of the middle part(~250m). In addition, the distributions of the Hack profile and SLK index vary spatially. In the western segments, rivers with up-convex Hack profiles and higher SLK abnormal values suggest that they are strongly affected by tectonic activity. Thus, the above-mentioned variations of geomorphic index values along MMS show a continuous eastward decreasing, which is displaying a similar trend as the late Quaternary long-term slip rate gradients along MMS. It demonstrates that quantitative geomorphologic analysis is of great indicative function on decoding geomorphologic responses to active deformation processes. Meanwhile, the spatial distribution of geomorphic index values and field geomorphologic investigations reveal that the major fault zone of MMS and its branch faults can be divided into 3 segments, and their activities also show an eastward decreasing. The HI and TR indicate that the turning point of tectonic activity intensity of MMS is near the township of Oulasuma. Therefore, we infer that the slip rate gradient decreasing along MMS might be caused by tectonic transformation and strain distribution of the major fault of MMS together with AWCF and GHF, which are composing a typical horsetail-shaped fault system and play a key role on tectono-geomorphic growth in the eastern part of EKLF.

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    RESEARCH ON COMPREHENSIVE STANDARDIZATION FOR SURVEYING AND PROSPECTING OF ACTIVE FAULT
    LI Yi-shi
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 455-463.   DOI: 10.3969/j.issn.0253-4967.2023.02.009
    Abstract203)   HTML14)    PDF(pc) (926KB)(169)       Save

    Active fault surveying and prospecting is the fundamental work for earthquake prevention and disaster reduction. China began to conduct quantitative research on active faults in the 1980s, and then carried out surveying and prospecting of active faults and seismic hazards assessment in several cities. The results provide a scientific basis for urban land planning, urban disaster prevention planning, construction project site selection and fault setbacks, potential seismic hazards investigation, earthquake emergency preparedness, etc.

    Standards research in surveying and prospecting of active faults began at the beginning of this century in pace with the development of professional work. Since 2013, the research on the technical system and standards system about surveying and prospecting of active faults was carried out, and a series of standards for technical methods and outcomes were compiled successively. Currently, 1 national standard and 9 sectors standards have been released, and 11 standards are in processing. The national standard GB/T 36072 “Surveying and Prospecting of Active Fault” stipulates the process, content, outcomes, and main technical methods. The 9 sectors standards cover techniques and methods consisting of remote sensing survey, fault geomorphological survey, paleo-seismic trenching, drilling, and fault strip mapping, and stipulate the requirements for the steps, technical indicators, and outcomes of the corresponding technical methods. These standards have become important technical support for active fault survey and prospecting and the main basis for operational supervision.

    However, there are still many gaps in the standards, and there are obvious contradictions between the supply and demand of the standards. At the same time, the compiling of standards for surveying and prospecting of active faults scattered in different periods and institutions, leading to the problems of function matching and technical indicators coordination among standards. This paper applies comprehensive standardization to surveying and prospecting of active faults, with the objectives to improve the work quality and the application benefit, by regarding the standardization object as a complete system, decomposing comprehensively the relevant elements in three aspects: business process, outcomes and application, and constructing the standard-complex of surveying and prospecting active faults. This is the first attempt to apply comprehensive standardization to the earthquake industry.

    The working process of surveying and prospecting of active faults can be decomposed into six steps: preparation and revision of implementation plan, determination of fault spatial distribution and parameters, identification of fault activity, analysis of the deep seismic-tectonic environment, assessment of seismic hazards of active faults, and determination of fault deformation zone width. The preparation and revision of the implementation plan comprise data collection, controlled detection, preliminary identification of fault activity, and revision of the implementation plan; the determination of fault spatial distribution and parameters include the implementation and on-site investigation of technical methods such as high-resolution remote sensing interpretation, geological and geomorphic investigation, fault geomorphological survey, geophysical exploration, drilling, paleo-seismic trenching, and dating. The relevant elements of the business process mainly include the work content, technical methods, and technical requirements for project implementation of these links, as well as the technical requirements for project implementation plan preparation and outcomes check and acceptance.

    The outcomes of surveying and prospecting active faults are divided into survey data, professional outcomes maps, reports, databases, etc. The relevant elements of the outcomes mainly include the technical requirements of the original data and the phased outcomes obtained from the analysis, professional outcomes maps, reports, and databases.

    The application of surveying and prospecting of active faults is oriented to meet the needs of disaster reduction, and its outcomes are applied to the practice of earthquake prevention and disaster reduction. Relevant elements of application mainly include technical requirements for fault classification and fault cataloging, three-dimensional modeling, hazard assessment, fault avoidance, data management, and information service system construction.

    Based on the analysis of relevant elements of business process, outcomes, and application, combined with the current status of existing standards, the framework structure of five sequences on surveying and prospecting of active faults standard-complex is put forward, namely, business foundation, project implementation, technical method, outcomes, and application, together with a detailed list of 41 standards. Among them there are 8 items of business foundation, 3 items of project implementation, 15 items of technology and methods, 10 items of outcomes, and 5 items of application.

    The standard-complex of surveying and prospecting of active faults covers the standards required by the entire business chain, and the standards are interconnected and coordinated. Taking the advantage of the complete set of standards will lay a good foundation for further improving the standardization level of surveying and prospecting of active faults and accelerating the progress of developing standards, and also provide a beneficial demonstration for the high-quality innovative and standardization development of other business areas of earthquake prevention and disaster reduction.

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    GRAIN SIZE AND MICROSTRUCTURE CHARACTERISTICS OF HOLOCENE MEGAFLOOD SLACK WATER DEPOSITS IN THE MIDDLE REACHES OF THE YARLUNG TSANGPO RIVER
    XU Bo, WANG Ping, WANG Hui-ying, GUO Qiao-qiao, SHI Ling-fan, SHI Yu-xiang
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 305-320.   DOI: 10.3969/j.issn.0253-4967.2023.02.001
    Abstract219)   HTML18)    PDF(pc) (6489KB)(168)       Save

    The terrain in southeastern Tibet is steep and the valleys are crisscrossed. Since the Quaternary, glacial ice and debris have blocked the course of the Yarlung Tsangpo River and its tributary river valleys to form giant dammed lakes, and the huge flood deposits formed by the dammed lake outburst floods are often associated with moraines, ice water deposits, lacustrine deposits, aeolian sand or other running water sediments to form complex river valley accumulation landforms. Different types of sediments in alpine and canyon areas are similar in morphology, structure and fabric, and are difficult to distinguish. Grain size and morphological characteristics are the most important structural characteristics of sediment, and the distribution rules are controlled by many factors such as sedimentary environment, physical properties of detrital material, transporting medium and transporting mode, etc., which is an important proxy index for restoring paleoclimate and inverting paleoenvironment. However, the relevant research on identifying sediment types in alpine valley area of southeast Tibet by grain size and morphology index is still in the exploratory stage. In order to understand the particle size characteristics and spatial differentiation laws of outburst flood sediments and the micromorphological characteristics of particle surfaces, we collected 33 samples of Holocene flood retention sediments preserved along the river within about 350km from the outlet of the Jiacha Gorge in the middle reaches of the Yarlung Tsangpo River to Pai Town, and measured them with Malvern 3000 laser diffraction particle size meter and Zeiss Signma scanning electron microscope, combined with digital geomorphology(DEM)data extracted river channel width and steepness coefficient. The features of spatial distribution law of particle size are analyzed, and the following understanding is obtained. The particle size of outburst flood retention deposits is characterized on the whole by fine-silty sand(2.57~5.18Φ)with poor sorting, positive skew and narrow peak state. Two end element models are obtained: The main peak of EM1 terminal element is 3.16Φ, with an average percentage content of 42.7%, which may represent the alluvial characteristics of higher energy of outburst floods in alpine valley areas, and the main peak of EM2 terminal elements is 2.06Φ with an average percentage content of 55.6%, which can be used to indicate the accumulation process of the outburst flood lag deposits. Affected by the width of the river, the EM1 content has a tendency to increase downstream, while EM2 has the opposite trend. The surface microstructure of quartz particles in the outburst flood lag deposits is mainly characterized by mechanical scratches, shell-like fractures, upturn cleavage and cleavage steps, with low structural maturity, mostly angular shape, and rare denudation pores of chemical origin. As a typical representative of climbing sand dunes in the valley area of the semi-humid monsoon area, the genesis of the dunes is of great guiding significance for revealing the source of sand dunes in the valley area of the alpine valley area, identifying paleoflood deposit and aeolian deposit, distinguishing aeolian deposit and paleoflood slackwater deposits on both sides of the riverbank, and windbreak and sand fixation engineering in the Yarlung Tsangpo River. By comparing the particle size and surface micromorphology characteristics of the known outburst flood deposits of the Yarlung Tsangpo River, we believe that the sand source of the Fozhang dunes is mainly from the outburst flood deposits and was transformed later by wind forces.

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    THE PRESENT CRUSTAL DEFORMATION CHARACTERISTICS OF THE HAIYUAN-LIUPANSHAN FAULT ZONE FROM INSAR AND GPS OBSERVATIONS
    JIANG Feng-yun, JI Ling-yun, ZHU Liang-yu, LIU Chuan-jin
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 377-400.   DOI: 10.3969/j.issn.0253-4967.2023.02.005
    Abstract304)   HTML21)    PDF(pc) (14678KB)(168)       Save

    The Haiyuan-Liupanshan tectonic belt is one of the most significant tectonic deformation areas in the northeastern Qinghai-Tibetan plateau with frequent strong earthquakes. It is an important opportunity to study the northeast extension of the Qinghai-Tibetan plateau and an ideal place to study the earthquake breeding process.

    The published GPS observations show that the southwest side of the Haiyuan fault may still be undergoing deformation caused by the crustal viscoelastic relaxation effect of the 1920 Haiyuan M8.5 earthquake. And the publicly published leveling data results show local vertical deformation of the crust in the area west of the Liupanshan fault is significant. According to the seismic geological data, there exist historical earthquake rupture gaps in the middle and south sections of the Liupanshan fault and the southeast section of the Xiangshan-Tianjingshan fault in the Haiyuan-Liupanshan structural area, which have the background of strong earthquakes above M7.0. In view of the low spatial resolution of GPS and leveling observations, we need to use high-resolution crustal deformation fields to further study the crustal deformation characteristics of the above regions. Therefore, we further discuss the above issues in combination with InSAR observations.

    The Sentinel-1A/B SAR data of two orbits covering the Haiyuan-Liupanshan fault from 2014 to 2020 were processed to obtain the current crustal deformation field in the line-of-sight direction. Furthermore, the high-density regional crustal deformation field was obtained by integrating InSAR and published GPS observations of the horizontal crustal movement velocity field on a time scale of 20 years. By comparing the observations of GPS, leveling and InSAR and high-resolution three-dimensional deformation integrated GPS-InSAR field, the characteristics of crustal deformation and strain field in the region are analyzed and discussed. The main conclusions are as follows:

    (1)GPS and InSAR observations show that the post-seismic viscoelastic relaxation effect of the 1920 Haiyuan M8.5 earthquake may still be pronounced on the south side of the Haiyuan fault, but this conclusion is still speculative and needs to be confirmed by further observations;

    (2)The high-resolution horizontal deformation field from GPS-InSAR shows that the decrease of the sinistral slip rate of the Haiyuan fault along the fault strike mainly occurs in the Middle East section. In contrast, the decrease of the middle and west sections is not significant, which may be related to the transformation of the left-lateral strike-slip to thrust nappe structure between the Haiyuan fault and the Liupanshan fault.

    (3)GPS vertical and leveling observations both show that the vertical crustal deformation characteristics in the middle and south sections of the Liupanshan fault are similar to the vertical deformation of the Longmenshan fault before the Wenchuan earthquake. Considering the similar structural characteristics of the Liupanshan fault and the Longmenshan fault, and combining with the seismic and geological data, we believe that the Liupanshan fault may be in the relatively late stage of the earthquake breeding process. It can also be recognized by the high-resolution horizontal deformation and strain field derived from GPS-InSAR data. According to the fault motion parameters obtained in our study and the existing seismic and geological data, it is estimated that the maximum moment magnitude of an earthquake in the middle-south section of Liupanshan Mountain is approximately 7.5.

    (4)The areas with rapid maximum strain accumulation in the study region are mainly concentrated in the vicinity of the Haiyuan fault and the left lateral shear zone between the Haiyuan fault and the Xiangshan-Tianjingshan fault. The dilatation strain rate west of the Liupanshan fault shows prominent compressive deformation characteristics corresponding to the nappe deformation in the Liupanshan tectonic area. The strain rate field in the southeast section of the Xiangshan-Tianjingshan fault is smaller than that of the surrounding area. There is a strain mismatch phenomenon, which may be related to the preparation for strong earthquakes. From the perspective of rotational deformation, the study area presents multiple deformation units, among which counterclockwise rotation corresponds to left-lateral strike-slip deformation(the left-lateral shear belt from the Haiyuan fault to the Xiangshan-Tianjingshan fault). In contrast, clockwise rotation corresponds to right-lateral strike-slip deformation(the right-lateral shear belt in the western margin of Ordos and Longxi block).

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    GEOLOGICAL DEFORMATION OF THE TUOLI FAULT IN THE WEST JUNGGAR SINCE THE LATE QUATERNARY
    YUAN Hao-dong, LI An, HUANG Wei-liang, HU Zong-kai, ZUO Yu-qi, YANG Xiao-ping
    SEISMOLOGY AND GEOLOGY    2023, 45 (1): 49-66.   DOI: 10.3969/j.issn.0253-4967.2023.01.003
    Abstract215)   HTML46)    PDF(pc) (11448KB)(168)       Save

    In the Cenozoic, under the influence of the collision of the India-Eurasia plate and the northward pushing after that, deformation occurred in the interior of the continent, and the crustal deformation is mainly absorbed by the thickening of the crust and the strike-slip movement of the fault. The GPS velocity field shows that the area north of Tianshan absorbs the shortening with a rate of~2mm/a. How the shortening with these rates is absorbed is a topic worthy of study. The West Junggar, located to the north of the Tianshan Mountains and developed with the inclined parallel strike-slip fault system is an important area of crustal shortening. The inclined parallel strike-slip fault system includes the east Tacheng Fault, Tuoli Fault and Daerbute Fault. Hence, the structural deformation of the Tuoli Fault in the late Quaternary is significant for understanding the structural deformation and crustal shortening absorption mode in the north of Tianshan Mountains.

    In this study, two branches were found extending along the Tuoli Fault in the direction of NE based on remote sensing image interpretation. Field investigation to the two branch faults shows that many marker landforms were dislocated in the study area, including gullies and terrace riser. The two faults cross through the terraces developed in the Kapusheke River and the Tiesibahan River in this area, forming offset terrace riser. Because the terrace riser is in the retained bank of the river, the upper-layer terrace model is used to calculate the fault’s slip rate. The gullies are mainly distributed on the T3 terrace of the Kapushek River on the west branch fault. The horizontal dislocation of these gullies ranges from 10m to 37.5m, and the largest horizontal dislocation is located in the No. 8 gully, which is (37.5-4.1/+2.7)m. Since the actual value of the fault movement rate must be greater than the rate obtained by the sub-gully offset, we choose the maximum offset of the gully on the landform surface in calculating the slip rate. We used OSL(Optical Stimulated Luminescence)to date the age of the landform and used UAV(Unmanned Aerial Vehicle)photogrammetry technology to extract high-precision DEM of the study area. Then, we calculate the movement rate of the Tuoli Fault since the late Quaternary from the dislocations and the age of landmark landforms such as gullies and terraces. The results show that the Tuoli Fault comprises two branch faults in the east and the west, both of which are left-lateral horizontal strike-slip. The east branch fault produced a (89±31)m and (39±13)m horizontal dislocation on the T3 and T2 terrace of the Kapusheke River, respectively. Combined with the (52.9±5.1)ka of the T3 terrace age and (23.4±1.5)ka of the T2 terrace age, the horizontal slip-rate of (1.7±0.8)mm/a is calculated for the eastern branch fault. The western branch fault produced a horizontal dislocation of (34.0±6.8)m on the T2 terrace of the Tiesibahan River and 37.5(-4.1/+4.1)m of the gully on the T3 terrace of the Kapusheke River. Combined with (18.8±1.3)ka of the T2 terrace age, we obtained a sinistral slip rate of 1.8(+0.5/-1.3)mm/a for the western branch fault. The sinistral slip rate of two branch faults of the Tuoli Fault is similar to the sinistral slip rate of the east Tacheng Fault in the previous research results. This study result indicates that these parallel left-lateral strike-slip faults in the West Junggar area conform to the characteristics of the bookshelf faults structural model, and most of the compression shortening in the West Junggar area is absorbed by the parallel strike-slip movement of the fault system. So this fault system has played an important role in controlling the NS shortening of the crust in this region.

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    COSEISMIC DISPLACEMENT MEASUREMENT AND DISTRIBUTED DEFORMATION CHARACTERIZATION: A CASE OF 2021 MW7.4 MADOI EARTHQUAKE
    SHAO Yan-xiu, LIU-ZENG Jing, GAO Yun-peng, WANG Wen-xin, YAO Wen-qian, HAN Long-fei, LIU Zhi-jun, ZOU Xiao-bo, WANG Yan, LI Yun-shuai, LIU Lu
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 506-523.   DOI: 10.3969/j.issn.0253-4967.2022.02.014
    Abstract647)   HTML26)    PDF(pc) (7392KB)(167)       Save

    The coseismic displacements are required to characterize the earthquake rupture and provide basic data for exploring the faulting mechanism and assessing seismic risk in the future. Detailed field investigation is still an important way to acquire reliable coseismic displacements comparing to geodetic measurements. Combining with previous research on other earthquakes, this study tries to discuss distributed deformation along the strike rupture and its implications. The MW7.4 Madoi earthquake ruptured the southeast section of the Kunlun Shankou-Jiangcuo Fault on May 22, 2021, in Qinghai Province. It is a typical strike slip event, and its epicenter locates at~70km south of the East Kunlun Fault, which is the north boundary of the Bayan Har block. Field investigation results show that the surface rupture extends along the piedmont. The deformation features mainly include compression humps, extensional and shear fissures, and scarps. After the earthquake, we used the unmanned aerial system to survey the rupture zone by capturing a swath of images along the strike. The swath is larger than 1km in width. Then we processed the aerial images by commercial software to build the orthoimage and the digital elevation model(DEM)with high resolutions of 3~5cm. We mapped the surface rupture in detail based on drone images and DEM along the western section. Meanwhile, we also got the commercial satellite images captured before the earthquake, on 2nd January 2021. The images were processed with geometrical rectification before comparison. The spatial resolution of satellite images before earthquake is about 0.5m.
    At the south of the Eling Hu(Lake), the clear offset tire tracks provide an excellent marker for displacement measurement. We located the positions of tracks precisely based on remote sensing images, and compared between the tracks lines after earthquake and the corresponding positions before earthquake, then extracted distance difference, which is defined as coseismic displacements. The results show that the total displacement is about 3.6m, which contains the distributed deformation of about 0.9m. The off-fault deformation is about 33% of the on-fault and about 25% of the total deformation. The ratios are similar to previous studies on earthquake worldwide. The fault zone width is probable about 200m. The total horizontal displacement measured by this study is similar to the slip in depth by InSAR inversion, which implies that there is no slip deficit at the west rupture section of the earthquake.
    The results also present the asymmetry of distributed deformation that most distributed deformation occurs at the south of the surface rupture zone. Comparing with other earthquakes in the world, it is likely that the asymmetrically distributed deformation is common in strike-slip earthquakes and the asymmetric feature is not related to the property of the material. The characteristics of distributed deformation might be related to fault geometry at depth or local stress state. More work is needed to resolve this question in the future. This study implies that we probably underestimated the slip rates resulting from ignoring distributed deformation in the past. In order to avoid underestimation of slip rates, we can correct the previous results by the ratio of distributed deformation to total slip. It is also suggested that the study sites should be on the segment with narrow deformation and simple geometry.

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    PALEOEARTHQUAKES AND VERTICAL SLIP RATES ON THE HUAI RIVER-NÜSHAN LAKE SEGMENT OF FAULT F5 IN THE MIDDLE SECTION OF THE TANLU FAULT ZONE
    YANG Yuan-yuan, LI Peng-fei, LU Shuo, SHU Peng, PAN Hao-bo, FANG Liang-hao, ZHENG Hai-gang, ZHAO Peng, ZHENG Ying-ping, YAO Da-quan
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1365-1383.   DOI: 10.3969/j.issn.0253-4967.2022.06.002
    Abstract482)   HTML49)    PDF(pc) (11724KB)(166)       Save

    The Anqiu-Juxian Fault(F5)in the middle part of Tanlu fault zone is the most important seismically active fault in eastern China. The Fault F5 is divided into the Anqiu-Juxian section, the Juxian-Tancheng section and the Xinyi-Sihong section, each of which is an independent rupture unit. There are no historical records about earthquakes with magnitude above 5 in the Xinyi-Sihong section, but it is revealed that there are Holocene paleoseismic events, so this section is a significant gap segment of surface rupture of historical earthquakes. In recent years, an important progress in the study of neotectonic activity of Xinyi-Sihong section of F5 is to find that it extends southward to the region between Huai River and Nüshan Lake in Anhui Province, with a length of about 20km. The fault spreads on the gentle slope on the edge of Cretaceous red sandstone uplift(hillock)along the line from Fushan to Ziyangshan, and the latest activity can date back to the early Holocene. At present, there is a clear understanding of the geometric distribution, structural characteristics and activity nature of the Huai River-Nüshan Lake section of F5(F5-HRNL), but the paleoseismic research is relatively weak, the revealed paleoseismic events are relatively sporadic, and the research results are from single trench, so there is a lack of comprehensive and comparative analysis from multiple trenches. In addition, the study on slip rate has not been carried out in this section, which affects the understanding of the overall activity level of the fault. Therefore, based on the previous work, paleoseismic research is carried out by excavating trenches in key locations, and more reliable paleoseismic events are determined through comprehensive comparative analysis of multiple trenches. The vertical slip rate of the fault is calculated by measuring the height of the fault scarp near the trench and combining with the dating data of relevant strata. Based on the paleoseismic research results of the F5-HRNL and combined with the data of other disciplines, the seismic risk of this fault section is analyzed. The results of this study enrich the understanding of the overall activity characteristics of F5 in the Tanlu fault zone in the Late Quaternary, and provide new data for medium- and long-term earthquake prediction in the border area of Jiangsu and Anhui Provinces.
    In this study, a new trench was excavated at the foot of Fushan Mountain on the south bank of the Huai River, named Santangnan trench, for the special research on ancient earthquake events. The trench reveals that four paleoseismic events have occurred on F5, and the latest event occurred since the late Late Pleistocene, that is, since(15.7±2.0)ka BP, but the trench failed to constrain the age of each event. Based on the trenching work and combined with the previously published trench research data, the paleoseismic events in the F5-HRNL are further constrained by using the progressive constraining method. The results show that at least five paleoseismic events have occurred in the F5-HRNL since the late Middle Pleistocene. The first three events occurred in the late Middle Pleistocene to the late Late Pleistocene, all of which were thrust in nature and manifested as gently dipping thrust faults, reverse faulting colluvial wedges and structural wedges in the trench; the latest two events occurred since the late Late Pleistocene, both of which were extensional in nature and manifested as splitting wedges in the trench; the age of the latest two events are constrained at 20.36~(18.7±0.3)ka BP and 10.92~7.83ka BP respectively.
    At present, the research on the slip rate of F5 mainly focuses on the horizontal slip rate on the Shandong Province section, where the water systems are relatively developed and the deformation is obvious. The vertical slip rate of the fault is rarely reported. Stable and continuous fault scarps are developed in local segments of the F5-HRNL, and trenches are excavated across the scarps, which provides support for the calculation of vertical slip rate of this section. Through UAV topographic mapping, a high-precision digital elevation model near the scarp is constructed, the topographic profile across the scarp is extracted, and the vertical displacement of the fault is discussed. Based on the results of Quaternary stratum dating and paleoseismic event analysis in the trench near the scarp, the starting time of vertical displacement of the scarp is determined. The calculation shows that the vertical slip rate of the F5-HRNL is about 0.05mm/a in the Ziyangshan area and about 0.07mm/a in the Doushan area, indicating that this fault section is weakly active as a whole.
    The Sihong-Mingguang section of F5 is from the south of Chonggang Mountain in Sihong County, Jiangsu Province to the north of Nüshan Lake in Mingguang City, Anhui Province, with a total length of about 65km. The latest paleoseismic event revealed in this section is about 8 000 years ago. Based on the research results of paleoearthquakes and combined with the research data of other disciplines, it is considered that the F5 Sihong-Mingguang section is the surface rupture gap section of historical earthquakes, a long time has elapsed since the latest ancient earthquake, and the current small earthquakes are not active, the locking degree is high, and it is likely to accumulate stress, and there is a risk of strong earthquakes of magnitude 7 or above.

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    EVIDENCE OF LATE QUATERNARY TECTONIC ACTIVITY OF THE BEIDA SHAN FAULT, SOUTHERN MARGIN OF THE ALASHAN BLOCK
    ZUO Yu-qi, YANG Hai-bo, YANG Xiao-ping, ZHAN Yan, LI An, SUN Xiang-yu, HU Zong-kai
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 355-376.   DOI: 10.3969/j.issn.0253-4967.2023.02.004
    Abstract184)   HTML36)    PDF(pc) (27392KB)(165)       Save

    The southern Alashan block is located at the crustal front of the northern Tibetan plateau. It was initially considered as a relatively stable area with weak tectonic activity. In recent years, an increasing number of studies have shown that the Alashan block has undergone significant tectonic deformation since the Cenozoic. Multiple active faults with a horse-tail distribution are developed in the southern margin of the Alashan block. However, there is still controversy over the tectonic deformation patterns of these active faults. One view is that the fault system in the southern margin of Alashan is the result of the eastward extension of the Altyn Tagh Fault and belongs to the tail structure of the strike-slip fault. Another view is that the fault system in the southern Alashan block is the result of the revival of the pre-existing fault caused by the northward compression and thrust of the Tibetan plateau. Therefore, deciphering fault’s kinematics and slip rates since the late Quaternary in the southern Alashan block is crucial to understand the tectonic deformation pattern of the block and its response to Tibet’s northward growth. In this paper, combined with interpretations of remote sensing images and field investigations, we documented the Quaternary activity of the Beida Shan Fault, one of the major faults in the southern Alashan block, along the segment developed in Quaternary alluvium.

    The Beida Shan Fault is a sinistral strike-slip fault with paralleled north and south branches that displaced the late Quaternary alluvial fans and terraces, forming offset gullies and fault scarps. According to the geometric distribution characteristics, activity and the landforms along the fault, we divided the fault into three segments: the Langwa Shan segment, the northern branch of the Jiapiquan Shan segment, and the southern branch of the Jiapiquan Shan segment. The fault is east-west trending, and the offset geomorphic features along the fault reveal that there are differences in the activity of different segments. The Langwa Shan segment is 10km long and developed at the junction of bedrock and alluvial fan. The fault trace is straight, and a series of gullies and ridges offset by the fault indicate that it is a sinistral strike-slip fault. The Jiapiquan Shan segment is 35km long and divided into two parallel north and south branches with a spacing of about 1.5km. The north branch fault strikes NE on the east side of Langwa Shan and has an angle of about 30° with the south branch fault. After extending about 2km to the northeast direction and entering the north side of Dahong Shan, the fault turns to the EW direction and is parallel to the south branch fault. It is distributed along the boundary between the bedrock and the alluvial fan with the south or north fault scarps and the secondary branch faults. To the east, the north branch fault is developed in bedrock, which is mainly characterized by offset gullies and ridges. The southern branch fault offset multi-stage alluvial fan, forming fault scarps of different heights and left-lateral offset gullies of different scales, and the exposed fault profiles show high angle reverse faults, which dip south or north, indicating that this segment is sinistral strike-slip.

    Based on the 1.5m resolution DEM data obtained from UAV-SfM, we measured the horizontal displacement of fault landforms using the LaDiCaoZ software developed by Zielke et al.(2012) on the MATLAB platform. Combined with field survey data, we obtained the left-lateral horizontal displacements of 70 sites along the Beida Shan Fault. The sinistral offset of~1m is not included in slip distribution statistics due to limitations of the quantity and data accuracy. Statistical analysis of the displacements reveals that the left-lateral displacements along the fault are concentrated between 3m to 20m, with the majority in two pronounced peaks at 5.3m and 10.1m. The 5.3m peak contains the most data points, with 17 displacements data, accounting for 24% of the total, while the 10.1m peak contains 6 data points, accounting for 9% of the total. This indicates that the Beida Shan Fault has experienced multiple seismic events involving the displacement and rupture of stratigraphic layers on the surface.

    An~8km-long surface rupture is discovered on the south fault branch, and it is represented by of fault scarps and of tens of centimeters 1~2m left-lateral displacement of small gullies. Fresh surface rupture and left-lateral offset gullies indicate the latest fault activity. Using the previously dated alluvial fan ages in Taohuala Shan, ~30km south of the Beida Shan, we calculated the late Pleistocene sinistral slip rate of 0.3~0.6mm/a along the Beida Shan Fault, which is consistent with the slip rate of the Taohuala Shan Fault estimated by Yu et al.(2017). Compared with the fault slip rate accommodated in the Hexi Corridor area and regional GPS rates, the southern Alashan block plays a significant role in absorbing deformation in response to the northern Tibetan growth.

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    A PRELIMINARY STUDY OF THE SHALLOW EXPLORATION AND QUATERNARY ACTIVITIES OF THE FENGQIU SEGMENT OF THE XINXIANG-SHANGQIU FAULT
    TIAN Yi-ming, YANG Zhuo-xin, WANG Zhi-shuo, SHI Jin-hu, ZHANG Yang, TAN Ya-li, ZHANG Jian-zhi, SONG Wei, JI Tong-yu
    SEISMOLOGY AND GEOLOGY    2023, 45 (1): 139-152.   DOI: 10.3969/j.issn.0253-4967.2023.01.008
    Abstract235)   HTML22)    PDF(pc) (9154KB)(159)       Save

    Xinxiang-Shangqiu Fault starts from Yuhekou in the west and extends eastward into Anhui Province through Xinxiang, Yanjin, Fengqiu, Lankao, Minquan, Shangqiu and Xiayi, with a total length of about 400km and a general strike of NWW. It is a regional concealed fault in Henan Province and a boundary fault between northern North China depression and southern North China depression.

    This study focuses on the Fengqiu section of Xinxiang-Shangqiu Fault, which is the boundary structure between the Kaifeng sag, Neihuang uplift and Dongpu sag. Controlled by the NE-NEE trending Changyuan Fault and Yellow River Fault at its east and west end, this fault section has a length of about 30km and controls the Mesozoic to early Cenozoic sedimentation in the Kaifeng sag and the south side of Dongpu sag.

    In this paper, the shallow structural characteristics and Quaternary activities of Fengqiu section of the Xinxiang-Shangqiu Fault are revealed by the combination of reflection seismic exploration and drilling detection. Two shallow seismic exploration profiles and one composite drilling geological section are arranged across the fault.

    The results of shallow seismic exploration show that the Fengqiu section of Xinxiang-Shangqiu Fault is NWW trending. It is a north-dipping normal fault accompanied by several nearly parallel normal faults, and the fault is still active since the Quaternary.

    In the composite drilling geological section at Yaowu, the latest faulted stratum is a clay layer between borehole YW5 and YW7, and the buried depth of the upper breakpoint is between 57.00~61.50m. Combined with the dating results of the collected samples, it is comprehensively judged that the latest activity age of Fengqiu section is the middle of late Pleistocene. Since the middle of late Pleistocene, the whole region is in a relatively stable tectonic period. It is verified that the comprehensive detection method of shallow seismic exploration with drilling can effectively find out the accurate location of hidden faults.

    The zone with strong vertical differential movement is often the zone where earthquakes occur. The vertical differential movement between Kaifeng sag and Neihuang uplift is very strong, and the difference reaches nearly 1 000 meters since Neogene. Moreover, the structural pattern of the main strong earthquakes in the North China Plain is characterized by zoning in NE direction and segmentation in NW direction, especially at the intersections of NWW-trending faults and NE-trending faults. The Xinxiang-Shangqiu Fault intersects with a series of NE-NEE trending faults, including Tangdong, Changyuan, Yellow River and Liaolan faults from west to east. The Fengqiu section is at the intersection with the Changyuan Fault and the Yellow River Fault, and is located in the Fengqiu M6.5 potential seismic source area of the North China plain seismic belt. The intersection of two groups of Quaternary active faults is a favorable place for the preparation and generation of moderate and strong earthquakes. Therefore, the research results provide seismological basis for the site selection of major engineering projects, urban planning and construction in this area, and have reference value for discussing the geodynamic issues such as deep and shallow structural relationship and structural evolution of Xinxiang-Shangqiu Fault.

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    THE CHARACTERISTICS OF MAJOR FAULTS AND STRESS FIELD IN WEIHE-YUNCHENG BASIN CONSTRAINED BY SEISMIC ACTIVITY AND FOCAL MECHANISM SOLUTIONS
    YU Zhan-yang, SHEN Xu-zhang, LIANG Hao, ZHENG Wen-jun, LIU Xu-zhou
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 395-413.   DOI: 10.3969/j.issn.0253-4967.2022.02.008
    Abstract760)   HTML16)    PDF(pc) (8440KB)(157)       Save

    In this paper, the seismic phase bulletin of 14381 earthquakes from January 1, 2009 to June 30, 2018 in the Weihe-Yuncheng Basin and its adjacent region were selected and analyzed. After removing the records with incomplete event information and insufficient station information, 11856 seismic events remained. A basic requirement for the double difference location method is that the distance between the pairs of seismic events is much smaller than the distance between the events and the stations and the linear scale of the velocity inhomogeneous body on the wave propagation path, so that the travel time difference between two earthquakes and the same station is only determined by the relative position between the two seismic events and the velocity of the seismic wave. In this case, the error caused by insufficient understanding of crustal structure can be effectively reduced and the result of relocation can be more accurate. Due to the large area, the whole study region was divided into three smaller parts for relocation of the events in order to reduce the influences of local structures. 8106 seismic events recorded by 52 stations were relocated using the double-difference location algorithm. It is found that the results constrained by the grid searching method are basically consistent with those obtained by other methods. The reliability of focal mechanism is affected by the number of initial motion and the azimuth distribution of the station. Therefore, when inversion of focal mechanism solution is carried out, earthquakes with more than 10 clear initial motion phases are selected, and the maximum azimuth gap between two stations with clear initial motion is required to be less than 90°. The azimuth coverage of the initial motion on the source sphere was measured according to azimuth and take-off angle distributions, and the focal mechanism solutions with poor coverage were eliminated. The contradiction ratio of focal mechanism solutions is less than 0.2. The average difference of b-axis of the best fitting solutions is less than 20°. Finally, the focal mechanism solutions of 346 seismic events with ML≥2 were determined with initial motion of P and S waves. Normal type and strike-slip type earthquakes are widely distributed, accounting for more than 60% of all seismic events, and most of them are concentrated near fault zones. Before the formal inversion, the study area was divided into 1°×1° grids, and a series of damping coefficients were set to obtain the trade-off curve between the residual error of data fitting and the length of the stress field inversion model. The crustal stress field of 1°×1° grid in Weihe-Yuncheng Basin was obtained based on focal mechanism solution and stress tensor damping inversion method, and a certain number of depth profiles vertical to the faults were constructed for the analysis. The results show that compared with the original locations of seismic phase bulletin, the distribution of seismic events after relocation is more concentrated along the fault strike in plane. Vertically, they are densely distributed along the fault plane. There are more earthquakes in and around Shanxi graben, but the magnitude is generally small. The seismic activity in Weihe rift is relatively weak. Before the relocation, the focal depth distribution was concentrated in 5~10km, but after the relocation, the focal depth distribution changed significantly. The earthquakes were concentrated in the range of 10~25km, the overall focal depth was concentrated in the range of 20km, and a small number of earthquakes occurred in the range of 25~35km. The focal depth in the basin is relatively shallow with depth range of 5~15km. The focal depth at both ends of the basin tends to deepen, and the deepest depth can reach about 30km, which is consistent with the results of previous studies. The results of the depth profiles show that most of the fault planes in the study area have a large dip angle, similar to the occurrence of the surface, and some fault planes are even nearly vertical. The motion properties of fault structure and focal mechanism indicate that the faults in the study area are mainly normal and strike-slip ones. The results of stress field inversion indicate that the R values, which indicate the stress state, of the other regions are all less than 0.5 except for some areas in the southeastern margin of the research area. The stress state of Weihe-Yuncheng Basin tends to be tensile, and the maximum horizontal principal stress direction is nearly EW in Weihe rift and NNE and NEE in southern Shanxi rift, which is basically consistent with previous studies.

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    GEOCHEMICAL CHARACTERISTICS OF SOIL GAS IN ACTIVE FAULT ZONE IN NORTHWEST YUNNAN AND ITS ENLIGHTENMENT TO FAULT ACTIVITY
    WANG Bo, ZHOU Yong-sheng, ZHONG Jun, HU Xiao-jing, ZHANG Xiang, ZHOU Qing-yun, LI Xu-mao
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 428-447.   DOI: 10.3969/j.issn.0253-4967.2022.02.010
    Abstract390)   HTML13)    PDF(pc) (6573KB)(155)       Save

    14 survey lines, with a total of 167 measuring points, were laid out in the northern section of the Red River Fault, the Longpan-Qiaohou Fault, the Heqing-Eryuan Fault, the eastern piedmont fault of Yulong Mountains, and the Lijiang-Jianchuan Fault in the northwest of Yunnan Province, China. Cross-fault soil gas radon, hydrogen, and carbon dioxide have been measured on the above-mentioned faults. The concentration intensity and distribution characteristics of soil gas in the study area were calculated and analyzed. The results show that:
    (1)The concentrations and distribution patterns of soil gas radon and hydrogen vary greatly in different faults. The concentrations of radon vary from 6.18Bq/L to 168.32Bq/L, while that of hydrogen are between 7.72ppm to 429ppm, and carbon dioxide are from 0.73% to 4.04%.
    (2)The average results of soil gas measurement show that the concentrations of radon are higher than 40kBq/m3 in the sampling sites of Yinjie, Niujie, Gantangzi, while the concentrations of radon in En’nu and Tiger Leaping Gorge measuring lines are smaller; The concentrations of hydrogen are higher than 60ppm in the sampling sites of Yangwang village, Houqing, Dawa, Yangcaoqing and Tiger Leaping Gorge, while the concentrations in Gantangzi and Niujie measuring lines are smaller.
    (3)The spatial distribution characteristics of soil gas concentration in faults in northwest Yunnan are obvious, and the intensity of radon and hydrogen concentrations in different active fault zones vary greatly. The intensities of radon and hydrogen concentration are higher and have good consistency in Yinjie and Yangwang village measuring lines located in the northern section of the Red River Fault, the Houqing survey line located in Longpan-Qiaohou Fault, Dawa survey line in the Lijiang-Jianchuan Fault and Yangcaoqing in the south of Chenghai Fault. The soil gas concentration in such sample sites is high and the degassing ability is strong, indicating the different activity characteristics of different segments of the above faults to some extent.
    Under the action of tectonic stress, the fault will slip and the rock properties and material structure of the fault will change, thus causing changes of underground material, gas transport channel and transport mode, which is characterized by the change of the concentration and distribution characteristics of escaped soil gas. Combined with the active characteristics of faults, slip rate and geomorphological features, the characteristics of concentration and spatial distribution of two soil gases(radon and hydrogen)are discussed, and the following conclusions are obtained.
    (1)There is a large difference in the concentration of escaped soil gas from different faults in the study area, indicating that the content of soil gas is controlled by regional geochemical background values, and there are certain differences in the gas concentration of different sections of the same fault, indicating that the local concentration/flux change is greatly affected by the transport.
    (2)The concentration of fault soil gas is related to fault activity, and for different faults, the higher the degree of fault activity is, the higher the concentration of fault gas will be. From the point of fault gas concentration characteristics, the concentrations in the survey lines in the northern section of the Red River Fault and Heqing-Eryuan Fault in the study area vary greatly, suggesting that the fault segmentation is obvious. Compared with other faults, the northern section of the Red River Fault has a higher concentration of soil gas, indicating that the fault is more active. However, there is no simple linear relationship between the soil gas concentration and the fault slip rate, and it may also depend on its material source, transport channel structure, etc.
    (3)The concentration of soil hydrogen at the outcrop of faults(especially normal faults and strike-slip faults)is generally higher, which shows that hydrogen has better indicative significance in revealing the location of fault rupture, and the distribution pattern of soil gas radon concentration is a good indicator for analyzing the characteristics of fault movement.

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    EMPIRICAL QUANTITATIVE ANALYSIS OF STRENGTH AND SEISMOGENIC DEPTHS FOR THE BRITTLE-DUCTILE TRANSITION OF CONTINENTAL FAULT ZONE
    LEI Hui-ru, ZHOU Yong-sheng
    SEISMOLOGY AND GEOLOGY    2023, 45 (1): 29-48.   DOI: 10.3969/j.issn.0253-4967.2023.01.002
    Abstract256)   HTML38)    PDF(pc) (5275KB)(155)       Save

    The strength properties of fault rocks at shearing rates spanning the transition from crystal-plastic flow to frictional slip play a central role in determining the distribution of crustal stress, strain, and seismicity in a tectonically active region. Since the end of the 20th century, many experimental and modelling works have been conducted to elucidate the variation of the strength profile and mechanism of brittle-ductile transition(BDT)with temperature, pressure, and sliding rate. We review the substantial progress made in understanding the physical mechanisms involved in lithospheric deformation and refining constitutive equations that describe these processes. The main conclusions obtained from this study are as follows:

    (1)The mechanical data and microstructure of friction and creep experiments indicated the transition from brittle to plastic deformation with the increasing crust depth, which not only controls the ultimate strength of the crustal profile but also limits the lower limit of the seismogenic zone. Moreover, based on the variation of rock characteristics, temperature, normal stress and sliding rate, the brittle-ductile transition zone distributes at different depths in the crust. The strength profile consisting of friction law and flow law is widely used to describe the strength and seismicity of the continental crust. However, this profile model is oversimplified in the BDT zone because this area involves a broad region of semi-brittle behavior in which cataclastic and ductile processes occur. At the same time, the model also lacks characterization of the transient dynamic properties of faults. Rate-and-state friction(RSF)law stipulates that the occurrence of slip instabilities(i.e. earthquake)can be linked with the velocity dependence of friction. Therefore, the RSF equations, when applied to the kilometer-scale of fault zones, models incorporation RSF equations can reproduce several important seismological observations, including earthquake nucleation and rupture, earthquake afterslip, and aftershock duration. However, these key microphysical processes of fault gouge evolution are unknown to this model.

    (2)During numerical model-fitting experimental observations, the Friction-to-flow constitutive law merges crustal strength profiles of the lithosphere and rate dependency fault models used for earthquake modelling on a unified basis, which is better than controlling the boundary of BDT using the Mohr-Coulomb criterion, Von Mises criterion and Goetze’s criterion. The Friction-to-flow constitutive law can predict the steady-state and transient behavior of the fault, including the response of shear stress, sliding rate, normal stress, and temperature, in addition to simulating the transition of fault sliding stability from velocity-weakening to velocity-strengthening. It also solved seismic cycles of a fault across the lithosphere with the law using a 2-D spectral boundary integral equation method, revealing dynamic rupture extending into the aseismic zone and rich evolution of interseismic creep, including slow slip before earthquakes. However, these constitutive models do not base on microphysical behavior. Furthermore, at low to intermediate temperatures, the ductile rheology of most crystalline materials are different from those at high temperatures.

    (3)A recent microphysical model, which treats fault rock deformation as controlled by competition between rate-sensitive(diffusional or crystal-plastic)deformation of individual grains and rate-insensitive sliding interactions between grains(granular flow), predicts both transitions well, called the CNS model. Unlike the numerical model, this model quantitatively reproduces a wide range of(transition)frictional behaviors using input parameters with direct physical meaning, which is closer to the natural strength of the fault. This mechanism-based model can reproduce RSF-like behavior in microstructurally verifiable processes and state variables. However, the major challenge in the CNS model lies in capturing the dynamics of micro- and nanostructure formation in sheared fault rock and considering the different processes of rock deformation mechanisms.

    Since it is microphysically based, we believe the modelling approach can provide an improved framework for extrapolating friction data to natural conditions.

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    THE QUATERNARY NORMAL FAULTING AND RECENT CO-SEISMIC SURFACE RUPTURE AND RELATED SEISMOLOGICAL SIGNIFICANCE ALONG THE ARU CO GRABEN SYSTEM IN NORTHERN NGARI, TIBET
    WU Zhong-hai, Baima Duoji, YE Qiang, HAN Shuai, SHI Ya-ran, Nima Ciren, GAO Yang
    SEISMOLOGY AND GEOLOGY    2023, 45 (1): 67-91.   DOI: 10.3969/j.issn.0253-4967.2023.01.004
    Abstract293)   HTML38)    PDF(pc) (16718KB)(155)       Save

    The Qinghai-Tibetan plateau, with an average altitude of about 5 000m, is one of the most intense regions of intraplate deformation in the globe during the Quaternary. However, the very weak field investigation of active faults and incomplete historical earthquake data in the northern Qinghai-Tibet Plateau limit the in-depth understanding of the deformation mechanism of active tectonics and the characteristics of related strong earthquakes in the Qinghai-Tibetan plateau. Based on the comprehensive geological, remote sensing, and seismic data, the active faults in northern Ngari are interpreted in detail, and the Quaternary activity of the normal faults along the western boundary of Kunchuke Co graben in the southern section of the Aru Co graben system, the newly discovered co-seismic surface ruptures, its magnitude and seismogenic time are analyzed. The newly active fault images show that high-density active fault system dominated by the near east-west extension deformation was developed in the north Ngari. The Quaternary active fault system mainly includes near north-south normal faults and the conjugated strike-slip faults composed of the NW and NE strike-slip faults. The density of the normal faults is significantly higher than that of the strike-slip faults in the region. Based on the comprehensive analysis of the Aruko graben system and the latest co-seismic surface rupture along the western boundary of Kunchuke Co Graben. We present two main conclusions. 1)The Aru Co graben system, with a total length of 210 to 220km, is one of the largest extensional fault depression structures in northern Ngari. The graben system contains four secondary graben and half-graben distributed in left-step echelon distribution from south to north and shows obvious segmented activity characteristics. Meima Co-Aru Co graben is the most intense extensional deformation section along the Aru Co graben system during the Quaternary period. The left echelon pattern of the secondary graben in the graben system indicates that there is a right-lateral shear deformation component along the NW-trending graben system in the region. 2)The newly discovered co-seismic surface ruptures along the boundary fault of the western margin of Kunchuke Co Graben in the southern section of the Aru Co graben are typical normal fault-type ruptures. The surface rupture is distributed along the NNW-trending, with an outcrop length of nearly 400m, a maximum vertical displacement of about 0.8m, and an average vertical displacement of about 0.30.4m. Comprehensive historical earthquake records, the freshness of co-seismic surface ruptures, and the magnitude results based on the classic “surface displacement and magnitude” statistical formula, we concluded that the Kunchuke Co surface rupture should be a result of the 1955 MW6.5 earthquake event, which epicenter of the instrument was located in eastern Nawu Co of Gègyai county, with a focal depth of 35km and small length and displacement. The deep focal depth is a major cause of lead to the co-seismic surface rupture is obviously small-scale. This small-scale surface rupture event on active faults suggests that irregular or random local fault rupture behavior should be paid attention to in the study of the earthquake recurrence model of active faults.

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    NEW PROGRESS IN PALEOEARTHQUAKE STUDIES OF THE JIANGSU SEGMENT OF THE ANQIU-JUXIAN FAULT IN THE TANLU FAULT ZONE
    ZHANG Hao, LI Li-mei, JIANG Xin, ZHANG Dong, XU Han-gang
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 880-895.   DOI: 10.3969/j.issn.0253-4967.2023.04.005
    Abstract860)   HTML19)    PDF(pc) (17177KB)(154)       Save

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

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

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

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

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