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    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
    Abstract576)   HTML26)    PDF(pc) (11099KB)(426)       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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    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
    Abstract230)   HTML12)    PDF(pc) (23227KB)(424)       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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    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
    Abstract275)   HTML22)    PDF(pc) (12117KB)(377)       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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    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
    Abstract310)   HTML11)    PDF(pc) (14665KB)(366)       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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    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
    Abstract273)   HTML14)    PDF(pc) (12092KB)(252)       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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    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
    Abstract317)   HTML30)    PDF(pc) (12803KB)(249)       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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    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
    Abstract302)   HTML12)    PDF(pc) (13089KB)(242)       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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    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
    Abstract315)   HTML8)    PDF(pc) (13890KB)(241)       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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    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
    Abstract297)   HTML14)    PDF(pc) (14700KB)(218)       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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    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
    Abstract289)   HTML15)    PDF(pc) (13190KB)(217)       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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    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
    Abstract388)   HTML29)    PDF(pc) (7598KB)(191)       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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    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
    Abstract312)   HTML11)    PDF(pc) (15760KB)(187)       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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    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
    Abstract343)   HTML19)    PDF(pc) (16086KB)(186)       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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    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
    Abstract223)   HTML10)    PDF(pc) (15635KB)(183)       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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    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
    Abstract344)   HTML19)    PDF(pc) (3676KB)(143)       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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    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
    Abstract247)   HTML11)    PDF(pc) (5321KB)(126)       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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    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
    Abstract469)   HTML19)    PDF(pc) (7392KB)(119)       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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    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
    Abstract489)   HTML15)    PDF(pc) (8440KB)(116)       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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    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
    Abstract223)   HTML35)    PDF(pc) (7558KB)(116)       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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    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
    Abstract165)   HTML18)    PDF(pc) (7110KB)(114)       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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    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
    Abstract278)   HTML11)    PDF(pc) (6573KB)(114)       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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    ZHOU Bing-rui, PAN Bo, YUN Sung-hyo, CHANG Cheol-woo, YAN Li-li
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 831-844.   DOI: 10.3969/j.issn.0253-4967.2022.04.001
    Abstract416)   HTML34)    PDF(pc) (6476KB)(109)       Save

    Changbaishan-Tianchi volcano(CBS-TC), located in Jilin Province on the border between China and North Korea, is the largest composite volcano around China, which is still active. The eruption stages of this large Quaternary composite volcano can be roughly divided into 2.0~1.48Ma shield forming stage, 1.48~0.05Ma cone forming stage and the explosive eruption stage since 50000 years ago. Its great eruption activities(the Millennium Eruption)from 946AD to 947AD and magmatic disturbances from 2002 to 2005 have attracted great attention of the government and scholars.

    Predecessors have done a lot of researches on Tianchi volcano, including its eruption periods, distribution of eruptive products, disaster assessment and so on. Geophysical data show that there are anomalies in the lower part, indicating the existence of magma chambers or conduits, but the accurate boundary and depth of magma chambers need to be further explored. The study of petro-geochemistry shows that the products of shield forming stage of Tianchi are mainly potassic trachy-basalts. The MgO# of these basic magma is lower than that of the primary magma in Northeast China, indicating that they are the evolved magma undergoing the process of fractional crystallization. In the past, the cone forming stage was considered to have the characteristic of “bimodal” eruptions, that is, the cone forming eruptions of high SiO2 trachytic/comenditic magma was accompanied by the low SiO2 basaltic magma, which formed small cinder cones on the edifice. In recent years, some drilling data show that there are thick basaltic trachy-andesite and trachy-andesite strata under the cone, indicating that the products of the cone forming stage of Tianchi include early basaltic trachy-andesite, medium trachy-andesite and late trachyte. Their SiO2 and Na2O+K2O contents are increasing with the degree of evolution. Since the late Pleistocene, Tianchi volcano has entered the stage of explosive eruptions with strong caldera forming effect. The eruptive products are mainly comenditic/trachytic airborne pumice, ignimbrite and so on. However, there are still many disputes about the magmatic evolution of CBS-TC, especially the evolution process from basalt to trachy-andesite, trachyte and comendite. In this study, we did abundant field geological investigation and collected rock samples of each eruptive stage of CBS-TC, and carried out whole-rock geochemical analysis. The results show that major elements of these samples have continuous linear trends with increasing of SiO2 content in magma, and the distribution of rare earth elements and trace elements is also consistent, which indicates a continuous evolution process. Meanwhile, compared with intermediate-basic magma, the trachyte and comendite magma in Tianchi has a characteristic of high Th/La and 87Sr/86Sr values, indicating that the magma has also experienced assimilated contamination by crustal materials. In order to verify this fractional crystallization with assimilation(AFC)process of Tianchi magma, the author uses petro-thermodynamic simulation(MELTS model)to calculate the magma evolution. The condition parameters used in the simulation include temperature, pressure, oxygen fugacity, water content, etc. Those parameters are considered as close as possible to the real situation in the magma system. The conditions of pressure and water content are still controversial, which are limited by this simulation. It is found that the evolution of Tianchi magma tends to have occured under the conditions of low pressure(2kbar)and high water content(≥0.5wt%), and about 10% granitic assimilates were mixed in the late stage of evolution, which is consistent with the previous research on the location of magma chambers and melt inclusions. The simulation results are consistent with the trends of tested major elements of Tianchi volcano. To sum up, we found that besides fractional crystallization, assimilation and contamination of shallow crustal granite also play an important role in the evolution of basalt to comendite.

    In this paper, the magmatic evolution of Tianchi volcano has been studied systematically, during which the method of petro-thermodynamic simulation combined with geochemical analysis is used. A series of new understandings have been obtained, including the eruption sequence, magmatic evolution, and contamination processes of Tianchi volcanic rocks. This analysis procedure provides a certain reference for the future study. The conclusions help to better understand this largest active volcano in China, and provide new ideas for interpretation of volcanic monitoring data, which helps prevent volcanic disasters. The study also provides references for the regional construction planning of the government.

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    DENG Wen-ze, LIU Jie, YANG Zhi-gao, SUN Li, ZHANG Xue-mei
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 1059-1070.   DOI: 10.3969/j.issn.0253-4967.2022.04.015
    Abstract301)   HTML22)    PDF(pc) (7098KB)(108)       Save

    At 02:04a.m. on May 22th, 2021, a MS7.4 earthquake struck Madoi County, Qinghai Province, China. The depth of this earthquake is 17km. The epicenter locates at 34.59° north latitude and 98.34° east longitude. It is another major earthquake occurring on a secondary fault within the Bayan Har block in the northern central Tibet Plateau during the past 30 years. Fast finite fault inversion and detailed focal mechanism inversion of the Madoi earthquake can help us better understand the seismogenic environment and its relationship with the faults and thus provide the scientific basis for post-earthquake emergency management and disaster assessment.

    In this study, firstly, we use W-phase method to determine focal mechanism of the main shock within 30 minutes after the origin time. The W-phase solution indicates that the main shock is a high-dip strike-slip event and the estimated centriod depth is 11km, the strike/dip/rake of two nodal planes of the optimum double couple model are 102°/81°/11° and 194°/79°/171°. Secondly, as of June 10th, the China Earthquake Networks Center has reported 57 aftershocks with magnitude larger than 3.0, the distribution of aftershocks indicates a mainly NWW direction. We obtained focal mechanisms of moderate aftershocks with MS≥4.0 inverted from regional stations in Qinghai, Tibet, Sichuan and Gansu Provinces with the method of full waveform fitting, 12 out of 15 aftershocks are of strike-slip which is consistent with the background tectonics, and the existence of two thrust and one normal type events probably indicates that the rupture process of the main shock was affected by structure in the crust. Finally, combined with the geological background and solution of focal mechanism, we select the nodal plane with strike 102°/dip 81°/ rake 11° as the real fault plane. We use finite fault inversion method to invert the rupture process of Madoi earthquake with teleseismic waveform data. The source time function shows that the total scalar moment M0 is 1.73×1020N·m(or moment magnitude MW7.45 ), which is consistent with the result of GCMT. The rupture process has lasted 45 seconds, the energy releasing was slow in the primary 5 seconds, the majority energy released during 10~30s after the main shock, then, the rupture was weakening and the fault was healing gradually. The slip and aftershock distribution of Madoi earthquake indicate an asymmetry bilateral rupture mode. The average rake is~3°, indicating a mainly left-lateral slip. The rupture area is estimated as 140km in length and 15km in depth, the slip distribution on SE and NW of epicenter shows obvious segmentation characteristics. The peak coseismic slip is estimated to be 400cm at 0~20km along strike in SE direction at shallow depth. The rupture of the earthquake did break through the ground surface which possibly causes seismic disaster. On the SE side of the main shock, the slip distribution shows a development into deep crust, while on the NW side, the slip distribution shows a more complicate mode. Over all, our results suggest that the Madoi main shock ruptured on a left-lateral strike-slip fault with high-dip along NWW direction in the Bayan Har block. The rupture length along strike is approximately 140km, slightly less than the length of aftershock distribution and field investigation due to the clear bifurcation geometry at both ends. Focal mechanism result of aftershocks shows that most of them are strike-slip but with variety in strike and dip, indicating the complex seismogenic environment in the fault zone. The slip distribution along strike and depth is highly heterogeneous, indicating that the rupture model has more complicated geometry in the lower crust than the shallow crust which controls the variability of slip distribution.

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    WANG Wen-xin, SHAO Yan-xiu, YAO Wen-qian, LIU-ZENG Jing, HAN Long-fei, LIU Xiao-li, GAO Yun-peng, WANG Zi-jun, QIN Ke-xin, TU Hong-wei
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 524-540.   DOI: 10.3969/j.issn.0253-4967.2022.02.015
    Abstract322)   HTML12)    PDF(pc) (8145KB)(104)       Save

    Exact characteristics of surface rupture zone are essential for exploring the mechanism of large earthquakes. Although the traditional field surface rupture investigation methods can obtain high-precision geomorphic data in a local area, it is difficult to rapidly get an extensive range of high-precision topographic and geomorphic data of the entire fault due to its limited measurement range and low efficiency. In addition, manual measurement is of tremendous workload, high cost, time-consuming and laborious, and the subjective differences in the judgment standards during the manual operation process may also cause the measurement results to be inconsistent with the actual terrain characteristics. In recent years, the development of photogrammetry technology has provided another more effective technical means for the rapid acquisition of high-precision topographic and geomorphic data, which has dramatically changed the way of geological investigation, improved the efficiency of fieldwork. At the same time, it also makes it a reality to reproduce the 3D tectonic features of field tectonic deformation indoors.
    Structure from Motion(SfM)multi-view mobile photogrammetry technology is widely concerned for its convenience, fast and low-cost acquisition of high-resolution 3D topographic data in a working area of tens-kilometers scale. The emergence of this method has greatly improved the automation degree of photogrammetry. The technology obtains image sets by motion cameras, uses a feature matching algorithm to extract homonym features from multiple images(at least three images), determines the relative positional relationship of cameras during photography, and continuously optimizes by the nonlinear least square algorithm. Finally, the pose of cameras is automatically solved, and 3D scene structure is reconstructed. The technology can restore the original 3D appearance of the object in the computer by a set of digital images with a certain degree of overlap. In the applications of terrain mapping, this technology only needs to combine a small number of ground control points(GCPs)to quickly establish digital orthophoto maps(DOMs)and digital elevation models(DEMs)with high-precision. In this way, low altitude remote sensing platforms such as small and medium-sized UAVs have provided a foundation for SfM photogrammetry technology.
    After the Madoi MW7.4 earthquake occurred on May 22, 2021, our research team rushed to the site as soon as possible and conducted the rapid photogrammetry of the entire coseismic surface rupture zone in a short period with the use of the CW-15 VTOL fixed-wing UAV. We completed the collection of topographic data in an area with ~180km length and ~256km2 area and collected 34302 aerial photographs. We used Agisoft PhotoScan TM software to process the images and generate DOMs quickly. The DOM resolution of the entire surface rupture was 2~7cm/pix, most of which were 3~5cm/pix. Then we used GIS software to vectorize the surface rupture. The centimeter-scale high-resolution DOMs could clearly display the coseismic surface rupture’s spatial distribution and the relative width. On this basis, the surface rupture could be accurately interpreted, and related parameters such as coseismic offsets could be extracted. In this study, the horizontal offsets measured by orthophoto images were basically consistent with the field measurement results, which proved the authenticity and reliability of the data obtained by the UAV photogrammetry method.
    In order to obtain more detailed surface rupture vertical offset data, we used DJI Phantom 4 Pro V2.0 UAV to collect terrain information of several areas with the most significant rupture deformation. The DEM resolution obtained could reach centimeter-scale, and the accuracy was greatly improved. The high-resolution topographic and geomorphic data obtained by this method could accurately identify tiny fault features, clearly display sub-meter-level vertical offset features, significantly improve the accuracy of offset measurement, and achieve high-resolution 3D reconstruction of fault geomorphic.
    In addition, we selected typical surface ruptures in the field, such as compressional stepovers, tensional cracks, and pressure ridges, and collected their 3D structural features using the iPhone 12 Pro LiDAR scanner. The 3D Scanner application was used to optimize the image, completely restore the “real object” in 3D to realize the indoor reconstruction of the 3D structure of surface ruptures and pressure ridges. The augmented reality(AR)imaging models could truly reflect the characteristics and details of surface ruptures, forming the same effect as field observations. This technology, which creates 3D models of close-range environments without any prior preparation, provides a novel, economical, and time-saving method to rapidly scan morphological features of small and medium-sized landforms(from centimeters to hundreds of meters)at high spatial resolution. This is the fastest and most convenient way to collect 3D models in field geological investigation without using external equipment, which provides a new idea for future geological teaching and scientific research.
    Although photogrammetry technology still has some limitations, such as the short flight time of the flight platform, being easily affected by factors such as weather and altitude, and unsatisfactory aerial photography in densely vegetated areas, it is believed that these problems will be solved with the advancement of technology. Once solved, photogrammetry will become an essential technical means in quantitative and refined research on active tectonics.

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    LU Ben-tian, LI Zhi-gang, LIANG Hao, YANG Jing-jun, ZHENG Wen-jun
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 961-975.   DOI: 10.3969/j.issn.0253-4967.2022.04.009
    Abstract365)   HTML14)    PDF(pc) (7104KB)(103)       Save

    As an important part of the land geomorphic unit, river is one of the main geological forces to shape the surface morphology. The fluvial geomorphic development characteristics are extremely sensitive to tectonic activities and record rich tectonic deformation information in geological history. Therefore, through the information extraction and quantitative analysis of bedrock river, we can reverse the relevant information about the tectonic evolution history. By extracting topographic information, comprehensively analyzing the spatial differences of fluvial geomorphological parameters, sieving the influencing factors such as tectonic, climatic and lithological characteristics, and quantifying the intensity of tectonic activity have become an important research tool for the segmental differences of active faults.

    The Northern Zhongtiao Mountains Fault is an active fault that controls the uplift of the Zhongtiao Mountains and subsidence of the Yuncheng Basin, and can be divided into the Hanyang, Yongji, Yanhu and Xiaxian sections from south to north. The activity of each section of the fault is closely related to the shaping of the present-day topography of the Zhongtiao Mountains, and it is a typical area for applying quantitative analysis of fluvial landform to the study of the segmentation differences along the fault. So we can effectively study the distribution characteristics of tectonic activity in the fault zone through the river geomorphological features of Zhongtiao Mountains. In this paper, by extracting information on the river topography of the bedrock mountain watershed system on the northern slopes of the Zhongtiao Mountains, parameters such as the normalized steepness index ksn, slope S, geometric features of the stream longitudinal profile of the drainage system, the location of the knickpoints and the amount of variant incision between upstream and downstream of the knickpoints are obtained. The results show that the bedrock channels on the northern slopes of the Zhongtiao Mountains has experienced accelerated incision in the longitudinal direction, and that the spatial variation of geomorphological parameters such as the normalized steepness index ksn, slope S and fluvial incision in the lateral direction is dominated by tectonic uplift, with high values in the Hangyang-Yongji section and decreasing in a segmental manner towards the west, which is consistent with the topographic relief of the Zhongtiao Mountains, but contradicts the high slip rate area and the Cenozoic subsidence centre(the Salt Lake).

    The geomorphic response to the slip rate is inconsistent with the topographic relief of the Zhongtiao Mountains, which is high in the west and low in the east. The high value area of geomorphic parameters reveals that the present active tectonic area of the Northern Zhongtiao Mountains Fault is located in the Hanyang-Yongji segment in the south, rather than the salt lake segment with high activity rate. The reason may be related to the migration of part of the activity of Huashan piedmont fault along the NE-trending hidden fault of Huayin Shouyang to the Hanyang Yongji segment of Zhongtiao Mountains. It suggests that the tectonic activity center of the Northern Zhongtiao Mountains Fault moves westward. Compared with the structural deformation caused by the change of sedimentary center, the time scale of river geomorphology response to structural deformation is shorter, and the landform is transformed most rapidly, which leads to the inconsistency between the geomorphological parameters and structural activities of the fault at the Northern Zhongtiao Mountains Fault.

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    WANG Xiao-shan, WAN Yong-ge
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 363-377.   DOI: 10.3969/j.issn.0253-4967.2022.02.006
    Abstract356)   HTML15)    PDF(pc) (6374KB)(100)       Save

    The occurrence of earthquake is closely related to the crustal stress field. Earthquakes are caused by the failure of faults, driven by tectonic stress build-up in the Earth’s crust. The change of the stress field before a large earthquake is directly related to the earthquake preparation process. In order to understand the relationship between the tectonic stress field and the low-level seismicity of the Longmenshan Fault and adjacent region before the 2008 Wenchuan earthquake, the composite focal mechanism method based on P wave first motions of small and medium earthquakes is used to determine the tectonic stress field before the Wenchuan earthquake and analyze the temporal and spatial characteristics of the composite focal mechanisms.
    Accurate earthquake location is a necessary factor to determine the focal mechanism and the stress field, especially to invert the focal mechanism and the stress field using P wave first motion of the near-field and local earthquake. Firstly, we estimated the hypocentral location and its uncertainty of a large number of small and medium earthquakes in Sichuan, China with a relatively accurate earthquake location method by considering the arrival time uncertainty. Secondly, the azimuth and take-off angle of the P wave first motion of a large number of small and medium earthquakes were calculated, whose focal mechanisms usually cannot be determined from small amount of P wave first motions, and the different weight values were given to the P wave first motion according to the hypocentral distance. Then we determine the composite focal mechanisms on the 0.5°×0.5° grid point in Sichuan area before the Wenchuan earthquake by using the composite focal mechanism method. The results show that the principal compressive stress(P)axes and principal tensile stress(T)axes of the composite focal mechanisms have obvious zoning characteristics, divided roughly by the Longmenshan Fault, the Xianshuihe Fault, and the Huayingshan Fault. The direction of the compressive axis of the northern Sichuan block from the west of the Longmenshan fault zone to the Longriba Fault is near EES-WWN, and that of the extension axis is nearly vertical, which results in the movement pattern of thrusting with right-lateral strike-slip in the Longmenshan fault zone and promoted the accumulation of stress field before the Wenchuan earthquake. The composite focal mechanisms in the south of the Xianshuihe Fault show a strike-slip pattern, which perfectly explains the sliding behavior of a series of major strike-slip earthquakes on the Xianshuihe Fault. The southeast segment of Huayingshan Fault presents a thrust pattern, which is consistent with the paleostress model proposed by predecessors. Thirdly, in order to understand the temporal variation of the crustal stress field before the Wenchuan earthquake, we calculate the focal mechanism rotation angles(FMOAs)of the annual composite focal mechanisms taking the Wenchuan earthquake as the time end to the focal mechanism of the Wenchuan earthquake obtained by different authors and institutions before the Wenchuan earthquake. It is found that the FMOAs of all the focal mechanisms of different authors and institutions reached its minimum value and were lower than its standard deviation 1 year before the Wenchuan earthquake. In view of the large rupture scale of the Wenchuan earthquake, we calculate the FMOAs of the annual composite focal mechanisms to the focal mechanisms of the Yingxiu-Hongkou initial rupture segment and Beichuan rupture segment before the Wenchuan earthquake. The results show that the FMOA of the Yingxiu Hongkou section decreased obviously, which indicates that this method can predict the location of future earthquake to some extent. Finally, in order to verify the uniqueness of convergence of stress field before the Wenchuan earthquake, we calculated the FMOAs of the annual composite focal mechanisms to the focal mechanisms of the other four reference points except the location of the Wenchuan earthquake in Sichuan area, and the results did not show the phenomenon that the stress direction of the four points tends to be consistent.
    Above all, the temporal and spatial variation characteristics of the FMOAs of the stress field show that the focal mechanism and location of the Wenchuan earthquake are closely related to the convergence of the composite focal mechanism around the epicenter before the Wenchuan earthquake, which illustrates that the convergence tendency of the stress field to the Wenchuan earthquake rupture may provide a new idea to explore large earthquake precursor from tectonic stress field.

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    PENG Fei, WANG Wei-jun, XIONG Ren-wei, LÜ Xiao-jian, YAN Kun, SUN Xin-zhe, GENG Shuang, KOU Hua-dong
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 561-577.   DOI: 10.3969/j.issn.0253-4967.2022.03.001
    Abstract347)   HTML16)    PDF(pc) (11673KB)(99)       Save

    Earthquake sources, wave propagation effects and site effects directly affect the structural damage during earthquakes. Among these factors, site effects amplify and prolong the strong vibrations, playing a very important role in many great earthquakes such as the 1985 M8.1 Mexican earthquake, the 2015 MW7.8 Gorkha, Nepal, earthquake and the 2016 MW7.8 Kaikōura, New Zealand, earthquake. Microtremor is a random, natural and permanent complex vibration composed of body waves and surface waves, in which the energy of surface waves accounts for more than 70% of the total energy. Due to the multiple reflection and refraction of the wave, microtremor accumulates information reflecting the inherent characteristics of the soil layer of the site during the propagation process. Microtremor H/V spectral ratio method is an effective way to assess the site effects. Compared to the traditional seismic surveys, the low-cost convenient observation and rapid surface detection are the advantages of this method. Its results can be used as basic data for future earthquake hazard evaluation and urban construction planning.

    Siyang in Jiangsu Province is located in Tanlu seismic zone. In the history, there were some large earthquakes on the Tanlu earthquake zone. Among them, the Tancheng M8.5 earthquake is about 110km from our study area, so there is a certain risk of earthquake disaster in this area. It is necessary to analyze the regional site effect and the distribution characteristics of the shallow sedimentary interfaces in detail. Site amplification effect is an important factor to aggravate earthquake hazard, which is closely related to the shallow structure. Based on 217 microtremor observations, we use H/V spectral ratio method to study the seismic site effect and the shallow sedimentary structure of Siyang. The results of H/V peak frequency distribution show that the resonance frequency of seismic site in Siyang study area is between 0.6~1.8Hz with obvious fluctuations. The corresponding shallow sedimentary thickness is between 30m and 200m, which gradually deepens on the east and west sides with a shallow central region. In particular, the central urban area is 30~70m thick and the southeast corner is the thickest. The shallow deposits show an obvious deep and shallow alternating band distribution in the NNE direction, consistent with the location and strike of the Haisi fault zone. The sedimentary structure of the soil layer obtained in this paper is basically the same as the geological structure, which can be verified with the results of the reflection seismic exploration profile. The comparison with two seismic exploration profiles for shallow reflection in the area shows that the bedrock shape obtained by the microtremor H/V spectral ratio method is reliable. Therefore, the sedimentary structure and site effect characteristics obtained by this method can provide useful reference for the microzoning of seismic risk in Siyang.

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    ZHAO Ling-qiang, HU Ya-xuan, WANG Qing-liang, ZHU Yi-qing, CAO Cong, LI Zhong-wei, QI Wei, WEN Yu-long
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 845-858.   DOI: 10.3969/j.issn.0253-4967.2022.04.002
    Abstract384)   HTML16)    PDF(pc) (6689KB)(99)       Save

    The Longgang volcano group, located about 150km west of the Tianchi volcano in Changbaishan, is one of the typical monogenic volcanoes formed in China since the Quaternary. The volcano group has the characteristics of high-density distribution and multi-center explosive eruption. At present, more than 160 low-level craters, volcanic cones and caldera lakes have been discovered. The eruption of Longgang volcano group is characterized by multi-cycle, multi-period and multi-stage eruption. In recent years, a large number of studies have shown that Jinlongdingzi volcano in the northwest of Longgang volcanic group underwent a large-scale eruption about 1600 years ago, and this volcanic group now has potential eruption risk. By exploring the electrical structure of the crust and upper mantle in the volcanic area, the structure of the underground magma system can be imaged, which provides key data for volcanic eruptive hazard modeling and further enriches our understanding of the formation mechanism of continental monogenetic volcano in Northeast China. In this paper, the data of a magnetotelluric profile with broadband dense measuring points with a length of more than 160km from Meihekou city in the west to the Changbaishan in the east, passing through the core area of Longgang volcano and Jinlongdingzi volcano, are used for phase tensor decomposition and two-dimensional inversion to obtain the deep electrical structure characteristics along the profile. Whether there are high-level magma chambers in the crust in Longgang volcanic area is discussed. The analysis shows that high-resistivity structures are distributed at different depths in the crust beneath the Longgang volcanic group and its adjacent area, and the high-resistivity structures are deeper under the early volcanic group, which are speculated to be related to the consolidation of magma. There are some obvious large-scale low-resistivity structures under the high-resistivity structures. These low-resistivity structures correspond to the distribution depth of high-resistivity structures in the upper crust of the region and have various depths from west to east. On the whole, these low-resistivity structures may be interconnected at the lower crust and mantle scales and show a trend of continuing to extend to the east and west sides of the study area. It is supposed that these low-resistivity structures are the magmatic system of the middle and lower crust, and the crustal uplift and seismic activity in the study area may be related to the magmatic activity. There may be a magma channel beneath the newly erupted Jinlongdingzi volcano(below 10km), connecting the magma system of the middle and lower crust, and the magma above 10km may have been consolidated. C3 area with a wide range of magma occurrence at a depth of about 30km is located in the east of Longgang volcanic area, which relatively corresponds to the depth and location of magma occurrence obtained from the inversion of previous deformation data. The deformation data reveal that the crustal uplift rate above the region is large, and the seismic data reveal that the region is seismically active, which is a region worthy of keeping an eye on the magmatic activity. The low-resistivity structures of the middle and lower crust found in the eastern part of the section show that they continue to extend to the eastern Changbaishan Tianchi volcanic area. Combined with previous magnetotelluric and seismological research results, it is speculated that the Longgang volcanic group and the Changbaishan volcano may share one magmatic system in the middle and deep parts. The results obtained can provide geophysical basis for volcanic eruption risk prediction and disaster evaluation in the Longgang volcano group.

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    WANG Lei, XU Hong-tai, WANG Zhi-cai, YANG Chuan-cheng, ZHANG Jian-min, WANG Dong-lei, XIA Nuan, CAI Ming-gang, LU Ren-qi, REN Zhi-kun
    SEISMOLOGY AND GEOLOGY    2022, 44 (5): 1156-1171.   DOI: 10.3969/j.issn.0253-4967.2022.05.005
    Abstract287)   HTML28)    PDF(pc) (10869KB)(93)       Save

    The Anqiu-Juxian Fault is an important seismogenic fault in the eastern China, along which many strong earthquakes occurred in history. From north to south, the fault can be divided into Anqiu section, Juxian-Tancheng section and Xinyi-Sihong section. According to the spatial distribution, occurrence and activity characteristics of the fault, the Anqiu section also can be divided into five sub-sections, which are the north of Changyi sub-section, the Changyi-Nanliu sub-section, the Anqiu-Mengtuan sub-section, the Qingfengling sub-section and the Mengyan sub-section. Since the late Quaternary, the activity of the Anqiu-Juxian Fault can be divided into two branches, namely, the west branch F5-1 and the east branch F5-2. There is a hidden area of the fault around Fumaying village, Weifang, Shandong Province. In order to find out the fault features of the hidden area on the Anqiu-Changyi segment of the fault, the geological-geomorphological investigation, shallow artificial seismic prospecting, combined drilling, trenching and OSL dating were carried out. Through the above work, we obtained the following understandings: 1)The results of geological and geomorphological survey and shallow seismic profiles show that the Meicun-Shuangguan segment of Anqiu-Juxian Fault can also be divided into F5-1 and F5-2. The branch F5-2 of the fault is hidden in the Quaternary layers, and the west branch F5-1 exposes at the east slope of the hills. The area between the two branches of the fault is the Fumaying hidden area in this paper. 2)The combined drilling section in the Fumaying hidden area shows that the east branch F5-2 passes through between the drillings Z4 and Z5, and the upper breakpoint can be inferred to extend to the interior of the layer w2 of Heituhu formation of Holocene series, buried at the depth of 4.2~6.9m. The shell was sampled as 14C dating sample at the bottom of Heituhu formation, and the result from the sample No. 14C-1 is(9.79±0.03)kaBP, indicating that the latest active age of the east branch is the Holocene. 3)Between the two branches, a long strip-shaped Quaternary basin is formed along the east branch fault. The Quaternary at the west side of the fault developed well, and the layers of lower Pleistocene, middle Pleistocene, upper Pleistocene and Holocene can be seen in the drilling cores; Only the upper Pleistocene and Holocene deposited at the east side. This phenomenon indicates that the Fumaying Basin deposited in the early-middle Quaternary and has the characteristics of the faulted basin. 4)There are different activities in different periods between the two branches of the Anqiu-Juxian Fault. The Fumaying hidden segment of the east branch F5-2 was active obviously in the early-middle Quaternary, and still active since the late Quaternary. According to the geological and geomorphological survey, there is a set of the late Pleistocene yellow silt strata offset by the west branch F5-1, indicating that the latest active age of the west branch is the late Pleistocene.

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    LI Zhao, FU Bi-hong
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1421-1447.   DOI: 10.3969/j.issn.0253-4967.2022.06.005
    Abstract183)   HTML9)    PDF(pc) (28982KB)(93)       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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    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
    Abstract230)   HTML17)    PDF(pc) (11724KB)(92)       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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    ZHANG Guo-qing, ZHU Yi-qing, LIANG Wei-feng
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 578-589.   DOI: 10.3969/j.issn.0253-4967.2022.03.002
    Abstract223)   HTML9)    PDF(pc) (4016KB)(90)       Save

    As part of the frontal edge of the Tibetan plateau, the eastern Tibetan plateau is featured by large-scale active fault systems, intense tectonic movement, and has experienced many devastating earthquakes, which have attracted high attention. The Fubianhe Fault is located inside the Songpan Block of eastern Tibetan plateau, and to its east is the Longmenshan Fault, which is a strong earthquake-prone zone. However, there are less earthquakes having occurred in the area around the Fubianhe Fault, and whether the area around the Fubianhe Fault has potential of strong earthquakes needs to be analyzed based on the crustal stress pattern. In this study, we calculate the Bouguer gravity anomalies by using two profiles with hybrid gravity and GPS observations, analyze the difference between measured gravity anomalies with the EGM2008 model, as well as the crustal density structures and isostatic additional stress(IAS)around Fubianhe Fault based on the Bouguer gravity anomalies. We analyzed the uplift mechanism of eastern Tibetan plateau based on the inverted IAS. At last, we discussed the medium-strong seismic risk in the eastern Xiaojin County of Sichuan Province, based on the IAS, geological active faults, historical earthquakes, and the regional gravity changes from 2018 to 2021. The main conclusions obtained in this study are as follows:

    (1)The measured free-air gravity anomalies near the Maerkang-Xiaojin range from -230mGal to 180mGal, which is less systematic than the EGM2008 model results, with the difference standard deviation being 57mGal. The measured gravity anomalies would be used to analyze the regional characteristics in the eastern Tibetan plateau, due to the poor accuracy of EGM2008 in this region. The crustal density structure and Moho depth are inversed based on the measured gravity anomalies, and the Moho depth beneath the Maerkang-Xiaojin is approximately 60km.

    (2)We estimated the isostatic depth in the study region based on the Airy isostatic theory, and the Moho depth beneath the study area is approximately 60km, which is generally deeper than depth of isostatic interface. We calculate the IAS in the study region based on the Moho depth and isostatic depth, and the result shows that the maximum IAS is approximately 20MPa, and the direction of IAS is upward in the whole, which indicates that the crustal uplift in the eastern Tibetan plateau attributes to compressing uplift, which is caused by the Tibetan plateau eastward extrusion and that the Sichuan Basin is inserted downward into the Songpan Block. The gravity profile crossing through Maerkang shows that there are fewer earthquakes in the east and more earthquakes in the west of the Fubianhe Fault. The IAS in the west of Fubianhe Fault is smaller than that in the east. This phenomenon is considered to be due to the difference in stress release in the crust by the earthquakes, with the Fubianhe Fault as the boundary. The relationship between the IAS and earthquakes across the Xiaojin profile is similar with that of the Maerkang profile. In addition, the IAS profile crossing through Xiaojin shows that there is an obvious high gradient zone in the east of Xiaojin, we suggest that there is a concealed fault located in the IAS gradient zone, which needs to be further explored in combination with other observation means.

    (3)The IAS change gradients appeared in the eastern Xiaojin County, which is located in the earthquake-prone arc crest zone of Xiaojin arc geological structure belt. The Jiaochang arc geological structure belt is located in the northeastern Xiaojin arc geological structure belt, and the 1933 M7.5 Diexi earthquake and 1941 M6 Heishui earthquake occurred in the arc crest zone of Jiaochang arc geological structure belt. The Jintang arc geological structure belt is located in the southwestern Xiaojin arc geological structure belt, and the 1941 M6 kangding earthquake occurred in the arc crest zone of Jintang arc geological structure belt. While, there are no medium-strong earthquakes in the arc crest zone of Xiaojin arc geological structure belt. Besides, regional gravity changes from 2018 to 2021 around the study region show obvious four quadrant spatial distribution in the gravity gradient belt area, with the gravity changes reaching approximately 90 microgals. Based on the results obtained above, we suggest that there exists medium-strong earthquake risk in the eastern Xiaojin County.

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    ZHAO Ling-qiang, ZHAN Yan, WANG Qing-liang, SUN Xiang-yu, HAN Jing, CAO Cong, ZHANG Song, CAI Yan
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 686-700.   DOI: 10.3969/j.issn.0253-4967.2022.03.008
    Abstract341)   HTML13)    PDF(pc) (4532KB)(89)       Save

    In the early Autumn of 1303AD, a large earthquake with a tremendous impact occurred in the northeast of Hongtong County, Shanxi Province, and this earthquake was the first major earthquake of M8 identified by seismogeologists through the study of historical records. The magnitude of the earthquake was large, and the isoseismal line was distributed in the NNE direction. The meizoseismal area was mainly located in the densely populated Fenwei fault-depression zone, so it caused great economic and property losses and casualties at that time, and left a lot of historical data. Most scholars have identified the seismic rupture of this earthquake as the Huoshan piedmont fault, but the current research methods are focused on geological methods such as seismogeological surveys and trenching. At present, in addition to seismogeological investigation and research, there is an urgent need for detailed geophysical exploration of the fine structure and seismogenic environment of the 1303 Hongtong earthquake area and the deep structure of the Huoshan piedmont fault. The phase tensor decomposition techniques and NLCG three-dimensional inversion were used to process the data of a MT profile, which is 160km in length and across the 1303 M8 Hongtong earthquake area, combined with the present-day crustal vertical motion data(including GPS and leveling data)and the latest geological and geophysical survey results in and around the study area. The results show that the Huoshan piedmont fault is an obvious large electrical boundary zone in the study area. In the middle and deep part, it is a low resistivity belt, which runs through the whole scale of the crust. The fault is a NNE-trending dextral normal fault, which may be the basement fault dividing Ordos block and North China block, extending from the surface to 40km underground. The Lishi Fault also shows as an obvious electrical boundary zone, which may be a large-scale fault system in the study area. With the Huoshan piedmont fault as the boundary, the Ordos block and North China block on the east and west sides of the fault show different electrical structural characteristics. The Ordos block in the west shows a stable tectonic environment, while the lithosphere in the North China block in the east is seriously damaged and has a trend of thinning. The results of magnetotelluric survey support the point that the Huoshan piedmont fault is the seismogenic fault of Hongtong earthquake in 1303. The earthquake might occur in the low resistivity zone under the Huoshan piedmont fault, and the focal depth may be between 10~20km. We believe that the seismogenic environment of the 1303 Hongtong earthquake may be controlled by multiple factors, such as the northeastward extrusion of the Qinghai-Tibet Plateau and the possible overall counterclockwise movement and uplift of the Ordos block, which led to an obvious right-slip movement of the Huoshan piedmont fault near the Linfen Basin. The upwelling of soft fluvial material in the lower and middle crust of the eastern part of the Linfen Basin caused the regional extension of the North China craton, leading to dip slip of the Huoshan piedmont fault, which may be the main controlling factor for the generation of this earthquake.

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    WANG Yu-qing, FENG Wan-peng, ZHANG Pei-zhen
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 313-332.   DOI: 10.3969/j.issn.0253-4967.2022.02.003
    Abstract398)   HTML20)    PDF(pc) (7645KB)(89)       Save

    Conjugate faults are a pair of faults developed under the identical regional tectonic stress fields with cross-cutting structures and opposite shear senses. They have been applied to restore the ancient regional tectonic stress fields, and the mechanics of local crust during its formation can be reflected by their dihedral angle. The ~60° intersecting conjugate fault occurs under brittle environment as proposed by the Anderson theory, while the 110° intersecting conjugate fault could be formed under the conditions of ductile environment as explained by maximum effective moment(MEM)criterion. In addition, there is another kind of conjugate faults with ~90° intersecting angle, which have been observed globally, but the mechanism of their formation still remains unsolved.
    Conjugate faults have been intensively studied using traditional geological methods and laboratory rock experiments. Interferometric synthetic aperture radar(InSAR), as an important geodetic mapping tool with an unprecedented precision and spatial resolution, provides a potential for investigating conjugate faults by exploring three-dimensional geometric structures. In this study, we investigated the 2019 Mw≥6.4 Philippines earthquake sequence as an example to link the present deformation characteristics of the ruptured conjugate faults to the regional tectonic stress.
    From October to December 2019, four MW≥6.4 earthquakes occurred in Mindanao, Philippines. The epicenters were located in the Philippine Sea plate, at the junction of the Eurasian plate, the Pacific plate and the Indian Ocean plate. Affected by three-sided subduction, the plate boundaries are almost convergent boundaries with active tectonic movement and frequent seismic activities. The target earthquake sequence occurred in Mindanao where the Philippine Sea plate collided with the Sunda plate. According to the GCMT earthquake catalog, this earthquake sequence shows similar focal mechanisms to the eight MW≥5.0 earthquakes in the study area before this earthquake sequence from 1992, which will have certain implications for the research on local mechanical background.
    This study collected both C-band Sentinel-1 TOPS and L-band ALOS-2 SAR images in ascending and descending tracks to retrieve surface deformation of the earthquake sequence. Four Sentinel-1 interferograms and three ALOS-2 interferograms were obtained using an InSAR open source package: GMTSAR. Based on the latest global atmospheric model, ERA5, the atmospheric phase delay correction was conducted, and the standard deviations(SDs)of the used Sentinel-1 and ALOS-2 interferograms before and after correction were reduced from 1.94cm and 3.55cm to 1.93cm and 3.46cm, respectively. The improved InSAR deformation products were used for earthquake fault modelling with a geodetic inversion package PSOKINV, which is based on the elastic half-space dislocation model, also called “Okada Model”. The obtained faults were further divided into several sub-faults with small patch-sizes to determine the accumulated distributed slip. The predicted interferograms from the obtained slip models can fit the original interferograms well, and the SDs of the residuals of Sentinel-1 and ALOS-2 interferograms were 1.55cm and 3.36cm, respectively, which were lower than the noise levels of the original InSAR data.
    The inversion results show that the four earthquakes mainly resulted from the ruptures of one dextral strike-slip fault(F1)of strike 48.8°, dip 74.5° and slip angle -174.1°, and the other sinistral strike-slip fault(F2)of strike 318.2°, dip 68.9° and slip angle 9.6°. The surface intersection of the two faults is nearly orthogonal, while the minimum spatial rotation angle between the two slip vectors is 29.28°. The latter indicates that two slip vectors are not completely conjugate in the seismological sense. The angle bisector of F1 and F2 is basically consistent with the azimuth of the regional principle compressive stress derived from seismic data, which also agrees with the horizontal components of the GPS velocities observed in the island. Given that the oblique direction of converging between the Philippine Sea and Sunda plates, a clear rotation of the regional stress conditions could have happened across the Philippine strike-slip fault.
    Furthermore, 4790 aftershocks in the study area from October to December 2019 recorded by the local seismic network show that the aftershocks are evenly distributed above a depth of 31km, which is the depth of the Moho based on previous studies. Therefore, the seismogenic faults of the earthquake sequence could have extended to the Moho boundary, indicating that it is likely that they may have formed in the ductile mechanical environment originally. The Coulomb stress change(CSC)analysis indicates that the rupture of one branch of the conjugate faults can release stress on the both fault planes in the vicinity of their interaction, and pose positive CSC in the far fields simultaneously, in which CSC on itself is larger. Meanwhile, combined with 14 sets of conjugate faults collected globally in this study, L-shaped characteristics of the conjugate faults turn to be common. The phenomenon having different rupture lengths and slip magnitudes for each fault branch in a set of conjugate faults is likely related to the significant variations of the fault physical properties.

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    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
    Abstract290)   HTML13)    PDF(pc) (16789KB)(89)       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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    GONG Meng, LÜ Jian, ZHENG Yong, XIE Zu-jun, SHENG Shu-zhong, ZHANG Xing-mian
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 1011-1028.   DOI: 10.3969/j.issn.0253-4967.2022.04.012
    Abstract233)   HTML22)    PDF(pc) (8737KB)(85)       Save

    The South China block, located in the east of the Eurasian plate, mainly consists of the Yangtze block and the Cathaysia block. The South China block is bounded by the eastern margin of the Qinghai-Tibet Plateau in the west, the Qinling-Dabie orogenic belt in the north, and its eastern boundary extends from the southeast coast to the north, through the Taiwan Strait, and then along the Ryukyu Island arc to the west direction. The neotectonic movement of the South China block is intense. It is not only the continental margin with the most active crustal growth and continental accretion, but also the tectonic belt with the most intense core-mantle mass transfer and the coupling zone of the inner layers of the Earth. Therefore, the crust-mantle velocity structure of the South China block and its formation and evolution have always been a hot topic in earth science research.

    In this paper, we collected continuous vertical component broadband seismic data between January 1, 2010 and December 31, 2012 from the regional networks of 609 stations and used ambient noise tomography method to inverse the three-dimensional S-wave velocity structure of South China block and its adjacent area. Firstly, the seismograms are cut into daily segments and decimated at a sampling rate of 1Hz. After the removal of the mean, trend, and instrument response, a 3~150s band-pass filter is applied. In order to reduce the effect of earthquakes and instrumental irregularities on cross-correlations, we normalized the seismograms with a time-frequency normalization method. Then, we computed daily cross-correlations for each station pairs and stacked all of them by using normalized linear stacking method to obtain cross-correlation functions. Next, the phase velocity dispersion curves of Rayleigh surface wave were extracted by frequency-time analysis method. Finally, the three-dimensional S-wave velocity structure of the study area was obtained by using nonlinear Bayesian Monte Carlo inversion method.

    The results show that the S-wave velocity distribution has a good correlation with surface geological and tectonic features, and could clearly reveal the lateral velocity variation in the crustal. The shallow S-wave velocity in basin and graben area presents low velocity anomaly due to the influence of sedimentary layer. The high velocity anomaly exists in the middle and lower crust of Jianghan Basin and Sichuan Basin, indicating that the middle and lower crust of these basins are cold and hard. Due to the phenomenon of arching existing in the upper mantle of Sichuan Basin, the S-wave velocity of the crust and mantle is relatively high in the upper mantle, meanwhile, the S-wave velocity in the center of the basin is higher than that in the edge. Although both the Yangtze block and Cathaysia block are located in the South China block, their upper mantle S-wave velocity structures are quite different due to their different evolutionary processes. The high S-wave velocity of the Yangtze block indicates the internal structure of the block is relatively stable, while the low S-wave velocity of the Cathaysia block indicates the strong magmatic activity during its evolution. The crust-mantle S-wave velocities in the west of the southwest boundary of the South China block show low velocity anomalies, which may indicate the existence of asthenosphere in the middle and lower crust of the eastern margin of the Qinghai-Tibet Plateau. The S-wave velocity structures of the eastern and western parts of the Qinling-Dabie orogenic belt are quite different, and the crustal thickness transition zone is the boundary of the S-wave velocity structure, which is high in the east and low in the west. The crust-mantle S-wave velocity of Ordos block is relatively high, indicating that the inner structure of ordos block is relatively stable. However, the S-wave low velocity anomaly in the upper mantle at the southwest corner of the Ordos Basin may indicate that the heat flow of the upper mantle of the North China Craton has begun to “invade” the Ordos lithosphere.

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    WEI Yan-kun, CHEN Xiao-li
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 590-603.   DOI: 10.3969/j.issn.0253-4967.2022.03.003
    Abstract256)   HTML9)    PDF(pc) (6988KB)(84)       Save

    Seismic landslide is a kind of natural disaster in which the slope is unstable and slips under the action of earthquake. Unlike landslides triggered by factors such as rainfall, strong earthquakes in mountainous areas tend to trigger a large number of landslides over a wide area, which can cause more casualties and economic property losses than the earthquake itself in many cases. Moreover, the occurrence of earthquake-induced landslides is characterized by abruptness and concealment, so it is difficult to spot monitoring and prevention. In order to reduce the loss of earthquake-induced landslide disaster, scientists have developed a variety of prediction and evaluation methods for earthquake landslide hazard based on different theories and models through long-term research. The MS7.4 earthquake, which occurred at 2:04 a.m. on 22 May 2021 in Maduo, Qinghai(34.59°N, 98.34°E), provided an opportunity to test the validity of the different models. On the one hand, based on the simplified Newmark displacement model, the susceptibility of seismic landslide in Maduo earthquake area is calculated. Furthermore, the seismic landslide risk is evaluated by combining with the seismic intensity distribution map after Maduo earthquake. On the other hand, based on the discrimination analysis method, the empirical model obtained from the Niigata earthquake in Japan is used to predict the earthquake landslide in Maduo earthquake area. The research results show that: Based on the rapid assessment of earthquake-induced landslide risk by simplified Newmark displacement model, the potential high-risk areas are mainly concentrated in the intensity area of Ⅷ, Ⅸ and Ⅹ which are greatly affected by the intensity of ground motion. On the whole, with the weakening of the impact of ground motion, the landslide risk decreases gradually, this is in good agreement with the actual situation. As an empirical model, discrimination analysis method is relatively dependent on a specific environment. When it is used out of its own environment, it is necessary to verify the universality of empirical formula, re-understand the relationship between various impact factors, and adjust the weight of each factor. The difference between the two methods in the prediction results is mainly in the seismic intensity Ⅵ region. In the areas with intensity VII and above, the risk zoning obtained by the two methods is generally consistent. Due to the differences in the research models adopted by the two methods, there are some differences in the distribution of seismic landslide hazard areas with different risk levels in the prediction results, especially in the Ⅵ intensity region. Intensity Ⅵ region is wide with more mountainous areas, and steep slopes are distributed in most of the areas. As a result, the discriminant analysis results in this area are more influenced by slope and curvature value, so there are more highly dangerous areas in the prediction results. However, the simplified Newmark method is greatly affected by the ground motion. Because this region is far away from the epicenter and the impact of ground motion is weak, so the main prediction results of this region show more low risk areas. However, in the intensity Ⅶ and above areas, the risk zoning of the two methods was generally consistent, and the prediction effect was good. In general, it can be seen from the prediction results that these two methods reflect their effectiveness to some extent. However, due to the different factors and fewer constraints, there are some differences in the results. In the seismic landslide risk assessment based on the discriminant analysis method, objective and complete landslide samples need to be fully analyzed, which is also a problem faced by the prediction method based on empirical model. As a physical model, Newmark model does not depend on the specific environment, although it has the problem in accuracy of input parameters, it is more objective and reasonable in the calculation results. In this paper, a simple evaluation and analysis of the Maduo earthquake was conducted based on the Newmark model method, which only considered the impact of slope itself and ground motion, but did not take into account hydrological factors, human activities, geomorphic factors and other conditions. Meanwhile, the Newmark evaluation method needs to obtain relatively clear rock-soil physico-mechanical properties and ground motion parameters, but it is difficult to obtain accurate data of each slope in practice, so there are still defects and deficiencies in regional risk assessment using this model. Compared with other traditional prediction methods based on statistical analysis, the physical meaning of this method is clearer, and it has irreplaceable advantages in combination with ground motion parameters. As a qualitative method, the discriminant analysis method uses the empirical formula derived from other earthquake cases to predict landslides. Engineering geological conditions are different in different earthquake regions, so the controlling factors of earthquake-induced landslide are not the same and the influence weight of each factor is different to some extent. Both qualitative and quantitative methods have their own advantages and disadvantages in the study of regional seismic landslide hazard prediction. It would take a long time to achieve accurate prediction of earthquake landslides.

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    ZHANG Xiu-li, XIONG Jian-guo, ZHANG Pei-zhen, LIU Qing-ri, YAO Yong, ZHONG Yue-zhi, ZHANG Hui-ping, LI You-li
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1403-1420.   DOI: 10.3969/j.issn.0253-4967.2022.06.004
    Abstract193)   HTML22)    PDF(pc) (9391KB)(84)       Save

    Slip rate is an important parameter for the quantitative study of active fault and can be used to reflect the mode and intensity of fault activity. However, the selection of geomorphic surface, the acquisition of displacements, and the limitation of chronologic methods result in challenges to constrain the slip rate. A series of boreholes and geochronology studies revealed a continuous sedimentary sequence of the Quaternary in the Yuncheng Basin in the southern Shanxi Graben System. Multiple late Quaternary river terraces have developed and been preserved in the northern piedmont of the Zhongtiao Shan. The activities of the north Zhongtiao Shan Fault resulted in the elevation difference between the strata in the Yuncheng Basin and the river terraces. In this study, we chose the geomorphic units of the Xiaolicun River and combined them with the results of boreholes in the Yuncheng Basin to constrain the slip rates of the north Zhongtiao Shan Fault since the Late Pleistocene. Based on field observation and remote sensing image interpretation, we established the distribution and sedimentary characteristics of four terraces and the latest alluvial fan of the Xiaolicun River. Two main faults(F1 and F2)and a series of fractures or branch faults have been identified in these sedimentary strata. The high-resolution DEM of the faulted landform of the Xiaolicun River was obtained using UAV photogrammetry technology. Combined with a stratigraphic outcrop survey, the landform and sedimentary section across the fault were constructed. The abandonment ages of the terraces T4, T3, T2, and T1 have been determined as(214.3±13.9)ka, (118.5±6.4)ka, (59.6±2.4)ka, and(10.9±0.5)ka by OSL dating, respectively. The chronological results of the AMS 14C dating show that the alluvial fan north of F2 was deposited at 35~1ka. Based on these results, this study established the relationship between the geomorphic evolution of the Xiaolicun River and the activities of the north Zhongtiao Shan Fault. Since the late Middle Pleistocene, F1 had been active, accompanied by the abandonment of the T4. At~120ka, the terrace T3 was formed, F1 was no longer active, but F2 began to be active and raise T3 and T4 in the footwall. Since then, the Xiaolicun River has undergone rapid incision and formed T2 and T1. The continuous activities of F2 maintained T4-T1 in an uplifted state and formed a series of fractures in the alluvial fan. Based on this evolutionary relationship, T4, T3 and their corresponding strata in the boreholes of the Yuncheng Basin were used to constrain the slip rate of the north Zhongtiao Shan Fault in this study. After determining the depth in boreholes corresponding to the abandoned ages of T4 and T3, subtracting the influence of the surface slope and the activities of the southern Salty Lake Fault, and considering the depth error caused by climate change, the vertical displacements of the north Zhongtiao Shan Fault since the two periods were obtained with the vertical slip rate of(0.31±0.05)mm/a and(0.34±0.04)mm/a, respectively. Our results indicate that the slip rates of the north Zhongtiao Shan Fault since the late Middle Pleistocene are greater than those since the Late Pliocene and Quaternary.

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    LI Dong-chen, REN Jun-jie, ZHANG Zhi-wen, LIU Liang
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1484-1502.   DOI: 10.3969/j.issn.0253-4967.2022.06.008
    Abstract124)   HTML14)    PDF(pc) (11560KB)(84)       Save

    Field investigations of large earthquakes indicate that earthquakes with a magnitude greater than 6.5 often produce seismic surface rupture zones ranging from thousands of meters to tens of kilometers on the earth’s surface. The geometric structures of surface ruptures contain the kinematic characteristics of seismogenic structures, which can not only provide critical quantitative data for analyzing the spatial distribution law of co-seismic displacement of active faults and the width of active fault deformation zone, but also have an important significance for understanding the kinematic mechanism and deformation law of seismogenic faults.
    At present, the conventional methods to obtain earthquake surface ruptures mainly include the field geological survey and visual interpretation of the remote sensing image. Although these two methods can get the rough geometry of coseismic surface ruptures, they both have certain limitations. The reliability of the field geological survey method is high. However, intra-continental earthquakes often occur in places with complicated topography, and lots of sites are difficult to reach, leading to incomplete data and failure to draw detailed features of the fracture zone. Meanwhile, the field geological survey is often time-consuming and laborious. Although the visual interpretation of remoting sensing images can be used to interpret surface fractures in areas that cannot be reached by the geological field survey, the result accuracy is vulnerable to the impacts of interpreters’ experience. The whole process of interpretation is still time-consuming and labor-intensive and the extraction results are mostly linear surface ruptures, so it is difficult to accurately obtain fine features such as the width of the surface rupture zone. Therefore, the automatic extraction of fine structures of seismic surface ruptures, especially micro-rupture surfaces, is an urgent problem in active tectonic studies.
    The remote sensing images obtained through satellite platforms have low resolution and are susceptible to weather factors, and the extracted surface rupture fineness is not enough. The UAV platform, on the other hand, is low-cost to use, can fly at a low altitude, is not affected by clouds and fog, and can acquire images with a centimeter-level resolution, which provides conditions for extraction the fine structure of surface ruptures of large earthquakes. Thus, to solve the problem that it is difficult to obtain surface ruptures of large earthquakes quickly, this study proposes an object-oriented “Rough segmentation and Fine extraction” method based on object-oriented and color segmentation theories of color space, which realizes the semi-automatic extraction of features of seismic surface rupture zone. The processing workflow of the method is as follows: First, the original image is cropped by the custom irregular raster cropping method designed in this study to obtain ROI(the Region of Interest). Second, the color space of ROI is converted into HSV, and the HSV color space of ROI is segmented into surface rupture candidate area by using brightness and hue tone values. And then, the surface rupture candidate area is processed by expansion operation of binary mathematical morphology. Third, the surface rupture candidate area is transformed into a series of sub-area objects by the contour tracking method. Fourth, the surface rupture is refined using the spectral standard deviation, spectral mean and the length-width ratio of the smallest surrounding rectangle as characteristic parameters. Finally, the results are output as the vector surface of surface ruptures.
    The effectiveness of the proposed method is analyzed by taking the high-resolution UAV image data of the MS7.4 Maduo earthquake in Qinghai Province as an example. The results show that the proposed method can effectively remove the noises such as the river channel similar to the characteristics(i.e., the color and shape features)of the surface rupture and extract the delicate structures of the surface rupture zone quickly and accurately, except that several poor extraction results were caused by the limitation of image resolution and the destruction of surface rupture caused by river erosion. The extraction results are highly reliable and can be used to extract quantitative parameters of surface ruptures in large earthquakes. Thus, the semi-automatic extraction method of seismic surface ruptures established in this study can provide a feasible scheme for the rapid extraction of delicate structures from surface ruptures and analysis of surface deformation characteristics after a large earthquake.

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    ZHANG Bo-xuan, ZHENG Wen-jun, CHEN Jie, HE Xiao-hui, LI Qi-lei, ZHANG Dong-li, DUAN Lei, CHEN Gan
    SEISMOLOGY AND GEOLOGY    2022, 44 (5): 1313-1332.   DOI: 10.3969/j.issn.0253-4967.2022.05.014
    Abstract153)   HTML16)    PDF(pc) (13664KB)(82)       Save

    The MS5.8 Mang’ai earthquake occurred in northwest Qaidam Basin on June 16, 2021. There are the Santai reverse fault-fold belt, Xiaoqulin reverse fault-fold belt, Lenghu reverse fault-fold belt and Eboliang reverse fault-fold belt developed in the seismic area from north to south, which converge along the direction of the Altun Fault in the northwest and spread out in the southeast basin. As the boundary between the southern margin of Qilian Shan and the northern margin of Qaidam Basin, the Mesozoic and Cenozoic tectonic deformation is very complex. The study of seismogenic capacity and mechanism of fold-related faults in this area is beneficial to further understand the strain distribution pattern and tectonic deformation mechanism between the southern Qilian Shan and the northern margin of Qaidam Basin.
    This earthquake occurred in the eastern section of the NW-trending Lenghu anticline in the northwestern margin of Qaidam Basin. The focal depth of the earthquake is about 13km determined by using CAP method, and the focal mechanism solution is thrust type. The parameters of the double-couple nodal planes obtained in this paper are similar to those obtained by GFZ and USGS, respectively. Combined with surface geology, satellite images and seismic reflection profile interpretation, it is considered that the earthquake occurred in the southeast of the Lenghu reverse fault-fold belt, and the seismogenic structure may be one of two deep concealed thrust faults dipping north and south, respectively, which control the growth of the anticline under the east of Lenghu No.7 anticline.
    According to the position of the growth strata interpreted by the seismic reflection profile, the latest rapid deformation in the eastern section of Lenghu anticline began around the sedimentary period of the Shizigou Formation in the Late Cenozoic, and the activity has continued till now.
    This MS5.8 Mang’ai earthquake only partially ruptured the underlying thrust fault of the east of Lenghu No.7 anticline, but did not rupture to the surface, so it was a typical folding earthquake.
    According to the empirical formula of the rupture area-magnitude of reverse fault, we have estimated the upper limit of the maximum magnitude of many Quaternary active anticlines in the seismic region. All of these structures have the structural condition for generating MW5.9~7.2 earthquakes, and may cause strong earthquakes of magnitude 7 or above due to cascading earthquake rupture. Therefore, the seismic risk of the earthquake area in the future cannot be ignored, and it is urgent to carry out more detailed research on the activity behavior of these structures.

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