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    ZHANG Chi, LI Zhi-min, REN Zhi-kun, LIU Jin-rui, ZHANG Zhi-liang, WU Deng-yun
    SEISMOLOGY AND GEOLOGY    2022, 44 (1): 1-19.   DOI: 10.3969/j.issn.0253-4967.2022.01.001
    Abstract587)   HTML103)    PDF(pc) (22131KB)(560)       Save

    Due to the collision between the Indian plate and the Eurasian plate, the Tibetan plateau has experienced violent uplift and strong intraplate deformation inside the plateau, which has a great impact on the tectonic evolution of the surrounding areas. The northeastern edge of the Tibetan plateau is the forefront of the northeastward expansion of the Tibetan plateau, which is the ideal place to study the deformation of the plateau as well as the far-field deformation associated with continental collision between the Eurasia and India plates. In recent years, scholars have gained a certain understanding of the characteristics of late Quaternary tectonic activity in the northeast margin of Tibetan plateau. Within the northeastern margin of Tibetan plateau, there are two major fault systems: One is the near EW-trending left-lateral strike-slip fault system, including the Kunlun, Haiyuan and western Qinling faults, the other one is the NNW-trending right-lateral strike-slip fault system, including the Elashan and Riyueshan faults. They are sub-parallel to each other. Since the Riyueshan Fault is one of the major right-lateral strike-slip faults in the northeastern margin of Tibetan plateau, its activity is of great significance for understanding the plateau expansion. Previous studies mainly focused on its northern part which is believed to be active during Holocene. However, its southern part is believed to be active during late Pleistocene, but not active since Holocene. Therefore, there are little studies focusing on the late Quaternary activities of the southern part of the Riyueshan Fault. Hence, our understanding about the characteristics of the late Quaternary activity is insufficient. During our preliminary field survey along the southern Riyueshan Fault, we found distinct deformation of Holocene landforms, such as the young alluvial fan, terrace risers and channels, which indicate its late Quaternary activity. In this study, we firstly analyze the fault geometry of the southern Riyueshan Fault based on high-resolution Superview-1 remote sensing images and carry out field verification. Based on fault geometry characteristics, fault strike orientation etc., we divided the southern Riyueshan Fault into two segments from north to south. One is the Guide segment(generally trending in NW 20°)and the other is the Duohelmao segment(generally striking in NS). During our field investigation, we found two typical sites for slip rate studies, the Rixiaolongwa site on the Guide segment and the Niemari site on the Duohemao segment, respectively. We collected high-resolution images using UAV, and then generated high-resolution DEM of these two sites. By measuring the offsets and corresponding dating results of multi-level terrace risers, we obtained the displacements of the three-level and two-level terraces at Rixiaolongwa and Niemari site, respectively. Then we collected the OSL and 14C samples on different terrace risers to constrain the age of each terrace. In the Rixiaolongwa area, the corresponding offsets of T1, T2 and T3 terraces are(26.3±3.1)m, (32.7±7.1)m and(38.6±8)m, and the age sequence is(7840±30)a BP, (9 350~10 700)a BP and(11.9±1.3)ka BP, respectively. In the Nimari area, the corresponding offsets of T1 and T2 terraces are(6.3±0.7)m and(9.7±1.7)m, and the ages are(2 860±30)a BP and(3 460±30)a BP, respectively. By applying Monte Carlo method, we obtained the corresponding slip rates of(3.37+0.55/-0.68)mm/a and(2.69+0.41/-0.38)mm/a for the Guide and Duohemao segment, which is comparable to the previously suggested slip rate of northern Riyueshan Fault. Finally, we discussed the role of the Riyueshan Fault in the tectonic deformation of northeastern Tibetan plateau.

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    XU Xi-wei, WEN Xue-ze, YE Jian-qing, MA Bao-qi, CHEN Jie, ZHOU Rong-jun, HE Hong-lin, TIAN Qin-jian, HE Yu-lin, WANG Zhi-cai, SUN Zhao-min, FENG Xi-jie, YU Gui-hua, CHEN Li-chun, CHEN Gui-hua, YU Shen-e, RAN Yong-kang, LI Xi-guang, LI Chen-xia, AN Yan-fen
    SEISMOLOGY AND GEOLOGY    2008, 30 (3): 597-629.  
    Abstract3808)      PDF(pc) (49676KB)(3295)       Save
    Field investigations show that the MS8.0 Wenchuan earthquake of 12th May 2008 ruptured two NW-dipping imbricate reverse faults along the Longmenshan Fault zone at the eastern margin of the Tibetan Plateau.This earthquake generated a 240km long surface rupture along the Beichuan-Yingxiu Fault characterized by right-lateral oblique faulting and a 90km long surface rupture along the Guanxian-Jiangyou Fault characterized by dip-slip reverse faulting.Maximum vertical and horizontal dispacements of 6.2m and 4.9m,respectively,were observed along the Beichuan-Yingxiu Fault,whereas a maximum vertical displacement of 3.5m occurred along the Guanxian-jiangyou Fault.This co-seismic surface rupture pattern,involving multiple structures,is among the most complicated of recent great earthquakes.Its surface rupture length is the longest among the co-seismic surface rupture zones for reverse faulting events ever reported.Aftershocks recorded by local network clearly outline the hanging wall of the Beichuan-Yingxiu Fault and indicate that the fault dips about 47? to the west.Industry seismic lines,in addition to surface ruptures and aftershocks,allow us to build a 3D model for the rupture geometry that shows crustal shortening is the dominant process along the Longmen Shan to accommodate long-term deformation.Oblique thrusting accomplished by the earthquake indicates that the east-southeastward extrusion of Tibet Plateau accommodates,in part,the continuing penetration of the Indian plate into the Eurasian plate,and this extrusion is transformed at the eastern margin of the Tibetan Plateau into crustal thickening and shortening along the Longmenshan Fault zone that is responsible for the growth of high topography in the region.
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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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    GAI Hai-long, LI Zhi-min, YAO Sheng-hai, LI Xin
    SEISMOLOGY AND EGOLOGY    2022, 44 (1): 238-255.   DOI: 10.3969/j.issn.0253-4967.2022.01.015
    Abstract693)   HTML31)    PDF(pc) (22330KB)(375)       Save

    At 01:45 on January 8, 2022, Beijing Time, an MS6.9 earthquake occurred in Menyuan County, Haibei Prefecture, Qinghai Province, with a focal depth of 10km. The microscopic(instrument)epicenter is located at 37.77°N latitude and 101.26°E longitude in the intersection between the Toleshan fault zone and the Lenglongling fault zone in the northern Qilian-Qaidam block. The epicenter is 54km away from Menyuan County in Qinghai, 99km away from Qilian County, 100km away from Haiyan County, 83km away from Minle County in Gansu Province, 83km away from Yongchang County, and 141km away from Xining City. When the earthquake occurred, Menyuan County and Xining City, the capital of Qinghai Province, were strongly felt, and Yinchuan, Lanzhou, Xi'an and many other places were felt. At the same time, affected by the earthquake, the Lanxin high-speed rail line, an important railway transportation hub of the Belt and Road, was suspended. This earthquake is the largest earthquake in the world since 2022. It is also another earthquake of magnitude 6.0 or above in Qinghai Province following the Maduo MS7.4 earthquake on May 22, 2021. Besides, this earthquake is the event with the highest magnitude and the longest surface rupture in the region after the two M6.4 Menyuan earthquakes of August 26, 1986 and January 21, 2016. Therefore, this earthquake has attracted much attention from the society. The coseismic surface rupture distribution, combination characteristics, development properties and coseismic displacement of this earthquake were identified in time to help to have a correct understanding of the earthquake seismogenic structure, rupture process, and assessment of short-term earthquake hazards. It is also of great significance for major project route selection, earthquake fortification and rescue and disaster relief. On the basis of the on-site seismic geological investigation, based on the interpretation and analysis of high-resolution satellite remote sensing images, and combined with the low-altitude photogrammetry of unmanned aerial vehicles(DJI PHANTOM 4RTK), the author obtained the coseismic rupture data of five typical sites along the surface rupture zone generated by the earthquake. Using Agisoft Metashape Professional software to process the aerial photos of each section indoors, a high-resolution orthophoto map(DOM)was generated. At the same time, the five typical earthquake surface rupture sections were described in detail in ArcGIS Pro software based on the orthophoto map. Preliminary research shows that the surface rupture zone of the Menyuan MS6.9 earthquake is more than 22km long and consists of the main rupture of the northern branch and the secondary rupture of the southern branch. The north branch main rupture zone is distributed in the middle-western segment of the Lenglongling Fault of central Haiyuan fault zone, with a length of more than 18km and an overall strike of 295°. The maximum co-seismic horizontal displacement is located in the middle of the rupture zone at Liuhuangou(37.799°N, 101.2607°E), which is about 3.1m and gradually decays towards both ends. The secondary rupture of the southern branch is distributed on the local segments of the eastern Toleshan Fault in the central-western Haiyuan fault zone, with a length of about 4km and a strike of 275°, constituting a secondary branch rupture zone arranged in a left-stepped en-echelon pattern to the western segment of the main rupture zone. There are en-echelon extensional stepovers between the two rupture zones of the north and south branches. The whole surface rupture zone is mainly composed of linear shear cracks, oblique tension cracks, tension-shear cracks, compressional bulges and other structural types. The coseismic surface rupture has the characteristic of typical left-lateral strike-slip motion with a thrust component, and the maximum vertical dislocation is 0.8m.

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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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    LIANG Kuan, HE Zhong-tai, JIANG Wen-liang, LI Yong-sheng, LIU Ze-min
    SEISMOLOGY AND GEOLOGY    2022, 44 (1): 256-278.   DOI: 10.3969/j.issn.0253-4967.2022.01.016
    Abstract656)   HTML24)    PDF(pc) (24460KB)(364)       Save

    At 1:45 on January 8, 2022, a MS6.9 earthquake occurred in Menyuan County, Haibei Prefecture, Qinghai Province. The epicenter(37.77°N, 101.26°E)is located in the western segment of the Lenglongling Fault of the Qilian-Haiyuan fault zone, with a focal depth of 10km. The earthquake is located in the northwest of the MS6.4 Menyuan earthquake on January 21, 2016. According to the survey results of China Earthquake Administration, the highest intensity of this earthquake is IX degree, and the long axis of the isoseismic line is NWW-striking. The earthquake caused serious damage to the Daliang Tunnel between Haomen Station and Junmachang Station, and the Lanxin high-speed railway was interrupted. After the earthquake, the distribution of the earthquake surface rupture zone was quickly determined by interpreting the GF-7 satellite post-earthquake images, and the field surface rupture investigation was carried out at the epicenter site in the first time. The field investigation mainly includes the identification of surface rupture zones, the investigation of rupture characteristics, the survey of fault geomorphology, the high-precision aerial photogrammetry of typical rupture points, the identification and measurement of coseismic dislocation, and the investigation of earthquake disasters. Aerial photogrammetry realizes real-time difference through UAV linked network RTK, and takes high-definition photos from multiple angles. Pix4D software is used to complete calculation and point cloud encryption, etc. DSM (Digital Surface Model) and DOM (Digital Orthophoto Map) are generated for surface rupture space reproduction and feature measurement and analysis. According to the interpretation of high-resolution remote sensing images by GF-7 satellite and field investigation, the surface rupture of MS6.9 Menyuan earthquake can be divided into NW-striking western segment of Lenglongling Fault and EW-striking eastern segment of Tuolaishan Fault. The two surface ruptures are 291° and 86.9°, respectively, and their lengths are not less than 26km and 3.5km respectively. We made detailed observation and measurement on the Jingyangling site, Daogou site, east Daogou site, Shixiamen site, the seven sites along the Liuhuanggou on the Lenglongling Fault, and the Yangchangzigou site on the Tuolaishan Fault. The surface rupture zone is mainly a complex coseismic surface deformation zone formed by the combination of multiple types of fractures, such as tensional fracture, tensional shear fracture, compression bulge and seismic depression, and characterized by sinistral strike-slip motion and partly by thrusting. Generally, the NW-striking ruptures exhibit left-lateral strike-slip characteristics, while NW-striking branch ruptures exhibit a small amount of right-lateral strike-slip characteristics. At Shixiamen site, four pasture fences were continuously offset left-laterally by 2.0~2.15m. At the Daliang Tunel site, the rut was offset left-laterally by 2.77m measured by UAV, which is the largest co-seismic left-lateral displacement of this earthquake. Based on high-resolution remote sensing image interpretation, field investigation, InSAR inversion of focal mechanism, fault rupture model and small earthquake precision location, it is determined that the earthquake occurred at the deep intersection of the Tuolaishan Fault and Lenglongling Fault, and the main seismogenic structure is the western segment of Lenglongling Fault(strike 112°, dip 88°). The Tuolaishan Fault on its west side ruptured simultaneously at the east end. According to the distribution characteristics of the surface ruptures and the field investigation of this earthquake, we believe that the Lenglongling Fault continues to extend westward after passing through the Liuhuanggou No. 1 site until the Jingyangling site, and the NWW-striking Lenglongling Fault has a “Y”-shaped contact relationship with the EW-striking Tuolaishan Fault. The 1986 MS6.4 earthquake occurred at the northwestern end of the Lenglongling North Fault, which protrudes in an arc toward NE, and the 2016 MS6.4 earthquake occurred at the southeastern end of the fault. Affected by the left-lateral strike-slip movement of the Lenglongling Fault, the small block bounded by the Lenglongling Fault and the Lenglongling North Fault also moves in the direction of SEE relative to the northern block. Therefore, the 1986 MS6.4 earthquake showed tensile properties, and the 2016 MS6.4 earthquake showed compression properties. The seismogenic structure of the Menyuan MS6.9 earthquake is the Lenglongling Fault, so the earthquake is mainly characterized by left-lateral strike-slip. The MS6.4 earthquake in 1986, MS6.4 earthquake in 2016 and MS6.9 earthquake in 2022all occurred in the western section of Lenglongling Fault. Three strong earthquakes of M>6 occurred in a short period of time, indicating that this area is still an accumulation area of stress and deformation, and has the potential risk of large earthquakes.
    Due to the limitation of the data range of the Gaofen-7 satellite image and the inconvenience of traffic caused by the icing of the river, the location of the easternmost end point of the rupture and the exact length of the rupture have not been determined in this field investigation. We hope that follow-up studies will be carried out to confirm the rupture length when weather conditions are appropriate.

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    ZHANG Zhi-wei, LONG Feng, ZHAO Xiao-yan, WANG Di
    SEISMOLOGY AND GEOLOGY    2022, 44 (1): 170-187.   DOI: 10.3969/j.issn.0253-4967.2022.01.011
    Abstract554)   HTML18)    PDF(pc) (10401KB)(351)       Save

    Based on the focal mechanism solutions of 2 600 ML≥3.0 earthquakes in Sichuan and Yunnan area from January 2000 to March 2017, the focal mechanism quantitative classification and stress field inversion are carried out for the sub blocks and fault zones with relatively dense focal mechanisms. Using the focal mechanism solutions of 727 ML≥4.0 earthquakes from January 1970 to March 2017, the regional stress tensor damping method is used to inverse the spatial distribution of principal compressive stress in Sichuan and Yunnan area before and after Wenchuan MS8.0 and Lushan MS7.0 earthquakes, and the temporal and spatial evolution characteristics of current stress field are discussed.
    The focal mechanisms are distributed mainly in Longmenshan fault zone, Xianshuihe-Anninghe-Zemuhe-Xiaojiang fault zone, Mabian-Yanjin fault zone, Lijiang-Xiaojinhe fault zone, the central Yunnan block, the west Yunnan block and the southwest Yunnan block in Sichuan and Yunnan area. The focal mechanism is mainly strike slip type in Sichuan and Yunnan area, but there are local differences. The Longmenshan fault zone is dominated by thrust type earthquakes, while in the Mabian-Yanjin fault zone, there are relatively more strike slip and thrust type earthquakes. The types of earthquakes in Sichuan Basin are complex, and there is no obvious dominant type. In general, the focal mechanisms of the Longmenshan fault zone and Sichuan Basin earthquakes are affected by strong earthquake and other factors, and the focal mechanism types have good inheritance in Sichuan and Yunnan area.
    The stress field in Sichuan and Yunnan area has obvious subarea characteristics, and it rotates clockwise from north to south. The compressive stress in Longmenshan fault zone and Sichuan Basin shows nearly EW direction. It shows NWW direction in the eastern boundary of Sichuan and Yunnan rhombic block and NNW direction in the inner part of rhombic, while it shows NNE direction in the western and southern Yunnan blocks. The principal compressive stress in Sichuan is more complex than that in Yunnan. The principal compressive stress direction in Sichuan experiences EW-NW-EW rotation from west to east, the dip angle is steep in the west and slow in the east, and the stress regime also experiences the transition from normal faulting to strike-slip to thrust. The principal compressive stress direction in Yunnan is NNE in the west and NNW in the east, forming an inverted “V” shape in space, the stress regime is mainly strike-slip and the dip angle is horizontal.
    Before and after the Wenchuan MS8.0 and Lushan MS7.0 strong earthquakes, the stress field in the Longmenshan fault zone changed greatly, followed by the Sichuan Basin and its surrounding areas, and there was no obvious change in other areas of Sichuan and Yunnan. The stress field in the Longmenshan fault zone experienced a complete transformation process from basic stress field to variable stress field to basic stress field.

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    XU Xi-wei, WU Xi-yan, YU Gui-hua, TAN Xi-bin, LI Kang
    SEISMOLOGY AND GEOLOGY    2017, 39 (2): 219-275.   DOI: 10.3969/j.issn.0253-4967.2017.02.001
    Abstract808)   HTML    PDF(pc) (20050KB)(1652)       Save
    High-magnitude earthquake refers to an earthquake that can produce obvious surface ruptures along its seismogenic fault and its magnitude M is at least equal to 7.0. Prediction and identification of locations, where the high-magnitude earthquakes will occur in potential, is one of the scientific goals of the studies on long-term faulting behavior of active faults and paleo-earthquakes, and is also the key problem of earthquake prediction and forecast. The study of the geological and seismological signatures for identifying M≥7.0 earthquake risk areas and their application is an important part of seismic prediction researches. It can not only promote the development of earthquake science, especially the progress of earthquake monitoring and forecasting, but also be positive for earthquake disaster prevention and effective mitigation of possible earthquake disaster losses. It is also one of the earthquake science problems which the governments, societies and the scientific communities are very concerned about and need to be addressed.
    Large or great earthquakes, such as the 2008 Wenchuan earthquake(M8.0), the 2010 Yushu earthquake(M7.1), the 2013 Lushan earthquake(M7.0)and the 2015 Gorkha earthquake(MW7.8), have unceasingly struck the Qinghai-Tibet Plateau and its surrounding areas, which have been attracting attention of a large number of geoscientists both at home and abroad. Owing to good coverage of the seismic networks and GPS sations, a lot of high-quality publications in seismicity, crustal velocity structure, faulting beihavior have been pressed, which gives us a good chance to summarize some common features of these earthquakes. In this paper, seismogenic structural model of these earthquakes, faulting behavior of seismogenic faults, crustal mechanical property, recent straining environment and pre-earthquake seismicity are first analyzed, and then, five kinds of common features for the sismogenic faults where those earthquakes occurred. Those five kinds of commom features are, in fact, the geological and seismological signatures for identifying M≥7.0 earthquake risk areas. The reliability of the obtained sigatures is also discussed in brief. At last, based on the results of 1:50000 active fault mapping, and published seismic tomography and fault-locking studies, an experimental identification of the risk areas for the future large/great earthquakes in the North China and the Qinghai-Tibet Plateau is conducted to test the scientificity and applicability of these obtained sigantures.
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    SONG Xiang-hui, WANG Shuai-jun, PAN Su-zhen, SONG Jia-jia
    SEISMOLOGY AND EGOLOGY    2021, 43 (4): 757-770.   DOI: 10.3969/j.issn.0253-4967.2021.04.002
    Abstract525)   HTML194)    PDF(pc) (5271KB)(646)       Save

    On May 22, 2021, an MS7.4 earthquake occurred in the Madoi area of Banyan Har block, with a focal depth of about 8km. The seismogenic fault is deduced as the Jiangcuo Fault, a branch of the east Kunlun strike-slip fault. Different with previous strong earthquakes which located at the boundary faults around the Bayan Har block, the Madoi MS7.4 earthquake occurred inside the block and about 70km away from the boundary fault. Furthermore, there is a contradiction between the small strike-slip component of the seismogenic fault and the large earthquake magnitude. The above phenomena indicate that the Madoi earthquake may have special seismotectonic background and seismogenesis. Strong earthquakes in Tibetan plateau are always closely related to the deep crustal structure and dynamic process. Therefore, it is of great significance to study the crustal structure and the distribution of deep faults in the Madoi area in order to reveal the deep tectonic background and genesis of the Madoi MS7.4 earthquake. To research the deep seismotectonic environments of the MS7.4 Madoi earthquake, we reinterpret the deep seismic sounding(DSS)results in Madoi area. The DSS profile reveals fine crustal structure beneath the Madoi area, and divides the crust into 3 crustal layers. From the crustal velocity structure of the Madoi and adjacent area, we found the generation of the Madoi earthquake is closely connected with the deep structure and crustal medium. Through analysis on the velocity structures, we get the following understanding: 1)There is an interface in the upper crust of the Madoi area, which represents the velocity changing from 5.8km/s to 5.6km/s and divides the upper crust into two layers. The upper layer is composed of high velocity structure, indicating a brittle medium environment, while the lower layer consists of low velocity zone and provides the strain accumulation condition for the Madoi earthquake. In addition, the transition between local high velocity zone(HVZ)and the normal crust in the focal area provides an ideal medium environment for earthquake preparation. 2)A wedge-shaped low velocity zone(LVZ)exists in the lower crust south of Madoi, which provides an environment for the movement of weak materials from the SW to NE direction. However, the high-velocity lower crust beneath Madoi area resists the crustal flow and thus transforms the horizontal movement to vertical upwelling, resulting in the stress concentration of the upper crust beneath Madoi area, which may provide dynamic for the preparation of the Madoi MS7.4 earthquake. 3)The Jiangcuo Fault merges into the East Kunlun Fault in the deep crust, forming a reverse thrust fault structural style dominated by the East Kunlun strike-slip fault. As a branch of the East Kunlun Fault, the strike slip of the Jiangcuo Fault is the adjustment results of strain and movement of the East Kunlun Fault. Moreover, the Jiangcuo Fault and adjacent faults constitute the horsetail-shaped fault zone, combined with the imbricated thrust fault zone profile, reflecting the compressive stress of Modoi area that facilitates the strain concentration. Therefore the occurrence of the Madoi earthquake is related to the left-lateral strike-slip movement of the East Kunlun Fault and the special imbricated thrust fault assemblages. On the other hand, the upwelling of the lower crustal flow and the corresponding sliding of the upper crust may be related with the occurrence of the Madoi earthquake. In conclusion, the Madoi MS7.4 earthquake is closely related to the ideal medium environment of the upper crust, the lower crustal flow and vertical upwelling beneath Madoi area, as well as the left-lateral strike-slip of the East Kunlun Fault.

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    Wen Xueze
    SEISMOLOGY AND GEOLOGY    2000, 22 (3): 239-249.  
    Abstract1951)      PDF(pc) (3430KB)(1842)       Save
    Based on several types of data, segmentation and its cause, earthquake ruptures along the Xianshuihe-Anninghe-Zemuhe fault zone in western Sichuan are analyzed from various aspects. The fault zone is divided into 12 rupture-segments for characteristic earthquakes. Along this fault zone, the persistent and non-persistent boundaries of ruptures have almost the same number ratio of 2%. The persistent and important boundaries can be identified by criteria such as geometry, structure and active behavior of faults. They terminate the propagation of earthquake ruptures by partly volume-changing there. The non-persistent boundaries can be identified by criteria such as extension and recurrence behavior of earthquake ruptures, spatial difference of current active behavior of faults, composition of both relatively small-scale geometric barriers and release barriers. Locations of the non-persistent boundaries may vary with time. Relatively small overlap occurs between adjacent ruptures if the time interval between them is shorter, and relatively large overlap appears between adjacent ruptures if the time interval between them is longer.
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    LI Zhen-yue, WAN Yong-ge, JIN Zhi-tong, YANG Fan, HU Xiao-hui, LI Ze-xiao
    SEISMOLOGY AND GEOLOGY    2020, 42 (5): 1091-1108.   DOI: 10.3969/j.issn.0253-4967.2020.05.005
    Abstract410)   HTML    PDF(pc) (4980KB)(482)       Save
    Based on the rupture model of Mainling M6.9 earthquake in Tibet on November 18, 2017, the spatial distribution of static Coulomb failure stress change at different depths are calculated respectively according to two different receiving fault selection schemes. The one scheme is that we set the parameters of receiving fault at different position to be consistent with the main shock; The other scheme is on the assumption that fault's orientation is randomly distributed under the ground, and we select the receiving fault which is most prone to slide under the influence of coseismic stress field produced by main shock. Therefore, the geometrical orientation of receiving fault will vary with space. According to the above two results of static Coulomb failure stress change, we discussed the static Coulomb stress influence produced by the main shock to short-term aftershocks and the Medog M6.3 earthquake in Tibet on April 24, 2019, respectively. The result shows that: 1)When the parameters of receiving fault are same with the main shock, the proportion of aftershocks in the positive zone of static Coulomb failure stress change is small at each depth. The focal mechanisms of aftershocks in the positive zone of static coulomb fracture stress are deemed similar to the main shock. We thought that they are motivated by the continuous rupture of the main shock. 2)Most of the aftershocks are in the negative zone of static Coulomb failure stress change at each depth. We inferred that this phenomenon which may be on account of the focal mechanisms of these aftershocks is quite different with the main shock. From the result of receiving fault to choose the most prone to slide under the coseismic stress field produced by main shock, we can clearly see that all the aftershocks are within the zone of static Coulomb failure stress change greater than the trigger threshold of 0.01MPa at different depths. It indicates that all the aftershocks are likely to be triggered. It was speculated that the aftershocks in the negative zone of static Coulomb failure stress change occurred in the crushed zone caused by violent rupture of the main shock. There are countless cracks in the crushed zone, and the orientation of these cracks is abundant. Perhaps, because most aftershocks occurred on these various cracks, their focal mechanisms are quite different from the main shock. The value of elastic constants will be reduced significantly in the crushed zone. All the results in this paper also indicate that considering the elastic constants difference between in and out of the source region is beneficial to accurately estimate the static Coulomb stress influence between earthquakes in the source region. 3)Different institutes and authors used different data and methods to get several different focal mechanisms of the Medog earthquake. According to these results, we calculated a central focal mechanism solution, which has a minimum difference with these focal mechanisms. On the basis of this central focal mechanism solution, the static Coulomb stress influence of the Mainling earthquake to the Medog earthquake is calculated quantitatively. Result indicates that the magnitude of static Coulomb failure stress change generated by the Mainling earthquake is quite small on both two nodal planes of the central focal mechanism solution of the Medog earthquake, this means that the Medog earthquake is independent of the Mainling earthquake.
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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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    HU Zong-kai, YANG Xiao-ping, YANG Hai-bo, LI Jun, WU Guo-dong, HUANG Wei-liang
    SEISMOLOGY AND GEOLOGY    2019, 41 (2): 266-280.   DOI: 10.3969/j.issn.0253-4967.2019.02.002
    Abstract408)   HTML    PDF(pc) (13132KB)(428)       Save
    The Bolokonu-Aqikekuduke fault zone(Bo-A Fault)is the plate convergence boundary between the middle and the northern Tianshan. Bo-A Fault is an inherited right-lateral strike-slip active fault and obliquely cuts the Tianshan Mountains to the northwest. Accurately constrained fault activity and slip rate is crucial for understanding the tectonic deformation mechanism, strain rate distribution and regional seismic hazard. Based on the interpretation of satellite remote sensing images and topographic surveys, this paper divides the alluvial fans in the southeast of Jinghe River into four phases, Fan1, Fan2, Fan3 and Fan4 by geomorphological elevation, water density, depth of cut, etc. This paper interprets gullies and terrace scarps by high-resolution LiDAR topographic data. Right-laterally offset gullies, fault scarps and terrace scarps are distributed in Fan1, Fan2b and Fan3. We have identified a total of 30 right-laterally offset gullies and terrace scarps. Minimum right-lateral displacement is about 6m and the maximum right-lateral displacements are(414±10)m, (91±5)m and(39±1)m on Fan2b, Fan3a and Fan3b. The landform scarp dividing Fan2b and Fan3a is offset right-laterally by (212±11)m. Combining the work done by the predecessors in the northern foothills of the Tianshan Mountains with Guliya ice core climate curve, this paper concludes that the undercut age of alluvial fan are 56~64ka, 35~41ka, 10~14ka in the Tianshan Mountains. The slip rate of Bo-A Fault since the formation of the Fan2b, Fan3a and Fan3b of the alluvial-proluvial fan is 3.3~3.7mm/a, 2.2~2.6mm/a and 2.7~3.9mm/a. The right-lateral strike-slip rate since the late Pleistocene is obtained to be 3.1±0.3mm/a based on high-resolution LiDAR topographic data and Monte Carlo analysis.
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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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    HE Xiang, DU Xing-xing, LIU Jian, LI Yi-hao, LI Qun
    SEISMOLOGY AND GEOLOGY    2022, 44 (1): 76-97.   DOI: 10.3969/j.issn.0253-4967.2022.01.006
    Abstract548)   HTML14)    PDF(pc) (13184KB)(240)       Save

    The Wuwei Basin is located in the eastern part of the Hexi Corridor Basin group. It belongs to the frontier of uplift and expansion of the northeastern margin of the Qinghai-Tibet Plateau. The Quaternary sedimentary strata in the Wuwei Basin are hundreds of meters thick, which records the geological information of uplift and expansion of the northeastern margin of the Qinghai-Tibet Plateau during the Quaternary period. In order to study the sedimentary process of the Wuwei Basin since Quaternary and the characteristics of the uplift and extension of the northeastern margin of the Qinghai-Tibet Plateau, the sedimentary stratigraphy and stratigraphic chronology methods are applied to analyze the Quaternary sedimentary strata in the Wuwei Basin. Firstly, the Quaternary sedimentary sequence of the Wuwei Basin is established by studying the stratum characteristics and OSL ages of the Quaternary sedimentary strata. Secondly, through the analysis of the paleocurrent direction and zircon U-Pb isotopic age of the sedimentary gravel, the paleocurrent direction of different deposition periods and sediment source are restored. Finally, based on the above analysis, the sedimentary process of Quaternary strata in the Wuwei Basin and its response to the uplift and extension of the Qinghai-Tibet Plateau are discussed.
    The Quaternary sedimentary strata in the Wuwei Basin are the lower Pleistocene Yumen conglomerate, the middle Pleistocene Jiuquan gravel bed and the Upper Pleistocene-Holocene Gobi gravel bed from bottom to top. The Gobi gravel bed can be subdivided into the Upper Pleistocene gravel bed, the Upper Pleistocene-Holocene loess layer and the Holocene gravel bed. In the early Pleistocene, the Yumen conglomerate’s source material is mainly Mesozoic and Paleozoic rocks. The main provenance area of the Yumen conglomerate is located in the Qilian Mountains south to the Wuwei Basin. The main sedimentary area of the Yumen conglomerate is located in the Zoulang Nan Shan located in the southern part of the Wuwei Basin and the southern part of the northern fault basin. In the middle Pleistocene, the Jiuquan gravel bed’s source material is mainly Cenozoic and Mesozoic rocks. In the early sedimentary stage of the Jiuquan gravel bed, the main provenance area is located in the Zoulang Nan Shan and the main sedimentary area is located in the northern part of Wuwei Basin. In the late sedimentary stage, the main provenance area of the Jiuquan gravel bed is located in the Zoulang Nan Shan and the Fenmen Mountain located in the northwestern margin of the Wuwei Basin, and the main sedimentary area is located in most of the northern fault basin and the surrounding area of the Fenmen Mountain. Since late Pleistocene, the Gobi gravel bed’s source material is mainly early Paleozoic rocks. The main provenance area of the Gobi gravel bed is located in the Zoulang Nan Shan, the Fenmen Mountain and the Longshou Mountain, and the main sedimentary area is located around the source area.
    The uplift boundary of the northeastern margin of the Qinghai-Tibet Plateau continued to expand towards the northeast direction. The uplift boundary was located in the Qilian Mountains south to the Wuwei Basin in the early Pleistocene. It extended northward to the Zoulang Nan Shan and the Fenmen Mountain in the middle Pleistocene, and reached the Longshou Mountain north to the Wuwei Basin in the late Pleistocene. The main provenance and sedimentary areas of the Quaternary sediments in the Wuwei Basin show the migration characteristics from south to north, which indicates the uplift and expansion of the northeastern margin of the Qinghai-Tibet Plateau. The uplift time was early in the south and late in the north, and the uplift intensity was strong in the south and west and weak in the north and east.

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    PENG Yan-ju, LÜ Yue-jun, HUANG Ya-hong, SHI Chun-hua, TANG Rong-yu
    SEISMOLOGY AND GEOLOGY    2009, 31 (2): 349-362.   DOI: 10.3969/j.issn.0253-4967.2009.02.016
    Abstract2037)      PDF(pc) (434KB)(6153)       Save
    Two kinds of site classification methods commonly used in earthquake engineering are analyzed in this paper.One is standard methods stipulated in seismic codes,and used to determine the site effects on seismic parameters for the seismic resistance of structures,the site classification methods and site indexes in seismic codes of China,United States,Europe and Japan are presented,and the problems about site index are discussed,such as the calculation method and depth of shear wave velocity,the choice of initial layer,the grade of overburden thickness,etc.Then some suggestions are put forward for the new generation of seismic code in China.The other kind of site classification methods is used to predict site effects on a large scale for a regional seismic hazard prediction.The popularly studied methods based on geology,topography and geomorphology are described in detail.The common character of this kind of methods is to find an easily obtained macro index,and to summarize the rules between the macro index and the site index in seismic codes(shear wave velocity in most cases),and then the regional site category zonation can be delineated.The response spectrum methods of ground motion are also presented here,such as RSS(Response Spectral Shape)and HVSR(Horizontal-Vertical Spectral Ratio),they can be used in areas with abundant ground motion records.Finally the advantage,limitation and applicable scope of these methods are discussed.
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    YE Yi-jia, TAN Xi-bin, QIAN Li
    SEISMOLOGY AND EGOLOGY    2022, 44 (1): 115-129.   DOI: 10.3969/j.issn.0253-4967.2022.01.008
    Abstract432)   HTML11)    PDF(pc) (6436KB)(214)       Save

    River networks, the backbone of most landscapes on Earth, record abundant information on the perturbations of tectonics, climate, and sea levels. Based on the fluvial shear stress model, the bedrock incision has a positive, monotonic correlation with fluvial shear stress. The correlation coefficient between the fluvial shear stress and the erosion rate is called rock erodibility, which mainly depends on lithology and erosion process. Therefore, the fluvial shear stress can be used to calculate the erosion rate, when the rock erodibility is known. It also can be used to calculate the rock erodibility, when the erosion rate is known.
    With the improvement of DEM and satellite image resolution, we can extract the corresponding geomorphic parameters(such as channel gradient, discharge, and channel width)from the DEM and satellite image, and then calculate the fluvial shear stress. In this study, we calculated the fluvial shear stress values at 240 points along two channels in the Longmen Shan area. The Minjiang River flows through Wenchuan-Maoxian Fault and then enters Pengguan Massif. Along the Chu River, the lithology is relatively consistent, all are sedimentary rocks, including a ~5km reach distributed along the Shuangshi-Dachuan Fault. The results show that: 1)The average fluvial shear stress is ~401Pa at the footwall of the Wenchuan-Maoxian Fault, while it is ~280Pa along the Wenchuan-Maoxian Fault; 2)the average fluvial shear stress is ~161Pa and ~104Pa at the hanging wall and footwall of Shuangshi-Dachuan Fault, respectively, while it is ~50Pa along the Shuangshi-Dachuan Fault. Fault activities cause the rock more fractured, which causes the fluvial shear stress along the fault to decrease significantly.
    Then, according to the empirical relationship between fluvial shear stress and erosion rates of sedimentary rocks(E=3.2(τ*-0.03)mm/a) and granitoid (E=1.6(τ*-0.03)mm/a) in the Longmen Shan area, we transform the fluvial shear stress for 139 survey points to erosion rate. The average erosion rate at the footwall of the Wenchuan-Maoxian Fault(i.e. the Pengguan massif) is ~0.43mm/a, and those at the hanging wall and footwall of the Shuangshi-Dachuan Fault is ~0.49mm/a and ~0.28mm/a, respectively.
    Moreover, according to the known erosion rates and fluvial shear stress, we calculated the rock erodibility of each survey point. The average erodibility is 13.9mm/a along the Shuangshi-Dachuan Fault(Chu River, sedimentary rocks)and 3.2mm/a along the Wenchuan-Maoxian Fault(Minjiang River, granitoid). The results indicate that: 1)The erodibility of the sedimentary rocks along the fault increases by ~3 times caused by the fault activity, while that of granitoid increases by ~1 time; 2)the fault activity can increase the rock erodibility within ~2km from the active fault in sedimentary rocks and within ~5km from the active fault in granitoid, respectively. This study indicates that fault activities could significantly increase the rock erodibility along the fault, and further influence the evolution of river networks around the fault.

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    GU Qin-ping, KANG Qing-qing, ZHANG Peng, MENG Ke, WU Shan-shan, LI Zheng-kai, WANG Jun-fei, HUANG Qun, JIANG Xin, LI Da-hu
    SEISMOLOGY AND GEOLOGY    2020, 42 (5): 1129-1152.   DOI: 10.3969/j.issn.0253-4967.2020.05.007
    Abstract382)   HTML    PDF(pc) (16177KB)(277)       Save
    The middle-southern segment of the Tan-Lu fault zone and its adjacent area is located in the joint zone of the North China craton and Yangtze craton. It is a natural test ground for studying the problems of intracontinental collision, continental convergence and growth, geodynamics and lithospheric deformation. Although early research involved the central-south section of the Tan-Lu fault zone and its neighboring areas, it is difficult to carry out a detailed discussion on the S-wave velocity and azimuthal anisotropy in the middle and south section of the Tan-Lu fault zone and its adjacent areas, due to different research purposes and objects, the limitation in selecting research scope or the lack of resolution.
    To obtain more detailed crust-mantle velocity structure and azimuthal anisotropy distribution characteristics in the study area, this paper uses waveform data recorded by 261 fixed wideband seismic stations in the middle-southern segment of the Tan-Lu fault zone and its adjacent zone for two consecutive years. The phase velocity dispersion curve of Rayleigh surface wave with 5~50s period was extracted by time-frequency analysis. Then, the study area was divided into 0.25°×0.25°grids, and the two-dimensional Rayleigh phase velocity and azimuthal anisotropy distribution image in the area was retrieved using the Tarantola method.
    The phase velocity and azimuthal anisotropy distribution images of 6 representative periods were analyzed. These images reveal the lateral heterogeneity of the crust-mantle velocity structure and spatial differences in azimuthal anisotropy in the middle-southern segment of the Tan-Lu Fault and its adjacent areas. The results show that the distribution characteristics of phase velocity have a good correspondence with geological tectonic units. In the shallow part of the earth's crust, the basins covered by thick unconsolidated sedimentary layers and the bedrock exposed orogenic belts show low and high velocity anomalies, respectively. With the increase of the period(15~20s), the influence of the shallow sedimentary layer is weakened, and the high-speed anomaly appears in some plain areas such as the Hehuai Basin and Subei Basin. The distribution of phase velocity in the lower crust and upper mantle(25~30s)is affected by the thickness of the crust, which is inversely related to the burial depth of Moho surface. For example, the Dabie orogenic belt with a thickness of 40km changes from a short period high-speed to a low-speed distribution.
    Due to the differences in the tectonic environment of each geological structural unit in the study area, the azimuthal anisotropy of Rayleigh waves has obvious spatial differences. In general, the strength of anisotropy increases with increasing period(depth), and the direction of fast wave is more regular and followable. Based on the consistent distribution of low velocity and azimuthal anisotropy from the shallow crust to the lithospheric mantle in the Subei Basin, we believe that there may be a strong crust-mantle coupling phenomenon. The results obtained by different seismic anisotropy observation methods are different manifestations of anisotropy. However, due to the one-sided and low-resolution problems of single observation method, it is necessary to carry out joint inversion or comprehensive multiple observation methods.
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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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    CHANG Hao, CHANG Zu-feng, LIU Chang-wei
    SEISMOLOGY AND EGOLOGY    2021, 43 (6): 1435-1458.   DOI: 10.3969/j.issn.0253-4967.2021.06.006
    Abstract630)   HTML20)    PDF(pc) (19495KB)(298)       Save

    The relationship between large-scale landslides and active faults has attracted much attention. From the point of view of active tectonics and disaster geology, the late Quaternary activity of the Jinsha River fault zone is investigated and studied, and the relationship between large-scale landslides and activity of the Jinsha River fault zone is emphatically analyzed. The Jinsha River fault zone was formed during the closure of the Paleotethys Ocean. According to the distribution of the 5km-wide ophiolitic melange zone, the ultramafic rock zone, and the local migmatization and progressive metamorphism around the Variscan intermediate acid intrusive rock mass distributed along the fault, it is inferred that the fault zone was once a strongly active superlithospheric fault zone with obvious compressive properties. The Jinsha River fault zone is a large-scale, long-term active suture structure, with many branches, forming a 50km wide structural fracture zone. Affected by the eastward compression of the Tibet Plateau, it has changed into a strike-slip fault zone characterized by dextral shear since Pliocene. In the study area, the fault landforms are clear along the Zengdatong, Xulong, Nizhong, Lifu-riyu, Langzhong and Guxue faults, which are mainly manifested as straight fault trough, linear ridge, fault scarp, and directional aligned fault facets. Results of field geological and geomorphological investigation and chronology show that the late Pleistocene and Holocene deposits are faulted, indicating the faults are active during the late Quaternary and dominated by dextral strike-slip with an average horizontal slip rate of 3.5~4.3mm/a in Holocene. The study area is located in the middle and north of the world-famous Jinsha River suture of the north-south structural belt in Sichuan, Yunnan and Tibet, and the geological structural conditions are very complex. The main structural line is distributed in NS direction, interwoven with NE and NW faults and fold axes in network shape, and the structure is complex. Strong neotectonic movement, huge topographic elevation difference, steep mountains, dry-hot valleys microclimate and other factors have caused serious internal dynamic geological disasters on both banks of Jinsha River. The landslide in the area has the characteristics of high frequency, large scale and serious damage. There are 23 large-scale and super large-scale landslides in the main stream and its tributaries of Jinsha River within the 38km-long section from Narong to Rongxue. Most of them are super large-scale landslides with a volume of more than 10 million cubic meters, even have a volume of more than 100 million cubic meters. Most of the landslides are located within 1km on both sides of faults, and many of them are developed on the fault zone. The occurrence of these large-scale landslides is closely related to the long-term activity, evolution history and complex structure of Jinsha River fault zone along the river, as a result, the rock mass structure gets fragmented and the continuous tectonic activity becomes the main cause of landslides. Active faulting is the fundamental controlling factor for the occurrence of large landslides along the river, especially for large landslides, and is an important internal dynamic condition for the formation of landslides. Further analysis of the fault structure shows that landslide is closely related to the movement evolution history of Jinsha River fault zone. Special structural combination parts(mechanical mechanism)such as closely adjacent faults, acute angle area of fault intersection, right turning parts of the faults and the intersection area between the main faults and the transverse faults are the key sites where the tectonic stress is easy to concentrate, thus conducive to generating large-scale landslides. Many large landslides occur in these structural parts. The controlling effect of active faults on landslides is not only embodied in the process of large earthquakes, but also can lead to the intensive occurrence of large and super large landslides in a natural state(non seismic action). This research has positive scientific significance for understanding the formation mechanism and development law of landslides on both sides of Jinsha River, and for understanding the relationship between fault activities and large landslides.

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    HUANG Shuai-tang, CHANG Xiang-de, MA Jian, HU Wei-hua, REN Jing, LIU Jian-ming, ZHANG Wen-xiu, LAI Ai-jing
    SEISMOLOGY AND EGOLOGY    2022, 44 (1): 20-34.   DOI: 10.3969/j.issn.0253-4967.2022.01.002
    Abstract350)   HTML29)    PDF(pc) (11493KB)(212)       Save

    Thrust fault is the basic model of crustal deformation and also one of the major structural forms of orogenic belts, indicating the tectonic environment of compression. Most of the catastrophic earthquakes that affect human activity occur within the plates. In the interior of the plate, reverse faults are likely to develop as long as there is compressive stress in the regional sense or under some local tectonic conditions. It is considered that the NS compression resulting from collision of the Indian plate and the Eurasian plate is the main cause for the formation of the present tectonic framework in both north and south sides of Tianshan Mountains. The continuous crustal shortening and thickening has made the Quaternary active structures in the front margins of Tianshan Mountains well developed. Meanwhile, the new nappe structures in front of Tianshan Mountains are also the main sites for the preparation of medium-strong earthquakes in the Tianshan Mountains area, and their seismogenic mode is mostly in the forms of blind fault ramp-decollement plane-surface fault ramps.
    The northern Tianshan inverse fault-fold belt is located at the junction between the northern foothill of Tianshan Mountains and Junggar Basin, where the Kusongmuqike piedmont fault is located in the south of Jinghe County, and is an important active thrust fault belt in the western northern Tianshan Mountains. In recent ten years, there were many earthquakes with magnitude 5.0 or above occurring in the eastern section of the fault zone. A detailed study of the geometric distribution and tectonic geomorphologic features is helpful to understand the tectonic deformation characteristics and regional strain distribution in the Tianshan area since the late Quaternary. The results of high-resolution remote sensing image interpretation, UAV aerial survey and differential GPS terrain profile survey combined with field geological survey show that the eastern segment of the Kusongmuqike piedmont fault is composed of two secondary reverse faults. Among them, the south branch, the Xinlongkou Fault, is composed of 5 en echelon-arranged sub-faults, with an overall trend of NW, dipping S, steep dip angle, and a length of about 48km. The fault offset the two-stage piedmont alluvial-pluvial fan and 5 river terraces, the activity time of terrace T1/T2 and fan3 is the latest, and the fault scarps are 3.6m to 4.7m high, being the product of concurrent fault activities. The vertical displacement of terrace T3 and T4 is 13.5m and 20.3m, respectively, and the vertical displacement of terrace T5 is roughly the same with that of the surrounding pluvial fan2, which is about 30m. On the fan1, there is no tectonic deformation observed in places where the fault passes through, and the initial landforms are retained on the surface. The north branch, the Hydrographic Station Fault, is distributed in an intermittent manner. The overall strike of the fault is near EW, with a total length of about 44km, and the fault offset multi-stage alluvial-pluvial fans. On the alluvial-pluvial fan of Fan3, two near-parallel normal scarps are developed in the northern margin of the alluvial-pluvial fan, while other faults cut through the alluvial-pluvial fan and the surface gully, forming steep reverse scarps on the surface. According to the cumulative height of the normal scarps, the maximum vertical displacement is 17.2m and the minimum vertical displacement is 0.3m, the scarp height is concentrated between 4.7~9.9m. On the reverse fault scarps, the maximum vertical displacement is 7.8~9.8m, the minimum scarp height is 2.4~3.1m, and the scarp height concentrates between 3.3~9.2m. Several sub-faults are developed scatteredly between the two sets of faults, with scarp heights ranging 0.5~1.0m. As far as the scarp height distribution is concerned, its vertical displacement shows a distribution law of decreasing from west to east. These results may contribute to the further understanding of the strain partitioning pattern in the western part of the northern Tianshan.

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    XU Fang, LU Ren-qi, WANG Shuai, JIANG Guo-yan, LONG Feng, WANG Xiao-shan, SU Peng, LIU Guan-shen
    SEISMOLOGY AND EGOLOGY    2022, 44 (1): 220-237.   DOI: 10.3969/j.issn.0253-4967.2022.01.014
    Abstract388)   HTML16)    PDF(pc) (11771KB)(198)       Save

    On February 3, 2020, an earthquake with a magnitude MS5.1 occurred in Qingbaijiang District, Chengdu City, Sichuan Province. The epicenter is located in the north segment of the Longquan Shan fault zone in the western Sichuan Basin. This fault zone locates in the forebulge of the foreland thrust belt of the Longmen Shan fault zone in the southeast margin of Tibetan plateau and is the east boundary of the western Sichuan foreland basin at the same time, so it has special tectonic significance. There are two branch faults in the north segment of Longquan Shan fault zone, which are distributed on the east and west sides, respectively, and the epicenter distance is almost similar to the two faults. At present, the seismogenic fault, earthquake genesis and dynamic source of the earthquake are not clear. As this earthquake is a moderate earthquake event, it is usually very uncertain to interpret it with structural geological or seismological data alone. Therefore, this study attempts to carry out a comprehensive study on the Qingbaijiang MS5.1 earthquake by performing cross fusion of multi-disciplinary data, adopting the multi-constraint method from geophysics, seismology and geodesy, and combining with structural geology and fault related fold theory. We collected three seismic reflection profiles located in the north segment of the fault zone to reveal the basic structural characteristics underground. The detachment layer in the middle-lower Triassic Jialingjiang-Leikoupo formation is developed at the depth of 4~6km below the anticline, and two obvious opposite thrust faults are developed on the two wings of the anticline, which are breakthrough fault-propagation fold deformation. The east branch thrust fault gradually rises from the detachment layer of Leikoupo formation to the surface, and the west branch thrust fault is exposed on the surface and connected with the detachment layer downward. The waveform data recorded by 14 fixed stations within 150km from the epicenter of Sichuan seismic network are studied and collected. The focal depth, focal mechanism and moment magnitude of the earthquake are obtained by using CAP waveform inversion method. The focal depth is 5km, indicating that the earthquake is related to shallow fault activity, the focal mechanism is 18°/32°/100° for nodal plane I and 186°/59°/84° for nodal plane Ⅱ, the moment magnitude is 4.64. Using the travel time data of P and S seismic phases, the Qingbaijiang earthquake sequence is relocated by HypoSAT location method and double difference location method. It is concluded that the epicenter position of the main earthquake is 30.73°N and 104.48°E. From February 4 to June 26, 2020, a total of 61 aftershock events were relocated, with magnitude 0≤ML≤3.0 and depth ranging from near surface to 15km. The 61 aftershocks spread about 5km in the NW-SE direction and have conjugate distribution in NW and NE directions, which may be related to the small thrust fault developed on the east branch of Longquan Shan Fault. Aftershocks have a good linear distribution in NE direction, which is closer to the east branch of the north segment of Longquan Shan fault zone, and the distribution direction is also consistent with the fault strike. On the seismic reflection profile, the aftershock projection is densely distributed along the east branch fault. The occurrence of the east branch fault is consistent with the focal mechanism nodal plane I, which is a low angle thrust fault dipping to NW. The InSAR coseismic deformation field near the epicenter is extracted by using the Sentinel data of orbit 55 and orbit 62 collected from ESA, including 8 single view complex images of orbit 55 and orbit 62, respectively. The surface deformation caused by this earthquake is in the middle of two thrust faults, and the maximum coseismic deformation can reach 4cm. The deformation caused by the earthquake is uplifting in the northwest and depressing in the southeast of the epicenter. The largest depression is located between the epicenter and the east branch fault. The thrust activity of the east branch fault is more in line with the above surface deformation characteristics. In this study, the seismotectonics of the 2020 Qingbaijiang MS5.1 earthquake is analyzed in detail using multi-disciplinary and multi-constraint method. The east branch fault in the north segment of the fault zone is determined as the seismogenic fault, and the possible seismic dynamic background is discussed. This result provides a scientific basis for fault activity analysis and seismic risk assessment in Longquan Shan area and has a great significance for further exploring the expansion and growth of Longmen Shan in the southeast margin of Tibetan plateau toward Sichuan Basin.

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    CHEN Gui-hua, MIN Wei, SONG Fang-min, JIAO De-cheng, XU Hong-tai
    SEISMOLOGY AND GEOLOGY    2011, 33 (4): 804-817.   DOI: 10.3969/j.issn.0253-4967.2011.04.006
    Abstract1323)      PDF(pc) (1913KB)(2202)       Save

    The co-seismic rupture is one of the important contents in active tectonic mapping.As the late Quaternary landform is a basic recording medium for the recent deformation of active fault,such as the co-seismic rupture,it is quite useful to acquire the activity information of the active fault from various landforms.We implemented a field work along the southeastern segment of the Xianshuihe Fault,mapped the rupture and excavated some trenches.The preservation characteristics of the surface rupture of the 1786 Moxi earthquake were discussed for the glacial area of the Tibetan plateau,the fluvial and flooding area and seriously eroded area at the margin of the Tibetan plateau,respectively.The cracks and offsets were preserved continuously in the glacial landforms such as the moraines and glacial outwashes along Kangding to Yajiageng segment.As the landforms in the fluvial and flooding area were unstable under strong erosion and rapid deposition,the surface rupture can be discovered in the trenches excavated in Yuejinping village and Ertaizi village with gaps for some previous earthquakes.There was no deposition from the erosion landform to record the surface rupture.We can only infer the earthquake effected area and the ruptured fault from the indirect relationship between landslides and the earthquake strong motion or the fault rupturing.Based on the integrated analysis with the geometry and tectonic setting of the southeastern segment of the Xianshuihe Fault,the Kangding-Tianwan segment of the Xianshuihe Fault was taken as the seismogenic fault of the 1786 Moxi earthquake,and the total length of the rupture is about 80 kilometers.

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    DENG Qi-dong, ZHU Ai-lan, GAO Xiang
    SEISMOLOGY AND GEOLOGY    2014, 36 (3): 562-573.   DOI: 10.3969/j.issn.0253-4967.2014.03.002
    Abstract624)      PDF(pc) (5689KB)(10533)       Save

    Strike-slip fault are the active faults that are most closely related to large earthquakes. The study on how a large earthquake develops and occurs on strike-slip faults is an issue much concerned with the seismologists. As it is shown by structural geology studies, strike-slip faults are a complex tectonic system, which represents combination of various types of deformation under the shearing forces. Based on the research cases of various strike-slip fault zones both at home and abroad, this paper investigates and summarizes the geometry, kinematics and evolution processes of continuous or discontinuous strike-slip faults and analyzes the hinge role of the strike-slip faults. It is found that the hinge axis area is subject to intense compression, and the area is locked, where stress is concentrated, strain is localized, and earthquakes nucleate and develop. When the locked hinge axis is broken through, unstable sliding will occur along the strike-slip fault, producing sudden big displacement, accompanied with large earthquake. In the stepover zones of discontinuous strike-slip faults, earthquakes of corresponding size and type will develop and occur according to the relevant stress fields and rupture mechanics.

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    REN Guang-xue, LI Chuan-you, SUN Kai
    SEISMOLOGY AND GEOLOGY    2022, 44 (1): 46-62.   DOI: 10.3969/j.issn.0253-4967.2022.01.004
    Abstract286)   HTML24)    PDF(pc) (12043KB)(192)       Save

    Karlik Tagh and other lesser ranges of the easternmost Tian Shan are natural laboratory for studying the fault architecture of an active termination zone of an intraplate mountain belt. The Karlik Tagh is located at the easternmost Tian Shan which is active due to the collision of India plate and Eurasian plate in Cenozoic and this range represents the geomorphological and structural end of Tian Shan. Therefore, studying the geometry and kinematics of active faults distributed at this area has important implications for understanding the dynamics features of the end porting of the Cenozoic orogenic belt. This paper is focused on the North Karlik Tagh Fault(KTNF), which is an important active structure at the easternmost Tian Shan. This fault extends about 180km and is gently distributed between the Yiwu Basin and the north of Karlik Tagh. Based on remote sensing and detailed field research, we propose to subdivide the NKTF into 2 segments based on its variation in strike and motion characteristics.
    At the west of the NKTF, the west segment is mainly distributed at south of Yanchi County and extends intermittently about 61km. The fault trace along Yanchi segment is obvious and expressed by several linear fault scarps on the foreland alluvial fan surfaces north of Karlik Tagh. Outcrop on a channel wall shows that the fault dips SW and thrusts directly to the NE. Topographic profiles across the scarps have shown that the minimum vertical offset is(1.3±0.5)m, which can be caused by a single earthquake rupture. The maximum vertical offset is(7.3±0.3)m. An OSL dating sample was obtained at 70cm below the T1 terrace surface. And we get the deposition age of(7.0±1.4)ka. Based on the OSL dating of deformed T1 terrace and the vertical displacement of(1.3±0.5)m of T1 and vertical displacement of(2.5±0.2)m of T2, a vertical slip rate of 0.19~0.35mm/a can be calculated. This vertical rate is slightly larger than that of the North Hami Basin Fault, which is consistent with the S-directed tilt of the Karlik Tagh.
    At south of Xiamaya town, the east segment of NKTF changes its strike and bends to NE, extending nearly 95km. Toward the east, this fault is connected with the west end of Gobi-Tianshan fault system(GTSFS)at the border of China and Mongolia. There are clear evidences of recent activity of this fault, including well-preserved scarps and offset streams on the alluvial sediments. And this fault segment is very obvious because of linear features on the Google Earth image. About 23km southeast of Xiamaya town, the fault trace runs across a north-flowing river, causing remarkable sinistral offset of the T3/T2 terrace ridge with the maximum displacement of(172±20)m. At about 10km northeast of this river, the NKTF passes through a massif with steep slope on the south and gentle slope on the north. Field observation of a hand-dug outcrop has shown that this fault dips N156°E. In addition, the fault also displays reverse faulting component and dislocates the gravel-bearing silt sedment by about 2.0m.
    At north of Karlik Tagh, several NW-trending faults can be interpreted on the satellite image. These faults extend short and form a clear boundary between bedrock and Quaternary sediments. Although there are no obvious deformations in the sediment such as diluvial fans or river terraces in the valley, the good linear characteristics on both sides of the valley indicate that these faults have been active since Quaternary. Because these faults are nearly parallel to the western segment of the northern margin of the Karlik Mountains, and there is no geomorphological evidence of horizontal movement of the faults, it can be inferred that the faults on both sides of the Adak Valley are mainly dominated by vertical movement.
    The Karlik Tagh North Fault, together with, the north margin faults of Hami Basin and other NW-trending secondary faults in the north side of Karlik Mountain constitute the horse-tail end structure of Gobi-Tianshan sinistral strike-slip fault system, which regulates and absorbs the sinistral deformation of Gobi-Tianshan fault system and these faults present a positive flower structure in the cross-section. The uplift of Karlik Tagh is controlled by NW thrust fault and NEE left-lateral strike-slip fault, and this range is a typical transpressional mountain in the easternmost Tian Shan.

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    ZHANG Lei, LIU Yao-wei, REN Hong-wei, GUO Li-shuang
    SEISMOLOGY AND GEOLOGY    2016, 38 (3): 721-731.   DOI: 10.3969/j.issn.0253-4967.2016.03.017
    Abstract481)      PDF(pc) (1714KB)(605)       Save

    Identifying the source of the observed fluid anomalies is a major tast in verifying the anomalies in seismic subsurface fluid research. The stable hydrogen and oxygen isotopes are proved to be effective to trace the underground fluid origin and its development. In this study, we summarized the basic principles, water sampling and testing techniques in recognizing the fluid anomalies by using the stable hydrogen and oxygen isotopes. We also enumerated the related applications in analyzing the sudden water level increase and the rapid shifting from limpid water to murky. The stable hydrogen and oxygen isotopes analysis can be used to verify the macroscopic underground fluid anomalies, such as subsurface water temperature, water level and chemical component changes, and the wide use of this method in seismic subsurface fluid research will be helpful to identify the tectonic or non-tectonic related influencing factors to the fluid anomalies.

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    ZHAO Qi-guang, SUN Ye-jun, HUANG Yun, YANG Wei-lin, GU Qin-ping, MENG Ke, YANG Hao
    SEISMOLOGY AND GEOLOGY    2021, 43 (3): 630-646.   DOI: 10.3969/j.issn.0253-4967.2021.03.010
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    The Gaoyou-Baoying MS4.9 earthquake on July 20, 2012 occurred in the Gaoyou Sag in the Subei Basin. This earthquake was a relatively rare medium-strength earthquake in the weak seismicity region of eastern China. Although studies on the seismogenic structure of this earthquake have been conducted previously, the seismogenic structure itself is still under debate and needs to be further studied. This paper uses the methods such as distribution of seismic intensity, precise positioning of earthquake sequence, focal mechanism, regional tectonic stress, seismic exploration, etc. to comprehensively study the seismogenic structure of this earthquake.
    The characteristics of earthquake sequence show that the seismic structure is a high dip-angle fault spreading along the NNE direction, dipping ESE. The result of focal mechanism solutions shows that the strike of one of the two nodal planes is NNE, and the fault plane shows high dip angle. The earthquake is mainly characterized by strike-slip motion. Through the seismic exploration lines(GYL1, GYL2)laid at the epicenter area of the earthquake, a fault structure is identified, which strikes nearly NNE and dips near ESE. This fault is located between the Linze sag and the Liubao low uplift, coinciding with the distribution of the Liuling Fault, the boundary fault in the northwest of the Gaoyou Sag, so it can be judged that all the detected breakpoints belong to the Liuling Fault. The “Y-shaped” breakpoints detected by the two seismic exploration lines are characterized by high dip angle. There is a very obvious wave group disorder area at the distance of 6 500~9 000m on the GYL1 seismic exploration line. This area is about 2.5km in width displayed on the post-stack migration profile and shows an uplifting trend. The disordered uplifting of wave group is caused by intrusion of soft material into the structural breakage and weakness, squeezed by horizontal stress. The GYL2 post-stack migration profile shows obvious uplift appearing in the reflection wave group(Tg)on the top of the bedrock. This arc-shaped uplift also reflects the effect of strong compression of horizontal stress.
    In order to further discuss the seismogenic structure of the Gaoyou-Baoying MS4.9 earthquake, we used the focal mechanism data to invert the modern tectonic stress field in the Northern Jiangsu-South Yellow Sea Basin where the earthquake occurred. The maximum principal stress in this area is NE-SW, while the minimum principal stress is NW-SE; both of them are nearly horizontal, and the intermediate principal stress is nearly vertical. According to Zoback's rule for dividing the types of dislocation in the direction of the force axis, the distribution of principal stresses in the Northern Jiangsu-South Yellow Sea Basin is equivalent to a strike-slip dislocation.
    To sum up, the stress characteristics reflected by the Liuling Fault are consistent with the horizontal forces on the P-axis and T-axis shown by the focal mechanism solution results, and also consistent with the horizontal state of the stress in the tectonic stress field in this region. The above characteristics indicate that the development of the Liuling Fault is affected and controlled by modern tectonic activities. At the same time, the characteristics of the strike and dip of the seismic fault reflected by the methods of seismic intensity investigation, precise earthquake positioning, focal mechanism solution and seismic exploration, etc. are consistent with each other. Therefore, the occurrence of this earthquake may be the result of continuous stress accumulation and sudden instability and rupture of the NNE-trending Liuling Fault under the long-term compression of the NE-direction principal stress.
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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
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    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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    YI Gui-xi, WEN Xue-ze, ZHANG Zhi-wei, LONG Feng, RUAN Xiang, DU Fang
    SEISMOLOGY AND GEOLOGY    2010, 32 (2): 282-293.   DOI: 10.3969/j.issn.0253-4967.2010.02.011
    Abstract1764)      PDF(pc) (4181KB)(2162)       Save
    Based on seismic data of the regional network of the last 34 years,we have analyzed current faulting behaviors of major fault zones in Mabian area,southern Sichuan,and preliminarily identified the risky fault-segments on which potential strong and large earthquakes may occur in future,with the method combining the spatial distribution of b-values with activity background of historical strong earthquakes and current seismicity.Our results mainly show:(1)The spatial distribution of b values displays significant heterogeneity in the study area,which reflects the spatial difference of cumulative stress level along various fault zones and segments in the area;(2)Three anomalously low b-value areas with different sizes exist on Mabian-Yanjin Fault zone,these anomalies can be identified as asperities under relatively high cumulated stress levels,in which,two asperities,located at north of Mabian county and Lidian town in western Muchuan county,and near Yanjin at the south end of the fault zone,respectively,may be the potential seismogenic sources of large earthquakes in Mabian area in the near future,and the third asperity with a small size located at southern Suijiang may be the potential strong-earthquake source;(3)An asperity at south-western segment of Longquanshan Fault may be the site of potential moderate to strong earthquakes;and(4)the asperity on the segment between Huangmu town in Hanyuan county and Longchi town in Emeishan city on Jinkouhe-Meigu Fault has potential for moderate to strong earthquakes.
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    XU Xi-wei, GUO Ting-ting, LIU Shao-zhuo, YU Gui-hua, CHEN Gui-hua, WU Xi-yan
    SEISMOLOGY AND GEOLOGY    2016, 38 (3): 477-502.   DOI: 10.3969/j.issn.0253-4967.2016.03.001
    Abstract783)      PDF(pc) (6533KB)(1389)       Save

    Living with disaster is an objective reality that human must face especially in China. A large number of earthquake case studies, such as the 2008 Wenchuan earthquake, 2010 Yushu earthquake, 2014 Ludian earthquake, have demonstrated that earthquake heavy damage and casualties stem from ground-faulting or rupturing along seismogenic active fault, near-fault high ground accelerations and building catastrophic structural failure. Accordingly, avoidance of active faults may be an important measure to effectively reduce earthquake hazard, which may encounter in the future, but how to avoid an active fault and how much a setback distance from the active fault is required to ensure that the ground faulting and rupturing has no any direct impact on buildings. This has been the focus of debate both for domestic and foreign scholars. This paper, first of all, introduces the definition of active fault. Then, quantitative analyses are done of the high localization of earthquake surface ruptures and relationship between the localized feature of the coseismic surface ruptures and building damages associated with the measured widths of the historical earthquake surface rupture zones, and an average sstatistic width is obtained to be 30m both for the earthquake surface rupture zones and heavy damage zones along the seismogenic fault. Besides, the widths of the surface rupture zones and spatial distribution of the building damages of the 1999 Chi-Chi earthquake and 2008 Wenchuan earthquake have also been analyzed to reveal a hanging-wall effect:Width of surface rupture zone or building damage zone on the hanging-wall is 2 or 3 times wider than that on its foot-wall for a dip-slip fault. Based on these latest knowledge learnt above, issues on avoidance object, minimum setback distance, location requirement of active fault for avoidance, and anti-faulting design for buildings in the surface rupture zone are further discussed. Finally, we call for national and local legislatures to accelerate the legislation for active fault survey and avoidance to normalize fault hazard zoning for general land-use planning and building construction. This preventive measure is significantly important to improve our capability of earthquake disaster reduction.

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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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    ZHAO Bo-ming, XU Xi-wei
    SEISMOLOGY AND GEOLOGY    2008, 30 (4): 839-854.  
    Abstract2992)      PDF(pc) (11489KB)(2666)       Save
    Complex spatial distribution of seismic motion near faults has always been a concern to scientists,and it still remains as an uncertain problem due to insufficiency of events and information.The paper presents the main cases of seismic disaster by field investigations of MS 8.0 Wenchuan earthquake,analyzes and discusses the relationship among earthquake fault,ground motion and earthquake disaster near the fault fractured zone,based on previous research of source rupture processes and source inversion of Wenchuan earthquake.Intensive deformation and ground surface rupture along the earthquake fault have caused obvious damage to buildings,so it is necessary to introduce the safety distance away from active fault and other measures.Possible reasons for buildings near surface rupture zone having withstood the strong earthquake are as follows:other than their performance of seismic resistance,firstly,most of them locate at hard sites or on bedrock in the surface rupture zone,and secondly,the effective stress drop and low rupture velocity may exist in the shallow asperities,resulting in a relatively lower ground motion at the period about 1 sec.
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    LI Cui-ping, TANG Mao-yun, GUO Wei-ying, HUANG Shi-yuan, WANG Xiao-long, GAO Jian
    SEISMOLOGY AND GEOLOGY    2019, 41 (3): 603-618.   DOI: 10.3969/j.issn.0253-4967.2019.03.005
    Abstract364)   HTML    PDF(pc) (4822KB)(335)       Save
    The Wulong MS5.0 earthquake on 23 November 2017, located in the Wolong sap between Wenfu, Furong and Mawu faults, is the biggest instrumentally recorded earthquake in the southeastern Chongqing. It occurred unexpectedly in a weak earthquake background with no knowledge of dramatically active faults. The complete earthquake sequences offered a significant source information example for focal mechanism solution, seismotectonics and seismogenic mechanism, which is helpful for the estimation of potential seismic sources and level of the future seismic risk in the region. In this study, we firstly calculated the focal mechanism solutions of the main shock using CAP waveform inversion method and then relocated the main shock and aftershocks by the method of double-difference algorithm. Secondly, we determined the seismogenic fault responsible for the MS5.0 Wulong earthquake based on these calculated results. Finally, we explored the seismogenic mechanism of the Wulong earthquake and future potential seismic risk level of the region.
    The results show the parameters of the focal mechanism solution, which are:strike24°, dip 16°, and rake -108° for the nodal plane Ⅰ, and strike223°, dip 75°, and rake -85° for the nodal plane Ⅱ. The calculations are supported by the results of different agencies and other methods. Additionally, the relocated results show that the Wulong MS5.0 earthquake sequence is within a rectangular strip with 4.7km in length and 2.4km in width, which is approximately consistent with the scales by empirical relationship of Wells and Coppersmith(1994). Most of the relocated aftershocks are distributed in the southwest of the mainshock. The NW-SE cross sections show that the predominant focal depth is 5~8km. The earthquake sequences suggest the occurrence features of the fault that dips northwest with dip angle of 63° by the least square method, which is largely consistent with nodal planeⅡof the focal mechanism solution. Coincidentally, the field outcrop survey results show that the Wenfu Fault is a normal fault striking southwest and dipping 60°~73° by previous studies. According to the above data, we infer that the Wenfu Fault is the seismogenic structure responsible for Wulong MS5.0 earthquake.
    We also propose two preliminary genetic mechanisms of "local stress adjustment" and "fluid activation effect". The "local stress adjustment" model is that several strong earthquakes in Sichuan, such as M8.0 Wenchuan earthquake, M7.0 Luzhou earthquake and M7.0 Jiuzhaigou earthquake, have changed the stress regime of the eastern margin of the Sichuan Basin by stress transference. Within the changed stress regime, a minor local stress adjustment has the possibility of making a notable earthquake event. In contract, the "fluid activation effect" model is mainly supported by the three evidences as follows:1)the maximum principle stress axial azimuth is against the regional stress field, which reflects NWW-SEE direction thrusting type; 2)the Wujiang River crosscuts the pre-existing Wenfu normal fault and offers the fluid source; and 3)fractures along the Wenfu Fault formed by karst dissolution offer the important fluid flow channels.
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