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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
    Abstract722)   HTML32)    PDF(pc) (11099KB)(439)       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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    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
    Abstract555)   HTML26)    PDF(pc) (16086KB)(229)       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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    ZHOU Bing-rui, PAN Bo, YUN Sung-hyo, CHANG Cheol-woo, YAN Li-li
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 831-844.   DOI: 10.3969/j.issn.0253-4967.2022.04.001
    Abstract473)   HTML39)    PDF(pc) (6476KB)(116)       Save

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

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

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

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    ZHAO Ling-qiang, HU Ya-xuan, WANG Qing-liang, ZHU Yi-qing, CAO Cong, LI Zhong-wei, QI Wei, WEN Yu-long
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 845-858.   DOI: 10.3969/j.issn.0253-4967.2022.04.002
    Abstract469)   HTML17)    PDF(pc) (6689KB)(104)       Save

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

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    LIU Zhong-yin, CHEN Xiao-bin, CAI Jun-tao, CUI Teng-fa, ZHAO Guo-ze, TANG Ji, OUYANG Biao
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 802-820.   DOI: 10.3969/j.issn.0253-4967.2022.03.015
    Abstract417)   HTML10)    PDF(pc) (9259KB)(72)       Save

    Magnetotelluric(MT)three-dimensional inversion has the advantages of simple data preprocessing, the model is close to actual situation, and the inversion result is more reliable and stable. It is one of the most advanced research topics and would take the place of the dominant two-dimensional inversion definitely. With the improvement of computing capability of computers and the breakthrough in inversion methods, great progress was made in MT three-dimensional inversion in recent years, from the theoretical research and test of this method at the beginning to the current application to practical data interpretation. For the great computation amount of MT three-dimensional inversion, current MT three-dimensional inversion algorithm programs are all implemented in parallel way and it is recommended to do three-dimensional inversion calculations on supercomputing system to make better use of computing resources and improve the inversion efficiency.

    Different from the MT three-dimensional inversion algorithm programs which have basically realized the utility function, the practical application of MT three-dimensional inversion is still in an early stage. Users should be familiar with the use of multiple software and fulfill the function manually with the help of the software as follows: generating the files required for the inversion program, connecting to the supercomputer to upload data, inputting the command to perform the inversion, etc. The process of manually connecting and operating calculations is the most primitive cloud computing. All processes need to be done manually, which would cause not only heavy workload and the complicated operation, but also the problems for the long-term effective preservation and management of complex inversion data.

    To conquer this, we develop independently a three-dimensional magnetotelluric inversion cloud computing system, toPeak, using Delphi language. This paper introduces some main features of toPeak. To begin with, system design and analysis are carried out in combination with the current situation and system structure and functions are realized. The main idea is to realize a set of cloud computing system platform based on server-client(C/S), on the basis of perfect inversion data management, integrate the most advanced three-dimensional magnetotelluric inversion algorithm program in the cloud, and connect through the Internet to realize all the system functions of three-dimensional magnetotelluric inversion. Then, the different parts of toPeak are introduced separately, including design structures and designs. The server is deployed on the supercomputer system(supercomputing)to receive the data for inversion tasks, configure and manage the storage of the inversion result data. Combined with the Internet connection, the server and the Internet together constitute a computing cloud. The client is deployed on the users’ windows operating system, including Windows visual data integration processing software and Internet operation middleware. The client is designed on the basis of object-oriented programming ideas, with data as the core, using data engineering objects to encapsulate and store all MT data, process and interpret the results, realize data processing inversion and other operations around this data project, and display the process and results of these processing and inversion in graphics using visualization technology. Internet operation middleware connects the client and server based on the SSH protocol to realize data processing and inversion, transmission and command sending and receiving. Furthermore, the whole work flow of inversion using toPeak and parts of procedure of it are shown. At last, some inversion results from toPeak are displayed. toPeak has realized the full functions require for implementing three-dimensional inversion and can grid, process and select, inverse and explain the data. It is a good tool for the practical use of three-dimensional inversion.

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    ZHANG Su-xiang, SHENG Shu-zhong, XI Biao, FANG Li-hua, LÜ Jian, WANG Gan-jiao, ZHANG Xiao
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1615-1633.   DOI: 10.3969/j.issn.0253-4967.2022.06.015
    Abstract407)   HTML9)    PDF(pc) (8791KB)(88)       Save

    With the continuous increasing density of the seismic network and the improvement of the seismograph observation capability, the number of observed seismic events has increased dramatically and the location accuracy has been continuously improved. Therefore, obtaining fault geometry and its parameters from massive seismic data has become an essential method for seismogenic structure research. At present, in the research of obtaining faults and their parameters based on seismic data, there are two main methods of selecting data: One is to select seismic data empirically based on the understanding of fault structures and the spatial distribution of seismic data, and then fit the fault plane from these data. However, it depends on prior information, i.e. the knowledge of existing fault structures and the linear distribution of earthquakes, and it is difficult to process relatively poor linear trends. The other is based on the spatial clustering of seismic data, which adopts unsupervised clustering technology in machine learning to select data. This method avoids the dependence on experience and is more suitable for fault segment data obtained from massive seismic data. Fault parameters can be inversed by fault segment data to determine the fault structure and give its quantitative parameters. However, the current clustering technique for obtaining fault parameters has some limitations, such as the selection of the optimal parameters being difficult, data with different densities being dealt with by the same parameters, and poor method generality. In order to automatically identify faults and obtain fault parameters based on the spatial distribution of earthquakes, and avoid the aforementioned limitations, a new method based on the improved DBSCAN algorithm is presented in this study.
    The method proposed in this study uses the k-average nearest neighbor method(K-ANN)and the mathematical expectation method to generate the candidate sets of eps and minPts threshold parameters, which are selected as optimal parameters based on the density hierarchy stability. Considering the spatial density differences of seismic events on different faults and the same fault, this study performs layer-by-layer density clustering from high density to low density. First, the above steps achieve the automatic selection of optimal parameters for clustering and identifying fault segments. Secondly, the fault parameters of the identified fault segments are calculated by the combination of the simulated annealing(SA)global search method and the local search method of Gaussian Newton(GN). Then, the adjacent similar fault segments are merged. Finally, the faults and their parameters are obtained.
    The reliability of the automatic fault identification method was verified by synthetic data and the double-difference location catalog of Tangshan area, China. The following results were obtained: Ⅰ. The improved DBSCAN algorithm can automatically identify the fault segments, which is verified by the application of synthetic data and the double-difference location data of the Tangshan area. Ⅱ. Based on the double-difference location data of the Tangshan area, eight fault segments were identified using the improved DBSCAN algorithm. The specific names of the 8 faults are as follows: Douhe fault segment, Weishan-Fengnan fault segment, Luanxian-Laoting fault segment, Lulong fault segment, Xujialou-Wangxizhuang fault segment, Luanxian fault north segment, Leizhuang fault segment, and Chenguantun fault segment, and their strike and dip angle are 229.1°, 230.4°, 132.2°, 31.7°, 191.3°, 31°, 229.5°, 84.9°, and 51.6°, 88.4°, 89.3°, 88.6°, 88.4°, 88.2°, 73.8° and 85.4°, respectively. The parameters of the first five faults are mostly consistent with those of previous research results. The last three faults are the newly identified faults in this study based on the seismic catalog, and the parameters of two of them have been confirmed by previous research results or focal mechanism parameters on the faults.
    In a word, the improved DBSCAN algorithm in this study can realize fault segment automatic identification, but there are still some problems that need to be improved urgently. In the follow-up research, we will continue to improve the automatic fault identification method and increase its ability of automatic fault identification so as to provide more accurate fault data for related research.

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    ZHAO Ling-qiang, ZHAN Yan, WANG Qing-liang, SUN Xiang-yu, HAN Jing, CAO Cong, ZHANG Song, CAI Yan
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 686-700.   DOI: 10.3969/j.issn.0253-4967.2022.03.008
    Abstract405)   HTML13)    PDF(pc) (4532KB)(92)       Save

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

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    ZHANG Xin, FAN Ye, YE Qing, QIAN Yin-ping
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 718-735.   DOI: 10.3969/j.issn.0253-4967.2022.03.010
    Abstract401)   HTML15)    PDF(pc) (6755KB)(48)       Save

    The grounding current of HVDC(high voltage direct current)converter station causes the most significant interference in the observation of geoelectric field, which usually causes a step change as 0.5~100mV/km within a range of hundreds of kilometers near the grounding pole. However, it is difficult to determine which converter station’s grounding current causes the step change. Taking Hainanzhou-Zhumadian line, Zhalute-Qingzhou line and Baoji-Deyang line as examples, we obtain the response data of three typical disturbances, and calculate the step change using the data of 58 geoelectric stations around these three lines. To compare the interference situation, we referred the extremely low frequency data of Dashan station for comparison with their original curves.

    First, we explained the different response modes of the stations at different locations to the ground current. This means that when the stations locate near a ground electrode, in the middle of the two poles and near to one side of the ground electrode between the two poles, the corresponding three response types are: step response, pulse response and pulse+half step response, respectively. In particular, stations located in the middle of two poles but close to one pole are mainly affected by the near pole and less importantly affected by the other pole. Consequently, step response appears in the near pole stations, step recovery appears in the far pole stations, and the final result is in the form of step+pulse.

    Then, we use daily variation amplitude to correct the order variable of HVDC interference and locate the position of grounding pole by the principle that the potential difference of multiple stations has directivity, and then determine the source of grounding current and the approximate location of converter station. The step change after diurnal change correction shows a certain trend, which is shown as the quadratic attenuation of the source-station distance. The fitting of the step change observed by a wide range of geoelectric stations confirms this trend. The locating results have good directive effect on the grounding poles’ positions of the Hainanzhou-Zhumadian line, Zhalute-Qingzhou line and Baoji-Deyang line, and by combining the step change synthesis vector of multiple stations, we can simultaneously determine the approximate location of the converter station. In addition, the amplitude of step change after daily variation correction can suggest the site of the ground electrode, which can supplement the locating results.

    Furthermore, we build the quantitative diffusion model of the grounding current to show the law of potential distribution of large input current, and determine the interference range and the variation trend. The simulation results show that the potential difference decreases rapidly within 50km near the grounding pole; the potential difference reduction effect is not strong in far-field exceeding 200km and basically maintains a gentle trend. Based on observation data of 58 geoelectrical stations and another station of extremely low frequency, the response characteristics of grounding current to the surrounding stations are identified, which may serve for the data correction of HVDC interference in the future.

    Results of the influence of grounding current on geoelectric and geomagnetic field can be further extended to the study of seismic electromagnetic signal. Electromagnetic stations are usually set up near the active fault zone in an attempt to detect electromagnetic signals generated by strong earthquakes. Relying on the observation data, researchers can present a preliminary prediction of strong earthquakes under certain conditions, and provide a spatial range and time scale of the earthquakes. However, the explanation of how the electromagnetic signal near the source propagates to the observation stations is not very satisfactory. In particular, there are anomalies appearing in some distant stations, while no anomalies appear in the nearby stations. It means the differential response is obvious. Moreover, some prediction is generally not logical and physical, which means the abnormal signal may not come from earthquake activity but some other sources. Therefore, it is necessary to study how the signal propagates from the source to the station and why it causes differential response.

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    LU Ben-tian, LI Zhi-gang, LIANG Hao, YANG Jing-jun, ZHENG Wen-jun
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 961-975.   DOI: 10.3969/j.issn.0253-4967.2022.04.009
    Abstract401)   HTML14)    PDF(pc) (7104KB)(116)       Save

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

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

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

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    PENG Fei, WANG Wei-jun, XIONG Ren-wei, LÜ Xiao-jian, YAN Kun, SUN Xin-zhe, GENG Shuang, KOU Hua-dong
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 561-577.   DOI: 10.3969/j.issn.0253-4967.2022.03.001
    Abstract394)   HTML22)    PDF(pc) (11673KB)(111)       Save

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

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

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    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
    Abstract377)   HTML14)    PDF(pc) (14700KB)(227)       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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    HAN Bing, TANG Ji, ZHAO Guo-ze, WANG Li-feng, DONG Ze-yi, FAN Ye, SUN Gui-cheng
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 753-770.   DOI: 10.3969/j.issn.0253-4967.2022.03.012
    Abstract373)   HTML11)    PDF(pc) (8002KB)(68)       Save

    With the support of the wireless electro-magnetic method(WEM)project, the control source extremely low frequency(CSELF)continuous observation network, which includes 30 electromagnetic stations in Beijing capital area(BCA)and the southern section of the North-South Seismic Belt in China, was built for recording the artificial and nature source singles. The natural source observation of the network was started in July 2013 and December 2013 in batches and the electromagnetic field was recorded continually with a sampling rate of 16Hz. Until now, the co-seismic electromagnetic signals have been recorded repeatedly in several stations. In this paper seven co-seismic electromagnetic signals recorded at Jinggu station and co-seismic electromagnetic signals associated with two strong earthquakes recorded at different stations surrounding the epicenter are studied.

    It is found that the variation of the EM filed is similar to the seismogram, and the amplitude of the co-seismic EM signal is much larger than the background signal generated by earth induction, and the intensity of the vertical magnetic field is about ten times as big as the horizontal electromagnetic field. For co-seismic EM signals recorded at the same station, the relationship between the amplitude of electromagnetic field and the magnitude of the earthquake is basically linear in logarithmic domain. Meanwhile, the amplitude of electromagnetic field is also affected by focal depth of the earthquake and distance between the stations and the epicenter. When the epicenter distance is close, the amplitude of the co-seismic signal caused by the earthquake with shallow focal depth is higher. When the focal depth is similar, the amplitude of electromagnetic co-seismic signal caused by the earthquake closer to the station is larger.

    For the co-seismic EM signals associated with a same earthquake recorded by different stations, the larger the epicenter distance is, the later the signal appears and the longer the duration is. However, the signal amplitude is not only affected by the epicenter distance, but also related to the near-surface medium at the observation point. The electromagnetic co-seismic signals observed at Dali station which is the farthest away from the epicenter of Jinggu earthquake show the characteristics of large amplitude, long duration, and low dominant frequency. This may be related to the electrical structure near the surface of Dali Platform. The electromagnetic field signals of the 5 components of Jinggu, Muding and Dali stations before and after the Jinggu earthquake of magnitude 5.9 were transformed by wavelet transform. Finally, the wavelet spectrum with the horizontal axis as time and the vertical axis as frequency was obtained to indicate the time-frequency changes of the abnormal electromagnetic signals of the same seismic wave. According to the wavelet analysis and combining with the time series before and after the Jinggu earthquake of MS5.9, the energy enhancement mainly occurs in the shear wave and surface wave periods, while the P-wave is not obvious in the wavelet energy spectrum due to its small amplitude, and only some weak enhancement with scattered frequency can be observed. The main frequency of electromagnetic co-seismic signal is between 1Hz and 2Hz. At the beginning of the co-seismic signal, there are high frequency components, and the high frequency gradually decreases with the increase of epicenter distance. Moreover, compared with electric field, magnetic field can record more abundant high-frequency information. This may have to do with different dominant mechanisms for electric and magnetic field generation.

    In this paper, several earthquakes recorded at Jinggu station and electromagnetic co-seismic phenomena caused by two strong earthquakes at Jinggu station are summarized and analyzed. The results show that the variation of co-seismic electromagnetic signal is very complicated, and its starting time, duration, amplitude, and frequency range have some rules, but some stations show their particularity under multiple seismic events, so it is difficult to discuss the mechanism of its generation. However, in terms of observation phenomena, the electromagnetic field variation data observed continuously by extremely low frequency stations give us a more comprehensive understanding of the Earth’s electromagnetic field itself and the electromagnetic signals related to earthquakes. The accumulation of more seismic-related electromagnetic phenomena and the support of theoretical simulation can deepen the understanding of electromagnetic field variation before, during and after the earthquake.

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    ZHANG Hao, WANG Jin-yan, XU Han-gang, LI Li-mei, JIANG Xin, ZHAO Qi-guang, GU Qin-ping
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1448-1468.   DOI: 10.3969/j.issn.0253-4967.2022.06.006
    Abstract364)   HTML15)    PDF(pc) (16789KB)(101)       Save

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

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    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
    Abstract354)   HTML11)    PDF(pc) (15760KB)(201)       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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    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
    Abstract352)   HTML10)    PDF(pc) (13890KB)(246)       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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    DENG Wen-ze, LIU Jie, YANG Zhi-gao, SUN Li, ZHANG Xue-mei
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 1059-1070.   DOI: 10.3969/j.issn.0253-4967.2022.04.015
    Abstract351)   HTML22)    PDF(pc) (7098KB)(115)       Save

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

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

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    YANG Yuan-yuan, LI Peng-fei, LU Shuo, SHU Peng, PAN Hao-bo, FANG Liang-hao, ZHENG Hai-gang, ZHAO Peng, ZHENG Ying-ping, YAO Da-quan
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1365-1383.   DOI: 10.3969/j.issn.0253-4967.2022.06.002
    Abstract350)   HTML21)    PDF(pc) (11724KB)(112)       Save

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

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    YANG Jing, CHEN Xiao-bin, ZHAO Guo-ze
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 771-785.   DOI: 10.3969/j.issn.0253-4967.2022.03.013
    Abstract348)   HTML6)    PDF(pc) (4743KB)(48)       Save

    The electromagnetic(EM)method using controlled-source extremely low-frequency(CSELF)waves is a new technology based on the large-power alternating electromagnetic field generated by an artificial procedure. The biggest advantage of this technology is that it has a long transmitting antenna(tens to hundreds of kilometers)and a large transmitting current(hundreds of amps)and can emit strong and stable electromagnetic waves, covering millions of square kilometers. It can be applied to earthquake monitoring, surveys for mineral resources and treatment of waste nuclear material as well as marine and land communication and detection to ionospheric structure in space. At present, domestic theoretical research on CSELF is not mature enough. This paper has carried out a more detailed study on the spatial propagation characteristics of the electromagnetic(EM)of controlled-source extremely low frequency(CSELF).

    The large-power CSELF EM waves cover almost all sections of space which can be divided into near, far and waveguide zones according to their propagation characteristics. The propagation of electromagnetic waves in the near and far zone is mainly manifested as the distribution and induction of the conductive currents, and the displacement current and effects of the ionosphere and spheric structure of the Earth can be neglected. The propagation theory of CSELF EM wave is similar to CSAMT in the near and far zones, and it can be described by the theory of quasi-stable field which is analogous to that of the classical theory of EM sounding. In this paper, we collated and verified the field strength calculation formulas in the existing literature. While in the waveguide zone, EM waves appear mainly as the displacement current, and the displacement current and effects of the ionosphere and spheric structure of the Earth must also be considered. The electromagnetic field is mainly the radiation field, and it runs in a way completely different from what the classic theory describes. Using the achievements of communication technology for reference, this paper presents the approximate calculation formula of CSELF EM wave of the earth-air-ionosphere spherical cavity model. Based on the field strength calculation formulas of the three regions obtained above, this paper has designed a piece of visualized software for calculation of the CSELF EM field in three coordinate systems(Cartesian, cylindrical and spherical coordinates). Finally, according to the calculation results, the spatial propagation characteristics of CSELF in the near area, far area and waveguide area are analyzed.

    The results show that the decay of CSELF EM field intensity is rapid in the near and far zone, but slightly slow in the far zone, which reflects the spatial distribution characteristics of the induced field in the lossy medium and the radiation field in the dielectric medium. The electric field enters the waveguide zone earlier than the magnetic field. Under the earth model, there is an increase in the field strength in the waveguide area near the antipole of the dipole source which shows completely different EM waves propagation characteristics in horizontal formation model. According to the calculation results of the CSELF EM field in near and far zones under the three coordinate systems, it is found that in the Cartesian coordinate system, the horizontal components have two zero lines and are distributed in four quadrants. While the vertical component field has only one zero line and are distributed in two half planes. In the cylindrical and spherical coordinate systems, all field components have merely one zero line and are characterized by half-plane distribution. The location of the zero line should be avoided as much as possible in the layout of field observation stations. We can choose different coordinate systems to solve this problem. In addition, it is also recognized that in the frequency domain EM sounding based on the horizontal electric dipole source, the far-field sounding mainly depends on the magnetic field rather than the electric field. Furthermore, it is recognized that in the frequency domain electromagnetic sounding method based on the horizontal electric dipole, the horizontal component of the electric field in the near zone is proportional to the resistivity of the medium, and has nothing to do with the frequency; the vertical component is proportional to the frequency and has nothing to do with the dielectric resistivity; the magnetic field has no relationship with the frequency and the dielectric conductivity. In the far zone, the horizontal component of the electric field is basically independent of frequency, and the vertical component of the electric field is related to both frequency and earth conductivity. However, due to the difficulty of observation, it is generally not used in the actual sounding. The three components of magnetic field in the far zone are all related to the frequency and the earth’s conductivity, so the far-field sounding mainly depends on the magnetic field rather than the electric field.

    Since CSELF antennas are generally very long(tens to hundreds of kilometers), the antenna can no longer be regarded as an electric dipole when measuring in the near and far zones, but should be regarded as a long wire source composed of multiple electric dipoles. In this paper, the electric dipole theory is still used for analysis, which has certain limitations that need to be overcome by further in-depth research.

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    WANG Lei, XU Hong-tai, WANG Zhi-cai, YANG Chuan-cheng, ZHANG Jian-min, WANG Dong-lei, XIA Nuan, CAI Ming-gang, LU Ren-qi, REN Zhi-kun
    SEISMOLOGY AND GEOLOGY    2022, 44 (5): 1156-1171.   DOI: 10.3969/j.issn.0253-4967.2022.05.005
    Abstract345)   HTML32)    PDF(pc) (10869KB)(104)       Save

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

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    XIE Tao, YU Chen, WANG Ya-li, LI Mei, WANG Zhong-ping, YAO Li, LU Jun
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 701-717.   DOI: 10.3969/j.issn.0253-4967.2022.03.009
    Abstract336)   HTML4)    PDF(pc) (4997KB)(71)       Save

    A MS6.6 earthquake occurred at the junction area of Minxian and Zhangxian, Gansu Province, on July 22, 2013. Before the earthquake, the apparent resistivity observed at Tongwei station showed abnormal anisotropic changes. Electrical resistivity is an important physical property for sedimentary rock-soil. The continuous load of compressive stress, by causing crack growth and directional alignment, would tend to increase the connectivity of these crack films. Build-up of strain at the locked fault segment and its vicinity area before an earthquake ought to be accompanied by change in resistivity. Laboratory measurements of resistivity on rock specimens under deformation to failure under uniaxial and triaxial compression show that resistivity of water-bearing rocks declines as the stress exceeds about half of the fracture stress. The decline rate increases considerably near the stage of final fracture. The magnitude of resistivity change in axial direction is usually greater than that in the transverse direction. In-situ experiments taken on field soil using Schlumberger arrays also showed decline change in apparent resistivity under compression stress loading. Monitoring arrays in different directions at the same set of array usually have different magnitudes of change, i.e. anisotropic changes. The array perpendicular to or near perpendicular to the P axis has the maximum magnitude of change, while the magnitude of change is the minimum or even unnoticeable when the array is parallel to or sub-parallel to the P axis.

    It can be expected from the above experiment results that absolute stress level is often needed to discuss the relationship between crack variation and stress. However, it is difficult to obtain successive absolute stress-strain measurement at present for a large tectonic region. On the other hand, the general quantitative mathematic relationship between the stress level and micro-crack activity is not clear. One alternative compromise way is to obtain the qualitative spatial distribution characteristic of the stress-strain accumulation required to produce the coseismic slip using the fault virtual dislocation model. In this paper, we use the fault virtual dislocation model to analyze the changes in the apparent resistivity data of Tongwei station before the earthquake. In the model, the coseismic sliding displacements of the earthquake are loaded in the same magnitude but opposite directions, in order to calculate the stress-strain distribution required to generate these coseismic dislocations before the earthquake. The areas of compression enhancement or relative expansion before an earthquake can be displayed. It should be noted that results from the virtual dislocation model are the changes of stress or deformation, not the absolute state of stress-strain. Northeast margin of Tibetan plateau is in compressive tectonics as a whole. The compression areas from the virtual dislocation model can be seen as areas with compression enhancement before the earthquake. However, for the extension areas from the model, we cannot distinguish them between true extension areas and compressive areas. They can be regarded as relative extension areas where the original tensile effect is strengthened or the original compressive effect is released to some extent.

    The results show that the Tongwei station is located at the compression stress and strain accumulation area before the occurrence of the earthquake, which coincides with the decreases of the apparent resistivity data. On the other hand, the focal mechanism solution shows that the azimuth of the principal compressive stress of this earthquake is 65°. The angle between the P axis and the N20°W direction of Tongwei station is 85°, and the angle from the EW direction is 25°. Before the earthquake, the decrease amplitude of the N22°W is 1.04%, and the decrease amplitude of the EW' is 0.37%. The anisotropic changes observed in the two directions are consistent with the results given by the experiment results, theoretical models and the summary of earthquake examples. Therefore, it can be considered that there may be a mechanical relationship between the changes in the apparent resistivity of the Tongwei station and the seismogenic process.

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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
    Abstract329)   HTML18)    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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    LI Qian, SONG Qian-jin, FENG Shao-ying, JI Ji-fa, DUAN Yong-hong, HE Yin-juan, QIN Jing-jing
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 1029-1045.   DOI: 10.3969/j.issn.0253-4967.2022.04.013
    Abstract328)   HTML12)    PDF(pc) (10314KB)(81)       Save

    The research area involved in this paper is the middle-southern segment of Liaocheng-Lankao fault zone(Lanliao fault zone)and its adjacent area. In order to study the fine crustal structure image and the tectonic features of the faults in this tectonic zone, we conducted a 70km-long deep seismic reflection profile along EW direction in Puyang City, Henan Province and got clearer lithospheric structure image along the profile.

    As regards data acquisition, we applied the geometry with 30m group interval, 1 160 recording channels and more than 90 folds. Seismic wave exploding applies the 30kg shots of dynamite source with the hole depth of 40~50m. In addition, in order to ensure the signal-to-noise ratio of the deep reflector, explosive quantity of dynamite source is increased to 96kg every 1 000m interval. In data processing, the most important thing is to improve the signal-to-noise ratio. Data processing methods mainly include one-dimensional time-varying filtering combined with two-dimensional filtering, tomographic static correction, residual static correction, deconvolution, normal moveout correction(NMO), dip moveout correction, common mid-point(CMP)stack and post-stack denoising, post-stack migration, etc.

    The section with high signal-to-noise ratio has been obtained. There are obvious characteristics of reflection wave groups in the crust, which reflects abundant information about geological structure. On this section, according to this study, the characteristics of deep and shallow structure and crustal reflection structures on both sides of the Lanliao fault zone are obviously different. The crust in this area is composed of brittle upper crust and ductile lower crust. There are rich reflective layers and clear tectonic framework in the upper crust. In the western area of Lanliao fault zone, there is a set of dense reflectors with strong energy, which reflects the sedimentary interface of different times since Mesozoic in the basin. The basement slope with gentle dip to the east is the bottom boundary of the “dustpan-shaped” sedimentary depression. The reflected wave of the crystalline basement presents a group of strong reflection wave groups with good continuity in the eastern area of Lanliao fault zone, which are parallel unconformities on the Ordovician strata of Paleozoic or older strata. There are some secondary faults in the hanging wall of Lanliao Fault, which together with the Lanliao fault zone control the tectonic framework of “dustpan-shaped” sedimentary depression, the Dongpu sag. The reflection structure of the lower crust is relatively simple. On the whole, it is mainly arc reflection with strong energy and short duration.

    The depth of Moho surface beneath the central-southern Lanliao fault zone in this area is 31.7~34.8km, where the fault is characterized by a strong reflection band with piecewise continuous distribution in horizontal direction and a duration of about 0.3~0.8s in vertical direction. And it is a transition zone with a certain thickness after geological deformation, rather than a sharp first-order discontinuity, which is consistent with the research results of Li Songlin et al.(2011). This profile reveals 2 deep faults(FD1 and FD2)that offset the Moho surface, extend down to the top of the upper mantle and create conditions for the upwelling of hot materials from asthenosphere and the energy exchange in this area. It may also be the cause of arc reflection in the lower crust.

    The deep seismic reflection profile shows that faults in the upper crust are well developed. Lanliao Fault is the largest boundary fault in this area, which controls the formation and evolution of the “dustpan-shaped” sedimentary depression and plays an important role in the filling of Paleogene strata in the sag. Pucheng Fault FP1 and Weixi Fault FP3 are developed in the hanging wall of Lanliao Fault, which are basement normal faults in the same direction as Lanliao Fault and control the structural framework of the depression. Pucheng Fault, Weixi Fault and Lanliao Fault constitute a domino fault system, which makes the basement of the depression incline to the SEE direction. In addition, a reverse secondary normal fault(Changyuan Fault FP2)is developed in the hanging wall of Lanliao Fault, which intersects with Weixi Fault FP3 at TWT 3.0s. These faults and Lanliao faults jointly control the basic structural pattern of the sedimentary sag.

    The deep and shallow tectonic framework in this area is controlled by the shallow faults in the upper crust and the deep faults in the lower crust. Deep faults(FD1 and FD2)create conditions for the upwelling of hot materials from asthenosphere, while shallow faults play an important role in the formation and evolution of basin structures.

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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
    Abstract322)   HTML23)    PDF(pc) (12117KB)(393)       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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    LI Zong-xu, HE Ri-zheng, JI Zhan-bo, LI Yu-lan, NIU Xiao
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 992-1010.   DOI: 10.3969/j.issn.0253-4967.2022.04.011
    Abstract321)   HTML14)    PDF(pc) (7913KB)(87)       Save

    The paper collects the seismic waveforms of the MS5.6 earthquake that occurred in southern Nima, central Tibe on July 24, 2009 recorded by Tibet seismic network and the mobile seismic networks of the orresponding period, i.e. Western Tibet/Y2 and TITAN. The seismic waveform data were preprocessed by rglitches, rmean, rtrend, taper, transfer and filtering. Then we hand-picked the arrival times of the P-and S-waves(0.05~2Hz for P wave, and 0.05~0.5Hz for S wave). The Hypo2000 method was applied to accurately relocate the earthquake.

    Because the earthquake occurred in the hinterland of Tibetan plateau, there are few local seismic stations available. Since the seismic stations and seismic phase information used in processing by different institutions are different, the epicenter location and focal mechanism determined by various institutions are different. Compared with the result(31.30°N, 86.10°E)relocated by Tibet seismic network, our result(31.08°N, 86.05°E)is more reliable due to the uniform distribution of stations used in our study, which is roughtly identical to the GCMT result(31.05°N, 86.10°E)inverted by the moment tensor method.

    Based on the relocated result, we apply the Cut-and-Paste(CAP)inversion method to invert the focal mechanism and focal depth. The waveform is decomposed into Pn1 and surface wave to perform cross-correlation fitting of theoretical waveform and actual waveform, respectively. To suppress the noise and influence of the source region medium, the bandpass filter is selected as 0.05~0.15Hz for body wave and 0.05~0.1Hz for surface wave. We set the earthquake source time function as 5s and search for the best focal depth at the depth of 1~30km, and the search step is 1km concerning the magnitude of the earthquake. The result shows that the earthquake has a best-fitting focal depth of 19.3km from the mean sea level and is of strike-slip faulting(the nodal plane Ⅰ: 220°/82°/-17° and nodal plane Ⅱ: 314°/73°/-171°).

    The shear stress and normal stress of the two nodal planes of the earthquake are calculated according to the stress field characteristics of the earthquake area. The generation of the earthquake is consistent with the stress field characteristics of NS compression and EW extension in the region. Referring to the near-EW strike-slip fault zone constrained by the EW-trending Wozang Fault and the NWW-trending Zhala Fault in the 1︰250000 regional geological survey map near the epicenter area, it is inferred that the earthquake is of EW-trending dextral strike-slip faulting.

    Most of the earthquakes that occurred along the 31°N belt near this earthquake area are EW-trending strike-slip ones, even in the interior of the Tangra-Yumco Rift. Considering the physical properties beneath Tibetan plateau, the low-velocity and high-conductivity layers are widely distributed in the depth range of 20km to 30km in the thick crust. According to surface geology and deep structures revealed by regional geophysics(receiver function, magnetotellurics, and tomography)of the region, the earthquake occurred on the top of the brittle-ductile transition zone with a low seismic velocity between the middle and upper crust beneath the south boundary faults of the Seng-ge Kambab-Lhaguo Tso-Yongzhu-Jiali ophiolite mélange zone(SYMZ), 30km away from the Tangra-Yumco Rift to the west. The occurrence of the earthquake indicates that SYMZ, which formed in the Late Jurassic, was reactivated in an EW-trending strike-slip manner during the quick uplift of the plateau. This cognition is of great significance to understand the geodynamic mechanisms of the EW-trending extension within the Tibetan plateau.

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    LI Meng-yuan, JIANG Hai-kun, SONG Jin, WANG Jin-hong
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 625-648.   DOI: 10.3969/j.issn.0253-4967.2022.03.005
    Abstract317)   HTML12)    PDF(pc) (7564KB)(62)       Save

    A significant seismic swarm occurred in Yigong, Bomi, Tibet, in July and August of 2020. 25 earthquakes with ML≥4.0 occurred during about 30 days and the magnitude of the maximum earthquake reached ML4.9(hereinafter referred to as the Bomi swarm). The proportion of large to small earthquakes in Bomi swarm is unbalanced, the number of earthquakes with larger magnitude is somewhat higher, and the proportionality coefficient, b value, of Gutenberg-Richter relationship is about 0.3, obviously smaller than the average b value of 1.0 of the whole seismic sequence. The seismicity of Bomi swarm has two dense stages, one is from July 19 to August 1 and another is from August 8 to 18, few earthquakes occurred between these two stages. For spatial distribution of earthquakes, the main areas of earthquake distribution in these two stages are almost overlapped. However, comparing with the previous stage, the southern boundary of the dense distribution of earthquakes in the latter stage has an extending trend to SE direction. The focal mechanism and the centroid depths of 20 earthquakes with ML≥4.0 have been calculated by CAP method. Results show that the centroid depths are shallow, most of them are distributed in the range of 3~4km. Viewing from the focal mechanism, taken July 27, 28 as the time boundary, the focal mechanisms before that time are mainly thrust with strike-slip component, the strike directions of nodal planes are inconsistent. After that time, the focal mechanism shows a good consistency with near EW-trending tensile rupture.

    The retroactive statistical results on historical earthquake catalogue have shown that earthquakes in Bomi region mostly occurred during July and August, indicating the obvious seasonal characteristics, and earthquakes mainly concentrated in a very small area(about 15km×20km)in space. The magnitude of maximum earthquake in each year is generally stable in the range of ML4.5~5.0, the annual average seismic energy release is roughly equivalent to one earthquake with ML4.9. It should be pointed out that swarms or significant earthquakes do not occur every year. During a total of 51 years from 1970 to 2020, significant swarms or earthquakes with ML≥4.0 occurred only in 18 years, accounting for about 35% of total time period.

    The correlation between seasonal meteorological factors and the seismicity in Bomi region is studied in this paper and the results show that there is a close but very complex relationship between them. Generally, the seismicity in Bomi region is closely related to the rainfall intensity and precipitation process in the first half of the year. The swarms mainly occurred during the periods with the peak precipitation, and generally followed the end of the first significant precipitation process in the year. The contrastive analysis shows that the strength of the seismicity is qualitatively proportional to the starting time of precipitation above designated scale, the days of precipitation above designated scale during the first half year, as well as the increasing rate of precipitation from April to June. Specificly, the earlier the starting time of precipitation above designated scale, the more the number of days with precipitation above designated scale in the first half of the year, the longer the time interval from the starting of the precipitation above designated scale to the seismicity, the higher the increasing rate of the monthly average precipitation from April to June, and the more the expected rainfall in June, the higher the seismicity level of this year will be.

    Bomi swarm is located to the north of Jiali fault zone and obviously off the Jiali fault zone. The seismicity in Bomi region is not the result of the fault activity of the Jiali fault zone, nor is related to the aftershock activity of Milin M6.9 earthquake in 2017, which occurred about 44km south of Jiali fault zone, since there is no obvious tectonic correlation among of them. Viewing from the geographical terrain, the seismicity in Bomi region mainly concentrated in the middle part of the NE-trending Lequ Zangbo River and its branches on both sides. Due to the lower terrain, it becomes an area for fast convergence of water from surrounding regions in the summer, which provides the basic conditions for fluid-triggered earthquakes in July and August every year. The lithology in the earthquake densely distributed area is mainly quartz sandstone and siltstone with relatively higher permeability, which is convenient for fluid penetration and leads to the pore pressure increasing in shallow crustal medium, thus, is liable to trigger seismicity. The local area with dense earthquake distribution in Bomi region is truncated and confined by several faults. The faults may act as a “water-retaining wall”, which has a certain confining effect on water infiltration and diffusion. On the other side, the faults, especially for normal faults, have better fluid conductivity, which is convenient for fluid infiltrating rapidly. Under the action of both the gravity and load pressure of the surface water, the fluid infiltrates rapidly along the fracture zone and the sandstone-like rock medium with good permeability, resulting in the rapid increase of the pore pressure in the underground cracks, faults and porous medium, therefore leading to the decrease of the strength for faults or cracks, and consequently triggering the seismicity. Considering the contribution of accumulated precipitation, groundwater level change, as well as warming and snowmelt to surface water level uplift in the first half of the year, the temporal variation of pore pressure at different depths are simulated by the numerical methods under the simplified conditions. The simulation results support the mechanism explanation on seismicity in Bomi region proposed in the paper.

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    SHI Zhi-wei, BAI Zhi-da, DONG Guo-chen, WANG Xu
    SEISMOLOGY AND GEOLOGY    2022, 44 (5): 1087-1106.   DOI: 10.3969/j.issn.0253-4967.2022.05.001
    Abstract312)   HTML43)    PDF(pc) (12038KB)(78)       Save

    Marine pillow basalts are widely developed, while large-scale continental pillow basalts are especially rare in China. The continental pillow basalt newly discovered in Chahar Right Back Banner, Inner Mongolia, is mainly tholeiite and a part of the Hannuoba basalt in Pliocene. In the accumulation sequence, from the bottom to the top, there are grayish-white calcareous mudstone of deep lacustrine, pillow basalt, stomatal basalt, and massive basalt. The pillow basalt has the thickness of about 10~12m and is mainly composed of black pillow body and yellow quenched clastics. The pillow bodies are preserved well and rare in China with complete structure. In detail, most of them are cylindrical, long ellipsoidal, of different sizes, about 0.8~1.5m long, and the largest pillow is about 2m long. Most of the cross sections are nearly circular, with a diameter of about 0.6m, up to 1m. The pillow bodies have obvious concentric layered structure, which can be divided into crust, middle layer and core. The degree of crystallization gradually becomes better from outside to inside. The crust is glassy, the middle layer is mesocrypt structure, and the core has relatively good crystallization, which is of intergranular-intersertal structure. Radial and discontinuous concentric ring fractures often occur in the pillow bodies, of which radial fractures are the most developed. The number of fractures varies from 10 to 20, with a width of 3~5mm. Most of them are filled with calcium and silica. In terms of composition, the pillow body is mainly olivine tholeiite, with porphyritic texture, stomatal-almond and massive structures. Phenocrysts are mainly plagioclase, clinopyroxene and olivine. Plagioclase is in the shape of self-shaped and plated strip, with the size of 1~3mm, the length-width ratio of 3︰1 to 5︰1, and the content of polysynthetic twin is 10%~15%; Clinopyroxene is short columnar, with a size of 0.6~1mm and a content of 5%~8%; Olivine is granular, with the size of 1~2mm and the content of 3%~5%. The matrix is composed of glass-based interlaced structure and intergranular-intersertal structure. It is mainly composed of microcrystalline plagioclase, pyroxene and glassy, accounting for 70%~85%. The basalt has SiO2 of 52.84% and(Na2O+K2O)of 5.46%, belonging to calc alkaline rock(Rittman index σ=3.0<3.3), with obvious fractionation of light and heavy rare earth elements(LREE/HREE=17.52, LaN/YbN=24)and weakly Eu negative anomaly(δEu=0.89), enriching large ion lithophile elements(Rb, Sr, Ba, etc.). The pillow bodies are mainly filled with calcareous cemented basaltic quenched clastics, including agglomerate, breccia, and tuff grades, mainly orange basaltic glassy quenched breccia. The determination of tuffaceous quenched clastics enriches the genetic types of volcanic ash. The cements are mainly calcareous and siliceous precipitated by hydrochemistry, a small amount of clay minerals and gypsum can be seen locally. The quenched breccia also contains some calcareous mudstone fragments. It shows that both marine and continental facies can form pillow basalt, and water is a necessary condition, but its formation is not related to water depth, and it is mainly controlled by the temperature and velocity of lava. When the temperature of underwater basaltic magma is between 1150℃ and 1000℃, pillow structure is easy to form, but it is difficult to form pillow lava below 1000℃, and relatively slow velocity is conducive to the formation of pillow body. Continental pillow basalts are usually distributed around craters, which belong to near-crater deposits. They are of definite significance of facies, which is of great practical significances for remodeling the morphology of continental volcanic edifice and studying the volcanic eruption process.

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    JIANG Cong, JIANG Chang-sheng, YIN Xin-xin, WANG Rui-jia, ZHAI Hong-yu, ZHANG Yan-bao, LAI Gui-juan, YIN Feng-ling
    SEISMOLOGY AND GEOLOGY    2022, 44 (5): 1333-1349.   DOI: 10.3969/j.issn.0253-4967.2022.05.015
    Abstract312)   HTML7)    PDF(pc) (1490KB)(64)       Save

    Induced earthquakes and the corresponding seismic risk are rising concerns for the smooth implementation of new industrial activities such as the exploitation of unconventional oil and gas resources and has attracted broad attention from both the public and academia. As a result, many associated scientific problems need to be further examined. The magnitude-frequency distribution(FMD)is fundamental to seismicity characterization, where systematic study of b values for induced earthquakes could reveal the regional accumulated stress, subsurface structural characteristics, as well as seismic risks of induced seismicity.
    In this study, we systematically reviewed the values, spatial-temporal heterogeneity, physical mechanism, dominating factors, as well as the application status of b values on hydraulic fracturing induced earthquakes in the past ten years. Multiple case analysis shows that the b value varies over a wide range(0.6~2.9)and exhibits large spatiotemporal heterogeneity. Felt earthquakes are often preceded by decreased b values and often occur in the regions with relatively low b backgrounds. In addition, fault activation or felt induced earthquakes are often accompanied by b values less than 1.0, despite that b>1.0 are commonly observed in the process of fracture expansion. Thus, the b value is promising for estimation of the state of faults(i.e., maturity and criticality).
    This study further assessed the factors that may affect b values, including objective factors such as in-situ stress field, fault geometry, fault maturity and focal depth, as well as subjective factors associated specific construction conditions, such as injection volume and injection rate. We then summarized multiple possible physical mechanisms, including the pore pressure, in-situ differential stress, maximum shear stress, and the non-uniformity of geological conditions. Although the discussed factors and physical mechanisms imply the multiple complexities associated with the b value that may challenge the effectiveness of its utilization, the b value remains a preliminary but effective evaluation for first-order estimation induced-earthquake hazard. For example, in the cases of deep hypocenter, high differential stress, high fault maturity, developed initial fracture network or bedding, or when the pore pressure, fault geometry and in-situ stress field meet the conditions of high probability fault slip tendency, the b value is often less than 1.0. In fact, due to the limited understanding of the seismic risk induced by hydraulic fracturing, b value still serves as a key parameter for the seismic risk analysis such as earthquake rate prediction and maximum magnitude prediction, as well as risk control technologies such as “Traffic Light System”(TLS), the b value has been widely used in hydraulic fracturing.
    Finally, we discussed the misunderstandings and challenges of b-value estimations for hydraulic fracturing induced earthquakes. For instance, b values calculated from different methods are less comparable and the quality of seismic catalogues, especially the reliability of magnitude measurement, also impact the accurate estimation and physical interpretation of b value. In addition, the mutation point of when the fault is about to reach its critical stage cannot be accurately identified through the temporal evolution of the b value alone. Even in cases where the mutation point is identified, the shut-down of current industrial operation does not guarantee the prevention of a subsequent felt event. Such challenges limit the effective utilizations of b values toward mitigating the seismic hazard associated with hydraulic fracturing induced earthquakes.
    After clarifying the consensus and controversial scientific issues, we speculate that the b value for induced earthquakes may serve as one preliminary criterion for the evaluation of reservoir reconstruction, the estimation of reservoir stress state and the mitigation of induced seismicity hazard. Our study summarized and evaluated the b-value characteristics for hydraulic fracturing induced earthquakes. The paper could serve as a scientific reference for the industrial, regulating and research communities that are interested in non-conventional energy exploration and/or seismic safety supervision.

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    FAN Ye, TANG Ji, MIAO Jie, YE Qing, CUI Teng-fa, DONG Ze-yi, HAN Bing, SUN Gui-cheng
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 669-685.   DOI: 10.3969/j.issn.0253-4967.2022.03.007
    Abstract309)   HTML10)    PDF(pc) (6006KB)(73)       Save

    On July 12, 2020, an MS5.1 occurred in Guye, Hebei Province, and as the largest earthquake in the capital circle in recent years, its unique geographical location has attracted more attention. During an earthquake, the electromagnetic properties of underground media will change, so dense electromagnetic observation stations were arranged in the capital circle. In this study, the data of geoelectric resistivity, geoelectric field, and extremely low frequency(ELF)observation within 400km of the Guye earthquake are analyzed using a combination of time-domain waveform analysis, sliding Fourier analysis with annual variation removed, normalized variation rate method(NVRM), and geo-electric azimuthal method. After eliminating the influencing factors such as operation status, observational environment, and the spatial electromagnetic effect, we analyzed the characteristics of electromagnetic phenomena that may be related to the Guye earthquake preliminarily and found that there was a variation process of “trend decrease—accelerated decrease—postseismic recovery” observed in 6 geoelectric resistivity stations and that the normalized variation rate exceeded the threshold value of ±2.4 in 7 stations within one year before the earthquake. In Luanxian station, the intensity of the geoelectric field in the north-south and north-western directions decreased and then rose back before the earthquake. In addition, the azimuth shifted to the direction of the Guye earthquake in the preseismic period, and then returned to the direction of the Luanxian-Laoting Fault. The ELF stations in Wen'an and Fengning precisely recorded the coseismic change of the 16Hz natural magnetic field, in which the variation of the vertical component is twice larger than that of the horizontal component. Under the condition of large subsurface structure difference beneath the stations, the observed electric values from the two stations are distinctively different; moreover, the coseismic disturbance is submerged by the background noise. The subsurface electric structure was obtained by interpolating and inversing the data collected from the ELF stations in the capital area, which indicates that the Guye earthquake occurs near the boundary of the electric property changes. Meanwhile, it shows high electric resistivity in the northern area, low electric resistivity in the southwestern area, and partially low electric resistivity in Baodi and Wen'an, which is consistent with the location of the abnormally stronger ground motion. Regarding the spatial selectivity of the anomalies, we believe it may be related to the direction of the two main conjugated structures in the capital area, which lie in NEE and NW direction, respectively. And the study also enlightens researchers that the investigation of the mechanism of seismic electromagnetic anomaly should start from the coseismic phenomenon, and then focus on the aspects of seismic signal source and propagation path, because the extremely low-frequency observation band is wide and the coseismic electromagnetic signals can be clearly recorded. There are many effective ways to extract electromagnetic signals related to earthquakes from strong interference background, such as making retrospective analysis of moderately strong earthquakes in time, summarizing the electromagnetic anomaly characteristics of different earthquake events, densifying the electromagnetic observation layout appropriately, so that the abnormal information can be mutually corroborated and a variety of means for fusion and comparative analysis can be developed.

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    WEI Yan-kun, CHEN Xiao-li
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 590-603.   DOI: 10.3969/j.issn.0253-4967.2022.03.003
    Abstract288)   HTML11)    PDF(pc) (6988KB)(94)       Save

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

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    ZHANG Yun-yun, WANG Pei-jie, CHEN Xiao-bin, ZHAN Yan, HAN Bing, WANG Li-feng, ZHAO Guo-ze
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 786-801.   DOI: 10.3969/j.issn.0253-4967.2022.03.014
    Abstract281)   HTML3)    PDF(pc) (6871KB)(53)       Save

    Magnetotelluric(MT)is a method of detecting electrical structures. The natural field source signal is weak, and there are many factors that affect the impedance estimation results, such as dead band, near-field interference, and random noise, so it is difficult to obtain accurate electromagnetic response in strong interference area. The stable and reliable impedance estimation is the premise for the follow-up inversion and interpretation. In order to suppress noise and improve the accuracy of impedance estimation, researchers have proposed various new data processing methods. However, these data processing methods are not widely used due to insufficient stability and poor applicability. The classic remote-reference method and robust estimation method are still the most widely used methods. This paper analyzes the characteristics of the strong interference data and the applicable scope of various data processing methods, combined with the processing effect of the measured magnetotelluric data in the strong interference area in eastern China, and summarizes a set of data processing strategies suitable for the strong interference area.

    The remote-reference method can effectively suppress coherent noise. It is essential in data processing in strong interference areas. Usually, the results will be improved after processing by remote reference. The remote-reference site should be selected at a place far enough away from the measuring point without interference.

    Robust estimation can highlight high-coherence signals and suppress low-coherence signals. In the dead band, the coherence of the natural field signal is higher than that of the background noise signal, so the robust estimation processing can improve the data processing result of the dead band. The intensity and coherence of the long-lasting near-field interference signal is higher than that of the natural-field signal. The robust estimation process will treat the near-field interference as the desired signal and suppress the natural source signal. Therefore, data containing long-term strong near-field interference is not suitable for using robust estimation but non-robust estimation. For data that is not well processed by the two methods, we can try a combination of the two. By carefully selecting the power spectrum obtained by the two methods, it is possible to improve the processing result.

    Increasing the number of data segments can provide more sets of power spectra for selection, and also increase the probability of obtaining higher quality power spectra. Through careful selection of multiple power spectra, it is more likely to obtain better processing results than when the number of segments is smaller.

    During the day when there is a lot of human activity, the interference signal is strong. And at night, the interference signal is weak. The measured data well proves this point, so we should extend the acquisition time at night as much as possible, and the data processing should also focus on the night data.

    In general, it is more likely to obtain better data with longer acquisition time. Research on synthetic data shows that the maximum valid period of magnetotelluric theoretical data is 1/8 of the data duration. The measured data results of Fengning Station also support this conclusion. The longer the data acquisition time is, the more effective power spectra can be obtained, and the more likely it is to select a better quality spectrum from them, and obtain a stable impedance estimation result. Therefore, the data collection time should be adjusted reasonably according to the interference situation during the observation to ensure the stability of the impedance estimation result.

    Magnetotelluric data processing methods are not invariable, and different data processing methods should be adopted according to the actual situation. When the better data processing method is not yet mature, flexible application of existing method is the necessary means for magnetotelluric data processing.

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    YANG Wen-jian, ZHAO Bo, YU Hong-mei, XU Jian-dong, PAN Bo, WANG Xi-jiao
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 859-875.   DOI: 10.3969/j.issn.0253-4967.2022.04.003
    Abstract277)   HTML12)    PDF(pc) (11876KB)(86)       Save

    As one of the largest Quaternary volcanic clusters in China, the volcanic activities of Qiongbei are characterized by multi-stage and multi-cycle. However, the eruption era of Eman volcanic rocks located in the northwest of Qiongbei volcanic cluster is still controversial. In this paper, we present a comprehensive study of volcanic geology and geomorphology, whole-rock major elements, K-Ar geochronology of volcanic rocks and 14C geochronology of conch, in order to reveal the epoch of volcanic activity and eruption characteristics of Eman volcanic field. According to the field geological survey, it is found that Eman volcanic field has many craters, such as Bijialing, Chunliling, Bingmajiao, Longmenjilang, Longmen Pharos, and Zhangwu. The main types of eruptions are effusive eruption, phreatomagmatic explosive eruption and weakly magmatic explosive eruption. Lava flows almost cover the entire volcanic field, with an area of about 26.3km2, which are mainly formed by the eruptions of Bijialing and Chunliling volcanoes in the central-south of the volcanic field. Among them, Bijialing volcano consists of five volcanoes, with steep-slope cones and grayish-black block lavas. Chunliling volcano is located in the east of Bijialing, with gentle slope cone, few lava outcrops and spherical weathering. However, the distribution of base-surge deposits, spattering deposits and scoria is relatively small, and limited to the vicinity of Longmenjilang to Wucaiwan and Zhangwu Village. They were formed by phreatomagmatic explosive eruptions of Bingmajiao and Zhangwu volcano, as well as weakly magmatic explosive eruptions of Longmenjilang and Longmen Pharos volcanoes. Moreover, compared with the Holocene Shishan and Late Pleistocene Daotang basalts in Haikou, Eman volcanic rocks have a wider range of silicon(SiO2=51.39%~55.00%)and alkali(K2O+Na2O=3.51%~8.48%)content. Nevertheless, they are general intermediates, mainly composed of basaltic andesite, basaltic trachyandesite and trachyandesite, and experienced fractional crystallization of olivine and clinopyroxene in the process of magmatic evolution. Combining the weathering degree of volcanic rocks(spherical weathering, red clay layer), volcanic geology and geomorphology(cone morphology, slope), petrology and petrogeochemistry(difference of major elements, olivine phenocryst alteration), K-Ar age of volcanic rocks(0.12~0.44Ma)and 14C age of conch((43.27±0.67)kaBP), we conclude that the eruption era in Eman volcanic field belongs to the Middle and Late Pleistocene.

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    CHI Hai-jiang, WEN Jia
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 821-830.   DOI: 10.3969/j.issn.0253-4967.2022.03.016
    Abstract268)   HTML4)    PDF(pc) (2125KB)(31)       Save

    The extremely low frequency(ELF)ground exploration project is one of the major national science and technology infrastructure construction projects approved by the National Development and Reform Commission in the Eleventh Five-year Plan. It uses the new artificial source extremely low frequency electromagnetic technology(CSELF)to carry out resource exploration, earthquake prediction and other cutting-edge scientific research. After several years of active preparation, 30 extremely low frequency seismic electromagnetic monitoring stations have been preliminarily built in the capital circle and the north-south seismic belt in Sichuan-Yunnan region to jointly record MT data from natural field sources and CSMT data from artificial sources. The latest generation of ADU-07e magnetotelluric observation system produced by the company Metronix of Germany is selected for observation. Its output data format, transmission mode and analysis method are quite different from the precursory instruments commonly used in seismic stations in the Tenth Five-year Plan. This paper analyzes the problems that have occurred since the installation of the instrument at Huailai seismic station. According to the actual situation and experience, practical software is developed, which has significantly improved the observation method.

    The main problems include: 1)The observers cannot grasp the operation status of the instrument in time; 2)The amount of data generated by observation is large and there are a large number of files; 3)The instrument and server are highly secure and professional, which is difficult for station personnel to master; 4)Log filling and submission are cumbersome and error prone.

    The development of practical monitoring software, which can monitor the observation status and observation data in real time on site or remotely, has important practical value for the long-term operation and maintenance of the station. The software functions mainly include: quasi real-time inspection of the working conditions of observation instruments, fault alarm, inspection, transmission and generation of preprocessed EDI files in the server, automatic filling and submission of logs, etc. This paper introduces the parameter configuration, function application and application method of each module in detail.

    The paper introduces the key programming statements such as control, dynamic library, macro language command, reading meteorological instrument data and alarm service, which realize software functions by VB language.

    The practical application effect of the software in Huailai seismic station is introduced, and it is found that the observation data quality and operation effect of the station is improved and the expected design purpose is achieved with the software.

    Extremely low frequency observation is a transformation from magnetotelluric sounding to long-term continuous monitoring to meet the need for earthquake prediction. It is a transformation from short-term manned observation to long-term unattended observation, which is a test of instrument performance and maintenance. Using software, the complicated instrument inspection is changed into “one key” threshold judgment, which improves the anomaly processing ability and response speed of the instrument. The file naming, data supplementary transmission, preprocessing, log submission and other operations under Linux system are turned into shortcut buttons under windows, which are standardized and convenient, suitable for the working conditions and popularizing and application in the stations.

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    ZHANG Wei-heng, CHEN Jie, LI Tao, DI Ning, YAO Yuan
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1351-1364.   DOI: 10.3969/j.issn.0253-4967.2022.06.001
    Abstract266)   HTML42)    PDF(pc) (7558KB)(128)       Save

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

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    GONG Meng, LÜ Jian, ZHENG Yong, XIE Zu-jun, SHENG Shu-zhong, ZHANG Xing-mian
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 1011-1028.   DOI: 10.3969/j.issn.0253-4967.2022.04.012
    Abstract262)   HTML25)    PDF(pc) (8737KB)(88)       Save

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

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

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

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    HAO Hong-tao, WANG Qing-hua, ZHANG Xin-lin, WEI Jin, WU Gui-ju, HU Min-zhang
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 876-894.   DOI: 10.3969/j.issn.0253-4967.2022.04.004
    Abstract262)   HTML23)    PDF(pc) (4630KB)(70)       Save

    The long-term variation of gravity field can provide an important reference for studying the regional dynamic background. In 1980s, the Western Yunnan Earthquake Prediction Study Area was established by the State Seismological Bureau for the purpose of monitoring and predicting earthquakes. Since 1984, gravity monitoring in the Western Yunnan Earthquake Prediction Study Area has been carried out continuously by Yunnan Earthquake Agency and Institute of Seismology of China Earthquake Administration. In this research, long-term gravity variations in the Western Yunnan Earthquake Prediction Study Area are obtained by using 62 campaigns relative gravity data and absolute gravity data of Xiaguan station, Lijiang station and Eryuan station from 1986 to 2014 in this area. On this basis, we first analyze the relationship between gravity variation and tectonic activity background in combination with fault distribution and historical seismicity. Then, the mechanism of gravity variation is discussed combined with the vertical crustal deformation, crustal structure and dynamic background. The main results are as follows.

    1)After a fine processing of the gravity data, gravity variation rates of 87 gravity stations are obtained, among which 77 stations show negative change. This indicates that the long-term gravity variation background in the study area is dominated by negative changes. The annual average rate of 87 gravity stations is about -1.24×10-8m/s2, which is consistent with the mean gravity variation rate in the Qinghai-Tibet Plateau and its adjacent areas by absolute gravity observation from the existing research result.

    2)In terms of spatial distribution, the intensity of gravity variation is closely related to the distribution of fault zones and historical strong earthquakes. In the area along the north section of Honghe Fault and Longpan-Qiaohou Fault, gravity variation shows strongly negative anomalies, and the frequency of historical strong earthquake activity is the highest. This indicates that the gravity variation clearly reflects the strong activity background of the fault zone on the west boundary of the central Yunnan secondary block. In the east area of Chenghai Fault, the spatial distribution of gravity variation is symmetrically positive and negative in a four-quadrant pattern. In 2003, the Dayao MS6.2 and MS6.1 earthquakes occurred in the central area of the four-quadrant, and their coseismic gravity variations caused by strike-slip dislocation are consistent with observed four-quadrant characteristics. Therefore, the observed gravity variations reflect the shear stress background in this region. While in the southwest of Yunnan, which is located in the west of the Honghe Fault, the magnitude of gravity variations and frequency of seismic activity decreased significantly in comparison with that of the Honghe fault zone and its eastern region. This indicates that the Honghe fault zone, which is the boundary of Sichuan and Yunnan rhombus block, has an obvious boundary control effect on the gravity field variations and tectonic activity in the study area.

    3)Vertical displacement velocities of GNSS stations in the study area are collected, and then the vertical displacement gravity effect and observed gravity variations of 13 GNSS stations are analyzed. The result shows that ground surface vertical movement is dominated by uplift, and the gravity effect of surface vertical displacement is basically consistent with the observed gravity variation in the direction. This indicates that observed gravity variation reflects the uplift background of crustal vertical movement in this area. The magnitude of the average observed gravity variation is about 0.784×10-8m·s-2/a after removing the denudation-caused gravity change rate, while the average gravity effect of vertical displacement is about -0.252×10-8m·s-2/a, accounting for just about 30% of the observed gravity variation. Therefore, observed gravity variation cannot be explained completely by vertical displacement, and mass loss in the study area is also an important factor causing gravity variation.

    4)The Western Yunnan Earthquake Prediction Study Area is located in the middle and lower crustal flow channel in the Qinghai-Tibet Plateau. The overall negative trend of gravity variation may be caused by Moho surface subsidence and surface uplift induced by the crustal flow. The spatial distribution details of gravity variation have a good correlation with the density distribution of the middle and upper crust. Therefore, we speculated that spatial distribution details of gravity variation are caused by activity of specific fault and local material distribution changes.

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    LIAO Gui-jin, YE Dong-hua, DENG Zhi-hui, LI Chong, TANG Guo-ying, HU Wei-ming
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 895-908.   DOI: 10.3969/j.issn.0253-4967.2022.04.005
    Abstract260)   HTML7)    PDF(pc) (7347KB)(66)       Save

    Gravity anomalies occurred at Lufeng, Bomei, Nantang, Jiazi, Xixi and other measuring points in the Guangdong Shantou mobile gravity measurement network before and after the Lufeng ML4.3 earthquake on September 24th, 2015, and the area was named as Lufeng gravity anomaly area, where the accumulated change of gravity in two years was greater than three times of the RMS errors of the observations and medium-term decline and reverse turning rise appeared. Meanwhile, gravity anomalies also appeared in the measurement points of Chendian and Heping, so the area was named as Chendian gravity anomaly area, where the gravity showed continuous monotonic increase. The two gravity anomaly areas were adjacent to each other, but the nature of the gravity anomalies might be different, there might be seismic gravity anomalies and ground subsidence gravity anomalies. In order to analyze the development trend of later earthquakes, it is necessary to determine the nature of gravity anomaly. The method of data analysis and field verification was used to distinguish the nature of gravity changes in each gravity anomaly area. The results mainly show that: 1)the Lufeng gravity anomaly area is of seismic gravity anomaly, while the Chendian gravity anomaly area is of ground subsidence gravity anomaly. Through the characteristic analysis of seismic gravity anomalies and ground subsidence gravity anomalies, we had a better understanding of the correlation between seismic gravity field evolution and seismic development, which helped to extract the precursor information of seismic gravity changes and predict earthquake. 2)The gravity changes at the observation points of Lufeng, Bomei, Nantang, Jiazi and Xixi in Lufengthe gravity anomalies area began to decline synchronously in August 2014 and rose synchronously in August 2015, forming a positive gravity anomaly area with regional synchronous decline and reverse turning rise. Through anomaly investigation and verification, no interference source to the observation environment was found, and the Lufeng gravity anomaly area was of seismic gravity anomalies. It was the manifestation of the evolution process of the seismic gravity field during the preparation of the Lufeng ML4.3 earthquake on September 24, 2015. 3)The gravity changes at Heping and Chendian observation points in Chendian gravity anomalies showed a continuous monotonic increase resulting from the pumping of a large amount of groundwater in Chendian and Heping, which led to ground subsidence, house and ground cracking, so the gravity anomaly was related to land subsidence. It is determined that the nature of the anomaly was land subsidence gravity anomaly. The gravity change caused by ground subsidence at Chendian from March 1995 to July 2016 was 292μgal, and the gravity change caused by ground subsidence at Heping from March 2006 to July 2016 was 137μgal. In the analysis of seismic gravity anomalies, the gravity changes at Heping and Chendian should deduct the gravity changes caused by ground subsidence. 4)For obvious gravity anomalies, the background conditions of the anomalies should be understood in detail, such as geological conditions, the use of domestic water and industrial water, ground subsidence, ground fissures, house fissures, etc., and the source of the anomaly should be found. It is necessary to collect water level observation data and hydrological observation data in gravity anomaly area for trend analysis. 5)According to the comparative analysis of the characteristics of time series gravity variation curve of the gravity anomaly points in the gravity anomaly area, the gravity anomalies of the observation points in Chendian gravity anomaly area showed a long-term continuous monotonic increase, while that in the Lufeng gravity anomaly area showed a medium-term decline and reverse turning rise. By analyzing the gravity variation of the adjacent observation points in the abnormal area, and analyzing the difference of the geological conditions and the field survey data, we can basically judge the nature of the gravity anomalies.

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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
    Abstract257)   HTML10)    PDF(pc) (15635KB)(187)       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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    LIN Xu, LIU Hai-jin, LIU-ZENG Jing, WU Zhong-hai, LI Zhao-ning, CHEN Ji-xin, LI Ling-ling, HU Cheng-wei
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 944-960.   DOI: 10.3969/j.issn.0253-4967.2022.04.008
    Abstract253)   HTML11)    PDF(pc) (5562KB)(50)       Save

    The collision of the Indian plate with Eurasia in the early Cenozoic era drove the emergence of the Tibetan plateau. At the same time, the subduction of the western Pacific plate towards Eurasia resulted in the stretching and thinning of the lithosphere in eastern Asia, leading to a series of faulted basins and marginal seas. The macro-geomorphic pattern of East Asia was finally established under the control of these two tectonic domains. In this case, the Yellow River, which originated from the Tibetan plateau and flowed through the Loess Plateau and the North China Plain, carried a huge amount of detrital material into the Bohai Sea, which played an important role in the regional geochemical cycle, environmental change, sedimentary flux and the diffusion of detrital material in the shelf sea. Therefore, tracing sediment sources in the Yellow River Basin is of great importance for understanding the coupling relationship between uplift and denudation in the northeastern Tibetan plateau, East Asian monsoon evolution, and detrital material accumulation. However, the Yellow River Basin spans multiple climatic and tectonic zones with different provenance areas, so it is particularly critical to select appropriate provenance tracing methods.

    Although K-feldspar is more vulnerable to chemical weathering than zircon, it is a widely distributed rock forming mineral and can best represent the provenance characteristics of a certain area. The non-clay minerals in the Yellow River Basin are mainly composed of quartz and feldspar. At the same time, the Pb isotope ratios(206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb)of K-feldspar in different blocks are much different from those of Nd and Sr isotope systems and are often used to construct regional Pb isotope geochemistry province, continental crust evolution, and reconstruct paleocurrent direction, etc. In recent years, detrital K-feldspar Pb isotopic composition has been successfully used to trace the provenance of the Indus, Yangtze and Mississippi Rivers. But this method has not been carried out in the Yellow River Basin. Therefore, we systematically analyzed the detailed K-feldspar Pb isotopic compositions from the Yellow River Basin, and compared the results with the potential source areas to determine the specific source areas. It also can provide basic comparative data for future studies on the formation age of the Yellow River and material source areas of the Loess Plateau and deserts in the northwestern China.

    We analyzed 15 samples from the Yellow River Basin and obtained 967 in-situ Pb isotopic results of K-feldspar grains by laser erosion inductively coupled plasma mass spectrometer(LA-MC-ICP-MS). K-feldspar grains in the samples from the Yellow River are angular, subangular and subcircular, with diameters ranging from 20μm to 300μm. The 206Pb/204Pb and 208Pb/204Pb ratios of K-feldspar grains from the source of the Yellow River to Lanzhou city range from 20 to 16 and 42 to 36. However, some ratios of 206Pb/204Pb and 208Pb/204Pb of K-feldspar grains from the Lanzhou city range from 23 to 19 and 40 to 37, respectively. The 206Pb/204Pb ratio of most K-feldspar samples in Bayannur city is greater than 19, and the maximum value is 24.79, while this ratio from Hequ and Hancheng cities located in the middle reaches of the Yellow River is less than 18.5. The 206Pb/204Pb ratios of the Mesozoic sandstone near the Hequ city range from 16 to 15. The 206Pb/204Pb and 208Pb/204Pb ratios of K-feldspar grains from the Weihe River, which is the largest tributary of the Yellow River, range from 19 to 17 and 40 to 37. The 206Pb/204Pb and 208Pb/204Pb ratios of K-feldspar grains in the Fenhe River, Yiluohe River, Kaifeng and Lijin cities range from 21 to 14 and 42 to 33. The comparison results of 206Pb/204Pb and 208Pb/204Pb ratios show that the Pb isotopic compositions of K-feldspar grains in the upper Yellow River, Daxiahe River and Huangshui River are significantly different from those in the Lanzhou city. The Pb isotopic composition of K-feldspar grains from the Yellow River from the Lanzhou city is consistent with that in the Bayannur city, which is influenced by similar eolian provenance. K-feldspar grains from the Yellow River and Fen River in the Jinshan Gorge are mainly from the Loess Plateau. By contrast, the K-feldspar grains in the Weihe River are mainly derived from the Qinling Mountains. The Pb isotopic compositions of K-feldspar grains in the Kaifeng and Lijin cities of the lower Yellow River are different to those in the upper Yellow River and the North China Plate, but similar to those in the middle reaches of the Yellow River. The Loess Plateau plays a leading role in the source of K-feldspar gains in the middle and lower reaches of the Yellow River.

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    ZHANG Guo-qing, ZHU Yi-qing, LIANG Wei-feng
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 578-589.   DOI: 10.3969/j.issn.0253-4967.2022.03.002
    Abstract250)   HTML12)    PDF(pc) (4016KB)(96)       Save

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

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

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

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

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    LI Zhao, FU Bi-hong
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1421-1447.   DOI: 10.3969/j.issn.0253-4967.2022.06.005
    Abstract246)   HTML11)    PDF(pc) (28982KB)(108)       Save

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

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