Most Read articles

    Published in last 1 year |  In last 2 years |  In last 3 years |  All

    In last 2 years
    Please wait a minute...
    For Selected: Toggle Thumbnails
    NEW PROGRESS IN PALEOEARTHQUAKE STUDIES OF THE JIANGSU SEGMENT OF THE ANQIU-JUXIAN FAULT IN THE TANLU FAULT ZONE
    ZHANG Hao, LI Li-mei, JIANG Xin, ZHANG Dong, XU Han-gang
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 880-895.   DOI: 10.3969/j.issn.0253-4967.2023.04.005
    Abstract954)   HTML21)    PDF(pc) (17177KB)(173)       Save

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

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

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

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

    Table and Figures | Reference | Related Articles | Metrics
    TYPICAL CASE ANALYSIS ON SETBACK DISTANCE FOR URBAN BURIED ACTIVE FAULT: AN EXAMPLE SITE ALONG THE TANLU FAULT ZONE IN XINYI CITY
    CAO Jun, LI Yan-bao, RAN Yong-kang, XU Xi-wei, MA Dong-wei, ZHANG Zhi-qiang
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 1071-1085.   DOI: 10.3969/j.issn.0253-4967.2022.04.016
    Abstract943)   HTML72)    PDF(pc) (11099KB)(513)       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.

    Table and Figures | Reference | Related Articles | Metrics
    THE 2022 M6.8 LUDING EARTHQUAKE: A COMPLICATED EVENT BY FAULTING OF THE MOXI SEGMENT OF THE XIANSHUIHE FAULT ZONE
    LI Chuan-you, SUN Kai, MA Jun, LI Jun-jie, LIANG Ming-jian, FANG Li-hua
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1648-1666.   DOI: 10.3969/j.issn.0253-4967.2022.06.017
    Abstract777)   HTML74)    PDF(pc) (16086KB)(338)       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.

    Table and Figures | Reference | Related Articles | Metrics
    THE CHARACTERISTICS OF MAJOR FAULTS AND STRESS FIELD IN WEIHE-YUNCHENG BASIN CONSTRAINED BY SEISMIC ACTIVITY AND FOCAL MECHANISM SOLUTIONS
    YU Zhan-yang, SHEN Xu-zhang, LIANG Hao, ZHENG Wen-jun, LIU Xu-zhou
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 395-413.   DOI: 10.3969/j.issn.0253-4967.2022.02.008
    Abstract769)   HTML17)    PDF(pc) (8440KB)(167)       Save

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

    Table and Figures | Reference | Related Articles | Metrics
    GEOLOGICAL DISASTERS AND SURFACE RUPTURES OF JANUARY 23, 2024 MS7.1 WUSHI EARTHQUAKE, XINJIANG, CHINA
    ZHANG Bo-xuan, QIAN Li, LI Tao, CHEN Jie, XU Jian-hong, YAO Yuan, FANG Li-hua, XIE Chao, CHEN Jian-bo, LIU Guan-shen, HU Zong-kai, YANG Wen-xin, ZHANG Jun-long, PANG Wei
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 220-234.   DOI: 10.3969/j.issn.0253-4967.2024.01.013
    Abstract726)   HTML9)    PDF(pc) (14676KB)(494)       Save

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

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

    Table and Figures | Reference | Related Articles | Metrics
    COSEISMIC DISPLACEMENT MEASUREMENT AND DISTRIBUTED DEFORMATION CHARACTERIZATION: A CASE OF 2021 MW7.4 MADOI EARTHQUAKE
    SHAO Yan-xiu, LIU-ZENG Jing, GAO Yun-peng, WANG Wen-xin, YAO Wen-qian, HAN Long-fei, LIU Zhi-jun, ZOU Xiao-bo, WANG Yan, LI Yun-shuai, LIU Lu
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 506-523.   DOI: 10.3969/j.issn.0253-4967.2022.02.014
    Abstract662)   HTML26)    PDF(pc) (7392KB)(173)       Save

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

    Table and Figures | Reference | Related Articles | Metrics
    NEW UNDERSTANDING OF THE MAGMA EVOLUTION OF CHANGBAISHAN-TIANCHI VOLCANO BASED ON MELTS SIMULATION
    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
    Abstract623)   HTML84)    PDF(pc) (6476KB)(164)       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.

    Table and Figures | Reference | Related Articles | Metrics
    THE CHARACTERISTICS OF DEEP ELECTRICAL STRUCTURE IN LONGGANG VOLCANIC AREA, JILIN PROVINCE
    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
    Abstract592)   HTML40)    PDF(pc) (6689KB)(126)       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.

    Table and Figures | Reference | Related Articles | Metrics
    THE SEISMOGENIC STRUCTURE OF THE 1303 HONGTONG M8 EARTHQUAKE INFERRED FROM MAGNETOTELLURIC IMAGING
    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
    Abstract590)   HTML32)    PDF(pc) (4532KB)(119)       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.

    Table and Figures | Reference | Related Articles | Metrics
    DETECTION OF SHALLOW SEDIMENTARY STRUCTURE IN SIYANG, JIANGSU PROVINCE BY MICROTREMOR H/V SPECTRAL RATIO METHOD
    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
    Abstract573)   HTML48)    PDF(pc) (11673KB)(137)       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.

    Table and Figures | Reference | Related Articles | Metrics
    PRESENT DEFORMATION OF~90° INTERSECTING CONJUGATE FAULTS AND MECHANICAL IMPLICATION TO REGIONAL TECTONICS: A CASE STUDY OF 2019 MW≥6.4 PHILIPPINES EARTHQUAKE SEQUENCE
    WANG Yu-qing, FENG Wan-peng, ZHANG Pei-zhen
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 313-332.   DOI: 10.3969/j.issn.0253-4967.2022.02.003
    Abstract571)   HTML23)    PDF(pc) (7645KB)(147)       Save

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

    Table and Figures | Reference | Related Articles | Metrics
    LATE QUATERNARY ACTIVE TECTONICS OF THE NORTH ALTYN FAULT
    YE Yu-hui, WU Lei, WANG Yi-ping, LOU Qian-qian, CHEN Li-qi, GAO Shi-bao, LIN Xiu-bin, CHENG Xiao-gan, CHEN Han-lin
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 297-312.   DOI: 10.3969/j.issn.0253-4967.2022.02.002
    Abstract542)   HTML38)    PDF(pc) (7598KB)(241)       Save

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

    Table and Figures | Reference | Related Articles | Metrics
    GEOMETRIC STRUCTURE CHARACTERISTICS OF XINYI SEGMENT OF ANQIU-JUXIAN FAULT
    ZHANG Hao, WANG Jin-yan, XU Han-gang, LI Li-mei, JIANG Xin, ZHAO Qi-guang, GU Qin-ping
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1448-1468.   DOI: 10.3969/j.issn.0253-4967.2022.06.006
    Abstract540)   HTML37)    PDF(pc) (16789KB)(182)       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.

    Table and Figures | Reference | Related Articles | Metrics
    THE DESIGN AND APPLICATION OF TOPEAK: A THREE-DIMENSIONAL MAGNETOTELLURIC INVERSION CLOUD COMPUTING SYSTEM
    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
    Abstract528)   HTML24)    PDF(pc) (9259KB)(123)       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.

    Table and Figures | Reference | Related Articles | Metrics
    GEOLOGICAL CHARACTERISTICS AND ERUPTION HAZARDS TYPES OF BINGMAJIAO: A COASTAL VOLCANO IN EMAN, HAINAN
    ZHAO Yong-wei, LI Ni, CHEN Zheng-quan, WANG Li-zhu, FENG Jing-jing, ZHAO Bo
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 281-296.   DOI: 10.3969/j.issn.0253-4967.2022.02.001
    Abstract524)   HTML32)    PDF(pc) (12803KB)(282)       Save

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

    Table and Figures | Reference | Related Articles | Metrics
    RESEARCH ON SHALLOW STRUCTURAL CHARACTERISTICS IN THE BANQUAN SEGMENT OF ANQIU-JUXIAN FAULT ZONE BASED ON SHALLOW SEISMIC REFLECTION PROFILING
    QIN Jing-jing, LIU Bao-jin, WANG Zhi-cai, FENG Shao-ying, DENG Xiao-juan, HUA Xin-sheng, LI Qian
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 349-362.   DOI: 10.3969/j.issn.0253-4967.2022.02.005
    Abstract514)   HTML20)    PDF(pc) (3676KB)(217)       Save

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

    Table and Figures | Reference | Related Articles | Metrics
    AUTOMATIC FAULT IDENTIFICATION METHOD BASED ON IMPROVED DBSCAN ALGORITHM AND ITS APPLICATION TO TANGSHAN AREA
    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
    Abstract511)   HTML26)    PDF(pc) (8791KB)(111)       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.

    Table and Figures | Reference | Related Articles | Metrics
    THE RESPONSE OF FLUVIAL LANDFORM TO THE EVOLU-TION OF FAULT STRUCTURE IN THE NORTHERN ZHONGTIAO MOUNTAINS FAULT
    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
    Abstract503)   HTML36)    PDF(pc) (7104KB)(157)       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.

    Table and Figures | Reference | Related Articles | Metrics
    CHARACTERISTICS OF THE CRUSTAL STRESS FIELD AND ITS DIRECTION CONVERGENCE BEFORE THE WENCHUAN EARTHQUAKE
    WANG Xiao-shan, WAN Yong-ge
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 363-377.   DOI: 10.3969/j.issn.0253-4967.2022.02.006
    Abstract501)   HTML19)    PDF(pc) (6374KB)(137)       Save

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

    Table and Figures | Reference | Related Articles | Metrics
    PRELIMINARY STUDY ON FAULTED LANDFORMS AND AGES OF RECENT STRONG EARTHQUAKE ACTIVITY ON THE KARAKORUM FAULT IN NGARI, TIBET
    XU Wei, LIU Zhi-cheng, WANG Ji, GAO Zhan-wu, YIN Jin-hui
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 925-943.   DOI: 10.3969/j.issn.0253-4967.2022.04.007
    Abstract500)   HTML30)    PDF(pc) (14700KB)(269)       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.

    Table and Figures | Reference | Related Articles | Metrics
    QUATERNARY TECTONIC FEATURES OF THE FUMAYING HIDDEN AREA OF THE ANQIU-JUXIAN FAULT
    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
    Abstract497)   HTML38)    PDF(pc) (10869KB)(147)       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.

    Table and Figures | Reference | Related Articles | Metrics
    THE INFLUENCE OF HVDC TRANSMISSION ON GEOELECTRIC FIELD AND LOCATING THE GROUNDING POLES
    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
    Abstract493)   HTML33)    PDF(pc) (6755KB)(69)       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.

    Table and Figures | Reference | Related Articles | Metrics
    PALEOEARTHQUAKES AND VERTICAL SLIP RATES ON THE HUAI RIVER-NÜSHAN LAKE SEGMENT OF FAULT F5 IN THE MIDDLE SECTION OF THE TANLU FAULT ZONE
    YANG Yuan-yuan, LI Peng-fei, LU Shuo, SHU Peng, PAN Hao-bo, FANG Liang-hao, ZHENG Hai-gang, ZHAO Peng, ZHENG Ying-ping, YAO Da-quan
    SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1365-1383.   DOI: 10.3969/j.issn.0253-4967.2022.06.002
    Abstract492)   HTML50)    PDF(pc) (11724KB)(171)       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.

    Table and Figures | Reference | Related Articles | Metrics
    PRELIMINARY ANALYSIS FOR RUPTURE PROCESS OF THE MAY 22TH, 2021, MADOI(QINGHAI) MS7.4 EARTHQUAKE
    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
    Abstract467)   HTML46)    PDF(pc) (7098KB)(142)       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.

    Table and Figures | Reference | Related Articles | Metrics
    CHARACTERISTICS AND PROCESSING OF MAGNETOTELLURIC DATA UNDER STRONG ELECTROMAGNETIC INTERFERENCE ENVIRONMENT
    HAN Jing, ZHAN Yan, SUN Xiang-yu, ZHAO Guo-ze, LIU Xue-hua, BAO YU-xin, SUN Jian-bao, PENG Yuan-qian
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 736-752.   DOI: 10.3969/j.issn.0253-4967.2022.03.011
    Abstract457)   HTML26)    PDF(pc) (15760KB)(250)       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.

    Table and Figures | Reference | Related Articles | Metrics
    DISCUSSION ON COSEISMIC SURFACE RUPTURE LENGTH OF THE 2021 MW7.4 MADOI EARTHQUAKE, QINGHAI, CHINA
    YAO Wen-qian, WANG Zi-jun, LIU-ZENG Jing, LIU Xiao-li, HAN Long-fei, SHAO Yan-xiu, WANG Wen-xin, XU Jing, QIN Ke-xin, GAO Yun-peng, WANG Yan, LI Jin-yang, ZENG Xian-yang
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 541-559.   DOI: 10.3969/j.issn.0253-4967.2022.02.016
    Abstract447)   HTML15)    PDF(pc) (13089KB)(288)       Save

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

    Table and Figures | Reference | Related Articles | Metrics
    ANALYSIS OF ELECTROMAGNETIC CO-SEISMIC PHENOMENA OBSERVED IN CSELF STATIONS
    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
    Abstract439)   HTML25)    PDF(pc) (8002KB)(93)       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.

    Table and Figures | Reference | Related Articles | Metrics
    STUDY ON THE LATE QUATERNARY ACTIVITY OF THE WEST XIADIAN FAULT IN BEIJING PLAIN
    SHEN Jun, DAI Xun-ye, XIAO Chun, JIAO Xuan-kai, BAI Qilegeer, DENG Mei, LIU Ze-zhong, XIA Fang-hua, LIU Yu, LIU Ming
    SEISMOLOGY AND GEOLOGY    2022, 44 (4): 909-924.   DOI: 10.3969/j.issn.0253-4967.2022.04.006
    Abstract435)   HTML57)    PDF(pc) (12117KB)(462)       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.

    Table and Figures | Reference | Related Articles | Metrics
    CRUSTAL VELOCITY STRUCTURE BENEATH THE SOUTHERN LIAONING PROVINCE DERIVED FROM DOUBLE DIFFERENCE TOMOGRAPHY
    WANG Liang, JIAO Ming-ruo, QIAN Rui, ZHANG Bo, YANG Shi-chao, SHAO Yuan-yuan
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 378-394.   DOI: 10.3969/j.issn.0253-4967.2022.02.007
    Abstract434)   HTML16)    PDF(pc) (14665KB)(400)       Save

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

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

    Table and Figures | Reference | Related Articles | Metrics
    DEEP STRUCTURES OF THE MIDDLE-SOUTHERN SEGMENT OF LANLIAO FAULT ZONE REVEALED BY DEEP SEISMIC REFLECTION PROFILE
    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
    Abstract432)   HTML26)    PDF(pc) (10314KB)(133)       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.

    Table and Figures | Reference | Related Articles | Metrics
    DISCOVERY AND SIGNIFICANCE OF CONTINENTAL PILLOW BASALT IN CHAHAR RIGHT BACK BANNER, CENTRAL INNER MONGOLIA
    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
    Abstract425)   HTML95)    PDF(pc) (12038KB)(104)       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.

    Table and Figures | Reference | Related Articles | Metrics
    PROBING THE SUBSURFACE ELECTRIC STRUCTURE FOR CSELF NETWORK IN CAPITAL CIRCLE REGION
    DONG Ze-yi, TANG Ji, ZHAO Guo-ze, CHEN Xiao-bin, CUI Teng-fa, HAN Bing, JIANG Feng, WANG Li-feng
    SEISMOLOGY AND GEOLOGY    2022, 44 (3): 649-668.   DOI: 10.3969/j.issn.0253-4967.2022.03.006
    Abstract424)   HTML25)    PDF(pc) (13890KB)(276)       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.

    Table and Figures | Reference | Related Articles | Metrics
    RAPID EXTRACTION OF FEATURES AND INDOOR RECON-STRUCTION OF 3D STRUCTURES OF MADOI MW7.4 EARTHQUAKE SURFACE RUPTURES BASED ON PHOTOGRAMMETRY METHOD
    WANG Wen-xin, SHAO Yan-xiu, YAO Wen-qian, LIU-ZENG Jing, HAN Long-fei, LIU Xiao-li, GAO Yun-peng, WANG Zi-jun, QIN Ke-xin, TU Hong-wei
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 524-540.   DOI: 10.3969/j.issn.0253-4967.2022.02.015
    Abstract421)   HTML13)    PDF(pc) (8145KB)(134)       Save

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

    Table and Figures | Reference | Related Articles | Metrics
    THE FOCAL MECHANISM AND TECTONIC SIGNIFICANCE OF THE MS5.6 EARTHQUAKE ON JULY 24, 2009 IN NIMA, TIBET
    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
    Abstract417)   HTML30)    PDF(pc) (7913KB)(116)       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.

    Table and Figures | Reference | Related Articles | Metrics
    A CENTENNIAL PUZZLE OF THE EVOLUTION OF THE YANGTZE RIVER: RETROSPECTION AND PROGRESSES
    GUO Ru-jun, WEI Chuan-yi, LI Chang-an, ZHANG Yu-fen, LI Ya-wei, SUN Xi-lin, ZHANG Zeng-jie, LENG Yong-hui, SU Jian-chao, LI Guo-nai, LÜ Ling-yun, CHEN Xu, DING Zhi-qiang
    SEISMOLOGY AND GEOLOGY    2023, 45 (1): 1-28.   DOI: 10.3969/j.issn.0253-4967.2023.01.001
    Abstract414)   HTML61)    PDF(pc) (9173KB)(237)       Save

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

    Table and Figures | Reference | Related Articles | Metrics
    APPARENT RESITIVITY VARIATION OF TONGWEI SEISMIC STATION BEFORE THE MINXIAN-ZHANGXIAN MS6.6 EARTHQUAKE IN 2013
    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
    Abstract413)   HTML11)    PDF(pc) (4997KB)(92)       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.

    Table and Figures | Reference | Related Articles | Metrics
    CALCULATION OF SPATIAL DISTRIBUTION OF CSELF ELECTROMAGNETIC FIELD
    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
    Abstract413)   HTML14)    PDF(pc) (4743KB)(69)       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.

    Table and Figures | Reference | Related Articles | Metrics
    SPATIOTEMPORAL HETEROGENEITY AND APPLICATION OF b VALUES IN HYDRAULIC FRACTURING INDUCED SEISMICITY
    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
    Abstract410)   HTML9)    PDF(pc) (1490KB)(108)       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.

    Table and Figures | Reference | Related Articles | Metrics
    GEOCHEMICAL CHARACTERISTICS OF SOIL GAS IN ACTIVE FAULT ZONE IN NORTHWEST YUNNAN AND ITS ENLIGHTENMENT TO FAULT ACTIVITY
    WANG Bo, ZHOU Yong-sheng, ZHONG Jun, HU Xiao-jing, ZHANG Xiang, ZHOU Qing-yun, LI Xu-mao
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 428-447.   DOI: 10.3969/j.issn.0253-4967.2022.02.010
    Abstract399)   HTML13)    PDF(pc) (6573KB)(169)       Save

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

    Table and Figures | Reference | Related Articles | Metrics
    DETAILED MAPPING OF THE SURFACE RUPTURE NEAR THE EPICENTER SEGMENT OF THE 2021 MADOI MW7.4 EARTHQUAKE AND DISCUSSION ON DISTRIBUTED RUPTURE IN THE STEP-OVER
    HAN Long-fei, LIU-ZENG Jing, YAO Wen-qian, WANG Wen-xin, LIU Xiao-li, GAO Yun-peng, SHAO Yan-xiu, LI Jin-yang
    SEISMOLOGY AND GEOLOGY    2022, 44 (2): 484-505.   DOI: 10.3969/j.issn.0253-4967.2022.02.013
    Abstract398)   HTML16)    PDF(pc) (12092KB)(294)       Save

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

    Table and Figures | Reference | Related Articles | Metrics