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    ZHANG Hao, LI Li-mei, JIANG Xin, ZHANG Dong, XU Han-gang
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 880-895.   DOI: 10.3969/j.issn.0253-4967.2023.04.005
    Abstract860)   HTML19)    PDF(pc) (17177KB)(154)       Save

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

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

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

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

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

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

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

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    JIANG Feng-yun, JI Ling-yun, ZHU Liang-yu, LIU Chuan-jin
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 377-400.   DOI: 10.3969/j.issn.0253-4967.2023.02.005
    Abstract304)   HTML21)    PDF(pc) (14678KB)(168)       Save

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

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

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

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

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

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

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

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    ZHAO De-zheng, QU Chun-yan, ZHANG Gui-fang, GONG Wen-yu, SHAN Xin-jian, ZHU Chuan-hua, ZHANG Guo-hong, SONG Xiao-gang
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 570-592.   DOI: 10.3969/j.issn.0253-4967.2023.02.016
    Abstract267)   HTML22)    PDF(pc) (7303KB)(203)       Save

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

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

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    LIU Qing, LIU Shao, ZHANG Shi-min
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 321-337.   DOI: 10.3969/j.issn.0253-4967.2023.02.002
    Abstract241)   HTML44)    PDF(pc) (15181KB)(230)       Save

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

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

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

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    LI Xiao-ni, YANG Chen-yi, LI Gao-yang, FENG Xi-jie, HUANG Yin-di, LI Chen-xia, LI Miao, PEI Gen-di, WANG Wan-he
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 484-499.   DOI: 10.3969/j.issn.0253-4967.2023.02.011
    Abstract240)   HTML11)    PDF(pc) (8781KB)(123)       Save

    The Weinan Tableland Piedmont fault is an important near-EW-trending Holocene active fault in the southeastern margin of the Weihe Basin, which is closely related to the occurrence of the 1556 Huaxian M8 earthquake. The northern branch of the fault, the northern branch fault in front of the Weinan tableland, passes through the urban area of Weinan. Therefore, finding out the distribution, shallow structure, late Quaternary activity, and seismic capacities of the northern branch fault are of great significance for local earthquake prevention and reduction. The Weihua fault zone, which is composed of F1 and F2 faults, generally strikes near east-west and has a gentle wave shape on the plane. It is a group of active normal faults rising in the south and descending in the north belt one. The Wei-Hua fault zone can be divided into two segments, east and west, and according to its spatial location and geometric distribution, strike change and the difference in geology and landforms on both sides. The eastern section is distributed in front of Huashan Mountain and is called Huashan Piedmont Fault(F2); the western section is distributed in Piedmont of Weinan tableland and is called Weinan Piedmont Fault(F1). There is a large sub-parallel branch fault about 2km to the north of the Piedmont Weinan tableland fault(F1)in the west section, which is called the branch fault on the north side of the Piedmont Weinan tableland. It is also the boundary fault between the Weinan tabland and the Gushi Sag. The Weinan tableland Piedmont Fault(F1)starts from the Weinan Xihekou in the west and extends eastwards through the Fenghe River to Mayukou, Huaxian County, with a length of about 54km; it strikes NWW from the Mayukou to Chishui River, and nearly EW from the Chishui River to the Fenghe River, the west of the Minhe River is NE to NEE, and it is mostly distributed in the form of broken lines or oblique rows. The fault plane dips northward with a dip angle of 60°~70°. The latest activity of the fault is manifested in the latest terraces and alluvial-pluvial fans faulting the Holocene strata, river valleys, and gullies; along the main fault, and a series of stepped normal faults on the north and south sides, a Holocene steep ridge belt with a width of between tens of meters and hundreds of meters, the Holocene strata are vertically faulted by 6~7m, and the vertical slip rate since the Late Pleistocene is about 0.29mm/a. In this paper, the shallow location and structural characteristics of the branch fault on the north side of the front of the Weinan tableland are determined through the combined profile detection of shallow seismic exploration and drilling, and evidence of the new activity of the fault is provided. The shallow seismic exploration results of the four survey lines all reveal the existence of a branch fault on the northern side of the front of the Weinan tableland, as well as the distribution location and cross-sectional structural characteristics of the fault new understanding. The results show that the branch fault on the north side of the Weinan Tableland Piedmont fault is a parallel branch of the main fault in front of the Weinan tabland. The branch fault on the north side of the front of the Weinan tableland is located at the front edge of the second-level terrace of the Weihe River in front of the Weinan tableland. The south end of the road, the mouth of the river, Zhangbaozi, and the outside of the north gate, have a length of at least 22km. The main section of the fault is inclined to the north, with a dip angle of about 70°~80° and a break distance of 6~20m at the upper breaking point, so it is a normal fault. Mainly concealed active faults, which have at least faulted the strata from the Middle Pleistocene to the late Pleistocene in the upward direction. In the four seismic sections, it appears as a normal fault zone with a width of 200~1 800m, including the main and secondary normal faults. Stepped structures and small grabens; secondary faults also fault up at least the Late Pleistocene strata. The combined geological profile of the Chongye Road borehole revealed that the main fault on the north side of the Weinan tableland had been faulted with many landmark strata of the Late Quaternary, and the latest fault occurred after 19ka; the average vertical activity rate since the middle of the Late Pleistocene between 0.07~0.26mm/a. Combined with phenomena such as fault ridges developed along the surface of the fault, it is judged that the fault was active in the Holocene. The branch fault on the north side of the front of the Weinan tableland has had strong activity since the late Quaternary, which means that the fault, as one of the branches of the southeastern boundary zone of the Weihe fault basin-the Weihua fault zone-obviously bears part of the deformation of the belt At the same time, the fault is located in the historically strong earthquake-prone area of the southeastern boundary of the Weihe fault basin, and it cannot be ruled out that it once participated in the rupture of the 1556 Huaxian M8 earthquake. Considering that the branch fault on the north side of the Weinan tableland passes through the urban area of Weinan, its potential seismic hazard and hazard are urgent research topics.

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

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

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    YANG Chen-yi, LI Xiao-ni, FENG Xi-jie, HUANG Yin-di, PEI Gen-di
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 464-483.   DOI: 10.3969/j.issn.0253-4967.2023.02.010
    Abstract227)   HTML11)    PDF(pc) (7081KB)(121)       Save

    The northern Qinling fault zone is an important active structure in the southern margin of the Weihe Graben Basin, containing many branch faults, of which the near EW striking Taochuan-Huxian Fault is located on the northern side of the fault zone, and the eastern segment is buried in the Weihe Graben Basin. Shallow seismic exploration has been carried out on the middle part of the buried segment of this fault, and the fault inferred to be a late Pleistocene fault with normal strike-slip movement, but the age and rate of the latest activity have not been determined. By conducting new shallow seismic and drilling joint exploration, we further study the shallow structure, the geometric distribution, the latest activity era and the slip rate in the Quaternary in the two segments of the Taochuan-Huxian Fault. The profile of shallow seismic exploration line TB1 reveals that the west segment of the Taochuan-Huxian Fault with NEE trend can extend at least 20km westward from Taochuan Town. The main fault plane dips to N, and the normal-slip movement has faulted the Quaternary bottom boundary and the underlying crystalline basement in the Taibai Basin. The vertical offset of the Quaternary bottom boundary is about 300m, and the remnants of the old thrust structure are still preserved in the fault zone. The shallow seismic reflection lines ZZ1 and YX1-2 reveal the location of the eastern Taochuan-Huxian Fault with the EW striking buried in the Quaternary of the Weihe Graben Basin in Zhouzhi and Huxian. The main fault plane dips to N, and the fault zone is represented by a fault depression zone of about 6km wide and a stepped structure of about 4km wide respectively. The fault up-breakpoints on both profiles offset the bottom boundary of the Holocene in the Weihe Graben Basin. The drilling joint profile exploration applied at Tanjiazhai in Zhouzhi County and Xiashimasi in Meixian County show that the Taochuan-Huxian Fault is distributed in the junction of the southern Weihe Granben Basin and the Qinling Mountains, where the Holocene marker layer S0 has been vertically offset by 4~5m, yielding an average vertical slip rate of 0.4~1.3mm/a. Combined with the results of shallow seismic surveys, it is well demonstrated that the eastern segment of the Taochuan-Huxian Fault(buried in the Weihe Graben Basin)shows Holocene activity, and it is significantly more active than the western segment(the Taibai Basin segment). This may be due to the fact that the eastern segment has been incorporated into the Weihe Graben Basin and has become part of the primary active tectonic zone on the block boundary, while the western segment has not been incorporated. Spatially, the eastern segment of the Taochuan-Huixian Fault is subparallel to the middle-eastern segment of the North Qinling Fault, which is capable of generating strong earthquakes of magnitude 7 or higher. As an important branch of the North Qinling Fault, the Taochuan-Huixian Fault may also be under the same strong seismic background. These two faults probably jointly control the important active boundary of the southern margin of the Weihe Graben Basin. Future research in seismology and geology of these two faults should be strengthened, including their interrelationships at depth, their roles in vertical and horizontal movement distribution, and their seismogenic capacity and potential seismic hazard. In particular, the activity of the Taochuan-Huoxian Fault since the late Quaternary has only recently received attention, and the level of seismo-geological research on the fault is generally low. In this paper, we conducted preliminary studies on the location, shallow tectonic structure, activity segmentation, latest activity and Holocene vertical slip rate of this fault. Future research on the seismogenic structure of the Taochuan-Huoxian Fault needs to be strengthened in order to deepen and improve the understanding of the fault activity and to provide a basis for analyzing the seismic hazard of this fault.

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    XU Bo, WANG Ping, WANG Hui-ying, GUO Qiao-qiao, SHI Ling-fan, SHI Yu-xiang
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 305-320.   DOI: 10.3969/j.issn.0253-4967.2023.02.001
    Abstract219)   HTML18)    PDF(pc) (6489KB)(168)       Save

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

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    XU Jian-hong, CHEN Jie, WEI Zhan-yu, LI Tao
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 811-832.   DOI: 10.3969/j.issn.0253-4967.2023.04.001
    Abstract214)   HTML26)    PDF(pc) (5260KB)(139)       Save

    A scarp is a common step-like landform in nature, which consists of a gently sloping plane connected to the upper and lower geomorphic surfaces of differing elevations. Common scarps include fault scarps, terrace scarps, lake shoreline scarps, shoreline scarps, volcanic ash cinder cones, etc. Scarps are often used as strain markers because of their linear characteristics and are favored in the study of active tectonics. However, it is difficult to directly constrain their ages. Instead, they are usually constrained by the ages of the upper and lower geomorphic surfaces. The scarp developed in loose deposits is controlled by a long process of low-energy degradation after a short collapse. This process can be modeled by the diffusion equation because the process can be considered as a slope process under the transport-limited condition. Under this condition, the slope can provide enough loose material for transport, that is, the material transport capacity is less than the material supply capacity. If process assumptions are sufficiently valid and rate constraints can be calibrated independently, the true age of scarps can be obtained. This method is called morphologic dating. This method has been included in many textbooks published overseas, but there have very little research on this method in China. Both linear and nonlinear models have been developed to describe scarp degradation. Linear diffusion models assume that the diffusion coefficient is a constant, whereas nonlinear transport models generally define the diffusion coefficient as a nonlinear function related to the topographic gradient. Compared to the linear transport models, nonlinear transport models can better explain the phenomenon of rapidly increasing deposition flux as the gradient approaches a critical value. In this paper, we review the study history of scarp degradation analysis and the concept model of scarp degradation. We focus on the establishment of the nonlinear model, the role of the different parameters in profile evolution, determining the best-fit age using a full-scarp nonlinear modeling procedure, and so on. Furthermore, we introduce the model of the nonlinear age chart, including the effect of far-field slope on morphologic dating of scarp-like landforms and two examples of the application of the chart, which shows that this method can correctly evaluate the ages of single-event scarps. Finally, we discuss the extension of the concept and method of the scarp degradation model, the applicability of the model, and repeated fault scarp morphological analysis. For nonlinear diffusion models, in addition to n equal to 2, two parameters (critical gradient (Sc) and diffusion constant (k)) need to be constrained. The critical gradient can be obtained from the young scarps in the study area, which roughly represents the initial state of scarp evolution, typically 0.6 to 0.7(30° to 35°). The diffusion constant needs to be characterized by a known age scarp. The slopes of the upper and lower geomorphic surfaces have an obvious influence on the morphology of a degraded scarp. These discussions indicate that both linear and nonlinear models can be used for the degradation analysis of single-event scarps, but a nonlinear diffusion model is recommended for young single-event scarps. The constant slip rate nonlinear model can be used to simulate the evolution history of<10ka high-slip rate active fault scarp. The multiple-event scarp model requires careful evaluation of the fault location and the amount of displacement per event. There are several assumptions in the scarp topography diffusion modeling, which require practice to verify its reliability. With advances in surveying technology, it is now possible to rapidly obtain high-resolution terrain data over broad areas from which numerous topographic profiles can be efficiently extracted. This provides a broad application prospect for scarp degradation analysis and morphologic dating.

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    YOU Zi-cheng, BI Hai-yun, ZHENG Wen-jun, PENG Hui, LIANG Shu-min, DUAN Lei, QIN Yi-gen
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1057-1073.   DOI: 10.3969/j.issn.0253-4967.2023.05.002
    Abstract208)   HTML28)    PDF(pc) (10517KB)(225)       Save

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

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    LI Yi-shi
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 455-463.   DOI: 10.3969/j.issn.0253-4967.2023.02.009
    Abstract203)   HTML14)    PDF(pc) (926KB)(169)       Save

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

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

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

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

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

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

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

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

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    ZUO Yu-qi, YANG Hai-bo, YANG Xiao-ping, ZHAN Yan, LI An, SUN Xiang-yu, HU Zong-kai
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 355-376.   DOI: 10.3969/j.issn.0253-4967.2023.02.004
    Abstract184)   HTML36)    PDF(pc) (27392KB)(165)       Save

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

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

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

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

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    ZHANG Ling, MIAO Shu-qing, YANG Xiao-ping
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 422-434.   DOI: 10.3969/j.issn.0253-4967.2023.02.007
    Abstract180)   HTML17)    PDF(pc) (7262KB)(132)       Save

    Digital topographic analysis, an important means in the research of active tectonics and tectonic geomorphology, has increasingly become one of the principal tools in the identification of active tectonic features and understanding of the development of the earth’s surface process. Indoor interpretation of surface fault trace plays a key role in the digital topographic analysis as it can provide the foundation for setting priorities and defining strategies in the subsequent field investigation. At present, the extraction of fault traces is often realized by assisting the traditional visual interpretation through the image enhancement method. The relevant subjective assessments lead to the amount of work and usually cause different results due to the differences in the interpretation experience of actual operators. At the same time, the field of quantitative research on geomorphic parameters is evolving very rapidly with the advances in the popularity of high-resolution digital topographic data. Therefore, intelligent and automatic extraction of surface fault traces has gradually become a promising research direction. The methods based on machine learning often rely heavily on the good programming foundation of the operator, which is a visible technical barrier. We present a semi-automated method using an ArcGIS toolbox with a set of tools to extract surface fault traces based on geomorphic constraints. The Hutubi and Dushanzi faults are two typical thrust faults located on the northern piedmont of the Tianshan Mountains and are chosen as examples. Excellent exposure of the surface fault traces in these two regions permits detailed mapping of fault traces and deriving shape factors of faults with high-resolution DEMs(digital elevation models). Additionally, they are two of the most-studied thrust faults in this area. Large-scale geological and geomorphological mappings of them and numerous achievements have been published. This creates possibilities for us to conduct comparison analysis on different major methods. Based on typical morphology characteristics of fault scarps, appropriate geomorphic parameters are selected. In practice, reverse fault scarps are distinctly defined into forward and backward ones according to whether their dip is the same as that of the neighboring geomorphic surfaces. Based on two sets of geomorphic constraints,two approaches are then illustrated, including slope calculation, gully extraction, data density analysis and process modeling. Through a detailed comparison of the final extraction results and previous visual interpretations of remote sensing data and field geomorphic investigations, the validity of the method proposed in this study is proven. This method provides a set of tools with user-friendly interfaces to realize step-by-step interpretation and emphasizes the importance of field-based geomorphic constraints at the same time. Moreover, many subtle fault traces which have not been recognized before are simultaneously revealed in the Dushanzi research area. The high-resolution DEMs guarantee the realization of picking out finer bits of fault information. Compared to traditional ways of working, the method has the advantage of automatically delineating reverse fault traces on the earth’s surface. This advantage can significantly reduce the efforts to manually digitize geomorphic features and improve efficiency. But many basic manual adjustment options for recognizing target characteristics also need to be set in extraction, because the distinguishing criterion of fault scarp and surrounding geomorphic landforms vary among different areas. In different specific circumstances, users can manually adjust relevant parameters for the extraction during the modeling process. Generally speaking, the more detailed constraints, the more confidence in the final delineation of fault traces. Subjective judgments are therefore particularly critical for conducting extraction under complex backgrounds. But improving the degree of automation of the whole process is still an important study direction. Future work is thus recommended to employ machine learning and explore appropriate evaluation methods to search for the optimal solution of intermediate parameters.

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    XU Ying-cai, GUO Xiang-yun
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 1006-1024.   DOI: 10.3969/j.issn.0253-4967.2023.04.012
    Abstract175)   HTML19)    PDF(pc) (13832KB)(131)       Save

    The 2022 MS6.0 Maerkang earthquake swarm in Sichuan Province is the first rare strong swarm activity with high frequency, concentrated spatial and temporal distribution, strong explosive and strong magnitude in Maerkang area in the eastern segment of Bayan Har block in China seismic network records. It is also another significantly strong earthquake event in Bayan Har block after the MS7.4 Maduo earthquake on May 22, 2021. The MS6.0 Maerkang earthquake on June 10, 2022 not only broke the 33-year record without MS≥6.0 earthquakes within 100km of the epicenter, but also broke the historical record without MS≥6.0 earthquakes within 50km of the epicenter. The earthquake swarm is mainly located in the nearly “T” shaped conjugate fault structure area composed of the NW strike Maerkang fault and NE strike Longriba fault in the Bayan Har block. This area is a relatively rare region for moderate and strong earthquakes in the history. Therefore, it is of great significance to analyze and discuss the possible seismogenic faults of the Maerkang strong earthquake sequence for the study of seismogenic structures and the risk of strong earthquakes in the weak seismic region of Bayan Har block.

    The earthquake swarm was relocated by double-difference method, and focal mechanisms and centriod depths of MS≥3.6 earthquakes were calculated by using gCAP inversion method. Then the relationship between the stress system in the Malkang area and these earthquake focal mechanisms was analyzed, and fault plane was fitted by using relocation results. Maerkang earthquake swarm is mainly distributed along NW direction, and the initial rupture depth is 9.8km on average. Depth profiles show that earthquakes are mainly concentrated at depth between 0km to 15km. The most earthquakes of early-stage occurred in 48 hours. The mid-stage and late-stage earthquakes are located less than 15km in depth and move to the northwest of the epicenters. Initial rupture depth of the largest MS6.0 earthquake is 12.5km, which is almost at the bottom of the dense area. The focal mechanism of MS6.0 earthquake is 150° in strike, 79° in dip, and 7° in rake on nodal plane Ⅰ, and 59° in strike, 83° in dip, and 169° in rake on nodal plane Ⅱ, with the centroid depth of 9km. Other focal mechanisms of MS≥3.6 earthquake are strike-slip types. Dips of nodal plane of focal mechanism range from 71° to 86°, and there exist different dip directions for one strike of every nodal plane. All azimuths of P axis are in NWW direction, and the plunges are nearly horizontal. The focal mechanisms of MS≥3.6 earthquakes show that the tectonic environment is very favorable for NE or NW strike faults to generate the strike-slip movement. Centriod depths range from 5 to 9km, which are lower than the average depth of 9.8km of relocation, indicating that these earthquakes mainly ruptured from deep to shallow. The relative shear stress of the NW nodal plane are significantly greater than that of the NE nodal plane, and the normal stress of the NW nodal plane was smaller than that of the NE nodal plane, indicating more possibility of strike-slip dislocation on the NW nodal plane. The fault plane fitting results reveal that there are obviously two nearly parallel and nearly NW strike earthquake belts in the epicenter area. Fitted fault plane parameters of the belt in the north branch show the strike 333°, the dip 88°, the slide -22°, and the belt in the south branch show the strike 331°, dip 88°, and slide -23°. It is indicated that the fault properties of these two earthquake belts are basically the same, revealing that most of earthquake activities of the swarm may be controlled by at least two parallel structures near the Maerkang fault with the NW strike, dip 88° and left-lateral strike-slip. Combined with the existing regional geological structure, it is inferred that the Maerkang earthquake swarm may be induced by the NW and NE strike conjugate faults, and the NW strike faults control most of the earthquake activities.

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    WAN Yong-ge, WANG Yu-ru, JIN Zhi-tong
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 1025-1040.   DOI: 10.3969/j.issn.0253-4967.2023.04.013
    Abstract175)   HTML17)    PDF(pc) (7335KB)(86)       Save

    The fine crustal stress field plays an important role in geodynamics. The 2021 Yangbi earthquake sequence occurred in an area with densely deployed seismic stations. Before the mainshock, there occurred multiple 3-4 magnitude earthquakes. The mainshock was followed by strong aftershocks, MS5.0 and MS5.2, occurring 7 and 36 minutes later respectively. The earthquake sequence is a typical example of a “foreshock-mainshock-aftershock” earthquake sequence. The abundant seismic data of the 2021 Yangbi earthquake sequence provide many seismic focal mechanisms for the fine stress field analysis in the study region.

    To study the relationship of the stress field, fault structure, and earthquake dynamics in the Yangbi earthquake source region, the central focal mechanism solution algorithm is selected for the earthquakes with several focal mechanisms to ensure the accuracy of the focal mechanism data, and 93 precisely determined focal mechanism data are determined. The overall stress field in the source region is determined as a compressive stress axis of nearly NS direction and extensional stress axis of nearly EW direction. Then, to reveal the heterogeneity of the stress field in the source region, according to the location of the earthquake sequence, the focal mechanism solutions are divided into 6 regions by using the moving window strategy and obtain the stress field in each sub-region. To verify the inversion results are not caused by the selection of a specific partition mode, we used two different partition methods to discuss the stress field inversion experiments: 1)change the number of sub-regions from 6 to 8, the number of focal mechanisms in each subregion is still 23, and moving the 15 focal mechanisms in each iteration; 2)the number of the sub-region is still 6, change the number of focal mechanism to 28 in each subregion. It can be found that although the different partition strategies are changed, the characteristics of the obtained stress field will not change. Finally, the earthquake dynamics revealed in the heterogeneous stress area are analyzed.

    The results show that the compressive stress axis changed from NNW-SSE direction in the northwest of the Yangbi earthquake focal area to NNE-SSW direction in the southwest region, with the rotation angle of 23°; And the stress shape factor in the northwest part of the rupture zone is always larger than that in the southeast region. Combined with the geodynamics studies of crustal motion map, tomography from seismic data, hydrographic net distribution, and topography of the study region, it is speculated that the change of the stress field in the northwest and the southeast is caused by the combined action of the blocked southward movement of the material in the northern part of the fracture area and the NNE extension in the shallow part of the study area due to the low angle NNE subduction of the Indo-Burma arc. The horse-tail-like fault distribution in the southeast of the Yangbi earthquake fault zone and the mountain and river alignment around the Yangbi earthquake are consistent with the predicted stress deflection and stress shape factor change. These studies are of significance for understanding the characteristics of fault activity and earthquake dynamics in study regions.

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    JIANG Yu-han, WANG Zi-si, LIU Jia-qi, LIANG Hui, ZHOU Qi-chao, GAO Xiao-qi
    SEISMOLOGY AND GEOLOGY    2023, 45 (3): 622-637.   DOI: 10.3969/j.issn.0253-4967.2023.03.002
    Abstract175)   HTML23)    PDF(pc) (2106KB)(137)       Save

    Large-scale observation network has been set up in China, including the observations of groundwater dynamics, geothermal water, and geochemical parameters, and long-term observation data has been obtained for underground fluids. Hydrogen observation is considered to be one of the methods that are most likely to make a breakthrough in the aspect of earthquake precursor monitoring and prediction, thus, plays an important role in earthquake monitoring and forecast in China. Many scholars have carried out research on the relationship about hydrogen and earthquake precursors, and proved that abnormal hydrogen concentrations are related to and have certain correlations earthquake activities. The main objects of hydrogen observation in China include the escaping gas from fault soil and the escaping gas from deep wells and hot springs near the fault. Different analytical methods are used for different types of hydrogen, and the main methods include gas chromatograph analysis and digital high-precision hydrogen analyzer analysis. Through years of observation practice, a large number of typical examples have been obtained in China. The relationship between the abnormal hydrogen concentration and the earthquake has a correspondence. The main manifestation is that the hydrogen concentration increases several times or even tens or hundreds of times in a few months or a few days before the earthquake. It is mainly divided into two cases: First, it rises rapidly to several times in a short time before the earthquake. The concentration reaches about hundreds of times the background value in more than ten to a few days immediately before the earthquake, and then the earthquake occurs. The concentration quickly declines and restores the background value after the earthquake. Second, the hydrogen concentration continues to increase in fluctuation, and decreases after reaching the maximum value, then, the earthquake occurs after recovery. This kind of anomaly is short in time, mostly, they are imminent or medium and short-term abnormalities. Therefore, the hydrogen response to the earthquake precursor is an important short-imminent earthquake prediction indicator, and can be used as an important approach to explore the short-impending earthquake prediction.
    The hydrogen in the crust mainly comes from biochemical and chemical actions. The hydrogen on the surface layer of the crust is mainly produced by microbial decomposition of organic matter and mineral salts. It is regularly symbiotic with gases such as methane and carbon dioxide. The hydrogen in the crustal fault belts, especially in the active fault zones, also comes from the failure and deformation of rock. The formation mechanism of hydrogen in the crust can be summarized into 3 categories: 1)Under normal circumstances, the hydrogen content is very low, and most of them exist in the pores of the rock and soil layer in a free state, or are adsorbed on the surface of the rock. When the external conditions remain unchanged, the gas is in a balanced state; when the environment changes, especially the underground stress changes, the cracks develop continuously under the action of tectonic stress, resulting in interconnecting each other, and subsequently, the deep hydrogen also changes and emits to the ground surface, including the imminent rupture stage in the earthquake preparation and rock oscillation; 2)The chemical reactions occur between the crushed rock's fine particles and water, generating hydrogen; 3)The temperature gradient causes the hydrogen attached in the crack to escape.
    In short, hydrogen is a better method for studying earthquake reflecting ability among the underground fluid observation methods. Representative earthquake cases are obtained from observations of both dissolved hydrogen in the water or soil hydrogen. This observation item plays an important role and has practical significance in the geochemical observation means. In the observation of earthquake underground fluids, hydrogen observations can provide data support for future earthquake risk zoning and earthquake tendency tracking and analysis.

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    WEI Chuan-yi, YIN Gong-ming, WANG Xu-long, WANG Duo, JI Hao, LIU Chun-ru, LI Xin-xiu
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1041-1056.   DOI: 10.3969/j.issn.0253-4967.2023.05.001
    Abstract174)   HTML10)    PDF(pc) (4654KB)(147)       Save

    As the most recent period of the geological record, the Quaternary climate change, tectonics and river drainage evolution have been well recorded by Quaternary sediment. Establishing the timing of these geological changes, and of their effects on the earth's environment, is a key element in Quaternary research. Because of dating range limit of quartz OSL dating and 14C dating, lacking of tephra for K-Ar dating, and strict restrictions for 26Al/10Be cosmogenic nuclide dating, the samples older than 200ka were critical but difficult in Quaternary dating, while electron spin resonance(ESR)dating method could provide absolute age for late Pliocene and Pleistocene samples. Previous studies show that quartz Al center and Ti-Li center are the most suitable signals for sediment ESR dating, and have been successfully applied into middle-late Pleistocene sediment dating. However, the application of those two centers ESR chronology into early Pleistocene or pre-Quaternary sediment remains confusion.

    In this study, early Pleistocene Jingyuan gravel layer sediment deposited at Yellow river were collected for ESR dating. The results of comprehensive comparative analysis of high resolution magneto-stratigraphy and deep-sea oxygen isotope curve of loess-paleosol sequences and high credible 26Al/10Be cosmogenic nuclide dating age make the Jingyuan gravel layer as the ideal material to evaluate the dating range, especially lower dating range, of the quartz Ti-Li center and Al center, respectively. The results show that:

    (1)The quartz Ti-Li center and Al center signal intensity of Jingyuan gravel layer was not saturated within 11 000Gy and 130 00Gy additional gamma ray dose, respectively; combined with the long thermal lifetimes of the quartz Ti-Li center(8×106a)and Al center(7.4×109a), guarantee the ESR dating range for million years.

    (2)The single saturation exponential function and “EXP+LIN” functions could provide more accuracy fitting result of equivalent dose of quartz Ti-Li center and Al center, respectively, and the fitting goodness is greater than 0.98.

    (3)The average ESR dating results of quartz Ti-Li center and Al center of Jingyuan gravel layer is~(1.67±0.15)Ma and~(1.65±0.69)Ma, respectively, which is consistent with the previously well-known age within the error range.

    To better understand the lower dating limit of the quartz ESR dating method, based on the previous analysis of the ESR signal thermal stabilities, we discuss the maximum saturation of the ESR signals and ESR signals' sensitivity. Combined with the fitting goodness evaluation of various fitting functions, we propose that the quartz Ti-Li center and Al center ESR dating method could provide reliable chronological constrains on the sand lens of early Pleistocene gravel layer. The results of our study not only provide a solid theorical foundation for the application of quartz ESR dating method for late Pliocene and early Pleistocene fluvial sediments, but also demonstrate a typical practice example of the ESR method on dating late Cenozoic sediments.

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    FU Ying, HU Bin, ZHAO Min, LONG Feng
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 987-1005.   DOI: 10.3969/j.issn.0253-4967.2023.04.011
    Abstract170)   HTML16)    PDF(pc) (7950KB)(118)       Save

    On June 1, 2022, a MS6.1 earthquake occurred in Lushan, Sichuan Province, western China, which is approximately 10km from the Lushan MS7.0 event on April 20, 2013. To understand if the earthquake has the same seismogenic structure as the Lushan MS7.0, we relocated the event in the Lushan area using the multi-stage locating method based on the seismic phase arrival data of the Sichuan Seismic Network from April 20, 2013, to July 1, 2022. A total of 6992 ML≥1.0 earthquakes were acquired, with a relative locating error of 0.5km and 0.7km in the horizontal and vertical directions, respectively, with a travel time residual(RMS)of 0.18s. The results show that the MS6.1 event is located at 102.943°E, 30.382°N with an initial-rupture focal depth of 15.6km, lying on the NW side of the 2013 Lushan MS7.0 event. The sub-surface rupture length of the long and short axis is 10 and 8km, measured from the dense aftershock area in NE-SW and NW-SE directions, respectively. The NE-SW profile in the Lushan area shows that the depth of Lushan MS7.0 earthquake in 2013 was about 15km, similar to that of Lushan MS6.1 and MS4.5 on June 1, 2022. The MS6.1 earthquake sequence, located at the NE end of the long axis, shows no evidence to break through the rupture termination point of the Lushan MS7.0 earthquake and enters the Dayi seismic gap, which is bounded by the 2008 Wenchuan MS8.0 and 2013 Lushan MS7.0 aftershock regions. The short-axis profile shows that the MS6.1 earthquake sequence occurred on a new back-thrust fault in the pre-existing seismogenic structure of the 2013 Lushan MS7.0. The new structure dips SE and ruptures in a slight arc protruding into the NW, parallel to the northern segment of the seismogenic structure of the 2013 Lushan MS7.0 earthquake with a horizontal distance of about 5km. The new and old structures connect at the detachment base to the main segment of the 2013 Lushan MS7.0 earthquake.

    We also inverted the focal mechanism of the Lushan MS6.1 earthquake using the CAP(Cut and Paste)method. The result indicates that the centroid depth of the MW5.7 main event is 14km which is very close to the initial-ruptured depth of 15km calculated by the phase arrival times. The best double couple parameters are 221°/40°/105° for nodal plane Ⅰ and 22°/52°/78° for nodal plane Ⅱ. The parameters are in order of the strike, dip, and rake angles. Combined with the realization of the NE-striking, SE-dipping seismogenic structure characteristics determined by the accurate locating of the earthquake sequence, it can be quickly confirmed that the nodal plane Ⅱ is the fault plane.

    Based on the accurate locating results, focal mechanism solutions, and geodynamic background of the focal area, it is inferred that the seismogenic structure of the Lushan MS6.1 earthquake is induced by the thrust dislocation of a NE-SW trending and SE inclining thrust fault in the southern section of Longmenshan fault zone. Finally, we discussed the relationship between MS7.0 and MS6.1 in the Lushan area. The two could be considered a unique sequence: the mainshock and the maximum aftershock, respectively, regarding spatial relationship and tectonic correlation. However, the time interval of these two earthquakes significantly overextends the statistical relationship between the principal earthquake and the maximum aftershock. Furthermore, considering the effects of the Coulomb stress change produced by the earthquakes repeated at the end of the Dayi gap, Lushan earthquake further enhanced the stress level in the Dayi seismic gap located in its northern segment.

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    LIU Wei, BAI Xi-min, LÜ Shao-jie, SHI Zhe-ming, QI Zhi-yu, HE Guan-ru
    SEISMOLOGY AND GEOLOGY    2023, 45 (3): 652-667.   DOI: 10.3969/j.issn.0253-4967.2023.03.004
    Abstract166)   HTML17)    PDF(pc) (3448KB)(96)       Save

    Groundwater, as one of the most active components of the earth's crust, has a sensitive reflection to crustal stress as well as solid deformation. Previous studies have shown that fluctuations in barometric pressure cause corresponding dynamic changes in well water level, and the response of well water level to barometric pressure signal can reveal a lot of hydrogeological information such as groundwater movement law and aquifer water storage mechanism, and can also provide a new way to estimate the hydraulic parameters of the aquifer. The method of using well water level in response to barometric pressure to estimate the hydraulic parameters is in-situ, low cost, and low disturbance, which has certain advantages compared with traditional field hydrogeological tests.
    We analyze the water level and barometric pressure data of the monitoring wells in the fault zone, and the change characteristics of the permeability of the fault zone can be obtained. The seismicity of the Qujiang fault is strong, and studying its permeability and evolution characteristics plays an important role in understanding the seismicity in this area. Therefore, in this study, we took the Gaoda well in the Qujiang fault zone in Yunnan Province as the object of study, and collected minute level data of well water level and barometric pressure from February 2019 to July 2020, based on the response of the well water level to the barometric pressure signal in different periods to calculate the hydraulic parameters of the aquifer using barometric response function and analyzed the change characteristics of the permeability of the fault zone after the earthquake. At the same time, compared and analyzed the parameter estimation results with the results calculated by previous methods using well water level in response to earth tide and slug test. The results show that:
    (1)The aquifer permeability of the Gaoda well ranges from 8.89×10-15 to 11.10×10-15m2 and the transmissivity ranges from 2.44×10-6 to 3.05×10-6m2/s during different observation periods, and the overall variation is not significant and fluctuates within a certain range, indicating the aquifer permeability of the Gaoda well did not change significantly after the earthquake, and the permeability of the Qujiang fault zone was relatively stable. Meanwhile, previous studies on the tidal analysis of the Jiangchuan well near the southern section of the Xiaojiang fault zone, which is 16.6km away from Tonghai, showed that the permeability of its aquifer did not change significantly after the Tonghai 5.0 earthquake in 2018, indicating that the permeability of the southern section of the Xiaojiang fault zone and the Qujiang fault zone are relatively stable, and the hydraulic characteristics of the two have a certain similarity, and the comparison result between the two wells is referential to some extent.
    (2)The hydraulic parameters of the aquifer calculated based on the response of well water level to the barometric pressure are somewhat different from those calculated by previous authors using earth tide responses and slug tests, and the obtained parameters are slightly lower than those obtained by earth tide responses and slug tests, this may be due to the different factors considered by different methods and the different degrees of fracture development in the part of the fault zone where the well is located, resulting in a certain degree of heterogeneity in the aquifer, causing differences in the results obtained by different methods, which reflect the differences in the spatial scales, the applicability of the models, and the parameter range represented by the aquifer hydraulic parameters inferred by the response models of different well-aquifer systems. In addition, the barometric pressure signal acts in a wider frequency band, more parameters are inferred by the model, and its response can be recorded under different hydrogeological conditions, making the response model to barometric pressure more widely applicable.

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    ZHAO Peng, LI Jun-hui, TAO Yue-chao, SHU Peng, FANG Zhen
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 338-354.   DOI: 10.3969/j.issn.0253-4967.2023.02.003
    Abstract163)   HTML18)    PDF(pc) (13253KB)(174)       Save

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

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

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

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    SHEN Hua-liang, YANG Yao, ZHOU Zhi-hua, RUI Xue-lian, LIAO Xiao-feng, ZHAO De-yang, LIANG Ming-jian, CHEN Meng-die, GUAN Zhi-jun, REN Hong-wei
    SEISMOLOGY AND GEOLOGY    2023, 45 (3): 689-709.   DOI: 10.3969/j.issn.0253-4967.2023.03.006
    Abstract161)   HTML9)    PDF(pc) (9916KB)(69)       Save

    Maoya hot spring, as a famous earthquake monitoring site, is seldomly studied in terms of its genesis and deep geothermal process. In this paper, we investigated the chemical and isotopic composition of thermal water in Maoya and Maohuo in Litang to elucidate the hydrochemical characteristics and genesis of the geothermal waters.
    The study results show that Maoya hot springs and Maohuo hot spring are of the Na-HCO3 type as a result of dissolution processes involving feldspars from the reservoir rocks due to the water-CO2-rock interaction during the deep circulation of the geothermal waters. According to the diagram of Cl- and Na+ concentrations of the geothermal water samples, Cl- in Maoya hot spring originates from the mixing of granodiorite and basalt aqueous solutions in the process of water rock interaction, while Cl- in Maohuo hot spring mainly originates from granodiorite aqueous solutions. The stable isotope δD and δ18O composition of geothermal waters indicates that they are recharged by meteoric precipitation. The Maoya hot springs have the characteristics of higher concentration of ion components and slightly oxygen drifting compared with the Maohuo hot spring, indicating that they have a deeper circulation depth and experience a stronger water-rock interaction. In addition, the ratio of Cl-that comes from deep source in Maoya hot springs is higher than that in Maohuo hot spring.
    The high temperature geothermal water formed by deep circulation of meteoric water is mixed by the shallow cold water during the ascending process. We employed SiO2 geothermometer and Si-enthalpy model to estimate the temperature of shallow reservoir after mixing with cold water and the temperature of deep reservoir and the mixing ratio of cold water, respectively. The results suggest that the temperature of shallow reservoir in Maoya thermal field is in the range of 75~103℃ and the temperature of deep reservoir in Maoya thermal field is about 235℃ and the mixing ratio of cold water ranges from 87% to 94%. Based on the temperature of deep reservoir, we calculated the depth of the geothermal cycle in Maoya area, which is close to 5km.
    The heat source triggering the formation of this geothermal system mainly originates from mantle and partial melting body of the crust. In addition, Cenozoic granitoid magmatic residual heat and upper crust radioactive heat can also provide additional heat sources. During the process of surface cold water circulation from shallow to deep, on the one hand, it forms deep geothermal water through normal geothermal gradients, and on the other hand, the mantle fluid upwelling below the Litang Basin and partial melting in the middle crust further heat the groundwater to form a high-temperature deep reservoir. The deep geothermal water is transported to the surface along the Litang Fault under the effect of hydrostatic pressure and hydrothermal convection. During ascending process, the first mixing of groundwater with superficial cold water occurred due to the presence of structural cracks in the crust, and the temperature of the mixing water is about 100℃. When the geothermal water migrates to the near surface, it mixes with the pore water and bedrock fissure water in the basin for the second time, and the mixing proportion of cold water increases(about 90%). Finally, it emerges to the surface, forming a group of medium-low temperature hot springs.

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    HAN Xiao-fei, SHI Shuang-shuang, DONG Bin, XUE Xiao-dong, FAN Xue-fang
    SEISMOLOGY AND GEOLOGY    2023, 45 (3): 795-810.   DOI: 10.3969/j.issn.0253-4967.2023.03.011
    Abstract159)   HTML16)    PDF(pc) (16671KB)(82)       Save

    Active fault deformation zones are commonly referred to as fault failure zones. The width of the deformation zone is generally several meters to tens of meters, representing the strongest range of fault activity deformation and the degree of exposure of future surface fracture zones, that is, the severely damaged strip area, which is a key avoidance object for construction projects.
    Modern ground buildings(structures)generally have underground engineering that requires excavation of foundations ranging from a few meters to several tens of meters. Hidden faults that cannot be exposed by foundation excavation and whose buried depth is less than 60m may also form fractures on the surface, but the location of the fractures is difficult to determine. At the same time, the long-term creep of the hidden faults and the historical multiple periods of seismicity have formed significant plastic deformation or weak displacement areas near the surface. Therefore, studying the range of hidden fault deformation zones of Quaternary sedimentary layers can provide scientific basis for scientific avoidance of active faults.
    The local changes of the Fenhe River channel(surface deformation survey)reflect the stages and stages of tectonic activity in the Taiyuan Basin, mainly including the early stage of the third episode of the Xishan Mountains, the Huangkun movement, and the Gonghe movement, and there are adjustments in two modes of movement: strike slip and tension; Through the precise processing and interpretation of 15 shallow seismic survey lines, and combined with some geological deep hole profiles, the stratigraphic age of the seismic stratigraphic profile was marked, revealing the multi-stage expansion activity of the Tianzhuang fault inverted terrace, and further demonstrating the correctness of the basin's third-phase expansion activity revealed by the changes in the Fenhe River channel.
    The Qinghai-Tibetan movement(3.4~1.66Ma BP)in the third episode of Xishan formed the front edge of Tianzhuang fault in Pliocene and the strike slip compression tectonic activity before the Qinghai-Tibetan movement formed the rear edge section. The Huangkun movement(1.2~0.7Ma BP)formed the front edge section of Tianzhuang fault in Middle Early Pleistocene Late Early Pleistocene and the tectonic activity between Qinghai-Tibetan and Huangkun movement formed the rear edge section of Middle Early Pleistocene, The Gonghe movement(after 0.15Ma)formed the front section of the Tianzhuang fault F2-Qh(creep slip)and the rear section of the Tianzhuang Fault $\mathbb{F}_{1-\mathbb{Q}_{\mathbb{P}}^3} $ in the middle Pleistocene uplift tectonic activity. The section formed by the Qinghai-Tibetan movement was the oldest, the section formed by the Huangkun movement was the second, and the section formed by the Gonghe movement was the latest. The dislocation activity of the section formed by the Gonghe movement occurred in the middle Pleistocene and Late Pleistocene, and the Holocene was dominated by slow seismicity, Shown as weak creep movement, the front edge section of the Tianzhuang Fault F2-Qh(creep)and the rear edge section of $\mathbb{F}_{1-\mathbb{Q}_{\mathbb{P}}^3} $ are active sections that require attention in urban planning.
    In the structural history of the Tianzhuang fault, as a branch of Jiaocheng fault in the NEE direction, together with Jiaocheng fault, controlled the sedimentary process of the basin before the middle Pleistocene in the Qingxu sag. Through this work, the response of the Tianzhuang fault to the Qinghai-Tibetan movement, the Huangkun movement, and the Communist movement has been systematically revealed. Finally, it is considered that the evaluation of the deformation zone formed by the Tianzhuang fault since the latest Gonghe movement(Late Pleistocene)is of practical significance for engineering earthquake resistance and avoiding faults. Through the measurement of fault gas profiles and evaluation of fault gas anomaly zones of the Cross Tianzhuang Fault in Xizancun and Malianying Road, combined with the detailed stratigraphic faulting revealed by the joint drilling profiles of the Cross Tianzhuang Fault in Xizancun and Malianying Road, the extension of the main active section F2-Qh on both sides of the front edge of the Tianzhuang Fault since the Republican Movement has been determined to be 30m, which is the deformation zone range. The extension of the main active section on both sides of the rear edge of the Tianzhuang Fault is 55m, which is the deformation zone range, Considering the specific needs of engineering seismic fortification, it is advisable to conduct seismic fortification and engineering design based on the deformation zone range on both sides of these two main sections.

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    WANG Liao, XIE Hong, YUAN Dao-yang, LI Zhi-min, XUE Shan-yu, SU Rui-huan, WEN Ya-meng, SU Qi
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 401-421.   DOI: 10.3969/j.issn.0253-4967.2023.02.006
    Abstract158)   HTML13)    PDF(pc) (22292KB)(146)       Save

    On January 8th, 2022, an MS6.9 earthquake occurred around Menyuan County(37.77°N, 101.26°E), Qinghai Province. The epicenter is located in the northeastern part of the Tibetan plateau, where the western section of the Lenglongling Fault meets the eastern section of the Tolaishan Fault. In order to know the spatial distribution of coseismic surface rupture zone as soon as possible, and determine the seismogenic structure, the post-earthquake GF-7 remote sensing images of the Menyuan MS6.9 earthquake were analyzed. Moreover, combining the interpretation of the GF-7 images and the field investigation, the distribution of the co-seismic surface rupture was determined and the typical coseismic landforms, and the image recognition features of various co-seismic landforms are interpreted and summarized. The results show that the earthquake produced two major surface rupture zones with a left-stepped oblique spatial arrangement. The main northern branch rupture distributes on the west side of the Lenglongling Fault, with a length of about 22km and a strike of 100°N~120°E, the secondary rupture of the southern branch distributes along the eastern section of the Tuolaishan Fault, with a length of about 4km and a strike of N90°E. The total length of the two rupture zones is about 26km.

    Along the rupture zones, a series of typical left-lateral strike-slip coseismic landforms were formed, such as tensional fractures, tensional-shear fractures, pressure ridges, pressure bulges, left-lateral strike-slip gullies, as well as left-lateral strike-slip roadbeds, etc. We divided the surface rupture into six segments to conduct detailed observation and analysis, that is, the west of Daohe segment, Liuhuanggou segment, Honggou segment, Yongan River segment and Yikeshugou segment, from west to east among the main rupture zone of the north branch, as well as the secondary rupture zone of the south branch. In general, each co-seismic landform has its distinctive image characteristics, and we obtained them from the interpretation and summarization of the GF-7 images. The shear fractures located at the two ends of the main rupture and in the areas where the surface rupture is weak are zigzaggy on the remote sensing images, while the shear fractures located in the areas where the surface rupture is intense are shown as dark, wide and continuously smooth stripes; thrust scarps are represented on remote sensing images as shaded, narrow and slightly curved strips; the pressure ridges and pressure bulges exhibit black elliptical feature on the images that are parallel or at a smaller angle to the main rupture; tensional-shear fractures are displayed as black strips arranged in en echelon with a 30°~45° intersection angle with the main shear rupture, and their linear features are not as straight as those of shear ruptures yet are still distinct; the coseismic scarps formed on the ice are manifested in the images as traction bend and texture change. Based on the GF-7 images, the cumulative dislocations of typical sinistral landforms along the co-seismic surface rupture on Lenglongling Fault are interpreted and futher compared with the previous study. This is the first time of application of GF-7 to the strong earthquake geohazards monitoring since it was officially launched in August 2020. From this study, it can be seen that with its high resolution, GF-7 can be used to accurately identify faulted features. Not only it could provide information of the geometric roughness, complexity and segmentation of the fracture, but also can record clear dislocations of the landforms. The study of the GF-7 images in the 2022 Menyuan earthquake has showed that the GF-7 images can provide strong data support for the geology and geological hazard studies.

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    WANG Ming-liang, ZHANG Yang, XU Shun-qiang, XU Zhi-ping
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 536-552.   DOI: 10.3969/j.issn.0253-4967.2023.02.014
    Abstract154)   HTML9)    PDF(pc) (5569KB)(74)       Save

    In order to study the deep electrical structure and its formation mechanism of different tectonic units in the central and southern part of the North China Depression, especially the southern North China Depression with NW-NNW trending tectonic lines and the northern North China Depression with NE-NNE trending tectonic lines, as well as the deep tectonic background of three destructive earthquakes in the study area, a 110km long magnetotelluric(MT)sounding profile across the main structural units in the study area was deployed to study the deep fine electrical structure in the central and southern part of the North China Depression by using high-density and broadband MT method with the support of the active fault exploration project in Kaifeng City. The profile is NS-trending as a whole, starting from Tongxu County of Kaifeng City in the south, passing through Xiangfu District of Kaifeng City, Fengqiu County and Changyuan County of Xinxiang City, and terminating at Banpodian Township of Huaxian County of Anyang City in the north, with a total length of 110km and an average point distance of about 3km. The observation points in the survey area of Kaifeng City are dense, with a point distance of 2km. From south to north, the whole profile crosses two first-order tectonic units of the southern North China Block and the northern North China Block and four second-order tectonic units of the Taikang Uplift, the Kaifeng Depression, the Dongpu Depression and the Neihuang Uplift. In MT data processing, in addition to the remote reference and robust techniques, the multi-point and multi-frequency tensor decomposition was employed to determine the regional electric strike, and the NLCG 2D inversion was performed on TE and TM data. And finally, the deep electrical structure is obtained.

    The result shows that, with Xinxiang-Shangqiu Fault as the boundary, the deep electrical structure on its north is relatively simple than that on the south. The electrical structure of Neihuang Uplift and Dongpu Depression in the northern North China Depression is relatively simple, and its resistivity structure is characterized by vertical segmentation and divided into low resistivity and high resistivity zones corresponding to the crust of the area consisting of sedimentary cover and crystalline basement of hard block with good basement integrity. At the same time, the high resistivity zone is very thick, which could represent the unified crystallization basement in the North China Block region. The deep electrical structure of the tectonic units in the southern North China Depression on the south of Xinxiang-Shangqiu Fault is relatively complex, showing a three-layer structure of low-high-low resistivity in the vertical direction and alternating high and low resistivity in the horizontal direction. For example, the resistivity of the crust below the Taikang Uplift shows a low-high-low three-layer structure, that is, a low resistivity zone above the depth of 2km, a high resistivity zone at the depth between 2km and 15km, and a low resistivity zone below 15km. This may be related to the mutual subduction and collision between the Yangtze plate and the southern margin of the North China plate, while the northern North China Depression is less affected by the Yangtze plate and Qinling-Dabie orogeny due to the control of the boundary Xinxiang-Shangqiu Fault.

    The two destructive earthquakes of 1342 and 1918 in Tongxu, Henan Province are located in the intersection area of high and low resistivity zones beneath the Taikang Uplift, and the Fengqiu earthquake of 1737 in Henan Province is located near the gradient zone of high and low resistivity in the crust.

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    ZOU Jun-jie, HE Hong-lin, ZHOU Yong-sheng, WEI Zhan-yu, SHI Feng, GENG Shuang, SU Peng, SUN Wen
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 833-846.   DOI: 10.3969/j.issn.0253-4967.2023.04.002
    Abstract153)   HTML21)    PDF(pc) (6000KB)(133)       Save

    Bedrock normal fault scarps, as classical topographic features and geomorphological markers along mountain range fronts, form in consolidated bedrock due to faulting in extensional settings. They generally preserve more complete records of paleo-earthquakes than fault scarps in unconsolidated sediments. With the development of technologies such as fault surface morphology measurement and terrestrial cosmogenic nuclide dating, bedrock fault planes have become a nice object for paleo-earthquake study in bedrock areas. The reconstruction of paleo-seismic history from a bedrock fault scarp in terms of the times, co-seismic slips and ages by a combination of quantitative morphological analysis, TCNs dating and other physical/chemical index has been proven feasible by several previous studies.

    However, this success heavily relies on a suitable site selection along the bedrock fault scarp because erosional processes can exhume the bedrock fault surface, and the sedimentary processes can bury the bedrock fault surface. Namely, non-tectonic factors such as gully erosion, sediment burial, and anthropogenic activity make bedrock fault planes difficult to record and preserve paleo-seismic information.

    Therefore, to successfully extract paleo-seismic information from the bedrock area, it is necessary to select suitable study points along the bedrock fault scarp in advance. Traditional survey and mapping methods are time-consuming and labor-intensive, and it is difficult to understand bedrock fault scarps. The resolution of satellite images cannot obtain the fine structure of bedrock fault scarps. Small unmanned aerial vehicle(sUAV), combined with Structure-from-Motion(SfM)photogrammetry has emerged over the last decade. It is used as an established workflow in acquiring topographic data by filling the spatial gap between traditional ground-based surveys and satellite remote sensing images. As a low-altitude photogrammetry technology, it can quickly obtain high-precision three-dimensional surface structures of bedrock fault scarps.

    In this paper, taking the Majiayao bedrock fault scarp at the northern foot of Liulengshan in Shanxi Rift as an example, the high-precision and three-dimensional topographic data of the bedrock fault was obtained by using sUAV combined with SfM photogrammetry technology. The high-resolution and high-precision images of tectonic landforms can be obtained conveniently and efficiently by sUAV survey. The sUAV-obtained photos can be further processed by the SfM photogrammetry for generating a digital 3D structure of the bedrock fault scarp with true or shaded color.

    The non-tectonic factors such as rock collapse, sediment burial, and gully erosion along the bedrock fault scarp are identified by interpreting the 3D model of the bedrock fault scarp. The profile shape characteristics of the erosion, burial and tectonic fault scarps are summarized through fine geomorphological interpretation and fault profile analysis. For the erosion profile, the hanging wall slope is down-concave, showing that the fault surface below the ground surface has been partially exposed. For the bury profile, the hanging wall slope shows an obvious concave-up shape, indicating that the lower part of the bedrock fault surface has been partially buried by the colluvium. For the tectonic profile, the hanging wall slope shows a smooth and stable slope, showing the exhumation of bedrock fault scarp is controlled purely by tectonics. Finally, the study sites suitable for paleo-earthquake study on bedrock fault surfaces were selected, showing the important role of sUAV aerial survey technology in the selection of paleo-earthquake study sites in bedrock areas.

    This study illustrates that based on the high-precision three-dimensional surface structure of the bedrock fault plane from sUAV aerial survey, the existence of non-tectonic factors such as gully erosion, sedimentary burial and bedrock collapse can be clearly identified. These non-tectonic sites can be excluded when selecting suitable sites for paleo-earthquake study indoors. The shape analysis of bedrock fault scarp is also helpful to determine whether the bedrock fault surface is modified by surface process and suitable for paleo-seismic study. The sUAV aerial survey can play an important role in paleoseismic research in the bedrock area, which can accurately select the study points suitable for further paleo-seismic work in the bedrock area.

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    WANG Bo, CUI Feng-zhen, LIU-ZENG Jing, ZHOU Yong-sheng, XU Sheng, SHAO Yan-xiu
    SEISMOLOGY AND GEOLOGY    2023, 45 (3): 772-794.   DOI: 10.3969/j.issn.0253-4967.2023.03.010
    Abstract151)   HTML14)    PDF(pc) (8810KB)(113)       Save

    An MS7.4 earthquake occurred in Madoi County, Guoluo Tibetan Autonomous Prefecture, Qinghai Province of China at 02:04 (Beijing Time) on May 22, 2021. A total of seven 800~3 000m trans-fault survey lines were targeted laid along different parts of the seismic surface rupture zone(the west, mid-west, mid-east, and the east), one month after the earthquake when the detailed field investigation of the coseismic displacement and the spread of the seismic surface rupture zone had been carried out. The soil gases were collected and the concentrations of Rn, H2, Hg, and CO2 were measured in situ.
    The results show that the maximum value of Rn, H2, Hg and CO2 concentrations in different fracture sections of the surface rupture was 2.10~39.17kBq/m3(mean value: 14.15kBq/m3), 0.4×10-6~720.4×10-6(mean value: 24.93×10-6), 4~169ng/m3(mean value: 30.72ng/m3)and 0.73%~4.04%(mean value: 0.59%), respectively. In general, the concentration of radon is low in the study area, which may be related to the thick overburden and the lithology dominated by sandstone. The concentration characteristics of hydrogen and mercury released from soil have good consistency, and the concentrations are higher at the east and west ends of the surface rupture zones but were lower in the middle of the rupture zone. This is consistent with the field investigation showing that the earthquake-induced surface rupture zone and deformation are more concentrated in the western section, while the eastern section has a large amount of seismic displacement.
    The fault strikes at the east and west ends of the Madoi MS7.4 earthquake surface rupture have deviated from the NW direction to a certain extent, and there also exits two branching faults and rupture complexities at the east end of the main fault of the Madoi earthquake. In the west end of the surface rupture, i.e., the south of Eling Lake, the fault strike turns to EW direction. We laid two survey lines(line 2 and line 3)at the west end of the rupture, the concentration of Rn, H2 and Hg escaped from line 3 is the lowest one among all lines while the gas concentration of line 2 is significantly higher. In the vicinity of line 3, the field geological survey did not find the cracked and exposed surface rupture, and only a small number of liquefaction points were distributed near the Eling Lake. The soil gas concentrations and morphological characteristics were consistent with the field phenomena. At the east end of the rupture zone, the soil gas morphological characteristics of the south and north fault branches were inconsistent: the soil gas of the south branch showed a single-peak type which was more similar to that at the west end, but the gas concentration pattern of the north fault branch showed a multiple-peaks type. This phenomenon is consistent with the characteristic shown in the surface fracture mapping, that is, the deformation zone of the rupture where is wider.
    To find out the source of soil gas and the possible influencing factors of soil gas concentrations in the study area, the carbon isotope and helium isotope of the collected gas samples were analyzed. The value of 3He/4He shows that the noble gas in the study area is mainly an atmospheric source, but the results of δ13C and CO2/3He show that the soil gas along the surface rupture of the Madoi earthquake has the mixed characteristics of atmospheric components and crustal components, which to a certain extent reflects the cutting depth of main fault-Jiangcuo fault may be shallow, and it is speculated that the surface rupture caused by Madoi MS7.4 earthquake may be confined to the shallow crust.

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    YANG Xiao-lin, YANG Jin-ling, SU Li-na, FENG Jing, WANG Jun
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1170-1186.   DOI: 10.3969/j.issn.0253-4967.2023.05.008
    Abstract150)   HTML27)    PDF(pc) (5264KB)(75)       Save

    To date, more than 60 four-component borehole strainmeters have been deployed in China to provide more data on geodynamics and earthquake precursors. In practice, the strain signals recorded by the four-component borehole strainmeters are greatly disturbed by the effect of barometric pressure at different frequencies. Therefore, precisely deciphering the frequency-dependent strain response to barometric pressure changes in the frequency domain is very significant for continuous high-resolution borehole strainmeter measurements. However, no much related research has been conducted in this day. For these more than 60 four-component borehole strainmeters being operated in Mainland China, the study of properties(e.g., Young's modulus and Poisson's ratio)of borehole surrounding rock are seldom probed via rock mechanical testing has not been given enough attention, compared that with in the USA and Japan, consequently, which makes it difficult in the understanding of the driving mechanism of atmospheric effects.

    Beginning at 2006, a YRY-4 type borehole strainmeter was installed approximately at 50m depth in the Jiangning area to precisely monitor the earthquake precursors and tectonic movements in Jiangsu Province. For daily observation, the atmospheric effect is quite obvious in different frequency bands. Therefore, we applied transfer function to uncover features, such as barometric pressure coefficient and phase shift, of frequency dependence observed at Jiangning station in Jiangsu Province in high-(>8cpd), intermediate-(0.5~8cpd), and low-frequency(0.1~0.5cpd)bands. Furthermore, the double bush mechanical model was adopted to estimate the parameters of elastic modulus and Poisson's ratio for the borehole surrounding rock.

    The results show that: 1)The values of coherence in the frequency band(0.1~30cpd)are higher than in others. 2)The barometric pressure responses were very stable and valid in the low-frequency band, and remained stable in the high-frequency band(8~30cpd), but significantly fluctuated in the intermediate-frequency band. 3)If the impacts of the diurnal and semidiurnal tidal waves are neglected, the spectra of barometric pressure response coefficients for the four borehole sensors and areal strain were quasi-linear and stable. 4)In the high-frequency band, the spectra of phase responses for borehole strains behaved exponentially, strongly depending on frequency, with the average phase delay of about 24.2°. 5)The estimated elastic modulus and Poisson's ratio were 33.9GPa and 0.27 according to the averaged barometric-pressure response coefficients of areal strain, respectively, in which the estimated parameters showed good agreement with the results obtained via rock mechanical testing.

    These above findings will be useful for separating the nonlinear barometric responses from the four-component borehole strainmeter records, as well as for estimating the mechanical parameters of the borehole surrounding rock. In the future, more and more rock cores taken from boreholes may disappear with time. So exploring the potential values of transfer function is an important work in the field of borehole strain study. In the near future, we will reconstruct the rock mechanical parameters of borehole surrounding rock for other unstudied sites in mainland China.

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    CHEN Han-lin, WANG Qin-cai, ZHANG Jin-chuan, LIU Rui-feng
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1233-1246.   DOI: 10.3969/j.issn.0253-4967.2023.05.012
    Abstract150)   HTML25)    PDF(pc) (7565KB)(127)       Save

    In this paper, we relocated earthquakes occurred from April 2013 to July 2022 in Lushan seismic zone, inversed focal mechanism solution of the Lushan MS6.1 earthquake on June 1, 2022 and discussed the seismogenic structure of the Lushan MS6.1 earthquake and its relationship with the MS7.0 earthquake in April 2013.

    The results of the focal mechanism solution show that the Lushan MS6.1 earthquake in 2022 is a thrust earthquake. The strike, dip and azimuth of nodal plane Ⅰ are 228°, 46° and 104° and for nodal plane Ⅱ are 28°, 46° and 76° respectively. The results of earthquake relocation show that the focal depth of the Lushan MS6.1 earthquake sequence is shallow in the north and deep in the south, the fault length is about 10km. The focal depth is mainly concentrated between 10km to 19km. The fault dip is southeast with an angle of 60°. The initial rupture point of the main shock of the Lushan MS6.1 earthquake is at a depth of 20km, located at the deepest part of the fault. The fault ruptured from deep to shallow. The Lushan MS7.0 earthquake occurred on April 2013 strikes northeast and dips northwestward, but there exists a reverse fault in the aftershock sequence that has the same direction of strike but the opposite direction of dip. This reverse fault is consistent with the strike and dip of the MS6.1 earthquake occurred in June 2022. It appears as two parallel faults in the profile. In addition to the reverse fault on the west side, the embryonic of another reverse fault seems to appear on the east side of the middle of earthquake sequence. These faults are about 10km away from the surface. The distribution of earthquakes in two northwest-oriented depth profiles shows that the dip angles of the main shock and the reverse fault of the MS7.0 earthquake is different at different locations, and these faults are not simple straight planar sections. From one year after occurrence of the MS7.0 earthquake to occurrence of the MS6.1 earthquake, the seismic activity on the main fault decreased but the seismic activity on the reverse fault on the west side of the MS7.0 earthquake sequence was more active during this period, most of the seismic activity occurred near the reverse fault that is parallel to the MS6.1 earthquake fault.

    By analyzing the seismogenic structure and seismic activity characteristics of the Lushan seismic zone, we concluded the Lushan MS6.1 earthquake on June 1, 2022 is caused by a blind thrust fault with strike towards northeast and dip towards southeast, located 10km away from the surface. It has the opposite directions of strike and dip of the Longmenshan Fault. The epicenters of the Lushan MS7.0 earthquake in April 2013 and the MS6.1 earthquake in June 2022 are located near the surface exposure traces of the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault, respectively. However, according to the analysis of the relocation aftershock depth in profile, the aftershock extension to the surface does not coincide with the surface exposure positions of the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault. Therefore, the seismogenic faults of these two earthquakes are not the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault, but two blind reverse faults. The Shuangshi-Dachuan Fault near the MS6.1 earthquake sequence and the main shock fault of the 2013 MS7.0 earthquake are thrust faults dipping northwest, while the Lushan MS6.1 seismogenic fault has opposite direction of dip. The seismogenic fault of the Lushan MS6.1 earthquake and the main thrust fault of the 2013 MS7.0 earthquake, which strikes northeast and dips northwest with the reverse thrust fault of the hanging wall, which strikes northeast and dips southeast, together form a double layer Y-shaped structure. These faults are all blind thrust faults and belong to the Qianshan-Shanqian Fault system in the southern segment of the Longmenshan fault zone. The seismogenic structure in the Lushan seismic zone is a complex fault system composed of one main northeast strike fault with dipping northwest, and three faults dipping southeast.

    From one year after occurrence of the Lushan MS7.0 earthquake to the occurrence of the Lushan MS6.1 earthquake, most of earthquakes in the Lushan seismic zone occurred near a reverse fault which is parallel to the Lushan MS6.1 earthquake seismogenic fault. These earthquakes are located in the area where the coulomb stress change caused by the MS7.0 earthquake acts as loading effect. The Lushan MS6.1 earthquake sequence is mainly distributed in the area where the coulomb stress change plays an unloading role caused by the Lushan MS7.0 earthquake. The research results showed that the coulomb rupture stress caused by the Lushan MS7.0 earthquake on the seismic nodal plane of the MS6.1 earthquake has a restraining effect on the MS6.1 Lushan earthquake.

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    CHEN Yi, ZHAO Bin, XIONG Wei, WANG Wei, YU Peng-fei, YU Jian-sheng, WANG Dong-zhen, CHEN Wei, QIAO Xue-jun
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1074-1091.   DOI: 10.3969/j.issn.0253-4967.2023.05.003
    Abstract146)   HTML16)    PDF(pc) (10431KB)(141)       Save

    Located in the eastern boundary of the Qinghai-Tibetan plateau, the Xianshuihe fault zone is one of the most active left-lateral strike-slip faults in Chinese mainland. As the southern boundary of the Bayanhar block, the Xianshuihe Fault accommodates the southeastward transport of material toward southeastern Asia. Earthquakes have occurred frequently along this fault, especially in the northwestern segment. More than 20 earthquakes with MW>6.0 have ruptured since 1700. The most recent MW>7 earthquake was the Luhuo earthquake in 1973, and the most recent MW>6 earthquake was the MW6.6 Luding earthquake in 2022. As one of the most active faults in mainland China, the present slip pattern of the Xianshuihe Fault, especially the shallow creep characteristics along its northwestern segment, has attracted much attention.

    The primary goal of determining slip rates of active faults using geodetic data is to quantify the seismic potential of the faults. Illuminating the long-term slip rate and shallow creep distribution of faults is the basis for calculating the seismic moment rate and evaluating the seismic potential. Due to the backwardness of early measurement methods, the seismic deformation along the Xianshuihe Fault was previously based on geologic, cross-fault short baseline and leveling surveys. With the application of GPS in tectonic geodesy, more and more GPS stations are installed near active faults, which provide accurate constraints on the long-term slip rates of the fault. Subsequently, the appearance of InSAR technology has brought a beneficial supplement to GPS, providing high spatial resolution surface velocity maps, which have been widely used to measure deep and shallow creep along active faults. It is the key to accurately characterize the fault slip behavior and evaluate the seismic potential.

    In this study, 119 Sentinel-1 satellite descent data from December 2014 to December 2021 were processed to obtain the average line-of-sight(LOS)velocity field of the northwestern segment of the Xianshuihe Fault based on the small baseline InSAR method. Then the elastic screw dislocation model was used to fit the fault normal InSAR LOS velocity profiles to estimate the long-term slip rates and shallow creep rates. Combined with the viscoelastic earthquake cycle model, the effects of the earthquake recurrence period, and rheology of the lower crust and upper mantle on slip rate estimation in Luhuo segment are analyzed. The main results are as follows:

    (1)The average InSAR LOS velocity field is in the northwestern segment of the Xianshuihe Fault during 2014—2021 has been obtained with a large range and high spatial resolution. The velocity field results show an obvious velocity gradient across the surface trace of the Xianshuihe Fault, which is consistent with the left-lateral strike-slip characteristics of the Xianshuihe Fault.

    (2)To investigate the slip rate variation along the northwestern segment of the Xianshuihe Fault, we used the two-dimensional elastic screw dislocation model to fit the 14 fault-normal velocity profiles selected along the northwestern segment of the Xianshuihe Fault and estimated the long-term slip rates and shallow creep rates using the Markov Chain Monte Carlo(MCMC)method. The results show that the overall slip rates of the NW segment of the Xianshuihe Fault range from 7.2mm/a to 11.0mm/a, and gradually decrease from west to east. The shallow creep rate ranges from 0.3mm/a to 3.1mm/a. The high creep rate appears mainly at Xialatuo and the segment from Daowu to Songlinkou. The shallow creep rates in other places are close to zero, implying that the fault is completely locked.

    (3)According to historical earthquake records, the recurrence interval of the Luhuo segment is set to be 150 years, 200 years, and 400 years, and the viscosity of the lower crust and upper mantle is set to be 5.0×1018Pa·s, 1.0×1019Pa·s, and 5.0×1019Pa·s. The slip rate of the Luhuo segment is estimated to be (7.91±0.3)~(9.85±0.4)mm/a using the MCMC method, which is slightly lower than the (10.14±0.5)mm/a obtained by the pure elastic model. In addition, when the earthquake recurrence interval is 150 years and the viscosity of the lower crust and upper mantle is 5.0×1019Pa·s, we simulate the fault-normal velocity at 5 years, 20 years, 75 years, and 125 years after the 1973 Luhuo earthquake, and find that in any period of the seismic cycle, the estimation of fault slip rate will be biased to some extent if the viscoelastic contribution of the lower crust and upper mantle is ignored.

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    LI Ji-ye, YAN Rui, ZHANG Si-meng, HU Lan-bin, MENG Ling-lei, ZHOU Chen
    SEISMOLOGY AND GEOLOGY    2023, 45 (3): 668-688.   DOI: 10.3969/j.issn.0253-4967.2023.03.005
    Abstract146)   HTML11)    PDF(pc) (4797KB)(83)       Save

    Underground fluid plays a vital role in the process of earthquake preparation and occurrence. Water level observation wells with good pressure and sealing are usually called crustal strain-sensitive indicators. In groundwater micro-dynamic observation, information such as earthquake precursor anomalies can be identified by using tidal response characteristics of well water level. In this paper, the water level data of Yanshou Station, Tonghe Station, Zhaodong Station, Gannan Station and Suihua Beilin Station, which can be used for tidal analysis in 18 water level observation wells in the Heilongjiang area since 2016, are selected, and the tidal factors of diurnal waves in the diurnal wave group of water level tidal response of each well are obtained by using Vinidkov harmonic analysis method. Based on the tidal analysis of the whole-point observation data in Heilongjiang and its surrounding areas without earthquakes above MS4.0, with no obvious interference of water level and high accuracy and continuity of water level, the maximum tidal factor of the diurnal wave is extracted as the background value of diurnal wave of well water level tidal response at each station. Combined with the significant earthquakes around each station, the abnormal variation characteristics of diurnal wave height before and after Ningjiang MS5.0, MS5.7 and MS5.1 earthquakes in Songyuan, Jilin Province are extracted.
    When the earthquake preparation reaches the final stage, the triggering effect of the external environment will become the key factor. This research shows that the horizontal tidal force of the day and month is closely related to the occurrence of large earthquakes, and the tidal force of the earth's tide before moderate and strong earthquakes has obvious modulation and triggering effect on seismicity. In the study of the relationship between earthquakes and tidal triggering, it is the most intuitive and effective method to analyze the degree of modulation triggering of earthquakes in a certain region and the modulation anomaly characteristics of small earthquakes near the epicenter before earthquakes. Using ML3.0 earthquakes in Heilongjiang Province and its adjacent areas, this paper selects a time window length: 1 year, time step length: 3 months, space window length: 150km, space step length: 0.5°×0.5°, and a lower limit of the number of earthquakes as 5, calculates the spatial anomaly area of modulation ratio one year before the earthquake, extracts the stress modulation anomaly near the focal area before the moderate and strong earthquake in Songyuan, Jilin Province, and further discusses the relationship between diurnal wave anomaly and small earthquake modulation in the process of earthquake preparation and occurrence.

    The results show that: 1)The background change of diurnal wave tide factor of well water level tide response is relatively stable, the anomaly is easier to identify and has a high signal-to-noise ratio. 2)Before the Ningjiang earthquake in Songyuan, Jilin Province, the diurnal wave anomaly of the tidal response of the well water level was synchronous and morphologically consistent, mainly represented by the matching anomaly of three or more stations. 3)The Ningjiang earthquake in Songyuan, Jilin, occurred within 2.6 months after the end of the matching anomaly of the Sunday wave height, with the shortest of only 7 days, and has obvious short-term and imminent characteristics. The duration and amplitude of the anomaly are related to the magnitude of the earthquake. 4)Before the Ningjiang earthquake in Songyuan, Jilin Province, there was a low-value anomaly in the modulation ratio of small earthquakes with ML≥3.0 in the focus area, which was mainly characterized by short-impending features. It may have the same physical meaning as the lower or lower b-value before the earthquake.
    The diurnal wave anomalies in the tidal response of the well water level reflects the change in the stress state within the structure. The modulation ratio of small earthquakes can better reveal that the tectonic stress in the focal area reaches or approaches a critical state. The combined analysis of the two helps identify and capture short-term and imminent anomalies of earthquake precursors. Studying the tidal response of well water level and the modulation of small earthquakes may be an effective way to explore earthquake precursor information related to tidal force triggering during earthquake preparation and occurrence.

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    CHEN Kun, GAO Meng-tan, YU Yan-xiang, XU Wei-jin, DU Yi, LI Xue-jin, LU Dong-hua
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 435-454.   DOI: 10.3969/j.issn.0253-4967.2023.02.008
    Abstract146)   HTML12)    PDF(pc) (7698KB)(104)       Save

    Using the Monte Carlo random sampling method, a set of probabilistic seismic hazard analysis calculation programs that integrates our country’s traditional planar potential seismic source zone and three-dimensional fault sources is developed. The program is not only suitable for our country’s traditional regional area sources, but also considers the rupture scale of earthquakes and is compatible with the probabilistic seismic hazard calculation of three-dimensional fault sources. The algorithm developed in this paper efficiently realizes the three-dimensional simulation of the seismic event set of the fault source and introduces the earthquake rupture scale into the probabilistic seismic hazard analysis calculation in China, which significantly improves the rationality of the seismic hazard calculation in the near-fault area. In order to improve the execution efficiency of the program, the algorithm adopts the method of filling grid points in the planar potential seismic source zone in advance and randomly simulating the uniform distribution of seismic events in the planar potential seismic source zone. For the seismic hazard calculation of elliptical attenuation relationship, the algorithm uses pre-constructed three-dimensional matrices of the distance of the ellipse minor axis under different magnitudes, distances, and different angles between sites and the ellipse long axis direction of potential seismic source zone, and directly obtains the corresponding distance of ellipse minor axis through table look-up and interpolation. The algorithm developed in this paper avoids the problem of low computational efficiency in the iterative approximation of the distance of the ellipse minor axis. The mathematical expression of the three-dimensional fault source is based on the Frankel fault plane form of the 2002 edition of the National Seismic Hazard Map of the United States. The surface track and average dip Angle of the fault are used to create the rectangular fault plane, in which the dip direction of each rectangle is always perpendicular to the strike of its local fault segment. To maintain the coordination between the rupture area and the magnitude, the rupture of the earthquake occurring on the fault plane should not exceed the fault plane or the combination of fault planes. If the boundary of the rupture plane is outside the fault boundary, the entire rupture plane will move so that the boundary of the entire rupture plane matches the boundary of the fault plane. Using the probabilistic seismic hazard program of the Seismic ground motion parameters zonation map of China(2015)and the algorithm developed in this paper, the regional seismic hazard of the study area including Changsha-Zhuzhou-Xiangtan of the urban agglomeration in Hunan Province with moderate to strong seismic activity are calculated. Seismic hazard at different probability levels(return periods of 50.8, 475 and 2 475 years, respectively)for the Changde near-fault sources and Zhuzhou sites are also computed. The comparative study shows that the procedure of the Seismic ground motion parameters zonation map of China(2015)underestimates the seismic hazard near the three-dimensional fault source, and the degree of underestimation becomes more significant as the probability level decreases. Considering the influence of the earthquake rupture scale at the low exceedance probability level, the decomposition results of the seismic hazard for sites near fault show that the contribution of the seismic hazard is different from that of the traditional method of the Seismic ground motion parameters zonation map of China(2015), which mainly focuses on the earthquake of high magnitude. However, earthquakes of all magnitudes on the fault source can contribute to the seismic hazard, but the proportion of high magnitudes is the largest. Finally, an example verifying the probabilistic seismic hazard program(data set 1 case 10)from the Pacific Earthquake Engineering Research Center(PEER)is used to verify the reliability of the algorithm developed in this paper.

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    ZHOU Jie-yuan, ZHOU Qing, RAN Hong-liu
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 914-935.   DOI: 10.3969/j.issn.0253-4967.2023.04.007
    Abstract145)   HTML12)    PDF(pc) (8795KB)(109)       Save

    Earthquake catalog is the foundational data for analyzing seismic activity, assessing seismic hazard, and studying earthquake prediction. The majority of historical earthquake records are sourced from historical documents, with a significant portion of these records found in local gazetteers. Compiling historical literature is an essential way in analyzing seismic activity because historical accounts of earthquakes often provide more detailed and accurate information than geological data. Among these sources, analyzing relevant content in local gazetteers, such as the historical development of local governance, military garrison, official records, and descriptions of disasters and auspicious events, plays a crucial role in seismic activity research. This article aims to acquire historical earthquake records by consulting local gazetteers, folk books, and other historical sources containing natural, social, and political records. These records serve as historical foundations for analyzing the completeness of seismic data records.

    The border region between Sichuan, Yunnan, and Tibet is located in the northwest secondary block of the Sichuan-Yunnan block, which is one of the areas with frequent strong earthquakes in China. The Xianshuihe fault zone and the Jinshajiang fault zone are the northeastern and northwestern boundary faults of the Sichuan-Yunnan block, respectively. They are large-scale and highly active fault zones formed due to the eastward escape of the Tibetan plateau caused by the relative movement between the Indian and Eurasian plates. Previous studies on active tectonics have shown that major earthquakes with magnitudes of 8 and above, as well as over 80% of strong earthquakes with magnitudes of 7, mainly occur in the boundary zones of active blocks with intense structural deformation and high stress accumulation. Moreover, the known active faults in the study area, such as the Batang fault and Litang fault, are also major faults that significantly have influence on the occurrence strong earthquakes. The Sichuan-Yunnan-Tibet adjacent region is home to significant infrastructure, including the Sichuan-Tibet railway and hydropower stations. Analyzing the completeness of earthquake data in the border region of Sichuan, Yunnan, and Tibet can contribute to the assessment of fault hazards and the analysis of regional seismic activity trends. This, in turn, can help minimize the damage caused by earthquakes to critical infrastructure and further enhance the safety and security of people’s lives and properties.

    This study reviewed the local gazetteers of 44 counties in the border region between Sichuan, Yunnan, and Tibet, and summarized the establishment and historical evolution of each county. Based on the analysis of the road evolution from Sichuan to Tibet and from Yunnan to Tibet, we examined the significant roles of important transportation hubs and nodes, such as stations, pond flood, and grain platforms, in regarding of recording earthquakes. Combining various historical sources and previous research on the completeness of earthquake data in the region, we conducted a comprehensive analysis to determine the probable starting years for the availability of seismic records of magnitude 7 and above in the Xianshuihe area and the three parallel rivers area. Additionally, based on the data of the length and short axis of isoseismal lines from 88 earthquakes, an elliptical model was used to derive the seismic intensity attenuation relationship for the Sichuan-Yunnan block. By placing the fitted isoseismal lines of magnitude 6 and 7 earthquakes in the study area, we analyzed their impact range, providing a spatial dimension basis for the completeness analysis of seismic data.

    This article provides a comprehensive analysis and demonstration of the complete starting years of seismic data in the border region between Sichuan, Yunnan, and Tibet from both temporal and spatial perspectives. The results indicate that due to the establishment of grain stations and Tangxun along the Sichuan-Tibet road, as well as the appointment of officials, several counties in the Xianshuihe area, including Kangding, Luhuo, Garzê, Litang, and Yajiang, were developed between 1719 and 1736. At the same time, there are relatively abundant historical documents related to earthquakes in the Xianshuihe area. Local chronicles, reports from governors and resident ministers, written records in Tibetan temples, and accounts from lamas have documented earthquake surveys, disaster assessments, and relief efforts. By combining these historical sources with the analysis of intensity attenuation relationships in the Sichuan-Yunnan block, the affected areas of earthquakes with magnitudes 6 and 7 can be determined that the period from 1719 to 1736 marks the starting years with complete M≥7 earthquake data in the Xianshuihe area. The towns of Batang, Mangkang, and Changdu in the three parallel rivers area are also significant nodes and hubs along the road to Tibet. They were established with administrative institutions and granaries between 1719 and 1728, and the road network extensively covered Tangxun in the region. In considering the seismic records and historical sources in the three parallel rivers area, as well as referencing the recording capabilities of granaries, administrative institutions, and Tangxun in the Xianshuihe area, and estimating the potential recorded seismic magnitudes based on the intensity attenuation relationships of the Sichuan-Yunnan block, it can be suggested that the period from 1719 to 1728 is a possible starting point for complete earthquake data with M≥7 in the three parallel rivers area. In areas farther away from the road to Tibet, such as Jiangda, Gongjue, Baiyu, Xinlong, and the northern regions of Batang and Litang, as well as the large contiguous regions of Derong, Xiangcheng, Daocheng, and Jiulong, the eastern boundary is the Xianshuihe fault zone, while the area between the two zones is divided by the northeast-oriented Batang fault. Previous seismic geological investigations have found that within the aforementioned regions, the influence of the Jinshajiang fault zone extends along the Batang-Derong-Benzilan line. In remote areas away from the road and with sparse population, the possibility of individual earthquakes with magnitudes above 7 occurring but being missed cannot be ruled out. However, in other areas not located on active fault zones, it can be considered unlikely to experience earthquakes with magnitudes above 7. Based on the analysis of the data, the starting years of earthquakes with a magnitude of 6 and above should be the same as those of earthquakes with a magnitude of 7 and above. However, according to the analysis of the average occurrence rate of earthquakes per year, there is a significant lack of records for earthquakes of magnitude 6 and above. This may be due to the sparsely populated and vast nature of the Tibetan region during historical times, limited administrative capabilities of officials, and lack of earthquake historical records and documents. Therefore, it is not possible to determine the exact starting year for complete data on earthquakes of magnitude 6, which would be the same as for earthquakes of magnitude 7 and above.

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    LIU Bai-yun, ZHAO Li, LIU Yun-yun, WANG Wen-cai, ZHANG Wei-dong
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 500-516.   DOI: 10.3969/j.issn.0253-4967.2023.02.012
    Abstract139)   HTML18)    PDF(pc) (6044KB)(92)       Save

    At 2:04 on May 22, 2021, an earthquake of M7.4 occurred in Maduo County, Golog Prefecture, Qinghai Province, with the focal depth of 17 kilometers, the epicenter at 34.59°N and 98.34°E. This earthquake was the largest after the Wenchuan earthquake in China. The epicenter of the earthquake is 38km away from Maduo county seat and 385km from Xining, the provincial capital. The earthquake caused some houses to collapse and some damage to roads in the epicenter. But due to the sparse population in the epicenter area, the earthquake did not cause casualties.

    Seismologist believe that the earthquake is the result of the continuous activity of the boundary fault of the Bayankala block, which is geographically located in the north of the Qinghai-Tibet Plateau and is the hub for the transformation of the direction of the crustal movement of the plateau. In recent years, many destructive earthquakes occurred inside the block. This earthquake is another strong earthquake after the M7.1 Yushu earthquake in Qinghai in 2010. According to the analysis of this earthquake briefing, the fault zone that induced this earthquake is speculated to be the Maduo-Gande fault zone or the Kunlun Mountains Pass-Jiangcuo fault zone.

    In order to find out which fault is the seismogenic structure and the distribution of the seismogenic structure of this earthquake, we relocated the dense earthquakes by double-difference method based on the data of 1357 aftershocks in the Maduo M7.4 earthquake area recorded by 72 fixed stations of the digital seismic network of Gansu and its adjacent seismic network and 12 portable seismographic stations during the May 22 to May 27, and obtained the source parameters for 1289 earthquakes. The accurately located small earthquakes distribute along both sides of the Kunlun Mountains Pass-Jiangcuo Fault, which is NNW-trending obviously. It shows that the seismogenic structure of this earthquake is the Kunlun Mountains Pass-Jiangcuo Fault, rather than the Maduo Gande Fault as considered previously by some scholars. This is consistent with the research results of surface fracture zone, magnetotelluric detection, InSAR coseismic deformation and relocation of other aftershocks. Most earthquakes distribute at the depth range of 0~15km of the crust after the relocation, and the result shows that the focal depths are more concentrated. The relocation also shows that the east and west ends of the main fault have bifurcations. It may be that the complex stress distribution triggered two new branch faults during the occurrence of the great earthquake, and the overall fault shows a “tree-type” structure. The west branch trends 306°and intersects the main fault at 21°. The east branch is nearly EW trending and connected with the east section of the main fault.

    Generally, the earthquakes are closely related to active tectonics, large earthquakes and its aftershocks usually occur on fault zones with obvious activity. The distribution of small earthquakes is related to the complex underground stress state and the complex structure of the fault zone. We can inverse the shapes and positions of the fault planes using spatial distribution of hypocenters of mainshock and the corresponding aftershocks, according to the principle that clustered earthquakes occur near the faults. Six rectangular regions are selected according to the distribution characteristics of relocated aftershocks and by reference to the distribution of geological faults and earthquake rupture zones. We obtained the detailed parameters of fault plane in each region by using the simulated annealing algorithm and the Gauss-Newton algorithm according to the source information after the relocation in 6 rectangular areas. On this condition, rake angle of the fault plane is further inferred from regional tectonic stress parameters. The results show that the main fault is a large, high dip angle, sinistral strike-slip fault with thrust component, striking 285°~290° and about 146km long. It extends from Tanggema Township of Maduo in the southeast(34.49°N, 98.91°E)to Gazejialong Township in the northwest(34.81°N, 97.54°E). The movement characteristics of the newly generated western segment 2 show dextral strike slip and thrust, which is diametrically opposite to that of the main fault. This shows the complexity of the earthquake rupture process, and further research is needed on the tectonic mechanics and deep structures that produce this special rupture.

    Compared with the focal mechanism solutions obtained by domestic and foreign authorities, the fault plane parameters obtained in this paper are similar to them, indicating that our conclusions are reliable. Besides, the spatial distribution of inverted fault plane is basically identical to that of the rupture zone derived from post-earthquake investigation in the earthquake area.

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    WANG Jian, ZHANG Xin-lin, TAN Hong-bo, HU Min-zhang, WU Gui-ju, LI Zhong-ya, ZHANG Ming-hui
    SEISMOLOGY AND GEOLOGY    2023, 45 (2): 553-569.   DOI: 10.3969/j.issn.0253-4967.2023.02.015
    Abstract138)   HTML6)    PDF(pc) (6043KB)(69)       Save

    Using the gravity observation data of Mulanshan short gravity baseline field in 2018 and 2022, we established a high-precision short gravity baseline field of Mulanshan based on the relative gravity joint measurement method under the control of absolute gravity. We also analyzed and discussed the accurate calibration of the monomial coefficient of the relative gravimeter during the construction of the gravity short baseline field, the distribution of gravity values in the gravity baseline field of Mulanshan and the contribution of various environmental factors in the gravity variation results, these results show that:

    (1)Maximum gravity segment difference of Mulanshan calibration baseline is 102.176mGal from G01 to G03 stations, and the average accuracy of gravity value of each measuring station reaches 4.8μGal. The geological structure of the Mulanshan baseline is stable, and the gravity change of measuring stations is not obvious. From 2018 to 2022, the gravity variation range of measuring stations was 5.9~12.8μGal, with an average of 9.5μGal, and the average uncertainty was ±5.7μGal. The gravity field mainly showed a positive change. The variation range of gravity in each measurement section is -4.8~6.9μGal, with an average of(1.8±8.6)μGal. The change of the surrounding environment has a certain impact on the gravity field, and the contribution of the new buildings near the G01 and G02 to the gravity change is 3.6μGal and -0.51μGal, respectively. These gravity changes of measuring stations in the IOS and Mulanshan baseline caused by vertical surface movement are(2.17±0.44)μGal and(1.67±0.45)μGal. The gravity effect caused by the change of surface water storage is(1.07±0.84)μGal, which cannot be ignored. Compared with observation results, the gravity change of each measuring station and section after correction is reduced, and the average gravity change values are reduced by 38.2% and 50.8%, respectively. The corrected gravity change results are more accurate. Due to the cumulative effect of errors in the correction process, the uncertainty of gravity change results after correction increases accordingly, and the uncertainty of gravity change results of measuring station and measuring section increases by 2.5% and 2.8%compared with observation results, respectively. Combined with the gravity change results of the measuring station and the measuring section, we can effectively extract abnormal information in gravity dynamic change results.

    (2)There are differences in monomial coefficients of different gravity sections of the relative gravimeter. The results of CG-6 and CG-5 relative gravimeters are relatively consistent, and there is no systematic deviation between the two gravimeters. The difference in the monomial coefficient between the Wuhan-Yichang section(sub-section)and the Wuhan-Lücongpo section(total section)is 4.809‰, which has a great influence on the gravity observation results. The monomial coefficient needs to be accurately measured. The difference of the monomial coefficient in the sub-section is negatively correlated with the proportion of the gravity segment difference in the sub-section to the total section; the monomial coefficient of the total section is a weighted average result of each sub-section, and the proportion of gravity segment difference in sub-section to total section is the corresponding weight factor. Accurate calibration of the monomial coefficient of the relative gravimeter is a technical guarantee to obtaining high-precision gravity observation results. The gravity segment difference of sub-segments cannot cover the gravity range of the measurement area due to smaller segment difference, which will lead to the extrapolation of the monomial coefficient, so it cannot effectively calibrate the monomial coefficient of the relative gravimeter applicable to the whole measurement area. The total section can cover the gravity range of the measurement area, and the monomial coefficient is the ratio between the segment difference measured by the relative gravimeter and the known segment difference, and its calibration accuracy is inversely proportional to the gravity segment difference, so when using the total section as a reference for calibration of the monomial coefficient of the relative gravimeter, accuracy of the calibration can be guaranteed and precision of the calibration can be improved, so calibration result of the monomial coefficient using the total section is more accurate. The existing widely used relative gravimeters(such as LCR, CG-5, BURRIS, CG-6, and so on)have time-varying characteristics of the monomial coefficient, weakening the errors caused by changes of the monomial coefficient is essential to improve the accuracy of observations, and corresponding calibration is required before each period of gravity observation. The monomial coefficient of the relative gravimeters needs to be calibrated using a large segment difference, and the segment difference(or the accumulated segment difference)should be greater than 300mGal.

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    WANG Kai-ming, YU Da-xin, ZHAO Li-jun, LI Wen-yi, YE Qing-dong
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1187-1199.   DOI: 10.3969/j.issn.0253-4967.2023.05.009
    Abstract138)   HTML9)    PDF(pc) (4725KB)(81)       Save

    The self-noise levels of different seismometer models directly affect the quality of seismic observations and further limit earth science research based on seismic records. We conducted a test of the self-noise level of seven types of seismometers at the Malingshan seismic station, in which the instrument types included short-period, broadband, very-broadband and ultra-broadband. Three seismometers of each type were set up, and the observation period was from November 22, 2018, to March 26, 2019. In this paper, based on continuous seismic waveforms from seven models of seismometers, the self-noise power spectral density(PSD)of the seismometers was calculated by using the three sensor correlation analysis method, and the probability density distribution of the self-noise PSD of the seven models of seismometers was obtained by using the PDF representation. Based on the mode values of the PDF distribution, the self-noise models of the three channels(UD, EW and NS)of the seven models of seismometers are given respectively.

    For the ultra-broadband seismometer CMG-3T-360, in the microseism band(0.1Hz to 1Hz), the self-noise of the horizontal components(EW and NS)is higher than that of the vertical components(UD)and is consistent with the trend of the seismic noise, which may be attributed to misalignment of the horizontal direction between seismometers. In the low frequency band(<0.03Hz), the self-noise level of the horizontal component is higher than that of the vertical component, and small changes in the barometric pressure may lead to higher incoherent noise in the horizontal direction of the sensor at long periods. Compared with the vertical direction, the horizontal direction of the seismometer is more susceptible to air disturbances. At a frequency of 0.005Hz, the instrument self-noise of the horizontal component is close to the seismic background noise, and the instrument self-noise of the horizontal component is the main source of noise recording. Installing a heat and wind shield can effectively reduce the seismometer self-noise in the low frequency band. When using the CMG-3T-360 to observe long-period seismic signals, a shield with both thermal insulation and windproof function is required.

    The self-noise level of the short-period seismometer JS-S02 is lower than that of TDV-33S and lower than that of the New Low Noise Model(NLNM)between 0.15Hz and 7Hz. In the UD and EW channels, the self-noise level of TDV-33S is lower than the NLNM model between 0.17Hz and 0.5Hz. The higher instrument self-noise further limits the extraction of long-period seismic signals in the digital recordings of short-period seismometers.

    For the broadband seismometer TDV-60B and the very broadband seismometer TDV-120VB, the self-noise levels are basically consistent in the vertical direction and also are higher than those of the broadband seismometer JS-60 and the very broadband seismometer JS-120. In the horizontal direction, the two self-noise levels in the microseism band and the low frequency band show different characteristics, i.e., the self-noise levels of TDV-60B are lower than those of TDV-120VB in the microseism band and higher than those of TDV-120VB in the low frequency band. When the frequency is lower than 0.03Hz, the self-noise levels of TDV-60B and TDV-120VB on the horizontal channels are close to those of JS-60 and JS-120, respectively.

    For JS-60 and JS-120, in the vertical direction, the self-noise levels of both are close to CMG-3T-360 in the microseism band. The self-noise level of JS-120 on the vertical channel is lower than 5dB away from CMG-3T-360 in the low frequency band and lies within the 68%confidence interval of the PSD; in the high frequency band(>2Hz), it is higher than CMG-3T-360 confidence interval of the PSD. In the horizontal direction, the self-noise levels of JS-60 and JS-120 are lower than those of CMG-3T-360 between 0.15Hz and 1Hz and in the microseism band, respectively. The self-noise levels of JS-60 on the horizontal channel show good agreement with those of CMG-3T-360 in the high frequency band. The self-noise of JS-120 on NS channel is higher than CMG-3T-360 confidence interval in the low frequency band. When extracting long-period seismic signals, a seismometer with lower noise level in the low frequency band should be selected as much as possible.

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    GUO Ting-ting, XU Xi-wei, YUAN Ren-mao, YANG Hong-zhi
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1200-1218.   DOI: 10.3969/j.issn.0253-4967.2023.05.010
    Abstract138)   HTML19)    PDF(pc) (10497KB)(135)       Save

    When the strong earthquakes occur, the deformation and rupture of overlying soil caused by the dislocation of focal faults is one of the important reason for the destruction of ground structures. In the process of a strong earthquake or large earthquake, the deformation reaction and failure of the overlying soil of underground concealed faults are very complicated. To study and analyze the characteristics and influencing factors of surface deformation and fracture of the overlying soil layer, in this paper, the influences of fault dip Angle, fault displacement, and overlying soil thickness on surface deformation and fracture of overlying soil are analyzed by the finite element numerical simulation method comprehensively. The results show that: 1)With the increase of fault vertical dislocation of 1m to 4m, the surface equivalent strain gradually increases, the surface rupture is more likely to occur, and the surface rupture width also wider. With the increase of the thickness of the overlying soil layer from 20m to 60m, and the increase of the fault inclination from 30°, 45°, 70° to nearly 90°, the surface equivalent strain is gradually smaller, the surface rupture is more likely to occur, and the surface fracture width becomes smaller, which means that the amount of dislocation required for the same rupture state needs to increase. 2)When the vertical dislocation of the fault is about 3.3%for the thickness of the overlying soil, the surface rupture occurs only as the fault dip angle is 30°, no surface rupture occurs as the dip angle is 45° and 70°. When the vertical dislocation of the fault is about 5% of the thickness of the overlying soil, the surface rupture occurs only as the fault dip angle is 30° and 45°, no surface rupture occurs as the dip angle is 70° and approaching 90°. When the vertical dislocation of the fault is about 6.6% of the thickness of the overlying soil, the surface rupture occurs as the fault dip angle is 30°、 45° and 70°, and surface rupture is expected to occur as the dip angle is approaching 90°. When the vertical dislocation of the fault is about 10% of the thickness of the overlying soil, the surface rupture occurs as the fault dip angle is 30°, 45°, 60°and approaching 90°. 3)When the amount of vertical dislocation and the thickness of the overlying soil are certain, the ratio of surface rupture width between the hanging wall and footwall which is less affected by fault dip ranges from 3︰1 to 3︰2~1︰1 with the increase of fault dip Angle from 30°, 45° to 70°. When the fault inclination Angle is 30°, with a decrease of vertical dislocation of 4m to 1m, or the increase of overlying soil layer thickness of 20m to 60m, the ratio of surface rupture width between hanging wall and footwall is slightly larger from about 3︰1. When the fault inclination Angle is 45°, with the decrease of vertical dislocation of 4m to 1m, or the increase of overlying soil layer thickness of 20m to 60m, the ratio of surface rupture width between hanging wall and footwall is slightly larger from about 2︰1. When the fault inclination Angle is 75°, with the decrease of vertical dislocation of 4m to 1m, or the increase of overlying soil layer thickness of 20m to 60m, the ratio of surface rupture width between hanging wall and footwall is slightly larger from about 3︰2~1︰1. Under the above fault dip conditions, the ratio of surface rupture width between hanging wall and footwall is less affected by the amount of vertical dislocation and the thickness of overlying soil. As the inclination is approaching 90°, the ratio of surface rupture width between the hanging wall and footwall is about 1︰1, which is not affected by the vertical dislocation and the thickness of the overlying soil layer. 4)The deformation and fracture of the overlying soil layer first began with the soil fracture at the interface of the fault bedrock and soil. With the increase in the amount of dislocation, a fracture point appeared on the surface when the fault dip Angle was 30°, 45°, and 70°. However, when the dip Angle of the fault was close to 90°, there were two initial rupture points on the surface. With the increase of vertical dislocation or the decrease of the overlying soil layer thickness, the overlying soil layer through fracture is finally formed.

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    WANG Jiang, CHEN Zhi, ZHANG Fan, ZHANG Zhi-xiang, ZHANG Su-xin
    SEISMOLOGY AND GEOLOGY    2023, 45 (3): 735-752.   DOI: 10.3969/j.issn.0253-4967.2023.03.008
    Abstract136)   HTML6)    PDF(pc) (4120KB)(71)       Save

    Based on the analysis and processing of field mobile observation data, soil gas Rn and CO2 degassing characteristics in the main fault zones of Xiong'an New Area(XNA)and its effects on the regional environment was preliminarily studied. Repeated observations of soil gas concentrations and fluxes in 2020 and 2021 for the seven measurement lines on the buried faults show that strong degassing characteristics exist in the main fault zones of XNA.
    The range of variation of the mean soil gas Rn fluxes for each profile is from 71.44 to 335.35mBq/m2·s, and the range of variation of the mean CO2 fluxes is from 25.96 to 78.23g/m2·d; the range of variation of the mean Rn concentration intensity is from 0.91 to 2.30, and the range of variation of the mean CO2 concentration intensity is from 1.13 to 2.61, which are comparable to the degassing intensity of soil gas in other typical fault and earthquake zones in the world. Except for the flux of Rn in the Niudong branch fault 1 and CO2 in the Niudong branch fault 2, the average values of Rn and CO2 fluxes in 2021 are higher than those in 2020.The maximum variation of soil gas Rn flux is 116%in the Niudong Fault, and the maximum variation of CO2 flux is 370%in the Niudong Fault; the variation of soil gas Rn concentration intensity is 119%in the Niudong Fault, but there is no significant variation of concentration intensity in other faults in both periods.
    The observation and analysis found that the areas of high soil gas concentration anomalies on the three seismic profiles in XNA are highly coincident with the distribution of deep faults, showing a concentrated degassing phenomenon along the fault zones. The AA' seismic profile on the west side exposes three hidden fractures, which are the Taihangshan Fault, Rongcheng Fault, and one unnamed fault on the west side. The AA' soil gas Rn and CO2 concentration profiles, corresponding to the location of the upward trend line of the Rongcheng Fault and the unnamed fault on the west side, show the characteristics of simultaneous single-peak type high-value anomalies of Rn and CO2 concentrations. This phenomenon may indicate that the Rongcheng Fault and the unnamed fault on the west side are still highly active.
    The BB' soil gas profile shows two areas of high soil gas concentrations respectively in the east and west. The western high value area shows synchronous peak anomalies of Rn and CO2 concentrations, and the location of the anomalies is basically consistent with the upward extension direction of the Xushui-Dacheng Fault. In the east, only single-peak anomalies in CO2 concentration are observed, and the magnitude of the anomalies is more significant than that in the western section, but there is no corresponding fault. Therefore, the synchronous peak anomalies of Rn and CO2 concentrations in the western section should be related to the fault activity of Xushui-Dacheng Fault, while the single-peak anomalies of CO2 concentrations in the eastern section may be non-tectonic.
    The CC' seismic profile contains the Taihangshan Fault, Niudong Fault, Niudong branch fault 1, Niudong branch fault 2, and several secondary faults. The peak anomalies of soil gas Rn concentration in the soil gas profile are detected at different degrees and near the locations corresponding to the upward extension trend lines of Niudong Fault, Niudong branch fault 1, and Niudong branch fault 2. All three peak anomalies of soil gas Rn concentration may be related to the activities of Niudong Fault, Niudong branch fault 1, and Niudong branch fault 2. However, the single-peak CO2 concentration anomaly is detected only above the Bazhou Depression, and the anomalous area basically overlaps with the area where the Bazhou Depression is located. The Bazhou Depression is one of the main areas of concentrated population in XNA, and the biological activity is relatively strong. According to the results of the existing study that soil gas CO2 generated by biological activities may also be the main recharge source of CO2 gas released from fault zones in the basin, the large-scale CO2 concentration anomalies detected above the Bazhou Depression may also be generated by biological activities in the basin.
    The results show that strong degassing characteristics exist in the main fault zones of XNA. The environmental effects of gas release from the main fault zones in XNA are evaluated by combining with the comprehensive prevention and control standards for indoor gas environmental pollution. The highest value of radon gas release in the main fault zone of XNA reaches 675mBq/m2·s in the Rongcheng Fault, and 395.70mBq/m2·s and 334.84mBq/m2·s in the Nudong branch faults respectively, the results indicate that it is necessary to carry out comprehensive radon prevention treatment for the buildings above the Rongcheng fault and Nudong fault zone. The preliminary estimation results of CO2 release show that the daily contribution of CO2 degassing from the main fault zones of XNA to the atmosphere is about 1 622.56t, and the annual contribution is as high as 0.59×106t. Attention should be given to its impact on the regional environment.
    The research results in this paper are significant for urban planning, environmental management and comprehensive assessment of the environmental impact of gas release from the fault zone in XNA.

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    LÜ Fang, MU Hui-min, LI Yan, GUO Wen-feng, YAO Lin-peng, GONG Jing-zhi
    SEISMOLOGY AND GEOLOGY    2023, 45 (3): 638-651.   DOI: 10.3969/j.issn.0253-4967.2023.03.003
    Abstract135)   HTML13)    PDF(pc) (4651KB)(82)       Save

    Hydraulic parameters of aquifers are important parameters for studying the rule of groundwater movement, and also key parameters for regional groundwater resource evaluation, groundwater flow numerical simulation and groundwater quantitative calculation. Aquifer hydraulic parameters also play an important role in the study of groundwater dynamic characteristics and their relationship with earthquakes. Among all kinds of hydrological responses caused by earthquakes, the change in well water level is the most common. Stress accumulation in the process of earthquake preparation, static stress caused by fault dislocation after the earthquake, and dynamic stress caused by seismic wave propagation will lead to the change of crust-media structure at local or regional scales, which will inevitably lead to the change of media characteristics(such as permeability)of the aquifer in the rock mass. As a result, the well water level will change. Therefore, the study of aquifer hydraulic parameters is of great significance to understand the role of groundwater in the process of earthquake preparation and occurrence to understand the hydrological response mechanism related to earthquakes.
    China has the largest seismic underground fluid observation network in the world, but most of the observation wells are transformed from geological and petroleum wells, lack complete basic data, and most of the hydraulic parameters of the aquifer are unknown. For underground water level real-time monitoring wells or seismic precursor information observation wells, hydraulic parameters can be obtained by traditional pumping test methods when the well is completed, but it is difficult to implement in order to ensure the continuity of observation data after the well is put into use. The hydraulic parameters of the-well-aquifer system often change with the change in the regional groundwater environment. In order to more accurately interpret the dynamic changes of the observed well water level, we need to obtain the hydraulic parameters of the-well-aquifer system under the current state. As a single well hydraulic test, the slug test has the characteristics of convenient operation, short test time, and low disturbance to aquifer, which can easily and relatively quickly obtain the hydraulic parameters of well-aquifer. With the advent of the digital water level measuring instrument of second sampling rate and its widespread use in the observation of underground fluid, the slug test method is more widely used in the determination of hydraulic parameters of the underground fluid aquifer.
    In this paper, the water-level time response data of 8 wells in Shanxi area were obtained by using the Slug test method, and the water conductance coefficient of the observed aquifer was estimated by selecting the corresponding data analysis model according to the attenuation type of water level. The coefficient of transmissivity of each well-aquifer is compared and analyzed by statistics of the co-seismic response of each well and discusses the reliability of the Slug test estimation results and the applicability of the method. The following conclusions are obtained: 1)The co-seismic response of wells with large transmissivity is usually of vibration type. 2)The hydrogeological parameters of the well-aquifer can be obtained dynamically by the Slug test, and the subtle changes of aquifer medium state can be captured to interpret the dynamic changes of well water level more accurately. 3)With the popularization of water level high-frequency sampling observation instruments, the Slug test has been well used in the parameter determination of high permeability medium in underground fluids.
    The results show that the slug test method can quickly and accurately obtain the hydraulic parameters of the well-aquifer, capture the change of aquifer medium state in time, interpret the dynamic change of well water level more accurately, and conduct quantitative analysis of the abnormal change of water level.

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    MA Si-yuan, XU Chong, CHEN Xiao-li
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 896-913.   DOI: 10.3969/j.issn.0253-4967.2023.04.006
    Abstract134)   HTML14)    PDF(pc) (10815KB)(91)       Save

    Earthquake-induced landslides, as an important secondary geological disaster, typically occurring during or shortly after an earthquake, have the characteristics of large quantity and scale, wide distribution, complex mechanism, serious casualties and economic losses, and long-duration post-earthquake effect. Rapidly and accurately obtaining the spatial distribution and potential hazard assessment of coseismic landslide following an earthquake is critical for emergency rescue and resettlement planning. Currently, the most commonly-used coseismic landslide hazard assessment methods include the data-driven machine learning methods and the Newmark method based on mechanics mechanism. The 2022 MW5.8 Lushan earthquake provides a valuable window for us to carry out rapid emergence assessment of earthquake-induced landslides with different evaluation models. In this study, a new generation of China's earthquake landslide hazard model(hereinafter referred to as Xu2019 model)and a simplified Newmark model are used to carry out the rapid landslide assessment of Lushan event. The Xu2019 model selects 9 earthquake-induced landslide inventories around China as training samples and uses a total of 13 influencing factors such as elevation, relative elevation, slope angle, and aspect, and etc. to generate a near real-time evaluation model for coseismic landslides based on the LR method. The model can rapidly assess coseismic landslides towards a single earthquake event according to the actual PGA distribution. For Newmark model, the cumulative displacement(Dn)is calculated by the critical acceleration(ac)and PGA maps. For the landslide inventory of this earthquake event, we completed the landslide inventory covering the entire affected area based on high-resolution optical satellite images(Planet)with 3m resolution acquired on 6 July 2022. Based on the coseismic landslide inventory including 2 352 landslides with an area of 5.51km2, the accuracy and applicability of the two models are estimated. The results show that the landslide area calculated based on Xu2019 model is 5.07km2, which is very close to the actual landslide area, and the predicted area calculated based on Newmark model reaches 21.3km2. From the perspective of the spatial distribution of the prediction results, the distribution of the predicted high failure probabilities of the two models is roughly same, with the high probability values mainly located on the left side of the seismogenic fault. However, the difference lies in the low probability predictions of the northwest region of Baoxing county by the Xu2019 model. A zoomed-in view of a specific area comparing the spatial distribution of predicted landslide probabilities with the landslide abundance area shows that most actual landslide are concentrated in the medium to high failure probability areas predicted by the Xu2019 model, with only a few sporadic events occurring in the low probability zone. On the other hand, the Newmark model primarily identifies high instability probability regions in steep slope areas, which correspond closely to the actual landslide and collapse occurrences. However, the predicted hazard level of the northwest region i.e. the landslide highly developed area is obviously low by Xu2019 model, while the prediction result based on Newmark model for the southwest region is obviously overestimated. In terms of the LR model, the prediction results are very close to the actual landslide distribution, and the majority of the landslides are essentially located in areas with a high failure probability, indicating that the model has a relatively high prediction accuracy. The ROC curve is used to assess the model's accuracy. The results suggest that the model based on Xu2019 outperforms the Newmark model, with a prediction accuracy of 0.77, while the prediction accuracy of the Newmark model is 0.74. Overall, both two models have good practicability in the rapid evaluation of cosesimic landslide. However, the Newmark model needs multi parameter input, and these parameters themselves and the way of human acquisition are uncertain, which results in that the model evaluation is greatly affected by subjectivity.

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