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20 December 2022, Volume 44 Issue 6

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Research paper

LATE QUATERNARY SHORTENING RATE OF THE SANSUCHANG ANTICLINE, SOUTHERN LONGMEN SHAN FORELAND THRUST BELT

ZHANG Wei-heng, CHEN Jie, LI Tao, DI Ning, YAO Yuan

2022, 44(6):  1351-1364.  DOI: 10.3969/j.issn.0253-4967.2022.06.001

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

PALEOEARTHQUAKES AND VERTICAL SLIP RATES ON THE HUAI RIVER-NÜSHAN LAKE SEGMENT OF FAULT F₅ IN THE MIDDLE SECTION OF THE TANLU FAULT ZONE

YANG Yuan-yuan, LI Peng-fei, LU Shuo, SHU Peng, PAN Hao-bo, FANG Liang-hao, ZHENG Hai-gang, ZHAO Peng, ZHENG Ying-ping, YAO Da-quan

2022, 44(6):  1365-1383.  DOI: 10.3969/j.issn.0253-4967.2022.06.002

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

SCHMIDT HAMMER EXPOSURE AGE DATING OF ANCIENT EARTHQUAKE-INDUCED BEDROCK LANDSLIDES AND ROCK AVALANCHES IN THE NORTHERN MARGIN OF QINLING MOUNTAINS

SHI Wen-fang, XU Wei, YIN Jin-hui, ZHENG Yong-gang

2022, 44(6):  1384-1402.  DOI: 10.3969/j.issn.0253-4967.2022.06.003

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It is difficult to use traditional trenching and field geological investigation to yield the age of paleoseismic events along active fault in western mountainous areas of China where the geomorphic trace mark and sediments are often eroded or altered by human activities. The recurrence interval of paleoearthquake possesses greater uncertainty. It is necessary to yield ages of paleoearthquake event from different ways and examine the reliability of paleoearthquake results. In these regions, an earthquake with magnitude greater than 7 can produce rock avalanches around 200~400km away from the epicenter, such as the Wenchuan earthquake in 2008, due to their structure setting of strong neotectonic activity and the higher topographic relief. Therefore, the seismic bedrock landslide and rock avalanche can record the occurrence time, intensity and damage of strong earthquake in the mountainous area. This provides a new way to assess the frequency and intensity of paleoearthquake occurring in the intraplate continental areas(such as the north-south seismic zone)where strong earthquakes recurred for hundreds to thousands of years based on the seismic landslide records. Identifying ancient earthquake bedrock collapse relics in Quaternary deposits and accurately determining their ages will not only help broaden the study on the recurrence history of active fault, but also assess the earthquake risk in mountainous area.
As shown by previous studies, the Schmidt-hammer exposure-age dating(SHD)method is a relatively simple, rapid, cheap and non-destructive in-situ exposure age dating method. In this study, ancient earthquake bedrock landslides and rock avalanches with known historical records distributed on the Qinling northern piedmont fault and the Huashan piedmont fault are used to preliminarily establish the rock weathering factor with age calibration curve. The rebound values of rock surface at dozens of sampling sites of each rock avalanche and landslide are measured by Schmidt hammer and analyzed statistically. The weathering factor of the exposed rock of each rock avalanche and landslide is calculated and the solution of SHD method is discussed. The reliability of SHD is evaluated according to the measured data and the records of historical age. The main conclusions are as follows:
(1)The Schmidt hammer rebound value of rock surface at three ancient earthquake bedrock landslides and rock avalanches is negatively correlated with their historical ages. The older the historical record age, the lower the average rebound value of the rock, and vice versa. Based on the statistical analysis of weathering factors of rocks of bedrock landslides and rock avalanches, a preliminary age calibration curve is obtained as T=(19 723±888)×f_w-(2 145±166). This curve can be used to infer the bedrock landslides and rock avalanches of more than 5×10² a BP, and it provides a new relative dating method for the ancient bedrock landslide and rock avalanches within the age of 3 000a BP in the northern margin of the Qinling Mountains.
(2)Under the climatic and lithological conditions of the northern margin of the Qinling Mountains, the relative ages of bedrock landslide and rock avalanches can discriminate the interval of millennium scale according to the rock rebound value measured by Schmidt hammer. However, it cannot distinguish the difference in weathering degree of the bedrock landslide and rock avalanches with the interval of less than 500 years.
(3)The Schmidt hammer rebound value measured repeatedly on fresh rocks shows that the fluctuation range of the rebound values is small, within the value of 0-3, which is helpful to rapidly select qualified sampling sites for terrestrial in-situ cosmogenic nuclide dating(TCND). Thus, the Schmidt hammer value can be used to evaluate whether the rock samples have the problem of nuclide inheritance induced by complex exposure history such as post-exposure and secondary transportation. This would introduce greater objectivity to the sample selection and possibly require less samples, thus reducing the costs; meanwhile, it will improve the dating efficiency and ensure the reliability of TCND. Therefore, SHD method is a valuable complementary method to TCND.
(4)Under the climatic and lithological conditions in the northern margin of the Qinling Mountains, the rebound value decreases by (25%±1%) for rocks after weathering for 2ka, by (16%±1%) for 1ka, and by (15%±1%) for 0.5ka.

STUDY ON THE SLIP RATE OF THE NORTH ZHONGTIAO SHAN FAULT SINCE THE LATE MIDDLE PLEISTOCENE

ZHANG Xiu-li, XIONG Jian-guo, ZHANG Pei-zhen, LIU Qing-ri, YAO Yong, ZHONG Yue-zhi, ZHANG Hui-ping, LI You-li

2022, 44(6):  1403-1420.  DOI: 10.3969/j.issn.0253-4967.2022.06.004

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

QUANTITATIVE ANALYSES OF GEOMORPHOLOGIC FEATURES IN RESPONSE TO LATE QUATERNARY TECTONIC ACTI-VITIES ALONG THE MAQIN-MAQU SEGMENT, EAST KUNLUN FAULT ZONE

LI Zhao, FU Bi-hong

2022, 44(6):  1421-1447.  DOI: 10.3969/j.issn.0253-4967.2022.06.005

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

GEOMETRIC STRUCTURE CHARACTERISTICS OF XINYI SEGMENT OF ANQIU-JUXIAN FAULT

ZHANG Hao, WANG Jin-yan, XU Han-gang, LI Li-mei, JIANG Xin, ZHAO Qi-guang, GU Qin-ping

2022, 44(6):  1448-1468.  DOI: 10.3969/j.issn.0253-4967.2022.06.006

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

STRUCTURAL DEFORMATION CHARACTERISTICS OF BO-A FAULT IN THE SOUTHWESTERN MARGIN OF TURPAN BASIN

MA Jian, WU Guo-dong, LI Jun, HUANG Shuai-tang

2022, 44(6):  1469-1483.  DOI: 10.3969/j.issn.0253-4967.2022.06.007

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The Bolokenu-Aqikekuduke Fault(Bo-A Fault)is a large-scale right-lateral strike-slip fault zone, which starts in Kazakhstan in the west, enters China along the NW direction, passes eastward through Alashankou, Lake Aibi and the southwestern margin of Turpan Basin, and terminates in the Jueluotage Mountain, with a total length of about 1 000km. At present, researches on the fault mainly focus on the area from Lake Alakol to Jinghe.
Through satellite images, it can be found that the Bo-A Fault enters the southwestern margin of the Turpan Basin in the SE direction, and offset various landforms such as river terraces and alluvial fans, forming clear linear features on the surface, which indicates that there have been obvious activities since late Quaternary in this fault section. However, no detailed research has been carried out on the tectonic deformation characteristics of the Bo-A Fault in this area. The active characteristics of the faults in the southwestern margin of the Turpan Basin are studied, and the results are helpful to understand the role of the Bo-A Fault in the Cenozoic tectonic deformation of the Tianshan Mountains.
The study area is located in the southwestern margin of the Turpan Basin, where three stages of alluvial-proluvial fans are developed. The first-stage alluvial-proluvial fan is called Fan3, which was formed earlier and its distribution is relatively limited, formed roughly in the early late Pleistocene; The second-stage alluvial-proluvial fan is called Fan2, which is the most widely distributed geomorphological surface in the study area. The geomorphic surface in this period was roughly formed from the late Pleistocene to the early Holocene. The third-stage alluvial-proluvial fan is called Fan1, which belongs to the Holocene accumulation, most of which are located at the outlet of gullies near the mountain passes, forming irregular fan-shaped inclined surfaces.
To the west of Zulumutaigou, the fault offset the Fan3 alluvial-proluvial fan, forming dextral dislocation and fault scarp of the gully on the surface. The measurement shows that the amount of the dextral dislocation produced by the fault is between 22m and 40m. The height of the scarp is 3.9~4.2m. The section exposed by the fault shows that the Paleozoic bedrock thrust northward onto the Quaternary gravel layer, and the fault fracture width is about 1m, which reflects that the Bo-A Fault also has a certain thrust component. On the east bank of Zulu Mutaigou, the fault offset the Fan3 alluvial-proluvial fan, and the measurement results show that the offset of the gully is between 46.3m and 70.2m. To sum up, the movement mode of the Bo-A Fault in the study area is dominated by dextral strike-slip.
On the Fan2 alluvial-proluvial fan at the northwest of Zulu Mutaigou, there are two secondary faults arranged in a right-step en-echelon pattern, forming high scarps with a height of 1.6~3.9m on the surface. Trench profiles reveal that both faults are SW-dipping thrust faults, thrusting from south to north, and they are preliminarily judged to be formed by the expansion of the Bo-A Fault into the basin.
There are mainly three stages of alluvial-proluvial fans developed in the study area. Although no specific dating results have been obtained in this work, we believe that the age of the Quaternary landforms in the study area is the same as that in the Chaiwopu Basin, which is only separated by a mountain. Quaternary geomorphological ages are basically the same. Through geomorphological comparison, we believe that the age of Fan2 alluvial-proluvial fan is 12~15ka, and the age of Fan3 alluvial-proluvial fan is 74ka. It is estimated that the dextral slip rate of the Bo-A Fault is about 1mm/a since the formation of Fan3, and the vertical movement rate of the fault is about 0.13~0.32mm/a since the formation of Fan2.
According to GPS observations and geological data, the NS-direction shortening rate in the East Tianshan area can reach 2~5mm/a. Through this study, it can be found that the Bo-A Fault also plays a role in regulating the near-NS-trending compressive stress in the East Tianshan area by accommodating the compression strain inside the Tianshan Mountains mainly through the NWW-directed right-lateral strike-slip motion. In addition, in the study area, the youngest fault scarp is located on the Fan2 alluvial-proluvial fan at the north of the main fault. It is preliminarily judged that the latest activity of the Bo-A Fault has a tendency to migrate from the mountain front to the basin.

RESEARCH ON SEMI-AUTOMATIC EXTRACTION METHOD OF SEISMIC SURFACE RUPTURES BASED ON HIGH-RESOLUTION UAV IMAGE: TAKING THE 2021 M_S7.4 MADUO EARTHQUAKE IN QINGHAI PROVINCE AS AN EXAMPLE

LI Dong-chen, REN Jun-jie, ZHANG Zhi-wen, LIU Liang

2022, 44(6):  1484-1502.  DOI: 10.3969/j.issn.0253-4967.2022.06.008

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Field investigations of large earthquakes indicate that earthquakes with a magnitude greater than 6.5 often produce seismic surface rupture zones ranging from thousands of meters to tens of kilometers on the earth’s surface. The geometric structures of surface ruptures contain the kinematic characteristics of seismogenic structures, which can not only provide critical quantitative data for analyzing the spatial distribution law of co-seismic displacement of active faults and the width of active fault deformation zone, but also have an important significance for understanding the kinematic mechanism and deformation law of seismogenic faults.
At present, the conventional methods to obtain earthquake surface ruptures mainly include the field geological survey and visual interpretation of the remote sensing image. Although these two methods can get the rough geometry of coseismic surface ruptures, they both have certain limitations. The reliability of the field geological survey method is high. However, intra-continental earthquakes often occur in places with complicated topography, and lots of sites are difficult to reach, leading to incomplete data and failure to draw detailed features of the fracture zone. Meanwhile, the field geological survey is often time-consuming and laborious. Although the visual interpretation of remoting sensing images can be used to interpret surface fractures in areas that cannot be reached by the geological field survey, the result accuracy is vulnerable to the impacts of interpreters’ experience. The whole process of interpretation is still time-consuming and labor-intensive and the extraction results are mostly linear surface ruptures, so it is difficult to accurately obtain fine features such as the width of the surface rupture zone. Therefore, the automatic extraction of fine structures of seismic surface ruptures, especially micro-rupture surfaces, is an urgent problem in active tectonic studies.
The remote sensing images obtained through satellite platforms have low resolution and are susceptible to weather factors, and the extracted surface rupture fineness is not enough. The UAV platform, on the other hand, is low-cost to use, can fly at a low altitude, is not affected by clouds and fog, and can acquire images with a centimeter-level resolution, which provides conditions for extraction the fine structure of surface ruptures of large earthquakes. Thus, to solve the problem that it is difficult to obtain surface ruptures of large earthquakes quickly, this study proposes an object-oriented “Rough segmentation and Fine extraction” method based on object-oriented and color segmentation theories of color space, which realizes the semi-automatic extraction of features of seismic surface rupture zone. The processing workflow of the method is as follows: First, the original image is cropped by the custom irregular raster cropping method designed in this study to obtain ROI(the Region of Interest). Second, the color space of ROI is converted into HSV, and the HSV color space of ROI is segmented into surface rupture candidate area by using brightness and hue tone values. And then, the surface rupture candidate area is processed by expansion operation of binary mathematical morphology. Third, the surface rupture candidate area is transformed into a series of sub-area objects by the contour tracking method. Fourth, the surface rupture is refined using the spectral standard deviation, spectral mean and the length-width ratio of the smallest surrounding rectangle as characteristic parameters. Finally, the results are output as the vector surface of surface ruptures.
The effectiveness of the proposed method is analyzed by taking the high-resolution UAV image data of the M_S7.4 Maduo earthquake in Qinghai Province as an example. The results show that the proposed method can effectively remove the noises such as the river channel similar to the characteristics(i.e., the color and shape features)of the surface rupture and extract the delicate structures of the surface rupture zone quickly and accurately, except that several poor extraction results were caused by the limitation of image resolution and the destruction of surface rupture caused by river erosion. The extraction results are highly reliable and can be used to extract quantitative parameters of surface ruptures in large earthquakes. Thus, the semi-automatic extraction method of seismic surface ruptures established in this study can provide a feasible scheme for the rapid extraction of delicate structures from surface ruptures and analysis of surface deformation characteristics after a large earthquake.

COMPARATIVE STUDY ON THERMAL ANOMALIES DETECTION METHOD BEFORE STRONG EARTHQUAKE: TAKING THE 2014 M_W6.9 YUTIAN EARTHQUAKE AS AN EXAMPLE

WU Wei-ying, SHAN Xin-jian, QU Chun-yan, LI Xin-yan

2022, 44(6):  1503-1520.  DOI: 10.3969/j.issn.0253-4967.2022.06.009

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The reliability of anomaly extracting methods is crucial for pre-seismic thermal anomalies research. However, there is a lack of relevant researches. We compared two commonly used anomaly extracting methods, Z-score(ZS)and Robust satellite technology(RST)method, taking the 2014 Yutian earthquake as a typical example and the 2008 Wenchuan earthquake as a validation. The four aspects of extracted results are compared qualitatively and quantitatively, including the extraction effect, sensitivity to slight change, suppression of background information and indication of seismic information in the actual earthquake case. Moreover, the extracted results of validation case are used to validate the reliability of typical case results. Many intermittent anomalies in surface temperature and outgoing longwave radiation appeared before Yutian earthquake. The frequency of anomalies increases with the proximity of earthquake. The spatial distribution of surface temperature and outgoing longwave radiation anomalies gradually concentrated around the fault zone at the same time. The largest surface temperature and outgoing longwave radiation anomalies occurred one month before Yutian earthquake. The difference between the extraction results of ZS and RST method is mainly manifested in the frequency and amplitude of anomaly changes. The frequency and amplitude of anomaly changes obtained by RST method are higher than those obtained by ZS method. To further explore the reason for these differences, we further evaluate the two methods quantitatively by combining the data of two non-seismic years before and after Yutian earthquake respectively. The sensitivity of anomaly extraction method represents its ability to identify the slight changes of thermal parameters caused by the seismogenic process. The two methods are sensitive to slight changes, but the RST method is better than ZS method. The background information represents normal variation in surface temperature and outgoing longwave radiation caused by non-seismic factors. Suppression of background information determines the accuracy of extraction results. The comparison results show that both methods have certain suppression effect to background information, but the ZS method is better. The spatial distribution of pre-seismic thermal anomalies is an important index for predicting earthquake information(e.g. time of occurrence and location of epicenter). The results of quantitative comparison through normalized distance index show that for surface temperature data, ZS method is slightly better than RST method in indicating the location of epicenter. However, RST method is better for outgoing longwave radiation data. The maximum value of normalized distance index of ZS method occurred closer to the origin time of earthquake. We used the same quantitative evaluation method for the validation earthquake case. The verification results show that in addition to the sensitivity to anomaly changes, the comparison results of the two earthquake examples are similar in terms of the ability to suppress background information and indicate earthquake information. The difference is that ZS method has a better ability to suppress background information and RST method is better in indicating earthquake epicenter in the verification earthquake example. The main reason for the difference in extraction effect between the two methods is that the RST method averages the ground feature classification, and the difference between the observed value and the average value of the classification makes the RST method have a certain amplification effect on the weak signal. The difference between the typical earthquake case and the verification earthquake case is mainly due to the different complexity of the object types in the regions. Based on the above research results, we believe that ZS method and RST method have certain ability to extract pre-seismic anomalies. However, comparatively speaking, the RST method also has a good effect on the extraction of anomalies caused by other factors, and there is uncertainty in the ground feature classification. We believe that ZS method is a more appropriate and simple anomaly extraction method in the general seismic anomaly change extraction.

STUDY ON THE DEEP STRUCTURAL CHARACTERISTIC OF MAIN ACTIVE FAULTS IN HENAN PROVINCE AND ITS ADJACENT AREAS

XU Zhi-ping, ZHANG Yang, YANG Li-pu, XU Shun-qiang, JIANG Lei, TANG Lin, LIN Ji-yan

2022, 44(6):  1521-1538.  DOI: 10.3969/j.issn.0253-4967.2022.06.010

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There are many first-order intersecting tectonic units and different strike faults developed widely in Henan Province, and many historical earthquakes with magnitude 6 and above occurred, which have brought great losses to people’s lives and property. In order to effectively reduce the risk of earthquake disaster in Henan Province and understand the deep seismogenic environment, we have carried out a systematic study on the deep structural characteristics of these active faults. Firstly, based on the high-precision Bouguer gravity anomaly data of Henan Province and its adjacent areas, we obtained the characteristics of gravity anomaly fields at different spatial scales in the study area by using the multi-scale wavelet analysis method. Then the detailed characteristics of different orders wavelets of Bouguer gravity anomaly field in the study area and its relationship with regional structure were analyzed. We found that within 14km of the crust, the regional tectonic activity has an obvious control effect on the trend of gravity anomaly zone. The trend of gravity anomaly zones is obviously different in different tectonic units in the study area. In the north of Henan, the trend of gravity anomaly zones is NE, which is consistent with the regional tectonic trend. The horizontal density difference is obvious. In the south of North China depression and Qinling-Dabie uplift area, the trend of gravity anomaly zones is NW, NWW and EW. In the differential uplift area of western Henan, the trend of gravity anomaly zones is NE. At the 27km depth of the crust, most gravity anomalies are in a clumpy shape, and the consistency between the trend of the gravity anomaly and the regional structure decreases, indicating the differences in regional tectonic stress effect and formation process at different depths of the crust. For example, under the northward compression from Qinling-Dabie uplift, the crust structure in the south of North China depression is different, and the difference gradually decreases from shallow to deep. At the same time, with the increasing of depth, the boundary between Qinling-Dabie uplift and southern North China depression moves to the Pingdingshan and Luohe. Our results show that the regional deep faults have an obvious control over the distribution of gravity anomalies, and the linear transition zone of gravity anomalies often corresponds to the deep faults. In order to obtain the distribution characteristics of active faults in Henan Province and adjacent areas, we analyzed the wavelet multi-scale decomposition of Bouguer gravity anomaly and identified 38 faults. Based on the seismic and geological results, we interpreted the 38 faults, including10 shallow faults in the upper crust with a depth of less than 8km, 15 faults at the bottom of the upper crust with a depth of 12~14km and 13 faults in the lower crust with a depth of 27km. In the study area, the deep faults control the boundary of the first-order tectonic units, such as Liaocheng-Lankao Fault, Tangxi Fault, Xinxiang-Shangqiu Fault, etc., and many moderately strong earthquakes occurred in these faults in history. At last, we analyzed the deep tectonic environment of historical earthquakes with magnitude 6 and above in Henan Province. The results show that the historical earthquakes with magnitude 6 in Xuchang locate near the boundary zone of second-order tectonic units. Other historical earthquakes with M6.0 locate below the secondary uplift or depression controlled by deep and large faults in the crust, such as Puyang earthquake which locates in the Dongpu depression. It can be concluded that the intersections of gravity anomalies zones with different trends, the deep seated fault-controlled intra-crust low gravity anomaly areas, and the intersections of deep seated fault with different strikes are the deep tectonic background and favorable locations for generating earthquakes with magnitude 6 and above in Henan Province. The results of analysis of the characteristics of major deep active faults in Henan Province expanded our understanding of the tectonic environment of the study area and provided a geophysical basis for earthquake prevention and disaster reduction in Henan Province in the future.

A NOVEL EXTRACTION METHOD OF PRE-EARTHQUAKE GRACE GRAVITY ANOMALY INFORMATION BASED ON MAXIMUM SHEAR STRAIN

SONG Dong-mei, WANG Hui, SHAN Xin-jian, WANG Bin, CUI Jian-yong

2022, 44(6):  1539-1556.  DOI: 10.3969/j.issn.0253-4967.2022.06.011

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The occurrence of earthquakes is closely related to the crustal tectonic movement and the migration of earth mass, which consequently cause the changes of the earth‘s gravitational field. Global time-varying gravity field data obtained by GRACE gravity satellite can be used to detect pre-seismic gravity anomalies. For example, gravity signals caused by several large earthquakes, such as the 2005 M_W8.6 Indonesia earthquake, the 2010 M_W8.8 Chile earthquake and the 2011 M_W9.0 Japan earthquake, have been successfully extracted using GRACE data. However, previous studies on GRACE satellite-based seismic gravity changes focused more on the dynamics of the co-seismic gravity field than on the pre-seismic gravity anomalies which are of great significance for the early warning of earthquakes. Moreover, the commonly adopted difference disposal of the gravity field with the gravity field of adjacent months or the average gravity field of many years when obtaining gravity anomalies cannot effectively remove the inherent north-south stripe noise in GRACE data. On the contrary, it is more likely to cause the annihilation of the medium-high order information in GRACE gravity field model, which results in the loss of some gravity information related to tectonic activities. To explore the pre-seismic gravity anomalies in a more refined way, this study proposes a method of characterizing gravity variation based on the maximum shear strain of gravity, inspired by the concept of crustal strain. In other words, the gravity strain tensor is obtained by further calculating the second-order gradient of the increment of disturbance potential after the removal of hydrological disturbance, and then the maximum shear strain of gravity is ultimately generated to characterize the pre-earthquake tectonic activities. Then, to better understand the seismogenic process of the fault zone by further extracting the pre-earthquake anomalous changes, the data of the maximum shear strain time series are analyzed in this study by means of the offset index K to describe the gravity anomaly. Because the maximum shear strain is calculated by the second-order gradient of GRACE gravity field, this method can suppress the stripe noise better than the difference disposal, thus effectively improving the sensitivity of gravity anomaly detection. The exploratory experiments are carried out in the Tibetan plateau and its surrounding area, which locates among the Pacific Ocean, the Indian Ocean and Eurasia, with the highest altitude, most complex topography and frequent strong earthquakes. Ultimately, the Wenchuan earthquake and Nepal earthquake were used as an example to complete the extraction of pre-earthquake gravity anomaly information by the above method, and the pre-earthquake tectonic activity of the fault zones was analyzed. The results show that a large area of gravity anomalies consistent with the spatial distribution of the fault zone appeared on the Longmenshan fault zone during the half a year before the earthquake, and the maximum anomalous value appeared within 50km from the epicenter, while no anomalies appeared during the non-earthquake period. In addition, compared with the traditional methods, the proposed method has a better ability to extract anomaly information of gravity field, which provides a new idea for understanding the dynamic mechanism of large earthquakes using GRACE data.

ANALYSIS OF GRAVITY VARIATION CHARACTERISTICS AND ITS SEIMOGENIC MEANING BEFORE AND AFTER YANGBI M_S6.4 EARTHQUAKE

HUANG Jiang-pei, CAO Ying, LIU Dong, ZHENG Qiu-yue, CHEN Zheng-yu, WANG Qing-hua

2022, 44(6):  1557-1573.  DOI: 10.3969/j.issn.0253-4967.2022.06.012

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On May 21, 2021, an earthquake with a magnitude of 6.4 occurred in Yangbi, Yunnan, China, which broke the calm of earthquakes above M_S6.0 in Yunnan Province for nearly seven years since 2015. This paper collects the relative gravity observation data of Yunnan from 2015 to 2021, and adopts the parameters estimation method based on Bayesian principle to obtain the station-value series of multi-year time-varying gravity field. Through years of time-varying gravity data, and according to the guiding idea of “Seeking the source from the field and combining the field and the source”, the series data are divided into different “time-space domains”. The “time domain” takes the epicenter as the center to analyze the one-year difference changes of the measuring points within a radius of 200km since 2015 and the multi-year cumulative change deduced backward from the origin time of earthquakes. The “space domain” takes the epicenter as the center, collects the point value change sequence of the measuring points with the radius of 200km, 100km, 50km and 25km respectively, and uses the method of “layered processing and step-by-step analysis” to analyze the characteristics of gravity change before the earthquake. The research results show that the geological active period of this earthquake is 2017-2018, the geological activity is relatively stable in 2018-2020 and there was the phenomenon of “escape” of underground materials in 2020 to 2021, and the earthquake occurred in the “escape” period. The abnormal gravity near the epicenter is characterized by a four-quadrant distribution, the positive and negative changes of gravity in the three years before the earthquake were more significant, reaching 80×10^-8m/s², the change of gravity in the two years before the earthquake was small, but the four-quadrant distribution characteristics were still maintained. After the earthquake, the gravity field in the study area changed inversely, and the magnitude of change reached the cumulative change of 3 years before the earthquake. This earthquake occurred near the center of the four quadrants of the gravity anomaly and coincided with the northern segment of the Honghe fault zone, so it is inferred that the Honghe Fault is the seismic structure of the Yangbi earthquake. The research methods and results of this paper can provide a reference for earthquake examples for the study of time-varying gravity field variation characteristics and the analysis and interpretation of pre-earthquake gravity signals, as well as for exploring and predicting the areas of future mid-strong earthquakes.
This paper systematically analyzes the characteristics of gravity changes in different “time-space domains” of the 2021-05-21 Yangbi M_S6.4 earthquake, and attempts to analyze its seismogenic meaning. The following conclusions can be drawn:
(1)The gravity anomaly appeared before the Yangbi earthquake and lasted for a long time. The phenomenon of energy accumulation appeared four years before the earthquake.
(2)The variation of gravity point value within the radius of 200km is basically consistent, and the variation of gravity point value within the radius of 100km is highly consistent, indicating that the range of gravity anomaly during the earthquake preparation period reached more than 200km, which is consistent with the research results of previous empirical statistics.
(3)The geological activity associated with this earthquake started at about 4 years before the earthquake, then there was a calm period of up to two years. Half a year before the earthquake, there was an overall decline in the value of gravity points near the epicenter, which is in line with the common sense in seismology that “seismic energy gathers first and then diffuses, and the earthquake occurs in the diffusion period”, but there is a calm period before this earthquake.
(4)The “four quadrant” phenomenon appeared and gradually increased in the variation trend diagram of gravity field in one year, two years and three years before the earthquake.
(5)The “four quadrants” of the variation trend of the gravity field before the earthquake are generally consistent with the focal mechanism solution and the variation of the coseismic displacement field issued by the USGS and Seismology research group, indicating that the phenomenon of “four quadrants” variation in the gravity field will appear before earthquake of strike-slip fault geological structure, and the gravity variation can reveal to a certain extent the earthquake-generating mechanism before earthquake.
(6)The change trend of gravity after the earthquake is completely opposite to that before the earthquake, which is the release and recovery of compression and accumulation of underground materials before the earthquake. Therefore, it is considered that this earthquake can effectively release the earthquake preparation energy in this area, and it is unlikely that earthquakes of M_S≥6.0 will occur again in a short time.

EVIDENCE AND REFINEMENT OF GEOMAGNETIC DIURNAL INDUCED CURRENT ANOMALY BASED ON HARD BODY SEIS-MOGENIC MODEL: TAKING THE 2016 ZADOI M6.2 EARTH-QUAKE, THE 2017 JIUZHAIGOU M7.0 EARTHQUAKE, AND THE 2017 MILIN M6.9 EARTHQUAKE AS EXAMPLES

DAI Yong, WU Ying-yan, FENG Zhi-sheng, YAO Li, JIANG Chu-feng, SUN Jun-song, ZHANG Xin, FENG Li-li, LI Jun-hui

2022, 44(6):  1574-1596.  DOI: 10.3969/j.issn.0253-4967.2022.06.013

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This paper studies the linear concentrated distribution of geomagnetic diurnal induced current and the spatial distribution characteristics of short-term in-situ recurrence anomalies in the 1~3 years before the 2016 Zadoi M6.2 earthquake, the 2017 Jiuzhaigou M7.0 earthquake and the 2017 Milin M6.9 earthquake. The main conclusions are as follows:
(1)The overlapping segment anomalies occurring within 1~3 years before strong earthquakes usually have the phenomenon of seismic gaps. The overlapping segment gap is a large high-resistance body and also a hard body(hard inclusion)in the seismogenic model of hard body. Overlapping segment gap and seismic gap are the detection results of hard body with different depth distribution range by different physical detection methods. The distribution range of hard body is delineated by seismic gap in the upper and middle crust and overlapping segment gap in the middle-lower crust and upper mantle. The overlapping segment gaps occurred within 1~3 years before strong earthquakes, which are the anomalies in the third stage of the seismogenic model. The seismic gap before strong earthquakes has different stages. For example, background gaps are formed decades of years before strong earthquakes, and the gaps formed again about 1 year before the earthquakes.
(2)The overlapping segment anomalies occurring within 1~3 years before strong earthquakes reflect the formation of short-time high conductive current channels in the high conductive layers among high resistance bodies. These short-time high conductive current channels are caused by the mutually detached slip events with up-arching property among the high-resistivity bodies located in the middle-lower crust and upper mantle, resulting from the upwelling of deep thermal fluid. They are the events in which the energy in the middle-lower crust and upper mantle migrates to the hard body in the seismogenic model of hard body, while the seismic gap events are the ones in which the energy in the middle-upper crust migrates to the hard body before the earthquake.
(3)Based on the results of seismic high-pressure fluid experiments in recent years, and combined with the mechanism of overlapping segment seismic anomalies, it is considered that each sealed high-pressure fluid in the seismogenic fault of the source body will not rupture at the same time in the impending earthquake stage. The original free water in the fault, the sealed high-pressure fluid broken in the earlier stages, and the high-pressure thermal fluid upwelling into the fault in the deep may make the seismogenic fault of the source body full of free water, and may form a high-conductivity current channel in the fault with impending earthquake significance. The high-conductivity current channel may be a real impending earthquake anomaly. Obviously, it is found that the high-conductivity current channel in the fault in the impending earthquake stage has practical significance for the short-term and impending earthquake prediction.
(4)The detachment slip events detected from the overlapping segment anomalies are located below the strong earthquake source, which is similar to the phenomenon that slow earthquake zone is located below the earthquake zone. Although the relationship between slow earthquakes and earthquake above them is unclear, some scholars believe that slip events produce stress accumulation on the surface of locked plates. A slip event may trigger a destructive earthquake, that is, a high-incidence period of intermittent tremors and slips can produce a peak period of seismicity. The above views on slow earthquakes are similar to the relationship between the linear overlapping segment anomalies of induced current associated with geomagnetic diurnal variation and earthquakes. The detachment slip events detected from the overlapping segment anomalies may be similar to the inter-plate slow earthquake or slow slip involving the upwelling and migration of thermal fluid under the continent, but this speculation needs to be demonstrated based on the research results of seismology.

GEOCHEMICAL CHARACTERISTICS OF SOIL GAS IN THE FAULT ZONES OF NORTH TIANSHAN, XINJIANG

JIANG Yu-han, GAO Xiao-qi, YANG Peng-tao, LIU Dong-ying, SUN Xiao-long, XIANG Yang, ZHU Cheng-ying, WANG Cheng-guo

2022, 44(6):  1597-1614.  DOI: 10.3969/j.issn.0253-4967.2022.06.014

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The soil gas concentration and escape rate value can sensitively and objectively reflect the underground state of stress, strain and tectonic activity. In addition, abnormal phenomena of fault soil gas often occur before and after seismic activity. It is often used to identify the active state of fault zones, explore hidden faults and assess earthquake risk.
As an important geochemical method, the soil gas measurement is an important geochemical method to reveal fault properties and fault activities and other tectonic activities. In this study, we laid out 8 measurement lines of soil gas along the Borokonu-Aqikkuduk Fault, the Kusongmuchike piedomont fault, the Dushanzi-Anjihai Fault, the Horgos-Tugulu Fault, the Kashi River Fault and the Nalati Fault in two earthquake risk areas of the north Tianshan Mountains in Xinjiang, namely, the “North Tianshan Wenquan-Jinghe M7 earthquake risk area” and the “Wusu-Hejing M6 earthquake risk area”. From 2017 to 2020, a total of 6~7 phases of measurements were carried out to make clear the distribution characteristics of Rn, CO₂ and Hg concentrations along these faults. There have been many moderate/strong earthquakes near the above-mentioned faults, and it is of great significance of soil gas measurement on these faults for us to gain a deep understanding of the fault activity characteristics and earthquake risk.
In this paper, the spatial distribution characteristics of fault soil gas are analyzed based on the multi-period measurement results, and the activity of the fault zone and the regional earthquake risk are discussed respectively. The results show that: 1)The Rn concentrations are more stable than that of CO₂ and Hg along each measurement line, which can be used as an effective indicator gas for analyzing the distribution of fault zones, indicating the location of fault fractures and judging the activity of faults. Since there are many interference factors of CO₂ and Hg concentrations, they can be used as an auxiliary means. In most cases, the distribution of Rn concentrations on other measuring lines is of single-peak shape, indicating that the soil gas concentration is higher at the outcrops of the fault. However, the concentrations of Rn in the Kusongmuchike piedmont fault and the Horgos-Tugulu Fault are higher, and the distribution curve of Rn concentrations shows multiple high-value forms, indicating that there are other fractures and broken positions on the fault zone besides the fault exposure position. 2)The highest Rn concentrations on the measuring lines of the Kusongmuchike piedmont fault, the Nalati Fault and the Horgos-Tugulu Fault are 99 802Bq/m³, 80 549Bq/m³, 78 834Bq/m³, which are not only higher than the Rn concentration of other measurement lines in the same period, but also higher than the highest Rn concentration of 58 205Bq/m³ in the Hutubi North Fault. The fault activity is relatively stronger. 3)The earthquake risk of Wenquan-Jinghe area is relatively low, with relatively high regional stress accumulation. In addition, the fault activity in this area is intensive, and moderate to strong earthquakes are more likely to occur, so there is a certain earthquake risk.
In a word, it is of great scientific significance to carry out the activity detection and seismic risk assessment of the main active faults in the northern Tianshan area of Xinjiang. In the future, monitoring and in-depth research on the geochemistry of fault soil gas in the “North Tianshan Wenquan-Jinghe earthquake risk zone” is of great significance for judging the earthquake risk in the north Tianshan area. The results of this paper provide geochemical data for analyzing the characteristics of gas released by the fault zone in the northern Tianshan area of Xinjiang, and for guiding the selection and layout of seismic stations, as well as for seismic situation tracking and anomaly ascertainment.

AUTOMATIC FAULT IDENTIFICATION METHOD BASED ON IMPROVED DBSCAN ALGORITHM AND ITS APPLICATION TO TANGSHAN AREA

ZHANG Su-xiang, SHENG Shu-zhong, XI Biao, FANG Li-hua, LÜ Jian, WANG Gan-jiao, ZHANG Xiao

2022, 44(6):  1615-1633.  DOI: 10.3969/j.issn.0253-4967.2022.06.015

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

GEOMETRY OF SEISMOGENIC FAULTS OF THE 2021 YANGBI EARTHQUAKE SEQUENCE DETERMINED BY FUZZY CLUSTERING ALGORITHM

ZHANG Li-juan, WAN Yong-ge, WANG Fu-chang, JIN Zhi-tong, CUI Hua-wei

2022, 44(6):  1634-1647.  DOI: 10.3969/j.issn.0253-4967.2022.06.016

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The rupture process of earthquake generally involves multiple fault activities. The seismogenic fault is generally not a single fault plane, but a combination of multiple fault planes. Based on the principle that clustered small earthquakes often occur near the fault plane, and assuming that the hypocenters obey three-dimensional normal distribution around the center of the sub-fault planes, the three-dimensional spatial structure of the Yangbi earthquake fault in Yunnan Province is estimated based on the fuzzy clustering algorithm. The results in this paper are estimated from the perspective of data analysis. The results will be more accurate if the comprehensive analysis can be carried out in combination with geological, geophysical exploration and other means. The fuzzy clustering analysis is mainly carried out for regions with dense seismic source data. Because the program compiled by this method runs fast on an ordinary computer and can be calculated many times in a short time, the best result can be obtained. In this study, the shape of fault zone can be quickly calculated and analyzed, the shape and spatial distribution of branch fault zone is roughly consistent with the seismic distribution, which verifies that this method has certain predictive effect and application value.
Firstly, GK(Gustafson, Kessel)fuzzy clustering method is used to obtain the partition matrix for all sub classes of hypocenter, then the outliers are removed by using the partition matrix and appropriate threshold, and the subclasses containing fault planes are extracted. Finally, the parameters of each fault plane(including position, strike and dip)with 95%confidence level are determined. It is inferred from the results that the hypocenters are distributed along the fault zone almost parallel to the Weixi-Qiaohou Fault and gradually divided into three fault branches to southeast direction. The east branch dips to southwest, which is the main fault, corresponding to two sub fault planes, with strike of 134.22°, 132.65°and dip angle of 87.14°, 81.96°, respectively; the west branch nearly parallels to the east branch with strike and dip of 129.45°and 74.77°, respectively. Except for the three main faults, a blind fault near the Weixi Qiaohou fault zone is identified in this study, with a strike of 235.66°and dip of 66.30°. In this study, we determined the fault structure of the Yunnan Yangbi earthquake sequence by fuzzy clustering algorithm, which is independent of other methods by using seismic wave data, geodetic data and geological data. It is of significance for tectonic and geodynamic studies.
This data analysis algorithm can be applied to the shape analysis and prediction of fault zone by a large number of such source data. In consideration of earthquake prediction and earthquake disaster assessment, the knowledge of fault network structure in the vicinity of large earthquakes will also help to test different assumptions about stress transfer effects.

Special topic on Sichuan Luding M6.8 earthquake

THE 2022 M6.8 LUDING EARTHQUAKE: A COMPLICATED EVENT BY FAULTING OF THE MOXI SEGMENT OF THE XIANSHUIHE FAULT ZONE

LI Chuan-you, SUN Kai, MA Jun, LI Jun-jie, LIANG Ming-jian, FANG Li-hua

2022, 44(6):  1648-1666.  DOI: 10.3969/j.issn.0253-4967.2022.06.017

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The September 5, 2022, M6.8 Luding earthquake occurred along the southeastern segment of the Xianshuihe fault zone. Tectonics around the epicenter area is complicated and several faults had been recognized. Focal mechanisms of the main shock and inversions from earthquake data suggest that the earthquake occurred on a northwest-trending, steeply dipping strike-slip fault, which is consistent with the strike and slip of the Xianshuihe fault zone. We conducted a field investigation along the fault sections on both sides of the epicenter immediately after the earthquake. NW-trending fractures that were recognized as surface ruptures during the earthquake, and heavy landslides along the fault section between Ertaizi-Aiguocun village were observed during the field investigations. There are no surface ruptures developed along the fault sections north of the epicenter and south of Aiguocun village. Thus it can be concluded that there is a 15.5km-long surface rupture zone developed along the Moxi Fault(the section between Ertaizi and Aiguo village). The surface rupture zone trends northwest and shows a left-lateral strike slip, which is consistent with the strike and motion constrained by the focal mechanism. The coseismic displacements were measured to 20~30cm. Field observations, focal fault plane, distribution of the aftershocks, GNSS, and InSAR observation data suggest that the seismogenic structure associated with the M6.8 Luding earthquake is the Moxi Fault that belongs to the southeastern segment of the Xianshuihe fault zone. Slip along the segment south of the epicenter generated this earthquake, and also triggered slip along a northeast-trending fault and the northwestern section of the Moxi Fault in the epicenter. So, the M6.8 Luding earthquake is an event that is nucleated on the section south of the epicenter and then triggered an activity of the whole fault segment.