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20 December 2021, Volume 43 Issue 6

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Research paper

GRAVITY EVIDENCE OF META-INSTABLE STATE BEFORE THE 2008 WENCHUAN EARTHQUAKE

GUO Shu-song, ZHU Yi-qing, XU Yun-ma, LIU Fang, ZHAO Yun-feng, ZHANG Guo-qing, ZHU Hui

2021, 43(6):  1368-1380.  DOI: 10.3969/j.issn.0253-4967.2021.06.002

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The fault will experience different stress states in the process from compression to final rupture(earthquake outbreak). Researchers have defined the fault in meta-instability stage as the stress changes from quasi-static release to irreversible quasi-dynamic release. Meta-instability stage is the last stage of instability before earthquake and means earthquake is sure to break out. Various rock deformation experiments in laboratory show that there are observable meta-instability stage and evident synergism activity of physical field. Observing the evolution characteristics and laws of relevant physical parameters is fundamental and helpful for identifying short-term signals indicating earthquakes. In this paper, based on the theoretical characteristics of meta-instability stage obtained in laboratory and the results of repeated absolute and relative gravity measurements in Longmen Mountains area during 1996—2007, we analyze the characteristics of gravity variation in the meta-instable state before the 2008 Wenchuan M_S8.0 earthquake and propose a basis or method for determining the meta-instability fault using gravity data. The results are as follows:
(1)Gravity field variations of Longmenshan fault zone before the Wenchuan M_S8.0 earthquake showed a normal-state change during 1996 to 2001, regional anomaly occurred in 2001—2004, then an obvious reverse change appeared in 2004—2007, but the change was small and the faults were in a locking state for a year before the earthquake. The 2008 Wenchuan M_S8.0 earthquake occurred at the high gravity change belts and the epicenter was located near the zero isoline of the gravity converting from positive value to negative value. The patterns of the gravity variation with time match well with the linear steady-state, the deviation from linear steady-state and the meta-instability state in the process of steady-state loading to instability stage of rock experiments in the laboratory. The dynamic changes of gravity reflect the local gravity anomaly near the epicenter, thus help to judge the instability location.
(2)The time series of gravity variation of some stations adjacent to the epicenter are obtained. Gravity time-series variations are disordered and irregular in the east side of the fault zone near the Sichuan Basin, but increased and decreased simultaneously since 2002 on the west of the fault zone in the observation stations over the Houshan Fault. The 2008 Wenchuan M_S8.0 earthquake occurred on the Houshan Fault and the distribution of aftershocks was consistent with the strike of the fault. This shows the Houshan Fault is the main instability position of Longmenshan fault zone. The instability of the fault led to the consistent change of the gravity field of the measuring stations in this area. The characteristic of this change satisfies the conditions for determining the degree of synergism which is an indicator for identifying the stress state. Combined with other research results we can confirm that Houshan Fault is in a critical state of meta-instability stage since 2002.
(3)The spatio-temporal variations of gravity in the measuring stations on the west of the Longmenshan fault zone showed a synergism activity process before the 2008 Wenchuan M_S8.0 earthquake. It began with the gravity rise centered on station 607 and 612 in 2002 and expanded to station 606, 607 and 612 in 2006. Station 606 and 607 are located near Yingxiu Town, closest to the epicenter of Wenchuan earthquake and also in the instability area.
In summary, the retrospective research on the regional gravity field before the 2008 Wenchuan M_S8.0 earthquake using the meta-instability fault theory indicates that the evolution of the time-varying gravity field corresponds well to the processes of rock deformation and instability in the laboratory experiment, and the time-series variation of gravity stations generally reflects the synergism characteristics of physical field, which is a key indicator to identify a fault in a meta-instable stress state. Seismicity evolution varies in different tectonic positions, so more earthquake cases should be investigated profoundly when we are trying to find the gravity anomaly related to the meta-instability stage. Next, we will carry out a profound study, such as gravity observation and analysis of fixed-point stations, intensive absolute gravity measurement, and research on gravity time series changes of measuring stations near the epicenter before and after strong earthquakes.

EVALUATION OF RYUKYU-MANILA TRENCH TECTONIC BACKGROUND AND SEISMOGENIC ABILITY

LI Zheng-fang, ZHOU Ben-gang, XIAO Hai-bo

2021, 43(6):  1381-1397.  DOI: 10.3969/j.issn.0253-4967.2021.06.003

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After the Fukushima nuclear accident caused by the “3·11” earthquake tsunami in Japan, whether the coastal nuclear power stations in China are liable to similar earthquake tsunami impact has been widely concerned by the whole society. According to the previous results of earthquake tsunami impact assessment conducted by professional departments on coastal nuclear power plants, China's coastal areas do not have the conditions for the occurrence of large-scale earthquake tsunami, but in order to fully learn from the experience and lessons of the Fukushima nuclear accident caused by Japan's “3·11” earthquake tsunami, definite conclusions have been drawn on the offshore tsunami and its impact on nuclear power plants in the early assessment work of potential tsunami impact of coastal nuclear power stations in China, combined with the structural background, historical seismic data and tsunami impact analysis. However, whether the earthquakes in the Ryukyu trench, Manila trench and other areas can generate tsunami has not been systematically considered. Therefore, in this paper, the seismogenic capacity of the Ryukyu trench and Manila trench is evaluated based on the seismotectonic background and relevant seismic source parameters.
Both Ryukyu and Manila trench belong to the west Pacific plate subduction zone, while the Japan's “3·11”earthquake is also located in the west Pacific plate subduction zone. Therefore, whether the former has the same tectonic background and conditions as the “3·11” earthquake does is the key factor to assess whether the Ryukyu trench and Manila trench have the same potential for M9 earthquake. Based on the analysis of a large number of data, this paper evaluates the tectonic background, segmentation characteristics and maximum potential earthquake generating capacity of the Ryukyu trench and the Manila trench. The Ryukyu trench and Manila trench are located in the west of the Philippine Sea plate. There are also subduction zones distributing in the east of the Philippine Sea plate from Izu-Ogasawara trench, Mariana trench to Yap Palau-Ayu trench. Since the Ryukyu trench-Manila trench subduction zones are not in the direct contact zone between the Pacific plate and the Eurasian plate, the plate tectonic setting is obviously different from the low-angle subduction zone where the Japan's March 11 earthquake locates. From the perspective of tectonic system, the Ryukyu trench belongs to the subduction tectonic system of trench-island arc-back arc basin. The island arc and trench are retreating eastward, showing the characteristics of weak coupling. The overall scale of the Manila trench is small, and affected by the “slab window” in the subduction slab formed by the ancient spreading ridge, the length of these two trench zones is much smaller than that of the subduction zones where M_W≥9 earthquakes have occurred.
Based on the comprehensive analysis of the differences in trench structure, earthquake data and etc., the Ryukyu trench can be divided into 6 rupture segments, and the section of the Manila trench concerned in this study can also be divided into 6 rupture segments. At the same time, the possibility of combined rupture of the rupture segment is considered from a conservative standpoint. The rupture segments RL5 and RL6 of the Ryukyu trench, RM2 and RM3 of the Manila trench all have the possibility of combined rupture, and rupture segments RM4, RM5 and RM6 also have the possibility of combined rupture. To sum up, the comprehensive estimation result of the maximum potential earthquake in the subduction zone is magnitude 8.5 in the Ryukyu trench and magnitude 8.8 in the Manila trench.

PALEOEARTHQUAKE CHARACTERISTICS IN DUNHUANG SEGMENT OF THE SANWEISHAN FAULT

LIU Xing-wan, YUAN Dao-yang, YAO Yun-sheng, ZOU Xiao-bo

2021, 43(6):  1398-1411.  DOI: 10.3969/j.issn.0253-4967.2021.06.004

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The Sanweishan Fault is located in the front of the northwest growth of the northern margin of Tibetan plateau, a branch fault of the Altyn Tagh Fault which extends to the northwest. The latest seismic activity of the Sanweishan Fault reflects the tectonic deformation characteristics of the northern plateau. Meanwhile, it is of great significance for the seismic risk assessment of Dunhuang and its adjacent areas to understand the characteristics of earthquake recurrence. The Sanweishan Fault runs along the western piedmont of the Sanwei Shan, with a total length of 175km. The fault is characterized by left-lateral strike-slip and reverse faulting, with local normal fault features. Based on the geometry, the fault can be divided into three segments, i.e. the Shuangta-Shigongkouzi, the Shigongkouzi-Shugouzi and the Shugouzi-Xishuigou segment from east to west. Previous studies about the paleoearthquakes on the Sanweishan Fault mainly focus on the middle and east segments of the fault, while the west segment of the fault has been less studied. Meanwhile, the available research does not involve the recurrence characteristics and possible magnitude of the paleoearthquakes. Based on high-resolution satellite images, we found that the main fault has grown toward the basin and formed fault scarps in the western segment of the Sanweishan Fault. We have carried out a detailed study on these fault scarps. Based on trench excavation and chronological study on the latest fault scarps, this paper determines the sequence of the paleoseismic events on the fault and discusses the recurrence characteristics and possible magnitude of earthquake for the Sanweishan Fault.
In the western segment of the fault, through satellite image interpretation and field investigation, we found new fault scarps distributed on the alluvial fan in front of the mountain near Gedajing. We called it Dunhuang segment of the Sanweishan Fault. The activity characteristics of the fault scarps may reflect the latest seismic events in the western part of the Sanweishan Fault. Different from the sinistral strike slip of the main Sanweishan Fault, this fault segment shows the characteristics of thrust with low angle. According to the differential GPS survey, the height of the fault scarp is approximately 2.2m. The paleoseismic trench was excavated across the fault scarp. Based on the analysis of paleoseismological trenching and optical stimulated luminescence dating, two paleoseismic events are determined. Event E1 occurred at approximately(35.1±3.7)~(36.7±4.1)ka; event E2 occurred at approximately(76.5±8.8)~(76.7±8.3)ka. According to the strata offset and corresponding age, the vertical slip rate of the Sanweishan Fault is determined to be(0.03±0.01)mm/a, with a corresponding shortening rate of(0.09±0.01)mm/a. Together with the previous results, we consider that the Sanweishan Fault is characterized by segmentation. The middle and east segments may have the ability of independent rupture, and also the characteristics of cascading rupture with the Dunhuang segment. According to the existing results, we conclude that the recurrence interval for cascading rupture behavior on the Sanweishan Fault is approximately 40ka, which shows a characteristic of low slip rate and long-term recurrence. The best estimated magnitude is inferred to be in the range between M_W7.1 and M_W7.5 based on the empirical relationships between moment magnitude and rupture length.

FEATURES OF ANCIENT LANDSLIDES AND THEIR SEISMIC-GEOLOGICAL SIGNIFICANCE ALONG THE SOUTHERN SEGMENT OF XIAOJIANG FAULT IN THE SOUTHEASTERN YUNNAN, CHINA

GAO Fan, HAN Zhu-jun, YUAN Ren-mao, DONG Shao-peng, GUO Peng

2021, 43(6):  1412-1434.  DOI: 10.3969/j.issn.0253-4967.2021.06.005

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Historical records with time information are useful for determining the time of earthquake events, while the investigation of historical damage phenomena such as earthquake-triggered landslides can help determine the magnitude of historical earthquakes by analyzing the correlation among historical earthquake-caused landslides, historical earthquakes and related active faults. A series of small basins were developed along the southern segment of the Xiaojiang Fault(XJF), with relatively flat and open topography and concentrated human activities. In most of the southern segment of the XJF, the terrain is relative flat, but some landslide accumulations are still clear, which are obviously different from the surrounding settings and are easy to be identified. Based on remote sensing interpretation and field investigations, landslides with different scales have developed in more than 10 locations along the southern segment of the XJF. Some of them are large with a volume of more than 1 million m³, and some are small with a volume of less than 100 000m³. They are the ancient landslides with a stable state. These landslides are mainly distributed in basins and their border areas with gentle terrain slopes. They are likely to be earthquake landslides rather than rainfall induced. The main scarp angles of these landslides are relatively concentrated, most of which are between 29~31 degrees, indicating that these landslides are caused by one geological event. We use light detection and ranging(LiDAR) measurement technology to obtain the digital elevation model(DEM)data of the landslide development section. The generated three-dimensional topographic shadow map presented in this paper suggests that there is a close relationship between these landslides and the latest surface ruptures of the southern segment of the XJF, indicating that these landslides should be triggered by the latest seismic event along the southern segment of XJF. The fault section was faulted in the latest earthquake events on the surface, triggering clusters of landslides. Based on the age test results of samples from the trench on the landslide body and historical literature data, the co-seismic landslides were triggered in 1606AD. According to the latest research results of the earthquake surface rupture zone in the southern segment of the XJF and empirical formula, combined with the comparative analysis on the intensity of geological disasters and the number of casualties of different earthquake cases, the authors re-assess the magnitude of the 1606 Jianshui earthquake and find that the magnitude of this historical earthquake could not be less than 7½(≥7.5). It means that the southern segment of the XJF, as a part of Xianshuihe-Xiaojiang fault(XSH-XJF) system, shows strong activity and has the ability to generate large earthquakes. GPS observations have verified that the crustal material on the southeastern margin of the Tibetan plateau rotates clockwise around the Eastern Himalaya Syntaxis(EHS), which requires a continuous left-lateral strike-slip fault system as the eastern boundary. The results presented in this paper are useful for deeper study of such an eastern boundary.

THE RELATIONSHIP BETWEEN ACTIVITY OF JINSHA RIVER FAULT ZONE AND LARGE-SCALE LANDSLIDES: A CASE STUDY OF THE SECTION BETWEEN NARONG AND RONGXUE ALONG THE JINSHA RIVER

CHANG Hao, CHANG Zu-feng, LIU Chang-wei

2021, 43(6):  1435-1458.  DOI: 10.3969/j.issn.0253-4967.2021.06.006

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

CHARACTERISTICS AND IMPLICATIONS OF SEISMIC ACTIVITY AROUND MINSHAN ACTIVE BLOCK IN EASTERN MARGIN OF QINGHAI-TIBET PLATEAU

LI Jia-ni, HAN Zhu-jun, LUO Jia-hong, GUO Peng

2021, 43(6):  1459-1484.  DOI: 10.3969/j.issn.0253-4967.2021.06.007

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Minshan active block is located in Bayan Har block of Qinghai-Tibet Plateau. It is bounded by the Huya Fault and Minjiang Fault on the east and west sides of the block. In less than 100 years, there have been four earthquakes with M_S≥7.0 occurring along the eastern and western boundary faults, namely, the Diexi earthquake with M7.5 in 1933, two Songpan earthquakes with M_S7.2 in 1976, the Jiuzhaigou earthquake with M_S7.0 in 2017, and several earthquakes with M6.0~6.9. Such intensity and frequency of seismicity on either side of a relatively small intraplate active block is rare. Because the landforms along the active fault are mostly relatively gentle valleys with dense population and there is large terrain difference between the two sides of the valleys, each of the major earthquakes and the large-scale landslides it triggered were liable to cause serious casualties and property losses.
Therefore, how does the destructive seismic activity around the active block migrate in space, and is it closely related to the segmentation and coalescence of active faults?And what are the temporal development characteristics of major earthquake activities and earthquake sequences?The discussion of these questions will not only deepen our understanding of the location and time of future destructive earthquakes, but also promote the development of the hypothesis of active block theory. Compared with the Bayan Har block, the Minshan active block located in the eastern margin of the Qinghai-Tibet Plateau provides a unique experimental field for studying the temporal and spatial regularity of earthquake occurrence in the active block.
In this paper, 39 076 small earthquakes in Minshan active block and its adjacent areas from 2000 to 2019 were relocated using the double-difference location method, and 48, 110 seismic events in the study area were obtained by combining the earthquake catalogues recorded by instruments in the same area from 1972 to 1999. For the major earthquakes since the 1933 Diexi M7.5 earthquake, a thorough analysis was made on the spatial distribution characteristics of earthquake sequences in different periods, especially on the basis of formation of small earthquake bands, and the results show that: Since the Diexi M7.5 earthquake in 1933, the four M≥7.0 earthquake sequences are all distributed along the boundary zone of Minshan active block in space, indicating that the active block plays a controlling role in the process of large earthquake preparation. In terms of the determination of seismogenic structure, the strike of the seismogenic fault of the 1976 Songpan M_S7.2 earthquake is basically the same with that of the 2017 Jiuzhaigou M_S7.0 earthquake, but differs by 60°~70° with that of the 1976 Pingwu M_S7.2 earthquake. So, it is more reasonable that the seismogenic faults of these three major earthquakes belong to two earthquake rupture segments, among them, the seismogenic fault of Jiuzhaigou M_S7.0 earthquake in 2017 and Songpan M_S7.2 earthquake in 1976 is the NW-trending Shuzheng Fault, and that of the 1976 Pingwu M_S7.2 earthquake is the north segment of the Huya Fault. From the perspective of seismicity, the seismogenic fault of the 1933 Diexi earthquake should be the southern segment of Minjiang Fault. The 2017 Jiuzhaigou M_S7.0 earthquake occurred in the gap between the 1976 Songpan M_S7.2 earthquake and the Minjiang Fault. There are probably two seismic hazard areas around Minshan active block, which are located in the southern segment of Huya Fault and the middle segment of Minjiang Fault. The large earthquakes around Minshan block probably belong to foreshock-main shock-aftershock type. Therefore, from the perspective of earthquake prediction, it is suggested to strengthen monitoring of these two seismic gaps.

EXTENSION AND STRUCTURAL FEATURE OF THE BURIED SEGMENT OF TAOCHUAN-HUXIAN FAULT IN THE WEIHE BASIN

ZHANG En-hui, SHI Ya-qin, ZHANG Yi, LI Miao, LI Gao-yang, PEI Gen-di, WANG Wan-he

2021, 43(6):  1485-1506.  DOI: 10.3969/j.issn.0253-4967.2021.06.008

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Weihe Basin, which is wide in the east and narrow in the west, deep in the south and shallow in the north, is one of the typical Cenozoic grabens in Asia continent, connecting the Ordos block in the north, Qinling fold belt in the south, adjacent to the arcuate fault belt in the northeast margin of Tibet Plateau in the west and the Shanxi rift zone in the east. The Weihe Basin has experienced strong faulting and sedimentation since early Cenozoic, with many buried active faults developed. The nearly E-W-trending Taochuan-Huxian Fault is one of these faults. The middle-deep depth seismic profiling shows that the buried segment of Taochuan-Huxian Fault in Weihe Basin is located between the Qinling north margin fault and the Weihe Fault and it is a fundamental fault that cuts through the Palaeozoic stratum and divides the Xi'an depression into two parts. To explore and know the location and structural characteristics of the Taochuan-Huxian fault segment hidden in the Weihe Basin and its activity in the Late Quaternary is of important significance for the researches of seismo-tectonic structure and seismic hazard of strong earthquakes in the study region. For this purpose, we deployed 7 profiles for shallow seismic reflection surveys, relied on the “Xingping Active Fault Project”. Based on these surveys, we determined the existence and hidden positions of the Taochuan-Huxian Fault and its branches in the Weihe Basin by combining with the data from some existing seismic reflection profiles of shallow-depths and middle-deep depths. Our research suggests that the Taochuan-Huxian Fault(F₈)is connected to the southern margin fault of the Taibai Basin in the west, and eastward, passes through the northern margin of the Qinling Mountains and enters into the Weihe Basin at the town of Tangyu, Zhouzhi County, and then is concealed under the loose sediment in the Weihe Basin. The strike direction of this fault is northeast when crossing obliquely through the town of Zhouzhi County, then gradually turns to a nearly east-west direction between Zhouzhi and Huxian, showing a northward convex bend in the fault trace buried in the basin. Further eastward, the Taochuan-Huxian Fault(F₈)connects to the Tieluzi Fault near the town of Yinzhen, Huxian County. In addition, a buried antithetic fault(DF₃)(also a secondary branch)of the buried Taochuan-Huxian Fault(F₈)is found between the north of Zhouzhi and the north of Huxian, and it extends roughly parallel to F8 under the loose sediment. This research also reveals that in the central portion of the Weihe Basin, the northern margin fault of the Qinling Mountains, the Weihe Fault and the Taochuan-Huxian Fault, together with their branch faults, constitute a large-scale fault zone with the tectonic feature of negative flower structure, as known from the interpreted cross-sections; among them, the F₈ and DF₃ faults and their secondary strands consist of a relatively small-scale negative flower structure. By combining with relevant information such as that from a composed cross-section using geological logs of multiple boreholes, and so on, we concluded that, within the study region of this research, the fault zone with the buried F₈ fault as its principal fault was active at least in the late Pleistocene, and hence is an active fault zone. Finally, the reason is discussed in this article for the faults, mentioned above, in the Weihe Basin that show the tectonic pattern of negative flower structure, instead of that of stair-stepping or ladder structure, and one possible interpretation is proposed that the dominant motion of these active faults are not normal faulting, but sinistral strike-slip faulting. Since the Cenozoic, the subduction of the Indian plate to the Eurasian plate caused the Tibet Plateau to be pushed out to the northeast and blocked by the Ordos block. Because of obstruction in the north, the material flows eastward along Qinling Mountains in the south, resulting in the extrusion shearing effect on the Weihe Basin in the middle. In addition, recent seismic and geological studies have discovered that many active faults in Weihe Basin and its edges are obviously of sinistral strike-slip, which also proves that the movement of these active faults in the basin is not dominated by normal faulting, but sinistral strike-slipping.

SEGMENTATION OF SURFACE RUPTURE AND OFFSETS CHARACTERISTICS OF THE FUYUN M8.0 EARTHQUAKE BASED ON HIGH-RESOLUTION TOPOGRAPHIC DATA

LIANG Zi-han, WEI Zhan-yu, ZHUANG Qi-tian, SUN Wen, HE Hong-lin

2021, 43(6):  1507-1523.  DOI: 10.3969/j.issn.0253-4967.2021.06.009

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The spatial distribution and deformation characteristics of the coseismic surface rupture zone are the direct geomorphological expressions of deep fault activities on the surface, which not only record the information of seismic rupture and fault movement but also reflect regional stress and crustal movement. Therefore, prompt investigation on the surface rupture zone after the earthquake is helpful to understand tectonic activities of the seismogenic fault. However, fieldwork is limited by hazardous environments and secondary disasters in the earthquake zone. High-precision geomorphological observation technology can obtain unprecedented high temporal and spatial resolution of the earth's surface features without being restricted by natural conditions, and provide high-quality data for identifying historical earthquake surface ruptures, extracting surface coseismic displacement, and geological mapping of active structures, thus help to understand the rupture processes deeply. The photogrammetric method based on SfM(Structure from Motion)technology provides an effective technical way for fast acquisition of high-resolution post-earthquake topographic data and obtaining 3D geomorphic characteristics in a short time without the limitation of topography. Fuyun Fault is located on the southwest edge of the Altai Mountains. Fuyun M8.0 earthquake occurred in 1931 and produced a coseismic surface rupture zone with obvious linear characteristics. There also developed a large number of right-lateral gully offset, extrusion uplifts, pull-apart basins and a series of tectonic landforms related to strike-slip activities, which are still well preserved after several decades. In this study, the surface rupture zone of the 1931 Fuyun earthquake is selected as the study area. Based on aerial photogrammetry SfM method, a digital elevation model (DEM) with a resolution of 1m is generated, which can reflect micro-structural geomorphology and is suitable for fine geomorphology research in a small area. Combined with the shadow and color change of DEM data, the surface deformation characteristics such as seismic cracks and seismic mole tracks are identified, the surface rupture tracks are drawn in detail, and the surface rupture zone of Fuyun earthquake is segmented through the distribution of its geometric and tectonic geomorphological features. Using gullies as geomorphological markers, the smallest regional offset is regarded as the coseismic offset in the 1931 earthquake. We finally identified the right-lateral horizontal offset of gully along the rupture zone and measured it with software. The results show that the Fuyun earthquake surface rupture zone can be divided into 4 sections from north to south, each of which has different length, connected by compression uplift or pull-apart basin. The main type of surface rupture is shear crack, and there are also transpressional cracks, tension cracks, and tectonic geomorphological expressions such as mole track, ridge, and pull-apart basin. Based on the measurement of the horizontal offset of 194 groups of gullies, it is found that the average coseismic offset in the 1931 earthquake is(5.06±0.13)m, which is equivalent to the coseismic offset produced by similar magnitude earthquake. The area where the local absence or sudden change of coseismic offset occurs also has a good corresponding relationship with the geometry of stepover, which reflects the geometric location of the stepover to a certain extent. The results fill up the gap of the fine morphology of the Fuyun earthquake surface rupture zone and further demonstrate the good application value of high-resolution topographic data in the study of active structures.

DISCUSSION ON SEISMICITY CHARACTERISTICS OF WEIXI-QIAOHOU FAULT IN THE NORTHERN SECTION OF RED RIVER FAULT ZONE BASED ON THE DENSE SEISMIC ARRAY OBSERVATION

WANG Zhi-wei, MA Sheng-li, LEI Xing-lin, WANG Kai-ying

2021, 43(6):  1524-1536.  DOI: 10.3969/j.issn.0253-4967.2021.06.010

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Based on the seismic data from temporary stations and regional stations in the northwestern area of Yunnan, the paper performs high-resolution detection and high-precision location on continuous waveforms recorded from February 25, 2018 to July 31, 2019 using waveform correlation methods and analyzes the seismicity characteristics of the Weixi-Qiaohou Fault in the northern section of the Red River fault zone. Studies have shown that the Weixi-Qiaohou Fault exhibits weak seismic activity currently, except for some special fault locations(such as terraces, intersections, etc.), but there may be a hidden steep-dip right-lateral strike-slip fault along the west side of the fault. Small earthquakes are frequent along the fault. The distribution of seismic activity and focal mechanism solutions indicate that this fault is a right-lateral strike-slip fault with a steep dip. Statistical parameters, such as seismic frequency, energy release rate and b-value, indicate that the seismic activity in the Weixi-Qiaohou Fault and its surrounding areas is relatively stable, and the regional stress enhancement is not obvious. The b-value is relatively high in most areas, and low b-value areas are mainly distributed in some special fault locations(such as terraces, intersections, etc.), but the scale is generally small. The statistical results of the ETAS model show that more than 40% of seismic activity may be affected by external factors such as deep fluid disturbance and remote strong earthquake triggering. This shows that the role of external trigger mechanisms in seismic activity cannot be ignored. The external triggering seismic activity factors are related to the disturbance of deep fluid activity and the dynamic triggering of long-distance strong earthquakes. Therefore, we believe that the Weixi-Qiaohou Fault is currently not active, but on the hidden branch fault to its west, small earthquake activity is clustering and has a tendency to increase. So, when assessing the seismic risk of the fault, comprehensive analysis shall be made on the activity of the main fault and the branch fault to its west.

STUDY ON PRESENT GRAVITY CHANGE AND DEEP CRUST DEFORMATION IN THE NORTHERN AND MIDDLE SECTIONS OF THE RED RIVER FAULT ZONE

WANG Jian, SHEN Chong-yang, SUN Wen-ke, TAN Hong-bo, HU Min-zhang, LIANG Wei-feng, HAN Yu-fei, ZHANG Xin-lin, WU Gui-ju, WANG Qing-hua

2021, 43(6):  1537-1562.  DOI: 10.3969/j.issn.0253-4967.2021.06.011

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Results of surface geological survey and deep geophysical exploration indicate that there are significant lateral differences in the crustal structure and deformation of the northern and middle sections of the Red River fault zone. In order to detect the current material migration and deformation characteristics in the crust along the Red River fault zone, we analyzed and removed the gravity changes caused by vertical surface movement, surface water circulation, denudation, and glacial isostatic adjustment effects based on mobile gravity observation data of 3 profiles in the northern and middle section of the Red River fault zone from 2013 to 2019, and obtained the trend of gravity change caused by the migration of materials in the deep crust. Based on recent gravity changes and crustal structure models, the deformation characteristics of Moho surface along the northern, middle, and middle-southern sections of the Red River fault zone are inverted. The results of the study are as follows:
(1)Average gravity change caused by vertical crustal movement is(-0.11±0.21)μGal/a, (0.22±0.21)μGal/a and(0.16±0.21)μGal/a in the northern, middle and middle-southern sections of the Red River fault zone, respectively. The surface crust of the Red River fault zone and its adjacent areas uplifts globally with a rate of((0.92±1.17)mm/a), which is identical to the background trend of uplift of Qinghai-Tibet plateau. Gravity change caused by the surface water reserves cannot be ignored, and the magnitude of the change is -10~10μGal. Gravity change trends on both sides of the Red River fault zone are accordant, but differences in the middle section are higher than that in the northern section.
(2)Recent gravity change of the Red River fault zone has segmental characteristics: The northern section of the Red River fault zone shows a negative gravity change trend with a rate of(-0.39±1.30)μGal/a. Bounded by the Red River fault zone, gravity change in northeastern side of the northern section of the Red River fault zone is negative, while the southwestern side shows positive change, with a gravity change rate increasing with(3.1±0.55)μGal/a·100km relative to the northeastern side, reflecting the constant mass accumulation in the process of deep material flow after crossing the Red River fault zone and then blocked by the Lancan River rigid block under the background of eastward material flow in the Qinghai-Tibet Plateau. Gravity change in the middle section of the Red River fault zone is(0.16±1.57)μGal/a, indicating a low-speed positive change trend. Gravity change in the middle Red River fault zone is lower than that in both sides, which reflects deep boundary control of the Red River fault zone. Recent gravity change rate gradually decreases with(-1.01±0.58)μGal/a·100km from the southwest to the northeast, which indicates more mass accumulation in the northeastern side. Middle-southern section of the Red River fault zone is the junction area between the IndoChina/Sichuan-Yunnan rhomboid and South China block, its positive gravity change trend(with(0.29±1.25)μGal/a on average)reflects the characteristics of mutual lateral compression and material accumulation between blocks. Magnitude of gravity change in northeastern Red River fault zone is greater than that in southwest. Gravity change decreases from southwest to northeast with an average rate of(-0.21±0.48)μGal/ a·100km.
(3)Combining the results of gravity changes caused by deep crustal material migration and Moho density interface model, we can get the recent Moho deformation information. Results indicates that depth of the Moho is generally increasing from southeast(about 36km)to northwest(about 50km), with the Red River fault zone as the boundary. Moho depth in the eastern side is generally deeper than that of the western side, and crustal structure on both sides of the Red River fault zone has significant lateral difference. Moho beneath the Red River fault zone uplifts continuously with an average rate of 0.54cm/a in recent period. Average deformation rate of the northern, middle, and middle-southern section of the Red River fault zone is -0.06cm/a, 1.36cm/a and 0.32cm/a, reflecting the effect of regional unbalanced tectonic movement to a certain extent. Moho beneath the northern section changes gradually from sinking to uplift from northeast to southwest. Moho of the middle section shows uplift in the northeast and sinking in the southwest. The middle-southern section's deformation rate is lower than that in the northern and middle-southern section, and the difference is small between the two sides. Deformation rate in the Red River fault zone is significantly lower than that in its both sides, which shows a strong boundary control effect on deep crustal deformation. The results can not only provide new constraint for fault activity study of the southeastern margin of Tibetan plateau, but also provide evidence to the study of strong earthquake preparation background in the northern and middle section of the Red River fault zone.

P-WAVE VELOCITY CHANGES IN HYPOCENTER REGION OF THE 2014 JINGGU M_S6.6 EARTHQUAKE USING TIME-LAPSE TOMOGRAPHY BASED ON DOUBLE-DIFFERENCE TOMOGRAPHY

CAO Ying, QIAN Jia-wei, HUANG Jiang-pei, ZHANG Guo-quan, FU Hong

2021, 43(6):  1563-1585.  DOI: 10.3969/j.issn.0253-4967.2021.06.012

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Various studies have reported on temporal changes of seismic velocities in the crust before and after earthquake. New time-lapse seismic tomographic scheme based on double-difference tomography can measure the temporal changes of seismic-wave velocities in the Earth and can offer a higher spatial resolution. The result is less affected by different data distribution and quality in different time periods. On October 7, 2014, an M_S6.6 earthquake occurred in Jinggu County, Pu'er City, Yunnan Province, and then on December 6, 2014, two strong aftershocks with magnitude M_S5.8 and M_S5.9 occurred successively. In order to obtain the high spatial resolution P-wave velocity changes in the hypocenter region of the 2014 Jinggu M_S6.6 earthquake, firstly, we used the seismic data in the hypocenter region of the Jinggu earthquake recorded by the Yunnan digital seismic network from January 1, 2008 to December 31, 2017 to invert the high-resolution three-dimensional P-wave velocity structure in the hypocenter region of the Jinggu earthquake by combining the absolute and relative arrival times using the double-difference tomography method. The inversion results show that the aftershock sequence is distributed at the junction between P-wave high-velocity anomaly area and low-velocity anomaly area. This may be the reason why the depth distribution of aftershocks is shallow in NW and deep in SE, and the number of aftershocks decreases fast in NW and slow in SE. The faults that intersect the Lancangjiang Fault are in the low-velocity anomaly zone, so the low velocity anomaly may be related to the fluid in the faults. Then, according to the technical route, this 3D velocity structure was taken as the initial model to invert for the 3D velocity structure of the five periods, and the 3D P-wave velocity structures of the five periods were obtained by using double differential tomography. Finally, the three-dimensional P-wave velocity model of the five periods was taken as the initial model and the new time-lapse tomography was used to obtain the spatial and temporal distribution of the P-wave velocity changes between different periods. In addition, combining the results with the existing geological and geophysical research results, the characteristics and mechanism of P-wave velocity changes are explored, our results indicate that:
(1)The maximum decrease in P-wave velocity at the shallow depth near the epicenter of the main earthquake is 0.2%, which occurred two months after the main earthquake and was caused mainly by rock failure.
(2)There is a P-wave velocity rising zone at a depth of 5km to 15km which is not affected by the rupture of the main earthquake. The existence of this zone caused the P-wave velocity change in the focal area to be discontinuous in depth. It is speculated that the reason for the existence of this zone is that there is a brittle-ductile transition zone with high-strength and high-resistance medium at this depth range. After the occurrence of the M_S5.8 and M_S5.9 aftershocks on December 6, the direction of distribution of aftershocks changed significantly, and also the focal depths showed a deepening trend. The distribution of the two strong aftershocks and their aftershocks were mainly located in the brittle-ductile transition zone, thus affecting the medium within a depth range of 5 to 15km, resulting in decrease of P-wave velocity with a 3.8%decline. It shows that the two strong aftershocks above magnitude 5 have an impact on the brittle-ductile transition zone, and the occurrence characteristics of aftershocks are usually consistent with the characteristics of P-wave velocity change.
(3)About three years after the Jinggu main earthquake, the amplitude of P-wave velocity increase is much larger than that of the previous P-wave velocity drop in the focal area. The P-wave velocity exceeded the pre-earthquake level. This indicates that the area experienced not only a post-earthquake seismic velocity recovery process, but also other physical processes. Combining with the results on strain field change obtained by the GPS data, it is inferred that the significant increase of P-wave velocity in this area is attributed to the superposition between the P-wave velocity increase due to the stress accumulation before the September 8, 2018, Yunnan Mojiang M_S5.9 earthquake and the post-earthquake seismic recovery process. So the P-wave velocity increase in this area is a complex process.

INSAR COSEISMIC DEFORMATION AND TECTONIC IMPLICATIONS FOR THE 2020 M_W6.3 NIMA EARTHQUAKE IN XIZANG

QIU Jiang-tao, JI Ling-yun, LIU lei, LIU Chuan-jin

2021, 43(6):  1586-1599.  DOI: 10.3969/j.issn.0253-4967.2021.06.013

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The Qinghai-Tibet Plateau has always been one of the areas with frequent strong earthquakes in China. On July 23, 2020, an M_W6.3 earthquake occurred in Nima, Tibet in a half-graben basin of the Yibug Caka-Riganpei Co fault zone in the central Qiangtang block. After the earthquake, many institutions have calculated the focal mechanism solutions based on seismic waves. Although there are differences in the source location and the parameters of the seismic fault, they all show that it is a normal fault earthquake. This is inconsistent with the strike-slip character of the Yibug Caka-Riganpei Co Fault. In addition, this earthquake is another strong earthquake that occurred on Yibug Caka-Riganpei Co Fault after the Gaize M_S6.9 event on January 9, 2008. Therefore, by studying this earthquake, we can better determine the tectonic movement of the seismogenic fault, so it has great significance for understanding the seismogenic mechanism and risk of the central Qiangtang block.
The average elevation in the central Qiangtang is more than 4 800m, the environment is harsh and it is difficult to carry out the field survey of the earthquake. At the same time, GNSS sites near the epicenter are extremely sparse. Therefore, the InSAR technology, which has been successfully applied to several earthquakes in Qinghai-Tibet Plateau, and the Sentinel-1 SAR data, which can be downloaded free of charge, are used to study this earthquake.
Firstly, we obtain the coseismic deformation field based on GAMMA software and select SRTM data with 30m resolution(http://gdex.cr.usgs.gov/gdex/)as the reference DEM. In order to improve the orbit error in interferogram, the precise orbit data provided by ESA(https://qc.sentinel1.eo.esa.int/aux_poeorb/)is used for correction. The adaptive filtering method with filtering function based on local fringe spectrum is used to filter the interferograms, and the filtering windows are set to 128×128 and 32×32, respectively. This iterative filtering window setting from large to small can greatly improve the coherence of the interferogram. Unwrapping phase method uses minimum cost flow(MCF)technology and irregular triangular mesh(TIN). In the ascend and descend InSAR deformation field, we can observe that both deformation trends are basically consistent. The earthquake caused an elliptic settlement area(~12km long and~8km wide)in the basin, and the maximum settlements in line-of-sight direction are -0.298m and -0.238m in the ascend and descend InSAR deformation field, respectively. It can also be observed that the basin has a small amount of horizontal sliding relative to the mountains on both sides. Based on the ascending and descending deformation field, the deformation pattern of the earthquake accords with the main characteristics of normal-fault event, which is consistent with seismological results.
Secondly, taking the focal mechanism solutions published by GCMT and NEIC as initial reference values, the seismic fault parameters are determined based on the elastic half-space dislocation model and InSAR deformation field. Then, according to the linear relationship between the slip and the deformation on the fault plane, SDM method is used to invert the coseismic slip distribution on the fault. In the inversion, since the average Poisson's ratio in Qiangtang is significantly higher than that in the normal crust, the Poisson's ratio is set at 0.29. The results show that: 1)The coseismic slip is dominated by normal dip-slip motion, with small amount of strike-slip component, and the slip is mainly distributed at the depths of 3~12km, with the maximum slip of approximately 1.1m at the depth of 7km. The causative fault did not rupture the surface; 2)The seismic fault is the west branch fault of the Yibug Caka-Riganpei Co Fault, with a strike of ~30°, a dip of ~68°, a slip of ~-73°.
The Yibug Caka-Riganpei Co Fault is still active today, and it is generally a left-lateral strike-slip fault. The Yibug Caka Lake in the epicentral area is a pull-apart basin controlled by the strike-slip fault. The rupture pattern of the Nima earthquake is similar to that of the Gaize M_S6.9 earthquake in 2008, both of which are normal dip-slip caused by accumulation of tensile stress. This is different from the strike-slip character of Yibug Caka-Riganpei Co Fault, indicating that there is extensional stress accumulation in the Yibug Caka-Riganpei Co strike-slip fault and the central part of the Qiangtang block is under an extensional stress regime. Most shallow earthquakes in Qiangtang block occur at the junctures of active faults. Therefore, more attention should be paid to this kind of areas in the future research on seismic risk of Qiangtang block.

INFLUENCE OF ROCK INTEGRITY ON QUANTITATIVE ANALYSIS OF EXTERNAL LOAD DISTURBANCE: A CASE STUDY OF XIAOXINZHUANG STRAIN, TIANJIN

LEI Sheng-xue, LIU Jian-bo, YAN Wei, SONG Tian, LI Hao, LI En-jian, ZHU Bing-qing, LI Ying-nan

2021, 43(6):  1600-1613.  DOI: 10.3969/j.issn.0253-4967.2021.06.014

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Fixed point deformation observation, with a broad application prospect, plays an important role both in monitoring crustal deformation and capturing possible seismic precursors. Due to advantages of high precision, broad bandwidth and relative good continuity, fixed-point deformation observation, together with InSAR and GPS, has become one of the major methods in crustal deformation researches.
During the daily observation, it is usual to encounter various load disturbances, such as heavy rain, reservoir, river, building constructions etc. However, when quantitatively evaluating the effects of such load disturbance, a number of studies were conducted without taking into account the rock mass integrity in the study area, i.e. simply using general rock mechanical parameters rather than specific rock mass mechanical parameters, which therefore greatly affect the final interpretation and the elimination of disturbance, or even the identification of seismic precursory anomalies.
In this study, the strain survey data at Xiaoxinzhuang seismic station in Jixian County of Tianjin are taken as an example to illuminate that the integrity or fragmentation of rock mass in each specific site should be considered when conducting quantitative analysis, only thus can the influence of external load on deformation observation be evaluated more accurately.
Firstly, a detailed joint statistic of the rock mass around Xiaoxinzhuang and Yuqiao reservoir was carried out; then the elastic modulus of rock mass of the study area was derived from the equation between rock mass volumetric joints and elastic modulus; finally the theoretical compressive strain at Xiaoxinzhuang station produced by Yuqiao reservoir was calculated by an irregular load model, and the influence of Yuqiao reservoir on the strain of Xiaoxinzhuang station was also evaluated.
Geological surveys, combined with numerical simulation indicate that: 1)The exposed rocks are dominated by Middle Proterozoic dolomites and dolomites with minor limestone at Xiaoxinzhuang seismic station and its vicinity, and the corresponding volumetric joint counts of rock mass are more than 32, the highest values of 41 and 39 occurred at Ligezhuang and south of Yuqiao reservoir, respectively, which means the bedrocks are moderately fragmented; 2)The elastic modulus of rock mass in the study area is only 0.10~0.19 times(an averaged value of 0.14)of that of integral rock, which means that under the same stress the effect of external load disturbance will be magnified about 5~10 times(an averaged value of 7 times); 3)During the impoundment period of Yuqiao reservoir from September to November in 2019, the theoretical compressive strain at Xiaoxinzhuang station in the north-south direction induced by Yuqiao reservoir impounding is about(93.76~200.91)×10^-8, calculated by an irregular load model, accounting for about 10%~21% of the actual observed strain; 4)Therefore, it is concluded that the earlier claimed anomaly is not an earthquake precursory anomaly, but probably the effect of water level change of Yuqiao reservoir.

PRESENT SLIP AND STRESS DISTRIBUTION OF BLOCK BOUNDARY FAULTS IN THE SICHUAN-YUNNAN REGION

WAN Yong-kui, SHEN Xiao-qi, LIU Rui-feng, LIU Xia, ZHENG Zhi-jiang, LI Yuan, ZHANG Yang, WANG Lei

2021, 43(6):  1614-1637.  DOI: 10.3969/j.issn.0253-4967.2021.06.015

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Previous studies have shown that M≥8 earthquakes and more than 80% M≥7 earthquakes occurred in the boundary zones of active blocks. Therefore, studies on the slip rate and stress distribution of the boundary faults can provide the basis for assessing the risk of strong earthquake. It also can help us understand the regional tectonic deformation, motion and dynamic process. Based on current cognition of the division of active block and fault system in the Sichuan-Yunnan region, we build a two-dimensional finite-element contact model, which includes ten small blocks and the primary block boundary faults, such as East Kunlun Fault, Minjiang Fault, Huya Fault, Xianshuihe-Xiaojiang Fault and Red River Fault. Slip rate and stress distribution of the primary block boundary faults are obtained by using long-term GPS observation data from 1991 to 2015 and “block-loading” method. This loading method can reflect interaction between the block and the boundary. Compared with the direct loading of GPS results, it can avoid local distortion caused by the large single-point error. Comparing GPS observation results with simulation results, the residual error less than 1mm accounts for 66%, and the error less than 2mm accounts for 86%. The direction angle residual error less than 5° accounts for~56%, and that less than 10° accounts for 82%, which means that simulation results of this study are reasonable. In addition, by collecting the relevant information on seismic activity and focal mechanism solutions in the Sichuan-Yunnan region, and combining with the simulation results, we discuss the relationship between slip rate distribution, transfer and stress transformation in large left-lateral strike-slip fault zones, the tectonic mechanism with normal fault type, as well as the probable cause of the seismic discrepancy between the northern and southern segments of the Red River Fault. The main conclusions are as follows:
(1)As the strike of the left-lateral strike-slip East Kunlun and Xianshuihe-Xiaojiang fault zones turns sharply from NW to near north-south-direction, the strike-slip component is partially absorbed by the fault-bend parts and then converted into strain accumulation, resulting in high stress distribution in the fault-bend areas. Among them, the area from the easternmost end of East Kunlun Fault to Huya Fault absorbs a strike-slip rate of~0.15mm/a. The accumulative rates of compressional stress are 3 711.7Pa and 699.3Pa, respectively. And the area from southeastern end of Xianshuihe Fault to Anninghe and Daliangshan Faults absorbs a strike-slip rate of~1mm/a. The accumulative rates of compressional stress are 3 051.7 Pa and 2 844.6 Pa, respectively.
(2)Affected by the left-lateral shear of Xiaojiang Fault, the south-central segment of the Red River Fault is dominated by right-lateral strike-slip with weak compression. The right-lateral strike-slip rate is 1.20~2.68mm/a. The right-lateral strike-slip rate of north segment of Red River Fault is 0.71~1.54mm/a. This indicates that right-lateral strike-slip in the northern segment of Red River Fault is caused by traction of the south-central segment. The Red River Fault constitutes a right-lateral shear deformation zone arranged in right-step en echelon pattern with the Jinsha River Fault and Deqin-Zhongdian Fault. In the vicinity of Deqin-Zhongdian Fault, the Yulong snow mountain eastern piedmont fault, the southern segment of the Lijiang-Xiaojinhe Fault and the Ninglang-Yongsheng-Binchuan Fault, form a tectonic pull-apart zone. The normal focal mechanisms are predominantly distributed within this zone. This deformation pattern is not consistent with imbricated thrust conversion-limited extrusion model, which suggests that the current movement mode of Jinsha River and Lijiang-Xiaojinhe fault zones and their effect on regional deformation may have changed.
(3)The north segment of the Red River Fault appears to be slightly tensional, while the south segment is weakly compressional. According to Coulomb's criterion, the shear stress required for fault rupture in the northern section should be lower than that in the southern section. As a result, the north section is more likely to rupture and the seismic activity is significantly stronger than that of the south-central part.

STUDY ON ATTENUATION CHARACTERISTICS OF SEISMIC WAVES AND SEISMIC SOURCE PARAMETERS IN THE NORTH-EAST MARGIN OF QINGHAI-TIBET PLATEAU

ZANG Yang, YU Yan-xiang, MENG Ling-yuan, HAN Yan-yan

2021, 43(6):  1638-1656.  DOI: 10.3969/j.issn.0253-4967.2021.06.016

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The northeastern margin of the Qinghai-Tibet Plateau located near the middle and northern section of the north-south seismic belt in China is a place where historical seismicity is extremely high and many strong earthquakes of magnitude above 7 occurred, including the August 8^th 2017 M7.0 Jiuzhaigou earthquake and the May 22^th 2021 M7.4 Maduo earthquake. There are many active faults in this region, most of them are NWW trending, among which the Maqin-Maqu segment on the east verge of the East Kunlun Fault is especially dangerous because a seismic gap exists on it. The 2017 M7.0 Jiuzhaigou earthquake and the 2021 M7.4 Maduo earthquake which occurred near the seismic gap are still not able to remove the danger of a future strong earthquake along the Maqin-Maqu segment. The seismic activity of a certain region is closely related to the variation of regional stress field and the physical properties of underground medium. With the development of the broadband digital seismic networks, source parameters, attenuation characteristics of seismic waves and site responses can be deduced from three-component seismic signals of regional small and medium earthquakes. Study results on the current state of regional stress and underground medium properties can provide basic information for the establishment of strong ground motion attenuation model and the assessment of seismic hazard in the study area.
In this study, the geometric attenuation model, Q value, source parameters of 444 earthquakes and the site responses of 118 stations are obtained near the northeast margin of Qinghai-Tibet Plateau using the joint inversion method based on the three-component S-wave data of small and medium-sized earthquakes occurring during 2010 to 2019. The results shows that the geometric attenuation model in the study area conforms to a form of 3-segment piecewise function, while the inelastic attenuation model meets Q(f)=401.8×f^0.2963 based on this geometric attenuation model. Among the site responses of 118 stations, 88 site responses show characteristics of rock site, 20 site responses show amplification effect, particularly in the high frequency, and 10 site responses show the overall value below 1, which may be affected by the spatial anisotropy of velocity structure and Q value. Through the correlation study of local magnitude, moment magnitude, stress drop and apparent stress, it is found that the moment magnitude has a linear correlation with the local magnitude as a whole, and the local magnitude has a positive correlation with the earthquake stress drop and apparent stress under the same moment magnitude. There is a significant statistical correlation between stress drop and apparent stress and an obvious linear correlation in logarithmic coordinate system. Under the same moment magnitude, the ratio of the apparent stress to the stress drop is higher in the earthquake with lower local magnitude, which means the seismic rupture is more sufficient and the radiation energy is relatively small. While the ratio of apparent stress to stress drop is relatively low in the earthquake with higher local magnitude, indicating the energy loss during fault rupture is relatively small and the radiation energy is relatively high.

Application of new technique

EARTHQUAKE BUILDING DAMAGE DETECTION USING UAV THERMAL INFRARED REMOTE SENSING IMAGES

FAN Xi-wei, NIE Gao-zhong, DENG Yan, AN Ji-wen, XIA Chao-xu

2021, 43(6):  1657-1670.  DOI: 10.3969/j.issn.0253-4967.2021.06.017

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Remote sensing is widely used for large-scale land surface information acquisition, such as land surface temperature estimation, land use and land cover classifications. Compared with in-situ field investigation, remote sensing has many advantages of efficiency and cost saving of human and material resources. As for the seismic risk reduction, the building area extraction, building height and other parameters acquisition, building structure type classification, and identification of post-earthquake building collapse, earthquake induced landslide, quake lake and other secondary disasters are the most important parts of remote sensing applications.
The high- or mid-spatial resolution visible images of centimeters to meters acquired from airborne and spaceborne instruments are widely used for damage building identifications, such as the April 14th Yushu earthquake in 2010. But the visible images cannot be used for nighttime land surface information acquisition. Thus, the active remote sensing techniques such as SAR or LiDAR are effective substitution for nighttime earthquake disaster information acquisitions. But the SAR and LiDAR data are not shown in traditional way which are similar with the visible images, and can only be interpreted by professional researchers. Note that some of the earthquakes occurred during the nighttime, for example the July 28th Tangshan earthquake occurred at the local time of 3:42. The nighttime earthquake disaster information acquisition is as important as the daytime. Considering the thermal infrared(TIR)sensor can receive thermal radiances emitted by surface features with the wavelength of 3 to 100μm and independent on solar radiations, this study tries to use TIR data acquired during nighttime to identify earthquake building damage types.
This study takes the Beichuan earthquake site of the Wenchuan earthquake in 2008 as the research area. To acquire TIR image of the study area, the UAV of DJI M200 version 2 with the payload of ZENMUSE XT2 is used. The XT2 has two cameras, with one traditional RGB visible camera and one TIR camera with the center wavelength of 10μm and spectral range of 7.5~13.5μm. The focal plane of XT2 have pixels of 4 000×3 000 for the visible camera and 640×512 for TIR camera. The pixel size of the TIR camera focal plane is 17μm and the focal length is 13mm. The M200 used in this study was flying at a height of 120m above ground. Thus the spatial resolution of the corresponding TIR image is about 15.7cm for vertical view condition. The TIR images of old town and new town of Beichuan county-seat are acquired with M200.
To evaluate the accuracy of identification of the damaged buildings using UAV TIR data, the visible images of the study areas are also acquired during the daytime and taken as reference. After mosaicing the daytime visible images and nighttime TIR images, respectively, all the buildings in the study area are classified into three damage types by artificial interpretation: slight damage, moderate damage, and destruction. By comparing the damaged building types by visible remote sensing data as the true value, the confuse matrix are constructed for the TIR data estimated building damage types. It is found that the UAV TIR remote sensing data can be used for the identification of damaged buildings at night, and the overall accuracy is 0.86, among which the user accuracy of the three damage types are all larger than 0.8. The mapping accuracy is 0.95, 0.67, and 0.84, for slight damage, moderate damage, and destruction types, respectively. The accuracy of moderate damage is 0.67. This is because the TIR image has only one channel and cannot show colors. Thus the moderate damage buildings are confused with trees and other features.

Scientific research flash

NEW INSIGHT ON THE HOLOCENE ACTIVITY OF THE EASTERN MARGINAL FAULT OF DAXING UPLIFT, BEIJING PLAIN

LI Zheng-fang, LI Yan-bao, ZHOU Ben-gang, ZHU Guo-jun, LIU Bao-jin, WU Jian

2021, 43(6):  1671-1681.  DOI: 10.3969/j.issn.0253-4967.2021.06.018

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The eastern marginal fault of Daxing Uplift is located in the southeast of the Beijing Plain, which is a boundary fault that controls the Daxing Uplift and the Langgu Sag. It intersects obliquely with the NNE-trending Xiadian Fault in the north where a magnitude 8 earthquake occurred in 1679. The overall strike of the fault is northeast, dipping southeast. Previous studies have suggested that the youngest stratum of the fault is the Mid Pleistocene of the Quaternary and it is not an active fault since the Late Quaternary. Based on high-precision shallow seismic exploration data, this study carried out high-density composite drilling geological section surveys and obtained evidence of obvious activity of the fault since the Late Quaternary. The fault is shown as an active normal fault in the composite drilling geological section. The top of the footwall of the fault is the 7m-thick silty clay marker layer buried at the depth of 74m and the top of the hanging wall is 102m deep, the amount of dislocation is about 28.0m. Fault slip surfaces were found in the cores of two of the boreholes, with depths of 54.2m and 39.4m, respectively. The buried depths of the top surface of the marker layer in the two boreholes with a horizontal distance of 2m are 8m and 10m, respectively, the dislocation amount is 2m. Combined with the observation of core deformation characteristics of the two boreholes, it is believed that the buried depth of the upper breakpoint of the fault may be shallower. This research has changed the understanding that the fault zone on the eastern margin of the Daxing Uplift is not active. This new discovery not only has great application value for understanding the risk of large earthquakes of this fault zone and the risk of earthquake disasters in Beijing, but also has scientific significance for the study of fault development and evolution and the deep-shallow coupling process in North China since the late Cenozoic. The main knowledge gained is as follows: 1)Through high-precision shallow seismic exploration, it is found that the Neogene and above strata in the study area generally show an inclined morphology which is deep in the south and shallow in the north. The strata below the Neogene are in angular unconformity contact with the bottom interface of the Neogene, and the depth of the shallowest upper breakpoint is about 38~43m. 2)The combined drilling geological section exploration reveals rich dislocation information of stratigraphic markers and further confirms the existence of active faults by borehole stratigraphic correlation. In the drill cores, fault slip surfaces were observed in the late Pleistocene strata at the depth of 39.4m, 51.5m and 54.2m, respectively. The stratigraphic comparison of the boreholes 5^# and 9^# with a hole spacing of 2m further reveals a fault throw of about 2m in the stratum at the buried depth of 8~10m, thus, it is inferred that the depth of the upper breakpoint on the fault may be 8m or shallower. According to the stratigraphic age data of adjacent boreholes in this area, it is considered that the fault is a Holocene active fault. The specific age of the latest activity and its activity parameters will be further studied through the subsequent borehole chronological tests and large-scale trench excavation.