Table of Content

20 February 2022, Volume 44 Issue 1

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CHARACTERISTICS OF LATE QUATERNARY ACTIVITY OF THE SOUTHERN RIYUESHAN FAULT

ZHANG Chi, LI Zhi-min, REN Zhi-kun, LIU Jin-rui, ZHANG Zhi-liang, WU Deng-yun

2022, 44(1):  1-19.  DOI: 10.3969/j.issn.0253-4967.2022.01.001

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Due to the collision between the Indian plate and the Eurasian plate, the Tibetan plateau has experienced violent uplift and strong intraplate deformation inside the plateau, which has a great impact on the tectonic evolution of the surrounding areas. The northeastern edge of the Tibetan plateau is the forefront of the northeastward expansion of the Tibetan plateau, which is the ideal place to study the deformation of the plateau as well as the far-field deformation associated with continental collision between the Eurasia and India plates. In recent years, scholars have gained a certain understanding of the characteristics of late Quaternary tectonic activity in the northeast margin of Tibetan plateau. Within the northeastern margin of Tibetan plateau, there are two major fault systems: One is the near EW-trending left-lateral strike-slip fault system, including the Kunlun, Haiyuan and western Qinling faults, the other one is the NNW-trending right-lateral strike-slip fault system, including the Elashan and Riyueshan faults. They are sub-parallel to each other. Since the Riyueshan Fault is one of the major right-lateral strike-slip faults in the northeastern margin of Tibetan plateau, its activity is of great significance for understanding the plateau expansion. Previous studies mainly focused on its northern part which is believed to be active during Holocene. However, its southern part is believed to be active during late Pleistocene, but not active since Holocene. Therefore, there are little studies focusing on the late Quaternary activities of the southern part of the Riyueshan Fault. Hence, our understanding about the characteristics of the late Quaternary activity is insufficient. During our preliminary field survey along the southern Riyueshan Fault, we found distinct deformation of Holocene landforms, such as the young alluvial fan, terrace risers and channels, which indicate its late Quaternary activity. In this study, we firstly analyze the fault geometry of the southern Riyueshan Fault based on high-resolution Superview-1 remote sensing images and carry out field verification. Based on fault geometry characteristics, fault strike orientation etc., we divided the southern Riyueshan Fault into two segments from north to south. One is the Guide segment(generally trending in NW 20°)and the other is the Duohelmao segment(generally striking in NS). During our field investigation, we found two typical sites for slip rate studies, the Rixiaolongwa site on the Guide segment and the Niemari site on the Duohemao segment, respectively. We collected high-resolution images using UAV, and then generated high-resolution DEM of these two sites. By measuring the offsets and corresponding dating results of multi-level terrace risers, we obtained the displacements of the three-level and two-level terraces at Rixiaolongwa and Niemari site, respectively. Then we collected the OSL and ¹⁴C samples on different terrace risers to constrain the age of each terrace. In the Rixiaolongwa area, the corresponding offsets of T1, T2 and T3 terraces are(26.3±3.1)m, (32.7±7.1)m and(38.6±8)m, and the age sequence is(7840±30)a BP, (9 350~10 700)a BP and(11.9±1.3)ka BP, respectively. In the Nimari area, the corresponding offsets of T1 and T2 terraces are(6.3±0.7)m and(9.7±1.7)m, and the ages are(2 860±30)a BP and(3 460±30)a BP, respectively. By applying Monte Carlo method, we obtained the corresponding slip rates of(3.37+0.55/-0.68)mm/a and(2.69+0.41/-0.38)mm/a for the Guide and Duohemao segment, which is comparable to the previously suggested slip rate of northern Riyueshan Fault. Finally, we discussed the role of the Riyueshan Fault in the tectonic deformation of northeastern Tibetan plateau.

Research paper

A STUDY ON THE SCARPS ALONG THE EASTERN SECTION OF THE KUSONGMUQIKE PIEDMONT FAULT AT THE NORTHERN FOOT OF TIANSHAN

HUANG Shuai-tang, CHANG Xiang-de, MA Jian, HU Wei-hua, REN Jing, LIU Jian-ming, ZHANG Wen-xiu, LAI Ai-jing

2022, 44(1):  20-34.  DOI: 10.3969/j.issn.0253-4967.2022.01.002

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Thrust fault is the basic model of crustal deformation and also one of the major structural forms of orogenic belts, indicating the tectonic environment of compression. Most of the catastrophic earthquakes that affect human activity occur within the plates. In the interior of the plate, reverse faults are likely to develop as long as there is compressive stress in the regional sense or under some local tectonic conditions. It is considered that the NS compression resulting from collision of the Indian plate and the Eurasian plate is the main cause for the formation of the present tectonic framework in both north and south sides of Tianshan Mountains. The continuous crustal shortening and thickening has made the Quaternary active structures in the front margins of Tianshan Mountains well developed. Meanwhile, the new nappe structures in front of Tianshan Mountains are also the main sites for the preparation of medium-strong earthquakes in the Tianshan Mountains area, and their seismogenic mode is mostly in the forms of blind fault ramp-decollement plane-surface fault ramps.
The northern Tianshan inverse fault-fold belt is located at the junction between the northern foothill of Tianshan Mountains and Junggar Basin, where the Kusongmuqike piedmont fault is located in the south of Jinghe County, and is an important active thrust fault belt in the western northern Tianshan Mountains. In recent ten years, there were many earthquakes with magnitude 5.0 or above occurring in the eastern section of the fault zone. A detailed study of the geometric distribution and tectonic geomorphologic features is helpful to understand the tectonic deformation characteristics and regional strain distribution in the Tianshan area since the late Quaternary. The results of high-resolution remote sensing image interpretation, UAV aerial survey and differential GPS terrain profile survey combined with field geological survey show that the eastern segment of the Kusongmuqike piedmont fault is composed of two secondary reverse faults. Among them, the south branch, the Xinlongkou Fault, is composed of 5 en echelon-arranged sub-faults, with an overall trend of NW, dipping S, steep dip angle, and a length of about 48km. The fault offset the two-stage piedmont alluvial-pluvial fan and 5 river terraces, the activity time of terrace T1/T2 and fan3 is the latest, and the fault scarps are 3.6m to 4.7m high, being the product of concurrent fault activities. The vertical displacement of terrace T3 and T4 is 13.5m and 20.3m, respectively, and the vertical displacement of terrace T5 is roughly the same with that of the surrounding pluvial fan2, which is about 30m. On the fan1, there is no tectonic deformation observed in places where the fault passes through, and the initial landforms are retained on the surface. The north branch, the Hydrographic Station Fault, is distributed in an intermittent manner. The overall strike of the fault is near EW, with a total length of about 44km, and the fault offset multi-stage alluvial-pluvial fans. On the alluvial-pluvial fan of Fan3, two near-parallel normal scarps are developed in the northern margin of the alluvial-pluvial fan, while other faults cut through the alluvial-pluvial fan and the surface gully, forming steep reverse scarps on the surface. According to the cumulative height of the normal scarps, the maximum vertical displacement is 17.2m and the minimum vertical displacement is 0.3m, the scarp height is concentrated between 4.7~9.9m. On the reverse fault scarps, the maximum vertical displacement is 7.8~9.8m, the minimum scarp height is 2.4~3.1m, and the scarp height concentrates between 3.3~9.2m. Several sub-faults are developed scatteredly between the two sets of faults, with scarp heights ranging 0.5~1.0m. As far as the scarp height distribution is concerned, its vertical displacement shows a distribution law of decreasing from west to east. These results may contribute to the further understanding of the strain partitioning pattern in the western part of the northern Tianshan.

THE LATE QUATERNARY ACTIVITY AND DISPLACEMENT RATE OF FUSHAN FAULT IN SHANXI

YAN Xiao-bing, ZHOU Yong-sheng, LI Zi-hong, HU Gui-rang, REN Rui-guo, HAO Xui-jing

2022, 44(1):  35-45.  DOI: 10.3969/j.issn.0253-4967.2022.01.003

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An earthquake of M6½ occurred near Fushan County in the 9th year of Dading Period of the Jin Dynasty(in 1209), which caused a large number of casualties and property losses. Many experts and scholars speculated that the Fushan Fault might be its seismogenic structure, but no in-depth research has been conducted, which greatly hinders the development of earthquake prevention and disaster reduction in the region. The Fushan Fault is located on the east side of the Linfen fault basin in the Shanxi fault depression zone. It is a boundary fault between the Linfen fault basin and the uplift area of the Taihangshan block. Predecessors have done little research on the Fushan Fault. This paper carries out a quantitative study on the late Quaternary activity and displacement rate of the Fushan Fault. First, we carried out remote sensing interpretation, fault surface excavation, collection and testing of fault geomorphological samples in the area of Qianjiao village of Fushan Fault. It is determined that the Fushan Fault starts from Hanzhuang village, Beihan Town in the north, extends to the southwest through Yushipo village, Fenghuangling village, Baozishang village, Zhaojiapo village in Beiwang town, Nanwang village, Zhuge village, Qianjiao village, Guojiapo village, Qiaojiapo village in Tiantan town, Dongguopo village and Zhaishang village in Zhangzhuang town, Lijiatu village and Zhujiashan village in Xiangshuihe town, and terminates in Chejiazhuang village in Xiangshuihe town, with a total length of 24km. The formation age of geomorphological bodies was obtained. It is determined that the latest stratum dislocation event of the fault is later than 7ka, and the fault is a Holocene active fault and has the ability to generate earthquakes of magnitude 7 and above. A total of two phases of stratum dislocation events have occurred on the Fushan Fault since 17ka BP(Late Quaternary): The first-phase event E1 occurred between 17ka and 7ka BP, producing a displacement of 2.04m, the average displacement rate of the Fushan Fault is 0.20mm/a; the second-phase event E2 occurred since 7ka BP, producing a displacement of 3.93m, and the average displacement rate of the Fushan Fault is 0.56mm/a. The displacement rate of the fault has been increasing since the Late Pleistocene. The future seismic hazard of this fault is worthy of attention. This paper also uses land-based LiDAR scanning to obtain the topographic data of the fault plane on the Qiaojiapo village bedrock section of the Fushan Fault(4.5km away from the Qianjiao village section). The isotropic variogram method was used to calculate the fractal dimensions of the fault surface morphology, and the morphological weathering zone was divided, and two phases of ancient seismic events of the Fushan Fault since the Late Quaternary were determined, which are, from old to new, the first-phase event E1 which caused a co-seismic displacement of 3.18m, and the second-phase event E2 which caused a co-seismic displacement of 2.51m. Studies have shown that the bedrock fault plane fractal method is an effective method for studying ancient seismic events in the bedrock area, and its ancient seismic period division is consistent with that of the sedimentary coverage area. Finally, this paper discusses the seismogenic structure of the 1209 Fushan earthquake with magnitude of 6½, and believes that the seismogenic structure of the Fushan earthquake is most likely to be the Fushan Fault. However, due to the lack of a lower age limit and that the only upper limit age is far away from the historical earthquake time, it is necessary to conduct a more detailed investigation and research on the fault to determine whether there can be a revelation of ancient earthquake events with a younger age and comparable magnitude.
This study has greatly improved Fushan County’s risk prevention and control, and territorial planning capabilities.

LATE QUATERNARY ACTIVITY AND TRANSPRESSIONAL DEFORMATION OF THE KARLIK TAGH NORTH FAULT, EASTERNMOST TIANSHAN

REN Guang-xue, LI Chuan-you, SUN Kai

2022, 44(1):  46-62.  DOI: 10.3969/j.issn.0253-4967.2022.01.004

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Karlik Tagh and other lesser ranges of the easternmost Tian Shan are natural laboratory for studying the fault architecture of an active termination zone of an intraplate mountain belt. The Karlik Tagh is located at the easternmost Tian Shan which is active due to the collision of India plate and Eurasian plate in Cenozoic and this range represents the geomorphological and structural end of Tian Shan. Therefore, studying the geometry and kinematics of active faults distributed at this area has important implications for understanding the dynamics features of the end porting of the Cenozoic orogenic belt. This paper is focused on the North Karlik Tagh Fault(KTNF), which is an important active structure at the easternmost Tian Shan. This fault extends about 180km and is gently distributed between the Yiwu Basin and the north of Karlik Tagh. Based on remote sensing and detailed field research, we propose to subdivide the NKTF into 2 segments based on its variation in strike and motion characteristics.
At the west of the NKTF, the west segment is mainly distributed at south of Yanchi County and extends intermittently about 61km. The fault trace along Yanchi segment is obvious and expressed by several linear fault scarps on the foreland alluvial fan surfaces north of Karlik Tagh. Outcrop on a channel wall shows that the fault dips SW and thrusts directly to the NE. Topographic profiles across the scarps have shown that the minimum vertical offset is(1.3±0.5)m, which can be caused by a single earthquake rupture. The maximum vertical offset is(7.3±0.3)m. An OSL dating sample was obtained at 70cm below the T1 terrace surface. And we get the deposition age of(7.0±1.4)ka. Based on the OSL dating of deformed T1 terrace and the vertical displacement of(1.3±0.5)m of T1 and vertical displacement of(2.5±0.2)m of T2, a vertical slip rate of 0.19~0.35mm/a can be calculated. This vertical rate is slightly larger than that of the North Hami Basin Fault, which is consistent with the S-directed tilt of the Karlik Tagh.
At south of Xiamaya town, the east segment of NKTF changes its strike and bends to NE, extending nearly 95km. Toward the east, this fault is connected with the west end of Gobi-Tianshan fault system(GTSFS)at the border of China and Mongolia. There are clear evidences of recent activity of this fault, including well-preserved scarps and offset streams on the alluvial sediments. And this fault segment is very obvious because of linear features on the Google Earth image. About 23km southeast of Xiamaya town, the fault trace runs across a north-flowing river, causing remarkable sinistral offset of the T3/T2 terrace ridge with the maximum displacement of(172±20)m. At about 10km northeast of this river, the NKTF passes through a massif with steep slope on the south and gentle slope on the north. Field observation of a hand-dug outcrop has shown that this fault dips N156°E. In addition, the fault also displays reverse faulting component and dislocates the gravel-bearing silt sedment by about 2.0m.
At north of Karlik Tagh, several NW-trending faults can be interpreted on the satellite image. These faults extend short and form a clear boundary between bedrock and Quaternary sediments. Although there are no obvious deformations in the sediment such as diluvial fans or river terraces in the valley, the good linear characteristics on both sides of the valley indicate that these faults have been active since Quaternary. Because these faults are nearly parallel to the western segment of the northern margin of the Karlik Mountains, and there is no geomorphological evidence of horizontal movement of the faults, it can be inferred that the faults on both sides of the Adak Valley are mainly dominated by vertical movement.
The Karlik Tagh North Fault, together with, the north margin faults of Hami Basin and other NW-trending secondary faults in the north side of Karlik Mountain constitute the horse-tail end structure of Gobi-Tianshan sinistral strike-slip fault system, which regulates and absorbs the sinistral deformation of Gobi-Tianshan fault system and these faults present a positive flower structure in the cross-section. The uplift of Karlik Tagh is controlled by NW thrust fault and NEE left-lateral strike-slip fault, and this range is a typical transpressional mountain in the easternmost Tian Shan.

RESEARCH ON THE CHARACTERISTIC OF QUATERNARY ACTIVITIES OF THE ZHENJIANG SECTION OF MUFUSHAN-JIAOSHAN FAULT

ZHANG Peng, WANG Yong, FAN Xiao-ping, XU Kui, LIU Jia-bin

2022, 44(1):  63-75.  DOI: 10.3969/j.issn.0253-4967.2022.01.005

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Running across the Zhenjiang and Nanjing area, the Mufushan-Jiaoshan Fault is an important near EW-trending fault in Nanjing and Zhenjiang area. It extends from Mufu Mountain through Yanziji, Qixia Mountain, and Longtan to Jiao Mountain of Zhenjiang, with a total length of about 75km. The overall trend of the Mufushan-Jiaoshan Fault is nearly east-west, dipping to the north, the southern side of the fault is Ningzhen Mountain, the north side is the hollow land along the river and the Yangzhou low hilly plain. The fault is divided into the western and eastern sections by the NW-trending fault near Xiashu Town in Jurong, namely the Mufushan-Qixiashan section and the Zhenjiang section.
Due to the long-term activity of the Mufushan-Jiaoshan Fault, the northern part of the Mufu Mountain, Qixia Mountain and other complex anticlines suffered large-scale fault depression, forming the Yizheng Sag in the north and the Ningzhen Uplift in the south of the Yangtze River. There is a significant differential up-and-down movement of the fault block along the fault. In the Yizheng Sag, there are huge deposits of the Upper Cretaceous, as well as the thicker Paleogene and Neogene, indicating that the Mufushan-Jiaoshan Fault is a long-term active normal fault. On the Bouguer gravity anomaly map and aeromagnetic anomaly map, the expressions of the Mufushan-Jiaoshan Fault are very obvious, indicating that the fault has a large cutting depth and is a large-scale fault.
There have been many destructive earthquakes in the Nanjing-Zhenjiang area, most of which occurred at the intersection of NW-trending faults and near-EW-trending Mufushan-Jiaoshan Fault. In particular, the Yangzhou M6 earthquake in 1624 had a great impact, and the Mufushan-Jiaoshan Fault is possibly the seismogenic structure of this earthquake. With the planning and construction of a series of Yangtze River crossing passages across the fault in Nanjing and Zhenjiang, whether the Mufushan-Jiaoshan Fault is an active fault and whether it has a greater earthquake risk also becomes the focus of attention in this area.
It is of great significance to study the nature, characteristics and the latest active times of the Mufushan-Jiaoshan Fault for the prevention and reduction of earthquake disaster in Zhenjiang city and Nanjing city. Previous work mainly focused on the Nanjing section, and judged that its latest activity age is late Middle Pleistocene; there has not been a systematic study on the fault in the Zhenjiang section, and its latest activity age is still unclear. Based on the project of “Urban active fault exploration and seismic risk assessment in Zhenjiang City”, we carried out a series of shallow seismic explorations along the Mufushan-Jiaoshan Fault in the Zhenjiang section, and on this basis, representative points were selected to carry out drilling joint profiling to study the Quaternary activity characteristics of the Mufushan-Jiaoshan Fault. The results are of great significance for urban earthquake disaster reduction, urban planning and land use.
The results of shallow seismic exploration show that the Zhengjiang section of the Mufushan-Jiaoshan Fault is dominated by normal faulting, and the trend is NEE, dipping to the north, with a dip angle of about 50°~60° and a displacement of 3~7m on the bedrock surface. All breakpoints of Mufushan-Jiaoshan Fault show that only the bedrock surface was dislocated rather than the interior stratum of Quaternary.
On the Qiaotou village site, there is no sign of dislocation in the stratum above the Middle Pleistocene, the lower part of Middle Pleistocene Xiashu formation has been dislocated, the displacement of the bottom boundary of the Middle Pleistocene on both sides of the fault is 3.2m. According to the characteristics of dislocated stratum, the latest active age of Mufushan-Jiaoshan Fault is late Middle Pleistocene. There is no evidence of activity since late Pleistocene. The fault activity is dominated by normal faulting on the Jinshan site, and there is no evidence of faulting in the Holocene. Based on the comprehensive analysis, the latest active age of the Zhenjiang section of the Mufushan-Jiaoshan Fault is the late Middle Pleistocene, and there is no evidence of activity since the late Pleistocene. According to the dating results, the latest activity time is after(222±22)ka and before the late Pleistocene.
Affected by the erosion of the Yangtze River, the Quaternary in the study area is dominated by the Holocene, the Lower Pleistocene is absent, and the Middle Pleistocene is absent or thin. Therefore, the stratum displacement identified by drilling is mainly developed in the bedrock and the bottom of the Quaternary, resulting in the uncertainty of identifying the latest displacement of the fault, and it is difficult to identify the precise magnitude of the displacement. This is the shortcoming of this work.
Mufushan-Jiaoshan Fault is a major fault with strong seismic risk in the Nanjing-Zhenjiang area, especially at the intersection between the fault and the NW-trending fault, which has the seismogenic environment of destructive earthquake. It is necessary to attach great importance to the prevention of earthquake damage in the relevant area.

SEDIMENTARY PROCESS AND TECTONIC SIGNIFICANCE OF WUWEI BASIN DURING THE QUATERNARY

HE Xiang, DU Xing-xing, LIU Jian, LI Yi-hao, LI Qun

2022, 44(1):  76-97.  DOI: 10.3969/j.issn.0253-4967.2022.01.006

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The Wuwei Basin is located in the eastern part of the Hexi Corridor Basin group. It belongs to the frontier of uplift and expansion of the northeastern margin of the Qinghai-Tibet Plateau. The Quaternary sedimentary strata in the Wuwei Basin are hundreds of meters thick, which records the geological information of uplift and expansion of the northeastern margin of the Qinghai-Tibet Plateau during the Quaternary period. In order to study the sedimentary process of the Wuwei Basin since Quaternary and the characteristics of the uplift and extension of the northeastern margin of the Qinghai-Tibet Plateau, the sedimentary stratigraphy and stratigraphic chronology methods are applied to analyze the Quaternary sedimentary strata in the Wuwei Basin. Firstly, the Quaternary sedimentary sequence of the Wuwei Basin is established by studying the stratum characteristics and OSL ages of the Quaternary sedimentary strata. Secondly, through the analysis of the paleocurrent direction and zircon U-Pb isotopic age of the sedimentary gravel, the paleocurrent direction of different deposition periods and sediment source are restored. Finally, based on the above analysis, the sedimentary process of Quaternary strata in the Wuwei Basin and its response to the uplift and extension of the Qinghai-Tibet Plateau are discussed.
The Quaternary sedimentary strata in the Wuwei Basin are the lower Pleistocene Yumen conglomerate, the middle Pleistocene Jiuquan gravel bed and the Upper Pleistocene-Holocene Gobi gravel bed from bottom to top. The Gobi gravel bed can be subdivided into the Upper Pleistocene gravel bed, the Upper Pleistocene-Holocene loess layer and the Holocene gravel bed. In the early Pleistocene, the Yumen conglomerate’s source material is mainly Mesozoic and Paleozoic rocks. The main provenance area of the Yumen conglomerate is located in the Qilian Mountains south to the Wuwei Basin. The main sedimentary area of the Yumen conglomerate is located in the Zoulang Nan Shan located in the southern part of the Wuwei Basin and the southern part of the northern fault basin. In the middle Pleistocene, the Jiuquan gravel bed’s source material is mainly Cenozoic and Mesozoic rocks. In the early sedimentary stage of the Jiuquan gravel bed, the main provenance area is located in the Zoulang Nan Shan and the main sedimentary area is located in the northern part of Wuwei Basin. In the late sedimentary stage, the main provenance area of the Jiuquan gravel bed is located in the Zoulang Nan Shan and the Fenmen Mountain located in the northwestern margin of the Wuwei Basin, and the main sedimentary area is located in most of the northern fault basin and the surrounding area of the Fenmen Mountain. Since late Pleistocene, the Gobi gravel bed’s source material is mainly early Paleozoic rocks. The main provenance area of the Gobi gravel bed is located in the Zoulang Nan Shan, the Fenmen Mountain and the Longshou Mountain, and the main sedimentary area is located around the source area.
The uplift boundary of the northeastern margin of the Qinghai-Tibet Plateau continued to expand towards the northeast direction. The uplift boundary was located in the Qilian Mountains south to the Wuwei Basin in the early Pleistocene. It extended northward to the Zoulang Nan Shan and the Fenmen Mountain in the middle Pleistocene, and reached the Longshou Mountain north to the Wuwei Basin in the late Pleistocene. The main provenance and sedimentary areas of the Quaternary sediments in the Wuwei Basin show the migration characteristics from south to north, which indicates the uplift and expansion of the northeastern margin of the Qinghai-Tibet Plateau. The uplift time was early in the south and late in the north, and the uplift intensity was strong in the south and west and weak in the north and east.

THE LATEST DEFORMATION STYLE ALONG THE NORTH CHINA BLOCK REVEALED BY HIGH-RESOLUTION TOPOGRAPHIC DATA: A CASE STUDY ON XIADIAN FAULT

LI Zhan-fei, XU Xi-wei, MENG Yong-qi, ZHAO Shuai, SUN Jia-jun, CHENG Jia, LI Kang, KANG Wen-jun

2022, 44(1):  98-114.  DOI: 10.3969/j.issn.0253-4967.2022.01.007

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Beijing locates in the North China active tectonic block, where the NW- and NE-trending active faults are widely distributed, such as the Nankou-Sunhe Fault, the Shunyi-Liangxiang Fault, and the Xiadian Fault. Historically, large earthquakes frequently occurred along these faults, especially in the intersection of these two sets of faults, e.g. the 1679 Sanhe-Pinggu earthquake(M~8). Thus, it is of great significance to quantitatively study the faults’ basic parameters, including the fault trace, slip distribution, and rupture behavior, for accurate assessment of seismic hazard of Beijing area.
The Xiadian active fault locates at the eastern boundary of Beijing, near the Beijing Municipal Administrative Center. The 1679 Sanhe-Pinggu earthquake(M~8)occurred on this fault. Previous studies on this area have revealed clearly the bedrock geology, fault geometry, seismicity distribution as well as co-seismic deformation of this earthquake, which greatly improves the understanding of the activity behavior of the Jiadian active fault.
However, the previous studies have focused on the surface rupture of the 1679 earthquake, the complete rupture geometry and slip distribution have not yet been constructed, due to the restriction of high-resolution topographic data. Furthermore, the triangular slip distribution has widely occurred along active faults, especially along the typical normal faults. Whether the fine slip distribution of Xiadian Fault conforms to the case or not is still unclear.
In order to explore all those issues above, using low-costing high-resolution(0.5m)satellite images, we derived 1.0m grid size DEM to quantitatively explore the surface rupture along the Xiadian Fault. Detailed mapping and offset measurements revealed 5 left-stepping branches(~3km), with a total length of 12.3km for the coseismic rupture of the 1679 Sanhe-Pinggu earthquake. Slip distributions along the fault exhibit the arc-shaped geometry, and the maximum and average vertical offsets are ~3.2m and ~1.8m, respectively.
Such triangular shaped slip distribution has also been found along other typical normal faults, for instance, the Wairarapa Fault in New Zealand, the Afar Fault in East Africa, and Owens Valley Fault in California. Modeling of these measurements revealed 2 earthquakes with co-seismic vertical offset of ~1.8m and 1.7m, respectively. Reasonably, the maximum ~3.2m vertical offset possibly represents the cumulative vertical offset of 2 earthquakes, including the 1679 Sanhe-Pinggu earthquake.
Based on the relationships among the surface rupture length, average offsets, as well as moment magnitudes, the calculated size is comparatively small. Based on the cutting shape of the 2 sets of faults and the upper crust imaging by shallow seismic reflection profile, we propose that the current right-lateral shear deformation of the fault is decoupled with the existing extensional structures, and this hypothesis has been verified by the current focal mechanism solution.

QUANTIFYING EROSION RATE AND ROCK ERODIBILITY FROM FLUVIAL SHEAR STRESS:AN EXAMPLE FROM LONGMEN SHAN

YE Yi-jia, TAN Xi-bin, QIAN Li

2022, 44(1):  115-129.  DOI: 10.3969/j.issn.0253-4967.2022.01.008

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River networks, the backbone of most landscapes on Earth, record abundant information on the perturbations of tectonics, climate, and sea levels. Based on the fluvial shear stress model, the bedrock incision has a positive, monotonic correlation with fluvial shear stress. The correlation coefficient between the fluvial shear stress and the erosion rate is called rock erodibility, which mainly depends on lithology and erosion process. Therefore, the fluvial shear stress can be used to calculate the erosion rate, when the rock erodibility is known. It also can be used to calculate the rock erodibility, when the erosion rate is known.
With the improvement of DEM and satellite image resolution, we can extract the corresponding geomorphic parameters(such as channel gradient, discharge, and channel width)from the DEM and satellite image, and then calculate the fluvial shear stress. In this study, we calculated the fluvial shear stress values at 240 points along two channels in the Longmen Shan area. The Minjiang River flows through Wenchuan-Maoxian Fault and then enters Pengguan Massif. Along the Chu River, the lithology is relatively consistent, all are sedimentary rocks, including a ~5km reach distributed along the Shuangshi-Dachuan Fault. The results show that: 1)The average fluvial shear stress is ~401Pa at the footwall of the Wenchuan-Maoxian Fault, while it is ~280Pa along the Wenchuan-Maoxian Fault; 2)the average fluvial shear stress is ~161Pa and ~104Pa at the hanging wall and footwall of Shuangshi-Dachuan Fault, respectively, while it is ~50Pa along the Shuangshi-Dachuan Fault. Fault activities cause the rock more fractured, which causes the fluvial shear stress along the fault to decrease significantly.
Then, according to the empirical relationship between fluvial shear stress and erosion rates of sedimentary rocks(E=3.2(τ^*-0.03)mm/a) and granitoid (E=1.6(τ^*-0.03)mm/a) in the Longmen Shan area, we transform the fluvial shear stress for 139 survey points to erosion rate. The average erosion rate at the footwall of the Wenchuan-Maoxian Fault(i.e. the Pengguan massif) is ~0.43mm/a, and those at the hanging wall and footwall of the Shuangshi-Dachuan Fault is ~0.49mm/a and ~0.28mm/a, respectively.
Moreover, according to the known erosion rates and fluvial shear stress, we calculated the rock erodibility of each survey point. The average erodibility is 13.9mm/a along the Shuangshi-Dachuan Fault(Chu River, sedimentary rocks)and 3.2mm/a along the Wenchuan-Maoxian Fault(Minjiang River, granitoid). The results indicate that: 1)The erodibility of the sedimentary rocks along the fault increases by ~3 times caused by the fault activity, while that of granitoid increases by ~1 time; 2)the fault activity can increase the rock erodibility within ~2km from the active fault in sedimentary rocks and within ~5km from the active fault in granitoid, respectively. This study indicates that fault activities could significantly increase the rock erodibility along the fault, and further influence the evolution of river networks around the fault.

IDENTIFYING FAULT LINEAMENT BASED ON ALOS-PALSAR DEM: A CASE STUDY FROM THE WEST QINLING MOUNTAINS

ZHANG Bo, WANG Ai-guo, TIAN Qin-jian, GE Wei-peng, JIA Wei, YAO Yun-sheng, YUAN Dao-yang

2022, 44(1):  130-149.  DOI: 10.3969/j.issn.0253-4967.2022.01.009

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The most significant feature of active faults on remote sensing images is fault lineament. How to identify and extract fault lineament is an important content of active fault research. The rapid development of remote sensing technology has provided people with extremely rich remote sensing data, and has also created the problem of how to choose suitable data for fault interpretation. In the traditional fault interpretation, people pay more attention to high-resolution optical images and high-resolution DEM, but optical remote sensing images are greatly affected by factors such as weather condition, vegetation and human impacts, and the time and economic costs for obtaining high-resolution DEM are relatively high. Due to the low resolution, the medium-resolution DEM(such as Aster GDEM, SRTM1, SRTM3, etc.)is generally used to automatically extract structural lineament, and then analyze the overall regional structural features, but it is rarely used to visually interpret active faults. ALOS-PALSAR DEM is generated from SAR images acquired by the phased array L-band synthetic aperture radar mission sensor of the Japanese ALOS satellite. It is currently a free DEM with the highest resolution(resolution of 12.5m)and the widest coverage. Based on ALOS-PALSAR DEM and ArcGIS 10.4 software, this paper generates a hillshade map and visually interprets the fault lineaments in the West Qinling Mountains. When generating a hillshade map, we set the light azimuths to be oblique or orthogonal to the overall trend of the linear structures, the light azimuths to be consistent with the slope direction of the hillslope, and the light dips to be a medium incident angle. Based on the hillshade map generated from ALOS-PALSAR DEM, this paper summarizes the typical performance and interpretation markers of fault lineaments on the hillshade map(generated by DEM), and visually interprets the V-shaped fault system in West Qinling Mountains where the research on fault geometry is limited based on the interpretation markers. The results of the research are as follows: First, this study discovers a number of fault lineament zones, including the fault lineament located between the Lintan-Dangchang Fault and the Guanggaishan-Dieshan Fault, the NE-directed fault lineament zone between the Lixian-Luojiapu Fault and the Liangdang-Jiangluo Fault, and the arc-shaped dense fault lineament zones distributed south of the Hanan-Daoqizi Fault and the Wudu-Kangxian Fault; Second, this study completes the geometric distribution images of the known active faults, such as the western and eastern sections of the Lintan-Dangchang Fault, the western and eastern sections of the Liangdang-Jiangluo Fault; Third, fault lineaments in the West Qinling Mountains exhibit a “V” shape, with two groups of fault lineaments trending NW and NE, whose tectonic transformation mainly consists of two kinds: mutual cutting and arc transition. The Lintan-Dangchang Fault cuts the Lixian-Luojiapu Fault, the Lintan-Dangchang Fault and the Guanggaishan-Dieshan Fault are connected with the Liangdang-Jiangluo Fault in arc shape, and the Tazang Fault is connected with the Hanan-Daoqizi Fault in arc shape. The research results show that ALOS-PALSAR DEM has an outstanding capability to display fault lineaments due to its topographic attributes and strong surface penetration. In circumstances when the surface is artificially modified strongly, the spectrum of ground objects is complex and the vegetation is dense, the ALOS-PALSAR DEM can display fault lineament that cannot be displayed on optical remote sensing images, indicating that the medium-resolution DEM is an effective supplement to high-resolution optical remote sensing images in the fault lineament interpretation. The research results are of great significance for improving the geometric image of the V-shaped fault system in the West Qinling Mountains. It is also the basis for further research on fault geometry, kinematics, regional geodynamics and seismic hazard.

ADAPTIVELY SMOOTHED SEISMICITY MODEL FOR CHINA SEAS AND ADJACENT REGIONS

WU Guo, RAN Hong-liu, ZHOU Qing, XIE Zhuo-juan

2022, 44(1):  150-169.  DOI: 10.3969/j.issn.0253-4967.2022.01.010

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Ocean earthquakes pose a serious threat to the security of the economic construction in coastal areas and the marine resource exploitation of China, so building proper seismicity models for China seas and adjacent regions is one of the focuses of the next generation seismic zoning map of China. Different from mainland China, there is lack of multidisciplinary data for the sea areas, such as seismic geology and geophysical exploration, thus the earthquake catalogue is the most important basic data for analyzing its seismic activity characteristics. Recently, a unified catalogue was compiled by researchers for China seas and neighboring regions, which provides a basis for further work.
The Smoothed Seismicity Model (SSM) is a classic model based on earthquake catalogue and has become a basic model for the United States National Seismic Hazard Map. The Adaptively Smoothed Seismicity Model (ASSM) is an improved version of SSM. Compared with SSM, ASSM has optimized the value algorithm of the kernel function, thereby improving the model’s capability on mid- and long-term earthquake forecast. Due to the outstanding performance of ASSM in the CSEP project, it has become a research hotspot for seismologists. Based on a further improved ASSM algorithm and the newly compiled catalogue, this study established for the first time an adaptively smoothed seismicity model for China seas and adjacent areas.
First, similar to most studies on mid-to-long-term seismicity models, we removed the foreshocks and aftershocks from the catalogue. Then we used seismic zones as the unit to estimate the start and end time of completely recorded earthquakes in different magnitude intervals. Furthermore, the maximum likelihood method was used to estimate the seismic activity parameters such as a-value and b-value for every seismic zone. On this basis, an improved adaptively smoothed algorithm was used to build the model. The function of probability gain per earthquake was applied to evaluate the performance of models under different parameter settings. Finally, our model was compared with the traditional SSM, and the advantages and limitations of our model were analyzed.
Results show that: Compared with SSM, our model has a better performance on the probability gain function, and this advantage is not affected by the minimum magnitude(M_min)of the input earthquakes. When M_min is set as M4.0, our model achieves its best performance. The performance of the model does not necessarily improve as M_min decreases or the number of earthquakes increases. This indicates that we need to comprehensively consider the distribution of earthquakes in the study area and the integrity of the earthquake catalogue when selecting parameters. The algorithm used in this study can make full use of seismic data with varying record level over time and space, and has a certain application value in seismic hazard analysis and mid- and long-term earthquake forecast. Meanwhile, our model can be used as one of the basic models to analyze the seismic hazard for China’s maritime areas, and provide technical support for the compilation of seismic zoning maps in China’s sea areas. In addition, the completeness analysis results and seismic activity parameters obtained by this study can also provide references for other peer research.
Since ASSM is only based on historical and instrumental earthquake catalogues, it has certain limitations. For example, it cannot calculate the upper limit of the magnitude and reflect the distribution of faults, nor can it consider the time-dependence of the recurrence of large earthquakes. Therefore, this model can be used alone to describe the occurrence probability of small to moderately strong earthquakes, and it can also be used as one of the important factors to determine the spatial distribution functions for potential seismic source zones. However, when conducting seismic hazard analysis, it is recommended to combine the results of other disciplines such as geology and geodesy to form a hybrid model, so as to further improve the applicability and effectiveness of the model.

STUDY ON FOCAL MECHANISM SOLUTION AND STRESS FIELD CHARACTERISTICS IN SICHUAN AND YUNNAN AREA

ZHANG Zhi-wei, LONG Feng, ZHAO Xiao-yan, WANG Di

2022, 44(1):  170-187.  DOI: 10.3969/j.issn.0253-4967.2022.01.011

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Based on the focal mechanism solutions of 2 600 M_L≥3.0 earthquakes in Sichuan and Yunnan area from January 2000 to March 2017, the focal mechanism quantitative classification and stress field inversion are carried out for the sub blocks and fault zones with relatively dense focal mechanisms. Using the focal mechanism solutions of 727 M_L≥4.0 earthquakes from January 1970 to March 2017, the regional stress tensor damping method is used to inverse the spatial distribution of principal compressive stress in Sichuan and Yunnan area before and after Wenchuan M_S8.0 and Lushan M_S7.0 earthquakes, and the temporal and spatial evolution characteristics of current stress field are discussed.
The focal mechanisms are distributed mainly in Longmenshan fault zone, Xianshuihe-Anninghe-Zemuhe-Xiaojiang fault zone, Mabian-Yanjin fault zone, Lijiang-Xiaojinhe fault zone, the central Yunnan block, the west Yunnan block and the southwest Yunnan block in Sichuan and Yunnan area. The focal mechanism is mainly strike slip type in Sichuan and Yunnan area, but there are local differences. The Longmenshan fault zone is dominated by thrust type earthquakes, while in the Mabian-Yanjin fault zone, there are relatively more strike slip and thrust type earthquakes. The types of earthquakes in Sichuan Basin are complex, and there is no obvious dominant type. In general, the focal mechanisms of the Longmenshan fault zone and Sichuan Basin earthquakes are affected by strong earthquake and other factors, and the focal mechanism types have good inheritance in Sichuan and Yunnan area.
The stress field in Sichuan and Yunnan area has obvious subarea characteristics, and it rotates clockwise from north to south. The compressive stress in Longmenshan fault zone and Sichuan Basin shows nearly EW direction. It shows NWW direction in the eastern boundary of Sichuan and Yunnan rhombic block and NNW direction in the inner part of rhombic, while it shows NNE direction in the western and southern Yunnan blocks. The principal compressive stress in Sichuan is more complex than that in Yunnan. The principal compressive stress direction in Sichuan experiences EW-NW-EW rotation from west to east, the dip angle is steep in the west and slow in the east, and the stress regime also experiences the transition from normal faulting to strike-slip to thrust. The principal compressive stress direction in Yunnan is NNE in the west and NNW in the east, forming an inverted “V” shape in space, the stress regime is mainly strike-slip and the dip angle is horizontal.
Before and after the Wenchuan M_S8.0 and Lushan M_S7.0 strong earthquakes, the stress field in the Longmenshan fault zone changed greatly, followed by the Sichuan Basin and its surrounding areas, and there was no obvious change in other areas of Sichuan and Yunnan. The stress field in the Longmenshan fault zone experienced a complete transformation process from basic stress field to variable stress field to basic stress field.

THE BURIED CO-SEISMIC RUPTURE EXPLORATION USING SEISMIC REFLECTION PROFILE: A CASE STUDY OF THE M_S6.2 SOUTH TAIWAN STRAITS EARTHQUAKE IN 2018

WANG Sun, QIU Xue-lin, ZHAO Ming-hui, YAO Dao-ping, ZHANG Yi-feng, YAN Pei, JIN Zhen

2022, 44(1):  188-204.  DOI: 10.3969/j.issn.0253-4967.2022.01.012

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Co-seismic near-surface rupture is one of the keys to the recognition of earthquake fault and the defense to seismic hazard. However, conventional investigation methods such as outcrop mapping and trenching, are often disturbed by the variation of capping formation. Besides, it’s difficult to apply these methods under the sea water. Drawing on the idea of time-lapse seismic techniques in the petroleum industry, we suggest identifying the buried co-seismic ruptures using two reflection seismic data sets acquired before and after earthquake, respectively. In this paper, a case study is presented.
On Nov. 26^th, 2018, a M_S6.2 earthquake occurred in the south Taiwan Straits. The focal mechanism of this event is dextral strike slip with a slight dip-slip component, and the aftershock distribution is E-W oriented. West of the epicenter, multi-channel seismic profiling was carried out twice under the direction of Fujian Earthquake Agency in 2017 and 2019. To avoid the influence of the difference in acquisition conditions, before comparing we reprocessed and cross-equalized the two data sets with the same de-noise method, illumination, migration algorithm and velocity field. The profile correlation in 20~50Hz shows that the dominant reflecting wave groups are coincident with the time-lapse ones, which means the two sections are in phase.
The comparison results show that: at about 25km west of the epicenter, the reflection profile met the earthquake fault inferred by focal mechanism, and the morphology of Fault F₁ did not change significantly after the earthquake, but at depths greater than 400m, the remarkable reflections near the strike-slip fault plane changed significantly. From 2017 to 2019, the strongest reflection in the hanging wall reduced in amplitude and shifted from near horizontal to a jagged fold in shape, besides, the polarity of two remarkable reflections reversed, and a piece of the basement reflection in the heading wall close to the fault plane subsided about 8milliseconds measured in two-way travel time. The remarkable reflections on the rest of the profile were aligned accurately as the control. These phenomena can be interpreted as a fluid migration through sandstone fractures model perfectly, which is also consistent with the petrological features in the study area. Since the gap between the two acquisition dates is only 26 months and there was no human activity affecting subsurface structure in the vicinity, the pore fluid migration is inferred to be related to the 2018 event.
This study demonstrates that although the vertical co-seismic displacement is smaller than the resolution of reflection seismic profiles in most cases, the near-surface fluid migration which accompanied the co-seismic rupture may cause significant impedance changes near the fault plane, and such changes can be reliably identified on time-lapse seismic profiles. Compared with the conventional investigation methods for co-seismic ruptures, the time-lapse seismic method can overcome the displacement absorption of capping formation and expand the identifiable scope of co-seismic ruptures. This method is practicable, especially for marine earthquake researches, because the acquisition repeatability and surface consistence of the marine reflection seismic data are relatively better than that of the land reflection seismic data. This study provides a new idea for recognizing the earthquake fault and slip distribution of shallow source earthquakes, which is especially important for the study of marine earthquakes with fewer geology and geodesy data available.

A PRELIMINARY STUDY ON 3D VELOCITY STRUCTURE OF RONGCHANG AND ITS ADJACENT AREA

LI Cui-ping, TANG Mao-yun, GUO Wei-ying, WANG Xiao-long, DONG Lei

2022, 44(1):  205-219.  DOI: 10.3969/j.issn.0253-4967.2022.01.013

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An M_S4.9 earthquake occurred at 08:17 on the December 27, 2016 in Rongchang District, Chongqing, and the epicenter is located in the north central section of Huayingshan basement fault system on the eastern margin of Sichuan Basin. The seismicity shown in the historical earthquake catalogue was originally very weak in this area. Since the late 1980s, due to the impact of waste water reinjection in the natural gas field, earthquakes of magnitude ≥4.0 occurred frequently and 14 earthquakes with M_S≥4.0 have occurred, the largest of which was Rongchang M_S5.0 earthquake in 1997. In this paper, the fine three-dimensional P-wave velocity structures and relocation results of seismic events in Rongchang and its surrounding areas are inversed by double difference tomography method, based on the P-wave and S-wave arrival time data of 1786 seismic events recorded by Chongqing regional fixed network, mobile network and Zigong local network from January 2008 to June 2020.
The results show that: 1)The distribution of high-velocity and low-velocity zones within 4km depth is significantly different from that below 7~13km depth. The P-wave high-velocity zone at 4km depth is mainly distributed in Renyi-Rongchang area, where there are four water injection wells, a major concentration area of continuous water injection in Rongchang since 2008. The range of Renyi-Rongchang high velocity zone significantly gets narrowed at the 7km depth and is obviously different from that at the shallow surface. The velocity structures on the east and west sides of Huayingshan basement fault vary greatly from 7 to 13km. The P-wave velocity structures of different sections across Huayingshan basement fault all indicate that the depth of the interface between the sedimentary cover and crystalline basement is 12km in Rongchang area, which is basically consistent with the previous research results and the characteristics of seismic reflection profiles in Rongchang area. The inversed velocity structures well mirror the shape of Luoguanshan fold, and further confirm the reliability of our results. 2)The lateral difference of P-wave velocity structure in the shallow layer of Rongchang area varies greatly. There is a high-velocity zone near the Luo2^# water injection well at the axis of Luoguanshan anticline and the depth distribution is 3~7km. The hidden fault in the north wing of Luoguanshan anticline with buried depth of 1.7km is developed near well Luo2^#, and the high velocity zone is distributed along the dip of the hidden fault, which may indicate that the hidden fault may be the main channel for wastewater infiltration. The depth of wastewater infiltration is up to 7km, resulting in a large velocity difference between the two sides of the fault. The M_S4.9 earthquake on December 27, 2016 and the M_S4.0 earthquake on December 28, 2016 are just distributed in the velocity transition zone. Obvious high-velocity body was not found below 3km in Luo4^# water injection well, which may be related to the cessation of water injection in Luo4^# well in February 2001. 3)The results of seismic relocation indicate that earthquakes are mainly distributed in the axis of the strongly deformed Luoguanshan anticline, showing obvious stripe distribution in NE direction, and the focal dominant depth is 0~6km. Based on the focal mechanism solution and the regional seismotectonic environment, it is believed that the seismogenic fault of earthquakes above M_S4.0 on the south side of Guangshun transverse fault should be the hidden fault on the south wing of Luoguanshan, while the seismicity on the north side of Guangshun transverse fault may be related to the hidden fault on the north wing of Luoguanshan.

STUDY ON THE SEISMOTECTONICS OF THE QINGBAIJIANG M_S5.1 EARTHQUAKE IN SICHUAN PROVINCE IN 2020 BY MULTIPLE CONSTRAINT METHOD

XU Fang, LU Ren-qi, WANG Shuai, JIANG Guo-yan, LONG Feng, WANG Xiao-shan, SU Peng, LIU Guan-shen

2022, 44(1):  220-237.  DOI: 10.3969/j.issn.0253-4967.2022.01.014

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On February 3, 2020, an earthquake with a magnitude M_S5.1 occurred in Qingbaijiang District, Chengdu City, Sichuan Province. The epicenter is located in the north segment of the Longquan Shan fault zone in the western Sichuan Basin. This fault zone locates in the forebulge of the foreland thrust belt of the Longmen Shan fault zone in the southeast margin of Tibetan plateau and is the east boundary of the western Sichuan foreland basin at the same time, so it has special tectonic significance. There are two branch faults in the north segment of Longquan Shan fault zone, which are distributed on the east and west sides, respectively, and the epicenter distance is almost similar to the two faults. At present, the seismogenic fault, earthquake genesis and dynamic source of the earthquake are not clear. As this earthquake is a moderate earthquake event, it is usually very uncertain to interpret it with structural geological or seismological data alone. Therefore, this study attempts to carry out a comprehensive study on the Qingbaijiang M_S5.1 earthquake by performing cross fusion of multi-disciplinary data, adopting the multi-constraint method from geophysics, seismology and geodesy, and combining with structural geology and fault related fold theory. We collected three seismic reflection profiles located in the north segment of the fault zone to reveal the basic structural characteristics underground. The detachment layer in the middle-lower Triassic Jialingjiang-Leikoupo formation is developed at the depth of 4~6km below the anticline, and two obvious opposite thrust faults are developed on the two wings of the anticline, which are breakthrough fault-propagation fold deformation. The east branch thrust fault gradually rises from the detachment layer of Leikoupo formation to the surface, and the west branch thrust fault is exposed on the surface and connected with the detachment layer downward. The waveform data recorded by 14 fixed stations within 150km from the epicenter of Sichuan seismic network are studied and collected. The focal depth, focal mechanism and moment magnitude of the earthquake are obtained by using CAP waveform inversion method. The focal depth is 5km, indicating that the earthquake is related to shallow fault activity, the focal mechanism is 18°/32°/100° for nodal plane I and 186°/59°/84° for nodal plane Ⅱ, the moment magnitude is 4.64. Using the travel time data of P and S seismic phases, the Qingbaijiang earthquake sequence is relocated by HypoSAT location method and double difference location method. It is concluded that the epicenter position of the main earthquake is 30.73°N and 104.48°E. From February 4 to June 26, 2020, a total of 61 aftershock events were relocated, with magnitude 0≤M_L≤3.0 and depth ranging from near surface to 15km. The 61 aftershocks spread about 5km in the NW-SE direction and have conjugate distribution in NW and NE directions, which may be related to the small thrust fault developed on the east branch of Longquan Shan Fault. Aftershocks have a good linear distribution in NE direction, which is closer to the east branch of the north segment of Longquan Shan fault zone, and the distribution direction is also consistent with the fault strike. On the seismic reflection profile, the aftershock projection is densely distributed along the east branch fault. The occurrence of the east branch fault is consistent with the focal mechanism nodal plane I, which is a low angle thrust fault dipping to NW. The InSAR coseismic deformation field near the epicenter is extracted by using the Sentinel data of orbit 55 and orbit 62 collected from ESA, including 8 single view complex images of orbit 55 and orbit 62, respectively. The surface deformation caused by this earthquake is in the middle of two thrust faults, and the maximum coseismic deformation can reach 4cm. The deformation caused by the earthquake is uplifting in the northwest and depressing in the southeast of the epicenter. The largest depression is located between the epicenter and the east branch fault. The thrust activity of the east branch fault is more in line with the above surface deformation characteristics. In this study, the seismotectonics of the 2020 Qingbaijiang M_S5.1 earthquake is analyzed in detail using multi-disciplinary and multi-constraint method. The east branch fault in the north segment of the fault zone is determined as the seismogenic fault, and the possible seismic dynamic background is discussed. This result provides a scientific basis for fault activity analysis and seismic risk assessment in Longquan Shan area and has a great significance for further exploring the expansion and growth of Longmen Shan in the southeast margin of Tibetan plateau toward Sichuan Basin.

Special topic on the Qinghai Menyuan M_S6.9 earthquake

PRELIMINARY INVESTIGATION AND RESEARCH ON SURFACE RUPTURE CHARACTERISTICS OF THE 2022 QINGHAI MENYUAN M_S6.9 EARTHQUAKE

GAI Hai-long, LI Zhi-min, YAO Sheng-hai, LI Xin

2022, 44(1):  238-255.  DOI: 10.3969/j.issn.0253-4967.2022.01.015

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At 01:45 on January 8, 2022, Beijing Time, an M_S6.9 earthquake occurred in Menyuan County, Haibei Prefecture, Qinghai Province, with a focal depth of 10km. The microscopic(instrument)epicenter is located at 37.77°N latitude and 101.26°E longitude in the intersection between the Toleshan fault zone and the Lenglongling fault zone in the northern Qilian-Qaidam block. The epicenter is 54km away from Menyuan County in Qinghai, 99km away from Qilian County, 100km away from Haiyan County, 83km away from Minle County in Gansu Province, 83km away from Yongchang County, and 141km away from Xining City. When the earthquake occurred, Menyuan County and Xining City, the capital of Qinghai Province, were strongly felt, and Yinchuan, Lanzhou, Xi'an and many other places were felt. At the same time, affected by the earthquake, the Lanxin high-speed rail line, an important railway transportation hub of the Belt and Road, was suspended. This earthquake is the largest earthquake in the world since 2022. It is also another earthquake of magnitude 6.0 or above in Qinghai Province following the Maduo M_S7.4 earthquake on May 22, 2021. Besides, this earthquake is the event with the highest magnitude and the longest surface rupture in the region after the two M6.4 Menyuan earthquakes of August 26, 1986 and January 21, 2016. Therefore, this earthquake has attracted much attention from the society. The coseismic surface rupture distribution, combination characteristics, development properties and coseismic displacement of this earthquake were identified in time to help to have a correct understanding of the earthquake seismogenic structure, rupture process, and assessment of short-term earthquake hazards. It is also of great significance for major project route selection, earthquake fortification and rescue and disaster relief. On the basis of the on-site seismic geological investigation, based on the interpretation and analysis of high-resolution satellite remote sensing images, and combined with the low-altitude photogrammetry of unmanned aerial vehicles(DJI PHANTOM 4RTK), the author obtained the coseismic rupture data of five typical sites along the surface rupture zone generated by the earthquake. Using Agisoft Metashape Professional software to process the aerial photos of each section indoors, a high-resolution orthophoto map(DOM)was generated. At the same time, the five typical earthquake surface rupture sections were described in detail in ArcGIS Pro software based on the orthophoto map. Preliminary research shows that the surface rupture zone of the Menyuan M_S6.9 earthquake is more than 22km long and consists of the main rupture of the northern branch and the secondary rupture of the southern branch. The north branch main rupture zone is distributed in the middle-western segment of the Lenglongling Fault of central Haiyuan fault zone, with a length of more than 18km and an overall strike of 295°. The maximum co-seismic horizontal displacement is located in the middle of the rupture zone at Liuhuangou(37.799°N, 101.2607°E), which is about 3.1m and gradually decays towards both ends. The secondary rupture of the southern branch is distributed on the local segments of the eastern Toleshan Fault in the central-western Haiyuan fault zone, with a length of about 4km and a strike of 275°, constituting a secondary branch rupture zone arranged in a left-stepped en-echelon pattern to the western segment of the main rupture zone. There are en-echelon extensional stepovers between the two rupture zones of the north and south branches. The whole surface rupture zone is mainly composed of linear shear cracks, oblique tension cracks, tension-shear cracks, compressional bulges and other structural types. The coseismic surface rupture has the characteristic of typical left-lateral strike-slip motion with a thrust component, and the maximum vertical dislocation is 0.8m.

SURFACE RUPTURE CHARACTERISTICS OF THE MENYUAN M_S6.9 EARTHQUAKE ON JANUARY 8, 2022, QINGHAI PROVINCE

LIANG Kuan, HE Zhong-tai, JIANG Wen-liang, LI Yong-sheng, LIU Ze-min

2022, 44(1):  256-278.  DOI: 10.3969/j.issn.0253-4967.2022.01.016

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At 1:45 on January 8, 2022, a M_S6.9 earthquake occurred in Menyuan County, Haibei Prefecture, Qinghai Province. The epicenter(37.77°N, 101.26°E)is located in the western segment of the Lenglongling Fault of the Qilian-Haiyuan fault zone, with a focal depth of 10km. The earthquake is located in the northwest of the M_S6.4 Menyuan earthquake on January 21, 2016. According to the survey results of China Earthquake Administration, the highest intensity of this earthquake is IX degree, and the long axis of the isoseismic line is NWW-striking. The earthquake caused serious damage to the Daliang Tunnel between Haomen Station and Junmachang Station, and the Lanxin high-speed railway was interrupted. After the earthquake, the distribution of the earthquake surface rupture zone was quickly determined by interpreting the GF-7 satellite post-earthquake images, and the field surface rupture investigation was carried out at the epicenter site in the first time. The field investigation mainly includes the identification of surface rupture zones, the investigation of rupture characteristics, the survey of fault geomorphology, the high-precision aerial photogrammetry of typical rupture points, the identification and measurement of coseismic dislocation, and the investigation of earthquake disasters. Aerial photogrammetry realizes real-time difference through UAV linked network RTK, and takes high-definition photos from multiple angles. Pix4D software is used to complete calculation and point cloud encryption, etc. DSM (Digital Surface Model) and DOM (Digital Orthophoto Map) are generated for surface rupture space reproduction and feature measurement and analysis. According to the interpretation of high-resolution remote sensing images by GF-7 satellite and field investigation, the surface rupture of M_S6.9 Menyuan earthquake can be divided into NW-striking western segment of Lenglongling Fault and EW-striking eastern segment of Tuolaishan Fault. The two surface ruptures are 291° and 86.9°, respectively, and their lengths are not less than 26km and 3.5km respectively. We made detailed observation and measurement on the Jingyangling site, Daogou site, east Daogou site, Shixiamen site, the seven sites along the Liuhuanggou on the Lenglongling Fault, and the Yangchangzigou site on the Tuolaishan Fault. The surface rupture zone is mainly a complex coseismic surface deformation zone formed by the combination of multiple types of fractures, such as tensional fracture, tensional shear fracture, compression bulge and seismic depression, and characterized by sinistral strike-slip motion and partly by thrusting. Generally, the NW-striking ruptures exhibit left-lateral strike-slip characteristics, while NW-striking branch ruptures exhibit a small amount of right-lateral strike-slip characteristics. At Shixiamen site, four pasture fences were continuously offset left-laterally by 2.0~2.15m. At the Daliang Tunel site, the rut was offset left-laterally by 2.77m measured by UAV, which is the largest co-seismic left-lateral displacement of this earthquake. Based on high-resolution remote sensing image interpretation, field investigation, InSAR inversion of focal mechanism, fault rupture model and small earthquake precision location, it is determined that the earthquake occurred at the deep intersection of the Tuolaishan Fault and Lenglongling Fault, and the main seismogenic structure is the western segment of Lenglongling Fault(strike 112°, dip 88°). The Tuolaishan Fault on its west side ruptured simultaneously at the east end. According to the distribution characteristics of the surface ruptures and the field investigation of this earthquake, we believe that the Lenglongling Fault continues to extend westward after passing through the Liuhuanggou No. 1 site until the Jingyangling site, and the NWW-striking Lenglongling Fault has a “Y”-shaped contact relationship with the EW-striking Tuolaishan Fault. The 1986 M_S6.4 earthquake occurred at the northwestern end of the Lenglongling North Fault, which protrudes in an arc toward NE, and the 2016 M_S6.4 earthquake occurred at the southeastern end of the fault. Affected by the left-lateral strike-slip movement of the Lenglongling Fault, the small block bounded by the Lenglongling Fault and the Lenglongling North Fault also moves in the direction of SEE relative to the northern block. Therefore, the 1986 M_S6.4 earthquake showed tensile properties, and the 2016 M_S6.4 earthquake showed compression properties. The seismogenic structure of the Menyuan M_S6.9 earthquake is the Lenglongling Fault, so the earthquake is mainly characterized by left-lateral strike-slip. The M_S6.4 earthquake in 1986, M_S6.4 earthquake in 2016 and M_S6.9 earthquake in 2022all occurred in the western section of Lenglongling Fault. Three strong earthquakes of M>6 occurred in a short period of time, indicating that this area is still an accumulation area of stress and deformation, and has the potential risk of large earthquakes.
Due to the limitation of the data range of the Gaofen-7 satellite image and the inconvenience of traffic caused by the icing of the river, the location of the easternmost end point of the rupture and the exact length of the rupture have not been determined in this field investigation. We hope that follow-up studies will be carried out to confirm the rupture length when weather conditions are appropriate.