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20 June 2021, Volume 43 Issue 3

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2021, 43(3):  0-0. 

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Research paper

ANALYSIS ON THE SEISMIC CHARACTERISTIC DISPLACEMENT OF ANQIU-JUXIAN FAULT BASED ON DEXTRAL HORIZONTAL DISLOCATION OF GULLY

JI Hao-min, LI An, ZHANG Shi-min

2021, 43(3):  471-487.  DOI: 10.3969/j.issn.0253-4967.2021.03.001

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The Tanlu fault zone(TLFZ)is the largest strike-slip fault system in eastern China, which is composed of five main faults in Shandong and Jiangsu Provinces. Among them, the Anqiu-Juxian Fault(AJF)is the only fault with obvious activity since the late Quaternary, and it is also the seismogenic structure of the Anqiu M7 earthquake in 70BC. It is of great significance to understand the tectonic activity of the TLFZ by analyzing the co-seismic displacement of this earthquake and studying the long-term activity behavior of the fault. According to the spatial distribution characteristics and seismic activity, the northern segment of the AJF between Juxian and Changyi(NAJF)is divided into four sub-segments, which are, from south to north, the Juxian-Mengyan segment, the Qingfengling segment, the Anqiu-Mengtong segment and the Changyi-Nanliu segment, respectively. However, paleoearthquake studies in the NAJF are not ideal, and only suggested that this segment was active in the Holocene. In addition, there is also no competent evidence of coseismic displacement in the previous researches.
In this study, we interpreted the geomorphic trace of the fault through remote sensing images and found that there were a large number of gullies where dextral horizontal dislocations are discovered, which are concentrated in the Anqiu-Mengtong segment and Qingfengling segment. Later, we used the high-resolution UAV-SfM photogrammetry technology to map the typical geomorphic areas from Anqiu to Juxian in the field investigation, and obtained the DEM of areas with offset gullies. Then we measured the offsets of the gullies by the measurement software, LaDiCao_v2, and acquired 79 horizontal dislocations. Combined with 5 measurement results from the previous research, we finally obtained 84 horizontal dislocations, including 26 data in the Anqiu-Mengtong segment and 58 in the Qingfengling segment. According to the statistical results of the cumulative offset probability distribution(COPD), the horizontal displacements in the Anqiu-Mengtong segment mainly concentrated in 5 intervals with the peak values of 5m, 10.4m, 15.5m, 20.6m and 25m, respectively; the horizontal displacements in the Qingfengling segment mainly concentrated in 4 intervals with the peak values of 5m, 9.7m, 16m and 19.7m, respectively. The bigger data is of less statistical significance due to large time span and small amount. The smallest dextral horizontal displacements of gullies on these two segments are both about 5m, and the larger offsets are also multiples of 5m. In addition, as the increase of the interval peak value, the number of gullies in the interval decreases. Therefore, the minimum dislocation of 5m should represent the latest activity event of these two secondary faults and be the coseismic displacement of the earthquake; the large dislocations represent the cumulative displacements of multiple seismic events, which reveal the characteristic displacement of about 5m for the two secondary faults. However, due to the unclear paleoearthquake sequence, it is also unclear whether these sub-segments were active at the same time. In addition, based on the statistical analysis on the strike-slip seismic events, there are a series of empirical formulas among the coseismic displacement, magnitude, and surface rupture length about the strike-slip faults. We used the coseismic displacement of 5m to infer the magnitude and surface rupture length of the Anqiu earthquake, and the results show that the earthquake magnitude mostly ranges from 7.5 to 7.7 and the surface rupture length is about 100km. According to previous historical records, when the 70BC Anqiu earthquake struck, the quake was felt strongly in the city of Xi 'an, hundreds of kilometers away. Therefore, combined with the calculation results and the fact that only the 70BC Anqiu earthquake was recorded in the NAJF, if the coseismic displacement of 5m was caused by the Anqiu earthquake, its magnitude may be undervalued, and the actual magnitude should be above 7.5. At the same time, the latest paleoearthquake event on Juxian-Mengyan segment is(2 140±190)a BP ago, close to the Anqiu earthquake in 70BC. Therefore, due to the calculation results of the surface rupture length of 100km, the Anqiu earthquake may have caused the cascade rupture of Anqiu-Mengtong, Qingfengling, and Juxian-Mengyan segments. Or the characteristic displacement of 5m indicates another paleoearthquake event, and the seismogenic fault of the 70BC Anqiu M7 earthquake is the Changyi-Nanliu segment, because there are more evidences of Holocene activity observed in this segment. However, since there has been no strong earthquake in this segment for more than 2 000a and various evidences have indicated that this segment has the ability of generating strong earthquake, high attention should be paid to the seismic risk in this area in the future.

GEOMORPHIC ANALYSIS OF STRIKE-SLIP FAULTING AT THE TOP OF ALLUVIAL FAN: A CASE STUDY AT AHEBIEDOU RIVER ON THE EASTERN MARGIN OF TACHENG BASIN, XINJIANG, CHINA

MIAO Shu-qing, HU Zong-kai, ZHANG Ling, YANG Hai-bo, YANG Xiao-ping

2021, 43(3):  488-503.  DOI: 10.3969/j.issn.0253-4967.2021.03.002

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The top of the piedmont alluvial fan has the characteristic of fan-shaped terrain and gradually descending terrain in the downstream direction. Faulting of various natures will result in different geomorphic features of alluvial fan surface. The variation of slope aspect and height of the pure sinistral fault scarp at the top of the alluvial fan is analyzed firstly under the three conditions, namely, the fault plane is vertical, the fault plane inclines toward the upper stream of the river, and the fault plane inclines toward the downstream of the river. We have also analyzed the variation of slope aspect and height of the fault scarps at the top of the alluvial fan under different fault inclination conditions of inverse sinistral strike-slip fault and the sinistral strike-slip normal fault. The seven geomorphic types we analyzed above cover the geomorphic features caused by the activity of strike-slip faults at the top of alluvial fans, which can help us to analyze the formation of the landforms. Based on drone-measured terrain data, Google satellite images and field investigations, we found that the Dongbielieke Fault, which strikes northeast-southwest and is located in the eastern margin of the Tacheng Basin, Xinjiang, almost vertically passes through the Ahebeidou River which develops from southeast to northwest. The direction of central axis of the alluvial fan at Ahebedou River is northwest, with a north-facing slope. The fault activity has caused the development of an uphill-facing scarp that has a height of~5.2m and a slope aspect facing southeast on the top of the alluvial fan at the Ahebiedou River section of the Dongbielieke Fault. And on the piedmont alluvial fan 1km away on both sides of the river bed, the sinistral fault scarps have a northwest-facing slope aspect and a height of 1~5m. The river terraces are divided into five levels, the T2 on the left bank, T4 on the right bank and T5 terraces on the left and right banks of Ahebeidou River were affected by fault activity. Sinistral offsets and southeast-facing fault scarps were developed on the geomorphic surface. By using DispCalc_Bathy_v2, a script based on Matlab, we get the offsets of the river terraces from the high-resolution DEM data obtained by using UAV photogrammetry technology. The sinistral horizontal offsets of T2 on the left bank, T4 on the right bank and T5 terraces on the left and right banks of Ahebeidou River are(10.1±0.2)m, (10.6±0.7)m, (29.1±0.2)m and(20.0±0.7)m, respectively. The vertical displacements are(1.5±0.1)m, (3.6±0.3)m, (4.7±0.2)m and(5.2±0.1)m, respectively. The asymmetrical development of terrains on both sides of the river is affected by topography and fault activity. The terraces on the lower elevation right bank of the river are misplaced into the channel by sinistral strike-slip faulting to receive more erosion, so the offsets we measured on the left bank of the river are more reliable than that on the right bank. Through field surveys, we found two fault outcrops, indicating that the fault plane is inclined to the southeast. The young river terrace T2 was effected by faulting and a uphill-facing scarp was developed, which indicates that the latest faulting was of sinistral strike-slip with a normal component, but the fault scarp's aspect changed twice within a short area of two kilometers, which is not consistent with the geomorphological type caused by the strike-slip faulting on the top of the alluvial fan as we previously analyzed. According to the landform features and the strike-slip fault geomorphic model, a model for the geomorphic surface development of the Ahebiedou River section is established. In this model, we think the Dongbielieke Fault was an inverse sinistral strike-slip fault after the formation of an older phase geomorphic surface S1 in the area. The early fault activity formed a northwest-facing fault scarp at S1, the height of the scarp is about 10m. Then the alluvial fan(Fan1)began to develop, and the material brought by the flowing water deposited and buried the fault scarp at the exit of piedmont, resulting in the disappearance of the existing fault scarp in the piedmont. Then the characteristic of fault changed into left-lateral strike-slip with a normal component. The activity of normal fault with the fault plane dipping to SE would form a fault scarp facing SE on the geomorphic surface. With the gradually cutting of the river, river terraces began to form on both sides of the river, and the corresponding geomorphic features were formed under the influence of fault activities. A fault scarp with a slope facing southeast formed at both banks of the river's mountain outlet with a height of about 5.2m through several fault activities, and sinistral horizontal offsets of river terraces increased at the same time. And the height of the pre-existing northwest-facing scarp 1~2km away from both banks of the river's mountain outlet decreased to about 5m, which can be observed in the field. The eventual geomorphic surface is characterized by the features of downhill-facing scarp-no scarp-uphill-facing scarp-no scarp-downhill-facing scarp from southeast to northeast.

THE LATE QUATERNARY AND PRESENT-DAY ACTIVITIES OF THE KOUZHEN-GUANSHAN FAULT ON THE NORTHERN BOUNDARY OF WEIHE GRABEN BASIN, CHINA

YANG Chen-yi, LI Xiao-ni, FENG Xi-jie, ZHU Lin, LI Miao, ZHANG En-hui

2021, 43(3):  504-520.  DOI: 10.3969/j.issn.0253-4967.2021.03.003

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The Kouzhen-Guanshan Fault trends in near E-W direction and obliquely cuts the active NEE-striking northern boundary fault zone of the Weihe Graben Basin, a fault zone that constitutes the boundary between Weihe Graben Basin and the Ordos block. Medium to small earthquakes occur frequently along the fault. Since the 1980s, a series of researches have been carried out on this fault, and certain cognition has been gained on its geometry, kinematics, tectonic evolution, recent activity and seismogenic capacity. However, most of the eastern segment of the fault is concealed in the Quaternary sediments of Weihe Graben Basin, and the corresponding research and attention are less. By conducting new field geological surveys and combining data from fault-crossing leveling and creepmeter observation, we studied the activities of the Kouzhen-Guanshan Fault during the late Quaternary and in the recent decades, supplemented the geological evidence of fault activity in the late Quaternary, and analyzed the characteristics and differences of tectonic activities on the western and eastern segments of the fault. Our research provides new insights as follows: 1)For the Kouzhen-Guanshan Fault, previous geological surveys were mainly carried out in the western segment with a focus on studying the vertical movement. It is considered that the fault activity has been stronger in the western segment and weaker in the eastern segment since the late Pleistocene. Our field investigation of three geologic cross-sections on the eastern bank of the Shichuan River in the eastern segment provides the understanding of the geological activity on the eastern segment. It reveals that the eastern segment of the Kouzhen-Guanshan Fault has a vertical motion component since the late Pleistocene, where the late Pleistocene stratum has been vertically offset by 8.8m, yielding a vertical slip rate of >0.13mm/a. At places between the central and western segments of the fault, the offset gullies were gradually cut down after the accumulation of loess layer L₁, and the age of S₁ at the bottom of L₁ can represent the lower limit of the left-lateral dislocation age of these gullies. The horizontally-faulted geomorphic features produced in the late Pleistocene have an average left-lateral displacement of 34m, which yields a left-lateral strike-slip rate of >0.49mm/a. These suggest that the Kouzhen-Guanshan Fault is a normal-sinistral oblique-slip one dipping steeply to the south; it would also be a growing transfer fault to adjust the non-uniform horizontal extension between segments of the Weihe Basin by obliquely cutting the northern boundary fault zone of the Basin. 2)Creeping movement is found to occur continuously on two connecting segments of the Kouzhen-Guanshan Fault at least in the last more than 30 years. Fault-crossing leveling observation for more than 30 years has been carried out on the Kouzhen and Jingyang sites on the western segment of the fault, respectively, and fault-crossing creepmeter observation has been carried out for nearly 7 years at Jingyang site, both of which have detected the present activity characteristics of the western segment of the fault. Among them, the two fault-crossing leveling observation time series show that the trends of vertical creep movement are basically the same since 1986. The creepmeter observation at Jingyang site shows that the fault has experienced continuously normal-sinistral creeping, and the horizontal-transverse stretching alternates with sinistral creeping since 2012. At Kangcun site on the western segment of the fault, fault-crossing leveling observation has been carried out for nearly 20 years. For the western segment, the fault creep is relatively stable with time and shows normal-sinistral oblique-creep faulting with the rates of 0.16~0.76mm/a for the vertical component, 0.42~0.78mm/a for the sinistral-creep component, and 0.15~0.26mm/a for the horizontal-transverse stretching component, respectively. Although technical means to observe or detect horizontal deformation are absent on the eastern segment of the fault, the campaign leveling surveys suggest that the fault creep on this segment has an average rate of 1.59mm/a for the vertical component(relative decline in the southern part of the fault)and shows a time series pattern of “step-like” or “episodic” creep, and the fault creep here with a rate as high as 13mm/a during the “step-like” period(2011 to 2014)may represent one slow slip event. 3)The present vertical creeping velocity of the eastern and western segments of the fault is different. The creep rate of the eastern segment is higher than that in the west, which may reflect the eastern segment of the fault is closer to the core of Weihe Graben Basin in space. This inference can be derived from the evidence that the new activity of the fault zone in the northern margin of Weihe Graben Basin, the development of ground fissures belt and seismicity along the Kouzhen-Guanshan Fault are all stronger in the eastern segment. 4)Both the seismicity and the cause of ground fissures belt along the Kouzhen-Guanshan Fault are closely related to the motion of normal-sinistral oblique-creep on this fault, which is controlled by the fault activity and should be the reflection of the surface macroscopic deformation of creeping. 5)The observed creeping movement on the Kouzhen-Guanshan Fault, especially, the phenomenon of “episodic” creep(rarely reported in China)in the vertical motion component on the eastern segment of the fault, proves that slow slip or creep may also occur on faults in tectonically active tensional environments of mainland China. There is obvious difference of normal creep faulting in the eastern and western segments of the fault. It is further necessary to study the differences in the friction properties of the fault segments reflected by the differences in the creep characteristics of these two segments, as well as seismic tectonic and seismic precursory implications of creeping with different characteristics. We therefore suggest strengthening the monitoring of the fault motion and the study of potential seismic hazards. 6)Regarding the “step-like” or “episodic” creep of the fault, the existing research mainly comes from the strike-slip fault. It is found that the present vertical motion component of the Kouzhen-Guanshan Fault shows obvious “step-like” or “episodic” creep characteristics. Therefore, it is necessary to study the relationship between the creeping effect and the phenomenon of seismicity and ground fissures alone the fault. In the future, we intend to combine the microseismic activity and fault friction theory to study the possible mechanism of the “episodic” creep, as well as the tectonic and seismic precursory implications of slow slip events similar to those observed at Kangcun site during 2012—2014.

TECTONIC GEOMORPHIC FEATURES AND GEOLOGICAL SIGNIFICANCE OF THE SHIDIQUAN ANTICLINE IN THE NORTHERN MARGIN OF THE QAIDAM BASIN

DONG Jin-yuan, LI Chuan-you, ZHENG Wen-jun, LI Tao, LI Xin-nan, REN Guang-xue, LUO Quan-xing

2021, 43(3):  521-539.  DOI: 10.3969/j.issn.0253-4967.2021.03.004

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In the process of intense compression and shortening of the orogenic belt, a series of thrust faults and folds related to reverse faults developed in the piedmont. Determining the kinematic characteristics of these reverse faults and folds is of great significance for understanding the deformation mode of the orogenic belt. The Qilian Shan is located on the northeastern margin of the Tibetan plateau and is the front edge of the plateau expansion. The area has undergone strong tectonic activity since the Late Quaternary, with developed active structures and frequent earthquakes. There are a series of piedmont thrust faults and thrust related folds in the northern and southern margins of Qilian Shan. Compared with a large number of research results of active folds in Tian Shan area, the study of active folds in Qilian Shan is relatively weak. In the northern margin of the Qilian Shan, in addition to the study of individual active folds, most previous studies focused on the thrust faults in the northern margin of the Qilian Shan and the Hexi Corridor, and obtained the active characteristics of these faults. In the southern margin of Qilian Shan, that is, the northern margin of the Qaidam Basin, some studies have been carried out on paleoearthquakes and slip rate of the fault in the southern margin of Zongwulong Shan. However, the study on the late Quaternary folds in this area is relatively weak and there are only some sporadic works.
Shidiquan anticline is located in the intermountain basin surrounded by Zongwulong Shan and Hongshan in the northern margin of Qaidam Basin. It forms the first row fold structure in front of Zongwulong Shan with Huaitoutala and Delingha anticline. Constraining the tectonic geomorphic features of the Shidiquan anticline is of great significance for studying the crustal shortening in the northern margin of the Qaidam Basin and the expansion of the Qilian Shan to the Qaidam Basin. In this paper, the tectonic and geomorphic characteristics of Shidiquan anticline are obtained by means of geological mapping, high-precision differential GPS topographic profile survey, geological profile survey and cosmogenic nuclide dating. Field investigation shows that Shidiquan anticline is an asymmetric fold with steep south limb and gentle north limb, and is controlled by a blind reverse fault dipping northward. The age of the alluvial fan3 obtained from cosmogenic nuclide dating is(158.32±15.54)ka. This age coincides with the Gonghe Movement, indicating that the formation of Shidiquan anticline responds to the Gonghe Movement in the northeast margin of Tibetan plateau. The uplift rate of Shidiquan anticline since 158ka is(0.06±0.01)mm/a, and the shortening rate is(0.05±0.01)mm/a. The folding effect of Shidiquan anticline indicates that the folding of the intermountain basin in the northern margin of the Qaidam Basin, similar to the thrust shortening of the piedmont fault, plays an important role in regulating the shortening of the foreland crust.

STUDY ON THE RECENT DEFORMATION CHARACTERISTIC AND STRUCTURAL DEFORMATION MODEL OF THE SOUTH-EASTERN MARGIN OF ORDOS BLOCK

LIU Rui-chun, ZHANG Jin, GUO Wen-feng, CHEN Hui, ZHENG Ya-di, CHENG Cheng

2021, 43(3):  540-558.  DOI: 10.3969/j.issn.0253-4967.2021.03.005

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The southeastern margin of the Ordos block is a key area for dynamic transformation from collision and compression in the western part of the Chinese mainland to extension in the east, and also is the junction of the NE-SW trending structure in the north and near the E-W trending structure in the south of the North China block. The tectonic activity in the southeastern margin of the Ordos block is intense. In this region, the Houma-Yuncheng section is a noteworthy area for medium- and long-term large earthquake risk determined by China Earthquake Administration, which involves three tectonic units: Linfen Basin, Yuncheng Basin and Emei Platform. The potential seismogenic faults include the Hancheng Fault, the southern margin fault of Emei Platform and the piedmont fault of Zhongtiao Mountains. Because the neotectonic movement in this region is mainly dominated by strong differential movement, it is important to estimate the fault kinematics parameters based on the high-resolution vertical crustal movement observation constraints.
Fault locking depth and slip rate are important indicators to judge the risk of future earthquakes. When the accumulation time of fault seismic moment and fault length are given, the larger fault locking depth and higher slip rate will cause the greater energy accumulation and stronger future earthquakes risk of the fault. Based on the traditional leveling and GPS data, previous studies found that the southeastern margin of the Ordos block is perhaps experiencing strong tectonic movement. However, the measuring point density of the above technical means is difficult to satisfy the quantitative study of the current activity characteristics of specific faults. Therefore, the interferogram stacking technique is used to obtain the spatial high-resolution InSAR average deformation rate field of the study area based on the Radarsat -2 wide-mode image in this paper firstly. At the same time, the three-component velocity of GPS continuous station in the study area is projected into the radar line of sight direction. After unifying the reference datum, comparative analysis was conducted to evaluate the accuracy and reliability of InSAR results. The results show that the standard deviation of the difference between the short-term InSAR and the long-term GPS observation values is 2.7mm. The annual crustal deformation field obtained by using the interferogram stacking technology in the study area has a high accuracy, which can reflect the characteristics of regional crustal movement. It also indicates that the regional crustal short-term deformation is consistent with the long-term deformation. Secondly, the dip-slip fault dislocation model and particle swarm optimization(PSO)were used to invert the main fault slip rate and locking depth, the inversion was repeated 1 000 times, and the optimum estimate of parameters was obtained by statistical analysis of results and uncertainty. The fault slip rate and locking depth data approximately obey the normal distribution, and the stability is good; the dip angles of faults are skewed but concentrated. The above results show that the fault movement parameters obtained from InSAR deformation field inversion are reliable and can be used for regional tectonic movement analysis. Finally, based on the data of regional geological structure, fault slip rate, fault locking depth and present seismic activity, this paper analyzes the variation characteristics of InSAR deformation field, and discusses the fault tectonic movement mode, future seismic risk and regional tectonic deformation pattern in the southeastern margin of Ordos. The results show that the tectonic and nontectonic deformations are superimposed on the southeastern margin of Ordos. Tectonic deformation mainly occurs near active faults, which is related to fault slip rate and closure depth. Nontectonic deformation mainly occurs in the Quaternary strata inside the basin, which is related to the thickness of the aquifer and the amount of groundwater extraction, and the maximum can reach 5cm/a. The slip rate of the fault at the northern foot of the Zhongtiao Mountains and the northern margin of the Emei Platform is 0.37mm/a and 0.74mm/a, and the blocking depth is 3.4km and 4.3km, which are relatively shallow. It may indicate that the fault was not completely closed after the last strong earthquake and is dominated by shallow seismic activity. The slip rate of the fault on the southern margin of the Emei Platform is 0.47mm/a, and the closure depth is 0.95km, indicating that the faults are mainly creepy. The counterclockwise rotation of the Ordos block and the eastward extrusion and escape of the Qinling Mountains formed a quasi-triple junction structural area on the southeastern margin of Ordos, characterized by strike-slip-extension transition.

THE LATEST ACTIVITY OF SUDIAN FAULT ON THE BORDER BETWEEN CHINA AND MYANMAR AND ITS TECTONIC SIGNIFICANCE

CHANG Zu-feng, CHANG Hao, MAO Ze-bin, LUO Lin, WANG Qi

2021, 43(3):  559-575.  DOI: 10.3969/j.issn.0253-4967.2021.03.006

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The Sudian Fault extends in nearly NS direction and crosses the border between China and Myanmar, with a length of about 100km. Historically, many earthquakes have occurred along the fault. However, restricted by traffic, climate and other factors, there has been little research on the late Quaternary activity of the fault for a long time. On the basis of results of field geological and geomorphological investigation, trenching and geochronology, the movement characteristics of the fault in late Quaternary, the latest active age and sliding rate are analyzed in this paper. The Neotectonic activity of the Sudian Fault is obvious. Beaded Quaternary basins in areas of Sudian, Mengdian, Huangcaoba and Longzhong have developed along the fault. Many boiling springs and gas springs are distributed linearly in the area of Humeng in the south section of the fault. The fault controls the Lama River and Zhanda River obviously. Fault landforms are mainly characterized by clear fault scarps, straight linear ridges and fault valleys. Mengdian pull apart basin is developed in the middle segment of Sudian Fault. In the Zuojiapo area of the western margin of the basin, there is a clear linear ridge about 1.7km long and a parallel fault valley which is close to the west side of the linear ridge. Trench excavation was carried out in this fault valley(24.97°N, 97.93°E). Zuojiapo trench reveals that three faults have developed in Quaternary deposits. At the position of 2~3m(from west to east)on the S wall of the trench, a fault dislocated all the strata(unit②~unit⑥)below the modern loam layer(unit①). These strata are obviously offset and some of them are cut off. The ¹⁴C age of the displaced unit ④(tested by BETA laboratory, USA)is(7 680±30)a, two ¹⁴C ages of the displaced unit ③are(6 970±30)a and(5 860±30)a, and the ¹⁴C age of the displaced unit ②is(1 260±30)a. The fault developed at 21m in the east section of S-wall of the trench has offset the lower bedrock(unit⑧), the middle gravel layer(unit⑤and unit⑦), the upper dark gray gravelly clay layer(unit④)and the peat interlayer(unit④'). In the peat interlayer(unit④'), there is obvious structural deflection deformation, and its ¹⁴C age is(350±30)a. There is another fault developed at 26~27m in the east section of S-wall of this trench, which cuts off the light yellow and light gray gravelly clay(unit ②), gray black gravelly clay(unit③), gray white sandy gravel(unit⑤), yellow gravelly silty clay(unit ⑥), yellow clay gravel(unit ⑦)and hornblende schist and quartz schist of Gaoligongshan group(unit ⑧). The fault shows obvious normal fault property, and the maximum offset is 1.3m. A 10cm wide schistosity zone is developed and gravels are arranged along the fault plane. The ¹⁴C ages of the faulted upper stratum(unit ③)are(1 100±30)a and(870±30)a. The N-wall also reveals the existence of faults, corresponding to the S-wall of the trench. These faults and dislocated strata fully indicate that the fault was active during the Holocene. According to field investigation, the Sudian Fault is mainly characterized by horizontal dextral strike-slip movement. For example, in Mengnong tea field, obvious synchronous dextral displacement occurred in three gullies along the fault. From south to north, the displacements of the three gullies are 40m, 42m and 45m, respectively. Shutter ridge landform is developed at the gully mouth. In the lower part of the northernmost gully, there is a pluvial fan, and the ¹⁴C age of the bottom of the pluvial fan is(13 560±40)a, which is less than the formation age of the gully, but roughly represents the formation age of the gully, indicating that the Sudian Fault is mainly characterized by horizontal dextral strike-slip movement. In Sudian area, the Mengga River is right-laterally offset 1 050~1 100m by the fault. At 1.7km north of Sudian, a diluvial fan is right-laterally offset 18~22m. There are fault scarps with a height of 1~1.5m developed on the alluvial fan, Quaternary faults and bedrock fault scarps with a height of about 8m developed on its extension line. The three points of the scarps, Quaternary faults and bedrock scarps are in a straight line, which absolutely shows the reliability of the dislocation of the alluvial fan. An organic carbon sample is obtained 1.8m below the alluvial fan, and its ¹⁴C test age is (6 210±30)a. This age should be close to the formation age of the pluvial fan, indicating that the fault underwent obvious horizontal dextral strike-slip movement during the Holocene. In the Sadung Basin, Myanmar, a river is offset about 380m right-laterally, forming a hairpin bending landform. Due to the continuous collision between the Indian plate and the Eurasian plate, the Indosinian block in the southeastern margin of the Tibet Plateau around the Eastern Himalayan Syntaxis escaped southerly, and the western Yunnan became the most intense part of the south extrusion. During the southerly escapement of the Indosinian block, the right-lateral strike-slip movement of Sudian Fault and other faults striking near SN plays a role in adjusting and absorbing the block strain. Under the action of current NNE tectonic stress field, the intersection of the dextral strike-slip Sudian Fault striking NS and the sinistral strike-slip Dayingjiang Fault striking NE is the key part of tectonic stress concentration, which will be the seismic risk area to be focused in the future. The research result of late Quaternary activity of the fault is of great practical significance for the correct understanding and reasonable assessment of the medium to long-term strong earthquake risk in this area, and for the mitigation and prevention of the earthquake disaster in the border area.

PROGRESS AND RESEARCH OF PALEOALTITUDE RECON-STRUCTION OF CENOZOIC BASINS IN THE SOUTHEASTERN TIBET PLATEAU

TANG Mao-yun, LIU-ZENG Jing, LI Cui-ping, WANG Wei, ZHANG Jin-yu, XU Qiang

2021, 43(3):  576-599.  DOI: 10.3969/j.issn.0253-4967.2021.03.007

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The elevation evolution history of the southeastern Tibet Plateau is of great significance for examining the deformation mechanism of the plateau boundary and understanding the interior geodynamic mechanics. It provides an important window to inspect the uplift and deformation processes of the Tibet Plateau, and also an important way to test two controversial dynamic end-element models of the Plateau boundary. In recent years, some breakthroughs have been made in the study of paleoaltitudes in the southeastern Tibet Plateau, which allows us to have a clearer understanding of its evolution process and dynamic mechanism. By reviewing and recalculation of the latest achievements of paleo-altitude studies of the basins in the southeastern Tibet Plateau from north to south, including the Nangqian Basin, Gongjue Basin, Mangkang Basin, Liming-Jianchuan-Lanping Basin, Eryuan Basin, Nuhe Basin and Chake-Xiaolongtan Basin, we discuss the surface elevation evolution framework of the Cenozoic geomorphology and dynamics in the southeastern Tibet Plateau. The results show as follows:
(1)There was an early Eocene-Oligocene quasi plateau with an altitude of at least 2.5km from the north to middle of the southeastern Tibet Plateau(north of Dali), while the surface elevation in the south(south of Dali to Yunnan-Guizhou Plateau)was relatively low, even close to sea level. Until Miocene, the north to middle of the southeastern Tibet Plateau reached the present altitude, while the southern part of the Tibet Plateau showed a differential surface uplift trend, which established the present geomorphologic pattern. But it cannot be completely ruled out that this trend was probably caused by the accuracy of the calculation results.
(2)The quantitative constraints on the uplift process of the southeastern Tibet Plateau during Cenozoic provide certain constraints for the dynamic mechanism of geomorphic evolution in the southeastern Tibet Plateau. The northern and central parts of the southeastern Tibet Plateau can be well explained by the plate extrusion model. In this model, the collision and convergence between India and Eurasia plate or Qiangtang block and Songpan-Ganzi block resulted in the shortening and thickening of the upper crust in the region, and making the early stage(early Eocene)surface uplift. Subsequently, due to delamination or the continuous convergence between the Qiangtang block and the Songpan-Ganzi block resulting in the shortening and thickening of the crust, the plateau continued to grow northward and rose to its present altitude around Miocene. In the Eocene, the area from the south of the southeastern Tibetan plateau to the Yunnan-Guizhou Plateau mainly showed a low altitude. It seems that it may be in the peripheral area not affected by the shortening and thickening of the upper crust during the early stage India-Eurasia plate collision or plate extrusion and escape. In addition, as proposed by the lower crustal channel flow model, the lower crust material made the low-relief upland surface extending thousands of kilometers in the region uplift gradually towards the southeast, which seems to explain the low elevation landform of the region in the early stage, but it could not explain the whole uplift process of the southeastern Tibet Plateau. Therefore, a single dynamic model may not be able to perfectly explain the Cenozoic complex uplift process of the southeastern Tibet Plateau, and its process may be controlled by various dynamic processes.
(3)According to the paleoaltitude reconstruction results, if most areas of the ancient southeastern Tibet Plateau, especially the area to the north of Jianchuan Basin, had been uplifted in a certain scale and became part of the early plateau in the early Cenozoic, and reached to the current surface altitude around Miocene, the widely rapid surface erosion in this area since Miocene probably would be a continuous lag response to the finished surface uplift process, and the lag time may correspond to the sequential response process of surface uplift, the decline of river erosion base level and the gradual enhancement of river erosion capacity. Therefore, it is not proper to regard the rapid denudation and rapid river undercutting as the starting time of plateau uplift, as proposed in the previous thermochronological study.

COSEISMIC DEFORMATION FIELD AND FAULT SLIP MODEL OF THE M_W6.0 PAKISTAN EARTHQUAKE CONSTRAINED BY SENTINEL-1A SAR DATA

JIA Rui, ZHANG Guo-hong, XIE Chao-di, SHAN Xin-jian, ZHANG Ying-feng, LI Cheng-long, HUANG Zi-cheng

2021, 43(3):  600-613.  DOI: 10.3969/j.issn.0253-4967.2021.03.008

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In the global scale, ten destructive earthquakes with magnitude larger than 7 happen on average each year. Yet the number of small earthquakes with limited or even no damage but recordable by seismographs(magnitude between 2.5 and 4.5)is over one million per year. In between, there are hundreds to thousands of earthquakes with moderate to strong magnitude(magnitude between 5.5 and 6.5)with notable destructiveness. The massive moderate to strong earthquakes are often less noticed or even overlooked, with only very few exceptions which caused human casualties and/or structure damages due to the very shallow focal depths. For medium earthquakes, the traditional seismology means can obtain the source mechanism solution of earthquake, but because of the inherent fuzziness of the source mechanism, it cannot distinguish the fault plane from the auxiliary nodal planes, because earthquakes of this magnitude usually do not produce surface rupture, and the result error is large, so it is not suitable for the study of medium and small earthquakes. It is of fundamental significance to further study the source fault of the moderate earthquakes, and more independent methods other than traditional seismology, such as satellite geodesy are needed. As one of the most applied satellite geodesy technique, interferometry of SAR(InSAR)satellite images are commonly used to obtain coseismic deformation related to earthquakes. InSAR has very high spatial sampling, though the temporal sampling is very low, which is several days to over a month depending on the satellite revisit span. The precision of coseismic deformation by InSAR can reach 2~3cm, which is good enough to obtain the surface deformation caused by a moderate earthquake. It is noted that InSAR coseismic measurements can detect 1-dimensional(1D)deformation along Line-of-Sight(LOS)direction. With multiple observing modes including descending and ascending, the InSAR deformation data is very useful for identifying surface ruptures, and for source fault plane discrimination. As a new geodetic observation technology, InSAR uses the elastic dislocation model to obtain source parameters, and the inversion results of fault parameters and slip distribution are more reliable. On September 24th, 2019, an M_W6.0 earthquake hit New Mirpur, Pakistan. The nearest known fault to the epicenter is the Main Frontal Thrust on its south side. We used the Sentinel-1A SAR imagery(TOPS-model)to reconstruct the InSAR coseismic deformation fields generated by the 2019 M_W6.0 Pakistan earthquake along the ascending and descending tracks. The ascending and descending deformation fields indicate that coseismic deformation is asymmetric by a trend of NW-SE in the south secondary fault of the Himalayan frontal thrust fault, with a maximum LOS displacement of~0.1m. The structures of ascending and descending deformation are highly consistent with each other, but the LOS displacement of southern side is obviously larger than the northern side. The continuous change of interference fringes between uplift and subsidence areas shows that there is no coherent phenomenon caused by excessively large deformation gradient or surface rupture, which indicates that the seismic fault rupture did not reach to the ground surface. Two initial fault models constrained by InSAR deformation, with a southwest-dipping and northeast-dipping fault, were utilized in the inversion. We finally determined the northeast-dipping fault as the seismogenic fault by joint inversion of ascending and descending observations, combined with tectonic setting. Our fault model suggests that an obvious slip concentrated area is located in the depth of 2~4km, with a peak slip of~0.64m and a mean rake angle of~125°. The north-dipping thrust motion with a small amount of strike-slip component dominated the faulting. The earthquake occurred in the low-dipping subduction zone between the Indian and Eurasian plates. The dip angle of the fault plane is relatively low. When the fault is ruptured, the upper wall thrust southwards and the north wall subducted northwards. Due to the compressional nappe structure, the front end of the upper wall was uplifted and the back end was stretched to become the subsidence area. Seismogenic fault is the south secondary fault of the Himalayan frontal thrust fault inferred from our coseismic fault model and rupture kinematic features. Active faults on the land have caused many large destructive earthquakes, resulting in surface faults and promoting the development of tectonic landforms. The detailed observation of coseismic surface rupture not only provides basic information for understanding the earthquake itself and estimating the earthquake recurrence period, but also helps to interpret the tectonic and geomorphic features in other areas. Since the M_W6.0 earthquake in Pakistan in 2019, no studies have been reported yet on this earthquake using InSAR technology, so the study of this earthquake provides a rare opportunity to assess the seismic risk of active thrust faults and to study the seismicity of northern Pakistan.

NEW DISCOVERY OF XIARIHA FAULT ZONE AROUND DULAN AREA, QINGHAI PROVINCE AND ITS TECTONIC IMPLICATIONS

HA Guang-hao, REN Zhi-kun, LIU Jin-rui, LI Zhi-min, LI Zheng-fang, MIN Wei, ZHOU Ben-gang

2021, 43(3):  614-629.  DOI: 10.3969/j.issn.0253-4967.2021.03.009

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The deformation pattern in the northeastern margin of Tibetan plateau is characterized by NE compression, clockwise rotation and eastward extrusion, forming the NNE trending dextral strike-slip faults which further divide the region into several sub-blocks. The deformation of Qaidam secondary block is dominant by northwestward extrusion and rotation, which is controlled by the Elashan and East Kunlun faults. However, the deformation style of Dulan area, the junction of these two faults, remains unclear. We discovered a new active fault zone with a length of 60~70km west to Elashan Fault during our recent field geological survey around Dulan area, named Xiariha fault zone(XFZ), which is a dextral strike-slip fault zone trending NW, consisting of the Xiariha and Yingdeerkang faults. According to the remote sensing interpretation and field investigation, it is found that the Xiariha fault zone showed distinct linear characteristics, reverse scarp, sag pond and ridge dislocation on the satellite images and displaced multi-levels of alluvial fans and river terraces. According to previous studies, the exposed age of T1 terraces is Holocene in the Elashan area, which is located at east of Dulan. During the field investigation, we used the unmanned aerial vehicle(UAV)to get the fine geomorphology features along the XFZ. Also, to define the active era, we tried to find the fault section of the XFZ that could provide the information of the contact between the fault and late Quaternary strata. Based on the high-resolution DEM obtained by UAV, the offset of T1 is about 2.5m, indicating its activity in Holocene compared with the Elashan area. Along the XFZ, the fault displaced late Quaternary strata revealed on the section. The geomorphic features and fault section show that the XFZ is a late Pleistocene to Holocene active fault. The Dulan area is located at the convergence of East Kunlun Fault and Elashan Fault, the southeastern end of Qaidam secondary block, which is affected by the regional NE and SW principal compressive stress and shear stress. Under this circumstance, the Qaidam block is experiencing extrusion and rotation and there are a series of NW-trending dextral strike-slip faults parallel to the Elashan Fault and EW-trending sinistral strike-slip faults parallel to the East Kunlun Fault, such as Reshui-Taosituo River Fault, developed in the Dulan area. Therefore, we suggest that the Xiariha Fault and the nearly EW trending, Holocene sinistral Reshui-Taosituo River Fault adjust the extrusion rotation deformation jointly at the southeast end of the Qaidam block under the control of the Elashan Fault and the East Kunlun Fault, respectively. Meanwhile, the new discovery of Xiariha Fault and its activity in Holocene is not only of great significance to understand the regional tectonic deformation model, but also leads to a great change in the understanding of regional seismic risk because of its capabliliby of generating strong earthquakes. Therefore, it is urgent to carry out further research work in this area, improve the understanding of regional strain distribution mode, and provide reference for regional seismic safety issues.

A STUDY ON THE SEISMOGENIC STRUCTURE OF GAOYOU-BAOYING M_S4.9 EARTHQUAKE

ZHAO Qi-guang, SUN Ye-jun, HUANG Yun, YANG Wei-lin, GU Qin-ping, MENG Ke, YANG Hao

2021, 43(3):  630-646.  DOI: 10.3969/j.issn.0253-4967.2021.03.010

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The Gaoyou-Baoying M_S4.9 earthquake on July 20, 2012 occurred in the Gaoyou Sag in the Subei Basin. This earthquake was a relatively rare medium-strength earthquake in the weak seismicity region of eastern China. Although studies on the seismogenic structure of this earthquake have been conducted previously, the seismogenic structure itself is still under debate and needs to be further studied. This paper uses the methods such as distribution of seismic intensity, precise positioning of earthquake sequence, focal mechanism, regional tectonic stress, seismic exploration, etc. to comprehensively study the seismogenic structure of this earthquake.
The characteristics of earthquake sequence show that the seismic structure is a high dip-angle fault spreading along the NNE direction, dipping ESE. The result of focal mechanism solutions shows that the strike of one of the two nodal planes is NNE, and the fault plane shows high dip angle. The earthquake is mainly characterized by strike-slip motion. Through the seismic exploration lines(GYL1, GYL2)laid at the epicenter area of the earthquake, a fault structure is identified, which strikes nearly NNE and dips near ESE. This fault is located between the Linze sag and the Liubao low uplift, coinciding with the distribution of the Liuling Fault, the boundary fault in the northwest of the Gaoyou Sag, so it can be judged that all the detected breakpoints belong to the Liuling Fault. The “Y-shaped” breakpoints detected by the two seismic exploration lines are characterized by high dip angle. There is a very obvious wave group disorder area at the distance of 6 500~9 000m on the GYL1 seismic exploration line. This area is about 2.5km in width displayed on the post-stack migration profile and shows an uplifting trend. The disordered uplifting of wave group is caused by intrusion of soft material into the structural breakage and weakness, squeezed by horizontal stress. The GYL2 post-stack migration profile shows obvious uplift appearing in the reflection wave group(T_g)on the top of the bedrock. This arc-shaped uplift also reflects the effect of strong compression of horizontal stress.
In order to further discuss the seismogenic structure of the Gaoyou-Baoying M_S4.9 earthquake, we used the focal mechanism data to invert the modern tectonic stress field in the Northern Jiangsu-South Yellow Sea Basin where the earthquake occurred. The maximum principal stress in this area is NE-SW, while the minimum principal stress is NW-SE; both of them are nearly horizontal, and the intermediate principal stress is nearly vertical. According to Zoback's rule for dividing the types of dislocation in the direction of the force axis, the distribution of principal stresses in the Northern Jiangsu-South Yellow Sea Basin is equivalent to a strike-slip dislocation.
To sum up, the stress characteristics reflected by the Liuling Fault are consistent with the horizontal forces on the P-axis and T-axis shown by the focal mechanism solution results, and also consistent with the horizontal state of the stress in the tectonic stress field in this region. The above characteristics indicate that the development of the Liuling Fault is affected and controlled by modern tectonic activities. At the same time, the characteristics of the strike and dip of the seismic fault reflected by the methods of seismic intensity investigation, precise earthquake positioning, focal mechanism solution and seismic exploration, etc. are consistent with each other. Therefore, the occurrence of this earthquake may be the result of continuous stress accumulation and sudden instability and rupture of the NNE-trending Liuling Fault under the long-term compression of the NE-direction principal stress.

ANALYSIS ON FORMATION MECHANISM OF TYPICAL VARIATIONS OF GEO-RESISTIVITY AT BAOCHANG STATION

DAI Yong, GAO Li-xin, YANG Yan-ming, WEI Jian-min, Ge-gen

2021, 43(3):  647-662.  DOI: 10.3969/j.issn.0253-4967.2021.03.011

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Fixed-electrode quasi-Schlumberger arrays are mainly used in geo-electric observation of China earthquake networks. The distance between power supply poles is generally about 1km. The detection depth is estimated to be within 0.705km by conventional geophysical and electrical methods in homogeneous medium. The resistivity at seismic station for precursor information monitoring reflects the overall electrical characteristics within the detection range below the polar distribution area, which is also known as apparent resistivity or geo-resistivity. Due to the small distance between power supply poles, small detection depth and great influence from shallow layer, there are usually annual, diurnal and step variations in geo-resistivity curves. Because of the above variations, the characteristics of abnormal variations before earthquakes are usually not obvious, or even annihilated. In this paper, taking Baochang station as an example, the causes of long-term, annual, diurnal and step variations are analyzed by inversion and numerical simulation. Baochang station is located in Baochang Town, Taipusi Banner, Xilin Gol League, Inner Mongolia. Its geographical coordinates are 41.9°N and 115.3°E. The regional geological structure is the eastern segment of Inner Mongolia axis, the fourth-order structural unit. The nearest fault structure is the Chifeng-Kaiyuan Fault, which is the northern boundary fault of North China fault-block region. The resistivity of geo-electric survey area at Baochang station basically presents horizontal distribution characteristics, and the type of electric sounding curve is KH. The inversion results show that the vertical profile of the survey area is divided into four layers: the first layer is frozen soil layer with depth from 0m to 1m, the second layer is sand gravel layer with depth from 1m to 6.5m, the third layer is aquifer with depth from 6.5m to 71.5m, and the fourth layer is quartz porphyry layer with depth greater than 71.5m. When power supply electrode distance AB is 560m and measuring electrode distance MN is 80m, the one dimensional influence coefficients of NS and EW direction in the third layer are all over 0.9, which is one order of magnitude larger than those in the other three layers. This indicates that the variation of resistivity in the range of 7m to 71m can effectively reflect the variation of geo-resistivity. Since 1993, the geo-resistivity at Baochang station has been declining for a long time in NS and EW direction, and the variation rate shows obvious anisotropic characteristics, which is mainly the result of the continuous effect of regional stress on the resistivity of the third layer. There is a normal annual variation pattern of “high in winter and spring, low in summer and autumn” in both directions of geo-resistivity at Baochang station, resulting mainly from the seasonal variation of temperature and rainfall on the resistivity of the first layer. The normal diurnal variation of geo-resistivity at Baochang station is characterized by “high in the morning, low in the afternoon and night”, which is mainly caused by the influence of temperature on surface resistivity. Similar diurnal variation also exists in the hourly value curves of geo-resistivity at the stations of Xiaomiao, Ganzi, Wujiahe, Qingguang and Baodixintai. The geo-resistivity step variation of Baochang station has the characteristics of “low frequency in winter and spring, high frequency in summer and autumn”, and most of them coincide with rainfall, pumping, embedding steel strand, etc. The results of experiments and numerical simulation show that the above factors are the main interference sources of the geo-resistivity step variations.

Application of new technique

RESEARCH ON IDENTIFICATION OF SEISMIC EVENTS BASED ON DEEP LEARNING: TAKING THE RECORDS OF SHANDONG SEISMIC NETWORK AS AN EXAMPLE

ZHOU Shao-hui, JIANG Hai-kun, LI Jian, QU Jun-hao, ZHENG Chen-chen, LI Ya-jun, ZHANG Zhi-hui, GUO Zong-bin

2021, 43(3):  663-676.  DOI: 10.3969/j.issn.0253-4967.2021.03.012

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In order to realize the rapid and efficient identification of earthquakes, blasting and collapse events, this paper applies the Convolutional Neural Network(CNN)in deep learning technology to design a deep learning training module based on single station waveform recording of single event and a real-time test module based on multiple stations waveform recording of single event.
On the basis of ensuring that the data is comprehensive, objective and original, the three-component waveforms of the first five stations that recorded the P-wave arrival time of each event are input, and the current mainstream convolutional neural network structures are used for learning test. The four main convolutional neural network structures of AlexNet, VGG16, VGG19 and GoogLeNet are used for learning training, and the learning effects of different network structures are compared and analyzed. The results show that in the training process of various convolutional neural network structures, the accuracy rate and the cost function curve of the training set and the test set of each network are basically the same. The accuracy rate increases gradually with the increase of the training times and exceeds 90%, and finally stabilizes around a certain value. The cost function curve decreases rapidly with the increase of the training times, and eventually the stability does not change near a relatively small value. At the same time, over-fitting occurred in all convolutional neural network structures during training, except for AlexNet. In the end, the cost function of each type of structural training set and test set is finally lower than 0.194, and the recognition accuracy of each type of structure for training sets and test sets is over 93%. Among them, the recognition accuracy of AlexNet network structure is the highest, the accuracy of the training set of AlexNet network structure is as high as 100%, the test set is 98.51%, and no overfitting occurred; the accuracy of VGG16 and VGG19 network structure comes second, and the recognition accuracy of GoogLeNet network structure is relatively low, and the trend curves of the accuracy and cost function in training and test set of each network in the training process are basically the same. Subsequently, in order to test the event discrimination efficiency of the CNN in deep learning in the real-time operation of the digital seismic network, we select the trained AlexNet convolutional neural network to perform event type determination test based on the waveform recording of multiple stations of a single event. The final result shows that the types of a total of 89 events are accurately identified in the 110 events with M ≥0.7 recorded by Shandong seismic network, and the accuracy rate is about 80.9%. Among them, the accuracy rate of natural earthquake is about 74.6%, that of explosion is about 90.9%, and that of collapse is 100%. The recognition accuracy of collapse and explosion events is relatively high, and it basically reaches or exceeds the recognition accuracy of manual determination in the daily work of the seismic network. The accuracy of natural earthquake identification is relatively low. Among the 18 misidentified natural earthquakes, up to 13 events were judged as blasting or difficult to identify due to distortion of waveforms recorded by some stations(They are determined to be explosion and earthquake each by the records of two of the five stations). If sloughing off the recognition type error events caused by waveform distortion due to the background noise interference that overwhelms the real event waveform or waveform drift, the recognition accuracy of earthquake will become 91.4%, and the recognition accuracy of all events will increase from 80.9%to 91.7%, which is basically equivalent to the recognition accuracy of manual judgment in the daily work of the seismic network. This indicates that deep learning can quickly and efficiently realize the type identification of earthquake, blasting and collapse events.

Special topic on the Yunnan Yangbi M_S6.4 and Qinghai Maduo M_S7.4 earthquakes

COSEISMIC DEFORMATION FIELD, SLIP DISTRIBUTION AND COULOMB STRESS DISTURBANCE OF THE 2021 M_W7.3 MADUO EARTHQUAKE USING SENTINEL-1 INSAR OBSERVATIONS

HUA Jun, ZHAO De-zheng, SHAN Xin-jian, QU Chun-yan, ZHANG Ying-feng, GONG Wen-yu, WANG Zhen-jie, LI Cheng-long, LI Yan-chuan, ZHAO Lei, CHEN Han, FAN Xiao-ran, WANG Shao-jun

2021, 43(3):  677-691.  DOI: 10.3969/j.issn.0253-4967.2021.03.013

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InSAR coseismic deformation fields caused by the Maduo M_W7.3 earthquake occurring on May 22, 2021 were generated using the C-band Sentinel-1A/B SAR images with D-InSAR technology. The spatial characteristics, magnitude of coseismic deformation and segmentation of the seismogenic fault were analyzed. The surface rupture trace was depicted clearly by InSAR observations. In addition, the coseismic slip distribution inversion was carried out constrained by both ascending and descending InSAR deformation fields and relocated aftershocks to understand the characteristics of deep fault slip and geometry of the seismogenic fault. The regional stress disturbance was analyzed based on coseismic Coulomb stress change. The results show that the Maduo M_W7.3 earthquake occurred on a secondary fault within the Bayan Har block which is almost parallel to the main fault trace of the Kunlun Fault. According to field investigation, geological data and InSAR surface rupture traces, the seismogenic fault is confirmed to be the Kunlunshankou-Jiangcuo Fault. The rupture length of seismogenic fault is estimated to be~210km. The NWW direction is followed by the overall displacement field, which indicates a left-lateral strike-slip movement of seismogenic fault. The maximum displacement is about 0.9m in LOS direction observed by both ascending and descending InSAR data. The inversion result denotes that the strike of the seismogenic fault is 276°and the dip angle is 80°. The maximum slip is about 6m and the average rake is 4°. The predicted moment magnitude is M_W7.45, which is overall consistent with the result of GCMT. An obvious slip-concentrated area is located at the depth of 0~10km. The coseismic Coulomb stress change with the East Kunlun Fault as the receiver fault shows that the Maduo earthquake produced obvious stress increase near the eastern segment of the East Kunlun Fault. Thus the seismic risk increases based on the high interseismic strain rate along this segment, which should receive more attention. In addition, the coseismic Coulomb stress change with the Maduo-Gande Fault as the receiving fault indicates that the Maduo earthquake produced an obvious stress drop near the western part of the Maduo-Gande Fault, which indicates that the Maduo earthquake released the Coulomb stress of the Maduo-Gande Fault, and its seismic risk may be greatly reduced. However, there is a stress loading effect in the intersection area of the Maduo-Gande Fault and the Kunlunshankou-Jiangcuo Fault. Considering that aftershocks of Maduo earthquake will release excess energy, the greater earthquake risk may be reduced.

COSEISMIC SURFACE DEFORMATION AND SLIP MODELS OF THE 2021 M_S6.4 YANGBI(YUNNAN, CHINA)EARTHQUAKE

WANG Shao-jun, LIU Yun-hua, SHAN Xin-jian, QU Chun-yan, ZHANG Guo-hong, XIE Zhao-di, ZHAO De-zheng, FAN Xiao-ran, HUA Jun, LIANG Shi-ming, ZHANG Ke-liang, DAI Cheng-long

2021, 43(3):  692-705.  DOI: 10.3969/j.issn.0253-4967.2021.03.014

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Due to the ongoing collision between Indian and Eurasian plates, the internal blocks of the Tibet plateau are experiencing eastward extrusion. Resulting from the blocking of the Sichuan Basin along the eastern boundary of the Bayanhar block, the plateau begins to rotate clockwise around the eastern syntaxis, and continues to move toward the IndoChina Peninsula. Such process forms the Hengduan Mountains with thousands of gullies in the Sichuan-Yunnan region, and generates major earthquakes across the entire Red River Fault, where infrastructures and residents are seriously threatened by the frequent earthquakes. InSAR observations feature a high spatial resolution and short intervals, ranging from several days to over a month, depending on the satellite revisit period.
On May 21, 2021, an earthquake struck the Yangbi city. This event provides a rare opportunity to look at the local tectonic and seismic risk in the north of the Red River Fault. We processed the Sentinel-1 SAR data with D-InSAR technology and generated the surface deformation caused by the Yangbi M_S6.4 earthquake occurring on May 21, 2021. Due to the abundant vegetation and moisture in Yunnan, significant atmospheric noise needs to be corrected for the derived InSAR displacement field. The results show a maximum deformation of~0.07m in line-of-sight for ascending track and~0.08m for descending track. The quality of interferogram on the ascending track is low, and only one of the quadrans can be distinguished, the rest of the interferogram is regarded as phase noise. However, the descending interferogram contains two deformation regions, with its long axis roughly along the NW-SE direction. The northeast part of interferogram moves towards the satellite, while the southwest part moves away from the satellite. The InSAR interferograms pattern shows a right-lateral strike-slip movement. Then, we combined coseismic displacement data obtained from the Global Navigation Satellite System(GNSS)and InSAR(both the ascending and descending)to invert the coseismic slip model of the Yangbi earthquake. The inversion test shows that our data cannot give strong constraints for the dip orientations, and the two slip models with opposite dip orientation can explain the observations within the noise level. No matter what the dip orientation is, the slip models show that the coseismic slip concentrated at depth of 2~10km, with a maximum slip of~0.8m, which corresponds to a moment magnitude of M_S6.4, and is consistent with body-wave-based focal mechanism. But the relocated aftershocks in 3 hours immediately after the mainshock reveal a SW-dipping fault plane 10km away to the west of Weixi-Qiaohou-Weishan Fault, we therefore conclude that the Yangbi earthquake ruptured a SW-dipping dextral fault, which is previously unknown. To analyze the effects of the Yangbi earthquake on the seismic risk of the regional dextral faults, we estimated the Coulomb stress change caused by our preferred slip model. The Coulomb stress at 7.5km depth is negative, indicating stress unloading, while the Coulomb stress at 15km depth is positive, indicating slightly loading, but still less than the empirical triggering threshold. The results indicate that Yangbi earthquake partially relieved the strain accumulated on the nearby faults, thus restraining the seismic risk of these faults.

THE SEISMOGENIC FAULT OF THE 2021 YUNNAN YANGBI M_S6.4 EARTHQUAKE

LI Chuan-you, ZHANG Jin-yu, WANG Wei, SUN Kai, SHAN Xin-jian

2021, 43(3):  706-721.  DOI: 10.3969/j.issn.0253-4967.2021.03.015

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The May 21, 2021 Yangbi M_S6.4 earthquake occurred at the western boundary of the Chuandian tectonic block in southeast Tibetan plateau. The structural background is complex, with multiple active faults distributed around the epicenter area. Focal mechanism and seismic waveform inversion reveal that this earthquake is right-lateral strike-slip type with a NW-trending rupture plane. This accords with the strike and motion directions of the Weixi-Qiaohou and Red River faults along the western boundary of the Chuandian block.
We made a careful field investigation along the Weixi-Qiaohou Fault and around the epicenter area, and did not find any obvious earthquake surface rupture. But we observed a NW-trending ground fissure zone near the epicenter area to the west of the Yangbi County. This zone is divided into two sections, the Yangkechang-Paoshuitian section in the northwest and the Xiquewo-Shahe section in the southwest. These sections have a length of 2.5~3km and 3~3.5km, respectively, and are separated by a ~6km gap. They are characterized by NW-trending ground fissures with a width of several meters to tens meters. The formation of these fissures is inferred to be related to the tectonic movement under the ground, and the fissures have the following features: 1)they are not affected by the topography and cut the slope and range upward; 2)they are continuous and concentrated in a zone with a strike of NW 310°~320°, which is consistent with the belt of aftershocks and differs from the gravity fissures that usually have no regular strikes; 3)they usually have a plane dipping towards upslope(southwest), opposite to the valley; 4)they present shear property, not tensional. This zone thus is interpreted to be the surficial expression of the seismogenic fault of the Yangbi M_S6.4 earthquake.
Moreover, satellite image and field observation suggest that a~30km long linear structure with a NW strike traverses the epicenter area, which may suggest an undiscovered fault. Relocation of small earthquakes shows that the aftershocks are concentrated in a NW-trending belt that is consistent with the linear structure. Furthermore, the fissure zone lies in the northeast side of the aftershock belt, which suggests that the earthquake fault dips SW. Such a dip direction coincides with that of the observed fissure plane, and also agrees with the results from the focal mechanism and InSAR inversion. Both the focal mechanism and the waveform inversion result suggest that the Yangbi earthquake is a right-lateral strike-slip type, which is consistent with the type of the observed ground fissures. No displacement is observed on the fissures, with is also consistent with the InSAR inversion results that suggest the rupture did not break the surface. In addition, there is no coseismic deformation observed along the Weixi-Qiaohou Fault, which may indicate this fault did not move during this earthquake.
Based on our field investigation, in combination with the focal mechanism, aftershock distribution, and InSAR and GNSS inversion results, the seismogenic fault for this Yangbi M_S6.4 earthquake is believed to be a NW-trending(310°~320°)fault with a length of~30km, named as the Yangkechang-Shahe Fault. According to the location, size, and motion of the fault, it is suggested that the Yangkechang-Shahe Fault is a secondary fault of the Weixi-Qiaohou fault system. This fault has a slightly SW-dipping plane, and is dominated by right-lateral strike-slip motion, which may be a younger fault developed during the westward expansion of the western boundary of the Chuandian block.

SEISMOGENIC FAULT AND COSEISMIC SURFACE DEFORMATION OF THE MADUO M_S7.4 EARTHQUAKE IN QINGHAI, CHINA: A QUICK REPORT

LI Zhi-min, LI Wen-qiao, LI Tao, XU Yue-ren, SU Peng, GUO Peng, SUN Hao-yue, HA Guang-hao, CHEN Gui-hua, YUAN Zhao-de, LI Zhong-wu, LI Xin, YANG Li-chen, MA Zhen, YAO Sheng-hai, XIONG Ren-wei, ZHANG Yan-bo, GAI Hai-long, YIN Xiang, XU Wei-yang, DONG Jin-yuan

2021, 43(3):  722-737.  DOI: 10.3969/j.issn.0253-4967.2021.03.016

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At 02:04 a.m. on May 22, 2021, a M_S7.4 earthquake occurred in the Maduo County, Qinghai Province, China. Its epicenter is located within the Bayan Har block in the north-central Tibetan plateau, approximately 70km south of the eastern Kunlun fault system that defines the northern boundary of the block. In order to constrain the seismogenic fault and characterize the co-seismic surface ruptures of this earthquake, field investigations were conducted immediately after the earthquake, combined with analyses of the focal parameters, aftershock distribution, and InSAR inversion of this earthquake.
This preliminary study finds that the seismogenic fault of the Maduo M_S7.4 earthquake is the Jiangcuo segment of the Kunlunshankou-Jiangcuo Fault, which is an active NW-striking and left-lateral strike-slip fault. The total length of the co-seismic surface ruptures is approximately 160km. Multiple rupture patterns exist, mainly including linear shear fractures, obliquely distributed tensional and tensional-shear fractures, pressure ridges, and pull-apart basins. The earthquake also induced a large number of liquefaction structures and landslides in valleys and marshlands.
Based on strike variation and along-strike discontinuity due to the development of step-overs, the coseismic surface rupture zone can be subdivided into four segments, namely the Elinghu South, Huanghexiang, Dongcaoarlong, and Changmahexiang segments. The surface ruptures are quite continuous and prominent along the Elinghu south segment, western portion of the Huanghexiang segment, central portion of the Dongcaoarlong segment, and the Huanghexiang segment. Comparatively, coseismic surface ruptures of other portions are discontinuous. The coseismic strike-slip displacement is roughly determined to be 1~2m based on the displaced gullies, trails, and the width of cracks at releasing step-overs.