SURFACE TRACKS AND SLIP RATE OF THE FAULT ALONG THE SOUTHERN MARGIN OF THE WUWEI BASIN IN THE LATE QUATERNARY

The fault along the southern margin of the Wuwei Basin, located in the eastern Hexi Corridor, NW China, plays an important role in the thrust fault system in the northern Qilian Mountains. The activities of this fault resulted in the generation of the Gulang earthquake(M_S8.0) in 1927. Based on remote sensing image interpretation, geological and geomorphic observations in the field and ¹⁴C geochronological dating results, we conducted a detailed research on the geometry and kinematics of the fault. According to the discontinuous geometric distribution and variable strike directions, we divide this fault into 5 segments: Kangningqiao Fault(F₁), Nanyinghe Fault(F₂), Shangguchengcun-Zhangliugou Fault(F₃), Tajiazhuang Fault(F₄)and Yanjiazhuang Fault(F₅). Results indicate that this fault, with a total of 60km long trace at the surface, has been active since the late Pleistocene. It behaves predominantly as a thrust fault and is accompanied with a locally sinistral strike-slip component along the Nanyinghe Fault(F₂). Intensive activities of this fault in Holocene have caused extensive occurrence of dislocated landforms along its strike. Some measured displacements of the dislocated geologic or geomorphic units, combined with the ¹⁴C dating results, yield a vertical slip rate of (0.44±0.08)mm/a on this fault in Holocene, and a sinistral strike-slip rate of (1.43±0.08)mm/a on the Nanyinhhe Fault (F₂) in late Pleistocene.     

滇西北通甸-巍山断裂中段的晚第四纪滑动速率

通甸-巍山断裂属于红河断裂带的分支断裂,目前对该断裂中段的晚第四纪活动特征研究较少。野外地质地貌调查和年代学研究结果表明,通甸-巍山断裂中段是以右旋走滑运动为主,兼有张性正断的全新世活动断裂,其最新活动时代距今约2.2ka。晚更新世中晚期以来断裂中段平均水平滑动速率为1.25mm/a,全新世晚期以来垂直运动趋于增强。该研究不仅为该断裂的地震危险性评价工作提供了基础资料,而且有助于理解川滇菱形块体西南边界构造变形的空间分配特点     

中甸-大具断裂南东段晚第四纪活动的地质地貌证据

中甸-大具断裂南东段位于哈巴和玉龙雪山北麓,属于川西北次级块体西南边界,断裂总体走向310°~320°,是一条重要的边界断裂。了解该断裂的活动性质、活动时代和滑动速率等对分析川西北次级块体运动,研究该断裂与玉龙雪山东麓断裂的交切关系等问题具有重要意义。文中基于1︰5万活动断层地质填图,对断裂沿线地层地貌、陡坎地貌、地表破裂、典型断层剖面以及河流阶地等进行了详细的研究。研究表明:1)中甸-大具断裂南东段按几何结构、断错地貌表现、断裂活动性可分为马家村—大具次级段和大具—大东次级段。2)通过野外地质调查发现,马家村—大具次级段断错了全新世冲洪积扇,形成了地表破裂,为全新世活动段;而大具—大东次级段虽然也断错了晚更新—全新世地层,但其断错规模及滑动速率均较小,由此认为其全新世以来活动较弱。3)通过分析断裂沿线断层陡坎、水平位错及地表破裂等地质地貌问题,认为马家村—大具次级段的活动性质为右旋走滑兼正断,其晚更新世以来的垂直滑动速率为0.4~0.8mm/a,水平滑动速率为1.5~2.4mm/a;大具—大东次级段以右旋走滑为主、正断为辅,其晚更新世晚期以来的垂直滑动速率为0.1mm/a。4)在大具盆地内发现的NW向地表破裂带的形成时代很年轻,不排除是1966年中甸6.4级地震或1996年丽江7.0级地震造成的地表破裂。     

NEW EVIDENCES OF HOLOCENE ACTIVITY IN THE JIANGSU SEGMENT OF ANQIU-JUXIAN FAULT OF THE TANLU FAULT ZONE

Anqiu-Juxian Fault is an important fault in the Tanlu fault zone, with the largest seismic risk, the most recent activity date and the most obvious surface traces. It is also the seismogenic fault of the Tancheng M8 1/2 earthquake in 1668. There are many different views about the southern termination location of surface rupture of the Tancheng earthquake and the Holocene activity in Jiangsu segment of this fault. Research on the latest activity time of the Jiangsu segment of Anqiu-Juxian Fault, particularly the termination location of surface rupture of the Tancheng earthquake, is of great significance to the assessment of its earthquake potential and seismic risk. Based on trench excavation on the Jiangsu segment of Anqiu-Juxian Fault, we discuss the time and characteristics of its latest activity. Multiple geological sections from southern Maling Mountain to Chonggang Mountain indicate that there was an ancient seismic event occurring in Holocene on the Jiangsu segment of Anqiu-Juxian Fault. We suggest the time of the latest seismic event is about(4.853±0.012)~(2.92±0.3)ka BP by dating results. The latest activity is characterized by thrust strike-slip faulting, with the maximum displacement of 1m. Combined with the fault rupture characteristics of each section, it is inferred that only one large-scale paleo-earthquake event occurred on the Jiangsu segment of Anqiu-Juxian Fault since the Holocene. The upper parts of the fault are covered by horizontal sand layers, not only on the trench in the west of Chonggang mountain but also on the trench in Hehuan Road in Suqian city, which indicates that the main part of the Jiangsu segment of Anqiu-Juxian Fault was probably not the surface rupture zone of the 1668 Tancheng M8 1/2 earthquake. In short, the Jiangsu segment of Anqiu-Juxian Fault has experienced many paleo-earthquake events since the late Pleistocene, with obvious activity during the Holocene. The seismic activities of the Jiangsu segment of Anqiu-Juxian Fault have the characteristics of large magnitude and low frequency. The Jiangsu segment of Anqiu-Juxian Fault has the deep tectonic and seismic-geological backgrounds of big earthquakes generation and should be highly valued by scientists.     

LATE QUATERNARY SINISTRAL STRIKE-SLIP ACTIVITIES OF SANWEI SHAN FAULT IN THE NORTH OF TIBETAN PLATEAU

Sanwei Shan Fault is located in the north of Tibet, which is a branch of eastern segment of Altyn Tagn fault zone. This fault is distributed along the boundary of fault facet and the Quaternary, with the total length of almost 150km. The fault is a straight-line structure read from the satellite image. Based on the spatial distribution of the fault, three segments are divided, namely, Xishuigou-Dongshuigou segment, Dongshuigou-West Shigongkouzi segment and West Shigongkouzi-Suangta segment, these three segments are distributed by left or right step.Though field microgeomorphology investigation along Sanwei Shan Fault, it has been found that two periods of alluvial-pluvial fans are distributed in front of Sanwei Shan Mountain, most of which are overstepped. Comparing the distribution of alluvial-pluvial fans with their formation age in the surrounding regions, and meanwhile, taking the results of optical stimulated luminescence(OSL) dating, it's considered that the formation age of the older alluvial-pluvial fans, which are distributed in northern Qilian Shan, inside of Hexi Corridor and western Hexi Corridor(including the Sanwei Shan piedmont fans), is between later period of late Quaternary and earlier period of Holocene. The gullies on the older fan and ridges have been cut synchronously. The maximum and minimum sinistral displacement is 5.5m and 1.7m, but majority of the values is between 3.0~4.5m. Taking the results from the OSL dating, we conclude that the minimum sinistral strike-slip rate is(0.33±0.04) mm/a since 14 ka BP and(0.28±0.03) mm/a since 20 ka BP.     

A PRELIMINARY RESEARCH ON THE RIGHT-LATERAL STRIKE-SLIP CHARACTERISTICS AND THE STRUCTURAL SIGNIFICANCE OF THE NORTHERN KUANTANSHAN FAULTS,HEXI CORRIDER,BASED ON HIGH-RESOLUTION IMAGERY

The sinistral strike-slip characteristic of the Altyn Tagh Fault gradually disappears near the Jiuxi Basin at the west end of Hexi Corridor, and the Kuantanshan Fault and the northern marginal fault of Heishan on its east are thrust structures. There are two faults distributed in the north of Kuantanshan, namely, the Taerwan-Chijiaciwo Fault and the Ganxiashan Fault, both are featured with obvious activity. Predecessors thought that the Taerwan-Chijiaciwo Fault is a thrust fault with low movement rate, but there is few detailed study on its horizontal motion. Is there horizontal strike-slip movement in the northern marginal fault of Kuantanshan? This issue has an important significance to further explore the structural transformation mode between the Altyn Tagh strike-slip faults and the northern thrust faults in the north margin of Qilianshan. Using high resolution remote sensing images and field work, such as combining with UAV SfM photogrammetry, the paper studies the strike-slip characteristics of the Taerwan-Chijiaciwo Fault and Ganxiashan Fault on the northern margin of Kuantanshan, and get two preliminary understandings:(1) The northern marginal fault of Kuantanshan is an active right-lateral strike-slip fault with thrust component, the horizontal to vertical dislocation ratio is about 3-4 times. Based on the statistics of dislocation amount of the gullies and terraces along the north marginal Kuantanshan fault, it is preliminarily estimated that the late Pleistocene right-lateral strike-slip rate is about 0.2-0.25 mm/a and the Holocene right-lateral strike-slip rate is about 0.5-1.5 mm/a. (2) The main driving force to the tectonics at the western end of Hexi Corridor, where the northern marginal fault of Kuantanshan locates, comes from the northward extrusion of the Qilian Mountains, which results in the right-lateral strike-slip of the northern marginal fault of Kuananshan and the thrust movement of several faults inside the Jiuxi Basin. The effect of the Altyn Tagh Fault on other tectonic structures is not obvious in this region.     

东昆仑断裂带东段玛曲断裂新活动特征及全新世滑动速率研究

东昆仑活动断裂是青藏高原东北部一条重要的NWW向边界断裂。 玛曲断裂位于东昆仑断裂带的最东段。 根据野外考察结果认为玛曲断裂全新世以来活动强烈, 主要表现为左旋走滑运动, 并伴有正倾滑运动性质。 断错地貌特征明显, 断裂过玛曲县城以后, 沿黑河南岸穿过若尔盖草地向东, 直至岷山北端求吉附近。 通过两处断错地貌的全站仪器实测和测年资料讨论了玛曲断裂新活动特征和全新世滑动速率, 玛曲断裂全新世早期以来的平均水平滑动速率为6.29～5.71 mm/a, 全新世晚期以来的平均水平滑动速率为4.19～4.03 mm/a。     

滇西南地区孟连断裂晚第四纪走滑速率的厘定

通过卫星影像解译和野外实地调查,获得滇西南地区孟连断裂的几何特征和活动性参数。孟连断裂总体走向NE-NEE向,不具有明显的分段性,连续性较好。断裂从单侧控制着沿线的勐滨、孟连和勐马三个新生代盆地的发育。断裂沿线地貌以线性较好的断层谷、断层崖和断层陡坎为主,并发育多级左旋位错的河流、冲沟和阶（台）地等,观测到的最小左旋位错约为7 m。采用高精度Li-DAR测量方法,对4处典型水平位错地貌进行精细测量,根据获得的相应地貌面年代,得到孟连断裂晚第四纪以来平均左旋走滑速率为2.2±0.4 mm/a。其结果与滇西南地区其他NE向左旋走滑断裂滑动速率相当,反映了区域构造活动的整体协调性。根据跨断层地质体最大左旋位错量9.5±1.8 km,估算断裂开始左旋走滑的时代为距今4.7±1.6 Ma左右,即中新世中晚期。     

龙陵-瑞丽断裂(南支)北段晚第四纪活动性特征

遥感影像解译和野外地质地貌调查表明,龙陵-瑞丽断裂(南支)北段是以左旋走滑为主兼张性正断的区域性活动断裂。根据一些断错地貌点的大比例尺填图、实地测量及其年代学分析,确定了该断裂为全新世活动断裂,断裂晚更新世以来的平均水平滑动速率为2.2mm/a,平均垂直滑动速率为0.6mm/a;全新世以来的平均水平滑动速率为1.8～3.0mm/a,平均垂直滑动速率为0.5mm/a。断裂晚更新世以来的滑动速率在不同的时间尺度上变化不大,反映了该断裂晚更新世以来的活动强度比较平稳     

HOLOCENE SLIP RATE ALONG THE NE-TRENDING QIXIANG CO FAULT IN THE CENTRAL TIBETAN PLATEAU AND ITS TECTONIC IMPLICATIONS

The two mainstream deformation models of the Tibet plateau are continental escape model and crustal thickening model, the former suggests that the NW-trending Karakoram Fault, Gyaring Co Fault, Beng Co Fault and the Jiali Fault as the Karakoram-Jiali fault zone is the southern border belt and that the dextral strike-slip rate is estimated as up to 10~20mm/yr. However, research results in recent years show that the slip rates along those faults are significantly less than earlier estimates. Taylor et al. (2003)suggest that the conjugate strike-slip faults control the active deformation in the central Tibet. The lack of research on the slip behavior of the NE-trending faults in the central Tibet Plateau constrains our understanding of the central Tibet deformation model. Thus, we choose the NE-direction Qixiang Co Fault located at the north of the Gyaring Co Fault as research object. Based on the interpretation of satellite images, we found several faulted geomorphic sites. Using RTK-GPS ground control point and unmanned aerial vehicle (UAV)topographic surveying, we obtained less than 10cm/pix-resolution digital elevation model (DEM)in the Yaqu town site. We used the LaDiCaoz_v2.1 software to automatically extract the left-lateral offset of the largest gully on the terrace T₂ surface, which is (21.3±7.1)m, and the vertical dislocation of the scarp on the terrace T₂ surface, which is (0.9±0.1)m. The age of both U-series dating samples on the terrace T₂ is (4.98±0.17)ka and (5.98±0.07)ka, respectively. The Holocene left-lateral slip rate along Qixiang Co Fault is (3.56±1.19)mm/a and the vertical slip rate is (0.15±0.02)mm/a. The kinematic characteristics of the sinistral strike-slip with normal slip coincide with the eastward motion of the central Tibet plateau, and its magnitude is in agreement with its conjugate Gyaring Co Fault, suggesting that the deformation pattern of the central Tibetan plateau complies with the conjugate strike-slip faults mode.     

SLIP RATES OF THE RIYUE MT. FAULT AT DEZHOU SEGMENT SINCE LATE PLEISTOCENE

The Riyue Mt. Fault is a secondary fault controlled by the major regional boundary faults (East Kunlun Fault and Qilian-Haiyuan Fault). It lies in the interior of Qaidam-Qilianshan block and between the major regional boundary faults. The Riyue Mt. fault zone locates in the special tectonic setting which can provide some evidences for recent activity of outward extension of NE Tibetan plateau, so it is of significance to determine the activity of Riyue Mt. Fault since late Pleistocene to Holocene. In this paper, we have obtained some findings along the Dezhou segment of Riyue Mt. Fault by interpreting the piedmont alluvial fans, measuring fault scarps, and excavating trenches across the fault scarp. The findings are as follows:(1) Since the late Pleistocene, there are an alluvial fan f_p and three river terraces T₁-T₃ formed on the Dezhou segment. The abandonment age of f_p is approximately (21.2±0.6) ka, and that of the river terrace T₂ is (12.4±0.11) ka. (2) Since the late Pleistocene, the dextral strike-slip rate of the Riyue Mt. Fault is (2.41±0.25) mm/a. In the Holocene, the dextral strike-slip rate of the fault is (2.18±0.40) mm/a, and its vertical displacement rate is (0.24±0.16) mm/a. This result indicates that the dextral strike-slip rate of the Riyue Mt. Fault has not changed since the late Pleistocene. It is believed that, as one of the dextral strikeslip faults, sandwiched between the the regional big left-lateral strike-slip faults, the Riyue Mt. Fault didn't cut the boundary zone of the large block. What's more, the dextral strike-slip faults play an important role in the coordination of deformation between the sub-blocks during the long term growth and expansion of the northeast Tibetan plateau.     

THE TECTONIC ACTIVITY CHARACTERISTICS OF AWANCANG FAULT IN THE LATE QUATERNARY,THE SUB-STRAND OF THE EASTERN KUNLUN FAULT

It is well known that the slip rate of Kunlun Fault descends at the east segment, but little known about the Awancang Fault and its role in strain partitioning with Kunlun Fault. Whether the sub-strand(Awancang Fault) can rupture simultaneously with Kunlun Fault remains unknown. Based on field investigations, aerial-photo morphological analysis, topographic surveys and ¹⁴C dating of alluvial surfaces, we used displaced terrace risers to estimate geological slip rates along the Awancang Fault, which lies on the western margin of the Ruoergai Basin and the eastern edge of the Tibetan plateau, the results indicate that the slip rate is 3mm/a in the middle Holocene, similar to the reduced value of the Kunlun Fault. The fault consists of two segments with strike N50° W, located at distance about 16km, and converged to single stand to the SE direction. Our results demonstrate that the Awancang fault zone is predominantly left-lateral with a small amount of northeast-verging thrust component. The slip rates decrease sharply about 4mm/a from west to east between the intersection zone of the Awancang Fault and Kunlun Fault. Together with our previous trenching results on the Kunlun Fault, the comparison with slip rates at the Kunlun fault zone suggests that the Awancang fault zone has an important role in strain partitioning for east extension of Kunlun Fault in eastern Tibet. At the same time, the 15km long surface rupture zone of the southeast segment was found at the Awancang Fault. By dating the latest faulted geomorphologic surface, the last event may be since the 1766±54 Cal a BP. Through analysis of the trench, there are four paleoearthquake events identified recurring in situ on the Awancang Fault and the latest event is since (850±30)a BP. The slip rate of the Awancang Fault is almost equivalent to the descending value of the eastern part of the east Kunlun Fault, which can well explain the slip rate decreasing of the eastern part of the east Kunlun Fault(the Maqin-Maqu segment)and the characteristics of the structure dynamics of the eastern edge of the Tibet Plateau. The falling slip rate gradient of the eastern Kunlun Fault corresponds to the geometric characteristic. It is the Awancang Fault, the strand of the East Kunlun Fault that accommodates the strain distribution of the eastward extension of the east Kunlun Fault. This study is helpful to seismic hazard assessment and understanding the deformation mechanism in eastern Tibet.     

NEW DISCOVERY OF RESHUI-TAOSTUO RIVER FAULT IN DULAN,QINGHAI PROVINCE AND ITS IMPLICATIONS

The 40km-long, NEE trending Reshui-Taostuo River Fault was found in the southern Dulan-Chaka highland by recent field investigation, which is a strike-slip fault with some normal component. DEM data was generated by small unmanned aerial vehicle(UAV)on key geomorphic units with resolution<0.05m. Based on the interpretation and field investigation, we get two conclusions:1)It is the first time to define the Reshui-Taostuo River Fault, and the fault is 40km long with a 6km-long surface rupture; 2)There are left-handed dislocations in the gullies and terraces cut by the fault. On the high-resolution DEM image obtained by UAV, the offsets are(9.3±0.5) m, (17.9±1.5) m, and(36.8±2) m, measured by topographic profile recovery of gullies. The recovery measurements of two terraces present that the horizontal offset of T₁/T₀ is(18.2±1.5) m and the T₂/T₁ is (35.8±2) m, which is consistent with the offsets from gullies. According to the historical earthquake records, a M5 3/4 earthquake on April 10, 1938 and a M_S5.0 earthquake on March 21, 1952 occurred at the eastern end of the surface rupture, which may be related to the activity of the fault. By checking the county records of Dulan and other relevant data, we find that there are no literature records about the two earthquakes, which is possibly due to the far distance to the epicenter at that time, the scarcity of population in Dulan, or that the earthquake occurred too long ago that led to losing its records. The southernmost ends of the Eastern Kunlun Fault and the Elashan Fault converge to form a wedge-shaped extruded fault block toward the northwest. The Dulan Basin, located at the end of the wedge-shaped fault block, is affected by regional NE and SW principal compressive stress and the shear stress of the two boundary faults. The Dulan Basin experienced a complex deformation process of compression accompanying with extension. In the process of extrusion, the specific form of extension is the strike-slip faults at each side of the wedge, and there is indeed a north-east and south-west compression between the two controlling wedge-shaped fault block boundary faults, the Eastern Kunlun and Elashan Faults. The inferred mechanism of triangular wedge extrusion deformation in this area is quite different from the pure rigid extrusion model. Therefore, Dulan Basin is a wedge-shaped block sandwiched between the two large-scale strike-slip faults. Due to the compression of the northeast and southwest directions of the region, the peripheral faults of the Dulan Basin form a series of southeast converging plume thrust faults on the northeast edge of the basin near the Elashan Fault, which are parallel to the Elashan Fault in morphology and may converge with the Elashan Fault in subsurface. The southern marginal fault of the Dulan Basin(Reshui-Taostuo River Fault)near the Eastern Kunlun fault zone is jointly affected by the left-lateral strike-slip Eastern Kunlun Fault and the right-lateral strike-slip Elashan Fault, presenting a left-lateral strike-slip characteristic. Meanwhile, the wedge-shaped fault block extrudes to the northwest, causing local extension at the southeast end, and the fault shows the extensional deformation. These faults absorb or transform the shear stress in the northeastern margin of the Tibet Plateau. Therefore, our discovery of the Dulan Reshui-Taostuo River Fault provides important constraints for better understanding of the internal deformation mode and mechanism of the fault block in the northeastern Tibetan plateau. The strike of Reshui-Taostuo River Fault is different from the southern marginal fault of the Qaidam Basin. The Qaidam south marginal burial fault is the boundary fault between the Qaidam Basin and the East Kunlun structural belt, with a total length of ~500km. The geophysical data show that Qaidam south marginal burial fault forms at the boundary between the positive gravity anomaly of the southern East Kunlun structural belt and the negative gravity anomaly gradient zone of the northern Qaidam Basin, showing as a thrust fault towards the basin. The western segment of the fault was active at late Pleistocene, and the eastern segment near Dulan County was active at early-middle Pleistocene. The Reshui-Taostuo River Fault is characterized by sinistral strike-slip with a normal component. The field evidence indicates that the latest active period of this fault was Holocene, with a total length of only 40km. Neither remote sensing image interpretation nor field investigation indicate the fault extends further westward and intersects with the Qaidam south marginal burial fault. Moreover, it shows that its strike is relatively consistent with the East Kunlun fault zone in spatial distribution and has a certain angle with the burial fault in the southern margin of Qaidam Basin. Therefore, there is no structural connection between the Reshui-Taostuo River Fault and the Qaidam south marginal burial fault.     

GEOLOGICAL EVIDENCES OF LATE QUATERNARY ACTIVITY OF MOTUO FAULT IN EASTERN HIMALAYAN SYNTAXIS

Motuo Fault locates at the east of Namjagbarwa Peak in eastern Himalayan syntaxis.Based on the remote sensing interpretation,the previous work,and with the field investigation,this paper obtains the spatial distribution and movement characteristics of Motuo Fault in China,and geological evidences of late Quaternary activity.Two trenches in Motuo village and Dongdi village located in Yalung Zangbo Grand Canyon reveal that the Motuo Fault dislocates the late Quternary stratum and behaves as a reverse fault in Motuo village and normal fault in Dongdi village.Motuo Fault is dominated by left-lateral strike-slip associated with the faulted landforms,with different characteristics of the tilting movement in different segments.The trench at Didong village reveals the latest stratum dislocated is~2780±30 a BP according to radiocarbon dating,implying that Motuo Fault has ruptured the ground surface since late Holocene.The movement of left-lateral strike-slip of Motuo Fault is related to the northward movement process of Indian pate.     

LATE-QUATERNARY ACTIVITY OF THE YALAHE FAULT OF THE XIANSHUIHE FAULT ZONE,EASTERN MARGIN OF THE TIBET PLATEAU

Complex geometrical structures on strike-slip faults would likely affect fault behavior such as strain accumulation and distribution, seismic rupture process, etc. The Xianshuihe Fault has been considered to be a Holocene active strike-slip fault with a high horizontal slip rate along the eastern margin of the Tibetan plateau. During the past 300 years, the Xianshuihe Fault produced 8 earthquakes with magnitude≥7 along the whole fault and showed strong activities of large earthquakes. Taking the Huiyuansi Basin as a structure boundary, the northwestern and southeastern segments of the Xianshuihe Fault show different characteristics. The northwestern segment, consisting of the Luhuo, Daofu and Qianning sections, shows a left-stepping en echelon pattern by simple fault strands. However, the southeastern segment(Huiyuansi-Kangding segment)has a complex structure and is divided into three sub-faults: the Yalahe, Selaha and Zheduotang Faults. To the south of Kangding County, the Moxi segment of the Xianshuihe Fault shows a simple structure. The previous studies suggest that the three sub-faults(the Yalahe, Selaha and Zheduotang Faults of the Huiyuansi-Kangding segment)unevenly distribute the strain of the northwestern segment of the Xianshuihe Fault. However, the disagreement of the new activity of the Yalahe Fault limits the understanding of the strain distribution model of the Huiyuansi-Kangding segment. Most scholars believed that the Yalahe Fault is a Holocene active fault. However, Zhang et al.(2017)used low-temperature thermochronology to study the cooling history of the Gongga rock mass, and suggested that the Yalahe Fault is now inactive and the latest activity of the Xianshuihe Fault has moved westward over the Selaha Fault. The Yalahe Fault is the only segment of the Xianshuihe Fault that lacks records of the strong historical earthquakes. Moreover, the Yalahe Fault is located in the alpine valley area, and the previous traffic conditions were very bad. Thus, the previous research on fault activity of the fault relied mainly on the interpretation of remote sensing, and the uncertainty was relatively large. Through remote sensing and field investigation, we found the geological and geomorphological evidence for Holocene activity of the Yalahe Fault. Moreover, we found a well-preserved seismic surface rupture zone with a length of about 10km near the Yariacuo and the co-seismic offsets of the earthquake are about 2.5~3.5m. In addition, we also advance the new active fault track of the Yalahe Fault to Yala Town near Kangding County. In Wangmu and Yala Town, we found the geological evidence for the latest fault activity that the Holocene alluvial fans were dislocated by the fault. These evidences suggest that the Yalahe Fault is a Holocene active fault, and has the seismogenic tectonic condition to produce a large earthquake, just like the Selaha and Zheduotang Faults. These also provide seismic geological evidence for the strain distribution model of the Kangding-Huiyuansi segment of the Xianshuihe Fault.     

LATE QUATERNARY ACTIVITY OF FAULTS IN THE EPICENTER AREA OF JINGGU M6.6 EARTHQUAKE

The 2014 Jinggu M6.6 earthquake attacked the Jinggu area where few historical earthquakes had occurred and little study has been conducted on active tectonics. The lack of detailed field investigation on active faults and seismicity restricts the assessment of seismic risk of this area and leads to divergent view points with respect to the seismotectonics of this earthquake, so relevant research needs to be strengthened urgently. In particular, some studies suggest that this earthquake triggered the activity of the NE-trending faults which have not yet been studied. By the approaches of remote sensing image interpretation, structural geomorphology investigation and trench excavation, we studied the late Quaternary activity of the faults in the epicenter area, which are the eastern margin fault of Yongping Basin and the Yixiang-Zhaojiacun Fault, and drew the conclusions as follows: (1)The eastern margin fault of Yongping Basin originates around the Naguai village in the southeastern margin of Yongping Basin,extending northward across the Qiandong, Tianfang, and ending in the north of Tiantou. The fault is about 43km long, striking near SN. The linear characteristic of the fault is obvious in remote sensing images. Structural geomorphological phenomena, such as fault troughs, linear ridges and gully dislocations, have developed along the faults. There are several dextral-dislocated gullies near Naguai village, with displacements of 300m, 220m, 146m, 120m and 73m, respectively, indicating that the fault is a dextral strike-slip fault with long-term activity. In order to further study the activity of the fault, a trench was excavated in the fault trough, the Naguai trench. The trench reveals many faults, and the youngest strata offseted by the faults are Holocene, with ¹⁴C ages of(1 197±51)a and(1 900±35)a, respectively. All those suggest that it is a Holocene active fault. (2)The Yixiang-Zhaojiacun Fault starts at the southeast of the Jinggu Basin, passes through Xiangyan, Yixiang, Chahe, and terminates at the Zhaojiacun. The total length of the fault is about 60km, and is a large-scale NE-trending fault in the Wuliangshan fault zone. Four gullies are synchronously sinistrally dislocated at Yixiang village, with the displacements of 340m, 260m, 240m and 240m, indicating that the fault is a long-term active sinistral strike-slip fault. A trench was excavated in a fault trough in Yixiang village. The trench reveals a small sag pond and a fault. The fault offsets several strata with clear dislocation and linear characteristic. The thickness of strata between the two walls of fault does not match, and the gravels are oriented along fault plane. The offset strata have the ¹⁴C age of(2 296±56)a, (3 009±51)a, and(4 924±45)a, respectively, and another two strata have the OSL age of(1.8±0.1)ka, (8.6±0.5)ka respectively, by which we constrained the latest paleoearthquake between(1.8±0.1)ka(OSL-Y01)and(378±48)a BP(CY-07). This again provides further evidence that the fault is a Holocene fault with long-term activity. (3)Based on the distribution of aftershocks and the predecessor research results, the 2014 Jinggu M6.6 earthquake and the M5.8, M5.9 strong aftershocks are regarded as being caused by the eastern margin fault of Yongping Basin, which is part of the Wuliangshan fault zone. The seismogenic mechanism is that the stress has been locked, concentrated and accumulated to give rise to the quakes in the wedge-shaped area near the intersection of the SN and NE striking faults, which is similar to the seismogenic mechanism in the southwest of Yunnan Province.     

GEOLOGICAL AND GEOMORPHIC EXPRESSIONS OF LATE QUATERNARY STRIKE-SLIP ACTIVITY ON JINTA NANSHAN FAULT IN NORTHERN EDGE OF QING-ZANG BLOCK

Jinta Nanshan Fault is an important fault in northeast front of Qing-Zang Plateau, and it is crucial for determining the eastern end of Altyn Tagh Fault. However, there is still debate on its significant strike-slip movement. In this paper, we study the Late Quaternary activity of Jinta Nanshan Fault and its geological and geomorphic expressions by interpreting aerial photographs and high-resolution remote sensing images, surveying and mapping of geological and geomorphic appearances, digging and clarifying fault profiles and mapping deformation characteristics of micro-topographies, then we analyze whether strike-slip activity exists on Jinta Nanshan Fault. We get a more complete fault geometry than previous studies from most recent remote sensing images. Active fault traces of Jinta Nanshan mainly include 2 nearly parallel, striking 100°~90° fault scarps, and can be divided into 3 segments. West segment and middle segment form a left stepover with 2~2.5km width, and another stepover with 1.2km width separates the middle and east segment. We summarize geomorphic and geologic evidence relating to strike slip activity of Jinta Nanshan Fault. Geomorphic expressions are as follows:First, fault scarps with alternating facing directions; second, sinistral offset of stream channels and micro-topographies; third, pull-apart basins and compressive-ridges at discontinuous part of Jinta Nanshan Fault. Geologic expressions are as follows:First, fault plane characteristics, including extremely high fault plane angle, unstable dip directions and coexistence of normal fault and reverse fault; second, flower structures. Strike-slip rate was estimated by using geomorphic surface age of Zheng et al.(2013)and left-lateral offset with differential GPS measurements of the same geomorphic surface at field site in Fig. 4e. We calculated a strike-slip rate of (0.19±0.05)mm/a, which is slightly larger than or almost the same with vertical slip rate of (0.11±0.03)mm/a from Zheng et al.(2013). When we confirm the strike-slip activity of Jinta Nanshan, we discuss its potential dynamic sources:First, eastern extension of Altyn Tagh Fault and second, strain partitioning of northeastward extension of Qilian Shan thrust belt. The first one is explainable when it came to geometric pattern of several E-W striking fault and eastward decreasing strike slip rate, but the former cannot explain why the Heishan Fault, which locates between the the Altyn Tagh Fault and Jinta Nanshan Fault, is a pure high angle reverse fault. The latter seems more explainable, because oblique vectors may indeed partition onto a fault and manifest strike-slip activity.     

LATE QUATERNARY ACTIVITY AND PALEOSEISMIC RUPTURE BEHAVIOR FOR THE SOUTHEAST SECTION OF THE GANZI-YUSHU FAULT

The Ganzi-Yushu Fault, the boundary of Bayan Har active tectonic block, Qiantang active tectonic block and Sichuan-Yunan active tectonic block, is a sinistral strike-slip fault zone with intensive Holocene activity. Thus, the study of activity characteristics and rupture behavior of paleoearthquakes in the late Quaternary on the Ganzi-Yushu Fault is of fundamental importance for understanding the future seismic risk of this fault. The southeast section of Ganzi-Yushu Fault is made up of three segments of Ganzi, Manigange and Dengke, where a M_S7.3 earthquake in 1866, a M_S7.7 earthquake in 1854 and a M_S7.3 in 1896 occurred, respectively. There is still lack of in-depth study on the active features and the cascading rupture possibility of these segments, which hindered the evaluation of seismic risk for the southeast section of Ganzi-Yushu Fault. By the means of field geological survey and micro topography measurement, this paper studied the geological and geomorphological features of the southeast section of the Ganzi-Yushu Fault. The results show that the Ganzi and Dengke segments show obvious extension movement, in addition to the left-lateral movement. For Manigange segment, the characteristics of the movement are mainly left-lateral strike-slip and thrusting, and the maximum vertical displacement of the Holocene strata is greater than 2m. In part areas, the movement is normal faulting, which perhaps relates to the left stepping zone in the local stress environment. Therefore, combining the research results such as the fracture distribution in different motion characteristics, rupture behavior of paleoearthquakes, and the distribution of historical earthquake surface ruptures, we divide the southeast section of Ganzi Yushu Fault into Ganzi, Manigange and Dengke segment, and consider the Yakou and the Dengke Basin as the stepovers and the segments' boundaries. As the small scale of impermanent barriers including Dengke Basin and the ridge near Yakou, of which the width is about 1~2km, they may be broken through in great earthquake rupture in future. A trench was excavated in Zhuqing township to investigate the paleoearthquakes on the Manigange segment, radiocarbon dating was employed and 3 paleoseismic events were revealed in the Zhuqing trench, which are the seismic events occurring respectively at 3875~3455BC, after 775BC, and the latest one that ruptured the surface. Compared with the previous results of paleoseismology in the southeast section of Ganzi-Yushu Fault, it is found that the paleoseismic events in the Manigange segment are obviously different with that in Ganzi segment and Dengke segment. Due to the lack of sufficient data on the southeast section of the Ganzi-Yushu Fault, it still needs further discussion whether the cascade-rupturing between these segments exists.     

震源机制解分类与川滇及邻近地区最新变形特征

以位错理论为依据探讨了地震分类的理论基础,利用美国哈佛大学1977—2008年的震源机制解资料,采用地震三角形分类法,研究了中国川滇及邻近地区震源机制解124例,从地壳脆性变形的角度分析了川滇次级块体的变形形式。结果表明:整体上川滇及邻区的走滑断层、逆冲断层和正断层具有明显的分区性特征,受青藏高原SE方向的挤压,沿着鲜水河断裂带、安宁河断裂带、则木河断裂带和小江断裂带产生了大的剪切位移和变形带;同时,受缅甸弧挤压和四川盆地的阻挡,在缅甸弧前端和龙门山断裂带等地形成了强烈的挤压区,在云南大部分区域形成了扇形剪切应力变形区;而沿鲜水河断裂带、安宁河断裂带、则木河断裂带和小江断裂带所产生的大的剪切位移和变形直接作用在红河断裂带上,造成红河断裂带呈右旋向SE方向错动,引起其后延金沙江断裂至丽江-小金河断裂之间形成大的应力拉张区,构成了现今川滇及邻区地壳变形的最新格局     

THE ACTIVITY OF WESTERN LISHAN FAULT SINCE THE LATE PLEISTOCENE

Along the northern piedmont of Mt. Lishan, the characteristics and locations of the active normal Lishan fault in west of Huaqing Pool provide important evidences for determining the seismotectonic environment, seismic stability evaluation of engineering in the eastern Weihe Basin. After reviewing the results from high-density resistivity method, seismic profile data, geological drillhole section and trenching in west of the Huaqing Pool, it is found that the strike of western normal Lishan Fault changes from EW direction at the eastern part to the direction of N60°W, and the fault consists of two branches, dipping NE with a high dip angle of~75°. The artificial shallow seismic profile data reveals that the attitude of strata near Lishan Fault mainly dips to south, which is presumed to be related to the southward tilt movement of Mt. Lishan since the Cenozoic. The section of geological drillhole reveals that since the late middle Pleistocene, the displacement of the paleo-soil layer S₂ is about 10m. And the maximum displacement of western Lishan Fault recorded in the paleo-soil layer S₁ reaches 7.8m since the late Pleistocene. In addition, evidences from trench profile show that the western Lishan Fault was active at least 3 times since Malan loess deposition with ¹⁴ C dating age(32 170±530)Cal a BP. The multiple activities of the Lishan Fault result in a total displacement about 3.0m in the Malan loess layer L₁. The latest activity of the western Lishan Fault produced a displacement of about 0.9m in the early Holocene loess layer L₀((8 630±20)Cal a BP)and caused obvious tensile cracks in the Holocene dark leoss layer S₀((4 390±20)Cal a BP). Briefly, we have obtained a vertical movement rate of about 0.11~0.19mm/a since the Holocene((8 630±20)Cal a BP)in the western extension of the Lishan Fault, the recurrence interval of earthquakes on the fault is about(10.7±0.5)ka, and the co-seismic surface rupture in a single event is inferred to be about 0.9m.