ACTIVE CHARACTERISTICS OF THE SANWEISHAN FAULT IN THE NORTHERN MARGIN OF THE TIBETAN PLATEAU DURING LATE PLEISTOCENE

The Sanweishan fault is located in the northern margin of the Tibetan plateau. It is a branch of the Altyn Tagh fault zone which extends to the northwest. A detailed study on Late Quaternary activity characteristics of the Sanwei Shan Fault can help understanding the strain distribution of the Altyn Tagh fault zone and regional seismic activity and northward growth of the Tibetan plateau. Previous research on this fault is insufficient and its activity is a controversial issue. Based on satellite images interpretation, field investigations and geological mapping, this study attempts to characterize this feature, especially its activity during Late Quaternary. Trench excavation and sample dating permit to address this issue, including determination of paleoseismic events along this fault. The results show that the Sanweishan fault is a large-scale active structure. It starts from the Shuangta reservoir in the east, extending southward by Shigongkouzi, Lucaogou, and Shugouzi, terminates south of Xishuigou, with a length of 175km. The fault trends in NEE, dipping SE at angles 50°~70°. It is characterized by left-lateral strike-slip with a component of thrust and local normal faulting. According to the geometry, the fault can be divided into three segments, i.e. Shuangta-Shigongkouzi, Shigongkouzi-Shugouzi and Shugouzi-Xishuigou from east to west, looking like a left-or right-step pattern. Plenty of offset fault landforms appear along the Sanweishan Fault, including ridges, left-lateral strike-slip gullies, fault scarps, and fault grooves. The trench study at the middle and eastern segments of the fault shows its activity during Late Pleistocene, evidenced by displaced strata of this epoch. Identification marks of the paleoearthquakes and sample dating reveal one paleoearthquake that occurred at(40.3±5.2)~(42.1±3.9) ka.     

阳关断裂带变形样式及其对阿尔金断裂扩展的指示意义

阳关断裂位于青藏高原北部阿尔金断裂系向北扩展的前缘位置,对其几何学和运动学的深入研究,有助于理解青藏高原向大陆内部扩展的机制。文章通过卫星影像解译、探槽开挖、差分GPS及无人机测量等对阳关断裂开展了详细研究。结果显示：阳关断裂东段发育多条正反向断层陡坎,断层陡坎高度在0.4～8 m之间,平均约2.2 m,探槽揭示断裂倾角约60°,形成高角度逆断层,局部发育正断层;西段断裂向北西前缘扩展,形成一组弧形分布的断层陡坎,陡坎高度多在0.9～2.4 m,平均约1.9 m。同时自南向北,逆冲断层陡坎形态由多级陡坎转为单一陡坎。对探槽剖面分析,显示断裂断错晚更新世冲洪积砾石层,发育的断层倾角较缓,以低角度逆冲为主要特征,约26°,有的甚至沿地层向前推覆。结合前人的研究成果,阳关断裂可能为本区阿尔金向北扩展的北边界,与三危山断裂共同协调吸收了阿尔金断裂东段的部分应变量。     

THE PALEOSEISMIC SURFACE RUPTURE AT SOUTH OF CENTRAL ALTYN TAGH FAULT AND ITS TECTONIC IMPLICATION

In this study, we described a 14km-long paleoearthquakes surface rupture across the salt flats of western Qaidam Basin, 10km south of the Xorkol segment of the central Altyn Tagh Fault, with satellite images interpretation and field investigation methods. The surface rupture strikes on average about N80°E sub-parallel to the main Altyn Tagh Fault, but is composed of several stepping segments with markedly different strike ranging from 68°N~87°E. The surface rupture is marked by pressure ridges, sub-fault strands, tension-gashes, pull-apart and faulted basins, likely caused by left-lateral strike-slip faulting. More than 30 pressure ridges can be distinguished with various rectangular, elliptical or elongated shapes. Most long axis of the ridges are oblique(90°N~140°E)to, but a few are nearly parallel to the surface rupture strike. The ridge sizes vary also, with heights from 1 to 15m, widths from several to 60m, and lengths from 10 to 100m. The overall size of these pressure ridges is similar to those found along the Altyn Tagh Fault, for instance, south of Pingding Shan or across Xorkol. Right-stepping 0.5~1m-deep gashes or sub-faults, with lengths from a few meters to several hundred meters, are distributed obliquely between ridges at an angle reaching 30°. The sub-faults are characterized with SE or NW facing 0.5~1m-high scarps. Several pull-apart and faulted basins are bounded by faults along the eastern part of the surface rupture. One large pull-apart basins are 6~7m deep and 400m wide. A faulted basin, 80m wide, 500m long and 3m deep, is bounded by 2 left-stepping left-lateral faults and 4 right-stepping normal faults. Two to three m-wide gashes are often seen on pressure ridges, and some ridges are left-laterally faulted and cut into several parts, probably owing to the occurrence of repetitive earthquakes. The OSL dating indicates that the most recent rupture might occur during Holocene.
Southwestwards the rupture trace disappears a few hundred meters north of a south dipping thrust scarp bounding uplifted and folded Plio-Quaternary sediments to the south. Thrust scarps can be followed southwestward for another 12km and suggest a connection with the south Pingding Shan Fault, a left-lateral splay of the main Altyn Tagh Fault. To the northeast the rupture trace progressively veers to the east and is seen cross-cutting the bajada south of Datonggou Nanshan and merging with active thrusts clearly outlined by south facing cumulative scarps across the fans. The geometry of this strike-slip fault trace and the clear young seismic geomorphology typifies the present and tectonically active link between left-lateral strike-slip faulting and thrusting along the eastern termination of the Altyn Tagh Fault, a process responsible for the growth of the Tibetan plateau at its northeastern margin. The discrete relation between thrusting and strike-slip faulting suggests discontinuous transfer of strain from strike-slip faulting to thrusting and thus stepwise northeastward slip-rate decrease along the Altyn Tagh Fault after each strike-slip/thrust junction.     

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.     

PALEOSEISMOLOGY ON THE YEMAHE SEGMENT OF THE YEMAHE-DAXUESHAN FAULT REVEALED BY TRENCH STUDY

A series of NWW striking faults are obliquely intersected by the NEE striking Altyn Tagh fault zone in the western Qilian Mountains. These faults were mostly active in late Quaternary and play an important role in accommodating regional lateral extrusion by both reverse and sinistral slip. Detailed studies on late Quaternary activity, tectonic transformation, paleoseismology, and strain partitioning not only significantly affect our recognition on seismogenic mechanism and zones of potential large earthquakes, but also provide useful information for exploring tectonic deformation mechanism in the northern Tibetan plateau. The Danghenanshan Fault, Yemahe-Daxueshan fault, and Altyn Tagh Fault form a triplet junction point at southwest of Subei county. The Yemahe-Daxueshan fault is one important branch fault in the western Qilian Mountains that accommodated eastward decreasing slip of the Altyn Tagh Fault, which was active in late Holocene, with a length up to 170km. Based on geometry and late Quaternary activity, the Yemahe-Daxueshan fault was subdivided into 3 segments, i.e. the Subei fault, Yemahe fault and Daxueshan Fault. The Yemahe Fault has the most prominent appearance among them, and is dominated by left-lateral slip with a little normal component. The heights of fresh scarps on this fault are only several tens of centimeters. We dug 2 trenches at the Zhazhihu site, and cleaned and reinterpreted one trench of previous studies. Then we interpreted trench profiles and paleoseismic events, and collected ¹⁴C and Optical Stimulated Luminescence samples to constrain event ages. Finally, we determined 3 events on the Yemahe fault with ages(6 830±30) a BP-(6 280±40) a BP, (5 220±30) a BP, (2 010±30) a BP, respectively. The elapsed time of most recent earthquake is(2 010±30) years before present, which is very close to the recurrence interval, so the possibility of major earthquakes on the Yemahe fault is relatively large.     

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.     

2014年于田7.3级地震的发震构造及动力学背景的初步分析

2004年2月12日新疆维吾尔自治区于田县发生了Ms7.3级地震,其发震断裂为阿尔金断裂带西南段的贡嘎错断裂带.由于地处高山无人区,存在区域历史地震漏记,但1970年以来5级以上地震活动是完整的,近20年来强震活动增强.综合分析认为,2008年于田Ms7.3地震可能加速了本次地震的发生.根据经验统计关系估计,2014年于田地震的同震地表破裂为30-40km,最大水平位错量为1.0-1.5m,地震的复发周期为300-400年.通过阿尔金断裂上前人资料和区域构造的综合分析,认为2014年于田地震是在青藏高原向北东运动背景下左旋走滑的阿尔金断裂向南西端扩展的结果.     

阿尔金断裂带东段距今20ka以来的滑动速率

阿尔金断裂带作为青藏高原北部边界 ,其走滑量和走滑速率一直为地学界所关注 ,对这样一条大陆内部巨型走滑断裂带的滑动速率进行研究 ,对于了解阿尔金断裂带左旋走滑和青藏高原北部隆升之间的耦合关系 ,具有重要意义。在阿尔金断裂带东段的疏勒河口以西 ,阿尔金断裂错断了几条规模相近的河流阶地和洪积扇 ,形成典型的走滑断层断错地貌。通过对这些典型断错地貌点的地貌观测和年代学研究 ,得到阿尔金断裂带东段石堡城以东疏勒河以西自 2 0kaBP以来的滑动速率约为 4～ 5mm/a。自 50kaBP以来 ,阿尔金断裂带东段断层平均滑动速率具有较高的时间、空间一致性 ,约为 4～ 6mm/a ,表明利用河流阶地和洪积扇位错作为断层走滑位移标志计算断层滑动速率 ,具有较高的可信度     

VISCOELASTIC STRESS TRANSFER BETWEEN 2008 AND 2014 YUTIAN M7 EARTHQUAKES,XINJIANG

Two earthquakes with magnitude larger than 7.0 occurred in 2008 and 2014 on the southwestern end of the Altyn Tagh Fault, which is located in the northwestern borderland of Tibetan plateau. Occurrences of these two earthquakes provide important insights into regional geodynamics and potential seismic risk. Layered viscoelastic model is employed in the paper to study the interaction between these two events. We find that most of aftershocks were triggered by coseismic stress produced by the 2008 Yutian earthquake, and the effect of this earthquake is insignificant on the occurrence of the 2014 Yutian earthquake. However, stress transfer by viscoelastic relaxation of postseismic deformation is in favor of occurrence of the 2014 Yutian earthquake. The coseismic and postseismic stress transfer produced by the 2014 Yutian earthquake leads to stress increasing on the western segment of the Altyn Tagh Fault. Since the occurrence time of the last major earthquake on the western segment of the Altyn Tagh Fault is tens of years ago, it should have accumulated large moment deficit on the fault segment. The Altyn Tagh Fault should be considered as a fault with high potential seismic risk.     

用阶地测量方法探讨阿尔金断裂中段全新世滑动速率

通过分析高精度数字化SPOT卫星影像 ,结合野外考察和年代学测试 ,对阿尔金南缘走滑断裂带的 3个典型走滑断层断错地貌点进行了研究。在安南坝沟 ,阿尔金南缘走滑断裂带一主要分支自 (9.36± 0 .73)kaBP以来的左旋滑动速率为 (7.5± 1.7)mm/a ;在七个泉子阿尔金南缘走滑断裂带有 4条分支 ,其中 1条规模较小的断层分支自 (13 86± 1 0 7)kaBP以来的左旋滑动速率为 (2 .3±0 5 )mm/a ,由此推断七个泉子附近断裂带全新世以来的滑动速率为 (6 .9± 1.5 )～ (9.2± 2 .0 )mm/a ;约马克其断裂带自 (4 .73± 0 .38)kaBP以来的左旋滑动速率为 (10 .6± 3.0 )mm/a。综合以上各点结果 ,阿尔金南缘走滑断裂带中段 88°30′E与 93°0 5′E之间全新世以来的水平滑动速率为 7～ 11mm/a ,与最新的GPS观测结果非常接近     

2014年新疆于田7.3级地震发震构造和震前地震活动过程讨论

2014年2月12日在新疆于田发生7.3级地震,震中位于阿尔金断裂西段,这是继2008年3月21日于田7.3级地震后在塔里木盆地南侧发生的第2次7级地震。这次于田7.3级地震的余震主体沿NE向分布,余震区的西南段呈近SN向分布;绝大部分余震与前震在余震区西南密集分布,强余震(全部的5级以上地震和81%的4级地震)绝大多数都分布在这个区域,第1天的余震主要在这个区域呈近SN向分布,余震由西向东扩展。在这次于田地震的近SN方向上曾在1982、2011以及2012年先后发生过几次6级左右的地震,而这次地震填补了其中的空段。文中从区域构造环境、地震震源机制解和余震分布特征等方面,分析这次地震的发震过程,认为地震发生在硝尔库勒盆地南缘的分支断裂,受阿尔金断裂带构造应力影响,硝尔库勒盆地受到局部近EW向的拉张作用力,首先沿近SN向破裂,这个构造部位的解锁,促进阿尔金断裂左旋错动,产生NE向破裂,应力向东传递;文中还对有历史记录以来,阿尔金断裂上7级地震的发震构造及其对阿尔金断裂带的影响进行了讨论。     

2008年于田Ms7.3地震地表破裂带特征及其构造属性讨论

新疆于田M<‘s>7.3地震发生在西昆仑块体与昆仑-柴达木-祁连块体之间的阿尔金断裂西南端NE向张剪切段邻近区域,也是阿尔金断裂、康西瓦断裂和昆仑断裂带西端玛尔盖茶卡断裂等交会部位,对理解青藏高原的变形及其动力学演化过程具有十分重要的作用.高分辨率卫星影像解译和野外考察表明,于田地震在阿什库勒火山群南部玉龙喀什河源头近...     

COSEISMIC SURFACE RUPTURES AND SEISMOGENIC MUJI FAULT OF THE 25 NOVEMBER 2016 ARKETAO MW6.6 EARTHQUAKE IN NORTHERN PAMIR

The M_W6.6 Arketao earthquake,which occurred at 14:24:30 UTC 25 November 2016 was the largest earthquake to strike the sparsely inhabited Muji Basin of the Kongur extension system in the eastern Pamir since the M 7 1895 Tashkurgan earthquake.The preliminary field work,sentinel-1A radar interferometry,and relocated hypocenters of earthquake sequences show that the earthquake consists of at least two sub-events and ruptured at least 77km long of the active Muji dextral-slip fault,and the rupture from this right-lateral earthquake propagated mostly unilaterally to the east and up-dip.Tectonic surface rupture with dextral slip of up to 20cm was observed on two tens-meter long segments near the CENC epicenter and 32.6km to the east along the Muji Fault,the later was along a previously existing strand of the Holocene Muji fault scarps.Focal mechanisms are consistent with right-lateral motion along a plane striking 107°,dipping 76° to the south,with a rake of 174°.This plane is compatible with the observed tectonic surface rupture.More than 388 aftershocks were detected and located using a double-difference technique.The mainshock is relocated at the Muji Fault with a depth of 9.3km.The relocated hypocenters of the 2016 Arketao earthquake sequence showed a more than 85km long,less than 8km wide,and 5~13km deep,NWW trending streak of seismicity to the south of the Muji Fault.The focal mechanism and mapping of the surface rupture helped to document the south-dipping fault plane of the mainshock.The listric Muji Fault is outlined by the well-resolved south-dipping streak of seismicity.The 2016 Arketao M_W6.6 and 2015 Murghob M_W7.2 earthquakes highlight the importance role of strike-slip faulting in accommodating both east-west extensional and north-south compressional forces in the Pamir interior,and demonstrate that the present-day stress and deformation patterns in the northern Pamir plateau are dominant by east-west extension in the shallow upper crust.     

阿尔金走滑断裂带昌马段的电性结构样式及构造意义

阿尔金断裂带东段走滑速率沿断裂走向方向存在明显的流失现象,有关阿尔金断裂带的影响范围及走滑速率变化的机制需要有更多的深部结构证据来提供支撑.本文以阿尔金断裂带昌马段为窗口,获取了4条横穿阿尔金断裂带及相邻地区的大地电磁测深剖面.二维电性剖面显示在阿尔金断裂带北侧中上地壳以连续的高阻体为主,而南侧祁连山内部的深部电性结构在横向上有较为复杂的变化.这一点与区域构造背景相对应,即北侧的塔里木盆地东缘依然具有较好的整体性,南侧的祁连山是青藏高原北缘生长的最前端,变形强烈.在断裂带的结构特征上,阿尔金断裂带沿走向方向的切割深度在昌马盆地西侧发生了显著的降低,与阿尔金断裂带相对应的电性边界在这里向南偏移了约15 km,对应F18断裂,并与昌马盆地相接.祁连山北部的断裂带,包括昌马断裂、旱峡—大黄沟断裂总体呈现出低角度南倾的样式,切过高阻异常体的顶部.虽然昌马盆地可以起到连接断裂带的阶区的作用,将部分阿尔金断裂的走滑分量转移到盆地南侧的昌马断裂上,但是昌马断裂的走滑速率从西向东是增加的,东侧的走滑速率甚至大于阿尔金断裂沿走向方向的流失分量.我们认为在青藏高原北部主要断裂带的活动还是受印度—欧亚板块碰撞引起的远程挤压效应的影响,包括阿尔金断裂以及祁连山内部系列断层都处于斜向挤压应力环境.在这种基本构造模式下,阿尔金断裂、断裂F18、昌马盆地、昌马断裂构成了一个局部的走滑速率分解-转换-吸收体系,对局部应力状态产生影响.     

LATE QUATERNARY ACTIVITY OF THE CENTRAL SEGMENT OF THE DARI FAULT AND RESTUDY OF THE SURFACE RUPTURE ZONE OF THE 1947 M73/4 DARI EARTHQUAKE,QINGHAI PROVINCE

Bayan Hara Block is one of the most representative active blocks resulting from the lateral extrusion of Tibet Plateau since the Cenozoic. Its southern and northern boundary faults are characterized by typical strike-slip shear deformation. Its eastern boundary is blocked by the Yangze block and its horizontal movement is transformed into the vertical movement of the Longmen Shan tectonic belt, leading to the uplift of the Longmen Shan Mountains and forming a grand geomorphic barrier on the eastern margin of the Tibet Plateau. A series of large earthquakes occurred along the boundary faults of the Bayan Hara Block in the past twenty years, which have attracted attention of many scholars. At present, the related studies of active tectonics on Bayan Hara Block are mainly concentrated on the boundary faults, such as Yushu-Ganzi-Xianshuihe Fault, East Kunlun Fault and Longmen Shan Fault. However, there are also some large faults inside the block, which not only have late Quaternary activity, but also have tectonic conditions to produce strong earthquake. These faults divide the Bayan Hara Block into some secondary blocks, and may play important roles in the kinematics and dynamics mechanism of the Bayan Hara Block, or even the eastern margin of the Tibet Plateau. The Dari Fault is one of the left-lateral strike-slip faults in the Bayan Hara Block. The Dari Fault starts at the eastern pass of the Kunlun Mountains, extends eastward through the south of Yalazela, Yeniugou and Keshoutan, the fault strike turns to NNE direction at Angcanggou, then turns to NE direction again at Moba town, Qinghai Province, and the fault ends near Nanmuda town, Sichuan Province, with a total length of more than 500km. The fault has been considered to be a late Quaternary active fault and the 1947 M73/4 Dari earthquake was produced by its middle segment. But studies on the late Quaternary activity of the Dari Fault are still weak. The previous research mainly focused on the investigation of the surface rupture and damages of the 1947 M73/4 Dari earthquake. However, there were different opinions about the scale of the M73/4 earthquake surface rupture zone. Dai Hua-guang(1983)thought that the surface rupture of the earthquake was about 150km long, but Qinghai Earthquake Agency(1984)believed that the length of surface rupture zone was only 58km. Based on interpretation of high-resolution images and field investigations, in this paper, we studied the late Quaternary activity of the Dari Fault and the surface rupture zone of the 1947 Dari earthquake. Late Quaternary activity in the central segment of the Dari Fault is particularly significant. A series of linear tectonic landforms, such as fault trough valley, fault scarps, fault springs and gully offsets, etc. are developed along the Dari Fault. And the surface rupture zone of the 1947 Dari earthquake is still relatively well preserved. We conducted a follow-up field investigation for the surface rupture zone of the 1947 Dari earthquake and found that the surface rupture related to the Dari earthquake starts at Longgen village in Moba town, and ends near the northwest of the Yilonggounao in Jianshe town, with a length of about 70km. The surface rupture is primarily characterized by scarps, compressional ridges, pull-apart basins, landslides, cleavage, and the coseismic offset is about 2~4m determined by a series of offset gullies. The surface rupture zone extends to the northwest of Yilonggounao and becomes ambiguous. It is mainly characterized by a series of linear fault springs along the surface rupture zone. Therefore, we suggest that the surface rupture zone of the 1947 Dari earthquake ends at the northwest of Yilonggounao. In summary, the central segment of the Dari Fault can be characterized by strong late Quaternary activity, and the surface rupture zone of the 1947 Dari earthquake is about 70km long.     

The Rotational Structure along the Eastern Segment of the Altyn Tagh Fault and Its Implications in Dynamics

As a result of the left-lateral strike-slipping of the Altyn Tagh fault in Neotectonic period, a contra-rotational structure, namely the Zhaobishan vortex structure, has developed at the juncture of the main Altyn Tagh fault and the northern fringe fault of the Qilian Mountains.Preliminary analysis on the deformation and evolution of the Zhaobishan vortex structure. In combination with the previous data, suggests that the tectonic transform between the Altyn Tagh fault and the northern fringe fault of the Qilian Mountains attributes to the deformation of the rotational structure. The existence of a series of rotational structures along the Altyn Tagh fault and on the northeastern edge of the Qinghai-Xizang(Tibet) plateau indicate that as the substance in the northern Qinghal-Xizang (Tibet) plateau moves clockwise around the eastern tectonic knot of the Himalayas, rotational structures become the principal mode on the northern marginal zone of the Plateau of transforming and absorbing tectonic deformation.     

青藏高原东北缘主要断裂带的库仑应力演化及其强震危险性

自1920年海原发生M8.5地震以来,青藏高原东北缘接连发生了1927年古浪M8.0地震、1932年昌马M7.6地震等一系列大地震,使其进入了强震活动的丛集期。为了探究青藏高原东北缘这一系列地震间的相互作用及区域地震危险性,建立青藏高原东北缘的三维Maxwell黏弹性有限元模型,模拟了区域自1920年以来17次M6.7以上地震的同震及震后库仑应力演化。结果显示：研究区自1920年海原M8.5大地震之后,后续的16次地震中,有13次地震发生在库仑应力变化为正的区域,说明了地震间的相互作用可能是导致区域地震丛集的主要原因之一。系列地震发生后,阿尔金断裂、柴达木盆地断裂西段、东昆仑断裂中段、鄂拉山断裂北段、共和盆地断裂南段、日月山断裂南段、庄浪河断裂、礼县—罗家堡断裂、成县盆地断裂西段、文县断裂西段、龙首山断裂南段、六盘山断裂东段、西秦岭北缘断裂东段、海原断裂西段和祁连断裂东段位于库仑应力变化为正的区域,且大部分断裂或断裂段的累积库仑应力变化超过了0.01 MPa,它们未来的地震危险性较高。     

NEW ACTIVITY CHARACTERISTICS AND SLIP RATE OF THE EBOMIAO FAULT IN THE SOUTHERN MARGIN OF BEISHAN,GANSU PROVINCE

The Ebomiao Fault is a newly discovered active fault near the block boundary between the Tibetan plateau and the Alashan Block. This fault locates in the southern margin of the Beishan Mountain, which is generally considered to be a tectonically inactive zone, and active fault and earthquake are never expected to emerge, so the discovery of this active fault challenges the traditional thoughts. As a result, studying the new activity of this fault would shed new light on the neotectonic evolution of the Beishan Mountain and tectonic interaction effects between the Tibetan plateau and the Alashan Block. Based on some mature and traditional research methods of active tectonics such as satellite image interpretation, trenches excavation, differential GPS measurement, Unmanned Aircraft Vehicle Photogrammetry(UAVP), and Optical Stimulated Luminescence(OSL)dating, we quantitatively study the new activity features of the Ebomiao Fault.
Through this study, we complete the fault geometry of the Ebomiao Fault and extend the fault eastward by 25km on the basis of the 20km-fault trace identified previously, the total length of the fault is extened to 45km, which is capable of generating magnitude 7 earthquake calculated from the empirical relationships between earthquake magnitude and fault length. The Ebomiao Fault is manifested as several segments of linear scarps on the land surface, the scarps are characterized by poor continuity because of seasonal flood erosion. Linear scarps are either north- or south-facing scarps that emerge intermittently. Fourteen differential GPS profiles show that the height of the north-facing scarps ranges from (0.22±0.02)m to (1.32±0.1)m, and seven differential GPS profiles show the height of south-facing scarps ranging from (0.33±0.1)m to (0.64±0.1)m. To clarify the causes of the linear scarps with opposite-facing directions, we dug seven trenches across these scarps, the trench profiles show that the south-dipping reverse faults dominate the north-facing scarps, the dipping angles range from 23° to 86°. However, the south-facing scarps are controlled by south-dipping normal faults with dipping angles spanning from 60° to 81°.
The Ebomiao Fault is dominated by left-lateral strike-slip activity, with a small amount of vertical-slip component. From the submeter-resolution digital elevation models(DEM)constructed by UAVP, the measured left-lateral displacement of 19 gullies in the western segment of the Ebomiao Fault are(3.8±0.5)~(105±25)m, while the height of the north-facing scarps on this segment are(0.22±0.02)~(1.32±0.10)m(L₃-L₇), the left-lateral displacement is much larger than the scarp height. In this segment, there are three gullies preserving typical left-lateral offsets, one gully among them preserves two levels of alluvial terraces, the terrace riser between the upper terrace and the lower terrace is clear and shows horizontal offset. Based on high-resolution DEM interpretation and displacement restoration by LaDiCaoz software, the left-lateral displacement of the terrace riser is measured to be(16.7±0.5)m. The formation time of the terrace riser is approximated by the OSL age of the upper terrace, which is (11.2±1.5)ka BP at (0.68±0.03)m beneath the surface, and(11.4±0.6)ka at (0.89±0.03)m beneath the surface, the OSL age (11.2±1.5)ka BP at (0.68±0.03)m beneath the surface is more close to the formation time of the upper terrace because of a nearer distance to sediment contact between alluvial fan and eolian sand silt. Taking the (16.7±0.5)m left-lateral displacement of the terrace riser and the upper terrace age (11.2±1.5)ka, we calculate a left-lateral strike-slip rate of(1.52±0.25)mm/a for the Ebomiao Fault. The main source for the slip rate error is that the terrace risers on both walls of the fault are not definitely corresponded. The north wall of the fault is covered by eolian sand, we can only presume the location of terrace riser by geomorphic analysis. In addition, the samples used to calculate slip rate before were collected from the aeolian sand deposits on the north side of the fault, they are not sediments of the fan terraces, so they could not accurately define the formation age of the upper terrace. This study dates the upper terrace directly on the south wall of the fault.
Since the late Cenozoic, the new activity of the Ebomiao Fault may have responded to the shear component of the relative movement between the Tibetan plateau and the Alashan Block under the macroscopic geological background of the northeastern-expanding of the Tibetan plateau. The north-facing fault scarps are dominated by south-dipping low-angle reverse faults, the emergence of this kind of faults(faults overthrusting from the Jinta Basin to the Beishan Mountain)suggests the far-field effect of block convergence between Tibetan plateau and Alashan Block, which results in the relative compression and crustal shortening. As for whether the Ebomiao Fault and Qilianshan thrust system are connected in the deep, more work is needed.     

Did the Altyn Tagh fault extend beyond the Tibetan Plateau?

The pre-Miocene northeastern termination of Altyn Tagh fault is a critical outstanding problem for understanding the mechanics of Cenozoic deformation resultant from the Indo-Asian collision and mechanisms of Tibetan Plateau formation. Structures beyond the widely accepted NE end of the Altyn Tagh fault, near the town of Yumen, are needed in order to accommodate strike-slip deformation related to plate-like lateral extrusion tectonics, but structures with the necessary slip magnitudes and histories have not been identified. We report on a series of newly recognized and documented E to ENE-striking faults within the Alxa block, NE of the Tibetan Plateau, that are visible on remotely sensed images and confirmed by field studies. These structures are demonstrably left-lateral faults based on offset geology and kinematic indicators such as striae and s-c fabrics in fault gouge. The faults have post-Cretaceous offsets of at least tens to possibly > 150 km, but limited post-Miocene displacement, constrained by offset sedimentary basins. These characteristics suggest that strike-slip faults of the Alxa region have a similar structural history as the central-eastern Altyn Tagh fault and can provide a mechanism for accommodating Oligocene-Early Miocene extrusion along the Altyn Tagh fault.     

昌马盆地内发现活动正断层

昌马盆地为祁连山西端的山间盆地,前人一直关注其周边断裂(如昌马断裂)的构造变形,盆地内部变形则鲜有研究。基于遥感解译和野外考察、探槽开挖、差分GPS和放射性碳(¹⁴C)测年等方法,发现昌马盆地西北部的一条活动断层。断层长约4 km,总体走向NEE,倾向SE,倾角陡立,断层地貌表现为陡坎、复陡坎、断层沟槽等,陡坎高度0～5.6 m,由WS向NE逐渐增大。断层运动性质以正断为主,最新活动时代为全新世,并识别出2期古地震事件：6 670～6 885 a B.P.和26 330～26 915 a B.P.。研究结果表明,在青藏高原东北缘向NE方向挤压扩展的背景下,祁连山造山带发生NW-SE向伸展,导致其西端受到SE向拉张作用而形成正断层。