阿尔金断裂带康西瓦段晚第四纪以来的左旋滑移速率及其大地震复发周期的探讨

在高分辨率Ikonos卫星影像（1m分辨率）分析基础上,结合野外考察和定量测量,详细研究了阿尔金断裂带西段康西瓦段三十里营房地区晚第四纪以来的变形特征,在三十里营房东侧塔尔萨依吉勒尕河下游地区,断裂切割了一系列发育完好的冲积扇和阶地。6级不同阶地陡坎和邻近冲积扇面上冲沟的左旋位错量分别为251±4m,250±5m,198±4m,22±2m,12±1m和约6m。T2表面的放弃年龄约10.9±0.2ka（10Be）所限定的位错量22²00m,得到左旋滑移速率为2¹8mm/a;而T4阶地的最大累积位错可能达500m,暗示的左旋滑移速率约4⁵mm/a。最近一次大地震造成的最新地貌左旋水平位错量约6m,该地震同震地表破裂带沿喀拉喀什河谷延伸长达100km,估算为Mw7.4地震,约12m的位错量可能是公元975¹020年（AMS14C）以来最近两次大地震的累积同震地表位错,约6m的特征滑移量暗示该地段发生类似约Mw7.4地震的复发周期约370⁵00 a。这些结果表明,在青藏高原北缘,阿尔金断裂带西段为大型的左旋走滑断裂,它吸收了印度/欧亚大陆碰撞产生的较大部分应变,并使高原西部物质向东运移。     

Relationship between the Pre-existing Active Kunlun Fault and Co-seismic Surface Ruptures Produced by the 2001 Mw 7．8 Central Kunlun Earthquake，China

Field investigations allow to constrain the co-seismic surface rupture zone of ～400km with a strike-slip up to 16．3 m associated with the 2001Mw 7．8 Central Kunlun earthquake that occurred along the western segment of the Kunlun fault,northern Tibet．The co-seismic rupture structures are almost duplicated on the pre-existing fault traces of the Kunlun fault．The deformational characteristics of the co-seismic surface ruptures reveal that the earthquake had a nearly pure strike-slip mechanism．Theg eologic and topographice vidence clearly shows that spatial distributions of the co-seismic surface ruptures are re-stricted by the pre-existing geological structures of the Kunlun fault．     

西昆仑康西瓦断裂带西延特征及其构造意义

青藏高原西北部康西瓦走滑断裂带(Karakax fault)为一条经过长期演化且现今仍在活动的重要大型断裂带,该断裂对该地区形成演化起到至关重要的控制作用。目前大多学者们认为该断裂在东段沿喀拉喀什河谷大致呈东西走向延伸,后在其西段麻扎地区向北西方向延伸。然而,通过详细的野外地质调查在该断裂带西段的麻扎地区新发现了一条NEE-SWW向的断裂,将之命名为麻塔断裂。实测地质剖面和显微构造分析发现麻塔断裂与康西瓦断裂具有相似的几何学和运动学特征,同样经历了早期右旋逆冲的韧性走滑变形和后期左旋脆性走滑变形,理应划分为一条断裂,前者是后者自麻扎向西的延伸部分。麻塔-康西瓦断裂共同参与调节了自古生代以来板块碰撞拼合在青藏高原西北部的构造变形,现今西昆仑-帕米尔地区的构造地貌格局正是康西瓦和喀喇昆仑等大型断裂新生代活动而形成的。     

基于D—InSAR的九寨沟地震同震形变场反演分析

利用覆盖九寨沟地区的RadarSat—2数据与Sentinel—1A数据,采用精轨数据进行定轨,消除轨道误差,并结合合成孔径差分(D-InSAR)方法中的双轨差分技术,获取2017年8月8日Mw7. 0级地震的同震形变场。结果表明,视线方向(LOS)最大沉降量约为20 cm,隆起量达9 cm。基于弹性半空间形变模型反演该地震的断层滑动分布,得出该地震断层滑动以左旋走滑为主,走向为330°,倾角为32°,滑动角为-170°,同震滑动分布主要集中在4～12 km深度处,最大滑动量位于9 km处,约为6. 14 m,平均滑动量为0. 57 m。反演获得的地震标量矩为4. 06E+18N·m,震级Mw约为6. 4,深度为19. 5 km。     

帕米尔东北缘-西昆仑的构造地貌及其构造意义

帕米尔东北缘-西昆仑位于青藏高原西北部,受三条大型断裂:康西瓦断裂、主帕米尔-铁克里克断裂和公格尔断裂的制约.通过野外考察、卫星遥感图像解译、ASTER GDEM高程数据的分析,对上述三条断裂及整个区域进行构造地貌研究,并探讨其构造意义.结果表明:康西瓦断裂为左行走滑断裂;主帕米尔-铁克里克断裂为逆冲断裂;公格尔断裂和塔什库尔干断裂分别为右行、左行走滑正断层,连接两者的是塔合曼正断裂.通过ASTER GDEM高程数据的高程分布、局部高程差和坡度分析,表明帕米尔东北缘-西昆仑至塔里木盆地存在三级特征地貌(塔里木盆地、塔里木盆地南缘山前褶皱逆冲带和帕米尔东北缘-西昆仑);西昆仑地区受印度/亚洲板块碰撞而产生垂向物质运动,由于三条大型断裂控制在西侧断裂附近存在水平方向的物质运动,垂直和水平两种运动的存在促使靠近康西瓦和公格尔断裂形成高山地貌.     

Co-seismic Faults and Geological Hazards and Incidence of Active Fault of Wenchuan Ms 8.0 Earthquake,Sichuan,China

There are two co-seismic faults which developed when the Wenchuan earthquake happened. One occurred along the active fault zone in the central Longmen Mts.and the other in the front of Longmen Mts.The length of which is more than 270 km and about 80 km respectively.The co-seismic fault shows a reverse flexure belt with strike of N45°-60°E in the ground,which caused uplift at its northwest side and subsidence at the southeast.The fault face dips to the northwest with a dip angle ranging from 50°to 60°.The...     

Late Quaternary activity and dextral strike-slip movement on the Karakax Fault Zone, northwest Tibet

Field observations and interpretations of satellite images reveal that the westernmost segment of the Altyn Tagh Fault (called Karakax Fault Zone) striking WNW located in the northwestern margin of the Tibetan Plateau has distinctive geomorphic and tectonic features indicative of right-lateral strike-slip fault in the Late Quaternary. South-flowing gullies and N–S-trending ridges are systematically deflected and offset by up to ~ 1250 m, and Late Pleistocene–Holocene alluvial fans and small gullies that incise south-sloping fans record dextral offset up to ~ 150 m along the fault zone. Fault scarps developed on alluvial fans vary in height from 1 to 24 m. Riedel composite fabrics of foliated cataclastic rocks including cataclasite and fault gouge developed in the shear zone indicate a principal right-lateral shear sense with a thrust component. Based on offset Late Quaternary alluvial fans, ¹⁴C ages and composite fabrics of cataclastic fault rocks, it is inferred that the average right-lateral strike-slip rate along the Karakax Fault Zone is ~ 9 mm/a in the Late Quaternary, with a vertical component of ~ 2 mm/a, and that a M 7.5 morphogenic earthquake occurred along this fault in 1902. We suggest that right-lateral slip in the Late Quaternary along the WNW-trending Karakax Fault Zone is caused by escape tectonics that accommodate north–south shortening of the western Tibetan Plateau due to ongoing northward penetration of the Indian plate into the Eurasian plate.     

东昆仑断裂带库赛湖段晚第四纪古地震研究*

对东昆仑断裂带库赛湖段进行了断错地貌填图和古地震探槽揭露研究。除2001年昆仑山口西8.1级地震外,共揭露出9次古地震事件,它们的年龄分别为31900±1923aB.P. , 27990±1681aB.P. , 23635±1427aB.P. , 20345±1225aB.P. , 16865±1018aB.P. , 12935±774aB.P. , 9730±592aB.P. , 6955±425aB.P.和3100±201aB.P.;古地震重复间隔分别为3910±2554a,4355±2205a,3290±1881a,3480±1593a,3930±1279a,3205±975a,2775±728a,3855±470a和3100±201a。研究结果表明,库赛湖段晚第四纪古地震活动具有准周期性,其平均重复间隔为3544±416a。发生在距今3100年前的倒数第1次古地震事件的离逝时间与重复间隔非常接近,这意味着2001年11月14日发生在库赛湖段的8.1级大地震为该断裂地震活动在准周期上的再现。高的滑动速率和长周期复发间隔表明库赛湖段活动习性以重复发生大地震为特征。     

2008年汶川大地震小鱼洞地表破裂带精细填图

5·12汶川Mw7.9级地震为罕见的、地壳尺度位移配分于多条平行断裂的板内逆冲走滑型地震。在2条北东走向、近平行的主要地表破裂间,发育北西走向的小鱼洞地表破裂。介绍了对小鱼洞北西向地表破裂的精细填图。小鱼洞地表破裂空间上位于灌县-江油与映秀-北川断裂间,全长约8km,总体走向310°,为南西盘抬升、逆冲兼具左旋走滑性质。地表破裂在南东端走向变化较大,从300~310°变为南北向,并与灌县-江油地表破裂带的磁峰段相连。小鱼洞地表破裂的垂向位错自北西往南东方向递减,北西端陡坎高度最大3.4m,南东端则小于0.2m,衰减梯度约为0.5m/km。左旋走滑位移测量点较少,集中在中段的小鱼洞镇附近,所测最大左旋走滑位移约为2.2m,一般走滑位错与同处垂直位错具有同步变化的特征。小鱼洞断裂近地表的倾角较缓,为30°±15°。结合已有地貌、地球物理和地质研究结果,提出小鱼洞断裂是向下与灌县-江油断裂交会的侧向断坡,位于映秀-北川断裂中南段间的断面倾角差异的撕裂部位,连接映秀-北川和灌县-江油断裂。在运动学上,认为小鱼洞断裂是以斜向断坡为几何形态的撕裂断裂,调节了北东走向的主断裂的运动学横向差异。小鱼洞断裂上的同震位移矢量与N70°、80°E的区域主压应力场方向匹配。这一方向与龙门山高原边界斜交。     

青藏高原东南部第四纪右旋剪切运动

通过对藏东南嘉黎断裂和滇西北断裂实地考察研究，表明青藏高原南部不存在统一的边界走滑断裂。嘉黎断裂的西段位于青藏高原南部，是一个南北挤压作用下的东西向伸展构造区，发育近南北向的地堑系，嘉黎断裂西段是这些地堑之间的转换断层，具有较高的右旋走滑速率。滇西北断裂与红河断裂构成川滇菱形块体的西南边界，该块体具有向东南逃逸和顺时针旋转运动。     

Paleoseismology of the Palu–Lake Hazar segment of the East Anatolian Fault Zone, Turkey

The East Anatolian Fault Zone (EAFZ) is among the most important active continental transform fault zones in the world as testified by major historical and minor instrumental seismicity. The first paleoseismological exploratory trenching study on the EAFZ was done on the Palu–Lake Hazar segment (PLHS), which is one of the six segments forming the fault zone, in order to determine its past activity and to assess its earthquake hazard.The results of trenching indicate that the latest surface rupturing earthquakes on this segment may be the Ms=7.1+ 1874 and Ms=6.7 1875 events, and there were other destructive earthquakes prior to these events. The recurrence interval for a surface rupturing large (M>7) earthquake is estimated as minimum 100±35 and maximum 360 years. Estimates for the maximum possible paleoearthquake magnitude are (Mw) 7.1–7.7 for the Palu–Lake Hazar segment based on empirical magnitude fault rupture relations.An alluvial fan dated 14,475–15,255 cal years BP as well as another similar age fan with an abandoned stream channel on it are offset in a left-lateral sense 175 and 160.5 m, respectively, indicating an average slip rate of 11 mm/year. Because 127 years have elapsed since the last surface rupturing event, this slip rate suggests that 1.4 m of left-lateral strain has accumulated along the segment, ignoring possible creep effects, folding and other inelastic deformation. A 2.5 Ma age for the start of left-lateral movement on the segment, and in turn the EAFZ, is consistent with a slip rate of 11 mm/year and a previously reported 27 km total left-lateral offset. The cumulative 5–6 mm/year vertical slip rate near Lake Hazar suggests a possible age of 148–178 ka for the lake. Our trenching results indicate also that a significant fraction of the slip across the EAFZ zone is likely to be accommodated seismically. The present seismic quiescence compared with the past activity (paleoseismic and historic) indicate that the EAFZ may be “locked” and accumulating elastic strain energy but could move in the near future.     

Co-seismic displacements, folding and shortening structures along the Chelungpu surface rupture zone occurred during the 1999 Chi-Chi (Taiwan) earthquake

A nearly 100-km-long surface rupture zone, called Chelungpu surface rupture zone, occurred mostly along the pre-existing Chelungpu fault on the northwestern side of Taiwan, accompanying the 1999 Chi-Chi Ms 7.6 earthquake. The Chelungpu surface rupture zone can be divided into four segments based on the characteristics of co-seismic displacements, geometry of the surface ruptures and geological structures. These segments generally show a right-step en echelon form and strike NE–SW to N–S, and dip to the east with angles ranging from 50 to 85°. The co-seismic flexural-slip folding structures commonly occurred in or near the surface rupture zone from a few meters to a few hundreds of meters in width, which have an orientation in fold axes parallel or oblique to the surface rupture zone. The displacements measured in the southern three segments are approximately 1.0–3.0 m horizontally and 2.0–4.0 m vertically. The largest displacements were measured in the northern segment, 11.1 m horizontally and 7.5 m vertically, respectively. The amount of co-seismic horizontal shortening caused by flexural-slip folding and reverse faulting in the surface rupture zone is generally less than 3 m. It is evident that the co-seismic displacements of the surface rupture zone are a quantitative surface indicator of the faulting process in the earthquake source fault. The relations between the geometry and geomorphology of the surface rupture zone, dips of the co-seismic faulting planes and the striations on the main fault planes generated during the co-seismic displacement, show that the Chelungpu surface rupture zone is a reverse fault zone with a large left-lateral component.     

First tectonic-geomorphology study along the Longmu–Gozha Co fault system,Western Tibet

The Longmu–Gozha Co left-lateral strike-slip fault system (LGCF) is located in remote western Tibet, forming a triple junction with both the Altyn Tagh fault (ATF) and the Karakorum fault (KF), the two major strike-slip faults in the region. The Ashikule, Gozha Co and Longmu Co faults are clear and distinct left-stepping en-echelon faults, together forming the LGCF system. Although poorly documented, quantifying its activity remains a key problem to understand the kinematics and the tectonic history of the westernmost Tibetan Plateau. Indeed, the Karakax fault (NW segment of the ATF), LGCF and KF together control the tectonics of western Tibet which itself controls the extrusion of Tibet towards the east, with the LGCF acting as a natural boundary for eastward motion of the Tibetan Plateau due to India's northward impingement. The LGCF system shows clear and impressive morphological indications of left-lateral active shear, that we quantify using field measurements (terrestrial LIDAR) along with ¹⁰Be surface-exposure dating. Our data suggest a slip-rate < 3 mm/yr, consistent with geodetic and block model studies. While it is on the order of the Karakax fault slip-rate (~ 2 mm/yr), it is smaller than those along the ATF and KF (> 9 and > 8 mm/yr, respectively), yielding a few mm/yr of extension accommodated most likely in the Ashikule graben and surroundings, located between the ATF and Karakax faults. Numerous evidences of recent tectonic-related events are present in the vicinity, such as the 1951 volcanic eruption as well as the 2008 and 2014 Ms 7.3 Yutian earthquakes, attesting of its high activity. In addition, the LGCF's en-echelon geometry and identical direction with the ATF, as well as smaller geological offsets and lower slip-rate compared to those on the surrounding faults, suggest that this segment of the ATF may be the most recent.     

Parameters of Coseismic Reverse- and Oblique-Slip Surface Ruptures of the 2008 Wenchuan Earthquake,Eastern Tibetan Plateau

On May 12th,2008,the M_w7.9 Wenchuan earthquake ruptured the Beichuan,Pengguan and Xiaoyudong faults simultaneously along the middle segment of the Longmenshan thrust belt at the eastern margin of the Tibetan plateau.Field investigations constrain the surface rupture pattern, length and offsets related to the Wenchuan earthquake.The Beichuan fault has a NE-trending rightlateral reverse rupture with a total length of 240 km.Reassessment yields a maximum vertical offset of 6.5±0.5 m and a maximum right-lat...     

Successive deformation episodes along the Lungmu Co zone, west-central Tibet

Field study, thermochronology and geochemistry of the east Lungmu Co (LMC) range highlight some of the geological events that shaped western Tibet. The LMC fault zone has long been interpreted as the boundary between the Tianshuihai terrane of Laurasian affinity and the Qiangtang block of Gondwanian affinity. In the LMC range, the Paleozoic series is intruded by the Mangtsa leucogranite whose zircon have a U/Pb age of 116.9 ± 1 Ma and by mafic rocks with U/Pb zircon ages ranging from 116.9 ± 1 to 95.1 ± 1.7 Ma. Geochemistry of the mafic rocks indicates that they have been emplaced in a supra-subduction zone setting, probably the north dipping Nujiang suture zone. ⁴⁰Ar/³⁹Ar micas ages of the granite indicate that cooling below ~ 350 °C occurred between 105 and 85 Ma. ⁴⁰Ar/³⁹Ar K-feldspar data suggest a fast cooling event at 60-55 Ma, which we relate to the reactivation of the LMC suture zone as a thrust at the onset of the India-Eurasia collision. The last, and still active, deformation event corresponds to left-lateral strike-slip faulting along the ENE-WSW LMC fault.     

Detrital zircon provenance evidence for large-scale extrusion along the Altyn Tagh fault

The question of whether or not the Altyn Tagh fault is a large-scale extrusion boundary is critical for understanding the role of lateral extrusion in accommodating the Indo-Asian convergence and in building the Tibetan Plateau. Oligocene conglomerate clasts in the eastern Xorkol basin are low-grade slate, phyllite, sandstone, dacite and carbonate, and associated paleocurrent indicators evince sediment derivation from the opposing side of the Altyn Tagh fault. Matching these clasts with similar basement rocks in the North Qilian and Tuolainanshan terranes requires post-Oligocene left-lateral offset of 380 ± 60 km on the eastern segment of the Altyn Tagh fault, suggesting large-scale extrusion along the fault in the Cenozoic (Yue, Y.J., Ritts, B.D., Graham, S.A., 2001b. Initiation and long-term slip history of the Altyn Tagh fault. International Geological Review 43, 1087–1094.). In order to further define this piercing point, the detrital zircon pattern of Oligocene sandstone from the Xorkol basin and the zircon ages of basement on the southern side of the fault were established by ion microprobe dating. Characterized by strong peaks between 850 and 950 Ma and the absence of Paleozoic and Mesozoic ages, the detrital zircon age pattern of the Oligocene sandstone matches the age distribution of zircon-bearing rocks of the Tuolainanshan terrane. This match requires 360 ± 40 km of post-Oligocene left-lateral displacement on the eastern segment of the Altyn Tagh fault, supporting as well as refining the previously reported lithology-based cross-fault match. At least one of the following three extrusion scenarios must have existed to accommodate this large offset: (1) northeastward extrusion along the Altyn Tagh–Alxa–East Mongolia fault, (2) eastward extrusion along the Altyn Tagh–North Qilian–Haiyuan fault, and (3) northeastward extrusion of northern Tibet as a Himalaya-scale thrust sheet along the North Qilian–Haiyuan fault. We prefer the first scenario inasmuch as rapidly growing evidence for Cenozoic strike-slip activity on the Alxa–East Mongolia fault and mid-Miocene exhumation of northern Tibet supports it.     

四川汶川Ms 8.0级地震同震变形特征和分段性

汶川地震发育2条地表破裂带,一条沿中龙门山活动断裂带分布,另一条沿前龙门山活动断裂带分布,前者长超过200km,后者长约80km。同震变形在地表表现为逆冲膝折带,走向N45～60°E,形成公路路面隆起和农田陡坎。逆冲膝折带西北侧抬高,东南侧下降。在剖面上冲断带倾向北西,倾角50～60°。膝折带两侧相对高差沿映秀－北川断裂一般为2.5～3.0m,沿都江堰－汉旺断裂为1.5～1.1m。沿中龙门山活动断裂带,同震变形运动方式具有明显的分段性,映秀－擂鼓镇段,表现为逆冲,走滑现象不明显;北川－青川段既有逆冲又有右旋走滑分量。沿前龙门山活动断裂带,同震变形运动方式主要表现为逆冲,走滑位移和分段性不明显。     

Growth, linkage, and termination processes of a 10-km-long strike-slip fault in jointed granite: the Gemini fault zone, Sierra Nevada, California

Field-based structural analysis of an exhumed, 10-km-long strike-slip fault zone elucidates processes of growth, linkage, and termination along moderately sized strike-slip fault zones in granitic rocks. The Gemini fault zone is a 9.3-km-long, left-lateral fault system that was active at depths of 8–11 km within the transpressive Late-Cretaceous Sierran magmatic arc. The fault zone cuts four granitic plutons and is composed of three steeply dipping northeast- and southwest-striking noncoplanar segments that nucleated and grew along preexisting cooling joints. The fault core is bounded by subparallel fault planes that separate highly fractured epidote-, chlorite-, and quartz-breccias from undeformed protolith. The slip profile along the Gemini fault zone shows that the fault zone consists of three 2–3-km-long segments separated by two ‘zones’ of local slip minima. Slip is highest (131 m) on the western third of the fault zone and tapers to zero at the eastern termination. Slip vectors plunge shallowly west-southwest and show significant variability along strike and across segment boundaries. Four types of microstructures reflect compositional changes in protolith along strike and show that deformation was concentrated on narrow slip surfaces at, or below, greenschist facies conditions. Taken together, we interpret the fault zone to be a segmented, linked fault zone in which geometrical complexities of the faults and compositional variations of protolith and fault rock resulted in nonuniform slip orientations, complex fault-segment interactions, and asymmetric slip-distance profiles.     

青藏高原中部第四纪左旋剪切变形的地表地质证据

在青藏铁路的格尔木—拉萨段进行的活动断裂调查发现,在沱沱河—五道梁之间宽约150km的地段内发育了多条由北西西向次级断层左列分布构成的北西西向和北西向左旋张扭性断裂带,在断裂带之间则发育"S"型的北东向裂陷盆地和雁列分布的菱形裂陷盆地,盆地边界断裂也为左旋张扭性质。上述断裂带和裂陷带主要形成于第四纪,它们构成了宽约150km的不均匀的左旋简单剪切变形域,该变形域的整体活动性较弱,属于弱的不均匀剪切变形域。但其中的二道沟断陷盆地是个例外,该盆地边界断裂的垂直活动速率约为0 5mm/a,左旋活动速率介于0 8～1 0mm/a之间。而在整个左旋剪切变形带累计的左旋走滑速率不会超过6mm/a,它们所调节的昆仑山与唐古拉山之间的地壳南北缩短量也可能仅占总缩短量的15%～30%。上述弱剪切变形域与强烈左旋走滑的昆仑断裂系共同构成了高原中部的左旋剪切变形带,它们在印度板块与欧亚板块强烈碰撞的构造动力学背景下,起着调节青藏高原南北向缩短的重要作用。     

Dextral‐Slip Thrust Faulting and Seismic Events of the Ms 8.0 Wenchuan Earthquake,Longmenshan Mountains,Eastern Margin of the Tibetan Plateau

Abstract: Dextral-slip thrust movement of the Songpan-Garzê terrain over the Sichuan block caused the Ms 8.0 Wenchuan earthquake of May 12, 2008 and offset the Central Longmenshan Fault (CLF) along a distance of ~250 km. Displacement along the CLF changes from Yingxiu to Qingchuan. The total oblique slip of up to 7.6 m in Yingxiu near the epicenter of the earthquake, decreases northeastward to 5.3 m, 6.6 m, 4.4 m, 2.5 m and 1.1 m in Hongkou, Beichuan, Pingtong, Nanba and Qingchuan, respectively. This offset apparently occurred during a sequence of four reported seismic events, EQ1–EQ4, which were identified by seismic inversion of the source mechanism. These events occurred in rapid succession as the fault break propagated northeastward during the earthquake. Variations in the plunge of slickensides along the CLF appear to match these events. The Mw 7.5 EQ1 event occurred during the first 0–10 s along the Yingxiu-Hongkou section of the CLF and is characterized by 1.7 m vertical slip and vertical slickensides. The Mw 8.0 EQ2 event, which occurred during the next 10–42 s along the Yingxiu-Yanziyan section of the CLF, is marked by major dextral-slip with minor thrust and slickensides plunging 25°–35° southwestward. The Mw 7.5 EQ3 event occurred during the following 42–60 s and resulted in dextral-slip and slickensides plunging 10° southwestward in Beichuan and plunging 73° southwestward in Hongkou. The Mw 7.7 EQ4 event, which occurred during the final 60–95 s along the Beichuan-Qingchuan section of the CLF, is characterized by nearly equal values of dextral and vertical slips with slickensides plunging 45°–50° southwestward. These seismic events match and evidently controlled the concentrations of landslide dams caused by the Wenchuan earthquake in Longmenshan Mountains.