Possible flexural accommodation on the eastern edge of the Altiplano in relation to focussed erosion in the Rio La Paz drainage system

It has been proposed that erosional unloading by focused incision can be accommodated by flexural rebound, which in turn, potentially exerts a positive feedback on erosion. This paper presents morphometric data from the La Paz drainage basin in the Bolivian Andes to illustrate the effects of such a feedback mechanism between erosion and crustal bending. The Rio La Paz is a prominent system with the headwaters on the Altiplano and a trunk stream segment that cuts across the Cordillera Real thereby connecting large portions of the Altiplano with the Amazon Basin. The flexural feedback model explains why all drainages beyond the watershed disperse their waters to the Altiplano. It also provides an explanation for the presence of the highest peaks just next to the location where the La Paz River cuts into the bedrock across the Cordillera Real.     

青藏高原巴塘断裂带地震滑坡危险性预测研究

全新世以来青藏高原东部巴塘断裂带活动强烈,地形地貌和地质构造复杂,历史地震频发,并诱发大量滑坡灾害。基于巴塘断裂带地震滑坡长期防控的需要,在分析区域地质灾害成灾背景和发育分布特征的基础上,采用Newmark模型完成了巴塘断裂带50年超越概率10%的潜在地震滑坡危险性预测评价,并完成地震滑坡危险性区划。结果表明:巴塘断裂带及其临近的金沙江断裂带区域、金沙江及其支流沿岸具有较高的潜在地震滑坡危险性,地震滑坡危险区具有沿断裂带和大江大河等峡谷区分布的总体趋势,受活动断裂和地形地貌影响显著;距离断层越近、坡度越大的斜坡,地震滑坡危险性越高;规划建设中的川藏铁路经巴塘县德达乡、白玉县沙马乡,向西北延伸,跨越金沙江,可以穿越较少的地震滑坡危险区,金沙江水电工程规划建设需加强潜在地震滑坡危害研判及防控。巴塘断裂带潜在地震滑坡危险性评价结果可为区域城镇开发和重大工程规划建设的地震滑坡长期防控提供科学参考。     

Seismic profiles across the middle Tan-Lu fault zone in Laizhou Bay,Bohai Sea,eastern China

The stratigraphic architecture, structure and Cenozoic tectonic evolution of the Tan-Lu fault zone in Laizhou Bay, eastern China, are analyzed based on interpretations of 31 new 2D seismic lines across Laizhou Bay. Cenozoic strata in the study area are divided into two layers separated by a prominent and widespread unconformity. The upper sedimentary layer is made up of Neogene and Quaternary fluvial and marine sediments, while the lower layer consists of Paleogene lacustrine and fluvial facies. In terms of tectonics, the sediments beneath the unconformity can be divided into four main structural units: the west depression, central uplift, east depression and Ludong uplift. The two branches of the middle Tan-Lu fault zone differ in their geometry and offset: the east branch fault is a steeply dipping S-shaped strike-slip fault that cuts acoustic basement at depths greater than 8 km, whereas the west branch fault is a relatively shallow normal fault. The Tan-Lu fault zone is the key fault in the study area, having controlled its Cenozoic evolution. Based on balanced cross-sections constructed along transverse seismic line 99.8 and longitudinal seismic line 699.0, the Cenozoic evolution of the middle Tan-Lu fault zone is divided into three stages: Paleocene–Eocene transtension, Oligocene–Early Miocene transpression and Middle Miocene to present-day stable subsidence. The reasons for the contrasting tectonic features of the two branch faults and the timing of the change from transtension to transpression are discussed.     

Seismic profiles across the middle Tan-Lu fault zone in Laizhou Bay, Bohai Sea, eastern China

The stratigraphic architecture, structure and Cenozoic tectonic evolution of the Tan-Lu fault zone in Laizhou Bay, eastern China, are analyzed based on interpretations of 31 new 2D seismic lines across Laizhou Bay. Cenozoic strata in the study area are divided into two layers separated by a prominent and widespread unconformity. The upper sedimentary layer is made up of Neogene and Quaternary fluvial and marine sediments, while the lower layer consists of Paleogene lacustrine and fluvial facies. In terms of tectonics, the sediments beneath the unconformity can be divided into four main structural units: the west depression, central uplift, east depression and Ludong uplift. The two branches of the middle Tan-Lu fault zone differ in their geometry and offset: the east branch fault is a steeply dipping S-shaped strike-slip fault that cuts acoustic basement at depths greater than 8 km, whereas the west branch fault is a relatively shallow normal fault. The Tan-Lu fault zone is the key fault in the study area, having controlled its Cenozoic evolution. Based on balanced cross-sections constructed along transverse seismic line 99.8 and longitudinal seismic line 699.0, the Cenozoic evolution of the middle Tan-Lu fault zone is divided into three stages: Paleocene–Eocene transtension, Oligocene–Early Miocene transpression and Middle Miocene to present-day stable subsidence. The reasons for the contrasting tectonic features of the two branch faults and the timing of the change from transtension to transpression are discussed.     

Cretaceous to Cenozoic sequential kinematics in the forearc–arc transition: effects of changing oblique plate convergence and the San Andreas system with implications for the La Paz fault (southern Baja California, Mexico)

We studied metasediments and mylonitic arc granitoids from the forearc–arc transition of southern Baja California, Mexico. Thin section analyses and field evidence show that metamorphism of the forearc–arc transition is of the high T/P active margin type. The heat was provided by Cretaceous arc intrusions. Field observations and thin section analyses, including the time/temperature deformation path, demonstrate that the study area was first affected by dextral, ductile shearing followed by ductile, sinistral, possibly transpressive strike-slip parallel to the magmatic arc during the Cretaceous. Both intervals are related to changing oblique plate convergence and, thus, identified as trench-linked strike-slip effects. The geometric relationship between arc-dipping foliation, stretching lineation and shear sense indicates that the arc may have been pressed onto the rocks of the study area during sinistral shearing. The sinistral interval lasted up until regional cooling (Early Cenozoic?). Because the La Paz fault is closely associated with the forearc–arc transition, it must have the same Cretaceous to Early Cenozoic kinematic history. The northern segment of the La Paz fault is a modern, brittle, strike-slip fault interpreted as a dextral synthetic fault of the San Andreas system which opened the Gulf of California (Mar de Cortés/Golfo de California). We found no evidence for Miocene Basin and Range extension.     

Probabilistic seismic hazard assessment for Lake Van basin,Turkey

The seismic hazard for the Lake Van basin is computed using a probabilistic approach, along with the earthquake data from 1907 to present. The spatial distribution of seismic events between the longitudes of 41–45° and the latitudes of 37.5–40°, which encompasses the region, indicates distinct seismic zones. The positions of these zones are well aligned with the known tectonic features such as the Tutak-Çald?ran fault zone, the Özalp fault zone, the Geva? fault zone, the Bitlis fault zone and Karl?ova junction where the North Anatolian fault zone and East Anatolian fault zone meet. These faults are known to have generated major earthquakes which strongly affected cities and towns such as Van, Mu?, Bitlis, Özalp, Muradiye, Çald?ran, Erci?, Adilcevaz, Ahlat, Tatvan, Geva? and Gürp?nar. The recurrence intervals of M _s ≥ 4 earthquakes were evaluated in order to obtain the parameters of the Gutenberg–Richter measurements for seismic zones. More importantly, iso-acceleration maps of the basin were produced with a grid interval of 0.05 degrees. These maps are developed for 100- and 475- year return periods, utilizing the domestic attenuation relationships. A computer program called Sistehan II was utilized to generate these maps.     

Discontinuity of the Mozumi–Sukenobu fault low-velocity zone, central Japan, inferred from 3-D finite-difference simulation of fault zone waves excited by explosive sources

We delineate shallow structures of the Mozumi–Sukenobu fault, central Japan, using fault zone waves generated by near-surface explosions and detected by a seismometer array. Two explosive sources, S1 and S2, were placed at a distance of about 2 km from the array, and the other two, S3 and S4, were at a distance of about 4 km. Fault zone head waves and fault zone trapped waves following direct P wave arrivals were clearly identified in the seismograms recorded by a linear seismometer array deployed across the fault in a research tunnel at a depth of 300 m. Synthetic waveforms generated by a 3-D finite-difference (3-D FD) method were compared with observed fault zone waves up to 25 Hz. The best fitting model indicates a 200-m-wide low-velocity zone extending at least to shot site S1 located 2 km east of the seismic array with a 20% decrease in the P wave velocity relative to the wall rock. The width of the low-velocity zone is consistent with the fault zone defined by direct geological observation in the research tunnel. However, the low-velocity zone should disappear just to the east of the site S1 to explain the observed fault zone waves for shot S3 and S4 located 4 km east of the seismometer array. Yet the observation and the simulation show notable trapped wave excitation even though shots S3 and S4 are outside the fault zone. These results indicate that (1) the effective waveguide for seismic waves along the fault does not exist east of source site S1 although the surface traces of the fault are observed in this region, and (2) considerable trapped waves can be excited by sources well outside the fault zone. These results highlight the along-strike variability in fault zone structure.     

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

Abstract: 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 vertical offset of the co-seismic fault ranges 2.5–3.0 m along the Yingxiu-Beichuan co-seismic fault, and 1.5–1.1 m along the Doujiangyan-Hanwang fault. Movement of the co-seismic fault presents obvious segmented features along the active fault zone in central Longmen Mts. For instance, in the section from Yingxiu to Leigu town, thrust without evident slip occurred; while from Beichuan to Qingchuan, thrust and dextral strike-slip take place. Main movement along the front Longmen Mts. shows thrust without slip and segmented features. The area of earthquake intensity more than IX degree and the distribution of secondary geological hazards occurred along the hanging wall of co-seismic faults, and were consistent with the area of aftershock, and its width is less than 40km from co-seismic faults in the hanging wall. The secondary geological hazards, collapses, landslides, debris flows et al., concentrated in the hanging wall of co-seismic fault within 0–20 km from co-seismic fault.     

汶川地震断裂作用研究新认识

2008年汶川地震后,人们不得不思考问题是:大地震是如何发生的?下一次大地震什么时候发生?也就是涉及地质学家和地球物理学家一直未解决的科学问题:断层是如何破裂的?震后断裂是如何愈合的?我们试图通过对汶川地震断裂带结构、断裂摩擦行为和断裂愈合过程的研究来回答这些问题。本文将介绍通过对地表露头和汶川地震断裂科学钻探一号孔(WFSD)岩心中汶川地震主滑移带的详细研究,以及钻孔中长期温度监测来分析有关汶川地震断裂动态弱化和摩擦行为,并结合钻孔中长期水文监测计算所得断裂带渗透率变化,分析震后断裂愈合过程,进而探讨和认识汶川地震断裂作用所涉及的上述问题。经过详细研究,确定了汶川地震断裂带(映秀—北川断裂带)宽105~240 m、具有五个不同断裂岩组合的内部结构,是一条经常发生大地震、具多种弱化机制的断裂带;发现了汶川地震不仅具有同震石墨化作用,而且测量到目前世界上最低的动态摩擦系数(≤0.02),同时首次记录到大地震后断裂快速愈合信息。这些研究结果不仅直接回答了一直困扰在地震地质和地震物理学领域几十年的关键问题,而且对完善地震断裂理论和认识汶川地震机制具有极其重要的意义,为防震减灾提供了理论依据。     

Fault zone geometry of a mature active normal fault: A potential high permeability channel (Pirgaki fault, Corinth rift, Greece)

We present results from petrophysical analysis of a normal fault zone with the aim of defining the flow pathways and their behavior during seismic and interseismic periods. Data are obtained on porosity geometry, strain structure and mineralogy of different domains of a normal fault zone in the Corinth rift. Data point out a close relationship between mineralogy of the clayey minerals, porosity network and strain structures and allow definition of a macroscopic anisotropy of the flow parameters with a strong control by microscopic ultracataclasite structures. The Pirgaki fault zone, developed within pelagic limestone, has a sharp asymmetric porosity profile, with a high porosity volume in the fault core and in the damage zone of the hanging wall. From porosity volumes and threshold measurements, a matrix permeability variation of 6 orders of magnitude could be expected between the protolith and the fault core. Modifications of this pathway during seismic and interseismic phases are depicted. Healing of cracks formed during seismic slip events occurred in the fault core zone and the porous network in the damage zone is sealed in a second step. The lens geometry of the fault core zone is associated with dissolution surfaces and open conduits where dissolved matter could move out of the fault core zone. These elementary processes are developed in particular along Riedel's structures and depend on the orientation of the strain surfaces relative to the local stress and depend also on the roughness of each surface type. P-surfaces are smooth and control shearing process. R-surfaces are rough and present two wavelengths of roughness. The long one controls localization of dissolution surfaces and conduits; the short one is characteristic of dissolution surfaces. The dissolved matter can precipitate in the open structures of the hanging wall damage zone, decreasing the connectivity of the macroscopic conduit developed within this part of the fault zone.     

塔里木盆地巴楚隆起西段边界断层联接及变形特征

在前人研究的基础上, 利用最新的地震和钻井资料, 分析了巴楚隆起西段与麦盖提斜坡过渡区色力布亚、康塔库木两条边界断层的构造特征.色力布亚和康塔库木断层均表现为基底卷入型逆冲断层, 两条断层之间通过软联接的形式发生相互作用.色力布亚断层沿其走向往南东方向位移量逐渐减小并终止, 康塔库木断层沿其走向往南东方向位移量逐渐增大.构造转换带传递了色力布亚断层和康塔库木断层的位移量, 使巴楚隆起西段边界断层总位移量保持一致.该转换带属于同向倾斜型构造转换带, 现今处于转换带复杂阶段.转换带内发育次级断层和走向裂缝, 有利于形成裂缝型油气藏, 也是油气运移的有利通道, 是巴楚地区未来油气勘探的重要目标.      

Imaging the heterogeneous source area of the 2003 M6.4 northern Miyagi earthquake, NE Japan, by double-difference tomography

A shallow M6.4 inland earthquake occurred on 26 July 2003 in the northern part of Miyagi Prefecture, northeastern Japan. This earthquake was a typical inland thrust earthquake, a type that is common in NE Japan. We obtained a detailed seismic velocity structure in the focal area of this earthquake by the double-difference tomography method. Arrival-time data came from temporary seismic stations deployed above the mainshock fault plane. Both the P-wave and S-wave velocities in the hanging wall were lower than those in the footwall. Aftershocks were aligned along a zone where the seismic velocity changes rapidly. This is consistent with the interpretation that the 2003 northern Miyagi earthquake occurred along a fault that acted as a normal fault in the Miocene and has been reactivated as a reverse fault under the present compressional stress regime. The large slip area by the main shock rupture (asperity) corresponds to an area with relatively high P- and S-wave velocities. A zone with low V_p/V_s was detected along the aftershock area. One of the possible causes of this low-V_p/V_s zone is the existence of high-aspect-ratio pores that contain water. Hypocenters of the main shock, largest foreshock, and largest aftershock are also located within the low-V_p/V_s zone.     

Late-Cenozoic pull-apart basins, Boconó fault zone, Venezuelan Andes

Several areas along the Boconó fault zone are characterized by elongate, almond-shaped basins containing thick alluvial sequences, mainly of Quaternary age, and bounded by faults with normal Quaternary displacements. These areas are separated by segments characterized by narrow fault traces and right-lateral displacements. The fault-bounded basins are interpreted as pull-apart basins that originated at releasing bends along the fault zone. The size of the La González pull-apart basin suggests that Pliocene (?)-Quaternary right-lateral slip on the Boconó fault zone was of the order of 7–9 km.     

营口—潍坊断裂带对辽东湾坳陷东部凸起的形成及构造分段的控制作用——来自物理模拟实验和断层几何学特征的证据

辽东湾坳陷是渤海湾盆地的重要组成部分,其发育受营口—潍坊断裂带控制。其东支的两条分支断层大体沿其次级构造单元辽东湾东部凸起的两侧延伸。本文应用大量地震测线的构造几何学特征分析,结合构造物理模拟试验,认为辽东湾坳陷东部凸起是在先期存在的低凸起的基础上,在渐新世东营组沉积期因营口—潍坊断裂带的右行走滑形成的。以Lz205测线和Lz185测线之间为界可分为类似的两段,各自经历了一个自南而北的右旋走滑伸展—剪切走滑—右旋走滑挤压的变化过程。这种变化由北东向的基底断裂与由之派生的新生代内的北北东内断裂间的关系造成,在两者的交汇点(如Lz151)或走向发生转折处(如Lz255)右旋走滑作用发生变换,其南侧拉张下沉,正断层发育,表现为右旋走滑伸展,北侧则挤压隆升,表现为右旋走滑挤压。南、北两侧相反的力学性质是走滑活动派生的局部差异应力场造成的。构造物理模拟试验证实了右旋走滑伸展—剪切走滑—右旋走滑挤压的变化过程。     

Analysis of the Wenchuan Ms8.0 Earthquake’s Co-seismic Stress and Displacement Change by Using the Finite Element Method

The variation of in situ stress before and after earthquakes is an issue studied by geologists. In this paper, on the basis of the fault slip dislocation model of Wenchuan Ms8.0 earthquake, the changes of co-seismic displacement and the distribution functions of stress tensor around the Longmen Shan fault zone are calculated. The results show that the co-seismic maximum surface displacement is 4.9 m in the horizontal direction and 6.5 m in the vertical direction, which is almost consistent with the on-site survey and GPS observations. The co-seismic maximum horizontal stress in the hanging wall and footwall decreased sharply as the distance from the Longmen Shan fault zone increased. However, the vertical stress and minimum horizontal stress increased in the footwall and in some areas of the hanging wall. The study of the co-seismic displacement and stress was mainly focused on the long and narrow region along the Longmen Shan fault zone, which coincides with the distribution of the earthquake aftershocks. Therefore, the co-seismic stress only affects the aftershocks, and does not affect distant faults and seismic activities. The results are almost consistent with in situ stress measurements at the two sites before and after Wenchuan Ms8.0 earthquake. Along the fault plane, the co-seismic shear stress in the dip direction is larger than that in the strike direction, which indicates that the faulting mechanism of the Longmen Shan fault zone is a dominant thrust with minor strike-slipping. The results can be used as a reference value for future studies of earthquake mechanisms.     

沂沭断裂带及其近区地震事件地层的时空分布及意义*

沂沭断裂带纵贯山东省中部,属郯庐断裂带中段。在沂沭断裂带及其近区新元古代—新生代的沉积地层中,到目前,已识别出25个地震事件层位。这些地震事件层位的名称取自不同年代或年龄的含地震记录的岩石地层。大多数地震记录是震积岩,少部分为震火山岩,它们的时空分布支持该断裂带生成—活动与发展历史分2个阶段: 古郯庐断裂带阶段(新元古代—古生代)和中—新生代阶段。新元古代初鲁中至苏皖北部NNE向韧性剪切带的形成,沟通了秦岭大别与苏鲁洋间的NEE走向的转换断层,可能是沂沭断裂带或古郯庐断裂带的成因机制。在纵向上,古郯庐断裂带阶段形成了8个地震事件层位,其中5个地震事件层位较密集地分布于南华系至中下寒武统;中—新生代阶段形成了17个地震事件层位,其中12个层位较密集的分布于白垩系—古近系。因此,南华纪—早中寒武世、白垩纪—古近纪分别为2个发展阶段的强地震事件频繁发生时段。在这2个发展阶段,该断裂带地震活动的动力来源不同: 古郯庐断裂带阶段主要源于华北与华南板块的相向运动与碰撞;中—新生代阶段主要源于太平洋板块向欧亚大陆板块下俯冲。在横向上,有15个(占60%)地震事件层位分布在此断裂带内或由该断裂带内向两侧延伸,这体现了沂沭断裂带一直是研究区内发震构造的主体。所有地震事件地层分布于该断裂带纵中轴线两侧150~180,km以内的同沉积盆地,这证明该深大断裂带的两侧近区是强构造地震活动区。作者关于地震事件层位的时空分布的论述和图解,展示了该断裂带自形成以来的地震作用的过程与历史,清晰地勾绘出了这条长期活动地震带的影响范围,这不仅对分析此类深大活动断裂带及其附近由地震引发的软沉积物变形与地震作用具有重要意义,而且对评价此类地震带对地表和建筑物的地震破坏效应也具有重要意义。     

广西防城-灵山断裂带北东支灵山段活动性初探

防城-灵山断裂北东支灵山段为北部湾经济区内地震活动最为频繁的地区。为了查明防城-灵山断裂带北东支灵山段的断裂活动变形特征,尤其是在活动性特征的关键部位控制断裂构造的产状特征,断裂产状深部变化、断裂带规模及展布方向,本文通过遥感解译、地质地貌调查以及浅层地震和地质雷达勘探等物探方法对广西防城-灵山断裂带北东支灵山段的平面展布特征以及活动性进行了初步探讨。结果表明该断裂带呈北东向展布并被多条北西向断裂错断,东西边界分别为灵山断裂和石塘断裂。灵山断裂错断了晚更新世洪积扇阶地,河流流经灵山断裂后发生右旋偏转;石塘断裂表现为宽约250 m的破碎带,次级断裂密集发育。物探结果显示防城-灵山断裂北东支灵山段的东西两支在晚更新世以来均具有较强的活动性。     

西藏羌塘裂谷带假玄武玻璃的发现及其岩石学特征与构造意义

假玄武玻璃是地震过程中断层滑动产生摩擦熔融并快速冷却的产物，被誉为"地震化石"，记录了断层带内地震发生时的物理-化学过程的相关信息，是认识断裂活动历史的重要物质组成。本文以青藏高原腹地羌塘地块SN向正断层——东依布茶卡断层中的假玄武玻璃和碎裂岩等为研究对象，借助偏光显微镜、扫描电镜、粉末X射线衍射分析、原位XRF分析等多种方法对其进行了详细的结构、构造、矿物成分及化学元素分布的分析。野外调查表明该断裂带中假玄武玻璃呈深灰-褐色，与超碎裂岩和碎裂岩相伴生，以断层脉、注入脉和网状脉的形式产出，厚度由几毫米至一厘米不等。显微结构观察显示，假玄武玻璃中残留有摩擦熔融形成的特征构造，如石英的港湾状熔蚀边、蜂窝状气孔构造以及局部发育的长柱状微晶、束状微晶集合体等，指示其为断层滑动摩擦熔融的产物。此外可见假玄武玻璃作为碎屑出现在碎裂岩和角砾岩中，且后期的碎裂岩中包含早期形成的碎裂岩的角砾等，这些特征表明在该断裂带内大地震事件多次重复发生。以该地区正断层初始活动时限13.5Ma、碎裂岩和假玄武玻璃形成深度为10km以下计算，这些断裂岩石从地下深部折返的速率至少为~0.74mm/yr。目前国际上鲜有报导正断层形成的假玄武玻璃，这是我们首次在羌塘地块SN向裂谷正断层中发现熔融成因的假玄武玻璃，对认识区域地震活动及断裂带演化过程具有重要意义。     

Surface Rupture of the 1515 Yongsheng Earthquake in Northwest Yunnan,and Its Seismogeological Implications

The 1515 M7? Yongsheng earthquake is the strongest earthquake historically in northwest Yunnan. However, its time, magnitude and the seismogenic fault have long been a topic of dispute. In order to accurately define those problems, a 1:50000 active tectonic mapping was carried out along the northern segment of the Chenghai–Binchuan fault zone. The result shows that there is an at least 25 km–long surface rupture and a series of seismic landslides distributed along the Jinguan fault and the Chenghai fault. Radiocarbon dating of the ~(14) C samples indicates that the surface rupture should be a part of the deformation zone caused by the Yongsheng earthquake in the year 1515. The distribution characteristics of this surface rupture indicate that the macroscopic epicenter of the 1515 Yongsheng earthquake may be located near Hongshiya, and the seismogenic fault of this earthquake is the Jinguan–Chenghai fault, the northern part of the Chenghai–Binchuan fault zone. Striations on the surface rupture show that the latest motion of the fault is normal faulting. The maximum co–seismic vertical displacement can be 3.8 m, according to the empirical formula for the fault displacement and moment magnitude relationship, the moment magnitude of the Yongsheng earthquake was Mw 7.3–7.4. Furthermore, combining published age data with the ~(14) C data in this paper reveals that at least four large earthquakes of similar size to the 1515 Yongsheng earthquake, have taken place across the northern segment of the Chenghai–Binchuan fault zone since 17190±50 yr. BP. The in–situ recurrence interval of Mw 7.3–7.4 characteristic earthquakes in Yongsheng along this fault zone is possibly on the order of 6 ka.     

The geological background of the May 12, 2008 Ms 8.0 Wenchuan catastrophic earthquake,Longmen Shan,Western China

The Longmen Shan fault zone is located at the particular boundary between the Triassic Songpan-Ganzi orogen of the Qinghai-Tibetan Plateau and the stable Sichuan basin of the Yangtze platform. There are four major active faults and three tectonic nappes in this region. According to an analysis of neotectonics and historical earthquakes, the Longmen Shan fault zone presents a high level of seismic hazard. The rupture system that hosted the Wenchuan earthquake is characterized by thrust and dextral strike-slip movement.