汶川8级大地震同震破裂的特殊性及构造意义——多条平行断裂同时活动的反序型逆冲地震事件

汶川地震是有仪器记录以来发生的世界上最大的板内逆冲型地震之一。野外调查表明,沿北东走向的龙门山断裂带上,至少有两条逆冲断裂同时参与汶川地震的同震破裂过程,即北川断裂和安县灌县断裂（彭灌断裂）。倾向北西的高角度北川逆冲断裂上的地表破裂长度大于200 km,可能达225 km。运动方式在南部表现为以北西盘抬升的逆冲为主,往北东转为逆冲右旋走滑,走滑分量与垂向陡坎高度相当,陡坎高度最大值约为11 m。在彭灌断裂上,地表破裂表现为北西盘抬升的近纯逆冲性质的破裂,破裂长度达70 km,陡坎最高达3~3.5 m。汶川地震是世界上第一次明确记录到多条平行断裂参与同震破裂的逆冲型地震,而且因发震断层是龙门山断裂带内部的高角度逆冲断裂,而非断裂带前锋的低角度逆冲断裂,所以汶川地震属于反序型逆冲断裂活动。这与1999年我国台湾7.5级集集地震和2005年克什米尔7.6级地震类似,说明反序型逆冲地震具有普遍性。汶川地震这一震级大、破裂长的逆冲地震事件是对目前流行的青藏高原下地壳流动的变形假说提出的严峻挑战,同时也表明加强青藏高原东缘南北地震带上其他滑动速率较低但同样具有发生大地震可能性的活动断裂的滑动速率和古地震定量研究的紧迫性,因为这一地区人口密度与东部相当,但发生强震的频率更高。     

斜向逆冲断层相关褶皱的正演模型与实例分析

斜向逆冲作用在自然界普遍存在,研究斜向逆冲断层相关褶皱的构造几何学特征,识别断层相关褶皱是否存在斜向逆冲有重要意义。文章采用Trishear 4.5、Gocad以及Trishear3D软件构建一系列不同滑移量的断层转折褶皱和断层传播褶皱的二维正演剖面,通过连接一系列不同排列方式的二维剖面建立了三种不同逆冲滑移方向的断层转折褶皱和断层传播褶皱的假三维模型,通过不同假三维模型的比较分析来探讨斜向逆冲断层相关褶皱的构造几何学特征。研究发现,斜向逆冲断层相关褶皱区别于正向逆冲断层相关褶皱的特征主要有两点：① 正向逆冲断层相关褶皱层面等高线图上的最高点与后翼等高线中点的连线以及水平切面上的核心点与后翼中点的连线方向均与断层走向垂直,而斜向逆冲断层相关褶皱的最高点以及核心点与后翼中点的连线方向均与断层走向斜交,并且最高点与后翼等高线中点的连线方向或者核心点与后翼中点的连线方向均与逆冲滑移方向一致;② 在褶皱平行断层走向纵剖面上,正向逆冲断层相关褶皱各个层面最高点的连线是直立的,而斜向逆冲断层相关褶皱各个层面最高点的连线发生倾斜。通过这两个特征可以判别褶皱是否存在斜向逆冲以及逆冲的方向。将模型分析结果运用到四川盆地西南部三维地震勘探资料所覆盖的邛西背斜和大兴西背斜的实例中。研究结果表明,两个背斜均存在右旋斜向逆冲,逆冲方向与各自断层走向的夹角均为70°左右,邛西背斜和大兴西背斜的逆冲方向分别是NE79°和NE77°左右,这与龙门山南段晚上新世以来的主应力方向以及反演的汶川地震最大主应力方向一致。     

Evidence for pre-seismic crustal movement in China and Japan

Some recent Chinese observations on precursory crustal movements detected by geodetic measurements such as short-range levelling or base-line survey across the active fault corroborate the three gamma phases model of Fujita and Fujii: Gamma 1 phase is pre-slip along the imminent faulting, gamma 2 is coseismic slip and gamma 3 is post-slip along the main or subsidiary faulting after an earthquake. Typical of these are the results of short-range levelling across the earthquake fault that ruptured during the 1976 Tangshan earthquake (M_s= 7.8). Associated with the occurrence of the Tangshan earthquake or the subsequent Ninghe earthquake (M_s= 6.9), remarkable gamma 2 phases were observed. At this same location abnormal gamma 1 or gamma 3 phases were detected from the Lulong 1982 earthquake (M_s= 6.2) that originated from the north end of the Tangshan Fault and was one of the aftershocks of the Tangshan earthquake. These phases are interpreted by analogy with the recent experimental results of stick-slip motion of the frictional sliding of rock.     

关于汶川地震发震机制

2008年5月12日汶川8级地震的发震断层是四川龙门山逆冲带的前锋灌县—安县断层,或此断层附近新产生相同产状的断层。发震断层走向NE倾向NW,逆冲兼右行平移。汶川地震的发震机制是印藏陆-陆碰撞后,印度次大陆活塞状嵌入欧亚板块造成青藏高原东部向SEE方向近水平挤压,在龙门山冲断带前锋向东南逆冲到四川盆地,构造应力积累和释放的结果。汶川地震演示了一个青藏高原东缘龙门山隆起的构造模型,即其经由龙门山冲断带的地壳冲断作用和缩短作用而隆升。这与Burchfiel的模型不同,该模型认为龙门山上升是由于韧性下地壳流受到四川盆地高强度地壳阻挡而上涌所致。这两种模型可能各有其适用时间阶段,然而本文的模型是不可或缺的,因汶川地震已显示了它的真实性。     

Source mechanisms and tectonic significance of historical earthquakes along the nankai trough,Japan

Two recent and three historical earthquakes which occurred along the Nankai trough, marking the northern plate boundary between the Philippine Sea and the Asian Plate, are studied mainly on the basis of the data of crustal deformations and tsunami waves. These earthquakes are the 1946 Nankaido, the 1944 Tonankai, the 1854 Ansei I, II and the 1707 Hoei earthquakes. They are all interpreted as low-angle thrust faults at the plate boundary, with the oceanic side underthrusting northwestward against southwestern Japan. The fault parameters of the historical earthquakes are assumed here to be common to those of the recent two earthquakes, except for the magnitude of dislocation.The entire fault region, which extends for 530 km from western Shikoku Island in the west to the Tokai district in the east, is divided into four fault planes, which are denoted the planes A, B, C and D, from west to east, respectively. Then, the five earthquakes may be attributed to the planes A, B, C and D, in the following manner: the Nankaido earthquake, A + B; the Tonankai earthquake, C; the Ansei II earthquake, A + B; the Ansei I earthquake, C + D; and the Hoei earthquake, A + B + C + D.The latest cycle of earthquake migration seems incomplete as proved by the recent inactivity in D. Consequently, the future major earthquake next to occur is expected there, off the Tokai district. Eight further ancient earthquakes from A.D. 684 to 1605 are also discussed. Taking the results of the foregoing studies into consideration, their sequence is well interpreted by the four migration cycles. Topographical data, tilt of coastal terraces and location of hinge lines, prove that the thrusting has continued all along the extension of the Nankai trough for at least 300,000 years.     

塔西北柯坪剪切挤压构造

塔里木西北的柯坪地区存在着再变形的逆冲岩席。研究表明塔里木盆地西北边界断层－阿合奇断层为一巨型左行走滑断层。它在新生代的总走滑量达３０４ｋｍ,具有与塔里木盆地东南边界阿尔金断层相同量级的走滑量。阿合奇断层与阿尔金断层造成了阿合奇－西昆仑－西南塔里木－阿尔金断层剪切挤压构造系统。     

Seismicity wedge beneath the Upper Rhine Graben due to backwards Alpine push?

The distribution of hypocentres in the Upper Rhine Graben area is re-examined, and discussed with respect to the present day tectonic framework. Most earthquakes occur within a N60° striking wedge, located on top of a Moho dome. This wedge is limited by the surface and at depth, by a plane which, in the south of the dome, coincides with the SE dipping Conrad discontinuity. In depth, the seismicity shows a normal distribution the maximums of which are located on a surface dipping 6° towards SE, parallel to the south-eastward dipping Conrad and Moho. This surface outcrops along the north-western edge of the uplifted crystalline Vosges and Black-Forest. The main shocks in earthquake swarms in the region often occur in the vicinity of this surface and along pre-existing N–S to NE–SW Variscan or Tertiary faults and show focal mechanisms of strike-slip. In contrast, part of the aftershocks show focal mechanisms of reverse faulting associated with SE–NW striking compression. The seismic wedge and the north-westward rising seismic surface suggest initiation of crustal ramp, which starts at the south-eastern rim of the Conrad dome and which may become a thrust plane if SE–NW compression continues. In the south-eastern edge of the graben and above the south-eastern ridge of the Moho dome, where evidences for extension have been found, we identify clustering of hypocentres along a surface that strikes N150°, parallel to the main compression and dipping towards NE. Dominant normal faulting mechanisms along this surface suggests initiation of a normal, probably listric fault. At depth, the onset of the future fault plane is located on top of the NW–SE striking ridge of the lower crust and Moho, which act as a an indenter. In addition to thrusting of the whole wedge towards NW, increasing of NW–SE compression would lead to the formation of a half graben at the place of the present Sierentz depression.     

Characteristics of the 1912 co-seismic rupture along the North Anatolian Fault Zone (Turkey): implications for the expected Marmara earthquake

Evidence of right‐lateral offsets associated with the 1912 earthquake (M_w 7.4) along the North Anatolian Fault (Gaziköy–Saros segment) allow us to survey (using DGPS) the co‐seismic and cumulative slip distribution. The damage distribution and surface breaks related with the earthquake show an elongated zone of maximum intensity (X MSK) parallel to the fault rupture on land but this may extend offshore to the north‐east and south‐west. Detailed mapping of the fault using topographic maps and aerial photographs indicates the existence of pull‐apart basins and pressure ridges. At several localities, the average 1912 offset along strike is 3.5–4 m and cumulative slip is 2–6 times that of individual movement. The fault rupture geometry and slip distribution suggest the existence of three subsegments with a combined total length of 110–120 km, a fault length and maximum slip similar to those of the 1999 Izmit earthquake. The amount of slip at the north‐easternmost section and in the coastal region of the Sea of Marmara reaches an average 4 m, thereby implying the offshore extension of the 1912 rupture. The results suggest that the 1912 event generated up to 150 km of surface faulting, which would imply a M_w 7.2–7.4 earthquake and which, added with rupture lengths of the 1999 earthquakes, help to constrain the remaining seismic gap in the Sea of Marmara.     

四川芦山2013年Ms7.0地震发震构造初步研究

2013年4月20日8时2分,四川龙门山断裂带的雅安芦山发生Ms7.0级地震,震中位于芦山县太平镇和双石镇之间,震源深度13～14km,震中最大烈度达IX级。震中区野外调查发现,尽管房屋建筑损坏较严重,但这次地震没有产生明显的地表破裂构造,仅见少量的地裂缝和喷砂冒水现象。高分辨率遥感图像解译、主余震分布、震源机制解等综合分析认为,该地震是龙门山断裂带西南段一次独立的破裂事件,属于逆冲型地震,沿双石-大川断裂中南段发生破裂,主破裂面西倾,倾角33°～43°,推断芦山地震与龙门山构造带底部滑脱带(13～19km)断坡构造活动有关。历史上,沿双石-大川断裂发生至少2次Ms6～6.5级地震,由此认为芦山地震是龙门山断裂带西南段特征型地震,与汶川地震不同。原地地应力测量和监测数据表明这是汶川地震后龙门山断裂带西南段应力释放的结果。     

Crustal structure, fault segmentation, and activity of the Median Tectonic Line in Shikoku, Japan

We conducted a seismic tomographic analysis to estimate the crustal structure beneath the Shikoku and Chugoku regions in Japan. The Philippine Sea slab (PHS slab) subducts continuously in a SE–NW direction beneath this region, and the crustal structure is complex. Furthermore, the Median Tectonic Line (MTL), one of the longest and most active arc-parallel fault systems in Japan (hereafter, the MTL active fault system), is located in this area, and the right-lateral strike–slip movement of this fault system is related to the oblique subduction of the PHS slab. The MTL active fault system has ruptured repeatedly during the last 10 000 years, and has high seismic potential. Our tomographic analysis clarified the heterogeneous crustal structure along the MTL active fault system. This fault system in Shikoku can be divided into two segments, an east segment and a west segment, on the basis of the velocity structure. This segmentation model is consistent with other such models that have been determined from geological and geomorphological data such as fault geometry, slip rate, and faulting history. This consistency suggests that the surface characteristics of the MTL active fault system are related to structural properties of the crust. In particular, a prominent low-velocity (low-V) zone is present in the lower crust beneath the east segment. Our tomographic images show that the lower crust structure beneath the east segment is obviously different from that of the other segment. Furthermore, this low-V zone may indicate the presence of fluid, possibly related to dehydration of the PHS slab. As the presence of fluid in the lower crust affects the activity of the fault, stress accumulation and the fault failure mechanism may differ between the two segments of the MTL active fault system.     

Earthquake mechanisms and tectonics in the Assam-Burma region

Eleven new focal mechanisms from earthquakes in the Assam-Burma region have been determined using P-wave first-motion directions reported in the Bulletins of the International Seismological Centre (Edinburgh). Out of them, eight mechanisms indicate thrust faulting, two normal faultings and one strike-slip faulting. In the thrust type of mechanism solutions, sense of motion on the shallow dipping of the two nodal planes is consistent with underthrusting beneath the arc-like mountain ranges. Seismic slip vectors strike in almost northerly direction along the eastern Himalayas and in almost easterly direction along the Burmese arc. A predominance of thrust faulting is consistent with geological evidences of thrusting and uplift in the Himalayas and the Assam-Burma region.     

Two Phases of Cenozoic Deformation in Northeastern Tibet: Thrusting Followed by Strike-Slip Faulting

TWO PHASES OF CENOZOIC DEFORMATION IN NORTHEASTERN TIBET: THRUSTING FOLLOWED BY STRIKE-SLIP FAULTING     

塔里木盆地西部的新生代断裂活动

研究区以塔里木盆地西部的巴楚隆起为核心,包括其南侧的麦盖提斜坡和北侧的北部坳陷(阿瓦提凹陷)。这里新生代断裂异常发育。以鸟山-古董山地区为重点,通过精细的地震剖面解释,在塔里木盆地西部识别出5期新生代断裂活动,分别发生于:白垩纪末-古近纪初(ca.65Ma)、中新世末-上新世初(ca.5Ma)、上新世末-更新世中期(ca.3~1.5Ma)、更新世中-晚期(ca.1~0.1Ma)和全新世(ca.0.01Ma)。中寒武统和古近系膏盐层构成了研究区的2个主滑脱面。ca.65Ma的冲断受控于基墨里中间大陆与亚洲大陆碰撞的远程效应;ca.5Ma、ca.3~1.5Ma、ca.1~0.1Ma和ca.0.01Ma的断裂活动(包括滑脱断层、冲断层、走滑断层和正断层)均受控于印度-亚洲碰撞的远程效应。晚新生代(中新世以来)的断裂活动以脉动式冲断为特征,两次相邻脉动式冲断之间为时间不等的短暂的构造间歇期;间歇期构造平静,甚至可以形成正断层。     

Seismogenic structures along continental convergent zones: from oblique subduction to mature collision

We summarize seismogenic structures in four regions of active convergence, each at a different stage of the collision process, with particular emphases on unusual, deep-seated seismogenic zones that were recently discovered. Along the eastern Hellenic arc near Crete, an additional seismogenic zone seems to occur below the seismogenic portion of the interplate thrust zone—a configuration found in several other oblique subduction zones that terminate laterally against collision belts. The unusual earthquakes show lateral compression, probably reflecting convergence between the subducting lithosphere's flank and the collision zone nearby. Along oblique zones of recent collision, the equivalence between space and time reveals the transition from subduction to full collision. In particular, intense seismicity beneath western Taiwan indicates that along the incipient zone of arc–continent collision, major earthquakes occur along high-angle reverse faults that reach deep into the crust or even the uppermost mantle. The seismogenic structures are likely to be reactivated normal faults on the passive continental margin of southeastern China. Since high-angle faults are ineffective in accommodating horizontal motion, it is not surprising that in the developed portion of the central Taiwan orogen (<5 Ma), seismogenic faulting occurs mainly along moderate-dipping (20–30°) thrusts. This is probably the only well-documented case of concurrent earthquake faulting on two major thrust faults, with the second seismogenic zone reaching down to depths of 30 km. Furthermore, the dual thrusts are out-of-sequence, being active in the hinterland of the deformation front. Along the mature Himalayan collision zone, where collision initiated about 50 Ma ago, current data are insufficient to distinguish whether most earthquakes occurred along multiple, out-of-sequence thrusts or along a major ramp thrust. Intriguingly, a very active seismic zone, including a large (M_w=6.7) earthquake in 1988, occurs at depths near 50 km beneath the foreland. Such a configuration may indicate the onset of a crustal nappe, involving the entire cratonic crust. In all cases of collision discussed here, the basal decollement, a key feature in the critical taper model of mountain building, appears to be aseismic. It seems that right at the onset of collision, earthquakes reflect reactivation of high-angle faults. For mature collision belts, earthquake faulting on moderate-dipping thrust accommodates a significant portion of convergence—a process involving the bulk of crust and possibly the uppermost mantle.     

Earthquake evidence for compressive stress in the southeast Australian crust

Earthquakes in SE Australia are usually caused by compressive stresses acting in the crust, and are associated with steeply dipping faults. Sometimes the faulting is predominantly strike‐slip, as for the Bowning earthquakes of 1977 and some of the Dalton/Gunning earthquakes; and sometimes it is high‐angle thrust faulting, as for the 1961 Robertson and 1973 Picton earthquakes. No surface expression of the faults associated with any recent earthquakes in SE Australia has been reported.

The directions of the pressure axes, from all the earthquakes for which focal mechanisms have been determined, do not form a consistent pattern. This suggests that the faulting associated with earthquakes in SE Australia is dominated by the geometry of pre‐existing crustal faults or zones of weakness.

In situ stress measurements have not been made near the epicentral areas of the larger recent earthquakes, because of the absence of competent, near‐surface rocks coupled to the crust. However, in the western part of the Lachlan Fold Belt the in situ stress results indicate that the maximum pressure axis is approximately E‐W. The evidence from the focal mechanisms does not preclude the persistence of this stress regime farther to the east, and a regional compressive stress in the crust with an azimuth of about 120° is consistent with most of the earthquake focal mechanisms and the in situ stress measurements throughout SE Australia.     

阿尔金断裂双层花状构造的厘定

阿尔金断裂带主要经历了两阶段的构造演化,形成不同层次且具有不同特点的典型花状构造型式。宏观断层构造组合、断层擦痕产状的变化以及节理组合充分反映出晚期阶段浅部层次花状构造的存在,早期阶段中部层次花状构造由反映区域应变型式的糜棱状岩石中其它应变标志性组合的存在表现出来。结合ArAr记年与矿物对测温结果,表明前者形成于距今46Ma,是一次规模最强烈的变形叠加事件;后者由距今91～89Ma的韧性走滑运动形成,并伴有断层带内的低级变质作用(温度270～500℃,压力0.05～0.45GPa)。两阶段双层花状构造并存充分说明区域性走滑作用过程及其调节作用的多阶段性与继承性,其出现部分地调节了由印度板块和欧亚板块陆—陆碰撞过程中区域性地壳物质的向东蠕散和隆升过程。     

Compressive Tectonics around Tibetan Plateau Edges

Various earthquake fault types, mechanism solutions, stress field, and other geophysical data were analyzed for study on the crust movement in the Tibetan plateau and its tectonic implications. The results show that numbers of thrust fault and strike-slip fault type earthquakes with strong compressive stress near NNE-SSW direction occurred in the edges around the plateau except the eastern boundary. Some normal faulting type earthquakes concentrate in the Central Tibetan plateau. The strikes of fault planes of thrust and strike-slip faulting earthquakes are almost in the E-W direction based on the analyses of the Wulff stereonet diagrams of fault plane solutions. This implies that the dislocation slip vectors of the thrust and strike-slip faulting type events have quite great components in the N-S direction. The compression motion mainly probably plays the tectonic active regime around the plateau edges. The compressive stress in N-S or NE-SW directions predominates earthquake occurrence in the thrust and strike-slip faulting event region around the plateau. The compressive motion around the Tibetan plateau edge is attributable to the northward motion of the Indian subcontinent plate. The northward motion of the Tibetan plateau shortened in the N-S direction encounters probably strong obstructions at the western and northern margins.     

以精定位背景地震活动与震源机制解研究郯庐断裂带中南段现今活动习性

郯庐断裂带是中国东部板内一条规模最大的强构造变形带与地震活动断裂带,其断裂结构与历史地震活动性具明显的分段活动性。文中通过沿郯庐断裂带中南段的历史地震活动性、精定位背景地震活动性与震源机制解分析,讨论了断裂带的深部几何结构与现今活动习性。现今地震活动在中段主要沿1668年郯城MS 8地震破裂带线性分布,线性条带在泗洪-诸城间延伸约340 km长,为1668年地震长期缓慢衰减的余震序列活动。大震地表破裂遗迹与精定位地震分布都揭示出郯庐断裂带中段的两条全新世活动断裂昌邑-大店断裂与安丘-莒县断裂以右阶斜列的形式共同参与了1668年郯城MS 8地震破裂。精定位震源剖面刻画出两条断裂结构面呈高角度相背而倾,其中昌邑-大店断裂倾向SE,安丘-莒县断裂倾向NW,两条断裂在深部没有合并汇聚。余震活动所代表的1668年地震震源破裂带是郯庐断裂带中现今尚未闭锁的安全段落,对应于高b值段。而未发生破裂的安丘以北段,小震活动不活跃,b值低,现今可能已成为应力积累的闭锁段。震源机制解揭示的断裂应力状态在中段以NE向主压应力为主,表现为右旋走滑活动性质,且存在少量正断分量,南段转为以NEE至近EW向为主,存在少量的逆冲分量。在中段与南段的转折处宿迁-嘉山段,主压应力方向垂直断裂带走向呈NWW向,反映出局部以挤压为主的应力特征,其中泗洪-嘉山段也是历史地震未破裂段,现今小震活动不活跃,因此该段可能更易于应力积累。精定位小震活动在郯庐断裂与北西向断裂相交汇处聚集,反映出北西向断裂的新活动性,以及郯庐断裂带现今的逆冲作用。在断裂带南端,精定位背景地震活动沿与其相交汇的襄樊-广济断裂带东段呈北西向线性分布,表明了该段的现今活动性。     

Structural evolution of the contact between two Penninic nappes (Zermatt-Saas zone and Combin zone,Western Alps) and implications for the exhumation mechanism and palaeogeography

The boundary zone between two Penninic nappes, the eclogite-facies to ultrahigh-pressure Zermatt-Saas zone in the footwall and the blueschist-facies Combin zone in the hanging wall, has been interpreted previously as a major normal fault reflecting synorogenic crustal extension. Quartz textures of mylonites from this fault were measured using neutron diffraction. Together with structural field observations, the data allow a refined reconstruction of the kinematic evolution of the Pennine nappes. The main results are: (1) the contact is not a normal fault but a major thrust towards northwest which was only later overprinted by southeast-directed normal faulting; (2) exhumation of the footwall rocks did not occur during crustal extension but during crustal shortening; (3) the Sesia-Dent Blanche nappe system originated from a continental fragment (Cervinia) in the Alpine Tethys ocean, and the Combin zone ophiolites from the ocean basin southeast of Cervinia; (4) out-of-sequence thrusting played a major role in the tectonic evolution of the Penninic nappes. An erratum to this article can be found at     

A crustal thrust system in an intracratonic tectonic environment

An intracratonic thrust belt, developed during the early Carboniferous in central Australia, deformed the Amadeus Basin and its basement, the Arunta Block. This belt is characterized by a marked structural asymmetry (vergence) and by the deposition of a thick molasse basin on the foreland. A review of existing field data shows that décollement tectonics produced folding, thrusting, faulting and back-faulting of the sedimentary sequence. Thin-skinned tectonics extend into the basement to produce recumbent folds and têtes plongeantes of nappe structures rooted in steeply dipping mylonite zones of greenschist to amphibolite grade. Minimum horizontal shortening displacements are 50–100 km resulting in a 50–70% contraction of the upper part of the basement. The structures and shortening are best explained by a crustal duplex, characterized by a crustal-scale thrust system, i.e. a sole thrust and imbricate faults, responsible for an isostatic bending of the underthrust slab. The observed Bouguer anomaly profiles support this crustal model. The dynamic evolution of this thrust belt on the scale of the crust is of thin-skin type.