3D modeling of earthquake cycles of the Xianshuihe fault,southwestern China

We perform 3D modeling of earthquake generation of the Xianshuihe fault, southwestern China, which is a highly active strike-slip fault with a length of about 350 km, in order to understand earthquake cycles and segmentations for a long-term forecasting and earthquake nucleation process for a short-term forecasting. Historical earthquake data over the last 300 years indicates repeated periods of seismic activity, and migration of large earthquake along the fault during active seismic periods. To develop the 3D model of earthquake cycles along the Xianshuihe fault, we use a rate- and state-dependent friction law. After analyzing the result, we find that the earthquakes occur in the reoccurrence intervals of 400–500 years. Simulation result of slip velocity distribution along the fault at the depth of 10 km during 2694 years along the Xianshuihe fault indicates that since the third earthquake cycle, the fault has been divided into 3 parts. Some earthquake ruptures terminate at the bending part of the fault line, which may means the shape of the fault line controls how earthquake ruptures. The change of slip velocity and displacement at 10 km depth is more tremendous than the change of the shallow and deep part of the fault and the largest slip velocity occurs at the depth of 10 km which is the exact depth of the seismic zone where fast rupture occurs.     

Finite element modelling of frictional instability between deformable rocks

Earthquakes are recognized as resulting from a stick–slip frictional instability along faults. Based on the node‐to‐point contact element strategy (an arbitrarily shaped contact element strategy applied with the static‐explicit algorithm for modelling non‐linear frictional contact problems proposed by authors), a finite element code for modelling the 3‐D non‐linear friction contact between deformable bodies has been developed and extended here to analyse the non‐linear stick–slip frictional instability between deformable rocks with a rate‐ and state‐dependent friction law. A typical fault bend model is taken as an application example to be analysed here. The variations of the normal contact force, the frictional force, the transition of stick–slip instable state and the related relative slip velocity along the fault between the deformable rocks and the stress evolution in the total bodies during the different stages are investigated, respectively. The calculated results demonstrate the usefulness of this code for simulating the non‐linear frictional instability between deformable rocks. Copyright © 2003 John Wiley & Sons, Ltd.     

郯庐断裂走滑活动与辽河盆地构造古地理格局

渐新世晚期 ,郯庐断裂的右行走滑活动控制了辽河盆地的构造古地理格局。研究认为 ,岩石圈断块沿郯庐断裂的走滑活动与大型板块构造的活动方式有一定的相似之处 ,即伴随着郯庐断裂的右行走滑 ,从断裂的增压弯曲部位到断裂的释压拉张部位将发生岩石圈断块的汇聚与离散现象 ,由此造成了走滑断裂带上增压弯曲部位与释压拉张部位局部应力场性质的不同 :增压弯曲部位应力相对集中 ,岩石圈断块发生汇聚、挤压、隆升 ;而释压拉张部位由于应力释放 ,岩石圈断块发生离散、伸展、沉降。岩石圈断块的隆升与沉降造成了渐新世晚期辽河盆地构造古地理格局的巨大差异。     

Rupture progression along discontinuous oblique fault sets: implications for the Karadere rupture segment of the 1999 Izmit earthquake, and future rupture in the Sea of Marmara

Large earthquakes in strike-slip regimes commonly rupture fault segments that are oblique to each other in both strike and dip. This was the case during the 1999 Izmit earthquake, which mainly ruptured E–W-striking right-lateral faults but also ruptured the N60°E-striking Karadere fault at the eastern end of the main rupture. It will also likely be so for any future large fault rupture in the adjacent Sea of Marmara. Our aim here is to characterize the effects of regional stress direction, stress triggering due to rupture, and mechanical slip interaction on the composite rupture process. We examine the failure tendency and slip mechanism on secondary faults that are oblique in strike and dip to a vertical strike-slip fault or “master” fault. For a regional stress field well-oriented for slip on a vertical right-lateral strike-slip fault, we determine that oblique normal faulting is most favored on dipping faults with two different strikes, both of which are oriented clockwise from the strike-slip fault. The orientation closer in strike to the master fault is predicted to slip with right-lateral oblique normal slip, the other one with left-lateral oblique normal slip. The most favored secondary fault orientations depend on the effective coefficient of friction on the faults and the ratio of the vertical stress to the maximum horizontal stress. If the regional stress instead causes left-lateral slip on the vertical master fault, the most favored secondary faults would be oriented counterclockwise from the master fault. For secondary faults striking ±30° oblique to the master fault, right-lateral slip on the master fault brings both these secondary fault orientations closer to the Coulomb condition for shear failure with oblique right-lateral slip. For a secondary fault striking 30° counterclockwise, the predicted stress change and the component of reverse slip both increase for shallower-angle dips of the secondary fault. For a secondary fault striking 30° clockwise, the predicted stress change decreases but the predicted component of normal slip increases for shallower-angle dips of the secondary fault. When both the vertical master fault and the dipping secondary fault are allowed to slip, mechanical interaction produces sharp gradients or discontinuities in slip across their intersection lines. This can effectively constrain rupture to limited portions of larger faults, depending on the locations of fault intersections. Across the fault intersection line, predicted rakes can vary by >40° and the sense of lateral slip can reverse. Application of these results provides a potential explanation for why only a limited portion of the Karadere fault ruptured during the Izmit earthquake. Our results also suggest that the geometries of fault intersection within the Sea of Marmara favor composite rupture of multiple oblique fault segments.     

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。     

基于实验结果讨论断层破裂与强震物理过程的若干问题

基于断层摩擦滑动实验、含凹凸体断层的变形破坏实验、断层撕裂扩展的实验、交叉断层的变形实验等多种实验结果并结合前人的工作 ,讨论了与断层破裂与强震物理过程相关的若干问题。研究表明 ,断层的整体滑动引起其两侧块体弹性应变的释放 ,是强震发生的原因 ,因此构造活动区具有较大尺度、结构连续且简单、介质均匀的断层 (或断层段 )是产生强震必备的构造条件 ,深部新生断层(盲断层 )向上撕裂扩展产生强震 ,尚需“弱层”提供“解耦”条件以便断层发生整体滑动。强震孕育过程中包含着凹凸体的破裂 ,断层面上凹凸体的尺度、强度及数量决定着前震活动的特征、强震动态破裂过程以及前兆现象。由断层分割的块体通过边界断层的交替滑动、以“框动”的方式运动 ,因此块体周边的断层上强震活动具有交替性。     

An extended finite element framework for slow‐rate frictional faulting with bulk plasticity and variable friction

We present an extended finite element (FE) approach for the simulation of slow‐rate frictional faulting in geologic media incorporating bulk plasticity and variable friction. The method allows the fault to pass through the interior of FEs without remeshing. The extended FE algorithm for frictional faulting, advocated in two recent articles, emanates from a variational equation formulated in terms of the relative displacement on the fault. In the present paper we consider the combined effects of bulk plasticity and variable friction in a two‐dimensional plane strain setting. Bulk plasticity is localized to the fault tip and could potentially be used as a predictor for the initiation and propagation of new faults. We utilize a variable velocity‐ and state‐dependent friction, known as the Dieterich–Ruina or ‘slowness’ law, formulated in a slip‐weakening format. The slip‐weakening/variable friction model is then time‐integrated according to the generalized trapezoidal rule. We present numerical examples demonstrating the convergence properties of a global Newton‐based iterative scheme, as well as illustrate some interesting properties of the variable friction model. Copyright © 2009 John Wiley & Sons, Ltd.     

汶川地震映秀-北川地表破裂带虹口乡段精细填图、位移特征和地震构造分析

同震地表破裂带是地震破裂最直观的表现。地表破裂带的精细填图对理解地震破裂过程及发震机理有着重要意义。我们对5.12汶川地震映秀-北川同震破裂带南部的虹口乡区段开展了较为精细的填图工作,该区段地表破裂带的平面几何和同震位移特征沿走向均显示出公里尺度的横向变化和复杂性。根据同震破裂不连续性特征可把地表破裂分为4段,自南向北依次为深溪沟段、庙坝段、高原新村段和八角庙段。其中,深溪沟段和八角庙段同震垂直位错较大,达.5～6m;庙坝段同震垂直位移量普遍较低,1～2m或更低;  在高原新村附近,地表破裂带分为近平行的南北两支,北支以右旋走滑为主,南支以南东盘逆冲抬升为主,这与与映秀-北川同震破裂总体北西盘抬升的性质相反。本文对高原新村附近的反向陡坎进行了深入的探讨,并综合前人的研究结果,认为是破裂在从深部往上扩展的过程中,在浅表部位顺着发育在虹口东南部的大鱼洞-龙溪飞来峰后缘的具南东倾向的断面继承而成。与北川沙坝附近的反向陡坎一样,说明老断裂带构造几何特征对单次地震破裂的扩展有牵引作用。最后,讨论了八角庙有切割关系的两组断层擦痕的地震学和地质学意义,指出可能指示了映秀-北川断裂面上局部区段震前的初始应力水平较低。     

Evidence for anthropogenic surface loading as trigger mechanism of the 2008 Wenchuan earthquake

Two and a half years prior to China’s M7.9 Wenchuan earthquake of May 2008, at least 300 million metric tons of water accumulated with additional seasonal water level changes in the Minjiang River Valley at the eastern margin of the Longmen Shan. This article shows that static surface loading in the Zipingpu water reservoir induced Coulomb failure stresses on the nearby Beichuan thrust fault system at <17 km depth. Triggering stresses exceeded levels of daily lunar and solar tides and perturbed a fault area measuring 416 ± 96 km². These stress perturbations, in turn, likely advanced the clock of the mainshock and directed the initial rupture propagation upward towards the reservoir on the "Coulomb-like" Beichuan fault with rate- and state-dependent frictional behavior. Static triggering perturbations produced up to 60 years (0.6%) of equivalent tectonic loading, and show strong correlations to the coseismic slip. Moreover, correlations between clock advancement and coseismic slip, observed during the mainshock beneath the reservoir, are strongest for a longer seismic cycle (10kyr) of M > 7 earthquakes. Finally, the daily event rate of the micro-seismicity (M ≥ 0.5) correlates well with the static stress perturbations, indicating destabilization.     

Observed source parameters for dynamic rupture with non-uniform initial stress and relatively high fracture energy

We have conducted dynamic rupture propagation experiments to establish the relations between in-source stress drop, fracture energy and the resulting particle velocity during slip of an unconfined 2 m long laboratory fault at normal stresses between 4 and 8 MPa. To produce high fracture energy in the source we use a rough fault that has a large slip weakening distance. An artifact of the high fracture energy is that the nucleation zone is large such that precursory slip reduces fault strength over a large fraction of the total fault length prior to dynamic rupture, making the initial stress non-uniform. Shear stress, particle velocity, fault slip and acceleration were recorded coseismically at multiple locations along strike and at small fault-normal distances. Stress drop increases weakly with normal stress. Average slip rate depends linearly on the fault strength loss and on static stress drop, both with a nonzero intercept. A minimum fracture energy of 1.8 J/m² and a linear slip weakening distance of 33 μm are inferred from the intercept. The large slip weakening distance also affects the average slip rate which is reduced by in-source energy dissipation from on-fault fracture energy.Because of the low normal stress and small per event slip (∼86 μm), no thermal weakening such as melting or pore fluid pressurization occurs in these experiments. Despite the relatively high fracture energy, and the very low heat production, energy partitioning during these laboratory earthquakes is very similar to typical earthquake source properties. The product of fracture energy and fault area is larger than the radiated energy. Seismic efficiency is low at ∼2%. The ratio of apparent stress to static stress drop is ∼27%, consistent with measured overshoot. The fracture efficiency is ∼33%. The static and dynamic stress drops when extrapolated to crustal stresses are 2–7.3 MPa and in the range of typical earthquake stress drops. As the relatively high fracture energy reduces the slip velocities in these experiments, the extrapolated average particle velocities for crustal stresses are 0.18–0.6 m/s. That these experiments are consistent with typical earthquake source properties suggests, albeit indirectly, that thermal weakening mechanisms such as thermal pressurization and melting which lead to near complete stress drops, dominate earthquake source properties only for exceptional events unless crustal stresses are low.     

Effects of cohesive zones on small faults and implications for secondary fracturing and fault trace geometry

Fracture mechanics theory and field observations together indicate that the shear stress on many faults is non-uniform when they slip. If the shear stress were uniform, then: (a) a physically implausible singular stress concentration theoretically would develop at a fault end; and (b) a single curved ‘tail fracture’ should open up at the end of every fault trace, intersecting the fault at approximately 70 °. Tail fractures along many small faults instead range in number, commonly form behind fault trace ends, have nearly straight traces and intersect a fault at angles less than 50 °. A ‘cohesive zone’, in which the shear stress is elevated near the fault end, can eliminate the stress singularity and can account for the observed orientation, shape, and distribution of tail fractures. Cohesive zones also should cause a fault to bend. If the cohesive zone shear stress were uniform, then the distance from the fault end to the bend gives the cohesive zone length. The nearly straight traces of the tail fractures and the small bends observed near some fault ends implies that the faults slipped with low stress drops, less than 10% of the ambient fault-parallel shear stress.     

Reduction of friction on geological faults by weak-phase smearing

Most common crustal rock types display friction coefficients of 0.6 or higher, but some faults must be frictionally weak as they slip when the stress state is unfavourably-oriented (i.e. the resolved shear stress is low for a given normal stress across the fault surface). A role for low-friction minerals and high pore fluid pressures, either separately or in combination, is frequently invoked to explain such slip, but volume fractions of dispersed weak phases often seem to be present in fault gouge in amounts too small to produce significant mechanical weakening. By means of mechanical tests on synthetic fault gouge and microstructural study of run products, we show that the effective area of an embedded weak phase (graphite) on a slip plane can be substantially increased by mechanical smearing, and that the enlarged area of the weak phase on the slip plane follows a linear mixing law. This allows a relatively small volume fraction of the initially dispersed weak phase to have a disproportionately large effect, provided the smearing is concentrated into a narrow planar slip zone or into an interconnected network of them.     

Uplift and subsidence from oblique slip: the Ganos–Marmara bend of the North Anatolian Transform, western Turkey

Bends that locally violate plate-motion-parallel geometry are common structural elements of continental transform faults. We relate the vertical component of crustal motion in the western Marmara Sea region to the NNW-pointing 18° bend on the northern branch of the North Anatolian Fault (NAF-N) between the Ganos segment, which ruptured in 1912, and the central Marmara segment, a seismic gap. Crustal shortening and uplift on the transpressive west side of the bend results in the Ganos Mountain; crustal extension and subsidence on the transtensional east side produce the Tekirdağ Basin. We propose that this vertical component of deformation is controlled by oblique slip on the non-vertical north-dipping Ganos and Tekirdağ segments of the North Anatolian Fault. We compare Holocene with Quaternary structure across the bend using new and recently published data and conclude the following. First, bend-related vertical motion is occurring primarily north of the NAF-N. This suggests that this bend is fixed to the Anatolian side of the fault. Second, current deformation is consistent with an antisymmetric pattern centered at the bend, up on the west and down on the east. Accumulated deformation is shifted to the east along the right-lateral NAF-N, however, leading to locally opposite vertical components of long- and short-term motion. Uplift has started as far west as the landward extension of the Saros trough. Current subsidence is most intense close to the bend and to the Ganos Mountain, while the basin deepens gradually from the bend eastward for 28 km along the fault. The pattern of deformation is time-transgressive if referenced to the material, but is stable if referenced to the bend. The lag between motion and structure implies a 1.1–1.4 Ma age for the basin at current dextral slip rate (2.0–2.5 cm/year). Third, the Tekirdağ is an asymmetric basin progressively tilted down toward the NAF-N, which serves as the border fault. Progressive tilt suggests that the steep northward dip of the fault decreases with depth in a listric geometry at the scale of the upper crust and is consistent with reactivation of Paleogene suture-related thrust faults. Fourth, similar thrust-fault geometry west of the bend can account for the Ganos Mountain anticline/monocline as hanging-wall-block folding and back tilting. Oblique slip on a non-vertical master fault may accommodate transtension and transpression associated with other bends along the NAF and other continental transforms.     

Surface rupture of the Cariaco July 09, 1997 earthquake on the El Pilar fault, northeastern Venezuela

This paper discusses the surface rupture of the Cariaco July 09, 1997 Ms 6.8 earthquake in northeastern Venezuela – located at 10.545°N and 63.515°W and about 10 km deep. The field reconnaissance of the ground breaks confirms that this event took place on the ENE–WSW trending onshore portion of the dextral El Pilar fault (between the Gulfs of Cariaco and Paria), which is part of the major wrenching system within the Caribbean–South America plate boundary zone. Dextral slip along this fault was further supported by the structural style of this rupture (en echelon right-lateral R shears connected by mole tracks at restraining stepovers) and by larger geometric complexities (pop-ups at Las Manoas and Guarapiche), as well as by the focal mechanism solutions determined for the event by several authors. This 1997 surface ruptre comprised two distinct sections, from west to east: (a) a main very conspicuous, continuous, 30-km-long, rather straight, 075°N-trending alignment of en echelon surface breaks, with a rather constant, purely dextral coseismic slip of about 25  cm, but reaching a maximum value of 40 cm slightly northwest of Pantoño; and (b) a secondary discontinuous, 10-km-long, boomerang-shaped rupture, with a maximum coseismic slip of 20 cm at Guarapiche. The onshore extent of the surface rupture totalled 36 km, but may continue westward underwater, as suggested by the very shallow aftershock seismicity. This aftershock activity also clearly defined the steep north dip of the fault plane along the western rupture, suggesting tectonic inheritance on this major fault.From many locals' accounts, the rupture seems to have propagated from Pantoño to the west (highly asymmetric bidirectionality). This suggests that earthquake nucleation happened at or near the Casanay–Guarapiche restraining bend and rupture quickly propagated westward, allowing only a small fraction to progress eastwards beyond the bend. Additionally, the large fraction of after-slip (or creep) released is to be related to such restraining bend, which seems to have partly locked slip during rupture.     

基于地应力实测数据分析郯庐断裂带中段滑动趋势

郯庐断裂带潜在滑动趋势对于区域地壳稳定性评价具有重要的意义。对郯庐断裂带中段(新沂至渤海段)附近12个钻孔共65条实测地应力数据分析,得出该区段最大和最小水平主应力与垂直应力之比Kmax和Kmin的范围,结果表明郯庐断裂带中段主要表现为逆断层的应力状态,局部地区呈现出有利于走滑断层的特征。研究区内应力比K值由西向东逐渐增大,并在东北部达到最大。通过计算得出断层面上的剪应力与有效正应力比值小于Byerlee准则所定义的摩擦系数,说明目前郯庐断裂带中段处于相对稳定状态;同时发现北西向断裂面上的剪应力与有效正应力比值比北东向断裂的大。区内应力积累指标μm的计算结果和地震的分布特征表明,郯庐断裂带中段北西向断裂上地震活动频繁,应力积累程度低;而北东向断层上应力积累程度高,特别是处于郯庐断裂带中段东北部的北东向断层,在应力集中的区域出现地震空白区,推测为断层的锁固段。最终认为在现今应力场下郯庐断裂带中段更易沿北西向的断裂滑动。     

香山天景山断裂带断层泥的微观研究及工程地震评价中的意义

断层运动是复杂多变的, 其运动在一条巨大的断裂带上经常以区段为活动单位, 为服务于工程建设和强震的预测, 划分出活断层运动区段的必要性越来越迫切。本文通过对香山天景山断裂带断层泥的微观特征研究, 结合断裂的几何特征综合分析, 认为该断裂的中段以粘滑活动为主, 东西两段主要是蠕滑。     

玉树地震序列重新定位及其地震构造研究

对玉树地震序列自2010年4月11日至9月15日由台网记录到的1 832个地震采用双差地震定位法进行重新定位,获得了1 670个地震重新定位的震源参数。重新定位后的震源深度主要分布在15 km以内。重新定位后的Ms 7.1级主震发生在无地表破裂段,余震活动向两侧破裂扩展。余震沿地表破裂带基本呈线性分布,剖面上显示为近垂直的结构面,在北西端无地表破裂出露处,出现近垂直于断裂方向较宽的北东向地震密集带。震源机制解显示的主压应力方向斜交地表破裂带,地表破裂与震源破裂都表现为纯左旋走滑的错动性质,而在北西端主压应力方向偏转为近垂直于断裂带的方向,此处较宽的北东向地震密集带可能由近东西与南北两个方向的共轭破裂所组成。余震的后期活动与发展并不局限于主震形成的破裂带内,更多的受局部应力调整被触发而产生新的破裂。     

A dislocation model of aseismic fault slip under nonuniform friction

A model is proposed for studying the mechanical behaviour of faults during their interseismic periods. The model considers a plane fault surface in an elastic medium, subject to a uniform shear stress which increases slowly with time. A1-D friction distribution is assumed on the fault, characterized by asperities and a weaker zone. The traction vector on the fault plane has an arbitrary orientation: in particular, it can be nonperpendicular to the asperity borders. Aseismic fault slip takes place when the applied stress exceeds the frictional resistance: slip starts in weak zones and is confined by asperities, where it propagates at increasing velocity. Propagation into asperities is characterized by a dislocation front, advancing perpendicularly to the asperity border. Fault slip does not take prate in the direction of traction, except when traction is perpendicular or parallel to the asperity border. The propagation of such aseismic dislocations produces a stress redistribution along the fault and can play a key role in determining the conditions which give rise to earthquakes.     

汶川Ms8级地震形成的擦痕形迹及其意义探讨

2008年5月12日14时28分(北京时间)中国四川省汶川县境内发生里氏80级地震。北川-映秀断裂是汶川Ms8级地震的发震断裂之一,以逆(右行)走滑活动为主,延伸长约220 km,活动强度大于其他两条断裂。主震在北川-映秀地表破裂带局部地点形成清晰的擦痕,它的位置和规模可能是深部破裂滑动量峰值在地表的响应。根据断层擦痕反映的应力场及位移与破裂长度关系等特征,将北川-映秀破裂带分为虹口段、北川段、南坝段。虹口段长度约22~45 km,同震位移量约45 m,σ1呈NWW-NW向,至少经历了三期活动,第三期逆冲活动强度最大,改造前两期逆(右行)走滑活动,它们的形成可能均与本次主震相关;北川段长约90~100 km,同震位移量约35 m,σ1呈NWW向近水平,活动强度弱于虹口第三期;南坝段长约35~50 km,同震位移量约25 m,以NWW向近水平挤压为特征,σ1方位角与中、南段相差约180°,活动强度与南段前两期相近。