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。     

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.     

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.     

Onset of natural terrain landslides modelled by linear stability analysis of creeping slopes with a two-state variable friction law

This paper further examines the possibility of modelling landslide as a consequence of the unstable slip in a steadily creeping slope when it is subject to perturbations, such as those induced by rainfall and earthquakes. In particular, the one-state variable friction law used in the landslide analysis by Chau is extended to a two-state variable friction law. According to this state variable friction law, the shear strength (τ) along the slip surface depends on the creeping velocity (V) as well as the two state variables (θ₁ and θ₂), which evolve with the ongoing slip. For translational slides, a system of three coupled non-linear first-order ordinary differential equations is formulated, and a linear stability analysis is applied to study the stability in the neighbourhood of the equilibrium solution of the system. By employing the stability classification of Reyn for three-dimensional space, it is found that equilibrium state (or critical point) of a slope may change from a ‘stable spiral’ to a ‘saddle spiral with unstable plane focus’ through a transitional state called ‘converging vortex spiral’ (i.e. bifurcation occurs), as the non-linear parameters of the slip surface evolve with its environmental changes (such as those induced by rainfall or human activities). If the one-state variable friction law is used in landslide modelling, velocity strengthening (i.e. dτ_ss/dV > 0, where τ_ss is the steady-state shear stress) in the laboratory always implies the stability of a creeping slope containing the same slip surface under gravitational pull. This conclusion, however, does not apply if a two-state variable friction law is employed to model the sliding along the slip surface. In particular, neither the region of stable creeping slopes in the non-linear parameter space can be inferred by that of velocity strengthening, nor the unstable region by that of velocity weakening. Copyright © 1999 John Wiley & Sons, Ltd.     

Slip-weakening and interactive dynamics of an heterogeneous seismic source

A theoretical 3D model of a fault region includes a slip-dependent friction, tectonic loading from the sides, and deterministic, continuous time formulation of governing equations. The model reproduces such properties of real faults as earthquake nucleation, earthquake complex rupture and nonregular recurrence. In particular, it is observed that the style of faulting changes from one event to another. Since all parameters related to the constitutive law are fixed during computer simulations, it is concluded that interactions between fault segments are responsible for such behavior. Neither the constitutive law nor fault spatial heterogeneities solely create complexity; rather it is the whole interactive dynamics of the system that determines the character of its evolution. Results are illustrated by time variations of global (i.e., related to the state of the whole fault) functions, such as energy release rate, seismic moment release rate, tectonic stresses, and local characteristics, such as driving and cohesive stresses, slip rates, slip displacements and mutual relations between them.     

A thermo‐mechanical model for the catastrophic collapse of large landslides

In this work, a new thermo‐mechanical model is developed, applicable to large‐scale, deep‐seated landslides consisting of a coherent mass sliding on a thin clayey layer. The considered time window is that of catastrophic acceleration, starting at incipient failure and ending when the acquired displacement and velocity are such that the sliding material begins to break up into pieces. The model accounts for temperature rise in the slip zone due to the heat produced by friction, leading to water expansion, thermoplastic collapse of the soil skeleton, and subsequent increase of pore water pressure. The model incorporates the processes of heat production and diffusion, pore pressure generation and diffusion, and an advanced constitutive law for the thermo‐mechanical behavior of soil. An analysis of the Vajont landslide is presented as an example. A sensitivity analysis shows that friction softening is the mechanism most affecting the timescale of the final collapse of a slide, but also that the mechanism of thermal pressurization alone can cause a comparably catastrophic dynamic evolution. It is also shown that, all other factors being equal, thermo‐mechanical collapse will cause thicker slides to accelerate faster than shallow ones. Copyright © 2010 John Wiley & Sons, Ltd.     

基于非均匀特征的岩石蠕滑与黏滑变形演化研究

基于岩石摩擦滑动变形场的非均匀特征,开展了岩石蠕滑和黏滑两种形式的摩擦滑动规律研究。采用双面摩擦模型试验方法,以数字散斑相关方法作为观测手段,分别研究了岩石蠕滑和黏滑变形过程中围岩位移场等值线、变形场非均匀统计指标、变形能密度与滑动面滑动速度演化及规律。结果表明:围岩位移场等值线分布能较好地反映滑动面阻滑特征,其中强阻滑区域的位移等值线呈弧状且分布较稀疏,弱阻滑区域的位移场等值线呈平行状且分布较密集;岩石蠕滑过程中,非均匀统计指标呈现波动增长特征,岩石黏滑过程中,非均匀统计指标呈现出突变跳跃特征。岩石蠕滑过程中不同区域的滑动速度均呈加速与减速滑动的交替状态,滑动面上各点滑动趋势不同;岩石黏滑过程中不同区域的滑动速度均出现突变。岩石蠕滑过程中不同区域上的变形能密度均处于积累与释放的波动状态,岩石黏滑过程中不同区域变形能密度产生突变。     

SPH procedures for modeling multiple intersecting discontinuities in geomaterial

A methodology is developed in SPH framework to analyze the behavior of preexisting multiple intersecting discontinuities or joints in rock material. The procedure does not require any additional unknowns to represent discontinuities and to capture velocity jump across them. Instead, a discontinuity is represented by a set of joint particles placed along the discontinuity plane, in which relative velocity and traction vector is evaluated, obeying the Mohr–Coulomb friction law with zero tension constrain. For failure of continuous rock material, the Drucker–Prager yield criterion with tensile cracking is employed in the elastic‐plastic constitutive model. Free‐sip, no‐slip, and symmetric boundary conditions are also implemented in SPH framework for proper representation of physical system. The paper analyzes behavior of a rock sample having a discontinuity plane under uniaxial loading and compares velocity and stress with a theoretical solution derived considering effective vertical stiffness of the joint planes. The efficacy of the proposed method is successfully demonstrated by solving another two problems of jointed rock mass under uniaxial and gravitational loading conditions.Copyright © 2014 John Wiley & Sons, Ltd.     

近断层强地震动预测中的有限断层震源模型

提出了近断层强地震动预测中建立活断层上设定地震有限断层震源模型的方法和步骤.首先,根据地震地质和地震活动性调查以及地球物理勘探等资料,确定活断层的空间方位和滑动类型; 然后,根据地震定标律确定活断层的宏观震源参数; 第三,将高强体模型与k平方滑动模型相结合,产生断层破裂面上的混合滑动分布.在此基础上,预测了与1994年Northridge地震断层类型、矩震级(M_w6.7)基本一致的设定地震的有限断层震源模型.最后,将预测的有限断层震源模型与基于地震学的、使用动力学拐角频率的地震动随机合成方法相结合,预测了1994年Northridge地震近断层12个基岩台站的加速度时程,并和实际记录进行了对比.结果表明,用上述方法和步骤建立的有限断层震源模型是可行、实用的.      

Dynamic sliding of tetrahedral wedge: The role of interface friction

The role of interface friction is studied by slow direct shear tests and rapid shaking table experiments in the context of dynamic slope stability analysis in three dimensions. We propose an analytical solution for dynamic, single and double face sliding and use it to validate 3D‐DDA. Single face results are compared with Newmark's solution and double face results are compared with shaking table experiments performed on a concrete tetrahedral wedge model, the interface friction of which is determined by constant velocity and velocity stepping, direct shear tests. A very good agreement between Newmark's method on one hand and our 3D analytical solution and 3D‐DDA on the other is observed for single plane sliding with 3D‐DDA exhibiting high sensitivity to the choice of numerical penalty value. The results of constant and variable velocity direct shear tests reveal that the tested concrete interface exhibits velocity weakening. This is confirmed by shaking table experiments where friction degradation upon multiple cycles of shaking culminated in wedge run out. The measured shaking table results are fitted with our 3D analytical solution to obtain a remarkable linear logarithmic relationship between friction coefficient and sliding velocity that remains valid for five orders of magnitude of sliding velocity. We conclude that the velocity‐dependent friction across rock discontinuities should be integrated into dynamic rock slope analysis to obtain realistic results when strong ground motions are considered. Copyright © 2011 John Wiley & Sons, Ltd.     

Shear localization,velocity weakening behavior,and development of cataclastic foliation in experimental granite gouge

Microstructural aspects of room-temperature deformation in experimental Westerly granite gouge were studied by a set of velocity stepping rotary-shear experiments at 25 MPa normal stress. The experiments were terminated at: (a) 44 mm, (b) 79 mm, and (c) 387 mm of sliding, all involving variable-amplitude fluctuations in friction. Microstructural attributes of the gouge were studied using scanning (SEM) and scanning transmission electron microscopy (STEM), image processing, and energy dispersive X-ray (EDX) analyses. The gouge was velocity weakening at sliding distances >10 mm as a core of cataclasites along a through-going shear zone developed within a mantle of less deformed gouge in all experiments. Unlike in experiment (a), the cataclasites in experiments (b) and (c) progressively developed a foliation defined by stacks of shear bands. The individual bands showed an asymmetric particle-size grading normal to shearing direction. These microstructures were subsequently disrupted and reworked by high-angle Riedel shears. While the microstructural evolution affected the effective thickness and frictional strength of the gouge, it did not affect its overall velocity dependence behavior. We suggest that the foliation resulted from competing shear localization and frictional slip hardening and that the velocity dependence of natural fault gouge depends upon compositional as well as microstructural evolution of the gouge.     

Numerical Investigations of the Dynamic Shear Behavior of Rough Rock Joints

The dynamic shear behavior of rock joints is significant to both rock engineering and earthquake dynamics. With the discrete element method (DEM), the dynamic direct-shear tests on the rough rock joints with 3D (sinusoidal or random) surface morphologies are simulated and discussed. Evolution of the friction coefficient with the slip displacement shows that the 3D DEM joint model can accurately reproduce the initial strengthening, slip-weakening, and steady-sliding responses of real rock joints. Energy analyses show that the strengthening and weakening behavior of the rock joint are mainly attributed to the rapid accumulation and release of the elastic energy in the joint. Then, effects of the surface roughness and the normal stress on the friction coefficient and the micro shear deformation mechanisms, mainly volume change and asperity damage, of the rock joint are investigated. The results show that the peak friction coefficient increases logarithmically with the increasing surface roughness, but decreases exponentially with the increasing normal stress. In addition, the rougher rock joint exhibits both higher joint dilation and asperity degradation. However, high normal stress constrains the joint dilation, but promotes the degree of asperity degradation significantly. Lastly, the effects of the 3D surface morphology on the shear behavior of the rock joint are investigated with a directional roughness parameter. It is observed that the anisotropy of the surface roughness consequently results in the variation of the peak friction coefficient of the joint corresponding to different shearing directions as well as the micro shear deformation mechanisms, e.g., the extent of joint dilation.     

断层导波研究加利福尼亚兰德斯和海克特曼恩地震断层带的四维特征(英文)

美国加利福尼亚州兰德斯和海克特曼恩地区于1992年和1999年先后发生7.4级和7.1级地震,分别在地面产生80km和40km长的断裂带。震后在断裂带布置的密集地震站台记录到明显的断层导波(fault-zone guided waves)。这些导波由断层带内的余震和人工震源激发产生,走时在S波之后,但具有比体波更强的振幅和更长的波列,并具有频散特征。通过对2～7 Hz断层导波的定量分析和三维有限差分数字模拟,获得了震深区断裂带的高分辨内部构造图像以及岩石的物理特性。数字模拟结果表明这些断裂带上存在被严重破碎了的核心层,形成低速、低Q值地震波导。核心破碎带宽约100～200 m,其内地震波波速降为周围岩石的40％～50％,Q值约为10～50。根据岩石断裂力学观点,这一低速、低Q值带可被解释为地震过程中处于断层动态断裂前端的非弹性区(或称之为破碎区,相干过程区)。在兰德斯和海克特曼恩断裂带测得的破碎区宽度与断裂带长度之比约为0.005,基本上符合岩石断裂力学预期的结果。观察到的断层导波还显示兰德斯和海克特曼恩地震中多条断层发生滑移和破碎。兰德斯地震时多条阶梯形断层相继断裂;而在海克特曼恩地震中,断裂带南北两端均出现分枝断裂,深处的分枝断裂较地表出现的破裂状况更为复杂。由三维有限元模拟的动态断裂过程表明,?     

Fractal dimension of molten surfaces as a possible parameter to infer the slip-weakening distance of faults from natural pseudotachylytes

High-velocity friction experiments on gabbro and monzodiorite, using a rotary-shear high-velocity friction apparatus, have revealed that frictional melting and progressive growth of a molten layer along a fault cause slip weakening, eventually reaching a nearly steady-state. The melting surface at the host rock/molten layer interface is initially very flat, but it becomes more complex and rounded in shape towards the steady state owing to the selective melting of minerals with lower melting points and the Gibbs–Thomson effect. This change in the melting-surface topography can be quantitatively expressed by the fractal dimension D, as determined by the divider method, from about 1.0 near the peak friction to around 1.1 near the steady-state friction. The ultimate fractal dimension at steady-state friction tends to decrease with increasing heat production rate presumably due to more rapid and uniform melting. A systematic correlation of D with mechanical behavior of the fault during frictional melting may provide a way of estimating slip-weakening distance and heat production rate at steady-state friction by measuring D for natural pseudotachylytes on slip surfaces with different displacements. The weakening distance is of vital significance in relation to fault instability and the heat production rate is related to the fault strength. The experimental studies point to ways to estimate these difficult quantities for natural faults.     

Effect of fault bend on the rupture propagation process of stick-slip

An experimental study of stick-slip is performed to examine the effect of a fault bend on the dynamic rupture propagation process. A granite sample used in the experiment has a pre-cut fault that is artificially bent by an angle of 5.6° at the center of the fault along strike, and accordingly the fault consists of two fault segments. The rupture propagation process during stick-slip instability is investigated by analyzing the records of shear strain and relative displacement measured with strain gauge sensors together with the hypocenters of AE (acoustic emission) events detected with piezoelectric transducers. The observed rupture propagation process of typical stick-slip events is as follows. (1) The dynamic rupture started on a fault segment is stopped near the fault bend. (2) The rupture propagation is restarted near the bend on the other fault segment 10.8 ms to 3.5 s after the stop of the first rupture. The delay time of the second rupture decreases with an increase in the slip amount of the first rupture or a decrease in the normal stress acting on the fault segment where the second rupture started. (3) The restarted rupture is not arrested by the presence of a fault bend, and slip occurs over the entire fault. We theoretically analyze the stress concentration near the fault bend to find that the normal stress produced by the preceding slip near the fault bend plays an important part in controlling the rupture propagation. A numerical simulation based on a rate- and state-dependent friction law is performed to interpret physically the retarded rupture in the experiment. The observed time interval of 10.8 ms to 3.5 s between the first rupture and the second is explained by the numerical simulation, suggesting that the rate- and state-dependence of rock friction is a possible mechanism for the retarded rupture on the fault.     

Numerical Analysis of Blast-Induced Stress Waves in a Rock Mass with Anisotropic Continuum Damage Models Part 1: Equivalent Material Property Approach

Summary This paper uses the concept of anisotropic damage mechanics to analyze dynamic responses of a granite site under blasting loads. An anisotropic continuum damage model is suggested to model rock mass behavior under blasting loads. The effects of existing cracks and joints in the rock mass are considered by using equivalent rock material properties obtained from both field and laboratory test data. The anisotropic damage accumulations are simulated by continuous degradation of equivalent material stiffness and strength during loading process and are calculated using the exponential function with respect to the principal tensile strain in three directions. The suggested models are programmed and linked to an available computer program Autodyn3D through its user's subroutine capability. Stress wave propagation and damage zone in the rock mass induced by underground explosions are simulated. Numerical results of damaged area, peak particle velocity and acceleration attenuation as well as acceleration time histories and Fourier spectra are compared with those from independent field tests.     

块系岩体滑移失稳中低摩擦效应的理论与试验研究

深部岩体具有块状层次结构,深部动载造成岩块发生相互间的振动脱离产生低摩擦效应,从而极易诱发原先处于平衡状态的岩体的动力变形破坏。在前人研究基础上,将块系岩体振动简化为等效质量-黏弹性模型,引入岩石摩擦滑移速率弱化模式,最终得到块系岩体滑移失稳计算模型。通过计算分析块系岩体自身特性及外荷载特性对岩块间低摩擦效应的影响。理论计算表明：水平静力及外扰动保持不变,增大岩块间弹性系数或者减小黏性系数,更容易引发岩体低摩擦滑移。随着冲击扰动、水平拉力幅值的增加,岩块的水平残余位移量值增加,当它们幅值超过一临界值时,岩块发生自持续滑移失稳运动。冲击扰动诱发岩块间不可逆位移、动力滑移失稳的临界能量与剪切力水平密切相关,在较大的剪切内力条件下,极其微弱的动力扰动即可诱发较大的岩块间不可逆位移甚至岩块的动力滑移失稳,随着剪切内力的减小,诱发岩块滑移失稳的能量阈值不断增大,当剪切内力低于岩块动摩擦强度时,单次冲击扰动只能诱发岩块间的不可逆位移。初步开展扰动诱发含初应力紫砂岩块体滑移试验,试验结果与理论计算基本符合,证明该模型的可行性。     

带有复合摩擦阻尼系统的基础隔震结构分析

针对常规摩擦阻尼器的起滑力难以确定的问题,提出了复合摩擦阻尼系统（CFDS）,它由非线性硬弹簧和摩擦阻尼器串联而成。当地震较小时,摩擦阻尼器处于附着状态,非线性硬弹簧发挥作用,防止隔震层位移过大;当地震较大时,非线性硬弹簧的变形则保持不变,摩擦阻尼器处于滑动状态,通过摩擦阻尼耗散能量。建立了带有复合摩擦阻尼系统的基础隔震结构的运动方程,指出摩擦阻尼器处于附着和滑移2种状态下边界条件。据此研究了常规强震和近断层脉冲型地震动作用下复合摩擦阻尼装置对橡胶隔震结构的减震效果。仿真分析表明：安装复合摩擦阻尼系统,在显著降低隔震层位移的同时,对上部结构的层间位移和加速度均有一定的减震效果;并且,在整个运动过程中,摩擦阻尼器会在附着和滑移2种状态之间来回切换。详细研究了复合摩擦阻尼系统的３个参数对隔震结构的减震效果。结果表明：在不同地震波作用下,由于摩擦阻尼系统参数的变化导致隔震结构地震响应的变化趋势是不同的,因此最优参数的取值与输入地震波类型和地面加速度峰值有关,并且不存在使隔震层位移和上部结构加速度同时达到最小的最优参数,因此需要设定合适的控制目标以确定适当的阻尼系统参数。     

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.     

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.