Granular Controls on Periodicity of Stick-Slip Events: Kinematics and Force-Chains in an Experimental Fault

It is a long-standing question whether granular fault material such as gouge plays a major role in controlling fault dynamics such as seismicity and slip-periodicity. In both natural and experimental faults, granular materials resist shear and accommodate strain via interparticle friction, fracture toughness, fluid pressure, dilation, and interparticle rearrangements. Here, we isolate the effects of particle rearrangements on granular deformation through laboratory experiments. Within a sheared photoelastic granular aggregate at constant volume, we simultaneously visualize both particle-scale kinematics and interparticle forces, the latter taking the form of force-chains. We observe stick-slip deformation and associated force drops during an overall strengthening of the shear zone. This strengthening regime provides insight into granular rheology and conditions of stick-slip periodicity, and may be qualitatively analogous to slip that accompanies longer term interseismic strengthening of natural faults. Of particular note is the observation that increasing the packing density increases the stiffness of the granular aggregate and decreases the damping (increases time-scales) during slip events. At relatively loose packing density, the slip displacements during the events follow an approximately power-law distribution, as opposed to an exponential distribution at higher packing density. The system exhibits switching between quasi-periodic and aperiodic slip behavior at all packing densities. Higher packing densities favor quasi-periodic behavior, with a longer time interval between aperiodic events than between quasi-periodic events. This difference in the time-scale of aperiodic stick-slip deformation is reflected in both the kinematics of interparticle slip and the force-chain dynamics: all major force-chain reorganizations are associated with aperiodic events. Our experiments conceptually link observations of natural fault dynamics with current models for granular stick-slip dynamics. We find that the stick-slip dynamics are consistent with a driven harmonic oscillator model with damping provided by an effective viscosity, and that shear-transformation-zone, jamming, and crackling noise theories provide insight into the effective stiffness and patterns of shear localization during deformation.     

A Continuum Damage–Breakage Faulting Model and Solid-Granular Transitions

We present a thermodynamically-based formulation for mechanical modeling of faulting processes in the seismogenic brittle crust using a continuum damage–breakage rheology. The model combines previous results of a continuum damage framework for brittle solids with continuum breakage mechanics for granular flow. The formulation accounts for the density of distributed cracking and other internal flaws in damaged rocks with a scalar damage parameter, and addresses the grain size distribution of a granular phase in a failure slip zone with a breakage parameter. The stress–strain relation and kinetics of the damage and breakage processes are governed by the total energy function of the system, which combines the energy of the damaged solid with the energy of the granular material. A dynamic brittle instability is associated with a critical level of damage in the solid, leading to loss of convexity of the solid energy function and transition to a granular phase associated with lower energy level. A non-local formulation provides an intrinsic length scale associated with the internal damage structure, which leads to a finite length scale for damage localization that eliminates the unrealistic singular localization of local models. Shear heating during deformation can lead to a secondary finite-width internal localization. The formulation provides a framework for studying multiple aspects of brittle deformation, including potential feedback between evolving elastic moduli and properties of the slip localization zone and subsequent rupture behavior. The model has a more general transition from slow deformation to dynamic rupture than that associated with frictional sliding on a single pre-existing failure zone, and gives time and length scales for the onset of the dynamic fracturing process. Several features including the existence of finite localization width and transition from slow to rapid dynamic slip are illustrated using numerical simulations. A configuration having an existing narrow slip zone with localized damage produces for appropriate loading conditions an overall cyclic stick–slip motion. The simulated frictional response includes transitions from friction coefficient of ~0.7 at low slip velocity to dynamic friction below 0.4 at slip rates above ~0.1 m/s, followed by rapidly increasing friction for slip rates above ~1 m/s, consistent with laboratory observations.     

沉管隧道穿越纵向非均匀场地振动台试验研究

为了研究穿越纵向非均匀场地的沉管隧道地震反应规律,进行了沉管隧道穿越砂土-黏土场地和均匀砂土场地两种工况的振动台试验。沉管隧道模型材料主要为微粒混凝土和镀锌钢丝,接头材料为橡胶,模型缩尺比为1/30,采用层叠剪切箱装填黏土和砂土构成纵向非均匀场地,输入荷载为不同峰值的El Centro波和Kobe波。采集土层和隧道不同观测点处的加速度和应变等数据并进行分析,研究在不同性质土层中的沉管隧道地震反应的特点,分析纵向非均匀场地对于沉管隧道地震反应的影响。试验结果表明砂土和黏土不同的动力特性会导致沉管隧道地震反应各异:穿越非均匀场地沉管隧道加速度反应、应变反应和管节间相对位移反应明显异于均匀场地沉管隧道,且在输入不同峰值的地震波下呈现不同规律。试验结果可供沉管隧道抗震设计参考。     

Correlated Crust and Mantle Strain Field in China

INTRODUCTION Moststudiesonactiveblockshavebeenfocusedonidentificationofblockboundariesandtheiractivity;inotherwords,mostoftheworkwaslimitedtothehorizontalmovementoftheblocks.Inreality,theblocksarenotonlysurroundedbyactivefaultsofhorizontalmotion,butalsoco…     

Soil creep and convex‐upward velocity profiles: theoretical and experimental investigation of disturbance‐driven sediment transport on hillslopes

The movement of unconsolidated materials near the Earth's surface is often driven by disturbances that occur at a range of spatial and temporal scales. The nature of these disturbances ranges from highly variable, such as tree turnover, to periodic and predictable, such as frost heave or creep. To explore the effect of probabilistic disturbances on surface processes, we formulated a granular creep model with analogy to rate process theory (RPT) used for chemical reactions. According to the theory, individual particles must be energized to a height greater than adjacent particles in order for grain dilation and transport to occur. The height of neighbouring particles (which is akin to activation energy in chemical reactions) varies with slope angle such that energy barriers get smaller in the downslope direction as slopes steepen. When slopes approach the friction‐limited angle of repose, the height of energy barriers approaches zero and grains ?ow in the absence of disturbance. An exponential function is used to describe the probability distribution of particle excitation height although alternative distributions are possible. We tested model predictions of granular dynamics in an experimental sandpile. In the sandpile, acoustic energy serves as the disturbance agent such that grains dilate and shear in response. Particle velocities are controlled by the frequency of energy pulses that result in grain displacement. Using tracer particles, we observed a convex‐upward velocity pro?le near the surface of the sandpile, consistent with predictions of our RPT‐based velocity model. In addition, we depth‐integrated the velocity model to predict how ?ux rates vary with inclination of the sandpile and observed non‐linear ?ux–gradient curves consistent with model predictions. By varying the acoustic energy level in the experimental sandpile, we documented changes in the rate of grain movement; similar changes in modelled velocities were achieved by varying the exponent of the particle excitation probability distribution. The general agreement between observed and modelled granular behaviour in our simple laboratory sandpile supports the utility of RPT‐based methods for modelling transport processes (e.g. soil creep, frost heave, and till deformation), thus enabling us to account for the probabilistic nature of disturbances that liberate sediment in natural landscapes. Copyright © 2004 John Wiley & Sons, Ltd.     

瑞利阻尼对砂土场地非线性地震响应的影响分析

在地震荷载作用下,自由场地会产生土体侧向变形和地表响应放大现象。由于土体的高度非线性,计算自由场地地震响应时,不同的阻尼比及剪切模量取值是造成其计算结果与试验结果相差较大的原因之一。目前动力计算常采用瑞利阻尼方法,其系数取值会在一定程度上影响计算结果。选用两模态简化瑞利阻尼系数计算方法,分析土体阻尼比及控制频率的取值对计算结果的影响,对比离心机模型试验,利用开源有限元平台OpenSees,采用适合于土体动力分析的多屈服面本构模型(PDMY),建立剪切梁模型模拟三维自由场地,并分析瑞利阻尼参数对自由场地地震响应和侧向变形计算结果的影响。结果表明,针对相对密度为60%的Nevada干砂,阻尼比为4%、控制频率比为5时,场地响应计算结果与试验结果较为符合。综合分析显示场地非线性响应时域计算时,应特别注意选用的瑞利阻尼参数值。     

The Mechanical Coupling of Fluid-Filled Granular Material Under Shear

The coupled mechanics of fluid-filled granular media controls the physics of many Earth systems, for example saturated soils, fault gouge, and landslide shear zones. It is well established that when the pore fluid pressure rises, the shear resistance of fluid-filled granular systems decreases, and, as a result, catastrophic events such as soil liquefaction, earthquakes, and accelerating landslides may be triggered. Alternatively, when the pore pressure drops, the shear resistance of these geosystems increases. Despite the great importance of the coupled mechanics of grain–fluid systems, the basic physics that controls this coupling is far from understood. Fundamental questions that must be addressed include: what are the processes that control pore fluid pressurization and depressurization in response to deformation of the granular skeleton? and how do variations of pore pressure affect the mechanical strength of the grains skeleton? To answer these questions, a formulation for the pore fluid pressure and flow has been developed from mass and momentum conservation, and is coupled with a granular dynamics algorithm that solves the grain dynamics, to form a fully coupled model. The pore fluid formulation reveals that the evolution of pore pressure obeys viscoelastic rheology in response to pore space variations. Under undrained conditions elastic-like behavior dominates and leads to a linear relationship between pore pressure and overall volumetric strain. Viscous-like behavior dominates under well-drained conditions and leads to a linear relationship between pore pressure and volumetric strain rate. Numerical simulations reveal the possibility of liquefaction under drained and initially over-compacted conditions, which were often believed to be resistant to liquefaction. Under such conditions liquefaction occurs during short compactive phases that punctuate the overall dilative trend. In addition, the previously recognized generation of elevated pore pressure under undrained compactive conditions is observed. Simulations also show that during liquefaction events stress chains are detached, the external load becomes completely supported by the pressurized pore fluid, and shear resistance vanishes.     

Coupled shear‐bending formulation for seismic analysis of stacked wood shear wall systems

A summary of the development of a new coupled shear‐bending model for analysis of stacked wood shear walls and multi‐story wood‐frame buildings is presented in this paper. The model focuses on dynamic response of light‐frame wood structures under seismic excitation. The formulation is intended to provide a more versatile option than present pure shear models in that the new model is capable of accurately capturing the overall lateral response of each story diaphragm and separates the inter‐story shear deformation and the deformation associated with rotation of the diaphragm due to rod elongation, which is an analogue to the bending deformation in an Euler–Bernoulli beam model. Modeling the coupling of bending and shear deformation is shown to provide more accurate representation of stacked shear wall system behavior than a pure shear model, particularly for the upper stories in the assembly. The formulation is coupled with the newly developed evolutionary parameter hysteretic model for wood shear walls. Existing data from a shake table test of an isolated three‐story wood shear wall were used to verify the accuracy of the model prediction. The numerical results agreed very well with shake table test measurements. The influence of a continuous rod hold‐down system on the dynamic behavior of the three‐story stacked wood shear wall was also successfully simulated. Copyright © 2009 John Wiley & Sons, Ltd.     

基于位移的结构静力弹塑性分析方法的研究

本文系统介绍了基于位移的一般结构静力弹塑性(pushover)分析方法、自适应结构静力弹塑性(pushover)分析方法,指出了基于力的结构静力弹塑性(pushover)分析方法与基于位移的结构静力弹塑性(pushover)分析方法的差别。应用各种结构静力弹塑性(pushover)分析方法对一7层和15层框架剪力墙结构进行了计算与比较分析。算例结果表明,应用基于位移的一般结构静力弹塑性(push-over)分析方法对结构的抗震性能进行评估时,不受高阶振型的影响,结果更加准确合理。     

Identification and imaging of soil and soil-pile deformation in the presence of liquefaction

1 Introduction Soil and soil-structure systems exhibit complex response patterns during earthquake-induced liquefaction. These patterns depend on geotechnical properties, in-situ stress conditions and interaction with subsurface structural elements. Seismic records of full- scale systems during case histories provide a valuable source of information on the associated response mechanisms. However, the response of these systems is commonly monitored at sparsely distributed locations only, mostly…     

A simple displacement model for response analysis of EPS geofoam seismic buffers

A simple displacement-type block model is proposed to compute the compression–load–time response of an idealized seismic buffer placed against a rigid wall and used to attenuate earthquake-induced dynamic loads. The seismic buffer is modelled as a linear elastic material and the soil wedge shear surface by a stress-dependent linear spring. The model is shown to capture the trends observed in four physical reduced-scale model shaking table tests carried out with similar boundary conditions up to a base excitation level of about 0.7g. In most cases, quantitative predictions are in reasonable agreement with physical test results. The model is simple and provides a possible framework for the development of advanced models that can accommodate more complex constitutive laws for the component materials and a wider range of problem geometry.     

强震蕴育过程中区域垂直形变场多种动态信息演化特征

以区域精密水准重复观测资料为依据,用定性与定量相结合的方法,研究了发生于我国大陆板内的１９９６年丽江７．０级等几次强震过程中区域垂直形变速率、剪切变形率、扩张变形率和体积变化率等形变场多种信息的动态演化特征。结果显示：强震蕴育过程中,区域垂直形变场在空间和时间上均会出现不同程度的非均匀态特征。在此基础上,结合区域构造,进一步研究了判定强震危险区的形变前兆场多种信息异常综合标志,得出了一些用大地形变进行强震预测的初步结论。     

A Wavelet-based Seismogram Inversion Algorithm for the In Situ Characterization of Nonlinear Soil Behavior

We present a full waveform inversion algorithm of downhole array seismogram recordings that can be used to estimate the inelastic soil behavior in situ during earthquake ground motion. For this purpose, we first develop a new hysteretic scheme that improves upon existing nonlinear site response models by allowing adjustment of the width and length of the hysteresis loop for a relatively small number of soil parameters. The constitutive law is formulated to approximate the response of saturated cohesive materials, and does not account for volumetric changes due to shear leading to pore pressure development and potential liquefaction. We implement the soil model in the forward operator of the inversion, and evaluate the constitutive parameters that maximize the cross-correlation between site response predictions and observations on ground surface. The objective function is defined in the wavelet domain, which allows equal weight to be assigned across all frequency bands of the non-stationary signal. We evaluate the convergence rate and robustness of the proposed scheme for noise-free and noise-contaminated data, and illustrate good performance of the inversion for signal-to-noise ratios as low as 3. We finally employ the proposed scheme to downhole array data, and show that results compare very well with published data on generic soil conditions and previous geotechnical investigation studies at the array site. By assuming a realistic hysteretic model and estimating the constitutive soil parameters, the proposed inversion accounts for the instantaneous adjustment of soil response to the level and strain and load path during transient loading, and allows results to be used in predictions of nonlinear site effects during future events.     

土体动剪切模量及其衰减特性现场试验研究

如何获取土体动模量原位衰减曲线是研究天然土层在循环动荷载下的动变形特性课题中亟待解决的难题。为此,本文自制了一套地震动参数现场试验系统,依据动剪切模量原位测试原理获得了不同深度土体在小应变范围内的动模量原位衰减曲线。试验数据表明:当剪应变小于4×10^－5时,土体变形处于线性阶段;当剪应变大于4×10^－5时,土体归一化动剪切模量G/G_max衰减加快,以塑性变形为主。同时,当向土层施加的垂向静荷载越大,归一化动剪切模量值G/G_max与对数剪应变值logγ_d之间的关系会逐渐上移,但直线段斜率值不变。相关成果可为试验场地的动模量取值提供一定参考。     

Submerged granular channel flows driven by gravity

The paper presents a rheological model for gravity driven granular flows saturated with water. The model adopts the kinetic theory for the collisional regime, which is dominant near the free surface, while for the frictional regime a specific model is proposed, which matches the Coulombian condition at the boundary with the loose static bed. The solution for the frictional regime is based on the observation that the frictional and the collisional regimes are not stratified but coexist across the flow depth.The model is able to predict the distribution along the depth of velocity, concentration, granular temperature, shear and normal stresses. In particular, it is possible to discriminate between the collisional and the frictional components of the normal and shear stresses.The results of the model are compared with the data of a laboratory investigation on a steady, uniform, highly concentrated saturated granular flow, composed of spheres with a uniform diameter of 6 mm.Another important issue addressed in the paper concerns the balances of the kinetic energy of the granular phase. The model is able to describe the mechanisms of production, diffusion and dissipation of kinetic energy, relevant to both the mean component of the flow and the fluctuating component (i.e., the collisional component). Also in this case the comparison with the experimental data is reasonably good. Near the static loose bed, the model predicts that the flux of the diffused fluctuating energy exceeds an order of magnitude the locally dissipated flux of fluctuating energy. This suggests that the motion of the grains, even at concentrations close to that of packing, is always accompanied by a certain degree of granular temperature.     

Modelling of material non-linearities in steel structures subjected to transient dynamic loading

This paper is concerned with the modelling of the behaviour of steel under cyclic and dynamic loading conditions. After a general discussion regarding the requirements for accurate and efficient modelling in dynamics, two models are described and implemented. The bilinear stress-strain constitutive relationship with kinematic hardening is widely used in many computer codes, hence is used for ‘control’ purposes. The multisurface plasticity model is said to exhibit the important qualities of strain hardening, softening and relaxation to a mean stress. This model is described in detail and notes on model parameter evaluation are given. A number of validation examples are presented, due to the complexity of implementation of the multisurface formulation. This is followed by comparisons between the bilinear response predictions and those of the multisurface model for cyclic and dynamic tests on beam-columns. It is concluded that in the absence of material test data under cyclic loading, the bilinear model provides acceptably accurate response predictions. However, the multisurface model provides a significantly closer fit to experimental results, due to its ability to model a yield plateau and a non-linear strain-hardening regime as well as cyclic degradation. It can also be used for new types of steel where no distinct yield point is observed.     

福建沿海、台湾海峡GPS观测分析及地球动力学特征研究

利用 3期GPS联测结果所获得的福建沿海地壳水平运动信息 ,采用ITRF94全球框架为基础的GPS测站地壳运动模型及其处理软件 ,对所获得的观测数据进行处理和精度分析。得到福建省高精度的GPS测站大地坐标、边长及其位移矢量 ,其精度达到 1 7×10 - 8。计算了福建地壳运动速率、主应变率 ,东西与南北向线应变率、面应变率、剪应变率、大地转动率和最大剪应变率等值线并给出了它们的分布图象。根据多年形变和现今GPS观测资料 ,分析福建地壳垂直运动与水平运动 ,显示区域应力场优势分布特征。最后 ,对福建沿海及台湾海峡地壳动力学特征作了初步的探讨     

2014年新疆于田MS7.3地震前 地壳变形特征研究

利用1999—2007年和2009—2013年两期GPS速度场资料, 采用最小二乘配置方法分别计算了2008年和2014年新疆两次于田M_S7.3地震前新疆及周边地区的主应变率、 面应变率及最大剪应变率, 分析了该区域的变形动态特征, 并结合速度剖面分析方法给出了震源区的构造变形特征. 速度场及应变率场动态结果表明: 新疆天山地区的地壳变形特征整体表现为由南向北缩短, 相对运动速率表现为由南向北、 由西向东逐渐减小; 震源区东侧的左旋剪切变形明显大于西侧; 2008年与2014年两次于田M_S7.3地震的震源区均处于拉张与挤压变形的过渡地带, 易于强地震的发生; 2008年于田M_S7.3地震的张性兼有少量剪性破裂的发生使得阿尔金断裂的左旋剪切变形增强. GPS速度场剖面分析结果表明, 2014年于田M_S7.3地震前震源区西侧的变形宽度大于东侧, 剪切应变积累程度西侧高于东侧. 综合分析认为, 震源周边构造区应变积累的差异性有利于强震的孕育, 2008年于田M_S7.3地震对2014年于田M_S7.3地震可能有促进作用.      

四川盆地场地土动力学参数统计研究

四川盆地地震构造环境相对稳定,主要遭受近场中强地震和外围大地震的影响。盆地内第四系主要为河流相的砂卵石层夹粘土、粉土层,分布于成都平原及盆地内外河流两岸。这种地层在土层地震反应计算时往往产生一个峰值,具有显著的放大作用。本文收集共计123组土动力学参数的实验资料,统计分析了淤泥质土、粉质粘土、粘土、粉土、细砂和全风化泥岩等6种土类实测土动力学参数,给出了它们在不同深度下的动剪切模量比和阻尼比的统计值。并且将统计值和"94"规范值与袁晓铭推荐值[9]进行了对比分析,结果表明在盆地内使用规范值和推荐值会对工程的抗震设防产生不利影响。该研究成果对盆地内各类工程建设的工程场地地震安全性评价工作具有一定的借鉴和参考价值。     

渤海海域典型场地土的动力学特征

文中通过收集已完成的近40个海洋平台的工作报告中的钻孔资料,通过室内振动三轴试验,整理出372渤海原状土样动力学测试的原始数据,进行归纳分析,利用数理统计学的方法给出渤海海域内不同类型土体、不同埋深的动剪切模量比和阻尼比随剪应变变化的平均曲线及推荐值。并以三种典型土类为例,将统计值与规范值、推荐值进行比较分析,得到了非常有价值的结论。本文的结果可供渤海海上工作无试验资料和今后修订渤海海上土动力学参数规范值时参考使用,对工程抗震和地震安全性评价也具有一定的借鉴意义。