The Structure and Mechanical Significance of Off-Fault Fracture Damage

对结晶岩中的出土断层的研究表明，大多数滑动发生在极细粒度断层泥（有时会变成粘土）的核心内非常尖锐的“突出滑动面”上。较粗的断层角砾岩的较宽的层包围着这个断层泥层。在角砾岩带之外，围岩破裂，破裂密度随距离减小至区域背景水平。用动态滑动脉冲模型检验了这一断裂带结构是在突出滑动面上传播的众多地震的“过程带”中形成的假说。Rice和P.I. (Rice例如，2003）被用于计算裂纹尖端应力场，并且损伤力学模型被用于计算该应力场中的断裂和破碎的空间范围和方向。分析滑动脉冲模型的灵感来自地震学滑动反演，发现在大地震中滑动传播更多的是作为位错（称为“希顿脉冲”），而不是作为一个不断增长的裂缝。该模型使用物理上合理的模型参数解释了希顿（1990）的观测结果，并得出了与先前独立估计值一致的断裂能估计值。使用Ashby和Sammis（1990）开发的损伤力学模型计算了损伤和角砾化的空间范围。它取决于几个模型参数，包括：初始损伤的大小和密度，区域应力场的方向，破裂速度，滑移脉冲长度，总滑移，和应力降。观测约束包括角砾岩带的宽度、围岩中的断裂损伤以及它如何随着距滑动面的距离而减小，以及角砾岩内无数小滑动面上的方向和滑动矢量。该提案的第二个主要目标是绘制自然断层带角砾岩中的这些滑动面及其滑动矢量，并测量围岩中的断裂方向和损伤，以与滑动脉冲/损伤模型预测进行比较。其他潜在的可观察到的动态影响的滑动脉冲/损伤模型进行了探讨，包括可能性的滑动（和时间延迟）需要形成断层外断裂损伤可能解释为什么特征位移，直流电，从地震观测推断的数量级大于那些在实验室中测量。另一个感兴趣的领域是核心渗透率（附近的滑动面）建立在破裂尖端过程区，这可能会影响动态摩擦。这种渗透率是最近的模型中的一个重要参数，用于水对动态摩擦的影响（Rice，AGU abst. 2003年）。最后，探讨了利用近断层宽频带地震仪直接观测损伤过程的可能性。在Ashby-Sammis损伤模型中计算每个增长缺陷的地震矩。虽然每一个都很弱，频率很高，但传播过程区中所有缺陷的综合效应产生了一个可观察的信号，与使用这种方法计算的地下爆炸源的信号相当（约翰逊和Sammis，2001年）。

Studies of exhumed faults in crystalline rock indicate that most of the slip has occurred on a very sharp "prominent slip surface" within a core of extremely fine-grained fault gouge, which is sometimes altered to clay. A wider layer of coarser fault breccia surrounds this gouge layer. Beyond the brecciated zone, the wall rocks are fractured, with the fracture density decreasing with distance to a regional background level. A dynamic slip pulse model is used to test the hypothesis that this fault zone structure is formed in the "process zone" of numerous earthquakes propagating on the prominent slip surface. The analytical dynamic slip pulse model recently developed by Rice and the P.I. (Rice et al., 2003) was used to calculate the crack tip stress field and a damage mechanics model was used to calculate the spatial extent and orientation of fracturing and fragmentation within that stress field. The analytical slip-pulse model was inspired by seismological slip inversions that find slip in large earthquakes propagates more as a dislocation (dubbed a "Heaton pulse") than as a growing crack. This model explains Heaton's (1990) observations using physically reasonable model parameters, and it yields an estimate of the fracture energy that is consistent with previous independent estimates. The spatial extent of damage and brecciation was calculated using the damage mechanics model developed by Ashby and Sammis (1990). It depends on several model parameters including: the size and density of initial damage, the orientation of the regional stress field, the rupture speed, the slip pulse length, the total slip, and the stress drop. Observational constraints include the width of the breccia zone, the fracture damage in the wall rock and how it decreases with distance from the slip surface, and the orientations and slip vectors on the myriad of small slip surfaces within the breccia. A second major objective of this proposal was to map these slip surfaces and their slip vectors in the breccia of natural fault zones and to measure the fracture orientation and damage in the wall rock for comparison with the slip-pulse/damage model predictions. Other potentially observable dynamic implications of the slip-pulse/damage model were explored, including the possibility that the slip (and time delay) required to form the off-fault fracture damage may explain why the characteristic displacements, Dc, inferred from seismological observations are orders of magnitude larger than those measured in the laboratory. Another area of interest is the core permeability (near the slip surface) established in the rupture-tip process zone, which may influence dynamic friction. This permeability is an important parameter in recent models for the effect of water on dynamic friction (Rice, AGU abst. 2003). Finally, the possibility that the damage process may be directly observed using near-fault broadband seismographs was explored. The seismic moment was calculated for each growing flaw in the Ashby-Sammis damage model. While each is weak and very high frequency, the integrated effect of all flaws in a propagating process-zone produce an observable signal, comparable to that calculated using this method for an underground explosion source (Johnson and Sammis, 2001).