The Structure and Mechanical Significance of Off-Fault Fracture Damage

对结晶岩中出土断层的研究表明，大多数滑动发生在极细颗粒断层泥核心内的一个非常尖锐的“突出滑动面”上，有时会变成粘土。一层更宽的更粗的断层角砾岩包围着这层泥。在角砾岩带之外，围岩是裂隙的，裂隙密度随着距离区域背景水平的距离而减小。利用动态滑动脉冲模型检验了断裂带构造是在突出滑动面上多次地震传播的“过程带”中形成的假设。裂纹尖端应力场的计算采用了莱斯和P.I.最近提出的动态滑移脉冲解析模型(莱斯等人，2003)，损伤力学模型则用来计算应力场中断裂和破碎的空间范围和方向。分析滑移-脉冲模型的灵感来自于地震学的滑移反转，它发现在大地震中滑移更多地以位错(被称为“热脉冲”)的形式传播，而不是作为不断增长的裂缝。该模型使用物理上合理的模型参数解释了Heaton(1990)的观测结果，并给出了与以前的独立估计相一致的断裂能估计。用Ashby和Sammis(1990)发展的损伤力学模型计算了损伤和角砾化的空间范围。它依赖于几个模型参数，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离滑动面的距离而减小，以及角砾岩内无数个小滑动面上的方向和滑动矢量。这一建议的第二个主要目标是绘制天然断裂带角砾岩中的这些滑动面及其滑动矢量图，并测量围岩中的断裂方向和损伤，以便与滑动脉冲/损伤模型预测进行比较。探讨了滑动-脉冲/损伤模型的其他潜在可观测的动力学含义，包括形成断层外断裂损伤所需的滑动(和时间延迟)可能解释为什么从地震学观测推断的特征位移DC比实验室测量的特征位移DC大几个数量级。另一个令人感兴趣的领域是在破裂尖端过程区建立的岩心渗透率(靠近滑动面)，这可能会影响动态摩擦。在水对动摩擦力影响的最新模型中，这个渗透率是一个重要的参数(莱斯，AGU Abst。2003年)。最后，探讨了利用近断层宽带地震仪直接观测震害过程的可能性。计算了Ashby-Sammis损伤模型中每个不断增长的缺陷的地震矩。虽然每个缺陷都很弱且频率很高，但传播过程区中所有缺陷的综合效应产生了一个可观测的信号，与使用这种方法对地下爆炸源计算的信号相当(Johnson和Sammis，2001)。

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).