The Structure and Mechanical Significance of Off-Fault Fracture Damage

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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).