Evolution of Fault Strength from Microscopic Asperity Scale to Macroscopic Fault Zone Scale
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Fault strength is of key importance to geological and geophysical processes over a vast range of scales, from the microscopic interactions at asperities to the macroscopic behavior of plates. In this dissertation, I present my work on the evolution of fault strength. I first use a micromechanical model of flash heating that describes how shear resistance evolves at the asperity scale as a result of distributed deformation over a weak layer that grows during the brief lifetime of each asperity contact. The model predicts that after the initial rate-weakening stage, the friction becomes rate-strengthening. A comparison with published experimental data from a range of mineral systems shows good agreement with the model predictions. The parameter choices that ensure good model fits to the laboratory friction data are consistent with a priori estimates for the onset of asperity melting at high contact normal stresses. Next, considering the role of friction in fluid-saturated gouge, a linear stability analysis shows that rate-strengthening friction favors broader shear zone widths that lower strain rate for a given total slip rate. However, geologic and laboratory observations suggest that finite shear zones can persist even with rate-weakening friction. I describe a model that incorporates the interactions between variations in pore pressure of saturated porous media and the localization-pressurization phenomenon. During co-seismic slip in a plane-strain configuration, the stress variation caused by poroelasticity promotes the mechanical instability of previously undeformed regions. The frictional strength varies throughout the mechanically unstable region. To maintain momentum balance during slip, I argue that multiple transient slip events must take place to accommodate the overall macroscopic shear. I introduce a strain-rate function that describes the overall influence on energy dissipation and fault strength as the model shear zone thickness expands. The model is used to predict the evolution of shear zone thickness, temperature, pore pressure, and fault strength during model earthquakes along a mature fault. These two components of my dissertation build from the very small scale of asperities, to granules, and finally to the finite shear zones that are observed in the fields. This dissertation includes previously published and unpublished co-authored material.