The Role of Grain Size and Effective Normal Stress on Localization and the Frictional Stability of Simulated Quartz Gouge

For earthquakes to occur on tectonic faults deformation localizes onto discrete slip surfaces within the gouge material of the fault core. Here, we investigate how localization is promoted in quartz gouge, a material known to exhibit unstable stick-slip behavior, by assessing the role of grain size and effective normal stress (sigma over bar n) on microstructural evolution and frictional behavior. We observe stable sliding, slow-slip instability, and stick-slip with increasing displacement. We find that initially fine-grained gouge and high sigma over bar n promotes unstable stick-slip behavior, whereas slow-slip is more prevalent at low sigma over bar n. Microstructural analyses of the sheared gouges show that stick-slip behavior occurs after localization of deformation on to discrete Y-shears, which undergo a significant cataclastic grain size reduction relative to the bulk of the gouge, a process enhanced by high sigma over bar n. Our results show that localization and instability are promoted by high effective normal stress and small grain size.