Stability and localization of rapid shear in fluid-saturated fault gouge: 2. Localized zone width and strength evolution

Field and laboratory observations indicate that at seismic slip rates most shearing is confined to a very narrow zone, just a few tens to hundreds of microns wide, and sometimes as small as a few microns. Rice et al. (2014) analyzed the stability of uniform shear in a fluid-saturated gouge material. They considered two distinct mechanisms to limit localization to a finite thickness zone, rate-strengthening friction, and dilatancy. In this paper we use numerical simulations to extend beyond the linearized perturbation context in Rice et al. (2014), and study the behavior after the loss of stability. Neglecting dilatancy we find that straining localizes to a width that is almost independent of the gouge layer width, suggesting that the localized zone width is set by the physical properties of the gouge material. Choosing parameters thought to be representative of a crustal depth of 7 km, this predicts that deformation should be confined to a zone between 4 and 44 mu m wide. Next, considering dilatancy alone we again find a localized zone thickness that is independent of gouge layer thickness. For dilatancy alone we predict localized zone thicknesses between 1 and 2 mu m wide for a depth of 7 km. Finally, we study the impact of localization on the shear strength and temperature evolution of the gouge material. Strain rate localization focuses frictional heating into a narrower zone, leading to a much faster temperature rise than that predicted when localization is not accounted for. Since the dynamic weakening mechanism considered here is thermally driven, this leads to accelerated dynamic weakening.