- Mature fault cores contain a finely ground powder termed fault gouge where most displacement occurs. If saturated, the gouge's resistance to shear changes due to pressurization of pore fluid that occurs when the gouge compacts or dilates. If gouge compacts at the onset of shear, fluid pressure increases, causing frictional contacts to weaken. More commonly, granular material dilates upon shear, causing frictional resistance to increase due to decreased pressure (dilation hardening). Depending on the relative rates of fluid flow and deformation, these local pressure deviations can be large enough to effect gouge strength even in drained (permeable) fault zones. To understand the conditions under which this occurs, we use a grain-scale numerical model of saturated fault zones.