Frictional heating on a fault plane causes pore fluids to pressurize. When the permeability of the fault zone materials is sufficiently low, fluid pressure on the fault can approach the normal stress, though it can never be exceeded due to the feedback between pore pressure and frictional strength. However, if slip occurs at a boundary between materials of different permeabilities, such as fault gouge and a damage zone, the highest pressure develops within a few millimeters of the fault in the lower permeability material, rather than at the fault surface. The pressure increase off the fault can reach or exceed the normal stress given a large enough permeability contrast, because there is no direct feedback between this pressure and the frictional heating at the fault surface. High fluid pressures off the fault might result in the slip shifting into the lower permeability material, where the frictional strength has been reduced.