We represent a well-developed fault by a layer of granular material (fault gouge) confined between two competent fault blocks. Slip on such a fault involves plastic shearing of the fault gouge. That is, the fault gouge behaves as a Coulomb material, and the plastic flow is accomplished by slip on the two sets of Coulomb shears appropriate to the stress state and the frictional properties of the gouge. Neither set of Coulomb shears is coplanar with the fault zone. Generally, the initial plastic flow is self arresting, and increased shear stress is required to drive further shear. But as the shear stress is increased a stress state is reached within the gouge in which the plane of maximum shear stress is parallel to the plane of the fault zone. Then the fault gouge shears at whatever rate is required to keep the shear stress across the fault from increasing further as was demonstrated experimentally by Mandl et al. [1977>.

In a mature fault previous slip cycles have presumably left the fault gouge in a stress state such that the plane of maximum shear is parallel to the fault plane. Then, with additional shear loading, unlimited shear deformation is possible at the shear corresponding to the onset of Coulomb failure, and the fault appears to slip as if friction on the plane of the fault were less than on planes of other orientations. Slip on the fault does not imply that the fault plane coincides with a Coulomb shear as is sometimes assumed. ¿American Geophysical Union 1992