Any planar or curvilinear interface in rock may potentially slide to form a fault if the shear stress resolved across it overcomes the frictional resistance. It is, of course, required that the interface be weaker than the surrounding matrix. The process of shear failure by sliding of weak planes is often referred to as Coulomb or Mohr-Coulomb failure (Jaeger and Cook, 1979). As mentioned earlier under 'Mechanisms and Mechanics of Faulting,' weaknesses in rocks are of different origins: Some are depositional while others are deformational, i.e., formed later than the depositional features but earlier than the slip event leading to faulting. Among the depositional features that are commonly sheared to form faults are bedding planes, dune boundaries, and basin boundaries. These are known to be bedding-plane faults and this term is retained here to keep the continuity with the previous literature on the subject.

Weaknesses in rock due to previously formed structures are as common as those due to depositional processes. These include joints, pressure solution seams, cleavages, deformation bands, and previously formed faults. In the case of reactivation of previous faults, particularly with a different sense of slip, the process is commonly called 'inversion.' In order to distinguish between faults whose formation mechanism is not known from those faults which formed by shearing of joints or pressure solution seams, the word 'sheared' is used as a prefix, such as 'sheared joints.'