Mechanisms of faulting
Because of friction and the rigidity of the constituent rocks, the planes cannot glide or flow past each other easily, and so occasionally all movement stops. Thus, stress builds up, and when it reaches a level that exceeds the strain threshold, the accumulated potential energy is released into the fault.
Strain occurs accumulatively or instantaneously, depending on the liquid state of the rock; the ductile lower crust and mantle accumulate deformation gradually via shearing, whereas the brittle upper crust reacts by fracture – instantaneous stress release – resulting in motion along the fault. A fault in ductile rocks can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain-release causes earthquakes.