Fault Mechanics

Abstract

The past 15 years have seen a revolution in fault mechanics. Before the 1990s, the field was limited to the Andersonian fault mechanics, which offered no mechanism for fault propagation and growth. Linear elastic fracture mechanics, which had been applied to calculating the stress fields around faults, also did not offer a mechanism for fault growth. The crack tip opening angle model, an elastic-plastic numerical model that allows for inelastic yielding in a volume surrounding the crack tip, is consistent with all observations concerning fault growth. It predicts the linear scaling between fault displacement and length and the observation that faults propagate by the breakdown of a brittle process zone that widens with fault length. The process zone consists primarily of intergranular dilatant microcracks that are oriented parallel to the maximum principal stress of the fault tip stress field and whose density increases exponentially as the fault is approached. These properties are consistent with theory and laboratory experiments. The theory is also consistent with the observation that fault tip displacement tapers are linear and scale-independent and increase with rock strength. Fault interactions through their stress fields are not understood except for the simple case of subparallel faults. These interactions result in the development of fault populations with well-defined statistical properties. These populations evolve with increasing brittle strain from power law to exponential size distributions and finally to evenly spaced system-size faults. Fault rocks and structures are described as a function of depth, from the upper schizosphere through the plastosphere. The strength of faults is shown to be consistent with Byerlee friction, and the weak San Andreas Fault hypothesis is critiqued.