For typical values of rock friction, active faults may be defined as favorably oriented for frictional reactivation, unfavorably oriented, or severely misoriented, depending on their attitude in the prevailing stress field. Continued reactivation of unfavorably oriented faults requires fluid pressures (Pf) elevated well above hydrostatic values to prevent the formation of new, more favorably oriented faults. In cases of severe misorientation, a necessary prefailure condition is that the least principal stress becomes effectively tensile, that is Pf > σ3. As a consequence, low-stress hydrofracture dilatancy is likely to develop in regions of rupture nucleation on such faults prior to failure. Triggering of shear failure through the buildup of fluid pressure to meet this specific condition then becomes probable.
Steep reverse faults require slightly supralithostatic fluid pressures for continued reactivation. Vein systems associated with ancient high-angle reverse faults demonstrate that this prefailure condition was met repeatedly. Failure episodes on such faults involve fault-valve action, where transient postseismic rupture permeability promotes upward fluid discharge from overpressured portions of the crust, followed by self-sealing and reaccumulation of fluid pressure. In compressional regimes, where steep reverse faults and severely misoriented strike-slip faults are currently active, there is direct evidence in several instances for fluid pressure levels approaching lithostatic values, and rupturing is occasionally followed by postseismic discharge. Where fault-valving occurs, recurrence intervals depend on strength reductions from increasing fluid pressure as well as on the rate of tectonic stress accumulation, and may be highly irregular.