IMPLICATIONS OF FAULT-VALVE BEHAVIOR FOR RUPTURE NUCLEATION AND RECURRENCE

Earthquakes in the shallow crust are generally thought to arise from the frictional instability of existing faults under shear stress. In simple recurrence models, failure on such faults is assumed to occur when tectonic shear stress (tau) rises to some constant critical level. However, frictional strength (tau(f)) may also vary substantially through the interseismic period, in which case recurrence intervals between successive earthquakes are related to the time-dependence of tau(f) as well as tau. In part, the manner in which tau(f) changes with tau depends on the coupling between normal stress and shear stress on the fault, and is related to the mode of faulting and the manner of fault loading. Time-dependence of cohesive strength and static friction may also play a role. However, fluid pressure cycling as a consequence of fault-valve behaviour (where the fault transects a suprahydrostatic gradient in fluid pressure) may give rise to very large variations in fault strength. Geological evidence suggests that valve-action may be especially important in the lower regions of the seismogenic zone, where large ruptures tend to nucleate, the largest fluid pressure fluctuations being associated with faults that remain active as a consequence of fluid overpressure though severely misoriented for reactivation in the prevailing stress field. Extensive hydrofracture dilatancy is likely to develop prefailure in the vicinity of such faults. Changes in frictional strength over the interseismic period as a result of fault-valve activity (DELTAtau(f)) may greatly exceed the shear stress drop at failure (1 < DELTAtau < 10 MPa, typically). In such circumstances, recurrence intervals between successive events can be highly variable. A rich variety of recurrence behaviour becomes possible, depending on the relative magnitudes of DELTAtau(f) and DELTAtau, and the coupling between shear stress accumulation and changing fluid pressure levels through the interseismic period.