Frequency dependence of delayed and instantaneous triggering on laboratory and simulated faults governed by rate-state friction

Earthquake triggering by transient stresses is commonly observed; however, some aspects remain unexplained. The first is the often-observed delay between the triggered earthquakes and the triggering waves, and the second is the unexpected effectiveness of transient stressing in the seismic frequency band. Previous theoretical and laboratory studies have suggested that seismic transients should have little impact on faults if the duration of the transient is smaller than the timescale for nucleation of slip. We reexamine the dynamics of stress triggering during stick-slip sliding on a laboratory fault and make three important observations that pertain to earthquake triggering. (1) Delayed triggering (clock advance) occurs for both bare granite surfaces and granular gouge prior to the onset of instantaneous triggering. (2) Triggering occurs much earlier in the stick-slip cycle than expected for a simple Coulomb stress threshold. (3) Shorter-period (higher stressing rate) pulses are more effective at triggering than longer-period pulses of the same stress amplitude. We use numerical simulations to show that rate-state friction can explain each of the observed features but not all three simultaneously. Only the Ruina slip law for state evolution, in which faults must slip to heal, can reproduce early-onset and stressing rate-dependent triggering. The laboratory and numerical experiments show that faults can remain relatively weak over much of the seismic cycle and that the triggered response depends on a competition between healing and weakening during triggered slip. Transient stressing at seismic frequencies may be more effective at triggering earthquakes than previously recognized.