Potential for earthquake triggering from transient deformations

We report on laboratory experiments in which stick-slipping shear surfaces are subject to transient stressing to simulate earthquake triggering by seismic waves. Granular layers and bare granite surfaces were sheared in a servo-controlled deformation apparatus in double-direct shear. The seismic waves from an earthquake and tectonic load were simulated by superimposing a loading rate sinusoid on a constant shear loading rate. The dependence of triggered stick-slip failure on fault stress state and architecture was analyzed. Fault architecture was evaluated by varying gouge layer thickness (2-6 mm) and studying bare granite surfaces. We compare the shortened recurrence times for faults under transient loading conditions to the consistent recurrence intervals under constant loading rate. Our results imply that triggering depends on oscillation amplitude and frequency, as well as properties of the fault. Larger-amplitude dynamic stresses reduce stick-slip recurrence intervals for granular layers, whereas failure times for granite surfaces are uncorrelated with oscillation amplitude. Granular layers have shorter recurrence rates at higher frequency, whereas the recurrence intervals of granite surfaces are lengthened or unaffected by high-frequency oscillations. Higher frequencies can inhibit failure when fault slip exceeds a critical distance, D-c, prior to peak velocity and encourages failure if Dc is achieved postpeak velocity. Increasing velocity temporarily strengthens faults, whereas velocity reduction further weakens and promotes failure, as predicted by the rate-and-state friction laws. Our results may explain variations in earthquake triggering thresholds and imply that high-frequency thresholds may not be constant, as has been previously proposed.