Effects of shear velocity oscillations on stick-slip behavior in laboratory experiments

We report on laboratory friction experiments in which simulated faults are exposed to shear velocity oscillations at different amplitudes and frequencies. Granular layers are sheared in a servo-controlled biaxial apparatus at constant normal stress and background shear velocity, with a shear velocity sinusoid superimposed. Correlation between oscillations and stick-slip events is determined by the timing of dynamic failure with respect to the osciliation phase. Schuster's test is used to calculate the statistical likelihood of phase recurrence. We find that correlation of failure with the shear stress oscillation depends on osciliation frequency and amplitude at low frequencies and solely on oscillation amplitude at high frequencies. The frequency boundary between these two regimes is proportional to the inverse of the time needed to displace the frictional critical slip distance. We evaluate changes in failure characteristics, including failure strength, recurrence time between events, creep, and phase of the oscillation at failure, to assess the effects of stressing rate oscillations. Failure occurs at maximum shear stressing rate at low frequencies and lags peak stressing rate in the high-frequency regime. Friction at the onset of dynamic failure decreases with increasing frequency. The distribution of events through time depends on the frequency of the shear oscillation; low-frequency oscillations produce bimodal distributions and high-frequency oscillations produce unimodal distributions. If the transition between the failure regimes depends on the critical displacement length as our experiments imply, the critical frequency will vary for faults with different gouge layer thicknesses and total displacement. Copyright 2007 by the American Geophysical Union.