Paul Johnson of LANL (in collaboration with co-workers at Penn State, UC Santa Cruz, Univ. Wisconsin, and the USGS) conducted laboratory studies of stick—slip in granular media with and without applied acoustic vibration. Glass beads were used to simulate granular fault zone wear material, sheared under constant normal stress, while subject to transient or continuous perturbation by acoustic waves. They show that small magnitude failure events, corresponding to triggered aftershocks, occur when applied sound-wave amplitudes exceed several microstrain. These events are frequently delayed and/or occur as part of a cascade of small events. Vibrations also cause large slip events to be disrupted significantly in time relative to those without wave perturbation. The effects are observed for many large-event cycles after vibrations cease, indicating a strain memory in the granular material. 

Stress-drop variation versus
recurrence for experiments conducted with and without vibration 

Dynamic stressing of tectonic faults may play a similar role in determining the complexity of earthquake recurrence, and improving our understanding of this role may lead to advances in seismic hazard assessment and earthquake forecasting.

For additional details, see:

Johnson, P.A., Savage, H., Knuth, M., Gomberg, J., and Marone, C., 2008, Effects of acoustic waves on stick-slip in granular media and implications for earthquakes. Nature, 451, 57-60, doi:10.1038/nature06440.