EFFECTS OF ACOUSTIC WAVES ON STICK—SLIP BEHAVIOR IN SHEARED GRANULAR MEDIA WITH IMPLICATIONS TO DYNAMIC EARTHQUAKE TRIGGERING AND SLOW SLIP

To better understand the physics of dynamic triggering and the influence of dynamic stressing on earthquake recurrence, we are conducting laboratory studies of stick—slip in granular media with and without applied acoustic vibrations. In our 3-D experiments, glass beads are used to simulate granular fault zone wear material, sheared in a double-direct configuration under constant normal stress, while subject to transient or continuous perturbations by acoustic waves. Under moderate normal stress, we observe both instantaneous and delayed triggering when vibration is applied. Vibrations also cause significant disruption in the recurrence rate. The effects of vibration are observed for many major-event cycles after vibrations cease, indicating a strain memory in the granular material. Vibration-induced disruption of periodic stick—slip is linked to failure of granular force chains. Under smaller normal stress, we observe vibration induced slow (silent) slip and tremor. In 2-D experiments we are applying photoelastic discs in stick—slip measurements in order to visualize the evolution of the force chain network. Photoelastic measurements provide insight into failure, and in particular small adjustments in the force chains network that presage failure. Applying larger amplitudes show repeatable instantaneous triggering and slow dynamics. A phenomenological model similar to Knopoff-Burridge shows the same general behaviors. Our results should lead to a new understanding of the importance of seismic energy on earthquake physics and more generally, we anticipate that it will have broad impact on unexpected material failure induced by moderate-amplitude elastic waves, including avalanches, landslide and failure of incipient damage in solids.