Seismic shaking during earthquakes is a potent reminder of the active tectonic processes shaping the surface of the Earth. In addition to their impacts on the surface, seismic waves can have an effect at depth, altering the conditions on fault zones and triggering other earthquakes. Here I study the effect of seismic waves on faults in order to probe the state of stress in the crust and to provide basic constraints on physical models of earthquake nucleation, propagation, and arrest. In the first section, I quantify the ability of seismic waves to trigger earthquakes, developing a new statistical metric based on changes in earthquake inter-event times. Triggering is identified in California at strain amplitudes down to 3×10-9, and the triggered rate change scales with seismic wave amplitude. This scaling, projected into the near field of moderate magnitude earthquakes (M 3-5.5), can account for 15-60% of observed aftershocks. In the second section, I build on the statistical methods of section 1 to assess whether a recent increase in the global rate of great (MW ≥ 8) earthquakes can be attributed to dynamic triggering from other great earthquakes. Triggered rate changes are measured at the sites of each of the 16 MW ≥ 8 events that occurred between 1998 and 2011. In only a few cases is triggering detected at sites separated by more than 10º, and systematic rate changes are too small to account for the large increase in earthquake rate. These two sections together place lower and upper bounds on the role of seismic waves in linking earthquakes across space and time. In the third section, I study the effect of seismic vibration on the sliding strength of an ongoing earthquake, using laboratory experiments to measure the effect of vibration on rapid granular shear flows. I find that noisy shear flows consisting of angular particles weaken and compact substantially at intermediate shear rates (0.1 - 10 cm/s). This compaction and weakening occurs in response to shear-generated acoustic vibration, and acts to counter shear-induced dilatational hardening. Acoustic compaction may be one of many processes contributing to co-seismic weakening of faults
