Dynamic weakening by acoustic fluidization during stick-slip motion
  The unexpected weakness of some faults has been attributed to the emergence of acoustic waves that promote failure by reducing the confining pressure through a mechanism known as acoustic fluidization, also proposed to explain earthquake remote triggering. Here we validate this mechanism via the numerical investigation of a granular fault model system. We find that the stick-slip dynamics is affected only by perturbations applied at a characteristic frequency corresponding to oscillations normal to the fault, leading to gradual dynamical weakening as failure is approaching. Acoustic waves at the same frequency spontaneously emerge at the onset of failure in absence of perturbations, supporting the relevance of acoustic fluidization in earthquake triggering. 
Ferdinando Giacco, Luigi Saggese, Lucilla de Arcangelis, Eugenio   Lippiello, Massimo Pica Ciamarra
cond-mat.stat-mech cond-mat.soft physics.geo-ph