  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.
