  We report results of 3D Discrete Element Method (DEM) simulations aiming at
investigating the role of the boundary vibration in inducing frictional
weakening in sheared granular layers. We study the role of different vibration
amplitudes applied at various shear stress levels, for a granular layer in the
stick-slip regime and in the steady-sliding regime. Results are reported in
terms of friction drops and kinetic energy release associated with frictional
weakening events. We find that larger vibration amplitude induces larger
frictional weakening events. The results show evidence of a threshold below
which no induced frictional weakening takes place. Friction drop size is found
to be dependent on the shear stress at the time of vibration. A significant
increase in the ratio between the number of slipping contacts to the number of
sticking contacts in the granular layer is observed for large vibration
amplitudes. These vibration-induced contact rearrangements enhance particle
mobilization and induces a friction drop and kinetic energy release. This
observation provides some insight into the grain-scale mechanisms of frictional
weakening by boundary vibration in a dense sheared granular layer. In addition
to characterizing the basic physics of vibration induced shear weakening, we
are attempting to understand how a fault fails in the earth under seismic wave
forcing. This is the well know phenomenon of dynamic earthquake triggering. We
believe that the granular physics are key to this understanding.
