A fundamental mystery in earthquake physics is ``how can an earthquake be
triggered by distant seismic sources?'' Here, we use discrete element method
simulations of a granular layer, during stick-slip, that is subject to
transient vibrational excitation to gain further insight into the physics of
dynamic earthquake triggering. Using Coulomb friction law for grains
interaction, we observe delayed triggering of slip in the granular gouge. We
find that at a critical vibrational amplitude (strain) there is an abrupt
transition from negligible time-advanced slip (clock advance) to full clock
advance, {\it i.e.}, transient vibration and triggered slip are simultaneous.
The critical strain is order of 10−6, similar to observations in the
laboratory and in Earth. The transition is related to frictional weakening of
the granular layer due to a dramatic decrease in coordination number and the
weakening of the contact force network. Associated with this frictional
weakening is a pronounced decrease in the elastic modulus of the layer. The
study has important implications for mechanisms of triggered earthquakes and
induced seismic events and points out the underlying processes in response of
the fault gouge to dynamic transient stresses