  Even the most regular stick-slip frictional sliding is always stochastic,
with irregularity in both the intervals between slip events and the sizes of
the associated stress drops. Applying small-amplitude oscillations to the shear
force, we show, experimentally and theoretically, that the stick-slip periods
synchronize. We further show that this phase-locking is related to the
inhibition of slow rupture modes which forces a transition to fast rupture,
providing a possible mechanism for observed remote triggering of earthquakes.
Such manipulation of collective modes may be generally relevant to extended
nonlinear systems driven near to criticality.
