  Shear banding and stick-slip instabilities have been long observed in sheared
granular materials. Yet, their microscopic underpinnings, interdependencies and
variability under different loading conditions have not been fully explored.
Here, we use a non-equilibrium thermodynamics model, the Shear Transformation
Zone theory, to investigate the dynamics of strain localization and its
connection to stability of sliding in sheared, dry, granular materials. We
consider frictional and frictionless grains as well as presence and absence of
acoustic vibrations. Our results suggest that at low and intermediate strain
rates, persistent shear bands develop only in the absence of vibrations.
Vibrations tend to fluidize the granular network and de-localize slip at these
rates. Stick-slip is only observed for frictional grains and it is confined to
the shear band. At high strain rates, stick-slip disappears and the different
systems exhibit similar stress-slip response. Changing the vibration intensity,
duration or time of application alters the system response and may cause
long-lasting rheological changes. We analyse these observations in terms of
possible transitions between rate strengthening and rate weakening response
facilitated by a competition between shear induced dilation and vibration
induced compaction. We discuss the implications of our results on dynamic
triggering, quiescence and strength evolution in gouge filled fault zones.
