  Shear banding is widely observed in natural fault zones as well as in gouge
layers in laboratory experiments. Understanding the dynamics of strain
localization under different loading conditions is essential for quantifying
strength evolution of fault gouge, energy partitioning during earthquakes and
characterizing rheological transitions and fault zone structure changes. To
that end, we develop a physics-based continuum model for strain localization in
sheared granular materials. The grain-scale dynamics is described by the Shear
Transformation Zone (STZ) theory, a non-equilibrium statistical thermodynamic
framework for viscoplastic deformation in amorphous materials. Using a finite
strain computational framework, we investigate the initiation and growth of
complex shear bands under a variety of loading conditions and identify
implication for strength evolution and ductile to brittle transition. Our
numerical results show similar localization patterns to field and lab
observations and suggest that shear zones show more ductile response at higher
confining pressures, lower dilatancy and loose initial conditions. Lower
pressures, higher loading rates and higher dilatancy favor a brittle response
and larger strength drops. These findings shed light on a range of mechanisms
for strength evolution in dry sheared gouge and provide a critical input to
physics-based multiscale models of fault zone instabilities.
