LAMMPS simulations scripts for the manuscript "A granular-physics-based view of fault friction experiments"

Rate- and State-dependent Friction (RSF) equations are commonly used to
describe the time-dependent frictional response of fault gouge to
perturbations in sliding velocity. Among the better-known versions are the
Aging and Slip laws for the evolution of state. Although the Slip law is
more successful, neither can predict all the robust features of lab data.
RSF laws are also empirical, and their micromechanical origin is a matter
of much debate.  Here we use a granular-physics-based model to
explore the extent to which RSF behavior, as observed in rock and gouge
friction experiments, can be explained by the response of a granular gouge
layer with time-independent properties at the contact scale. We examine
slip histories for which abundant lab data are available, and find that
the granular model (1) mimics the Slip law for those loading protocols
where the Slip law accurately models laboratory data (velocity-step and
slide-hold tests), and (2) deviates from the Slip law under conditions
where the Slip law fails to match laboratory data (the reslide portions of
slide-hold-slide tests), in the proper sense to better match those
data.  The simulations also indicate that state is sometimes
decoupled from porosity in a way that is inconsistent with traditional
interpretations of "state" in RSF.  Finally,
if the "granular temperature" of the gouge is suitably
normalized by the confining pressure, it produces an estimate of the
direct velocity effect (the RSF parameter a)  that is consistent
with our simulations, and in the ballpark of lab data.