Abstract
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.