Slide-hold-slide protocols and frictional healing in Discrete Element
Method (DEM) simulations of granular fault gouge
Abstract
The empirical constitutive modeling framework of Rate- and
State-dependent Friction (RSF) is commonly used to describe the
time-dependent frictional response of fault gouge to perturbations from
steady sliding. In a previous study (Ferdowsi & Rubin, 2020), we found
that a granular-physics-based model of a fault shear zone, with
time-independent properties at the contact scale, reproduces the
phenomenology of laboratory rock and gouge friction experiments in
velocity-step and slide-hold protocols. A few slide-hold-slide
simulations further suggested that the granular model might outperform
current empirical RSF laws in describing laboratory data. Here, we
explore the behavior of the same Discrete Element Method model in
slide-hold and slide-hold-slide protocols over a wide range of sliding
velocities, hold durations, and system stiffnesses, and provide
additional support for this view. We find that, similar to laboratory
data, the rate of stress decay during slide-hold simulations is in
general agreement with the “Slip law” version of the RSF equations,
using parameter values determined independently from velocity-step
tests. During reslides following long hold times, the model, similar to
lab data, produces a nearly constant rate of frictional healing with log
hold time, with that rate being in the range of ~0.5-1
times the RSF “state evolution” parameter b. We also find that, as in
laboratory experiments, the granular layer undergoes log-time compaction
during holds. This is consistent with the traditional understanding of
state evolution under the Aging law, even though the associated stress
decay is similar to that predicted by the Slip and not the Aging law.