Microphysical modeling of carbonate fault friction at slip rates
spanning the full seismic cycle
Abstract
Laboratory studies suggest that seismogenic rupture on faults in
carbonate terrains can be explained by a transition from high friction,
at low sliding velocities (V), to low friction due to rapid
dynamic weakening as seismic slip velocities are approached. However,
consensus on the controlling physical processes is lacking. We
previously proposed a microphysically-based model (the
‘Chen-Niemeijer-Spiers’ model) that accounts for the (rate-and-state)
frictional behavior of carbonate fault gouges seen at low velocities
characteristic of rupture nucleation. In the present study, we extend
the CNS model to high velocities (1mm/s≤ V ≤10m/s) by introducing
multiple grain-scale deformation mechanisms activated by frictional
heating. As velocity and hence temperature increase, the model predicts
a continuous transition in dominant deformation mechanisms, from
frictional granular flow with partial accommodation by plasticity at low
velocities and temperatures, to grain boundary sliding with increasing
accommodation by solid-state diffusion at high velocities and
temperatures. Assuming that slip occurs in a localized shear band,
within which grain size decreases with increasing velocity, the model
results capture the main mechanical trends seen in high-velocity
friction experiments on room-dry calcite-rich rocks, including
steady-state and transient aspects, with reasonable quantitative
agreement and without the need to invoke thermal decomposition or fluid
pressurization effects. The extended CNS model covers the full spectrum
of slip velocities from earthquake nucleation to seismic slip rates.
Since it is based on realistic fault structure, measurable
microstructural state variables and established deformation mechanisms,
it offers an improved basis for extrapolating lab-derived friction data
to natural fault conditions.