The mechanics of lubricated faults: Insights from 3-D numerical models

The weakening mechanisms occurring during an earthquake failure are of prominent
importance in determining the resulting energy release and the seismic waves excitation.
In this paper we consider the fully dynamic response of a seismogenic structure where
lubrication processes take place. In particular, we numerically model the spontaneous
propagation of a 3-D rupture in a fault zone where the frictional resistance is
controlled by the properties of a low viscosity slurry, formed by gouge particles and
fluids. This model allows for the description of the fault motion in the extreme case
of vanishing effective normal stress, by considering a viscous fault response and
therefore without the need to invoke, in the framework of Coulomb friction, the
generation of the tensile mode of fracture. We explore the effects of the parameters
controlling the resulting governing law for such a lubricated fault; the viscosity of
the slurry, the roughness of the fault surfaces and the thickness of the slurry film.
Our results indicate that lubricated faults produce a nearly complete stress drop
(i.e., a very low residual friction coefficient; mu ~ 0.01), a high fracture energy
density (E_G ~ few 10s of MJ/m^2) and significant slip velocities (vpeak ~ few 10s
of m/s). The resulting values of the equivalent characteristic slip-weakening distance
(d_0_eq = 0.1–0.8 m, depending on the adopted parameters) are compatible with the
seismological inferences. Moreover, in the framework of our model we found that
supershear ruptures are highly favored. In the case of enlarging gap height we can
have the healing of slip or even the inhibition of the rupture. Quantitative
comparisons with different weakening mechanisms previously proposed in the
literature, such as the exponential weakening and the frictional melting, are
also discussed.