  We investigate the propagation of fluid-driven fault slip on a slip-weakening
frictional interface separating two identical half-spaces of a
three-dimensional elastic solid. Our focus is on axisymmetric circular shear
ruptures as they capture the most essential aspects of the dynamics of
unbounded ruptures in three dimensions. In our model, fluid-driven aseismic
slip occurs in two modes: as an interfacial rupture that is unconditionally
stable, or as the quasi-static nucleation phase of an otherwise dynamic
rupture. Unconditionally stable ruptures progress through four stages.
Initially, ruptures are diffusively self-similar and the interface behaves as
if it were governed by a constant friction coefficient equal to the static
friction value. Slip then accelerates due to frictional weakening while the
cohesive zone develops. Once the latter gets properly localized, a finite
amount of fracture energy emerges along the interface and the rupture dynamics
is governed by an energy balance of the Griffith's type. In this stage, fault
slip transition from a large-toughness to a small-toughness regime. Ultimately,
self-similarity is recovered and the fault behaves again as having a constant
friction coefficient, but this time equal to the dynamic friction value. When
slow slip is the result of a frustrated dynamic instability, slip also
initiates self-similarly at a constant peak friction coefficient. The maximum
aseismic rupture size varies from a critical nucleation radius (shear modulus
divided by slip-weakening rate) to infinity near the limit that separates the
two modes of aseismic sliding. We provide analytical and numerical solutions
for the problem solved over its full dimensionless parameter space. Due to its
three-dimensional nature, the model enables quantitative comparisons with field
observations as well as preliminary engineering design of hydraulic stimulation
operations.
