How fault creep makes its way!

We model mechanics of an aseismic fault creep propagation and conditions when it may lead to the initiation of seismic slip. We do so by considering fault bounding medium to be elastically deformable and fault's interfacial strength to be slip rate- and state-dependent characterized by the steady-state rate-weakening. The fault is considered to be initially locked: a state of slip when interfacial slip velocity is considerably low and arbitrarily less than the steady-state sliding rate for given uniformly distributed prestress. We find solutions for creep penetration into the fault under geologically relevant loading scenarios (e.g., that of a plate-bounding strike-slip faulting driven by the slip at depth, or that of a rate-weakening patch of a fault loaded by a creep on an adjacent rate-strengthening part due to, e.g., anthropogenic fluid injection). In all the cases, the creep makes its way as a self-similar traveling front characterized by high stress owed to the direct effect; however, the remaining creep profile exhibits a near steady-state sliding. This may imply that a choice from a set of rules for the evolution of state variable—with identical linearizations about steady-state sliding—has no bearing on the creep penetration. Further, we find that the prestress, close to or far from steady-state sliding stress, controls the rate and manner of the creep penetration. We study slip propagation from an imposed dislocation accrued at a constant rate at one end of a homogeneous fault with the other end either at (1) the free surface of an elastic half-space or (2) strictly locked (buried) in the elastic full space. In both scenarios, no slip instability takes place over aseismic creep propagation distances relatable to the usual elasto-frictional nucleation lengthscale (e.g. Rubin & Ampuero 2005). Instead, in the first case creep propagation leads to the nucleation of the first and all subsequent dynamic events of the emerging cycle at/near the free surface after the creep traversed the entire length of the fault. In the second case, the creep front traverses nearly the entire length of the fault, but, instead of nucleating a dynamic event, the front arrests at some distance from the buried fault end, followed by the continual accumulation of aseismic slip without ever nucleating a dynamic event. These results may be owed to the physical and geometrical invariance of the considered homogeneous fault and may signal the essential role of fault strength heterogeneity, either that of the normal stress and/or frictional properties (Ray & Viesca, 2017, 2019), in defining its seismogenic character, i.e. under which conditions and where on the fault the earthquake slip instability can take place.

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