Partially creeping faults exhibit complex behavior in terms of which parts of the fault slip seismically versus aseismically. The specific geometry of creeping versus locked fault patches may pose constraints on rupture lengths on partially-creeping faults. We use the 3D finite element method to conduct dynamic rupture simulations on simplified partially-creeping strike-slip faults, to determine whether coseismic rupture can propagate into creeping regions, and how the presence and distribution of creeping regions affects the ability of rupture to propagate across the whole fault. We implement rate-state friction, in which locked zones are represented by rate-weakening behavior and creeping zones are assigned rate-strengthening properties. We model two simplified geometries: a locked patch at the base of a creeping fault and a creeping patch at the surface of a locked fault. In the case of a locked patch within a creeping fault, rupture does not propagate far past the edges of the locked patch, regardless of its radius. The case of a creeping patch within a locked fault is more complicated. The width of the locked areas around the creeping patch determine whether rupture is able to propagate around the creeping patch. Although rupture is always able to propagate at least a small distance into the creeping patch, if the width of the locked zone between the edge of the creeping patch and the end of the fault is too narrow, rupture stops. This simplified parameter study may be useful for understanding first-order behaviors of real-world partially-creeping strike-slip faults.

Recent geological observations imply that slow slip events (SSEs) occur in fault zones with a finite thickness of ~100s of meters. The bulk matrix of the fault zone deforms viscously, while pervasive frictional surfaces are distributed in the viscous matrix. In this theoretical study, we investigate the rupture behaviors of a frictional-viscous-mixing fault model and explore its potential to generate a single SSE. To simultaneously consider both the 10s-kilometer-scale rupture propagations and the 100s-meter-scale “frictional-viscous” features in the same model, we treat a fault zone as a zero-thickness “surface” embedded in an elastic medium. The “frictional-viscous” characteristics are parameterized into a constitutive relation, or “friction law”, where fault strength is partitioned into a frictional and a viscous component. For simplicity, the frictional strength is set to be slip weakening, while the viscous strength increases linearly as the bulk shear rate (slip rate) increases. We explore the rupture behaviors of the above model both analytically and numerically. We find that: 1. Final slip is proportional to the static stress drop and slipping area length, as in fast earthquakes. Peak slip rate increases with dynamic frictional stress drop, while a high viscous coefficient can significantly reduce slip rate, leading to slow slip behaviors. 2. Rupture propagation speed is mainly controlled by the radiation damping factor and viscous coefficient and can be significantly reduced compared to typical fast earthquakes when the viscous coefficient is high. 3. The slip rate decay time increases with the viscous coefficient and slipping area length, which eventually predicts M~T^3 scaling. When frictional stress drop is ~1MPa and the viscous coefficient is smaller than the radiation damping factor μ/(2β), the above models predict the fast slip behavior of regular fast earthquakes. Our model predicts slow slip behaviors in a wide range of parameter space when stress drop is low and viscous coefficient is high. In particular, a frictional strength drop of ~10 kPa and viscous coefficient of 10^4-10^5 μ/(2β) can simultaneously explain many independent characteristic rupture parameters of SSEs. Our model can be further tested with future geophysical, geological, and experimental data.