The Mechanics of Creep, Slow Slip Events, and Earthquakes in Mixed Brittle‐Ductile Fault Zones

Abstract

Geological observations show that fault zone composition varies and often accommodates a mixture of brittle and ductile deformation. There is growing evidence that the nature of this mixture may play an important role in determining whether the fault creeps steadily or slides in slow slip events (SSEs) and/or fast earthquakes. Using numerical experiments of slip events in a fault zone of finite thickness, we explore how the ratio of brittle to ductile material and the absolute friction change resulting from a variation in slip velocity, |b−a|, affect energy partitioning and slip behavior in brittle‐ductile mixtures. We treat brittle material as Mohr‐Coulomb elastoplastic and ductile material as Maxwell viscoelastic. We simulate velocity‐weakening (a−b<0) behavior in the brittle part of the mixture and velocity‐strengthening (a−b≥0) behavior in the ductile part using a rate‐and‐state formulation dependent on plastic strain accumulation. We show that: (1) mixtures can exhibit multiple slip behaviors including earthquakes and slow slip, (2) highly brittle mixtures do not tend to generate SSEs while weakly brittle mixtures can generate slow slip over a wider range of compositions, (3) structural features formed during simulated creep, SSEs, and earthquakes share notable similarities with structures observed in natural fault zones. We find that slip‐synchronous strengthening in the ductile portion of the mixture controls whether a rupture propagates as SSEs of yearlong durations. Shorter duration SSEs occur when the length of the plastic shear segments formed during slip is similar to the characteristic weakening distance for an earthquake.

Transient Slow Slip Characteristics of Frictional‐Viscous Subduction Megathrust Shear Zones

“…In the purely elastic-frictional, one-dimensional model case of Skarbek et al (2012), the transition from elastodynamic slip to slow slip was more abrupt and the range over which slow slip events could be expected was comparatively narrow. In the case of Lavier et al (2021), velocity-neutral conditions in the ductile matrix are similar to our threshold viscosity models, whereas increasing velocity-strengthening conditions simulates decreasing matrix viscosity. Their models produced behaviors similar to our below-threshold models shown in Figure 7, with aseismic creep dominating at low clast concentrations, transitioning to transient slip events when clast concentrations are increased to between 45% and 80% E (cf.…”

“…More recently, Lavier et al. (2021) explored the role of brittle‐ductile interactions in finite‐width shear zones, simulating ductile regions by matching velocity‐strengthening frictional properties to variations in viscosity. Each of these model frameworks predicted a transition in rupture behavior from elastodynamic to slow slip, similar to what we observe here.…”

“…Figure 4c in Lavier et al. [2021]). In the case of our models, however, these transitions in slip style can be generated simply by varying matrix viscosity, with no variations in the velocity‐dependent frictional properties within the shear zone required.…”

“…Skarbek et al (2012) similarly examined slip behaviors for RSF models with alternating velocity-weakening and strengthening patches, varying both the E a b  parameters and the patch distributions. More recently, Lavier et al (2021) explored the role of brittle-ductile interactions in finite-width shear zones, simulating ductile regions by matching velocity-strengthening frictional properties to variations in viscosity. Each of these model frameworks predicted a transition in rupture behavior from elastodynamic to slow slip, similar to what we observe here.…”

“…Their models produced behaviors similar to our below-threshold models shown in Figure 7, with aseismic creep dominating at low clast concentrations, transitioning to transient slip events when clast concentrations are increased to between 45% and 80% E (cf. Figure 4c in Lavier et al [2021]). In the case of our models, however, these transitions in slip style can be generated simply by varying matrix viscosity, with no variations in the velocity-dependent frictional properties within the shear zone required.…”