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

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, vertical bar b-a vertical bar, 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.

Automated summary

Geological observations show that fault zone composition varies, potentially accommodating a mixture of brittle and ductile deformation, with the ratio of these materials and friction changes affecting energy partitioning and slip behavior. The nature of this mixture plays a role in determining whether the fault creeps steadily or engages in slow slip events or fast earthquakes. Structural features observed in simulated creep, slow slip events, and earthquakes have similarities with those seen in natural fault zones, and slip-synchronous strengthening in ductile sections can control the propagation of yearlong slow slip events.