A mechanism for transient creep in crustal shear zones

Abstract

The brittle-to-ductile transition in faults and shear zones marks the change from frictional stick-slip behavior in the seismogenic zone to creep accommodated by plastic mechanisms at higher temperatures. Geodetic observations of slow slip events (SSEs) on plate-boundary shear zones indicate that transient periods of elevated aseismic slip rate occur beneath the seismogenic zone where dislocation creep is likely to be active. SSE duration on the San Andreas fault (California, USA) and major subduction zones decreases systematically with increasing depth and temperature. However, current flow laws for dislocation creep are unable to explain the occurrence of transients similar to SSEs. We develop a microphysical model of dislocation creep that shows how small perturbations in stress (of the order kPa) can lead to strain-rate transients orders of magnitude faster than the background during nominally steady-state flow. The model explains the first-order characteristics of SSEs, reconciling the geophysical observations of deep slow slip events with expectations from experimental rock mechanics. Given that these microphysical mechanisms are general to rock-forming silicate minerals, our results suggest that non-steady-state deformation is inherent to natural shear zones that deform by dislocation creep and that the transition from brittle to steady-state creep occurs over a broad depth range within which transients are predominant.