Shear localization in a mature mylonitic rock analog during fast slip

Highly localized slip zones developed within ductile shear zones, such as pseudotachylyte bands occurring within mylonitic fabric rocks, are frequently interpreted as evidence for earthquake nucleation and/or propagation within the ductile regime. To understand brittle/frictional shear localization processes in ductile shear zones and to relate these to earthquake nucleation and propagation, we performed tests with large changes in velocity on a brine‐saturated, 80:20 (wt %) mixture of halite and muscovite gouge after forming a mature mylonitic structure through frictional‐viscous flow. The direct effect a on shear strength that occurs in response to an instantaneous upward velocity‐step is an important parameter in determining the nature of seismic rupture nucleation and propagation. We obtained reproducible results regarding low‐velocity mechanical behavior compared with previous work, but also obtained new insights into effects of sudden increases in slip velocity on localization and strength evolution, at velocities above a critical velocity V c (∼20 μm/s). We found that once a ductile, mylonitic structure has developed in a shear zone, subsequent cataclastic deformation is consistently localized in a narrow zone. This switch to localized deformation is controlled by the imposed velocity and becomes most apparent at velocities above V c . In addition, the direct effect drops rapidly when the velocity exceeds V c . This implies that slip can accelerate toward seismic velocities almost instantly and without much loss of fracture energy, once V c is exceeded. Obtaining a measure for V c in natural faults is therefore of key importance for understanding earthquake nucleation and propagation in the brittle‐ductile transitional regime.