Laboratory gouge friction: Seismic‐like slip weakening and secondary rate‐ and state‐effects

We investigate experimentally the frictional response of a thick sample of simulated fault gouge submitted to very high shear displacements (up to 40 m) in an annular simple shear apparatus (ACSA). The frictional strength of our granular material exhibits velocity‐weakening consistent with classical rate‐ and state‐dependent friction laws. The length scale involved in the latter phenomenon is d c = 100 μm. However, the evolution of friction is largely dominated by a significant slip‐weakening active over decimetric distances ( L = 0.5 m). Interestingly, these decimetric frictional length scales are quantitatively compatible with those estimated for natural faults. During shearing, a thin and highly‐sheared layer emerges from the thick and slowly‐deforming bulk of the sample. Because of the intermittent and non‐local coupling observed between these two zones, we relate the large frictional length scales in our data to the slow structuring of the thick interface.