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THEORETICAL and experimentally based laws for seismic faulting contain a critical slip distance^1–5, D_c, which is the slip over which strength breaks down during earthquake nucleation. On an earthquake-generating fault, this distance plays a key role in determining the rupture nucleation dimension⁶, the amount of premonitory and post-seismic slip^7–10, and the maximum seismic ground acceleration^1,11. In laboratory friction experiments, D

_c has been related to the size of surface contact junctions^2,5,12; thus, the discrepancy between laboratory measurements of D_c (∼10⁻⁵m) and values obtained from modelling earthquakes (∼10⁻²m) has been attributed to differences in roughness between laboratory surfaces and natural faults⁵. This interpretation predicts a dependence of D_c on the particle size of fault gouge² (breccia and wear material) but not on shear strain. Here we present experimental results …