Extended Data Fig. 10: Additional evidence for velocity strengthening in quartz gouge.

Friction versus slip (a, b) and slip rate versus slip (c, d) for rupture sequence 1 at the edge of the gouge layer, \({x}_{1}=1{\rm{mm}}\) and \({x}_{2}=2{\rm{mm}}\). These plots show that friction increases during event A over slips of roughly 5 μm, while the slip velocity increases from roughly 0.5 m s⁻¹ to a peak value of over 1 m s⁻¹. In event B (red), substantial weakening of ~0.15 in friction coefficient occurs over slip scales of just 2 μm or so as slip rate increases. At \({x}_{1}=8{\rm{mm}}\) (Fig. 4a), weakening of 0.22 occurs over slip scales of approximately 5 μm, again indicating that weakening of 0.1 in friction occurs over slips of 2–3 μm. Hence the strengthening and weakening occur over similar slip scales indicating that both are sustained friction response. Note that if the strengthening were due to the direct effect of otherwise rate-weakening rate-and-state friction, then the strengthening would occur over negligible slips compared to the weakening.