Abstract Fine‐grained gouge found in fault cores controls earthquake generation through its frictional behavior and microstructural evolution. Here we present friction experiments on natural granitic gouges of various initial grain sizes at hydrothermal conditions. To obtain the velocity(V)‐dependence of friction of both fine‐ and coarse‐grained samples, velocity‐stepping tests were conducted between 0.01 and 100 μm/s at 25–600°C, 100 MPa pore pressure and 100 MPa effective normal stress. In the rate‐and‐state friction framework, we observed that fine‐grained gouge is strongly velocity‐weakening over a substantially broader range in temperature than coarse‐grained gouge at V of 1–100 μm/s. Velocity‐weakening may result from a combination of mechanical comminution (grain size reduction) and dilatant granular (i.e., cataclastic) flow, operating in competition with compaction and creep involving grain scale mass transfer by pressure solution. We also infer that temperature‐dependent grain growth of ultrafine particles plays a role in determining the upper temperature limit of velocity‐weakening.

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