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We report on laboratory experiments designed to illuminate grain‐scale deformation mechanisms within fault gouge. We vary particle size distribution, grain and surface roughness, and gouge layer thickness to better understand how grain sliding, rolling, dilation, and compaction affect the strength and stability of granular fault gouge. The experiments employed the double direct shear testing geometry and were run at room temperature, controlled humidity, and shearing rates from 0.1 to 3000 μm/s. Experiments were carried out under constant normal stress of 5 and 10 MPa and thus within a nonfracture loading regime where sliding friction for smooth, spherical particles is measurably lower than for rough, angular particles. We compare results from shear between smooth boundaries, where we hypothesize that grain boundary sliding is the dominant deformation mechanism, and roughened surfaces, where rolling …