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Roma sed fo line 1 e and imest and Shimamoto, 2003), in laboratory experiments (Frye and laboratory simulations of faults (Beeler et al., 1996) show Journal of Structural GeologyE-mail address: billi@uniroma3.it.Through the conventional sieving-and-weighting method, the grain size distribution of 20 samples of fault rocks was obtained. The distributions follow power-laws with fractal dimension (D) in the 2.00977–3.04008 range. Gouge D-values are proportional to the normalised thickness of the corresponding gouge zones. For a gouge D-valuez2.2, the thickness of the corresponding gouge zone is only about 3 % of the fault core thickness, whereas for a gouge D-valuez3.0, the thickness of the corresponding gouge zone is almost 90 % of the fault core thickness. Results from this study suggest that, with progressing fault displacement: (i) grain comminution in fault cores occurred mostly by early bulk fragmentation of grains and late grain abrasion; (ii) breccia zones were progressively incorporated into the adjacent gouge zones