The distribution of seismicity about strike-slip faults provides measurements of fault roughness and damage zone width. In California, seismicity decays with distance from strike-slip faults according to a power law ∼(1 + x2/d2)−γ/2. This scaling relation holds out to a fault-normal distance x of 3–6 km and is compatible with a “rough fault loading” model in which the inner scale d measures the half width of a volumetric damage zone and the roll-off rate γ is governed by stress variations due to fault roughness. According to Dieterich and Smith's 2-D simulations, γ approximates the fractal dimension of along-strike roughness. Near-fault seismicity is more localized on faults in northern California (NoCal, d = 60 ± 20 m, γ = 1.65 ± .05) than in southern California (SoCal, d = 220 ± 40 m, γ = 1.16 ± .05). The Parkfield region has a damage zone half width (d = 120 ± 30 m) consistent with the SAFOD drilling estimate; its high roll-off rate (γ = 2.30 ± .25) indicates a relatively flat roughness spectrum: ∼k−1 versus k−2 for NoCal, k−3 for SoCal. Our damage zone widths (the first direct estimates averaged over the seismogenic layer) can be interpreted in terms of an across-strike “fault core multiplicity” that is ∼1 in NoCal, ∼2 at Parkfield, and ∼3 in SoCal. The localization of seismicity near individual faults correlates with cumulative offset, seismic productivity, and aseismic slip, consistent with a model in which faults originate as branched networks with broad, multicore damage zones and evolve toward more localized, lineated features with low fault core multiplicity, thinner damage zones, and less seismic coupling. Our results suggest how earthquake triggering statistics might be modified by the presence of faults.