A fault zone is a complex brittle-frictional system that wears as slip occurs on it. It is formed of three main features, that evolve with fault growth (Figure 1): (i) the cataclastic core contains the cataclastic detritus of wear of the slipping surfaces of the fault. Its width (W C in Figure 1) increases linearly with fault displacement at a rate that depends on the strength of the wall rock (Scholz, 1987(Scholz, , 2019. For displacements greater than a few hundred meters, growth of the fault core levels off at a thickness of a few tens of meters (Scholz, 2019); (ii) Beyond the fault core lies a region of pervasive tensile fracturing which defines the "dilatant damage zone" (W D , Figure 1; e.g., Faulkner et al., 2011;Savage & Brodsky, 2011;Vermilye & Scholz, 1998). The fracture density in this zone dies off as a power law with distance from the fault (e.g., Ostermeijer et al., 2020, and references therein). The dilatant damage zone width increases linearly with fault displacement, and typically levels out at several hundred meters for fault displacements exceeding several hundred meters (Savage & Brodsky, 2011); (iii) Including and extending beyond the dilatant damage zone is what we call the "shear deformation zone" (W S ; Figure 1) which is defined by a region of enhanced seismicity, first pointed out by Powers and Jordan (2010). This zone shows a region of high seismic activity near the fault with a power law fall-off beyond a corner at W S1 to a full half-width of W S2 (Figure 1).