A compilation of many data sets indicates that the exponents for the cumulative length distribution of small faults of L < T, the brittle thickness, and for large faults of L > T, are both approximately 2. The same result was found by Zou and Fialko (2024), but that was fortuitous: their finding being contingent on two cancelling errors. If is 2 for both distributions, it indicates that the exponent B of the cumulative moment distribution is 2/3 for small faults and 1 for large faults, the same values as found for small and large earthquakes. The moment release rates obtained from both earthquake and fault data have also been found to correspond. This demonstrates that in continental regions the long-assumed correspondence between earthquakes and faults is exclusive. All tectonic earthquakes must occur on faults and faults must slip predominately in earthquakes. The relative contribution of small earthquakes/faults to strain is minor for a single large fault over the recurrence time of the fault-rupturing earthquake because that case is governed by the characteristic earthquake model. In contrast, over a larger region containing many moderate sized faults, for which the Gutenberg-Richter distribution holds, the relative contribution of small earthquakes/faults to strain may be significant. These different behaviors characterize mature smooth vs. immature segmented fault systems.

We use high-resolution earthquake locations to characterize the three-dimensional structure of active faults and how it evolves with fault structural maturity. We investigate the distribution of aftershocks of several recent large earthquakes that occurred on crustal strike slip faults of various structural maturity (i.e. various cumulative fault displacement, length, initiation age and slip rate). Aftershocks define a tabular zone of shear deformation surrounding the mainshock rupture plane. Comparing this to geological observations, we conclude that this results from the re-activation of secondary faults. We observe a rapid fall off of the number of aftershocks at a distance range of 0.06-0.25 km from the main fault surface of mature faults, and 0.7-1.5 km from the fault surface of immature faults. The total width of the active shear deformation zone surrounding the main fault plane reaches ~1.5 km and 2.5-6 km for mature and immature faults, respectively. We find that the width of the shear deformation zone decreases as a power law with cumulative fault displacement. Comparing with an existing dynamic rough fault model, we show that the narrowing of the shear deformation zone agrees quantitatively with earlier estimates of the smoothing of faults with displacement, both of which are aspects of fault wear. We compare this evolution of fault structure with several attributes of earthquakes, and find that earthquake stress drop decreases with fault displacement and hence with increased smoothness.