Fracture density in fault damage zones decays with perpendicular distance r from the fault core, commonly as a power law ρ(r) ~ r^(-n). The exponent n is widely used as a proxy for structural maturity, falling from ~0.8 on small isolated faults to 0.2–0.5 on faults with kilometre-scale displacement. Yet at comparable displacement n varies by more than a factor of two, a spread that persists even among faults of similar faulting depth, layer thickness and lithology. Cumulative displacement is a lossy variable that does not record how slip was delivered. Faults reaching the same displacement through few large events or many small increments follow different loading paths. Displacement also co-varies with roughness, gouge, and the number of strands. Geometry, wear and loading history therefore cannot be separated from field data alone.
We therefore ran the controlled experiment that the field cannot provide. Using in situ X-ray tomography, we tracked fracture growth around the laboratory faults with sawtooth geometry in two uniaxially loaded granite cores, holding geometry and lithology fixed and varying only the loading path between two end members: monotonic loading to failure and cyclic loading with an upper limit at 79% of peak strength. Damage decays as a power law throughout both paths, but the two diverge progressively; in the last scan before failure, n = 0.70±0.096 under monotonic loading, but n = 0.30±0.091 under cyclic loading. On a single interface with no secondary strands, the loading path alone spans the range of n that the field compilation develops over five orders of magnitude of displacement.
To identify the mechanism, we built a physics-based model that reproduces both paths. Under cyclic loading, crack tips whose stress intensity remains below toughness (KIc) still advance by subcritical growth, and extend damage into the low-stress region away from the interface, lowering n, whereas monotonic cracking stays confined to asperity contacts.
Revisiting the field compilation, faults of comparable lithology, kinematics and displacement but subject to more stress perturbation carry lower n, as the mechanism predicts. The structure of the damage zone therefore archives loading history as well as cumulative displacement, and the spread in n at fixed displacement records it.
 

fault damage zones,loading history,in situ micro-CT,cyclic loading,subcritical crack growth