Scaling of Permeability Within Faults Across Nine Orders of Magnitude of Displacement

Faults are ubiquitous structures, ranging in length from millimeters to thousands of kilometers, with significant variations in permeability that regulate regional fluid flow, solute transport, seismicity, and hydrothermal circulation within the crust. Measurement of in situ fault permeability is challenging due to drilling difficulties and the risk of hydraulic fracturing. Moreover, existing scaling laws of laboratory permeability or fracturing intensity within faults are site-specific, highlighting the need for universal laws. Furthermore, damage zone permeabilities (k_DZ) normalized to the protolith permeability (k_NDZ) are typically high, while normalized fault core permeability (k_NC) varies. We analyzed 752 in situ injection tests and 967 geomechanical experiments on seven faults with shear displacements (D) ranging from 1 to 5 m in the Asmari–Jahrum Formation (AJF), Iran. The AJF database was supplemented with 334 k_DZ and 64 k_NC datasets from the literature, covering 245 faults and spanning nine orders of magnitude in D. We quantified the hydraulic roles of fault cores as conduits (k_NC>1) or barriers (k_NC<1) based on porosity changes. We also developed k_NC scaling laws using displacement divided by fault core thickness within a fuzzy-logic framework. A universal k_NDZ law was established using distance from the fault core, damage zone thickness, and geomechanical parameters through kriging analysis. The universal material- and fault-dependent k_NDZ and k_NC laws indicate variations up to ten orders of magnitude in permeability. These findings enhance our understanding of fault hydrology and offer predictive tools for estimating fault permeability.