Seismic and Aseismic Slip Compete to Regulate Permeability Evolution During Fault Reactivation

Abstract Crustal permeability can be created through microearthquakes as observed at field and laboratory scales. The respective roles of seismic and aseismic slip in governing permeability evolution remain ill‐constrained. Here, we present observations of controlled pore pressure stepping experiments designed to distinguish the separate influences of seismic and aseismic slip on permeability evolution. We show that permeability first decreases with slip before subsequently increasing then finishing with a net increase in permeability. This behavior tracks with the net increases in seismic moment throughout the reactivation as a fraction of the overall combined (seismic plus aseismic) moment. We observe that the aseismic moment can account for ∼90% of the total moment. We develop a mechanistic model to recreate these non‐monotonic observations, incorporating shear dilation modulated by fracture compactions. Our results demonstrate that aseismic slip exerts an indispensable control on crustal permeability evolution, providing new constraints on fluid–fault interactions in the crust.