Evolution of frictional properties, permeability and elastic parameters due to surface roughness changes during fault maturation in laboratory shear experiments

The surface roughness and geometry of faults play a key role in determining their frictional behavior and poromechanical properties including permeability. These attributes and their interplay are important in geothermal reservoirs, where generating and maintaining permeability must be accomplished while limiting hazardous induced seismicity. Newly created fractures are rough, but their roughness changes with cumulative slip offset. Here, we describe systematically measured roughness changes with accumulated slip on laboratory faults and link it to changes in frictional behavior and along-fault permeability. We explore the contrasting responses of granitoid and gneiss samples taken from the UtahFORGE geothermal demonstration site. The gneiss develops striations along the fault, has friction of order 0.7 and shows velocity-strengthening behavior. Conversely, the granitoid fault retains isotropic roughness, with friction of ∼0.6 and velocity-weakening behavior. We link differences in friction to the radius of curvature of asperity contacts. Roughness changes can be partially tracked by P-wave speed and amplitude of fracture-crossing waves. We observe a directionality in P-wave velocities and amplitudes oblique to the fracture, maximized in the direction of the principal stress as dependent on friction coefficient. Such directionality in P-wave velocity may be indicative of in situ stress state of faults. Permeability shows a large dependence on sliding velocity and a minor increase with ongoing slip, due to the formation of connected fluid pathways. Altogether, our results indicate that as faults mature, the decrease in roughness due to asperity elongation reduces both friction coefficient and frictional instability, potentially reducing the risk of hazardous induced seismicity.