Permeability Development During Fault Growth and Slip in Granite

In tight crystalline rocks faults are known to be substantially more hydraulically conductive thanthe rock matrix. However, most of our knowledge relies on static measurements, or before/after failure data sets.The spatio‐temporal evolution of the permeability field during faulting remains unknown. Here, we determine atwhich stage of faulting permeability changes most, and the degree of permeability heterogeneity along shearfaults. We conducted triaxial deformation experiments in intact Westerly granite, where faulting was stabilizedby monitoring acoustic emission rate. At repeated stages during deformation and faulting we pauseddeformation and imposed macroscopic fluid flow to characterize the overall permeability of the material. Thepore pressure distribution was measured along the prospective fault to estimate apparent hydraulictransmissivity, and propagation of the macroscopic shear fault was monitored by locating acoustic emissions.We find that average permeability increases dramatically (by two orders of magnitude) with increasingdeformation up to peak stress, where the fault is not yet through‐going. Post‐peak stress, overall permeabilityincreases by a factor of three. However, at this stage we observed local heterogeneities in permeability by up tofactors of eight, ascribed to a partially connected fracture network. This heterogeneity decreases with faultcompletion at residual shear stress. With further slip on the newly formed fault, the average hydraulictransmissivity remains mostly stable. Our results show that permeability enhancement during shear rupturemostly occurs ahead of the rupture tip, and that strongly heterogeneous permeability patterns are generated inthe fault cohesive zone due to partial fracture connectivity.