Hydraulic diffusivity of fault gouge zones and implications for thermal pressurization during seismic slip

Abstract

Laboratory-determined permeability and compressibility data for natural fault gouge samples from the Median Tectonic Line (MTL) are presented and used to estimate hydraulic diffusivities in fault gouge zones. Bulk compressibility varies with effective pressure in a log-linear manner. Hydraulic diffusivity decreases significantly during the first isotropic loading partly due to a plastic compaction component, but does not significantly change during elastic unloading. Hydraulic diffusivity decreases with decreasing gouge grain size and is lowest in the very fine-grained centre of the fault zone, identified as the most recent principal displacement zone of the MTL. Previous models of fluid-controlled dynamic strength evolution during seismic slip are assessed using the data. The data suggest that the most recent principal displacement zone has a characteristic hydraulic diffusion length lower than the half width of the low-permeability zone. Hence pressurized fluid is unlikely to escape into the surrounding high-permeability fault rocks over the lifetime of an earthquake slip event, suggesting that thermal pressurization is likely to occur if the rupture plane is confined to the low-permeability gouge principal displacement zone.