The effects of variable thermal and hydraulic properties on thermal pressurization in localized fault zones

Thermal pressurization (TP) of pore fluids is predicted to be a dominant frictional weakening mechanism during earthquakes. The current prevailing models for frictional sliding with TP assume constant hydraulic and thermal properties (e.g., permeability) of the fault rocks. These models estimate that a fault where TP is activated should experience near-complete stress drops. However, the TP mechanism intrinsically changes the temperature and pore pressure in the fault zone. Consequently, the fault zone rocks physical, thermal and hydraulic properties change as well. We present a model for a localized fault zone, where slip is restricted to a plane, with temperature- and effective pressure-dependent thermal and hydraulic properties. We show that for very low permeability (<10-20 m2) and porosity (<2%) the fault exhibits more frictional weakening, compared to the constant properties case, further limiting the temperature-rise in the fault zone. When considering more permeable (~10-19 m2) and porous (17%) fault gouge we find that fault exhibits less frictional weakening and greater temperature-rise in the fault zone, than predicted in the constant properties case. We find that the parameters that control whether fluid pressurization or diffusion dominates, i.e., the pressurization factor and hydraulic diffusivity, evolve considerably during TP. Our analysis examines the complex poromechanical response of the fault zone during TP and explores the competition between fluid diffusion and pressurization.