The Pressure‐ and Temperature‐Dependence of Thermal Pressurization in Localized Faults.

Thermal pressurization (TP) is predicted to be an important dynamic frictional weakening mechanism during earthquakes. The prevailing models, though, assume that the physical, thermal and hydraulic properties of the fluid‐saturated rock in the fault zone are constant during TP (the constant case) which inherently involves temperature and pressure changes. We solve the governing equations for TP in their general form with pressure‐ and temperature‐dependent physical, thermal and hydraulic parameters of a saturated fault zone with a one‐dimensional numerical model (the variable case). The model considers slip on a plane at a constant rate, and so does not account for dynamic frictional rupture scenario. We test a wide range of medium permeabilities of 10−22–10−16 m2 and porosities 0.5%–17%, based on experimental data for Frederick diabase, Westerly granite and the Hanaore Fault gouge. We find that the predicted shear stress drop and temperature rise is similar between the two cases for low permeability (<10−20 m2) and low porosity (<1%) fault rocks, owing to an increase in the fluid pressurization factor and hydraulic diffusivity. Differences between the two models are evident in more permeable and porous fault rocks. The increase in hydraulic diffusivity during TP results in a diffusional length which scales with time0.7 in the variable case. In addition, our calculations show that it is important to apply the constant case model with the ambient initial conditions for the modeled fault zone and not time‐averaged values that aim to represent the temperature and pressure changes that occur during TP. Plain Language Summary: Thermal pressurization (TP) is a dynamic frictional weakening mechanism that is predicted to operate during earthquakes. TP activates when frictional heat causes the fluids in the pores of the rocks to pressurize due to differences in thermal expansivity between fluids and rocks. The elevated pore pressure decreases the frictional resistance during sliding. The current models for TP assume that the properties of the fault rocks and fluids are constant (the constant case), whereas in practice, these properties vary due to temperature and pressure changes during TP. We solve the general form of the governing equations for TP with a one‐dimensional numerical model with pressure‐ and temperature‐dependent fluid and rock properties (the variable case). We compare the two models for different fault rocks that span a wide range of hydraulic properties. Our results show that for low permeability (<10−20 m2) and low porosity (<1%) rocks the two cases are similar. However, for more permeable and porous rocks the two models differ with continuous fault slip. Furthermore, we find that it is important to apply ambient initial conditions in the constant case model, rather than some time‐averaged conditions that aim to compensate for the lack of sensitivity to pressure and temperature changes. Key Points: Changes in hydraulic diffusivity and pressurization factor during thermal pressurization (TP) balance each other in low permeability and low porosity fault rocksHydraulic diffusional length scales as time0.7 when considering TP parameters that depend on temperature and pressureThe constant case model should be considered with ambient initial conditions and not time‐averaged ones [ABSTRACT FROM AUTHOR]