Thermo‐poro‐mechanics of chemically active creeping faults. 1: Theory and steady state considerations

Abstract

AbstractIn this paper, we study the behavior of a fluid‐saturated fault under shear, based on the assumption that the material inside exhibits rate‐ and temperature‐dependent frictional behavior. A creeping fault of this type can produce excess heat due to shear heating, reaching temperatures which are high enough to trigger endothermic chemical reactions. We focus on fluid‐release reactions and incorporate excess pore pressure generation and porosity variations due to the chemical effects (a process called chemical pressurization). We provide the mathematical formulation for coupled thermo‐hydro‐chemo‐mechanical processes and study the influence of the frictional, hydraulic, and chemical properties of the material, along with the boundary conditions of the problem on the behavior of the fault. Regimes of stable‐frictional sliding and pressurization emerge, and the conditions for the appearance of periodic creep‐to‐pressurization instabilities are then derived. The model thus extends the classical mechanical stick‐slip instabilities by identifying chemical pressurization as the process governing the slip phase. The different stability regimes identified match the geological observations about subduction zones. The model presented was specifically tested in the Episodic Tremor and Slip sequence of the Cascadia megathrust, reproducing the displacement data available from the GPS network installed. Through this process, we identify that the slow slip events in Cascadia could be due to the in situ dehydration of serpentinite minerals. During this process, the fluid pressures increase to sublithostatic values and lead to the weakening of the creeping slab.