Relative plate motion in subduction zones transitions from frictional slip to viscous flow with increasing depth and temperature. The frictional-viscous transition can control the depth extent of megathrust earthquakes and episodic tremor and slip (ETS). Pore fluid pressure is a critical control on the transition, but models for its depth dependence are lacking. Here, we present a steady-state modeling framework to calculate the fluid pressure and shear stress profile along the subduction interface. We consider fluid production from dehydration reactions in the subducting oceanic lithosphere, calculated from thermodynamic equilibrium models. These fluids are channeled up-dip, though in some models we allow for fluid loss into the overriding plate. The fluid pressure is calculated from Darcy's law, with permeability depending on effective stress, temperature, and slip rate. We allow for both rate-state frictional sliding and thermally activated linear viscous flow, and solve for the partitioning of deformation between them. We apply the modeling framework to the Cascadia subduction zone. Our results show nearly uniform effective stress in the seismogenic zone, below which it decreases with depth. The frictional-viscous transition spans a wide range of depths, including the ETS source depth. Alternative models adding fluid loss at the mantle wedge corner or locally low-permeability rocks produce a non-monotonic frictional-viscous transition, which may explain the gap between the seismogenic zone and the ETS zone in Cascadia. Our results provide important insight into earthquake hazards and the mechanism of ETS in Cascadia, and the modeling framework is applicable to global subduction zones.
Ozawa et al. (Thu,) studied this question.