Metamorphic Dehydration, Fluid Pressure, and the Frictional‐Viscous Transition Along Subduction Megathrusts: Case Study in Cascadia and Implications for Slow Earthquakes

Abstract 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 updip, 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 and fluid pressure approaches lithostatic pressure. The frictional‐viscous transition spans a wide range of depths, and mixed frictional‐viscous deformation is predicted at the ETS source depth. A model with fluid leak‐off into the overlying plate produces a more heterogeneous effective stress, with a local minimum near the major dehydration depth. 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.