Splay Fault Permeability Governs Fluid–Structure Interaction in Accretionary Wedges

Abstract Interactions between upper plate deformation and plate interface seismicity in subduction zones remain poorly understood, but growing evidence indicates that fluid flow along splay faults modulates upper‐plate faulting. Field observations from two exhumed splay faults define end‐member scenarios where impermeable faults trap fluids in their footwall, whereas permeable faults channel fluids along them. Using finite element poroelastic models, with slip mode inferred solely from stress–pore‐fluid pressure patterns, we define two end‐member behaviors: (a) Impermeable, clay‐rich, mature splay faults favor footwall fluid flow, promoting low differential stress, dilation, and vein formation in the upper plate, while reducing pore‐fluid pressure and coupling the megathrust downdip of the intersection. (b) Permeable, less mature faults allow distributed upper‐plate fluid flow, increasing fluid flux, and differential stress, while maintaining plate interface overpressure and promoting creep. These models provide a framework for prism‐scale effects of splay fault permeability on shallow subduction zone deformation and seismicity.