Evolution of Pore Fluid Pressure in an Accretionary Outer Wedge Over Megathrust Earthquake Cycles: The Northern Cascadia Example

The evolution of pore fluid pressure ( ) in the most seaward portion of a subduction accretionary prism plays an essential role in the cycle of tsunamigenic earthquakes. Based on recent geophysical observations, here we propose a testable conceptual model for this evolution for northern Cascadia offshore of Washington. During a large subduction earthquake, stress increase on the near‐trench part of the megathrust causes compressive stress loading and increase in the outer wedge. Following the earthquake, drainage through pore space and permeable fault zones allows to return to the background preseismic state. Using simple dynamic Coulomb wedge and fluid pressure diffusion models, we demonstrate the feasibility of this conceptual model. With reasonable parameters, the models predict a low‐to‐moderate above hydrostatic in most of the interseismic period, and a coseismic elevation of by ∼15–20 MPa near the base of the outer wedge. The postseismic drainage timescale is estimated to be several to tens of years, consistent with the present‐day overall absence of active seafloor venting above the northern Cascadia outer wedge. The efficient postseismic drainage over many earthquake cycles may contribute to the over‐consolidation of the outer wedge.