The Evolution of Pore Pressure, Stress, and Physical Properties During Sediment Accretion at Subduction Zones

Abstract

AbstractWe study stress, pressure, and rock properties in evolving accretionary wedges using analytical formulations and geomechanical models. The evolution of the stress state from that imposed by uniaxial burial seaward of the trench to Coulomb failure within the wedge generates overpressure and drives compaction above the décollement. Changes in both mean and shear stress generate overpressure and shear‐induced pressures play a particularly important role in the trench area. In the transition zone between uniaxial burial and Coulomb failure, shear‐induced overpressures increase more than overburden and are higher than footwall pressures. This rapid increase in overpressure reduces the effective normal stress and weakens the plate interface along a zone that onsets ahead of the trench and persists well into the subduction zone. It also drives dewatering at the trench, which enables compaction of the hanging‐wall sediments and a porosity offset at the décollement. Within the accretionary wedge, sediments are at Coulomb failure and the pore pressure response is proportional to changes in mean stress. Low permeability and high convergence rates promote overpressure generation in the wedge, which limits sediment strength. Our results may provide a hydromechanical explanation for a wide range of observed behaviors, including the development of protothrust zones, widespread occurrence of shallow slow earthquake phenomena, and the propagation of large shallow coseismic slip.