Mechanical and hydrological effects of seamount subduction on megathrust stress and slip

Changing stresses and pore fluid pressures during subduction of seamounts, as simulated with a numerical model that couples mechanical and hydrological processes, help explain observed patterns of megathrust slip. Subduction of rough seafloor and seamounts is thought to impact a broad range of geodynamic processes, including megathrust slip behaviour, forearc fluid flow and long-term structural evolution in the overriding plate. Although there are many conceptual models describing the effects of seamount subduction, our quantitative and mechanistic understanding of the underlying deformation and fluid processes remains incomplete. Here we investigate the interplay between sediment consolidation, faulting and the evolution of stress and pore fluid pressure in response to seamount subduction, using a numerical model that couples mechanical and hydrological processes and is constrained by laboratory and field observations. Our results show that subducting topography drives marked spatial variations in tectonic loading, sediment consolidation and megathrust stress state. Downdip of a subducting seamount on its leading flank, enhanced compression and drainage lead to large fault-normal stress and overconsolidated wall rocks. A stress shadow in the seamount's wake leads to anomalously high sediment porosity. These variations help explain observed patterns of megathrust slip, with earthquakes and microseismicity favoured at the downdip edge of seamounts and aseismic or slow slip in the updip stress shadow.