Elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust, Nankai Trough subduction zone

Pore fluid pressure is a key parameter governing both the shear strength of faults and the conditions for seismic and tsunamigenic slip on subduction zone megathrusts. However, quantitative constraints on this parameter based on in situ data in these, or any, faults have proven elusive. Using seismic interval velocities derived from a three-dimensional seismic reflection survey, we compute porosity, fluid pressure, and effective stress at the plate interface in the Nankai Trough subduction zone (offshore southwestern Japan) to 20 km down the fault dip using well-constrained, locally calibrated empirical relationships between velocity, porosity, and effective stress. We show that the fault and immediately subjacent footwall are nearly undrained, implying that the subduction megathrust slips under a shear stress of