Upper-plate controls on subduction zone geometry, hydration and earthquake behaviour

Many characteristics of the incoming oceanic lithosphere, such as its age, rigidity, fabric orientation or sediment thickness, are often cited as important properties controlling the geometry, state of stress, dynamics and hazard potential of subduction zones, yet the links between upper-plate structures and subduction zone processes remain poorly understood. Here we report that high forearc wavespeeds (vP greater than 6.6 km s−1) beneath 8,000 km2 of Kii Peninsula are associated with the Kumano pluton. We show that the dense, high-rigidity Kumano pluton generates a large vertical load, which forces the incoming Philippine Sea Plate to subduct with a trajectory that is a factor of two steeper than adjacent regions. Beneath the region of maximum curvature and faulting of the Philippine Sea Plate, reduced mantle velocities (6.5–7.5 km s−1) within a 25-km-thick, 100-km-wide region at 5–30-km sub-Moho depths may reflect serpentinization (more than 40% antigorite) of the subducting mantle and enhanced porosity from bending stresses. We further report that great (larger than Mw 8) earthquakes nucleated from the flanks of the Kumano pluton in 1944 and 1946. Our study demonstrates the profound impact of upper-plate structures on the geometry, hydration state and segmentation of large megathrust earthquakes at subduction zones.