Along-trench variations in the seismic structure of the incoming Pacific plate at the outer rise of the northern Japan Trench

To investigate along-trench variations in the seismic structure of the incoming oceanic plate and their effect on water transportation by the oceanic plate, we conducted a wide-angle seismic survey of a trench-parallel transect 270 km long on the outer rise of the northern Japan Trench. The resulting seismic structure models show that the central part of the transect is characterized by rough topography, thick oceanic crust, low seismic velocities, and high V-p/V-s ratios, suggesting pervasive fracturing and high water content (hydration) there. These observations are consistent with the presence of an ancient fracture zone associated with ridge propagation. The trenchward extension of this fracture zone corresponds to an area of low interplate seismicity, low seismic velocities, and high V-p/V-s ratio around the depth of the subduction interface. Our results suggest that this ancient scar on the oceanic plate influences along-trench variations in interplate seismic coupling through its effect on water transportation.