High‐Density Seismic Refraction Imaging of Plate‐Boundary Structures in the Slow Earthquake Gap Zone off Western Kii Peninsula, Nankai Trough

Abstract Despite recent observations of slow earthquakes along the Nankai subduction zone, none have been reported in the central Nankai Trough between the Kii Channel and Cape Shionomisaki. In November 2018, a very dense array of 96 ocean‐bottom seismometers were deployed by JAMSTEC to acquire active‐source seismic refraction dataset (supplemented by a multichannel seismic reflection profile) from the seaward side of the subduction trough to the accretionary prism off Cape Shionomisaki. We applied traveltime tomography to the refraction data to constrain the P wave velocity down to the upper mantle, coordinating with a migrated seismic reflection profile to confirm the depth of the Moho and interpret shallower structural features. From a comparison with a transect across the Kumano basin, we conclude that structural and physical differences between these two locations, especially the geometry of the subducting plate surface, lead to different slow earthquake activities.