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

Slow earthquakes are considered to be the key to understanding the nature of subducting plate boundaries and their seismic coupling. Along the Nankai Subduction zone, many slow earthquakes have been observed and their distribution shows along-strike variation. In the central part of the Nankai Trough, many very low frequency earthquakes (VLFEs) have been observed recently, but rare cases have been reported in the central Nankai Trough between the Kii Channel and Cape Shionomisaki. To investigate the structural features and its possible relationship to the slow events, 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 multichannel seismic reflection data) from the seaward side of the subduction trough to the accretionary prism off Cape Shionomisaki. We applied travel time tomography and graph-space optimal transport based full waveform inversion to the OBS data, and reconstruct the P-wave velocity down to the upper mantle, coordinating with a migrated seismic reflection profile to interpret subsurface structural features. From a comparison with a transect across the adjacent Kumano Basin, where there are dense VLFEs, we find that structural and physical differences between the two locations lead to different seismic activities.