Seismic imaging of the Southwest Japan arc from the Nankai trough to the Japan Sea

► High-resolution tomography under SW Japan forearc and back-arc is determined. ► Suboceanic earthquakes are relocated precisely using sP depth phases. ► Structural heterogeneity affects seismogenesis of the megathrust zone. ► Subducted seamounts and ridges may form asperities in the megathrust zone.