The Iyo-nada Sea is the inland sea in southwestern Japan and is located at the western end of the Median Tectonic Line (MTL) that extends east-west through southwestern Japan. We conducted seismic reflection and refraction surveys to understand geological structures and to evaluate activity of faults at the Iyo-nada Sea. We recognized three geological structures accompanied with the MTL in this survey. Those are the boundary between the Ryoke granites and the Sambagawa metamorphic rocks (R/S boundary), the boundary between the Izumi Group and the Sambagawa metamorphic rocks (Iz/S boundary) and the MTL active fault system (MTLAFS). The R/S boundary in the Iyo-nada Sea strikes southwest, coinciding with the linear trace of the MTLAFS. On the other hand, the Iz/S boundary extends southwest along a curved coastline of the Sadamisaki Peninsula and is not concordant with the trace of the MTLAFS. Additionally, the Iz/S boundary in the central Iyo-nada Sea is overlaid with sedimentary layers (Neogene to Quaternary) based on shallow seismic survey results. These results indicate that the Iz/S boundary is not an active fault in the central Iyo-nada Sea.
