Fig. 1

Locations of the seafloor fiber-optic cable and hypocenter distribution during the observation period. The bold line represents the cable route of the Comprehensive Seafloor Monitoring System off Cape Muroto, with the red section indicating the 50-km-long DAS observation section. The two small open triangles along the cable show the ocean-bottom seismometer locations. The triangles with different colors are the stations that were used for a comparison with the DAS observations: MURH and KTGH (Hi-net), UMJF (F-net), and MRA04 and MRG29 (DONET2). Red circles indicate the epicenters of earthquakes that occurred during the January 27–31, 2020 (JST), period, as determined by the Japan Meteorological Agency