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Although more than 80 percent of earthquakes in Japan occur on the seafloor, seismic network on the seafloor is quite few and insufficient. As a step to increase seafloor seismic network, Comprehensive Seafloor Monitoring System was deployed in Nankai Trough off Cape Muroto in March 1997. The prototype system is a combination of observatories with a cable and without a cable. The former system comprises of two seismometers, two Tsunami pressure gauges, seafloor observatory with multiple sensors and 125 km long optical cable. The data are sent in realtime to the land station at Muroto and they are also transmitted to JAMSTEC in Yokosuka and Meteorological Agency of Japan. The latter system, which could be deployed at any place, comprises of seafloor observatory with imultiple sensors and four long-term digital ocean bottom seismometers. The data could be recovered once every month by releasing pop-up buoys to the surface through the satellite. The system with a cable was deployed on the land ward slope of Nankai Trough off Muroto at water depths between 1286 m and 3572 m. The system without cable will be deployed 200 km off Muroto in Shikoku Basin at a depth of 4300 m in early 1998. Five similar systems will be deployed until the year of 2002.

A project of submarine cabled seafloor observatory network development has been carried out in Japan since 2006. This research has aimed to establish the technology of a large scale real-time seafloor research infrastructure development for earthquake, geodetic and tsunami observation. The project scheduled to install 20 sets of cabled earthquake and tsunami observatory to an active seismogenic zone with mega-thrust earthquakes called Nankai trough. This paper describes design concept of a novel ocean observation system and the key technologies for reliable and extendable real-time seafloor monitoring.