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The DONET (Dense Oceanfloor Network System for Earthquakes and Tsunamis) observatory was developed by JAMSTEC (Japan Agency for Marine-Earth Science and Technology). Having a 300-km primary cable, it was installed in 2011 in the Nankai Trough off the south coast of Honshu, Japan. Unlike the OOI and the ONC observatories, DONET is focused exclusively on earthquake, geodetic, and tsunami observations. DONET2 is an extended version of DONET with a 450-km long primary cable, installed in 2015 to the southwest of the original observatory. Both observatories feature modular construction similar to the OOI and ONC observatories, and have multiple clusters of four or five instruments (circles; generally standalone seismometers and bottom pressure sensors) distributed around nodes (stars).
The S-Net (Seafloor observation Network for Earthquakes and Tsunamis along the Japan Trench) observatory (Kanazawa, T., Japan Trench earthquake and tsunami monitoring network of cable-linked 150 ocean bottom observatories and its impact to earth disaster science, Underwater Technology Symposium, 2013 IEEE International) was developed by NIED (National Research Institute for Earth Science and Disaster Prevention) and installed in 2018 (Tanioka, Y., Gusman, A, Near-field tsunami inundation forecast method assimilating ocean bottom pressure data: A synthetic test for the 2011 Tohoku-oki tsunami, Physics of the Earth and Planetary Interiors, 2018). The array, located offshore of northeast Honshu and east of Hokkaido, incorporates 5800 km of seafloor cable with 150 repeater-like nodes spaced at 30-km intervals, as well as six cable landings. Each S-Net node contains four accelerometers for long and short period seismic measurements, as well as two pressure sensors (for redundancy) for tsunami detection and seafloor level monitoring.