[S-TT37] Seismic monitoring and processing system

[STT37-P01] Submarine installation work for the N-net offshore system

*Takayuki Miyoshi¹, Kenji Uehira¹, Masashi Mochizuki¹, Azusa Nishizawa¹, Tomotake Ueno¹, Shin Aoi¹, Masanao Shinohara^1,2, Tetsuya Takeda¹, Takashi Kunugi¹, Toshikazu Tanada¹ (1.NIED, 2.ERI, University of Tokyo)

Keywords:N-net, Submarine installation work, Nankai trough earthquake

Disastrous great earthquakes and tsunamis have occurred repeatedly with time intervals of approximately 90-150 years along the Suruga and Nankai troughs. Since the seafloor observation cable network was constructed off the Tokai district by the Meteorological Research Institute of Japan Meteorological Agency in 1978 in order to observe earthquakes under the seafloor, some submarine networks were installed in the source region of the anticipated Nankai trough earthquake. Nankai Trough Seafloor Observation Network for Earthquakes and Tsunamis (N-net) will be located in the westernmost part of the region. N-net consists of two cable systems: an offshore system and an inshore system, connecting Muroto geopark landing station in Kochi Prefecture and Kushima landing station in Miyazaki Prefecture. The National Research Institute for Earth Science and Disaster Resilience (NIED) conducted the installation work of the offshore system from October, 2023 to January, 2024 using a cable ship. We installed 18 inline observatories, two branching units, two terminal units, approximately 900 km length of submarine cables, and the other devices during the work. We used KDDI cable infinity (KCI; gross tonnage 9,766 tons) as a cable ship. KCI has cable tanks, drum cable engines, cable sheaves, a cable burying machine, a ROV, and the other equipment in order to install the submarine cable. The main outline of installation work includes landing the submarine cable to connect it to a land station, sweeping, laying the submarine cable and inline observatory with/without burying. We buried the submarine cable and the inline observatory beneath the seafloor where the water depth is shallower than 1,000 m. One inline observatory closest to the Kushima land station was buried during this work. In order to shorten the construction period in the coastal area, the cable landing was also carried out for the inshore system. Electronic power was supplied from the ship during the mechanical, electrical, and optical checks on board the ship. The check of the inline observatory was carried out after loading, before installation, and after landing the observatory on the sea bottom, after working of the Lay Block, and before the Final Splice. All tests were involved analyzing data obtained from each sensor of the target inline observatory. We decided whether to proceed to the next process of the work based on the results obtained from each test. There were no major problems in any of the tests, and we were able to connect 18 observation devices with submarine cables from the Muroto Geopark landing station to the Kushima landing station. The N-net offshore system will be operated and the data will be provided after quality checks.