11:30 AM - 11:45 AM
[SCG53-04] Development of N-net observation system and construction of offshore system
Keywords:N-net, Nankai Trough earthquake, seafloor observation network for earthquakes and tsunamis, earthquake observation, tsunami observation, early warning
Early detection of the occurrence of great earthquakes in the ocean is important for earthquake disaster mitigation. NIED is currently constructing a new cable-type seafloor observation network for earthquakes and tsunamis, N-net, in the sea area from off Kochi to the Hyuga-nada (e.g., Aoi et al., JpGU2023). Along the Japan and Kuril Trenches, NIED operates S-net, and along the Suruga and Nankai Troughs, JMA and NIED operate the Tokai/Tonankai cable ocean bottom seismographs systems and DONET, respectively. In the offshore areas of the Pacific Ocean, cable-type seafloor observation networks have been established by various organizations. These observation networks cover the sea area where there is concern about the occurrence of a huge subduction zone earthquakes, but a blank area remains in the western half of the source area of the anticipated Nankai Trough earthquake, and the completion of the N-net will eliminate this blank area.
In order to transmit observed data via submarine cables and supply power to observatories, a new land station was built in Kushima, Miyazaki Prefecture, and the Muroto Geopark land station in Kochi Prefecture, which is used for DONET, significantly increase power, air conditioning, communications, and other functions to share the station with N-net. N-net consists of two observation systems, an offshore system and an inshore system, with a total of 36 observatories and four branching unit and terminating unit connected to both land stations via an optical submarine cable approximately 1,650 km in length.
The basic design of N-net is the same as that of S-net, with two sets each of strong-motion accelerometers, velocity type seismographs, and water pressure gauges equipped in each observatory on the seafloor, while major improvements have been made in some points. The pressure gauges manufactured by Paroscientific, which has been used for many tsunami observations, uses a Bourdon tube to increase sensitivity, but the Bourdon tube has a movable part, so noise can be induced due to vibration and attitude change during earthquakes. In order to reduce such influences as much as possible, a pressure gauge equipped with a silicon vibration type pressure sensor using MEMS (Micro Electro Mechanical Systems) technology has been newly developed and adopted by N-net. In addition, the pressure transmission system was developed by designing a new pressure-gauge housing and changing the viscosity of the polybutene that transmits the water pressure on the seafloor to the water pressure sensor. In addition, a rotation reduction device was introduced to reduce the effect of the rotation of the observatory, which is cylindrical in shape and may rotate slightly during earthquakes. Various tests were conducted to ensure the performance and long-term reliability of the instruments. In particular, the newly developed pressure gauge and pressure-gauge housing were carefully tested, including a year-long test using 10 units. One of the features of N-net is that it is a hybrid system that combines an inline system and a plug-in node system, and the influence of both systems on each other was considered to be minimized as much as possible.
The offshore system is scheduled to be manufactured and laid in the ocean by FY2023, and the inshore system is scheduled to be laid in the ocean in FY2024. The major components of the offshore system have been individually tested and confirmed to be in good working order. Furthermore, after all of these were electrically and optically connected and underwent a system assembly test, which is a test of the entire system as it would be installed on the seafloor, they were loaded onto a cable ship, and the offshore system was laid at sea from October 2023 to January 2024 (Miyoshi, et al, JpGU2024 ). Currently, a close examination of the observation data acquired for testing in the offshore system is being conducted.
In order to transmit observed data via submarine cables and supply power to observatories, a new land station was built in Kushima, Miyazaki Prefecture, and the Muroto Geopark land station in Kochi Prefecture, which is used for DONET, significantly increase power, air conditioning, communications, and other functions to share the station with N-net. N-net consists of two observation systems, an offshore system and an inshore system, with a total of 36 observatories and four branching unit and terminating unit connected to both land stations via an optical submarine cable approximately 1,650 km in length.
The basic design of N-net is the same as that of S-net, with two sets each of strong-motion accelerometers, velocity type seismographs, and water pressure gauges equipped in each observatory on the seafloor, while major improvements have been made in some points. The pressure gauges manufactured by Paroscientific, which has been used for many tsunami observations, uses a Bourdon tube to increase sensitivity, but the Bourdon tube has a movable part, so noise can be induced due to vibration and attitude change during earthquakes. In order to reduce such influences as much as possible, a pressure gauge equipped with a silicon vibration type pressure sensor using MEMS (Micro Electro Mechanical Systems) technology has been newly developed and adopted by N-net. In addition, the pressure transmission system was developed by designing a new pressure-gauge housing and changing the viscosity of the polybutene that transmits the water pressure on the seafloor to the water pressure sensor. In addition, a rotation reduction device was introduced to reduce the effect of the rotation of the observatory, which is cylindrical in shape and may rotate slightly during earthquakes. Various tests were conducted to ensure the performance and long-term reliability of the instruments. In particular, the newly developed pressure gauge and pressure-gauge housing were carefully tested, including a year-long test using 10 units. One of the features of N-net is that it is a hybrid system that combines an inline system and a plug-in node system, and the influence of both systems on each other was considered to be minimized as much as possible.
The offshore system is scheduled to be manufactured and laid in the ocean by FY2023, and the inshore system is scheduled to be laid in the ocean in FY2024. The major components of the offshore system have been individually tested and confirmed to be in good working order. Furthermore, after all of these were electrically and optically connected and underwent a system assembly test, which is a test of the entire system as it would be installed on the seafloor, they were loaded onto a cable ship, and the offshore system was laid at sea from October 2023 to January 2024 (Miyoshi, et al, JpGU2024 ). Currently, a close examination of the observation data acquired for testing in the offshore system is being conducted.