13:45 〜 14:00
[SCG55-24] S-net観測網を用いた混濁流検出の試み
キーワード：混濁流、S-net、重力流
Turbidity currents are gravity-driven flows formed by sediment particles entrained through slope failures and floods. They serve as a major conduit for mass transport to the deep-sea floor and play a role in the carbon cycle. It is well documented that turbidity currents damaged submarine cables during the 1929 Grand Banks earthquake and the 2011 Tohoku-Oki earthquake, indicating that they pose a significant natural hazard to marine infrastructure. Understanding their causes and behavior is crucial for interpreting seafloor morphodynamics and studying paleoseismic history.
However, the unpredictable nature of turbidity currents limits observations to only a few submarine canyons. Furthermore, equipment failures have occurred due to powerful flows, underscoring the high risk associated with planned observations. Several instruments successfully recorded turbidity currents a few months after mooring systems were deployed only in a limited number of submarine canyons such as Monterey, Congo, and Gaoping. Thus, existing measurements may have recorded the behaviors of turbidity currents in very exceptional environments. The frequency of turbidity currents in general land shelf and submarine valleys remains questionable.
To this end, this study uses the S-net data to explore the feasibility of detecting the passage of turbidity currents offshore Tohoku Region, Japan. The S-net seafloor observation network for earthquakes and tsunamis has been operating from off Nemuro to off Boso since August 2016. S-net is equipped with seismometers, pressure gauges, and thermometers, with data transmitted in real-time to land-based stations. Because turbidity currents can generate rapid changes in ocean bottom pressure gauges, S-net instruments may detect similar pressure variations.
Daily average pressure values collected on the 15th of each month from 2016 to 2024 were analyzed to determine when events occurred. Monthly fluctuations were generally within ~0.02 MPa, yet changes greater than 0.1 MPa were observed at several stations. These stations were commonly located in topographic depressions at water depths exceeding 1,500 m and had been installed in exposed conditions. Temperature and seismometer data showed an approximate 0.01°C increase and amplified acceleration in the 0.01–0.05 Hz range in certain directions, coinciding with the pressure changes. These features are thought to result from warm water being advected downstream and that water mass transported the instrument. Turbidity currents and debris flows are possible phenomena that could account for such transport and temperature increases. After an anomalous pressure change, temporary pressure rises of up to ~0.01 MPa were recorded four or five times in some events, interpreted as pulse-like loading from a density flow.
Such events were observed ~30 times during 7.5 years of monitoring. Further insights into the properties of the gravity-flow sediments could be gained through core sampling and acoustic surveys near the S-net stations. Moreover, additional works are needed to clarify the generation mechanisms behind these gravity flows.
However, the unpredictable nature of turbidity currents limits observations to only a few submarine canyons. Furthermore, equipment failures have occurred due to powerful flows, underscoring the high risk associated with planned observations. Several instruments successfully recorded turbidity currents a few months after mooring systems were deployed only in a limited number of submarine canyons such as Monterey, Congo, and Gaoping. Thus, existing measurements may have recorded the behaviors of turbidity currents in very exceptional environments. The frequency of turbidity currents in general land shelf and submarine valleys remains questionable.
To this end, this study uses the S-net data to explore the feasibility of detecting the passage of turbidity currents offshore Tohoku Region, Japan. The S-net seafloor observation network for earthquakes and tsunamis has been operating from off Nemuro to off Boso since August 2016. S-net is equipped with seismometers, pressure gauges, and thermometers, with data transmitted in real-time to land-based stations. Because turbidity currents can generate rapid changes in ocean bottom pressure gauges, S-net instruments may detect similar pressure variations.
Daily average pressure values collected on the 15th of each month from 2016 to 2024 were analyzed to determine when events occurred. Monthly fluctuations were generally within ~0.02 MPa, yet changes greater than 0.1 MPa were observed at several stations. These stations were commonly located in topographic depressions at water depths exceeding 1,500 m and had been installed in exposed conditions. Temperature and seismometer data showed an approximate 0.01°C increase and amplified acceleration in the 0.01–0.05 Hz range in certain directions, coinciding with the pressure changes. These features are thought to result from warm water being advected downstream and that water mass transported the instrument. Turbidity currents and debris flows are possible phenomena that could account for such transport and temperature increases. After an anomalous pressure change, temporary pressure rises of up to ~0.01 MPa were recorded four or five times in some events, interpreted as pulse-like loading from a density flow.
Such events were observed ~30 times during 7.5 years of monitoring. Further insights into the properties of the gravity-flow sediments could be gained through core sampling and acoustic surveys near the S-net stations. Moreover, additional works are needed to clarify the generation mechanisms behind these gravity flows.