Hydroacoustic and tsunami observations by a seafloor fiber optic strain meter

The authors have developed a strain meter using a 200-m fiber optic cable detecting tiny strain change by optic interferometry technique, i.e., measurement of optic phase change with respect to a reference fiber (Araki et al., 2022). Three fiber optic strain meters have been deployed with forming like an array at the seafloor in the Nankai Trough seismogenic zone, and real-time dataset are acquired via the DONET seafloor observatory network. A series of submarine earthquakes in October 2023 occurred near Torishima Island, a volcanic island of the Izu-Ogasawara (Izu-Bonin) Islands had generated unexpected tsunamis, remaining their earthquake magnitude unknown. At a series of the earthquakes, fourteen hydroacoustic signals, i.e., T-phases, followed by a dispersive tsunami were detected by the fiber optic strain meters. Amplitudes of T-phases and the tsunami detected by the fiber strain meters were 0.07 to 0.2 micro-strain and 0.003 micro-strain, respectively. Likewise, a DONET pressure gauge located about 3 km away from the fiber optic strain meters detected pressure fluctuations with their amplitude of 100 to 500 Pa and 140 Pa, respectively. It has been confirmed that the frequency contents of T-phases and the tsunami detected by the fiber optic strain meters consistent with those of the nearby DONET pressure gauge. The fact that a fiber optic strain meter detected a tsunami signal suggests that hydro-static loading associated with tsunami can cause strain change at the seafloor.