[S-VC34] Monitoring and assessment of volcanic activities

[SVC34-04] Long-range underwater acoustic observations of the recent 2021 eruption at Fukutoku-Okanoba

*Masayuki Obayashi¹, Metz Dirk¹ (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:IMS, Fukutoku-Okanoba, submarine volcano, hydrophone

A hydrophone station of the International Monitoring System (IMS) recorded underwater sound waves associated with the August 2021 eruption at Fukutoku-Okanoba, an active submarine volcano in the southern Bonin Arc. Since low-frequency sound trapped in the Sound Fixing And Ranging channel travels hundreds to thousands of kilometers before dissipating, acoustic data recorded by the IMS station - which is located at a distance of more than 2,680 km from the volcano - can be used to continuously monitor activity at Fukutoku-Okanoba from afar and in near-real time. On 12 August 2021, Fukutoku-Okanoba, a shallow submarine volcano located circa 5-km offshore the island of Minami-Ioto in the southern Bonin Arc, entered a phase of intense activity. As IMS hydrophone triplets are typically moored in a near-horizontal configuration, the delay times between elements can be used to invert for the direction of arrival. Figure 1 shows that shortly after 21:00 UTC on 12 August 2021, back azimuths of the time windowed data recorded by the H11S triplet begin to stabilize at 288°, indicating the presence of a coherent acoustic source in a direction consistent with the geodesic path towards Fukutoku-Okanoba. Thousands of arrivals from the direction of the volcano are recorded over the following days, suggesting that acoustically detectable activity occurred more or less continuously until at least mid-day on 15 August, after which rates drop to less than a dozen events per day. These observations highlight the potential of hydroacoustic measurements as a tool for remotely detecting and studying submarine volcanic eruptions in the global ocean.