Hydroacoustic monitoring of submarine lava flows: the eruption of Fani Maoré volcano offshore Mayotte, Indian Ocean

With the vast majority of our planet volcanism occurring in the Oceans, moored networks of hydrophones have become efficient tools to monitor, and therefore better understand, underwater volcanic eruptions. As these instruments can record sounds from thousands of kilometers away, they are particularly relevant to detect earthquakes and volcanic eruptions remotely, allowing better geohazards assessment. In 2018, a major seismic and volcanic crisis gave rise to the Fani Maoré underwater volcano at 3500 m below sea level, ~50 km east offshore Mayotte, in the North Mozambique channel. The MAHY hydroacoustic network has been deployed in October 2020 to monitor the eruption and investigate the oceanic soundscape in the area. It consists of four hydrophones moored in the SOFAR channel in the water column around the volcano. Since their deployment, the hydrophones have continuously recorded all low-frequency sounds (0-120 Hz), particularly hundreds of impulsive signals resulting from the interaction between hot lava and cold seawater. An automatic detection method of these specific signals on spectrograms has been developed to investigate in details the spatio-temporal evolution of the new lava flows. This technique could be used worldwide to remotely detect and monitor active submarine eruptions in the absence of local monitoring networks or regular near-bottom seafloor surveys.