Magnetization structure of Nishinoshima volcano, Ogasawara island arc, obtained from magnetic surveys using an unmanned aerial vehicle

Nishinoshima, a volcanic island in the Ogasawara (Bonin) island arc, has intermittently erupted since November 2013 after a quiescence of nearly 40 years. The three-dimensional (3D) magnetization structure of this growing volcanic island was investigated for the first time by acquiring aeromagnetic data with an unmanned aerial vehicle (UAV) system and subjecting the data to a 3D regularized inversion. The aeromagnetic surveys, in September 2018 and June 2019, covered an area of about 3 km × 3 km that included the emergent part of Nishinoshima volcano. We extracted the magnetic anomaly induced by the magnetization structure from the observations and estimated the magnetization structure from the anomaly by applying the 3D inversion, which combines L1 and L2 norm regularizations. We conducted a cross-validation to simultaneously determine optimum values of a regularization parameter and a hyperparameter. We found that Nishinoshima volcano had an average magnetization of about 3.0 A/m and that two more strongly magnetized bodies existed as of 2019 beneath the summit crater and the northeastern flank at depths several hundred meters below the surface. These features may represent massive bodies of old chilled magma. This study demonstrated the utility of this relatively safe and inexpensive observation method and this data analysis method for investigating the magnetic structure of remote volcanic islands. Repeated future surveys of this type may enable us to monitor volcanic activities that affect the magnetization structure of volcanoes.