| Summary: | Abstract Nishinoshima is an active oceanic island arc volcano situated approximately 1000 km south of Tokyo, Japan. Since 2016, marine electromagnetic observations using ocean bottom electromagnetometers have been conducted around the island to investigate the electrical structure beneath the volcano for the first time. In contrast to the original purpose of the experiment, the data collected at five sites deployed in 2016–2017 showed distinct time variations in the magnetic field and the tilt of the volcano’s slope. These time variations occurred coincidentally in mid-November 2016; this was during a quiet period between eruptions in 2015 and in 2017. The independence between the observed total magnetic force and tilt data was verified, highlighting that these variations were not artificial rather, associated with volcanic activity that did not invoke an eruption. Sources for demagnetization and deflation were estimated beneath the volcanic slope in the northeast of Nishinoshima Island, assuming a magnetic dipole and a spherical volume change, respectively. The resultant dipole moment and the volume change were too large to maintain simple source assumptions. However, the limited available data only enabled quantitative discussion under simple model settings, suggesting that the source mechanisms were more complex. The observations from this study demonstrate that if deployed strategically, ocean bottom electromagnetometers are useful to monitor island volcano activities.
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