Two independent signals detected by ocean bottom electromagnetometers during a non-eruptive volcanic event: Ogasawara Island arc volcano, Nishinoshima

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.

Probability distributions of model parameters obtained by grid search – application to the geomagnetic anomaly source estimation problem

“…We applied the method to real data observed around the Nishinoshima volcano (Baba et al, 2020). In this area, geomagnetic anomaly changes were observed in November 2016 with 4 ocean bottom electro-magnetometers (OBEM) installed around the island (Fig.…”

“…4). Baba et al (2020) interpreted these changes as underground demagnetization and estimated the source location to be northeast of the volcano based on a conventional grid search approach. We reanalyze the observed magnetic variations shown in Fig.…”

“…Because grid search is convenient to code and is a robust global search method, it is widely used in geophysical studies, especially for nonlinear problems. For example, grid search has been applied to determine source locations and characteristics (e.g., geometry, size, and amplitude of physical properties) causing crustal deformation (e.g., Himematsu et al, 2020), magnetic anomalies (e.g., Tanaka, 1993;Hashimoto, 2022;Baba et al, 2020), and seismic activities (e.g., Ogiso and Yomogida, 2012). The residual between observed data and the model-predicted response, such as root mean square misfit (RMSm), is usually used as a criterion of model goodness.…”

Ocean Bottom Electromagnetometer Carried From Bonin to Ryukyu Islands by Sea Currents

“…We initiated OBEM observations around Nishinoshima in 2016 to estimate the electrical conductivity structure of the island (Baba et al, 2020). In September 2018, we deployed six more OBEMs around Nishinoshima from the Japan Meteorological Agency (JMA) weather ship Keifu Maru (cruise KS18-07).…”

“…About nine months later, in June 2019, we attempted to retrieve the six OBEMs during cruise KS19-05 of the Keifu Maru. In this particular operation, we recalibrated the positions of the OBEMs because it was known that some of them had moved, perhaps due to volcanic activity and/or slope collapse (Baba et al, 2020). We recovered four OBEMs but were unable to recover the other two (JM4 and JM6) during that cruise.…”