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.

中文翻译：

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在一次非喷发火山事件中海底电磁力计检测到的两个独立信号：西之岛小笠原岛弧火山

西之岛是一座活跃的海洋岛弧火山，位于日本东京以南约 1000 公里处。自2016年以来，首次在全岛开展了海底电磁波观测，首次对火山下方的电气结构进行了研究。与实验的最初目的相反，在 2016 年至 2017 年部署的五个站点收集的数据显示，磁场和火山坡度的倾斜度存在明显的时间变化。这些时间变化巧合地发生在 2016 年 11 月中旬；这是在 2015 年和 2017 年喷发之间的安静时期。 观察到的总磁力和倾斜数据之间的独立性得到了验证，强调这些变化不是人为的，而是，与未引发喷发的火山活动有关。假设磁偶极子和球形体积变化，分别在西之岛东北部的火山坡下估计退磁和放气的来源。由此产生的偶极矩和体积变化太大，无法维持简单的源假设。然而，有限的可用数据只能在简单的模型设置下进行定量讨论，表明来源机制更为复杂。这项研究的观察结果表明，如果战略部署，海底电磁力计可用于监测岛屿火山活动。分别假设磁偶极子和球形体积变化。由此产生的偶极矩和体积变化太大，无法维持简单的源假设。然而，有限的可用数据只能在简单的模型设置下进行定量讨论，表明来源机制更为复杂。这项研究的观察结果表明，如果战略部署，海底电磁力计可用于监测岛屿火山活动。分别假设磁偶极子和球形体积变化。由此产生的偶极矩和体积变化太大，无法维持简单的源假设。然而，有限的可用数据只能在简单的模型设置下进行定量讨论，表明来源机制更为复杂。这项研究的观察结果表明，如果战略部署，海底电磁力计可用于监测岛屿火山活动。