A phreatic eruption suddenly occurred at Motoshirane (Kusatsu-Shirane volcano, Japan) at 10:02 JST on January 23, 2018. A member of the Japan Self-Defense Force was killed by volcanic blocks during training in Motoshirane, and 11 people were injured by volcanic blocks or fragments of broken glass. According to a field survey, ash fall was confirmed in Minakami, about 40 km east-northeast from Motoshirane. Although the eruption was not captured by a distant camera, the eruption plume/cloud was captured by three of the Japan Meteorological Agency’s operational weather radars. These radars observed the echo propagated to the northeast in the lower troposphere, and to the east in the middle troposphere. This is generally consistent with the observed ash fall distribution. Using the modified probabilistic estimation method, the maximum plume height was estimated to be about 5580 ± 506 m (1σ) above sea level. Estimates of the erupted mass based on the range of plume heights from radar observations and the duration of volcanic tremor during the eruption (about 8 min) do not match that obtained from a field survey (3.0–5.0 × 10⁷ kg). This discrepancy confirms that estimates of erupted mass based on plume heights must account for eruption style parametrically, which can only be constrained by case studies of varied eruption styles.

日本标准时间2018年1月23日10时02分，本白根（日本草津白根火山）突然发生潜水喷发。 日本自卫队一名成员在本白根训练时被火山块炸死，11人受伤由火山块或碎玻璃碎片组成。根据实地调查，在元白根东北偏东约 40 公里的水上确认了火山灰坠落。虽然远处的相机没有捕捉到喷发，但日本气象厅的三个运行中的天气雷达捕捉到了喷发羽流/云。这些雷达观测到在对流层低层向东北传播，在对流层中层向东传播的回波。这通常与观察到的灰降分布一致。使用改进的概率估计方法，估计最大羽流高度约为海平面以上 5580 ± 506 m (1σ)。根据雷达观测的羽流高度范围和喷发期间火山震颤的持续时间（约 8 分钟）对喷发质量的估计与实地调查（3.0-5.0 × 10⁷ 公斤）。这种差异证实，基于羽流高度的喷发质量估计必须以参数方式考虑喷发风格，这只能受到不同喷发风格案例研究的限制。