We estimate the mass and energy budgets for the 2018 phreatic eruption of Mt. Motoshirane on Kusatsu–Shirane volcano, Japan, based on data obtained from a network of eight tiltmeters and weather radar echoes. The tilt records can be explained by a subvertical crack model. Small craters that were formed by previous eruptions are aligned WNW–ESE, which is consistent with the strike of the crack modeled in this study. The direction of maximum compressive stress in this region is horizontal and oriented WNW–ESE, allowing fluid to intrude from depth through a crack with this orientation. Based on the crack model, hypocenter distribution, and MT resistivity structure, we infer that fluid from a hydrothermal reservoir at a depth of 2 km below Kusatsu–Shirane volcano has repeatedly ascended through a pre-existing subvertical crack. The inflation and deflation volumes during the 2018 eruption are estimated to have been 5.1 × 10⁵ and 3.6 × 10⁵ m³, respectively, meaning that 1.5 × 10⁵ m³ of expanded volume formed underground. The total heat associated with the expanded volume is estimated to have been ≥ 10¹⁴ J, similar to or exceeding the annual heat released from Yugama Crater Lake of Mt. Shirane and that from the largest eruption during the past 130 year. Although the ejecta mass of the 2018 phreatic eruption was small, the eruption at Mt. Motoshirane was not negligible in terms of the energy budget of Kusatsu–Shirane volcano. A water mass of 0.1–2.0 × 10⁷ kg was discharged as a volcanic cloud, based on weather radar echoes, which is smaller than the mass associated with the deflation. We suggest that underground water acted as a buffer against the sudden intrusion of hydrothermal fluids, absorbing some of the fluid that ascended through the crack.

我们估计了 2018 年火山喷发的质量和能量预算。Motoshirane 在日本草津 - 白根火山，基于从八个倾斜仪和天气雷达回波网络获得的数据。倾斜记录可以用近垂直裂缝模型来解释。由先前喷发形成的小火山口排列成 WNW-ESE，这与本研究中模拟的裂缝走向一致。该区域中最大压应力的方向是水平的和定向的 WNW-ESE，允许流体从深处通过具有这种定向的裂缝侵入。根据裂缝模型、震源分布和 MT 电阻率结构，我们推断来自草津 - 白根火山下方 2 公里深度的热液储层的流体通过预先存在的近垂直裂缝反复上升。⁵和 3.6 × 10 ⁵ m ³，分别意味着 1.5 × 10 ⁵ m ³的膨胀体积在地下形成。与膨胀体积相关的总热量估计≥ 10 ¹⁴  J，类似于或超过山的尤加马火山口湖释放的年热量。Shirane 和过去 130 年中最大的喷发。尽管 2018 年潜水喷发的喷发质量很小，但 Mt. 就草津-白根火山的能量收支而言，本白根是不可忽视的。0.1–2.0 × 10 ^{7 的}水质量 根据天气雷达回波，kg 以火山云的形式排放，它小于与通缩相关的质量。我们认为，地下水可以作为缓冲热液突然侵入的缓冲液，吸收了一些通过裂缝上升的流体。