Since the beginning of the twenty-first century, volcanic unrest has occurred every 2–5 years at Hakone volcano. After the 2015 eruption, unrest activity changed significantly in terms of seismicity and geochemistry. Like the pre- and co-eruptive unrest, each post-eruptive unrest episode was detected by deep inflation below the volcano (~ 10 km) and deep low frequency events, which can be interpreted as reflecting supply of magma or magmatic fluid from depth. The seismic activity during the post-eruptive unrest episodes also increased; however, seismic activity beneath the eruption center during the unrest episodes was significantly lower, especially in the shallow region (~ 2 km), while sporadic seismic swarms were observed beneath the caldera rim, ~ 3 km away from the center. This observation and a recent InSAR analysis imply that the hydrothermal system of the volcano could be composed of multiple sub-systems, each of which can host earthquake swarms and show independent volume changes. The 2015 eruption established routes for steam from the hydrothermal sub-system beneath the eruption center (≥ 150 m deep) to the surface through the cap-rock, allowing emission of super-heated steam (~ 160 ºC). This steam showed an increase in magmatic/hydrothermal gas ratios (SO₂/H₂S and HCl/H₂S) in the 2019 unrest episode; however, no magma supply was indicated by seismic and geodetic observations. Net SO₂ emission during the post-eruptive unrest episodes, which remained within the usual range of the post-eruptive period, is also inconsistent with shallow intrusion. We consider that the post-eruptive unrest episodes were also triggered by newly derived magma or magmatic fluid from depth; however, the breached cap-rock was unable to allow subsequent pressurization and intensive seismic activity within the hydrothermal sub-system beneath the eruption center. The heat released from the newly derived magma or fluid dried the vapor-dominated portion of the hydrothermal system and inhibited scrubbing of SO₂ and HCl to allow a higher magmatic/hydrothermal gas ratio. The 2015 eruption could have also breached the sealing zone near the brittle–ductile transition and the subsequent self-sealing process seems not to have completed based on the observations during the post-eruptive unrest episodes.

自二十一世纪初以来，箱根火山每2-5年发生一次火山动乱。2015年爆发后，动荡活动在地震活动和地球化学方面发生了显着变化。像爆发前和爆发前的动荡一样，每次爆发后的不稳定事件都是通过火山下方（〜10 km）的深层充气和深低频事件检测到的，这可以解释为从深处反映了岩浆或岩浆流体的供应。爆发后动荡时期的地震活动也有所增加。然而，在动乱期间，喷发中心下方的地震活动明显较低，尤其是在浅水区（约2 km），而在距震源中心约3 km的破火山口边缘下方则观察到零星的地震群。这项观察和最近的InSAR分析表明，火山的热液系统可能由多个子系统组成，每个子系统都可以容纳地震群并显示独立的体积变化。2015年的喷发建立了从喷发中心下方的热液子系统（深度≥150 m）通过盖岩层到地表的蒸汽路径，从而允许释放过热蒸汽（〜160ºC）。这种蒸汽显示出岩浆/热液气体比率（SO₂ / H ₂ S和HCl / H ₂ S）在2019年的骚乱事件中; 但是，地震和大地测量都没有显示出岩浆供应。喷发后动荡时期的SO ₂净排放量（仍保持在喷发后时期的正常范围内）也与浅埋侵入不一致。我们认为，爆发后的动荡事件也是由新近产生的岩浆或深部岩浆流体触发的。但是，破裂的盖层岩层无法在喷发中心下方的热液子系统内进行随后的增压和强烈的地震活动。新产生的岩浆或流体释放出的热量使水热系统中以蒸汽为主的部分干燥，并抑制了SO _2的洗涤和HCl，以允许更高的岩浆/热液气体比率。2015年爆发的火山喷发也可能突破了脆性-韧性过渡附近的密封带，根据火山爆发后的动荡时期的观察结果，随后的自密封过程似乎还没有完成。