![]() ![]() 8), while long-term changes in spring water compositions and ground level were also documented (e.g., Sano et al. In retrospect, useful precursors were identified seconds to minutes before the eruption (e.g., Kato et al. ![]() Following the eruption, numerous studies focused on detecting potential precursors. It consisted of three phases starting with pyroclastic density currents flowing 2.5 km away from the craters, followed by increased ash injection into the atmosphere and ejection of ballistics, and finally, muddy hot water flowing from the craters 7. The Mt Ontake eruption was initiated without any clear surface precursor 6. It is therefore paramount to better investigate such eruptions by integrating new methodologies to fully understand the preparatory processes at play and improve our ability to forecast these eruptions. Despite their frequent occurrence, they remain poorly understood and have recently caused human fatalities the 2019 White Island eruption in New Zealand also killed more than 20 people 5. The absence of forecasting signals 4 challenges volcanologists’ knowledge. These non-magmatic eruptions, so-called phreatic, hydrothermal 1, hydro-volcanic 2 or steam-driven eruptions 3, do not release juvenile material but remain among the most difficult to forecast, therefore representing a potential risk. In a country optimally equipped to face natural disasters, the eruption claimed the life of more than 50 hikers, becoming the worst volcanic disaster in Japan in almost a century. 1) highlighted our limited ability to anticipate small non-magmatic eruptions. The 2014 eruption at Mt Ontake in Japan (Fig. Our results shed light onto previously undetected pressurized fluids using stations located above the volcano-hydrothermal system and hold great potential for monitoring. ![]() ![]() These, in turn, later triggered detectable deformation and earthquakes. We use various novel approaches such as covariance matrix eigenvalues distribution, cutting-edge deep-learning models, and ascribe such velocity pattern as reflecting critically stressed conditions in the upper portions of the volcano. By taking advantage of continuous seismic recordings, we uncover an intriguing sequence of correlated seismic velocity and volumetric strain changes starting 5 months before the eruption a period previously considered as completely quiescent. Tens of scientific studies attempted to identify a precursor and to unravel the processes at work but overall remain inconclusive. Such an eruption occurred at Mt Ontake in 2014, claiming the life of at least 58 hikers in what became the worst volcanic disaster in Japan in almost a century. A large fraction of volcanic eruptions does not expel magma at the surface. ![]()
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