The Onset, Middle, and Climax of Precursory Hydrothermal Intrusion of the 2018 Phreatic Eruption at Kusatsu‐Shirane Volcano

Triggering intrusions of phreatic eruptions are often observed as seismic and ground deformation signals on a time scale of minutes. The current understanding of hydrothermal intrusions still needs improvement to obtain insight into the eruption scale from the observables. We examine local geophysical data from the precursory hydrothermal intrusion of the 2018 phreatic eruption of Kusatsu‐Shirane volcano. To achieve an integrated intrusion model, we divide analyzing time window into the onset, middle, and climax. Focusing on the transient response of tilt data for the sudden pressurization, we estimate a vertical tensile opening (1.7 × 103 m3/s in 40 s) at 1.1 km depth for the intrusion onset. Pressurization can represent the start of vapourization. Very long period (VLP, 0.033–0.1 Hz) seismic signals are adopted to constrain the middle and climax phases. We obtained two sequential semi‐horizontal tensile crack oscillation sources with peak volume changes of 3.6 × 104–1.9 × 105 m3 at 0.3–0.6 km depths. The second VLP source acted as a final trigger of the eruption to cause depressurization in the shallow portion of the intruded region, which is constrained as having reached 0.1 km depth by surface deformation. Simultaneously, we find another depressurization originated from depth in the climax due to a decrease in the hydrothermal intrusion rate. Through comparison with the 2014 Ontake phreatic eruption, the total inflation volume may correlate with eruption scales. Intruded hydrothermal fluid and local structure characteristics also may have to be considered to evaluate the eruptions scales from inferred signal source intensity. Plain Language Summary Phreatic eruptions, emitting non‐fresh magma fragments, are driven by underground water and gas mixtures intruding into the subsurface. Local geophysical observations often detect intrusions as seismic and ground deformation signals. Those observation data potentially provide insight into eruption and hazard scale evaluations. However, such an interpretation is still challenging for the current understanding. This paper examines seismic and ground deformation data associated with the precursory intrusion of a phreatic eruption at Kusatsu‐Shirane volcano in Japan, revealing a detailed hydrothermal intrusion process. Using ground deformation data, we have found that an aqueous intrusion started at 1.1 km depth. This starting process may represent the vapourization onset of underground water and gas mixtures. B