However, for continuously active, seismically noisy, and “wet” volcanoes 25, tremor precursors prior to phreatic or phreatomagmatic eruptions are difficult to distinguish, or only identified in retrospect 2, 6, 8, 26, 27. Continuous volcanic seismic signals are interrogated to understand fluid movement (e.g., magma degassing), conduit fracture/bubble processes that precede eruptions, and pressurization of a hydrothermal system 22, 23, 24. Seismic data analysis is standard practice at volcano observatories to track magmatic processes, the volcano state, and the possibility of future eruptions 17, 18, 19, 20, 21. In extreme cases, they may form in seconds to hours 16 due to mineralized zones (alunite, anhydrite, smectite) that open and close in response to pressure variations. In active hydrothermal systems beneath lakes, seals commonly form over weeks to months 15 with mineral precipitation (sulfur, sulfates, and silica). The timescales of seal formation depend on a range of mineralization, pressurization, and fluid-rock properties 14. Hydrothermal seals and their failure can play an important role in phreatic eruptions because diminished vent permeability drives pressurization whereas a rapid increase can drive sudden decompression 6, 12, 13. Restriction of gas flow through the upper vent system leads to localized shallow pressurization, creating the conditions for an explosive eruption.

Permeability barriers are especially important if gas input increases during magmatic unrest. Containment arises through sealing of a liquid water, magmatic gas, or steam reservoir under diverse conditions, e.g., low-permeability layers that form below crater lakes or within crater basins (clay/silt deposits, liquid sulfur 6) hydrothermal alteration that changes rock texture/permeability 7 pore-blockage by elemental sulfur or hydrothermal precipitates 8, 9, 10 and/or active sealing by pressure applied to compressible clays 11. Phreatic eruptions are usually driven by expansion and steam-flashing of suddenly released hot pressurized fluids 5. Real-time monitoring for this precursor may improve short-term eruption warning systems at certain volcanoes. Applying this model to the 2019 eruption at Whakaari (New Zealand), we describe pressurization of the system in the week before the eruption, and cascading seal failure in the 16 h prior to the explosion. We interpret this as formation of a hydrothermal seal that enables rapid pressurization of shallow groundwater. The precursor manifests in the displacement seismic amplitude ratio between medium (4.5–8 Hz) and high (8–16 Hz) frequency tremor bands, exhibiting a characteristic rise in the days prior to eruptions. Here, we describe a general seismic precursor signal for gas-driven eruptions, identified through correlation analysis of 18 well-recorded eruptions in New Zealand, Alaska, and Kamchatka. Even with real-time geophysical monitoring, forecasting sudden eruptions is difficult, because their precursors are hard to recognize and can vary between volcanoes. Volcanic eruptions that occur without warning can be deadly in touristic and populated areas.

Wag seismac