Large-scale thermal unrest of volcanoes for years prior to eruption

Identifying the observables that warn of volcanic eruptions is a major challenge in natural hazard management. A potentially n important, but under-investigated, observable is the release of heat through volcano surfaces, which represents a major energy source at quiescent volcanismvolcanoes. However, it remains unclear whether surface heat emissions respond to pre-eruptive processes and vary before eruption. Here we show through a statistical analysis of satellite-based long-wavelength (10.780–11.280 μm) infrared data that the last magmatic and phreatic eruptions of five different volcanoes were preceded by subtle but significant long-term (~years), large-scale (10s tens of km2), increases of in their radiant heat flux (up to ~1 ℃ of in median radiant temperature). This pre-eruptive thermal unrest is found through a new statistical analysis of satellite-based long-wavelength (10.780–11.280 μm) infrared data, and is attributed to the enhancement of underground hydrothermal activity. Large-scale thermal unrest is detected even before eruptions that were impossible not to anticipated through from other geophysical/geochemicalvolcano monitoring methods, such as (e.g., the 2014 phreatic eruption of Ontake (, Japan) and; the 2015 magmatic eruption of Calbuco (, Chile). We attribute large-scale thermal unrest to the enhancement of underground hydrothermal activity, and suggest that ),such analysis of satellite-based infrared observations can improve thus opening new horizons to better constrainconstraints on the thermal budget of volcanoes, detections of pre-eruptive conditions, and and assessments of volcanic alert levels.