Anatomy of thermal unrest at a hydrothermal system: Case study of the 2021-2022 crisis at Vulcano

Abstract

Hydrothermal systems can generate phreatic and/or phreatomagmatic explosions with little warning. Understanding the temporal and spatial evolution of geophysical and geochemical signals at hydrothermal systems is crucial for detecting precursors to unrest and to inform on hazard. Thermal signatures of such systems are poorly defined because data records are often too short or punctual compared to activity timescales, which can be decadal. La Fossa system of Vulcano has been monitored since the 1980s and entered a period of unrest in 2021. We assessed the thermal signature using ground- and satellite-based data with temporal and spatial scales ranging from minutes to days. While continuously-recording stations provided continuous but point-based measurements, fumarole field vent surveys and ASTER and VIIRS images allowed lower temporal resolution but synoptic records to be built. By integrating this multi-resolution data set, precursory signals to the unrest could retrospectively be placed ranging from February to June 2021. Intensity of unrest increased during summer 2021, with an onset over a few days in September 2021. By September, seismic, CO2, SO2 and geochemical metrics also indicated unrest, leading Civil Protection to raise the alert level to yellow on October 1. Heat flux, having been 4 MW in May 2019, peaked at 90 MW in September, and increased to 120 MW by March 2022. This ranked Vulcano as one of the highest intensity hydrothermal systems like Reykjanes, well ahead of Yellowstone and Nysiros We thus convolved our thermal data sets with all other monitoring data to validate a Vulcano Unrest Index (VUI) that can be potentially applied to any hydrothermal system. The VUI highlighted four stages of unrest, none of which were clear in any single data set: baseline, precursory, onset and unrest. Onset was characterized by sudden release of fluids, likely caused by failure of sealed zones that had become pressurized during the precursory phase that began possibly as early as February 2021. Unrest has been ongoing for more than 18 months, and may continue for several more years. Our understanding of this system behavior has been due to hindsight, but demonstrates how multiparametric surveys can track and forecast unrest.