Non-eruptive transients and fluid flow processes driving volcano-tectonic crises at Vulcano, Italy

Although not all volcanic unrests lead to eruptions, it is commonly believed that magma rising through the shallow crust drives volcanic awakening. When eruptions do not occur, hydrothermal activity is often claimed to be responsible for inflation and deflation processes. Yet, a causal process explaining long-lasting non-eruptive unrest is still missing. Vulcano, the southernmost island of the Aeolian volcanic archipelago, Italy, entered in unrest in September 2021. The island experienced intense ground deformation, a sustained increase in fumarole temperatures, gas emissions, and shallow seismicity. CO${}_2$ diffuse soil degassing increased at the foothill of La Fossa cone, causing the evacuation of inhabitants. Very Long Period (VLP) seismic events with a daily rate of up to 450 events/day were found in the seismic records for the first time since the deployment of the broadband network in 2005. With the benefit of hindsight, new VLPs were also discovered hidden in the 2018 seismic records. Geodetic data show inflation occurring in 2021, suggesting the pressurization of the shallow portion of the magmatic plumbing system beneath Vulcano. A similar behaviour occurred also in 2018. However, a few aspects of these unrests are not fully compatible with traditional causative models invoking a shallow dike emplacement or with a hydrothermal scenario. In particular, the long-lasting transient character of VLPs during 2021–22 has never been encountered before in hydrothermal-driven unrests.

We propose that deep-seated fluid pressure, possibly promoted by a destabilizing event at depth, either of magmatic or tectonic origin, may have driven the unrests and be responsible for a discrete and transient release of lithostatic fluid pressures from the plumbing system. In particular, NE-striking normal faults highlighted by a high-resolution nodal ambient noise tomography seem to play a key role in modulating the transient character of the 2021 unrest. Once released, overpressure fronts travel across a rheologically complex domain causing VLPs. Once entering the hydrothermal system, fluids (e.g. H${}_2$O and CO${}_2$ dominated mixtures) phase-separate and expand. This pressurizes the shallow plumbing system leading to intense shallow microseismicity. Our model is supported by the long-lasting transient character of the VLP events occurring in swarms and reconciles multiple interdisciplinary observations impacting how we understand the interplay between tectonics, volcanism and natural hazards.