Multi‐Temporal InSAR, GNSS and Seismic Measurements Reveal the Origin of the 2021 Vulcano Island (Italy) Unrest

Abstract

La Fossa Caldera at Vulcano (Italy) has been showing signs of unrest since September 2021. To investigate this phenomenon, we conducted an analysis of geodetic and seismological data from July to December 2021. In particular, we analyzed Multi Temporal Interferometric Synthetic Aperture Radar and Global Navigation Satellite System data, showing a pronounced elliptical uplift signal, which we elaborated using analytical source modeling. Additionally, seismic data were used to identify seismicity associated with hydrothermal system activity and assess its temporal evolution. The results indicate that the observed deformation is consistent with the expansion of the hydrothermal system within the La Fossa Caldera. These findings align with the analysis of seismic data, revealing signals indicative of hydrothermal activity, such as Very Long Period events. The results suggest that the ongoing phenomenon since 2021 represents a hydrothermal unrest, similar to the one observed during the late 1970s to early 1990s.

The last 1100 years of activity of La Fossa caldera, Vulcano Island (Italy): new insights into stratigraphy, chronology, and landscape evolution

“…Rosi et al ( 2018 ) suggested that the explosion resulted from a massive and rapid ascent of deep fluids promoted by the activation of a regional, N-S trending crustal fault. The inferred correlations between impulsive phreatic explosions and the activity of the NS-trending regional fault are predicated on three observations: (i) the synchronicity of the event with the Rocche Rosse event (Lipari), whose vent aligns with the same fault (Pistolesi et al, 2021 ); (ii) the alignment of the two phreatic events of the Forge craters, situated on the northern slope of the La Fossa cone, with the same tectonic line; (iii) the correspondence between the location of the source of Very Long Period (VLP) signals, indicating gas transfer from the hydrothermal system, and the fumarole areas within the caldera (La Fossa cone, Faraglione) during recent hydrothermal unrest (Di Traglia et al 2023 ). The event ascribed to 1444 AD likely had the features of highly energetic lateral blast as the chronicles report blocks launched to distances of various km.…”

“…1891 ; De Astis et al 2013 ; Di Traglia et al 2013 ; Fusillo et al 2015 ; Rosi et al 2018 ; Malaguti et al 2022 ). Since then, the caldera, which hosts an important hydrothermal system (Fulignati et al 1998 ), has experienced several periods of unrest, the last one starting in August–September 2021 and still ongoing at the time of writing (September 2023), which culminated in a partial evacuation of the island because of strong degassing increase in inhabited areas (Aiuppa et al 2022 ; Di Traglia et al 2023 ). All these factors, combined with the remarkable vulnerability of the island due to uncontrolled urban development over the past 70 years and a significant seasonal variation of the exposed population, result in a high volcanic risk (Galderisi et al 2013 ; Biass et al 2016a , b ; Bonadonna et al 2022 ; Bonasia et al 2022 ).…”

Sulfur Impurities: The Overlooked Process in Volcanic Hazard Assessment

“…All these variations may be attributed to a substance with wide variations in viscosity, such as impure sulfur. Recent multidisciplinary studies [91,92] on the causes of the 2021 unrest envisaged a "prolate-spheroidal" source at depths of 598 m below the La Fossa cone, which was similarly oriented NW-SE; this "inflated" and "deflated", and was accompanied by degassing, seismicity, and ground deformation. This surficial body is likely to be native sulfur of low viscosity.…”

Imaging volcanoes during unrest: Ambient Noise Tomography of a transient plumbing system, Vulcano, Italy.

“…Since September 2021, La Fossa Caldera, the primary active volcanic feature of Vulcano, exhibited increased activity characterized by unprecedented and rapid intensification of previously unseen unrest indicators 7,[13][14][15][16] . The proposed conceptual models suggest either the intrusion of magma at about ∼4-5 km depth 16 or enhanced hydrothermal activity at a shallow depth 14 .…”

“…In the past, authors proposed the replenishment of the shallow hydrothermal system with magmatic fluids originating from the pulsating degassing process of a deeper, pressurized, and stationary magmatic body 3,4,6,7,13,[15][16][17][18][19] . The 2021 activity is marked by the largest rate of increase in fumarolic temperatures, accompanied by substantial emissions of magmatic gases (SO 2 and CO 2 -rich) and diffuse soil fluxes 5,[13][14][15][16] . Concurrently, a significant ground deformation pattern (e.g., an uplift) was observed at La Fossa, along with a significant increase in local seismicity and, for the first time on record, the detection of Very Long Period (VLP) seismic signals 13,14,16 .…”