Volcanic unrest episodes at Vulcano, Aeolian Islands (Italy), monitored by InSAR and GNSS

Vulcano is a composite volcanic edifice representing the southernmost emerged island of the Aeolian archipelago (Tyrrhenian Sea, Italy). Grown at the convergence of the Africa and Eurasian plates, Vulcano is part of a complex volcano-tectonic system characterized by a NNW-SSE fault system which controls the volcanism evolution of the Aeolian central branch and its continuous long-term deformation. Vulcano experienced many eruption episodes in historical times, the most recent of which occurred in 1888-1890. Since then, it has undergone repeated phases of unrest characterized by shallow seismicity, increased fumarolic activity, and sometimes ground deformation. The most recent unrest episode occurred from the summer till the end of 2021 with intense fumarolic activity mainly concentrated at La Fossa cone and some sectors of the caldera.  

Long-term tectonic movement and unrest phases cause measurable deformation which can provide important insights into the volcanic system behavior. In this work we leveraged two types of satellite-geodesy deformation data: GNSS and InSAR. We considered the time series of 6 continuous GNSS stations managed by the Osservatorio Etneo of INGV. The SAR dataset consists of Sentinel-1A images acquired from January 2016 to December 2023 along ascending and descending orbits, and processed through the Interferometric Point Target Analysis (IPTA) approach to retrieve ground deformation velocity and displacement time series.

Focusing on the 2016-2023 interval, we first validated the InSAR results with GNSS data, obtaining a general good agreement. The GNSS time series clearly show different phases of deviation from the long-term linear trend, particularly in 2018 and 2021. The 2021 period is associated with up to about 2 cm uplift and 1 cm of nearly radial pattern around La Fossa caldera. InSAR data are noisier, but also show transient signals, with a clear signal in 2021, associated with an elliptical deformation area of up to about 3 and locally 5 cm in Line of Sight at La Fossa caldera.

InSAR and GNSS data provide complementary information respectively about the near- and far-field deformation pattern associated with the 2021 unrest phase. We jointly inverted these data using a new modeling algorithm implementing elastic and inelastic (thermo-poroelastic) sources to retrieve the volcanic source of the ground deformation observed during the recent unrest phase. Results indicate as preferred model a spheroid/cylindrical source located below La Fossa cone, with cumulated volume and pressure variations in agreement with previous studies using only InSAR or GNSS data. We also analyzed the 2018 deformation through GNSS data, whose pattern reveal an additional unrest episode possibly located at the northern edge of La Fossa Caldera. We discuss the highlighted unrest episodes in the context of the more general volcano-tectonic deformation pattern affecting the island.