Localization and classification of the seismicity associated with the 2021 volcanic-hydrothermal unrest of Vulcano Island (Italy) and following years

This thesis tackled the classification and localization of the seismic signals detected by the INGV-OE monitoring network, made up of 6 permanent stations and 9 mobile stations arranged on Vulcano and Lipari islands, during the unrest of Vulcano island in 2021. In Chapter 1 the introduction and the aims of the research are presented along with the description of the geological framework of Vulcano and of the details of the 2021 unrest. In Chapter 2, to create a seismic catalogue of the 2021 unrest, we employed the continuous waveforms recorded by the permanent seismic stations of the INGV network located on Vulcano and in the southern sector of Lipari Island. In addition to this configuration, after the beginning of the unrest, 5 additional temporary stations were installed by INGV-OE to improve the monitoring system and to enhance earthquake detection and location quality. In order to detect the maximum number of small magnitude seismic events, we applied on the continuously recorded seismic traces a trigger algorithm based on the ratio between STA (short- time average) and LTA (long-time average). Then the events of December 2021 were visually inspected because in this mentioned period there was a great richness of clear data with a good signal-to-noise ratio, and the manual picking can improve the automatic location; the arrival times of the most energetic events were manually determined. This picking procedure involved the characterization of the first arrival (first break) of the P wave on the seismograms. The arrival times were used to locate seismic events of December 2021 by using 1D and successively 3D velocity models. In particular for 3D locations, the tomographic model of Chiarabba et al. (2004) was used to obtain more precise and reliable focal solutions. For the month of December 2021, a total of 369 events were located within the 3D velocity model by Chiarabba et al. (2004). These events are distributed mostly between 0 and 2 kilometers of depth and are concentrated below the La Fossa Cone. This main concentration of seismic events runs exactly along a sharp variation in Vp velocity, suggesting that volcanotectonic structures, like the intersection between the Caldera del Piano and Caldera La Fossa, are the main channel for the ascent of deep fluids. In addition, it is evident that a strong concentration of seismic events occurs at 1 km depth, in an area where there is a minimum of 4 Vp/Vs ratio, suggesting that the hydrothermal component is probably significant in this volume. In Chapter 3, the spectral analysis of seismograms associated with recorded events allowed to propose an event classification, identifying key features to distinguish them into three different groups: Low Frequency (LF), Hybrid (HYB), and High Frequency (HF) events. Template matching for the LF, HYB and HF events has been carried out on the continuous signal from 12th September 2021 to 31st December 2023. Considering the vertical component of seismograms detected at the IVCR station, a total of 35578 LF events, 419 HYB events and 254 HF events were detected over two years. Such numbers of events are one order of magnitude larger than those detected automatically and constitute an improved seismic catalogue that allows a much clearer definition of the seismicity patterns associated with the unrest. The quantitative distribution of the events over two years shows interesting patterns especially during the first months of the unrest, when an anticorrelation between LF and HF events can be observed. In addition, when compared with the CO2 flux, the dominance of the LF events characterizes the initial pressurization phase of the shallow hydrothermal system, followed by a transition to a fracturing phase dominated by HF events and strong CO2 degassing at surface. Finally, Chapter 4 draws some conclusions about the importance of the seismic record in describing the different phases of the Vulcano 2021 hydrothermal unrest and in general at active volcanoes.