Francesca Bianco

The traditional method of representing the seismicity in a region involves plotting earthquakes of various magnitudes on a map. Energy release, however, offers a different perspective for understanding seismicity, especially in volcanic areas where earthquakes are abundant and occur in the form of swarms. In this study, we analyzed earthquakes with a moment magnitude (Mw) greater than 1.5 that occurred between 2006 and 2023, which were relocated using a 3D velocity model, to examine the distribution of energy space density (ESD) in the Etna region. The ESD maps reveal the fundamental patterns of seismicity, which generally align with the local structural features. The shallow crust above 10 km plays a crucial role in energy distribution, with the highest values found in the eastern and southwestern sectors, along well-known structures bordering the unstable sector of the volcano. At deeper levels (down to 20 km), high ESD values are observed in the western sector and are related to the dynamics of magma movement at these depths. The ESD pattern improves the clarity of rock fracture density visualization, offering a more detailed view of the areas with the highest fracture density. These regions may indicate where fluid is stored beneath the volcano.

Seismic wave attenuation is a key feature of seismic wave propagation that provides constraints on the composition and physical state of the medium within the Earth. We separated intrinsic and scattering attenuation coefficients for the shallow crust and lower crust/upper mantle in the Mt. Etna area. For this purpose, the Multiple Lapse Time Window Analysis (MLTWA) was applied to two groups of earthquakes, well separated in depth. We also studied the spatial variation of the attenuation parameters by dividing the study area into four sectors around Etna. The results show an effective homogeneity of the propagation characteristics inside Etna and, in particular, some lateral variations and minor variations with depth. We observe that structural discontinuities and lithology control scattering losses at all frequencies, with higher scattering in the shallow crust. The intrinsic absorption shows no sensitivity to the presence of these main geological structures and is quite uniform for different depths. Furthermore, compared to the northern sector of the volcano, the southern one shows stronger scattering attenuation at low frequencies. This pattern correlates well with the high seismic activity along most of Etna’s active tectonic structures and ascending magmatic fluids that characterize this sector of the volcano. Although we only discuss the differences in the "average" scattering and inelastic properties of the investigated volumes, the results of this study are very informative about the characteristics of each region. Moreover, they suggest that a future study is necessary, providing a more detailed picture of the spatial distribution of seismic attenuation in the study area, through a 3D inversion of the attenuation parameters estimated along the single source-receiver paths.

After a period of mild eruptive activity, Stromboli showed between 2017 and 2018 a reawakening phase, with an increase in the eruptive activity starting in May 2017. The alert level of the volcano was raised from “green” (base) to “yellow” (attention) on 7 December 2017, and a small lava overflowed the crater rim on 15 December 2017. Between July 2017 and August 2018 the monitoring networks recorded nine major explosions, which are a serious hazard for Stromboli because they affect the summit area, crowded by tourists. We studied the 2017–2018 eruptive phase through the analysis of multidisciplinary data comprising thermal video-camera images, seismic, geodetic and geochemical data. We focused on the major explosion mechanism analyzing the well-recorded 1 December 2017 major explosion as a case study. We found that the 2017–2018 eruptive phase is consistent with a greater gas-rich magma supply in the shallow system. Furthermore, through the analysis of the case study major explosion, we identified precursory phases in the strainmeter and seismic data occurring 77 and 38 s before the explosive jet reached the eruptive vent, respectively. On the basis of these short-term precursors, we propose an automatic timely alarm system for major explosions at Stromboli volcano.

Earthquakes at Campi Flegrei have been low magnitude and sparse since 1985, denying onshore monitoring observations of their usual source for structural constraint: seismic tomography. Here we used continuous seismic records from 2011–2013 to reconstruct period‐dependent Rayleigh wave group velocity maps of the volcano. The Neapolitan Yellow Tuff rim faults bound high‐velocity intracrater products of historical eruptions, which act as a barrier for deep fluid migration. The anomaly with lowest group velocity is aseismic and corresponds to the portion of a fluid storage zone that was fractured during the 1984 volcanic unrest under Pozzuoli town. Earthquake locations show that fluids migrate from this area toward the Solfatara and Pisciarelli fumaroles along shallower low‐velocity fractures. The aseismic anomaly is likely fed by a deep‐seated offshore magmatic source. Its spatial relation with regional dynamics and observations from historical unrests mark the area as the most likely feeder pathway for fluid and magmatic inputs from depth.

The Campi Flegrei caldera is one of the highest risk volcanic fields worldwide, because of its eruptive history and the large population hosted within the caldera. It experiences bradiseismic crises: sudden uplift with low energetic seismic swarm occurrences. No seismicity is recorded out of these deformation rate changes. Therefore, a continuous seismic monitoring of the caldera is possible only by means of the ambient seismic noise. We apply a noise‐based seismic monitoring technique to the cross correlations of 5 year recordings at the mobile seismic network. The resulting relative velocity variations are compared to the temporal behavior of the geophysical and geochemical observations routinely sampled at Campi Flegrei. We discriminate between two kinds of crustal stress field variations acting at different timescales. They are related to a possible magmatic intrusion and to the gradual heating of the hydrothermal system, respectively. This study sets up the basis for future volcano monitoring strategies.