A multiparametric analysis of the recent unrest at Campi Flegrei, Italy

Understanding volcanic activity, especially unrest, is a challenging task. This complexity is magnified in Napoli (Southern Italy), where the presence of nearly a million people living on the Campi Flegrei (CF) caldera makes invasive monitoring activities impossible to be performed. Yet, the analysis of coda-waves from continuous ambient-noise recordings (Shapiro & Campillo, 2004) provides highly resolved in-time measurements of mechanical and structural variations in the crust without the need to be invasive, as also exploited in the geothermal field (Hillers et al., 2015; Sánchez-Pastor et al., 2023).
The CF caldera is one of the active hydrothermal systems of the Mediterranean region experiencing notable unrest episodes. Since 2005 a monotonic uplift phenomenon started with unsteadily accelerating seismicity (Bevilacqua et al., 2022). Subsurface rocks withstand a large strain and have high shear and tensile strength (Vanorio & Kanitpanyacharoen, 2015).  As a consequence, seismicity reaches magnitude ~ 4.0 only upon relatively large uplifts (~70-80 cm in the previous unrest (’80 years) and > 1 m in the recent one) contrary to what is generally observed for calderas exhibiting much lower deformation levels (Hill et al., 2003). The caprock above the seismogenic area has a pozzolanic composition and a fibril-rich matrix contributing to its ductility and increased resistance to fracture (Vanorio & Kanitpanyacharoen, 2015). However, specific conditions, e.g., an increase in pore pressure or/and chemical alterations, may lead to mechanical failure over time of the caprock and a change in the structural properties of subsurface rocks. In addition, magma pressure in the reservoir can weaken the volcanic edifice, causing decreases in Elastic moduli (Carrier et al., 2015; Olivier et al., 2019). In recent years, a quasi-elastic behavior and a stress memory effect of the upper crust of the CF caldera under increasing stress suggest a progressive mechanical weakening (Bevilacqua et al., 2024; Kilburn et al., 2017, 2023). 
Elastic models used to describe volcanic surface deformation would assume that accelerations in surface deformation are due to increases in reservoir pressure. Another possible cause for these accelerations is magma pressure in the reservoir weakening the volcanic edifice. Weakening models imagine crustal shear modulus to decrease with damage and therefore with time (Carrier et al., 2015; Olivier et al., 2019). In analogy to these models, we fixed the source of deformation (location and size) to values from the literature, and we inverted the observed deformation searching for changes in the crustal rigidity, modeling for the sill by Fialko et al.  (2001).
We performed a continuous analysis at CF between 2016 and 2024 to investigate the recent unrest characterized by a significant uplift and increased seismicity.  We compared seismic-waves velocity variations δv/v in relation to the deformation and other sources of changes controlling the mechanical and structural variations of crustal rocks, such as rain and temperature. For this purpose, we employ seismic ambient-noise interferometry to estimate δv/v (Shapiro & Campillo, 2004) at various local seismic stations from single-station autocorrelations and we quantify surface geodetic strain using data from a local GPS network (De Martino et al., 2021).