Temporal elastic properties changes and rock weakening at Campi Flegrei, Italy

Understanding volcanic activity remains a challenging task. So far, several conceptual geodetic models have been proposed to describe the inter-eruptive period, typically invoking either progressive rock damage or increasing overpressure within the magmatic (or gas) reservoir. Here, we adopted a combined seismo-geodetic framework to investigate volcanic unrest and to model surface deformation at the Campi Flegrei (CF) volcano, Italy. 

The CF caldera is one of the most active hydrothermal systems in the Mediterranean region and has experienced notable unrest episodes. Since 2005 a monotonic uplift phenomenon has been observed, accompanied by unsteadily accelerating seismicity (Bevilacqua et al., 2022). 

Subsurface rocks sustain large strains and exhibit high shear and tensile strength (Vanorio & Kanitpanyacharoen, 2015). Consequently, seismicity reaches magnitude ~ 4.0 only upon relatively large uplifts ~70–80 cm during the 1980s unrest and >1 m during the recent episode), contrary to what is generally observed for calderas exhibiting much lower deformation levels (Hill et al., 2003).

The caprock above the seismogenic zone is characterized by a fibril-rich matrix that enhances ductility and resistance to fracturing (Vanorio & Kanitpanyacharoen, 2015). However, changes in pore pressure and/or chemical alteration may ultimately induce mechanical failure and modify the structural properties of subsurface rocks. In addition, increased magma pressure within the reservoir can weaken the volcanic edifice, leading to reductions 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). Seismic tomography indicates that most of the observed seismicity is associated with a pressurized gas reservoir (De Landro et al., 2025), while advanced big-data-based earthquake locations exclude shallow magma migration (Tan et al., 2025). Furthermore, recent petrological and geochemical studies identified a weak layer that plays a key role in overpressure accumulation, driving both deformation and seismicity (Buono et al., 2025). The initiation and growth of a volcano-tectonic fault have also been hypothesized (Giordano et al., 2025).

In our study, we tracked the evolution of subsurface elastic properties by monitoring temporal changes in relative seismic wave velocities (δv/v) thanks to the coda wave interferometry of continuous ambient noise at local seismic stations. A progressive decrease in δv/v is detected in the area where we observe the highest concentration of seismicity and that we attribute to the rock-weakening tracked by the earthquake occurrences. By incorporating time-dependent elastic moduli changes in the geodetic inversion of surface displacement recorded by a local GPS network (De Martino et al, 2021), we retrieved a refined time evolution of reservoir overpressure.  Our results suggest the active contribution of elastic properties of geomaterials in controlling the volcanic dynamics.