Ambient Noise Tomography of Campi Flegrei caldera (Naples, Italy): High Frequency Phase-Velocity Anomalies beneath the Solfatara-Pisciarelli Hydrothermal System

Campi Flegrei caldera (Naples, Italy) is one of the most active volcanic systems worldwide, continuously monitored by the Istituto Nazionale di Geofisica e Vulcanologia - Osservatorio Vesuviano (INGV-OV). Intense hydrothermal activity, recurrent seismicity and significant episodes of ground uplift (bradyseism) peaking at the caldera centre over the last decades have been related to the dynamics of a complex magmatic-hydrothermal system. Previous studies indicate active fluid migration through the Solfatara-Pisciarelli hydrothermal system, as well as strong small-scale heterogeneities and gas accumulation and release in this area. In this work, we produced a time-dependent Rayleigh-wave tomography model of Campi Flegrei caldera using the Python Package SeisLib. We applied the method to three years of ambient noise data (from January 2022 to December 2024). This period corresponds to the most significant seismic unrest of the last 40 years, with a total of 37 seismic events with duration magnitude Md≥3.0 and a maximum magnitude of Md=4.4 on May 20, 2024. We used records of 18 seismic stations of the INGV-OV network, 17 from the IV network and 1 (CFB3) from the Medusa network. We processed the continuous seismic records using a standard ambient-noise processing workflow, including the removal of transient seismic swarms and band-pass filtering. Data were then resampled and cross-correlated for all available station pairs, knowing that cross-correlation of seismic ambient noise can be related to the surface-wave Green function between two points of observation. From these, we extracted Rayleigh-wave dispersion curves in order to produce phase-velocity maps at 0.25 Hz, 0.50 Hz, 0.75 Hz, 1.0 Hz, 1.25 Hz and 1.50 Hz. Here, we focused on the three highest frequencies (1.0 Hz, 1.25 Hz and 1.50 Hz), which provide the best resolution in the shallowest portion of the caldera. Inversions for Rayleigh-wave phase velocities reveal high-velocity anomalies in the Solfatara-Pisciarelli area, with values ∼100 m/s above average velocities, and sensitivity extending to a few hundred meters of depth. These velocities are consistent with the presence, below the Solfatara-Pisciarelli region, of a shallow hydrothermal system comprising an aquifer and shallow faults. The spatial distribution of the anomalies is also qualitatively consistent with geophysical models indicating the presence of a clay cap atop a highly resistive plume, constrained by faults, that feeds the fumaroles on the surface. High-frequency Rayleigh-wave phase velocities, obtained from the inversion, are also consistent with the presence of an elongated shallow zone of high rigidity. This transfer structure, formed by lateral stress accumulation in the crust, crosses the resistive plume that stores steam and gas beneath the Solfatara-Pisciarelli system. The results are also consistent with the interpretation that shallow faults in the Solfatara-Pisciarelli area act as preferential conduits for ascending gases and hydrothermal fluids.