Seismic Images of Pressurized Sources and Fluid Migration Driving Uplift at Campi Flegrei Caldera During 2020–2024

After the 1982–1984 subsidence phase, ground uplift at Campi Flegrei Caldera resumed in 2005, while volcanic-tectonic earthquakes have steadily increased in frequency and intensity since 2020, with a significant intensification observed since 2023. This rise in seismic activity enabled us to conduct a new tomography using data collected from 2020 to June 2024. In this work, we used 4,161 local earthquakes (41,272 P-phases and 14,683 S-phases) processed with the tomoDDPS code, considering 388,166 P and 107,281 S differential times to improve earthquake locations and velocity models. Compared to previous tomographic studies, our 3D velocity models provide higher-resolution images of the central caldera's structure down to ~4 km depth. Additionally, we separately inverted datasets for 2020–2022 (moderate seismicity) and 2023–2024 (intense seismicity), identifying velocity variations (ranging from 5% to 10%) between these periods. These changes observed in 2023–2024 support the existence of two pressurized sources at different depths. One, located at 3–4 km depth beneath Pozzuoli and offshore, could represent either a magma intrusion enriched in supercritical fluids or an accumulation of pressurized, high-density fluids—a finding that aligns with recent ground deformation studies and modeled source depths. Furthermore, the upward migration of magmatic fluids interacting with the geothermal system generated a secondary, shallower pressurized source at approximately 2 km depth (beneath the Solfatara-Piscirelli area). Overall, these processes are responsible for the recent acceleration in uplift, increased seismicity and gases from the fumarolic field, and changes in crustal elastic properties through stress variations and fluid/gas migration.