Rainwater accumulation model related to tectono-stratigraphic assessment for bradyseism at Campi Flegrei, Italy

The Campi Flegrei (CF) volcanic system near Naples, Italy, poses a significant hazard due to bradyseism — a slow vertical ground deformation resulting in either uplift or subsidence. Indeed, between January 2005 and January 2025, the urban area of Pozzuoli experienced approximately 1.4 m of uplift (GNSS RITE Station). The bradyseism is driven by a combination of hydrothermal and magmatic processes, whereby pressurized magmatic fluids generated by deep magma crystallization accumulate beneath an impermeable layer that regulates fluid exchange between upper hydrostatic and lower lithostatic systems. This study introduces a new perspective through a detailed reconstruction of the stratigraphic-tectonic architecture of the CF area that enables identification of structural controls on seismicity, deformation, and fluid migration, and the role of meteoric water. Seismicity beneath the Pozzuoli-Solfatara area occurs at shallower depths near the top of an anticline, whereas deeper earthquakes in Pozzuoli Bay occur in synclinal environments. The anticline beneath Pozzuoli facilitates hydrothermal fluid pressurization in two main reservoirs beneath two relatively impermeable units. The shallow reservoir, referred to as Unit C, is located at a depth of approximately 1.0 to 2.0 km and acts as a reservoir for meteoric water infiltration. The deeper reservoir, referred to as Unit A, occurs at a depth of about 2.0 and 4.0–4.5 km, where magmatic fluids generated by second boiling in the underlying magma accumulate. An impermeable unit of marine sediments, referred to as Unit B, is located at ∼ 2 km depth and separates Units A and C. The shallow reservoir is bounded at the top by a relatively impermeable unit mainly made up of pyroclastic deposits. We developed a simplified hydrogeological model using rainfall data dating back to 1950 to assess the role of meteoric water in bradyseism at CF. We found a strong correlation between subsurface water infiltration and vertical ground deformation observed at the Pozzuoli RITE Station, which corresponds to the crest of the anticline. Our results suggest that meteoric water contributes to interannual uplift fluctuations of up to ∼ 5 cm and accounts for over 20 % of the total uplift recorded between 2005 and 2025. Furthermore, a shortening of recharge time-lag — from about four years to three years since 2010 — indicates enhanced fracturing and infiltration rates. These findings highlight the previously underestimated role of meteoric water in driving deformation and seismicity at CF. Our results also suggest that geoengineering involving targeted surface drainage interventions could mitigate ongoing ground instability and seismic hazards in the region.