The recurrence of geophysical manifestations at the Campi Flegrei caldera

The Campi Flegrei caldera (CFc), Italy, exhibits distinct unrest patterns, including shallow seismicity following substantial strain accumulation, all within a densely populated area. Previous geophysical studies typically analyzed individual episodes, but by comparing two distinct unrest periods we identified recurring manifestations and V_P/V_S anomalies linked to a confined reservoir at 2- to 4-kilometer depth. Integrating rock physics experiments under hydrothermal conditions, 24 years of rainfall data, and subsurface hydrodynamics, we found increasing rainfall rates, which indicate reservoir recharge and pressurization. We show that hydrothermal water promotes caprock sealing through the formation of a fibrous microstructure. Our experiments further demonstrate that fluid accumulation rates directly influence deformation rates. Together, these processes drive gradual deformation, natural seismicity, and deepening earthquake foci. Recognizing these recurring patterns is crucial for understanding the caldera's unrest-driving mechanism, enabling us to offer actionable insights for hazard assessment and engineering solutions, such as intercepting water upstream to prevent drainage toward Pozzuoli.