Fig. 7: Mechanisms of slow subsidence and uplift, 1984-2022.

a Campi Flegrei’s long-term melt-rich zone feeds shallow intrusions (magenta), while degassing (arrows) maintains crust rich in CO₂ (green) below the low-permeability base (brown) of the hydrothermal system (blue). The surface cap for the hydrothermal system (olive)¹⁸ is overlain by the uppermost volcanic deposits (pink). Fractures beneath Solfatara and Pisciarelli (black dashed line leading to black triangle) allow the leakage of hydrothermal steam and CO₂. b Changes with time in the CO₂/H₂O ratio of gases collected from Solfatara-Pisciarelli (1984-2022)^22,72. Both. 1984–1986. Repeated intrusions of sills (1950–1984) leads to partial rupturing through the low-permeability zone in 1984 (double black lines) and an increase in the escape of trapped CO₂ towards Solfatara-Pisciarelli (a, left; b, pale yellow). 1986–2000. CO₂ pressure decays during withdrawal (a, middle, white), continuing slow subsidence and promoting a decrease in the CO₂/H₂O ratio. Subsidence closes fractures and enhances resealing by mineralisation. Young seals are intermittently broken to allow temporary surges in CO₂ escape (b, pale green). 2000-2015. The pressure drop increases the gas pressure gradient above the melt-rich zone and, hence, also the flux of ascending CO₂ (a, middle, arrows). CO₂ accumulates as the low-permeability horizon re-heals. Slow uplift begins with almost no seismicity until 2015. The CO₂ flux to the surface increases as the local pressure gradient becomes larger (b, blue). 2015-2022. The crust stretches beyond its value in 1984, triggering the return of persistent VT seismicity at shallow depth and continuing the approach to completing crustal rupture (a, double black lines) with a concentration of seismicity at shallow depth (a, b, orange).