Fig. 1: Unrest at Campi Flegrei since 1950.

a Ground movement has occurred across the central 80 km² (blue ellipse) of the caldera (dashed outline). Most of the VT seismicity since 1983 (Fig. 3) has been focussed around the Starza marine terrace and fault system (white curve) between Monte Nuovo and the fumarolic fields of Solfatara and Pisciarelli, which are next to each other. b From the surface downwards, the crust beneath the central region of the caldera consists of predominantly pyroclastic volcanic deposits (primary and reworked) to depths of as much as 2.3 km, overlying marine silts and clays, which become thermally altered (thermo-metamorphosed) at depths of about 2.5–3.0 km and then grade (at unknown depth) into bedrock that contains solidified magmatic intrusions and extends to the top of a melt-rich layer about 8 km below the surface (magenta¹⁰). The melt-rich zone feeds magma and gas (notably CO₂) to shallower depths. Measured temperatures of about 400 °C in the upper levels of the thermo-metamorphic horizon^16,19 favour silica mineralisation and formation of low-permeability horizons (grey band)¹⁹. Uplift between 1950 and 1984 is consistent with intrusions of magmatic sills to depths of about 3 km (pale magenta)¹⁵. Pore pressures are expected to be hydrostatic above the thermo-metamorphic zone, but to reach higher values (that may approach lithostatic) where magmatic gas accumulates underneath⁴⁸. Precipitation of Ca-rich minerals favours the formation of a stronger cap rock at depths less than 1.5 km¹⁸ (outlined by the blue dashes). Almost all the seismicity during the 1982-1984 unrest (red dots) occurred in the hydrothermal system between the cap rock and thermo-metamorphic horizons. Since 2005 (green dots), seismicity has also occurred through the cap rock in the vicinity of Solfatara and Pisciarelli (black triangle; see also Fig. 3) which may be favouring the escape of CO₂ (black arrow).