At the Phlegraean Fields therainwater contributes significantly toraising of the soiland therefore also to earthquakes linked to the phenomenon of bradyseism. To demonstrate the phenomenon, with a study published in the magazine Geoscience Frontierswere researchers from the Department of Earth, Environmental and Resource Sciences (DiSTAR) ofUniversity of Naples Federico II. It is calculated that well 20% of the total lift registered between 2005 and 2025 in the Pozzuoli area it is due to meteoric water that infiltrates the subsoil of the caldera. Here, between 1 and 2 km deepaccumulates and increases the pressure in the pores, contributing to soil deformation.
The Campi Flegrei and the effect of rain: the analysis of the structure under the caldera
To understand the role of rainwater in bradyseism at Campi Flegrei, researchers first had to reconstruct the conformation of the subsoil in detail of the caldera from a stratigraphic and tectonic point of view. From the reconstruction, the presence of one emerges under the Pozzuoli area where the uplift is greatest anticlinal foldthat is, a curvature of the rock layers convex upwards. This sort of “dome” is the result of compressive efforts, and favors the accumulation of fluids and the progressive increase in pressure. These fluids are found in two tanks: the most superficial, located one Between 1 and 2 km deepcollects meteoric water that infiltrates the surface and is delimited at the top by an impermeable layer of volcanic materials; the lower one, located Between 2 and 4 km deepcollects the fluids released by the underlying magma. At a depth of 2kmbetween one tank and another, there is one waterproof layer of marine sediments. The fluids of magmatic origin heat the meteoric ones, which thus expand and exert considerable pressure also due to the more superficial impermeable rock layer which hinders their escape. As a result, the ground deforms and seismicity increases.
The role of rainwater in bradyseism
The researchers analyzed historical precipitation data occurred between 1950 and 2025 and crossed them with those relating to the ground deformation recorded by the GNSS RITE station, installed in the area of maximum lifting. The comparison revealed an evident correlation between the infiltration of meteoric water and the vertical deformation of the caldera. Between January 2005 and January 2025 the center of Pozzuoli underwent a lift of approximately 1.4 mof which 20% was attributed by researchers toinfiltration of rainwater. According to their calculations, the latter can determine up to 5cm lift per yearsince its accumulation leads to an increase in pressure in the pores. One of the factors that favor the infiltration of large quantities of water into the subsoil is the presence in the area of volcanic vents capable of acting as a funnel collecting the water, which from here easily infiltrates into the ground through the fractures. Furthermore, it was noted that starting from 2010 the charging timei.e. how long it takes for precipitation to reach the aquifer, it got shorter going from 4 to 3 years. This means that the permeability of the subsoil is increased due to a greater fracturing linked to seismic activity. These results show that the role of meteoric water in bradyseism has so far been underestimated.
The authors of the study suggest the control of surface runoff of rainwater, which would reduce its infiltration into the ground. In particular, surface rainwater that falls into the interior could be drained Astroni craterlocated upstream of the Solfatara-Pisciarelli area. Such interventions, however, would have significant results only after a few years.
