To the Phlegraean Fields the distribution of earthquakes is changing: until two years ago seismicity was widespread throughout the caldera, while since then it has been concentrating in a precise area of the crust at the center of the caldera, along a plane that can be interpreted as a fault. Documenting this change is a new study resulting from the collaboration between the National Institute of Geophysics and Volcanology (INGV) and the University of Roma Tre, published in the journal Communications Earth & Environment. Such an evolution explains the progressive intensification of the bradyseism phenomenon from 2023 onwards central area of the caldera between Pozzuoli and Bagnoli, with increasingly frequent and strong earthquakes. The fault could be newly formedor it could be the activation of a fault pre-existing. In any case, this phenomenon can cause a increased risk at Campi Flegrei. In particular, it could lead to the occurrence of earthquakes of higher magnitude.
The concentrated distribution of earthquakes and the Campi Flegrei fault: the new INGV study
By analyzing a huge amount of data with innovative methodologies, the researchers observed that from 2023 the hypocenters of the earthquakes became concentrated along a floor below the central area of the caldera (over 50% of the earthquakes in recent years originated in this area). The researchers interpreted this distribution as evidence of the presence of a fault. It could be one newly formed fault or one pre-existing fault that has been reactivated. Its development indicates that a fragile behavior compared to an elastic type: it means that the pressure to which they are subjected, due to its duration and intensity, is producing deformations in the rocks that are no longer reversible and their breaking.
The rocks of the calderaTherefore, they are progressively weakening and showing failure. The presence of the fault, in particular, could lead to a change in the distribution and magnitude of the seismic risk of the caldera. In the future, earthquakes of higher magnitude could occur and the fault could also become a preferential route for the ascent of fluids of magmatic origin. Consequently, themonitoring activities will need to be adapted to this new situation, in order to improve risk prevention at the Campi Flegrei.
Widespread microseismicity in the Campi Flegrei caldera before 2023
The current uplift of the Campi Flegrei caldera due to the phenomenon of bradyseism has been underway for twenty years, i.e. since 2005. Since then, the ground uplift at the Rione Terra in Pozzuoli (the point of maximum deformation of the caldera) has reached approximately 149.5 cm in July 2025. The deformation has a bell shape, which has become more asymmetric in recent years. At the same time there was a progressive increase in seismicitywhich intensified in terms of both frequency and maximum magnitude. Since 2023 this increase has been particularly significant, with a magnitude reaching 4.6 on March 13, 2025the highest ever recorded since 1970, that is, since a seismic monitoring network existed in the Campi Flegrei. Until 2023 earthquakes, almost all superficial with a hypocenter located in the first 4 km of depth, occurred in a widespread way in the caldera area. Their origin, as well as that of the deformation of the ground, has been attributed toincrease in meteoric water pressure accumulated underground: these fluids, heated by the gases released by the magma present in depth, expand and exert considerable pressure on the rocks, which are subject to microfracturing.
