Birth and growth of a volcanotectonic fault during the current volcanic unrest at Campi Flegrei caldera (Italy)

The ongoing volcanic unrest at the highly populated Campi Flegrei caldera is accompanied by incremental seismic activity, ground uplift and geochemical anomalies that are raising severe public concern. Here we apply an unsupervised least root square method and a tailored Monte Carlo approach to the 2019-2024 earthquake hypocenter locations and show that since 2023 seismicity has progressively clustered along a preferential N249° ± 4°, 53° ± 1°plane near the center of the caldera, concentrating more than 50% of seismic events and most of the released seismic energy. This clustering is evidence for an internal process that drives the transition from diffuse micro-seismicity to the development of an extensional volcanotectonic fault in its initial stages. The nucleation (or reactivation) of this fault suggests that caldera rocks are experiencing conditions for critical failure and may have overcome elastic deformation resulted, until 2023, in an almost perfect axisymmetric uplift, thus representing an important change for volcanic and related hazards evaluation at Campi Flegrei. Spatial analysis of seismicity within the Campi Flegrei caldera suggests it has become progressively clustered since 2023 around a central plane which is consistent with the development of an extensional volcanotectonic fault