The 1 September 2025 geodetic event: a key phenomenon for understanding the unrest evolution at Campi Flegrei caldera (Italy)

On 1 September 2025, an Md 4.0 earthquake occurred within a seismic swarm at the Campi Flegrei caldera (Italy) and produced an unprecedented coseismic displacement. The resulting ground deformation, reaching approximately up to 4 cm, clearly outlined the directions of motion of a distinct crustal block and revealed an extensional displacement pattern. This deformation developed in an area where a geodetic anomaly (an uplift deficit, in particular), superimposed on the long-term background deformation field, was identified in previous studies. The spatial distribution and geometry of the deformation, retrieved through GNSS and DInSAR measurements, closely replicate those of the previously recognized anomaly in the Mt. Olibano–Accademia sector, thereby confirming the active involvement of this structural domain in the ongoing caldera dynamics. The sharp and well-defined displacement associated with the Md 4.0 earthquake allowed us to retrospectively identify smaller, analogous deformation episodes that occurred earlier in the unrest sequence but remained less distinct due to their limited amplitude. Altogether, these observations place new constraints on the mechanical behavior of the central–eastern sector of the Campi Flegrei caldera. They improve our understanding of how localized fracturing and faulting processes, within the shallow crust, interact with the broader deformation field driven by the current unrest phase.