Burst-like swarms during the ongoing unrest at Campi Flegrei Caldera - Flora Giudicepietro

Seminario di Flora Giudicepietro Burst-like swarms during the ongoing unrest at Campi Flegrei Caldera Recent years have been characterized by a significant intensification of unrest at the Campi Flegrei caldera, one of the most densely populated active volcanic systems in the world. Since the onset of the current unrest phase in 2000, the caldera has experienced progressive ground uplift, increasing seismicity, and enhanced hydrothermal degassing. However, from early 2021 onward, these phenomena have shown a marked acceleration, accompanied by the emergence of peculiar Volcano-Tectonic (VT) seismic sequences characterized by extremely short inter-event times. The seismicity recorded between 2021 and 2024 was investigated with the aim of exploring the relationships between seismic processes, deformation field evolution, and hydrothermal activity during the ongoing accelerating unrest. Particular attention was devoted to a class of seismic sequences that we define as burst-like swarms, following the classical volcanic seismology literature. These sequences are characterized by rapid clusters of earthquakes occurring within intervals of only a few seconds, making them difficult to recognize using standard seismic analysis approaches. Our analysis shows that these burst-like swarms are spatially concentrated in a sector encompassing both the main hydrothermal area and a recently identified geodetic anomaly located in the Mt. Olibano area, east of the city of Pozzuoli. This anomaly, which became clearly detectable in 2021, is characterized by a relative uplift deficit with respect to the surrounding areas, despite the overall bell-shaped radial deformation pattern that has characterized both past and present bradyseismic crises at Campi Flegrei. The temporal evolution of the unrest reveals a progressive acceleration in seismicity rate, earthquake magnitude, uplift velocity, and geochemical indicators such as soil CO2 flux. The appearance and increasing occurrence of burst-like swarms coincide with an increase of the acceleration and suggest a strong link between localized deformation processes, hydrothermal pressurization, and evolving seismogenic mechanisms. Similar seismic patterns have been associated at other volcanoes with critical unrest phases and, in some cases, with phreatic explosions. Our results therefore suggest that burst-like swarms may represent an important indicator of the current state of activity of Campi Flegrei and provide new insights into the interaction between hydrothermal processes, deformation anomalies, and seismicity in caldera unrest dynamics.