Burst-like swarms and periodical VT events in the accelerating unrest phase of Campi Flegrei caldera (2021-2024)

Abstract Since 2021, peculiar seismic sequences became evident and frequent in Campi Flegrei caldera (Italy), while deformation, seismicity and gas emission showed an acceleration. We distinguished burst-like swarms and periodical VT sequences. The earthquakes of both types of sequences resulted located in an area that includes the main hydrothermal field, and a zone affected by a geodetic anomaly, which clearly appeared in 2021. Burst-like swarms (max Md = 4.4) are accompanied by a pseudo-tremor, suggesting a mechanism involving near-continuous brittle failure. The periodical VT sequences are shallow and appear linked to the dynamics of the Mt Olibano lava dome, which deforms non-uniformly compared to the rest of the caldera and coincides with the geodetic anomaly. This peculiar seismicity, described in the Campi Flegrei for the first time in this study, has been associated with phreatic explosions and critical phases of unrest in other volcanoes, and currently characterizes the rapidly evolving state of activity of this high-risk volcano.