Accelerating upper crustal deformation and seismicity of Campi Flegrei caldera (Italy), during the 2000–2023 unrest

Campi Flegrei is the largest active caldera in Europe and it is home to more than 350,000 people. Since 2005, the caldera has shown accelerating ground inflation and an intensification of seismic activity. Here we quantify the decadal accelerating trend alongside oscillations of various frequencies and explore the relationships between deformation and seismic activity over the period 2000 to November 2023. Results highlight the major changes that have occurred in the last four years analyzed and reveal a parabolic increase in vertical uplift and a super-exponential (squared-exponential) rise in the number of earthquakes and seismic energy release. Inspection of data indicate a close temporal correlation between deformation rate and seismicity, and an exponential relationship, with an exponent increasing over time, between ground deformation and cumulative number of earthquakes. These relationships are consistent with a quasi-elastic behavior and a stress memory (Kaiser) effect of the upper crust of the caldera under an increasing stress suggesting a progressive mechanical weakening. Most importantly, they provide evidence of an accelerating sensitivity of seismic activity to caldera inflation and warn of the possibility of significant seismic events in case of continuation, with the same trends and relations, of the bradyseismic crisis in the next years. During the Campi Flegrei caldera unrest from 2000 to 2023, the upper crust had a quasi-elastic behavior and there was a parabolic increase of ground uplift and a superexponential increase in the number of earthquakes and seismic energy release.