Quantifying the link between ground deformation and seismicity during the ongoing unrest of Campi Flegrei caldera (Italy)

Campi Flegrei, the largest active caldera in Europe and home to over 350,000 people, has exhibited accelerating ground inflation and intensified seismic activity since 2005. Using monitoring data collected from 2000 to November 2023, we quantified the decadal accelerating trends, characterized oscillations of varying frequencies, and explored the relationships between deformation and seismic activity.

Analysis revealed a strong temporal correlation between deformation rates and seismic activity, expressed by an exponential relationship between ground deformation and the cumulative number of earthquakes. Since around 2010, the relationship between the cumulative number of earthquakes and vertical uplift has been better described by two exponential functions with increasing exponents over time, with the inflection in the period between 4/2020 and 9/2022.

This inflection effectively represents the rising intensity of seismic activity since about 2020 and is interpreted as a stress memory effect, attributed to reaching a stress level in the shallow crust comparable to the peak stress during the 1982–84 crisis. The recent exponential trend differs from the previously suggested linear relationship between these two variables and is interpreted as indicative of a progressive evolution in the quasi-elastic behavior of the shallow crust of Campi Flegrei caldera. Furthermore, this exponential-type relationship differs from the linear-type relationship observed during the 1982–84 crisis, suggesting that the two crises are driven by different forcing sources or mechanisms.

Crucially, these findings provide evidence of an accelerating sensitivity of seismic activity to caldera inflation. During 2024, the exponential correlation between vertical uplift and the cumulative number of earthquakes remained consistent. The Md 4.4 event on May 20, 2024, aligns closely with the hypothesized patterns, highlighting concerns about the possibility of new seismic crises should the bradyseism persist with these trends and relationships.