Preparatory Phase of Major Earthquakes During Campi Flegrei Unrest (2020–2024)

The ongoing unrest at Campi Flegrei is posing significant challenges for hazard assessment and risk mitigation. This densely populated region has experienced more than 20,000 shallow earthquakes since 2020, driven by hydrothermal fluid accumulation beneath a mechanically strong caprock. In this work, we analyze the seismic activity characterizing the ongoing crisis, also including the magnitude Md 4.4 occurred in May 2024 in correspondence of a cumulative caldera uplift of 130 cm at the RITE GPS station. Here, we demonstrate that the joint interpretation of micro-seismicity evolution and deformation trend reveals the preparatory phase of major earthquakes. Our analysis of 20 seismic sequences from 2015 to 2024 highlights for larger earthquakes a consistent preparatory phase, with a strong correlation between deformation rates, cumulative seismic moment, and earthquake magnitudes. Residual strain, representing the difference between deformation-driven input and seismic output, emerges as a critical parameter linearly associated with the maximum earthquake magnitudes. Unveiling the preparation phase of larger earthquakes in the densely populated Campi Flegrei area has the potential to significantly contribute to seismic risk mitigation. Preliminary, simple predictive models based on strain dynamics show promising results, leading the way to developing novel approaches to forecast earthquake magnitudes and event rates days in advance. Our insights provide a crucial foundation for improving seismic risk mitigation strategies at Campi Flegrei and other volcanic systems worldwide.