The unprecedented Md = 4.4, 2024, Campi Flegrei earthquake highlights the fluids contribution to the ongoing unrest

The Campi Flegrei caldera has been experiencing volcanic unrest since 2005, rising concern in the
population and in local and national authorities. On May 20, 2024, the largest local earthquake ever
instrumentally recorded up to that time produced substantial damage, forcing the evacuation of tens of
buildings west of the epicenter. At Campi Flegrei, M > 4 earthquakes are rare and their analysis is
crucial to understand the unrest dynamics and the relation between rupture and ground shaking
pattern, which is essential to mitigate the damage of future earthquakes. We analyse seismic
waveforms at local to regional distances to reconstruct the source geometry and kinematics. We
estimate millimetric to submillimetric coseismic surface subsidence– below the sensitivity of any
standard geodetic technique– which, compared to the general uplift, highlights the crucial role of deep
pressurized uids in earthquakes’ generation. Our results also indicate that rupture directivity and local
ampli cation determined the damage distribution.