Source characterization of the 20th May 2024 MD 4.4 Campi Flegrei caldera earthquake through a joint source-propagation probabilistic inversion

On May 20, 2024, an earthquake of magnitude MD 4.4 nucleated at shallow depth (2.6 km) in the Campi Flegrei caldera (Southern Italy), a densely populated area where an increase in seismic activity has been observed since 2019 attributable to an on-going unrest episode. While the magnitude was moderate, the event produced a strong ground shaking with an observed maximum peak ground acceleration of 3.58 m s-2, and several buildings were damaged. Here, we characterize the earthquake source using a probabilistic joint source-propagation spectral inversion in the Fourier space. We estimate a moment magnitude Mw = 3.70± 0.13 and a corner frequency fc = 1.11 ± 0.19 Hz. Assuming a circular rupture model, we estimate a source radius r = 400 ± 70 m and a stress drop ∆σ = 3.2 ± 2.2 MPa. The estimated stress drop suggests that future earthquakes in the hypocentral region, considering a possible rupture length of 3 km suggested by previous studies, can have magnitude increased by 1.2 ± 0.3 units with respect to May 20th event. A systematic source characterization of the recent seismicity in the caldera would help in estimating the expected ground motion from future large-magnitude events.