Fig. 2: Seismic recordings in the Campi Flegrei caldera.

a–c Seismograms and spectrograms of examples of generic very short inter-event time sequences. d Seismogram and spectrogram of the earthquake of 27 April 2024, 03:44:56 UTC (Md = 3.9) located in the Gulf of Pozzuoli (group B in Fig. 1a) reported as an example of a Campi Flegrei earthquake not included in a very short inter-event time sequence. e–h Seismograms and spectrograms of examples of burst-like swarms. These sequences also include earthquakes with relatively high magnitudes such as the one with Md 4.2 occurred on 27 September 2023 at 01:35 (UTC) (g) and the largest Campi Flegrei earthquake, with Md = 4.4, recorded on 20 May 2024 at 18:10 (UTC) (h). All examples are recorded by the multiparametric seismic station CSOB, component HNZ (vertical component of the accelerometer with a sampling rate of 200 samples per second).