## 3 Results
### Seismicity  
The observation of large amplitude signals that were associated with PDCs, in particular those that occurred on March 31, is the main peculiarity of the seismic data recorded during the March-April 2020 eruptive crisis at Stromboli. To gain insight into the nature of these signals, we compared the seismic recordings to the images of thermal cameras (Figure 8a), timed with the same reference system as the seismic network (UTC based on GPS). The signal due to a PDC consists of a landslide-type initial part and a near monochromatic phase (peak frequency around 3 Hz) with much larger amplitude. The spectrogram (Figure 8b) clearly highlights the transition between the two phases that occurs exactly at the time of the impact of the PDC on the coast. The landslide-type signal is shown in red in Figure 8c, whereas the near monochromatic phase is drawn in blue. This observation highlights that the PDCs flowing on the ground generate signals that are similar to those typical of landslides moving on the SdF. The material accelerates on the slope and then impacts on the coastline (or on the sea surface). The impact generates the large amplitude 1-3 Hz phase.  
We also focused on the seismic amplitude of STRE station, near the SdF where the PDCs flow, and STR1 station, which is relatively far from the SdF. The comparison between different seismograms produced in one h at STRE and STR1 stations (Figure 1), including the major PDCs that occurred on 31 March, is displayed in Figure 9 and indicates the explosion (E) and PDC signals. We calculated the ratio between the amplitude of the E3 explosion and PDC4 signals (top of Figure 9) that were recorded at the two stations (STRE: black; STR1: red). In order to evaluate the seismic amplitude, we calculated the average of the absolute values (RSAM [86]) of 30-s windows of both the explosion and PDC signals, starting from the onsets of the transients recorded by the two different stations (vertical component). Subsequently, we calculated the amplitude ratios of the explosion signals and the PDC signals. We obtained E3 ratio (STRE RSAAM)/(STR1 RSAAM) \(\approx\) 2 and PDC4 ratio (STRE RSAAM)/(STR1 RSAAM) \(\approx\) 4. This observation highlights the rapid decay of the seismic signal amplitude moving away from SdF slope and confirms that the source of the seismic phase with dominant frequency around 3 Hz is on the surface.  
Figure 7: Three-dimensional (3D) mapping of the deposit in the area obtained using the ML classifier employed between 8 October 2019 and 7 April 2020.  
Figure 8: Comparison of a PDC seismic signal, starting at 03:43:54, 31 March 2020 (labeled “1”) with the thermal camera images. (**a**) Impact of the PDC on the coast line recorded by the SPCT camera (see Figure 1 for camera location). (**b**) Seismogram and spectrogram of the PDC signal that occurred on 31 March at 03:43:54. The magenta arrows, labeled “1”, indicate the onset of the landslide-type seismic signal. The red arrows, labeled “II”, indicate the beginning of the 1–3 Hz frequency signal generated by the impact of the PDC on the coast line at 03:44:23 UTC. (**c**) 10 min of seismic signal highpass-filtered in frequencies \(>\)10 Hz (red) and bandpass-filtered in the 1–3 Hz band (blue) containing the recordings of some of the major PDCs that occurred on 31 March 2020. The amplitude of the highpass-filtered signal (red) is multiplied \(\times\) 20. The yellow box highlights the PDCs represented in panels (**a**) and (**b**).  
Figure 9: Comparison of one-h (2020-03-31T03: 00:00-2020-03-31T04: 00:00 UTC) of the vertical component seismic signals of the STRE (black line) and STR1 (red line) stations. The E and PDC labels  
followed by an integer represent the signals of the most significant explosions and PDCs respectively, recorded in the considered time interval. At the top of the figure, the seismograms of the PDC4 and E3 explosion are shown in detail.