## 2 Methods
### The INGV Seismic Network  
The seismic monitoring of Stromboli is based on a broadband seismic network [76] deployed on the island after the 2002-2003 effusive eruption. In the following years, the number of stations decreased due to the closure of some routes. Moreover, the seismic network was also damaged by the paroxysmal explosions of 2003, 2007, and 2019. The current configuration consists of seven stations (Figure 1, Table 2), managed by INGV-OV and INGV-OE. The data transmission is based on UHF radio links and the INGV wireless local area networking system. Seismic data are first transmitted to the centres in Stromboli and Lipari and then to the acquisition centres of the INGV-OV and the INGV-OE, in Napoli and Catania, respectively. Stromboli seismicity is typically characterized by explosion-generated signals [77; 78; 79; 80; 81] and by persistent volcanic tremor [82]. Seismic signals that are associated with landslides [83; 84], rolling blocks, PDCs, and lava flows were also recorded in the period 28 March-1 April 2020 (Figure 2). Although the explosion signals are transient events, compared to the recordings of tectonic and volcano-tectonic earthquakes, they (Figure 2a) are characterized by an emerging onset, so that a clear arrival of the P wave cannot be recognized on the seismogram. Moreover, they show a wide frequency band (0.05-10 Hz) containing a Very Long Period (VLP) event. The volcanic tremor (Figure 2b), typical of open conduit volcanoes, has a frequency content of between 1 and 3 Hz. The signals that are caused by the typical landslides in loose pyroclastic deposits on the SdF (Figure 2c) have a fusiform envelope and are characterized by a relatively high frequency (4-15 Hz). An increase in the occurrence of these signals was a short-term precursor of the flank effusive eruption on 2007; therefore, they have been studied in detail in various works, especially from the point of view of their automatic and early detection [83; 84]. Such signals are partly due to the morphogenetic processes of the SdF slope. Moreover, they can be related to the explosive activity that throws incoherent materials on the slope, which can be easily re-mobilized, thus generating landslides. These are also favored by effusive activity, as the lava front is often a source of incoherent material, which can move on the steep slope of the volcano's flank [84]. Additionally, the signals that are caused by the rolling of large blocks (Figure 2d), in some cases, can be associated with detachments occurring at the lava flow front. They are similar to the ones associated with landslides in loose pyroclastic deposits, however some impulsive phases can be recognized in the waveform, and the spectrogram shows a less gradual onset when compared to that of the landslides in loose pyroclastic deposits. In the example of Figure 2d, the association of the seismic signal with the rolling of large blocks was based on its comparison with the images of the cameras. But an experienced seismic analyst is able to distinguish these signals, even on the basis of the visual analysis of the seismogram alone. This type of signal can also originate from rock-falls along the cliffs of the Labronzo area (North edge of the SdF). Occasionally, they were recorded on the southern side of the island, where there are steep morphologies. The signals caused by the rolling of large blocks (Figure 2d) are similar to the ones that were associated with landslides, however some impulsive phases can be recognized in the waveform, and the spectrogram shows a less gradual onset as compared to that of the landslides. These signals can also be associated with detachments occurring at the lava flow front. The signals that are linked to hot avalanches or PDCs (Figure 2e) are