## 4 Discussion
On these occasions, the PDCs expanded on the sea surface for several hundred meters threatening a boat with tourists onboard ([[https://www.youtube.com/watch?v=RPKgS3](https://www.youtube.com/watch?v=RPKgS3) sPP1Y]([https://www.youtube.com/watch?v=RPKgS3](https://www.youtube.com/watch?v=RPKgS3) sPP1Y)). Lava flow velocities are normally much lower when compared to the mobility of PDCs, with speeds normally below 10 m s\({}^{-1}\) ([99], and references therein), because of their greater viscosity (e.g., [100]). Only rarely they pose a threat to people, with the exception of few cases of lava accumulated within a crater and suddenly drained by a fissure opening, such as on Nyiragongo [101; 102]. However, at Stromboli, lava flows can represent an indirect threat because they can erode the base of the summit cone and trigger summit collapses [14]. Table 4 shows the very different speed along the same SdF slope between lava flows, which had velocities between 2.8 and 5.2 m s\({}^{-1}\), and the PDCs, which displayed velocities between 12.9 and 40.3 m s\({}^{-1}\). Several authors [103; 104; 105] observed that abrupt slope changes or variations in magma supply affect the velocity of lava flows. When the slope is greater than 24\({}^{\circ}\), it can result in the detachment of blocks from the flow front to form talus and, when the slope is even greater, as in the case of the SdF on Stromboli that reaches 30-35\({}^{\circ}\)[44; 106], the tensional stresses overcome the tensional strength, so that the lava cannot flow any longer. Instead, it breaks into incandescent blocks [103] rolling down the slope and forming a PDC and a distal pile of talus. The monitoring of PDCs along the slope of steep volcanoes, like Stromboli, is therefore of crucial importance, because they unexpectedly evolve into flank collapses possibly triggering tsunamis, as happened at Stromboli in 2002 [16; 17; 96], and more recently at Anak Krakatau in 2018. On that occasion, the tsunami caused over 430 fatalities, injured 14,000 people, and displaced 33,000 more along the Sunda Strait [18; 107]. Interesting enough, the precursory phase of the Anak Krakatau flank collapse was characterized by an increase of the eruptive activity that lasted for 175 days, and the collapse was preceded by two seismic signals consistent with minor mass movements as well as a momentary quiescence [18; 107]. Luckily, the mass movements in the eruptive crisis of 28 March-1 April 2020 at Stromboli were of modest size, not comparable to the events that were described at Anak Krakatau. However, the PDC seismic recordings of Stromboli share some characteristics with the PDCs recorded on other volcanoes, such as Merapi [108] and Unzen [109]. In general, the PDC seismic signals have frequency content between 2 and 15 Hz, similar to that of signals due to landslides in loose clastic material. When PDCs originate from dome collapse, as in the case of Unzen [109], they may contain a lower frequency component caused by the collapse of the dome. In general, the onset of PDC seismic signals emerges with a gradual increase in amplitude, which then remains constant on average for the entire time of the event. These characteristics lead to the classification of PDC seismic signals as continuous and non-transient signals [110]. In addition to these characteristics, in the Stromboli PDC seismograms we were able for the first time to identify the seismic signals caused by the impact of PDCs on the coastline. They could be due to the development of a T phase that can be generated when a landslide enters the underwater environment ([89] and references therein). However, the very well defined peak frequency around 3 Hz (Figure 2e), within the tremor frequency band (Figure 2b), can also be attributed to the resonance of the shallow conduit located in the volcano edifice at a small depth below the SdF [77], which generates the volcanic tremor of Stromboli [82].