## 4 Application to Etna
### 3-5 December 2015  
Four lava fountain episodes with high volcanic plumes occurred between 3 and 5 December 2015. On 3 December, the EW levels of TC preceded both the IS and WR detection, allowing to consider the TC system as a valid pre-alert for explosive events (Figure 9). TC-EW2 was detected more than half an hour before the growth of the IP signal. We observed an advance of the \(v_{ex}\) respect to the \(Q_{M}\) of about 30 min. The maximum value of \(Q_{M}\) during the paroxysm was about 5 \(\times\) 10\({}^{6}\) kg/s, while the \(v_{ex}\) reached 200 m/s.  
On the morning of 4 December, the IS-EW1 (Figure 10) anticipated the lava fountain about two hours before, which lasted just over an hour (09:00 to 10:15 UTC) and was indicated by the EW levels of the three sensors in very close time windows. The \(v_{ex}\) and \(Q_{M}\) were almost overlapping over time, whereas the IP signal had not yet reached its maximum value over 250. In this case, the rapid growth of \(v_{ex}\) allowed directly defining the TC-EW2 (which coincides with TC-EW1). The IS signal lasted about 30 min longer than the WR and TC signals.  
On the evening of 4 December (Figure 11), the IS-EW1 alert remained for the whole day, although a slight increase of the IP signal was observed. The EW2 level was detected at 20:30 UTC, reaching a value of about 300 at 21:30 UTC. The WR-Q\({}_{M}\) high and EW2 levels from TC were observed in correspondence with the growth of the associated estimates of \(v_{ex}\) and \(Q_{M}\), achieving maximum values of 70 m/s and about 1.6 \(\times\) 10\({}^{6}\) kg/s, respectively. The trends of both \(Q_{M}\) and \(v_{ex}\) did not show a similar time extension. In fact, the \(v_{ex}\) was limited to a smaller time interval than that of \(Q_{M}\). Additionally, in this case, TC anticipated the EW levels of the other sensors.  
Figure 9: Time series of \(Q_{M}\) (kg/s), IP (-), and \(v_{ex}\) (m/s) are shown as blue, red, and magenta lines, respectively, for the 3 December 2015 Etna lava fountain. The EW level time steps are plotted as vertical dashed lines with different colors, identifying the associated sensors.  
The event on 5 December 2015 (Figure 12) was that in which the TC system was still confirmed as a good precursor. The misalignment between the WR and TC signals' is less than 30 min, also confirmed by the associated EW levels and the \(Q_{M}\) level. This difference was probably due to the contamination of the scene by the presence of meteorological  
Figure 11: Time series of \(Q_{M}\) (kg/s), IP (-), and \(v_{ex}\) (m/s) are shown as blue, red, and magenta lines, respectively, for the evening of 4 December 2015 Etna lava fountain. The EW level time steps are plotted as vertical dashed lines with different colors, identifying the associated sensors.  
Figure 10: Time series of \(Q_{M}\) (kg/s), IP (-), and \(v_{ex}\) (m/s) shown in blue, red, and magenta lines, respectively, for the morning of the 4 December 2015 Etna lava fountain. Values are plotted together with the EW1 and EW2 (vertical dashed lines with different colors).  
clouds that prevented the view of the summit crater from the TC. Both EW1 of WR and IS were identified very close in time. The \(v_{ex}\) trend reached a maximum value of about 160 m/s, but it was limited in time (14:20 to 15:10 UTC) with respect to \(Q_{M}\), which showed a value of about \(0.8\times 10^{6}\) kg/s and a longer time interval (14:20 to 17:20 UTC). The IP signal showed values lower than 200 during most of the event.