## 3 Results
### Seismicity Investigation
The magnitude of these events is comprised in the range between \(-0.5\) and \(3.5\), as also shown in the magnitude distribution in Figure 7B. The magnitude histogram distribution confirms the previous analysis shown in Figure 5, which estimated a completeness magnitude of about 1.5 at the end of the time series that coincides with the year analysed in Figures 6 and 7. Figure 5: The magnitude of completeness (Mc) in the function of the time. In some cases, it is possible to note that at a certain time, the seismicity is recorded mainly at a particular depth. This can affect and modulate the Mc trends depicted in Figure 5. In fact, Mc is intrinsically higher for the deeper seismicity, as the instrumentation is installed on the surface. Furthermore, it is outstanding to consider that the seismicity under La Palma volcano migrated from deeper locations of about 40 km 9 months before the eruption to 30 km, from 30 km to 20 km 7.5 months before and finally from 20 km until the wide under-surface seismicity observed from 10 days before the eruption. In addition, we depicted two gaps in the column of seismicity under La Palma volcano, a  
Figure 6: Earthquakes map and time series with magnitude and depth of the seismicity (M \(\geq\) Mc) recorded around La Palma island from one year before “La Palma” volcano eruption of 19 September 2021. Figure 7: **(A) Earthquakes 3D distribution from 19 September 2020 until 18 September 2021 (the day before the eruption). The colour represents the time the seismic event occurred. (B) Distribution of magnitude of the represented earthquakes. (C) Graph of the cumulative Benioff Strain of the earthquake that occurred only in La Palma Island. Earthquakes are represented as circles. The indication of a specific date highlights particular increases in seismicity. **  
deeper gap located between 13 km and 20 km depth and a shallower gap at less than 5 km depth. Such gaps could be two magmatic chambers under the volcano. In fact, inside a magmatic chamber, it is impossible that an earthquake could happen as the rock is melted. The shallow magmatic chamber is also known in the literature, for example, by [PERSON] et al. [27]. We can infer that firstly the magma migrated from the mantle in a deeper magmatic chamber at \(\sim\)15 km depth 9-7 months before the eruption, and then in the shallower magmatic chamber from about ten days before the volcanic eruption. The presence of a deep magmatic chamber was also documented by [PERSON] et al. [74]. For comparison, it is possible to see that some seismicity has been recorded under the other volcanos during this year. Still, there is not a temporal order related to the depth like for \"La Palma\" volcano, confirming that the seismicity recorded under La Palma is very likely a proxy of the magma uplift. The Cumulative Benioff strain of the events already selected in Figure 6 with M \(\geq\) 1.5 (i.e., greater or equal to the completeness magnitude) in La Palma Island (18.1\({}^{\circ}\)W \(\leq\) longitude \(\leq\) 17.5\({}^{\circ}\)W and 28.4\({}^{\circ}\)N \(\leq\) latitude \(\leq\) 28.9\({}^{\circ}\)N) have been calculated using the Equation (1) and finally plotted in Figure 7C.