## 3 Results  
### Seismicity Investigation  
In this section, the seismicity has been investigated from the earthquake catalogue provided by the Spanish National Institute of Geography extracted in the area around the Canarias Islands volcanic area, limited by 20\({}^{\circ}\)W \(\leq\) longitude \(\leq\) 12\({}^{\circ}\)W and 26\({}^{\circ}\)N \(\leq\) latitude \(\leq\) S31\({}^{\circ}\)N from 1980 until 19 September 2021. The selected period for the seismicity is the same as the one available for the atmospheric data. In a period that spans more than four decades, the seismic network has been significantly improved in terms of the number of seismic stations and the quality of the same sensors. Such improvements imply that, nowadays, the network can detect smaller seismicity, permitting better monitoring. In fact, the seismicity due to volcanic activity and not tectonic activity is generally associated with  
Figure 4: Solar conditions during the investigated period from 23 March 2021 until 19 September 2021. The two horizontal red lines represent thresholds calculated as mean \(\pm\) 2.0 standard deviations of the time series. lower magnitude events. Such fractures in the lithosphere can be induced by the uplift of the magma prior to an eruption and have been proved to be an effective method for predicting volcanic eruptions [72]. We checked the magnitude of completeness (Mc) by the free software ZMap [49] using the statistical criteria Mc90, i.e., the minimum magnitude determined with a 90% probability that all the events with magnitude equal to or greater than Mc have been properly detected. Figure 5 represents the variation of Mc with time. Some gaps are due to low or null seismicity detected in the investigated area, as we required a minimum of 20 seismic events to estimate Mc reasonably. In particular, we can confirm that Mc improved (become lower) in recent years: between 1980 and 1992 it oscillated around 2.7 (best estimation always lower than 3.0); from 1995 to 2001 it seems a trend comprised between 2.2 and 2.5; from 2003 to 2011 it was comprised between 1.4 and 2.0. From 2011 until the present, the Mc shows more oscillation of its value following particularly strong events, but generally is lower than 2.0 and often around 1.5. The Mc not only can change in function of the time, but also for geographical reasons. In fact, for example, sea and oceanic do not permit the installation of seismic stations, typically reducing the capability of detecting the smaller off-shore seismicity. Despite this, as in this work, we will investigate an emersed volcano, and we do not expect this aspect to affect our results significantly. Some other variations of the completeness magnitude could also be due to natural variations of the seismicity. In fact, after a larger magnitude event, Mc could increase as the seismic network could not distinguish smaller events inside the perturbations produced by the larger ones. Modern machine learning techniques partially addressed this problem, but they are out of the scope of this paper [73]. Furthermore, a variation of the depth of the seismicity can also affect the Mc, as it tends to increase among the depth. These could explain some smoothed variation in a short time of a few years like the ones from 1996 to 2000, probably due only to variation of depth of seismicity. The seismicity recorded one year before 19 September 2021 (excluded) in a box around the volcano island (17.1\({}^{\circ}\)E \(\leq\) longitude 18.1\({}^{\circ}\)E and 28.4\({}^{\circ}\)N \(\leq\) latitude 28.9\({}^{\circ}\)N) was extracted, and their map and time series are represented in Figure 6. Such events are used in the following to compute the Cumulative Benioff Strain. To better investigate the seismicity of the area, we plotted the earthquake hypococentres in the three-dimensional graph in Figure 7. The colour represents the time of occurrence of the recorded seismic events.