## 4 Seismic and Fault Temporal Evolution of Tajogaire 2021 Eruption  
For monitoring seismicity during the volcanic eruption, hypocenter data from the Instituto Geografico Nacional (IGN: [[https://www.ign.es/web/ign/portal/sis-area-sismicidad](https://www.ign.es/web/ign/portal/sis-area-sismicidad)]([https://www.ign.es/web/ign/portal/sis-area-sismicidad](https://www.ign.es/web/ign/portal/sis-area-sismicidad)); accessed on 15 January 2024) have been used. The seismic crisis prior to the eruption began in October 2017 with a seismic swarm of 300 earthquakes with a maximum magnitude of M \(<\) 2 [21; 35]. The seismic swarms that began in 2017 featured sets of earthquake clusters in the southern half of the island, but they could not fit into regular planes or structures. This seismicity was associated with the location of the magmatic chamber and its rise could take place in areas already subject to extension [1]. In September 2021, earthquakes of M \(>\) 3 at a depth of 9 km began to be recorded. Some of these hypocenters showed a NW-SE alignment at a depth of 9 km. This line was probably contained in a fault plane to which the rest of the hypocenters adapted. This plane could be parallel to the one inferred for the Jedey eruption (1585), since our  
Figure 4: Data points and results for the fault population analysis in La Palma Island (the dashed lines represent D\({}_{\rm ey}\)’s mean trajectories. ): (**A**) Atlantic Strain Field and (**B**) Local Strain Field. surface microfracturing data measured at this volcano (Figure 5) show a fault plane with a 100\({}^{\circ}\)/80\({}^{\circ}\) SW orientation and dip that controlled this eruption and the emission of volcanic material [2]. This fault plane could be compatible with a normal fault with a NW-SE direction of mean horizontal shortening (D\({}_{\text{ey}}\)), coinciding with the regional results of [16] and with the Atlantic Deformation Field. Another fault plane with very similar characteristics can be found in the Roques de Jedey with decametric-sized slickenside, also associated with the 1585 eruption (Figure 5A). Such NW-SE direction faults are part of the Tazacorte Fault (TZF) (Figure 6A). In this case, the cut of this plane with the surface would correspond to an area under extension further north, where the 2021 Tajogaite eruption took place (3D online earthquake viewer of the ICME-CSIC: [[https://info.igme.es/eventos/Erupcion-volcanica-la-palma/ssimicidad3d](https://info.igme.es/eventos/Erupcion-volcanica-la-palma/ssimicidad3d)]([https://info.igme.es/eventos/Erupcion-volcanica-la-palma/ssimicidad3d](https://info.igme.es/eventos/Erupcion-volcanica-la-palma/ssimicidad3d)); accessed on 15 January 2024). Figure 5: Normal fault planes of the Roques of Jedey, Jedey eruption (1585), and FPA analysis: (**A**) fault planes and slickensides; (**B**) Right-Dihedron Method solution; (**C**) Slip Model solution; (**D**) average direction of maximum horizontal shortening (D\({}_{\text{ey}}\)) obtained from the Slip Model. Figure 6: Spatial evolution of the hypocenter distribution during the 2021 eruption: (**A**) 12–19 September; (**B**) 19 September–1 October; (**C**) 1–10 October; (**D**) 10 October–13 December. See text for further explanation. Ref. [36] used the paths of the earthquake hypocenters to define the magmatic feeding system of this eruption based on gravity changes produced by mass variations. In this work, we use the spatial evolution of seismicity to study the movement and effects of these two faults MZF and TZF. So, we have divided the earthquakes recorded during the volcanic eruption, on the basis of the spatiotemporal variations in the distribution of hypocenters, into four time intervals, between 12 September and 13 December 2021:  
1. 12-19 September (Figure 6A): Seismicity prior to the eruption indicating the rise of magma in a dike towards the surface in an accelerated manner [37]. The hypocentral distribution fits the Tazacorte Fault plane (TZF) with a NW-SE orientation (N100\({}^{\circ}\) E/80\({}^{\circ}\) SW).