Large dyke intrusion and small eruption: The December 24, 2018 Mt. Etna eruption imaged by Sentinel‐1 data

On December 24th, Mt. Etna volcano underwent a seismic crisis beneath the summit and upper southern flank of the volcano, accompanied by significant ash emission. Eruptive fissures opened at the base of summit craters, propagating SE‐wards. This lateral eruption lasted until December 27th. Despite the small eruption, seismic swarm and ground deformation were very strong. Sentinel‐1 interferograms show a wide and intense ground deformation with some additional features related to volcano‐tectonic structures. We inverted DInSAR data to characterise the magma intrusion. The resulting model indicates that a large dyke intruded but aborted its upraise at about the sea level; however, this big intrusion stretched the edifice, promoting the opening of the eruptive fissures fed by a shallower small dyke, and activating also several faults. This model highlights that a big intrusion beneath a structurally complex volcano represents a main issue even if the eruption is aborted.

. Before noon, strombolian activity and fire fountains started from the NSEC (New South-East Crater, Acocella et al., 2016) along an eruptive fissure that propagated towards SE along the fracture formed during the 1989 eruption (SFS7 in Barreca et al., 2013), forming vents that fed a lava flow flowing in the Valle del Bove (VdB, Figure 1). After this initial explosive phase, the eruption rapidly evolved to an effusive phase feeding the flows that formed a small lava field on the western part of the VdB. Lava emission ended on December 27th. Despite the small eruption, both in terms of duration and extension of lava field, the seismic swarm accompanying the eruptive phase was very strong, in terms of number of events and their energy, with dozen of events with M > 3. The peculiarity of this seismic swarm was that many volcano-tectonic structures on the flanks of the volcano were active. Most of the earthquakes occurred below the upper SE flank, beneath the 1989 fracture on an area that already showed seismic and ground deformation activity during recent eruptions (Bonforte, Gambino, &Neri, 2009 andPuglisi, 2013). Furthermore, also the PFS -at NE -and Ragalna Fault System (RFS) (Neri, Guglielmino, & Rust, 2007) -at SW -were activated. Beside from the seismic swarm localised beneath the upper part of the volcano, a Mw = 4.9 earthquake struck the lower SE flank of Mt Etna on December 26th at 02:19 GMT. Seismic swarm lasted for many days, with a continuously decreasing energy, in terms of daily events and their magnitude. The observed seismic and volcanic phenomena suggest that the main evidence is that the magma intrusion occurred on December 24th produced a F I G U R E 1 Map of Mt. Etna volcano (on the top) and main fault systems from Barreca et al. (2013) too small eruption compared with the relevant seismic release of the whole volcano edifice. In this paper, we will analyse the ground deformation generated by this complex sequence of phenomena, as imaged by Sentinel-1 SAR satellite constellation, in order to detect, characterise and constrain the magma intrusion that triggered the eruption and seismic crisis.

| G ROUND DEFORMATION
The Sentinel-1A and 1B C-band SAR (Synthetic Aperture Radar) data were exploited to image the ground deformation accompanying the eruptive and seismic sequence occurring on Etna, as soon as they were available. The new Sentinel-1A/B (S1A/B) constellation, with a temporal gap of only 6 days, allows the rapid generation of interferograms; we used the 22-28 December pair acquired in TopSAR

| MODELLING
In order to define the volcanic source of the observed deformation, we simultaneously inverted the ascending and descending interferograms under the assumption of a homogeneous, isotropic, and elastic half-space by using 3 Okada's (1985) model plus a Yang, Davis, and Dieterich (1988) depressurizing source. In order to reduce the computational effort, we considered a subset of the whole interferograms relevant to the portion of the volcano with the largest deformation ( Figure 3).
The composite deformation pattern suggested a complex set of sources. In particular, we chose to search for: (a) a shallow and small dyke (Okada-source)-below the eruptive vents-able to produce the much denser fringes observed on the summit area ( Figure S1); (b) a wider and deeper dyke (Okada-source) able to produce the wide deformation field affecting the whole volcanic edifice ( Figure S2); (c) a third dislocation (Okada-source) on the lower SE flank reproducing the deformation observed across the FPF, in order to improve the general fit, minimizing the strong residuals over that area ( Figure S3); (d) a deflating source (Yang-source) able to modulate the wide fringe pattern to fit the observed one on the ascending and descending views ( Figure S4); this kind of source was often needed in studying ground deformation of past events on Mt. Etna (Bonaccorso, 1996;Bonforte et al., 2013;Puglisi et al., 2008).
Even if the main aim of this modelling is to obtain the parameters of the volcanic sources (pressure and dykes), it was necessary to add a dislocation corresponding to the Fiandaca fault-fixed from field data-in order to facilitate the minimization process of the inversion algorithm. Thus, the sources of the minor features described in the previous section were excluded because not related to volcanic sources and the weak effect on the deformation pattern.
To search the minimum of the residuals, we used an optimization routine based on the Genetic Algorithms (GA) approach, as modified by Nunnari, Puglisi, and Guglielmino (2005). The cost function d assumed is the Index of Agreement, and is defined according to Guglielmino et al. (2011).
The search grid parameters, and results of the GA search are shown in Table 1  we estimate a volume change in −29.5 × 10 6 mc.

| D ISCUSS I ON AND CON CLUS I ON
From December 22nd to 28th, the ground deformation, imaged by Sentinel-1 interferograms, depicts a wide displacement at Mt. Etna produced by an intrusion of a wide and deep dyke, splitting the entire volcano edifice. Furthermore, a smaller and shallower dike intruded below the summit area. The deformation pattern due to the volcanic intrusion is perturbed by local displacements due to the activation of the features drawn in Bonforte, Guglielmino, Coltelli, Ferretti, and Puglisi (2011), namely the PFS on the NE, the RFS on the SW and the 1989 fracture on the SE flanks. The most significant local deformation is on the low SE flank, due to a Mw = 4.9 earthquake along the FPF.
Modelling of this complex ground deformation pattern has been really challenging and at least an additional source (not-volcanic), roughly corresponding to the FPF, was necessary to improve the general fitness of the model. The active magmatic system detected by data inversion depicts a complex framework (Figure 4). Magma The volume involved in this vertical migration is ~30 x 10 6 m 3 (the relative difference between depleting source and deeper dyke volumes is negligible), which correspond to the order of magnitude of the 2001 eruption, when ~40 x 10 6 m 3 were erupted in about 3 weeks (Coltelli et al., 2007). Above this system, a smaller dyke cut the volcano edifice above the sea level, with a NW-SE orientation, involving a much smaller volume, compatible with the small eruption occurred.
In general, the interferograms and the resulting model describe the dynamics of the recent unrest of Mt. Etna. At the first order, a dyke intruded beneath the central-southern part of the volcano, splitting the edifice in two parts, similarly to what happened in 2001 . This wide and intense deformation was accommodated by the slip on some faults on the volcano's flanks.
It is noteworthy that the dynamics of the faults cutting the flanks of the volcano is often solicited by significant pressure increase in the plumbing system (Le Corvec, Walter, Ruch, Bonforte, & Puglisi, 2014;Bonforte, Bonaccorso, Guglielmino, Palano, & Puglisi, 2008).
In this way, we defined the magmatic sources triggering the succession of eruptive and seismic phenomena, resolving the discrepancy between the large ground deformation and the small eruption occurred. We can hypothesise that the erupted magma was drained from the upper plumbing system by the shallow radial dyke (likewise in 2004, Bonaccorso, Bonforte, Guglielmino, Palano, & Puglisi, 2006) and not from the deeper eccentric intrusion. Next studies on geochemical and petrologic data could confirm this hypothesis.
In any case, we can surely assess that a large volume of magma (~30 × 10 6 m 3 ) has not been erupted and stopped beneath the volcano, running out of its energy at about the sea level. According to Bonaccorso, Aoki, and Rivalta (2017) (Poland, Peltier, Bonforte, & Puglisi, 2017). The fault slip discharges the energy and may lead the intrusion to stop, eventually aborting the eruption. Furthermore, it forces to have a multi-hazard approach in the hazard assessment on complex volcanoes, even if the eruption is aborted.