**1. Introduction**

Multidecadal satellite data from different platforms have provided great support for volcano monitoring, and are enlarging our knowledge on the processes that drive the eruptions (e.g., [1–5]). For example, infrared sensors with low to moderate spatial resolution (i.e., >1 km) make it possible to detect and quantify almost continuously the heat flux sourced from remote volcanoes [6,7], and may now be used to estimate, in near real-time, the lava discharged rates and erupted volumes, during worldwide effusive eruptions [5,8–10].The availability of high spatial resolution data (i.e., 30–90 m) acquired from different platforms constitutes a further improvement of space-based thermal volcano monitoring [11], making it possible to localize with high accuracy the position of an eruptive vents or, more generally, to characterize the thermal contribution of different volcanic sectors inferring magnitude and spatial information (e.g., location and size) of small hot spots (i.e., fumaroles, hot cracks) [12–15]. High-temporal resolution thermal data make it possible to track fast and energetic events (such as lava flows) once they have started [9,10], but they not have sufficient spatial resolution to detect small and long-term precursory thermal anomalies.

On the other hand, ground-based monitoring networks, installed on active volcanoes, acquire, store and process several fundamental parameters (e.g., seismicity, infrasound) that, combined together, make a great contribution in the short-term forecasting of eruptive crises [16].

Open-vent basaltic volcanoes, such as Etna, continuously emit magmatic-related products into the atmosphere [17], and can be sporadically affected by more energetic phenomena, such as paroxysmal explosions or flank eruptions [18,19] that can be preceded by short- or long-term precursors [20]. Hence, the ability to combine multiple dataset and to decipher the processes occurring within a volcanic system at different timescales represents a challenge and a future mandatory prerequisite for hazard assessment [21,22].

In this view, the great impact related to the explosive ash-rich paroxysms experienced by the Etna volcano in the latter years, pushed the scientific community to improve multiparametric monitoring systems by investigating long- and short-term variations in the behavior of the Etna volcano [23–26]. For example, Ripepe and coauthors [22] successfully developed a fully-automated early warning system that is able to track and notify the transition from violent intermittent strombolian activity to LFs- related eruptive column, based on infrasonic and thermal data [22].

In this work, we combine space- and ground-based geophysical data in order to investigate the September-December 2018 eruptive phase of the Etna volcano (Sicily, Italy), with a focus on the longand short-term precursor signals that preceded the main effusive event of the 24 December 2018, and the successive resuming of the summit explosive activity.

Heat flux data, derived by MODIS sensor, are used to calculate and track the evolution of time averaged lava discharge rates (TADRs) and erupted volumes, while the high-spatial resolution potentiality of SENTINEL-2 images are used to locate the thermal activity at the multiple summit active vents of Etna. Infrasonic arrays and tremor amplitude measures are used to track the intensity, the frequency and the source of the explosive events occurring at summit craters.

The integration of satellite and ground-based data, give us an in-depth ability to record the shifting from open-vent conditions, represented by sustained summit strombolian activity, to the 24–26 December flank effusion promoted by a 2-km long feeder-dyke intrusion. This approach is particularly useful for a complex and multiple-crater volcano as Etna, with composite eruptive pathway and high variability in distribution and impact of future opening vents and effusive fissures.

#### **2. Background**

Etna is an open-vent basaltic volcano, characterized by a persistent degassing and frequent explosive activity, ranging from low strombolian explosions to lava fountaining and sub-plinian ash-rich paroxysms [27–29]. This activity occurs at several active vents located at three main sectors of the summit area, named: Bocca Nuova–Voragine (BN/VOR, also cited as Central Craters—CC; [29]), NorthEast Crater (NEC) and Southeast Crater–New Southeast Crater (SEC/NSEC) ([30,31]; see Figure 1).

This open-vent activity is periodically interrupted by effusive eruptions, occurring at the summit or along the flanks of the volcano, that drain portion of the shallow magmatic system of Etna [29,32–35]. While the so-called "summit eruptions" are fed by degassed magma likely stored at a shallow level

and rising through an open central conduit [29], the flank and fissural eruption seems to be fed by a deeper, gas-rich magma. These eccentric eruptions take place along three main "rift zones"—NE Rift, S Rift and W Rift—outlining the strict control played by the tectonic structural framework of the volcano on the opening of lateral vents [34,36]. The eccentric flank eruptions may be promoted by magma reaching the surface via new, secondary conduits and/or dike intrusions, successively being erupted from propagating lateral fissures with multiple aligned vents [29,36]. In this view, a main hazard source at the Etna volcano is related to the opening of distal eruptive fissures with lava flows that potentially threatened nearby villages, infrastructures and tourist facilities [37].

During the last decade, the Etna volcano has exhibited a migration in persistence and magnitude of eruptive activity from SEC toward the NSEC crater, due to a preferential NE–SW oriented dilatation in the summit sector promoted by NE volcano flank instability during inflation phases [38]. Since 2011, its activity has been characterized by the occurrence of several (54) Lava Fountain (LF) episodes occurring at NSEC and VOR, generally preceded by an hour- to day-long increase of strombolian activity [22,25,29,39], and recurrently accompanied by short-lived (0.5–9 h), low-volume (0.5–3 Mm3) lava flows. The 2011–2018 period was also characterized by 5 minor effusive eruptions (January–April 2014, July–August 2014, February 2017, March–April 2017, August 2018) not accompanied by lava fountain activity [40,41]. These events were characterized by duration from 5 to 75 days, and by limited erupted volumes (between 1 and 12 Mm3; [42–45],) remarking a clear difference from the voluminous eruptions occurred between 2001–2009 (having duration of 20–420 days and volumes of 30–60 Mm3; [46]).

The last effusive episodes occurred on 24 December 2018, and were characterized by the opening of an eruptive fissure at the base of the New South-East Crater on the western flank of Valle del Bove. The eruption produced gas and ash-rich plumes from the summit vents an intense Strombolian activity along the fissure, feeding several eastward lava flows [47]. Notably, this event was accompanied by an intense seismic swarm (more than 130 earthquakes in 3 h) culminating, on 26 December at 2:19 am (UTC), with a Mw 4.9 earthquake with the epicenter between the village of Lavinaio and Viagrande (CT), in correspondence of the Flandaca Fault. The earthquake was very shallow (1.2 km depth) and affected buildings, facilities and injured a dozen of people close to the epicentral areas along the lower south-eastern flank of Etna [48].

**Figure 1.** Shaded relief map of Etna summit area, with the indication of the main crater sectors and approximate maps of lava flows emplaced during the September–December 2018 activity. The location and the configuration of the MVT infrasonic array are also represented (see Method section). Blue arrows show the main localized back-azimuth during summit (7◦, BN/VOR sector) and lateral activity (20◦–40◦, eruptive fissure), respectively. The adopted satellite sensors are also presented.
