*4.2. Phase II (5 November–28 November 2018): Gradual Increase in Explosive Activity at Summit Craters*

Since early November, an intensification on summit thermal activity at NSE and NE craters was tracked by high-spatial resolution SENTINEL-2 images (Figure 3d). A clear increase in the number of hot pixels (Figure 2a) was in fact coupled with a VRP that reached a value above 50 MW, for the first time on 12 November, (see Figures 2a and 3d). This thermal increasing trend was confirmed by an increment in clustered medium infrasonic pressure transient (up to 1 Pa at MVT array; Figure 2c) associated to strombolian explosions. On 20 November 2018, a 15 h-long seismic swarm affected the western sector of the volcano [68].

#### *4.3. Phase III (29 November–24 December 2018): Strombolian and E*ff*usive Activity at Summit Craters*

A few days after the seismic swarm of 20 November, the Etna volcano showed a further increase of summit activity, with the beginning of a sporadic lava effusion from a vent opened at the eastern base of the NSE crater. SENTINEL-2 images firstly captured this overflow activity on 29 November (see Figure 3e) when emissions become more continuous. Similarly, on 29 November at 00:50 UTC, MODIS data firstly record VRP value above 100 MW (Figure 2a). This marked change in the thermal activity detected by space marks the transition from Phase II to Phase III. Satellite observations tracked the persistence of the lava overflows from the NSE Crater producing short lava flows towards East-South East direction (maximum length of ca. 0.7 km, i.e., 21 December; Figure 3e–h).

Overall, MODIS thermal alerts during December 2018 show a significant augmentation in terms of both frequency (from 15% to 75% of the overpass) and intensity (from 25 to 160 MW, on average). The further increase in VRP measured since 18 December (above the 200 MW; Figure 2a), corresponds to the occurrence of several lava flows outpouring from the vent(s) at the base of the NSE crater, as reported by field observations [69]. This heat flux increment is coherent with a general increase in the number of hot pixels detected by SENTINEL-2 images during the same period (Figure 2a).

In the Phase *III*, the effusive episodes at NSE was accompanied by the concurrent explosive activity at BN and NE craters (Figure 3e–h). This is also outlined by the increased thermal anomalies detected by thermal profiles obtained by SENTINEL-2 Thermal Index map processing (Figure 4a,b). Despite of a sustained activity at the others summit craters, no thermal anomalies were detected inside VOR throughout Phase *I, II* and *III* (Figures 3e–h and 4a,b). It is worth noting that the overall heightened eruptive activity at the summit craters was accompanied by only a moderate increase in frequency and amplitude of infrasonic detections (Figure 2b).

**Figure 4.** N–S and E–W thermal profile(s) acquired at summit crater sector of the Etna volcano during September 2018– January 2019 (**a**,**b**). The profiles are produced stacking each SENTINEL-2 image, analyzed in the SWIR bands and with at least one hot pixel anomaly, in two direction, from north to south (for each row) and from east to west (for each column; (**c**)). With white dotted lines are marked the Etna craters locations, thus letting to enlighten the presence of thermal activity over time for each vent(s). The hot pixel spread toward south and west in both profiles (red sign) indicate the occurrence of eruptive fissure feeding the lava flow of the 24 December.

#### *4.4. Phase IV (24 December–26 December 2018): Opening of Lateral Fissure and Distal E*ff*usive Activity*

The general increase of eruptive activity recorded during the Phases *II* and *III* culminated on 24 December when the geophysical network recorded a drastic change on infrasonic and seismic signals (Figure 5). At 06:00 UTC there was a sudden, almost complete cessation of infrasonic activity at the Central Craters (back-azimuth close to 7◦; see Figures 1 and 5b), followed at 08:24 UTC, by a sharp increase in volcanic tremor (Figure 5a,c). Intense grey- and reddish-ash plumes were observed from NE and BN craters [47] and accompanied the rapid rise of the tremor amplitude that reached a peak value at about the 11:00 UTC (Figure 5c). Hence, the volcanic tremor started to decline, while infrasonic activity resumed, but with back-azimuth consistent with a new source, located at SE from the summit craters (back-azimuth spanning from 20◦ to 40◦ angle; see Figure 5b). During this period, ground surveys reported the opening of a 2 km-long NNW–SSE eruptive fissure (EF) extending from the south-east base of the NSE crater, (at 3000 m a.s.l.) toward the upper portions of the west rim walls of the Valle del Bove (2400 m a.s.l.) [47]; cf. Figures 1 and 6). This episode was accompanied by violent strombolian explosions and ash emissions from distinct fissure segments, as well as by the emission of several lava flows units descending along the steep walls of the Valle del Bove [47]. The end of fissure propagation was marked by a new, sharp shift of infrasonic detections that, since 13:00 UTC were localized again at BN crater (Figure 5). However, SENTINEL-2 and MODIS images acquired, at 10:00 UTC and 11:50 UTC, respectively, did not show yet any clear increase in terms of intensity and extension of thermal anomaly at the summit craters, still indicating the occurrence of a multiple-branch overflow from NSE sector (Figures 5 and 6a). Interestingly, the SENTINEL-2 image shows a dark-grey plume, apparently sourced at the base of northwards side of Serra Giannicola Grande (Figure 6a), that could be possibly associated to a landslide generated in response to the ongoing fissure propagation and/or related to the intense seismic activity. On the evening of 24 December, at the 21:15 UTC, the MODIS image recorded a peak value of 2294 MW suggesting that an important effusive activity was ongoing (Figure 2a). However, the successive image, acquired just few hours later at 01:25 UTC of 25 December, detected a much lower thermal radiation (612 MW), likely resulting from a rapidly waning phase of lava emission. The decrease of thermal activity was, in turn, accompanied by a marked return of intense explosive activity at the Central Craters, as outlined by the infrasonic pressure measured at MVT array (Figure 5a). In particular, the peaking period of explosive activity recorded at BN sector on 25 December at 12:00 UTC (with infrasonic pressure up to 15 Pa) was coupled by VRP values close to 100 MW, suggesting that the feeding of lateral lava flow was drastically decreased or, at least, ceased.

The end of the lateral eruption was accompanied by the closure of the effusive crack, and by the reactivation of the explosive activity at the summit vents, as clearly testified by the SENTINEL-2 image acquired on 26 December at 10:00 UTC (see Figure 6b). This image captured the cooling of the southernmost lava flow unit (about 2.5 km in length), as well as a clear thermal anomaly at BN that, outline a sudden resumption of explosive activity at the summit craters (Figures 5b and 6b), as supported by infrasound detections.

Notably, during the ongoing eruption, at 02:19 UTC on 26 December, an earthquake of Mw 4.9 localized on the SE flank of the volcano and linked to the reactivation of the Fiandaca Fault, injured 28 people and damaged several buildings in the villages surrounding the fault [70].

**Figure 5.** Timeseries of the 23–24 December of: (**a**) amplitude of infrasonic detections; (**b**) direction of the infrasound activity, EF—eruptive fissure; BN—Bocca Nuova; (**c**) tremor amplitude recorded at ETN site. (**a**) and (**b**) are those recorded at MVT, see Figure 1 for location of ETN and MVT sites.

**Figure 6.** Focus on 24 (**a**) and 26 (**b**) December 2018, Phase *III* of the Etna volcano activity. SENTINEL-2 class images are processed in RGB colors (bands 12–11–8a), and to enhance the presence of hot targets (see Method section). The images resume the eastern summit portion of Etna edifice, covering from craters area to the upper portion of Valle of the Bove.
