*4.1. Phase I (1 September–4 November 2018): Low-Level Explosive Activity at Summit Craters*

After the short effusive eruption of 23–29 August 2018 a low activity persisted at the Etna volcano from September to mid-November (Phase I; Figure 2). This period was characterized by persistent degassing, and low-level strombolian activity at the Central Craters (www.ct.ingv.it). Thermal and geophysical observations during this phase outlined: (i) sporadic low to moderated MODIS-MIROVA thermal alerts (VRP < 20 MW; Figure 2a); (ii) tremor amplitude at background level (Figure 2b) and (iii) low-pressure infrasonic detections (<1 Pa at MVT array; Figure 2c) localized at the BN crater (back-azimuth about 7◦). In particular, SENTINEL-2 images show that the MODIS-detected thermal alerts were sourced by two distinct vents hosted within BN crater and from NSE sector with a sustained degassing from NE and Central Craters (Figure 3a–c).

**Figure 2.** 1 September 2018 –15 January 2019 time-series of thermal, seismic and infrasonic activity at the Etna volcano: (**a**) VRP (blue stem) and Number of Hot Pixel (red circles) as retrieved by MODIS-MIROVA and SENTINEL-2 images, respectively; (**b**) hourly-mean tremor amplitude expressed as RSAM measured at ETN station; (**c**) hourly-mean amplitude pressure of the infrasonic detections recorded at MVT array (see Figure 1) and cumulative number of detections at the same array. See Figure 1 for ETN and MVT location. All datasets are expressed as logarithmic scale, excepted for the Number of Infrasonic detections. Yellow stars represent the occurrence of the 20 November seismic swarm and the 26 December earthquake (Mw = 4.9), see text for details.

**Figure 3.** SENTINEL-2 class images acquired during the investigated Etna activity, in with summit craters are marked. The imagery is processed in RGB colors (bands 12–11–8a) to enhance the presence of hot targets (see Method section). Images represents the most representative shots (**a**–**n**) of the activity characterized Etna volcano during September–December 2018 period.
