**1. Introduction**

Several papers have shown that satellite remote sensing may play an important role for studying and monitoring thermal volcanic activity, due to global coverage, continuity, and high frequency of observation, particularly in remote areas where ground-based surveillance systems are often lacking (e.g., [1–5]).

Sensors such as TM (Thematic Mapper) and ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer), that have a repeat cycle of 16 days and offer channels in the SWIR (shortwave infrared) and TIR (thermal infrared) bands with a spatial resolution of 30–90 m, were widely used to investigate volcanic thermal anomalies (e.g., lava bodies, fumarole fields) [1,6–8]. HYPHERION, which is a hyperspectral imaging spectrometer providing VNIR (visible, near-infrared)/SWIR data at 220 wavelengths, was profitably used even for characterizing hot magmatic surfaces (e.g., [7,8]). AVHRR (Advanced Very High Resolution Radiometer) and MODIS (Moderate Resolution Imaging Spectroradiometer), acquiring data in the MIR (medium infrared) band and offering a good compromise between spatial and temporal resolution (1.1 km at the nadir; up to 6 h for AVHRR), represented key instruments for monitoring active volcanoes from space (e.g., [9–14]). SEVIRI (Spinning Enhanced Visible and Infrared Imager), like other geostationary satellite sensors, enabled the prompt identification of short-lived eruptive events (e.g., [15–19]), thanks to the high frequency of observation (15 min) and despite the low spatial resolution (3 km at the sub-satellite point).

Data from the above-mentioned sensors were exploited to retrieve the volcanogenic radiant flux, the time variations of which can be used as a proxy of the intensity changes of volcanic eruptions (e.g., [20–24]). This parameter enables the estimation of lava effusion rate, which is a critical parameter for numerical models that aim to predict lava flow paths (e.g., [25]).

In this paper, we present the results of satellite monitoring of Mt. Etna (Sicily, Italy) thermal activity between May and August 2016, integrated with ground-based structural and volcanological data. In particular, we investigate the eruptive events occurring in May and the fumarolic emissions recorded before the opening of a small degassing vent within the Voragine crater (VOR). The latter is one of Mt. Etna summit craters which has opened at the top of the central conduit, see Figure 1 [26,27].

**Figure 1.** Structural map of the summit area of the Mount Etna volcano, updated in August 2016.

These events were investigated by means of a satellite-based system developed at IMAA (Institute of Methodologies for Environmental Analysis) implementing the RSTVOLC algorithm [28]. This monitoring system integrates AVHRR and MODIS observations for monitoring Italian volcanoes in near-real time, generating hotspot products (i.e., JPG, Kml, and ASCII files) a few minutes after the sensing time [29]. The work aims at assessing the contribution that multi-platform satellite observations may provide for better monitoring Mt. Etna, complementing the information provided by traditional surveillance systems.

#### **2. The 2016 Mt. Etna Eruptive Activity**

Five months after the early-December 2015 eruptions [30,31], ash emissions resumed at the North-East crater (NEC) during the night of 15–16 May 2016, while a Strombolian activity started the day after [32]. On the morning of 18 May, a 20–30 m long, short-lived (a few minutes) eruptive fissure activated on the northern flank of the NEC displaying weak spattering, immediately followed by violent Strombolian activity which occurred at the VOR crater. At that time, an eruptive fissure also opened feeding a lava flow, which expanded on the high Western flank of Mt. Etna. Within a couple of hours, the Strombolian activity at VOR totally filled this crater and the adjacent Bocca Nuova crater (BN), finally overflowing the BN western rim. The eruptive activity ceased the following night but resumed on early 19 May for ~1 h. On 20 May, a strong explosive activity started again at VOR and a new fracture field opened between VOR and the New South-East crater (NSEC), feeding a lava flow that expanded toward the east in the high Valle del Bove depression. At the same time, a new lava overflow occurred from the western rim of the BN. After a break of about a day, mild Strombolian activity resumed during the night between the 22 and 23 May at NEC, while lava fountaining occurred on the 24–25 May at VOR which eventually caused the total filling and obstruction of NEC, VOR, and BN.

At the end of the 15–25 May eruptions, a ~N-S fractured area characterized the volcano's summit. This fracture field was ~400 m wide and ~2000 m long, extending from the northern flank of the NEC to the eastern flank of the NSEC cone, crossing the eastern rim of the VOR, see Figure 1. This fracture field is bounded towards the east by a graben several tens of meters wide that also caused the collapse of the southern portion of the pyroclastic cone of the NEC, radically changing its morphology. As stated before, in the central crater, which contains VOR and BN, see Figure 1, the eruptive activity emerged at VOR only. After the end of the Strombolian activity that occurred on the 23–25 May, the VOR showed conspicuous subsidence phenomena, evidenced by the formation of numerous sub-circular and concentric fractures (lunar cracks) placed around a weakly degassing vent positioned at the bottom of this crater. Moreover, the BN was totally obstructed by products that erupted in May 2016; however, soon after the end of the eruption a weak subsidence also began to affect this crater.

Late in the evening of 7 August 2016, the top of Etna showed almost continuous flashes. Since that moment, a new 20−30 m wide vent, placed on the inner eastern rim of VOR, emits a pulsating emission of incandescent gases up to over 600 ◦C, see Figure 2; video footage of this activity can be found as a supplementary material. The new degassing vent fits perfectly into the structural framework inherited from the eruption of May 2016, see Figure 3. It is located in the area where the graben, described before, intercepts the edge of the VOR; i.e., an area subject to open for the "pull" induced by the movement of the eastern flank of the volcano [33–37] and for the subsidence that affects the bottom of the VOR.

(**b**)

**Figure 2.** Mt. Etna activity of 10 August 2016 from the degassing vent opening within the Voragine crater (VOR) crater; (**a**) morning picture; (**b**) afternoon picture with evidence of the volcanic glow (credits: Marco Neri).

**Figure 3.** New degassing vent opened on 7 August 2016 and located along the fracture system formed during the May 2016 eruption (see Figure 1 for map view) (credits: Marco Neri).
