## 2 Methods
### GBInSAR  
Measuring surface deformation, exploiting the phase difference between two spaceborne SAR images (differential InSAR, DInSAR; [51]) makes it possible to recognize ground displacements along the satellite line of sight (LOS) direction on a centimeter-scale. Processing a long stack of images using multi temporal (MT) InSAR techniques allows for the detection of millimeter-scale displacements over long time frames through the reduction of error sources [52; 53]. GBInSAR has the additional advantage of producing frequent SAR images (on the order of seconds to minutes), resulting in very high frequency slope maps and time series. Moreover, the use of GBInSAR in the Ku-band (17.0-17.1 mm radar) can penetrate dust clouds, abundant especially during collapse events, and can work with variable light and atmospheric conditions [14]. The NE portion of the summit crater terrace at Stromboli and the northern portion of the SdF are monitored by two GBInSAR devices, which are located in a stable area N of the SdF (Figure 1). The first GBInSAR (GBInSAR NE400; Model: GB-InSAR LiSALab, Ellegi srl, Rovello Porro, Italy, [[http://lisolab.com/home/](http://lisolab.com/home/)]([http://lisolab.com/home/](http://lisolab.com/home/)); Revisiting time; 11 min; [54]) was installed in February 2003, during the 2002-2003 flank eruption, whereas the second device (GBInSAR NE190; Model: GB-InSAR LiSAmobile k09, Ellegi srl - LiSALab, Rovello Porro, Italy, [[http://lisolab.com/home/](http://lisolab.com/home/)]([http://lisolab.com/home/](http://lisolab.com/home/)); \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Label** & **Type and Model** & **Location** &
\begin{tabular}{c} **Distance from** \\ **the Caters** \\ \end{tabular} & **Optics** & **Field of View** \\ \hline \multirow{2}{*}{SPCT} & Thermal, FLIR & West SdF flank, & \multirow{2}{*}{1698 m} & \multirow{2}{*}{90\({}^{\circ}\)} & \(90^{\circ}\times 73^{\circ}\) \\  & A320 & 85 m a.s.l. & & & \(2150\times 1613\) m \\ \hline \multirow{2}{*}{SCT} & Thermal, FLIR & East SdF flank, & \multirow{2}{*}{1538 m} & \multirow{2}{*}{25\({}^{\circ}\)} & \(25^{\circ}\times 19^{\circ}\) \\  & A655 sc & 165 m a.s.l. & & & \(807\times 605\) m \\ \hline \multirow{2}{*}{SQV} & Visual, Sony & East SdF flank, & \multirow{2}{*}{1027 m} & \multirow{2}{*}{18\(\times\)} & \(48^{\circ}\) (wide \\  & FCB-EX480 CP & 390 m a.s.l. & & & \(\text{(tele end)}\times 2.8^{\circ}\) \\ \cline{1-1} \cline{4-4}  & & & & \(657\times 493\) m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of the INGV monitoring cameras and of their main features. Revisiting time; 2 min) was installed on 14 December 2014, after the flank eruption that took place that year. Radar images were obtained through sampling techniques; for this reason, particular frequency and spatial steps had to be selected in order to avoid ambiguity in range and cross-range [54]. The system is able to measure line-of-sight (LoS) ground displacement in the time interval between two acquisitions and the displacement is calculated from the phase difference between the back-scattered signals received at different times, through the cross-correlation between two SAR images. Range and cross-range resolution are, on average, \(2\times 2\) m, with a measurement precision being referred to the displacement of less than 1 mm [54]. The displacement rate is the result of the mathematical division between the displacement measured in an interferogram (referred to the difference between two SAR images) and the elapsed time between the two images, allowing for the identification of very low displacement rates (0.010-0.001 mm/h) related to the creep of the northern sector of the SdF or very fast displacement rates (up to 300 mm/h) associated with effusive vent opening [55]. The capability of InSAR to detect ground displacement depends on the persistence of phase coherence over appropriate time intervals, therefore a SAR coherence mask (threshold = 0.5) was set to mask the noisy areas of the interferogram [54].