Figure 4
From: Short-term detection of volcanic unrest at Mt. Etna by means of a multi-station warning system
(a) Example of warning flag for the lava fountains at Etna between July and September 2011. Lava fountains (here marked by a smoking volcano symbol) stemmed from NSEC. Colored lines depict the duration of the alert at each of the 11 seismic stations (listed in order of increasing distance from the crater). Small hatched boxes mark time spans with uncertain closing time. (b) Zoom of the eruptive episode on 25 July; the dashed and solid lines mark the moment when the warning system reaches the second and third alert level, respectively.
