Fig. 2. Dense gas plume rising from the New Southeast Crater (at left) and ash puff rising from the Bocca Nuova (at right) at 12:49 GMT on 28 January 2013. Photo taken from Milo (eastern flank of Etna) by Boris Behncke (INGV-Osservatorio Etneo)

Fig. 3. Frames extracted from video recorded by the high-sensitivity visual surveillance camera of the INGV-Osservatorio Etneo (Catania) on the Montagnola  (EMOH), during the 28 January 2013 eruptive episode at the Bocca Nuova and at the New Southeast Crater.

Etna: new eruptive episode at the Bocca Nuova. On the evening of 28 January 2013, the Bocca Nuova - the largest of the summit craters of Etna - has been the site of a new episode of vigorous Strombolian activity. This episode was similar to, though slightly less intense than, the episodes of 16 and 18 January, but it was accompanied by weak Strombolian activity at the New Southeast Crater (NSEC).

During the preceding days, the summit craters had been degassing strongly, in particular the NSEC, where for a brief period on the evening of 27 January, weak incandescence was visible. The crater had last produced an episode of Strombolian activity on 22-23 January 2013 (see 23 January 2013 update). On the morning of 28 January, gas emission from the NSEC was often rhythmic and pulsating; at times the gas puffs appeared "dirty" because they contained modest amounts of fine pyroclastic material (Fig. 1 and 2). The degassing was not accompanied by any noise to be heard at a distance of a few kilometers.

Starting at 07:46 GMT (=local time -1) on 28 January, the eruptive vent located in the southeastern portion of the crater floor of the Bocca Nuova, produced small puffs of dark volcanic ash, which rarely rose more than 150-200 m above the crater rim (Fig. 1 and 2). Initially, these ash emissions were separated by intervals of 5-10 minutes, but later they became more and more frequent, with one emission every 1-3 minutes in the late forenoon. Some of the emissions were accompanied by loud rumbling noises well audible at Piano del Vescovo, about 7.7 km southeast of the Bocca Nuova. Ash emissions continued at least until 14:00 GMT, when weather conditions deteriorated and precluded any visual observations.

Shortly after 17:00 GMT, the visual surveillance cameras of the INGV-Osservatorio Etneo showed the beginning of mild Strombolian activity within the Bocca Nuova (Fig. 3, top frame). Contemporaneously, the volcanic tremor amplitude recorded in the summit area started to rise, accelerating notably around 19:00 GMT and reaching a peak shortly thereafter. During the same time interval, the Strombolian activity within the Bocca Nuova intensified, with jets of incandescent bombs and scoriae that often rose several tens of meters above the crater rim.

From 17:40 GMT onward, weak and discontinuous Strombolian activity was visible at the NSEC (Fig. 3, second and third frames). Some explosions launched incandescent bombs up to 30 m above the crater rim. This activity reached its greatest intensity between 19:00 and 19:30 GMT, and from 19:40 GMT rapidly diminished to be followed by the emission of a conspicuous column of white vapor, which continued during the whole night and through the following day of 19 January.

At the Bocca Nuova, however, the intracrater Strombolian activity - which was presumably accompanied by the emission of lava onto the crater floor - continued until about 00:15 GMT on 29 January. Starting around 21:50, the activity had shown signs of gradually diminishing and more and more discontinuous, even though some explosions between 22:00 and 23:00 GMT were particularly violent (Fig. 3, bottom frame). The diminution of the vigor of the eruptive activity was accompanied by a rapid decrease in the volcanic tremor amplitude.

As for the Bocca Nuova, this episode - the fourth from this crater during this month, after those of 10-15, 16 and 18 January - was rather similar to its predecessors. The peak values attained by the volcanic tremor amplitude were slightly lower than those of the 18 January episode, and significantly lower than those of the 16 January episode. The single source of the activity was once more the intracrater cone leaning against the southeastern base of the inner crater wall; unfortunately, the harsh weather conditions in the past few weeks have precluded any field visit to the crater rim aimed at observing the morphological changes caused by the recent activity.

A new element in the eruptive episode of 28 January 2013 is the Strombolian activity at the NSEC, which accompanied during the first two-and-a-half hours of the episode the Strombolian activity within the Bocca Nuova.

This is indeed the first occurrence of simultaneous eruptive activity at two of Etna's summit craters since about 12 years; in contrast, during the period of summit activity of 1995-2001, it was rather normal to see contemporaneous activity at two, sometimes three and rarely all four of the summit craters. One particularly noteworthy event of this type was the sequence of paroxysmal episodes that occurred within a few hours on 4 September 1999: first a sub-Plinian explosive episode at the Voragine, followed by lava fountaining within the Bocca Nuova, and finally an episode of Strombolian activity at the (old) Southeast Crater, which culminated in the fracturing of its cone from the summit to the base.