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Etna and Stromboli update, 23 January 2013

Fig. 1. Strombolian activity at the New Southeast Crater on the evening of 22 January 2013, seen from Giarre, on the lower eastern flank of the volcano. Photo taken by Salvo Virzì and published here with kind permission of the author (original photo on Flickr)

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Fig. 2. Ejection of incandescent volcanic bombs from the eruptive vent located at the base of the southeastern inner crater wall of the Bocca Nuova on the evening of 18 January 2013. Some bombs are seen to have landed outside the crater at right; the height of the incandescent jet above the crater rim is about 100 m. At right one can barely make out the dark silhouette of the old Southeast Crater cone. Photo taken by Boris Behncke (INGV-Osservatorio Etneo)


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Fig. 3. Frames extracted from video recorded by the high-sensitivity visible surveillance camera of the INGV-Osservatorio Etneo (Catania) on the Montagnola  (EMOH), during the 22-23 January 2013 eruptive episode at the New Southeast Crater. Top frame shows early phase of activity; the second represents one of numerous explosions that threw incandescent bombs onto the cone's flanks, and the third frame shows the last strong explosion shortly before the cessation of the eruptive activity.

Etna: eruptive episodes at the Bocca Nuova and at the New Southeast Crater. During the past week, four eruptive episodes have occurred at two of the summit craters of Etna, two at the Bocca Nuova and two at the New Southeast Crater (NSEC); Fig. 1 shows a scene of the latest episode at the NSEC, which took place during the night of 22-23 January 2013.

After the temporary cessation of eruptive activity within the Bocca Nuova on the morning of 15 January (see the 15 January 2013 update), the same crater produced two further episodes of intense Strombolian activity on the evening of 16 January and on the evening of 18 January 2013. Both were characteriized by a sudden increase in the volcanic tremor amplitude, which especially in the earlier case rose considerably higher than the maximum levels recorded during the 10-15 January activity; however, the rather poor weather conditions rendered a direct observation of the phenomenon impossible, the only visible evidence of which was a bright glow illuminating the clouds covering the summit of the volcano. The activity started around 17:30 GMT (=local time -1) on 16 January, when the volcanic tremor amplitude showed a rapid increase. After reaching a peak around 20:00 GMT, the volcanic tremor amplitude started to diminish and returned to normal levels shortly after 22:00 GMT.

At 17:30 GMT on 18 January 2013, the volcanic tremor amplitude once more rose sharply, signaling the onset of yet another eruptive episode at the Bocca Nuova. Differently from the previous episode, visibility gradually improved while the activity was still underway, allowing to observe vigorous explosive activity (Fig. 2), which launched incandescent bombs well beyond the crater rim, and some clasts fell onto the southern outer slope of the central summit cone. After 22:00 GMT, both the eruptive activity and the volcanic tremor amplitude showed a rapid diminution, and at 23:20 GMT the episode was over.

In the early morning hours of 20 January 2013, again during rather inclement weather conditions, the volcanic tremor amplitude was seen to rise again, but this time the increase was much more pronounced at the "Belvedere" seismic station (EBEL, about 700 m from the NSEC) than at the "Cratere del Piano" station (ECPN), which stands closer to the Bocca Nuova. Contemporaneously, there were reports of a glow illuminating the cloud cover over the summit of Etna; seismic and infrasonic data allowed to infer that the activity was occurring at the NSEC, and consisted of mild Strombolian explosions, which ceased a few hours later.

The start of the latest eruptive episode was announced by a rapid rise of the volcanic tremor amplitude, also this time more conspicuous at EBEL, at 17:40 GMT on 22 January 2013. The first visible glow produced by the Strombolian activity within the NSEC was recorded by the high-sensitivity visible surveillance camera of the INGV-OV on the Montagnola (EMOH) at 17:56 GMT; shortly thereafter the activity was well visible (Fig. 3) and consisted of frequent Strombolian explosions, which launched incandescent volcanic bombs up to 100 m above the crater rim. The strongest explosions were followed by abundant fallout of coarse-grained pyroclastics onto the flanks of the NSEC cone.

Eruptive activity continued for nearly 12 hours with minor fluctuations, without at any time producing lava flows or appreciable quantities of volcanic ash. Bangs produced by the explosions were audible in nearby population centers on the eastern flank. About 05:00 GMT on 23 January 2013, the volcanic tremor amplitude began to show a diminution; the last explosion that was well visible in the video footage of "EMOH" occurred at 05:35 GMT (Fig. 3 bottom frame). During the following hour, a few small, sporadic puffs of vapor mixed with volcanic ash were observed, which were blown eastward by the strong wind.

The two episodes of Strombolian activity at the NSEC of 20 and 22-23 January 2013 represent the first true eruptive manifestation (that is, emission of new magmatic products) after a quiet interval of nearly 9 months at this crater; the most recent significant eruptive activity at the NSEC was the lava fountaining episode of 23-24 April 2012. During the past few months, signs of a possible reactivation of the crater were observed, starting with dull glows coming from within the crater on 22 November 2012 and following days, a series of small vapor and ash emissions on 25-27 December 2012, and a short-lived episode of intense glow on the evening of 3 January 2013.

Stromboli: cessation of effusive activity. As reported in the 15 January 2013 update, the intermittent emission of lava flows (overflows) from the crater terrace of Stromboli continued on the evening of 15 January 2013. During the night of 15-16 January, effusive activity ceased, and since then only very small lava overflows were observed on the evening of 17 January and during the night of 19-20 January.