Aeolian Islands, Italy (38.79 N, 15.21 E)

(This report is relative to the period 16.09.1995 - 29.02.1996)

Seismic activity during the last three months of 1995 showed little variation for what concerns the volcanic tremor intensity, which oscillated around an average of 4 V.s.

The number of recorded events per day showed low values (below 100) at the beginning of the observed period, reached a maximum of 337 events on 18 October, to decrease again until the beginning of November. This period was quite interesting for its very rapid transitions between days of very quiet activity (3,4,5 and 8,9 November) and days with a more consistent number of events (6,7 November). The absolute minimum of the period was reached on 8 November, with only 53 events recorded by the seismic station during a full day of regular operation. A period with a greater number of stronger events, i.e. either more energetic or less shallow, was recorded at the end of November and first days of December (max. 46 events on 4 December).

Visual observations were made by R. Carniel on 13 October. Abundant light fumarolic activity could be seen in different spots of the crater area. Referring to the tagging of active vents of crater map published on page 8 of Bulletin Vol. 20, n. 11/12, activity of vent 3/1 (front of SW crater 3) indicated a very shallow lava pond, not directly visible but inferred by continuous night glow and ejection of small spatters that in most cases were thrown only high enough to be seen from Pizzo sopra la Fossa, sometimes reached 3-4 meters above the rim and only quite rarely ended their trajectories outside the vent itself. The other active vent of the SW crater was the one at the rear, 3/2 in the cited map. It showed regular explosions, some of which quite spectacular, with ejecta reaching considerable heights.

Central crater 2 (i.e. vent 2/1) was completely quiet, not showing not only explosions such as the ones documented by Andy Harris and Nicki Stevens in September (Bull. Vol. 20, n. 11/12), but not even the gas jet activity that often characterizes this crater.

NE Crater (i.e. number 1) showed what remains of the cones built during July-August 1994 and seriously damaged by the explosion of 5 March 1995. The surviving cones (1/4 and 1/5) did not show any more the twin explosions seen during the summer of 1994. The only activity could be observed inside the 1/4 cone through a hole produced by partial collapse of the side in the direction of Pizzo. This activity consisted in continuous gas puffing, strong glowing which was visible also during the day, and very short blasts which only threw smoke out of the hole. The other active vent in the NE crater was 1/1, the one nearest to the Sciara. Although its eruptions were not so spectacular as the ones in 3/2, there was occasional emission of a black smoke cloud and ejection of sufficient material to trigger noteworthy movement of pyroclasts rolling down the Sciara del Fuoco.

Unfortunately the solar panels efficiency was not sufficient to assure a continuous acquisition during the winter months, hence the gaps in the plot. It is useful to point out that in case of partial operation during a day the number of recorded events and the tremor intensity is in any case significant, as it is normalized to the period of acquisition. On the contrary, as the stronger events can easily be clustered in a limited period of time, we chose not to rescale their number, so that the number shown in the graphs is always that of effectively recorded saturating events.

January and first half of February showed an increased activity, with an average number of 200-300 events per day and a tremor intensity reaching peaks exceeding 6 V.s. Stromboli guides report a big blast at 22:57 GMT of 16 February, which woke up people in the village. The red glowing pyroclasts were thrown into the sky just high enough to be seen from the village and went down the central part of Sciara del Fuoco. A black smoke column reached a height of 200-250 metres above the craters. Continuous visual observation of the summit for the following 30 minutes did not reveal any relevant activity. A field survey made by Istituto Internazionale di Vulcanologia, which recorded a remarkable increase of tremor amplitude at the seismic station of Ginostra, revealed that the bombs had fallen northward on a few hundred meters wide area.

The summit seismic station of University of Udine showed a general drop in seismic activity after the event, quite evident in the graph. This repeats the pattern already observed after the great explosions of 10 February 1993 (Bull. Vol. 18, n. 1, p.7) and 16 October 1993 (Bull. Vol. 18, n. 9, p.7-8) and after the small lava flow of May 1993 (Bull. Vol. 18, n. 4, p.13-14 and n. 9, p.7-8), when similar abrupt decays were observed. This kind of explosions seems therefore to mark significant turnpoints in the dynamics of Stromboli volcano. The following days show the rapid return of the strombolian activity to normality, accompanied by an increasing seismicity both in terms of tremor intensity and of number of events.

Timely information (including seismic activity) can now be found on "Stromboli On Line", a new web site which also gives information and links about other volcanoes, together with touristic and curious information addressed to a public of non-specialists. The pages, care of J. Alean and R. Carniel, are presented in German, English and Italian. The URL for the english homepage is given below.

To see the Figure, click here!

Fig. caption:

open bars = number of recorded events per day

solid bars = saturating events, i.e. with ground velocity exceeding 100 µm/s

line = daily average of tremor intensity on hourly 60-seconds samples.