Aeolian Islands, Italy (38.79 N, 15.21 E)

(This report covers the period 01.03.1996 - 15.05.1996)

Following the string explosion recorded at the craters on 16 February (Bulletin v. 21, n.2), the summit seismic station of the University of Udine showed a general drop in seismic activity, as seen previously after paroxysmal phases at Stromboli (Bulletin v. 18, n. 1 - n. 4 - n. 9). The new increase of the seismic activity, already evident in the graph published in Bulletin v. 21, n.2, continues throughout the first days of March, with an increasing average tremor intensity, while a decrease is evident in the number of recorded events and in the number of saturating events (see Fig. 1). After a fall in the tremor intensity on 9 and 10 March, all parameters start showing a general increasing trend that continues for all the rest of the month and throughout the first week of April.

After a brief drop of the tremor intensity (6-8 April) and of the number of events (10- 11 April), a new period of increasing trend begins: all parameters grow until 16 April, when a major change in the seismic behaviour can be observed.

The number of recorded events suddenly increases between 15 April (286 events) and 16 April (540 events); in the following days numbers become even greater, as the seismic station starts triggering almost every minute. Stromboli volcano guide N. Zerilli confirms that in those days NE Crater (i.e. Crater 1) started erupting almost continuously, with very shallow magma level and fountaining up to 40-50 metres above the crater. This was the source of a succession of moderate explosion-quakes that followed each other very rapidly (of the order of seconds), causing the high total of events recorded by the station. Impressive is also the number of more energetic events. Such strong explosive activity is still continuing today (15 May 1996).

Field observations were made by J. and P. Alean, R. Carniel and F. Iacop between 21. and 28. April 1996, well inside the period of highest activity. Unfortunately the weather conditions were not as good as the volcanic activity, so they could only go to the summit three times.

The most striking feature of the activity during the whole period was an almost continuous spattering at vent 1/2 in Crater 1 (cf. Fig. 2b). Ejecta with up to (and occasionally in excess of) 2 m in diameter were thrown up to a height of at least 50 meters. This unusual permanent activity was somewhat modulated with a period of maybe half an hour. For very short time periods (few minutes) it almost disappeared, then increased and sometimes reached an intensity which made it difficult to distinguish it from the more normal, larger Strombolian eruptions. The latter occurred in Crater 1 from 3 distinct vents. On 21. April we saw 20 eruptions at Crater 1 in 3.5 hrs, some of them reaching heights of about 200 m. Vent Nr. 1/4 (i.e. one of the cones; the numbering is, as far as possible, consistent with the map in Bulletin, v.20, n.11/12) produced remarkable smoke rings on 22 April. On 28 April, Crater 1 showed some 45 "normal" Strombolian explosions between 13:00 and 19:00 GMT, apart from the continuous spattering described above. During the days of our visit there was an intensive red glow emanating from Crater 1 which illuminated steam and clouds above it. This glow was one of the most intense we have ever seen during our visits to the volcano: it could often be seen even from Stromboli - S. Vincenzo village, and the view from Punta Labronzo was most impressive.

Apart from fumarolic emissions, Crater 2 remained inactive during all our observation time.

Crater 3 showed remarkable variety in the type of eruptions. On 21. April they were relatively small, albeit very noisy. Scoria did not reach altitudes of more than maybe 100m. It appeared as if the material were ejected from several individual vents or a fissure somewhere within the crater. A lot of ash was thrown out, occasionally producing impressive black mushroom shaped clouds. Vent 3/1 had grown to an impressive size since 1994. We only saw one eruption from it, on 21. April at 18:15 GMT; all the others were generated at the vents 3/2 or 3/3. On April 28. the activity of crater 3 had increased (55 eruptions between 13:00 and 19:00 GMT) and reached a most impressive level around 21:00 GMT: Eruptions then occurred at intervals of 1 to 5 minutes, most of them exceeding 200m in altitude. The ash content was clearly smaller then on 21. April and brown ash clouds had become rare.

Fig. 2 illustrates the change in crater morphology between October 1993 (Fig. 2a) and April 1996 (Fig. 2b). These sketches were drawn on the base of simple terrestrial stereo photographs taken with a 35mm single lens reflex camera. Stereo photos, to some extent, allowed us to better see through fumarolic emissions, which was, because of wind directions) more of a problem in 1993 than in 1996. The most striking morphological changes are probably the ones in Crater 1, which saw the construction of a series of cones and their subsequent destruction that lead to the present configuration near vents 1/3 and 1/4. Moreover, the zone around vent 1/2 in the foreground seems now more open towards the Sciara del Fuoco, thus allowing a better visibility of the crater from Punta Labronzo. As already partly pointed out, evident changes are also observed in vent 3/1, now better separated from Crater 2 on the rear and more connected to the rest of Crater 3, due to the prominent slumping of blocks between 3/1 and 3/3. Recent photographs of eruptions and crater morphology can be found on our web site STROMBOLI ON-LINE (http://www.ezinfo.ethz.ch/volcano/strombolihomee.html).

Fig 1. caption:

Seismicity recorded at Stromboli, 01.03.1996 - 15.05.1996.

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.

Courtesy of Roberto Carniel.

Fig 2. caption:

Sketches drawn on the base of terrestrial stereo photographs indicating morphological changes of the crater area between October 1993 and April 1996.

a) 17.10.93. ? indicate areas impossible to see because of fumaroles

b) 25.4.96. 3/2? and 3/3? vents are indirectly inferred from trajectory of ejecta

Courtesy of Jürg Alean