STROMBOLI

Aeolian Islands, Italy (38.79°N, 15.21°E)

(This report covers the period January 1997 - September 1998)

1997

As reported in last contribution to the GVN (Bulletin, v.22, no. 3), Stromboli volcano was showing moderate activity at the beginning of 1997. This period of moderate activity lasted roughly until May 1997, with a slightly decreasing trend in the volcanic tremor intensity and a slightly increasing number of recorded events (we remind that an event - or explosion-quake - is the seismic footprint of an eruption, while the tremor intensity is recorded bewteen the eruptions). During January to May the number of saturating events, i.e. the events that reach the saturation level for our summit seismic station, was always very low (most of the time it remained below 10 per day).

June and July showed an increased number of both events (sometimes exceeding 300 per day) and saturating events (maximum: 92 on 8 June 1997).

After a short lasting decrease of the activity in August, a new long term growing trend started in September and continued at least until November, with several days during which the triggering of the seismic station was almost continuous, and the tremor intensity reached quite high values (8 - 9 V.s). This kind of seismic behaviour is usually recorded in presence of almost continuous spattering at the vents.

Unfortunately, no seismic data were recorded between 24 November 1997 and 9 January 1998.

1998

By the time the seismic acquisition resumed, the activity had returned to moderate levels. The number of events per day was rapidly decreasing (from 280-290 on 10/11 January to less than 70 events on 14/15 January), together with the tremor intensity (from 6.2 on 11 January to 2.6 on 15 January). On 16 January 1998, at 11:30 local time, a strong explosion occurred in the crater area. This explosion may be classified among the paroxysms that have interested Stromboli several times in the last few years (last one occurring on 4 September 1996). These explosions do not put in danger the population in the villages of Stromboli and Ginostra, but may be dangerous for the safety of tourists visiting the summit part of the volcano, as flying bombs easily reach the usual places where tourists go to observe the eruptions. Another source of risk is given by the fires that, started by incandescent bombs, interest the vegetation on the volcano slopes over the villages. In the case of 16 January eruption, both these risks were strongly limited by the bad weather and rain that prevented tourists from climbing the volcano and promptly extinguished any fire in the vegetation.

A new rise in the overall seismic activity began a few days after the explosion. The maximum was reached during the days 16-20 February, with as many as 405 events in one day (19 February), when the tremor intensity also exceeded 8.3 V.s, and 43 saturating events recorded on 20 February. Then, a long term decreasing trend was observed until the end of April, although with the usual fluctuations. Several local minima were observed in the number of events (less than 100 per day), but also short lasting episodes of stronger volcanic tremor (7 March; 12-13 March 1998) and series of saturating events characterized by a stronger energy (e.g. 25-27 March).

During May and June 1998 the seismic activity showed a general increasing trend. During July two sharp local minima were observed. During the first 3 days of the month the number of events did not exceed 80 per day. The second minimum in the seismic activity was recorded during 22-24 July, when the number of events went below 50 per day, and the tremor intensity reached the absolute minimum of the year with 1.4 V.s. August showed a little reprise of the activity, which in any case remained moderate.

On 23 August 1998, at 17:26 local time, another powerful explosion occurred at the craters. The strong blast was heard throughout the island, and a column of ash and lapilli was shot over the craters. Incandescent bombs thrown out by the explosion fell on a vast area and in a wide angle, towards Vallonazzo, Labronzo, Forgia Vecchia, etc. At least another explosion followed the first one. Several fires were started in the vegetation on the upper volcano slopes (the biggest one, near Forgia Vecchia, was extinguished only the following day, 24 August). Fortunately, notwithstanding the high number of tourists on the island, noone was hurt by the explosion. A big, light ash cloud was reported after a first, darker column; small lapilli fell over the village of Ginostra. Bombs were found on the tourist path down to an altitude of 750 m a.m.s.l., and in other directions the bombs fell as low as 500 m a.m.s.l. The explosion also caused significant morphological changes to the rim of Crater 1 towards Semaforo Labronzo. The authorities immediately blocked public access to the upper part of the volcano. Photos of the eruptive cloud and the beginning of the seismic record of the explosion can be found at the "Special events" page of Stromboli On Line website, at the URL http://stromboli.net.

Another strong explosion, probably even more energetic than the previous one, happened on 8 September 1998 at 19:14 local time. A considerable atmospheric shock wave was reported from the village of Stromboli, with broken windows near San Bartolo. Ash and small lapilli fell near the village of Ginostra, several bush fires were started by the bombs and on the volcano slopes. Once again, fortunately no tourist was injured as a result of the eruption. Unfortunately, our seismic station was not operational, due to a technical problem. We therefore did not record this explosion. However, also for this eruption photos are available at the "Special Events" page of Stromboli On Line website, at the URL http://stromboli.net.

Fig 1. caption:

Seismicity recorded at Stromboli during 1997.

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:

Seismicity recorded at Stromboli during January-August 1998

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.

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Information contacts:

Roberto CARNIEL

Dipartimento di Georisorse e Territorio, Università di Udine

Via Cotonificio, 114

I-33100 UDINE

E-mail carniel@dgt.uniud.it

Jürg ALEAN

Kantonsschule Zürcher Unterland

CH-8180 Bülach, Switzerland

E-mail alean@stromboli.net

STROMBOLI ON-LINE website:

http://stromboli.net