STROMBOLI

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

(This report covers the period January 2000 - May 2000; all times are local)

During the first two months of 2000 the seismic activity recorded by the summit station of University of Udine (Figure 1) indicated low to moderate activity, with short periods characterized by a very low number of events (e.g. 1-7 January, less than 50 events/day).

After a short gap in the acquisition, on 17 February the tremor intensity showed quite a high value (8 V.s), but returned to more usual values (< 4 V.s) already the following day. An even more quiet period was observed during the first half of March, when there were never more than 50 events/day and some days (1,5-9, 11 March) were characterized by less than 20 events/day. The number of events returned to more usual values (100-120 per day) during the second half of March, and further increased to 140-160 per day in April. However, during all March and April the tremor intensity did not show any significant variation. Unfortunately, a technical problem caused the interruption of the acquisition at the beginning of May.

Between 10 and 19 May a field campaign was carried out on Stromboli by Universities of Udine, Florence, Hawaii and Open University. During these 10 days near-continuous seismic, thermal and infrasonic measurements were taken, a detailed activity log was constructed and the crater terrace map updated (Figure 2). The activity log totaled ~71 hours and was divided into nine 2-15-hour-long segments covering all days except May 13. It is worthwhile to remind the reader that the number of events per day recorded by the seismic station are not directly comparable to the number of eruptions logged by visual observation. This because some eruptions (e.g. the gas- or ash-dominated ones) may not produce a seismic signal detectable by a short period seismometer.

During the observation period the NE Crater was the location of up to 5 active vents (Figure 2). The western cluster (1/1) consisted of vents which emitted ejecta in three different directions: obliquely to the SW, obliquely to the NE, and symmetrically. This zone was the source of persistent, strong nighttime glow, and near-continuous sloshing/puffing sounds accompanied by regular emission of single bombs between strombolian events. The eastern cluster (1/2) consisted of two sources that would often erupt together. By night this zone was the source of weak, intermittent glow. These two activity centers (1/1 and 1/2) were located on the floors of two coalesced depressions within the main NE Crater construct (Figure 2). The SW Crater was the location of at-least two active vents (3/1), but no nighttime glow was observed. A zone of high temperature fumaroles was aligned along crater-bounding fractures at the east rim of the SW Crater (Figure 2).

During the period of visual observations a total of 916 strombolian events were recorded, with 383, 198, 335 events occurring at 1/1, 1/2 and 3/1, respectively. This gives an overall eruption rate of 13 events/hour, with a range of 6-20 events/hour and a typical repose of 5 minutes (maximum = 37 mins). Breaking these statistics down by vent, 1/1, 1/2 and 3/1 were respectively active with 5, 3 and 5 events/hour, with ranges of 0-12, 0-6 and 0-9 events/hour and mean reposes of 12, 22 and 12 minutes (maximum = 194, 136 and 117 mins).

During May 10-14, activity at 1/1 was characterized by emissions of molten ejecta, in heavily loaded, single shot fountains that reached heights of 50-350 m and landed as far as the eastern edge of the SW Crater. Fountain heights and ejected volumes appeared to increase through May 14, where maximum ejecta heights were 100-200 m between May 10-11 compared with 230-350 m on May 14. At this point rare bombs were landing within 50 m of Pizzo sopra la Fossa. Activity at 3/1 was characterized by 10-40-second-long emissions of gas and brown ash, with minor ejecta, to feed ash clouds rising up to 300 m above the vent before detaching. These emissions had, however, become increasingly gas dominated and lower (<50 m) by May 14.

Overnight on May 14-15 a change in activity occurred, where activity at all vents switched to gas dominated emissions that attained lower heights (<75 m). Events also became increasingly quiet, such that by the evening of May 15 some emissions were completely silent. Ejections from 1/1 continued to be short, 1-2 second long, single shot events, but these now contained low volumes of molten ejecta with minor ash that rose <75 m above the vent with gas being the dominant component of the ejected mixture. Activity at 3/1 continued to comprise of long-lasting (30-40-second-long) emissions but these now consisted almost entirely of gas and rose <100 m above the vent. This gas-dominated style of activity continued until the final day of observations on May 19. Although the frequency of events from all vents remained approximately stable at 11-17 events/hour during the activity change, an event frequency decrease at 1/1 from 6-7 events/hour during May 10-14 to 3-6 during May 15-19 was matched by an increase at 3/1 from 2-5 to 5-8.

The 1/2 vent cluster was the source of prolonged, pulsing emissions of incandescent ejecta, ash and gas, typically lasting 20-30 seconds. Emissions reached <100 m above the vent and ejecta was emitted in a diffuse spray, where emissions became increasingly gas dominated after May 14. Two closely spaced vents (within 10 m of each other) would often erupt together, with activity at the westernmost of the two vents usually beginning after the eastern vent had begun to erupt. These vents showed a relatively constant eruption frequency throughout the observation period of 2-4 events per hour.

No activity or glow was observed from the Central Crater, where the hornito observed during July 1999 (Bulletin v. 24, n. 6) remained (Figure 2). This was the source of low temperature fumarolic activity only. The Central Crater itself is now entirely filled and is not apparent as a crater, rather a saddle between the SW and NE Craters. The deep, funnel-shaped pit to the SE of Central Crater which developed during 1997-99 (Bulletin v. 24, n. 6) is now, however, a well established feature and appears, morphologically, to be an eastern extension of the SW Crater. It is, though, separated from the SW Crater by a low septum (Figure 2). A single, ~5-m-wide vent (3A) was active on the floor of this pit throughout the observation period, but with persistent degassing only. A new vent (3B) has established at the NW rim of the 3A pit since July 1999. This ~2-m-wide vent was active with degassing only and was not surrounded by any constructive feature (e.g. hornito, spatter rampart or crater features) suggesting eruptions from it are rare or absent. We obtained a maximum temperature of 639 °C from the gas-heated walls of 3B. The temperature of vent 3B was, however, variable and glow would wax and wane, where the vent temperature would fall below 550 °C (the lower detection limit of our thermal infrared thermometer) during low activity periods. By day these variations were obvious from increases and decreases in the vigor of degassing. Similar variations in glow, temperature and degassing were also observed from 3A, where the vent switched between periods of high temperature, vigorous degassing, and low temperature, weak degassing. The activity at the two vents appeared to be inversely correlated, as a decrease in temperature and degassing at 3B would often synchronize with an increase at 3A and vice versa.

Figure caption

Figure 1. Seismicity detected at the summit of Stromboli during 2000.

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.

The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

Figure 2. Sketch map of Stromboli's Crater Terrace drawn during 10-19 May 2000 from Pizzo sopra la Fossa and fitted to the map produced from our September 1995 EDM survey of the Crater Terrace (Bulletin v. 20, n. 11/12). For this survey we have changed our naming convention, where the craters previously assigned 1, 2 and 3 are now named NE, Central and SW Craters, respectively. Courtesy of Andy Harris.

Information contacts:

Andy Harris

Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, U.K., Email: harris@pgd.hawaii.edu

Fax: +001 808-956-6322, Tel: +001 808-956-3158

Roberto Carniel

Dipartimento di Georisorse e Territorio, Università di Udine, via Cotonificio 114, I-33100 Udine, Italy. Email: carniel@dgt.uniud.it

Fax: +39-0432-558700, Tel: +39-0432-558749

Website: STROMBOLI ON-LINE, http://stromboli.net

Maurizio Ripepe

Dipartimento di Scienze della Terra, Università di Firenze, via G. La Pira 4, I-50121 Firenze, Italy. Email: maurizio@ibogfs.cineca.it

John Bailey
Department of Geology and Geophysics, SOEST, University of Hawai' i, 2525 Correa Road, Honolulu, HI 96822, USA, Email: jbailey@soest.hawaii.edu