Report on Etna (Italy) — October 1984
Scientific Event Alert Network Bulletin, vol. 9, no. 10 (October 1984)
Managing Editor: Lindsay McClelland.
Etna (Italy) Earthquake swarm starts as lava production ends
Please cite this report as:
Global Volcanism Program, 1984. Report on Etna (Italy) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 9:10. Smithsonian Institution.
37.748°N, 14.999°E; summit elev. 3320 m
All times are local (unless otherwise noted)
After 172 days of activity, the eruption in the Southeast Crater stopped during the evening of 16 October, although activity continued at the Northeast and central craters. On 17-18 October, only violent ejections of ash at rather long intervals occurred from the small cone that had formed inside the Southeast Crater. The ash ejections stopped completely the morning of 18 October. During the last week of activity from the Southeast Crater, there was a gradual decrease in lava effusion, which was limited to vents along the S rim, and ash ejection from the small cone inside the crater. The lava flows were directed mainly toward the SE, generally stopping after only a few hundred meters.
"During the late morning of 16 October, strombolian activity began from a vent at the bottom of The Chasm. Activity was extremely violent during the evening. At times, lava fragments were ejected above the crater rim, falling back within a 100-m radius. The strombolian activity diminished during the night but continued, with alternating phases, through 17 October. During the morning of 18 October, the crater was obstructed by consolidated lava. It reopened 25 October with the ejection of old lava and ash, and more or less intense emission of gas has continued since then. Bocca Nuova alternated phases of slow emission of gas with periods of intense emission of vapor mixed, at times, with mainly reddish ash.
"Ejections of vapor mixed with reddish ash that had started at the beginning of September from the Northeast Crater intensified during the final phase of the Southeast Crater eruption. During the night of 19 October, weak strombolian activity from the Northeast Crater was recorded at irregular intervals. This activity was succeeded by almost continuous ejection of dark ash, followed during the night of 21 October by the collapse of the crater's summit area. This enlarged the central part of the crater and ejected old lava to a distance of about 500 m, mainly toward the W. Discontinuous and more or less intense emission of vapor mixed with reddish ash started 21 October from this crater, while continuous emission of dense white vapor occurred from a small vent to the N.
"During the afternoon of 16 October a seismic crisis began, with earthquakes mainly occurring in the middle and upper parts of the E flank. The strongest shocks (M ~3.5-4.5), which were felt, were all shallow (around a few kilometers)."
A few of the largest earthquakes in the swarm are described below. Date, time, epicenter, and some casualty and damage data are from Romolo Romano; intensity values and the remaining casualty/damage information are from press sources.
18 October, 1258: centered near Piano Pernicana (15.5 km NE of the summit): Intensity reached MM V-VII, causing ground fracturing and some cracking of roofs and walls.
19 October, 1843: centered near Zafferana Etnea (11.5 km SE of the summit): 1 person was killed and others injured. Intensity reached MM VII-VIII. More than 400 buildings were damaged, including 50% of the historic district, and about 500 people were left homeless. Damage also occurred in Milo (11 km ESE of the summit), Fornazzo (10.5 km E), Santa Venerina (14 km SE), and Giarre (16.5 km ESE).
25 October, 0211: centered near Fleri (14 km SE of the summit), where it reached MM VIII, injured 12-15 people, destroyed 80% of the houses, and left 900 homeless. Ground cracking was observed in the area. Mt. Ilice, a 350-m-high prehistoric cone roughly 1.5 km upslope from Fleri, lost about 20 m of height during the earthquake. The shock was also felt in Catania.
7 November, 0956, centered near Pedara (15.5 km SE of the summit), where a few buildings were damaged.
Geologic Background. Mount Etna, towering above Catania, Sicily's second largest city, has one of the world's longest documented records of historical volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km horseshoe-shaped caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.
Information Contacts: R. Romano, IIV; La Stampa, Torino; Corriere della Sera, Milano.