Report on Etna (Italy) — April 1987
Scientific Event Alert Network Bulletin, vol. 12, no. 4 (April 1987)
Managing Editor: Lindsay McClelland.
Etna (Italy) Phreatic explosions from Southeast Crater kills two, injures seven
Please cite this report as:
Global Volcanism Program, 1987. Report on Etna (Italy) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 12:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198704-211060.
37.748°N, 14.999°E; summit elev. 3320 m
All times are local (unless otherwise noted)
An explosion from Southeast Crater on 17 April ejected tephra that killed two people and injured seven others.
During the first half of March, Northeast Crater occasionally emitted gas and vapor, sometimes with ash. During the second half of the month only weak fumarolic activity occurred from Northeast Crater, but more or less intense gas emission episodes that occasionally ejected ash occurred from the central crater's Bocca Nuova. Low-level volcanic tremor occurred during the same period but no earthquakes were recorded. Ten microshocks (M 1.0) occurred 23-25 March.
Sudden increases in sporadic tremor, lasting ~ 10-15 minutes, began 1 April. Geologists attributed the tremor to deep phreatomagmatic explosions. Activity increased during the first few days of April, reaching a maximum of 10 episodes of sporadic tremor on the 6th. The episodes lasted 30-40 minutes each and occurred at ~ 2-hour intervals. On 8 April > 50 microearthquakes were recorded. Only weak gas emission occurred from the central crater's E vent through early April. However, on 8 April at 0835 a violent phreatic explosion from that vent fed a 1-km-high eruption column. Abundant tephra was strewn to 300 m from the crater rim, with maximum dispersion to the NE. Geologists believed that similar explosions had probably occurred during the second half of March. The 8 April explosion was followed by a long period of relative seismic quiescence when only weak sporadic tremor was recorded. Beginning 12 April, 4-5 episodes of sporadic tremor were recorded daily.
Following several days of forceful gas emissions from Southeast Crater, tremor duration increased to a maximum of 30 minutes on the morning of 17 April. At 1335 a moderate-intensity phreatic explosion launched tephra SSE, killing two and injuring seven of the ~ 30 tourists who, the press reported, were standing ~500 m from the crater. A similar explosion on 12 September 1979 had killed 9 tourists and injured 23 others near the central crater's Bocca Nuova (04:09). The 17 April ejecta appeared to be older volcanic material. Tephra fragments 150 m from the crater rim reached diameters of 15 cm and at 250 m were a maximum of 5 cm. IIV geologists suggested that conditions for the 8 and  April explosions resulted from the collapse of the internal vent walls and subsequent gas accumulation.
Immediately after the  April explosion an increase in tremor was noted. The next day, three episodes of periodic tremor were recorded, each lasting ~ 90-120 minutes. Levels of tremor 2-3x normal continued through the end of the month. Only small quantities of gas were emitted from the central crater during the days following the fatal explosion. Weak Strombolian activity [from Southeast Crater] was observed during the night of 25 April and the morning of 26 April [and 10-16 May]. Vapor emission resumed after the end of the Strombolian activity.
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, T. Caltabiano, D. Condarelli, O. Consoli, and G. Frazzetta, IIV; S. Gresta and C. Sturiale, Univ di Catania; La Republica, Rome; AP.