The following, from IIV, describes activity March-April 1989.

Summit activity. (S. Calvari, M. Coltelli, and M. Pompilio.) Vigorous Strombolian activity at the two central craters (Bocca Nuova and La Voragine), persisted through March and April. At Southeast Crater, Strombolian ejections became strong and continuous during the last 10 days of March, similar to those of 24 December-24 January and 29 January-22 February. Strong explosions continued through April. A small lava flow emerged from the base of Southeast Crater's cinder cone on 15 April at 2300 and flowed over the crater's S rim. Lava effusion stopped early the next morning. Degassing continued at Northeast Crater.

Seismicity. (E. Longo, A. Montaldo, M. Patanè, E. Privitera, and S. Spampinato.) A total of 71 tectonic earthquakes (M >1.0 and S-P <5 seconds) were recorded on the Serra Pizzuta Calvarina seismic station (~ 13 km S of the summit craters) in March, compared to 194 events during all of 1988. The highest seismic energy release and the largest number of events (9, with two of M 2.7) occurred on 17 March. Tectonic events were fewer and smaller (M<=2.7) in April than in the first three months of 1989. Most of the largest events (M>=2) were located on the W flank at depths of ~ 15-25 km. High-energy volcanic tremor episodes were recorded 9-16 April and represent the only seismic evidence of the Southeast Crater eruptive episode on 15 April. After the effusive episode, the spectral amplitude of tremor greatly decreased, but remained higher than in March.

SO₂ emissions. (T. Caltabiano and R. Romano.) During the first half of March, SO₂ emissions from the summit craters were similar to relatively high February values at ~ 6,000 t/d. During the last half of March, SO₂ emissions increased to ~ 8,000 t/d, then decreased to ~2,000 t/d. After the lava effusion of 15-16 April, SO₂ emission rates rose to ~12,000 t/d, but decreased to ~3,000 t/d during the remainder of the month.

Ground deformation. (O. Campisi, G. Falzone, B. Puglisi, G. Puglisi, and R. Velardita.) Ground deformation at the Serra Pizzuta Calvarina borehole tilt station showed no significant variations during March and April.

Geological Summary. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of 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 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. Santacroce, IIV.