The March 1981 Mount Etna eruption inferred through ground deformation modelling

The March 1981 eruption of Mt. Etna was characterised by the fast propagation of a long eruptive fissure (approximately 7 km) and by an unusually high eruption rate. The continuous tilt recorded by a bore-hole station located on the southern flank showed a small inflation 3-4 months before the eruption followed by a rapid deflation during the fissure opening. The discrete deformation measurements (electroptical distance measurements (EDM) and levelling) showed large horizontal changes in the volcano flanks and smaller vertical ones close to the eruptive fissure. We modelled the deformation pattern associated with the 1981 eruption using horizontal (57 data) and vertical (50 data) measurements. Two geometric constraints were considered for the eruptive fissure. The dislocation theory was used to model the intrusion by characterising it in two parts as a double tensile crack, which represents the initial deeper intrusion (first crack) and then the final shallower part associated with the eruptive fissure (second crack). An analytic inversion was performed obtaining a very good solution, and then, the robustness of this result was verified through an accurate trial and error search. The first deeper crack extends vertically from about 300 to 1700 m above sea level (a.s.l.). It was widely opened (opening 5 m), dipping W and radiating from the summit of the volcanic edifice in the NNW direction for approximately 3 lan. In the shallowest part (last 500 m), the intrusion propagated radially in the northern sector along the NNW direction (second crack), matching the surface location of the observed eruptive fissure. We interpret that the deeper crack formed during the 3-4 months preceding the eruption. (C) 1999 Elsevier Science B.V. All rights reserved.