Coseismic and Aseismic Tilt Variations on Mount Etna

In the last ten years (1990–1999), 21 discrete variations of continuous tilt signal have been recorded on Mount Etna, among which one episode was caused by the opening of the eruptive fracture. The remaining 20 anomalies can be classified into two categories: the first comprises 5 ‘‘instantaneous’’ tilt variations recorded in correspondence to the most energetic seismic events (ML ‡ 3.3) localized on the high western part of the volcano; the second consists of 15 transient anomalies ranging from some hours to 1–2 days, observed at different times at the various tilt stations, with no correlation to seismic events or other evident volcanic episodes. The aseismic variations propagate through the volcanic edifice with a velocity between 4.5–6.0 km/day. Modeling studies suggest that the deformation is generated by a tensile source located 3–6 km SW from Etna volcano summit and 5–10 km depth.