Fig. 1. Structural sketch map of eastern Sicily, including the Mt. Etna area, showing the main geodynamic domains and tectonic features. 1=main faults (ME=Malta Escarpment; TFS=Timpe Fault System; MKAS=Mt. Kumeta–Alcantara System; TLS=Tindari–Letojanni System; GMS=Giardini–Messina System), 2=Southern margin of Apennine Maghrebian Chain.

Fig. 2. Sketch map of the upper southern flank of Mt. Etna showing the main known eruptive fissures since 1900. The July–August 2001 fissures system is shown with dashed lines. PL=Piano del Lago.

Fig. 3. Daily frequency of earthquakes with M≥1.5 occurring in the whole area of the upper southern flank of Etna, and associated cumulative strain release, during the period June 30th 2000 to June 30th 2001. The dashed lines with roman numbers highlight significant changes in the gradient of cumulative strain release (see text).

Fig. 4. (a) Epicentral location of earthquakes recorded in the study area (bounded by dashed lines) during the period July 1st 2000 to June 30th 2001. Shaded areas highlight the zones with greater clustering of epicenters (A=summit; B=Valle del Bove; C=Mt. Frumento Supino; D=Piano del Lago). The figure also shows some representative focal mechanisms of the analysed sample; (b) projection of the hypocenters of the same earthquakes along a E–W cross-section of the study area (all depths are relative to sea level); (c) velocity model used for the interpretation of seismic data at Mt. Etna (modified from Hirn et al., 1991).

Fig. 5. Temporal variation in (a) latitude and (b) longitude values of the epicenters of the earthquakes belonging to cluster D recorded during the period January 1st to June 30, 2001. The plot highlights the progressive shift of epicentral locations from SSE to NNW towards the summit area of Mt. Etna.

Fig. 6. Location map of the fumarolic areas and the sampling sites of soil gas (open circles with diamonds) in the upper southern flank of Mt. Etna. Shaded areas indicate the approximate extension of steamy ground near Torre del Filosofo (TDF) and Belvedere (see text for details). The Vulcarolo steam emission is also indicated, as well as the site of anomalous soil degassing along the 1949 eruptive fissure (P7). Segments with capital letters indicate sampling lines of soil CO₂. Dashed lines across sampling profiles highlight the directions recognized from soil CO₂ anomalies. SEC=South–East Crater; VDB=Valle del Bove. Altitudes in metres.

Fig. 7. Values of CO₂ concentration in the soil measured along the sampling lines on the upper southern flank of Mt. Etna (letters refer to those indicated in Fig. 6). Values are expressed in ppm by volume (CO₂ STP) above the atmospheric CO₂ content (340 ppm vol), which is taken as zero. The vertical lines show the inferred axis of hidden active faults. The figure does not report sampling lines F and G because all soil CO₂ concentrations along them were found equal to the atmospheric content.

Fig. 8. (a) Time variation of the intensity of volcanic activity at Mt. Etna's summit vents during the period June 30th 2000 to June 30th 2001 based on an empirical scale from I (no activity) to V (paroxysmal activity with strong explosions and lava fountaining); (b) time variations of soil temperature at TDF, fumarole temperature at Belvedere and air temperature. The seasonal influence of air temperature on soil temperature at TDF is evident; (c) time variations of soil CO₂ concentrations (ppmv) at TDF and in the fumarole emissions at Belvedere sampling sites; (d) daily frequency of earthquakes with M≥1.5 occurring in the whole area of the upper southern flank of Etna, and associated cumulative strain release (J^1/2), during the studied period (the dashed grey lines indicate the two periods of significant increase in the gradient of cumulative strain release discussed in the text); (e) hypocentral depth of recorded earthquakes in the study area vs. time. Arrows with numbers in the plots indicate the onset of periods of anomalous changes in the observed parameters as a consequence of magma ascent and degassing (see text for explanation).

Table 1. List of the eruptions and the main directions of the relevant eruptive fissures (sorted by classes of orientation) occurring in the south–east sector of the volcano since 1900

Table 2. Summary of the volcanic activity of Mt. Etna from August 2000 to August 2001