Process of Magma Migration at Mt. Etna, Italy, Evidenced by Mapping the b-value Anomalies (July 17 - August 9, 2001 Lateral Eruption)

. During the period July 17 - August 9, 2001 a very intense eruption occurred in the upper southern flank of Mt. Etna in Sicily (Italy). The seismicity from August 1999 to August 2001 has been studied in order to map the intrusive process inside the volcano edifice by positive anomalies in the frequency-magnitude distribution (b-value) of earthquakes. The dataset, collected by the permanent network of System Poseidon, spread out over eastern Sicily, contains 1207 events with Md >= 1, concentrated in the upper 15 km of the crust within a volume of about 7 km radius. We have analyzed the b-value as a function of space and time, using a dense grid with 0.005° spacing sampling the 50 nearest earthquakes which occur in a maximum radius of 3 km. By this technique we find out, in the period November 5, 2000 - July 11, 2001 (when intense swarms occurred) a highly significant b-value positive anomaly (b = 1.86 +/- 0.11) respect to the background value (0.72 +/- 0.08). This anomaly, with an approximately NE-SW trend in agreement with earthquake alignment, located at 4 +/- 2 km depth in the southern flank, has been correlated with a dike emplacement. The increase of b-value observed during this period may infact reflect the increasing fracture density due to the intrusion activity. In the period July 12 - 18, 2001, preceding the eruption, the volume of high b anomaly migrated to 1 - 2 km depth in combination with another very intense swarm. The excellent correlation of these results with the high Vp - body revealed by tomographic images confirms the analysis of b-values as a very useful support in the resolution of volcanoes' feeding conduits and their migration in time.