[1] The pattern of b value of the frequency-magnitude relation, or mean magnitude, has been analyzed as function of space beneath Mt. Etna during the intense lateral eruption of July–August 2001. The data used (August 1999–August 2001), subdivided in the pre-eruptive and eruption period, show significant space variations in the b value. For each of the two periods, a volume of anomalously high b value can be observed in the southeastern flank of the volcano. The anomalies located at 3 ± 1 km (b = 1.84 ± 0.20), and at about 1 ± 1 km (b = 2.11 ± 0.29) depth are interpreted as two different steps of a magma migration process ended with the July–August 2001 eruption. All the available geophysical evidences such as geodetic deformation measurements, focal mechanism and tomographic studies support this interpretation.