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.