Mapping of the b value anomalies beneath Mt. Etna, Italy, during July–August 2001 lateral eruption

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

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the spatial variations of the b value
in the area studied are real circumstances, linked to the
magmatic processes ongoing in that period of time and
leading to the eruptive phenomena. Our detailed b value
tomography appears to be a good example in which an
intrusion activity may be also documented by increased
b values. Our analysis demonstrates clearly that the
parameter b is a sensitive and accurate indicator of the
magmatic processes that occurred during the investigation
period at Etna volcano. In summary, the anomalies observed
during the two period analyzed (August 1999–June 2001
and July–August 2001) appear to be the result of a
fundamental process connected to the magma ascending,
ending with a high velocity vertical dike emplacement,
which heralded to the July–August 2001 eruption.