Attenuation and Source Parameters of Shallow Microearthquakes

We estimated the attenuation laws of high-frequency seismic waves in
the shallow crust (depth 5 km) and earthquake source parameters by using a selected
data set of 320 shallow events (2.6 MD 4.2), recorded at Mt. Etna volcano
during the last two flank eruptions occurring in 2001 and 2002–2003. The quality
factor (Q) was estimated from spectra of P and S waves for 24 stations of the local
permanent network by applying a spectral ratio technique. The results show variations
in both QP and QS as a function of frequency, according to the power law
Q Q0 f n, with n ranging between 0.3 and 1.3 for P waves and between 0.2 and
0.9 for S waves. As typical of volcanic environments, strong azimuthal variations of
QP were also found, suggesting the presence of local strong lateral heterogeneities
and/or of fluid-filled cracked volumes. After correction for attenuation, we estimated
the source parameters (seismic moment, source radius, and stress drop) of a subset
of 66 shallow events, under the assumption of a circular dislocation. The estimated
seismic moments M0 range from 1013 to 1015 N m. The source radii (r) are confined
between 100 and 1000 m and stress drop (Dr) ranges between 0.2 MPa and about
4 MPa. Combining the source parameters obtained in this study with those calculated
by Patane` et al. (1997) for an old data set of smaller microearthquakes (109 M0
1014 N m) recorded in the same area, we re-evaluated the scaling relationship between
seismic moment (M0) and corner frequency ( fc) for the earthquakes with M0 ranging
between 1013 and 1015 N m. We confirm that microearthquakes at Mt. Etna seem not
to obey a scaling relationship, as generally observed for moderate to 3 M f 0 c
large earthquakes, as the slope of the scale dependence about 4.3 or higher. Assuming
that this dependence is real, within the uncertainty in the results, a departure from the
self-similarity exists for the volcanotectonic earthquakes at Mt. Etna.