Depth and Spatial Variation of the Shear Wave Attenuation Parameters in the Shallow Crust and Lower Crust/Upper Mantle of Mt. Etna (Italy)

Seismic wave attenuation is a key feature of seismic
wave propagation that provides constraints on the composition and
physical state of the medium within the Earth. We separated
intrinsic and scattering attenuation coefficients for the shallow crust
and lower crust/upper mantle in the Mt. Etna area. For this purpose,
the Multiple Lapse Time Window Analysis (MLTWA) was applied
to two groups of earthquakes, well separated in depth. We also
studied the spatial variation of the attenuation parameters by
dividing the study area into four sectors around Etna. The results
show an effective homogeneity of the propagation characteristics
inside Etna and, in particular, some lateral variations and minor
variations with depth. We observe that structural discontinuities
and lithology control scattering losses at all frequencies, with
higher scattering in the shallow crust. The intrinsic absorption
shows no sensitivity to the presence of these main geological
structures and is quite uniform for different depths. Furthermore,
compared to the northern sector of the volcano, the southern one
shows stronger scattering attenuation at low frequencies. This
pattern correlates well with the high seismic activity along most of
Etna’s active tectonic structures and ascending magmatic fluids that
characterize this sector of the volcano. Although we only discuss
the differences in the ‘‘average’’ scattering and inelastic properties
of the investigated volumes, the results of this study are very
informative about the characteristics of each region. Moreover,
they suggest that a future study is necessary, providing a more
detailed picture of the spatial distribution of seismic attenuation in
the study area, through a 3D inversion of the attenuation parameters
estimated along the single source-receiver paths.