Small scale shallow attenuation structure at Mt. Vesuvius, Italy.

We present a high resolution 3D model of S-wave attenuation (Q
−1
S ) for the volcanic structure of Mt. Vesuvius. Data from 959
waveforms relative to 332 volcano-tectonic earthquakes located close to the crater axis in a depth range between 1 and 4 km (below
the sea level) recorded at 6 three-component seismic stations were used for the inversion. We obtained the estimate of Q
−1
S for
each source–station pair using a single-station method based on the normalization of the S-wave spectrum for the coda spectrum at
12 s lapse time. This is a modification of the well known coda-normalization method to estimate the average Q
−1
S for a given area.
We adopt a parabolic ray-tracing in the high resolution 3D velocity model which was previously estimated using almost the same
data set; then we solve a linear inversion scheme using the L-squared norm with positive constraints in 900 m-side cubic blocks,
obtaining the estimate of Q
−1
S for each block. Robusteness and stability of the results are tested changing in turn the input data set
and the inversion technique. Resolution is tested with both checkerboard and spike tests. Results show that attenuation structure
resembles the velocity structure, well reproducing the interface between the carbonates and the overlying volcanick rocks which
form the volcano. Analysis is well resolved till to a depth of 4–5 km. Higher Q contrast is found for the block overlying the carbonate
basement and close to the crater axis, almost cohincident with a positive P-wave velocity contrast located in the same volume and
previously interpreted as the residual high density body related to the last eruptions of Mt. Vesuvius. We interpret this high-Q zone
as the upper part of carbonate basement in which most of the high energy seismicity take place. The low-Q values found at shallow
depth are interpreted as due to the high heterogeneity mainly caused by the mixing of lava layers and pyroclastic materials extruded
during the last eruptions.