A three-dimensional S wave attenuation tomography of Mt. Vesuvius has been ob-
tained with multiple measurements of coda-normalized S-wave spectra of local small
magnitude earthquakes. We used 6609 waveforms, relative to 826 volcano-tectonic
earthquakes, located close to the crater axis in a depth range between 1 and 4 km
(below the sea level), recorded at seven 3-component digital seismic stations. We
adopted a two-point ray-tracing; rays were traced in an high resolution 3-D velocity
model. The spatial resolution achieved in the attenuation tomography is comparable
with that of the velocity tomography (we resolve 300 m side cubic cells). We statisti-
cally tested that the results are almost independent from the radiation pattern. We
also applied an improvement of the ordinary spectral-slope method to both P- and
S-waves, assuming that the di¤erences between the theoretical and the experimental
high frequency spectral-slope are only due to the attenuation e¤ects.We could check
the coda-normalization method comparing the S attenuation image obtained with
the two methods. The images were obtained with a multiple resolution approach.
Results show the general coincidence of low attenuation with high velocity zones.
The joint interpretation of velocity and attenuation images allows us to interpret
the low attenuation zone intruding toward the surface until a depth of 500 meters
below the sea level as related to the residual part of solidi ed magma from the last
eruption. In the depth range between -700 and -2300 meters above sea level, the
images are consistent with the presence of multiple acquifer layers. No evidence of
magma patches greater than the minimum cell dimension (300m) has been found.
A shallow P wave attenuation anomaly (beneath the southern ank of the volcano)
is consitent with the presence of gas saturated rocks. The zone characterized by
the maximum seismic energy release cohincides with a high attenuation and low
velocity volume, interpreted as a cracked medium
