Abstract
In this paper we have determined apparent Q from body-wave dispersion
and a spectral ratio method for the Campi Flegrei area. A contour
map of azimuthal variations of apparent Q (ranging from Q = 4 to
Q = 40) reveals that the lower Q (higher attenuation) corresponds
to an area that includes the Solfatara Crater (with active fumaroles),
near the site of maximum uplift produced during the bradyseismic
crisis of 1984, and just over the site evidencing the presence of
a magmatic body. A strong azimuthal variation of the predominant
frequency of the first pulse of the P wave has been found and is
interpreted in terms of a vertical travel time in a surficial resonant
layer (\~ 1 km thick) over a halfspace. The attenuation maps, the
interpretation of the resonant frequencies, and the additional information
provided by the P-wave velocity suggest the presence of a circular
structure in the area. Not only does the distribution of the thickness
of the resonant layer (obtained through the estimation of the contribution
of different harmonics) show the circular structure, but it also
agrees with the presence of a thin layer of lighter sediments, determined
by means of a microgravimetric study, and with the total magnetic
map, previously obtained for this area. The results of the azimuthal
variation provide new evidence of the structure that can be used
to improve the knowledge of the area.
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