NEW EVIDENCE OF AZIMUTHAL VARIATIONS AT CAMPI-FLEGREI - ATTENUATION AND PREDOMINANT FREQUENCY

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) r eveals that the lower Q (higher attenuation) corresponds to an area th at includes the Solfatara Crater (with active fumaroles), near the sit e of maximum uplift produced during the bradyseismic crisis of 1984, a nd just over the site evidencing the presence of a magmatic body. A st rong azimuthal variation of the predominant frequency of the first pul se of the P wave has been found and is interpreted in terms of a verti cal travel time in a surficial resonant layer (similar to 1 km thick) over a halfspace. The attenuation maps, the interpretation of the reso nant frequencies, and the additional information provided by the P-wav e velocity suggest the presence of a circular structure in the area. N ot only does the distribution of the thickness of the resonant layer ( obtained through the estimation of the contribution of different harmo nics) show the circular structure, but it also agrees with the presenc e of a thin layer of lighter sediments, determined by means of a micro gravimetric study, and with the total magnetic map, previously obtaine d for this area. The results of the azimuthal variation provide new ev idence of the structure that can be used to improve the knowledge of t he area.