New Evidence of Azimuthal Variations at Campi Flegrei: Attenuation and Predominant Frequency

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.