SEISMIC SITE AMPLIFICATION FACTORS IN CAMPI-FLEGREI, SOUTHERN ITALY

The method of spectral ratios of S waves from local small earthquakes recorded at various sites in the Campi Flegrei volcanic area (Southern Italy) has been used to evaluate the seismic site response in the frequency range 1-15 Hz. The spectra of-the S-wave trains recorded at the short-period seismic stations are corrected for attenuation using attenuation parameters (quality factor Q) previously determined for the area using the coda method. A reference station has been used, selected on the basis of the highest number of recorded earthquakes and not on geological considerations. The relative uniformity of the geological conditions and the absence of hard-rock sites conditioned such choice. The method has been checked using the coda-wave method to estimate the spectral site response, and similar results were obtained. The results show a relative peak of the spectral ratio pattern centred at low frequency (2-3 Hz) and a minimum of 0.2 at 8 Hz for two stations located on yellow tuff. Such results are interpreted as being caused by a filtering effect produced by selective attenuation. Such a pattern could be of interest for seismic engineering purposes, in view of the abundance of tuff over a wide area in this densely inhabited region.