A physically sound way of using noise measurements in seismic microzonation, applied to the urban area of Napoli.

Noise measurements have been done in the urban area of Napoli (Italy), a densely and crowded populated city of southern Italy, with historical architectonic heritage often damaged by tectonic earthquakes. Noise has been recorded by Kinemetrics Q330 station with a 3 component Episensor broadband accelerometer. The H/V ratio between the mean of the Fourier spectra of the horizontal components and the spectrum of the vertical component has been computed for 20 s windows and averaged on about 150 windows of noise data. Noise has been measured at sites with detailed shear wave velocity profiles obtained from the non-linear inversion (hedgehog method) of the group velocity dispersion curve of Rayleigh fundamental mode, extracted with FTAN method from active seismic surveys. Ellipticities of all modes, Rayleigh waves and spectral amplifications, both with sophisticated algorithms like mode summation and finite difference techniques and simplified 1-D techniques, have been computed. It turns out that the fundamental and, at times, also higher modes Rayleigh waves are present at the resonance frequency of the soil cover. where the maximum spectral value of noise H/V shows resemblance to the main peak of computed spectral amplifications. Although the match at the resonance frequency, realistic computed amplifications must be assumed for seismic rnicrozonation. Hence a novel use of noise measurements is proposed which consists of a rapid, detailed mapping of the computed spectral amplifications and average physical properties, starting from the fit of the main peak frequency.