Site response obtained from array techniques applied to the seismic noise:Two examples in Italy

The vertical component of the seismic noise has been recorded in two differ ent sites near the towns of Mercato S. Severino and Benevento in Southern I taly by a small aperture array, in order to investigate the characteristics of the noise propagation and to study the site response. Three different a rray techniques have been applied in the two investigated sites: Beam Formi ng, High Resolution and Spatial Correlation methods. We used two simple arr ay geometry for localising possible noise sources and estimating local shal low structure using ambient noise. The cross shaped array results effective for determining the phase velocity of waves in the case when the noise is from a single localised source; the circular array, on the other hand, is s uccessfully used when the noise sources are distributed. The main results a re: the analysis of a coherent component of the noise recorded in the two s ites, interpreted as Rayleigh waves, results in reasonable velocity models; the noise recorded in the M.S.S. Plain is a space stationary signal, while the noise at the Benevento site is possibly produced by a stable noise sou rce located close to the array. Due to this evidence, the correlation metho d does not yield satisfactory results when applied to the Benevento site. T he 2-6 Hz spectral peaks of the noise recorded in the M.S.S. Plain can be i nterpreted as due to a site effect, considering the satisfactory agreement of the noise spectrum at those frequencies with the theoretical transfer fu nction computed on the basis of the velocity model deduced from the Rayleig h waves dispersion analysis.