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In the last decades, researchers focused their attention on using the Rayleigh waves dispersion characteristics to estimate the shear wave velocity profile of a site, since their dispersion characteristics can be extracted also from only one component recordings (namely the vertical ones) of active and passive seismic tests. In particular, inverse analysis of empirical surfacewaves dispersion curves from microtremor measurements (passive seismic test) is a very attractive tool since it allows to keep the cost of investigation relatively low and to avoid the use of active sources that might be prohibitive in urban areas. However, until now the full potential of seismic noise array methods was not fully exploited, and, in particular, the possibility of better constraining the subsoil structure by extracting also the Love wave part from microtremors recordings has not been fully investigated.

Microzonation studies aim to estimate amplification differences of the ground motion at close sites. To evaluate site responses several techniques have been utilized and compared in recent studies (e.g., Field and Jacob, 1995, Riepl et al., 1998). Two of the proposed methods, the Reference Site Method (RSM) (Borcherdt, 1970) and the Receiver Function Technique (RFT or H/V) (Langston, 1977; Lermo and Chavez-Garcia, 1993) are based on a spectral ratio scheme. In both these techniques the source and path contributions are removed from the seismic recordings by means of a deconvolution operation using a function free of site effects. An alternative method to calculate site responses is the Generalized Inversion Technique (GIT) (Andrews, 1986; Castro et al., 1990; Boatwright et al., 1991). The site responses, the source spectra and the quality factor Q are simultaneously obtained by inverting the shear wave spectra in the framework of the parametric approach introduced by Castro et al. (1990). Generally, GIT methods are applied to data recorded by regional seismic network and few studies deal with the small interstation distance characterizing the microzonation analyses. In this work, the reliability of the results obtained by applying the GIT to site responses estimate is assessed by means of numerical simulations. The synthetic data are generated looking at a real case. We concern with five sites in Fabriano (central Italy) and we use the data recorded during 11 day experiment in the contest of the UMBRIA-MARCHE microzonation Project (see http://seism.irrs.mi.cnr.it). Four stations were placed on soft soil and one on rock site. The interstation distance ranged between 100m and 1400m, even if the maximum distance between the stations on soft soil was 600m about. The synthetic data are computed solving the forward problem, considering the same source-station geometry of the real case and assuming a simplified model for the source functions and the Q factor. Moreover, we adopt the site effects detected by applying the H/V spectral ratio to the real data set. Finally, the synthetic data are perturbed with a 10% random noise. The inversions are performed by using three different methods: Simultaneous Reconstruction Technique (SIRT), Singular Value Decomposition (SVD) and Least Squares Method (LSQ). The results obtained with the synthetic data show that GIT provide reliable estimate of the site effects. Furthermore, the site amplification relevant to the three considered inversion scheme are very similar. If no a priori information about a reference site are available, a frequency independent scaling factor is unresolved. If this is the case, only relative amplification levels between close site can be retrieved. The GIT are also applied to the real data-set. The recorded events are corrected for attenuation using a Q(f) function obtained with a coda-Q analysis (Aki and Chouet, 1975; Sato 1977). The achieved results are compared with those obtained by the reference site method and the receiver function technique.

It has long been noticed that the effects of geology on ground shaking is an important aspect of earthquake engineering. In general site response estimate from instrumental recordings is accomplished by removing the source and the path effects from recorded seismograms using information relevant to bedrock ground motion. When ground motion recordings at the base of the soil layers (e.g. down-hole array) are not available, simplified tecniques that only requieres recordings at the top of the layers could be used. Basically these methods may be divided in to two main categories (Bard, 1994): the 'Reference Site' and the 'Non Reference Site' techniques. In this study we adopted a non reference site technique which requires to take the speetral ratio between the horizonthal and the vertical components (WV) of the signal recorded at surface. Many authors agree that this method, when applied to earthquake data (Receiver function method (Langston, 1977)) provide a reliable estimation of the site response. Nevertheless it seems to underestimate the absolute level of amplification with respect to those inferred by the reference site or down hole recording methods. Differently, other authors pointed out that WV ratio applied to microtremors (Nakamura method, (Nakamura, 1989)) allows a reliable evaluation of the fundamental resonance frequencies of soft deposit, but it fails in providing higher harmonies, Moreover the obtained amplitude value is generally lower with repsect to the amplification measured with other techniques. In this study we utilize short period recordings as weIl as microtremors to examine frequency dependent site effects. Data were recorded by a temporary network layed out during the earthquake sequence occurred in Central Italy (Umbria Marche) in september 1997. The emphasis of this preliminary analysis is both on a comparison of the results obtained by Nakamura and Receiver function methods and on assessing the influence of different deconvolutive approaches to estimate the speetral ratio.