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.


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