Site effects at the Campi Flegrei, preliminary results

Riassunto

A critical factor for seismic hazard estimation is a detailed evaluation of the site response. Seismic shaking is highly influenced by the local physical and mechanical properties of the rocks underlying a site. Shallow or thick sedimentary deposits can significantly amplify the earthquake ground motion. Seismologists and engineers have conducted numerous studies to quantify how the seismic energy is modified and how structures that overlie vulnerable soils will behave during strong ground shaking [Wills et al., 2000]. Different methods can be applied to estimate the site response from seismic records. The most frequently used is the spectral ratio method. This approach needs the identification of a reference site, usually characterized by outcropping lithoid rocks, and consists in computing the ratio between the spectrum at a site of interest with respect to the referen-
ce site spectrum [Borcherdt and Gibbs, 1976]. Both S waves [Field et al., 1995] and coda waves [Phillips and Aki, 1986] can be used for this application. One of the advantages of using the coda waves is that their propagation depends only on average regional properties of the medium and is independent on the source and recording site locations. Other methods, called inversion method [Andrews, 1986; Boatwright et al., 1991], cast the problem into a formalized inversion that is based on the assumption that the observed earthquake spectrum can be decomposed into a source, path and site terms. This method does not make any assumption on the source spectra and allows to determine the site transfer function with respect to a reference site or to the average response of all the other sites [Phillips and Aki, 1986]. These methods depend on the used...