Can site effects be estimated with respect to a distant reference station? Performance of the spectral factorization of coda waves

The reliability of the ground motion simulation is highly dependent on the quality of the site response (or site effects) evaluation. There are several methods to estimate the Site Amplification Factor (SAF) corresponding to Fourier Amplitude Spectra (FAS), either by using numerical simulation tools or empirical approaches. A widely used empirical method is the 'Standard Spectral Ratio' (SSR) technique based on the FAS ratio of the seismic record at a target site to the corresponding one at a nearby amplification-free 'rock' site (reference site). The main limitation of this method lies in the availability of a reference site relatively close to the target one. In this study a SAF estimation technique at a target site in relation to a distant reference site is presented and evaluated. This technique is based on the retrieval of the minimum phase Source Time Function (mpSTF) at a pair of examined sites (target-reference), with the Spectral Factorization analysis of Coda waves (SFC) proposed by Sebe et al. The so derived mpSTF are considered as a convolution of the actual source function, and of the SAF, so that the FAS ratio of the mpSTF, derived at one site (target) and at a distant reference site, should be an estimate of the target SAF. The latter is confirmed in this study. Under the conditions of a common STF at the examined sites and of similar coda waves excitation factor, the ratio of the FAS of the mpSTFs (target over reference site) can safely approach the actual SAF, at least when target-reference distance is up to similar to 60 km and provides encouraging results at longer distances. This technique was applied at 24 sites in western Greece in relation to 4 reference sites located at varying distances from the target ones (from 0.4 to 110 km). More than 700 STFs were calculated for 89 moderate magnitude earthquakes (3.9 <= M <= 5.1) located in this high seismicity area and SAFs were determined from each pair of target-reference stations using common seismic sources. Finally, the average SAFs were computed and compared to the ones computed by other methods (e.g. SSR, GIT and HVSR) demonstrating the reliability and robustness of the proposed technique in site effect estimation.