The SSRc method as a practical solution for evaluating site response using far apart stations: applications in Western Greece and South-Eastern France

The Standard Spectra Ratio (SSR) of S-wave earthquake ground-motion, is a commonly applied technique between a target and a nearby reference-site, that provides Site spectral-Amplification Factor (SAF[f]). The required “nearby reference-site” constitutes a significant restriction of the SSR technique, which however, has been partially overcome, by relaxing it, based on a recent study, using an analytical, time–consuming 10–step analysis, using the Spectral Factorization method on Coda-waves (SFC). A first effort on this direction, has been earlier proposed by Phillips and Aki (Bull Seismol Soc Am 76:627–648, 1986), based on a more sophisticated technique on coda–waves, using vertical-component data, considering however an undefined reference–site, as the average of all studied–stations. Here, a new simplified empirical technique of SAF[f] computation at a target site with respect to a distant reference one, is examined as a combination of SSR and SFC methods, satisfying at the same time their main advantages, i.e. the easy application of SSR and the use of longer target-reference site distance of SFC. This SSR technique based on coda-waves (SSRc), has been applied in the past in several cases but based on a nearby reference-station. Here, it is modified to make use of a distant reference–station from the target one as well and is demonstrated and applied for several stations in two areas of different seismicity level: the low-to-moderate and the high seismicity areas of southeastern-France and western-Greece, respectively. The results are encouraging in further applying this alternative SSRc technique, compared to those retrieved by different methodologies and datasets, supporting its valid and effective application.