Influence of Site Parameters on Fourier Amplification Application for 1D Linear Viscoelastic Method

Abstract

We focus on the effect of site parameters, also called site proxies, on the variation of the amplification factor. This latter, named Fourier Amplification Factor (FAF) is defined as the ratio of the Fourier transform of the seismic motion at surface and at bedrock. For this study, the wave propagation theory is used limited to 1D linear viscoelastic domain. At this effect, a set of FAF, is established for a set of 858 real profiles. From there, the site parameters are also derived, it is necessary to mention that the FAF can be computed in independent manner of seismic signals which it is applicable only on linear domain.
In Nuclear Power Industry application, the FAF is mostly used and can be approximated by limited number of site proxies. As the usual code practice implies a lower number of site proxies (generally 1, sometimes 2) as UBC97 or EC8, a sensitivity analysis is conducted to identify the "best performing" site parameters. The results show that by far, using the six site proxies lead to a better prediction of FAF. However if we have to use one single site parameter, results show that the best one is the overall resonance frequency (f0). In the case when we intend to use two site parameters built from the average shear wave velocity over the upper 30 m (Vs30) and the resonance frequency (f0) which are preferred and give an important variance reduction superior than 61%. In the result, a new formula has been established.