Estimation of the Local Seismic Amplification on an Industrialized Site in the French Rhône Valley

The so-called site effects caused by superficial geological layers may be responsible for strong ground motion amplification in certain configurations. We focus here on the industrialized Tricastin area, in the French Rhône valley, where a nuclear site is located. This area lies above an ancient Rhône Canyon whose lithology and geometry make it prone to site effects. This study presents preliminary measurements to investigate the local seismic amplification. We deployed three seismic stations in the area for several months: two stations were located above the canyon, the third one was located on a nearby reference rock site. The recorded seismicity was analysed using the Standard Spectral Ratio technique (SSR). The estimated amplification from weak motions reaches a value of 6 for some frequencies. These first results confirm the possibility of estimating seismic amplification using earthquakes recorded for less than one year, in this highly anthropogenic and industrialized environment, despite the local low-to-moderate level of seismicity. Noise-based SSR, that presents an obvious interest in such seismic context, shows also promising results in the area. To complement this empirical approach, we estimated the amplification using 1D wave propagation modelling. This numerical estimate is based on shear wave velocity profiles resulting from geophysical characterization campaigns. Comparison of the two approaches at low frequency, where numerical estimate is considered as the most representative, tends to suggest that edge-generated surface waves may have a strong influence in the local seismic response. This interpretation will be further investigated in the future.