Broadband site effect assessment: comparison between approaches based on earthquakes and microtremors on two sites

Abstract

In the last decades, the use of microtremors to assess linear site effects has received a significant attention. Various authors demonstrate the reliability of ambient noise processing techniques to assess the relative amplification generally between distant sites (>1-10 km) in the low frequency range (<1 Hz). In this study we attempt to evaluate the validity of such an approach at the scale of an industrial facility (≈300 m) and up to high frequency (≈10 Hz) on two sites: an industrial site located across a small sedimentary basin in the low-to-moderate seismicity context of Southeastern France (Provence) and another sedimentary basin in the seismically active island of Cephalonia in Greece. On both sites, transfer functions relative to a rock reference deduced from microtremors (SSRn) are compared to those obtained from earthquake recordings with the classical Standard Spectral Ratio (SSR) approach. We find a good agreement between both approaches until intermediate frequencies. We thus propose an approach based on microtremor recordings and on one neighboring available SSR computed for a station located inside the basin (SSRh). This new approach seems reliable to assess the spatial variability of the site amplification up to high frequency (microzonation). The upper frequency limit for both noise approaches (SSRn and SSRh) is discussed according to the inter-station distance and to the stations geological context.