Site amplification at the city scale in Basel (Switzerland) from geophysical site characterization and spectral modelling of recorded earthquakes

• Published in: Physics and chemistry of the earth. Parts A/B/C 2017-04, Vol.98, p.27-40
• Main Authors: Michel, Clotaire, Fäh, Donat, Edwards, Benjamin, Cauzzi, Carlo
• Format: Article
• Language: English
• Subjects: Ground motion amplification; Seismic network; ShakeMap; Site effects; Strong motion
• ISSN: 1474-7065; 1873-5193
• DOI: 10.1016/j.pce.2016.07.005

Hazard assessment at the city scale requires a detailed characterization of the effect of surface geology on ground motion (site effects). Though this analysis is commonly achieved using geophysical site characterization and site response modelling, we propose here a complementary analysis based on amplification functions retrieved from Empirical Spectral Modelling (ESM) of earthquake recordings. We applied this method to the city of Basel (Switzerland) that benefits from a detailed microzonation and a dense Strong Motion Network with 21 modern free-field stations. We first verified the accuracy of ESM amplification functions for this region and used them to determine the bedrock interface at a site with a detailed velocity profile. While the interface between Upper and Lower Tertiary was, until now, considered responsible for the fundamental frequency of resonance in the Rhine Graben, we found that the bedrock interface in fact lies at the Mesozoic limestone. We also investigated the second peak of the H/V ratios that is clustered in a particular area of the basin where amplification is found to be different. We successfully used the ESM amplification functions to verify the microzonation of 2006 and would strongly advise the installation of strong motion stations where such studies are performed in the future. Outside the Rhine Graben, where shallow sediments are found, we propose an amplification functional form based on ESM and the fundamental frequency of resonance. Finally, we combined all our findings and generated amplification maps of the response spectrum at any period of interest for earthquake engineering. This map is proposed for a high resolution real-time implementation in ShakeMap and will be used for seismic loss assessment. [Display omitted] •Site amplification is extracted from earthquake recordings in Basel.•New method to derive amplification maps from observations is proposed.•Strong-motion stations should be installed to verify a microzonation study.•The high-resolution map is used in ShakeMap to retrieve Intensity shortly after an earthquake.