Abstract

A predictive equation to obtain the vertical‐to‐horizontal ratio (V/H) of ground motion for rock sites has been established in a previous article. The method was based on the comparison between V/H of Fourier and response spectra of earthquakes with the quarter‐wavelength average velocity at discrete frequencies. We extend this approach to account for resonance phenomena at soft‐sediment sites. In order to do so, a new parameter is defined and included in the comparison with the V/H spectra. The new parameter is directly derived from the quarter‐wavelength velocity and represents the frequency‐dependent seismic impedance contrast at the site. We show that extending the correlation in this three‐dimensional space is beneficial to reconstruct V/H of the 5%‐damped response spectra at soft‐sediment sites (VS30<800  m/s) for which a shear‐wave velocity profile is available. In this study we analyze 220 sites of the Japanese KiK‐net strong‐motion network. These sites were selected from the entire network through comparison of the fundamental frequencies estimated from the recordings and by indirect modeling methods. From the analysis, two types of predictive equations are then established, the first based on frequency‐dependent and the second on frequency‐independent correlations. These can subsequently be used to reconstruct the V/H spectrum at any site with a known shear‐wave velocity profile. For both equations, uncertainties of the V/H models are provided, and a sensitivity study to magnitude–distance dependence is presented. Finally, we show an example of the application of the model at four selected soft‐sediment sites of the Swiss Seismic Network.

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