Ratios of vertical-to-horizontal (V/H) ground motion are important for the computation of scenario-compatible vertical design spectra. They are therefore a crucial aspect of seismic hazard analysis. We characterize the V/H ratio at rock sites in terms of the recording site’s average quarter-wavelength velocity. A predictive equation is presented, which can be used for reconstructing the expected V/H ratio of the 5%-damped response and Fourier spectra for rock sites, given a known shear-wave velocity profile. The equation is based on the regression analysis of two datasets: one from Switzerland and one from Japan. The two datasets allow us to analyze well-characterized hard-rock sites in Switzerland using small earthquakes (Mw>2), while the magnitude range is increased up to events of Mw 7.3 using strong-motion recordings from Japan’s KiK-net seismic network. It is shown that a correlation exists between the quarter-wavelength velocity at a given frequency and the V/H ground-motion ratio. Small differences, possibly due to velocity measurement bias or topographical and basin effects, existed when analyzing the individual regional datasets. Apart from near-source distances (R<30 km) for response spectra V/H, no clear magnitude- or distance-dependence of the V/H ratios is present in either of the datasets of earthquake recordings. We show that the same correlation exists for response spectra and Fourier spectra of recorded ground motion. A similar correlation was derived for ambient vibrations recorded at Swiss station sites, resulting in larger values of V/H at a given frequency and thereby indicating that V/H is sensitive to the composition of the wave field. Finally, uncertainties in the V/H models are presented. We separate the sources of uncertainty into source and site-specific components and present the single-site sigma uncertainty measure for site-specific hazard analyses.

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