ALBERT

Description: The definition of a reference bedrock condition representative of a region of interest is of great significance in seismic-hazard assessment. It is highly beneficial when ground-motion prediction equations are referenced to a specific site condition, particularly in the case of site-specific seismic-hazard analyses. When known, the effect of any given site with respect to the reference can then be applied to the predicted ground motion. However, the choice of a reference velocity profile is not straightforward, mainly due to the high variability of the velocity structure in the shallower layers. A new method to define the regional reference rock profile is proposed. The method relates quarter-wavelength average velocity at a site to frequency-dependent amplification. A reference bedrock velocity profile can then be directly defined in relation to expected amplification characteristics over a number of sites. We compare 27 quarter-wavelength velocity profiles from seismic station locations in Switzerland with empirical amplification functions derived from spectral modeling. From this comparison, a set of frequency-dependent calibration relationships is established. Assuming that the reference profile is defined by a lack of any relative amplification, the quarter-wavelength velocity profile that corresponds to unitary spectral amplification can be extracted. The reference velocity profile can then be obtained through an inversion procedure and defines the reference for the ground-motion prediction equation (GMPE). The proposed reference velocity profile is compared with previous reference velocity profiles. A good agreement is found between the different methods. Additionally, an estimation of the transfer function for the Swiss reference rock condition is provided. This can be used to correct recorded or estimated spectral amplitudes for the local response of the reference site. Finally, it is shown that the coefficients from the aforementioned correlations can be used to estimate a generic amplification function at any site with a known quarter-wavelength velocity profile.

Description: 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