Site‐response high‐frequency frontiers and the added value of site‐specific earthquake record‐based measurements of velocity and attenuation

One‐dimensional (1D) ground‐response analysis based on 1D velocity profiles and generic relationships between damping and velocity is frequently employed under the assumption that it can provide a reliable assessment of site effects. According to this hypothesis, epistemic uncertainty in site response primarily stems from inaccurately determined soil parameters. As shown by the recent development of ground‐motion models in the Fourier domain, the underlying site‐to‐site variability of site response rises sharply at intermediate to high frequencies (>3 Hz), and this increase cannot be compensated for when relying on coarse velocity profiles and generic relationships between damping and velocity. In order to solve this high‐frequency frontier in site response, we invert the spectral ratio of the horizontal‐to‐vertical components of strong motions based on the diffuse field concept for obtaining refined 1D velocity profiles and site‐specific damping profiles. These updated equivalent structures of both velocity and damping significantly improve the performance of 1D ground‐response analyses. This effect is particularly strong at intermediate frequencies (3–10 Hz), where both the deep sedimentary structure and the damping profile above the seismological bedrock have a strong influence on site response. While the use of refined 1D velocity and generic damping profiles for test sites of the Japanese KiK‐net already provides a reduction of the intermediate‐to‐high‐frequency residual and corrected site‐to‐site variability (and then an increase in the precision of predictions), the inclusion of site‐specific damping allows this value almost to be halved. The results show that many sites are indeed too complex to be modeled by classical 1D ground‐response analysis relying on generic damping relationships, while only site‐specific record‐based models allow a significant reduction in the bias.