Empirical Estimation of High‐Frequency Ground Motion on Hard Rock

Site effects for hard-rock sites are typically computed using analytical models for the effect of κ0, the high-frequency attenuation parameter. New datasets that are richer in hard-rock recordings allow us to evaluate the scaling for hard-rock sites (e.g., VS30 > 1500 m=s). The high-frequency response spectra residuals are weakly correlated with κ0, in contrast to the strong scaling with κ0 in the analytical models. This may be due to site-specific shallow resonance patterns masking part of the effect of attenuation due to damping. An empirical model is developed for the combined VS30 and κ0 scaling for hard-rock sites relative to a reference site condition of 760 m=s (i.e., correction factors that should be used for going from soft rock to hard rock, taking into account the net effect of VS and κ0). This empirical model shows high-frequency amplification that is more similar to the analytical prediction corresponding to a hard-rock κ0 of 0.020 s rather than the typical value of 0.006 s, which is commonly used for hard-rock sites in the central–eastern United States. Compared to the current analytical approach, this leads to a reduction of high-frequency (>20 Hz) scaling of about a factor of 2.