Ground Motions on Competent Rock Sites Have Low Aleatory Variability

The aleatory variability of response spectral amplitudes about the median values specified in ground-motion models is significantly lower for competent rock sites in central and eastern North America (CENA), in comparison to commonly assumed values derived from soil sites. Competent CENA rock is defined as sites with near-surface shear-wave velocity >800 m/s having measured site response peaks no greater than a factor of 3 in amplitude (0.5–30 Hz). At frequencies ≤5 Hz, the within-event aleatory variability for competent rock is ∼0.4 ln units at moderate magnitudes, compared to values of 0.5–0.6 reported for soil sites. Variability on rock sites increases at higher frequencies, exceeding that for soil sites at >15 Hz. Single-station total aleatory variability for competent CENA rock sites is ∼0.5–0.6 ln units from 1 to 10 Hz.