Site-Effects Model for Central and Eastern North America Based on Peak Frequency and Average Shear-Wave Velocity

We develop an empirical site-amplification model for central and eastern North America (CENA) that is parameterized using two site variables: (1) the peak site frequency (f(peak)) and (2) the time-averaged shear-wave velocity in the upper 30 m (V-S30). The model is developed by first deriving empirical site-amplification terms for seismographic sites in CENA. This is done by analyzing the residuals calculated from ground-motion amplitude data with respect to a selected regional ground-motion prediction equation that applies to hard-rock reference site conditions. An amplification model is then developed that explains the residuals in terms of the two site variables (f(eak) and V-S30). We consider two alternative cases. In the first case, we assume that V-S30 is the main modeling parameter and then model any remaining residual effects in terms of f(peak). In this case, we obtain a frequency-dependent V-S30 scaling term that is similar in form to that obtained in previous studies for sites in western North America (WNA). However, the scaling term is less significant in amplitude for CENA in comparison to that for WNA, suggesting that V-S30 is not as strongly indicative of site response. For the second alternative, assuming that f(peak) is the main site-effects parameter, a frequency-independent V-S30 scaling term is obtained for CENA, which is much smaller in amplitude compared to the V-S30 scaling effect derived in the first approach. This shows that, using f(peak) as the primary site-effects modeling parameter, we remove most of the V-S30 scaling effects that are implied by the data. Finally, we provide recommendations on the effective use of f(peak) and V-S30 to model site effects in CENA, differentiating between glaciated and nonglaciated sites. Glaciated sites show larger amplifications compared to nonglaciated sites, especially at intermediate-to-high frequencies, presumably due to the strong impedance contrast at the base of glaciated sites.