Using velocity profiles from sites in Japan, California, Turkey, and Europe, we find that the time-averaged shear-wave velocity to 30 m (m>Vm>m>Sm>30), used as a proxy for site amplification in recent ground-motion prediction equations (GMPEs) and building codes, is strongly correlated with average velocities to depths less than 30 m (m>Vm>m>Szm>, with m>zm> being the averaging depth). The correlations for sites in Japan (corresponding to the KiK-net network) show that m>Vm>m>Sm>30 is systematically larger for a given m>Vm>m>Szm> than for profiles from the other regions. The difference largely results from the placement of the KiK-net station locations on rock and rocklike sites, whereas stations in the other regions are generally placed in urban areas underlain by sediments. Using the KiK-net velocity profiles, we provide equations relating m>Vm>m>Sm>30 to m>Vm>m>Szm> for m>zm> ranging from 5 to 29 m in 1-m increments. These equations (and those for California velocity profiles given in Boore, 2004b) can be used to estimate m>Vm>m>Sm>30 from m>Vm>m>Szm> for sites in which velocity profiles do not extend to 30 m. The scatter of the residuals decreases with depth, but, even for an averaging depth of 5 m, a variation in logm>Vm>m>Sm>30 of ±1 standard deviation maps into less than a 20% uncertainty in ground motions given by recent GMPEs at short periods. The sensitivity of the ground motions to m>Vm>m>Sm>30 uncertainty is considerably larger at long periods (but is less than a factor of 1.2 for averaging depths greater than about 20 m). We also find that m>Vm>m>Sm>30 is correlated with m>Vm>m>Szm> for m>zm> as great as 400 m for sites of the KiK-net network, providing some justification for using m>Vm>m>Sm>30 as a site-response variable for predicting ground motions at periods for which the wavelengths far exceed 30 m.
