Regional Correlations of VS30 and Velocities Averaged Over Depths Less Than and Greater Than 30 Meters

Using velocity profiles from sites in Japan, California, Turkey, and Europe, we find that the time-averaged shear-wave velocity to 30 m (V-S30), 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 (V-Sz, with z being the averaging depth). The correlations for sites in Japan (corresponding to the KiK-net network) show that V-S30 is systematically larger for a given V-Sz 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 V-S30 to V-Sz for z 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 V-S30 from V-Sz 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 log V-S30 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 V-S30 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 V-S30 is correlated with V-Sz for z as great as 400 m for sites of the KiK-net network, providing some justification for using V-S30 as a site-response variable for predicting ground motions at periods for which the wavelengths far exceed 30 m.