Estimating Vs(30) (or NEHRP Site Classes) from Shallow Velocity Models (Depths < 30 m)

The average velocity to 30 m [Vs(30)] is a widely used parameter for classifying sites to predict their potential to amplify seismic shaking. In many cases, however, models of shallow shear-wave velocities, from which Vs(30) can be computed, do not extend to 30 m. If the data for these cases are to be used, some method of extrapolating the velocities must be devised. Four methods for doing this are described here and are illustrated using data from 135 boreholes in California for which the velocity model extends to at least 30 m. Methods using correlations between shallow velocity and Vs(30) result in significantly less bias for shallow models than the simplest method of assuming that the lowermost velocity extends to 30 m. In addition, for all methods the percent of sites misclassified is generally less than 10% and falls to negligible values for velocity models extending to at least 25 m. Although the methods using correlations do a better job on average of estimating Vs(30), the simplest method will generally result in a lower value of Vs(30) and thus yield a more conservative estimate of ground motion [which generally increases as Vs(30) decreases].