Should average shear-wave velocity in the top 30 m of soil be used to describe seismic amplification?

The average velocity of shear waves in the top 30 m of soil, νL, has become the parameter used by many engineering design codes and most recently by published empirical-scaling equations to estimate the amplitudes of strong ground motion. Yet there are few studies to determine whether this is a meaningful parameter to use—and whether estimates that do use it are reliable. In 1995, the authors studied this problem and concluded that νL should not be used. We reported then that an older site characterization in terms of the soil site parameter proposed by Seed et al. [1], sL, worked better because it included a measure of the thickness of the soil layers to considerably greater depths. Our report, however, made no difference; numerous papers continued to be published based only on scaling in terms of νL, and worse, they also ignored the geological site conditions. The purpose of this paper is to emphasize that the average shear-wave velocity in the top 30 m of soil should not be the only site parameter used to scale strong-motion amplitudes. While the search continues for the more meaningful site parameters to use in empirical scaling of strong earthquake ground motion, it is better to use sL to describe the amplification of seismic waves by soil deposits near the surface.