V S 30 Estimation from Shallow VS Profiles

ABSTRACT The time-averaged shear-wave velocity to a 30 m depth from the ground surface (VS30) is a critical parameter for site classification in seismic hazard analyses and building codes. Boore (2004) proposed a method to extrapolate VS30 from shallow velocity profiles (depths <30 m) using correlations between shallow average shear-wave velocities VSd and VS30, based on 135 California boreholes. Here, I expand this approach using over 2400 profiles from the VS Profile Database, enabling analysis of regional biases (e.g., by states in the U.S. and sedimentary basins) and methodological biases (e.g., borehole logging versus surface-wave dispersion). The expanded approach provides a minor improvement to the predictive accuracy of the original Boore (2004) method. I find that constant-velocity extrapolation underestimates VS30, while correlation-based methods reduce bias but vary by measurement method. Comparisons of colocated profiles (<5 m apart) do not show statistically significant differences in the VS30 between the velocity measurement methods. These findings are the basis to improve VS30 extrapolation accuracy from shallow velocity profiles.