We validate a model for the velocity structure of basin sediments in southern California against sonic log data and evaluate the bias and uncertainty of basin depth terms. Basin depth, as measured by depth to the 1.5 km/sec shear-wave isosurface, is underpredicted near the basin margins where the basin is relatively shallow and overpredicted near the middle of the basin. The standard deviation of the residuals is approximately 410 m when the bias trends are not considered and approximately 240 m when they are considered. The effects of the bias on estimated ground motions are likely small because the available empirical models utilized this same velocity model, and hence inherently incorporate the bias effects. The effect of uncertainty in basin depths can introduce uncertainty in ground motions of up to 0.1 (in natural log units), which is small compared to typical error terms in attenuation relations.