We describe a technique to derive first-order site-condition maps directly from topographic data. For calibration, we use global 30 arc sec topographic data and VS30 measurements (here VS30 refers to the average shear-velocity down to 30 m) aggregated from several studies in the United States, as well as in Taiwan, Italy, and Australia. VS30 values are correlated against topographic slope to develop two sets of parameters for deriving VS30: one for active tectonic regions where topographic relief is high, and one for stable shields where topography is more subdued. By taking the gradient of the topography and choosing ranges of slope that maximize the correlation with shallow shear-velocity observations, we can recover, to first order, many of the spatially varying features of site-condition maps developed for California. Our site-condition map for the low-relief Mississippi Embayment also predicts the bulk of the VS30 observations in that region despite rather low slope ranges.
We find that maps derived from the slope of the topography are often well correlated with other independently derived, regional-scale site-condition maps, but the latter maps vary in quality and continuity, and subsequently, also in their ability to match observed VS30 measurements contained therein. Alternatively, the slope-based method provides a simple approach to uniform site-condition mapping.
After validating this approach in regions with numerous VS30 observations, we subsequently estimate and map site conditions for the entire continental United States using the respective slope correlations.