A Next Generation VS30 Map for California Based on Geology and Topography

Correlations between geologic units and shear-wave velocity form the basis of a series of maps developed over the past 15 years to estimate the time-averaged shear-wave velocity in the upper 30 m (V-S30). The Wills et al. (2000) site-condition map for California was found to correlate with seismic amplification (Field, 2000) and was adopted as a standard depiction for many applications of seismic shaking estimates (ShakeMap, for example). Wills and Clahan (2006) modified that map to show simplified geologic units and corresponding V-S30 values. Preparation of this map raised a number of questions on how best to distinguish units within younger alluvium. Wills and Gutierrez (2011) found that a simple system based on surface slope could be used to subdivide the younger alluvium into three classes that have distinct V-S30 ranges. The classes defined by slope have approximately the same variability in V-S30 as the previously defined classes, but the total number of classes is reduced and the system can be easily applied to other tectonically active areas. We have now applied the system of Wills and Gutierrez (2011) to create a new map of California using the most detailed available geologic maps. Use of more detailed geologic maps, from 1: 250,000 scale to 1: 24,000 for much of California, results in a much more detailed and accurate depiction of the surficial geology and, we anticipate, a more detailed and accurate depiction of seismic amplification due to the near-surface materials.