NEW MAP OF AVERAGE SHEAR WAVE VELOCITY IN THE UPPER 30M FOR ESTIMATING POTENTIAL SEISMIC AMPLIFICATION

Correlations between geologic units and shear-wave velocity form the basis of a series of maps to estimate the time-averaged shear-wave velocity in the upper 30 m (V_S30), a proxy for seismic amplification. A new map by Wills et al. (2015) shows simplified geologic units and corresponding V_S30 values and applies a system to subdivide the younger alluvium based on surface slope. Young alluvium with surface slopes of less than 0.5% includes areas previously mapped as fine-grained alluvium, deep alluvium of the Imperial Valley, and deep alluvium. The mean V_S30 measured in these areas is 228 m/s, similar to V_S30 of areas mapped as “fine alluvium” by Wills and Clahan (2006). The standard deviation of V_S30 in this unit is similar to that for “fine alluvium”, showing that even though the criteria for mapping this unit do not require any subsurface data, material in these areas is consistently low velocity. Young alluvium with surface slopes between 0.5% and 2% includes areas mostly mapped as “deep alluvium” by Wills and Clahan. Mean V_S30 and standard deviation for moderately sloped young alluvium is also similar to that for deep alluvium. Young alluvium with surface slopes greater than 2% includes areas mapped by Wills and Clahan as thin alluvium and coarse alluvium as well as some areas of deep alluvium. The slope-based sub-units of young alluvium can be applied beyond the area of subsurface data. Previously, Wills and Clahan (2006) had classified the young alluvium throughout the Mojave Desert as coarse alluvium. Using the slope-based subdivisions of young alluvium results in patterns of the three categories of young alluvium that appear to reflect real variation in average grain size of the deposits. This new map uses 1:100,000 scale geologic maps, which generally preserve the details from 1;24,000 scale source maps when available. A major resource for southern California was the compilation of Quaternary deposits in southern California by Bedrossian and others (CGS Special Report 217). Use of these more detailed geologic maps for much of California, compared to the original map which relied on 1:250,000 scale mapping, results in a much more detailed and accurate depiction of the surficial geology in the new map and, it is anticipated, a more detailed and accurate depiction of seismic amplification because of the near-surface materials.