|Description||With the relatively sparse number of direct VS30 measurements in Southern California,
researchers have created various proxies to estimate this predictor of ground motion.
Utilizing 400 publicly available VS30 site characterization measurements, the goal of this
study is to evaluate the data and spatial uncertainty of VS30 predictions. We calculated
VSZ for depths of 10, 30, 50 and 100 m as well as Z500, Z1000, Z1500 for the 82 new site
measurements presented in this study. For all 400 publicly available site
characterizations, we made predictions based on the Southern California Earthquake
Center Community Velocity Model Version 4.0 (Magistrale et al., 2000) and a
topographic-slope proxy developed by Wald & Allen (2007). The subsets of our data set,
"rock" (44 sites), "soil" (118) and "basin" (238) are based on the site class map of Wills et
al. (2000). The mean values of basin and soil subset predictions based on the SCEC
CVM are within 5% and 15% respectively when compared to measurement means. Less
than half of the soil site predictions are within ±20% of measured values. Less than half
of the predictions based on topographic slope are within ±20% of the measured values.
Through statistical analysis, a log normal distribution best fits the complete dataset.
VS30 and spatial standard deviation maps created from natural-log transformed data show
a spatial standard deviation or uncertainty of >0.30 natural-log units in the Los Angeles
Basin and >0.38 ln units outside the basin in areas where predictions are made. The
spatial uncertainty does not reflect the epistemic uncertainty associated with the
measured values. Unlike proxies based on geologic correlation of units, and topographic
slope proxies, hazard maps created from indicator kriging show NEHRP hazard class E is
likely in the Los Angeles Basin. More dense measurements are feasible and can readily
decrease the uncertainty of predictions.||