Incorporating the Effects of Site Geology in CEUS Hazard Maps

The effects of local geology, including uncertainty, have been incorporated into several central and eastern U.S. seismic hazard products. The approach is to generate site amplification distributions on a grid for a given period using three dimensional geological/geophysical/geotechnical models (urban hazard maps) or reference profiles keyed to surface geology and depth-to-bedrock (regional hazard maps). A derived shear-wave velocity profile for a grid point is randomized along with dynamic soil properties and the choice of input ground motion to represent model uncertainties. A Monte Carlo approach is used to estimate the lognormal mean and standard deviation of the site amplification distribution. Probabilistic seismic hazard maps use the full site amplification distribution and scenario (deterministic) maps use the appropriate fractile site amplification (median or 84^th percentile). Computational efficiency and soil response program calibration are important aspects of this procedure.