Through Thick and Thin: The Effect of Sediment Deposits on Central US Ground-motion Predictions

Local geology and the presence of soils affects the frequency content, amplitude, and duration of ground shaking, and thus also seismic hazard estimates (both scenario and probabilistic). As part of the U.S. Geological Survey's (USGS) efforts to produce state-of-the-art seismic hazard maps in the central US (CUS), techniques have been developed for estimating site amplification distributions from local and regional 3D geological and geophysicsical observations (Gomberg et al., 2003; Cramer, 2003). The estimation of site amplification distributions as a function of input rock ground motion has been completed for the Memphis, TN urban area and is underway for the upper Mississippi embayment. These distributions are used to incorporate the effect of site geology into the USGS national seismic hazard maps for these regions. They also are being incorporated into CUS ShakeMaps and to analyses of seismograms at ANSS sites. Overall uncertainty and sensitivity of site amplification estimates to input data are also being investigated. For the 1 km thick sediments in the Memphis area, the overall standard deviation (sd) varies from 28% to 52%, depending on the input peak ground acceleration (pga) (0.05 - 1.0g). The largest contribution comes from the ground motion variability in the 16 M7-8 seismograms at rock sites that are scaled and used as input ground motions at the base of the soil column (max. sd of 36%). The next largest contributor is the uncertainty in the soil profiles (mostly in shear-wave velocity) derived from a 3D geological model (max. sd of 27%). The third largest contributor is the uncertainty in the dynamic properties of the soils (max. sd of 17%). The effect of equivalent-linear versus non-linear estimates of soil amplification at high frequencies is also being investigated.