Site-specific seismic-hazard maps and deaggregation in the western United States using the NGA models for ground-motion prediction

The 2008 National Seismic Hazard Mapping Project (NSHMP) update for the conterminous United States employs several new ground-motion prediction equations which include modern empirical models of linear and nonlinear site response to local and regional earthquakes. The recent availability of attenuation functions incorporating site conditions via Vs30 values permits the calculation of site-specific hazard maps for a wide range of spectral accelerations. I compare alternative site specific hazard maps using Vs30 values estimated according to the methods of Wills and Clahan (2006), Wald and Allen (2007), and Yong and others (in press). These maps are presented for 5-hertz (Hz) and 3-second spectral accelerations having 2 percent probability of exceedance in 50 years for central California and the western part of southern California.
Because these attenuations incorporate nonlinear site response for the larger ground motions, the site-specific probabilistic ground motions for the western United States can show either increases or decreases with respect to the firm-rock site condition. Furthermore, the ground motions on soil can be different from those that are predicted by applying National Earthquake Hazard Reduction Program recommendations for adjusting rock values to account for the soil column. One finding of this investigation is that at high spectral frequencies, strong differences in the site's Vs30 estimates often result in relatively small differences in probabilistic ground motion in western California or other tectonically active regions.
In addition, this report shows how incorporating geologic site condition information alters the values of the dominating magnitudes and distances in deaggregation-5-Hz values for a site near San Quentin, Calif., and 5-Hz and 1-Hz values for Harbor Island near Seattle, Wash. These deaggregations show that the modal event can shift from a larger closer source to a more distant, perhaps smaller source when nonlinear soil behavior is explicitly included in the hazard integral. The potential shift in the mode when considering the soil column's effect ought to be carefully considered by engineers who select scenario events based in part on the distribution in magnitude, distance, and epsilon space.