Seismic Hazard Maps for Seattle, Washington Incorporating 3D Sedimentary Basin Effects, Nonlinear Site Response, and Rupture Directivity - 5 % probability of exceedance in 50 years.
It is clear from recent seismological research that sedimentary basins strongly affect the amplitude and duration of earthquake ground motions and are responsible for certain damage patterns caused by earthquakes (e.g., Graves et al. 1998; Stephenson et al., 2006). Furthermore, it has been recognized for many decades that shallow geological units such as artificial fill amplify ground shaking and that the direction of rupture propagation on a fault (directivity; e.g. Wald et al., 1996) is an important cause of large-velocity ground-motion pulses that can damage buildings. As seismologists develop a predictive understanding of these phenomena, they can be quantitatively incorporated into probabilistic assessments of seismic hazard.
In this paper we include these effects in seismic hazard maps for Seattle, which we call urban seismic hazard maps. Urban seismic hazard maps provide more spatially-detailed information about seismic hazard than the national seismic hazard maps, which use a firm-rock site condition and ground-motion relations that don't explicitly include rupture directivity. However, these urban seismic hazard maps are not site-specific maps. They are based on models of sub-surface structure that are often interpolated from sub-surface data several hundreds of meters from a given site, as opposed to a site-specific study where a shear-wave velocity profile is measured at that particular location.
The Seattle maps presented in this document use the same set of faults and distributed-earthquake sources as in the 2002 national seismic hazard maps (Frankel et al., 2002a). The new maps include the effects of the Seattle Basin, the young alluvium and artificial fill, and rupture directivity on the Seattle fault. A summary of the methodology and preliminary maps were presented in Frankel et al. (2006).