Ground motion is effected by near surface soils, deep geologic structure such as sedimentary basins, and the mechanics of the earthquake its self. The NSHMP does a good job estimating the level and frequency of shaking at a given site but does not account for many conditions that impact ground motion in a region. More sophisticated modeling of all these effects goes into the production of Urban Seismic Hazard Maps where they exist.
This Seattle Urban Seismic Hazard Map shows shaking levels at 1 hertz fequency that have a 10% of being exceeded in the next 50 years. A high reolution version and other related maps are available from the USGS.