The long-period ground motions in the northern part of the area are dominated by these surface waves. Map view of Seattle region showing the coastlines. North is to the top of the map. Rectangle at south part of map is the surface projection of the portion of the Seattle fault which ruptures in the simulation. Now the colors are keyed to the intensity and direction of ground motion at the surface.
Blue colors represent ground motion to the north and red colors are ground motion to the south.
The intensity of the color corresponds to the amount of the ground motion. This is the S-wave from the earthquake. Then we see successive waves of ground motions which begin in the southern part of the Seattle basin and propagate to the north. These waves are the surface waves described in the vertical cross section. This energy is trapped by the unconsolidated material in the shallow part of the Seattle basin.