Following the initial S-wave are several pulses of ground motion which are trapped in the top 2 km or so of the Seattle basin. This is surface wave energy which is trapped by the relatively unconsolidated deposits near the surface. This trapped energy causes the long-period ground motions in Seattle to have a long duration of shaking of up to about 25 sec. 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.