North is to the left and south to the right. The black curved lines in the cross section represent the configuration of the Seattle basin. The upper curve is the top of the bedrock and the lower curve is the top of the basement rocks at the bottom of the Seattle basin. The Seattle fault is located in the lower right hand portion of the cross section, at about 8-11 km depth. The colors denote the ground velocity caused by the seismic waves that radiate from the fault, analogous to waves in a pond generated when a pebble is dropped.
Red values indicate the largest ground velocities.
This energy is concentrated in an area that includes downtown Seattle. This strong pulse is caused by the earthquake rupturing northward towards the downtown area and the focusing caused by the rigidity change with depth in the Seattle basin. Some of the S-wave energy is reflected downward by the surface. 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.