SHIPS - Ground Motions

SHIPS helps forecast future ground motions

SHIPS recordings will help us map and forecast the pattern of ground shaking during future earthquakes in the Puget Lowland area. The Seattle sedimentary basin beneath north Seattle and the glacial deposits that cover most of the Puget Lowland will either strengthen or weaken the ground shaking depending on the frequency of the shaking [Pratt et al., 2003; Barberopoulou et al., 2004, in press; Li et al., in press; and Pratt and Brocher, in press]. The results of these studies can be used by structural engineers to better design earthquake-resistant structures. Ten story buildings tend to have a natural frequency of 1 Hz, whereas 30 story buildings have a natural frequency of about 0.3 Hz.

Seattle sedimentary basin ground shaking

The maps above show how the Seattle sedimentary basin and shallower deposits affect ground shaking at different frequencies relative to bedrock sites. The accompanying graphs (below) show how the strength of ground shaking changes from west to east across the Puget Lowland. Each dot on the map is a SHIPS site, with the diameter of the dot being proportional to the strength of ground shaking at that site. At low frequencies appropriate for the largest structures (0.1 and 0.3 Hz), the basin amplifies the ground motions by factors of 6 to 12, as also shown on the graph for 0.3 Hz. The maps show that some amplification occurs outside of the basin, suggesting that a large part of the wave amplification is caused by the glacial deposits that extend beyond the basin margins.

amplification changes with longitude and frequency

At higher frequencies appropriate for smaller structures such as one to three story buildings (3 and 7 Hz), the Seattle basin appears to have little effect on ground shaking, and may actually decrease the strength of the high-frequency shaking (7 Hz). Note that the strongest shaking at these high frequencies occurs in the Duwamish River valley (DR) just south of downtown Seattle and in the Snoqualmie River valley (SR) just east of the Seattle basin. These two areas are covered by weak, unconsolidated deposits that strengthen and prolong the high-frequency seismic waves.