INSIGHTS ON AMPLIFICATION IN THE SEATTLE BASIN FROM THE “DRY” AND “KINGDOME” SHIPS SEISMIC REFRACTION EXPERIMENTS

The SHIPS (Seismic Hazards Investigations of Puget Sound) experiments are part of a continuing effort to define the geometry of sedimentary basins and faults beneath the densely populated Puget Lowland of Washington State. Here, we present velocity model results from the “Dry” and “Kingdome” SHIPS experiments, and we discuss the implications for amplification of weak ground motions by the Seattle basin. Observations of weak ground motions indicate that the Seattle basin, underlying Seattle and other urban centers in the Puget Lowland, amplifies low frequency (0.2 to 0.7 Hz) arrivals by a factor of 10 or more. The velocity models, which are well resolved to a depth of about 10 km, for the first time, define the eastern extent of the Eocene to Holocene age Seattle basin. The models show that the basin is broadly symmetric in east-west direction and that it reaches a maximum thickness of 6 to 7 km beneath Seattle along the “Dry” SHIPS profile. P-wave 3-D tomographic results show that the basin is about 70 km wide and contains sedimentary strata with velocities ranging from 1.8 to 4.5 km/s. The contact with underlying basement rocks is characterized by a steep gradient in the velocity field from 4.5 to 5.0 km/s.

We have compared our velocity model for the Seattle basin with coincident amplification curves for weak ground motions produced by the Mw7.6 Chi-Chi, Taiwan, earthquake. This comparison suggests that the geometry of sub-basins of Quaternary deposits in the upper 1 km of the basin may influence the variation in amplification across the basin. Specifically, the Quaternary deposits appear to thin on the east side of the basin, where amplifications are largest and the amplification increases eastward from 0.2 to 0.7 Hz energy. Also, a reduction in the velocity gradients in the upper part of the basement rocks beneath Seattle may mark the transition between Crescent Formation rocks to the west and pre-Tertiary Cascade basement rocks to the east. This transition may focus seismic waves entering the basin from below, causing increased amplifications over parts of the east side of the basin, which may also correlate with the eastward increasing of amplification. Seismicity within the Seattle basin along the velocity model lines up with this inferred basement contact and coincides at the surface with the location of proposed strike-slip faults.