Seismic amplification within the Seattle Basin, Washington State: Insights from SHIPS seismic tomography experiments

Abstract

Recent observations indicate that the Seattle sedimentary basin, underlying Seattle and other urban centers in the Puget Lowland, Washington, amplifies long-period (1-5 sec) weak ground motions by factors of 10 or more. We computed east-trending P- and S-wave velocity models across the Seattle basin from Seismic Hazard Investigations of Puget Sound (SHIPS) experiments to better characterize the seismic hazard the basin poses. The 3D tomographic models, which resolve features to a depth of 10 km, for the first time define the P- and S-wave velocity structure of the eastern end of the basin. The basin, which contains sedimentary rocks of Eocene to Holocene, is broadly symmetric in east-west section and reaches a maximum thickness of 6 km along our profile beneath north Seattle. A comparison of our velocity model with coincident amplification curves for weak ground motions produced by the 1999 Chi-Chi earthquake suggests that the distribution of Quaternary deposits and reduced velocity gradients in the upper part of the basement east of Seattle have significance in forecasting variations in seismic-wave amplification across the basin. Specifically, eastward increases in the amplification of 0.2- to 5-Hz energy correlate with locally thicker unconsolidated deposits and a change from Crescent Formation basement to pre-Tertiary Cascadia basement. These models define the extent of the Seattle basin, the Seattle fault, and the geometry of the basement contact, giving insight into the tectonic evolution of the Seattle basin and its influence on ground shaking.