THE INFLUENCE OF SEDIMENTARY BASINS ON GROUND SHAKING IN THE PUGET LOWLAND, WASHINGTON STATE

and the SHIPS Working Group

In the past five years, as part of the Seismic Hazard Investigations of Puget Sound (SHIPS) project, we have conducted two major crustal tomography experiments and two site response studies. Spectral ratios computed for 6 major teleseisms and 12 local earthquakes recorded during our experiments provide insight into the amplification of weak ground motions by the Everett, Seattle and Tacoma basins. Analyses of the shear-wave arrivals from teleseisms in the 0.1 to 1 Hz band (10 to 1 sec periods) consistently show large amplifications of seismic waves in the 0.2 to 0.7 Hz band by the Seattle basin. These amplifications reach a factor of 10 near the center of the basin. Potential causes of this amplification include resonance within the shallow (<1 km) basin sediments, focusing, and locally-induced basin surface waves. At higher frequencies (1 to 20 Hz), sites on all three basins consistently show amplification peaks at 1 to 5 Hz, and steadily decreasing amplification with increasing frequency. In contrast, spectral ratios at sites outside of the basins show a variety of shapes. Above about 10 Hz, the basins cause a net decrease in amplitude compared to bedrock sites. The amplitudes of the resonance peaks increase southward, with the amplification in the Everett basin being the lowest and the amplification the Tacoma basin being the highest. We can only speculate as to the cause of these differences, but possibilities include greater compaction of basin sediments and higher shear-wave velocities in the north because of thicker ice during the last glaciation, or changes in the thickness of the glacial deposits causing changes in the resonance frequencies. Attenuation (Q) within the Seattle basin has been estimated from refraction data through inversions of amplitudes in refraction data and an inversion of t* values estimated from the spectral slopes. Results for frequencies of 2 to 30 Hz show Qp ranging from 50 to 500 at shallow depths to values of 100 to 1000 near the bottom of the basin (8 km depth). S-wave attenuation factors (Qs) for the 2 to 8 Hz frequency range are 25 to 200 near the surface and 100 to 380 near the bottom of the basin.