Variability of site response in seattle, Washington

Ground motion from local earthquakes and the SHIPS (Seismic Hazards Investi gation in Puget Sound) experiment is used to estimate site amplification fa ctors in Seattle. Earthquake and SHIPS records are analyzed by two methods: (1) spectral ratios relative to a nearby site on Tertiary sandstone, and ( 2) a source/site spectral inversion technique. Our results show site amplif ications between 3 and 4 below 5 Hz for West Seattle relative to Tertiary r ock. These values are approximately 30% lower than amplification in the Duw amish Valley on artificial fill, but significantly higher than the calculat ed range of 2 to 2.5 below 5 Hz for the til-covered hills east of downtown Seattle. Although spectral amplitudes are only 30% higher in the Duwamish V alley compared to West Seattle, the duration of long-period ground motion i s significantly greater on the artificial fill sites. Using a three-dimensi onal displacement response spectrum measure that includes the effects of gr ound-motion duration, values in the Duwamish Valley are 2 to 3 times greate r than West Seattle. These calculations and estimates of site response as a function of receiver azimuth point out the importance of trapped surface-w ave energy within the shallow, low-velocity, sedimentary layers of the Duwa mish Valley. One-dimensional velocity models yield spectral amplification f actors close to the observations for till sites east of downtown Seattle an d the Duwamish Valley, but underpredict amplifications by a factor of 2 in West Seattle. A two-dimensional finite-difference model does equally well f or the till sites and the Duwamish Valley and also yields duration estimate s consistent with the observations for the Duwamish Valley. The two-dimensi onal model, however, still underpredicts amplification in West Seattle by u p to a factor of 2. This discrepancy is attributed to 3D effects, including basin-edge-induced surface waves and basin-geometry-focusing effects, caus ed by the proximity of the Seattle thrust fault and the sediment-filled Sea ttle basin.