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U.S. Geological Survey
Denver Federal Center
Box 25046 MS 966
Denver, Colorado 80225
(S.H., A.L., A.F., R.A.W., J.O., W.S., W.S.)
Pacific Engineering and Analysis
311 Pomona Avenue
El Cerrito, California 94530
(W.S.)
The Seattle fault poses a significant seismic hazard to the city of
Seattle, Washington. A hybrid, low-frequency, high-frequency method is used to
calculate broadband (020 Hz) ground-motion time histories for a M 6.5
earthquake on the Seattle fault. Low frequencies (<1 Hz) are calculated
using a deterministic approach and finite difference in a 3D velocity model.
High frequencies (>1 Hz) are calculated by a stochastic method that uses a
fractal subevent size distribution to give an
-2
displacement spectrum. Time histories are calculated for a grid of stations
and then corrected for the local site response using a classification scheme
based on the surficial geology. Average shear-wave velocity profiles are
developed for six surficial geologic units: artificial fill, modified land,
Esperance sand, Lawton clay, till, and Tertiary sandstone. These profiles
together with other soil parameters are used to compare linear,
equivalent-linear, and nonlinear predictions of ground motion in the frequency
band 015 Hz. Linear site-response corrections are found to yield
unreasonably large ground motions. Equivalent-linear and nonlinear
calculations give peak values similar to the 1994 Northridge, California,
earthquake and those predicted by regression relationships. Ground-motion
variance is estimated for (1) randomization of the velocity profiles, (2)
variation in source parameters, and (3) choice of nonlinear model. Within the
limits of the models tested, the results are found to be most sensitive to the
nonlinear model and soil parameters, notably the overconsolidation ratio.
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