Seismic Hazard Maps for Seattle, Washington, Incorporating 3D Sedimentary Basin Effects, Nonlinear Site Response, and Rupture
Directivity
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Resource Abstract:
This report presents probabilistic seismic hazard maps for Seattle, Washington, based on over 500 3D simulations of ground
motions from scenario earthquakes. These maps include 3D sedimentary basin effects and rupture directivity. Nonlinear site
response for soft-soil sites of fill and alluvium was also applied in the maps. The report describes the methodology for incorporating
source and site dependent amplification factors into a probabilistic seismic hazard calculation. 3D simulations were conducted
for the various earthquake sources that can affect Seattle: Seattle fault zone, Cascadia subduction zone, South Whidbey Island
fault, and background shallow and deep earthquakes. The maps presented in this document used essentially the same set of faults
and distributed-earthquake sources as in the 2002 national seismic hazard maps. The 3D velocity model utilized in the simulations
was validated by modeling the amplitudes and waveforms of observed seismograms from five earthquakes in the region, including
the 2001 M6.8 Nisqually earthquake. The probabilistic seismic hazard maps presented here depict 1 Hz response spectral accelerations
with 10%, 5%, and 2% probabilities of exceedance in 50 years. The maps are based on determinations of seismic hazard for 7236
sites with a spacing of 280 m. The maps show that the most hazardous locations for this frequency band (around 1 Hz) are soft-soil
sites (fill and alluvium) within the Seattle basin and along the inferred trace of the frontal fault of the Seattle fault
zone. The next highest hazard is typically found for soft-soil sites in the Duwamish Valley south of the Seattle basin. In
general, stiff-soil sites in the Seattle basin exhibit higher hazard than stiff-soil sites outside the basin. Sites with shallow
bedrock outside the Seattle basin have the lowest estimated hazard for this frequency band.
Citation
Title Seismic Hazard Maps for Seattle, Washington, Incorporating 3D Sedimentary Basin Effects, Nonlinear Site Response, and Rupture
Directivity