Seismic Hazard Maps for Seattle, Washington, Incorporating 3D Sedimentary Basin Effects, Nonlinear Site Response, and Rupture Directivity

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.
[Summary provided by the USGS.]