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Attenuation relationships were used to predict both PGA and spectral acceleration response
content of each type of earthquake. Once the PGA and spectral response of the ground motions
were determined, time history records of like events were selected for use in the dynamic ground
response analysis. An effort was made to match distance, magnitude, spectral content and PGA
for all input time histories with those of the predicted values, though the inherent variability in
earthquakes motions do not allow for complete agreement with the smoothed spectra provided
from the regression analyses on which the attenuation relationships are based.
8.4.1 Summary of Recent Seismic Hazard Investigations
Comparisons were made between the recommendations of the PGA on soil and rock from the
following sources: (1) the Geomatrix (1995) report to the ODOT titled, Seismic Design Mapping
of the State of Oregon, (2) the USGS’s National Seismic Mapping Project (USGS 2000), and (3)
the URS Greiner/Woodward Clyde Portland Microzonation Maps (Wong et al. 2000). These
recommendations are based on probabilistic, uniform hazard studies that combined the ground
shaking contributions from the interplate, intraplate, and crustal earthquake scenarios,
respectively, into one peak ground acceleration value for a given return period (Table 8.3). It
should be noted that investigations by Geomatrix and the USGS provide recommendations for
PGA on bedrock. The URS Greiner/Woodward Clyde report yields PGA values at the soil
surface. The recommendations put forth in the URS Greiner/Woodward Clyde report,
specifically the Portland area fault map shown in Figure 8.3, were incorporated into this
demonstration application for the sake of completeness.
Table 8.3: Comparison of Recommended PGA Values for the Site
GROUND MOTIONS ASSOCIATED WITH SPECIFIED
PROBABILITIES OF EXCEEDANCE
PGA ROCK
PGA soil
SOURCE 10% in 50 Years 2% in 50 Years 10% in 50 Years 2% in 50 Years
USGS 0.19g 0.38g N/A N/A
Geomatrix 0.19g 0.37g N/A N/A
URS Greiner /
Woodward Clyde
N/A N/A 0.20g – 0.25g 0.40g
The peak ground accelerations provided by the seismic hazard studies referenced here reflect the
contributions of all seismic sources in the region. These investigations yield uniform hazard data
in that the value of peak horizontal acceleration for a given exposure time represents the
combined, or “aggregate” hazard. The acceleration values cannot be directly attributed to a
single source zone or single earthquake of given magnitude. For many seismic analyses in
geotechnical and structural engineering, the earthquake magnitude is requisite input data. This is
particularly important for analysis methods that incorporate the duration of ground motions, the
number of significant cycles of shaking, or seismic energy. Soil liquefaction can be thought of as
a fatigue failure of soils; therefore, the intensity of the shaking and number of loading cycles are
necessary parameters in hazard evaluation. In order to use the acceleration values from hazard
studies a de-aggregation of the hazard associated with individual seismic sources is required, to
estimate the relative contribution of each seismic source and size of earthquake. This information
is available for the seismic hazard studies presented by Geomatrix and the USGS.
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