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have been recently developed (Wong et al. 2000). The three main faults around Portland
are the Oatfield, Portland Hills, and East Bank Faults (shown in Figure 8.3). The East
Bank Fault was used as the design fault in this study because of its close proximity to the
site (8 km).
8.2.2.4 Probabilistic Assessment of Bedrock Ground Motions
Probabilistic uniform hazard studies have been completed for the Portland area in recent
years that provide estimates of the peak ground acceleration values on rock (Geomatrix
1995; USGS 1999). These studies incorporate the relative contributions of the three
seismic sources in the Portland area to develop one peak ground acceleration value for
specified probabilities of exceedance in given time periods. Typically, values are
estimated for earthquakes with a 10% probability of exceedance in 50 years and a 2%
probability of exceedance in 50 years, which correspond to return periods of 475 and
2475 years, respectively. Figure 8.4 provides the PGA contours on bedrock with a 475-
year return period as estimated by Geomatrix (1995).
SITE LOCATION
Figure 8.3: Portland Area Faults (Wong et al. 2000)
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