The Portland Hills Fault: An Earthquake Generator or Just Another Old Fault?

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Several lines of indirect evidence and preliminary interpretations of recently collected seismic reflection data have led to the conclusion that the Portland Hills fault at the eastern base of the Portland Hills appears to be capable of generating large-magnitude earthquakes. Although no historical earthquake can be associated with the Portland Hills fault, small-magnitude seismicity in the past 20 years in the vicinity of the Portland Hills fault zone, which includes the Oatfield and East Bank faults, suggests that one or all of these structures may be seismogenic. The Portland Hills fault may be 40–60 km long, probably dips to the southwest beneath the Portland Hills, and may slip in a reverseoblique sense. Limited observations suggest that, on average, the intervals between large earthquakes are a few thousand to more than 10,000 years. Given its location in the midst of the Portland metropolitan area, rupture of the Portland Hills fault resulting in a large earthquake could be devastating. Future studies are required to characterize the earthquake potential of the fault in a more definitive manner and to provide an improved basis for predicting the hazards that would result from such a large earthquake.

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