Abstract

Portland, Oregon, lies in the southern half of an approximately rectangular basin measuring 30 by 50 km. Since 1969, there have been no earthquakes with M ≥ 4.0 in or on the margins of the Portland basin, but this level of seismicity may not be characteristic of the region. Using microseismicity data collected by the University of Washington regional short-period seismograph network for the period mid-1982 through 1989, we have determined P-wave focal mechanisms for four individual earthquakes and three groups of earthquakes. We have also relocated the 6 November 1962, MW = 5.2 Portland earthquake and analyzed regional surface-wave recordings of this event, using the seismic moment-tensor inversion technique. The results of these seismic analyses, along with geologic and other geophysical data, are integrated into a seismotectonic model of the Portland basin. The P-wave mechanisms are compatible with dextral strike-slip motion along approximately NW-striking fault zones bounding the eastern and western margins of the basin. We speculate that there is a dextral strike-slip fault zone, which we call the Frontal Fault Zone, along the eastern margin of the Portland basin. The western margin has been previously recognized as a zone of dextral strike-slip faulting, known as the Portland Hills Fault Zone. The epicenter of the 1962 earthquake is located between the two fault zones and lies approximately 15 km NE of downtown Portland. Our preferred mechanism is normal faulting on NE- or NNE-trending fault planes. These results support the hypothesis posed by previous investigators that the Portland basin is a pull-apart basin and are evidence for contemporary crustal extension between the Frontal and Portland Hills fault zones.

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