Abstract

High-resolution seismic reflection profiles were acquired to aid earthquake hazard assessment in the Portland-Vancouver urban area of Oregon and Washington, western North America. The profiles show (1) a strong reflector at the base of unconsolidated deposits; (2) the ancestral Columbia River channel where it has eroded into the unconformity at the base of the unconsolidated deposits; and (3) evidence consistent with late Pleistocene or Holocene faulting. The seismic data consist of marine profiles along 40 km segments of the Columbia and Willamette Rivers and two 1.5-km-long land profiles across the East Bank and Portland Hills fault zones. The marine profiles show a strong reflector as deep as 85 m that correlates with the unconformity at the base of unconsolidated, late Pleistocene and Holocene sediments penetrated in nearby drillholes. A ∼1.5-km-wide, up to 85-m-deep paleochannel filled by unconsolidated sediments marks the course of the ancestral Columbia River. Apparent vertical displacements of late Pleistocene or Holocene reflectors at the East Bank fault are consistent with recent faulting. The Portland Hills fault zone also shows what could be late Pleistocene to Holocene deformation, but other interpretations of these features are possible. No obvious faulting of the late Pleistocene unconformity is observed on our profiles across the inferred location of the Frontal Fault zone. The strong reflection from the unconformity and a large contrast in measured S-wave velocities between the unconsolidated sediments (∼250 m/sec) and the underlying strata (477 to 817 m/sec) indicates the shallow layer could amplify and trap seismic energy during an earthquake. These results indicate the East Bank and Portland Hills faults may represent significant seismic hazards to the Portland-Vancouver urban area and emphasize that further characterization of the shallow strata is crucial to estimating the shaking potential at sites above the Portland basin.

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