Late Pleistocene and Holocene tectonics of the Portland basin, Oregon and Washington, from high-resolution seismic profiling

High-resolution seismic reflection profiles were acquired to aid earthquake hazard assessment in the Portland-Vancouver urban area of Oregon and Washi ngton. western North America. The profiles show (1) a strong reflector at t he base of unconsolidated deposits;, (2) the ancestral Columbia River chann el where it has eroded into the unconformity at the base of the unconsolida ted 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 ac ross 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 a t the base of unconsolidated, late Pleistocene and Holocene sediments penet rated in nearby drillholes. A similar to 1.5-km-wide, up to 85-m-deep paleo channel filled by unconsolidated sediments marks the course of the ancestra l Columbia River. Apparent vertical displacements of late Pleistocene or Ho locene reflectors at the East Bank fault are consistent with recent faultin g. The Portland Hills fault zone also shows what could be late Pleistocene to Holocene deformation, but other interpretations of these features are po ssible. No obvious faulting of the late Pleistocene unconformity is observe d on our profiles across the inferred location of the Frontal Fault zone. T he strong reflection from the unconformity and a large contrast in measured S-wave velocities between the unconsolidated sediments (similar to 250 m/s ec) and the underlying strata (477 to 817 m/sec) indicates the shallow laye r 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 fu rther characterization of the shallow strata is crucial to estimating the s haking potential at sites above the Portland basin.