SEISMIC SOURCE CHARACTERIZATION OF FAULTS IN THE PORTLAND AND TUALATIN BASINS AND A PALEOSEISMIC STUDY OF THE GALES CREEK FAULT, OR

Portland, OR lies within the tectonically active forearc of the Cascadia subduction zone. Several, potentially hazardous, northwest striking faults in and around the Portland Basin are classified as Quaternary active by the USGS, but little is known about their Holocene activity. Geologic and geodetic studies in the Pacific Northwest (PNW) document ongoing clockwise rotation of the region since 16 Ma. Current models for crustal deformation in the PNW suggest NW trending faults accommodate dextral shear inferred from increasing clockwise rotation rates west of Portland. We compiled structural information to improve the seismic source characterization of these faults, and using empirical scaling relationships for fault length and earthquake magnitude, we find that many of the faults in the region are capable of generating earthquakes of magnitude 6 to 7. The Gales Creek fault (GCF), approximately 35 km west of Portland, has strong surface expression, and geophysical evidence points to a fault of at least 50 km length. We excavated a paleoseismic trench at a site on the northern GCF characterized by a prominent break in slope, in the form of a side-hill bench, to document the style and timing of surface deforming earthquakes. We interpret two surface rupturing earthquakes from stratigraphic and structural relationships in the trench. Radiocarbon samples from offset stratigraphy constrain these earthquakes to have occurred ~1,000 and ~7,200 calibrated years before present. The penultimate earthquake backtilted a buried soil into the hillslope creating accommodation space that was infilled by a colluvial deposit, and the most recent earthquake faulted and formed a fissure within the penultimate colluvial deposit. New earthquake timing from this northern site combined with existing data on the central GCF will better constrain the lateral extent of prehistoric surface rupturing earthquakes, and can be used to refine magnitude estimates for the GCF.