Multiple Holocene earthquakes on the Gales Creek Fault, northwest Oregon fore-arc

Several potentially hazardous northwest-striking faults in and around the Portland basin, within the fore-arc of Cascadia, are classified as Quaternary active by the U.S. Geological Survey, but little is known about their Holocene activity. We present new earthquake-timing constraints on the Gales Creek fault (GCF), a 73 km long, northwest-trending fault with youthful geomorphic expression located about 35 km west of Portland. We excavated a paleoseismic trench across the GCF in the populated northern Willamette Valley and document three surface-rupturing earthquakes from stratigraphic and structural relationships. Radiocarbon samples from offset stratigraphy constrain these earthquakes to have occurred approximately 1000, approximately 4200, and approximately 8800 calibrated years before the present. The penultimate earthquake back-tilted a buried soil into the hillslope creating accommodation space that was infilled by a colluvial deposit. The most recent earthquake faulted and formed a fissure within the penultimate colluvial deposit. Our results suggest that the GCF has a recurrence interval of approximately 4000 yr, and if the full 73 km length were to rupture, it would result in an Mw 7.1-7.4 earthquake, providing a significant seismic hazard for the greater Portland metropolitan area.