Multiple Holocene Earthquakes on the Gales Creek Fault, Northwest Oregon Fore-Arc

Abstract

ABSTRACT 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 ∼1000, ∼4200, and ∼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 ∼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.