THE GALES CREEK FAULT AT SCOGGINS DAM: AN ACTIVE FAULT IN THE NORTHWEST OREGON COAST RANGE

Late Holocene surface rupture has been recently documented by Redwine et al. (2019) and Horst et al. (2021) on the Gales Creek fault (GCF), ~35 km west of Portland, Oregon. The NW-striking GCF abuts the western margin of the Tualatin Valley, where it passes beneath Scoggins Dam and Henry Hagg Lake, 15 km SW of Hillsboro, Oregon. Bedrock and geophysical mapping show 12 km dextral offset of Eocene basement (Siletzia) and up to 9 km dextral offset of 16-Ma Miocene Columbia River Basalt Group, with long-term slip rates averaging ~0.6 mm/yr. The GCF is well-defined on lidar topography by linear valleys, shutter ridges, and right-lateral stream offsets of up to 1.5 km. Detailed geologic mapping and paleoseismic trenching of the GCF reveals Scoggins Dam lies within a complex, 1–2 km-wide right (releasing) step between two major NW-striking right-lateral fault strands, each with paleoseismic evidence for multiple Holocene earthquakes. NE-striking, left-lateral normal-oblique faults and NW-striking right-lateral faults occur within the stepover and are exposed along the shoreline of Henry Hagg Lake. Here, thick colluvium, soils, and weathered bedrock are stripped by wave action, providing 4 km of nearly continuous bedrock exposure during low reservoir levels. Some of these secondary stepover faults appear to offset soil horizons, although ubiquitous mass wasting complicates their interpretation. Based on scarp morphology, trenching, and outcrops, the revised location of the active, NW-striking Scoggins Creek strand of the GCF projects beneath the dam to where a ~3 m-wide shear zone was observed in bedrock during dam construction. Inversion of fault slip data in Eocene-Oligocene bedrock suggests that the maximum horizontal compressive stress has been oriented ~N13°E, consistent with right-lateral motion on the GCF and historic stress field orientations derived from earthquake focal mechanisms and borehole breakouts. Although the GCF is >60 km long and has evidence for progressive offset continuing into the late Holocene, it does not have an obvious geodetic signal, perhaps because of the overwhelming overprint from locking on the subjacent Cascadia subduction zone megathrust. Nonetheless, the geologic evidence indicates that the GCF is a significant source of seismic hazard to the greater Portland metro area.