Slow Dextral Slip on the Gales Creek Fault at Scoggins Dam: Hazard Implications for the Greater Portland-Vancouver-Hillsboro Metropolitan Area, Oregon and Washington
Abstract
Many active faults have been recently discovered in the forearc above the Cascadia subduction zone with the aid of lidar topographic mapping. However, most active faults appear to have low slip rates, <1 mm/yr, consistent with low rates of seismicity in the forearc crust. This is particularly true in the greater Portland, Oregon metro area, where seismicity is sparse, and late Holocene surface rupture has only been recently documented on the Gales Creek fault (GCF). The NW-striking GCF abuts the western margin of the populated Tualatin Valley (and basin), where it passes beneath Scoggins Dam and its reservoir, Henry Hagg Lake, ~15 km SW of Hillsboro, Oregon. Bedrock and geophysical mapping show ~12 km dextral offset of Eocene basement and up to 9 km dextral offset of Miocene strata, with long-term (post 16 Ma) slip rates averaging ~0.6 mm/yr. The GCF has a strong geomorphic signature with linear valleys, shutter ridges, and dextral stream offsets of up to 1.5 km. Detailed mapping and trenching of the GCF reveals that Scoggins Dam lies within a complex, 1-2 km-wide right (releasing) step between two major NW-striking dextral fault strands, each with paleoseismic evidence for multiple Holocene earthquakes. Numerous NE-striking, sinistral normal-oblique faults and NW-striking dextral faults that occur within the stepover are exposed along the shoreline of Henry Hagg Lake, where thick colluvial soils are stripped by wave action, providing ~4 km of nearly continuous bedrock exposure during low reservoir levels. Many of these secondary stepover faults appear to offset soil horizons, although ubiquitous mass wasting complicates their interpretation. Inversion of fault slip data in Eocene-Oligocene bedrock suggests that the maximum horizontal compressive stress has been oriented ~N13°E, consistent with dextral 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 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 it is a significant source of seismic hazard to the greater Portland metro area.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMNH0390009W
- Keywords:
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- 4319 Spatial modeling;
- NATURAL HAZARDS;
- 4327 Resilience;
- NATURAL HAZARDS;
- 4328 Risk;
- NATURAL HAZARDS;
- 4334 Disaster risk communication;
- NATURAL HAZARDS