Portland State University. Department of Geology
Date of Award
Master of Science (M.S.) in Geology
1 online resource (ix, 85 pages)
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 at least 16 Ma. Models for crustal deformation in the PNW suggest northwest trending faults accommodate dextral shear inferred from increasing clockwise rotation rates west of Portland. I compiled structural information to improve the seismic source characterization of these faults, and using empirical scaling relationships for fault length and earthquake magnitude, I find that many of the faults in the region are capable of generating earthquakes of magnitude 6 to 7.
The focus of this study is the Gales Creek fault (GCF) which is the longest northwest-trending fault in the study area with prominent geomorphic expression, located 35 km west of Portland. In addition to a seismogenic source characterization, I investigated the GCF through paleoseismic trenching to document the style and timing of surface deforming earthquakes. I interpret three surface rupturing earthquakes from stratigraphic and structural relationships in the trench. Radiocarbon samples from offset stratigraphy constrain these earthquakes to have occurred ~1,000, ~4,200 and ~8,800 calibrated years before present. The penultimate earthquake backtilted 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. My results suggest the GCF is active, and has experienced at least three surface-deforming earthquakes in the Holocene. New earthquake timing constraints presented here, combined with ongoing research 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.
Horst, Alison Elizabeth, "Seismic Source Characterization of Faults in the Portland and Tualatin Basins and a Paleoseismic Study of the Gales Creek Fault, OR" (2019). Dissertations and Theses. Paper 5267.
Available for download on Friday, September 11, 2020