Sponsor
Portland State University. Department of Geology
First Advisor
Ashley Streig
Term of Graduation
Summer 2026
Date of Publication
9-14-2026
Document Type
Thesis
Degree Name
Master of Science (M.S.) in Geology
Department
Geology
Language
English
Subjects
Active tectonics, Paleoseismology
Physical Description
1 online resource (vii, 107 pages)
Abstract
North-striking extensional fault systems in the Oregon Cascade Range accommodate regional clockwise rotation of the upper plate of the Cascadia subduction zone. However, the duration of activity and amount of extension is poorly constrained. High-resolution lidar topography (1-m-grid) of the Oak Grove Fork Clackamas River area 75 km SE of Portland, OR revealed previously unmapped faults that lengthen the extent of the multistrand Clackamas River fault zone (CRFZ) northward towards the Mount Hood fault zone, potentially providing a kinematic link between these fault systems. I use this lidar to estimate fault offsets at over 200 locations across more than 10 newly mapped fault scarps in the northern Clackamas River fault zone (nCRFZ). Combined with new geologic mapping, and paleoseismic age data I evaluate fault geometry and displacement through time. Offsets range from 0.7 to ~100 m across fault scarps that cut Pliocene-Quaternary volcanic and sedimentary bedrock and Late Pleistocene surficial deposits, including at least two generations of glacial deposits. I excavated a paleoseismic trench across the north-striking Anvil Lake fault, where the scarp displays 3 m of W-down offset. The trench exposed >2 m of glacial deposits in fault contact with conglomerate and pyroclastic bedrock. Colluvial wedge mapping indicates at least two surface-rupturing earthquakes. Radiocarbon ages from detrital charcoal and OxCal modeling constrain the timing of the two most recent surface-rupturing earthquakes as mid to late Holocene. Empirical scaling relationships between fault length and earthquake magnitude yield ~Mw 6.0 estimate if only the 7.5 km-long Anvil Lake fault ruptured. With the addition of ~14 km of newly mapped faults, the full CRFZ system is ~42 km-long from N to S and could generate a ~Mw 7.0 earthquake. CRFZ fault strands are spaced ~0.2 to 4 km apart suggesting surface-rupturing earthquakes involve complex multi-strand ruptures. I infer subsurface connectivity of many CRFZ fault strands, where they likely root into primary structures at depth. I calculate slip rates of 0.03-0.12 mm/yr along individual strands of the nCRFZ. However, the complex subsurface connectivity of the nCRFZ likely drives strain partitioning across the system, suggesting that system-wide slip rates may differ substantially from those measured on individual faults. These findings have implications for increasing hazards in future updates to the U.S. Geological Survey’s National Seismic Hazard Model and for improving the characterization of seismic hazard in the region.
Rights
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Persistent Identifier
https://archives.pdx.edu/ds/psu/45144
Recommended Citation
Schwarzbart, Sandy, "Lidar Mapping and Paleoseismic Investigation Reveal Holocene Activity and Northward Continuation of the Clackamas River Fault Zone, Oregon Cascade Range" (2026). Dissertations and Theses. Paper 7210.