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JGR Solid Earth


Upper-plate faulting in the Olympic Peninsula of Washington State results from relative motion of crustal blocks within the Cascadia forearc and earthquake cycle processes along the Cascadia subduction zone. We reconstruct fault slip rates since ∼14 ka on the Sadie Creek fault (SCF), north of the Olympic Mountains, using airborne lidar and field mapping of surficial deposits and landforms and optically stimulated luminescence and radiocarbon dating. The SCF is a ≥14 km-long northwest-striking, subvertical, dextral strike-slip fault with a subordinate dip-slip component. Laterally, offset debris flow channels cut into Late-Pleistocene and younger surfaces show dextral slip of 4.0–24.5 m and dip slip of 0.7–6.5 m. Re-evaluation of fault slip on the adjacent Lake Creek Boundary Creek fault (LCBCF) shows similar dextral (4.5–29.7 m) and dip slip (0.8–4.6 m). A deglacial age of 14 ka paired with the largest—and presumably oldest—slip measurements produce a minimum dextral slip rate of 1.3–2.3 mm/yr and dip-slip rate of 0.05–0.5 mm/yr. Similarities in geometry, kinematics, slip rate, and earthquake timing between the SCF and LCBCF suggest these faults represent one continuous geologic structure, the North Olympic fault zone. Geodetically constrained boundary element method models considering the effects of coseismic subduction zone stresses on upper plate structures produce comparable kinematics to those measured on the SCF and LCBCF, suggesting that the North Olympic fault zone acts as the main strain-accommodating structure in the northern Olympic Peninsula and may be modulated by stress transferred from subduction zone earthquakes.







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Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.


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