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Holocene tectonics and fault reactivation in the foothills of the north Cascade Mountains, Washington

Geosphere

By:
, , , , , , , ,
DOI: 10.1130/GES00880.1

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Abstract

We use LiDAR imagery to identify two fault scarps on latest Pleistocene glacial outwash deposits along the North Fork Nooksack River in Whatcom County, Washington (United States). Mapping and paleoseismic investigation of these previously unknown scarps provide constraints on the earthquake history and seismic hazard in the northern Puget Lowland. The Kendall scarp lies along the mapped trace of the Boulder Creek fault, a south-dipping Tertiary normal fault, and the Canyon Creek scarp lies in close proximity to the south-dipping Canyon Creek fault and the south-dipping Glacier Extensional fault. Both scarps are south-side-up, opposite the sense of displacement observed on the nearby bedrock faults. Trenches excavated across these scarps exposed folded and faulted late Quaternary glacial outwash, locally dated between ca. 12 and 13 ka, and Holocene buried soils and scarp colluvium. Reverse and oblique faulting of the soils and colluvial deposits indicates at least two late Holocene earthquakes, while folding of the glacial outwash prior to formation of the post-glacial soil suggests an earlier Holocene earthquake. Abrupt changes in bed thickness across faults in the Canyon Creek excavation suggest a lateral component of slip. Sediments in a wetland adjacent to the Kendall scarp record three pond-forming episodes during the Holocene—we infer that surface ruptures on the Boulder Creek fault during past earthquakes temporarily blocked the stream channel and created an ephemeral lake. The Boulder Creek and Canyon Creek faults formed in the early to mid-Tertiary as normal faults and likely lay dormant until reactivated as reverse faults in a new stress regime. The most recent earthquakes—each likely Mw > 6.3 and dating to ca. 8050–7250 calendar years B.P. (cal yr B.P.), 3190–2980 cal. yr B.P., and 910–740 cal. yr B.P.—demonstrate that reverse faulting in the northern Puget Lowland poses a hazard to urban areas between Seattle (Washington) and Vancouver, British Columbia (Canada).

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Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Holocene tectonics and fault reactivation in the foothills of the north Cascade Mountains, Washington
Series title:
Geosphere
DOI:
10.1130/GES00880.1
Volume
9
Issue:
4
Year Published:
2013
Language:
English
Publisher:
Geological Society of America
Contributing office(s):
Earthquake Science Center
Description:
26 p.
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
First page:
827
Last page:
852
Number of Pages:
26
Country:
United States
State:
Washington
County:
Whatcom County
Other Geospatial:
Cascade Mountains