Finding concealed active faults: Extending the southern Whidbey Island fault across the Puget Lowland, Washington

Journal of Geophysical Research B: Solid Earth
By: , and 

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Abstract

The southern Whidbey Island fault zone (SWIF), as previously mapped using borehole data, potential field anomalies, and marine seismic reflection surveys, consists of three subparallel, northwest trending strands extending ∼100 km from near Vancouver Island to the northern Puget Lowland. East of Puget Sound, the SWIF makes landfall between the cities of Seattle and Everett but is concealed beneath a thick mantle of young glacial deposits and vegetation. A ∼20‐km‐wide, northwest trending swath of subparallel, low‐amplitude aeromagnetic anomalies crosses this region of the Puget Lowland and is on strike with the SWIF. The most prominent aeromagnetic anomaly, the Cottage Lake lineament, extends at least 18 km and lies approximately on strike with the SWIF on Whidbey Island. Subtle scarps and topographic lineaments on Pleistocene surfaces, visible on high‐resolution lidar topography at a number of locations along the SWIF, lie on or near these magnetic anomalies. In the field, scarps exhibit northeast‐side‐up and vertical relief of 1 to 5 m. Excavations across several lidar scarps lying on or near magnetic anomalies show evidence for multiple folding and faulting events since deglaciation, most likely above buried reverse/oblique faults. Excavations in areas away from magnetic anomalies do not show evidence of tectonic deformation. In total, paleoseismological evidence suggests that the SWIF produced at least four earthquakes since deglaciation about 16,400 years ago, the most recent less than 2700 years ago.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Finding concealed active faults: Extending the southern Whidbey Island fault across the Puget Lowland, Washington
Series title Journal of Geophysical Research B: Solid Earth
DOI 10.1029/2007JB005060
Volume 113
Year Published 2008
Language English
Publisher Wiley
Contributing office(s) Earthquake Science Center, Geology, Minerals, Energy, and Geophysics Science Center
Description B05313, 25p.
Country United States
State Washington
Other Geospatial Southern Whidbey Island
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