Holocene Fault Scarps and Shallow Magnetic Anomalies Along the Southern Whidbey Island Fault Zone Near Woodinville, Washington

The southern Whidbey Island fault zone (SWIFZ), as previously mapped using borehole data, potential-field anomalies, and marine seismic-reflection surveys, consists of four subparallel, northwest-trending fault strands, extending ~100 km from near Vancouver Island to the Washington mainland. The vertical component of displacement changes both across and along the fault, and strike slip motion may be important as well. Abrupt uplift at a coastal marsh on south-central Whidbey Island suggests that the SWIFZ experienced a MW 6.5 7.0 earthquake between 3200 and 2800 years B.P. The SWIFZ has been hypothesized to extend southeastward beneath the mainland, making landfall between the cities of Seattle and Everett. Linear, lowamplitude aeromagnetic anomalies in this mainland region are on strike with the SWIFZ and may facilitate mapping the SWIFZ onshore. The most prominent of the residual aeromagnetic anomalies, the Cottage Lake aeromagnetic lineament, extends at least 16 km, lies approximately on strike with the SWIFZ on Whidbey Island, and passes within about 27 km of downtown Everett. Glacial deposits are slightly magnetic in this region, as indicated by magnetic susceptibility measurements and a ground-magnetic survey, and, in places, the Cottage Lake aeromagnetic lineament is associated with topographic lineaments. Spectral analysis and modeling experiments indicate that the source of the Cottage Lake aeromagnetic lineament extends to depths greater than 2 km and into Eocene sedimentary strata. Subtle scarps on Pleistocene surfaces are visible on high-resolution lidar topography at a number of locations in the mainland region, often closely associated with aeromagnetic lineaments. In the field, scarps exhibit northeast-side-up vertical relief of 1 to 5 m. One group of scarps extends a total distance of 18 km along the Cottage Lake aeromagnetic lineament, where they defines two strands: One strand lies east of Crystal Lake and is referred to here as the Crystal Lake lidar lineament, another strand follows the Little Bear Creek drainage and is called here the Little Bear Creek lidar lineament. Four excavations across these two lidar lineaments show evidence for multiple folding and faulting events since deglaciation, most likely above buried reverse/oblique faults. One trench exposed a normal fault, although it was not possible to determine whether slip was caused by glacial or tectonic processes. In this same trench, evidence for younger folding suggests that surface deformation during an earthquake formed the scarp seen on lidar maps near Crystal Lake. Subsequent scarp degradation buried a forest soil, from which charcoal samples indicated that the folding event occurred after 11,670 years B.P. Two trenches were also excavated along the Little Bear Creak lineament near Grace, Washington. One of these trenches showed evidence for multiple events, including both folding and faulting, with the youngest event disturbing late Holocene wetland soils. In total, paleoseismological evidence suggests that the SWIFZ produced at four events since deglaciation about 16,400 years ago, the most recent after 2.7 ka. Liquefaction features near Everett indicate that strong shaking has occurred there at least three times in the last 1100 years, although evidence that those earthquakes occurred on the SWIFZ has not yet been found.

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