The northwest-striking southern Whidbey Island fault zone (SWIF) was mapped previously using borehole data and potential-field anomalies on Whidbey Island and marine seismic surveys beneath surrounding waterways. Abrupt subsidence at a coastal marsh on south-central Whidbey Island suggests that the SWIF experienced a MW 6.5 to 7.0 earthquake about 3000 years ago. Southeast of Whidbey Island, a hypothesized southeastward projection of the SWIF would make landfall between the cities of Seattle and Everett. As part of systematic, ongoing studies by the U.S. Geological Survey, University of Washington, and other earth science organizations to evaluate potentially active faults and other earth hazards throughout the Puget Lowland, we test this hypothesis using aeromagnetic, lidar, and borehole data.
Linear, northwest-striking magnetic anomalies traversing the mainland region project southeastward toward the communities of Woodinville and Maltby, Washington. All of these magnetic anomalies are low in amplitude and best illuminated in residual magnetic fields. The most prominent of the residual magnetic anomalies extends at least 16 km, lies approximately on strike with the SWIF on Whidbey Island, and passes near Crystal and Cottage Lakes, about 27 km southeast of downtown Everett. In places, this magnetic anomaly is associated with topographic lineaments, but spectral analysis indicates that the source of the anomaly extends to depths greater than 2 km and cannot be explained entirely by topographic effects. The Alderwood #1 oil exploration well located on strike with the Cottage Lake aeromagnetic lineament shows evidence of deformation over a total depth range of 3000 m; some beds within this interval exhibit intense fracturing and shearing, although deformation within the well can only be constrained as post-early Oligocene and pre-Pleistocene. Boreholes acquired as part of a wastewater tunnel project show evidence of soil disturbance at locations where some topographic and aeromagnetic lineaments cross the tunnel alignment. Some of the disturbance is likely tectonic in origin, although other explanations are possible. Some of the soil disturbance demonstrably predates the 15-13 ka Fraser glaciation of the Puget Lowland; other samples have inconclusive ages and may be younger. Subtle scarps in Pleistocene surfaces are visible on high-resolution lidar topography at a number of locations along the Cottage Lake aeromagnetic lineament. Collectively, the scarps are parallel to the trend of the aeromagnetic lineament and extend a total distance of 18 km. In the field, scarps exhibit 1 to 5 m of north-side-up offset. The scarps provide targets for future paleoseismic trenching studies to test the hypothesis that they have a tectonic origin.