The Cottage Lake Aeromagnetic Lineament: A Possible Onshore Extension of the Southern Whidbey Island Fault, Washington

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.

[Summary provided by USGS.]