HR: 1330h
AN: S42A-0149    [PDF]
TI: The Cottage Lake Lineament, Washington: Onshore Extension of the Southern Whidbey Island Fault?
AU: * Blakely, R J
EM: blakely@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 989, Menlo Park, CA 94025 United States
AU: Weaver, C S
EM: craig@usgs.gov
AF: U.S. Geological Survey, Box 351310, University of Washington, Seattle, WA 98195 United States
AU: Sherrod, B L
EM: bsherrod@usgs.gov
AF: U.S. Geological Survey, Box 351310, University of Washington, Seattle, WA 98195 United States
AU: Troost, K G
EM: ktroost@u.washington.edu
AF: University of Washington, Department of Earth and Space Sciences, Box 351310, Seattle, WA 98195 United States
AU: Haugerud, R A
EM: rhaugerud@usgs.gov
AF: U.S. Geological Survey, Box 351310, University of Washington, Seattle, WA 98195 United States
AU: Wells, R E
EM: rwells@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 989, Menlo Park, CA 94025 United States
AU: McCormack, D H
EM: dmccormack@aspectconsulting.com
AF: Aspect Consulting, 811 First Avenue, Suite 480, Seattle, WA 98104 United States
AB: The northwest-striking southern Whidbey Island fault zone (SWIF) is reasonably well expressed by borehole data, marine seismic surveys, and potential-field anomalies on Whidbey Island and beneath surrounding waterways. Johnson et al. (1996) described evidence for Quaternary movement on the SWIF, suggested the fault zone is capable of a M 7 earthquake, and projected three fault strands onto the mainland between the cities of Seattle and Everett. Evidence for this onshore projection is scant, however, and the exact location of the SWIF in this populated region is unknown. Four linear, northwest-striking magnetic anomalies on the mainland may help address this issue. All of the anomalies are low in amplitude and best illuminated in residual magnetic fields. The most prominent of the magnetic anomalies extends at least 15 km, is on strike with the SWIF on Whidbey Island, and passes near Cottage Lake, about 15 km south of downtown Everett. The magnetic anomaly is associated with linear topography along its entire length, but spectral analysis indicates that the source of the anomaly lies principally beneath the topographic surface and extends to depths greater than 2 km. The anomalies are likely created by northwest-trending, faulted and folded Tertiary volcanic and sedimentary rocks of the Cascade foothills, which rise from beneath the Quaternary lowland fill to the southeast of the SWIF. High-resolution Lidar topography provided by King County shows subtle scarps cutting the latest Pleistocene glaciated surface at two locations along the magnetic anomaly; scarps are parallel to the anomaly trend. In the field, one scarp has 2 to 3 m of north-side-up offset; paleoseismic trench excavations are planned for Fall 2003 to determine their nature and history. Preliminary examination of boreholes, recently acquired as part of an ongoing sewer tunnel project, show anomalous stratigraphic and structural disturbances in the area of the magnetic anomalies. Analyses are underway to determine if faulting causes these disturbances. Taken together, the SWIF and the Cottage Lake lineament extend $>$50 km. Thus defined, the SWIF is spatially associated along its length with an arcuate (convex to the southwest) zone of deep ($>$10 km) crustal earthquakes, one of several arcuate alignments of epicenters between the SWIF and Olympia, Washington.
DE: 1517 Magnetic anomaly modeling
DE: 1545 Spatial variations (all harmonics and anomalies)
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 8110 Continental tectonics--general (0905)
SC: Seismology [S]
MN: 2003 Fall Meeting