HR: 0800h
AN: S51C-1022    [Abstracts]
TI: Holocene Fault Scarps and Shallow Magnetic Anomalies Along the Southern Whidbey Island Fault Zone Near Woodinville, Washington
AU: * Sherrod, B
EM: bsherrod@ess.washington.edu
AF: U.S. Geological Survey, Dept. of Earth and Space Sciences, Univ. of Washington, Box 351310, Seattle, WA 98195 United States
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
EM: craig@ess.washington.edu
AF: U.S. Geological Survey, Dept. of Earth and Space Sciences, Univ. of Washington, Box 351310, Seattle, WA 98195 United States
AU: Kelsey, H M
EM: hmk1@humboldt.edu
AF: Dept. of Geology, Humboldt State University, Arcata, CA 95521 United States
AU: Barnett, E
EM: eli@ess.washington.edu
AF: Dept. of Geology, Humboldt State University, Arcata, CA 95521 United States
AU: Wells, R
EM: rwells@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 973, Menlo Park, CA 94025 United States
AB: The southern Whidbey Island fault zone (SWIFZ), mapped previously using borehole data, potential-field anomalies, and marine seismic-reflection surveys, consists of four sub-parallel, northwest-trending fault strands, extending ~100 km from near Vancouver Island to the Washington mainland. The SWIFZ has been hypothesized to extend southeastward beneath the mainland, making landfall between the cities of Seattle and Everett. Linear, low-amplitude aeromagnetic anomalies in this mainland region are on strike with the mapped portion of the SWIFZ and may indicate that the fault continues southeast. The Cottage Lake aeromagnetic lineament, is most prominent, extends at least 16 km and is approximately on strike with the SWIFZ on Whidbey Island. 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 probably into Eocene sedimentary strata. Coastal marsh stratigraphy, lidar mapping, and fault scarp excavations help define recent activity along the SWIFZ. 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. Subtle scarps on Pleistocene surfaces are delineated by 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. Four excavations across two lidar scarps 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 glacial or tectonism caused this slip. 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 excavated across lidar features near Grace, Washington 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 2700 years ago.
DE: 1105 Quaternary geochronology
DE: 7221 Paleoseismology (8036)
SC: Seismology [S]
MN: Fall Meeting 2005