NEHRP Clearinghouse
- Title
- Holocene Fault Scarps and Shallow Magnetic Anomalies Along the Southern Whidbey Island Fault Zone Near Woodinville, Washington.
- File
- PB2006105335.pdf
- Author(s)
- Sherrod, B. L.; Blakely, R. J.; Weaver, C.; Kelsey, H.; Barnett, E.
- Source
- January 2005, 40 p.
- Identifying Number(s)
- USGS/OFR-2005-1136
- Abstract
- The southern Whidbey Island fault zone (SWIFZ), as previously mapped using borehole data, potential-field anomalies, and marine seismic-reflection surveys, consists of four sub-parallel, northwest-trending fault strands, extending approx. 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 M(sub W) 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, low-amplitude 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.
- Keywords
- Woodinville (Washington); Holocene fault scarps; Fault scarps; Southern Whidbey Island Fault Zone; Research; Magnetic anomalies; Aeromagnetic anomalies; Excavations; Earthquakes; Ground-magnetic survey; Prehistoric deformation; Paleoseismology; Shallow magnetic anomalies