HR: 16:00h
AN: G44A-01 INVITED     [Abstracts]
TI: Finding Active Faults in a Glaciated and Forested Landscape: the Southern Whidbey Island Fault, Washington
AU: * Blakely, R J
EM: blakely@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 989, Menlo Park, CA 94025
AU: Sherrod, B L
EM: bsherrod@ess.washington.edu
AF: U.S. Geological Survey, Univ. of Washington, Box 351310, Seattle, WA 98195
AU: Wells, R E
EM: rwells@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 989, Menlo Park, CA 94025
AU: Weaver, C S
EM: craig@ess.washington.edu
AF: U.S. Geological Survey, Univ. of Washington, Box 351310, Seattle, WA 98195
AB: The Puget Lowland, Washington, lies within the Cascadia forearc and is underlain by at least six seismically active and regionally significant crustal faults that together accommodate several mm/yr of net north-south shortening. The surface expression of pre-15-ka slip on Puget Lowland faults has been largely scoured away or covered by glacial deposits, and younger fault geomorphology is often concealed by vegetation and urban development. High-resolution aeromagnetic and lidar surveys, followed by geologic site investigations, have identified and confirmed late Holocene deformation on each of these mostly concealed but potentially hazardous faults. Most geomorphic features identified in lidar data are closely associated with linear magnetic anomalies that reflect the underlying basement structure of the fault and help map its full extent. The southern Whidbey Island fault (SWIF) is a case in point. The northwest-striking SWIF was mapped previously using borehole data and potential-field anomalies on Whidbey Island and marine seismic-reflection surveys beneath surrounding waterways. Gravity inversions and aeromagnetic mapping suggest that the SWIF extends at least 50 km southeast, from Vancouver Island to the Washington mainland, and transitions along its length from northeast-side-down beneath Puget Sound to northeast-side-up on the mainland. Abrupt subsidence at a coastal marsh on south-central Whidbey Island suggests that the SWIF experienced a $M_W$ 6.5 to 7.0 earthquake about 3 ka. Southeast of Whidbey Island, a hypothesized southeastward projection of the SWIF makes landfall between the cities of Seattle and Everett. Linear, northwest-striking magnetic anomalies in this mainland region do coincide with this hypothesized projection, are low in amplitude, and are 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 within about 27 km of downtown Everett. Glacial deposits are slightly magnetic, and, in places, the magnetic anomaly is associated with topographic lineaments. Spectral analysis indicates, however, that the source of the anomaly extends to depths greater than 2 km and into Eocene sedimentary strata. Subtle scarps on Pleistocene surfaces are visible on high-resolution lidar topography at a number of locations along the aeromagnetic lineament. Collectively, the scarps are coincident with the aeromagnetic lineament and extend a total distance of 18 km. In the field, scarps exhibit 1 to 5 m of northeast-side-up offset. Two trenches were excavated across one of the lidar scarps during the summer of 2004. Both trenches showed evidence for folding, presumably above a buried reverse/oblique fault tip with at least 2 m of vertical offset. One trench also exposed a normal fault, although it was not possible to determine whether slip was caused by glacial or tectonic processes. The radiocarbon age of a folded, buried soil indicates that the earthquake occurred after 12 ka. Thus, the SWIF has produced at least two slip events in the Holocene, one occurring $\sim$3 ka on Whidbey Island and the other $\sim$12 ka on the Washington mainland.
DE: 7230 Seismicity and seismotectonics
DE: 8105 Continental margins and sedimentary basins
DE: 1517 Magnetic anomaly modeling
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting