HR: 11:30h
AN: T32C-05    [Abstracts]
TI: Structure of the Seattle Fault Zone, Washington State, from vibroseis seismic reflection profiles
AU: * Liberty, L M
EM: lml@cgiss.boisestate.edu
AF: Boise State University, CGISS Department 1910 University Dr., Boise, ID 83725-1536 United States
AU: Pratt, T L
EM: tpratt@usgs.gov
AF: U.S. Geological Survey, School of Oceanography University of Washington, Seattle, Wa 98195 United States
AB: We acquired high-resolution vibroseis seismic reflection profiles east of Lake Washington in June 2004 to characterize the geology in the upper one km across a 15 km-wide swath of the Seattle fault zone. We collected more than 20 km of north-trending profiles to image the southern part of the Seattle Basin, the northern edge of the Seattle uplift, and the intervening Seattle fault zone. Between Lakes Washington and Sammamish, the profiles show flat-lying and hummocky glacial strata in the southern part of the Seattle Basin, overlying north dipping ($>$25 degrees), faulted reflectors. Disrupted reflectors in the southern end of the Seattle Basin suggest that active fault strands are present north of the topographic scarp in the Seattle fault near Vasa Park and Interstate 90, where shortening from a M7 earthquake uplifted strata A.D. 900-930. A nearly identical pattern of dipping reflectors appears along strike on a profile 5 km east of Lake Sammamish, suggesting that the entire length of the Seattle fault east of Lake Washington has a similar fault history and is unsegmented. Deformation in the fault zone east of Lake Sammamish implies a documented fault length of at least 50 km, and the eastern extent of the fault still undetermined. In the Seattle uplift south of the Seattle fault, steeply-dipping reflections ($>$50 degrees) within Eocene and older sediments match known attitudes in outcrops and can be used to constrain structural models of the fault motion. Further processing of these data should reveal the history, character, and continuity of faulting and help identify likely trench sites for paleoseismic studies.
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 8107 Continental neotectonics
DE: 0935 Seismic methods (3025)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting