HR: 0800h
AN: T11A-0356 INVITED     [Abstracts]
TI: Great Earthquakes of Variable Magnitude at the Cascadia Subduction Zone
AU: * Nelson, A R
EM: anelson@usgs.gov
AF: Alan R. Nelson, Geologic Hazards Team U.S. Geological Survey MS 966, PO Box 25046, Denver, CO 80225 United States
AU: Kelsey, H M
T11A-0356 AF: Harvey M. Kelsey, Department of Geology Humboldt State University, Arcata, CA 95521 United States
AU: Witter, R C
T11A-0356 AF: Robert C. Witter, William Lettis & Associates, Inc., 1777 Botelho Drive, Suite 267, Walnut Creek, CA 94596 United States
AB: Radiocarbon age ranges for evidence of great plate-boundary earthquakes and accompanying tsunamis at seven coastal paleoseismic sites, combined with relative amounts of coseismic subsidence and heights of tsunamis, show a 5000-year history of great earthquakes of variable magnitude at the Cascadia subduction zone. Evidence for rupture of most of the plate boundary (greater than 620 km) is only conclusive for the earthquake of 26 January 1700, but the earthquake about 1.6 ka also had a long rupture, substantial subsidence, and a high tsunami consistent with a magnitude 9 earthquake. Correlations suggest that earthquakes about 1.35 ka, 2.5 ka, and 3.4 ka also may have been close to magnitude 9. A rupture about 0.8 ka was limited to the northern and central parts of the subduction zone, and a northern rupture about 2.9 ka may have been similarly limited. Times of probable short ruptures in southern Cascadia include about 1.1 ka, 1.7 ka, 3.2 ka, 4.2 ka, 4.6 ka, and 4.7 ka. Rupture patterns suggest that the plate boundary in northern Cascadia usually breaks in long ruptures during the greatest earthquakes. Southernmost Cascadia is typified by short as well as long ruptures during great earthquakes of variable magnitude and more irregular recurrence than in northern Cascadia.
DE: 1105 Quaternary geochronology
DE: 1115 Radioisotope geochronology
DE: 7221 Paleoseismology (8036)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 9350 North America
SC: Tectonophysics [T]
MN: Fall Meeting 2005