HR: 11:50h
AN: T12B-07    [Abstracts]
TI: Middle to Late Holocene Earthquake Chronologies for the Cascadia Subduction Zone From Two Estuaries in Southern Oregon
AU: * Witter, R C
EM: witter@lettis.com
AF: William Lettis & Associates, Inc., 1777 Botelho Drive Suite 262, Walnut Creek, CA 94596 United States
AU: Kelsey, H M
EM: hmk1@humboldt.edu
AF: Department of Geology, Humboldt State University, Arcata, CA 95521 United States
AU: Nelson, A R
EM: anelson@usgs.gov
AF: Geologic Hazards Team, U.S. Geological Survey MS 966, Denver, CO 80225 United States
AB: Comparisons of regional Cascadia earthquake records to earthquake chronologies at the Coquille River and Sixes River estuaries in southern Oregon suggest that, in contrast to the {\bf M} 9 A.D. 1700 earthquake, some events ruptured shorter segments of the subduction zone and failed to trigger offshore turbidites. Between 2,000 and 4,700 years ago, earthquakes occurred on average every 350 to 415 years at the Sixes River. Over a similar time period at the Coquille River estuary, earthquakes occurred on average every 525 to 650 years. When compared to the average recurrence interval of tsunamis and strong shaking reported for Bradley Lake (240 to 280 years), these different return periods at sites within 30 km of one another suggest segmented rupture of the subduction zone. Corroborative evidence for segmented rupture includes 2-sigma age ranges for coseismically subsided soils at the Sixes River that fail to overlap age ranges for soils at the Coquille River estuary, and regional comparisons that suggest the penultimate earthquake recorded in southwestern Washington did not rupture south of Coos Bay, Oregon. Five earthquakes that occurred between 3,700 and 4,700 years ago are alternately recorded at the Sixes River and the Coquille estuary and each earthquake correlates in time with evidence for a tsunami or strong shaking at Bradley Lake. The alternating pattern of this earthquake sequence suggests that rupture termination of adjacent segments coincide as an inferred segment boundary at Cape Blanco. The Cape Blanco anticline, a transverse structural high that separates the Coquille estuary from the Sixes River, is inferred to be a possible segment boundary because it coincides with the boundary between two forearc basins that may reflect separate long-term asperities. We attribute faster relative sea-level rise at the Coquille estuary, accelerated by 0.5 to 1.3 mm/yr compared to the Sixes River, to differential uplift accommodated by one or both upper-plate structures, the Coquille fault and the Cape Blanco anticline, that separate the sites. This Holocene uplift rate agrees with long-term uplift rates derived from marine terrace studies. Variations in relative sea-level curves indicate that at least two plate-boundary earthquakes triggered slip on a blind upper-plate fault that underlies the Cape Blanco anticline. Differential uplift of 0.4 to 1.7 m resulted from an earthquake 3,560 to 3,390 cal yr BP. Another earthquake produced 1.9 to 3.9 m of differential uplift about 2,470 to 2,150 cal yr BP.
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 8150 Plate boundary--general (3040)
DE: 7215 Earthquake parameters
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting