HR: 10:35h
AN: G52A-02    [Abstracts]
TI: A Microfossil-Based Approach to Constraining Megathrust-Induced Coseismic Land Displacement of the 1700 Event in the Pacific Northwest
AU: * Hawkes, A D
EM: hawkesa@sas.upenn.edu
AF: University of Pennsylvania, Hayden Hall 240 South 33rd Street, Philadelphia, PA 19104, United States
AU: Horton, B P
EM: bphorton@sas.upenn.edu
AF: University of Pennsylvania, Hayden Hall 240 South 33rd Street, Philadelphia, PA 19104, United States
AU: Nelson, A
EM: anelson@usgs.gov
AF: USGS, P.O. Box 25046 Mail Stop 966, Golden, CO 80225-0046, United States
AU: Grand Pre, C
EM: grandpre@sas.upenn.edu
AF: University of Pennsylvania, Hayden Hall 240 South 33rd Street, Philadelphia, PA 19104, United States
AB: Continuing subduction of the Juan de Fuca plate beneath the North America plate in central western North America constitutes a major seismic hazard but the history of great earthquakes in the region remains unclear. The tsunami accompanying the last great earthquake along the Cascadia subduction zone was widely recorded in Japanese records as an "orphan" tsunami (no source earthquake was felt in Japan) that would have been on the evening of January 26th AD 1700. Models of the inferred tsunami indicate an earthquake magnitude between M8.7 and M9.0, suggesting a plate-boundary rupture of over 1000km causing up to 20m of slip. Relative sea-level changes along much of Cascadia's coast are thought to be dominated by an earthquake cycle of rapid coseismic subsidence during plate-boundary rupture followed by gradual interseismic relative sea-level rise due to upper plate strain accumulation lasting hundreds of years. To learn more about the deformation cycle of the AD 1700 and earlier great earthquakes, we apply transfer functions to modern foraminiferal datasets from coastal Oregon to interpret the fossil foraminiferal record of sea-level change in cores. We collected seven modern salt-marsh transects (162 samples) extending from mudflat to upland. Nine of our tidal marsh cores sample sediment spanning the AD 1700. Furthermore, sediment in 4m-long vibracores collected at each of Nehalem River and Siuslaw River estuaries span the times of three to five earlier great earthquakes. The AD 1700 earthquake is marked in all cores by a distinct lithological and biostratigraphical changes. Preliminary foraminiferal data show a sudden subsidence of 0.44m ± 0.07m during the AD 1700 earthquake in the Nehalem River core and between 0.16m and 0.36m ± 0.08cm at Siuslaw River core. The microfossil-based transfer function approach produces high-precision geological reconstructions of relative sea-level of sufficient resolution to better estimate vertical ground displacements associated with the earthquake deformation cycle.
DE: 0468 Natural hazards
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 0497 Wetlands (1890)
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 8036 Paleoseismology (7221)
SC: Geodesy [G]
MN: 2007 Fall Meeting