HR: 0800h
AN: S51C-1017    [Abstracts]
TI: Constraints on the Extent of Subduction Zone Locking Along the Central Oregon Coast From Leveling and Sea Level Observations
AU: * Burgette, R J
EM: rburgett@uoregon.edu
AF: Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403 United States
AU: Weldon, R J
EM: ray@uoregon.edu
AF: Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403 United States
AU: Livelybrooks, D
EM: dlivelyb@hendrix2.uoregon.edu
AF: Dept. of Physics, 1274 University of Oregon, Eugene, OR 97403 United States
AU: Schmidt, D A
EM: das@uoregon.edu
AF: Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403 United States
AU: Alba, S K
EM: salba@uoregon.edu
AF: Dept. of Physics, 1274 University of Oregon, Eugene, OR 97403 United States
AU: Wisely, B A
EM: bwisely@uoregon.edu
AF: Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403 United States
AB: The Cascadia megathrust is the greatest source of seismic hazard for the northwestern U.S. and southwestern Canada. Although regional-scale numerical models of the Cascadia subduction zone fit observations of interseismic uplift to the first order, there are significant discrepancies along the margin, particularly along the central coast of Oregon. To better resolve the pattern of uplift rate in central Oregon, we have greatly expanded the dataset of vertical geodesy through analyzing additional historical leveling lines, releveling key stretches of historical leveling lines, extending available historical tidal measurements by digitizing paper records, and reoccupying five historical tide gauge sites in a 22 km E-W profile along the Siuslaw River estuary. The NOAA-operated continuous tide gauge near Newport at South Beach (102 km from the trench) shows that the coast is subsiding at 0.7 ± 0.1 mm/yr. In contrast, numerical models (e.g. Wang et al., 2003) predict an uplift rate of approximately 1.7 mm/yr. The rate of subsidence is consistent with analysis of regional first-order leveling lines that have been tied to observations of sea level at other continuous tide gauges in Oregon and California. Repeated first-order leveling along Highway 20 east from Newport shows an essentially constant uplift rate inland, suggesting that both the locked and transition zones are located entirely offshore at this latitude. Just over 70 km to the south, at the coast near Florence (96 km from the trench), the uplift rate is approximately 2 mm/yr. Our leveling, coupled with analysis of USGS and NGS historical leveling lines east from Florence along Highway 126 provides a dense trench-perpendicular uplift rate profile in an area of the Cascadia margin that had not been previously examined. These leveling data give relative uplift rates, but their pattern shows that uplift rate increases dramatically near the coast, with tilts up to 0.2 mm/yr/km. This uplift rate gradient is much steeper than predictions from published numerical modeling, and suggests a narrower transition zone than inferred elsewhere. A narrow transition zone could explain the lack of observed uplift at Newport, where the geodetic benchmarks closest to the trench are ~ 6 km farther from the trench than at Florence. By tying our Florence leveling profile to a local sea level reference frame we will convert the values to absolute uplift rate. The leveling transect runs along the Siuslaw River estuary, which is tidal 22 km inland from the coast to Mapleton, and we are currently reoccupying five historic tide gauge sites between the entrance of the estuary and Mapleton. Using long term NOAA records to the north (South Beach) and south (Charleston) to define sea level rise and environmental noise, we have generated de-tided, de-noised daily values for the historical and modern observations that have standard deviations of 1-2 cm. Assuming no systematic errors, and given 535 days over 72 years at Florence and ~ 200 days over 31 years for the other sites, we should be able to determine uplift rate to ± 0.1 and ± 0.3 mm/yr, respectively.
DE: 1222 Ocean monitoring with geodetic techniques (1225, 1641, 3010, 4532, 4556, 4560, 6959)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 1295 Integrations of techniques
DE: 8104 Continental margins: convergent
DE: 9350 North America
SC: Seismology [S]
MN: Fall Meeting 2005