HR: 10:20h
AN: G52A-01 INVITED    [Abstracts]
TI: A More Accurate Vertical Velocity Field for Coastal Oregon Reveals Variations in Extent of Locking on the Cascadia Subduction Zone
AU: * Burgette, R J
EM: rburgett@uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, Eugene, OR 97403-1272, United States
AU: Weldon, R J
EM: ray@uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, Eugene, OR 97403-1272, United States
AU: Schmidt, D A
EM: das@uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, Eugene, OR 97403-1272, United States
AB: The pattern of uplift observed at the Earth's surface provides a strong constraint on the extent of locking at depth on subduction thrust faults. We have utilized the rich history of high quality leveling and sea level observations made in coastal Oregon and northernmost California over the last century to calculate more accurate, spatially- dense observations of uplift rate with realistic uncertainties. We have extended the water level time series of the six primary NOAA tide gauges from Crescent City, CA to Astoria, OR to include all observations 1925-2006, placed in a common reference frame at each gauge by local leveling. Analysis of the tidal leveling loops shows that tidal records are contaminated by instability of individual primary benchmarks, which have been used to define the local vertical datum. Some of the primary benchmarks changed elevation relative to nearby monuments up to 1.6 mm/yr at South Beach and Astoria. We corrected the tidal records and tied them to a stable reference monument at each gauge that has been releveled in regional lines. Sea level trends are refined with more precise inter-site differences, weighted by uncertainties that account for autocorrelation. The estimated standard errors are ~0.1 mm/yr for the six sites after the adjustment. We use a regional rate of sea level rise specific for the data interval of 2.3±0.2 mm/yr. This rate is consistent with the long record at Seattle, which lies outside the region of interseismic subduction uplift, and global geocentric reconstructions of regional sea level change. Regional releveling conducted by the NGS provides estimates of relative uplift rates along the north-south coastal route, and three lines extending east. At least three leveling epochs along most of these routes allow us to identify systematic errors in the data. We double the number of previously published high precision differences by making secondary ties between nearby monuments using tidal leveling, reset benchmarks, and 1941 elevation differences to span between 1930s and 1980s and minimize the impact of systematic errors in long 1941 lines. We attach the relative uplift rates from leveling to the rates at the tide gauges through a weighted adjustment that accounts for uncertainties in both the relative tidal and leveling uplift rates, as well as 0.1-0.3 mm/yr errors in tying the leveling to the tidal rates. Tidal and leveling uplift rates agree within error for all but one of the coastal segments, where previously unrecognized systematic leveling error affects the 1988 line. Down-dip profiles centered on the east-west leveling lines show that vertical velocities decrease from maxima near the coast to near zero in a smooth, concave-up pattern. However, along-strike, the pattern shifts east-west, with northern coastal Oregon uplifting up to 3.5 mm/yr faster than central Oregon, for the same distance from the trench. There is a ~1 mm/yr uplift rate increase near Cape Blanco, OR, with the southern area uplifting more slowly. We model the area with an elastic dislocation model of the subduction zone with smoothly varying locking depth along strike, which matches the observed data well. The vertical velocity field is a more direct probe of subduction locking than the horizontal field, which reflects both forearc motion and subduction strain accumulation. We will compare predictions of horizontal velocities from our model to GPS observations. Predicted uplift rate patterns for known active crustal structures and post-glacial isostatic adjustment do not appear consistent with the uplift rate data, suggesting that subduction zone locking is the only resolvable vertical signal.
DE: 0545 Modeling (4255)
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: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Geodesy [G]
MN: 2007 Fall Meeting