HR: 1340h
AN: G13A-0792    [Abstracts]
TI: Viscoelastic Deformation Model of the Western United States Instantaneous Velocity Field
AU: * Vergnolle, M
EM: mvergnolle@usgs.gov
AF: USGS, 345 Middlefield Road, MS977, Menlo Park, CA 94025 United States
AU: Pollitz, F F
EM: fpollitz@usgs.gov
AF: USGS, 345 Middlefield Road, MS977, Menlo Park, CA 94025 United States
AB: Constraints on long-term deformation characteristics of the continents are provided by geologic slip rates on major faults, paleomagnetic rotations, and estimates of instantaneous velocity as measured by Global Positioning System (GPS) or other geodetic measurements over a short time span. Geologic slip rates and paleomagnetic rotations represent the long-term velocity field (i.e., that averaged over a timescale much longer than the earthquake cycle on an individual fault) in the vicinity of the corresponding faults or blocks. On the other hand, instantaneous velocity measurements correspond to a timescale that is much shorter than a typical earthquake cycle on a major fault. Such measurements are gaining importance with ever-increasing coverage of continental regions with geodetic networks. In order to satisfactorily explain the instantaneous strain rate field, we require estimates of slip of significant historic earthquakes, the mechanics of tectonic loading, and the underlying rheology. These are also critical ingredients to understand the long-term deformation pattern. We present and extend an existing relationship between the long-term dislocation rates and instantaneous velocities, allowing separate treatments of faults with known slip history, creeping faults, and dislocation sources distributed between the faults. For faults with known slip history, the relationship depends on a viscoelastic Earth model and thus accounts explicity for viscoelastic cycle effects. For distributed dislocation sources, we calculate the average interseismic velocity produced by viscoelastic relaxation over an entire cycle. We apply this relationship to the GPS velocity field in the western United States in order to test the importance of the relaxation from historic events and characterize the pattern of distributed deformation and the tectonic forces imposed by the bounding Pacific and Juan de Fuca plates. By accounting for viscoelastic cycle effects integrated over the totality of dislocation sources in the western United States, the relative contributions of discrete (fault-like) and distributed dislocations may be quantified. Our modeling of the western US strain rate field shows that relaxation following major earthquakes (M $>$ 7.5) strongly shapes the present strain rate field over most of the plate boundary zone. Relaxation following minor earthquakes is well detected within smaller regions around secondary active seismic zones (e.g., Central Nevada Seismic Zone, East California Shear Zone).
DE: 8150 Plate boundary--general (3040)
DE: 1236 Rheology of the lithosphere and mantle (8160)
DE: 1243 Space geodetic surveys
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting