IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS014 Oral Presentation 2267
New constraints on the motion and boundaries of the Sierra Nevada Block
and Colorado Plateau from continuous GPS velocities
Dr. Corne Kreemer
Nevada Bureau of Mines and Geology University of Nevada, Reno IAG
William C. Hammond, Geoffrey Blewitt
We present a new GPS velocity solution for much of the western between the San Andreas Fault and
the Rocky Mountains . This solution uses data from ~500 sites, including those from the BARD, SCIGN,
EBRY, PANGA, BARGEN, CORS and PBO permanent arrays, as well as from our own semi-continuous
MAGNET network in the western Basin and Range.Using the velocity solution we obtain better
constraints for the motion of the Sierra Nevada block than was previously possible. Moreover, our
analysis shows that all of the Sierra Nevada west of the crest moves as a rigid entity. Geodetic velocities
in the Walker Lane are oriented parallel to the Sierra Nevada-North America small-circle, implying that
most of the deformation to be accommodated as shear. However, many normal faults (particularly
those bounding the eastern Sierran front) are oriented at ~45 degrees to the relative motion,
suggesting regional strain partitioning. Overall, the deformation pattern is controlled by the geometry of
the Sierras granite/metamorphic province as it moves along and away from the Basin and Ranges
western margin. For the Colorado Plateau (CP) a consistent pattern is emerging with sites on the CP
moving northwest, implying that the block is rigid and that the extension rate across the Rio Grande Rift
is ~1 mm/yr. Our results also imply ~1-2 mm/yr of motion across the Hurricane Fault system between
the CP and the Basin and Range. Anomalies to the observed velocity pattern exist near large urban
areas (e.g., Phoenix, Las Vegas) or along the Colorado River . The anomalous velocities are probably
due to loading effects from local ground-water variations. To obtain our solution we use the GIPSY-
OASIS II software and apply a new ambiguity resolution technique that allows for fast and efficient
processing of daily solutions for the entire network. Most of North Americas Intermountain-West
velocities relative to stable North America are very small and we would commonly need long time-series
to infer significant motions in order to avoid seasonal effects and other long-term transients. Here we
deploy a new regional filtering technique that allows us to include sites with relatively short observation
span (~1 year). We create a regional-filter for each site by using residuals of all sites within 500-2000
km of that site (except the site itself). Those residuals are the result of fitting a linear rate and annual
and semi-annual term to the unfiltered time-series. We solve for a linear rate simultaneously with
seasonal parameters, and we infer white-, flicker, and random-walk noise components for each
individual time-series and apply those in the estimation of the rate uncertainties. The ITRF2000
velocities are transformed into the Stable North America Reference Frame (SNARF) by using ~30
velocities across the North American continent common with those in the SNARF solution. Using SNARF
provides a clear demonstration that the Rio Grande Rift is separating CP from stable North America .
Keywords: gps velocities, sierra nevada, colorado plateau