A Reference Frame for PBO: What do we Have; What do we Need?
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A Reference Frame for PBO: What do we Have; What do we Need?
Geoff Blewitt Nevada Bureau of Mines & Geology,and Seismological Laboratory,University of Nevada, Reno, USA
Stable North America Reference FrameWorking Group
Plate Boundary Observatory
PBO Image the ongoing
tectonic deformation of North America
Physics of earthquakes, magmatic processes, plate boundary dynamics and evolution
~1000 Permanent GPS: 800-900 clustered sites,
5-50 km spacing 100 “backbone” sites,
~200 km spacing
PBO Backbone
Alaska + western U.S. ~20 existing GPS stations + 100 new
Eastern U.S. ~20 GPS at IRIS/USGS Global Seismographic Network sites
Where are we Now?
WUSC GPS velocity map[Bennett, Davis, Wernicke, Normandeau, 2002]
GPS strain rate magnitude[Blewitt, Coolbaugh, Sawatzky,Holt, Davis, Bennett, 2003]
PBO Needs
What are PBO reference frame needs? How can we meet those needs?
PBO stated requirements: PBO needs
“…that plate boundary deformation be adequately characterized over the maximum ranges of spatial and temporal scales common to active continental tectonic processes.”
[ES Facility Proposal] How broad is the plate boundary? Is there a “stable plate interior”?
to within potential GPS accuracy ~ 0.1 mm/yr would require accurate modeling of non-tectonic
deformation If so, where is this stable plate interior?
PBO will address these questions by Network design including broad GPS spatial coverage
across North America Research
How Broad is the Plate Boundary?
PBO “mini-proposal”[Holt, Blewitt, Bennett, 2000]
Questions: Is the Colorado Plateau
rotating? 8-13° in Mesozoic
Is accommodated by Rio Grande Rift?
Ignorance may lead to biases elsewhere
“Geology Plate Tectonics” Residual velocity between:
Strain rates inferred from Quaternary slip vectors integrated from Colorado Plateau to Pacific
[Shen-Tu 1999], [also Humphreys & Weldon 1994]
NUVEL-1A [DeMets et al. 1994] If real, possible mechanisms:
50% can be accounted by errorsin NUVEL-1A [Larson et al. 1997; DeMets and Dixon 1999;Kreemer et al., 2000]
Offshore faults? [Shen-Tu, 1999] Colorado Plateau?
[Holt, Blewitt, Bennett, 2000] Clockwise rotation ~0.1°/Myr 1-3 mm/yr across Rio Grande Rift Consistent with Cenozoic rates Consistent with VLBI [Ma and Ryan,1998]
Current Frame Stability?
The International GPS Service Network
Current Frame Stability Accuracy of ITRF2000
Approaching 1 mm/yr at best performing sites [Altamimi et al., 2001] Evidence that current GPS accuracy < 0.5 mm/yr
Comparison of IGS Analysis Center solutions Smoothness of velocity field [“total error”, Davis et al, 2003]. 0.14 mm/yr RMS, GIPSY-GAMIT, BARGEN [Hill et al., 2002]
BUT! Plate rotations are sensitive to stability of Euler’s Fixed Point at frame origin
“Chasles Effect” [Blewitt and Davies, 1995] Biased prediction of plate boundary strain from plate rotations [Lavallée, 1999]
North America – Pacific Plate motion is sensitive to station selection Direction of relative motion changes few degrees
with/without Fairbanks, Alaska [Kreemer et al., 2000] North America may have internal deformation
1-2 mm/yr in “stable North America” [Dixon et al.] Non-tectonic motions can be significant
~1 mm horizontal motion by hydrological loading Few mm horizontal secular motion due to PGR Seismo-isostatic strain at recently activated faults?
Practical Needs: Consistency
GPS site velocities in North America Are almost universally published in a reference
frame referred to by the authors as “stable North America”
Reference frame varies between groups By definition and by realization procedure Specific procedure to realize the frame is often not
prescribed in sufficient detail Systematic velocity differences exist
1-2 mm/yr (smooth) between group
Stable North AmericaReference Frame (SNARF)
Working group Appointed by UNAVCO Board, June 2003 And as part of IAG Working Group “NAREF”
Charge: Produce a standard reference frame and specify
standard procedures to realize such a frame to meet the highest precision needs of the scientific community Design frame (concepts, models, …) Realize a specific frame (select sites, geodetic solution) Specify procedures to attach to such a frame
Conclusions
PBO is developing a reference frame That accounts for non-tectonic deformations
Loading, PGR, … Stable to < 1 mm/yr
Identification of “stable plate interior” Site selection
Frame that is specific & easily implemented For scientific and precision survey applications
Toward a new “North American Datum” (NAD)
SNARF Working Group Members
Don Argus Frame origin, tectonics, site selection
Rick Bennett Testing and application to BARGEN
Geoff Blewitt Coordinate specs and recommendations
Eric Calais Intraplate deformation
Mike Cramer Testing and application to NAREF
Jim Davis Coordinate specs and recommendations
Tim Dixon Plate stability, site selection
Tom Herring Global GPS, ITRF, site selection
Kristine Larson P.I. (NSF proposal), ITRF, site selection
David Lavallée Global GPS, GPSVEL, seasonal loading
Meghan Miller Testing and application to PANGA
Jerry Mitrovica PGR models, site selection
Frank Webb Testing and application to SCIGN
Richard Snay National geodetic survey applications