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, Unive
rsity of Nevada, Reno, USA Stable North America
Reference FrameWorking Group
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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

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PBO Backbone

• Alaska western U.S.
• 20 existing GPS stations 100 new
• Eastern U.S.
• 20 GPS at IRIS/USGS Global Seismographic Network
  sites

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Where are we Now?

• WUSC GPS velocity mapBennett, Davis, Wernicke,
  Normandeau, 2002

• GPS strain rate magnitudeBlewitt, Coolbaugh,
  Sawatzky,Holt, Davis, Bennett, 2003

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PBO Needs

• What are PBO reference frame needs?
• How can we meet those needs?

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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

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How Broad is the Plate Boundary?

• PBO mini-proposalHolt, 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

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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
  1999Kreemer 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

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Current Frame Stability?

• The International GPS Service Network

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Current Frame Stability

• Accuracy of ITRF2000
• Approaching 1 mm/yr at best performing sites
  Altamimi et al., 2001
• Evidence that current GPS accuracy lt 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
  Eulers 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?

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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

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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

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Conclusions

• PBO is developing a reference frame
• That accounts for non-tectonic deformations
• Loading, PGR,
• Stable to lt 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)

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• 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