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Overview of the SNARF Working Group, its activities, and accomplishments

Stable North America Reference Frame Working Group (SNARF) Chair: Geoff Blewitt. Overview of the SNARF Working Group, its activities, and accomplishments. What is SNARF and Why is it Important?. Objective define a reference frame that represents the stable interior of North America Why?

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Overview of the SNARF Working Group, its activities, and accomplishments

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  1. Stable North America Reference Frame Working Group (SNARF) Chair: Geoff Blewitt Overview of the SNARF Working Group, its activities, and accomplishments

  2. What is SNARF and Why is it Important? • Objective • define a reference frame that represents the stable interior of North America • Why? • Appropriate frame to describe relative motions of sites spanning the N.A. - Pacific plate boundary • Facilitate geophysical interpretation • Facilitate inter-comparison of solutions • Standardization and documentation

  3. Why do we Need a Reference Frame? • GPS alone does not provide unambiguous coordinates • Can arbitrarily rotate your solution • Fixing the rotation can facilitate interpretation • Why not simply use, say, ITRF and NUVEL-1A? • Difficult to interpret N.A. deformations in ITRF • NUVEL-1A has known deficiencies • For example, African Rift not included • Glacial isostatic adjustment (GIA) is significant

  4. Velocities in ITRF – not appropriate for interpretation M. Craymer

  5. Velocities in NUVEL-1A M. Craymer

  6. Vertical Velocities: Not dominated by tectonics! GIA is the issue. M. Craymer

  7. GIA Predicted Velocities: Very sensitive to model parameters J. Davis, M. Tamisea, and T. Herring

  8. J. Davis, M. Tamisea, and T. Herring

  9. Example of problem we need to address: Horizontal GIA motions are sensitive to lateral heterogeneity in Earth’s structure. M. Tamisea

  10. Questions • Where does the plate boundary begin? • and why? • what is the extent of the stable plate interior? • and how tectonically stable is the plate interior? • is the Colorado Plateau still rotating? • and how active is the Rio Grande Rift? • extends to Bermuda, Greenland, Alaska, Siberia…? • is Alaska rigidly attached to North America? • empirical evidence is weak

  11. Questions • What is the vertical velocity field across North America? • what is GIA versus tectonic? • role of body forces and mantle dynamics? • Deceptively simple question: • Is the Basin and Range going up or down? • Not straightforward to determine using GPS • Reference frame dependent

  12. Questions • How can we design geodetic products that are stable over decadal time-scales and beyond? • will we be able to detect a >5-year transient? • can we detect the “ghosts” of historic earthquakes? • is tectonic activity “constant” (steady-state) ? • or does it switch on and off? • and migrate from one region to another? • can we confidently compare and relate geodetic rates to geologic rates?

  13. NSF Proposals • To support SNARF workshops • No salary – volunteer work • Proposal 1: Feb 2004 – Jan 2006 • Proposal 2: Jan 2006 – Dec 2007 • No new proposals – “operational” work supported by NRCAN, NGS, PBO • Goals • Tools and products to help users realize a stable North America-fixed frame • Provide the reference frame for PBO (Analysis Coordinator: Tom Herring) • SNARF operations and maintenance jointly by NGS (Richard Snay) and NRCan (Mike Craymer), under the auspices of the IAG “NAREF” projects

  14. SNARF Workshops • 2004-01, UNAVCO Inc., Boulder, CO. • 2004-05, Joint Assembly, Montreal, Canada. • 2005-03, EarthScope National Meeting, Santa Ana, NM.. • 2005-06, UNAVCO/IRIS Meeting, Stevenson, WA.. • 2006-03, UNAVCO Science Meeting., Denver, CO. • 2006-11, Natural Resources Canada, Ottawa, Canada. • 2007-03, EarthScope National Meeting, Monterey, CA • 2008-12, AGU, San Franscisco, CA • 2009-03, UNAVCO Science Workshop, Boulder, CO.

  15. Working Group Progress • Have identified and tackled the major issues: • GPS velocity field that is accurate (representative), and relatively dense to select a base model for GIA • Site selection criteria to define “frame” sites • geological considerations • monumentation and equipment • data quality and duration • Subset of “frame” sites used to define “datum” that can represent a non-rotating stable plate interior • Define products to be distributed for general use

  16. SNARF Products • First Release: SNARF 1.0 in June 2005 • rotation rate vector: (North America – ITRF2000) • gridded/site velocities from assimilation model • site epoch coordinates (X, Y, Z) and velocities • SNARF web page at www.unavco.org • Has been adopted by PBO Data Analysis Centers • products in Stable North America Reference Frame • In production-mode: October 2005 • SNARF 2.0 to be release April 2008 • ITRF2005, longer time series, improved models

  17. Example: UNR Solution: 3790 stations, 1994-2008 (now ~2600 daily)Ambiguity resolved (Ambizap) G. Blewitt and C. Kreemer

  18. UNR NA-NNR Frame: 45 sites, 2000-2008Horizontal Velocities Provides frame for daily transformations(GIPSY x-files) G. Blewitt and C. Kreemer

  19. UNR NA-NNR Frame: Horizontal Velocities (zoom)18 Core sites provide the NNR condition G. Blewitt and C. Kreemer

  20. UNR NA-NNR Frame: Vertical Velocities G. Blewitt and C. Kreemer

  21. UNR NA-NNR Frame: Vertical Velocities (zoom) G. Blewitt and C. Kreemer

  22. N Application of Daily X-Files (preliminary):Where Does the Plate Boundary Begin? G. Blewitt and C. Kreemer

  23. Lessons Learned • Pattern of GIA uplift (Hudson Bay) and peripheral bulge (Canada-US) clearly delineated • GIA can cause systematic error in pole of rotation that leads to ~1mm/yr velocity bias • Large variations and model sensitivity in GIA models • Method (currently) requires a data-model assimilation approach. • Plate interior (east of Rockies/Rio Grande Rift) is stable << 1 mm/yr • Vertical motions below peripheral bulge consistently << 1 mm/yr • Also across the Basin and Range, Sierra Nevada, and NA-Pacific transform (San Andreas,..) • ITRF2005 works very well • Bermuda is on stable North America (no apparent passive margin deformation) • Greenland, Alaska, Siberia motions are significant compared to frame stability • 1-3 mm/yr motions, perhaps due to mix of GIA, current ice (de-)loading, permafrost, … • Monument stability and jumps in time series remains an issue for many sites • But the best sites have 0.1 mm/yr monument stability (inferred by local baselines) • Daily transformations into SNARF are recommended versus rotation of velocity field • Example: UNR will provide daily x-files to transform fiducial-free positions into SNARF

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