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

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

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 1) 2004-01, UNAVCO Inc., Boulder, CO. 2) 2004-05, Joint Assembly, Montreal, Canada. 3) 2005-03, EarthScope National Meeting, Santa Ana, NM.. 4) 2005-06, UNAVCO/IRIS Meeting, Stevenson, WA.. 5) 2006-03, UNAVCO Science Meeting., Denver, CO. 6) 2006-11, Natural Resources Canada, Ottawa, Canada. 7) 2007-03, EarthScope National Meeting, Monterey, CA 8) 2008-12, AGU, San Franscisco, CA 9) 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, 2000-2008 Horizontal 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. Application of Daily X-Files (preliminary): Where Does the Plate Boundary Begin? N 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