1. Workshops for Establishing a Stable North American Reference Frame (SNARF) to Enable Geophysical and Geodetic Studies with EarthScope: Annual Report 2004-2005 Stable North America Reference Frame Working Group G. Blewitt, D. Argus, R. Bennett, Y. Bock, E. Calais, M. Craymer, J. Davis, T. Dixon, J. Freymueller, T. Herring, D. Johnson, K. Larson, M. Miller, G. Sella, R. Snay, M. Tamisiea

2. Outline ● What is SNARF and why is it important? ● The BIG questions ● SNARF Working Group – Who are we? – Progress to date ● SNARF products and future plans

3. 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

4. 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

5. Velocities in ITRF – not appropriate for interpretation

6. Velocities in NUVEL-1A – better, but SNARF will be even better!

7. Vertical Velocities: Not dominated by tectonics! GIA is the issue.

8. GIA Predicted Velocities: Very sensitive to model parameters

10. Example of problem we need to address: Horizontal GIA motions are sensitive to global Earth structure.

11. The BIG Questions (1) ● 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? – is Alaska rigidly attached to North America? ● empirical evidence is weak

12. The BIG Questions (2) ● 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 ● Totally reference frame dependent

13. The BIG Questions (3) ● What signals are natural vs. anthropogenic? – ground fluid withdrawal and aquifer deformation ● buoyancy forces on the aquifer matrix ● elastic deformation plus secular compaction – natural lithospheric flexure from fluid pressure ● atmospheric pressure ● seasonal regional-scale hydrological pressure ● global-scale change in Earth's shape from the water cycle – can we disentangle this from GIA and tectonics? – how to define an unbiased reference frame?

14. The BIG Questions (4) ● 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?

15. Working Group Progress to Date ● 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 represents the stable plate interior – Define products to be distributed for general use

16. Initial SNARF Products ● First Release of SNARF 1.0 in June 2005 – rotation rate vector: (North America – ITRF2000) – reference frame ● list of selected sites ● epoch coordinates (X, Y, Z) ● site velocities ● Will be adopted by PBO Data Analysis Centers – products in Stable North America Reference Frame – scheduled to be in production-mode: October 2005

17. Future Products ● Improved Frame – more sites, improved quality control – improved GIA – seasonal variation in site coordinates – loading models applied (e.g., atmospheric pressure) ● Extended Products – daily transformations into stable North America ● continental “regional filtering” of common-mode error – GPS software-specific products and tools ● GIPSY and GAMIT software

18. Future Plans ● Transition from Research to Operations – Currently in research-mode – by 2008, fully operational – 2006-2008 transition period (development, validation) ● Future operations – National agencies ● National Geodetic Survey (NGS) ● Natural Resources Canada (NRCan) – Incorporate SNARF into legal definition of “NAD”