1. 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 Frame Working Group

2. 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

3. PBO Backbone Alaska + western U.S.  ~20 existing GPS stations + 100 new Eastern U.S.  ~20 GPS at IRIS/USGS Global Seismographic Network sites

4. Where are we Now? WUSC GPS velocity map [Bennett, Davis, Wernicke, Normandeau, 2002] GPS strain rate magnitude [Blewitt, Coolbaugh, Sawatzky, Holt, Davis, Bennett, 2003]

5. PBO Needs What are PBO reference frame needs? How can we meet those needs?

6. 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

7. 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

8. “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 errors in 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]

9. Current Frame Stability? The International GPS Service Network

10. 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?

11. 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

12. Stable North America Reference 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

13. 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)

14. SNARF Working Group Members Don ArgusFrame origin, tectonics, site selection Rick BennettTesting and application to BARGEN Geoff BlewittCoordinate specs and recommendations Eric CalaisIntraplate deformation Mike CramerTesting and application to NAREF Jim DavisCoordinate specs and recommendations Tim DixonPlate stability, site selection Tom HerringGlobal GPS, ITRF, site selection Kristine LarsonP.I. (NSF proposal), ITRF, site selection David LavalléeGlobal GPS, GPSVEL, seasonal loading Meghan MillerTesting and application to PANGA Jerry MitrovicaPGR models, site selection Frank WebbTesting and application to SCIGN Richard SnayNational geodetic survey applications