A Reference Frame for PBO: What do we Have; What do we Need?

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 FrameWorking 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 errorsin 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 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

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 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 • David Lavallée Global GPS, GPSVEL, seasonal loading • Meghan Miller Testing and application to PANGA • Jerry Mitrovica PGR models, site selection • Frank Webb Testing and application to SCIGN • Richard Snay National geodetic survey applications