1. Original objective = quantify intraplate deformation –Pros: Larger number of sites High density of sites in some areas Minimal cost… –Cons: Density varies geographically Monument quality Data processing: –Combine 3 independent solutions (using Altamimi et al’s model and Catref software) Gamit (Purdue) Gipsy (U. Wisconsin) Latest IGS –Rescaling of covariance associated with each individual solution => final uncertainty reflects: Variance in original solution Level of agreement between solutions Solutions (position/velocities) produced every ~6 months –2006 JGR paper: September 2005 solution –Latest solution: November 2006 –Next solution: wait until new IGS orbits available –All solutions availables in SINEX format (just ask) 608 continuous GPS sites: most are “CORS” stations + IGS + NRCan + local networks (e.g., GAMA) Calais, E., J.Y. Han, C. DeMets, and J.M. Nocquet, Deformation of the North American plate interior from a decade of continuous GPS measurements, J. Geophys. Res., 111, B06402, doi:10.1029/2005JB004253, 2006.

2. Precision: < 0.5 mm/yr after 10 years Bulk of sites: 0.5-1 mm/yr Accuracy: ~ 0.8 mm/yr on horizontal components ~ 3 mm/yr on vertical components Precision and Accuracy Calais et al, JGR, 2006

3. Residual velocities w.r.t. “stable North America” (defined using sites east of 100W and south of 40N - wrms = 0.4 mm) Calais and DeMets, unpublished solution, Nov. 2006

4. What can we resolve? Glacial Isostatic Adjustment SW U.S. extension (incl. Rio Grande Rift)

5. Spatially filtered residual velocity field Calais et al, JGR, 2006

6. New Madrid GPS Velocity Field

7. NWCC - PTGV NWCC - RLAP

8. Conclusions Residual velocities (horizontal): –Stable NOAM wrms = 0.4 mm/yr –NMSZ wrms = 0.5 mm/yr Strain resolvable at the 1 mm/yr level -- with high-enough station density: e.g., GIA and extension in SW U.S. New Madrid Seismic Zone: –Unresolved problem at site RLAP  should be discarded from interpretations –No velocity significantly different from zero -- even at the 1-sigma level We know: < 1 mm/yr residual velocities over 100 km (95% confidence) in NMSZ  should be taken into account in hazard studies.