1. Current Reference Frame Treatment and Future Needs: Regional Arrays SNARF Workshop 27 January 2004 Rick Bennett Harvard-Smithsonian CfA …from the southwestern US perspective

2. North America plate frame realization for southwestern US Choose RF rotation rate so as to minimize velocities at sites in North America interior: I used ALGO, DRAO, NLIB, STJO, MDO1, PIE1, HEBE, PRDS, CAST, FRED, FERN

3. Southwestern US North America plate frame statistics SITE R.R. Sigma (all values in mm/yr) --------- ------ -------- ALGO 1.1 0.03 NLIB 0.4 0.04 DRAO 2.8 0.03 STJO 3.3 0.04 MDO1 0.3 0.03 PIE1 0.8 0.03 HEBE 0.4 0.03 PRDS 2.5 0.04 CAST 0.4 0.03 FRED 0.8 0.07 FERN 1.0 0.07 Number of sites = 11 Number sites with R.R. < 1 mm/yr = 6 Percentage with < 1 mm/yr = 55 The rate NRMS is 48 The rate WRMS is 1.6 mm/yr The component NRMS is 34 The component WRMS is 1.1 mm/yr R.R. = rate residual to realization of the plate frame.

4. Some southwestern US North America plate frame caveats 1)Data used to define the frame is evolving New NA stations Changing equipment Always more data 2)Inconvenient to compute geophysical models in the geodetically determined frame.

5. Example 1 Estimate relative motions of fault- bounded blocks accounting for elastic strain associated with locked faults. Half-space fault models do not accurately represent far-field plate rates. Observed velocities

6. Example 1 continued One solution is to define local no-net translation reference frame: Residual velocities

7. Example 2 Determine the relative motions between the Colorado Plateau (CP), central Great Basin (CGB), and Sierra Nevada-Great Valley (SNGV) regions.

8. Example 2 continued SNGV-CGBSNGV-CP Bennett et al., 2003

9. Motion of a deforming body in space Deformation term depends on reference frame selection.

10. Simple 2D Cartesian deformation analysis problem: Choose coordinate axes such that: Then the information matrix looks like this: Note the trade-off depending on network and/or model geometry. Motion Cont…

11. Summary BARGEN North America plate frame determined by minimizing velocities at 11 NA interior sites. Should distinguish between geodetic and geophysical reference frame issues –Geodesist Frame definition Frame realization –Geophysicist Account for differences between model and data frames, regardless of errors.