5/18/2994 G21D-04 Spring AGU 2004 1

Realization of a Stable North America Reference Frame

Thomas HerringDepartment of Earth Atmospheric and

Planetary, Sciences, MITtah@mit.edu

http://geoweb.mit.edu/~tah

5/18/2994 G21D-04 Spring AGU 2004 2

Overview

• Construction of a stable North America Reference Frame:–Secular motion field (alignment to GIA model)–Temporal evolution (day-to-day realization of frame)

• Concentrate on comparing analysis with Bernese and GAMIT software with fitting to GIA Models

5/18/2994 G21D-04 Spring AGU 2004 3

Acknowledgements

• Jim Davis, SAO - 216 Glacial Isostatic Adjustment (GIA) Models: 3 lithosphere thicknesses (120,96,71 km); 8 Upper Mantle viscosities (0.05-1x1021 Pa-s; 9 Lower Mantle viscosities (1-50x1021 Pa-s)

• Eric Calais, Purdue University, 29,000 processed networks of east-coast GPS data.

• SOPAC facility with ftp access to processed networks (up to 600 global and North America sites per day). ftp://garner,ucsd.edu/pub/hfiles

• CODE weekly sinex files submitted to IGS.

5/18/2994 G21D-04 Spring AGU 2004 4

Secular Field Construction

• Necessary steps:–Account for offsets from antenna type and radome changes.–Establish sites that have “linear” motion (this step is relative

easy with correlated noise models).–Determine a “stable” region of North America

• With stable region selected, GIA models can be evaluated• This is the most complicated (and unresolved) step. Fit to GIA models depends on choice of stations (examples shown)

–With stable North America defined, frame can be transferred to other linear motion sites (extrapolation issue: the stable region is small. Adding sites on other plates can make system more robust, but adds complication of motion of other regions).

5/18/2994 G21D-04 Spring AGU 2004 5

GIA Model Evaluation

• Basic approach:–Select sites that have linear motion. We do this by fitting a

correlated noise model and then selecting sites with small correlated noise (and long time series)

–Select from these sites, those who motions are expected to be due GIA only (not clear at moment)

–Determine the rotation/translation of the loosely combined GPS solution that best aligns it with the GPS model

–Examine 2-D (horizontal) and 3-D fits to GIA–Evaluate with different GPS analyses:

• CODE Bernese solutions (sinex files)• SOPAC analysis• SOPAC analysis+Purdue analysis

5/18/2994 G21D-04 Spring AGU 2004 6

Fit of COD to GIA: 9/1995-3/2004: 17 sites

This minimummoves with increasing LT

Lithosphere Thickness (LT) 71 km Details here depend geographic sites distribution

5/18/2994 G21D-04 Spring AGU 2004 7

Fit of SIO to GIA: 1/1996-3/2004: 28 Sites

Basic structure similar to CODE; similar spatial distribution of sites

5/18/2994 G21D-04 Spring AGU 2004 8

Fit of PUR to GIA: 1/1994-4/2004: 12 sites selected to be GIA sensitive

Sites selected in Canada and down the middle of the US

Min UM 1, LM 2x1021 Pa-s

5/18/2994 G21D-04 Spring AGU 2004 9

Comparison of PUR solution (red, 50% confidence ellipses) with GIA model 71 km LT, UM 1, LM 2x1021 Pa-s

Fit: 26-sitesN 0.6 mm/yrE 0.3 mm/yrU 1.9 mm/yr

5/18/2994 G21D-04 Spring AGU 2004 10

Comparison of PUR solution (red, 50% confidence ellipses) with GIA model 120 km LT, UM 1, LM 2x1021 Pa-s

Fit: 26-sitesN 0.5 mm/yrE 0.4 mm/yrU 1.8 mm/yr

5/18/2994 G21D-04 Spring AGU 2004 11

Comparison between COD, SIO and PUR for Reference Sites

Red: CODE RMS3-D 0.7 mm/yr;2-D 0.3 mm/yr

Blue: SIO RMS2/3-D 0.2 mm/yr

Black: PUR

5/18/2994 G21D-04 Spring AGU 2004 12

Motion estimates in “North America” for sigma less than 0.5 mm/yr and rates less than 2 mm/yr

NOTE: Scale change

5/18/2994 G21D-04 Spring AGU 2004 13

Time series of Site GDAC for North and Height

5/18/2994 G21D-04 Spring AGU 2004 14

Conclusions

• In general none of the GIA models fit the GPS observations at the expected noise level:–Fits are 0.5 mm/yr horizontal, 1.5 mm/yr vertical–Agreement between the GPS results is better than with the

GIA models• Coherent non-GIA residuals seem to present in the

results• Caveat: With a decade of GPS data, satellites

constellation has evolved and there are un-accounted satellite phase center variations that will effect the results