1. The Plausible Range of GIA Contributions to 3-D Motions at GPS Sites in the SNARF Network 2004 Joint AssemblyG21D-03 Mark Tamisiea 1, Jerry Mitrovica 2, Jim Davis 3 1 University of Colorado, 2 University of Toronto, 3 Harvard-Smithsonian CFA

2. 2004 Joint AssemblyG21D-03 Motivation Majority of studies using GPS data investigate tectonic deformations – the GIA signal is noise. Developing a GIA model that best reduces the variance between the model prediction and GPS data may well remove tectonic signals. Are there regions where the GIA signal is robust with respect to model variations?

3. 2004 Joint AssemblyG21D-03 1. Models Solid Earth Model Elastic Model – PREM – well known Viscosity Model – vigorously debated Example: 71 p5 5 Lithosphere – 71 km thick Upper Mantle Viscosity – 5 x 10 20 Pa s Lower Mantle Viscosity – 5 x 10 21 Pa s

4. 2004 Joint AssemblyG21D-03 Varying the Earth Model Use all combinations of: Lithospheric Thickness : 71, 96, 120 km Upper Mantle Viscosity : {2, 3, 5, 8} x 10 20 Pa s Lower Mantle Viscosity : {5, 8, 10, 20, 30} x 10 21 Pa s

5. 2004 Joint AssemblyG21D-03 Ice Models ICE-I – completely independent of Earth model –Peltier and Andrews, 1976 ICE-3G – model uses assumption of Earth model –Tushingham and Peltier, 1991 Insufficient geological data exists to fully constrain the ice sheet models. Thus, construction of the model must be supplemented by geodetic data and/or glaciological models.

6. 2004 Joint AssemblyG21D-03 Model Solutions Solve the model using the sea level equation: 1.Conserve mass 2.Apply ocean and ice loads to Earth to determine deformation 3.Adjust ocean load Refinements to sea level theory: 1.Time-variable coastlines 2.Water dumping 3.Feedback from rotational perturbation

7. 2004 Joint AssemblyG21D-03 Radial DisplacementModel: 71 p5 5 – ICE-3G 2. Model Results 1 mm/yr

8. 2004 Joint AssemblyG21D-03 1 mm/yr Horizontal DisplacementModel: 71 p5 5 – ICE-3G

9. 1 mm/yr 2004 Joint AssemblyG21D-03 Varying the Earth Model Radial Displacement

10. 2004 Joint AssemblyG21D-03 1 mm/yr Northward Displacement

11. 2004 Joint AssemblyG21D-03 1 mm/yr Eastward Displacement

12. 2004 Joint AssemblyG21D-03 Average Horizontal Velocities with 2  variation 1 mm/yr

13. 2004 Joint AssemblyG21D-03 1 mm/yr Using Additional Constraints Viscosity Model from Mitrovica and Forte, 2004 UM = (3 to 6) x10 20 Pa s TLM = (1 to 3) x 10 21 Pa s BLM = (8 to 30) x 10 21 Pa s

14. 2004 Joint AssemblyG21D-03 1 mm/yr Errors Introduced by Ice Models Difference between solutions using ICE-3G and ICE1

15. 2004 Joint AssemblyG21D-03 Conclusions The range of uncertainties in the Earth model produce large variations of the prediction of the GIA signal. Large variations caused by the ice sheet model are mainly confined to regions under or near the ice sheet at last glacial maximum. Model results for vertical rates may be useful in certain regions if additional constraints are added.

16. 2004 Joint AssemblyG21D-03