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The Plausible Range of GIA Contributions to 3-D Motions at GPS
Sites in the SNARF Network
2004 Joint AssemblyG21D-03
Mark Tamisiea1, Jerry Mitrovica2, Jim Davis3
1University of Colorado, 2University of Toronto,3Harvard-Smithsonian CFA
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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?
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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 1020 Pa s
Lower Mantle Viscosity – 5 x 1021 Pa s
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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 1020 Pa s
Lower Mantle Viscosity : {5, 8, 10, 20, 30} x 1021 Pa s
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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.
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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
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2004 Joint AssemblyG21D-03
Radial Displacement Model: 71 p5 5 – ICE-3G
2. Model Results
1 mm/yr
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2004 Joint AssemblyG21D-03
1 mm/yr
Horizontal Displacement Model: 71 p5 5 – ICE-3G
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1 mm/yr
2004 Joint AssemblyG21D-03
Varying the Earth ModelRadial Displacement
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2004 Joint AssemblyG21D-03
1 mm/yr
Northward Displacement
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2004 Joint AssemblyG21D-03
1 mm/yr
Eastward Displacement
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2004 Joint AssemblyG21D-03
Average Horizontal Velocities with 2 variation
1 mm/yr
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2004 Joint AssemblyG21D-03
1 mm/yr
Using Additional Constraints
Viscosity Model fromMitrovica and Forte, 2004
UM = (3 to 6) x1020 Pa s
TLM = (1 to 3) x 1021 Pa s
BLM = (8 to 30) x 1021 Pa s
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2004 Joint AssemblyG21D-03
1 mm/yr
1 mm/yr
Errors Introduced byIce Models
Difference between solutions using ICE-3G and ICE1
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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.
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2004 Joint AssemblyG21D-03