The Plausible Range of GIA Contributions to 3-D Motions at GPS Sites in the SNARF Network 2004 Joint...

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


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?


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


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


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.


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


Radial Displacement Model: 71 p5 5 – ICE-3G

2. Model Results

1 mm/yr


1 mm/yr

Horizontal Displacement Model: 71 p5 5 – ICE-3G

1 mm/yr


Varying the Earth ModelRadial Displacement


1 mm/yr

Northward Displacement


1 mm/yr

Eastward Displacement


Average Horizontal Velocities with 2 variation

1 mm/yr


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


1 mm/yr

1 mm/yr

Errors Introduced byIce Models

Difference between solutions using ICE-3G and ICE1


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.

The Plausible Range of GIA Contributions to 3-D Motions at GPS Sites in the SNARF Network 2004 Joint...

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