Thoughts on the GIA Issue in SNARF Jim Davis & Tom Herring Input from and discussions with Mark Tamisiea, Jerry Mitrovica, and Glenn Milne

Assumed Goal of SNARF Provide a reference frame for interpretation of GPS vector velocities in terms of tectonic processes Should incorporate known Glacial Isostatic Adjustment (GIA) Will be used with velocities and strain Could incorporate simple tectonic models Could incorporate non-secular components

Purpose of and Requirements for GIA Correction Non-tectonic signal Has long wavelength (inc. degree 1) contributions Account “correctly” (~0.1 mm/yr?) for main features of GIA: Size/location of main bulge, peripheral bulge; tangential motions

Review of GIA Predictions (1)Earth model --> Viscoelastic (i.e., time- dependent) Green’s function for deformation given time-dependent load (2)Ice-load history (3)Redistribution of ice-load (sea-level equation) (4)Incorporation of subtle effects (time-dependent continent margin, rotational feed-back, water- dumping)

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. Tamisiea et al., AGU

Methods for Accounting for GIA in SNARF (1)Ignore GIA (2)“Adopt” GIA model (3)Find combination of observations insensitive to GIA model that can uniquely determine SNARF (4)Ad-hoc GIA “background” model (new proposal)

(1) Ignore GIA Easy Don’t have to worry about errors in GIA predictions GIA source of significant vertical and tangential deformation Makes interpretation of GPS velocities more difficult

(2) Adopt Ice/Earth Models for GIA Predictions Fairly easy to find a combination of ice/Earth models that are “OK” match to GPS data Physical Adopted GIA model will have short- and long- wavelength errors Non-unique Will “crown” one particular set of models

Fit of COD to GIA: 9/1995-3/2004: 17 sites This minimum moves with increasing LT Lithosphere Thickness (LT) 71 km Details here depend geographic sites distribution Herring, AGU

2004 Joint AssemblyG21D-03 1 mm/yr Northward Displacement Tamisiea et al., AGU

(3) Combination Insensitive to GIA Great, if we can find one Probably can’t find one

(4) Ad-hoc reference “velocity background model” Start with “reasonable” GIA predictions Assimilate velocity field to match GPS or other (e.g., tide-gauge) observations Add “layers” of complexity (e.g., tectonic models, non-secular motions) TAH: –Phase I: Secular –Phase II: Non-secular, seasonal & blended –Phase III: More complex geophysical models

(4) Ad-hoc reference VBM Represents blend of model and observations, with appropriate weightings VBM can be used for GIA studies Can accommodate velocity variations of any wavelength wished Model complexities can be incorporated Does not rely on any particular Earth/ice model VBM can be produced on grid Subjective component Requires proof-of-concept Not global