The Hunting of the SNARF Giovanni F. Sella Seth Stein Northwestern University Timothy H. Dixon University of Miami "What's the good of Mercator's North Poles and Equators, Tropics, Zones, and Meridian Lines?" So the Bellman would cry: and the crew would reply "They are merely conventional signs!

Ideal Rigid Plate Motion described by a rotation pole and angular velocity Points rotate about pole along small circles and their rate increases as sin  No vertical motion Real Rigid Plate Internal deformation due to platewide (e.g. ridge push) regional (e.g. GIA) and local (e.g. density anomalies) effects causes both horizontal and vertical motion

Choice of GPS sites Stable plate vs rigid plate Geographically representative distribution of sites Stable plate (geologic) Use geologic a priori criteria to exclude sites not expected to be on stable plate >100 km from plate boundaries, seismicity, active faults (Avoid seismic cycle effects) Rigid plate (geodetic) Minimize any plate wide effects e.g. GIA, subsidence, intra plate deformation. Avoid any local effects e.g. fluid withdrawal or injection

Detecting GIA using GPS GPS vertical velocities with respect to ITRF2000 Large vertical signal observed ~10 mm/yr

Regionally Coherent Vertical Velocity Pattern Clear pattern of positive velocities in and around Hudson Bay that decreases going southwards to zero (hinge line), beyond which velocities are initially negative and then rise to near zero All sites north of the hinge line and those up to a 100 km south of it are interpreted as being GIA affected

Distribution of GPS Sites on North America with Seismicity CGPS - Continuous GPS 24hrs, 365 days EGPS - Episodic GPS 8-12hrs, 2-3 days, every 2-3 yrs If plate is: rigid, good geographic distribution and errors are accurate  2 =1 Rigid North America defined using 83 CGPS sites (black diamonds)  2 =1.08 include 46 GIA (red circles) affected sites 129 sites  2 =1.33

Residual horizontal velocities after removing the motion of rigid North America defined using an 83 site solution Rigid plate sites: Small magnitude with a random distribution Other sites: Larger magnitude with a clear pattern of northward and southward directed motions Horizontal Residual Velocities

Residual (intraplate) Velocities GIA signal: Vertical: Around Hudson Bay rapid uplift, slower subsidence south of Great Lakes Horizontal: Outward near HB and Western Canada (secondary ice load) Non-GIA signal Central US, small coherent uplift with randomly oriented horizontal Coherent patterns in GPS Velocities Vertical Velocities

Largest detectable motion within stable plate is GIA Essential to account for it How? Remove GIA model predictions - Helps or hurts? Omit GIA affected sites? Implication of GIA

GIA: Many models, many predictions We use Earth assumed laterally homogeneous with seismically realistic depth-varying density and elastic parameter profiles ICE-3G ice loading history from the Last Glacial Maximum (18,000 yrs to the present) 120 km thick elastic lithosphere, upper mantle viscosity of Pa s and lower mantle viscosity of 2 x Pa s (LM2) and 4.5 x Pa s (LM4.5) Load increases from 0 at 100,000 yrs to maximum at 18,000 yrs then decreases in 1,000-year increments

Vertical: predicted uplift (rebound) north of Great Lakes, subsidence to south, both models similar Horizontal: Orientation of vectors similar but large difference in the far field Model Predictions

Observed Vertical Good agreement between model & observations In Hudson Bay LM 4.5 (higher mantle viscosity) better fit minus Predicted Vertical Equals If perfect match then should be all white Vertical Velocities LM4.5 LM 2

Test removal of GIA predictions from horizontal Vel. LM2LM4.5 Rigid North America defined using 83 CGPS  2 =1.08 (Better) -5.0N,85.3N,0.195°/Myr,1.0  max,0.3  min include 46 GIA affected sites 129 sites  2 =1.33 (worse) Remove LM2 Predictions to 83 CGPS  2 =1.11 Remove LM4.5 Predictions to 83 CGPS  2 =1.41 GPS intraplate

LM2LM4.5 GPS intraplate Pole Positions wrt ITRF00 83 sites RIGID N E  /Myr 1.0  max 0.3  min GIA N E  /Myr 1.9  max 0.5  min 83 LM2 Corr N E  /Myr 1.0  max 0.3  min 83 LM4.5 Corr N E  /Myr 1.1  max 0.3  min

Horizontal Velocities LM2LM4.5GPS intraplate Near field: GPS horizontal velocities are larger than LM4.5 Far field: GPS horizontal velocities are small like LM2 but random LM4.5 very poor fit in the far field Reasons: Model assumptions of a laterally homogeneous earth are not valid Ice load history may be incorrect affecting near field GPS field too sparse in the North

Implications for SNARF Largest residual (intraplate) signal across stable North America is GIA GIA models generally predict observed GPS vertical velocities but do poorly in the horizontal Removing GIA model predictions does not reduce misfit In contrast omitting sites that appear affected by GIA reduces misfit Improved or other GIA model may do better

Pole Positions wrt ITRF00 83 sites RIGID N E  /Myr 1.0  max 0.3  min GIA N E  /Myr 1.9  max 0.5  min 83 LM2 Corr N E  /Myr 1.0  max 0.3  min 83 LM4.5 Corr N E  /Myr 1.1  max 0.3  min