The Hunting of the SNARF

Giovanni F. SellaSeth Stein

Northwestern University

Timothy H. DixonUniversity 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 PlateMotion 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 PlateInternal 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 sitesStable 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 GPS24hrs, 365 days

EGPS - Episodic GPS8-12hrs, 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

LakesHorizontal: Outward near HB and Western Canada (secondary ice load)

Non-GIA signalCentral US, small coherent uplift with randomly oriented horizontal

Coherent patterns in GPS VelocitiesVertical 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 1021 Pa s

and lower mantle viscosity of 2 x 1021 Pa s (LM2) and 4.5 x 1021 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

ObservedVertical

Good agreement between model & observations

In Hudson Bay LM 4.5 (higher mantle viscosity) better fit

minusPredicted Vertical

Equals

If perfect match then

should be all white

Vertical Velocities LM4.5

LM 2

Test removal of GIA predictions from horizontal Vel.

LM2 LM4.5

Rigid North America defined using 83 CGPS

2=1.08 (Better)

-5.0N,85.3N,0.195°/Myr,1.0max,0.3min

include 46 GIA affected sites 129 sites

2 =1.33 (worse)

Remove LM2Predictions to 83 CGPS

2=1.11

Remove LM4.5Predictions to 83 CGPS

2=1.41

GPS intraplate

LM2 LM4.5GPS intraplate

Pole Positions wrt ITRF00

83 sites RIGID -4.96 N -85.34 E 0.1953/Myr 1.0max 0.3 min

83 + 46 GIA -2.23 N -83.60 E 0.1989/Myr 1.9max 0.5 min

83 LM2 Corr -4.44 N -84.40 E 0.1996/Myr 1.0max 0.3 min

83 LM4.5 Corr -4.30 N -81.79 E 0.2011/Myr 1.1max 0.3 min

Horizontal Velocities

LM2 LM4.5GPS intraplate

Near field: GPS horizontal velocities are larger than LM4.5Far field: GPS horizontal velocities are small like LM2 but randomLM4.5 very poor fit in the far fieldReasons:Model assumptions of a laterally homogeneous earth are not validIce load history may be incorrect affecting near fieldGPS 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 -4.96 N -85.34 E 0.1953/Myr 1.0max 0.3 min

83 + 46 GIA -2.23 N -83.60 E 0.1989 /Myr 1.9max 0.5 min

83 LM2 Corr -4.44 N -84.40 E 0.1996 /Myr 1.0max 0.3 min

83 LM4.5 Corr -4.30 N -81.79 E 0.2011/Myr 1.1max 0.3 min