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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