1. Geodesy & Crustal Deformation
Geology 6690/7690
Geodesy & Crustal Deformation
25 Sep 2017
Borehole Strain; GPS Reference Frames
• Borehole strainmeters extremely sensitive to both strain &
local elastic heterogeneity => best to constrain timing
• Plate Velocity Modeling uses GPS velocities to build on
plate velocity models originally estimated from geologic
spreading rates (seafloor magnetic anomalies), relative
motion directions (transform fault azimuths & EQ slip vectors).
• Velocities at sites in the IGS reference network are part of
the GPS Reference Frame relating Earth-fixed to satellite
motion; reference frame effects are part of position error
• Translation rate of the GPS origin may partly reflect “real”
changes in Earth center-of-mass relative to center-of-figure,
but is within error so (likely) mostly reflects sampling geometry
Read for Wed 27 Sep: Wahr § (67-75)
© A.R. Lowry 2017

2. Read for Monday, Oct 2:
Herring et al. (2016) Plate Boundary Observatory and related
networks: GPS data analysis methods and geodetic
products, Reviews of Geophysics, 54(4),

3. Example time series including correction for an antenna
Sella et al., JGR 2002
Example time series
including correction
for an antenna
change at the site.

4. Compare with RMS from circa-2009 PBO solutions: The
RMS = 1.5 mm (3.0)
RMS = 1.7 mm (4.8)
RMS = 4.7 mm (8.2)
Compare with RMS from circa-2009 PBO solutions: The
difference is mostly in realization of the reference frame.

5. Different applications entail different model assumptions:
One common starting point is “plate motion”, which
assumes spherical motion of a rigid cap or plate on the
surface of the Earth ( no strain!)
Bird, Geochem. Geophys. Geosys., 2003

6. Frame is somewhat ambiguous in that we have to choose
a translation describing how plates move relative to the
center of the Earth… For GPS we use a no-net rotation
(NNR) frame, which is different from the hotspot
(mantle-fixed?) frame that we most often think of plate
tectonics in terms of.
It’s also useful to describe plate frames (e.g., North
America fixed):
Blewitt et al., J. Geodyn., 2013

7. Recent improvements enable us to look more closely at
nonlinear signals: The blue line here is a load model
derived from climatological data.

8. Left out sites based on <100 km distance from a Bird (2003) plate
Altamimi et al.
JGR 2012
Left out sites based on <100 km distance from a Bird (2003) plate
boundary (red); glacial isostatic adjustment from a model (blue);
large velocity residuals (black, > 3s or 3 mm).

9. Note however velocities can be
perturbed significantly beyond 100 km
from the plate boundary…

10. They use Kreemer et al.’s GPS-derived global strain rate map to
exclude other deforming zones.
Kreemer et al., G3, 2014

11. GRACE geoid rate (Detlef &
Ivins, J. Geodynamics, 2008)
GPS vertical velocities show
some general correspondence
to the GRACE measurements…
Difficult to see given differences
in projections here, but there is
a sampling issue with GPS.
Sella et al. (GRL, 2004) used
both campaign and continuous
GPS data; get better spatial
sampling than Calais et al….

12. But the effects of glacial isostatic adjustment on
horizontal velocity (especially
that which could be interpreted
as plate motion) is MUCH more
widespread than the vertical
perturbation.
Latychev et al, GRL, 2005