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 §3.1-3.2 (67-75) © A.R. Lowry 2017

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), 759-808

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.

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.

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

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

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

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).

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

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

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….

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