Researchers: C. Kreemer, W. C. Hammond, and G. Blewitt, University of Nevada, Reno
Written by Linda Rowan
17 September 2018
The stable North American plate shows internal deformation mostly because of the rising of the land surface since the demise of the extensive glaciers that weighed the surface down. Analysis of thousands of GPS observations over decades show that besides the upward bending the stable plate also undergoes horizontal contraction, mostly due to higher than expected inward velocities around the edges of the former ice sheets. Intraplate earthquakes do not correlate with the geodetic forcing, suggesting that other factors must play a role in causing those events.
Plate tectonic theory holds that tectonic plates move as rigid bodies and only show deformation at plate boundaries. The North American plate is considered to be stable and rigid from the central to eastern U.S. and Canada, eastern Mexico, Greenland and part of the Atlantic Ocean. Any bending or deformation of this part of the plate has been attributed to glacial isostatic adjustment (GIA). GIA refers to the rebound of the land surface due to the removal of glaciers. The ice sheets covered Canada and the northern part of the United States during the last ice age and now these regions are rising as the weight of the ice disappears.
One compelling way to measure the amount of deformation in a stable plate is by GPS measurements. GPS measures the horizontal and vertical motion of the plate over time at a specific location with millimeter precision. Here the authors use 3,271 GPS velocities collected daily between 1 January 1996 and 3 November 2017. Unlike past work, this study determines the horizontal and the vertical velocities of each site on the stable plate and presents a new strain rate field map using an algorithm called median estimate of local deformation (MELD).
The computed velocities and strain field mapping show that much of the stable plate is moving about 1 to 2 millimeters per year toward central Canada. This causes a previously undetected crescent zone of contraction around Canada. The motion toward the center of the rebounding land, where the thickest ice has been lost tens of thousands of years ago and the resulting contraction reveal an unexpected far-field reach of GIA. These observations offer new insight into the structure and dynamics of the crust and mantle of the stable plate. The other interesting result is that there is no correlation between historic intraplate seismicity and the current geodetic forcing. Therefore the search for an explanation of the intraplate seismicity continues with the likely causes related to smaller scale forces whereas the geodetic forcing described here is at a resolution of 100 kilometers or larger.
Kreemer, C., Hammond, W. C., & Blewitt, G. (2018). A robust estimation of the 3-D intraplate deformation of the North American plate from GPS. Journal of Geophysical Research: Solid Earth, 123, 4388–4412. https://doi.org/10.1029/2017JB015257 doi:10.1029/2017JB015257.
glacial isostatic adjustment, geodetic forcing, strain
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