Stable North American Plate Bending and Moving After Ice Loss According to GPS

Researchers: C. Kreemer, W. C. Hammond, and G. Blewitt, University of Nevada, Reno

Written by Linda Rowan
17 September 2018

Summary

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.

Observations

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).