North American GPS station velocities relative to international terrestrial reference frame

North American GPS station velocities relative to fixed NA plate

 

Source: This map is updated daily by C. DeMets of UW-Madison from GPS station velocities determined at the UW Department of Geosciences. The map is created using Generic Mapping Tools software.

Upper map: The station motions in the upper map show how continuous GPS stations in the United States and vicinity move with respect to an international Earth-based reference frame called ITRF2008. The systematic counterclockwise motion (rotation) of the stations illustrates the expected rotation of the North America tectonic plate about its pole of rotation, which lies just south of the edge of the map near 270 degrees east longitude. Mathematically, it is easy to select all GPS stations from the rigid interior area of the plate and use them to find the pole of rotation and rate of angular rotation that best describe the measured GPS station rates and directions (the GPS site velocities).

Lower map: Given the North America plate rotation described above, one can easily change the frame of reference for GPS sites shown in the map to one in which the North America plate interior does not remove (i.e. the influence of the rotation is removed). To do so, the best-fitting plate rotation is first used to estimate the plate's motion at the location of each GPS site in the United States. Those estimates are then subtracted from the measured velocity of the GPS site, thereby removing the influence of North America plate motion at each site. The remaining "residual" site velocities mostly show random and other noise that affects daily estimates of the location of each GPS antenna. However, the residual velocities can reveal patterns of movement within a tectonic plate interior. Examples include the slow southward movement of stations in Canada and the north-central and northeastern United States in response to ice sheet unloading of these areas at the end of the last ice age (roughly 10,000 years ago) and motions of stations near active plate boundary faults in the western United States.