Measuring Plate Motions

Four space geodetic techniques for plate motions include:

The measurements must be applied to a reference frame; the most commonly used now is ITRF (Reference 1, Reference 2).  This provides the motion at each station. The station velocites can be combined to find the Euler poles for each plate.  Then the Relative motion between two plates can be determined at their boundary.

GPS motions of western North America (1689 stations from SOPAC).  The number of stations has been decimated to show the pattern, and even then the large vectors west of the San Andreas plot all over each other.

The motion in the global reference frame take some thinking about.  The stable part of North America is moving west, away from the Mid-Atlantic ridge, although in a different reference frame North America could be moving east, just more slowly than Europe and Africa.  You also have to deal with the map projection, and its depiction of great and small circles, and the location of the Euler pole in relation to the geographic poles.

West of the San Andreas fault, the Pacific plate with San Diego, Los Angeles, and San Francisco is moving northwest, parallel to the plate boundary.
Horizontal speed, in m/year.  This was computed with the Pythogorean theorem from the N and E components of motion.
Vector motions.  This uses the N and E components of motion.
Vertical (Upward) component of motion.  These are all very small, except for the Great Central Valley.  This agricultural region was been rapidly withdrawing ground water, and has seen substantial ground subsidence.

Last revision 1/7/2017