Rigid North America and its western plate boundary zone

An important premise of the theory of plate tectonics is that that large regions of the earth's surface move coherently, such that most earthquakes, deformation, volcanism, and extreme topography occur at plate boundaries. This prediction is beautifully confirmed by GPS data. The red arrows show the direction and speed of each GPS site relative to the Pacific plate. The longer the arrow the faster the movement. The pattern of arrows shows that sites within the essentially rigid North American plate rotate coherently about a point in northern Canada (the Pacific-North America pole of rotation). Linear velocities vary as the sine of the angular distance from the pole of rotation, and directions follow small circles about it, like those shown by the dashed lines. The small deviations from this ideal behavior, about 1-2 mm/yr, occur in part due to deformation within the plate that causes intraplate earthquakes. In contrast, site motions within the western plate boundary zone deviate significantly from the rigid plate model, showing the much larger deformation and hence earthquakes there.

Stein, S. and G. Sella, Plate boundary zones: concept and approaches, in Plate Boundary Zones, American Geophysical Union Geodynamicsr Series 30, 1-26, edited by S. Stein and J. Freymueller, Washington, D. C., 2002.