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. |
