PLATE KINEMATICS (Copyright 2010, David T. Sandwell)
(Reference - The Solid Earth, C.M.R. Fowler, Cambridge University Press, 1990, Chapter 2)
Plate Motions on a Flat Earth
Plate tectonic theory describes the motions of rigid plates on a spherical earth. However, when considering the relative motions very close to the plate boundary, or at a triple junction, it is appropriate to use a flat earth approximation. We'll begin with the flat earth case and then move on to the spherical case. Consider 2 plates A and B, which have a subduction zone boundary between them such as the Nazca and South American plates. In this analysis all plate motions are relative so one can either consider plate B as fixed or plate A as fixed and draw the relative vector velocity between them as shown in the diagram below.
– velocity vector of plate A relative to plate B.
– velocity vector of plate B relative to plate A.
= Vxi + Vy j