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# 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.

A

B

A

B

VAB

fixed

fixed

VBA

V_{AB }

– velocity vector of plate A relative to plate B.

V_{BA }

– velocity vector of plate B relative to plate A.

V_{A B }

= −V_{BA }

V_{A B }

= V_{xi + Vy j }