Components of the velocity gradient field derived from the GPS velocity field (A, B and C) and the best-fitting model velocity field (D, E and F). Plots A and D represent the second invariant of the strain rate tensor (i.e. pure shear) and indicate the axes of maximum rates of contraction and extension with black and white line segments, respectively, with magnitudes proportional to the lengths of these segments. Plots B and E represent the first invariant of the strain rate tensor (i.e. areal strain rate) with C and E indicative of rates of contraction and extension, respectively, plots C and F represent the rotation rate tensor (i.e. the antisymmetric part of the velocity gradient tensor) with CW and CCW indicative of clockwise and counter-clockwise. Superimposed dots indicate GPS sites. Contours are plotted at intervals of 1 × 10⁻⁷ yr⁻¹, with blue for positive, red for negative and green for zero values.