Sensitivity analysis of inversions of GPS velocity data (Fig. 2) for the velocity gradient field. Results for size[C] (eq. 2, Figs 1a, 2a and 3a), size[S] (eq. 3, Figs 1b, 2b and 3b), the normalized root-mean-square misfit (eq. 4) of the ∇×∇v= 0 constraints (Figs 1c, 2c and 3c) and the GPS data equations (Figs 1d, 2d and 3d) for varying weights on damping (αd, along y axes), smoothing (αs, along x axes) and ∇×∇v= 0 constraints (γr, subplots 1, 2 and 3). Every contour plot represents 25 inversions.