Fig. 4

Phase tensor plots coloured by skew for AMT and BBMT datasets (annotated with period). Data unaffected by noise are predominantly 1D at short periods, as indicated by yellow circular ellipses. Distorted ellipses with low skew values are indicative of 2D data, while ellipses with red or blue colouration indicate 3D subsurface structure. a–e north block AMT data, increased data noise responsible for scattering at 1 s, 1.25 × 10⁻³ s and 1.25 × 10⁻⁴ s. f–j southern block AMT data. k–n data for BBMT sites, western sites are 3D at 1 s. Displayed ellipses were generated using one-third of the total BBMT sites and half of all AMT sites for clarity