This plot of seismic anisotropy shows two aspects of shear wave splitting.

The magnitude of the blue circle is proportional to the difference in time (in seconds) between the arrivals of the two incoming shear waves. A value of ‘null’ indicates either no data was taken for that location, or the difference in arrival times was small enough (less than 0.05 seconds) that the instrument could not detect a variation.

The black line through each blue circle indicates the fast polarization direction of the incoming shear waves.

The fast direction shows the approximate direction of flow in the mantle and/or the direction of maximum extension in the region.

For a useful animation describing shear wave splitting, visit this link (offsite)