Rayleigh waves produce retrograde ellipses. Therefore, the particle motion plot will show us if we have elliptical waves or transverse waves.

Therefore if we think we have FZTW, so if BBFK shows that the horizontal phase velocity of the supposed FZTW is close to that of the S waves, plus or minus errors, then that shows that we have a low velocity zone near the surface which is unrelated to the fault.