It is well known that the shear strain amplitude caused by an elastic wave can be estimated on the basis of measurements of the particle velocity. In this aspect, the difference between a vertically polarized plane shear wave and a plane Rayleigh wave is presented. A particular case is studied in which the vertical component of the velocity of a particle lying on the ground surface is known. The relation between the particle velocity and the strain amplitude is derived and a diagram for evaluation of this amplitude is given.