The vertical ground vibration at the surface of an elastic half-space caused by a point excitation will attenuate with distance from the excitation. Based on the classical wave theory

the vibrations are mainly governed by body waves （P and S waves） in the near region and by the surface wave （R wave） in the far field

respectively

which leads to different equations for predicting their vibration intensities varying with distance from the excitation. Through comparing the exact and approximate solutions of the Lamb’s problem

the borderline between near and far field of vertical ground vibration at the surface of an elastic half-space is investigated in this paper. Parametrical studies show a greater distance of the mentioned boderline from the excitation determined by the amplitude of displacement than by its phase angle

and that the result by the former will increase with increasing Poisson’s ratio of ground and with decreasing allowable error of computation while the result by the latter behaves less correlatively to the Poisson’s ratio of ground. Because the wave length of P increases larger than those of S and R waves

it is mainly the P wave that makes the borderline between near and far field farther for an increasing Poisson’s ratio or saturation of ground.