Through study of the characteristics of strong earthquake activities in the Western China, it is found that strong (M≥6) earthquakes form three groups of gaps with different directions, which are nearly equidistantly distributed. The distance between two gaps is about 5°,and the width of one gap is 1°. This paper gives a preliminary explanation about it by supposing the plateau boundary is being affected by periodical forces. It is considered that such phenomenon may be due to the stationary wave which is formed by the superposition of stress waves created by three groups of periodical boundary forces. Moreover the wave lengths of such stress waves are very similar to those of structural waves obtained by other researchers in the past.