The most popular means to obtain the direction from which microseismic waves approach are the tripartite method and the method based on the orbital motion of the ground particle. However, it has been clarified by the theoretical researches on the Rayleigh-type dispersive waves that even among the waves having apparently Rayleigh-type orbits, there can exist two kinds of waves, one undergoing retrograde motion and the other undergoing progressive motion.
In the present study, the writer examined the winter microseisms that were recorded during the period from November 1964 to February 1965. The directions of approach of these microseisms were determined by the tripartite method, and also the orbital motions of the ground surface particles were drawn on the basis of the three-component seismograms, so as to compare the results of the two different methods. In consequence, 221 waves were recognized to have typical Rayleigh-type orbits, about 40% of which have undergone retrograde motion and about 20% are of progressive motion. Thus, it has become clear that the directions from which microseismic waves approach should not be determined merely by their orbital motion, because it would involve a great danger.