On the long period character of shear waves

S and multiple S phases at moderate to large epicentral distances are frequently followed by normally-dispersed, long-period, wave trains for which surface particle motion is elliptical and progressive and in the plane of propagation of the SV wave. The character of such phases can be explained as the result of coupling between the incident shear waves and dispersive PL waves in the near-surface wave guide. A detailed study of shocks in Mexico and in Montana recorded at Resolute [Northwest Territories, Canada], and less detailed studies of other data support this hypothesis.