The dispersion of shear-coupled PL waves, PL(S) and PL(SS), has been studied for several paths on the Canadian Shield over the distance range 33.4° to 115.5° with the twofold aim of elucidating the manner in which these waves are generated and of examining their value in investigating the structure of the crust and upper mantle. In many cases these waves are composed of several wave trains quasi-stationary in period. These observations can be explained by considering the large amplitudes associated with cusps in the S-wave travel-time curve and the adjacent flat portion of the S-wave apparent velocity curve produced by velocity discontinuities in the upper mantle; little-dispersed PL(S) trains appear to be generated in regions where S waves of very high amplitudes impinge on the crust. In addition, the recovery of the periods of these individual wave trains in the PL(S) record can provide information on the crustal structure in a relatively localized region and on the ray parameter close to each cusp.

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