Certain normally and inversely dispersed wave trains appearing in the interval between P and S at moderate epicentral distances are identified as specific higher leaking modes. Observed periods of normally dispersed wave trains vary from 7 to 5 sec and from 12 to 9 sec; observed periods of inversely dispersed wave trains increase from about 15 to 35 sec. While the normally dispersed wave trains are generally observed at epicentral distances less than 20°, the inversely dispersed wave trains may be observed at distances up to about 50°. This latter type of wave, with group velocities between 7.0 and 8.5 km/sec, is shown to be controlled by upper mantle structure, and thus represents a possible tool for upper mantle investigations. These waves differ from waves of the Rayleigh and Love modes in that they are more sensitive to the compressional velocity structure than to the shear velocity structure of the waveguide.

PL and shear-coupled PL data which are identified with the fundamental leaking mode were obtained for two paths in South America. Crustal thicknesses for the path between Buenos Aires and Rio determined independently from PL, shear-coupled PL and Rayleigh wave data agree well with one another. An analysis of the method of obtaining phase and group velocity curves consistent with the shear-coupled PL data shows that this method and this type of data together provide a high degree of precision for the determination of phase velocity curves.

In the theoretical treatment, a rapid, approximate method, which is basically Haskell's (1962) method, was used to obtain dispersion curves for the leaking modes. The results of this method are virtually identical to those of Gilbert (1964) and those of Oliver and Major (1960). The main advantage of the present method is that it provides additional information on particle motion and on the relative amplitudes of the predicted arrivals as functions of phase velocity and period.

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