When the path between epicenter and station traverses only continental structure, the dispersion of the entire train of directly arriving seismic surface waves can be explained as the result of normal mode propagation in a crust-mantle system in which the velocity increases in some manner with depth within the crust. At least four modes, the Rayleigh mode, Sezawa's M2 mode, and the first two Love waves, may appear prominently on the seismogram. The characteristics of the higher-mode dispersion curves permit the explanation of the Lg phase of Press and Ewing, Båth's Lg1 and Lg2, and, in some cases, Caloi's Sa without recourse to a low-velocity layer in the crust or mantle. Speculation on changes in these curves for less simplified models indicates that the remaining cases of Sa as well as Leet's C or coupled wave may be explained by classical theory.
The occurrence of the higher-mode waves is widespread; they are found on the four continents for which data are available.
Higher-mode data, particularly when combined with information from the fundamental modes, make surface-wave dispersion, previously a useful tool, a much more potent method for the study of crustal structure.