Local changes in crustal structure from variations of phase velocity of Rayleigh waves are deduced by utilizing the prolonged, sinusoidal trains of dispersed Rayleigh waves characteristic of long oceanic propagation paths. Crests and troughs of these waves can be traced across a triangular array of seismograph stations when the station separations are not more than a few wave lengths. Phase velocity and direction of approach can be determined for waves of a given period. By comparing observed phase velocity with an experimentally determined phase-velocity curve representative of the average continental crust local crustal thickness is obtained.
Application of the method to Southern California indicates the following: (1) normal crustal thickness in the Peninsular Range Province; (2) increase of mean crustal thickness by about 50 per cent in the Sierra Nevada block; (3) significant decrease in crustal thickness, possibly as much as 50 per cent in the continental borderland.