The ratio of the vertical and horizontal components of the P phase provides a crustal transfer function which is independent of the frequency content of the source and is a function only of the angle of incidence of the rays and the crustal parameters of the site where the observation is taken.
This ratio of the vertical and horizontal spectra corresponds to the tangent of the apparent angle of emergence and as such this apparent angle is a function of the frequency. Spectra of this type have been obtained in the central United States and in La Paz, Bolivia, for large magnitude teleseisms. The observed curves have been compared with theoretical universal curves corresponding to one- and two-layer models.
Average results for several observations give a crustal thickness in the central United States of 42 kilometers and a mean P velocity of 6.6 kms/sec. For the Bolivian Andes at La Paz, the crustal thickness obtained is 64 kms and the mean P velocity 6.7 kms/sec. These results are in good agreement with similar determinations obtained by independent methods.