Contributions of theoretical seismograms to the study of modes, rays, and the Earth

Publisher Website
Google Scholar
Abstract
Theoretical seismograms, for both simplified and realistic models of the earth, can be calculated and analyzed to learn the relations between free oscillations and traveling waves. These studies can help to clarify the influence of the physical properties of the earth upon the generation, propagation, attenuation, and dispersion of elastic waves. Principal results include the equivalence of mantle and crustal Rayleigh waves to the fundamental radial modes of spheroidal free oscillations and the equivalence of G waves and ordinary Love waves to the fundamental radial modes of torsional free oscillations. In contrast, the higher radial modes produce all the direct, reflected, refracted, and diffracted body phases, as well as the higher‐mode surface waves. Stoneley‐wave propagation along the core‐mantle boundary introduces a large number of interesting phenomena into the propagation of the spheroidal overtones. The confinement of the Stoneley modes to the vicinity of this boundary is responsible for the discontinuities in the spectra and the relative difficulty in the excitation of long‐period P and SV diffracted arrivals, as compared with the corresponding SH phases. The trapping of body‐wave energy for sources within a velocity inversion is demonstrated. The particle motion ratio at the surface may be as useful as phase velocity in the understanding of the physical properties beneath a recording site; realistic curves of this parameter are presented for a wide range of periods and radial orders.