Mantle wave deviations from ‘pure-path’ propagation on aspherical models of the Earth by Gaussian beam waveform synthesis

Abstract

Departures from the classical approximations of geometrical optics and great circle propagation are investigated by surface wave ray-tracing on laterally heterogeneous models and computation of synthetic seismograms by the Gaussian beam method. Higher order deviations, such as time differences between odd and even orbits, residuals from great-circle average times, and fluctuations in the succession of eigenfrequencies in the spectra are sought for and compared with long-period great-circle observations. In the case of the Akita-Oki (Honshu) earthquake (1983) at the Geoscope station PAF, the deviations are of the order of the first order tomographic effect. They are interpreted by a transverse velocity gradient shifting the orbits. In other examples where great-circle propagation is nearly satisfied, geometrical effects of curvature are displayed. Frequency fluctuations due to lateral heterogeneity appear in the synthetic spectrum.