Free-oscillation coupling theory

Abstract

The free oscillations of an elastic, spherical earth model can be combined using variational or Galerkin techniques to form hybrid oscillations that represent the free oscillations of a rotating, anelastic and laterally-variable earth model. With supercomputers and/or array processors, it is feasible to synthesize realistic long-period seismograms from sums of these coupled free oscillations. Such calculations demonstrate pervasive coupling between toroidal (SH, Love) and spheroidal (P-SV, Rayleigh) motion in surface wave with periods Τ > 240s due to the earth’s rotation, and show these effects to be especially prevalent on north-south propagation paths. Surface-wave propagation effects, such as wavepacket amplitude anomalies, can also be modelled. Theoretical techniques have been developed to represent functionals of the seismogram (e.g. modal amplitude and frequency shift in the frequency domain, the oscillation envelope of an isolated multiplet in the time domain) in the presence of mode-mode coupling. These techniques promise, along with recent work on the coupling induced by anisotropy, a significant improvement in the accuracy of future inversions of the global long-period seismic data set.KeywordsSurface WaveLateral StructureGreat CircleFree OscillationSynthetic SeismogramThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.