Effect of sharp lateral heterogeneity on the Earth's normal modes

Abstract

When inverting normal mode data for global large-scale lateral heterogeneity, possible biases due to small-scale structure are commonly ignored. We conducted two experiments. In the first one, we calculated, with a recently developed first order scattering formalism, the effect of a simplified subduction zone model on the eigenfrequencies of the fundamental spheroidal mode branch. The frequency shifts induced by this single subduction zone appear to be smaller by only a factor of 3 to 4 than shifts observed on long-period seismograms, or than shifts induced by existing models of global upper mantle inhomogeneity. The second experiment involved a model of a hot spot plume on the core mantle boundary. Solving the variational problem with many coupling terms included, we calculated the effect of such a plume on the amplitude of some low angular order modes. The results suggest an effect at least as large as that due to available large scale mantle models. It thus appears, that relatively small-scale, sharp lateral structure in different depth ranges of the Earth may have an important effect on normal mode observations and that these effects may not be ignored in inversions of normal mode data.