Observation of Coriolis coupled modes below 1 mHz

We present observations of spectral energy at toroidal mode frequencies in vertical seismic recordings of the 1998 Balleny Islands earthquake. Since toroidal modes on a spherically symmetric, non-rotating Earth have horizontally polarized particle motion these observations call for an explanation. We first rule out local and instrumental effects as being responsible for the vertical-component signal of the toroidal modes 0T3 (0.59mHz) and 0T4 (0.77mHz). The global effects that we consider are general heterogeneous mantle structure, ellipticity of figure and rotation. We find that rotation through Coriolis coupling of low-order spheroidal and toroidal oscillations is the dominant mechanism.