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 t oroidal modes on a spherically symmetric, non-rotating Earth have horizonta lly polarized particle motion these observations call for an explanation. W e first rule out local and instrumental effects as being responsible for th e vertical-component signal of the toroidal modes T-0(3) (0.59 mHz) and T-0 (4) (0.77 mHz). The global effects that we consider are general heterogeneo us mantle structure, ellipticity of figure and rotation. We find that rotat ion through Coriolis coupling of low-order spheroidal and toroidal oscillat ions is the dominant mechanism.