Abstract
SUMMARY At frequencies below 1 mHz toroidal modes are more difficult to observe than spheroidal modes for two reasons. First, the comparatively rapid decay of the signal (low Q factor) of toroidal modes and the consequently short records which have to be analysed result in low resolution in frequency. Secondly, high noise on horizontal component seismometers due to their sensitivity to tilts induced by atmospheric disturbances makes it difficult to observe long-period motion. Here we present observations of the lowest order toroidal modes based largely on digital recordings of the 1989 Macquarie Rise event. We present what we believe to be the first unambiguous observation of the gravest toroidal mode oT2. We estimate multiplet degenerate frequencies and Q for the fundamental modes up to ,TI,, and the overtone lG. The frequencies of individual singlets are estimated for the modes o&-o&, and for the three multiplets o&-oco we have sufficient signal to estimate also the degree 2 aspherical structure coefficients.
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