Observed envelopes of coupled seismic free oscillations

On a rotating, aspherical earth, coupling between spheroidal and toroidal seismic free oscillations is manifested in the time domain as a slow alternation of the oscillation envelope between the transverse and the radial and/or vertical motion components. Using a narrow‐band smooth envelope estimator, we have observed this behavior in data from the M S = 8.1 Macquarie Ridge earthquake of 5/23/89. Rotational coupling is the principal cause of mixed‐type coupling for f < 4.2 mHz. However, envelope perturbations consistent with mixed‐type coupling are observed for 0 S 38 ‐ 0 T 36 and 0 S 37 ‐ 0 T 35 , which do not couple through rotation, and couple only weakly through smooth upper mantle velocity models. Such envelope perturbations can bias estimates of modal Q , and suggest that scattering may play a role in the attenuation of long‐period surface waves.