The radial modes 1S0--5S0 are clearly observable on several long-period seismic records from the 5/23/89 Macquarie Ridge earthquake. Observations of 0S0 are weaker, but stacking demonstrates significant excitation. Calculations suggest that, for spherical-earth models, no simple double couple in the uppermost mantle can explain the excitation amplitudes of 1S0--5S0 relative to 0S0. Calculations suggest that the anomalous amplitudes of 1S0--5S0 can be explained by coupling to other modes induced by plausible levels of deep-earth lateral structure. The coupling of 0S0 is much weaker, and its observed amplitude suggest that roughly 4% of the total moment release inferred from surface wave studies deviated from pure strike-slip geometry. This can be modeled by >8¿ deviations from &lgr;=0¿, Δ=90¿ in a simple double-couple point source, or by thrust subevents within the main shock.