Radial mode observations from the 5/23/89 MacQuarie Ridge Earthquake

The radial modes 1 S 0 – 5 S 0 axe clearly observable on several long‐period seismic records from the 5/23/89 Macquarie Ridge earthquake. Observations of 0 S 0 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 1 S 0 – 5 S 0 relative to 0 S 0 . Calculations suggest that the anomalous amplitudes of 1 S 0 – 5 S 0 can be explained by coupling to other modes induced by plausible levels of deep‐earth lateral structure. The coupling of 0 S 0 is much weaker, and its observed amplitude suggests 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 λ = 0°, δ = 90° in a simple double‐couple point source, or by thrust subevents within the main shock.