RADIAL MODE OBSERVATIONS FROM THE 5/23/89 MACQUARIE RIDGE EARTHQUAKE

The radial modes 1S0–5S0 axe 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 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. Copyright 1990 by the American Geophysical Union.