LONG-PERIOD SEISMIC RADIATION FROM THE MAY 23, 1989, MACQUARIE RIDGE EARTHQUAKE - EVIDENCE FOR COSEISMIC SLIP IN THE MANTLE

Long-period seismic source parameters of the May 23, 1989, Macquarie Ridge earthquake are determined using a surface wave inversion procedure that incorporates detailed source-time functions obtained from shorter-period body waves. The seismic source model obtained using this method is consistent with observations of both body waves and long-period Rayleigh waves from the earthquake. The Macquarie Ridge earthquake rupture has a centroid time (28 s) and a right-lateral strike-slip fault mechanism with a rake of 175-degrees on a vertical fault plane striking N38-degrees-E. This mechanism is consistent with P wave first motions of the event. Inversions performed for various earth models demonstrate that the choice of surface wave attenuation model, in particular, affects the estimates of centroid depth and seismic moment significantly. Allowing for uncertainty in attenuation, the long-period Rayleigh waves (periods from 150 to 300 s) indicate that the Macquarie Ridge earthquake had a seismic moment of 1.9 +/- 0.2 X 10(21) N m and a corresponding centroid depth of 21 +/- 6 km. The static stress drop calculated using the depth and seismic moment is 0.7 +/- 0.5 X 10(7) Pa (70 +/- 50 bars). The centroid depth, combined with the lack of resolvable directivity of the earthquake rupture, suggests that significant slip occurred beneath the Moho, which has a maximum depth of about 16 km in the epicentral region. The mantle slip component may have preferentially radiated long-period seismic energy given the shallow centroid depths and low moments determined for the event from the shorter-period body wave observations.