THE 23 MAY 1989 MACQUARIE RIDGE EARTHQUAKE - A VERY BROAD-BAND ANALYSIS

We use data from the GEOSCOPE global seismograph network to determine source parameters for the 23 May 1989 Macquarie Ridge earthquake. By moment tensor inversion of long‐period surface waves we obtain a seismic moment of 1.8–2.4 × 1021 N m, which makes this the largest seismic event to have occurred worldwide since the 1977 Sumba earthquake. The focal mechanism shows pure right‐lateral strike‐slip motion on a plane parallel with Macquarie Ridge and the Pacific‐Australian plate boundary. The mechanism is consistent with the strike‐slip motion predicted for this area from relative plate motion models. We invert broad band teleseismic P and SH waveforms to determine source complexity, using the results from the long‐period analysis as a priori soft constraints. The broad band waveforms show that the release of seismic energy was concentrated in the first 30 s of seismic rupture, but fault slip continued for more than 60 s. Lack of directivity in the long‐period and broad band waveforms indicates that the rupture propagated bilaterally or vertically from the nucleation point. Copyright 1990 by the American Geophysical Union.