THE 1989 MACQUARIE RIDGE EARTHQUAKE AND ITS CONTRIBUTION THE REGIONAL SEISMIC MOMENT BUDGET

The Macquarie Ridge earthquake (1989 May 23; Mw 8.2) occurred in a region of complex tectonics that marks the southern continuation of the plate boundary (Pac‐Aus) that runs through New Zealand. The first motion focal mechanism (from WWSSN and regional station polarities) indicates dominantly strike‐slip movement (strike 025°, dip 80°, rake 180°), probably dextral on the northeast striking nodal plane. The plate boundary south of New Zealand appears to be segmented and marked by linear bathymetric highs with distinctive trends. The distribution of shallow seismicity follows these bathymetric trends. The moment tensors for the largest earthquakes (Ms > 6.0) occurring on five distinctive segments of the plate boundary, for the period 1964–1989, have been summed and compared with the deformation predicted by the plate motions. Recurrence times of the largest earthquakes in each segment, determined by assuming that all deformation occurs seismically along the Macquarie Ridge Complex (MRC), generally show longer recurrence times than those determined from the historic record. These results suggest that during the last 70 years there has been more seismic moment release along the MRC than predicted by the plate motions. The preferred explanation is that this century is a seismically overactive period which is part of a natural episodicity of quiescence and overactivity. Alternative explanations include complex plate interactions resulting from proximity of the plate boundary to the instantaneous pole of rotation and/or the existence of a southeast Australian microplate. Copyright 1990 by the American Geophysical Union.