Fault parameters and tsunami excitation of the May 23, 1989, MacQuarie Ridge Earthquake

Abstract

The Macquarie Ridge earthquake of May 23, 1989, is one of the largest events in the last decade. Furthermore, it is the largest strike‐slip earthquake ever recorded instrumentally. We analyzed long‐period surface waves and body waves recorded at global networks (GDSN, IRIS, GEOSCOPE, and IDA) to estimate the fault parameters and computed tsunamis from the estimated parameters. The Centroid Moment Tensor (CMT) solution from surface waves shows that the mechanism is almost pure strike‐slip with one of the nodal planes parallel to the strike of the Macquarie Ridge. The seismic moment is 1.6 × 1021 Nm (× 1028 dyn·cm) and the corresponding moment magnitude is Mw =8.1. Teleseismic P and S waves from 10 stations with good azimuthal coverage are used to model the temporal and spatial distribution of the subevents. Four subevents are located sequentially from south to north along the ridge system in about 30 sec. All of them have a mechanism similar to the CMT solution. The fault length is estimated to be about 120 km from the subevent and the aftershock distributions. The rapture propagates from south to north at a relatively high speed. The average slip on the fault depends on the estimate of the fault width, but is probably larger than 9 m. A large strike‐slip earthquake like this event produces significant vertical displacements on the ocean bottom and excites tsunamis. Computation of tsunamis using the estimated fault parameters predicts that small tsunamis are expected at Australia and New Zealand. In fact, small tsunamis were observed on the southern coast of Australia.