Source Parameters of Seven Large Australian Earthquakes Determined By Body Waveform Inversion

Abstract

We determine source mechanisms and centroid depths for the seven largest earthquakes in and near the Australian continent in the past 20yr using teleseismic body waves. Six of the earthquakes were thrusts and one was strike-slip. All five earthquakes that occurred beneath the continent were shallower than 10 km (the ranges of centroid depths are constrained to be 1-5, 2-4, 0-3, 5-11, and 6-10km) and indicate nearly pure thrust faulting. The three shallowest events were associated with surface faulting. While the published first motion solutions for these three events included large amounts of strike-slip, our thrust mechanisms are more consistent with displacements observed at the surface of the Earth. The range of possible centroid depths for a strike-slip earthquake beneath the continental shelf off the northwest coast is 23-29 km, and that for a thrust earthquake in oceanic lithosphere beneath the Tasman Sea is 22-26 km below sea level. The pressure axis is consistently near-horizontal, and in the western half of the continent, five earthquakes indicate that the azimuth of the maximum compressive stress is E-ENE (azimuthal range of 66"-91"). On the basis of one earthquake, we conclude that the P-axis is oriented NNW beneath the Tasman Sea.