Stress and faulting in southeast Australia as derived from the strongest earthquakes in the region

In an attempt to clarify the state of stress in Southeast Australia, the spatial and dynamic behaviour of the strongest events in the region (with magnitude greater than 5.0) has been studied in the light of the most recent numerical modelling of the crust. Although the local seismicity has little obvious spatial correlation with the mapped structures, the strongest events line up in a very narrow strip with a NW–SE direction. Composite fault-plane solutions, based on data from all of these earthquakes, allows the general characteristics of the local stress field to be determined, and shows that the orientation of the main compressive stress lies in an E–W direction. Taking into account the number of inconsistent data (40%) for this composite solution and the fault-plane solutions for some individual events, an improved result can be obtained by domain analysis, which shows that northern and southern zones have different orientation of the compressional axes. Although the composite solution for the events in the northern domain shows the same E–W orientation of the main compression as for the whole region, the composite solution for the southern zone reveals a rotation of the compression axis to an azimuth of 120–130°. From the relation between the energy and seismic moment the maximum value of the stress-drop for the whole of SE Australia is estimated as ∼2.0 MPa.