Evidence from earthquake focal mechanisms, in situ stress measurements, and surface deformations indicate that the Australian continent is in a state of substantial horizontal compression. Reliable focal mechanism determinations are now available from eight earthquakes that have occurred in several parts of the continent since 1967. Each of these mechanisms indicates that the faulting associated with the earthquakes was caused by compressive stress acting close to horizontal. In situ measurements made in mines and tunnels, and close to the surface in quarry floors or on rock outcrops, also indicate horizontal compressive stress in all areas. Near the epicentres of the 1968 Meckering and 1970 Calingiri earthquakes, shallow over-coring measurements (<10 m) were carried out to compare the in situ observations with the earthquake focal mechanisms and surface faulting. The measurements were made in competent granite at seven locations along a 200 km north-south traverse. The results indicate a high regional compressive stress acting about 77°E of N. This agrees well with the 91° and 102°E of N directions for the pressure axes obtained respectively from the earthquake focal mechanisms. The highest stress (23 MPa) was measured at the site farthest north from the Meckering epicentre, and the lowest stress was close to that epicentre where the maximum principal stress was about 4 MPa. During 1978 and 1979 several sites in NSW were tested at depths ranging from 3 to 9 m. At each site the stress measured was compressive. In the eastern part of the State at Buckleys Lake, Jindabyne, Milton, and Moruya the axes of maximum compression were north-south, but in the west at Ardlethan, Mirrool, Berrigan, and Tocumwal the stresses are close to east-west, agreeing with the earlier results at Broken Hill and Cobar. The highest values of about 20 MPa were obtained in Silurian granite at Tocumwal. The results give principal stress orientation in different directions for different regions of the continent; it is therefore clear that simple models derived from plate tectonic concepts cannot be applied directly to explain the high observed stresses or their directions.
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