The crustal stress pattern in the Australian continent

Abstract

The state of stress in the crust is an important factor in the design and performance of underground excavations, the behaviour of oil reservoirs and in the design and construction of major civil works such as dams, bridges and road cuttings. A compilation of 'near surface' stresses, measured from tunnels, mines and rock outcrops has recently been conducted by Brown and Windsor (1990). This compilation has been extended to include the results from more than 40 earthquake focal mechanisms, and breakout data from 31 deep (>1 km) drill holes. The results of this compilation show that most of the crust is experiencing compressive stress and that in some regions, e.g. the southwest part of the continent, the levels of stress are very high (greater than 20 MPa at depths less than 10 m). The orientations of the axes of maximum compression vary considerably across the continent but within regions the stress direction appears consistent. For example, in the southeast part of the continent it is northwest-southeast, whereas in the southwest it is close to east-west. The results obtained within the upper 500 m exhibit more scatter than those obtained at greater depths. This is indicative of local perturbations in the stress field at shallow depth caused by tectonic decoupling, topography and weathering effects. A major factor in the continent-wide stress regime is probably the movement of the Australian Plate and the subsequent deformation as it collides along its northern margin. However, there are other factors influencing the situation because the observations do not fit a simple continent-wide pattern.