Abstract

Data obtained in the last 20 years show that tectonic models requiring the Australian continent to be stable and aseismic have to be modified. It is now apparent that the continent experiences significant levels of seismicity; large (M∼7) earthquakes occur from time to time; and the spatial and temporal distributions of earthquakes do not exhibit well defined patterns. We now know that these earthquakes are caused by compressive stresses in the crust, and that since 1967 five earthquakes have been associated with surface faulting. There are also indications of Recent prehistoric fault scarps in several localities. Evidence from focal mechanisms, borehole deformations in deep (>1 km) drill holes, and in-situ stress measurements indicates that compressive stresses in the crust exist continent-wide. In some regions the levels of stress are very high; in Western Australia, values >20 MPa have been observed at depths ∼10 m. The orientation of the axis of maximum compression varies considerably across the continent, but within specific regions the directions appear consistent. Thus, in the southeastern part of the continent the direction is northwest-southeast, whereas in the southwest it is almost due east-west. We can speculate that most earthquakes take place in the weaker upper-crust, but a model has not yet been developed which will assist in forecasting when and where the next large earthquake will occur.

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