In situ stress field of eastern Australia

New in situ data based on hydraulic fracturing and overcoring have been com piled for eastern Australia, increasing from 23 to 110 the number of in sit u stress analyses available for the area between and including the Bowen an d Sydney Basins. The Bowen Basin displays a consistent north-northeast maxi mum horizontal stress (sigma(H)) orientation over some 500 km. Stress orien tations in the Sydney Basin are more variable than in the Bowen Basin, with areas of the Sydney Basin exhibiting north-northeast, northeast, east-west and bimodal sigma(H) orientations. Most new data indicate that the over bu rden stress (sigma(V)) is the minimum principal stress in both the Bowen an d Sydney Basins. The Sydney Basin is relatively seismically active, whereas the Bowen Basin is relatively aseismic. Despite the fact that in situ stre ss measurements sample the stress field at shallower depth than the seismog enic zone, there is a correlation between the stress measurements and seism icity in the two areas. Mohr-Coulomb analysis of the propensity for failure in the Sydney Basin suggests 41% of the new in situ stress data are indica tive of failure, as opposed to 13% in the Bowen Basin. The multiple pre-exi sting structural grains in the Sydney Basin further emphasise the differenc e between propensity for failure in the two areas. Previous modelling of in traplate stresses due to plate boundary forces has been less successful at predicting stress orientations in eastern than in western and central Austr alia. Nonetheless, stress orientation in the Bowen Basin is consistent with that predicted by modelling of stresses due to plate boundary forces. Vari able stress orientations in the Sydney Basin suggest that more local source s of stress, such as those associated with the continental margin and with local structure, significantly influence stress orientation. The effect of local sources of stress may be relatively pronounced because stresses due t o plate boundary forces result in low horizontal stress anisotropy in the S ydney Basin.