The stress field near the sites of the Meckering (1968) and Calingiri (1970) earthquakes, Western Australia

A comparison of three methods of stress determination using overcoring tests, earthquake focal-mechanism solutions, and fault-displacement analysis, was carried out near the epicentres of the 1968 Meckering and the 1970 Calingiri earthquakes, Western Australia. Shallow overcoring measurements (3–10 m) were made at seven sites in competent granite, along a 200-km north—south traverse through the location of the Meckering earthquake. The in situ measurements indicate a high regional compressive stress, acting about N77°E. This compares with the direction of the P-axes from the Meckering and Calingiri earthquakes of N91°E and N102°E, respectively, and east-west orientation of major principal stress deduced from observations of surface faulting. The highest maximum principal stress, 23 MPa, was measured at the site farthest north from the Meckering epicentre (~ 90 km), and the lowest, 4 MPa, was measured near the epicentre. The magnitude of the stress increases with distance away from the epicentral area at about 0.2 kPa/m. The results were corrected for drilling water temperature, thermal rock stresses, and suction pore pressure. The total correction was less than 2 MPa at most sites.Estimates of shear stress release from the fault-scarp displacements for Meckering and Calingiri were of similar magnitude (~10 MPa) to the difference between observed shear stress at sites close to and sites remote from the Meckering earthquake.As a cause of the earthquake it is proposed that the fault plane is progressively weakened by alteration or weathering, and that a small long-term fluctuation of head of ground water was the trigger.The high stress at shallow depths, the growing evidence of east—west major principal stress in most of the Australian continent, and the existence of horizontal stress higher than gravity lithostatic stress may have a common explanation, in terms of the driving forces of plate tectonics. The Australian results do not agree with the north—south orientation suggested by crude plate-drift theories in which the major stress is in the direction of the drift, and do not appear to fit with orientations from other continents, in the Richardson et al. (1976) interacting plate model which assumes a velocity-dependent plate driving force. An analysis of driving forces at diverging plate boundaries was carried out and suggests no dominant force-velocity relationship, thus forces independent of plate velocity may control the stress orientation in the plate.