Aeromagnetic mapping of Precambrian geological structures that controlled the 1968 Meckering earthquake (Ms 6.8): Implications for intraplate seismicity in Western Australia

Occurring in the southwest of Western Australia, the 1968 Meckering earthquake (MS 6.8) resulted in the formation of an extensive surface rupture complex comprising faults with a range of orientations and reverse and dextral lateral offsets. Modeling of the seismological characteristics of the source showed that reverse failure occurred on a north–south striking, east-dipping surface, but how this is related to the local Precambrian bedrock geology is not clear.Interpretation of new aeromagnetic data has allowed concealed bedrock lithology and structure to be mapped in previously unachievable detail. These data show that the surface faulting correlates closely with linear magnetic anomalies, interpreted as dykes/faults and lithological contacts. The complicated pattern of surface faulting contrasts with the more simple seismological fault model, but can be explained in terms of the reactivation of northeasterly (dykes and faults) and northwesterly (stratigraphic) trending features in a stress regime with an east–west oriented maximum principal stress. Space problems created where these two trends converge led to the creation/reactivation of a linking north–south trending thrust fault which accommodated the greatest displacements recorded for the 1968 event.The district scale distribution of epicentres in the 3 years encompassing the Meckering event shows the same northeasterly and northwesterly trends as seen in the aeromagnetic data. The implied basement features controlling the seismicity will be prone to strike–slip failure in the regional, east–west oriented stress field. It is speculated that smaller events in the Meckering area will tend to be strike–slip and these account for most of the strain. Larger events, all known examples of which involve predominantly thrusting, are caused by stress build up where strike–slip faults with the two orthogonal trends intersect. This hypothesis provides an explanation for the lack of topography in the region, which is incompatible with the high level of seismic activity and a predominance of thrust faulting.