The Geological Setting of the 1968 Meckering Earthquake Based on the Interpretation of High-Resolution Aeromagnetic Data

Abstract

Estimates of seismic hazard are strongly influenced by the location and recurrence of large earthquake events. Understanding such events is inherently difficult because recurrence intervals for large events in a given intra-plate region are typically much greater than the period over which earthquakes have been instrumentally recorded. Consequently, there is often very little data with which to robustly estimate seismic hazard. One way of enlarging the dataset is to study pre-historic large events using palaeoseismological methods, and there are many probable palaeo-fault scarps in the southwest of Western Australia. Studying these, along with historic fault scarps, places constraints on the geological setting of large events. The Southwest Seismic Zone is a belt of intra-plate seismic activity within the southwest of Western Australia. The local geological environment comprises deeply weathered Archaean granitoids and supracrustals. Outcrop is extremely sparse. The geological setting of large seismic events is best determined using high-resolution aeromagnetic data. These data allow bedrock geology to be mapped, and in particular identify structures that have been reactivated. This is the first step in establishing whether particular areas are more prone to large events than others. The 1968 Meckering earthquake (MS 6.8) resulted in an extensive and complex pattern of surface faulting, comprising faults with a range of orientations with reverse and dextral lateral offsets. Interpretation of new aeromagnetic data show that the surface faulting correlates closely with linear magnetic anomalies, interpreted as dykes/faults and lithological contacts/faults. The geometry of the surface faulting can be explained in terms of the reactivation of northeasterly and northwesterly trending surfaces 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 vertical displacements recorded for the 1968 event.