Aeromagnetic mapping of Precambrian geological structures that controlled the 1968 Meckering earthquake (M s 6.8): Implications for intraplate seismicity in Western Australia Mike Dentith a, ⁎, Dan Clark b, 1 , Will Featherstone c,2 a School of Earth and Environment, The University of Western Australia, Nedlands, Western Australia 6009, Australia b Geoscience Australia, GPO Box 378, Canberra, Australian Capital Territory 2601, Australia c Western Australian Centre for Geodesy, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia abstract article info Article history: Received 23 December 2008 Received in revised form 1 July 2009 Accepted 1 July 2009 Available online 9 July 2009 Keywords: Aeromagnetics Faulting Intra-plate seismicity Meckering Western Australia Occurring in the southwest of Western Australia, the 1968 Meckering earthquake (M S 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. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. 1. Introduction Most attempts to explain modern occurrences of intra-plate seismicity invoke reactivation of faults within a zone of weakened lithosphere, often a palaeorift, created during previous tectonic events (Johnston and Kanter, 1990; Johnston et al., 1994). There is less agreement regarding reasons for failure in particular parts of the ‘zone of weakness’. Models invoking stress concentration at the intersection of faults (e.g. Talwani, 1999; Dentith and Featherstone, 2003; Bhatt et al., 2009) and adjacent to large intrusions (Stevenson et al., 2006) are popular. Celerier et al. (2005) explain neotectonic strain localisa- tion in the Flinders Ranges of South Australia in terms of enhanced upper crustal heat production weakening a suitably pre-structured crust. Models invoking general mechanical weakness as the result of lithological, structural and large-scale crustal and/or mantle inhomo- geneity have also been proposed (e.g. Stuart et al., 1997; Kenner and Segall, 2000; Sandiford and Egholm, 2008). It is likely that the range of models reflects the range of mechanisms which are operating. There has been much less research on ‘zones of weakness’ at the scale of individual intra-plate seismic events and the factors that might make a particular fault, or association of faults, “weak”, i.e. prone to reactivation. Historic surface ruptures provide the best opportunity to bridge this gap in knowledge by combining instrumental or historical seismological data and detailed mapping of the surface rupture geometry. In regions where basement outcrop is sparse and non-representative of the subsurface geology, geophysical mapping is an invaluable tool to Tectonophysics 475 (2009) 544–553 ⁎ Corresponding author. Fax: +61 8 6488 1037. E-mail addresses: mdentith@segs.uwa.edu.au (M. Dentith), Dan.Clark@ga.gov.au (D. Clark), W.Featherstone@curtin.edu.au (W. Featherstone). 1 Fax: +61 6249 9986. 2 Fax: +61 8 9266 2703. 0040-1951/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.07.001 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto