Graphite-assisted strain localization as a control on intracontinental earthquake distribution

Although Australia lacks active plate-boundary faults, it experiences intraplate earthquakes of local magnitude ≥6 roughly once per decade, concentrated in a few key regions. This localization is commonly attributed to the continental stress field transmitted from far-field plate interactions and to the focusing of that stress into preexisting zones of crustal weakness. These weaknesses are predominantly crustal-scale faults with long, multistage reactivation histories spanning billions of years. Their current seismogenic potential critically depends on the orientation of their strike and dip under the present-day stress regime. However, many comparable faults remain aseismic, implying that additional factors influence where strain ultimately localizes. Here, we used a 3-D resistivity model from a broadband magnetotelluric survey covering an ∼50 × 50 km area of the southern Gawler craton, South Australia, to investigate a region with unusually concentrated seismicity. The study area crosses strongly folded, graphitized Paleoproterozoic metasediments and the Kalinjala shear zone (KSZ), a long-lived structure forming part of the lithospheric boundary along the eastern margin of the Archean Gawler core. Earthquake-resistivity distributions show clear bimodality at 5−15 km depth, with earthquakes occurring in both conductive and more resistive midcrustal domains, and those in resistive crust more commonly sampling stronger lateral resistivity gradients. Together, these relationships suggest two scales of structural localization under the same regional stress field: reactivation of the KSZ-related crustal-scale fault system and local weakening along graphite-rich structures within the adjacent metasedimentary domain. Graphite may therefore influence where intracontinental strain is accommodated within inherited crustal architecture.