Fluid-assisted intra-plate seismicity at the edge of the Gawler Craton, South Australia

Abstract

The Australian continent, being void of active plate boundaries, is often perceived as seismically quiescent. However, earthquakes of moderate magnitude (M6+) occur on the continent around once per decade. Such intra-plate activity can pose a significant risk as these earthquakes can occur along fault lines that are either unknown or considered inactive, are often non-periodic, and poorly understood. Within Australia, the spatial distribution of intra-plate seismicity is non-uniform, instead tending to concentrate along certain weak zones of increased activity. One such region is the eastern margin of the Gawler Craton in South Australia, one of the oldest building blocks of the continent. Recently, several new temporary seismic arrays have been deployed in the region, transforming data coverage across southern Australia. In total, 139 new local earthquakes have been recorded, most of which went undetected by the national seismic network. Following relocation, the pattern of earthquakes becomes more localised and appears to coincide with the edge of the Gawler Craton. Further, a spatial association was found between earthquakes and mound springs, which act as the discharge point for groundwater migrating from the Great Artesian Basin. Enhanced fluid pressures (mantle-degassing) within permeable crustal scale fault systems, responding to a regional contractional strain field, appear to be a key driver of seismicity in the area.