Trends in Seismicity in the CSG Producing Region of the Surat Basin in Queensland

The Australian continent is generally in a state of compressive stress. Australian earthquakes, for which focal mechanisms have been calculated, are generally reverse-faulting events, consistent with a predominantly compressive tectonic regime. Earthquakes of moderate intensity have been reported in Queensland since the first decades of European settlement; the first reported earthquake occurring in Cape York in 1866. The Central Burnett region, just to the north of the Surat Basin (and on different terrain), remains one of the most active regions of the State, the most recent notable earthquake being in 2015 (Eidsvold, M=5.2 main-shock). Aftershocks of this event continued to be recorded some four years subsequent. Since 1937, a growing number of entities have operated seismic networks within the State, for varying purposes and with equipment of varying instrumental design and capabilities. Campaigns of seismograph installations in the late 1970s and early 1980s improved the detection threshold down to M=3; and as low as M=2.5 in parts of south-eastern Queensland (Cuthbertson and Jaume, 1996). Whilst additional seismograph networks have been periodically installed and operated since that time, the detectability threshold for very small magnitude earthquakes has remained approximately constant. Large amounts of previously uninterpreted data has been used in this study. The study considers the location of sensors and examines the Gutenberg-Richter frequency magnitude relation for Queensland and for the Surat Basin since 1986. It also discusses detection and resolution limits and provides a baseline understanding of natural seismicity and likely rates thereof.