On Wednesday 1 August 2012, slightly after 1.50 am local time (slightly after 3.50 pm on Tuesday 31 July, GMT), a small Earth tremor occurred between Gunning and the Mundoonen Nature Reserve in New South Wales, roughly 50 km north of Canberra, according to Geoscience Australia, who measured the quake as measuring 1.7 on the Richter Scale, and occurring at to shallow a depth for this to be measured accurately. A quake this small is highly unlikely to have caused any damage or casualties, and may not have been felt (or recognized) by anyone.
The location of the 1 August quake. Google Maps.
While New South Wales doe suffer occasional Earthquakes, these tend to be small and infrequent, making their precise causes hard to determine. The state is far from any tectonic plate margin that might provide an obvious cause. Many ancient sedimentary beds in southeast Australia are stacked at fairly steep angles, the result of fold mountain formation during the assembly of the supercontinent of Pangea (though the mountains themselves have largely eroded away), which can make the area slightly more quake-prone than would otherwise be expected. A quake this small and this shallow could even be the result of a large surface explosion, though no such event has been reported in the local media.
If you felt this quake you can report it to Geoscience Australia here. Witness reports help geologists build up their understanding of Earth-movements and their causes.
See also Earthquake in New South Wales, Earthquake shakes Macquarie Island, Melbourne shaken by Earthquake, Canberra shaken by mild Earthquake and Earthquakes on Sciency Thoughts YouTube.
Follow Sciency Thoughts on Facebook.