Small earthquake like this can be a good thing – says former seismologist

An earthquake measuring 3.8 on the Richter Scale was recorded south of Orange, near Newcrest’s Cadia Valley Operations on Sunday afternoon.

The earthquake occurred at 12.46pm Sunday and according to Geoscience Australia, was located about 2 kilometres west of the mine tailings dam and 10 kilometres underground.

All underground personnel in the mine were initially moved to refuge chambers before being safely moved to the surface.

Mining operations were temporarily suspended while Cadia geotechnical engineers conducted inspections to ensure the workplace was safe.

Normal mining resumed at about 1am Monday 23 July 2018. Processing activities continued uninterrupted. 

The earthquake caused no damage to the underground workings, the tailings storage facilities or other infrastructure.

In March, two small earthquakes measuring 2.7 were recorded near the mine just a day prior to a breakthrough in the wall of its northern tailings dam.

A significant earthquake in April 2017 measuring 4.3 saw the min evacuated and full production suspended for six months.

Ron Smith, a retired seismologist now living in Orange, spent 17 years with the Bureau of Mineral Sciences, now known as Geoscience Australia.

He said small earthquakes like that on Sunday are far from unusual to this area.

“The Cadia Mine area has had about 60 earthquakes recorded since 1960,” said Mr. Smith, whose work largely involved operating seismological stations throughout Antarctica.

“It’s a minor earthquake so it is nothing new and it is all due to this compressive stress being dissipated at a weakness at the boundary between the igneous volcanic and sedimentary limestone.”

The stress in the rock occurs due to the continental drift north, Mr. Smith said.

“What we've got is the whole continent is under compression because we've been pushed northwest at something like 7 centimetres a year and you get a natural distortion of the stress field when you get dissimilar rocks close together such as when you get a volcanic pipe pushing up through sedimentary rock which is what we've got here.

The release of the stress as earthquakes is really a good thing, said Mr. Smith.

“...because it means instead of building up until the stress is large enough to cause a damaging earthquake it’s releasing it in small bits — that is essentially what I think.”