It was the biggest shake-up to hit Orange in 50 years – a 4.3 magnitude earthquake lasting seconds.
The quake hit just 30 kilometres south of the city, five kilometres west of Newcrest’s Cadia Valley Operations, at 2.30am on Good Friday.
The tremor brought a halt to production as 56 people were shepherded to safety chambers, before later escaping to the surface at 9.30am once access points were deemed safe.
The exact cause is unknown, but Geoscience Australia’s senior duty seismologist Dan Jacksa said it was not related to the mine’s operation.
“The mine being there is really inconsequential to the earthquake,” Mr Jacksa said.
Mr Jacksa said mines were built near fault lines where minerals were located.
He said for structural damage to occur, the magnitude would have to be at least 4.5 to five. Even at the epicentre, damage was unlikely during a 4.3 magnitude earthquake.
“Twenty kilometres away, it would feel like a train or a truck going by, right near the epicentre, it would have been a loud bang,” he said.
Mr Jacksa said it was the third earthquake in Orange and the biggest in the last 50 years, with a 3.5 magnitude in 1977 and a 3.4 in 1982.
“You can’t predict earthquakes, there’s every possibility that there could be more in the future, but [Orange] is not very active seismic zone.”
Orange geologist Lawrence Sherwin said more information was needed to determine the cause.
“There are a lot of major fault lines running through Orange, they’re mostly inactive,” Dr Sherwin said.
He said water or minerialisation flows could track along the fault lines and around Orange, those flows were likely to have reacted with volcanic rock.
“That volcanic activity ended several hundred million years ago,” he said.
Dr Sherwin said the earthquake could have been the earth’s crust settling.
Asked whether the cause was related to mining activity, a CVO spokeswoman said a full investigation was underway into the cause.
“Our engineering and operational teams are currently assessing the damage,” she said.
“Until the mine has been thoroughly assessed and certified safe to re-enter, production has been suspended.”
She said it was too early to tell when production could resume, but minor seismic events were a natural part of Cadia East’s mining process.
“Cadia’s geotechnical engineers closely monitor seismic activity in the mine with a network of over 50 monitoring devices,” she said.