By Bridget Murphy and Alexander Lewis, ABC
Glencore's Bulga coal mine in the Hunter Valley. Photo: AFP / David Gray
Saturday's 4.5-magnitude earthquake at Muswellbrook, in the New South Wales Upper Hunter, was the latest in a series of moderate-sized events near the town in just over a fortnight.
A 4.7-magnitude quake was recorded on 23 August, then a 4.5-magnitude quake rocked the same area the next day.
The latest quake happened just before 6am on Saturday (Australian time), with Geoscience Australia receiving more than 1,000 reports from the public - some hundreds of kilometres from Muswellbrook.
These quakes have been described by experts as a part of a "swarm". So what is that? And what could be the cause?
Is mining behind the NSW earthquakes?
Muswellbrook is one of Australia's busiest coal towns and home to NSW's largest open cut mine, BHP's Mount Arthur.
The mine temporarily halted operations following Saturday's quake, and all workers onsite were OK.
But seismologists say it is hard to know if mining activity in the Hunter is the direct cause of these quakes.
Geoscience Australia is the government agency that researches and monitors earthquakes.
Dr Trevor Allen is a seismologist there.
"Earthquakes can occur anywhere across Australia," he said.
"There are plate boundary forces that are acting on the Australian continent, and that sets up stresses within the rocks within Australia.
"Over long periods of time, the rocks will become more stressed, and they can eventually break along zones of weakness, which we know are fault lines."
The link between mining and earthquakes
Allen said mining could potentially have contributed to that stress in recent weeks and months, but more investigation was needed to be sure.
He said earthquakes overseas have shown the link between mining and earthquakes.
"For example, 90 percent of the earthquakes in South Africa they suggest are mining related. And the reason for that is because in mining activities, you are removing large volumes of rock mass," he said.
"What's happening is that you're adjusting those stress conditions around where your mining activities are, and that can potentially induce earthquakes."
Research underway in Hunter Valley
Allen said more investigation was happening in the Upper Hunter, which was aiming to uncover more about the current earthquake sequence - not necessarily to see if mining was the cause.
"Geoscience Australia has installed temporary seismic monitoring equipment and we are receiving that data," he said.
"We hope to better analyse that data in the coming weeks and months to determine where those earthquakes are actually located.
"So we hope to get much better locations of those earthquakes, which will potentially be able to provide more information on this front."
What is an earthquake 'swarm'?
Allen referred to the latest event as part of an earthquake "swarm".
"So that's where we see a series of moderate-sized earthquakes of around about the same magnitude," he said.
"Each of those earthquakes will have their own aftershock sequence, where we tend to see the aftershocks decaying over time."
Are more earthquakes predicted in NSW?
Allen said the swarm could see more strong shakes in the near future, with "perhaps an elevated chance" of similar-sized earthquakes hitting the region.
"However, probably a stronger likelihood is that over the coming weeks and months, we will see a gradual decline in both the rate of the earthquakes occurring, and also hopefully the magnitudes of those earthquakes as well," he said.
"But we can expect aftershocks probably will occur for the coming weeks and months ahead."
After the two August quakes, seismologists correctly predicted the region would see more activity over the following weeks and months.
"I would say it is likely that we may have a similar-sized event, which would be the third main shock if you like," Geoscience Australia seismologist Hadi Ghasemi told us in August.
"Generally speaking, the larger aftershocks are at the earlier stages of the sequence, and their frequency starts to decay as time goes by."
What would happen if a bigger quake hit?
Muswellbrook is only a two-hour drive from Newcastle - the site of the 1989 earthquake that killed 13 people and injured more than 100 others.
It was a magnitude-5.6, and the 13 deaths were mostly due to building collapses.
Many Hunter locals are quick to draw comparisons to that quake.
But the Hunter Valley looks vastly different now than it did in 1989.
Newcastle, Australia's largest regional centre, is now the host of high-rise buildings with a population larger than Hobart or Darwin.
Dr Trevor Allen said modern buildings, like the ones now dotting the Novocastrian skyline, are built to very different standards to mid-20th-century structures, and would likely hold up better to a quake.
"Modern buildings are constructed to the Australian earthquake loading standard, and so there are specific detailing actions that are required in terms of designing and constructing those buildings," he said.
"Older buildings built certainly before the 1989 Newcastle earthquake, probably didn't have those same requirements.
"In particular, earlier structures [like] masonry buildings, we know that they are particularly vulnerable to earthquake ground shaking."
Masonry buildings that had not been reinforced were damaged in Muswellbrook during this latest series of quakes.
This story was originally published by ABC News.
