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Did coal mining cause the 'earthquake swarms' that hit the Hunter region of NSW?
Yesterday's 4.5-magnitude earthquake that struck the town of Muswellbrook in the Upper Hunter region of New South Wales was the latest in a series of moderate-sized events near the town in just two weeks.
A 4.7-magnitude earthquake was recorded on August 23, followed by a 4.5-magnitude earthquake in the same area the next day.
The latest quake struck just before 6am on Saturday, with Geoscience Australia receiving more than 1,000 reports from the public – hundreds of kilometres from Muswellbrook.
Earthquake hits Hunter region of NSW
Experts have described these earthquakes as part of a “swarm.” What is this swarm? And what might be behind it?
Is mining the cause of NSW earthquakes?
Muswellbrook is one of Australia's busiest coal towns and is home to New South Wales' largest open pit mine, BHP's Mount Arthur mine.
Mine operations were temporarily halted following yesterday's earthquake, and all workers on site were safe.
The Mount Arthur coal mine in the Upper Hunter is the largest open-pit mine in New South Wales. (ABC News: Tom Hancock)
But seismologists say it's difficult to know whether mining activity in the Hunter region is the direct cause of these quakes.
The Australian Geoscience Agency is the government agency that researches and monitors earthquakes.
Dr. Trevor Allen is a seismologist there.
“Earthquakes can happen anywhere in Australia,” he added.
There are plate boundary forces acting on the Australian continent, creating stresses within the rocks within Australia.
Dr. Trevor Allen says mining activities can cause earthquakes. (ABC News: Pedro Ribeiro)
“Over long periods of time, the rocks will become more stressed, and can eventually break along areas of weakness, which we know are fault lines.”
The relationship between mining and earthquakes
Dr Allen said mining may have contributed to the tension in recent weeks and months, but further investigation is needed to be sure.
He added that earthquakes that occurred abroad showed the relationship between mining and earthquakes.
“For example, it is estimated that 90% of earthquakes in South Africa are linked to mining. The reason for this is that during mining activities, large amounts of rock mass are removed,” he said.
“What happens is you're modifying those stress conditions around where your mining activities are, and that can lead to earthquakes.”
An earthquake that struck Muswellbrook in August caused bricks to fall from an old building. (ABC Newcastle: Jesmin Cheung)
Research is underway in the Hunter Valley.
Dr Allen said further investigations were underway in the Upper Hunter region, which were aimed at finding out more about the current earthquake sequence – not necessarily to find out if mining was the cause.
“Geoscience Australia has installed temporary seismic monitoring equipment and we are receiving that data,” he said.
“We hope to be able to better analyze this data in the coming weeks and months to determine where these earthquakes are actually occurring.
“So we hope to get much better locations for those earthquakes, which will be able to provide more information on that front.”
What is an earthquake swarm?
Dr. Allen referred to the recent event as part of a “swarm” of earthquakes.
“So we see a series of medium-sized earthquakes of roughly the same size,” he added.
“Each of these earthquakes will have its own sequence of aftershocks, where we tend to see aftershocks fade away over time.”
Loading…Are more earthquakes expected in NSW?
Dr Allen said the group could see more strong tremors in the near future, with a “high probability” of similar-sized earthquakes hitting the region.
“However, there is perhaps a stronger possibility that over the coming weeks and months, we will see a gradual decrease in the rate of earthquakes occurring, and hopefully also in the size of those earthquakes,” he added.
“But we can expect aftershocks to likely occur in the coming weeks and months.”
After the two earthquakes in August, seismologists correctly predicted that the region would see more activity in the following weeks and months.
Aftershocks are expected to rock the Upper Hunter region of New South Wales for several months.
“I think it’s likely we’ll see an event of similar magnitude, which would be the third mainshock, if you will,” Geoscience Australia seismologist Hadi Ghasemi told us last month.
“In general, larger aftershocks occur early in the sequence, and their frequency begins to decline over time.”
What would happen if a bigger earthquake happened?
Muswellbrook is a two-hour drive from Newcastle – the site of the 1989 earthquake that killed 13 people and injured more than 100.
The earthquake measured 5.6 magnitude, and the 13 deaths were mostly caused by building collapses.
Many Hunter residents were quick to draw comparisons to this earthquake.
The Newcastle Workers' Club had to be demolished after being badly damaged in the 1989 earthquake. (Newcastle District Library Collection)
But Hunter Valley looks very different now than it did in 1989.
Newcastle, Australia's largest regional centre, is now home to more high-rise buildings than Hobart or Darwin.
Modern buildings, such as those now dotting the Novo Castrian skyline, are built to a very different standard than those built in the mid-20th century, and are likely to withstand an earthquake better, said Dr Trevor Allen.
“Modern buildings are built to Australian seismic load standards, so there are specific detailed procedures required in terms of the design and construction of those buildings,” he said.
“Older buildings that were certainly built before the 1989 Newcastle earthquake, probably didn't have the same requirements.
In particular, the former structures [like] “Stone buildings, we know, are particularly vulnerable to ground shaking due to earthquakes.”
Unreinforced stone buildings in Muswellbrook were damaged during the recent series of earthquakes.
The ABC Emergency website has tips on how to prepare your home for the possibility of an earthquake.
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