‘Earthquake swarm’ no cause for concern, experts say after magnitude 4.5 quake in NSW Hunter region

A magnitude 4.5 earthquake has hit the Hunter region of New South Wales early Saturday morning, two weeks after an earthquake of a similar size struck the region.

Geoscience Australia told Guardian Australia the earthquake was part of an “earthquake swarm” in the region: a series of moderate to large earthquakes with their own aftershock sequences.

The latest earthquake struck near Muswellbrook, 125km north-west of Newcastle, just before 6am on Saturday, with more than 1,000 reports lodged with Geoscience Australia from those who felt the shake. It was felt in Sydney, down to Wollongong, up north around Tamworth and to Dubbo in the west.

Dr Trevor Allen, a senior seismologist at Geoscience Australia, said that the reports would only represent a “small fraction” of the overall number of people who felt the shake, which occurred 10km to the south-west of Muswellbrook.

About 2,500 customers in the local region were initially left without power after the quake; by 8.30am, less than 200 remained without power. Both NSW police and the State Emergency Service said they had not received any reports of injury or damage to infrastructure on Saturday morning.

Saturday’s earthquake comes two weeks after a magnitude 4.8 earthquake hit Denman, 25km away, on 23 August. The following day, another 4.5 magnitude earthquake hit near Muswellbrook.

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Allen said there had been more than 30 earthquakes in the region since 23 August as part of this swarm.

“This is quite common in Western Australia, in the Wheatbelt of south-western WA, but is actually quite rare in eastern Australia,” he said.

Allen said Saturday’s earthquake was the second-largest to occur in the recent sequence, after the magnitude 4.8 in August. In the lead up to this latest earthquake a series of small earthquakes, around 2.5 magnitude, had in hindsight indicated “activity was starting to ramp up again in the region”, he said.

While he said it was “impossible” to determine earthquake activity into the future, he suggested there was an “elevated chance” the region would continue to see earthquakes of this magnitude, “possibly even a little bit larger in the coming weeks to months”.

He said, however, it was still “more likely” earthquake activity in the region would decrease.

Allen said earthquakes can occur anywhere in Australia, as plate boundary forces stress the rocks within the Australian crust.

“Over long periods of time, what happens is the stress builds up on a particular faultline, and when that stress becomes too great, the rocks will break in an earthquake.”

The Liddell power station, one of seven sites where the Coalition has proposed a nuclear power station, is less than 20km away from Muswellbrook.

Allen stressed that a range of criteria set out by the International Atomic Energy Agency must be satisfied even before the approval stage for a nuclear site.

“These are quite strict, particularly when it comes to earthquakes,” he said. “There will have to be a range of geotechnical assessments that are undertaken, and those assessments will need to satisfy the Australian regulator [Arpansa] … that the site is fit for purpose for any sort of nuclear facility.”

The federal Labor MP for Hunter, Dan Repacholi, posted to social media on Saturday morning that “the ground has grumbled again with a decent shake”.

One commenter, whose husband works at the Mt Arthur coalmine, said the earthquake frightened workers: “The bang was huge right before everything shook.”

When another commenter raised the proposed nuclear site, Repacholi replied: “Seems those with the big mouths a few weeks back have gone very very quiet on it.”

The MP suggested in another comment that the Coalition was “reconsidering their plan”, and joked, “the beer coaster has been amended”.

Repacholi said on social media in May that every day, “voters in my patch are raising their opposition to having a nuclear power station built anywhere near them with me”.

Ted O’Brien, the shadow climate change and energy minister, told ABC radio last month that if advice suggested a nuclear plant should not proceed, “then a power plant would not proceed, full stop”.

Associate prof Edward Obbard, the director of the UNSW Nuclear Innovation Centre, said any structure – whether bridge or power station – is designed for the known seismic activity where it is built.

“Nuclear infrastructure like power stations has far, far higher seismic requirements than any other building,” he told Guardian Australia. “It’s not like you rule somewhere in or out based on whether you might get an earthquake, [you just] have to know whether there could be an earthquake, and then design what you build there to withstand it.”

Speaking on the recent earthquakes around Muswellbrook, Obbard said the seismic activity was “not zero, but it’s much, much less than a real earthquake zone”, pointing to Japan and the US west coast.

He said nothing about the recent earthquakes suggested a proposal should not go ahead for a nuclear site in the Hunter.

“I don’t think it’s the most critical aspect of siting a nuclear power station,” Obbard said, pointing to other major factors such as proximity to populations and workforce.