Australia is rocking. Or at least, last weekend it was.
From Friday to Monday, the country experienced several major earthquakes, the tremors felt far and wide from New South Wales to Western Australia.
On Friday afternoon, thousands of people reported feeling an earthquake over an area spanning hundreds of kilometres from south of Sydney to the NSW mid-north coast.
The 4.7-magnitude quake hit Denman, around three hours north of Sydney in the upper Hunter region. Locals said they felt their houses rattle. Some parents at various schools in the region were also contacted and told to come and collect their children.
Then there was a cluster of earthquakes recorded in WA's Wheatbelt over the past week, one of them sizing up to 4.4-magnitude.
It raises the question — just how normal is this? Are these quakes randomly occurring events? Or is there something happening beneath the surface we should be aware of?
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Are earthquakes actually common in Australia?
Jonathan Griffin is a Senior Seismologist at Geoscience Australia.
"Across Australia, we register about 100 magnitude-three earthquakes every year, and we get about one or two magnitude-five earthquakes every year and we get a magnitude-six earthquake every five to 10 years," he tells Mamamia's news podcast The Quicky.
"Australia is very sparsely populated and they can occur anywhere across the continent. And so they often occur where very few people are."
Griffin adds: "This has been a more interesting week than average."
Why does Australia have earthquakes?
Australia doesn't sit on the edge of a tectonic plate, like some other countries do. That's why we rarely experience devastatingly large earthquakes, but it doesn't mean we'll never have them.
"Australia is a very ancient continent, and while it's in a stable continental area now—we occupy the interior of a tectonic plate—that plate is moving northeast about seven centimetres a year," explains Griffin.
"As it does that it's crashing into the Pacific plate to the north...it's at those plate boundaries where most earthquakes occur."
As Griffin notes, Australia has a legacy of old fault lines that have developed over time as the continent moved. And these faults or 'weaknesses' can cause earthquakes.
"Crashing into other plates at the boundary causes stresses in the crust of Australia itself," he says. "The continent is being squeezed from the plate boundary.
"There's still weaknesses in the crust and by squeezing the crust, those weaknesses can fail. And that's what causes an earthquake [in Australia]."
What do coal mines have to do with earthquakes?
Near the Hunter River in New South Wales, you will catch glimpses of the colossal open-cut coal mines in the region. One of these mines is between two Hunter Valley towns — Muswellbrook and Denman. Both these towns felt Friday's earthquake and subsequent aftershocks the most.
The Mount Arthur Mine was the epicentre of that 4.7-magnitude earthquake.
Then followed the arguments over whether mining can cause seismic activity. Some suggest the digging of such an immense amount of dirt from that one spot which lies on a fault line could be earthquake-risky.
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Senior Seismologist Griffin says: "Human activity can induce earthquakes. The Australian continent is being stressed by these big plate boundary forces, and that's sort of the cause of our earthquakes. But we can then locally through various human activities affect the stress field."
Some of these human activities that can induce earthquakes include mining, hydraulic fracking and filling water reservoirs.
But we can't be too quick to directly correlate these human activities to any specific earthquakes that occur, says Griffin.
"We can't say, 'Yes, it was caused by mining' or 'No it wasn't.' To do that you need really high-level monitoring. While we can say that there have been plenty of earthquakes in the Hunter Valley and it's possible that at least some of them have been induced by mining, what we can't do is go and point to any particular event like these ones that happened over the weekend and say, 'Yes they definitely were due to mining.'"
Ultimately, tectonic forces are still the main cause of moderate earthquakes in Australia. We also cannot predict earthquakes.
"At Geoscience Australia we produce the national seismic hazard assessment, which is basically a model of where we think earthquakes could happen in the future," explains Griffin. "That's the kind of information that can then be used to design buildings and other infrastructure to be resilient to the level of shaking we think we might see."
Feature Image: Getty.