Melbourne residents were shaken by a 3.8- magnitude earthquake on Sunday night.
It was the largest earthquake with an epicentre within 40km of the Melbourne central business district for more than 120 years, according to Adam Pascale, the chief scientist at the Seismology Research Centre. The last was a 4.5-magnitude earthquake that occurred in 1902.
The strongest Victorian earthquake since detailed records began, however, occurred in 2021. That 5.9-magnitude quake had its epicentre in Woods Point, about 130km from Melbourne, but led to building damage in the capital city. It resulted from the rupture of a previously unknown faultline, which was discovered this year by Dr Sima Mousavi, a seismologist at the Australian National University, and her collaborators.
How common are earthquakes in Victoria compared with other states?
“We expect earthquakes to happen in Victoria, but not more than other states,” Mousavi said. “We get minor earthquakes all over Australia.”
The entirety of Australia sits on the Australian tectonic plate, which is classified as a stable continental plate. It is, however, under pressure from neighbouring plates, such as the Eurasian Plate to the north and the Pacific Plate to the east, resulting in faultlines that form within the Australian plate.
“Think of Australia like a pavlova,” Mousavi said. “You are pushing this pavlova from the sides – you can get cracks anywhere on the top of the pavlova.”
These intraplate faultlines are widespread across Australia, particularly in the south-east, Dr Behzad Fatahi, an associate professor of civil engineering at the University of Technology Sydney, said.
Geoscience Australia maintains a national map of known fault scarps.
“Wherever you see mountains, that’s a pretty good indication that you have a history of seismic activity,” Pascale said. Every week, there are roughly 30 to 40 earthquakes in the south-eastern states of South Australia, Victoria, New South Wales and Tasmania, he said. “Maybe once every couple of weeks, one of those will be felt.”
Could a very large earthquake occur in Australia?
The magnitude of an earthquake depends on the size of the faultline, Fatahi said. The longer a faultline, the greater the potential amount of friction and release of energy. “When you increase the magnitude of an earthquake, let’s say from five to six, or six to seven, the amount of energy that it releases increases 30 times.”
On average, Australia experiences two earthquakes a year with magnitudes greater than five, according to Fatahi. “We can expect, in Australia, earthquakes with magnitudes of seven to 7.5, but not very regularly,” he said.
Australia has intraplate faultlines that are long enough to host a 7.5-magnitude earthquake, Pascale agrees. Some are near major cities, but there is the potential for a large earthquake to occur “pretty much anywhere in Australia”.
“The one probably nearest to Melbourne that most people are aware of is the Selwyn fault that runs along the Mornington Peninsula. That’s one that is potentially large enough to host a [magnitude] seven-ish earthquake,” Pascale said. “Another [large] fault starts near Apollo Bay and reaches up through Geelong.”
What was Australia’s largest earthquake?
The Tasman Sea earthquake was a large 8.1-magnitude earthquake which in 2004 triggered a small tsunami when it struck a remote area of the southern Tasman Sea.
The strongest Australian earthquake on land occurred in 1988 at Tennant Creek in the Northern Territory. Three earthquakes struck in one day, and the largest had a magnitude of 6.6, according to Geoscience Australia.
“Sometimes they appear out of nowhere, because we’re looking at such a tiny timeframe in the scale of geological history,” Pascale said.
