Does the Melbourne earthquake indicate more tremors in Victoria?

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At a quarter past midnight, seismologist Adam Pascal sat upright in his bed as the ground shook. Pascal lives in Reservoir, in the northern suburbs of Melbourne, and knew immediately that the earthquake had struck close to home.

The 3.8-magnitude earthquake occurred at 11.41pm east of Sunbury, about 33 kilometers from Melbourne’s central business district and two kilometers below the surface. “That’s why it’s been felt so widely — the closer to the surface, the more shaking you experience,” says Pascal, chief scientist at the Center for Seismology Research. “It was in a built-up area.”

The earthquake is the largest in Melbourne in 120 years. It was about 100 times less powerful than the Mansfield earthquake of September 2021 that shook Melbourne and damaged many properties, but it was much closer to the city, which is why it was so widely felt.

What caused the earthquake? Has Melbourne become an earthquake hotspot? Does this mean there is a big one on the way?

Credit: Design by Cormac Lally, Maria Ersgovac

What caused the earthquake?

Victoria is pressed, quite literally.

Australia is fortunate enough to sit in the center of its tectonic plate, which means we have fewer devastating earthquakes than New Zealand straddles the border.

But our plate is under stress. We’re moving north about five or six centimeters a year. “This is faster than your fingernails grow, so it’s not trivial,” says Mark Quigley, associate professor of seismology at the University of Melbourne.

As we push north, the sides of the tectonic plate are under tremendous pressure. “All of these car crashes happen around the plate that transfers pressure to the interior,” says Quigley.

Combined with northerly pressure, this puts great stress on the inner rocks of Australia. And oftentimes, those compressed rocks slip at faults, causing an earthquake.

The Sunbury quake was likely the result of an error in the size of Flemington Racecourse, says Dr Brendan Duffy, technical director at GHD, an engineering services firm that provides earthquake risk assessments.

He said that most Victorian earthquakes are caused by reverse faults, where one side of a fault suddenly slips relative to the other.

The rocks would have moved about 10 centimeters. That sudden movement released energy in the form of seismic waves, which spread across and across Victoria.

Melbourne sits on solid, unbroken rock, so these waves can really travel.

Imagine hitting a block of wood with a hammer, Quigley says. If the wood is hard, you can feel the hammer hitting the other end. But if the wood is full of cracks, the blows don’t travel either.

Quigley says these earthquakes can happen anywhere in Victoria.

When fractures reach their stress limits and move suddenly, the entire landscape can become deformed when one side of the fault slides under the other, says Gary Gibson, a seismologist at the University of Melbourne.

The best example in Victoria, says Gibson, can be seen on the drive to Bacchus Marsh at Rowsley Scarp, where the flat landscape suddenly gives way to hills, triggered by a 5.0-magnitude earthquake in 1976.

More than 20,000 Victorians – and some Tasmanians – have submitted a ‘felt report’ to Geoscience Australia following Sunday night’s earthquake.

Is Melbourne a hotspot right now?

Sunday’s quake is the largest to strike within 40 kilometers of Melbourne since 1902, when a 4.5-magnitude quake in Port Phillip Bay rocked the city, according to the Earthquake Research Centre’s database.

The era recorded the earthquake two days later, under the headline shock earthquake in the sun. Reverend C. Geyer said he felt a strong shock for about 30 seconds, causing window panes to shake violently.

The Age reported “a lady in the town about the same time heard a heavy growling, which she described as like the sound made by heavily laden wagons crossing a wooden bridge”.

Newspaper article in The Age, published 24 December 1902, credit: The Age

The city sure is experiencing a lot of earthquakes at the moment. Woods Point, near Mansfield, was in 2021, but there was a smaller earthquake in east Melbourne earlier in May and Frankston was hit last September.

The list of Victorian earthquakes also states that most are located in regional Victoria – not the city itself.

The recent earthquake is part of a normal pattern of seismic activity. “We expect earthquakes of this magnitude to occur every 50 to 100 years somewhere within the Melbourne metro area. It’s a normal distribution of earthquake activity,” Pascal says.

Experts agree that this latest earthquake is not part of the construction of a “big earthquake”. “We’re relatively safe. But we’ll have it regularly,” says Gary Gibson. “It’s completely normal, completely normal.”

But earthquakes beget earthquakes, with each earthquake weakening other nearby fault lines. So while we don’t automatically go down the road to a big one—it’s possible but unlikely—we’ll get more.

“The more earthquakes you get, the more likely you are to experience a larger earthquake,” Quigley says.

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