What causes earthquakes in Victoria? And why did my sunburst tremors feel so strong?

An earthquake in Sunbury on the western edge of Melbourne kept thousands of Victorians awake on Sunday night.

It caused a social media storm, but thankfully no damages were reported.

Geoscience Australia says more than 25,000 people reported feeling tremors in the ten hours after the 11:41pm quake was recorded, as most Melburnians were asleep.

It is far from the 43,072 reported after the largest earthquake in Victoria in 2021.

The magnitude 5.9 quake was centered near the town of Rawson and was felt from Sydney to Hobart, west to Adelaide and, of course, Melbourne.

The unusual thing about Sunday night, however, was how shallow the quake was and how close it was to Melbourne.

Here’s what we know.

How common are earthquakes in Victoria?

Earthquakes are measured by magnitude, which indicates the amount of energy released by an earthquake.

Geoscience Australia says Australia records about 100 earthquakes above magnitude 3 each year.

She says the Australian tectonic plate is the fastest moving continental mass on Earth and collides with the Pacific and Eurasian plates, generating compressive stress in the interior of Australia.

Earthquakes occur when there is a sudden release of this pressure, which causes rocks to fracture along fault lines deep within the Earth.

Geoscience Australia seismologist Hugh Glanville says smaller tremors occur regularly in Victoria.

“We’ve had 29 earthquakes in the last 10 years, so there are about three small earthquakes on average per year in this local area,” says Glanville.

Why did so many people feel the Sunbury earthquake?

Most earthquakes in Australia occur at a depth of about 10 km, but the Sunbury earthquake occurred only 3 km underground.

“It’s very superficial and is probably why people feel this as strongly as they did and as widely as they did,” says Glanville.

“The source of the vibration is much closer to people, so they feel it more strongly.”

Another reason it caused such a stir was the time it was hit.

Most people were in a quiet house and a lot of them were in bed, which means it was louder and more noticeable, says seismologist Elodie Burless, a hazard seismologist at the Center for Seismology Research.

“It might look like a truck going by or a train,” she says.

Some callers to ABC Radio in Melbourne described hearing what sounded like a small explosion.

“It’s the earthquake’s waveforms that travel through the ground and then rise and produce the sound of an explosion when it reaches the surface,” Borliss says.

How devastating are Victorian earthquakes?

Geoscience Australia says nationwide, there is a magnitude 5 earthquake higher every one to two years.

Destructive earthquakes of magnitude 6.0 or higher occur approximately every 10 years.

Earthquakes of the Sunbury magnitude can cause strong shaking, Glanville says, but generally only cause minor damage, such as cracks in plaster.

“You won’t start damaging the walls and the buildings themselves until 4.5 and even 5 on the Richter scale,” he says.

The 2021 earthquake caused significant structural damage to several local heritage-listed buildings on Melbourne’s Chapel Street, including a century-old building that houses the burger chain.

“Of course, that in 2021 was the largest we’ve ever recorded in Victoria, so that was a very big exception in terms of seismicity,” says Glanville.

Stephen Curry, Lecturer in Geology at Union University, says the Sunbury earthquake is typical of tremors recorded in Victoria.

But it does approach the higher end of a powerhouse usually close to Melbourne.

Seismologists say the shallowness of the quake meant it was louder and that people in Melbourne felt the shaking stronger. (ABC News: Patrick Rocca)

“Because Victoria is a fair way from the edge of the continental plate that we’re on, the Australian tectonic plate, we don’t tend to have many earthquakes and the earthquakes we get are generally fairly small,” says Dr Carey.

It is common, he says, to record tremors in southern Gippsland, which is closest to the fault lines.

“You’ll get a jolt that might shake the windows or the glass in the closet, but that’s generally about as damaging as it gets,” he says.

Are earthquakes becoming more frequent?

Two minor tremors were recorded in Croydon, east of Melbourne, this month.

But Elodie Borleis of the Earthquake Research Center says they haven’t become more frequent.

“It’s not strange,” she says.

“A lot of them are so small that people won’t feel them, especially if it’s during business hours.

“We don’t report all the earthquakes that happen just because they’re so small that people don’t really realize they happen all the time.”

Will there be sequels?

Borliss says some aftershocks have already occurred.

“We had an aftershock about two minutes after the main shock that everyone felt,” she says.

“Its strength was 2.8 degrees.”

She says there may have been small shivers recorded over the following days.

How safe are you during a tremor or earthquake?

Just seconds before the Sunbury earthquake shook Melbourne, thousands of Android phone users received an emergency alert warning them of what was about to happen.

Point Cook resident Wally, who is connected to Melbourne’s ABC Radio, says he was one of them.

“It was very strange,” he says.

“I was kind of sitting there watching YouTube and other stuff on the phone, and just as it was about to happen, like about two or three seconds before that, the phone brings up this alert just to say there’s an earthquake coming.

“It was only three seconds and then bang, bang, bang and it happened.”

Android users got a warning alert on their phones seconds before the earthquake struck. (ABC News: Fraser Fyfe)

He says the letter included advice on what action to take to keep yourself safe.

iPhones do not provide earthquake warnings.

Dr. Carey says phone alert technology is in use around the world, including in Japan where his son recently received a warning seconds before an earthquake struck.

“The earthquake happened,” he says, “and an almost instant message was sent and reached the recipients as the earthquake was moving towards them.”

“These digital signals really respond to the earthquake so they send a message really quickly which is impressive.”

Dr Carey says Australia’s written records of earthquakes only go back to the early 20th century.

One Twitter user said the Sunbury earthquake caused these cracks in the roof of his house in South Morang in north-east Melbourne.

He says Australian building codes have improved dramatically since the 1989 magnitude 5.6 earthquake in Newcastle, which killed 13 people in New South Wales.

“On a global scale, it wasn’t a major earthquake but I think it makes the point that our history is very limited and most earthquakes we’ve had have been very small…so our building codes weren’t taking earthquakes into account,” says Dr. Carey.

“But this Newcastle earthquake more than 30 years ago has prompted planners to look again – or perhaps for the first time – at earthquake risk in Australia.”

This may explain why the 2021 earthquake, which was stronger, was not as devastating.

“There was a little bit of damage in 2021, but I think it was a 19th-century building,” he says.

What if you don’t feel it?

Well, you are in good company.

Dr. Carey has devoted the last 30 years of his career to the study of earthquakes and earth sciences.

He never felt an earthquake.

He did not feel a Sunbury earthquake and once slept through two magnitude 6 earthquakes on consecutive nights while leading a study tour of Taiwan.

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