Why are there more earthquakes in Western Australia than in the rest of the country?

It was a shaky start to the year for Western Australia.

Key points: Last year, more than half of Australia’s earthquakes occurred in the Western Australia region, with several factors contributing to the frequency of earthquakes, and there have been more than 100 earthquakes in the Greater Southern region this year.

Hundreds of people were shaken by a 4.7 magnitude earthquake in southwestern Australia at the start of 2022.

The earthquake occurred near Wajin and was the largest of more than 100 earthquakes recorded in that area so far this year.

Earthquakes are closely related to places on the edge of the Earth’s tectonic plates, such as New Zealand or Japan.

But the land under Australia’s feet is not as stable as many think – particularly in Washington.

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Why are some areas more vulnerable than others?

On average, Australia records 700 earthquakes annually, occurring across the country.

But some regions are more active than others.

WA is the most active in terms of size and number of earthquakes. One of the largest earthquakes recorded in Australia struck the wheat-growing community of Meering in Western Australia on October 14, 1968.

Meekering Hall after the 6.5 magnitude earthquake in 1968, one of Australia’s largest onshore earthquakes (Supplied: Western Australian State Library)

In 2021, Australia recorded 548 earthquakes, more than half of which occurred in Washington State, according to GeoScience Australia seismologist Hugh Glanville.

Mr Glanville said part of the reason Western Australia had the highest rate of earthquakes was because of the land mass compared to other states.

But he said Washington also has some other factors working against it, such as the state’s rock structure.

“It’s older crust across most of Western Australia, so it’s older rocks, some of which tend to be more fragile,” he said.

“So it can absorb stress a little differently than smaller rocks, and it can fracture more easily.”

WA also has many fault lines that are aligned in this way, compared to the direction the pressure is coming from, making it easier for them to break.

An earthquake measuring 5.4 on the Richter scale struck the Lake Muir region of Washington state, about 330 kilometers from Perth, in November 2018.

Farmer Mark Muir and his neighbor Rob de Campo discovered winding fissures across Mr. de Campo’s fields.

At the time, Geoscience Australia’s chief seismologist Jonathan Bathgate said there was “a little increased activity” around the Lake Muir earthquake, with about 600 having occurred in the following two months.

Mark Muir (left) and Rob de Campo stand on either side of what is believed to be an extended fault caused by a 5.7-magnitude earthquake (ABC Local: Anthony Pancia)

And WA also gets something known as earthquake swarms much more frequently than other states.

While a typical earthquake pattern involves a main shock and a series of aftershocks, an earthquake swarm includes dozens or even hundreds of moderate main shocks, each with their own aftershocks.

The basin surrounding Broome, in northwest Australia, is another active earthquake area.

The earthquake that shook Broome in July 2019 was on par with the largest ever recorded in Australia (Supplier: Esse Deves)

In July 2019, a 6.6-magnitude earthquake was felt in the Indian Ocean over large parts of Western Australia.

While it sure horrified Broome’s residents, the damage was limited to some roof shingles and stock that fell off supermarket shelves.

Mr. Glanville said its proximity to the Indonesian fault line made that region more prone to earthquakes.

Other active areas across Australia include the Adelaide Hills and the Great Dividing Range between Melbourne, Canberra, Newcastle, and Tennant Creek in the Northern Territory.

Recreating the GeoScience Australia National Seismic Hazard Assessment Map (ABC: Stefan Hammett)

This is where Australia’s largest earthquake was recorded in 1988, with a magnitude of 6.6.

What else causes earthquakes in Australia?

The boundaries of the Australian tectonic plates around the Indonesian archipelago and Papua New Guinea extend through the southwestern Pacific Ocean and New Zealand and terminate halfway between New Zealand and Antarctica.

The Australian tectonic plate collides with the Pacific and Eurasian plates (ABC: Stephan Hammat)

Like all tectonic plates, it is constantly moving, colliding with the Pacific plate to the north and east of Australia, and the Eurasian plate in the northwest.

This collision between the moving plates generates stress deep within the Earth’s crust, which eventually sets off as an earthquake.

Most large earthquakes occur at plate boundaries, in addition to volcanic activity, such as the recent devastating eruption in Tonga.

But this pressure also creates what are known as fault lines within the plate.

Mr. Glanville said you could think of it like Pavlova.

Fault lines within the plate are like cracks within Pavlova, says Hugh Glanville (Flickr: Loloeig)

“So if you hit or drop Pavlova, or you only have one sink, you’re going to get a lot of cracks around the edge, which is the edge of the board,” he said.

“But you do get some cracks inside.”

Is Australia at risk of a major earthquake?

The good news is that most of the earthquakes that hit Australia have a magnitude of 2 and 3, which will cause mild shaking but no damage.

You won’t start to see minor damage until it reaches 4 degrees and above, such as shattered dishes and windows and cracking plaster.

Above 5 degrees you’ll start to see damage to homes and buildings.

Earthquakes greater than 5 on the Richter scale occur every one to two years, and earthquakes greater than 6 occur approximately once every decade, according to GeoScience Australia.

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