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Why Australia keeps getting small earthquakes
Anthony Sharwood, Wednesday August 7, 2024 - 11:53 AEST
A magnitude 4.1 earthquake shook Victoria overnight, centred on the tiny town of Woods Point, population 33, in the foothills of the Victorian high country.
This was the strongest quake in Australia (not including the Australian territory of Macquarie Island) since the April 17, 2024 magnitude 4.2 quake in Jamestown, in South Australia's Mid North district, which was also felt in Adelaide, 200 km south of the epicentre.
Thankfully, there have been no reports thus far of damage from last night's Victorian quake, even though magnitude 4.1 is strong enough for structural damage and landslides, as Weatherzone's Ben Domensino explained in his story entitled 'How Common are Earthquakes in Australia?'
Image: Shaken, not stirred. The main street of Woods Point Victoria. Source: Creative Commons.
What were Australia's worst earthquakes?
Australia is not a noted zone for strong earthquakes. The country's deadliest seismic event was the 1989 Newcastle earthquake, a 5.6 magnitude quake which killed 13 and injured more than 160.
Australia's most powerful recorded earthquake was in 1988 at Tennant Creek in the Northern Territory. The 6.6 magnitude quake was the culmination of several days of strong tremors, and while no deaths or injuries were reported, the damage bill ran over a million dollars due mostly to ruptures of a gas pipeline.
How does earthquake magnitude work?
An earthquake's magnitude is a measure of the energy it releases, and it is ranked using a logarithmic scale.
According to Geoscience Australia, each unit of magnitude is roughly 30 times more powerful than the preceding level.
This means a magnitude 3.0 earthquake releases 30 times more energy than a magnitude 2.0 earthquake, while a magnitude 4.0 earthquake releases 900 times more energy than a magnitude 2.0 earthquake (30 x 30).
So obviously, this makes higher magnitude earthquakes many, many times more damaging than lower magnitude events.
Why does Australia get earthquakes when it’s not on the boundary between tectonic plates?
Earthquakes are largest and most frequent at the boundaries between tectonic plates, where two plates are colliding and/or sliding past one another, or even moving away from each other. That’s why New Zealand is so earthquake-prone, as you can see on the image below.
Image: The continent of Australia, including Tasmania, is wholly contained within the Australian Plate: Source: Creative Commons.
Away from plate boundaries in the relatively stable interior of continents like Australia, earthquakes are less common and are known as intraplate earthquakes, generally originating at a shallow depth.
Intraplate zones can still be quite volatile at times. As Weatherzone meteorologist Joel Pippard explains, there’s an intraplate tectonic stress fault that runs from NZ’s South Island to Victoria, which explains why Victoria experiences notable earthquakes of a magnitude around 4 to 5 once or twice each year.
"If normal tectonic plates are like two pieces of paper next to one other, they can freely move and bump into one another," Pippard explains.
"A stress fault would be the wrinkling of the paper in the middle due to the contact on the edge. In other words, the stress fracture helps diffuse the energy across a larger area, reducing the frequency of larger quakes.
On average, Australia experiences around 100 earthquakes of magnitude 3 or higher each year. Earthquakes above magnitude 5 happen about once every one-to-two years, while earthquakes measuring magnitude 6 and above only occur about once per decade.
- Weatherzone
© Weatherzone
2024
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