Why are there earthquakes in Australia?

This morning, residents in south-eastern Australia felt the earth shake beneath them for about 20 seconds.

The magnitude 5.8-6.0 earthquake was centred 10km below Mansfield, 129km from Melbourne. Two later shakes, understood to be aftershocks, registered at magnitudes of 4.0 and 3.1.

Gary Gibson, a seismologist at the University of Melbourne, told ABC Radio that it was the biggest earthquake experienced in Victoria since European settlement.

Mansfield mayor Mark Holcombe told ABC Breakfast that “it came right out of left field. We don’t have earthquakes, that I am aware of. None of the locals I spoke to this morning had that experience with earthquakes here before, so it is one right out of left field. It was just a really big crumble.”

Read more: Tectonic Timelapse

What causes earthquakes?

Earthquakes occur when continental plates shake or tremor as fault planes move. This happens because pressure below the crust builds up and eventually tries to escape and forces the plates to move to accommodate.

 There are three different ways the plates can move to cause an earthquake:

  • Strike-slip: Two sides of the plane move horizontally to each other
  • Normal: One fault tries to move downwards
  • Reverse: One fault moves on top of the other.
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Three types of plate movement. A) Strike-slip, B) Normal, C) Reverse. Credit: Wikimedia commons.

Why aren’t there many earthquakes in Australia?

Land masses along continental plates, such as New Zealand (Aotearoa), are riddled with major fault lines. This means they have frequent earthquakes because the plates aren’t connected.

Australia, however, is in the middle of a large continental plate, called the Indo-Pacific Plate. Here, there are far fewer small fault planes and no major fault lines. This means big earthquakes are uncommon.

The last earthquake of a similar size in Victoria was a 5.5-magnitude quake in 2009, but the largest earthquake recorded in Australia – magnitude-6.7 – was in 1988, south-west of Tennant Creek in the Northern Territory.

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Tectonic plate boundries. Credit: Wikimedia commons.

“In terms of seismicity and plate tectonic setting, Australia is classified as a stable continental plate, with intraplate stress fields generated due to presence of passive margins imposing forces at the plate boundaries,” says Behzad Fatahi, Associate Professor of Geotechnical and Earthquake Engineering at the University of Technology Sydney,

“On average, Australia experiences two earthquakes with magnitudes greater than 5 per year, which is a rather high level of seismicity for a stable continental region.

“In the past century, about 27,000 earthquakes were recorded in Australia, and over 90 per cent of these earthquakes occurred at a very shallow depth.”

Why do earthquakes happen in Australia?

There are still some fault planes in Australia, but they aren’t very large. These fault planes are more like cracks in the surface rather than two separate plates moving against each other.

“Aboriginal people tell of past earthquakes, for example the Awabakal people of Newcastle remember past occurrences recounted in their Dreaming stories,” says Dr Margaret Cook, a historian and history lecturer at University of the Sunshine Coast

The pressure below the crust is still trying to release and escape, which means Australia experiences frequent tiny earthquakes. Usually, they are so deep in the earth that we barely feel them.

“The 5.8 magnitude earthquake this morning in Mansfield, Victoria, with shocks spreading all the way south to Tasmania and north to New South Wales is a reminder that Australia faces seismic risk,” says Associate Professor Iftekhar Ahmed from the School of Architecture and Built Environment at the University of Newcastle.

“The Australian tectonic plate is colliding into the Pacific plate in the Pacific Ocean and generates compressive stress, which when built up is released suddenly, causing earthquakes.”

Read more: Triggered: new insights into deep earthquakes

When a big quake occurs, like this morning, it is usually because the pressure builds up to the point that it ‘pushes’ up the crust. This causes a reverse fault. If there’s enough pressure, this can even elevate the land, which, over time, could become a mountain. In fact, the mountains in Victoria were caused as pressure forced minor faults to move and elevate.

There are many mountains around Mansfield because it’s the location of a minor fault plane.

“The earthquake in Mansfield, Victoria is likely to have occurred along the Governor Fault – one of the deepest faults in Victoria – that separates the Melbourne Zone from the alpine region,” says Ben Mather, a plate tectonics expert at the University of Sydney.

You can help Geoscience Australia by making a report if you felt the earthquake here.

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