The magnitude-5.9 earthquake that rattled the nerves of Victorians and others living on Australia’s east coast on Wednesday is the largest recorded one to have hit the state. Here, experts help us to understand why and how it happened.
Geoscience Australia recorded the earthquake’s epicentre originating near Mansfield at around 9.15am. Another magnitude-4 tremor followed 20 minutes afterwards, and then a magnitude-3.1 earthquake at 9.54am.
The strength of the quake was greater than the devastating 1989 Newcastle earthquake, which was a recorded magnitude-5.6 tremor, and led to the death of 13 people, injuring 160 others.
— Earthquake Monitor (@EQAlerts) September 21, 2021
Speaking to The Mandarin, seismologist Dr Caroline Eakin said that earthquakes in Australia equivalent to the magnitude of the first were only expected to occur every six years or so. And while they have occurred in Australia before, it was quite unusual for them to occur somewhere so close to people and be widely felt.
“This is the largest magnitude earthquake in recorded history in the State of Victoria. It is certainly not something that’s common,” Eakin explains.
“There’s certain regions of Australia which get more earthquakes than other parts.”
“Across Australia, we know it is possible to get these magnitude-5, earthquakes, even a magnitude-6, earthquake, but we don’t know exactly where that will happen,” she says.
Eakin says small earthquakes in the range of magnitude-2 or -3 are more common (averaging approximately 100 earthquakes annually) in areas like the highlands of Australia’s south east, in the south west of the continent in Western Australia, and the Flinders Ranges in South Australia.
“Those are all regions where small magnitude earthquakes can be relatively common,” she says.
“There are zones of Australia, which tend to get, compared to other parts of [the country], more seismicity in a relative sense.”
Although these tremors are regarded as small, Eakin says they can still be felt but are unlikely to cause much damage. Unlike countries like Japan (which experienced a magnitude-9.1 earthquake and subsequent tsunami in 2011) and New Zealand, which sit on a tectonic plate boundary, Australia rarely experiences frequent or high-magnitude earthquakes.
According to Geoscience Australia, magnitude-5 earthquakes usually occur every one-to-two years, and magnitude 6.0 earthquakes every 10 years or so.
Everything is lining up consistently with today’s earthquake in Victoria. The strike-slip mechanism solution shows NW-SE maximum horizontal stress direction, consistent with the regional stress direction and neotectonic faults. More details are here: https://t.co/Ayyrbcgl8D pic.twitter.com/hv971UHvsK
— Mojtaba Rajabi (@RajabiMojtaba) September 22, 2021
Victoria’s most recent earthquake events were recorded at a shallow depth of 10km and about 38 kilometres from Mount Buller, near Mansfield. The earthquakes resulted in some brick structures collapsing but there have been no reports of injuries or deaths.
Experts say that this one was caused by Australia’s tectonic plate colliding with the Pacific plate in the Pacific Ocean; the collision generated a build up of compressive stress that, when released suddenly, resulted in the earthquake phenomenon.
“Imagine you have two blocks of material next to each other, and you apply a force to those two blocks — whether that’s compressive, you’re squeezing them, or you’re trying to push them past each other,” Eakin says.
“That stress builds up until it passes the friction between the material, and then the blocks will slip, or fracture or slide — like what happens in an earthquake — and then it releases that energy.”
While earthquakes are rare in Australia, Wednesday’s tremor demonstrated that the landmass does face seismic risk.
“We certainly can’t predict earthquakes in terms of knowing a date, a time, a magnitude and an exact location,” Eakin says.
Preparedness for these potentially catastrophic — however rare — events remains an important issue to consider, according to University of Newcastle Associate Professor Iftekhar Ahmed. The lecturer at the university’s school of architecture and built environment said that the benefit of mapping recorded earthquakes indicated the risk of certain areas and should form the basis of risk-planning.
“The oft quoted saying, ‘Earthquakes don’t kill people, buildings do’ points to the importance of safe buildings,” Ahmed says.
“This needs to be combined with strong public awareness and institutional training initiatives so that people and organisations are prepared on what to do during and after an earthquake.”
Behzad Fatahi, a geotechnical and earthquake engineering expert at the University of Technology Sydney, said that the intensity of Wednesday’s tremor in Victoria was estimated to be ‘very strong’ and had the potential to cause light to moderate damage to structures, buildings and infrastructure.
“The length of fault rupture for this earthquake is estimated to be about 1.4 km. Based on the initial data, energy released due to this earthquake was estimated to be twice that of the 1989 Newcastle earthquake in NSW,” Fatahi says.
“Residents must stay away from the cracked building, as aftershocks can further loosen the already compromised sections of structures. Residents should also stay away from temporary excavations particularly near existing buildings.”
— Simon Love (@SimoLove) September 21, 2021
Eakin, from the Australian National University’s school of earth sciences, has studied geophysics from institutions in the US, UK and more recently in Australia. Her research aims to understand the links between plate tectonics and earth’s surface.
Data collected from earthquake occurrences inform Geoscience Australia’s national seismic hazard assessment, Eakin says, adding that she would be interested to see what her own network of seismometers located in central Australia have picked up on the Mansfield tremors.
“Today’s event, not only is it interesting in terms of where it occurred and what that means, but also in terms of how those seismic waves will then have travelled across the continent and been recorded on many different seismometers across Australia.
“I’m interested in the architecture of the Australian continent, the deeper, upper-200 kilometres beneath the surface; and the patterns of the formation and history and structure of the continent,” she says.