What Did the Ground Say to the Earthquake

An earthquake affected south-east Australia on Tuesday with seismologists placing its magnitude at 5.9, making it the largest onshore earthquake in Victoria's recorded history. The epicentre was Mansfield, in the foothills of the Victorian Alps, but it was felt as far away as New South Wales and Tasmania. Despite this, only mild structural damaged occurred, and there were no reports of injuries. So what prompted the earthquake and why was its impact relatively mild?

Why did it happen?

Dr Adrian McCallum, a senior lecturer in geotechnical engineering at the University of the Sunshine Coast, says the continental plate on which Australia sits is moving north at about 7cm a year, building up compressive stress within the plate. This stress is occasionally released, resulting in an earthquake, typically along pre-existing faultlines.

Geological maps of Victoria show a large number of faults in Mansfield, McCallum says.

Dr Ben Mather, a plate tectonics expert at the University of Sydney, says the earthquake in Mansfield "likely occurred along the Governor Fault – one of the deepest faults in Victoria – that separates the Melbourne zone from the alpine region".

Associate Prof Iftekhar Ahmed, from the school of architecture and built environment at the University of Newcastle, says the alpine region used to be an area of volcanic eruptions hundreds of years ago. "So it's an area of seismic activity," he says. "This faultline runs on a north-south parallel, and if it's where this earthquake originated from, then that's why the wave was felt all along the eastern seaboard, all the way to Tassie".

The 5.6 magnitude Newcastle earthquake in 1989 caused serious devastation. Why didn't this one?

Ahmed says: "Earthquakes don't kill people, buildings do."

"Our buildings are built to a certain level of seismic resistance," he says. "Some of the older buildings aren't, but many have been retrofitted to bring them up to standard."

Dr Januka Attanayake, the research lead with the University of Melbourne's Earthquake Seismology Earth Sciences unit, says in the aftermath of earthquakes like the one on Tuesday, seismologists would usually install more seismometers to try to detect aftershocks, which can last for months but are usually much milder than the initial earthquake and not felt by people.

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"It's important work because if we can detect aftershocks we can detect the fault area that ruptured," he says. "We need to know this information for proper future hazard analysis. It helps us detect expected ground motions of earthquakes going forward. This is essential information for engineers building city structures in future, as we can, for example, say how much ground motion can be expected at a given location over the next 50 years."

Two aftershocks of 2.5 and 3.0 magnitude have already been detected.

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Photograph: Tim Robberts/Stone RF

Ahmed says while it is not possible to precisely predict earthquakes, mapping of seismic activity and recorded earthquakes indicate an area's risk, which is the basis for risk reduction measures, including the construction of buildings to a seismic standard.

Are earthquakes common?

Not of this magnitude in Victoria.

Attanayake says "it is probably the largest earthquake we have felt around Melbourne in the last 175 to 200 years". "This is the first earthquake of this magnitude I have seen here during my lifetime, and it has probably not been seen during the lifetime of several generations," he says.

But Victoria does record about 400 earthquakes of less than magnitude 2.5 every year, he says. "So earthquakes are not an exception."

Prof Adrian Russell, a geotechnical engineer at the University of New South Wales Centre for Infrastructure Engineering and Safety, says earthquakes are an important consideration for Australia.

"Earthquakes of magnitude 5.0 or greater occur about once every six months and are sufficient to cause widespread damage if near our oldest cities and infrastructure," he says. "For example, reinsurance companies rate an earthquake in Sydney in their 20 top risk exposures worldwide, not because the earthquake would be large, but because there would be widespread and yet fairly mild property damage."

What is protective in an earthquake?

Aside from properly designing buildings, there are steps people can take to avoid injury during an earthquake.

According to the SES, if you are inside: "Drop to the ground; take cover by getting under a sturdy table or other piece of furniture; and hold until the shaking stops.

"If there isn't a table or desk near you, cover your face and head with your arms and crouch in an inside corner of the building," the advice says. "Stay away from glass, windows, outside doors and walls, and anything that could fall, such as lighting fixtures or furniture."

What Did the Ground Say to the Earthquake

Source: https://www.theguardian.com/australia-news/2021/sep/22/victorian-earthquake-explained-why-did-it-happen-and-why-was-the-impact-relatively-mild

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