Natural Wonders

Geologist explains the cause of earthquake that rocked Melbourne

Melbourne was shocked by a powerful earthquake that rocked the city but a geologist has explained why we shouldn’t be surprised.

The dramatic tremors which rocked homes and buildings in Melbourne was likely caused by Australia’s continent edging north and crunching into Asia Pacific nations.

Curtin University’s structural geologist, Professor Chris Elders, said Australia is moving away from Antarctica at 7cm a year — rapid speed in geology terms — unsettling plates surrounding the nation and forcing the earth under our feet to move at regular intervals.

“The Indian Ocean is getting bigger and we’re colliding with Indonesia, Timor and New Guinea to the north,” he told NCA NewsWire.

“So all those boundaries produce stresses that get transmitted through the crust and then when they hit a weak fault, the fault moves and causes the earthquake.”

Wednesday morning’s quake, centred south of Mansfield in regional Victoria, was measured by a magnitude of 5.9, according to Geoscience Australia.

The remoteness and 10km depth of the quake below the earth’s surface meant the shake was less destructive than other movements of similar magnitude, given the tragedy in Christchurch was measured at 6.3.

Comparatively, the devastating New Zealand quake was closer to the earth’s surface and central to the city, while Australia’s most destructive earthquake was weaker than both but caused widespread death and injuries.

In 1989, a 5.6 magnitude quake rocked Newcastle and caused one of the most deadly natural disasters in Australian history — killing 13, injuring more than 160 and causing the equivalent of billions of dollars in damage.

The strongest measured quake in Australia was about 6.6 in Tennant Creek in 1988, though the Richter figure is misleading given the logarithmic scale — 6.8 is 10 times stronger than 5.8.

“The amount of damage that an earthquake will do depends on a whole range of different factors,” Prof Elders said.

“The most important is how close it is to a central population.

“Second thing will be obviously the ways in which the buildings themselves are constructed as to how solid the foundations are and how much the buildings can withstand the degree of shaking.

“And then the third factor is sometimes the bedrock itself — this was sort of softer and water laden sediments, as opposed to a harder rock that might also effect the amount of shaking the buildings experience.”

Australians are more acquainted with seasonal disasters such as fires, floods and cyclones, with earthquakes closely linked with California, Japan and the Pacific.

But the Curtin University academic said shakes across the country are surprisingly frequent.

He said quakes of the magnitude recorded in Victoria on Wednesday occurred about every five years, but there is a shake of three or more magnitude detected once a week.

“If you look at a map of the distribution of earthquakes, they’re occurring in all the states,” Prof Elders said.

“That is a reflection of the fact that Australia is a very ancient bit of continental crust. So there’s lots of fault lines and lines of weakness that kind of fail when the stresses build up.

“It’s a combination of the plate boundaries all the way around Australia, and the abundance of lines of weakness within the ancient Australian crust can fail and cause earthquakes to occur.”

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