'We can blame this earthquake on the Kiwis': Experts explain how the Victoria quake happened

Melbourne's earthquake on Wednesday, the biggest to hit the state of Victoria in 50 years, has been blamed on New Zealand. 

The seismic underarm was bowled by a geoscientist at Macquarie University in Sydney after the 5.9 quake hit at 11:15am (NZ time). 

"We can blame this earthquake on the Kiwis, to some extent," Paul Somerville told the Sydney Morning Herald

The apparent dramatic escalation in our two countries' rivalry thankfully didn't leave anyone dead. At 5.9, the Victoria quake was about two-and-a-half times weaker than that which hit Christchurch in 2011, hit twice as deep and was centred about 180km to the northeast of Melbourne, the state's largest city, rather than directly underneath. 

Quakes are far less common in Australia than in New Zealand. It's estimated quakes as strong as Wednesday's only happen once a decade or so.

"At plate boundaries you get magnitude 5, 6s probably every year. But that's not the case here in Australia," Januka Attanayake, a research fellow of earthquake seismology at the University of Melbourne, told The AM Show on Thursday. 

Most happen in the country's west, he said.

"If you look at the eastern half of the continent, we haven't had a magnitude 6 onshore - we've had magnitude 6s and greater offshore, but not onshore. This is probably the largest earthquake that we've observed in the eastern part of the continent onshore."

Australia's deadliest quake happened in New South Wales in 1989, killing 13 people - but it measured only 5.6, Australian Geographic saying the damage caused was "unusual for a relatively low magnitude quake". Every other onshore quake as big as the one which happened on Wednesday in recorded history happened in South Australia, Northern Territory or Western Australia. 

"At magnitude 5.8, this is the largest onshore earthquake in Victoria in recorded history," said Adam Pascale, chief scientist at Australia's Seismology Research Centre.

In contrast, New Zealand has had four quakes of magnitude 7 or greater in the past two years alone, if you include offshore tremors. In the past 10 or so years on land we've had the monster 7.8 Kaikōura earthquake of 2016, a 6.5 in Taupo in 2011 and of course, the devastating Christchurch quake which left 185 dead. 

"New Zealand is sitting right on top of the plate boundary, right? The Pacific plate is crashing into the Indian Ocean plate, and as a result of that you get a whole bunch of earthquakes every year," said Dr Attanayake.

"But that is not the case in Australia - Australia is sitting right in the middle of the Indo-Australian plate, so we don't get as many earthquakes."

While hundreds are recorded every year in the state, they can't be felt.

"But occasionally we get these very large events," said Dr Attanayake. The mechanics behind that are the same as what you'd see at a plate boundary - that is a build-up of stress along these pre-existing faults, and from time-to-time these faults give way and produce these large earthquakes."

So where do those faults come from? University of Sydney geoscientist Patrice Rey told the Sydney Morning Herald about 350 million years ago - before the dinosaurs -the area was "part of an ancient mountain belt". 

"These mountain beds left zones of weakness, in the form of faults and fractures.. Like if you break something, even if you manage to glue it again, usually it breaks again on the same spot."

Adrian McCullum, a lecturer of geotechnical engineering at the University of the Sunshine Coast in Queensland said there are a large number of faultlines around the quake's epicentre.

"It appears like an area where the release of compressive stress via an earthquake might be probable." 

So it's not entirely New Zealand's fault after all.