BY:EMMA YOUNG | OCTOBER-6-2011
Earthquakes don’t only occur near our neighbours Japan and New Zealand – they’re common in Australia too.
WHEN THE GROUND BEGAN to shudder, Gavin Corica didn’t worry about it – at least not at first.
“It’s not uncommon in Kalgoorlie to feel the ground vibrate because of the mining activity,” says the physiotherapist, who until recently had a practice in the West Australian town. “But then I thought that the morning was an unusual time for a blast – and then the ground started to shake sideways.”
A few seconds after that, it felt as though someone was driving a truck through the front of the building, he says. “I could hear all these sounds, like massive three-tonne boulders falling and crashing on the ground, and the lights went out. That’s when I, and the patient I was with, looked at each other and pretty much ran for the doorway.”
It was 8.17am on 20 April 2010, and an earthquake measuring magnitude 5.0 on the Richter scale was ripping apart the earth 10km south-west of Kalgoorlie. People reported feeling the earthquake up to 200km away, and, like Gavin’s building, many others within a 10km radius were badly damaged.
Most of the world’s earthquakes happen at so-called plate boundaries – parts of the planet where tectonic plates are pushing against one another – and about 80 per cent occur around the edge of the Pacific Plate (the ‘Rim of Fire’), affecting New Zealand, Japan, the west coast of North and South America and New Guinea.
A 6.3 earthquake with a shallow epicentre struck 10km south-east of Christchurch on New Zealand’s South Island in February 2011, turning much of the city centre to rubble and killing 181 people, while a huge, magnitude 9.0 quake (possibly the fourth most powerful ever recorded) struck off the coast of Japan in March creating a tsunami that killed an estimated 25,000 people, wiped entire towns off the map, and caused the largest nuclear disaster in history.
Australia doesn’t sit on the edge of a tectonic plate. However, the Indo-Australian plate, at the centre of which our continent lies, is being pushed to the north-east at about 7cm per year. It’s colliding with the Eurasian, Philippine and Pacific plates, causing stress to build up in the 25km-thick upper crust. This build-up of pressure within the plate can cause earthquakes in Australia.
In fact, Australia has more quakes than other regions that sit in the middle of plates and are considered relatively stable, such as the eastern USA. “The level of seismicity does seem to be significantly higher here,” says Professor Phil Cummins, an expert on quakes at Geoscience Australia (GA) and the Australian National University’s Centre for Natural Hazards. “But no-one really knows why that is.”
According to recent research by GA, there’s been about one earthquake measuring magnitude 2.0 or greaterevery day in Australia during the past decade. “There are likely to be many more smaller earthquakes that we cannot locate because they’re not recorded on a sufficient number of seismograph stations,” says Clive Collins, a senior GA seismologist.
Western Australia is a quake hotspot, with more quakes than all the other states and territories combined. But the GA data show that Adelaide has the highest risk of any capital. It’s suffered more medium-sized quakes in the past 50 years than any other (including one that struck in March 1954, just before the visit of Queen Elizabeth II) – and that’s because it’s being squeezed sideways.
In regions around plate boundaries, it’s possible to predict roughly when quakes are likely to happen, as scientists know where to look for any build-up of stresses. “We can’t predict with an accuracy that would be valuable for evacuation or early warning,” says Phil, “but we can forecast pretty well that certain parts of the plate boundary might be more likely to experience an earthquake in the next 10-20 years than others.” But for regions that sit in the middle of a plate, like Australia, quakes can strike anywhere, making prediction practically impossible.
Thankfully, most of our quakes are small, and go unnoticed, except by seismologists. But tremors of the size that terrified the residents of Kalgoorlie last August happen about every 1-2 years, and about every five years there’s a potentially devastating quake of magnitude 6.0 or more. The biggest quake ever recorded in Australia was in 1941, at Meeberrie in WA, with an estimated magnitude of 7.2 – but it struck a remote, largely unpopulated area.