An earthquake theory developed in the laboratory has just passed a huge test in the field, bringing accurate forecasts one step closer.

An article in this week's Nature, by Shinji Toda of the Geological Survey of Japan and colleagues, has confirmed a theory explaining the behaviour of earthquakes.

It is already known that an earthquake occurs when stresses on a crack in the Earth's crust overcome the forces of friction and cause the crack to slip. But how is it possible to predict when this will happen?

One theory, developed by US geologist Jim Dieterich, argues that the more sudden the stress the more earthquakes will occur. This helps explain earthquake 'swarms' — clusters of quakes over a short period of time.

Dieterich's theory also predicts that the higher the stress rate, the faster the decay of aftershocks.

According to seismologist Dr Phil Cummins of Geoscience Australia, Dieterich's theory had been controversial because it had been developed in the laboratory using samples of rock only a metre long at the most.

Toda and colleagues put Dieterich's theory to the test by studying the events leading up to and during a 'super swarm' of 7,000 earthquakes, which struck offshore from Tokyo over two months in 2000.

The researchers relied on measurements from a network of seismometers and Global Positioning System sensors. They tracked the movement of the ground over time to calculate the rate of stress change and compared this to the number of earthquakes and aftershocks.

Dr Cummins said the new study, which studied the stresses at work in an actual fault line 15 kilometres long and 8 kilometres deep, provided the best measurements yet of the rate of change of stress in a real life earthquake.

These measurements were good enough to confirm Dieterich's theory, he said, and should help convince some seismologists who had been sceptical of its usefulness in the field.

"It gives confidence to people that if you can measure the stress rate you can estimate the probability of larger earthquakes in a swarm," he told ABC Science Online.

"The lack of a coherent physical theory to describe the behaviour of faults has hampered research into earthquake prediction."

Significance

Dr Cummins likened the significance of the new study to the testing of Einstein's theory of relativity.

"Einstein's theory predicted red shift, but it took an actual observation of red shift to give people more confidence in his theory and to apply it to a larger range of problems."

Earthquake 'swarms' occur regularly around the world, even in Australia which has relatively low seismic activity.

In fact, said Cummins, a swarm has been ongoing since September 2000 around the town of Burakin in Western Australia.

While swarms typically include many low-intensity quakes, the fewer larger ones can cause damage, especially in built-up areas.

So will this new study help seismologists to predict the size of earthquake swarms in Australia?

Not immediately, said Dr Cummins. Seismologists in Australia are trying to make similar measurements to the Japanese study, but they face a number of challenges.

"While earthquake swarms in volcanic areas such as Japan are known to be caused by intrusions of magma into the Earth's crust, we don't know the cause of them in Australia," he said.

The problem is, if scientists don't know the cause of an earthquake, they don't know what is causing the stress — let alone where to start measuring for the rate of stress change.

So, the first step in understanding Australian earthquake swarms is to work out what is causing them.

Once this has been done, then Dieterich's theory may well be used with some confidence.

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