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Mega-quake clusters unlikely: study

Chile quake

The recent spate of catastrophic earthquakes may just be a random occurrence - or not (Source: Reuters/Ivan Alvarado )

Mega-quake clusters The global risk of catastrophic earthquakes has not risen in recent years, according to a new study.

"It could have just been random chance that we've had a few quite large earthquakes recently," says seismologist Professor Peter Shearer, of University of California, San Diego, lead author of the study that appears this week in the Proceedings of the National Academy of Science.

Since 2004, there have been three mega quakes that measured above 8.8 on the Richter scale - in Sumatra, Japan and Chile - and some scientists say history suggests these may have been linked.

They point to a similar spate of mega quakes over a period of 15 years during the 1950s and 60s.

"There's been speculation that there was some kind of global swarm of large earthquakes in the 50s and 60s," says Shearer.

"There was concern there was some kind of physical link and that whatever was causing them stopped and now the kind of clustering that we saw back then is starting up again."

But Shearer and colleague statistician Professor Philip Stark of University of California, Berkeley analysed the frequency of earthquakes since 1900 and say the findings show the risk of big earthquakes hasn't increased.

"If you do the statistical tests to evaluate whether this is likely due to random chance, you can't remove the null hypothesis, [that it's a random chance]," says Shearer.

But, he says the small number of earthquakes in the analysis means that it is difficult to use statistics alone to determine if there are clusters occurring.

So the researchers also looked at the possible explanations for how such clustering could be occurring.


While the shaking from a large earthquake is understood to trigger smaller aftershock quakes, two observations undermine this as a possible mechanism for mega quake clusters, says Shearer.

First, he says, there were no increases in smaller earthquakes at the same time as there was an increase in large quakes.

"The rate of magnitude 7 earthquakes in the last five to ten years is very close to the long-term average rate. It's hard to think of a mechanism that would trigger the large ones, but not at the same time trigger a lot of small earthquakes," says Shearer.

"The above average rate of activity that we've seen in the last few years seem to be only for the larger earthquakes."

Second, he says, evidence from the 1960s shows the large quakes occurred only after a series of smaller quakes rather than before them, undermining the aftershock hypothesis.

It has also been proposed that earthquake clusters can be caused by changes in the internal forces of the Earth's crust, but Shearer says this mechanism is not plausible either given that stress changes die out very rapidly with distance away from a quake.

"There is no plausible physical mechanism that would link a large earthquake in Chile with one in Japan, so it's most likely that these are truly random events which means that earthquake risk in an area like California is the same as it was 10 or 20 years ago. It has not changed," he says.

Shearer says people have a "tendency to see patterns in things" when there are none.

"Psychologists have studied this and shown that we tend not to judge randomness very well," he says.

'Can't rule out clusters'

Physicist Dr Felipe Dimer de Oliveira of Macquarie University and colleagues have also analysed the clustering of quakes and emphasises the data are too scarce for useful statistics.

"Some of statistical tests give only a 50 per cent chance of correctly detecting a cluster," says de Oliveira, who responded to the research by Shearer and Stark at a recent academic meeting in California.

De Oliveira, who works for the insurance industry-funded Risk Frontiers says it's still an open question whether mega-quake clusters really exist.

He says the best available evidence to support the idea that the apparent clusters are not random comes from the fact that 8.3-magnitude quakes have an average frequency of 20 to 30 per century and yet there was a 40-year break between the two groups of them.

"If you have a chance of around three events per decade, the chance that you will have four decades without seeing any events is really low," says de Oliveira.

De Oliveira says many scientists are sceptical of mega-quake clusters because they can't find a mechanism to support them, but he says it's too early to be sure.

"The truth is we don't know much about what happens underneath the Earth's crust. There can still be some mechanisms that we don't know about," he says.

"You can't rule out clusters."

Tags: earthquake, earth-sciences, geology, physics