'No evidence' of links between mega-earthquakes
[SANTIAGO] Scientists have rejected fears that a series of highly destructive large-scale earthquakes in the past few years, in countries bordering the Pacific and Indian oceans, signal an increased global risk of these deadly events.
Several vast earthquakes have taken place since 2004 — in Chile, Indonesia and Japan — leading some academics to express concern that they may be linked.
But a new study suggests that the pattern of earthquakes, although improbable, is likely to be random and that the risk of large earthquakes is no higher today than it was historically.
The conclusion of the study, published online in Proceedings of the National Academy of Sciences last month (19 December), challenges speculation that the above-average rate of earthquakes of magnitude 8 and above on the Richter scale in recent years reflects a change in the underlying rate of activity.
Seismologist Peter Shearer from the Scripps Institution of Oceanography at the University of California, San Diego, United States, and statistician Philip Stark at the university's Berkeley campus, examined records of the timings of quakes of magnitude 7 and above since 1900.
"We found no evidence that the recent global sequence of earthquakes and an earlier one over a 15-year period during the 1950s and 1960s is not simply a random occurrence, but we also cannot say with certainty that it is random," Shearer told SciDev.Net.
"The number of large earthquakes since 1900 (five of magnitude 9) is simply too small to reach a firm conclusion. It's also not surprising that most giant earthquakes occur around the Pacific, because the world's most active subduction zones border that ocean," he added.
Because empirical earthquake data do not settle the question conclusively, the researchers looked at the plausibility of a physical mechanism that could link large events that are far away, such as the recent earthquakes in Chile and Japan.
They concluded that the likelihood of the existence of such a mechanism was small for two main reasons.
First, no large quake has triggered another one at distances beyond 600 to 1,000 kilometres in the last 30 years, according to an earlier study published in Nature Geoscience.
Second, if changes underneath the Earth's crust had taken place, smaller earthquakes would also have increased during periods when several large ones occur, but this has not happened.
"Shearer's and Stark's paper rightly concludes that the grouping of mega-earthquakes can be considered to be random," Sergio Barrientos, director of the Seismological Service at the University of Chile, told SciDev.Net.
Mario Pardo, geophysicist and seismologist at the same university, agreed that the risk of large earthquakes has not changed in the last century and that it may simply be a coincidence when several happen in a short space of time.
Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1118525109 (2011)
Nature Geoscience doi: 10.1038/ngeo1110 (2011)