A house decimated by the 2010 earthquake in Chile.
What's the News: Enormous earthquakes are rare; there have been only seven quakes with a magnitude 8.8 or above since the start of the 20th century. Of those seven quakes, three of them have happened in the past seven years: off the coasts of Indonesia
in 2004, Chile
in 2010, and Japan
last month. Some researchers think this earthquake cluster marks the start of a period of megaquakes, while others believe that the earthquake cluster is simply a statistical fluke, with these unusually massive quakes just happening to occur within a short amount of time, according to recent analyses
(PDF) of Earth's earthquake history presented at the Seismological Society of America's annual meeting last week. Some Scientists Say:
In a 2005 paper, researchers Charles Bufe and David Perkins from the US Geological Survey identified a cluster of large earthquakes about 50 years ago: three earthquakes with magnitudes of at least 9.0 in Russia, Chile, and Alaska between 1952 and 1964. The random probability of this earthquake cluster---that is, the likelihood the three quakes occurred within that timeframe just by chance---was 4%, they found. From 1950 to 1965, the researchers observed, there were a total of seven earthquakes with a magnitude greater than 8.5---a string of events with a random probability of 0.2%.
Building on that analysis, the researchers recently examined earthquake data up through the Indonesian earthquake in 2004. The likelihood of the last half-century's earthquake distribution being random---with lots of strong quakes from 1952 to 1964, a period of relative calm, then seismic activity ramping up again in 2004---was 2%, they found.
Using their analysis, they estimated that the chances of a 9.0 or greater earthquake in the next ten years at 63%, as opposed to the 24% chance if massive quakes happen at random.
On the Other Hand:
Another study turned up no evidence that earthquake clusters were anything but chance. Another USGS seismologist, Andrew Michael, mined the seismic record for patterns, using a variety of magnitude thresholds and time scales to run multiple analyses. "I've run a large number of tests and can't find any reason to reject the idea that clustering is random," he told Nature News.
What's the Context:
These analyses aren't looking at aftershocks, which have long been known to follow a major earthquake, but at separate seismic events.
A recent study found that big earthquakes may trigger more quakes nearby, but don't seem to up the odds that a large earthquake will happen in a distant part of the globe.
It's not clear why massive quakes would cluster, if in fact they do. It could be that a monster earthquake reverberates through the planet, impacting how tectonic plates move. "Maybe on the very large scale we have a weakening of these very long fault zones that are on the verge of failure,” Bufe told ScienceNews.
Charles G. Bufe and David M. Perkins. "The 2011 Tohoku earthquake: resumption of temporal clustering of Earth’s megaquakes." Seismological Society of America meeting, Memphis, April 14, 2011.
Charles G. Bufe and David M. Perkins. "Evidence for a Global Seismic-Moment Release Sequence," Bulletin of the Seismological Society of America, June 2005. DOI: 10.1785/0120040110
Andrew J. Michael. "The recent rate of great earthquakes: global clustering or random variability?" Seismological Society of America meeting, Memphis, April 14, 2011.