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.