Seismologists rumble over quake clusters

Japan tremor may be part of a second grouping of great quakes

Japan’s catastrophic March 11 earthquake was the biggest ever recorded in Japan and the costliest natural disaster in history, but in a way it was nothing new. Three other quakes of magnitude 8.6 or greater have struck worldwide in the past seven years — after a gap of four decades.

People walk through rubble in Sendai, Japan, a month after the March 11 earthquake and tsunami that destroyed the city’s coastal districts. Some geologists argue that the event may be seismically linked to other recent major earthquakes. AP Photo, Kyodo News
| Since 1900, most earthquakes of magnitude 8.5 or greater have occurred in two clusters, one in the 1950s and ’60s and the second starting in 2004.

Two U.S. Geological Survey scientists contend that the Japan quake bolsters their idea that the planet is experiencing a spasm of great earthquakes, its second since 1900. Other scientists say that any apparent bunching is a statistical fluke that disappears if the data are analyzed in other ways. Researchers presented their opposing views on April 14 in Memphis, Tenn., at the annual meeting of the Seismological Society of America.

On the face of it, large quakes certainly do seem to have been popping off lately. The magnitude 9.1 Sumatra quake in 2004, which caused the disastrous Indian Ocean tsunami, was followed by a nearby magnitude 8.6 the following year. A magnitude 8.8 hit Chile in February 2010 and the Japan quake struck on March 11 of this year, leading many to question whether big quakes are becoming more frequent.

Scientists say there’s not enough evidence to support that idea. But big quakes may at least be coming in groups.

Some earthquake clusters happen naturally; after a large quake, aftershocks continue to rattle the area. And large quakes can also trigger separate tremors, smaller and close to the original quake (SN Online: 3/28/11).

But Charles Bufe of the USGS office in Golden, Colo., argues that Sumatra, Chile and Japan represent an unusual grouping.

In a 2005 paper, Bufe and his colleague David Perkins argued that a similar cluster occurred between 1952 and 1964, anchored by a magnitude 9.0 in Kamchatka, Russia, at its beginning and a magnitude 9.2 in Alaska at its end. The Kamchatka quake was the first magnitude 9 or greater since 1900, when scientists began recording seismic activity worldwide. This cluster also included the largest quake ever recorded, a magnitude 9.5 monster that struck Chile in 1960.

Bufe and Perkins used the same analysis — simulating the probabilities that the earthquake patterns could be explained by random events — to argue that the 2004 Sumatra quake kicked off a similar period. The chance of this particular series of events — an active period from 1952 to 1964, followed by a lull and then a start-up again in 2004 — occurring randomly is less than 2 percent, Bufe says.

Why quakes would cluster isn’t entirely known. Perhaps shaking from the biggest quakes sets off a large-scale ringing within the Earth that affects tectonic activity, Bufe proposes. “Maybe on the very large scale we have a weakening of these very long fault zones that are on the verge of failure.”

Over the next six years, Bufe calculates, the chance of another magnitude 9 quake occurring somewhere in the world is 63 percent — compared to 24 percent if the planet were not experiencing a cluster. “We think we’re in an increased hazard situation from these very large quakes,” he says.

Other researchers say Bufe has gone too far out on a statistical limb. Andrew Michael, a USGS researcher in Menlo Park, Calif., has done a separate analysis. “We don’t see clustering,” he says.

Rather than study only the highest magnitude quakes, Michael probed different magnitudes over different time periods. Among other tests, he looked at whether there was a statistically meaningful increase in seismicity worldwide after big earthquakes, and whether the time between big quakes followed a nonrandom pattern. He found that the tests could produce what looked like earthquake clusters, but that the clusters disappeared when different magnitudes and time periods were included. “If we use a range of magnitude cutoffs we find that the data are very well explained by the random model,” he says.

Bufe counters that because clustering may happen only for very large quakes, the pattern wouldn’t be seen when looking at lower magnitudes. But Michael argues that the number of magnitude 9 earthquakes is too small to draw any statistically significant conclusions. “At 9 you just don’t have enough data,” he says.

Richard Aster, a geophysicist at the New Mexico Institute of Mining and Technology in Socorro, agrees with Michael. He notes that the list of earthquakes since 1900 — which everyone must use for their studies — includes around 1,700 quakes greater than magnitude 7; about 70 greater than magnitude 8; and only 5 greater than magnitude 9.

Aster has done a separate analysis looking, in part, at the total amount of energy released by all quakes since 1900. Big earthquakes tend to dominate, simply because there haven’t been that many; the Japan quake accounts for about 5 percent of all global cumulative seismic energy released since 1900, he reported at the meeting.

Over the past two decades, the number of earthquakes greater than magnitude 7.5 has been increasing worldwide, Aster’s team found. But that increase could be due to natural random fluctuations as opposed to any actual trend in tremors worldwide.

Like Michael, Aster does not find quake groupings beyond the known effects of aftershocks and local quake triggering. “We have found that there’s no evidence for clustering at long scales, say trans-Pacific scales,” Aster says.

But Bufe is not backing down. “The only way out of this will be unfortunately waiting a long time until we see more large earthquakes,” says Michael. “That is the problem we face in seismology.”


In recent weeks seismologists have collected more details about how Japan’s destructive March 11 earthquake unfolded.

AP Photo, Kyodo News

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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