Seismologists Rumble Over Quake Clusters

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

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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.”

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