Seismologists rumble over quake clusters

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

By Alexandra Witze, 15:58 PM April 15, 2011

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.

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...

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