Japan aftershocks not shocking (excerpt from the Cosmic Blog)

By John Roach

The news today that Japan was hit by a magnitude 7.1 aftershock nearly a month after a magnitude 9.0 earthquake and tsunami devastated the northeast coast is grim, but not shocking to experts.

“It is not surprising as part of the aftershock sequence to see a magnitude 7 plus,” John Bellini a geophysicist with the U.S. Geological Survey told me today.

Generally, he said, for each lower magnitude you expect to see ten times the preceding magnitude. So, in this case, experts expect to see one magnitude 8 earthquake, up to 10 magnitude 7s, and many, many 6s, 5s, and 4s.

Hundreds of aftershocks
Indeed, hundreds of aftershocks have hit Japan since the March 11 earthquake, but few have been above 7.0. According to the USGS, an aftershock about 30 minutes after the main shock was a 7.9. There has been one other greater than 7.0 aftershock and many smaller ones. An 8 is still possible.

“You can still have it,” Bellini said, “but as time goes by it is less and less likely each day and we would expect to see fewer 7s and fewer 6s.” In fact, he added, “we’ve been seeing a lot less aftershocks for the last week than two weeks ago.”

John Rundle, an expert on earthquake dynamics at the University of California at Davis, said a large aftershock, especially near a major city such as Tokyo, was among his largest worries when we spoke about earthquake clustering last month.

Read more here...

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