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Japan aftershocks not shocking

AP Photo / Lee Jin-man

Hotel guests check their mobile phones for earthquake news after they evacuated the building following an aftershock, in Ichinoseki, Iwate Prefecture, Japan, Friday, April 8, 2011. Japan was rattled by a strong aftershock and tsunami warning Thursday night nearly a month after a devastating earthquake and tsunami flattened the northeastern coast.

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.

More to come?
The 7.9 aftershock about 30 minutes after the main shock, which was about 100 kilometers from Tokyo, could have been the 8 he was worried about, he told me today, but he doesn't think the city is in the clear.

"Tokyo has had a lot of seismic activity in the last month," he said. "The thing I worry about is one of those events being significantly larger than the ones they've been having."

New to his list of worries based on probability analysis is the southwestern Japan town of Nagasaki, which he says "has an elevated probability for a magnitude 7 earthquake in the next 12 months or so because it's had two magnitude 7s in 1968 and since the last one it has had about 10 magnitude 6s. So it would be about for a magnitude 7."

More on Japan earthquakes:

 


John Roach is a contributing writer for msnbc.com. Connect with the Cosmic Log community by hitting the "like" button on the Cosmic Log Facebook page or following msnbc.com's science editor, Alan Boyle, on Twitter (@b0yle).