xpost: Japan quake released 2,400 megatons TNT equivalent Energy

Understanding why Japan's recent quake defied expectations
By John Timmer | Published about 20 hours ago

Last week, Science released three papers and a perspective, all focused on understanding what happened during the March earthquake that struck Japan. Now officially termed the Tohoku-Oki quake, the event is estimated as a magnitude 9 quake—one of the biggest in recorded history—and it has triggered significant aftershocks. But it's not the size alone that has people worried; it's the fact that something this size occurred on a segment of fault that we didn't think was capable of producing a quake of this magnitude (an estimate that has had disastrous consequences at the Fukushima nuclear reactors). Understanding what happened and why can potentially tell us a lot more about risks elsewhere along this fault.

The quake occurred along a segment of fault that creates the Japan Trench, where the Pacific plate slides underneath the one that plays host to Japan. This subduction zone gives rise to Japan's volcanoes, and the pressure helps push Japan upwards, creating more of its topography. As with many faults, the two plates sporadically stick as they slide past each other, triggering large earthquakes when the strain is released. All told, the earthquakes have to release a strain that results from a relative motion of the plates that's estimated at about 8.5cm every year.

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It also has implications for Japan itself. For one, we shouldn't be confident that a lack of historic earthquakes means that the plates in the area are sliding past each other—one paper suggests that having a sea mount dragged down into the trench could have caused the long-term quiet that the Tohoku-Oki section had apparently experienced. Thus, we probably need to go back to examine other areas that have seemingly enjoyed periods of quiet near the trench border. Ominously, the authors note that one such area is a bit further to the south—and much closer to Tokyo.

Globally, the quake has also told us that it's possible for a relatively small fault segment to spawn a giant earthquake. A perspective quotes the US Geological Survey's David Wald as saying, "If you can get a 9 that is this compact, it increases the number of places you can a 9 where you may not have expected one." So, coastal nations around the globe might want to start re-evaluating the risks they face from nearby subduction zones.

http://arstechnica.com/science/news/2011/05/scrambling-to-understand-why-japans-recent-quake-defied-expectations.ars