Understanding why Japan's recent quake defied expectations

What's this mean? A few things. For one, the horizontal motion was the primary cause of the tsunami, but only provided a portion of the energy released in the quake. Thus, if a quake can generate that sort of motion on its own, then we could see much smaller quakes generating tsunamis that are far larger than we would have predicted.

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

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