It was the most powerful Earthquake in Japan's History ever and the 7th most powerful earthquake recorded on Earth!!
One controversial explanation is impact on earth from solar flares due to recent increased activity on the sun. When a large amount of energy is absorbed by the earths crust it has to be dissapated somehow and the earth responds with earthquakes (some simple Einstein rationale E=Mc2). Using further advanced data some people have applied this theory predicting earthquakes from recent powerful M & X class solar flares and their trajectories on earth.
Please note that these are still controversial theories and remain unaccepted by Physicists and meteorologists at the moment, but with NASA's new solar observatory, SOHO and more advanced solar flare technologies evolving, scientist need to take a serious an urgent re-look at this theory and see if the flare and earthquake predictions do infact have a correlating relationship. The theory could become a valid predictive model in the future for earthquakes, giving a 3-4 day warning period which is much better then then 30 minutes - 1 hour that people get at present.
So please keep an eye out when a big flare (M or X class) leaves the sun - give it 3-4 days and there may be an earthquake if it has an earthbound trajectory.
Astronometry Earthquake predictions:
Tsunami Hits destroys Japan Coast
1.5 Solar Flare (Earth Directed) - March 9, 2011 - 3 days to effect earth (the 11th)