The March 11 Japanese earthquake, at 9.0 magnitude, was caused by the Pacific plate’s movement under the Eurasian plate. It happened 32lm beneath the sea floor of the coast of Honshu, Japan. This is a convergent subduction zone boundary, which means the crust of one plate is being forced under another plate, and in turn shortens the crust in that area.
Convergent boundaries create the largest earthquakes on Earth and almost always involve the oceanic crust, resulting in the largest tsunamis. This is because the plates are going through much more complicated stress variables that make the rocks more likely to fail — i.e., break and move.
These can be anything from fluids, melts, thermal pressurization — heated liquids or gas — or episodic tremor and slip. Episodic tremor and slip is when pressure builds in fluctuating directions and eventually lets go of the stress.
During the earthquake, the Eurasian plate moved upward by 30-40m, and slipped over an area 300km long by 150km wide in the down-dip direction. In normal terms, this means that the area pushed under the upper plate was 300km long by 150km wide, resulting in 45 thousand square km of earth displaced in about three minutes. This is an incredibly large number in light of the background plate motion rates of centimeters/year — about as fast as a fingernail grows.
Some studies have been done that show Honshu moved eight feet to the east in this single event. Other sources have claimed that the length of the day has been shortened by eight minutes because of this event. But that claim is completely false, the earth was not made smaller — Japan was just moved to the east. This event will likely have a minor effect on latitude and longitude points on the ground.
Earthquakes like this have been happening for at least five million years, since the Pacific plate began to sink into the mantle. The island of Japan is a result of this action, because convergent boundaries melt the underlying rock and create volcanoes that grow high enough to rise above sea level. Geologists are still trying to understand exactly how and why such large earthquakes happen, but it is certain this will not be the last great earthquake to hit Japan.