Ground acceleration for the Tohoku quake

Tohoku, by the way, is Japanese for northeast, since that is the part of the island of Honshu where the earthquake occurred. It is also being referred to as the Sendai earthquake, Sendai being the largest big city nearby.

The primary hazard intrinsically associated with earthquakes is groundshaking. When the elastic energy stored in the crust is released by the earthquake and motion on the fault, the stored energy causes the ground to vibrate, or accelerate back and forth.  The figure below from the USGS shows the peak acceleration in the vicinity of the quake caused by the shaking earth.

The peak ground acceleration in the graph below was almost three times the Earth's gravitational acceleration (g).

By default, buildings are designed to withstand the vertical acceleration due to gravity, but not necessarily to withstand horizontal ground accelerations of the same magnitude. This is why earthquake-resistant buildings must be designed to deal with. Japan has done a good job with this, so the major damage in the quake was not caused by the groundshaking.