Tsunamis, such as the one generated by the magnitude 8.9-magnitude earthquake that struck Japan today (March 11), are often generated by massive ruptures beneath the Earth’s surface underneath the ocean floor.
When the earthquake ruptures along a fault line, the surface around that fault is pushed up and then dropped back down. (Not all undersea quakes generate tsunamis, as some occur so deep in the Earth's crust that they won't cause this push.) That movement displaces the entire water column above that chunk of the surface.
"This is the most common way to generate a tsunami," said Aggeliki Barberopoulou of the University of Southern California's Tsunami Research Center, who is monitoring the current tsunami as it affects California.