Let's look at how a tsunami occurs. The earth is covered with hard rock. This rock is actually made up of more than a dozen pieces that fit together a bit like a jigsaw. These are called plates. The problem is that these plates move, albeit very, very slowly. Four plates meet around Japan, and they are always pushing and shoving against each other. Oceanic plates slide under continental plates, but as plates are made of hard rock, this sliding does not go smoothly. The edge of the continental plate is dragged down as the oceanic plate tries to slide underneath it.
The movement of the plates near Japan is slow: only about 5-15 cm a year. But when a continental plate becomes too bent, it suddenly snaps up to its former shape. This is how a plate-boundary earthquake takes place. When this happens, seawater is pushed up from the ocean floor to the surface. The wave that is generated then spreads in all directions, just like the ripples that you see when you throw a pebble into a pond. This is a tsunami.
The speed of a tsunami is about 80 km per hour when the water is 50 m deep, about 250 kph at 500 m, and about 800 kph - as fast as a jet airplane - at 5,000 m. A tsunami slows down as it nears shore, but it would still be impossible to escape if you were to start running away only after seeing it.
A tsunami can suddenly grow very tall when it enters narrow places like an inlet or a gulf, and sometimes it will rush up the shore. The tallest known tsunami to ever hit Japan's Honshu Island was 38.3 m tall, and the world record is the 520 m tsunami that was observed in Alaska.