What was the speed of the tsunami after the earthquake in Japan on March 11 2011?

Britain's National Oceanography Centre said "the tsunami quickly roared through the Pacific Ocean at comparable speeds to a jumbo jet flying in the sky".

The speed of tsunami waves depends on the depth of the sea where the waves occurs, where the speed can reach hundreds of kilometers per hour if the tsunami forms in deep water. When it hits land it is "almost like a freight train that smashes everything in its path," said a professor of engineering at the University of Ottawa, "usually in a sequence of two or three waves".

The tsunami wave height can reach a few centimeters to several meters, but when it reaches shore the wave height can reach tens of meters due to a buildup of the water. When the tsunami reaches the coast of the mainland it could travel over land with a range of several hundred meters up to a few kilometers. In the case of Japan's recent tsunami it reached up to 10 km or (6 mi) inland.

Another Answer:

500 MPH is the commonly accepted speed of the energy wave in a tsunami in the open ocean. In this state, it typically only has an amplitude of a couple of feet. Most boats in the ocean will not even notice a tsunami as it goes past them. As it approachs land, however, it slows down and gains amplitude. This change depends on the topography of the ocean floor, but a typical approach speed is about 50 MPH with an amplitude closer to 30 to 60 feet, depending on the energy.