Tsunami Glossary 2013 Technical Series 85
A tsunami travels outward from the source region as a series of waves. Its speed depends upon the depth of the water, and consequently, the waves undergo accelerations or decelerations in passing respectively over an ocean bottom of increasing or decreasing depth. By this process, the direction of wave propagation also changes, and the wave energy can become focused or defocused.
In the deep ocean, tsunami waves can travel at speeds of 500 to 1,000 kilometers per hour. Near the shore, however, a tsunami slows down to just a few tens of kilometers per hour. The height of a tsunami also depends upon the water depth. A tsunami that is just a meter in height in the deep ocean can grow to tens of meters at the shoreline.
Unlike familiar wind-driven ocean waves that are only a disturbance of the sea surface, the tsunami wave energy extends to the ocean bottom. Near the shore, this energy is concentrated in the vertical direction by the reduction in water depth, and in the horizontal direction by a shortening of the wavelength due to the wave slowing down.