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 kilometres per hour. Near the shore, however,
a tsunami slows down to just a few tens of kilometres
per hour. The height of a tsunami also depends upon the
water depth. A tsunami that is just a metre in height in the
deep ocean can grow to tens of metres 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