Hitting the Water
Once the water pushes upward, gravity acts on it, forcing the energy out horizontally along the surface of the water. It's sort of the same ripple effect you get from throwing a pebble in the water except the energy is generated by a force moving out of the water rather than into it. The energy then travels through the depths and away from the initial disturbance.
The tremendous force created by the seismic disturbance generates the tsunami's incredible speed. We calculate the actual speed of the tsunami by measuring the water depth at a point in time when the tsunami passes by.
A tsunami's ability to maintain speed is directly influenced by the depth of the water. A tsunami moves faster in deeper water and slower in shallower water. So unlike a normal wave, the driving energy of a tsunami moves through the water as opposed to on top of it. Therefore, as a tsunami moves though deep water at hundreds of miles an hour, it is barely noticeable above the waterline. A tsunami is typically no more than 3 feet (1 meter) high until it gets close to shore.
Once a tsunami gets close to shore, it takes its more recognizable and deadly form.