Hitting the Water

Once the water has been pushed 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, but in reverse: The energy is generated by a force moving out of rather than into the water. The energy then moves through the depths of the water and away from the initial disturbance.

The tremendous force created by the seismic disturbance generates the tsunami's incredible speed. The actual speed of the tsunami is calculated by measuring the water depth at a point in time when the tsunami passes by. The speed is the square root of the product of acceleration of gravity and the quantity of water depth, or:

    t = square root (g x d)
    t = tsunami speed in meters per second
    g = acceleration of gravity (32 feet/10 meters per second/per second)
    d = quantity of water depth


Photo courtesy NOAA
An animation of the 2004 Indian Ocean tsunami

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. As a result, 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.


Photo courtesy USGS
Simulation of a theoretical tsunami in the Pacific

Tsunami Types
Tsunamis are categorized in one of two ways:
  • distant tsunamis
  • local tsunamis
This distinction is made based on the time it takes the tsunami to leave the source disturbance and reach land.

A distant tsunami travels more than 600 miles (1,000 km) from the source area before it reaches land. Distant tsunamis are more likely to occur in the Pacific Ocean and are capable of traveling across the entire ocean in less than one day. Since distant tsunamis make such long trips with a relatively constant speed, experts can predict their arrival with a fair degree of accuracy. This makes it easy to warn and evacuate people that could be affected by the wave.

A local tsunami travels toward nearby coastal lands within 60 miles (100 km) of the source. Local tsunamis are usually the result of submarine landslides and typically occur in a bay or harbor. Local tsunamis are particularly dangerous because they can reach land within a matter of minutes. This type of "sneak attack" makes it hard to warn the public about the tsunami's approach.