Some people may not understand the difference between a tsunami and a tidal wave. Tides are the normal rise and fall of saltwater levels against the coast. Tsunamis are not normal tides - they are extreme events. And a tsunami is not just one wave, but a series of waves. Some of the waves can be huge. Scientists say the first wave is often not the largest. That is usually the third or fourth wave. The waves can be from five minutes to one hour apart.

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BOB DOUGHTY: How is a tsunami wave caused? The land underneath the ocean is made up of tectonic plates. These large areas are always moving. Usually the plates just rub up against each other on a crack, also called a fault line. Sometimes, one plate subducts -- or slides under -- another plate. Over time, a huge amount of pressure builds up on the plate that has slid under the other. It suddenly springs up, resulting in an earthquake. The large subduction zone earthquakes are responsible for most of the ocean-wide tsunamis, such as the recent tsunami in Japan.

Scientists are able to measure the strength and position of earthquakes because there are hundreds of seismic monitoring stations around the world. If it is a strong quake and it happens near or in the ocean, computers quickly measure the length, depth and location of a quake. These measurements help to show how strong a resulting tsunami might be.

Waves hitting the coast of northern Japan after the March 11 earthquake