Ironically, "tsunami" is a Japanese word. It means, "harbor wave", but it is actually not a wave at all. It's a series of surges of water that move throughout the depth of the ocean, where a "wave" is a phenomenon that is usually confined nearer to the surface, and therefore as they come in don't have the force of the ocean's depth behind them. They flow circular allowing each mini surge of water to flow back out underneath. In a tsunami water just keeps coming, often with devastating results. There is sometimes little visual warning on the shore, before the surge of deadly water sweeps in. The visible wave may be as little as one foot in height, but when the surge of water reaches the continental shelf it slows and becomes compressed, and the only place for it to go is up and onshore.
I say "series" of surges because tsunami frequently are not just one surge. The surges may come one after another and be separated by as much as 500 miles. People have often been killed after returning to their homes thinking the water had left, only to see it return hours later. This will be an ongoing concern today in the Hawaiian islands as well as on the U.S. west coast. While it does appear that the initial tsunami impacts in Hawaii have not been devastating, only several hours more of time will tell the whole story.
Fortunately, tsunami lose momentum as they travel across the ocean, so the devastation in Japan will almost certainly not be repeated in the US. The closer to the earthquake epicenter a coastal region is, the more likely it will have devastating tsunami impacts. This graphic from NOAA shows estimated tsunami travel times from the main event and what corners of the Pacific will be affected when. As it travels, it will continue to become less severe.
What does all of this have to do with the earthquake? That part is actually simple, when the earth shakes it also shakes underwater. The movement (particularly the upward motion of the sea bottom) displaces a large volume of water which then races out in all directions from the point of origin (Reuters graphic from the 2005 tsunami below).
Think of what happens when you drop a pebble in the water, it creates waves that ripple out around the pebble until they disperse. This just happens from the bottom up, and it's a much larger pebble, that can impact an entire ocean.
Some interesting tsunami facts:
- Hawaii and even the west coast are more used to this then you may think. Hawaii averages one tsunami per year, with a damaging tsunami once every seven years. Damaging does not necessarily mean devastating, though, so they don't all make national news. You are hearing about this one because of what happened in Japan. Likewise, the U.S. mainland experiences a damaging tsunami about once every 18 years.
- Tsunami occur most often in the pacific because of the active Pacific rim earthquake zone. They are usually associated with earthquakes, but not always. A landslide or volcanic eruption can also trigger a tsunami. Scientists have also found evidence of asteroid collisions that have triggered tsunamis that swept around the earth several times long ago.
- Tsunami can travel as fast as 500 MPH, as fast or faster than a commercial jet.