A key difference between a tsunami and an average wind-generated wave is that the energy creating a tsunami travels through the water instead of moving above the water as it does in a wind-generated wave. As a practical matter, this means that a tsunami primarily travels underwater, where it can't be seen. For most of a tsunami's existence, it only appears a few feet (1 about meter) above the waterline and is barely noticeable. Depending on the force created by sudden displacement in the sea floor -- usually an earthquake -- the tsunami also can travel quite rapidly from a remote area of the ocean to a populated shore.
As a tsunami gets closer to shore and into shallower water, it begins to slow down. The shallow water and shoreline compress the tsunami's energy, pushing the water upward. So although the tsunami slows down as the water becomes shallower, its energy has to go somewhere and the height of the waves suddenly shoot up in what is called the "shoaling effect" [source: NOAA Physics].
If you are on the shoreline as a tsunami comes in, you might see some rise and fall of the beach water before the large waves appear. In some cases, the water along the shoreline may disappear altogether as it gets sucked into the tsunami's energy. Shortly after this happens, the bottom of the tsunami arrives at the shore. The maximum, or run-up, height that tsunamis can reach onshore is 98 feet (30 meters) above sea level, although there was an exception in Lituya Bay, Alaska, in 1958. A tsunami there produced a wave that was 1,722 feet (525 meters) high [source: NOAA Physics].
Although the wall of water usually appears as a series of waves instead of a single, monstrous wave, that's not always the case. A large vertical wave, called a bore, may appear with the churning front commonly associated with vigorous waves. Floods of water then follow the bores. Additional waves will likely follow, called the tsunami wave train, which can continue anywhere from 5 to 90 minutes after the first wave arrived.The other reason why tsunamis are so dangerous is that there often is very little warning time between when the event causing the giant waves and their appearance on shore occur. When a tsunami hit the coast of Japan in
March 2011, residents had only about 15 minutes to prepare. A method of splitting tsunami data collection models uses special buoys to estimate when waves will arrive and how high they might be, but emergency managers, meteorologists and others are working to improve the model's accuracy to provide real-time forecasting that better helps leaders make rapid decisions.
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