A tsunami is a series of ocean waves
that sends surges of water, sometimes reaching heights of over 100 feet (30.5
meters), onto land. These walls of water can cause widespread destruction when
they crash ashore.
These awe-inspiring waves are typically
caused by large, undersea earthquakes at tectonic plate boundaries. When the
ocean floor at a plate boundary rises or falls suddenly it displaces the water
above it and launches the rolling waves that will become a tsunami.
Most tsunamis, about 80 percent,
happen within the Pacific Ocean’s “Ring of Fire,” a geologically active area
where tectonic shifts make volcanoes and earthquakes common.
Tsunamis may also be caused by
underwater landslides or volcanic eruptions. They may even be launched, as they
frequently were in Earth’s ancient past, by the impact of a large meteorite
plunging into an ocean.
Tsunamis race across the sea at up to
500 miles (805 kilometers) an hour—about as fast as a jet airplane. At that
pace they can cross the entire expanse of the Pacific Ocean in less than a day.
And their long wavelengths mean they lose very little energy along the way.
In deep ocean, tsunami waves may
appear only a foot or so high. But as they approach shoreline and enter
shallower water they slow down and begin to grow in energy and height. The tops
of the waves move faster than their bottoms do, which causes them to rise
precipitously.
A tsunami’s trough, the low point
beneath the wave’s crest, often reaches shore first. When it does, it produces
a vacuum effect that sucks coastal water seaward and exposes harbor and sea
floors. This retreating of sea water is an important warning sign of a tsunami,
because the wave’s crest and its enormous volume of water typically hit shore
five minutes or so later. Recognizing this phenomenon can save lives.
A tsunami is usually composed of a
series of waves, called a wave train, so its destructive force may be
compounded as successive waves reach shore. People experiencing a tsunami
should remember that the danger may not have passed with the first wave and
should await official word that it is safe to return to vulnerable locations.
Some tsunamis do not appear on shore
as massive breaking waves but instead resemble a quickly surging tide that
inundates coastal areas.
The best defense against any tsunami
is early warning that allows people to seek higher ground. The Pacific Tsunami
Warning System, a coalition of 26 nations headquartered in Hawaii, maintains a
web of seismic equipment and water level gauges to identify tsunamis at sea.
Similar systems are proposed to protect coastal areas worldwide.
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