2. Introduction
One of the most powerful and feared natural disasters, the large and
destructive water wave known as a tsunami can challenge a jet airplane
for speed. Near the coast it can beat a three-story building for height.
These waves pack enough energy to destroy an entire coastal
community in moments. Underwater earthquakes and volcanic eruptions
are the most common causes of tsunamis.
The word tsunami is a Japanese word, represented by two
characters: tsu, meaning “harbor,” andnami, meaning “wave.” Although
tsunamis are often referred to as tidal waves, this is a
misnomer.Tides are generated by the gravitational effects of the moon,
sun, and planets. Tsunamis are caused by a sudden or impulsive
disturbance that vertically displaces the water column. They are not
connected to tides, though the tide level affects the impact a tsunami
has as it reaches the shore.
3. Tsunamis are most commonly generated by
underwater earthquakes, which are caused when the
seafloor abruptly deforms. The Earth’s outer shell is made
up of several slowly moving plates. Most earthquakes
occur near the plate boundaries as the plates move over,
along, or away from each other. Earthquakes release a
large amount of energy, and in the ocean the violent
shaking of the seafloor displaces the water. If a large
earthquake occurs near the surface of the ocean floor, the
resulting energy transfer to the water column produces
great waves. Most tsunamis result from earthquakes with a
magnitude greater than 6.5 on the Richter scale and that
occur less than 30 miles (50 kilometers) beneath the
seafloor. Volcanic eruptions, landslides, and other great
disturbances of the ocean water can also generate
tsunamis.
4. Wave Properties
A tsunami can travel great distances. After the earthquake, volcanic eruption, or
other cause, a series of waves spreads over the ocean surface in ever-
widening circles. In the deep water, a tsunami has an amplitude, or height, of
only about one or two feet (30 to 60 centimeters). A tsunami has a much longer
wavelength and period, however, than the wind-generated waves seen at the
beach. (The period is the time between the end of one wave and the beginning
of another.) While a wind-generated wave might have a length of approximately
500 feet (150 meters) and a period of 10 seconds, a tsunami may be 60 to 120
miles (100 to 200 kilometers) long and have a period ranging from five minutes
to more than an hour.
5. Tsunamis behave like shallow-water waves because the ratio between the
depth of the water and the length of the wave is very small. This property
of tsunamis relates directly to the tremendous speed they can achieve,
because a shallow-water wave travels at speeds related to the depth of
the water and the acceleration of gravity. The deeper the water, the faster
the wave. Where the ocean is 13,100 feet (4,000 meters) deep, for
instance, a tsunami can travel more than 400 miles (640 kilometers) per
hour.
The vast length of a tsunami also affects the force it carries. As the wave
travels, it loses energy. But the rate of energy loss is inversely
proportional to the length of the wave. This means that the longer the
wave, the less energy is lost as it travels, and therefore the more energy it
still has as it strikes the coastline.
6. As the wave approaches land, it slows down and its height increases.
Because the speed of the wave is proportional to the water depth, as the
water becomes more shallow, the speed of the wave diminishes. Closer to
shore, bottom friction and turbulence slow the wave even more. The force
of a tsunami as it reaches shore is still enormous, and it will continue to
travel until its energy is completely dissipated. This means that it may
travel inland several hundred feet, carrying with it boats, large rocks, and
other heavy debris. The flow of the water back to the sea can also be
extremely destructive.
All low-lying coastal areas are vulnerable to tsunamis. Because the waves
have very long periods, the danger posed by a tsunami can last several
hours. The first wave may not be the largest of the series. Occasionally a
wave’s trough—the part with the lowest water level—may arrive first,
temporarily causing the water to recede and exposing the seafloor. This
may attract curious people and fishers to the seafloor right before the
crest, or peak, of the wave arrives and drowns those in its path.
7. Warnings
The height of a tsunami is very small in deep water, so it is virtually imperceptible
on the open sea. However, various national and international warning centers issue
bulletins when there is a possibility of a tsunami. Geological agencies may issue an
alert if they observe a very powerful underwater earthquake. Meteorologists may
report unusual changes in the sea level. Agencies that monitor tsunamis use tide
gauges along coasts to detect changes in water level. Instruments deployed on the
seafloor can detect changes in water pressure, and deep-sea buoys transmit the
readings via satellite. The United States has two regional tsunami warning centers
for the Pacific Ocean—one at Palmer, Alaska, and one near Honolulu, Hawaii. The
latter also serves as a warning center for 26 countries lining the Pacific. This
international system is organized by the United Nations Educational, Scientific, and
Cultural Organization (UNESCO). After a catastrophic tsunami occurred in the
Indian Ocean in late 2004, UNESCO decided to oversee the creation of a tsunami
warning system for the Indian Ocean and eventually the world.
8. It is not possible to outrun a tsunami,
but there are precautions that can be
taken to minimize the possible
dangers of a tsunami. The
generation of a tsunami is rapid, and
because of their great speed, the
waves can reach shore very quickly.
Anyone who is at the beach or near
the ocean and feels the Earth shake
should immediately move to higher
ground, rather than wait for a
warning to be issued. It is not safe to
return to low-lying ground until an
official “all-clear” has been issued.
Those who are at sea when a
tsunami warning is issued should not
return to port. Tsunamis cause rapid
changes in water levels and create
dangerous currents in harbors and
ports.
9. Origin and development
In deep water a tsunami can travel as
fast as 800 km (500 miles) per hour.
The wavelengths are enormous, about
100 to 200 km (60 to 120 miles), but
the wave amplitudes (heights) are
very small, only about 30 to 60 cm (1
to 2 feet). The waves’ periods (the
lengths of time for successive crests
or troughs to pass a single point) are
very long, varying from five minutes to
more than an hour. These long
periods, coupled with the extremely
low steepness of the waves, enables
them to be completely obscured in
deep water by normal wind waves and
swell. A ship on the high seas
experiences the passage of a tsunami
as an insignificant rise and fall of only
half a meter (1.5 feet), lasting from five
minutes to an hour or more.
10. As the waves approach the coast
of a continent,
however, friction with the rising
sea bottom reduces the velocity of
the waves. As the velocity lessens,
the wavelengths become
shortened and the wave
amplitudes increase. Coastal
waters may rise as high as 30
meters (about 100 feet) above
normal sea level in 10 to 15
minutes. By a poorly understood
process, the continental shelf
waters begin to swing after the
rise in sea level. Between three and
five major oscillations generate
most of the damage, frequently
appearing as powerful “run-ups”
of rushing water that uproot trees,
pull buildings off their
foundations, carry boats far
inshore, and wash away entire
beaches, peninsulas, and other
low-lying coastal formations.
Frequently the succeeding outflow
of water is just as destructive as
the run-up or even more so. In any
case, swinging may continue for
several days until the ocean
surface reaches equilibrium.
11. Much like any other water waves, tsunamis are reflected and refracted by the
topography of the seafloor near shore and by the configuration of a coastline.
As a result, their effects vary widely from place to place. Occasionally, the first
arrival of a tsunami at a coast may be the trough of the wave, in which case the
water recedes and exposes the shallow seafloor. Such an occurrence took
place in the bay of Lisbon, Portugal, on November 1, 1755, after a large
earthquake; many curious people were attracted to the bay floor, and a large
number of them were drowned by the wave crest that followed the trough only
minutes later.
Notable tsunamis
One of the most destructive tsunamis took place on December 26, 2004, after an earthquake of
magnitude 9.1 displaced the ocean floor off the Indonesian island of Sumatra.
On March 11, 2011, seafloor displacement resulting from a magnitude-9.0 earthquake in the Japan
Trench of the Pacific Ocean created a large tsunami that devastated much of the eastern coast
of Japan’s main island of Honshu.
The most destructive tsunami was caused by the spectacular explosive eruption of
the Krakatoa (Krakatau) volcano on August 26 and 27, 1883. This series of blasts, which
submerged the island of Rakata between Sumatra and Java, created waves as high as 35 metres
(115 feet) in many East Indies localities, killing more than 36,000 people.
The largest earthquake ever recorded (magnitude 9.5) took place in 1960 off the coast of Chile; it
caused a tsunami that killed approximately 2,000 people in Chile, 61 people 15 hours later in
Hawaii, and 122 people 22 hours later in Japan.
