A tsunami (pronounced tsoo-nah-mee) is
a series of waves created when a body of water, such as an ocean, is rapidly
displaced on a massive scale.
Earthquakes, mass movements above or below water, volcanic
eruptions and other underwater explosions, landslides, large meteorite impacts
and testing with nuclear weapons at sea all have the potential to generate a
tsunami. The effects of a tsunami can range from unnoticeable to devastating.
The term tsunami comes from Japanese words meaning harbor ("tsu") and wave
Although in Japanese tsunami is used for both the singular
and plural, in English tsunamis is often used as the plural. The term was
created by fishermen who returned to port to find the area surrounding their
harbor devastated, although they had not been aware of any wave in the open
water. Tsunami are common throughout Japanese history; approximately 195 events
in Japan have been recorded.
A tsunami has a much smaller amplitude (wave height)
offshore, and a very long wavelength (often hundreds of kilometers long), which
is why they generally pass unnoticed at sea, forming only a passing "hump" in
A tsunami can be generated when
the plate boundaries abruptly deform and vertically displace the overlying
water. Such large vertical movements of the Earth's crust can occur at plate
boundaries. Subduction earthquakes are particularly effective in generating
Tsunami are surface gravity waves that are formed as the
displaced water mass moves under the influence of gravity and radiates across
the ocean like ripples on a pond.
In the 1950s it was discovered that larger tsunami than
previously believed possible could be caused by landslides, explosive volcanic
action, and impact events when they contact water. These phenomena rapidly
displace large volumes of water, as energy from falling debris or expansion is
transferred to the water into which the debris falls. Tsunami caused by these
mechanisms, unlike the ocean-wide tsunami caused by some earthquakes, generally
dissipate quickly and rarely affect coastlines distant from the source due to
the small area of sea affected. These events can give rise to much larger local
shock waves (solitons), such as the landslide at the head of Lituya Bay which
produced a water wave estimated at 50 – 150 m and reached 524 m up local
mountains. However, an extremely large landslide could generate a “megatsunami”
that might have ocean-wide impacts.
The geological record tells us that there have been massive
tsunami in Earth's past.
Signs of an approaching tsunami:
is often no advance warning of an approaching tsunami. However, since
earthquakes are often a cause of tsunami, an earthquake felt near a body of
water may be considered an indication that a tsunami will shortly
When the first part of a tsunami to reach land is a trough
rather than a crest of the wave, the water along the shoreline may recede
dramatically, exposing areas that are normally always submerged. This can serve
as an advance warning of the approach crest of the tsunami, although the warning
arrives only a very short time before the crest, which typically arrives seconds
to minutes later. Although in the 2004 tsunami in the Indian Ocean the sea
receding was not reported on the African coast or any other western coasts it
hit, when the tsunami approached from the east.
Warnings and prevention:
cannot be prevented or precisely predicted, but there are some warning signs of
an impending tsunami, and there are many systems being developed and in use to
reduce the damage from tsunami. In instances where the leading edge of the
tsunami wave is its trough, the sea will recede from the coast half of the
wave's period before the wave's arrival. If the slope is shallow, this recession
can exceed many hundreds of meters. People unaware of the danger may remain at
the shore due to curiosity, or for collecting fish from the exposed seabed.
Tsunami occur most frequently in the Pacific Ocean, but are a global phenomenon;
they are possible wherever large bodies of water are found, including inland
lakes, where they can be caused by landslides. Very small tsunami,
non-destructive and undetectable without specialized equipment, occur frequently
as a result of minor earthquakes and other events.