A
tsunami (pronounced /tsʊˈnɑːmi/)
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 the Japanese
words meaning harbor
("tsu", 津)
and wave ("nami", 波). 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 the ocean. Tsunami have been historically
referred to as tidal waves
because as they approach land, they take on the characteristics of a violent
onrushing tide
rather than the sort of cresting waves that are formed by wind action upon the
ocean (with which people are more familiar). Since they are not actually
related to tides the term is considered misleading and its usage is discouraged
by oceanographers.
Since not all tsunami occur in
harbors, however, that term is equally misleading, although it does have the
benefit of being misleading in a different language.
A Tsunami can
be generated when the sea floor abruptly deforms and vertically
displaces 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. Submarine
landslides (which are sometimes triggered by large earthquakes) as well as collapses of
volcanic edifices may also disturb the overlying water column as sediment and
rocks slide downslope and are redistributed across the sea floor. Similarly, a
violent submarine volcanic eruption can uplift the water column and form a tsunami.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.
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..
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. These tsunami
were so large that they caused landslides on the opposite coast triggering
another massive tsunami, or
"bounce back" tsunami.
An example today would be a landslide equivalent to everything west of Portland,
Oregon falling into the Pacific Ocean,
resulting in a tsunami that
would then hit the Chinese coast with enough force to erode the coast,
triggering a landslide large enough to send a tsunami that would in turn inundate the U.S. West Coast and
would wipe out Portland.
More abstracts about the Tsunami