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Weatherzone2280
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Tsunami
A
tsunami is a wave train, or series
of waves, generated in a body of water by an impulsive
disturbance that vertically displaces the water column.
Earthquakes, landslides, volcanic eruptions, explosions, and
even the impact of cosmic bodies, such as meteorites, can
generate tsunamis. Tsunamis can savagely attack coastlines,
causing devastating property damage and loss of
life.
What does "tsunami"
mean? Tsunami is a
Japanese word with the English translation, "harbor wave."
Represented by two characters, the top character, "tsu," means
harbor, while the bottom character, "nami," means "wave." In
the past, tsunamis were sometimes referred to as "tidal waves"
by the general public, and as "seismic sea waves" by the
scientific community. The term "tidal wave" is a misnomer;
although a tsunami's impact upon a coastline is dependent upon
the tidal level at the time a tsunami strikes, tsunamis are
unrelated to the tides. Tides result from the imbalanced,
extraterrestrial, gravitational influences of the moon, sun,
and planets. The term "seismic sea wave" is also misleading.
"Seismic" implies an earthquake-related generation mechanism,
but a tsunami can also be caused by a nonseismic event, such
as a landslide or meteorite impact.
How do earthquakes generate tsunamis?Tsunamis can be generated when the sea floor abruptly
deforms and vertically displaces the overlying water. Tectonic
earthquakes are a particular kind of earthquake that are
associated with the earth's crustal deformation; when these
earthquakes occur beneath the sea, the water above the
deformed area is displaced from its equilibrium position.
Waves are formed as the displaced water mass, which acts under
the influence of gravity, attempts to regain its equilibrium.
When large areas of the sea floor elevate or subside, a
tsunami can be created.
Large vertical movements of the earth's
crust can occur at plate boundaries. Plates interact along
these boundaries called faults. Around the margins of the
Pacific Ocean, for example, denser oceanic plates slip under
continental plates in a process known as subduction.
Subduction earthquakes are particularly effective in
generating tsunamis.
View A Tsunami
Simulation
Opens in New
Window |
The Simulation2
at the left is of the 1993 Hokkaido earthquake-generated
tsunami, developed by Takeyuki Takahashi of the Disaster
Control Research Center, Tohoku University, Japan, shows
the initial water-surface profile over the source area
and the subsequent wave propagation away from the
source. Areas in blue represent a water surface that is
lower than the mean water level, while areas in red
represent an elevated water surface. The initial
water-surface profile, as shown in this image, reflects
a large, long uplifted area of the sea floor lying to
the west (left) of Okushiri Island, with a much smaller
subsided area immediately adjacent to the southwest
corner of Okushiri.
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A tsunami can be
generated by any disturbance that displaces a large water mass
from its equilibrium position. In the case of
earthquake-generated tsunamis, the water column is disturbed
by the uplift or subsidence of the sea floor. Submarine
landslides, which often accompany large earthquakes, as well
as collapses of volcanic edifices, can also disturb the
overlying water column as sediment and rock slump downslope
and are redistributed across the sea floor. Similarly, a
violent submarine volcanic eruption can create an impulsive
force that uplifts the water column and generates a tsunami.
Conversely, supermarine landslides and cosmic-body impacts
disturb the water from above, as momentum from falling debris
is transferred to the water into which the debris falls.
Generally speaking, tsunamis generated from these mechanisms,
unlike the Pacific-wide tsunamis caused by some earthquakes,
dissipate quickly and rarely affect coastlines distant from
the source area.
As a tsunami leaves the deep water of the open ocean
and travels into the shallower water near the coast, it
transforms. You will discover that a
tsunami travels at a speed that is related to the water depth
- hence, as the water depth decreases, the tsunami slows. The
tsunami's energy flux, which is dependent on both its wave
speed and wave height, remains nearly constant. Consequently,
as the tsunami's speed diminishes as it travels into shallower
water, its height grows. Because of this shoaling effect, a
tsunami, imperceptible at sea, may grow to be several meters
or more in height near the coast. When it finally reaches the
coast, a tsunami may appear as a rapidly rising or falling
tide, a series of breaking waves, or even a bore.
What happens
when a tsunami encounters land?As a tsunami
approaches shore, we learn that it begins to slow and grow in
height. Just like other water waves, tsunamis begin to lose
energy as they rush onshore - part of the wave energy is
reflected offshore, while the shoreward-propagating wave
energy is dissipated through bottom friction and turbulence.
Despite these losses, tsunamis still reach the coast with
tremendous amounts of energy. Tsunamis have great erosional
potential, stripping beaches of sand that may have taken years
to accumulate and undermining trees and other coastal
vegetation. Capable of inundating, or flooding, hundreds of
meters inland past the typical high-water level, the
fast-moving water associated with the inundating tsunami can
crush homes and other coastal structures. Tsunamis may reach a
maximum vertical height onshore above sea level, often called
a runup height, of 10, 20, and even 30
meters.
View a Tsunami
Simulation
Opens in New
Window |
This simulation, produced by
Professor Nobuo Shuto of the Disaster Control Research
Center, Tohoku University, Japan, shows the 1923 Kanto
tsunami attacking a Japanese village. . Note that the
structures in this model are rigid - in a real-life
tsunami, coastal structures often are destroyed. (The
QuickTime movie presented here was digitized from a
video tape produced from the original computer-generated
animation.)
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Before and Photos of
a Tsunami
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Image Set
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