The
word is Japanese and means "harbor wave," because of the devastating
effects these waves have had on low-lying Japanese coastal communities. Tsunamis
are often incorrectly referred to as tidal waves, but a tsunami is actually a
series of waves that can travel at speeds averaging 450 (and up to 600) miles
per hour in the open ocean.
Sometimes
reaching heights of 40 meters (120 ft.) or more, tsunamis are the most dramatic
and destructive of waves. Underwater disturbances, such as volcanoes,
earthquakes and landslides, are the cause of these monster waves. The larger the
disturbance, the larger the tsunami will be.
In
the open ocean, tsunamis may be hard to spot. Long wavelengths can hide the size
of the wave, but just like other kinds of waves, changes occur when the wave
enters shallow water. The wavelength shortens, and the height increases. The
strength of the disturbance, the distance the wave travels and the shape of the
coastline combined determine the tsunami's height, and ultimately, its
destructiveness.
The phenomenon we call tsunami is a series of large waves of extremely long
wavelength and period usually generated by a violent, impulsive undersea
disturbance or activity near the coast or in the ocean. When a sudden
displacement of a large volume of water occurs, or if the sea floor is suddenly
raised or dropped by an earthquake, big tsunami waves can be formed by forces of
gravity. The waves travel out of the area of origin and can be
extremely dangerous and damaging when they reach the shore. The word
tsunami (pronounced tsoo-nah'-mee) is composed of the Japanese words "tsu"
(which means harbor) and "nami" (which means "wave"). Often
the term, "seismic or tidal sea wave" is used to describe the same
phenomenon, however the terms are misleading, because tsunami waves can be
generated by other, non seismic disturbances such as volcanic eruptions or
underwater landslides, and have physical characteristics different of tidal
waves. The tsunami waves are completely unrelated to the
astronomical tides - which are caused by the extraterrestrial, gravitational
influences of the moon, sun, and the planets. Thus, the Japanese word
"tsunami", meaning "harbor wave" is the correct, official
and all-inclusive term. It has been internationally adopted because
it covers all forms of impulsive wave generation
How
do earthquakes generate tsunamis?
By far, the most destructive tsunamis are generated from large, shallow
earthquakes with an epicenter or fault line near or on the ocean floor. These
usually occur in regions of the earth characterized by tectonic subduction along
tectonic plate boundaries. The high seismicity of such regions is caused
by the collision of tectonic plates. When these plates move past each
other, they cause large earthquakes, which tilt, offset, or displace large areas
of the ocean floor from a few kilometers to as much as a 1,000 km or more.
The sudden vertical displacements over such large areas, disturb the
ocean's surface, displace water, and generate destructive tsunami waves. The
waves can travel great distances from the source region, spreading destruction
along their path. For example, the Great 1960 Chilean tsunami was
generated by a magnitude 8.3 earthquake that had a rupture zone of over 1,000
km. Its waves were destructive not only in Chile, but also as far
away as Hawaii, Japan and elsewhere in the Pacific. It should be noted
that not all earthquakes generate tsunamis. Usually, it takes an
earthquake with a Richter magnitude exceeding 7.5 to produce a destructive
tsunami.
How do volcanic eruptions generate
tsunamis?How do submarine landslides, rock falls and underwater
slumps generate tsunamis?
Less frequently, tsunami waves can be generated from displacements of water
resulting from rock falls, icefalls and sudden submarine landslides or
slumps. Such events may be caused impulsively from the instability
and sudden failure of submarine slopes, which are sometimes triggered by the
ground motions of a strong earthquake. For example in the 1980's, earth
moving and construction work of an airport runway along the coast of Southern
France, triggered an underwater landslide, which generated destructive tsunami
waves in the harbor of Thebes. Major earthquakes are suspected to
cause many underwater landslides, which may contribute significantly to tsunami
generation. For example, many scientists believe that the 1998 tsunami ,
which killed thousands of people and destroyed coastal villages along the
northern coast of Papua-New Guinea, was generated by a large underwater slump of
sediments, triggered by an earthquake. In general, the energy of tsunami
waves generated from landslides or rock falls is rapidly dissipated as they
travel away from the source and across the ocean, or within an enclosed or
semi-enclosed body of water - such as a lake or a fjord. However, It
should be noted, that the largest tsunami wave ever observed anywhere in the
world was caused by a rock fall in Lituya Bay, Alaska on July 9, 1958. Triggered
by an earthquake along the Fairweather fault, an approximately 40 million cubic
meter rock fall at the head of the bay generated a wave, which reached the
incredible height of 520-meter wave ( 1,720 feet) on the opposite side of the
inlet. A initial huge solitary wave of about 180 meters (600 feet) raced
at about 160 kilometers per hour (100 mph) within the bay debarking trees along
its path. However, the tsunami's energy and height diminished rapidly away
from the source area and, once in the open ocean, it was hardly recorded by tide
gauge stations.
How does tsunami energy travel across the ocean and how far
can tsunamis waves reach?
Once a tsunami has been generated, its energy is distributed throughout the
water column, regardless of the ocean's depth. A tsunami is made up of a
series of very long waves. The waves will travel outward on the surface of
the ocean in all directions away from the source area, much like the ripples
caused by throwing a rock into a pond. The wavelength of the tsunami
waves and their period will depend on the generating mechanism and the
dimensions of the source event. If the tsunami is generated from a
large earthquake over a large area, its initial wavelength and period will be
greater. If the tsunami is caused by a local landslide, both its
initial wavelength and period will be shorter. The period of the tsunami
waves may range from 5 to 90 minutes. The wave crests of a tsunami can be
a thousand km long, and from a few to a hundred kilometers or more apart as they
travel across the ocean. On the open ocean, the wavelength of a tsunami
may be as much as two hundred kilometers, many times greater than the ocean
depth, which is on the order of a few kilometers. In the deep ocean,
the height of the tsunami from trough to crest may be only a few centimeters to
a meter or more - again depending on the generating source. Tsunami waves
in the deep ocean can travel at high speeds for long periods of time for
distances of thousands of kilometers and lose very little energy in the process.
The deeper the water, the greater the speed of tsunami waves will be.
For example, at the deepest ocean depths the tsunami wave speed will be as much
as 800 km/hr, about the same as that of a jet aircraft. Since the
average depth of the Pacific ocean is 4000 m (14,000 feet) , tsunami wave speed
will average about 200 m/s or over 700 km/hr (500 mph). At such high
speeds, a tsunami generated in Aleutian Islands may reach Hawaii in less than
four and a half hours. In 1960, great tsunami waves generated in Chile
reached Japan, more than 16,800 km away in less than 24 hours, killing hundreds
of people.
Magnitude 9.0 OFF THE WEST COAST OF NORTHERN SUMATRA
Sunday, December 26, 2004 at 00:58:53 UTC
Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver
The devastating megathrust
earthquake of December 26, 2004, occurred on the interface of the India and
Burma plates and was caused by the release of stresses that develop as the India
plate subducts beneath the overriding Burma plate. The India plate begins its
descent into the mantle at the Sunda trench, which lies to the west of the
earthquake's epicenter. The trench is the surface expression of the plate
interface between the Australia and India plates, situated to the southwest of
the trench, and the Burma and Sunda plates, situated to the northeast.
In the region of the earthquake,
the India plate moves toward the northeast at a rate of about 6 cm/year relative
to the Burma plate. This results in oblique convergence at the Sunda trench. The
oblique motion is partitioned into thrust-faulting, which occurs on the
plate-interface and which involves slip directed perpendicular to the trench,
and strike-slip faulting, which occurs several hundred kilometers to the east of
the trench and involves slip directed parallel to the trench. The December 26
earthquake occurred as the result of thrust-faulting.
Preliminary locations of larger
aftershocks following the megathrust earthquake show that approximately 1200 km
of the plate boundary slipped as a result of the earthquake. By comparison with
other large megathrust earthquakes, the width of the causative fault-rupture was
likely over one-hundred km. From the size of the earthquake, it is likely that
the average displacement on the fault plane was about fifteen meters. The sea
floor overlying the thrust fault would have been uplifted by several meters as a
result of the earthquake. The above estimates of fault-dimensions and
displacement will be refined in the near future as the result of detailed
analyses of the earthquake waves.
The world's largest recorded
earthquakes have all been megathrust events, occurring where one tectonic plate
subducts beneath another. These include:
the magnitude 9.5 1960 Chile
earthquake, the magnitude 9.2 1964 Prince William Sound, Alaska, earthquake, the
magnitude 9.1 1957 Andreanof Islands, Alaska, earthquake, and the magnitude 9.0
1952 Kamchatka earthquake. As with the recent event, megathrust earthquakes
often generate large tsunamis that cause damage over a much wider area than is
directly affected by ground shaking near the earthquake's rupture
The
great earthquake (Mw 9.0) of December 26 off Sumatra generated the tsunami in the Indian Ocean.According to U.S.G.S.,
the aftershocks extended as far as 1,000 km toward north (the red circles
indicate aftershocks occurred within 24 hours).Assuming that the aftershock area represents the tsunami source, the
tsunami propagation was computed.
The
tsunami propagation is also animated (up to 5 hours) from a 1200 km fault. The
red color means that the water surface is higher than normal, while the blue
means lower. It indicates that initial tsunami to the east (e.g., Phuket)
began with receding wave, while to the west (e.g., Sri Lanka)
large wave suddenly reached. The darker the color, the
larger the amplitude.The
tsunamis were larger in the east and west directions.
Tsunami Animation: Courtesy: National Institute of Advanced
Industrial Science and Technology, Japan
Data
compiled from The British Antarctic Study, NASA, Environment Canada,
UNEP, EPA and
other sources as stated and credited Researched by Charles
Welch-Updated dailyThis
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