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 It
sounds like
the stuff of a tragic fictional tale - a giant wave from out of nowhere
comes crashing onto shore, sweeping away whole communities as if they
were made of playing cards and tooth picks and leaving behind many
dead and injured citizens. Unfortunately, these killer waves are a
reality. They are called tsunami, which means "harbor wave"
in Japanese. Tsunamis are oftentimes confused with tidal waves, and
although, their severity has a link to tidal schedules, tsunamis are
strictly caused by seismic activity.
 Although
most tsunamis occur in the Pacific Ocean, they can occur in any ocean.
Tsunamis, which can move through the open ocean at speeds greater
than 500 mph, can savagely attack coastlines, causing devastating
property damage and loss of life. Areas at greatest risk are less
than 25 feet above sea level and within one mile of the shoreline.
Most deaths caused by a tsunami are because of drowning. Associated
risks include flooding, contamination of drinking water, fires from
ruptured tanks or gas lines, and the loss of vital community infrastructure
(police, fire, and medical facilities). Tsunamis can travel upstream
in coastal estuaries and rivers, with damaging waves extending farther
inland than the immediate coast. A tsunami can occur during any season
of the year and at any time, day or night.
 By
definition 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. Earthquakes, landslides, volcanic eruptions, explosions,
and even the impact of cosmic bodies, such as meteorites, can generate
tsunamis. Shortly after one of these events occur on the ocean floor,
the water is displaced, and a tsunami is born. As a tsunami leaves
the deep water of the open ocean and travels into the shallower water
near the coast, it transforms. A tsunami travels at a speed that is
related to the water depth - hence, as the water depth decreases,
the tsunami slows, and grows in height. A ship traveling in deeper
waters may not even notice a tsunami, due to its lenghened wavelength
(sometimes hundreds of miles long), and amplitude of only a few feet.
This also makes it unnoticable from the air.
 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.
From an initial tsunami generating
source area, waves travel outward in all directions much like the
ripples caused by throwing a rock into a pond. As these waves approach
coastal areas, the time between successive wave crests varies from
5 to 90 minutes. The first wave is usually not the largest in the
series of waves, nor is it the most significant. Furthermore, one
coastal community may experience no damaging waves while another,
not that far away, may experience destructive deadly waves. Depending
on a number of factors, some low-lying areas could experience severe
inland inundation of water and debris of more than 1,000 feet.
Twenty-four tsunamis
have caused damage in the United States and its territories during
the last 204 years. Just since 1946, six tsunamis have killed more
than 350 people and caused a half billion dollars of property damage
in Hawaii, Alaska, and the West Coast. Scientists throughout the world
are continually improving tsunami warning systems as technology advances.
One such system is called DART, an early detection system consisting
of a surface buoy and an ocean floor device that transmit
information up to a space satelite. As part of the U.S.National Tsunami
Hazard Mitigation Program (NTHMP), the DART Project is an ongoing
effort to develop and implement a capability for the early detection
and real-time reporting of tsunamis in the open ocean. DART is essential
to fulfilling NOAA's national responsibility for tsunami hazard mitigation
and warnings. Project goals are:
1) Reduce the loss of life and property in U.S. coastal communities.
2) Eliminate false alarms and the high economic cost of unnecessary
evacuations.
DART stations are sited in regions with a history of generating destructive
tsunamis to ensure early detection of tsunamis and to acquire data
critical to real-time forecasts.
All
information obtained from and Tsunami!
Web Site, www.fema.gov,
and Pacific
Marine Environmental Laboratory (PMEL)
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