Tsunami.
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Tsunami.
I
INTRODUCTION
Tsunami, Japanese word meaning "harbor wave," used as the scientific term for a class of abnormal sea wave that can cause catastrophic damage when it hits a
coastline. Tsunamis can be generated by an undersea earthquake, an undersea landslide, the eruption of an undersea volcano, or by the force of an asteroid crashing
into the ocean. The most frequent cause of tsunamis is an undersea earthquake. When the ocean floor is uplifted or offset during an earthquake, a set of waves is
created similar to the concentric waves generated by an object dropped into the water. Most tsunamis originate along the Ring of Fire, a zone of volcanoes and seismic
activity, 32,500 km (24,000 mi) long, which encircles the Pacific Ocean. Since 1819, about 40 tsunamis have struck the Hawaiian Islands.
A tsunami can have wavelengths, or widths (the distance between one wave crest to the next), of 100 to 200 km (60 to 120 mi), and may travel hundreds of
kilometers across the deep ocean, reaching speeds of about 725 to 800 km/h (about 450 to 500 mph). A tsunami is not one wave but a series of waves. In the deep
ocean, the waves may be only about half a meter (a foot or two) high. People onboard a ship passing over it would not even notice the tsunami. Upon entering shallow
coastal waters, however, the waves may suddenly grow rapidly in height. When the waves reach the shore, they may be 15 m (50 ft) high or more. Tsunamis can also
take the form of a very fast tide or bore, depending on the shape of the sea floor.
Tsunamis have tremendous force because of the great volume of water affected and the speed at which they travel. Just a cubic yard of water, for example, weighs
about one ton. Although the tsunami slows to a speed of about 48 km/h (30 mph) as it approaches a coastline, it has a destructive force equal to millions of tons.
Tsunamis are capable of obliterating coastal settlements.
Tsunamis should not be confused with storm surges, which are domes of water that rise underneath hurricanes or cyclones and cause extensive coastal flooding when
the storms reach land. Storm surges are particularly devastating if they occur at high tide. A cyclone and accompanying storm surge killed an estimated 500,000 people
in Bangladesh in 1970. Tsunamis are sometimes called tidal waves, but they have nothing to do with the gravitational forces that cause tides.
II
TSUNAMI WARNINGS AND PREPARATION
Tsunamis are rare and difficult to predict. Although scientists can rapidly detect an undersea earthquake through the use of seismometers, not all undersea earthquakes
will generate a tsunami. Other factors, such as the topography of the ocean floor at the epicenter of a quake, are involved.
Since 1949 scientists have attempted to establish a reliable warning system for tsunamis in the Pacific Ocean. That year the Pacific Tsunami Warning Center was
established in Ewa Beach, Hawaii. From 1949 to 2004, the center issued warnings for all five of the major ocean-spanning tsunamis that occurred in the Pacific Ocean.
However, it also issued 15 false alarms in that period.
In 1965 the International Tsunami Information Center (ITIC) was established by the Intergovernmental Oceanographic Commission of the United Nations Educational,
Scientific and Cultural Organization (UNESCO). The purpose of the ITIC, which is based in Honolulu, Hawaii, is to mitigate the hazards posed by tsunamis by helping
nations that rim the Pacific Ocean prepare for a tsunami. Both the Pacific Tsunami Warning Center and the ITIC are operated under the auspices of the United States
National Oceanic and Atmospheric Administration (NOAA). The NOAA also operates the Pacific Marine Environmental Laboratory in Seattle, Washington, a leading
research center for the study and monitoring of tsunamis.
The Pacific Marine Environmental Laboratory developed the first reliable scientific instrument for detecting tsunamis and quickly alerting scientists when a tsunami
occurs. The instrument, known as a tsunameter, is anchored on the ocean floor and measures changes in water pressure when a tsunami passes above. Six of these
instruments are deployed in the Pacific Ocean. When a tsunameter detects a tsunami, it sends acoustic signals to a buoy on the surface. The buoy converts the signals
to radio waves and relays the data to an orbiting satellite, which then alerts several warning centers, including the Pacific Tsunami Warning Center in Ewa Beach and the
West Coast Tsunami Warning Center in Palmer, Alaska. The entire process takes only about two minutes. NOAA scientists then study the signals to determine if a
tsunami warning is necessary.
The early warning system is known as the Deep-ocean Assessment and Reporting of Tsunamis (DART). In 2005 only the Pacific Ocean had a DART system. Reliable early
warning systems do not yet exist for other ocean basins. However, in late 2005 India's Meteorological Department announced that it was using 11 tidal gauges and
seismic monitors in the Indian Ocean to establish an interim warning system until a more comprehensive system could be completed. Once a tsunami warning is issued,
people in the affected areas are warned to seek high ground immediately and to stay away from coastlines until the alert has ended.
III
MAJOR TSUNAMI DISASTERS
The worst tsunami disaster in history occurred in December 2004 when a magnitude 9.0 undersea earthquake, centered in the Indian Ocean off the northwestern coast
of the Indonesian island of Sumatra, generated a tsunami that struck the coasts of 14 countries from Southeast Asia to northeastern Africa. The International
Committee of the Red Cross reported a death toll of more than 250,000 people as a result of the tsunami and the earthquake, with nearly two-thirds of the deaths
occurring in Indonesia. High death tolls were also reported in India, Sri Lanka, and Thailand. Geologists calculated that the ocean floor at the epicenter was thrust
upward 9 m (30 ft) as a result of the quake.
Previously the highest death toll from a tsunami was an estimated 60,000 people killed in 1755 when an earthquake generated a tsunami that struck coastal Portugal,
Spain, and Morocco and destroyed the city of Lisbon, Portugal. The last major tsunami to occur in the Indian Ocean happened in 1883 with the eruption of Krakatau
(also spelled Krakatoa). The resulting tsunami reached an estimated height of 30 m (100 ft), traveled 13,000 km (8,000 mi), and drowned about 34,000 people along
the coasts of Java and Sumatra. Another 2,000 people were fatally burned by hot volcanic ash.
In North America the worst known tsunami occurred in 1964 when an earthquake off the coast of Anchorage, Alaska, created a tsunami that killed 115 people in Alaska,
Oregon, and California. Scientists also believe that a magnitude 9.0 quake occurred along the Cascadia fault off the coast of Washington and Oregon in 1700 and
generated a massive tsunami. Scientists believe the quake and tsunami converted vast spruce tree forests into saltwater tidal flats.
Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
Tsunami.
I
INTRODUCTION
Tsunami, Japanese word meaning "harbor wave," used as the scientific term for a class of abnormal sea wave that can cause catastrophic damage when it hits a
coastline. Tsunamis can be generated by an undersea earthquake, an undersea landslide, the eruption of an undersea volcano, or by the force of an asteroid crashing
into the ocean. The most frequent cause of tsunamis is an undersea earthquake. When the ocean floor is uplifted or offset during an earthquake, a set of waves is
created similar to the concentric waves generated by an object dropped into the water. Most tsunamis originate along the Ring of Fire, a zone of volcanoes and seismic
activity, 32,500 km (24,000 mi) long, which encircles the Pacific Ocean. Since 1819, about 40 tsunamis have struck the Hawaiian Islands.
A tsunami can have wavelengths, or widths (the distance between one wave crest to the next), of 100 to 200 km (60 to 120 mi), and may travel hundreds of
kilometers across the deep ocean, reaching speeds of about 725 to 800 km/h (about 450 to 500 mph). A tsunami is not one wave but a series of waves. In the deep
ocean, the waves may be only about half a meter (a foot or two) high. People onboard a ship passing over it would not even notice the tsunami. Upon entering shallow
coastal waters, however, the waves may suddenly grow rapidly in height. When the waves reach the shore, they may be 15 m (50 ft) high or more. Tsunamis can also
take the form of a very fast tide or bore, depending on the shape of the sea floor.
Tsunamis have tremendous force because of the great volume of water affected and the speed at which they travel. Just a cubic yard of water, for example, weighs
about one ton. Although the tsunami slows to a speed of about 48 km/h (30 mph) as it approaches a coastline, it has a destructive force equal to millions of tons.
Tsunamis are capable of obliterating coastal settlements.
Tsunamis should not be confused with storm surges, which are domes of water that rise underneath hurricanes or cyclones and cause extensive coastal flooding when
the storms reach land. Storm surges are particularly devastating if they occur at high tide. A cyclone and accompanying storm surge killed an estimated 500,000 people
in Bangladesh in 1970. Tsunamis are sometimes called tidal waves, but they have nothing to do with the gravitational forces that cause tides.
II
TSUNAMI WARNINGS AND PREPARATION
Tsunamis are rare and difficult to predict. Although scientists can rapidly detect an undersea earthquake through the use of seismometers, not all undersea earthquakes
will generate a tsunami. Other factors, such as the topography of the ocean floor at the epicenter of a quake, are involved.
Since 1949 scientists have attempted to establish a reliable warning system for tsunamis in the Pacific Ocean. That year the Pacific Tsunami Warning Center was
established in Ewa Beach, Hawaii. From 1949 to 2004, the center issued warnings for all five of the major ocean-spanning tsunamis that occurred in the Pacific Ocean.
However, it also issued 15 false alarms in that period.
In 1965 the International Tsunami Information Center (ITIC) was established by the Intergovernmental Oceanographic Commission of the United Nations Educational,
Scientific and Cultural Organization (UNESCO). The purpose of the ITIC, which is based in Honolulu, Hawaii, is to mitigate the hazards posed by tsunamis by helping
nations that rim the Pacific Ocean prepare for a tsunami. Both the Pacific Tsunami Warning Center and the ITIC are operated under the auspices of the United States
National Oceanic and Atmospheric Administration (NOAA). The NOAA also operates the Pacific Marine Environmental Laboratory in Seattle, Washington, a leading
research center for the study and monitoring of tsunamis.
The Pacific Marine Environmental Laboratory developed the first reliable scientific instrument for detecting tsunamis and quickly alerting scientists when a tsunami
occurs. The instrument, known as a tsunameter, is anchored on the ocean floor and measures changes in water pressure when a tsunami passes above. Six of these
instruments are deployed in the Pacific Ocean. When a tsunameter detects a tsunami, it sends acoustic signals to a buoy on the surface. The buoy converts the signals
to radio waves and relays the data to an orbiting satellite, which then alerts several warning centers, including the Pacific Tsunami Warning Center in Ewa Beach and the
West Coast Tsunami Warning Center in Palmer, Alaska. The entire process takes only about two minutes. NOAA scientists then study the signals to determine if a
tsunami warning is necessary.
The early warning system is known as the Deep-ocean Assessment and Reporting of Tsunamis (DART). In 2005 only the Pacific Ocean had a DART system. Reliable early
warning systems do not yet exist for other ocean basins. However, in late 2005 India's Meteorological Department announced that it was using 11 tidal gauges and
seismic monitors in the Indian Ocean to establish an interim warning system until a more comprehensive system could be completed. Once a tsunami warning is issued,
people in the affected areas are warned to seek high ground immediately and to stay away from coastlines until the alert has ended.
III
MAJOR TSUNAMI DISASTERS
The worst tsunami disaster in history occurred in December 2004 when a magnitude 9.0 undersea earthquake, centered in the Indian Ocean off the northwestern coast
of the Indonesian island of Sumatra, generated a tsunami that struck the coasts of 14 countries from Southeast Asia to northeastern Africa. The International
Committee of the Red Cross reported a death toll of more than 250,000 people as a result of the tsunami and the earthquake, with nearly two-thirds of the deaths
occurring in Indonesia. High death tolls were also reported in India, Sri Lanka, and Thailand. Geologists calculated that the ocean floor at the epicenter was thrust
upward 9 m (30 ft) as a result of the quake.
Previously the highest death toll from a tsunami was an estimated 60,000 people killed in 1755 when an earthquake generated a tsunami that struck coastal Portugal,
Spain, and Morocco and destroyed the city of Lisbon, Portugal. The last major tsunami to occur in the Indian Ocean happened in 1883 with the eruption of Krakatau
(also spelled Krakatoa). The resulting tsunami reached an estimated height of 30 m (100 ft), traveled 13,000 km (8,000 mi), and drowned about 34,000 people along
the coasts of Java and Sumatra. Another 2,000 people were fatally burned by hot volcanic ash.
In North America the worst known tsunami occurred in 1964 when an earthquake off the coast of Anchorage, Alaska, created a tsunami that killed 115 people in Alaska,
Oregon, and California. Scientists also believe that a magnitude 9.0 quake occurred along the Cascadia fault off the coast of Washington and Oregon in 1700 and
generated a massive tsunami. Scientists believe the quake and tsunami converted vast spruce tree forests into saltwater tidal flats.
Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
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