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Tsunami Background

Major earthquakes occurring along subduction zones are especially hazardous, because they can trigger tsunamis (from the Japanese word tsunami meaning "harbor wave") and pose a potential danger to coastal communities and islands that dot the Pacific. Tsunamis are often mistakenly called "tidal waves" when, in fact, they have nothing to do with tidal action. Rather, tsunamis are seismic sea waves caused by earthquakes, submarine landslides, and, infrequently, by eruptions of island volcanoes.

Tsunamis in deep water can have a wavelength greater than 300 miles (500 kilometers) and a period of about an hour. This is very different from the normal California tube, which generally has a wavelength of about 300 feet (100 meters) and a period of about ten seconds. (The period of a wave is the time between two successive waves.)

Tsunamis are shallow-water waves, which means that the ratio between water depth and wavelength is very small. These shallow-water waves move at a speed equal to the square root of the product of the acceleration of gravity (9.8m/s/s) and the water depth. The deeper the water, the faster and shorter the wave is. For example, when the ocean is 20,000 feet deep, a tsunami travels at 550 miles per hour. At this speed, the wave can compete with a jet airplane, traveling across the ocean in less than a day.

Another important factor in considering tsunamis is the rate at which they lose energy. Because a wave loses energy at a rate inversely related to its wavelength, tsunamis can travel at high speeds for a long period of time and lose very little energy in the process.

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).

The worlds largest recorded earthquakes were all megathrust events and occur 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, Alaska earthquake, and the magnitude 9.0 1952 Kamchatka earthquake. As with the recent event, megathrust earthquakes often generate large tsunamis that can cause damage over a much wider area than is directly effected by ground shaking near the earthquake's rupture.

Offshore and coastal features can determine the size and impact of tsunami waves. Reefs, bays, entrances to rivers, undersea features and the slope of the beach all help to modify the tsunami as it attacks the coastline. When the tsunami reaches the coast and moves inland, the water level can rise many meters. In extreme cases, water level has risen to more than 15 m (50 ft) for tsunamis of distant origin and over 30 m (100 ft) for tsunami waves generated near the earthquake's epicenter. The first wave may not be the largest in the series of waves. One coastal community may see no damaging wave activity while in another nearby community destructive waves can be large and violent. The flooding can extend inland by 300 m (~1000 ft) or more, covering large expanses of land with water and debris.

Because of the small wave height a tsunami has during much of its journey through deep water, a reliable early detection system for tsunamis has yet to be developed. The Pacific Tsunami Warning Center (PTWC) in Hawaii is the regional operational center for tsunami information in the Pacific.

Unfortunately, this system is not very reliable. Seventy-five percent of all warnings issued since 1948 have been false. For example, Honolulu was evacuated in 1948 on a false alarm at a cost of more than $30 million dollars.

TsunamiReady is a National Weather Service (NWS) initiative that promotes tsunami hazard preparedness as an active collaboration among Federal, state and local emergency management agencies, the public, and the NWS tsunami warning system. This collaboration supports better and more consistent tsunami awareness and mitigation efforts among communities at risk. The TsunamiReady program is based on the NWS StormReady model.

As dangerous as tsunamis are they do not happen very often. There is no Atlantic Ocean or Indian Ocean counterpart to the Pacific warning network.

Something like the Andaman Tsunami could happen in America, on the Atlantic coast. The Cumbre Vieja volcano is located right next to the shoreline of the island's west coast, and the entire side of the volcano is judged to be unstable. It's been calculated that if that wall of the mountain tumbled into the sea, it would create a "splash" 3000 feet in height and tens of miles wide. A tsunami would then be generated that would be unrivaled in modern times. An enormous wave would head toward the U.S. (and parts of the Caribbean, Mexico, Central America and Canada) at a speed of 500 mph. In about nine hours the wave would slam the entire US east coast from Maine to Florida, obliterating everything in its path. Every coastal city large and small - probably every sign of civilization along the eastern seaboard of the country - would be wiped out, and the death toll and damage would be beyond imagination.

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Page last modified: 01-07-2011 18:34:15 ZULU