Physics of Tsunamis

Tsunamis are waves, or series of waves, created by a disturbance in the ocean. Most of the time this disturbance is by an earthquake but can be from meteorites, landslides, or even explosions. They are sometimes known as tidal waves but this is a misnomer because tsunamis have nothing to do with tides. While tsunamis are feared particularly in light of the December 26th tsunami, the physics behind them is fascinating.

One the largest tsunamis in recent history was the Cascadia tsunami in 1700. This occured when two continental plates (stretching from Vancouver to California) slipped causing a 9.0 magnitude earthquake. The resulting tsunami stretched across the entire Pacific and is recorded in Japanese history. Researchers have recently found the evidence in the Washington/Oregon area of the huge tsunami there. Both Japanese and Northwest Native American cultures tell of the tsunami. Japanese historians say that the tsunami was over 5 meters tall, and Native American legends tell of landslides and entire villages disappearing in the same time period.

Most of the information about tsunamis is from the last century and specifically the last 50 years; this is because this is when we have just started to understand the physics behind them.

Most of the major tsunamis that have occured are in the Pacific because there are much more earthquakes in the Pacific "Ring of Fire". The Pacific tectonic plate is very active and causes sucduction and slipping between plates, thus causing many earthquakes.

Another large tsunami occured in 1946. This was caused by a 7.8 earthquake on the Aleutian Islands that spread to Hawaii. There was little warning in Hawaii and a large death toll followed. The result of the tsunami was 165 deaths ...

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...56 before the advent of sonar and modern depth techniques. The scientists knew the amount of time it took for the tsunami to reach Hawaii from Alaska, and also the distance, so they thus knew the velocity. Since g is already known, they simply solved for the average ocean depth.

Another factor in the creation of tsunamis is the depth of the earthquake. If the earthquake occurs more than 100 km below the surface, a tsunami will not occur because there is not enough vertical displacement of the water.

Bibliography

"Physics for Scientists and Engineers", (2004), Serway and Jewett, Brooks/Cole Thomson Learning

"Furious Earth - The Science and Nature of Earthquakes, Volcanoes, and Tsunamis", (2000), Ellen Prager, McGraw-Hill Publishing

"The Official Tommy Tsunami and Ernie Earthquake Coloring Book", (1980), Jewell Hermon, Alaska Emergency Services