DESCRIPTION:
Tsunami - Seismic Sea Wave

A tsunami is a series of very long wavelength ocean waves caused by the sudden displacement of water by earthquakes, landslides, or submarine slumps. Ordinarily, tsunamis are produced only by earthquakes exceeding magnitude 7.5. In the open ocean, tsunami waves travel at speeds of 600-800 kilometers per hour, but their wave heights are usually only a few centimeters. As they approach shallow water near a coast, tsunami waves travel more slowly, but their wave heights may increase to many meters, and thus they can become very destructive.

Tsunamis are long-wavelength, long-period sea waves generated by an abrupt movement of large volumes of water. In the open ocean, the distance between wave crests can be greater than 100 kilometers, and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour, depending on water depth. Large subduction earthquakes causing vertical displacement of the sea floor and having magnitudes greater than 7.5 are the most common cause of destructive tsunamis. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.

Tsunami wave heights at sea are usually less than one meter, and the waves are not frequently noticed by people in ships. As tsunami waves approach the shallow water of the coast, their heights increase and sometimes exceed 20 meters. ...

Tsunamis are large, rapidly moving ocean waves triggered by a major disturbance of the ocean floor, which is usually caused by an earthquake but sometimes can be produced by a submarine landslide or a volcanic eruption. Tsunamis are also referred to as "tidal waves," but they have no relation to tides.

Ships at sea cannot detect a passing tsunami, nor can the waves be seen from aircraft. While passing through deep oceans, a tsunami consists of a series of waves that are only a few feet high and a hundred miles or more apart. These waves typically travel at speeds of about 600 mph. As they reach shallow water, the waves slow down but greatly increase in height, and the distance between them shrinks. When the tsunami finally strikes the coast, the waves may crest to heights of 100 feet and travel inland at speeds of 30 mph. A series of waves may reach the coast at intervals of 5 to 40 minutes; the first wave is frequently not the largest.

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. During a major earthquake, the seafloor can move by several meters and an enormous amount of water is suddenly set into motion, sloshing back and forth for several hours. The result is a series of waves that race across the ocean at speeds of more than 800 km per hour, comparable to those of commercial jetliners. The energy and momentum of these transoceanic waves can take them thousands of kilometers from their origin before slamming into far-distant islands or coastal areas.

To someone on a ship in the open ocean, the passage of a tsunami wave would barely elevate the water surface. However, when it reaches shallower water near the coastline and "touches bottom," the tsunami wave increases in height, piling up into an enormous wall of water. As a tsunami approaches the shore, the water near shore commonly recedes for several minutes -- long enough for someone to be lured out to collect exposed sea shells, fish, etc. -- before suddenly rushing back toward land with frightening speed and height.

When the sea floor is raised suddenly during a great earthquake, water is raised with it, the sea surface is tipped, and water rushes away; or, if the floor is dropped, water rushes in. An enormous mass of water is suddenly set in motion, and complex sloshing back and forth continues for many hours. The result is a train of water waves of a unique type, the seismic sea wave or "tsunami" (its Japanese name). The velocity of a water-wave form increases with its length, and in the deep ocean these uniquely long waves travel at about 800 kilometers (500 miles) per hour. The waves at sea are something like an hour apart at a given point, are only perhaps 30 centimeters (1 foot) high, and are virtually undetectable. As a wave approaches land, however, its bottom is slowed down by contact with the shallowing sea floor, whereas the top is slowed much less and catches up with the bottom. Where sea-floor topography and orientation are optimal for a tsunami from a given direction, the wave can hump up into a breaking wall of water 9 meters (30 feet) or more high, and rush onto shore to cause enormous destruction. Nearby coastal points, where the bottom configuration is different, may record the same wave only as a rapid surge and withdrawal of water, with much lower height.

Damaging earthquakes and sea waves (tsunami) may also be closely related to volcanoes and volcanic activity. Large earthquakes related to intrusion of magma into Hawaii's active rift zones of Mauna Loa and Kilauea have caused extensive damage on land and also triggered tsunami in 1868 and 1975 that devastated low-lying coastal areas (Tilling and others, 1976). Large landslides from Alaskan volcanoes near the sea have also generated tsunami that destroyed coastal villages (Kienle and others, 1987). Recent research in Hawaii has shown that much larger tsunami have, in the more distant past, washed as much as 366 meters up on some of the islands (Moore and Moore, 1984). These waves appear to have been generated by gigantic submarine landslides that removed large parts of the Hawaiian volcanoes (Moore and others, 1989). These colossal failures have also torn away and submerged subaerial parts of islands.

Link to: Hawaiian Volcano Observatory Website

The 1883 eruption of Krakatau Volcano, located in the Sunda Straits between the islands of Sumatra and Java, Indonesia, provides an excellent example of an eruption-caused tsunami. A series of tsunamis washed away 165 coastal villages on Java and Sumatra, killing 36,000 people. The larger tsunamis were recorded by tide gauges as far away as the southern coast of the Arabian Peninsula-more than 7,000 km from Krakatau!

More About Krakatau

Unzen is well know for Japan's greatest volcanic disaster. In 1792, about a month after lava stopped erupting from the volcano, a landslide from nearby Mount Mayuyama swept through ancient Shimabara City, entered the sea, and generated a tsunami that struck nearby areas. More than 15,000 people were killed by the landslide and tsunami. The amphitheater-shaped scar of the landslide is still clearly visible on Mount Mayuyama just above the city.

More About Unzen

Historically, tsunamis originating in the northern Pacific and in South America have caused more damage on the west coast of the United States than tsunamis originating in Japan and the South Pacific. The tsunami generated by the 1964 Alaska earthquake caused $85 million damage in Alaska, $10 million damage in Canada, $115,000 damage in Washington, $754,000 damage in Oregon, and $11 million damage in California (Wilson and Torum, 1972). ... In places, wave heights reached 4.5 meters. The wave heights varied considerably, depending on the local water depth and the shape of inlets. The 1964 tsunami destroyed a small bridge across the Copalis River (Grays Harbor County) by hurling log debris against supporting piles. The tsunami was also detected on the Columbia River as far as 160 kilometers from the ocean (Wilson and Torum, 1972). Besides causing property damage, the 1964 tsunami killed 103 people in Alaska, 4 in Oregon, and 12 in California. Newspaper accounts tell of narrow escapes along the Washington coast, but there were no fatalities there. ...

Past tsunamis have caused only minor damage in Washington. The damage caused in the state by the tsunami triggered by the 1964 Alaska earthquake occurred along small estuaries north of Grays Harbor. In some places south of Grays Harbor sand dunes protected developed areas from damage (Hogan and others, 1964). Parts of these dunes have since been cleared to enhance the view; some homes behind the dune area may be exposed to greater risk from tsunami damage in the future. ...

In addition to a tsunami generated by a distant earthquake, a magnitude 8 or greater subduction earthquake between the Juan de Fuca and North America plates might create a large local tsunami on the coast of Washington. Atwater (1987) and Reinhart and Bourgeois (1987) have found evidence they believe indicates that a tsunami from a nearby great subduction earthquake did affect the coast of Washington about 300 years ago. In general, local tsunamis are much more destructive than tsunamis generated from a distant source. In addition, they may occur within minutes of the earthquake or landslide that produces them, allowing little time for evacuation. Estimates of the effect of a local tsunami in Washington are speculative because we have no written record of a large, shallow earthquake near the coast. However, the sudden submergence of coastal areas that may accompany great earthquakes might increase the amount of land in Washington susceptible to tsunami damage.

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