Causes of Tsunamis

sunami: the Great Waves Tsunami is a Japanese word with the English translation, â€Å"harbor wave. † Represented by two characters, the top character, â€Å"tsu,† means harbor, while the bottom character, â€Å"nami,† means â€Å"wave. † In the past, tsunamis were sometimes referred to as â€Å"tidal waves† by the general public, and as â€Å"seismic sea waves† by the scientific community. The term â€Å"tidal wave† is a misnomer; although a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term â€Å"seismic sea wave† is also misleading. â€Å"Seismic† implies an earthquake-related generation mechanism, but a tsunami can also be caused by a nonseismic event, such as a landslide or meteorite impact. Tsunamis are a threat to life and property to anyone living near the ocean. For example, in 1992 and 1993 over 2,000 people were killed by tsunamis occurring in Nicaragua, Indonesia and Japan. Property damage was nearly one billion dollars. The 1960 Chile Earthquake generated a Pacific-wide tsunami that caused widespread death and destruction in Chile, Hawaii, Japan and other areas in the Pacific. Large tsunamis have been known to rise over 100 feet, while tsunamis 10 to 20 feet high can be very destructive and cause many deaths and injuries. What Cause Tsunamis? Tsunamis, also called seismic sea waves or, incorrectly, tidal waves, generally are caused by earthquakes, less commonly by submarine landslides, infrequently by submarine volcanic eruptions and very rarely by a large meteorite impact in the ocean. Submarine volcanic eruptions have the potential to produce truly awesome tsunami waves. The Great Krakatau Volcanic Eruption of 1883 generated giant waves reaching heights of 125 feet above sea-level, killing thousands of people and wiping out numerous coastal villages. Ring of Fire About two-thirds of the earth is covered by the waters of the four oceans. The Pacific Ocean is the world's largest, covering more than one third of the total surface area of our planet. The Pacific Ocean is surrounded by a series of mountain chains, deep ocean trenches and island arcs, sometimes called a â€Å"ring of fire. † The great size of the Pacific Ocean and the large earthquakes associated with the â€Å"ring of fire† combine to produce deadly tsunamis. Tsunamis on the Move Wave Height and Water Depth In the open ocean a tsunami is less than a few feet high at the surface, but its wave height increases rapidly in shallow water. Tsunamis wave energy extends from the surface to the bottom in the deepest waters. As the tsunami attacks the coastline, the wave energy is compressed into a much shorter distance creating destructive, live-threatening waves. In the deep ocean, destructive tsunamis can be small–often only a few feet or less in height–and cannot be seen nor can they be felt by ships. But, as the tsunami reaches shallower coastal waters, wave height can increase rapidly. Sometimes, coastal waters are drawn out into the ocean just before the tsunami strikes. When this occurs, more shoreline may be exposed than even at the lowest tide. This major withdrawal of the sea should be taken as a warning of the tsunami waves that will follow How Fast? Where the ocean is over 20,000 feet deep, unnoticed tsunami waves can travel at the speed of a commercial jet plane, nearly 600 miles per hour. They can move from one side of the Pacific Ocean to the other in less than a day. This great speed makes it important to be aware of the tsunami as soon as it is generated. Scientists can predict when a tsunami will arrive since the speed of the waves varies with the square root of the water depth. Tsunamis travel much slower in shallower coastal waters where their wave heights begin to increase dramatically. How Big? Offshore and coastal features can determine the size and impact of tsunami waves. Reefs, bays, entrances to rivers, undersea features and the slop 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 feet. In extreme cases, water level has risen to more than 50 feet for tsunamis of distant origin and over 100 feet 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 community destructive waves can be large and violent. The flooding can extend inland by 1000 feet or more, covering large expanses of land with water and debris.