What is a tsunami?
A tsunami (pronounced soo-nah-mee) is a series of waves of extremely long wave length and long period generated in a body of water by an impulsive disturbance that vertically displaces the water. The term tsunami was adopted for general use in 1963 by an international scientific conference. Tsunami is a Japanese word represented by two characters: "tsu" and "nami." The character "tsu" means harbor, and the character "nami" means wave. In the past, tsunamis were often referred to as "tidal waves." The term "tidal wave" is a misnomer. Tides are the result of gravitational influences of the moon, sun, and planets. Tsunamis are not caused by the tides and are unrelated to the tides; although a tsunami striking a coastal area is influenced by the tide level at the time of impact.
What causes tsunamis?
Tsunamis are most often caused by earthquakes and landslides. These push the water upward, sideways or downward to create the tsunami waves. Volcanic eruptions can also cause tsunamis. There is evidence than in the distant past, asteroids and comets striking the Earth have created enormous tsunamis.
How do earthquakes generate tsunamis?
Tsunamis can be generated when the sea floor abruptly shifts and vertically displaces the overlying water from its equilibrium position. Waves are formed as the displaced water mass attempts to regain its equilibrium. The main factor which determines the initial size of a tsunami is the amount of vertical sea floor deformation. Not all earthquakes generate tsunamis. To generate tsunamis, earthquakes must occur underneath or near the ocean, be large and create movements in the sea floor. All oceanic regions of the world can experience tsunamis, but in the Pacific Ocean there is a much more frequent occurrence of large, destructive tsunamis because of the many large earthquakes along the margins of the Pacific Ocean.
How do landslides, volcanic eruptions and cosmic collisions generate tsunamis?
Any disturbance that displaces a large water mass from its equilibrium position can generate a tsunami. Generally tsunamis caused by landslides or volcanic eruptions dissipate more quickly than Pacific-wide tsunamis caused by some earthquakes and rarely affect coastlines distant from the source.
How do tsunamis differ from other water waves?
Tsunami waves are shallow-water waves with long periods and wave lengths. (A wave is classified a shallow-water wave when the ratio between the water depth and its wavelength gets very small. The speed of a shallow-water wave is equal to the square root of the product of the acceleration of gravity (32ft/sec/sec or 980cm/sec/sec) and the depth of the water.) Shallow water waves are different from wind-generated waves (the waves many of us have observed on the beach). Wind-generated waves usually have period (time between two succesional waves) of five to twenty seconds and a wavelength (distance between two successional waves) of about 50 to 600 feet (15 to 200 meters) A tsunami can have a period in the range of 10 minutes to 1 hour and a wavelength in excess of 700 km (430 miles).
Tsunami vs. Tidal Wave?
The terms 'tsunami' and 'tidal wave' do not mean the same thing, though many use the words interchangeably. You will find the term 'tidal wave' (referring to tsunamis) is most often seen in reproductions of old news reports and older text books.
The wave motion of the tides.
In popular usage, any unusually high and therefore destructive water level along a shore. It usually refers to either a storm surge or tsunami.
A series of traveling waves of extremely long length and period, usually generated by disturbances associated with earthquakes occurring below the ocean floor.
Rogue wave vs tsunami?
A tsunami is caused by a sudden displacement of water. The most frequent cause is an underwater earthquake but, less frequently, tsunamis can be generated by volcanic eruptions, landslides, or even oceanic meteor impact. The length of these waves, from one crest to the next, can be up to 200 km long, and they travel in the deep ocean at speeds around 700 km/hr. Their height in the open ocean is very small, a couple of meters at most, so they pass under ships and boats undetected.
So called 'rogue waves' are a bit more mysterious, and not very well understood. They are very high waves, tens of meters, perhaps. They are very short compared to tsunamis, less than a 2000 m, perhaps. They arise unexpectedly in the open ocean, and the generating mechanism is a source of controversy and active research. Some theories:
Strong currents interact with existing swell to make them much higher.
They are just a statistical aberration that occurs when a bunch of waves just happen to be in the right spot at the right time, so that they add together to make one big wave.
If a storm 'prepares' the ocean, by making it very rough, and this is followed by a sudden intensification of the storm, then the wind can get a "better grip" on the ocean surface (i.e., wind energy is much more efficiently transferred to the water), and the monster waves can thus be created."
What is the current tool used to detect tsunamis?
Tsunamis are detected by open-ocean tsunami buoys and by coastal tide gages. These instruments report their information in real-time to tsunami warning centers (one center in Alaska, another in Hawaii, and a third to be installed soon in Puerto Rico).
Besides the direct observations, the amplitudes of tsunamis are also estimated from the size and type of earthquake that may have generated them. The warning centers receive earthquake (seismic) data from many sites as well as analyses of this information from earthquake centers. Since seismic waves travel much faster than tsunamis, the earthquake information is often available hours before the tsunamis are able to travel across the ocean. This is much help for people near the earthquake, however, since the local tsunami is there often within minutes. It is that easy to estimate the strength of a tsunami from the character of an earthquake. Hence, there are false alarms. It is hoped that the direct tsunami observations will reduce the number of these. Local tsunamis are also generated by underwater landslides and volcanoes. There is some research being done to better understand these types of tsunamis and to develop early detection methods for them.