Tsunami

Tsunami

Tsunami, or seismic sea waves, are a series of very long wavelength ocean waves generated by the sudden displacement of large volumes of water . The generation of tsunami waves is similar to the effect of dropping a solid object, such as a stone, into a pool of water. Waves ripple out from where the stone entered, and thus displaced, the water. In a tsunami, the "stone" comes from underneath the ocean or very close to shore, and the waves, usually only three or four, are spaced about 15 minutes apart.

Tsunami can be caused by underwater (submarine) earthquakes, submarine volcanic eruptions , falling (slumping) of large volumes of ocean sediment, coastal landslides, or even by meteor impacts. All of these events cause some sort of landmass to enter the ocean and the ocean adjusts itself to accommodate this new mass. This adjustment creates the tsunami, which can circle around the world. Tsunami is a Japanese word meaning "large waves in harbors." It can be used in the singular or plural sense. Tsunami are sometimes mistakenly called tidal waves, but scientists avoid using that term since they are not at all related to tides .

Tsunami are classified by oceanographers as shallow water surface waves. Surface waves exist only on the surface of liquids. Shallow water waves are defined as surface waves occurring in water depths that are less than one half their wavelength. Wavelength is the distance between two adjacent crests (tops) or troughs (bottoms) of the wave. Wave height is the vertical distance from the top of a crest to the bottom of the adjacent trough. Tsunami have wave heights that are very small as compared to their wavelengths. In fact, no matter how deep the water, a tsunami will always be a shallow water wave because its wavelength (up to 150 mi [240 km]) is so much greater than its wave height (usually no more than 65 ft [20 m]).

Shallow water waves are different from deep water waves because their speed is controlled only by water depth. In the open ocean, tsunami travel quickly (up to 470 mph [760 kph]), but because of their low height (typically less than 3 ft [1 m]) and long wavelength, ships rarely notice them as they pass underneath. However, when a tsunami moves into shore, its speed and wavelength decrease due to the increasing friction caused by the shallow sea floor.

Wave energy must be redistributed, however, so wave height increases, just as the height of small waves increases as they approach the beach and eventually break. The increasing tsunami wave height produces a "wall" of water that, if high enough, can be incredibly destructive. Some tsunami are reportedly up to 200 ft (65 m) tall. The impact of such a tsunami can range miles inland if the land is relatively flat.

Tsunami may occur along any shoreline and are affected by local conditions such as the coastline shape, ocean floor characteristics, and the nature of the waves and tides already in the area . These local conditions can create substantial differences in the size and impact of the tsunami waves, even in areas that are very close geographically.

Tsunami researchers classify tsunami according to their area of effect. They can be local, regional, or ocean-wide. Local tsunami are often caused by submarine volcanoes, submarine sediment slumping, or coastal landslides. These can often be the most dangerous because there is often little warning between the triggering event and the arrival of the tsunami.

Seventy-five percent of tsunami are considered regional events. Japan, Hawaii, and Alaska are commonly hit by regional tsunami. Hawaii, for example, has been hit repeatedly during this century, about every 5–10 years. One of the worst was the April 1, 1946, tsunami that destroyed the city of Hilo.

Pacific-wide tsunami are the least common as only 3.5% of tsunami are this large, but they can cause tremendous destruction due to the massive size of the waves. In 1940 and 1960, destructive Pacific-wide tsunami occurred. More recently, there was a Pacific-wide tsunami on October 4, 1994, which caused substantial damage in Japan with 11.5 ft (3.5 m) waves. However, waves of only 6 in (15 cm) over the normal height were recorded in British Columbia.

Tsunami are not only a modern phenomenon. The decline of the Minoan civilization is believed to have been triggered by a powerful tsunami that hit the area in 1480 b.c. and destroyed its coastal settlements. Japan has had 65 destructive tsunami between a.d.684 and 1960. Chile was hit in 1562 and Hawaii has a written history of tsunami since 1821. The Indian and Atlantic Oceans also have long tsunami histories. Researchers are concerned that the impact of future tsunami, as well as hurricanes, will be worse because of intensive development of coastal areas in the last 30 years.

The destructive 1946 tsunami at Hilo, Hawaii, caused researchers to think about the problem of tsunami prediction. It became clear that if scientists could predict when the waves are going to hit, steps could be taken to minimize the impact of the great waves.

In 1965, the Intergovernmental Oceanographic Commission of the United Nations Educational, Scientific, and Cultural Organization agreed to expand the United States' existing tsunami warning center at Ewa Beach, Hawaii. This marked the formation of the Pacific Tsunami Warning Center (PTWC), which is now operated under the U.S. Weather Service. The objectives of the PTWC are to "detect and locate major earthquakes in the Pacific basin; determine whether or not tsunami have been generated; and to provide timely and effective information and warnings to minimize tsunami effects."

The PTWC is the administrative center for all the associated centers, committees, and commissions of the International Tsunami Warning System (ITWS). Japan, the Russian Federation, and Canada also have tsunami warning systems and centers and they coordinate with the PTWC. In total, 27 countries now belong to the ITWS.

The ITWS is based on a world-wide network of seismic and tidal data and information dissemination stations, and specially trained people. Seismic stations measure movement of the earth's crust and are the foundation of the system. These stations indicate that some disturbance has occurred that may be powerful enough to generate tsunami. To confirm the tsunami following a seismic event, there are specially trained people called tide observers with monitoring equipment that enables them to detect differences in the wave patterns of the ocean. Pressure gauges deployed on the ocean can detect changes of less than 0.4 in (1 cm) in the height of the ocean, which indicates wave height. Also, there are accelerometers set inside moored buoys that measure the rise and fall of the ocean, which will indicate the wave speed. These data are used together to help researchers confirm that a tsunami has been generated. Tsunami can also be detected by satellite monitoring methods such as radar and photographic images.

The ITWS is activated when earthquakes greater than 6.75 on the Richter scale are detected. The PTWC then collects all the data, determines the magnitude of the quake and its epicenter. Then they wait for the reports from the nearest tide stations and their tide observers. If a tsunami wave is reported, warnings are sent to the information dissemination centers.

The information dissemination centers then coordinate the emergency response plan to minimize the impact of the tsunami. In areas where tsunami frequency is high, such as Japan, the Russian Federation, Alaska, and Hawaii, there are also Regional Warning Systems to coordinate the flow of information. These information dissemination centers then decide whether to issue a "Tsunami Watch," which indicates that a tsunami may occur in the area, or a more serious "Tsunami Warning," which indicates that a tsunami will occur. The entire coastline of a region is broken down into smaller sections at predetermined locations known as "breakpoints" to allow the emergency personnel to customize the warnings to account for local changes in the behavior of the tsunami. The public is kept informed through local radio broadcasts. If the waves have not hit within two hours of the estimated time of arrival, or, the waves arrived but were not damaging, the tsunami threat is assumed to be over and all Watches and Warnings are canceled.

One of the more recent changes in the ITWS is that the Regional Centers will be taking on greater responsibility for tsunami detection and warning procedures. This is being done because there have been occasions when the warning from Hawaii came after the tsunami hit the area. This can occur with local and regional tsunami that tend to be smaller in their area of effect. Some seismically active areas need to have the warning system and equipment closer than Hawaii if they are to protect their citizens. For example, the Aleutian Islands near Alaska have two to three moderate earthquakes per week. As of May 1995, centers such as the Alaska Tsunami Warning Center located in Palmer, Alaska, have assumed a larger role in the management of tsunami warnings.

In terms of basic research, one of the biggest areas of investigation is the calculation of return rates. Return rates, or recurrence intervals, are the predicted frequency with which tsunami will occur in a given area and are useful information, especially for highly sensitive buildings such as nuclear power stations, offshore oil drilling platforms, and hospitals. The 1929 tsunami in Newfoundland has been studied extensively by North American researchers as a model for return rates and there has been some dispute. Columbia University researchers predict a reoccurrence in Newfoundland in 1,000–35,000 years. However, some geologists argue that it may reoccur as soon as 100–1,000 years. These calculations are based on evidence from mild earthquakes and tsunami in the area. They also suggest that the 1929 tsunami left a sedimentary record that is evident in the soil profile, and that such records can be dated and used to calculate return rates. Research is currently ongoing to test this theory.

See also Seismology; Wave motions