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

tsunamis The word ‘tsunami’ is derived from the Japanese words meaning ‘harbour wave’. Often described as tidal waves, although they have nothing to do with tides, tsunamis are in fact generated by offshore earthquakes, submarine landslides, and occasionally by undersea volcanic activity. In each case, water disturbance is created by large-scale underwater displacement of sediment or rock on the sea bed, usually as a result of a fault or a landslide. The initial water movement is often characterized by a rapid drawdown or lowering of the sea surface at the coast as the water moves into the area of sea-bed displacement. Thereafter, large kinematic waves are propagated outwards from the zone of sea-bed disturbance, travelling across the ocean at very high velocities, often in excess of 450 km per hour, and possessing very long wavelengths and periods. At the coast, the flood level or run up associated with a tsunami is partly a function of the dimensions of the propagated waves, but it is also greatly influenced by the topography and bathymetry of the coastal zone. The waves can reach considerable elevations and can cause widespread destruction and loss of life.

The geological processes associated with tsunamis are poorly understood. Remarkably, most Earth science textbooks do not consider the geology and geomorphology of tsunamis. Tsunamis, however, produce unique coastal landforms that are characterized by the effects of erosion and deposition of large magnitude. They frequently deposit boulder accumulations, and in many coastal areas, run-up processes commonly result in the deposition of continuous and discontinuous sheets of sediment. Many earthquake-generated tsunamis are also associated with the flooding of coastlines caused by subsidence associated with the earthquakes (coseismic subsidence). On the sea bed, tsunamis can also result in the remobilization of sediments and the formation of distinctive strata (so-called tsunamites).

Until recently, knowledge of the long-term frequency of tsunami flooding along individual coastlines was based simply on whether or not the occurrence of former tsunamis was described in historical records. Thus, for example, the record of documented tsunamis for Hawaii extends only to 1850 ad. Before this date, the return frequency of tsunamis is not known, since no historical record exists, yet many tsunamis are likely to have occurred. In recent years, it has proved possible to investigate the occurrence of prehistoric tsunamis for individual areas because many ‘palaeotsunamis’ have been associated with the deposition of sediment in coastal areas subject to flooding. In this way it has been possible to calculate the number of tsunamis that have taken place in particular areas over timescales of 10⁴–10⁵ years. For example, geological research in Japan has provided a chronology of past tsunami flooding for the last 4000 years.

During the early 1990s a number of large tsunamis occurred in various parts of the world. Studies of the geomorphological processes associated with these floods have provided a unique opportunity to understand processes of tsunami erosion and deposition. The most important tsunamis that have taken place during this period include a major tsunami that struck the Pacific coastline of Nicaragua and Costa Rica during 1991 and a very destructive tsunami on the island of Flores, Indonesia on 12 December 1992. More recently, destructive tsunamis have struck on two occasions in Hokkaido in northern Japan and in the neighbouring Kurile Islands, Russia, as well as on three occasions in Java, Indonesia, and in the Philippines. Data on coastal flooding resulting from these tsunamis has bee used to develop models of individual tsunamis. In most instances, this type of numerical modelling has been used to reconstruct the properties of the offshore earthquakes and sea-bed faulting that generated the tsunami waves. In general, these studies have shown that the observed flood run-up at the coast is much greater than the run-up values predicated by the mathematical models.

Not all tsunamis are generated by offshore earthquakes. As mentioned above, many are triggered by submarine sediment slides. One of the world's largest areas of submarine slides is located west of the coast of Norway in the Norwegian Sea. This area, known as the Storegga area, has been the site of three exceptionally large underwater slides during the past 30 000 years. Each of these submarine slides is believed to have generated an exceptionally large tsunami. The best known of these is a slide that took place approximately 7000 years ago and involved the movement of approximately 1700 km³ (cubic kilometres) of debris from the continental slope to the abyssal plain east of Iceland. The tsunami generated by this slide produced flooding up to 10 m above former sea level along parts of the west coast of Norway; it also caused severe flooding along the northern and eastern coastlines of Scotland and as far south as the present location of Amsterdam.

What was probably the world's largest tsunami took place in the Pacific basin approximately 105 000 years ago. This tsunami was generated by a large underwater slide south of the island of Lanai in Hawaii. The tsunami is presently believed to have caused flooding up to approximately 360 m above former sea level on Lanai. It has also been proposed that the tsunami waves reached up to 20 m, above sea level along the New South Wales coastline of eastern Australia.

Recognition that underwater slides are capable of generating large tsunamis has, not surprisingly, rendered the task of mathematical modelling of tsunamis much more difficult. More importantly, it has made it very clear that it is a mistake to assume that all major tsunamis are solely the products of offshore earthquakes. The clearest example of this is the well-known meteorite impact that took place in central America approximately 65 million years ago (the so-called K/T impact) that is commonly associated with the extinction of the dinosaurs. A number of Earth scientists believe that this meteorite impact also generated an extremely large tsunami that caused severe flooding throughout many coastal areas. Some scientists have gone so far as to suggest that this tsunami might have caused flooding throughout the entire world at this time.

As a result of the loss of life and damage to property caused by tsunamis in the Pacific, attempts have been made to develop a tsunami warning system, the purpose of which is to alert the public in advance of the arrival of individual tsunamis. Tsunami warning systems did not exist for the Pacific prior to the Aleutian trench earthquake of 1 April 1946. A tsunami warning system network was eventually established by the United States Coast and Geodetic Survey with its headquarters on the Island of Oahu, Hawaii. The centre, known as the Pacific Tsunami Warning Centre (PTWC), became operational in 1948 and is now linked to over 30 seismological stations throughout the Pacific Basin. These provide data on Pacific earthquakes whose magnitude and epicentres make them tsunamigenic (capable of producing tsunamis). Once an earthquake has taken place, the PTWC issues a ‘tsunami watch’ to all receiving stations. In addition, the initial registration of a tsunami on tide gauges is relayed to the PTWC and, once the estimated times of tsunami arrival are computed for different coastal areas, a tsunami warning is issued.

The accuracy of PTWC tsunami warnings is exemplified by the Chilean earthquake and subsequent tsunami of 21 May 1960. Once the earthquake epicentre had been determined and tide-gauge data analysed, it was predicted that the tsunami would reach the Hawaiian islands 14 hours 56minutes after its generation off the Chilean coast. The prediction was that the first wave would strike Hilo, Hawaii at 9.57 p.m. It arrived one minute later.

Because tsunamis are less frequent outside the Pacific region, it might be argued that the cost-effectiveness of separate tsunami warning systems for other areas is very low. Until recently, this was the case in Portugal. On 1 November 1755 ad, one of the highest-magnitude earthquakes documented for Europe led to the complete destruction of the city of Lisbon, as well as causing widespread damage and loss of life across a coastal zone stretching from northern Portugal to southern Morocco. In Lisbon alone, some 50 000 people were killed. Most of them were drowned by the resulting tsunami, which reached a height of 17 to 20 m in many areas. In recent decades, two minor tsunamis have struck the coast of Portugal, one on 25 November 1941 and the other on 28 February 1969. The Portuguese authorities, aware of the risk posed by tsunamis, subsequently decided to instal seismometers and wave recorders west of Portugal with the aim of providing advance warning of any future tsunami comparable to the one that struck Lisbon in 1755. In most other areas of the world (apart from the Pacific), however, coastal populations have no protection from tsunamis. They rely instead on the expectation that no tsunamis will ever strike the particular coastlines in which they live.

Alastair G. Dawson

tsunami, a Japanese term (tsu, harbour, nami, wave) for waves triggered by earthquakes, landslides, volcanic eruptions, or large meteorites splashing down in the ocean. They are sometimes mistakenly called tidal waves but they are nothing to do with tides. They are very large scale versions of the ripples that radiate out when a stone is dropped into water. They occur most frequently in the Pacific Ocean where major earthquakes are common occurrences along the ocean's active margins where the oceanic plates are slipping down below the continents. So the disaster in the Indian Ocean on Boxing Day 2004 was unexpected. Associated with the major plate boundary between the Indian and Eurasian tectonic plates, it was triggered by a major earthquake over more than 100 kilometres (63 mls.) of seabed off the north-west coast of Sumatra. Its epicentre was in deep water at 3.32° N., 95.85° E. and measured 9.0 on the Richter Scale, the fourth largest earthquake recorded for a century. The displacement of the seabed represented about 130 years of seafloor spreading (see geological oceanography), hence the violence of the shock. The fault line is orientated north/south so the waves spread more east/west; fortunately for vulnerable countries like Bangladesh it did not travel far in a northerly direction, otherwise the subsequent loss of life would have been even more shocking. Another factor was that the earthquake occurred only just below the seabed; if it had been deeper within the Earth's crust the shock waves would have stayed within the rocks and would not have radiated out in the ocean.

A tsunami is not one but a series of waves. Over deep water each wave is only about a metre high, and the distance between wave crests is over 100 kilometres (63 mls.). They travel at speeds of up to 700 kph (437 mph), about as fast as a passenger jet flies, but are almost impossible to detect out in the open ocean. The waves from the 2004 tsunami took just fifteen minutes to reach Sumatra, 30 minutes to reach the Andaman Islands and 90 minutes to reach Thailand, so even if a tsunami warning system had been in place, as it has been for some time in the Pacific, it is unlikely that it would have saved many lives in those countries. A warning system, which at the time of writing is due to be installed in 2005, may have helped save life in Sri Lanka, where the wave took two hours to arrive and the Maldives where it took three and a half hours. It took seven hours to reach East Africa. Television gave some warning of its approach and only one life was lost on the Kenyan coast. But there was no warning for Somalia and hundreds died.

A tsunami can cross the Pacific in a less than a day. In 1960 a powerful underwater earthquake off Chile, measuring 8.6 on the Richter scale, generated a tsunami that killed many people locally. Fifteen hours later it came ashore at Hilo in Hawaii. Its height was 10.7 metres (35 ft) and despite warnings it killed 61 people. This was mainly because, after the first wave, people began to return to their homes only to be drowned when the second wave struck the shore. In Hilo alone this tsunami caused damage costing $US24 million.

As the tsunami waves reach shallow water, the drag of the seabed slows them so their heights build up in excess of 10 metres (33 ft), even so they can be still travelling at speeds of over 100 kph (63 mph). They come ashore as a great river of water, so their destructive power is awesome. If they are funnelled into harbours or bays they can resonate back and forth like sound in a musical instrument, which increases their destructiveness. One interesting observation of the impact of the 2004 Indian Ocean tsunami was that coastlines with intact mangrove swamps and offshore coral reefs were far less seriously affected than those where the mangroves had been cleared and the coral reefs degraded, so the degradation of coastal habits led to greater damage and loss of life (see also environmental issues).

Tsunamis give little warning of their approach, but often the sea ebbs away from shallow bays leaving boats high and dry before the arrival of the first destructive wave within about half an hour. This first wave is usually followed by several more, and, as was experienced in Thailand on Boxing Day 2004, it is not always the first of the waves that is the largest. This tsunami immediately killed over 200,000 people and rendered millions homeless and exposed them to serious health risks and economic ruin. Previous to that the most destructive tsunami on record resulted from the explosion of Krakatoa, a volcanic island in the Sunda Strait between the south of Sumatra and Java. A large volcano had collapsed in ad 416, forming an underwater caldera—a cauldron-like cavity—and leaving a few remnant islands, one of which was Krakatoa. On 27 September 1883, after a long series of volcanic eruptions, the island exploded. It is believed that the eruptions had cracked open the magma chamber beneath the volcano and water had flowed in. The resulting explosion had a force estimated to be 20,000 times that of the Hiroshima atom-bomb, and caused the island to collapse back into the caldera. The resulting tsunami that came ashore in Sumatra and Java reached an estimated 40 metres (130 ft) in height, and it wiped out all the coastal communities along the shoreline, killing an estimated 30,000 people.

Another destructive tsunami was the one which hit Japan in 1896. Fishermen fishing out at sea off Sanriku were unaware that it had passed beneath them, but when they returned to port they found 28,000 people had been killed by the waves. Between 1995 and 2005 about 2,000 people were killed by tsunamis around the Pacific.

Recently concern has been expressed about the lack of stability of Cumbre Vieja, a volcano on the island of La Palma in the Canaries. Some, but not all, geologists, have speculated that if this volcano erupts its flanks may collapse and triggered a major underwater landslip. This might generate a mega-tsunami that would have a devastating impact on the eastern seaboard of North America. No such mega-tsunami has been experienced during historical times, but about 5,000 years ago during the Bronze Age there is archaeological evidence that a giant wave over-topped a number of Scottish islands wiping out settlements at least 65 metres (200 ft) above sea-level. This was probably triggered by a massive failure of the continental shelf off south-west Norway.

M. V. Angel

Bibliography

Bryant, E. , Tsunami: The Underrated Hazard (2001).
Simon Winchester, S. , Krakatoa: The Day the World exploded, August 27 1883 (2003).

Tsunamis

A tsunami is a powerful wave, usually created by a large-scale motion of the ocean floor. Although they are almost imperceptible at sea, tsunami waves increase in height as they reach a coastline and are capable of causing great destruction. The term "tsunami" is taken from the Japanese words for "harbor" and "wave."

In the 1990s, eighty-two tsunamis were reported worldwide, taking more than four thousand lives and causing hundreds of millions of dollars in damage. Most tsunamis occur in seismically active regions such as the Pacific Ocean, but tsunamis can occur anywhere in the world where there are large bodies of water.

Mechanics of Tsunami Generation and Propagation

A tsunami can be caused by any disturbance that moves a large amount of water. The vast majority of tsunamis originate during undersea earthquakes when water is moved by the uplift or subsidence of hundreds of square kilometers of the sea floor. Landslides (which often accompany large earthquakes), volcanic eruptions and collapses, and explosions and meteor impacts can also disturb enough water to generate a tsunami.

Propagation.

The wind-generated waves usually seen breaking on the beach arrive every 10 to 15 seconds and have wave crests tens of meters apart. In contrast, tsunamis can have crests that are more than 20 minutes and hundreds of kilometers apart (see figure).

Most tsunamis are classified as long waves—that is, waves with long wavelengths relative to their water depth. They travel with speeds proportional to the square root of the water depth. In the deep ocean, their speed can be similar to that of a jet plane, as high as 700 kilometers per hour. Closer to shore, in shallow water, they slow down appreciably. At sea, the height of a tsunami wave is not usually distinguishable from the surrounding wind waves without sensitive measuring equipment because the wave often is only 1 to 2 meters high and hundreds of kilometers long.

Tsunamis generated by earthquake movement of the seabed can travel thousands of miles across the ocean without losing their energy. For example, in 1960 a tsunami generated in Chile, South America caused substantial damage nearly 14,500 kilometers (9,000 miles) away in Japan. Hawaii, in the middle of the Pacific Ocean, is particularly susceptible to tsunamis that travel across the ocean.

Unlike earthquake-caused waves, tsunamis generated by mechanisms like landslides and eruptions dissipate quickly and rarely affect coastlines far away from the source. Their local effects, however, can sometimes be just as damaging: in 1883, the tsunami caused by the eruption of the volcano Krakatau killed more than 36,000 people on the nearby islands of Java and Sumatra.

Landfall.

As tsunami waves approach the coastline, a large change takes place in their shape. Since the landward portion of the wave is in shallower water than the seaward portion, it travels relatively slower. This allows rear portions of the wave to "catch up" with the front of the wave, concentrating the wave energy into ever-higher growing crests as it approaches land. The shoaling waves can reach crest heights of tens of meters, either breaking and flowing onto the shore as violent bores or surging onshore as flood waves.

When a tsunami reaches the shore, the impact can destroy buildings and other coastal structures. The flowing water can move boats, vehicles and debris. Further destruction results when these objects collide like battering rams with anything in their path. Gas lines broken during the tsunami often cause fires that increase the tsunami damage. Tsunamis can flood low-lying areas, destroying crops with salt water and leaving behind sand and boulders.

Mitigation and Research

Efforts to protect people from tsunamis center on proper preparation of tsunami-prone areas. Many lives have been saved when residents of coastal communities were aware that earthquake shaking was a signal to evacuate to high ground.

Although certain tsunamis, such as those generated by landslides, arrive without warning, tsunami researchers are focusing on better predicting these locally destructive waves as well as the transoceanic ones. The tools that researchers use include seismic stations, deep-ocean pressure gauges, and physical and numerical models. Field surveys of recent tsunamis and geological investigations of ancient waves also help scientists and hazards planners design structures and plan communities so that casualties and damage can be reduced.

see also Human Health and the Ocean; Landslides; Waves.

Catherine M. Petroff

Bibliography

Folger, T. "Killer Waves, the Struggle to Predict Tsunamis." Discover Magazine May 1994, 66–73.

Gonzalez, F. "Tsunami, Predicting Destruction by Monster Waves." Scientific American May 1999, 56–65.

McCredie, S. "Tsunamis, the Waves that Kill." Smithsonian Magazine March 1994, 28–39.

Internet Resources

Tsunami! University of Washington. <http://www.geophys.washington.edu/tsunami>.

USC Tsunami Research Group. University of Southern California. <http://www.usc.edu/dept/tsunamis>.

Tsunami Research Program. National Oceanic and Atmospheric Association. <http://www.pmel.noaa.gov/tsunami>.

TSUNAMIS ARE NOT TIDAL WAVES

At one time, tsunamis were called tidal waves for the way that the water flowed on and offshore like a quickly rising and falling tide. But because tsunamis are not caused by the gravitational pull of the Moon and Sun, the term tidal wave is no longer used.

SEA OF JAPAN: 1993

A 7.8-magnitude earthquake in the Sea of Japan caused waves 5 to 10 meters high that swept up buildings and vehicles on the island of Okushiri. Although 239 people died from the Okushiri tsunami, many residents saved themselves by fleeing to high ground immediately after the earthquake.

tsunami , series of catastrophic ocean waves generated by submarine movements, which may be caused by earthquakes , volcanic eruptions, landslides beneath the ocean, or an asteroid striking the earth. Tsunamis are also called seismic sea waves or, popularly, tidal waves.

In the open ocean, tsunamis may have wavelengths of up to several hundred miles and travel at speeds up to 500 mi per hr (800 km per hr), yet have wave heights of less than 3 ft (1 m), which pass unnoticed beneath a ship at sea. The period between the crests of a tsunami's waves varies from 5 min to about 1 hr. When tsunamis approach shallow water along a coast, they are slowed, causing their length to shorten and their height to rise sometimes as high as 100 ft (30 m). When they break, they often destroy piers, buildings, and beaches and take human life. The wave height as they crash upon a shore depends almost entirely upon the submarine topography offshore. Waves tend to rise to greater heights along gently sloping shores, along submarine ridges, or in coastal embayments.

There is little warning of approach; when a train of tsunami waves approaches a coastline, the first indication is often a sharp swell, not unlike an ordinary storm swell, followed by a sudden outrush of water that often exposes offshore areas as the first wave trough reaches the coast. After several minutes, the first huge wave crest strikes, inundating the newly exposed beach and rushing inland to flood the coast. Generally, the third to eighth wave crests are the largest.

Since tsunamis principally occur in the Pacific Ocean following shallow-focus earthquakes over magnitude 6.5 on the Richter scale , one of the best means of prediction is the detection of such earthquakes on the ocean floor with a seismograph network (see seismology ). Tsunamis may be detected by wave gauges and pressure monitors, such as those emplaced as part of the U.S. Tsunami Warning System; established in 1949 and originally confined to the Pacific region, the system has been expanded to the Caribbean and the W North Atlantic. An early warning system for the Indian Ocean began operating in 2006. Measurement of sudden sea level changes from satellites are also used to warn of a potential tsunami.

One of the most destructive tsunamis to occur during historical times followed the explosive eruption of the volcano Krakatoa in the East Indies on Aug. 27, 1883, when over 36,000 people were killed as a result of the wave. Waves were up to 100 ft (30 m) high. Its passage was traced as far away as Panama. On Dec. 26, 2004, a 9.1 earthquake off NW Sumatra, Indonesia, caused a tsunami with waves as high as 65 ft (20 m) nearest the epicenter. Some 230,000 people are believed to have died. The waves devastated many areas in the E Indian Ocean basin, particularly the nearby coast of N Sumatra, the Andaman and Nicobar Islands, and the E and S coasts of Sri Lanka. Areas of SE India and SW Thailand were also hard hit. The 9.0 earthquake off NE Honshu, Japan, on Mar. 11, 2011, caused a tsunami that devastated nearby areas on the Honshu coast. The water overtopped 33 ft (10 m) seawalls and in some locations reached places as far as 5 mi (8 km) inland. Most of the more than 20,000 killed or missing as a result of the earthquake were lost to the tsunami. It is believed that a 0.6-mi-wide (1-km-wide) asteroid that struck the ocean SW of New Zealand about AD 1500 created a tsunami that reached heights of more than 425 ft (130 m).

tsunami (seismic sea wave) Ocean wave caused by a submarine earthquake, subsidence or volcanic eruption. Sometimes erroneously called a tidal wave, tsunamis spread radially from their source in ever-widening circles. Tsunamis travel across oceans at speeds up to 400km/h (250mph) and reach heights of 10m (33ft). On December 26, 2004 a massive Tsunami, resulting from an earthquake near to Sumatra in the Indian Ocean, killed more than 250,000 people, mostly in Indonesia (particularly western Sumatra), Sri Lanka, India and Thailand.

tsunami A seismic sea wave of long period, produced by a submarine earthquake, underwater volcanic explosion, or massive gravity slide of seabed sediment. In the open ocean, such waves are barely noticeable even though they may be travelling at 700 km/h, but on reaching shallow water they build up to heights of more than 30 m and can cause severe damage in coastal areas.

tsunami A seismic sea wave of long period, produced by a submarine earthquake, underwater volcanic explosion, or massive gravity slide of sea-bed sediment. In the open ocean such waves are barely noticeable even though they may be travelling at 700 km/h, but on reaching shallow water they build up to heights of more than 30 m and cause severe damage in coastal areas.

tsunami a long high sea wave caused by an earthquake or other disturbance; the word is Japanese, and comes from tsu ‘harbour’ + nami ‘wave’.

On 26 December, 2004, an undersea earthquake in the Indian Ocean resulted in a tsunami which devastated coastal regions of Indonesia, Thailand, India, and Sri Lanka, causing great loss of life.

tsu·na·mi / (t)soōˈnämē/ • n. (pl. same or -mis ) a long high sea wave caused by an earthquake, submarine landslide, or other disturbance. ORIGIN: late 19th cent.: from Japanese, from tsu ‘harbor’ + nami ‘wave.’

tsunami •chamois, clammy, gammy, Grammy, hammy, jammy, mammae, mammee, mammy, Miami, ramie, rammy, Sammy, shammy, whammy •acme, drachmae •Lakshmi •army, balmy, barmy, gourami, macramé, origami, palmy, pastrami, salami, smarmy, swami, tsunami, Yanomami •Clemmie, Emmy, jemmy, lemme, semi •elmy •Amy, cockamamie, flamy, gamy, Jamie, Mamie, samey •beamy, creamy, dreamy, gleamy, Mimi, preemie, seamy, steamy •gimme, shimmy, Timmy •pygmy • filmy •arch-enemy, enemy •synonymy • Jeremy • sashimi •blimey, gorblimey, grimy, limey, slimy, stymie, thymy •commie, mommy, pommie, pommy, tommy •dormy, stormy •foamy, homey, loamy, Naomi, Salome •polychromy