Tsunamis

A tsunami is a series of potentially catastrophic ocean waves caused by a sudden, high-volume displacement of water. They are also sometimes called tidal waves, although scientists discourage that name because such waves are unrelated to normal ocean tides. Instead, they are most commonly caused by the movement of the sea floor associated with large earthquakes or volcanic eruptions. Tsunamis have resulted in thousands of deaths, primarily around the Pacific Ocean.

Causes of Tsunamis

Giant ocean waves capable of devastating coastal areas have been known and feared throughout history, especially by communities around the Pacific Ocean. Once incorrectly called tidal waves or seismic sea waves, these destructive waves are now generally known as tsunamis (after the Japanese term for "harbor wave"). In the simplest terms, a tsunami is a series of waves usually caused by the violent movement of the sea floor.

The movement at the sea floor that causes the tsunami can be produced by three different types of violent geologic or seismic activity. By far the most common and, therefore, the most important of these is submarine faulting, which occurs when a section of the ocean floor is thrust upward or suddenly drops at a subduction zone fault. Such fault movements are accompanied by earthquakes. Probably the second most common cause of tsunamis is a sudden landslide by which a mass of rock and earth falls suddenly into a body of water. A tsunami may be generated by a landslide starting above sea level and then plunging into the sea or by a submarine landslide. The highest tsunami waves ever officially reported were produced by a landslide at Lituya Bay, a confined fjord in Alaska, on July 9, 1958. A massive rockslide at the head of the bay produced a tsunami wave that attained a high-water mark more than 500 meters above the shoreline.

The third major cause of tsunamis is nearshore or submarine volcanic activity. In most cases, the flank of a volcano is suddenly uplifted or depressed, producing a tsunami in much the same way as faulting activity does. However, tsunamis have also been produced by the actual explosion of submarine or shoreline volcanoes. In 1883, the violent explosion of the famous island volcano Krakatau sent tsunami waves as high as 40 meters, crashing ashore in Java and Sumatra, killing more than 36,000 people.

Although tsunamis caused by landslides or volcanic activity may become very large near their sources and may cause great damage there, they have relatively little energy. They decrease rapidly in size in the deep waters of the open ocean, becoming small or even unnoticeable at any great distance. As they approach land and enter shallower waters again, their size increases according to the kinetic energy that they have retained. The giant tsunami waves that cross entire oceans are almost all caused by submarine faulting associated with large subduction earthquakes. Most tsunamis occur in the Pacific Ocean because the Pacific Ocean basin is surrounded by a zone of very tectonically active features, such as deep ocean subduction trenches, explosive volcanic islands, and dynamic mountain ranges.

Characteristics of Tsunamis

Tsunami waves are very different from ordinary ocean waves, most of which are caused by the wind blowing over the water. Wind-generated waves rarely have a wavelength (distance from crest to crest) greater than 300 meters. Tsunami waves may have a wavelength of as much as 160 kilometers. Wind-generated ocean surface waves never travel at more than about 100 kilometers per hour and are usually much slower. In the deep water of an ocean basin, tsunami waves may travel at 800 kilometers per hour but are often only 50 centimeters high, thus passing unnoticed by ships at sea.

A tsunami is not a single giant wave but rather consists of a series of several waves, perhaps ten or more, that form what is called a tsunami wave train. These individual waves follow one behind the other, between five and ninety minutes apart. When tsunami waves move into shallower water and approach the shore, they start to change. The shape of the nearshore sea floor has an effect on how tsunami waves behave. The waves tend to be smaller near small, isolated islands, where the bottom drops away quickly into deep water. Near large islands, such as the main Hawaiian Islands, the waves are strongly influenced by the bottom; they bend around the land and may be reflected from the shoreline. The reflected waves may augment other waves and create extremely large wave heights in unexpected places. As video records of the 2011 tsunami that struck Japan indicate, the tsunami wave does not appear like a typical wind-driven wave breaking on the shore but more like an abrupt, massive increase in sea level as the water rushes inland.

Though it is not yet possible to accurately predict the production, size, or effects of tsunami waves as they arrive in coastal areas, prediction of the velocity of tsunamis is made possible by their great wavelength. To understand how the arrival time of a tsunami is determined, it is necessary to look at ocean waves in general. Oceanographers divide waves into various categories based on the relationship between wavelength and the depth of water through which they are passing. When the water depth is less than one-twentieth of the wavelength, the waves are known as shallow-water waves, and their speed is determined solely by the depth of water. Knowing the water depth, one can calculate the velocity of any shallow-water wave. Tsunami waves may have a wavelength of more than 160 kilometers, and if the water depth is less than one-twentieth of the 160-kilometer wavelength, or 8 kilometers, then the tsunami waves would be shallow-water waves. Most of the deep Pacific Ocean basin is less than 5 kilometers deep, so most tsunamis there are shallow-water waves. To determine the speed of such tsunamis, all that is necessary to know is the depth of the water through which they pass. In most of the deep Pacific Ocean, this speed works out to be around 725 kilometers per hour, but it will vary depending on the exact water depth along the path of the tsunami.

Effects of Tsunamis

As the waves approach the shore, frictional effects of contact with the sea floor begin to affect the tsunami waves, and they travel progressively more slowly, with the forward speed dropping to around 65 kilometers per hour. At this point, the wave height usually begins to increase dramatically. A tsunami wave that was 50 centimeters high at sea may reach a height of 10 meters or more at the shoreline.

Like storm waves, tsunami waves are often more severe on headlands, where the wave energy is concentrated. The presence of a well-developed coral reef off a shoreline also appears to have a strong effect on tsunami waves. The reef may serve to absorb a significant amount of the wave’s energy, reducing its height and the intensity of its impact on the shoreline. Unlike ordinary waves, however, tsunamis are often quite large in bays. In this way, tsunami waves somewhat resemble tides.

Another wave phenomenon may also be produced in bays when a tsunami strikes. The water in any basin, be it a small bay or a large sea, will tend to oscillate back and forth with a fixed period determined by the size and shape of the basin. This oscillation is known as a seiche. A tsunami wave may initiate a seiche, and if the following tsunami wave arrives in conjunction with the next natural oscillation of the seiche, the water may reach even greater heights than the tsunami waves alone by the wave property of constructive interference. Much of the great height of tsunami waves in bays may be explained by this constructive combination of a seiche wave and a tsunami wave arriving simultaneously.

The popular image of a tsunami wave approaching shore is a nearly vertical wall of water, similar to the front of a breaking wave in the surf. Actually, most tsunamis do not form such wave fronts; the water surface instead is very close to horizontal, and the surface itself moves up and down. The waves arrive like a very rapidly rising tide. Under certain circumstances, however, an arriving tsunami wave can develop an abrupt, steep front that will move inland at high speeds. This phenomenon, generally encountered only as a tidal phenomenon, is known as a bore.

Bores produced by tides occasionally occur in the mouths of rivers. Well-known examples occur on the Solway Firth and the Severn River in Great Britain, on the Petitcodiac River in Maine, near the mouth of the Amazon River in Brazil, and, most strikingly, on the Chientang River in China, where the bore may attain a height of nearly 5 meters. In place of the usually gradual rise of the tide, the onset of high tide is delayed by the outflow of the estuary and augmented by a funneling effect as the stream width becomes narrower. When the high tide does arrive, it comes in very quickly, and the rapidly moving wall of water is followed by a less steep, but still quite dramatic, rise in the water level, accompanied by swift upstream currents.

Because the height of tsunami waves is strongly influenced by the submarine topography and shape of the shoreline and by reflected waves, and because they may be further modified by seiches, tides, and wind waves, the actual inundation and flooding produced by a tsunami may vary greatly from place to place over even a short distance. Though the image of a bore is the most dramatic—and such a wall of water can raze nearly everything in its path—it is the flooding and backwash effect of a tsunami that typically cause the most damage. Two different terms are often used to describe the extent of tsunami flooding: "inundation" and "run-up." Inundation is the depth of water above the normal level and is usually measured from sea level at average low tide. Inundation may be measured at any location reached by the tsunami waves. Run-up is the inundation at the maximum distance inland from the shoreline reached by the tsunami waters.

The withdrawal of the tsunami waves can cause significant damage. As the water rapidly withdraws toward the sea, the force of its movement scours out bottom sediments, undermines the foundations of buildings, and carries almost everything in its path out to sea. Entire beaches have been known to disappear as the sand is carried out to sea by the withdrawing tsunami waves, as have thousands of people, cars, trucks, houses, boats, and any other material caught in the retreating backwash.

Study of Tsunamis

The best way to learn about tsunamis is to study the tsunami waves themselves. Much of this work is carried out in the Hawaiian Islands because of their susceptibility to tsunamis from all parts of the Pacific Ocean basin and because Hawaii is the headquarters of the Pacific Tsunami Warning System. Because tsunamis are, fortunately, not everyday events, it is imperative to collect the most information possible from each occurrence. When a tsunami is en route to the Hawaiian Islands, trained observers head toward preselected shoreline vantage points, where time-lapse surveillance cameras are set up to film the waves. Portable tsunami gauges are deployed from piers and in designated shoreline areas. These gauges sense the change in water pressure as the waves pass over them and record their measurements.

Patrol aircraft take to the air before the arrival of the first waves. The aircraft fly at an altitude of 300 meters in a racetrack pattern over critical shoreline areas, covering the same spot every fifteen minutes. Special cameras mounted in the belly of each plane document the arrival of each tsunami wave on the shores of the islands.

After a tsunami, a ground survey and damage assessment are conducted by a team from the US Army Corps of Engineers, which is joined by volunteers from the American Society of Civil Engineers. A post-tsunami aerial photographic survey is undertaken jointly by the Navy, the Coast Guard, and the Civil Air Patrol, using National Weather Service surveillance aircraft.

Laboratory techniques have also been employed to study tsunami waves. Much of the damage caused by tsunamis results from the run-up of the giant waves on shore. Various modeling techniques have been used to try to simulate the run-up phase. One such technique, called hydraulic modeling, uses a physical scale model. Hilo Bay, on the island of Hawaii, is particularly sensitive to tsunami waves. After the disastrous tsunami of 1960, a hydraulic model of the bay was constructed. The model measured 25 by 19 meters and represented the triangular shape of Hilo Bay. Model tsunami waves were produced by releasing water from large tanks according to a program. Experiments with this hydraulic model have shown that almost any wave that enters the bay either hits downtown Hilo directly or is bounced off the northeast coast into the town. Sometimes the direct and reflected waves interact constructively to produce especially large waves in the center of the bay.

Another type of model used to study tsunamis is the numerical model, which uses high-speed digital computers to calculate mathematical simulations of tsunami waves. The models, however, are only as accurate as the data on which they are based, and these data can only come from measurements and observations of actual tsunamis. An improvement in data collection has been the use of satellites to transmit data from remote tide stations.

Satellite-telemetered tide stations have been installed at some twenty-five sites across the Pacific. These stations operate on their own independent power sources, secure from electricity outages that may result from earthquakes. Sea-level measurements are made every two seconds and averaged over a three- or four-minute interval, and the data are routinely transmitted by satellite to the Pacific Tsunami Warning Center every three to four hours. In the event of a tsunami wave, however, an "event detector" almost instantaneously sends a message to the warning center over a special emergency satellite channel.

Beginning in the 1990s, even more sophisticated measuring devices were developed, including devices that do not have to be installed on land. A warning system was proposed that would be based on the detection of tsunamis by highly sensitive bottom-pressure gauges located on the midocean sea floor. To study how tsunami waves change upon entering shallow water, an observational program was conducted off the Galápagos Islands, with instruments at depths of 3,000 meters, 10 meters, and 1 meter. Such testing eventually led to the creation of Deep-ocean Assessment and Reporting of Tsunami (DART) technology placed on buoy stations. The 2004 Indian Ocean earthquake and tsunami led to the expansion of the DART system, with the United States completing its buoy array in 2008 and other countries establishing their own programs.

In addition to seafloor tsunami detectors, seismographs have been deployed on the ocean bottom. Japanese scientists successfully operated a permanent ocean-bottom seismograph system off the southern coast of central Honshu. Attached to their seismographs is a tsunami gauge. The most advanced tsunami warning systems use both shore-based and ocean-bottom earthquake and tsunami sensors that transmit real-time data via satellite to the warning center, where computers provide scientists with the information they need to issue confirmed tsunami alerts. These efforts have increased knowledge of tsunami wave generation, propagation, and run-up, though the waves are still not fully understood, and research continues. Scientists used these methods of study to better understand tsunami behavior after the 2022 eruption of Hunga Tonga-Hunga Ha’apai, an underground volcano in the island nation of Tonga that caused a tsunami. The continued study of such instances helps scientists better understand the science behind tsunamis and plan for future disaster mitigation techniques.

Principal Terms

• bore: an unusually large advancing wave of water leading a sudden rise in water level that may be produced by the rising tide, a tsunami, or a seiche in an estuarine stream
• seiche: an oscillation in a partially enclosed body of water, such as a bay or estuary
• tsunami warning: the second phase of a tsunami alert; it is issued after the generation of a tsunami has been confirmed
• tsunami watch: the first phase of a tsunami alert; it is issued after a large earthquake has occurred on the sea floor

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