Tsunamis and earthquakes

A tsunami is a series of traveling ocean waves of extremely long length and depth generated by violent underwater disturbances associated primarily with earthquakes. The mass movement of water represented by a tsunami poses a significant danger to low-lying coastal regions, particularly along the Pacific Rim.

Undersea Environment

While other catastrophic events can trigger a tsunami, undersea earthquakes are responsible for the majority. Many tsunamis occur around the Pacific Rim in regions designated by geologists as subduction zones, where the dense earthic crust of the ocean floor slips below the lighter continental shelf crust and into the earth's mantle. Subduction zones are most common along the west coasts of North and South America and the coasts of Japan, eastern Asia, and the Pacific island chains. Another subduction zone is located in the Caribbean Sea but is not considered as active as those in the Pacific.

Scientists believe the sudden movement in the sea floor during an earthquake disrupts the equilibrium of the overlying expanse of water, raising or lowering enormous amounts of it all the way from the seafloor to the surface. Once the seafloor settles into its new position, it has nowhere to go until the next disturbance. However, the water mass above it is still subject to the downward pull of gravity. Consequently, as the swell of water returns to its original position, the water around it is pushed up, creating a rippling effect or series of waves called a tsunami. The primary factor in determining the size of the tsunami is the amount of seafloor uplifted during the undersea disturbance. A side-to-side movement of the seafloor is unlikely to cause a severe tsunami. For example, the San Andreas fault does not generate tsunamis, since its primary movements are horizontal.

Tsunami waves are not to be confused with tidal waves, which are simply the movements of water associated with the rise and fall of tides generated by the gravitational pull of the sun and moon. Since they often are the result of a sudden movement in the earth's crust, tsunamis often are referred to as seismic sea waves. This can be somewhat misleading because the term “seismic” indicates an earthquake-related event, when other natural phenomena, such as landslides, volcanic eruptions, or even meteor strikes, can also generate tsunamis, albeit on a much less frequent basis.

Wave Movement

The waves that form a tsunami are different from the surface waves observable from a beach. The latter are produced by winds blowing over the surface of the sea, and their size is directly dependent on the strength of the wind that creates them. The distance between these waves can range from a few centimeters to nearly 300 meters, though the normal separation is about 9 to 18 meters. The speed of the common surface wave can range from a few kilometers per hour to more than 90 kilometers per hour, with the lapse of time between two successive waves running from about five to twenty seconds.

Unlike the common surface wave, the tsunami wave is categorized as a shallow-water wave because of its extensive wavelength, which can stretch up to 500 kilometers and run for a period of ten minutes to two hours at speeds up to 800 kilometers per hour, depending on the depth of water in which it is traveling. A wave is classified as a shallow-water wave when the ratio between the water depth and the wavelength becomes very small. The formula for making the determination is as follows. The speed of a shallow-water wave is equal to the square root of the product of the acceleration of gravity (9.8 centimeters per second per second) and the depth of the water in meters. For a depth of 100 meters, this gives a speed of 99 m/s, or 360 km/hr. This formula enables seismologists to alert coastal communities of a potential tsunami following an earthquake and highlights that the rate at which a wave runs out of energy is inversely related to its wavelength. Because of its long wavelength, a tsunami can travel thousands of kilometers across ocean waters without dissipating; in contrast, the average surface wave begins to lose its energy after a distance of a few kilometers. In 1960, an undersea earthquake off the coast of Chile generated a tsunami that had enough lasting power to kill 150 people in Japan, following an earlier strike on the Hawaiian Islands, where it killed 51 people.

Tsunamis are able to travel great distances and reverberate through an ocean for extended periods as they bounce between continents. At up to 800 kilometers per hour, a tsunami can travel across an ocean at about the same speed as a jet aircraft. The swell of the waves is of such magnitude that it takes only a few surges and collapses for it to span the sea. The tsunami is barely discernible to the naked eye since the crest of one of its waves represents only the tip of the vast amount of water that extends deep into the ocean. Because of the depth of the tsunami, its course can be altered by undersea mountain ranges, valleys, or other landforms in its path.

Wave Transformation

As a tsunami approaches land, it undergoes a major transformation. While its total energy level remains constant, it begins to slow from the friction it encounters in the shallow waters above the continental shelf. The process, called wave refraction, occurs when the tsunami's wave train travels in shallow water and begins to move at a slower pace than the portion still advancing in deeper water. As its speed decreases, the wave's height grows because of a “shoaling” effect in which the trailing waves pile onto the waves in front of them. At this stage, the tsunami takes on a more visible appearance, as its waves often reach heights of 9 meters or more. The incoming waves approach much like an incoming tide, except at a much faster pace. The maximum vertical height a wave reaches in relation to the sea is called a “run-up.” The maximum horizontal distance attained by a wave is termed an “inundation.” The contours of local reefs, bays, mouths of rivers, and undersea features, as well as the angles of beaches, can have a significant effect on the shape and impact of the tsunami as it nears land.

Tsunami waves normally do not curve and break like common surface waves. Survivors of tsunamis often describe them as walls or plateaus of water being driven by what appears to be the whole weight of the ocean behind them. Although greatly drained of their energy, the waves retain sufficient momentum to wash away nearly everything in their path, including buildings, houses, and trees. Tsunamis can cause enormous erosion, stripping beaches of sand and vegetation that has taken years to accumulate. The fact that a tsunami consists of a series of waves poses a hidden danger to coastal residents. In some cases, curious onlookers and homeowners have returned to an exposed area following the initial wave only to be overwhelmed by a succeeding one.

Damage from tsunamis generally falls into three categories—inundation of coastal structures resulting from rapid flooding, destruction of buildings and beaches caused by water velocities, and a combination of the two in which velocity and flooding result in a complete tidal-like inundation. The most destructive form of tsunami is one that transforms itself into a bore, a concentrated wave of great force created when the tsunami moves from deep water into a well-defined shallow bay or river. This was the case during the 1960 Chilean tsunami when one of its waves struck Hilo Harbor in Hawaii as a high-velocity bore.

Destructive tsunamis strike somewhere in the world an average of once or twice each year, with most occurring in the Pacific Basin. During the four-year period from 1992 to 1996, seventeen recorded tsunamis in the Pacific claimed 1,700 lives. Thirteen major tsunamis hit the Hawaiian Islands during the twentieth century; all were generated by earthquakes along the Pacific Basin. The largest recorded wave heights were nearly 16 meters on the islands of Hawaii and Molokai in 1946, as a result of an earthquake off the Aleutian Islands that registered 7.1 on the Richter scale. The waves from this tsunami crested at about fifteen-minute intervals.

Historical Record

Since scientists are unable to predict exactly when earthquakes will occur, they cannot determine the precise moment when a tsunami will be generated. However, with the aid of historical records and numerical models, they can predict where they are most likely to occur. In addition, scientists have deployed sensors on the floor of the Pacific Ocean along the Aleutian Islands and the Pacific Northwest to enhance advance-warning systems. The Pacific Tsunami Warning System (PTWS) in Hawaii, established after the devastation of the 1946 tsunami, monitors seismological and tidal stations throughout the Pacific Basin to evaluate potential tsunami-causing earthquakes for the purpose of determining their direction and issuing adequate advance warnings.

For confirmation of past tsunamis, scientists have turned to geological evidence. Based on a technique called dendrochronology (the dating of trees by counting the ring patterns in their trunks), researchers discovered in the 1980s that a cataclysmic earthquake that struck the Pacific Northwest was the triggering mechanism for a giant tsunami that inundated the coastal Japanese island of Honshu in 1700. For some time, scientists suspected that an earthquake, centered somewhere along the Cascadia subduction zone—a fault that stretches from British Columbia, Canada, to Northern California—was to blame. They found their evidence in the marshlands along the Washington and Oregon coasts. Traces of a thin, unbroken sheet of sand were detected nearly 1 kilometer inland, indicating that a wave at least 9 meters high was likely to have hit the local coast. Calculations indicate that it would have taken an earthquake up to a magnitude 8, striking at high tide, to have produced such a wave. However, the energy of an earthquake of at least magnitude 9 would have been necessary to send waves of sufficient size across the Pacific to inflict substantial damage in Honshu. The link between the two events was established more firmly when researchers discovered that annual growth rings of drowned cedar trees and damaged spruce revealed they had died or were damaged around the same time of the Pacific Northwest waves and Honshu floods. The trees were located in regions where scientists have discovered geological evidence of previous earthquakes.

In 1998, an earthquake off the north coast of Papua New Guinea, registering a magnitude of 7.1, produced a tsunami that killed close to three thousand people at Sissano Lagoon with waves reaching 12 meters in height. Earthquakes of this size normally do not generate significant tsunamis, but researchers concluded that the earthquake probably occurred in relatively shallow water near the ocean floor and thus was able to induce a tsunami larger than normal. A later hypothesis was that the earthquake likely produced an undersea landslide that helped generate the giant waves. The lagoon is separated from the ocean by a narrow strip of land and represents an especially vulnerable section of the coast. Since the lagoon blocked the route inland, the families of the fishermen living on the sand bar had no way of escaping the waves. The entire coastal strip was swept clean by the tsunami, except for some stilts that the residents used to raise their houses off the sand.

In 2004, a 9.1-magnitude subduction earthquake off the west coast of Sumatra triggered a tsunami in the Indian Ocean. It produced wave heights reaching 30 meters, inundating coastlines of 14 countries and killing 230,000 people. The 2011 Tohuku earthquake in the Pacific Ocean caused a tsunami that caused widespread destruction on Japan's northeast coast. More than 15,600 people were killed.

In 2011, a 9.1 magnitude earthquake struck the Pacific coast of Japan, causing tsunamis as high as 40.5 meters. Between 2020 and March 2023, earthquakes which caused tsunamis occurred in Greece, New Zealand, Indonesia, Tonga, and Mexico.

Earthquake Magnitude

Many questions remain to be answered regarding the relationship between earthquake activity and the formation of tsunamis. For many years, it was assumed that the magnitude of the earthquake, as registered on the Richter scale, was the major factor in determining the size of the tsunami. It was also believed that the shock would have to register somewhere near the 7.4 range to have a major impact on wave generation. However, scientists have learned that the largest earthquakes do not always create the greatest seismic waves. Some earthquakes can release energy in subterranean settings where the earth's crust reacts slowly and with less violent convulsions than might be the case elsewhere.

Another assumption long held by scientists was that the highest waves of a tsunami were generated immediately following the disturbance. In 1992, however, the highest tsunamis measured along the Northern California coast arrived nearly six hours after a nearby earthquake. During the same year, an earthquake with a magnitude of 7 produced a series of giant waves that devastated a 320-kilometer stretch of the Nicaraguan coast, killing nearly 170 people and injuring 500 others. The earthquake generated only moderate shaking but shifted an estimated 193-kilometer stretch of sea floor nearly 1 meter in about two minutes. A slow-paced event of this type is very efficient in producing great amounts of water to supply the tsunami. The unusually large waves in the Nicaraguan incident resulted from the relatively shallow depth of the disturbance and an accompanying subterranean landslide. The waves arrived at some coastal regions only twenty minutes after the earthquake and struck in the evening, when most of the fishing boats were in dock, destroying or damaging many of them. Numerous homes and two schools also were destroyed. As in the Papua New Guinea disaster, the initial waves were relatively weak, misleading residents about the potential danger.

An example of a landslide-induced tsunami occurred in Skagway, Alaska, in 1994, when a large accumulation of sediment along the eastern edge of the harbor was loosened by a drop in the tide. An estimated one-third of the landmass involved in the landslide was situated above the water. The collapse produced a wave nearly 12 meters in height at the shore, killing a construction worker and causing extensive damage to a railroad dock.

Perhaps the most destructive of the volcanic-induced tsunamis was the one that occurred in 1883, when the volcano Krakatau, located between the islands of Java and Sumatra in Indonesia, erupted in a series of violent undersea explosions. The upheaval destroyed Krakatau and created gigantic waves as high as 35 meters that inundated towns and villages on nearby islands, killing more than 36,000 people. In a desperate maneuver, the crew of the ship Loudon aimed the boat's bow into the oncoming tsunami and managed to ride out the waves. Scientists have debated whether it was the submarine explosions, the slumping of the cone into the crater, or the occasional surges of matter falling into the water that caused the event. Waves from the Krakatau tsunami were recorded as far away as South America and Hawaii. Nearly nine hours after the event, the waves smashed boats resting in the harbor at Calcutta, India. When the tsunami reached Port Alfred in South Africa, it was still nearly 0.5 meter high. The English Channel even recorded a minor surge.

Significance

By exploring the relationship between earthquakes and tsunamis, scientists continue to learn more about the conditions that lead to a tsunami. The bottoms of the oceans, where most tsunamis originate, are largely unexplored. Only in the last few decades of the twentieth century did researchers begin to accumulate data in sufficient amounts to assess the destructive potential of the phenomenon, particularly regarding the near-shore tsunamis similar to the one that struck Papua New Guinea.

Scientists have determined that the hazard to American shorelines, especially along the heavily populated West Coast, is greater than previously thought. The likelihood that the earthquake that triggered the Papua New Guinea tsunami also generated an underwater landslide carries great significance for coastal settlements located close to the offshore seismic faults stretching from Northern California to the Aleutian Islands and near Hawaii. These areas feature canyons that slope deep into the ocean, making them vulnerable to the kinds of underwater landslides that occur in conjunction with moderate earthquakes. How a relatively small disturbance generates such large waves remains a key area for research. They take place only minutes from shorelines underscores the importance of public awareness of a tsunami's potentially destructive force.

Researchers have concluded that the technology utilized in early warning systems, from monitoring seismographs to measuring water-level changes at tide-gauging stations, cannot significantly alter the advance warning time for near-coastal tsunamis as it can for the disturbances that originate in deeper seas. Instead, with the aid of computer-generated models and historical records, researchers seek to provide a basis for educational programs designed to alert the general public about the regions where tsunamis are most likely to occur and about the impact that can be expected.

Principal Terms

continental shelf: the gently sloping surface that extends between the shoreline and the top of the continental slope, which slopes steeply to the deep ocean bed

earthic crust: the thin, outermost layer of the earth that varies in thickness from about 30 to 50 kilometers

ocean wave: a disturbance on the ocean's surface, viewed as an alternate rise and fall of the surface

Richter scale: the measurement of the magnitude of seismic disturbance (an earthquake) using the amplitude of seismic waves, named after American seismologist and physicist Charles Richter

seismic sea wave: an enormous wave in the ocean generated by an earthquake under the floor of the ocean or along the seacoast

subduction zone: the zone, at an angle to the earth's surface, down which an upper layer of oceanic or continental plate descends

wave refraction: the process by which the direction of waves moving through shallow water is altered by local submarine conditions

wavelength: the distance between the crest of one water wave to the crest of the next

Bibliography

Associated Press. The Associated Press Library of Disasters. Grolier, 1998.

Clague, John, et al. At Risk: Earthquakes and Tsunamis on the West Coast. Tricouni Press, 2006.

Dudley, Walter, and Min Lee. Tsunami! 2d ed. U of Hawaii P, 1998.

Folger, Tim. “Waves of Destruction.” Discover, vol. 15, May 1994, pp. 66-73.

Montastersky, Richard. “Waves of Death.” Science News, vol. 154, Oct. 1998, pp. 221-223. www.sciencenews.org/archive/waves-death. Accessed 20 Apr. 2023.

O'Loughlin, K. F., and James F. Lander. Caribbean Tsunamis: A 500-Year History from 1498-1998. Kluwer Academic Publishers, 2010.

Prothero, Donald R. Catastrophes!: Earthquakes, Tsunamis, Tornadoes, and Other Earth-Shattering Disasters. Johns Hopkins UP, 2011.

Satake, Kenji, and Fumihiko Imamura, eds. Tsunamis 1992-1994: Their Generation, Dynamics, and Hazards. Birkhäuser, 1995.

Sutton, Gerard K., and Joseph A. Cassalli, eds. Catastrophe in Japan: The Earthquake and Tsunami of 2011. Nova Science Publishers, 2011.

Tsuchlya, Yoshito, and Nobuo Shuto, eds. Tsunami: Progress in Prediction, Disaster Prevention, and Warning. Springer, 2011.

"Tsunami Generation: Earthquakes." NOAA, 10 Apr. 2023, www.noaa.gov/jetstream/tsunamis/tsunami-generation-earthquakes. Accessed 16 Apr. 2023.