Tsunamis are displacements of water that increase sea height and typically result in both a sea surge and large waves as the displaced water moves into the shore. Even without large waves, flooding from a tsunami can drive water inland, often causing major destruction in coastal regions. A tsunami can be caused by any event that rapidly displaces a large amount of water. Most commonly, tsunamis occur as a result of underwater seismic activity, such as an earthquake.


Though tsunamis are commonly termed tidal waves, this is an erroneous term; these potentially catastrophic waves have nothing to do with the tides. Tides are the up and down movements of the sea surface at the shore, and they are caused by the gravitational attraction of the moon and sun on our marine waters. Tides rarely cause major damage unless they are associated with a storm. Tsunamis, on the other hand, are caused by the movements of Earth's crustal plates, and they can cause major loss of life and property.

It has been known for several hundred years that tsunamis are caused by seismic movements of the ocean floor. Tsunamis occur most commonly during submarine earthquakes, underwater landslides, meteorite impacts, and volcanic eruptions. The sudden movement of Earth's crust caused by an underwater earthquake, for example, displaces or moves the water above it. This movement causes a high-energy wave to form, which then passes rapidly through the water.

Compiled photo shows aerial views from a Kyodo News helicopter of the Shishiori district of Kesennuma in Miyagi Prefecture on March 12, 2011

Compiled photo shows aerial views from a Kyodo News helicopter of the Shishiori district of Kesennuma in Miyagi Prefecture on March 12, 2011, the day after the city in northeastern Japan was hit by a massive tsunami (upper), and on Feb. 23, 2018, with new housing estates and shops standing on elevated ground.
(Kyodo via AP Images)

Tsunamis are more common in the Pacific Ocean than in the other oceans of the world primarily because there is so much seismic activity at the perimeter of the Pacific Ocean, where crustal plates meet. Thus, this is the region where some of the world's most damaging tsunamis have occurred. Tsunamis also occur along the chain of Caribbean islands and in the Mediterranean. Both of these places, like the Pacific, are at the edges of Earth's crustal plates where earthquakes and other seismic activity are common. Areas that are 25 feet (7.6 m) or less above sea level and are within one mile of the coast are vulnerable to tsunamis.

A tsunami is composed of a very long series of waves (commonly referred to as a tsunami wave train), with a period (the time for one complete wave to pass a fixed point) ranging from ten minutes to two hours. These waves typically travel 500 to 600 miles per hour (800 to 1,000 km per hour). The speed of a tsunami is affected by water depth. In places where the ocean is 3.7 miles (6,000 m) deep, tsunamis can travel over 500 miles per hour (800 km per hour), but in shallow, coastal waters they slow down and their wave heights increase significantly.

It is almost impossible to feel a tsunami out at sea in deep water, but the form of the wave changes when it reaches shallow water. Because the water is shallower, the bottom of the wave begins to collide with the ocean bottom. The friction that results causes the wave to slow down from about 450 miles per hour (200 meters per second) in very deep ocean water to 49 miles per hour (22 meters per second) in water 164 feet (50 m) deep. While the front part of the wave has been reduced in speed, the part at sea is still moving in quickly. As a result, the energy of the wave is compressed. As the wave enters shallow water, like that in a bay, the crest rises. It quickly builds vertically as the wave moves onto the shore. This wall of water can be more than 100 feet (30.5 m) high in extreme cases. Because gravity is acting on this huge wall of water, it cannot support itself and crashes or breaks onto the land, similar to a normal breaker in the surf zone of a beach. However, the huge amounts of energy released by a breaking tsunami are many times greater and more destructive than an ordinary breaker, and the tsunami can destroy anything in its path. Hazards resulting from tsunamis include damage to buildings and structures, flooding, drinking water contamination, and gas-line or tank fires. The majority of deaths caused by tsunamis are a result of drowning.

In Japan, where some of the most destructive tsunamis have occurred, there have been cases in which whole fishing villages were devastated by tsunamis. Oddly, when these tsunamis occurred, fishermen at sea did not feel the wave as it passed right under them. They did not discover the disaster until they returned home and found their homes and villages destroyed. Because these villages were often located within shallow bays, and the fishermen, being at sea, did not experience the wave, they assumed that the tsunami arose within the bay or harbor. Therefore, these waves were referred to as tsunamis, which means “harbor wave” in Japanese.

Early warning systems

Tsunamis are capable of traveling hundreds of miles per hour in the ocean and rising over 100 feet (30.5 m) or more upon reaching the coast. About once in a decade, a major tsunami occurs. Of the tsunami events on record, 59 percent have occurred in the Pacific Ocean, 25 percent in the Mediterranean Sea, 12 percent in the Atlantic Ocean, and 4 percent in the Indian Ocean. Tsunamis can occur at any location along most of the U.S. coastline, but the most damaging tsunamis in the United States have affected the coasts of California, Oregon, Washington, Alaska, and Hawaii.

While scientists are not yet able to predict submarine seismic activity with much accuracy, they can easily measure such events when they occur, and they use this information to predict when destructive sea waves will occur. This early detection is extremely important in reducing loss of life and property. It became clear that a warning system was needed to monitor seismic activity throughout the Pacific Ocean after a very destructive tsunami hit Hawaii in 1946. As a result of the 1946 tsunami, the Pacific Tsunami Warning Center (PTWC) was established in 1948 which is the headquarters for the International Tsunami Warning System. The geographical and administrative center of this monitoring system is in Honolulu, Hawaii.

Under the early warning system, when an earthquake, underwater volcano, or landslide is sensed, scientists quickly pinpoint its location. If the seismic activity generates a wave, the change in the water height is measured at a nearby tide-measuring station, and the scientists can then accurately calculate the speed of the wave to determine when it will make landfall. The appropriate agencies can be alerted, and, if necessary, evacuations and other preparations can be made. This early warning system has been very successful in reducing the death toll associated with tsunamis. For example, when a large tsunami struck Hawaii in 1957, the early warning system prevented any tsunami-related deaths, despite the fact that the tsunami was over 26 feet (8 m) tall.

Before the warning systems existed, the first indication of an approaching tsunami was the rapid movement of water in a bay out to sea. This exposed areas of the bay bottom that were rarely or never exposed. The water that rushed offshore rose to build the huge crest of the wave that would crash down a few minutes later.

Despite the success of warning systems, early detection information sometimes results in warnings when no tsunamis occur. For example, not all seismic activity generates tsunamis. Many result from shallow focus earthquakes, where the actual point of crustal movement is close to the surface and major crustal movement is likely. Deep focus earthquakes, which can be very strong but often result in less crustal movement, are less likely to trigger tsunamis. It has been estimated that only one out of ten large underwater earthquakes causes damage. In addition, the chances of a tsunami hitting any one spot directly and causing major damage are relatively small because the energy in the form of the tsunami is not passed along equally in all directions. Finally, other factors may reduce or enhance the chances of a tsunami striking. For example, major tsunamis are rare in regions with wide continental shelves, such as the Atlantic Coast of the United States. A wide continental shelf is thought to both reflect the wave (with the energy being sent back out to sea) and absorb some of the energy of the wave through friction as it drags along the bottom. Thus, the early warning system, while essential, often gives false alarms.

Human impact of historic tsunamis

One of the most dramatic and destructive tsunamis occurred on August 27, 1883, when the volcanic island of Krakatoa, located in the Pacific Ocean between Sumatra and Java, Indonesia, exploded and disappeared in a massive volcanic eruption. The eruption was so immense that the sound of the volcanic explosion was heard 3,000 miles (4,827 km) away, and the dust that entered the atmosphere caused climate change and unusual sunsets for almost a year. The volcanic eruption generated a wall of water that reached over 98 feet (30 m) in height and caused catastrophic damage in coastal areas in the Sundra Strait. Of the more than 36,000 people who lost their lives due to the volcano, an estimated 90 percent were killed by the huge tsunami. The energy from the tsunami was still measurable after it had crossed the Indian Ocean, moved around the southern part of Africa, and headed north through the Atlantic Ocean into the English Channel.

In 1896, a major tsunami in Japan killed 27,000 people along the coast. In 1964, an earthquake in Alaska, and the resulting tsunami, caused major damage in some ports such as Kodiak and Seward. In addition, the tsunami traveled to the south, where four and a half hours later, despite warnings, it killed additional people and did major damage in Crescent City, California. A total of 119 people died, and damage to property amounted to over $100 million.

The tsunami of December 26, 2004 (known as the Indian Ocean Tsunami or Boxing Day Tsunami), which was generated by a very powerful earthquake off the west coast of Sumatra, is the worst tsunami in modern records, killing an estimated 230,000 people in 11 countries—with 170,000 deaths in Indonesia alone. There was no warning system in place for tsunamis in the Indian Ocean in 2004. Having an early warning system in place may have saved many lives. In 2006, the UN/International Strategy for Disaster Reduction (ISDR) project, Building Resilience to Tsunamis in the Indian Ocean, was established to strengthen methods of disaster risk reduction.

Since the Indian Ocean Tsunami in 2004, many smaller tsunamis have occurred. In March 2005, a tsunami resulting from an 8.7-magnitude earthquake struck the coast of Sumatra, killing 1,300 people. In 2006, a 3.3-yards (3-m) high tsunami generated by a 7.7-magnitude earthquake hit the West Java province of Indonesia, resulting in the deaths of an estimated 550 people. This tsunami caused damage in a 110-mile (177-km) coastal strip that had not been affected by the Indian Ocean Tsunami. Three minor tsunamis were reported in 2008, two in Indonesia and one in Japan.

On September 29, 2009, a tsunami raced ashore in the Samoan islands (composed of the Independent State of Samoa and American Samoa, a U.S. territory). That tsunami wiped out entire coastal villages and killed at least 150 people. The Pacific Tsunami Warning Center said a 8.3-magnitude earthquake, occurring at a depth of 20 miles (33 km) about 120 miles (190 km) from Apia in the Independent State of Samoa also sent small but measurable waves ashore as far away as New Zealand, Hawaii, and Japan.

Following the January 2010 earthquake in Haiti, the Pacific Tsunami Warning Center, which issues warnings worldwide, initially issued a tsunami warning for the area surrounding Haiti, but cancelled it shortly afterwards when no displacement of sea level was observed from the inland-centered earthquake.

The surge of water generated by the earthquake off the Chilean coast ultimately resulted in much smaller tsunami waves than anticipated as they traveled across the Pacific Ocean. In Hawaii tsunami waves of less than 3 feet (1 meter) came ashore. Later the same day in Hokkaido, Japan, tsunami-generated waves of about 12 inches (30 centimeters) were observed.

The largest earthquake ever recorded, registering 9.5 on the Richter scale, occurred in 1960 in the same area as the 2010 Chilean quake and resulted in a Pacific-wide tsunami that arrived unannounced and generated waves up to 82 feet (25 meters) in height along the South American coast. Thousands of miles away from the epicenter of the 1960 earthquake, the tsunami carried walls of water 35 feet (10.6 meters) high.

See also Earthquakes ; Fukushima Daiichi nuclear power station disaster ; Japanese earthquake (2011) .



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National Geographic Society. “Tsunami Safety Tips.” http://environment.nationalgeographic.com/environment/natural-disasters/tsunami-safety-tips.html (accessed May 3, 2012).

Max Strieb

  This information is not a tool for self-diagnosis or a substitute for professional care.