What Is a Tsunami? Definition and Explanation

Tsunami Diagram
A tsunami is a giant wave or series of waves produced by an earthquake, volcano, or other event that displaces a large amount of water.

A tsunami is a series of enormous ocean waves that results from the rapid displacement of a large volume of water. The waves often rise to a height of over 30 meters (100 feet). Unlike typical ocean waves, which are caused by the wind, tsunamis are primarily the result of geological activities.

Word Origin and Comparison with Other Terms

The word ‘tsunami’ is of Japanese origin, where ‘tsu’ means harbor and ‘nami’ means wave, essentially translating to “harbor wave.” This term is favored over alternatives like “tidal wave” or “seismic sea wave” because it captures the essence of the phenomenon more accurately.

  • Tidal Wave: Tsunamis are not influenced by the tides, so the term “tidal wave” is misleading.
  • Seismic Sea Wave: This term is closer to describing a tsunami but is somewhat restrictive, as seismic activity is just one of the causes.

Causes of Tsunamis

There are multiple causes for tsunamis, including:

  1. Underwater Earthquakes: Undersea earthquakes are the most common cause of tsunamis, where tectonic plates shift suddenly. Aftershocks may generate additional waves.
  2. Volcanic Eruptions: Explosive eruptions or the collapse of volcanic islands displace water, sometimes triggering a tsunami.
  3. Landslides: Some tsunamis results from either underwater landslides or from a land mass sliding into the ocean. An ice mass breaking off and falling into the ocean is another potential trigger.
  4. Meteorite Impacts: Although rare, a large enough meteorite impact on an ocean can generate a tsunami.
  5. Human Events: A tectonic weapon has the potential for inducing a tsunami. Most explosions do not generate large waves, but the 1917 Halifax Explosion produced an 18-meter high tsunami in the harbor.

Approximately 80% of tsunamis occur in the Pacific Ocean, but they can happen in any large body of water, including lakes. Shoreline topography is important, too. For example, Japan has experiences over a hundred tsunamis throughout history, while nearby Taiwan has only recorded two.

How a Tsunami Works

A tsunami starts with an event that displaces a large volume of water. The resulting waves spread outward radially, much like the pattern you see when you drop a rock into a pool. These wave move more quickly than wind waves and gain height when they reach shallow water. Unlike normal waves, tsunami waves rarely break. Instead, a tsunami appears as a wall of water or tidal bore.

  1. Initiation: Geological activity displaces a large volume of water.
  2. Propagation: The waves move outward in all directions from the point of origin.
  3. Amplification: As the tsunami approaches shallower waters, it gains height.
  4. Impact: The waves reach the shore, often with little warning, causing destruction.

A tsunami is a set of waves and not a single wave. It may feature multiple waves that arrive over a period of hours. The first wave is not always the highest one.

Tsunami Characteristics

The waves from a tsunami differ from ordinary waves:

  1. Long Wavelengths: Unlike regular waves, tsunamis have wavelengths that can extend up to 200 miles. In other words, the distance from the trough of one wave to the next might be miles or kilometers, rather than the typical 60–150 m (200–490 ft) wavelength of wind-caused waves.
  2. High Speed: They travel at speeds up to 500-800 km/h (310-500 mph). So, time is a critical factor in reducing the impact of the waves.
  3. Increase in Height: Tsunamis are often barely noticeable in deep water but increase dramatically in height as they approach shallower waters. So, a ship in deep water might be unaffected by a tsunami that causes devastation on shore.

Recognizing a Tsunami

How do you know when a tsunami is coming? Warning systems are the best protection, but watching the water and maybe surrounding wildlife also helps.


Before a tsunami hits, there is often a noticeable retreat of water from the shore, known as ‘drawback.’ This phenomenon serves as a natural warning signal. If you see the ocean retreating, head for high ground.

Warning Systems

Sophisticated early-warning systems, involving seismic sensors and ocean buoys, provide some advance notice. The notice ranges from minutes to hours, depending on the distance from the point of origin.

Animal Behavior

While not scientifically confirmed, there are numerous reports of animals acting unusually before tsunamis, possibly due to their sensitivity to vibrations or sounds humans can’t detect.

Time to Safety

The time to reach safety varies significantly, depending on how close the tsunami source is to the coastline. In some cases, people have just minutes.

Magnitude Scales

Two of the most common tsunami magnitude scales are the Imamura-Iida Intensity Scale and the Sieberg-Abraseys Scale.

  • Imamura-Iida Intensity Scale: This scale measures height and distance traveled.
  • Sieberg-Ambraseys Scale: This scale measures effects on both humans and landscapes.

Mitigating Future Damage

Scientists and policymakers are taking a multi-tiered approach to minimizing the impact of future tsunamis. While the events are not preventable, improving warning systems and public education and building structures to withstand the waves reduces the damage and loss of life.

  1. Improved Warning Systems: This include increasing the network of seismic and oceanographic sensors and establishing sirens and emergency evacuation routes.
  2. Engineered Structures: Building seawalls and breakwaters, as well as engineering buildings reduces the impact of the waves..
  3. Community Preparedness: Education and drills reduces the time it takes for people to take action and reach safety.

Major Historical Tsunamis

Here are 10 historically important tsunamis:

  1. Indian Ocean, 2004: One of the deadliest natural disasters in recorded history, this tsunami was triggered by a massive undersea earthquake off the coast of Sumatra, Indonesia. It resulted in over 230,000 deaths across 14 countries, including Thailand, Sri Lanka, and India.
  2. Tohoku, Japan, 2011: Triggered by a 9.0-magnitude earthquake, this tsunami led to the Fukushima nuclear disaster. Nearly 16,000 people were killed, and the event had extensive economic repercussions.
  3. Lituya Bay, Alaska, 1958: The highest tsunami wave ever recorded occurred in Lituya Bay, Alaska, with a wave reaching 1,720 feet. Triggered by a landslide, it had a relatively lower human toll but showcased the incredible power of tsunamis.
  4. Great Lisbon Earthquake and Tsunami, 1755: Occurring on All Saints’ Day, this event devastated Lisbon, Portugal, and affected much of Europe and North Africa. The tsunami wave traveled as far as the Caribbean.
  5. Krakatoa, Indonesia, 1883: The eruption of the Krakatoa volcano resulted in a tsunami with waves as high as 135 feet. The event was so powerful that it was heard 3,000 miles away, and it killed approximately 36,000 people.
  6. Messina, Italy, 1908: Triggered by an earthquake in the Strait of Messina, this tsunami killed an estimated 80,000 people in the cities of Messina and Reggio Calabria.
  7. Nankaido, Japan, 1707: This is one of the earliest well-documented tsunamis. It resulted from a massive earthquake and caused significant loss of life and property in Japan.
  8. Papua New Guinea, 1998: Caused by an undersea landslide, this tsunami resulted in waves up to 15 meters high and killed more than 2,200 people.
  9. Sanriku, Japan, 1896: Known for its incredibly high run-up heights, the tsunami resulted from an undersea earthquake and affected the Sanriku coast of Japan, killing over 22,000 people.
  10. Chile, 1960: Triggered by the most powerful earthquake ever recorded (magnitude 9.5), this tsunami affected the entire Pacific, causing deaths as far away as Hawaii, Japan, and the Philippines.

Each of these historical tsunamis serves as a stark reminder of the immense power and potential devastation that this natural phenomenon can cause. Understanding these events can help improve preparedness and response strategies for future tsunamis.

Tsunami Glossary

Understanding tsunamis is easier when you know the terms that scientists use when discussing them. Here is a list of tsunami vocabulary terms and their definitions:

  • Wave Train: A series of waves traveling together, separated by a relatively consistent distance, typically found in a tsunami event.
  • Run-up: The maximum vertical height a tsunami wave reaches as it moves inland from the coastline.
  • Tsunamigenic: Refers to any geological or cosmic event capable of producing a tsunami.
  • Wavelength: The distance between two corresponding points on adjacent waves, such as from crest to crest or trough to trough.
  • Wave Height: The vertical distance from the crest (top) of a wave to the trough (bottom).
  • Wave Period: The time it takes for a single wave to pass a fixed point.
  • Wave Frequency: The number of waves passing a fixed point per unit of time, often measured in Hertz (Hz).
  • Wave Speed: The speed at which a wave travels, often calculated by multiplying the wave’s frequency by its wavelength.
  • Amplitude: The maximum displacement of the water surface from its rest position, essentially half of the wave height.
  • Crest: The highest point of a wave.
  • Trough: The lowest point of a wave.
  • Drawback: The noticeable retreat of ocean water along the shore, exposing the sea floor, which often occurs just before a tsunami hits.
  • Shoaling: The process where a wave’s height increases as it enters shallower water.
  • Refraction: The bending of a wave as it moves into areas with different depths, often causing the wave to align more parallel with the shoreline.
  • Seismicity: Earthquake frequency, distribution, and magnitude within a specific region.
  • Subduction Zone: An area where one tectonic plate is pushed below another, often the site of tsunamigenic events.
  • Seismograph: An instrument that records the vibrations of the Earth, used to detect earthquakes and, by extension, potential tsunamis.
  • Seismic Waves: The waves of energy caused by the sudden breaking of rock within the Earth or an explosion, which are the primary cause of earthquakes.
  • Plate Tectonics: The scientific theory that describes the movement of Earth’s lithosphere (crust and upper mantle) as divided into several large and small pieces known as tectonic plates.
  • Aftershock: A smaller earthquake that occurs in the same general area during the days to years following a larger earthquake or “mainshock.”
  • Buoyancy: The ability of an object to float in water or another fluid, utilized in the design of tsunami-detecting buoys.


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