What is a tsunami and how does it occur: The 5 strongest tsunamis in history

On July 30, 2025, the most powerful earthquake since 2011, with a magnitude of 8.7, occurred off the coast of Kamchatka. It caused alarm throughout the Pacific region: residents of Kamchatka, the Kuril Islands, and several countries from the United States to Peru were warned of a tsunami threat. In Japan, an evacuation was announced on the Pacific coast. Although the waves in Kamchatka were not high (up to 32 cm), a tsunami remains a real threat even today. Let’s consider what a cataclysm is, why it occurs, and recall the five most destructive tsunamis that humanity has experienced.

What is a tsunami

A tsunami is a series of large ocean waves that occur when huge volumes of water suddenly shift. Most often, the cause is an underwater earthquake: when lithospheric plates move, the ocean floor can suddenly rise or fall, pushing the water above it. Such a vertical displacement of the bottom in a matter of seconds generates radial waves throughout the entire thickness of the water. Tsunamis can also occur for other reasons  —  during the eruption of underwater volcanoes, powerful collapses or landslides, less often from explosions or falling meteorites. The main condition is that the event must move a huge amount of water at once. 

In the open ocean, tsunami waves are not always noticeable: their height can be less than a meter, but their length is tens and hundreds of kilometers. The speed of tsunami propagation is colossal  —  up to 700–1000 km / h, which is comparable to the speed of a jet aircraft. Waves travel thousands of kilometers almost without losing energy. For example, the tsunami from the 1960 Chilean earthquake crossed the entire Pacific Ocean  —  it reached Hawaii in 15 hours and reached Japan almost 22 hours later.

When tsunamis approach shallow water near the coast, the wave height increases many times  —  up to tens of meters on the shore. That is when the tsunami shows its destructive power.

What does a tsunami look like

A tsunami wave hits the coast of Thailand, 2004 (Photo: David Rydevik/The Atlantic)

A telltale sign of an approaching tsunami is a sudden low tide before it hits. The water near the shore can initially recede unusually far, exposing the seabed for hundreds of meters. A few minutes later, the horizon darkens from the approaching wall of water. The first wave is not always the largest. Tsunamis usually come in a series: several waves can roll in over an hour or more, with subsequent waves often higher than the previous ones.

When a tsunami hits the shore, water from a height of up to 50 m falls to the ground, breaking trees and buildings. A shock wave moves in front of the wave front, like from an explosion  —  it is also capable of causing damage to buildings, property, and living creatures. A powerful stream floods the coastal area, destroying cars, houses, and infrastructure. During the 2011 tsunami in Japan, water washed away entire neighborhoods.

Houses in the city of Natori, washed away by the sea after the tsunami in Japan on March 11, 2011 (Photo: Reuters)

Even relatively low waves are extremely dangerous. Tsunamis carry enormous energy and mass of water. They flood the area far inland  —  sometimes kilometers from the shore. The flow, returning back to the sea, carries away collapsed structures, people and everything that was on the shore.

Satellite image of the coastline of Kalutara, Sri Lanka, taken on December 26, 2004, shortly after the tsunami hit. (Photo: AP Photo / DigitalGlobe)

What causes tsunamis

A destructive wave usually requires an earthquake of magnitude 7.0 or greater with an epicenter under the ocean floor or near the coast. Such a shock can cause an area of the seabed hundreds of square kilometers in area to shift vertically by several meters. The water above it, trying to fill the resulting “hole” or compensate for the elevation, forms a wave.

However, it is not only earthquakes that cause tsunamis. Volcanic eruptions can also be the cause. In 1883, the explosion of the Krakatoa volcano in Indonesia almost destroyed it  —  and simultaneously generated 20–30-meter tsunamis on the surrounding coasts. The waves washed away hundreds of settlements, killing more than 36 thousand people.

Large landslides and underwater collapses are another cause of tsunamis. If a huge mass of rock or ice suddenly falls into the ocean, it will displace water and create waves. Such mechanisms are associated, for example, with the mega-tsunami 524 m high in Lituya Bay (Alaska, 1958)  —  then a giant landslide descended into the fjord from a height of 910 m as a result of an earthquake. Fortunately, such cases are rare.

Nuclear explosions and meteorite falls can also theoretically cause tsunamis. But the vast majority of actually recorded tsunamis still occur due to earthquakes.

5 most destructive tsunamis in human history and their consequences

1. Indonesia, 2004

On December 26, 2004, one of the most destructive tsunamis of modern times occurred in the Indian Ocean. An underwater earthquake with a magnitude of 9.1 occurred off the coast of the Indonesian island of Sumatra. The fault zone that caused the tsunami stretched for 1,300 km.

The devastated area of Banda Aceh on the island of Sumatra in Indonesia after the powerful tsunami on December 26, 2004 (Photo: Getty Images)

In the open ocean, the tsunami reached 0.8 m in height, and near the coast it increased to 15 m, and in places where the water rose the most, it reached 30 m. The speed of the wave in the ocean was about 720 km/h. Just 15 minutes after the earthquake, the tsunami covered the northern part of the island of Sumatra. An hour and a half later, the waves hit the coast of Thailand, then Sri Lanka and India. In eight hours, the tsunami crossed the Indian Ocean, and within 24 hours, for the first time in the history of wave observations, it had traveled around the entire World Ocean. Even on the Pacific coast of Mexico, the wave height was 2.5 m. In total, the tsunami covered at least 14 countries.

Consequences: The official death toll exceeded 225,000 people. Millions of people were left homeless, and damage was estimated at $10 billion. This disaster prompted the global community to create a tsunami early warning system in the Indian Ocean. After the tragedy, seismic buoys and warning sirens were installed on the coasts, and December 26 was declared World Tsunami Remembrance Day.

Streets of Phuket moments after the tsunami hit the city on December 26, 2004 (Photo: AP Photo)

2. Japan, 2011

On March 11, 2011, the largest earthquake in Japanese history, measuring 9.0, struck off the northeastern coast of Honshu. It triggered the Tohoku tsunami, which became a national tragedy. Waves 30–40 m high destroyed houses, cars, and even planes at local airports.

A wave floods the city of Miyako after the earthquake in Japan on March 11, 2011 (Photo: Reuters)

Aftermath: The tsunami killed about 19,700 people. Thousands more were injured and left homeless. Particular damage was caused to the Fukushima-1 nuclear power plant: the power plant suffered a top-category accident associated with the flooding of backup cooling systems. A radiation release comparable to Chernobyl occurred  —  tens of kilometers around the plant became an exclusion zone. The economic damage is estimated at $200–300 billion. To this day, the tragedy of March 11, 2011 remains a painful memory for the Japanese.

A tsunami hits the residential area of Natori after a powerful earthquake in Japan on March 11, 2011 (Photo: Reuters)

3. Portugal, 1755

On the morning of November 1, 1755, All Saints’ Day, a series of tremors measuring 8.7 in magnitude almost wiped the capital of Portugal off the face of the earth. Minutes after the earthquake, a tsunami about 15 m high hit Lisbon. Many residents, fleeing from the destruction and fires, ran to the river and became victims of the approaching wave.

A 1755 engraving depicting the ruins of Lisbon amidst fires and tsunami waves engulfing ships in the harbor (Photo: Wikipedia)

Aftermath: Exact figures are unknown, but it is believed that between 30,000 and 90,000 people died as a result of the earthquake and tsunami. The damage to Lisbon was enormous: about 85% of buildings were destroyed or burned to the ground. The waves devastated the coastline of Portugal, and also caused damage in Spain and North Africa. In Morocco, up to 10,000 people died when a 20-meter tsunami hit. Echoes of the waves were noted as far away as England (waves reached 3 m) and the Caribbean Sea.

Lisbon ceased to exist in its previous form; the city was rebuilt almost from scratch. The catastrophe shook all of Europe, was reflected in literature (it was mentioned by Voltaire) and stimulated the development of seismology.

4. Indonesia, 1883

On August 27, 1883, the Krakatoa volcano between Java and Sumatra erupted. The force of the volcano’s explosion is estimated at 100–200 megatons in TNT equivalent, which is more than 10 thousand times greater than the power of the explosion that destroyed Hiroshima. The eruption literally split the volcano into many small islands and caused a giant tsunami. Waves up to 20–30 m high crashed onto the coastal areas of two neighboring islands. Entire villages and towns were swept off the face of the earth. According to historical chronicles, the water receded from the shore so much that it exposed the reefs, and then returned in several waves, taking with it all living things.

Lithograph from 1889 depicting the eruption of Krakatoa (Photo: Wikipedia)

Consequences: More than 36,000 people died on the coasts of Java and Sumatra. 295 settlements disappeared from maps — they were completely destroyed by the waves. Tsunamis from Krakatoa reached the Indian and Pacific Oceans, were recorded in South Africa, India, and the Arabian Peninsula. 

A new volcano, Anak-Krakatau, subsequently grew on the site of the destroyed volcano, which is also active.

5. Chile, 1960

On May 22, 1960, Chile experienced the most powerful earthquake ever recorded  —  a magnitude of 9.5. It generated a devastating tsunami along the entire coast of central and southern Chile. The height of the waves in some places reached 20–25 m, and dozens of settlements were wiped out from the coastal areas.

A flooded city in Chile on May 22, 1960 (Photo: Factroom)

The tsunami caused by the earthquake passed through the entire Pacific Ocean. After 15 hours, it reached the Hawaiian Islands, where it destroyed the center of the city of Hilo and killed 61 people. Almost 500 buildings were destroyed. The waves then reached Japan, where 138 people died, and the Philippines, where the death toll was another 32 people. The consequences of this earthquake affected several countries at once and acquired a global scale. 

Consequences: About 5.7 thousand people died in Chile, more than 2 million lost their homes. The experience of 1960 stimulated the creation of modern tsunami warning systems. At that time, the radio and emergency services of Hawaii managed to warn the population (after the first signals, many residents left the lowlands and thus saved themselves), but it was not possible to completely avoid casualties. Now networks of buoys and sensors across the Pacific Ocean can record tsunamis in the open sea and issue early warnings in potentially dangerous regions.

The aftermath of the Great Chilean Earthquake and the tsunami it caused (Photo: imago-images.com)

The main thing about tsunamis

• A tsunami is a series of waves that occur due to the sudden movement of large volumes of water. Most often, they are provoked by underwater earthquakes with a magnitude of >7, but other causes are also possible (volcanic eruptions, landslides, explosions).

• In the open ocean, tsunami waves are almost imperceptible, but they can spread at the speed of a passenger plane  —  hundreds of kilometers per hour

• When approaching the coast, the speed of the waves decreases, and their height increases sharply. Tsunamis can rise to 10–30 meters or more. Such waves crash onto land like rapid floods, crushing everything in their path.

• The deadliest tsunami in modern history occurred in the Indian Ocean in 2004. Powerful waves covered 14 countries, about 230 thousand people died. This disaster led to the creation of an international tsunami warning system.

• Signs of an approaching tsunami: a noticeable earthquake near the coast, an unexpected and sharp retreat of water that exposes the bottom far away, or, conversely, an unusually fast rise in water. If you notice this, you must immediately evacuate to higher ground.

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