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How do tsunamis form?

April 25, 2024

Tsunamis are rare but can cause mass destruction. Here’s what you need to know about the science behind them.

Rubble following a tsunami in Thailand in 2004
Thailand saw massive destruction after the 2004 tsunamiImage: Pornchai Kittiwongsakul/AFP/Getty Images

Although rare, Tsunamis can be very destructive. They are usually caused by earthquakes along tectonic plates under the ocean. This sudden underwater movement causes a large displacement of water, sending waves across the ocean. These are often small, but move at high speeds — up to 500 miles (800 kilometers) per hour.

These waves can be hundreds of miles long. It can take up to a day for a tsunami to travel through the ocean and reach the shore depending on where the underwater quake occurred.

The waves that make up a tsunami are typically not noticeable to the human eye until they are close to shore. They appear on land as a series of multiple waves in a row and can arrive as huge walls of water that can sweep away people, buildings, cars and trees. 

These waves can hit land in lengthy intervals that are hard to predict, Robert Weiss, a professor of Natural Hazards in the Department of Geosciences at Virginia Tech told DW in an email.

"The arrival pattern can be pretty complex because waves are complex features," he explained. "Just throw two stones in a pond and observe the complex wave pattern they form. There is no standard spacing and number that one can count on."

Earthquakes larger than 6.5 on the Richter scale generally cause tsunamis, experts say. This 1-10 scale classifies the magnitude of quakes using a device called a seismograph. Earthquakes with a magnitude of 1 are common and usually go unnoticed, while earthquakes with a magnitude of 10 are very rare but can cause catastrophic damage.

Less commonly, tsunamis are caused by underwater volcanic eruptions, landslides or asteroids hitting the ocean.

Where do tsunamis occur?

Tsunamis most commonly occur in the Pacific Ocean around the "Ring of Fire", a tectonic plate with a string of volcanoes and earthquakes. According to the US NOAA Tsunami Program, nearly 80% of tsunamis that occurred in the last century occurred there.

One of the most destructive tsunamis in history occurred in 2004 in Sumatra, Indonesia, when a 9.1-magnitude quake struck off the coast. The tsunami reached a height of 50 meters and caused damage 5 kilometers inland, killing about 230,000 people.

The 2011 earthquake and tsunami off the coast of Japan's main island of Honshu is also well known for its destruction — a 9.0-magnitude quake caused a tsunami that killed some 20,000 people.

Weiss told DW this tsunami also traveled around 5 kilometers inland. 

The Atlantic Ocean and the Caribbean Sea, the Mediterranean Sea and the Indian Ocean have also experienced Tsunamis.

Volcano, active
A recent volcano in Indonesia prompted a tsunami warningImage: Hendra Ambalao/AP/dpa/picture alliance

How do we know a tsunami is coming?

It is impossible to precisely predict when and where tsunamis will hit.

"The problem is that earthquakes, the most common cause of tsunamis, are not very predictable, nor is it possible to say where, when, how deep, and with what magnitude," Weiss said.

He said the time between the earthquake and the appearance of the tsunami on land is short. 

"Preparation for such events is critical, but it is hard to prepare for something that people rarely experience twice in their lifetime," he explained. "Large earthquakes and subsequent tsunamis may occur reasonably often on a global scale but seldom happen twice in the same region on the same destructive level. It is essential to understand that preparation is vital alongside good warning. Only having one or the other is not enough."

Global tsunami warning centers scan the oceans using observation systems to try to predict when earthquakes will strike underwater, and local authorities send out warnings when a threat is detected.

But the UN warns these systems aren't ubiquitous.

"Half of the world's countries lack adequate systems, and even fewer have adopted legislation to connect these systems to preparedness and response," the organization said in 2023. "The impact of tsunamis is not experienced evenly: the poor and those facing societal barriers often suffer the most."

Edited by: Ashutosh Pandey

Clare Roth
Clare Roth Editor and reporter focusing on science and migration