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How Tsunamis Form

Here's how tsunamis form, why they grow so much taller near shore, and how warning systems work.

How undersea earthquakes rapidly displace ocean water

Tsunamis are usually triggered by large earthquakes on the seafloor, where a sudden vertical shift in the seabed pushes the entire column of water above it. Underwater volcanic eruptions and large submarine landslides can also generate tsunamis.

Low but fast-moving in open water

In deep water, a tsunami's wave height is often under one meter, barely noticeable to a ship, but its wavelength is enormous and it travels at speeds of several hundred kilometers per hour.

Why wave height grows near the coast

As the seafloor becomes shallower near the coast, the wave slows down while the water behind it keeps piling up, causing the wave height to rise sharply, a process called shoaling. In bays or narrow coastal terrain, this can amplify wave height to several meters, or in extreme cases tens of meters.

How tsunami warning systems generally work

National weather agencies and international bodies such as the Pacific Tsunami Warning Center analyze the magnitude and location of undersea earthquakes to quickly assess tsunami risk and issue warnings to coastal areas.

Notable historical examples

The tsunami triggered by the 2004 Indian Ocean earthquake caused catastrophic damage across Southeast and South Asia, and the tsunami from the 2011 Tohoku earthquake struck Japan's northeastern coast, causing massive loss of life and property as well as the Fukushima nuclear accident.

Always follow official safety guidance

If you feel a strong earthquake near the coast or see the sea suddenly and rapidly recede, move to high ground immediately rather than waiting for an official warning, and follow the specific guidance issued by local weather and emergency management authorities.

Tsunamis behave very differently in deep water and near shore

A tsunami can be almost imperceptible far out at sea, yet grow dramatically as it approaches the coast, a distinctive trait that sets it apart from ordinary wind-driven waves. Understanding how tsunamis form helps explain why low-lying coastal areas are especially vulnerable.

Not every tsunami is caused by an earthquake

While the vast majority of tsunamis are triggered by undersea earthquakes, volcanic eruptions, large coastal or submarine landslides, and, rarely, meteorite impacts can also generate them. Regardless of the cause, the physics that make waves grow taller near shore stay the same.

Frequently Asked Questions

Are tsunamis always caused by earthquakes?

Most are caused by undersea earthquakes, but volcanic eruptions, large submarine or coastal landslides, and, rarely, meteorite impacts can also trigger tsunamis.

Is Korea's coastline completely safe from tsunamis?

Not entirely. Korea's East Sea can be affected by earthquakes off Japan's western coast, and tsunamis actually reached Korea's east coast in 1983 and 1993, causing damage.