Tsunamis: The Science Behind the Giant Waves

What Causes a Tsunami?

Tsunamis usually begin with sudden movements on the ocean floor.

• Underwater earthquakes at tectonic plate boundaries cause most tsunamis.
• Other causes include volcanic eruptions, underwater landslides, or even asteroid impacts.
• When the seafloor shifts, it pushes a huge amount of water upward, starting a wave.
• This wave travels across the ocean at speeds up to 500–600 mph, but it may only be a few inches high in deep water.

How Tsunamis Behave Near Shore

Tsunamis change shape and become more dangerous as they reach shallow water.

• As the wave enters shallow water, it slows down and grows taller—sometimes over 30 feet high.
• This happens because the water behind the wave keeps pushing forward, causing it to rise.
• Tsunamis often arrive as a series of waves, not just one big wave.
• The first wave might not be the strongest; larger waves can follow minutes later.
• In some cases, ocean water may first pull back from the shore, exposing the seafloor—this is a warning sign that a tsunami is coming.

Can We Predict Tsunamis?

Scientists use special tools and warning systems to detect and predict tsunamis.

• Although tsunamis cannot be predicted in advance, we can detect the events that cause them, like undersea earthquakes.
• Seismographs detect ground shaking, and DART buoys (Deep-ocean Assessment and Reporting of Tsunamis) sense changes in ocean pressure.
• Tsunami warning centers analyze this data and issue alerts within minutes.
• Coastal areas often have sirens or mobile alerts to warn people to evacuate.

Fun Facts

Here are some interesting facts about tsunamis:

• The word “tsunami” comes from Japanese and means “harbor wave.”
• Tsunamis can cross entire oceans in less than a day.
• The Indian Ocean tsunami in 2004 traveled as far as South Africa.
• The biggest tsunami ever recorded, with a wave height of 1,720 feet (525 meters), occurred in Lituya Bay, Alaska, in 1958.

Review

Let’s quickly recap what we learned about tsunamis:

• What causes most tsunamis? Earthquake
• What happens to a tsunami in shallow water? Rises
• How fast can a tsunami travel in deep water? 500 miles per hour
• What instruments sense changes in ocean pressure? DART Buoys