Tsunami are huge, destructive waves that follow an earthquake or disturbance that has happened in the Earth’s crust under the ocean.
The word is Japanese, meaning harbor wave; it’s pronounced “tsoo-nah-mee.”
They’re wrongly called tidal waves, since they have nothing to do with the tides. During an earthquake, cracks open up along the ocean floor.
When water rushes into the hole and then bounces back out onto the surface, it causes strong swells to form, which can travel over open ocean up to 400 or 500 miles (644 or 800 km) per hour.
When these swells reach the shallow waters near a coastline, they are slowed down by friction from the seabed.
Then they are transformed into very high waves, averaging 30 feet (9 m) high and often much higher.
Once ashore, these huge waves can crush structures along the coastline and cause immense flooding. Most tsunami occur in the Pacific Ocean.
Tsunami have hit the United States in Alaska and Hawaii.