If you’ve ever taken a bath or swum in a lake or pool, you’ve probably enjoyed creating your own waves. Unlike solid objects, water is fluid and easy to move around. The harder you push with your hands, the bigger the waves will be.

Now imagine what happens in the ocean when an earthquake occurs underground. If you’ve ever felt an earthquake or seen one on television, you know that they have the power to shake the ground and move large buildings.

When an earthquake occurs under the ocean floor, the earthquake’s massive energy is transferred to the water above it, creating a series of water waves called a tsunami. The word tsunami comes from the Japanese words tsu (harbor) and nami (wave).

In addition to earthquakes, tsunamis can be caused by other events that generate enough energy to displace a large volume of water, such as volcanic eruptions, landslides, meteors, and even underwater tests of nuclear devices. Given the frequent earthquake and volcanic activity in the Pacific Rim, tsunamis happen often in Japan.

Tragically, because of the huge amounts of water and energy involved, tsunamis can cause tremendous damage to coastal areas. On December 26, 2004, a massive earthquake in the Indian Ocean caused a series of devastating tsunamis along the coasts of the areas bordering the Indian Ocean. One of the deadliest natural disasters in history, waves up to 100 feet high crashed into coastal communities, killing over 230,000 people in 14 countries, including Indonesia, Sri Lanka, India, and Thailand.

You may recall the earthquake and tsunami that occurred in Japan on March 11, 2011. That tsunami resulted in mass destruction including a nuclear power plant that released radiation into the Pacific Ocean.   

Normal ocean waves created by the wind have an average wavelength (measured from crest to crest) of approximately 330 feet and an average height of about 6.6 feet. A tsunami, on the other hand, can have a wavelength of 120 miles or more. Traveling at up to 500 miles per hour in the deep ocean, a tsunami’s height might only be as little as 3 feet, making it almost impossible to detect, even for a ship in the area.

As a tsunami approaches a coastline, though, things begin to change dramatically. As it approaches shallow waters, an effect known as wave shoaling compresses and slows the wave to below 50 miles per hour. Wavelength also decreases dramatically, but amplitude — the height of the wave — increases greatly, leading to the unmistakable wall of water that causes such destruction along coastlines.

Approximately 80% of tsunamis happen in the Pacific Ocean, although they’re possible in any large body of water, including lakes. Tsunamis cannot be prevented and can rarely be predicted with precision. In the event of an earthquake, oceanographers, geologists, and seismologists often urge potentially-affected areas to issue tsunami warnings.

Regions that experience tsunamis regularly have developed tsunami warning systems to give as much advance warning as possible to people who live along the coastline. In Japan, for example, coastal communities are well-educated about earthquakes and tsunamis. Japanese shorelines feature tsunami warning signs and warning sirens atop nearby hills.

Although tsunamis cannot be prevented, some tsunami-prone countries use engineering lessons learned from earthquakes to reduce shoreline damage. Many Japanese coastal communities have built tsunami walls, floodgates, and channels to block or redirect water from incoming tsunamis.

One of the best ways to make a coast tsunami-proof, though, is quite natural: plant trees! Scientists noticed that some locations in the path of the 2004 Indian Ocean tsunami suffered little damage because trees, such as coconut palms and mangroves, absorbed the tsunami’s energy. In fact, one small Indian village saw little damage because the tsunami broke against a forest of over 80,000 trees that had been planted along the shoreline just two years earlier in an attempt to set a Guinness World Record.

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