Tsunami Dangers: What You Need To Know

Tsunamis are among the most powerful and destructive natural forces on Earth. These massive surges of water form through entirely different processes than regular ocean waves, and their size can range from barely noticeable ripples to waves taller than buildings.

The 8.8 magnitude earthquake that struck near the coast of Russia generated generally small waves. However, this isn’t always the case. Sometimes tsunamis can push water 100 feet high or more, and cause catastrophic damage and loss of life.

How Does A Tsunami Form?

Tsunamis usually begin with a sudden displacement of water, most often triggered by an undersea earthquake. When tectonic plates shift along a fault line beneath the ocean floor, they can cause part of the seabed to rise or fall abruptly. This movement displaces a vast amount of water in a short period of time, sending energy through the ocean in all directions.

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Other triggers include underwater landslides, volcanic eruptions and also onshore landslides where large volumes of debris fall into the ocean. The key is that the event must rapidly push or pull a large volume of water, which is what sets a tsunami in motion.

Tsunamis are unlike waves produced by wind and storms, because those only affect the surface. Rather, tsunami waves move through the entire depth of the ocean, from surface to seafloor. This makes them incredibly powerful and fast! In deep water, tsunamis can travel at 500 to 600 miles per hour, nearly as fast as a commercial airliner, though they may appear only a few feet high and go unnoticed by ships at sea.

They don’t break and crash on the coast like typical waves we see surfers taking advantage of, but rather an abrupt rise of the sea. The tide rises sharply and is nearly impossible to outrun. According to experts, if you see a tsunami coming, it’s too late to outrun it.

Why Size Varies So Much

The size of a tsunami depends on several key factors:

• Magnitude And Type Of Trigger: Larger earthquakes or landslides displace more water, creating more energy and potentially larger waves. The vertical movement of the seafloor is particularly effective at generating tsunamis, and greatly impacts the height of a tsunami.
• Ocean Depth And Seafloor Shape: In deep water, tsunami waves are long and low, but as they approach shore and the ocean floor rises, the wave slows down and grows dramatically in height—a process called “shoaling.” This is why some tsunamis that start small in the deep ocean become towering walls of water at the coast.
• Distance From The Source: A tsunami’s energy spreads out as it travels. Communities closer to the origin of the event (like the epicenter of an earthquake) are more likely to experience larger and more sudden waves.
• Coastal Geography: Narrow bays, steep shorelines and certain underwater features can amplify tsunami waves, making them more dangerous in some areas than others.

Not Just One Wave

A tsunami is usually a series of waves, not just one. The first wave isn’t always the largest, and waves can continue to arrive for hours after the initial surge. This is why tsunami warnings often stay in effect well after the first wave hits.

If you are along the coast and feel very strong shaking or unusual wave activity (such as the sea withdrawing far from shore), it’s critical that you seek higher ground immediately and stay far away from the coast. Also, follow any tsunami alerts that are issued and stay away from the shore until the threat has passed.

Has A Large Tsunami Ever Struck The U.S.?

Yes. According to the USGS, one of the most notable tsunamis that Hawaii has experienced was in 1946. It was caused by an earthquake and generated waves up to 55 feet, unfortunately killing 159 people.

Another major tsunami occurred following the 1964 earthquake in the Gulf of Alaska, which was a 9.2 magnitude quake. This caused damage and loss of life across many areas in the Pacific, including Alaska, Hawaii and the U.S. West Coast.

Tsunamis across the Atlantic Coast are more infrequent, however. One of the most catastrophic occurred in 1929 following a 7.3 magnitude quake near Newfoundland. It generated waves up to 23 feet high mainly in and around Newfoundland. Waves reached as far south as South Carolina.

One of the most well-known tsunamis in recent history was in 2004. A magnitude 9.1 earthquake struck off the coast of Sumatra, generating tsunami waves across the Indian ocean, as high as 100 feet in some locations, impacting Sri Lanka the greatest. It killed more than 225,000 people and caused catastrophic damage.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.