Tsunami Facts: Understanding a Fearsome Phenomenon

Tsunamis occur as a result of a sudden and significant displacement of the ocean.

• Most tsunamis are caused by undersea earthquakes.
• They can also be caused by volcano eruptions or landslides.
• Less commonly, they can be caused by fast-moving weather events that create ocean displacement.
• It is speculated that a large meteor or asteroid could bring on a tsunami. After such an event, a series of multiple waves forms and travels to the shore.

Tsunamis vary greatly in size, depending on factors like how deep the water was where they were formed, how far they are from the shore, the composition of the body of water in which they form, and the shape of the shoreline.

• When tsunamis first form, they are generally much wider than they are tall.
• As they move from deeper water to more shallow areas, the waves grow higher.
• Most tsunamis are less than 10 feet high when they impact the shoreline, but in extreme cases can be over 100 feet high.

Tsunamis can travel up to 500 miles per hour (mph) over deep water. As a result, a tsunami that starts far offshore can reach the shoreline in just a matter of minutes. Upon reaching shallow water, tsunamis slow down to move in the range of 20 to 30 mph.

Since a tsunami involves a series of waves rather than a single wave, it's important to consider the period of time in between waves. They can continue impacting an area for multiple days.

• Depending on a variety of factors, the period between waves in a tsunami can range between five minutes and one hour.
• After the initial impact of a tsunami, another wave should be expected to follow within this timeframe.
• Tsunamis don't all have the same number of waves.

Tsunamis are referred to as local, regional or distant, based on whether they form fairly close to shore or far out in the ocean.

• They can all produce catastrophic results, but local ones are considered the most dangerous because the impact can be expected within an hour of detection.
• There is much less time to evacuate or otherwise prepare for the impact of a local tsunami versus one that would take longer to arrive.
• Regional tsunami impact is expected within one to three hours, while distant tsunami impact is expected in three or more hours.

While tsunamis first impact inland areas along the shoreline, their impact can be felt far inland. Major tsunamis can travel inland up to 10 miles, with flooding occurring for many miles beyond that. For example, in 2011 Japan was impacted by a tsunami that flooded a 200 square mile area along the coast.

Weather authorities around the world continually monitor for tsunami risk. Numerous tsunami warning systems monitor seismic activity and other factors that could lead to the formation of a dangerous tsunami. The goal is to be able to warn people who may be impacted by a tsunami as early as possible. The U.S. tsunami warning system is operated by the National Oceanic and Atmospheric Administration (NOAA)/National Weather Service (NWS).

When a tsunami risk is identified, weather authorities issue an appropriate notification. Depending on the situation, they may issue an advisory, a watch or a warning.

• watch - A tsunami watch is issued when a seismic event that could cause a tsunami has been detected. It does not mean that there is a confirmed tsunami. People should prepare to take action, as a tsunami may later threaten the area.
• advisory - A tsunami advisory is issued in situations where there is expected, imminent or current risk that a tsunami would make it dangerous to be in or close to the water, but without significant inundation. Officials typically close and evacuate beaches, harbors and marinas, as well as direct maritime traffic to deep sea.
• warning - A tsunami warning is issued for areas with expected, imminent or current risk of widespread inundation from a tsunami. In addition to evacuations associated with an advisory, low-lying areas along the coast are likely to be evacuated.

While tsunamis can occur in any ocean or sea (or other major body of water), they are most common in the Pacific Ocean. Nearly 75 percent of tsunamis occur in the Pacific. This is because there so much volcanic and seismic activity near the Pacific Ring of Fire.

Because most tsunamis occur in the Pacific Ocean, the U.S. states most at risk of experiencing a tsunami are ones that touch it. As a result, within the U.S., the greatest tsunami threat is to Oregon, California, Washington, Hawaii, and Alaska. This risk for these states is classified as "high or very high" by NOAA/NWS. Additionally, several U.S. territories are classified as "high" risk of tsunami impact due to activity in the Atlantic Ocean or Caribbean Sea. They are: American Samoa, Guam, the Northern Mariana Islands, the U.S. Virgin Islands, and Puerto Rico.

The deadliest known tsunami occurred in December of 2004. It originated in the Indian Ocean with a 9.1 magnitude undersea earthquake off the coast of Sumatra in Indonesia. Within 20 minutes of the powerful megathrust earthquake, the first of several 100+ foot waves hit the shoreline. Waves impacted different parts of the shoreline for eight hours, impacting Sumatra, Thailand, India, Sri Lanka, and South Africa. The first wave killed 100,000 people, with a final toll of nearly 230,00 lives lost.

If you live in an area at risk of a tsunami, it's very important to have an emergency plan that includes a plan for evacuating and maintaining communication with family and loved ones.

• Select an inland evacuation destination so there will be a designated spot to meet up with family members.
• Identify safe routes to get to that destination from home, school or work and practice getting there.
• Pack disaster supply kits to keep them in family vehicles and other locations so you'll be able to evacuate quickly if needed.
• Promptly heed all official warnings, as there is often little time to act when a tsunami is near.

Now that you're learned a few interesting facts about tsunamis, it's easy to see just how dangerous this type of event really is. Since most tsunamis are triggered by earthquakes, this is a great time to learn some earthquake facts. Not only can this help expand your understanding of tsunamis, but you'll also develop a deeper understanding of the natural world. If you like learning about interesting water movement, you might also want to explore hurricane facts.